A secondary battery and an electronic device

Abstract

A secondary battery and an electronic device, the secondary battery comprising an electrode assembly and an electrolyte, the electrode assembly comprising a positive electrode tab, a negative electrode tab, and a separator, the separator being disposed between the positive electrode tab and the negative electrode tab, the positive electrode tab comprising a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, the positive electrode active material layer comprising a positive electrode active material, the positive electrode active material comprising a transition metal element, the transition metal element comprising at least one of Co, Ni, Mn, Fe, or V; the separator comprising a base film, the base film having a thickness of D µm, 3 ≤ D ≤ 7. The secondary battery comprises an electrolyte containing a nitrile group additive and a positive electrode containing a nitrile group additive, the mass percentage content of the electrolyte containing a nitrile group additive being x% based on the mass of the electrolyte, the mass percentage content of the positive electrode containing a nitrile group additive being y% based on the mass of the positive electrode active material layer, x + y = z, 0.3 ≤ z ≤ 3.3 and 0 < x ≤ 2.3, 0 < y ≤ 1.

Description

Technical Field

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

[0002] The trend towards larger capacity and higher energy density in secondary batteries (such as lithium-ion batteries) is becoming increasingly strong. For current secondary batteries, the separator is an indispensable component, serving to isolate the positive and negative electrodes and prevent short circuits; however, the separator itself does not contribute to capacity. Therefore, separators are being developed to be increasingly thinner, as thinner separators are an effective measure to improve energy density. On the other hand, the output voltage of secondary batteries is also trending towards increasing, thereby improving capacity and energy density. Summary of the Invention

[0003] The inventors of this application discovered that in systems used in laptops, due to the series-parallel assembly of secondary battery modules, there are high requirements for the consistency of output voltage and capacity of individual secondary batteries within the module. If the output voltage of the secondary battery is large and a thin separator is used, the separator's function of isolating the positive and negative electrodes is weakened, leading to self-discharge within the secondary battery. This affects the consistency of the secondary battery, causing series voltage difference problems during the charging and discharging process of the secondary battery module, and reducing the cycle performance of the secondary battery.

[0004] The purpose of this application is to provide a secondary battery and an electronic device to improve the energy density of the secondary battery and alleviate the self-discharge problem during the charging and discharging process.

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

[0006] This application provides a secondary battery comprising an electrode assembly and an electrolyte. The electrode assembly includes a positive electrode, a negative electrode, and a separator, with the separator disposed between the positive and negative electrodes. The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer includes a positive active material comprising a transition metal element, including at least one of Co, Ni, Mn, Fe, or V. The separator includes a base film with a thickness of D μm, where 3 ≤ D ≤ 7. The secondary battery includes an electrolyte containing a nitrile-based additive and a positive electrode containing a nitrile-based additive. Based on the mass of the electrolyte, the mass percentage of the nitrile-based additive in the electrolyte is x%, and based on the mass of the positive active material layer, the mass percentage of the nitrile-based additive in the positive electrode is y%, where x + y = z, 0.3 ≤ z ≤ 3.3, 0 < x ≤ 2.3, and 0 < y ≤ 1.

[0007] This application improves the energy density of a secondary battery by using a separator with a thin base film. It also uses nitrile-based additives in both the electrolyte and the cathode, and controls the mass percentages of the nitrile-based additives in both the electrolyte and the cathode within the aforementioned ranges. The nitrile-based additives in the electrolyte and the cathode work synergistically with the separator to reduce the dissolution of transition metal ions (such as Co ions) inside the secondary battery under high voltage conditions, thereby controlling the diffusion resistance and improving the self-discharge problem during the charging and discharging process of the secondary battery.

[0008] In some embodiments of this application, 1.4 ≤ z ≤ 3.3. This further improves the self-discharge problem during the charging and discharging process of secondary batteries.

[0009] In some embodiments of this application, the carbon-to-nitrogen ratio of the electrolyte containing nitrile-based additives is m1, where 1 ≤ m1 ≤ 6. The electrolyte containing nitrile-based additives includes at least one of the following: malononitrile, butadionitrile, glutaronitrile, adiponitrile, heptanonitrile, octadionitrile, sebaconitrile, 3,3'-oxodipropionitrile, hex-2-enadionitrile, trans-butenedionitrile, 2-pentenedionitrile, methylglutaronitrile, 4-acrylonitrile heptanonitrile, (Z)-but-2-enadionitrile, 2,2,3,3-tetrafluorobutadionitrile, ethylene glycol bis(propionitrile) ether, 1,3,5-pentanetricarbonitrile, 1,3,6-hexanetricarbonitrile, 1,2,6-hexanetricarbonitrile, 1,2,3-tris(2-acrylonitrileoxy)propane, or 1,1,3,3-propanetetracarbonitrile. Using the above-mentioned types of electrolyte containing nitrile-based additives is beneficial for improving the self-discharge problem during the charging and discharging process of secondary batteries and exhibits better room-temperature cycling performance.

[0010] In some embodiments of this application, the positive electrode nitrile-containing additive is a nitrile-containing polymer, the carbon-nitrogen atom ratio of the positive electrode nitrile-containing additive is m2, 3≤m2≤5, and the weight-average molecular weight of the positive electrode nitrile-containing additive is M. w 300≤M w ≤700000. Controlling the carbon-nitrogen atom ratio and weight-average molecular weight of the nitrile-based additives in the positive electrode within the above range is beneficial to improving the self-discharge problem during the charging and discharging process of secondary batteries and can reduce the impact of the addition of nitrile-based additives in the positive electrode on the manufacturing performance of the positive electrode sheet.

[0011] In some embodiments of this application, the cathode nitrile-based additive includes at least one of polyacrylonitrile, polybutadiene nitrile, or polyisobutylene nitrile. Using the above-mentioned types of cathode nitrile-based additives is beneficial for improving the self-discharge problem during the charging and discharging process of secondary batteries.

[0012] In some embodiments of this application, the output voltage of the secondary battery is greater than or equal to 4.48V and less than or equal to 4.6V. Controlling the output voltage of the secondary battery within this range can better balance energy density performance and self-discharge risk.

[0013] In some embodiments of this application, the output voltage of the secondary battery is less than or equal to 4.56V.

[0014] In some embodiments of this application, 0.035D 2 -0.562D + 2.253 ≤ y ≤ 0.027D 2 -0.482D+2.257. By adjusting the relationship between the thickness of the base film and the mass percentage of the nitrile-based additive in the positive electrode, it is beneficial to improve the self-discharge problem during the charging and discharging process of secondary batteries.

[0015] In some embodiments of this application, the material of the base film includes a polyolefin, which includes at least one of polyethylene or polypropylene.

[0016] In some embodiments of this application, the diaphragm further includes an insulating layer disposed on the surface of the base membrane. The insulating layer includes inorganic particles and a binder. The inorganic particles include at least one of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. The binder includes at least one of styrene-butadiene rubber, polyacrylic acid, polyvinyl alcohol, polyethylene glycol, polyacrylate, polyvinylidene fluoride, polyvinyl chloride, formaldehyde resin, or cyanoacrylate.

[0017] In some embodiments of this application, the base film has a multi-layer structure. This is beneficial because the layer interfaces of the multi-layer structure hinder the shuttle movement of transition metal elements between the positive and negative electrodes, thereby reducing the risk of self-discharge.

[0018] In some embodiments of this application, the positive electrode active material includes at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese oxide, lithium iron phosphate, or lithium iron manganese phosphate.

[0019] A second aspect of this application provides an electronic device comprising the secondary battery described in any of the foregoing embodiments. Therefore, the electronic device exhibits good cycle performance.

[0020] The beneficial effects of this application are:

[0021] This application provides a secondary battery and an electronic device. The secondary battery includes an electrode assembly and an electrolyte. The electrode assembly includes a positive electrode, a negative electrode, and a separator. The separator is disposed between the positive and negative electrode. The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer includes a positive active material, which includes a transition metal element, including at least one of Co, Ni, Mn, Fe, or V. The separator includes a base film with a thickness of D μm, where 3 ≤ D ≤ 7. The secondary battery includes an electrolyte containing a nitrile-based additive and a positive electrode containing a nitrile-based additive. Based on the mass of the positive active material layer, the mass percentage of the electrolyte containing the nitrile-based additive is x%, and the mass percentage of the positive electrode containing the nitrile-based additive is y%, where x + y = z, 0.3 ≤ z ≤ 3.3, 0 < x ≤ 2.3, and 0 < y ≤ 1. With the above configuration, a secondary battery with high energy density can be obtained, and the self-discharge problem during the charging and discharging process of the secondary battery can be improved. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this application clearer, the following embodiments are provided to further illustrate this application in detail. Obviously, the described embodiments are merely some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments in this application are within the scope of protection of this application.

[0023] It should be noted that, in the specific embodiments of this application, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries in this application are not limited to lithium-ion batteries.

[0024] The inventors discovered that using high output voltages (e.g., above 4.48V) and thin separators (e.g., base film thickness of 3μm to 7μm) to increase the energy density of secondary batteries can easily lead to voltage drop issues during charging and discharging. Analysis suggests the primary cause is chemical self-discharge. Chemical self-discharge mainly results from reversible redox products shuttling between the positive and negative electrodes via the electrolyte and separator. Current analysis indicates that these shuttle couples originate from transition metal elements in the positive electrode active material, such as Co. 2+ / Co 3+ Cobalt (Co) is element number 27, and it is generally represented by Co. 2+ and Co 3+ Co exists in the form of d7 and d6 electron configurations, respectively. 2+ Compared to Co 3+ The outermost d orbital has an extra electron, and this electron can be drawn from Co. 2+ Transferred to Co 3+However, excessively long constant-voltage times result in a higher concentration of cobalt ions with varying valence states in the electrolyte, leading to electron transfer and electronic conductivity. Furthermore, higher output voltage leads to more byproducts within the secondary battery; thinner separators result in shorter diffusion paths between the positive and negative electrodes, while thicker separators result in longer diffusion paths, thus affecting diffusion resistance and causing self-discharge. Therefore, Co dissolution content and diffusion resistance are two primary factors influencing chemical self-discharge. The presence of chemical self-discharge affects the consistency of the secondary battery, causing voltage drop issues during charging and discharging, and reducing cycle performance. In light of this, this application provides a secondary battery and an electronic device.

[0025] This application provides a secondary battery comprising an electrode assembly and an electrolyte. The electrode assembly includes a positive electrode, a negative electrode, and a separator, with the separator disposed between the positive and negative electrodes. The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer includes a positive active material comprising a transition metal element, including at least one of Co, Ni, Mn, Fe, or V. The separator includes a base film with a thickness of D μm, where 3 ≤ D ≤ 7. The secondary battery includes a nitrile-based additive in the electrolyte and a nitrile-based additive in the positive electrode. Based on the mass of the electrolyte, the mass percentage of the nitrile-based additive in the electrolyte is x%; based on the mass of the positive active material layer, the mass percentage of the nitrile-based additive in the positive electrode is y%; x + y = z, 0.3 ≤ z ≤ 3.3 and 0 < x ≤ 2.3, 0 < y ≤ 1. For example, D is 3, 4, 5, 6, 7, or any value within any two of the above ranges. The value of z is 0.3, 1, 1.5, 2, 2.5, 3, 3.3, or any value within any two of the above ranges. The value of x is 0.01, 0.5, 1.0, 1.5, 2.0, 2.3, or any value within any two of the above ranges. The value of y is 0.01, 0.2, 0.4, 0.6, 0.8, 0.9, 1, or any value within any two of the above ranges.

[0026] Through extensive research, the inventors discovered that when the positive electrode active material includes at least one transition metal element from Co, Ni, Mn, Fe, or V, electron transfer between ions of different valence states of these transition metal elements causes electronic conductivity, making the secondary battery prone to self-discharge. The higher the output voltage of the secondary battery, the more transition metal elements dissolve. A thinner base film reduces the battery's volume, increasing its volumetric energy density, but this also increases chemical self-discharge due to shorter diffusion paths of byproducts between the positive and negative electrodes. Conversely, a thicker base film increases the diffusion paths of byproducts between the positive and negative electrodes, reducing chemical self-discharge, but this also decreases the battery's volumetric energy density. In contrast, the carbon-nitrogen triple bond in the nitrile group (-CN) has a high bond energy, making it less susceptible to oxidation. Furthermore, the nitrile group has strong coordination ability, allowing it to bind to active sites on the electrode surface (such as the aforementioned transition metal ions) to mask active ions, reducing the electrode's decomposition effect on the electrolyte. This, in turn, inhibits electrolyte decomposition under high output voltage and high temperature, reducing the generation of byproducts. In this application, high temperature means a temperature greater than or equal to 45°C, and high voltage means an output voltage greater than or equal to 4.48V.

[0027] This application improves the energy density of secondary batteries by using a separator with a thin base film. It also uses nitrile-based additives in both the electrolyte and the cathode. However, when the sum of the mass percentages z of the nitrile-based additives in the electrolyte and the cathode is less than 0.3, the mass percentages x% of the nitrile-based additives in the electrolyte or y% of the nitrile-based additives in the cathode are too small. This results in a low nitrile content, limiting its binding ability to active sites (such as the aforementioned transition metal ions) on the electrode surface. Consequently, the improvement effect on internal chemical self-discharge of the secondary battery is limited. Furthermore, the improvement effect on electrolyte decomposition is limited, and the content of byproducts generated after electrolyte decomposition is high, which will affect the performance of the secondary battery (such as cycle performance, storage performance, and safety performance). When the sum of the mass percentages (z) of nitrile-based additives in the electrolyte and the cathode is greater than 3.3, and the mass percentages (x%) of nitrile-based additives in the electrolyte or the mass percentages (y%) of nitrile-based additives in the cathode are too high, the high electronegativity of the nitrile functional groups means that the carbon atoms attached to them typically carry active α-H atoms. Excessive nitrile content leads to an excessive amount of free active α-H atoms in the electrolyte, accelerating the consumption of the negative electrode additive (such as fluoroethylene carbonate) and affecting the room-temperature cycle performance of the secondary battery. Furthermore, besides forming a stable film on the electrode surface, excessive nitrile-based additives in the electrolyte will continue to react, consuming active lithium and forming an excessively thick interfacial film, affecting the performance of the secondary battery, such as cycle performance and charge rate. Excessive nitrile-based additives in the cathode will affect the cold-pressing compaction density of the cathode sheet, causing a loss in the energy density of the secondary battery. During the cold pressing process, problems such as strip breakage or film detachment may also occur, rendering the cathode sheet unusable.

[0028] This application controls the thickness of the separator, the mass percentage of nitrile-based additives in the electrolyte, and the mass percentage of nitrile-based additives in the cathode within the above-mentioned ranges. The nitrile-based additives in the electrolyte and the cathode work synergistically with the separator to reduce the dissolution of transition metal ions (such as Co ions) inside the secondary battery and to regulate the diffusion resistance. This improves the self-discharge problem during the high output voltage and high temperature charging and discharging process of the secondary battery.

[0029] In this application, unless otherwise specified, the electrolyte containing nitrile-based additives is added to the electrolyte, and the positive electrode containing nitrile-based additives is added to the positive electrode active material layer. The positive electrode containing nitrile-based additives are nitrile-based polymers.

[0030] The aforementioned "positive electrode active material layer disposed on at least one surface of the positive electrode current collector" refers to a positive electrode active material layer disposed on one surface of the positive electrode current collector, or a positive electrode active material layer disposed on two surfaces of the positive electrode current collector. Here, "surface" can be part or all of the surface of the positive electrode current collector. In some embodiments of this application, other material layers, such as adhesive layers, safety layers, and other active material layers, may also be disposed between the positive electrode active material layer and the positive electrode current collector.

[0031] In some embodiments of this application, secondary batteries can be assembled into secondary battery modules via series and parallel connections. With improvements in the self-discharge problem during the charging and discharging process of the secondary batteries, the consistency of output voltage and capacity among the individual secondary batteries in the secondary battery module can be enhanced. Consequently, the series voltage difference problem during the charging and discharging process of the secondary battery module is improved, and the cycle performance of the secondary batteries under high voltage and high temperature is enhanced.

[0032] This application does not impose any particular limitation on the number of secondary batteries contained in the secondary battery module; there can be two or more. The specific number can be selected by those skilled in the art based on the application and capacity of the secondary battery module, as long as the purpose of this application is achieved. This application also does not impose any particular limitation on the arrangement of the secondary batteries in the secondary battery module, as long as the purpose of this application is achieved. For example, multiple secondary batteries can be arranged sequentially along the length of the secondary battery module. Of course, other arrangements are also possible.

[0033] In some embodiments of this application, 1.4 ≤ z ≤ 3.3. Controlling the sum of the mass percentages z of the electrolyte-containing nitrile-based additive and the cathode-containing nitrile-based additive within the above range will be more beneficial in improving the self-discharge problem during the charging and discharging process of the secondary battery.

[0034] In some embodiments of this application, the ratio of carbon to nitrogen atoms in the nitrile-based additive in the electrolyte is m1, where 1 ≤ m1 ≤ 6. For example, m1 is 1, 2, 3, 4, 5, 6, or any value between any two of the above ranges. Furthermore, the electrolyte containing nitrile-based additives includes at least one of malononitrile, butadionitrile, glutaronitrile, adiponitrile, heptanonitrile, octadionitrile, sebaconitrile, 3,3'-oxodipropionitrile, hex-2-enadionitrile, trans-butenedionitrile, 2-pentenedionitrile, methylglutaronitrile, 4-acrylonitrile heptanonitrile, (Z)-but-2-enadionitrile, 2,2,3,3-tetrafluorobutadionitrile, ethylene glycol bis(propionitrile) ether, 1,3,5-pentanetricarbonitrile, 1,3,6-hexanetricarbonitrile, 1,2,6-hexanetricarbonitrile, 1,2,3-tris(2-acrylonitrile)propane (also known as 1,2,3-tris(2-cyanoxy)propane), or 1,1,3,3-propanetetracarbonitrile. The aforementioned types of electrolyte additives containing nitrile groups possess suitable carbon-to-nitrogen atom ratios and exhibit good stability. Using these additives facilitates the binding of nitrile groups to active sites on the electrode surface, reducing the dissolution of transition metal ions (such as Co ions) within the secondary battery and thus improving self-discharge during the charging and discharging process of the electrode assembly. Consequently, the consistency of output voltage and capacity among the individual secondary batteries in the secondary battery module is improved, the series voltage difference problem during charging and discharging is mitigated, and the cycle performance of the secondary battery under high voltage and high temperature is enhanced.

[0035] In some embodiments of this application, the carbon-to-nitrogen atom ratio of the nitrile-based additive in the positive electrode is m², where 3 ≤ m² ≤ 5, and the weight-average molecular weight of the nitrile-based additive in the positive electrode is M. w 300≤M w ≤700000. For example, m2 is 3, 4, 5, or any value between any two of the above ranges. M wThe values ​​are 300, 1000, 10000, 100000, 200000, 300000, 400000, 500000, 600000, 700000, or any value between any two of the above ranges. When the carbon-to-nitrogen atom ratio of the nitrile-based additive in the positive electrode is within the above range, and the nitrile-based additive has an appropriate amount of polar functional groups, it is easily soluble in organic solvents (such as N-methylpyrrolidone), which is beneficial for uniform distribution in the positive electrode sheet. Furthermore, the number of transition metal ions interacting with the nitrile functional groups will be reduced, and the transition metal ions are less likely to undergo side reactions with the electrolyte, thereby improving the high-temperature performance of the secondary battery. By controlling the carbon-to-nitrogen ratio and weight-average molecular weight of the nitrile-containing additive in the positive electrode within the aforementioned range, the solubility of the nitrile-containing polymer in the electrolyte is reduced, while its solubility in organic solvents, such as N-methylpyrrolidone, is improved. This ensures uniform mixing of the nitrile-containing polymer with the positive electrode active material, facilitating the binding of the nitrile groups in the additive to the active sites on the electrode surface. This reduces the dissolution of transition metal ions (such as Co ions) within the secondary battery, minimizes side reactions between the positive electrode active material and the electrolyte, and lowers the content of byproducts generated by the electrolyte. This improves the self-discharge problem during the charging and discharging process of the secondary battery. Consequently, the consistency of output voltage and capacity among the individual secondary batteries in the secondary battery module is improved, the series voltage difference problem during the charging and discharging process is mitigated, and the cycle performance of the secondary battery under high voltage and high temperature is enhanced.

[0036] This application does not impose any particular restrictions on the method of controlling the weight-average molecular weight of the cathode nitrile-based additive, as long as the purpose of this application can be achieved. For example, it can be achieved by directly purchasing cathode nitrile-based additives with different weight-average molecular weights from manufacturers.

[0037] In some embodiments of this application, the nitrile-containing additive in the positive electrode includes polyacrylonitrile (molecular formula: (C3H3N)). n Polybutadiene acrylonitrile (molecular formula: (C4H5N)) n ) or polyisobutylene nitrile (molecular formula: (C4H5N) n At least one of the above-mentioned positive electrode nitrile-based additives can be used to further improve the self-discharge problem during the charging and discharging process of secondary batteries. This improves the consistency of voltage and capacity among the individual secondary batteries in the secondary battery module, alleviates the series voltage difference problem during the charging and discharging process, and enhances the cycle performance of the secondary battery under high voltage and high temperature.

[0038] In some embodiments of this application, the output voltage of the secondary battery is greater than or equal to 4.48V and less than or equal to 4.6V. In some embodiments of this application, the output voltage of the secondary battery is less than or equal to 4.56V. With the output voltage within the above range, the secondary battery has a high energy density, and, in conjunction with the separator, electrolyte containing nitrile-based additives, and positive electrode containing nitrile-based additives of this application, its self-discharge problem during charging and discharging can be improved.

[0039] This application does not impose any particular restrictions on the method of regulating the output voltage of the secondary battery, as long as the purpose of this application can be achieved. The output voltage of the secondary battery can be regulated by adjusting the positive and negative electrode materials, electrolyte, etc. For example, the output voltage of the secondary battery can be regulated by controlling the doping and / or coating of the positive electrode active material lithium cobalt oxide, or by controlling the type of positive electrode active material. For example, the output voltage can be increased by doping lithium cobalt oxide with elements such as Mg, Al, Ti, etc., and these materials can be purchased on the market.

[0040] Furthermore, 0.035D 2 -0.562D + 2.253 ≤ y ≤ 0.027D 2 -0.482D+2.257. By adjusting the relationship between the thickness of the base film and the mass percentage of the nitrile-based additive in the positive electrode, a synergistic effect between the base film and the nitrile-based additive in the positive electrode is achieved. This reduces the dissolution of transition metal ions (such as Co ions) inside the secondary battery, regulates the diffusion resistance, and thus improves the self-discharge problem during the charging and discharging process of the secondary battery. Consequently, the consistency of voltage and capacity among the individual secondary batteries in the secondary battery module is improved, the series voltage difference problem during the charging and discharging process of the secondary battery module is mitigated, and the cycle performance of the secondary battery under high voltage and high temperature is enhanced.

[0041] In some embodiments of this application, the material of the base film includes a polyolefin, which includes at least one of polyethylene (PE) or polypropylene (PP).

[0042] In some embodiments of this application, the base membrane is a multi-layer structure. This application does not impose any particular limitation on the number of layers in the multi-layer structure, as long as the purpose of this application can be achieved. In some embodiments, the base membrane in the multi-layer structure has at least two layers, for example, two or three layers. When the base membrane is a multi-layer structure, different types of materials can be combined to leverage the unique properties of each material. For example, the base membrane includes, but is not limited to, PP / PE / PP and PP / PE.

[0043] This application does not impose any particular limitation on the porosity of the base membrane, as long as it achieves the purpose of this application. For example, the porosity of the base membrane can be between 25% and 60%.

[0044] In some embodiments of this application, to improve the strength, insulation performance, and obstruction of transition metal element shuttle of the diaphragm, the diaphragm further includes an insulating layer disposed on the surface of the base film. The insulating layer is disposed on at least one surface of the base film along its thickness direction. It is understood that in some embodiments of this application, the insulating layer is disposed on one surface of the base film along its thickness direction; in other embodiments, the insulating layer is disposed on two surfaces of the base film along its thickness direction. The insulating layer comprises inorganic particles and a binder. This application does not particularly limit the type of inorganic particles, as long as they can achieve the purpose of this application. For example, the inorganic particles include at least one of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. This application does not particularly limit the type of binder, as long as it can achieve the purpose of this application. For example, the adhesive includes at least one of styrene-butadiene rubber, polyacrylic acid, polyvinyl alcohol, polyethylene glycol, polyacrylate, polyvinylidene fluoride, polyvinyl chloride, formaldehyde resin or cyanoacrylate.

[0045] This application does not impose any particular limitation on the average volume particle size of the aforementioned inorganic particles. Those skilled in the art can select the appropriate size based on actual needs, as long as the purpose of this application can be achieved.

[0046] This application does not impose any particular limitation on the thickness of the insulating layer, as long as it achieves the purpose of this application. For example, the thickness of the insulating layer can be from 0.2 μm to 3 μm.

[0047] In some embodiments of this application, the positive electrode active material includes at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese oxide, lithium iron phosphate, or lithium iron manganese phosphate. Using the above-mentioned types of positive electrode active materials is beneficial for achieving higher capacity in the secondary battery.

[0048] This application does not impose any particular limitation on the type of positive electrode current collector, as long as it achieves the purpose of this application. For example, the positive electrode current collector may include aluminum foil, aluminum alloy foil, etc. In this application, the positive electrode active material may also contain non-metallic elements, such as at least one of fluorine, phosphorus, boron, chlorine, silicon, or sulfur, which can further improve the stability of the positive electrode active material. In this application, there is no particular limitation on the thickness of the positive electrode current collector and the positive electrode active material layer, as long as it achieves the purpose of this application. For example, the thickness of the positive electrode current collector is 5 μm to 20 μm, preferably 6 μm to 18 μm. The thickness of the positive electrode active material layer is 30 μm to 120 μm.

[0049] Optionally, the positive electrode active material layer may further include a positive electrode conductive agent and a positive electrode binder. There are no particular limitations on the types of the positive electrode conductive agent and the positive electrode binder in the positive electrode active material layer in the present application, as long as the objects of the present application can be achieved. There are no particular limitations on the mass ratio of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder in the positive electrode active material layer in the present application, and those skilled in the art can select according to actual needs as long as the objects of the present application can be achieved. For example, the mass ratio of the positive electrode active material, the positive electrode nitrile group-containing additive, the positive electrode conductive agent, and the positive electrode binder in the positive electrode active material layer is (90.00 - 99.00):(0.001 - 1):(0.495 - 4.75):(0.495 - 4.75).

[0050] There are no particular limitations on the negative electrode sheet in the present application, as long as the objects of the present application can be achieved. For example, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer. There are no particular limitations on the negative electrode current collector in the present application, as long as the objects of the present application can be achieved. For example, the negative electrode current collector may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam or copper foam, etc. The negative electrode active material layer of the present application includes a negative electrode active material. There are no particular limitations on the type of the negative electrode active material in the present application, as long as the objects of the present application can be achieved. For example, the negative electrode active material may include natural graphite, artificial graphite, mesocarbon microbeads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO x (0 < x < 2), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel-structured lithium titanate Li4Ti5O 12 、Li-Al alloy or metallic lithium, etc. At least one of them. In the present application, there are no particular limitations on the thickness of the negative electrode current collector and the negative electrode active material layer, as long as the objects of the present application can be achieved. For example, the thickness of the negative electrode current collector is 6 μm to 10 μm, and the thickness of the negative electrode active material layer is 30 μm to 130 μm. Optionally, the negative electrode active material layer may further include at least one of a negative electrode conductive agent, a thickener, and a negative electrode binder. There are no particular limitations on the types of the negative electrode conductive agent, the stabilizer, and the negative electrode binder in the negative electrode active material layer in the present application, as long as the objects of the present application can be achieved. There are no particular limitations on the mass ratio of the negative electrode active material, the negative electrode conductive agent, the thickener, and the negative electrode binder in the negative electrode active material layer in the present application, as long as the objects of the present application can be achieved. For example, the mass ratio of the negative electrode active material, the negative electrode conductive agent, the thickener, and the negative electrode binder in the negative electrode active material layer is (97 - 98):(0.5 - 1.5):(0.5 - 1.5):(1.0 - 1.9).

[0051] The electrolyte of this application also includes lithium salts and non-aqueous solvents. The lithium salts may include at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), or lithium difluoroborate. This application does not have particular limitations on non-aqueous solvents, as long as they achieve the purpose of this application; for example, they may include, but are not limited to, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. The aforementioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorocarbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (MEC). The aforementioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (also known as propylene carbonate, abbreviated PC), butyl carbonate (BC), or vinyl ethylene carbonate (VEC). Fluorinated carbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. Other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. This application does not impose any particular limitation on the content of lithium salt and non-aqueous solvents in the electrolyte, as long as the purpose of this application is achieved.

[0052] The secondary battery of this application also includes a packaging bag, in which the electrolyte and electrode assembly are contained. This application does not impose any particular limitation on the packaging bag; it can be any packaging bag known in the art, as long as it achieves the purpose of this application.

[0053] This application does not impose any particular limitation on the type of secondary battery, which may include any device in which an electrochemical reaction occurs. For example, secondary batteries may include, but are not limited to: lithium metal secondary batteries, lithium-ion secondary batteries (lithium-ion batteries), sodium-ion secondary batteries (sodium-ion batteries), lithium polymer secondary batteries, and lithium-ion polymer secondary batteries.

[0054] A second aspect of this application provides an electronic device comprising the secondary battery described in any of the foregoing embodiments. Therefore, the electronic device exhibits good cycle performance.

[0055] The electronic device described in this application is not particularly limited and can be any electronic device known in the prior art. For example, the electronic device may include, but is not limited to: laptop computers, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, household large-capacity batteries, and lithium-ion capacitors.

[0056] Example

[0057] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below.

[0058] Test methods and equipment:

[0059] Test method for base film thickness D:

[0060] The cross-section of the diaphragm in each embodiment or comparative example was obtained by scanning electron microscopy (SEM). The thickness of the base film was measured in the SEM image. Five points were randomly selected from each image, and the average thickness measured at the five points was calculated as the thickness D of the base film.

[0061] Output voltage testing method:

[0062] (1) The lithium-ion batteries of each embodiment and comparative example were first fully discharged. The discharge process was as follows: constant current discharge at 0.2C to 3.0V, and then left to stand for 30 minutes; constant current discharge at 0.2C to 3.0V (C is the working capacity of the lithium-ion battery);

[0063] (2) Charge with a current of 0.05C for 20 hours and record the voltage V at this time, which is the output voltage.

[0064] Test method for the mass percentage (x%) of nitrile-based additives in electrolytes:

[0065] Cut open the top seal of the lithium-ion battery, invert it and place it in a centrifuge tube, then centrifuge to remove the electrolyte; disassemble the lithium-ion battery to obtain the positive electrode sheet and weigh it, taking the mass M1 (g) of the positive electrode sheet.

[0066] The electrolyte obtained by centrifugation was serially diluted in a container with a fixed volume of V1 (mL) to different gradients, with a concentration range of 0.1 mg / L to 10 mg / L (the ratio of two adjacent concentrations is high concentration: low concentration = V1:1). The concentration n1 mg / mL in the sample was measured using gas chromatography-mass spectrometry. The content of nitrile additive in the electrolyte can be calculated as x = n1 × V1 / M1.

[0067] Test method for the mass percentage (y%) of nitrile-based additives in the positive electrode:

[0068] A 2cm × 2cm sample was cut from the positive electrode obtained by disassembling a lithium-ion battery, where 'a' is an integer. The sample mass M2 (g) was weighed and washed three times with dimethyl carbonate (DMC) to remove free lithium salts and soluble components from the solid electrolyte interphase (SEI) film. The sample was then placed in N-methylpyrrolidone and stirred for 5 hours. The mixture was filtered, and the supernatant was collected. The supernatant was separated by chromatography to obtain a dilute solution of pure-phase positive electrode containing nitrile-based additives.

[0069] The above-mentioned pure phase cathode nitrile-based additive dilute solution was serially diluted in a container with a fixed volume of V2 (mL) to different gradients, with a concentration range of 0.1 mg / L to 10 mg / L (the ratio of two adjacent concentrations is high concentration: low concentration = V1:1). The concentration n2 mg / mL in the sample was measured using gas chromatography-mass spectrometry, and the content of cathode nitrile-based additive y = n2 × V2 / M2 could be calculated.

[0070] Method for detecting the carbon-to-nitrogen atom ratio m1:

[0071] The carbon-to-nitrogen atom ratio m1 was detected using an inductively coupled plasma spectrometer (ICP).

[0072] Methods for detecting the carbon-to-nitrogen atom ratio m2:

[0073] The molar ratio of carbon (C) to nitrogen (N) in the nitrile-containing polymer was determined to be m2 using a typical elemental analysis method.

[0074] Weight-average molecular weight M w Detection methods:

[0075] The weight-average molecular weight M was determined using a laser light scattering instrument. W .

[0076] Cyclic performance testing methods:

[0077] The lithium-ion batteries of each embodiment and comparative example were placed in a 45°C constant temperature chamber and left to stand for 30 minutes to allow them to reach a constant temperature. The lithium-ion batteries that had reached a constant temperature were then charged at a constant current rate of 1C to 4.2V at 45°C, followed by constant voltage charging at 4.2V to 0.05C, left to stand for 5 minutes, and then discharged at a constant current rate of 0.7C to 3.0V; this constitutes one charge-discharge cycle. The capacity retention rate of the lithium-ion battery after 100 charge / discharge cycles was calculated. During the test, after every 50 cycles, the lithium-ion battery underwent an additional low-rate charge-discharge cycle to correct for the discharge behavior. During the low-rate charge-discharge cycle, all parameters were the same as the above charge-discharge cycle process, except for the 0.2C constant current discharge.

[0078] The capacity retention rate of a lithium-ion battery after cycling is calculated using the following formula: Capacity retention rate = Discharge capacity at 100th cycle / Initial discharge capacity × 100%. The capacity retention rate of the lithium-ion battery was measured through three sets of parallel experiments, and the average value of the three sets of parallel experimental data was taken as the capacity retention rate of the lithium-ion battery.

[0079] Test method for maximum differential pressure during circulation:

[0080] Preparation of lithium-ion battery modules:

[0081] Four lithium-ion batteries from each embodiment or comparative example are connected in series to obtain a lithium-ion battery module.

[0082] When the lithium-ion battery module in each embodiment and comparative example is charged to the end during the cycle, the voltage difference (V1-V2) between the highest voltage V1 and the lowest voltage V2 of the lithium-ion battery is recorded as the maximum cycle voltage difference.

[0083] Example 1-1

[0084] <Preparation of the positive electrode>

[0085] The positive electrode active material is lithium cobalt oxide, the conductive agent is conductive carbon black (Super P), and the positive electrode additive containing nitrile groups is polyacrylonitrile (m2=3, M). w =300000), binder polyvinylidene fluoride (PVDF, weight average molecular weight 800000 to 1200000) were mixed at a mass ratio of 94.43:2:1:2.57, and N-methylpyrrolidone (NMP) was added as a solvent. The mixture was stirred under vacuum until a homogeneous positive electrode slurry with a solid content of 70 wt% was obtained. The positive electrode slurry was then mixed at 97.38 g / m 2The coating material is uniformly applied to one surface of a 12μm thick aluminum foil for the positive electrode current collector. The foil is then dried at 120℃ for 1 hour to obtain a positive electrode sheet with a single-sided coating of the positive electrode active material layer (110μm thick). The above steps are then repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of the positive electrode active material layer. After cold pressing, cutting, and welding of tabs, a positive electrode sheet with dimensions of 74mm × 867mm is obtained.

[0086] <Preparation of Negative Electrode Sheets>

[0087] The negative electrode active material artificial graphite, conductive agent Super P, binder polyacrylic acid (weight average molecular weight of 5,000,000 to 14,000,000), and thickener sodium carboxymethyl cellulose (weight average molecular weight of 10,000,000 to 15,000,000) were mixed in a mass ratio of 83.5:10:5:1.5. Then, deionized water was added as a solvent, and the mixture was stirred under vacuum until a negative electrode slurry with a solid content of 75 wt% and a homogeneous system was obtained.

[0088] Polyacrylic acid (weight average molecular weight of 8,000,000) and conductive agent Super P were mixed at a mass ratio of 50:50. Then, deionized water was added as a solvent, and the mixture was stirred under vacuum until a base coating slurry with a solid content of 75 wt% and a uniform system was obtained.

[0089] First, a base coating slurry is applied to one surface of a 12μm thick copper foil used as the negative electrode current collector. This is then dried at 120℃ to obtain a negative electrode sheet with a 30μm thick base coating on one side. The above steps are repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided base coating. Next, a negative electrode slurry is applied to one surface of the copper foil with the base coating and dried at 120℃ to obtain a negative electrode sheet with a 130μm thick base coating on one side, containing a negative electrode active material layer. This process is repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided base coating. After cold pressing, cutting, and welding of tabs, a negative electrode sheet with dimensions of 76mm × 851mm is obtained for later use.

[0090] <Preparation of Electrolyte>

[0091] In a dry argon atmosphere, ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) were mixed in a mass ratio of 1:1:1 to obtain a basic organic solvent. Lithium hexafluorophosphate (LiPF6) was then added to the basic organic solvent, dissolved, and mixed thoroughly. Finally, malononitrile, a nitrile-based additive for the electrolyte, was added to obtain the electrolyte. Based on the mass of the electrolyte, the mass percentage of LiPF6 was 12.5%, the mass percentage of malononitrile was x = 2.3%, and the remainder was the basic organic solvent.

[0092] <Septum>

[0093] A 4μm thick polyethylene-based membrane (manufacturer: Zhuogao New Materials Technology Co., Ltd.) was used as the separator.

[0094] <Preparation of Lithium-ion Batteries>

[0095] The negative electrode, separator, and positive electrode prepared above are stacked and wound sequentially to obtain a wound electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, degassing, and edge trimming, a lithium-ion battery is obtained. The formation conditions are constant current charging at 0.02C to 3.3V, and then constant current charging at 0.1C to 4.5V.

[0096] Examples 1-2 to Examples 1-14

[0097] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0098] In the <Preparation of Positive Electrode Sheet>, the mass percentage of the positive electrode containing nitrile-based additives changes, and the mass percentage of the positive electrode active material changes accordingly, while the other components remain unchanged. The sum of the mass percentages of the positive electrode active material, conductive agent, positive electrode containing nitrile-based additives, and binder is 100%.

[0099] In the preparation of the electrolyte, the mass percentage of the nitrile-based additive in the electrolyte changes, the mass percentage of the basic organic solvent changes accordingly, and the other components remain unchanged. The sum of the mass percentages of the nitrile-based additive, the basic organic solvent, and the lithium salt in the electrolyte is 100%.

[0100] Examples 1-15

[0101] Except for the preparation of the positive electrode sheet, in which zinc oxide-coated lithium cobalt oxide is selected as the positive electrode active material (the mass of zinc oxide is 1 wt% of the positive electrode active material), and the output voltage of the secondary battery is adjusted to 4.5V, the rest is the same as in Examples 1-4.

[0102] Examples 1-16

[0103] Except for the preparation of the positive electrode sheet, in which Mg, Al, and Ti doped lithium cobalt oxide is selected as the positive electrode active material (the mass of each of Mg, Al, and Ti is 0.1 wt% of the positive electrode active material), and the output voltage of the secondary battery is adjusted to 4.56 V, the rest is the same as in Examples 1-4.

[0104] Examples 1-17

[0105] Except for the preparation of the positive electrode sheet, in which Mg-doped Se-coated lithium cobalt oxide is selected as the positive electrode active material (the molar amount of Mg is 1 mol of the positive electrode active material and the mass of Se is 0.2 wt% of the positive electrode active material), and the output voltage of the secondary battery is adjusted to 4.6V, the rest is the same as in Examples 1-4.

[0106] Examples 1-18 to Examples 1-26

[0107] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0108] In the <Preparation of Positive Electrode Sheet>, the mass percentage of the positive electrode containing nitrile-based additives changes, and the mass percentage of the positive electrode active material changes accordingly, while the other components remain unchanged. The sum of the mass percentages of the positive electrode active material, conductive agent, positive electrode containing nitrile-based additives, and binder is 100%.

[0109] In the preparation of the electrolyte, the mass percentage of the nitrile-based additive in the electrolyte changes, the mass percentage of the basic organic solvent changes accordingly, and the other components remain unchanged. The sum of the mass percentages of the nitrile-based additive, the basic organic solvent, and the lithium salt in the electrolyte is 100%.

[0110] Examples 2-1 to 2-6

[0111] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as in Examples 1-4.

[0112] Comparative Examples 1 to 7

[0113] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0114] In the <Preparation of Positive Electrode Sheet>, the mass percentage of the positive electrode containing nitrile-based additives changes, and the mass percentage of the positive electrode active material changes accordingly, while the other components remain unchanged. The sum of the mass percentages of the positive electrode active material, conductive agent, positive electrode containing nitrile-based additives, and binder is 100%.

[0115] In the preparation of the electrolyte, the mass percentage of the nitrile-based additive in the electrolyte changes, the mass percentage of the basic organic solvent changes accordingly, and the other components remain unchanged. The sum of the mass percentages of the nitrile-based additive, the basic organic solvent, and the lithium salt in the electrolyte is 100%.

[0116] The preparation and performance parameters of each embodiment and comparative example are shown in Table 1 and Table 2.

[0117] Table 1

[0118]

[0119]

[0120] Note: In Table 1, “\” indicates that there are no relevant preparation parameters.

[0121] As can be seen from Examples 1-1 to 1-26 and Comparative Examples 1 to 7, when the output voltage and the base film thickness of the separator in the secondary battery are within the range of this application, the secondary batteries in the examples improve the output voltage, use a separator with a thin base film, and introduce electrolyte-containing nitrile-based additives and positive electrode-containing nitrile-based additives. Furthermore, by controlling the mass percentage of electrolyte-containing nitrile-based additives, the mass percentage of positive electrode-containing nitrile-based additives, and the sum of the mass percentages of electrolyte-containing nitrile-based additives and positive electrode-containing nitrile-based additives within the range of this application, the secondary batteries exhibit higher 45°C cycle capacity retention and lower maximum voltage difference during 45°C cycles. This indicates that the self-discharge problem during the charging and discharging process of the secondary battery is improved, the consistency of voltage and capacity among the secondary batteries in the secondary battery module is improved, the series voltage difference of the secondary battery module is smaller, the series voltage difference problem during the charging and discharging process of the secondary battery module is improved, and the cycle performance of the secondary battery under high voltage and high temperature is improved. In contrast, when the secondary battery in the comparative example did not introduce a nitrile-based additive in the positive electrode or a nitrile-based additive in the electrolyte, or when at least one of x, y, or z was not within the scope of this application, the secondary battery exhibited a lower 45°C cycle capacity retention rate and a higher maximum 45°C cycle voltage difference. This indicates that the secondary battery has a significant self-discharge problem during charging and discharging, the voltage and capacity consistency between individual secondary batteries in the secondary battery module is worse, and the series voltage difference of the secondary battery module is larger.

[0122] As can be seen from Examples 1-4, 1-15 to 1-17, when the output voltage of the secondary battery of this application is within the range of this application, it has a high 45°C cycle capacity retention rate and a low 45°C cycle maximum voltage difference. This indicates that the self-discharge problem during the charging and discharging process of the secondary battery is improved, the consistency of voltage and capacity among the secondary batteries in the secondary battery module is improved, the series voltage difference problem during the charging and discharging process of the secondary battery module is improved, and the secondary battery has good cycle performance under high pressure and high temperature.

[0123] The mass percentage y of the positive electrode containing nitrile-based additives satisfies: 0.035D 2 -0.562D + 2.253 ≤ y ≤ 0.027D 2 When the content is -0.482D + 2.257, the self-discharge problem of the secondary battery during charging and discharging at high output voltage and high temperature can be further improved. From Examples 1-1 to 1-14 and Examples 1-18 to 1-26, it can be seen that the mass percentage y of the nitrile-based additive in the positive electrode satisfies: 0.035D. 2-0.562D + 2.253 ≤ y ≤ 0.027D 2 At -0.482D+2.257, the secondary battery exhibits higher capacity retention at 45℃ and lower maximum voltage difference at 45℃, indicating that the self-discharge problem during the charging and discharging process of the secondary battery has been improved, the consistency of voltage and capacity among the secondary batteries in the secondary battery module has been enhanced, the series voltage difference of the secondary battery module is smaller, the series voltage difference problem during the charging and discharging process of the secondary battery module has been improved, and the cycle performance of the secondary battery under high voltage and high temperature has been further improved.

[0124] Table 2

[0125]

[0126] The type of nitrile-based additive in the electrolyte typically affects the cycle performance of secondary batteries. As can be seen from Examples 1-4, 2-1, and 2-2, secondary batteries using nitrile-based additives in the electrolyte within the scope of this application exhibit higher 45°C cycle capacity retention, and the modules composed of these batteries show lower maximum voltage drop at 45°C. This indicates that the self-discharge problem during the charge and discharge process of the secondary battery is improved, the consistency of voltage and capacity among the individual secondary batteries in the secondary battery module is enhanced, the series voltage drop problem during the charge and discharge process of the secondary battery module is improved, and the secondary battery exhibits good cycle performance under high voltage and high temperature.

[0127] The type of nitrile-based additive in the cathode typically affects the cycle performance of a secondary battery. As can be seen from Examples 1-4, 2-3, and 2-4, the type of nitrile-based additive changes when the carbon-to-nitrogen ratio of the cathode nitrile-based additive changes accordingly. Secondary batteries using nitrile-based additives within the scope of this application exhibit higher 45°C cycle capacity retention and lower maximum voltage drop at 45°C in their constituent secondary battery modules. This indicates that the self-discharge problem during charging and discharging is improved, the voltage and capacity consistency among the individual secondary batteries in the module is enhanced, the series voltage drop problem during charging and discharging is mitigated, and the secondary battery exhibits good cycle performance under high voltage and high temperature.

[0128] The weight-average molecular weight of the cathode nitrile-based additive typically affects the cycle performance of secondary batteries. As can be seen from Examples 1-4, 2-5, and 2-6, secondary batteries using cathode nitrile-based additives with a weight-average molecular weight within the range of this application exhibit higher 45°C cycle capacity retention and lower maximum voltage drop at 45°C in their constituent secondary battery modules. This indicates that the self-discharge problem during charging and discharging is improved, the voltage and capacity consistency among individual secondary batteries in the module is enhanced, the series voltage drop problem during charging and discharging is mitigated, and the secondary battery demonstrates good cycle performance under high voltage and high temperature.

[0129] It should be noted that, in this document, the terms “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0130] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0131] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A secondary battery comprising an electrode assembly and an electrolyte, the electrode assembly comprising a positive electrode tab, a negative electrode tab, and a separator, the separator being disposed between the positive electrode tab and the negative electrode tab, the positive electrode tab comprising a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, the positive electrode active material layer comprising a positive electrode active material, the positive electrode active material comprising a transition metal element, the transition metal element comprising at least one of Co, Ni, Mn, Fe, or V; the separator comprising a base film, the base film having a thickness of D μm, 3 ≤ D ≤ 7; the secondary battery comprising an electrolyte nitrile group-containing additive and a positive electrode nitrile group-containing additive, the mass percentage content of the electrolyte nitrile group-containing additive being x% based on the mass of the electrolyte, the mass percentage content of the positive electrode nitrile group-containing additive being y% based on the mass of the positive electrode active material layer, x + y = z, 0.3 ≤ z ≤ 3.3, and 0 < x ≤ 2.3, 0 < y ≤ 1.

2. The secondary battery according to claim 1, wherein 1.4≤z≤3.3。 3. The secondary battery according to claim 1, wherein the electrolyte nitrile group-containing additive having a carbon-nitrogen atom number ratio of m1, 1 ≤ m1 ≤ 6, the electrolyte nitrile group-containing additive comprising at least one of malononitrile, succinonitrile, glutaronitrile, adiponitrile, pimelonitrile, suberonitrile, sebonitrile, 3,3'-oxydipropionitrile, hex-2-enedinitrile, crotononitrile, 2-pentenenitrile, methylglutaronitrile, 4-cyanomethylpentanenitrile, (Z)-but-2-enedinitrile, 2,2,3,3-tetrafluorosuberonitrile, ethyleneglycol bis(propionitrile) ether, 1,3,5-pentanetricarbonitrile, 1,3,6-hexanetricarbonitrile, 1,2,6-hexanetricarbonitrile, 1,2,3-tris(2-cyanato)propane, or 1,1,3,3-propanetetracarbonitrile.

4. The secondary battery according to claim 1, wherein The positive electrode contains a nitrile group-containing additive, which is a nitrile group-containing polymer, has a carbon-nitrogen atom number ratio of m2,3≤m2≤5, and has a weight average molecular weight of M w , 300≤M w ≤700000.

5. The secondary battery according to claim 4, wherein the positive electrode nitrile group-containing additive comprising at least one of polyacrylonitrile, polybutyronitrile, or polyisobutyronitrile.

6. The secondary battery according to claim 1, wherein the secondary battery having an output voltage greater than or equal to 4.48 V and less than or equal to 4.6 V.

7. The secondary battery according to claim 6, wherein the secondary battery having an output voltage less than or equal to 4.56 V.

8. The secondary battery according to claim 1, wherein 0.035D 2 -0.562D + 2.253 ≤ y ≤ 0.027D 2 -0.482D + 2.

257.

9. The secondary battery according to claim 1, wherein the base film comprising a polyolefin, the polyolefin comprising at least one of polyethylene or polypropylene.

10. The secondary battery according to claim 9, wherein the separator further comprising an insulating layer disposed on a surface of the base film, the insulating layer comprising inorganic particles and a binder; the inorganic particles comprising at least one of alumina, silica, magnesia, titania, hafnia, tin oxide, ceria, nickel oxide, zinc oxide, calcium oxide, zirconia, yttria, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate; the binder comprising at least one of styrene butadiene rubber, polyacrylic acid, polyvinyl alcohol, polyethylene glycol, polyacrylate, polyvinylidene fluoride, polyvinyl chloride, formaldehyde resin, or nitrile group-containing acrylate.

11. The secondary battery according to claim 9, wherein the base film being a multilayer film structure.

12. The secondary battery according to claim 1, wherein the positive electrode active material comprising at least one of lithium cobaltate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminate, lithium manganese oxide, lithium iron phosphate, or lithium iron manganese phosphate. 13.An electronic device comprising the secondary battery of any one of claims 1 to 12.

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