Secondary battery and electrical apparatus

Abstract

A secondary battery and an electrical apparatus. The secondary battery comprises a positive electrode, a negative electrode, and an electrolyte. The electrolyte comprises an ethoxy-containing additive and a nitrogen-containing additive. In a solid electrolyte interphase film, 3≤2WO+WN≤10. By controlling the type of the electrolyte in the secondary battery, and controlling the contents of nitrogen and oxygen elements in the solid electrolyte interphase film formed on the surface of a negative electrode active material layer to be within a specific range, a nitrogen-containing compound and an ethoxy-containing compound achieve a structurally synergistic effect, which improves the fast charging performance and low-temperature performance of the battery, suppresses the fracture of the SEI film during a high-temperature cycling process, and further prolongs the cycle performance of the secondary battery, especially the cycle life and storage performance at high temperatures.

Description

Secondary batteries and electrical appliances

[0001] This application claims priority to Chinese patent application 202510024396.1, filed on January 7, 2025. The entire contents of the aforementioned Chinese patent application are incorporated herein by reference. Technical Field

[0002] This application relates to the field of lithium-ion battery technology. Specifically, this application relates to a secondary battery and an electrical device. Background Technology

[0003] Lithium-ion batteries, with their superior energy density, excellent cycle life, and lack of memory effect, have become an indispensable energy supplier for 3C products such as mobile phones and laptops, and occupy a core position in the electric vehicle sector, driving the trend of green travel. In recent years, electric vehicles have made significant progress in energy density, cycle performance, safety, and cost control, to which lithium-ion batteries have played a crucial role. However, the long charging time of electric vehicles limits their widespread adoption and application in the market. Breakthroughs in fast charging technology are crucial for the future development of electric vehicles. They can not only shorten charging time and improve user experience, but also accelerate the market penetration of electric vehicles and promote a green energy revolution.

[0004] However, existing commercial lithium-ion batteries struggle to balance fast charging and cycle performance when facing challenges in high and low temperature environments. To improve fast charging capability and low-temperature adaptability, current technologies tend to use low-boiling-point carboxylic acid ester solvents to reduce electrolyte viscosity and increase ionic conductivity. However, while this approach improves certain battery performance indicators in the short term, it poses a serious threat to the battery's high-temperature safety and cycle life in the long run. Moreover, the frequent side reactions between carboxylic acid ester solvents and negative electrode materials not only reduce the battery's high-temperature safety performance but also exacerbate internal battery losses and shorten its lifespan.

[0005] Therefore, finding a solution that can meet the needs of fast charging, maintain stable battery operation under extreme temperatures, and extend battery cycle life has become one of the urgent tasks for technology researchers. Summary of the Invention

[0006] The main objective of this application is to provide a secondary battery and an electrical device, particularly a fast-charging, long-life secondary battery and an electrical device, to solve the problem that batteries in the prior art cannot simultaneously achieve high-temperature performance, low-temperature performance, fast charging, and cycle performance.

[0007] To achieve the above objectives, according to one aspect of this application, a secondary battery is provided, comprising a positive electrode, a negative electrode, and an electrolyte, wherein the electrolyte includes an ethoxylated additive and a nitrogen-containing additive; the negative electrode comprises a negative electrode active material layer and a solid electrolyte interface film located on the surface of the negative electrode active material layer, the solid electrolyte interface film containing an ethoxylated compound and lithium nitride, wherein the ethoxylated compound is derived from the ethoxylated additive and the lithium nitride is derived from the nitrogen-containing additive; the weight percentage of oxygen element from the ethoxylated compound in the solid electrolyte interface film is defined as W using X-ray photoelectron spectroscopy. O The weight percentage of nitrogen from lithium nitride in the solid electrolyte interface film is defined as W. N %, of which 3≤2W O +W N ≤10. Under the above conditions, lithium nitride and ethoxy compounds in the SEI film exhibit a good synergistic effect. Lithium nitride improves ion transport, while ethoxy compounds improve the flexibility of the SEI film, enhance the structural stability of the interfacial film during fast charging, improve the kinetic characteristics of lithium ion transport at the interface, reduce impedance, and improve the fast charging and low-temperature performance of the battery. Furthermore, they can further improve the density and flexibility of the SEI film, thereby effectively suppressing SEI film rupture during high-temperature cycling. This helps reduce the negative impacts of continuous side reactions between the electrolyte and the negative electrode, extending the cycle performance of the secondary battery, especially the cycle life and storage performance at high temperatures.

[0008] Furthermore, 0.05 < W O <5, preferably, 1≤W O ≤3; Under the above conditions, it is possible to more effectively promote the formation of a stable and flexible SEI film, which is more conducive to improving the fast charging performance and cycle stability of lithium-ion batteries; and / or, 1 < W N <10, preferably, 2≤W N ≤8, under the above conditions, can further enhance the lithium-ion conductivity and high-temperature stability of the SEI film, which is more conducive to improving the fast charging capability and cycle life of the battery.

[0009] Furthermore, the tortuosity of the negative electrode is defined as τ, where 0 < 2(W N ×W O )-τ<1. Under the above conditions, the lithium-ion conductivity in the electrode is faster, the electrolyte can be more fully wetted into the gaps, and there are fewer side reactions between the electrolyte and the electrode. This allows for a more effective balance between high-temperature performance, low-temperature performance, fast charging and cycle performance, resulting in better overall battery performance.

[0010] Furthermore, 2.5 < τ < 9. Under the above conditions, the secondary battery has a shorter ion transport path, faster transport efficiency, more uniform current distribution, more stable SEI film, faster heat dissipation, better mechanical and chemical stability, and better high-temperature performance, low-temperature performance, fast charging, and cycle performance.

[0011] Further, the ethoxylated additive includes one or more of ethoxyacrylate, ethoxyethylene carbonate, and ethoxyethylene sulfite; preferably, the ethoxyacrylate has the structure shown in general formula (I-1):

[0012] In general formula (I-1), n ​​is an integer from 1 to 4, and R 11 Selected from hydrogen, C1-C3 alkyl, vinyl carboxylate, or propenyl carboxylate, R 12 Selected from hydrogen and C1-C3 alkyl groups;

[0013] Preferably, ethoxyethylene carbonate has the structure shown in general formula (I-2):

[0014] In general formula (I-2), m is an integer from 1 to 4, and R 13 Selected from hydrogen, C1-C3 alkyl, vinyl carboxylic acid esters or propenyl carboxylic acid esters;

[0015] Preferably, ethoxyethylene sulfite has the structure shown in general formula (I-3):

[0016] In general formula (I-3), p is an integer from 1 to 4, and R 14 Selected from hydrogen, C1-C3 alkyl, vinyl carboxylic acid esters or propylene carboxylic acid esters; ethoxylated additives with the above structure can significantly improve lithium-ion transport and further improve the flexibility of the SEI film, and improve the structural stability of the interface film of the electrode during fast charging.

[0017] And / or, nitrogen-containing additives include one or more of nitrile compounds, phosphazenes, amides, organic nitrogen-containing lithium salts, inorganic nitrogen-containing alkali metal salts, nitrate esters and nitro esters;

[0018] Preferably, the nitrile compound has the structure shown in general formula (II-1): N≡C——R 21 ——R 22 General formula (II-1);

[0019] In general formula (II-1), R 21 Selected from C2-C10 alkylene groups or nitrile-substituted C2-C10 alkylene groups, R 22 Selected from hydrogen, nitrile, C1-C6 alkyl, or carboxylic ester groups;

[0020] Preferably, the phosphazene has the structure shown in general formula (II-2):

[0021] In general formula (II-2), R 23 Selected from C1-C6 alkyl or fluorinated C1-C6 alkyl, R 24 R 25 R 26 R 27 and R 28 Each is independently selected from hydrogen, fluorine, C1-C6 alkyl, or fluorinated C1-C6 alkyl, and R 24 R 25 R 26 R 27 and R 28 At least one of them is fluorine or a fluorinated C1-C6 alkyl group;

[0022] Preferably, the amide has the structure shown in general formula (II-3):

[0023] In general formula (II-3), R 29 R 210 R 211 Each is independently selected from hydrogen, C1-C6 alkyl, or fluorinated C1-C6 alkyl, and R 29 R 210 R 211 At least one of them is a fluorinated C1-C6 alkyl group;

[0024] Preferably, the organic nitrogen-containing lithium salt has the structure shown in general formula (II-4) or general formula (II-5):

[0025] In general formula (II-4), R 212 R 213 R 214 Each group is independently selected from hydrogen, fluorine, C1-C6 alkyl, fluorinated C1-C6 alkyl, or nitrile groups, and R 212 R 213 R 214 At least one of them is selected from fluorine, fluorinated C1-C6 alkyl or nitrile groups;

[0026] In general formula (II-5), R 215 R 216 Each is independently selected from fluorine, C1-C6 alkyl, or fluorinated C1-C6 alkyl, and R 215 and R 216 At least one of them is selected from fluorine or fluorinated C1-C6 alkyl groups;

[0027] Preferably, the inorganic nitrogen-containing alkali metal salt has the structure shown in general formula (II-6): R 217 NO t General formula (II-6);

[0028] In general formula (II-6), t is selected from 2 or 3; R 217 Selected from lithium, sodium, or potassium;

[0029] Preferably, the nitrate ester has the structure shown in general formula (II-7): R 218 NO r General formula (II-7);

[0030] In general formula (II-7), r is 3; R 218 Selected from C1 to C4 alkyl groups;

[0031] Preferably, the nitro ester has the structure shown in general formula (II-8): R 219 NO r General formula (II-8);

[0032] In general formula (II-8), s is 2; R 219 Selected from C1 to C4 alkyl groups. Nitrogen-containing additives with the above structure can further improve the lithium-ion transport properties and structural stability of the SEI film at high temperatures, thus making it more beneficial to improve the fast-charging performance and high-temperature cycle storage performance of the negative electrode.

[0033] Further, ethoxyacrylates include one or more of ethoxyethoxyethyl acrylate, 2-methoxyethyl acrylate, and triethylene glycol diacrylate; and / or, ethoxyvinyl carbonates include methyl ethoxyvinyl carbonate and / or methoxyethoxymethyl vinyl carbonate; and / or, ethoxysulfite includes ethoxymethyl vinyl sulfite; and / or, nitrile compounds include one or more of butadienenitrile, adiponitrile, glutaronitrile, hexanetrionitrile, ethylene glycol (bis)propionitrile ether, methoxypropionitrile, 2,3-dimethoxypropionitrile, and methyl cyanoacetate; and / or, phosphazenes include methoxypentafluorocyclotriphosphazene, trifluoromethoxypentafluorocyclotriphosphazene, etc. One or more of triphosphazene, ethoxypentafluorocyclotriphosphazene, and trifluoroethoxypentafluorocyclotriphosphazene; and / or, amides including trifluoroformamide or trifluoroacetamide; and / or, organic nitrogen-containing lithium salts including one or more of 4,5-dicyano-2-(trifluoromethyl)imidazolium lithium, (fluorosulfonyl)(perfluorobutylsulfonyl)imide lithium, bis(fluorosulfonyl)imide lithium, bistrifluoromethylsulfonylimide lithium, and bis(pentafluoroethylsulfonic acid)imide lithium; and / or, inorganic nitrogen-containing alkali metal salts including one or more of lithium nitrate, lithium nitrite, sodium nitrate, and sodium nitrite; and / or, nitrate esters including ethyl nitrate and / or propyl nitrate; and / or, nitro esters including nitromethane and / or nitroethane. The above-mentioned specific types of ethoxy-containing additives and nitrogen-containing additives can work synergistically to better construct a dense, flexible, and highly ionicly conductive SEI film, thereby significantly reducing side reactions during fast charging and improving battery performance under high and low temperature environments.

[0034] Furthermore, the electrolyte contains 0.05–4% by weight of ethoxylated additives; and / or, the electrolyte contains 0.5–10% by weight of nitrogen-containing additives; preferably, the weight ratio of ethoxylated additives to nitrogen-containing additives is (0.05–4):1. Under the above conditions, the synergistic effect of ethoxylated and nitrogen-containing additives can further optimize the SEI film structure, which is more conducive to improving the battery's fast-charging performance and cycle stability, while maintaining good safety and electrochemical efficiency.

[0035] Furthermore, the electrolyte may contain a mixture of ethoxyethoxyethyl acrylate and lithium bis(fluorosulfonyl)imide in a weight ratio of (0.05–4):1; or, a mixture of ethoxyethoxyethyl acrylate and ethoxypentafluorocyclotriphosphazene in a weight ratio of (2–4):1; or, a mixture of ethoxyethoxyethyl acrylate and ethyl nitrate in a weight ratio of (1–4):1. The synergistic effect of these electrolyte additives further facilitates the construction of an SEI film that combines high lithium-ion transport with structural stability, thereby significantly improving the fast-charging capability and high-temperature cycling performance of the secondary battery, reducing gas production, and enhancing battery safety.

[0036] Furthermore, the electrolyte also includes other additives, including one or more of the following: vinylene carbonate, ethylene carbonate, lithium difluorophosphate, tris(trimethylsilane) phosphate, tris(trimethylsilane) borate, fluoroethylene carbonate, difluoroethylene carbonate, trifluoropropylene carbonate, methyl ethyl 2,2,2-trifluorocarbonate, diethyl 2,2,2-trifluorocarbonate, tris(trifluoroethyl) phosphate, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether; the electrolyte In this process, the weight percentage of other additives is 0.1% to 10%; these other additives can participate in the formation of the SEI film, and in combination with ethoxylated and nitrogen-containing additives, construct a composite SEI film, which is more conducive to maintaining the structural integrity of the SEI film during fast charging, thereby further reducing electrolyte consumption and improving battery cycle life and safety; and / or, the positive electrode includes a positive electrode active material, which includes a lithium nickel transition metal oxide, the chemical formula of which is: LiNi x Co y A (1-x-y) O2, wherein A includes one or more of manganese, aluminum, magnesium, chromium, calcium, zirconium, molybdenum, silver and niobium, 0.5≤x≤1, 0≤y≤0.5, x+y≤1; the stable and highly conductive interface layer formed by the additives of this application can more effectively slow down the structural degradation of the above-mentioned positive electrode material during the charge and discharge process, thereby further extending the cycle life of the battery; and / or, the negative electrode active material in the negative electrode active material layer includes silicon-based materials, which include one or more of silicon, silicon alloys, silicon oxides and silicon-carbon compounds; preferably, the weight percentage of silicon-based materials in the negative electrode active material is 10-100%, and the additives of this application can interact with the above-mentioned negative electrode active materials to form a more stable SEI film, protecting the negative electrode material from the corrosion of the electrolyte, more effectively buffering the volume change of the silicon-based material during the charge and discharge process, and improving the conductivity of the negative electrode, thereby more conducive to improving the charge and discharge efficiency, cycle stability and safety of the battery.

[0037] According to another aspect of this application, an electrical device is provided, including the aforementioned secondary battery. The electrical device employing the secondary battery of this application can enhance its fast charging and cycle performance through an optimized SEI film, significantly improving overall energy density, power output, and durability, making it suitable for applications requiring high energy, fast charging, and long-term stable operation.

[0038] This application controls the type of electrolyte in the secondary battery and the nitrogen and oxygen content in the solid electrolyte interphase (SEI) film formed on the surface of the negative electrode active material layer within a specific range. Nitrogen from nitrogen-containing additives improves the lithium-ion transport and structural stability of the SEI film at high temperatures, thereby improving the fast-charging performance and high-temperature cycle storage performance of the negative electrode. Oxygen from ethoxy-containing additives, existing in the form of -(CH2-CH2O)-, enhances lithium-ion transport and improves the flexibility of the SEI film, thus ensuring the structural stability of the interfacial film during fast charging. Furthermore, the nitrogen-containing and ethoxy-containing compounds exhibit a good synergistic effect in structure, improving the kinetics of lithium-ion transport at the interface, reducing impedance, and enhancing the battery's fast-charging and low-temperature performance. They also further improve the density and flexibility of the SEI film, effectively suppressing SEI film rupture during high-temperature cycling, thereby reducing negative impacts such as continuous side reactions between the electrolyte and the negative electrode, and extending the cycle performance of the secondary battery, especially its cycle life and storage performance at high temperatures.

[0039] Based on the above improvements, the SEI film in the secondary battery of this application is dense and stable, effectively suppressing the continuous decomposition and gas generation of the electrolyte. The secondary battery has excellent fast charging performance, and at the same time, it has good cycle performance, storage performance and safety performance at high and low temperatures. Detailed Implementation

[0040] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present application will now be described in detail with reference to the embodiments.

[0041] For the sake of brevity, this application only specifically discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, just as any upper limit can be combined with any other upper limit to form an unspecified range. Furthermore, each individually disclosed point or single value can itself serve as a lower or upper limit and be combined with any other point or single value, or with other lower or upper limits, to form an unspecified range.

[0042] Unless otherwise stated, the terms used in this application have their common meanings as commonly understood by those skilled in the art. Unless otherwise stated, the values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (e.g., they can be tested according to the methods given in the embodiments of this application).

[0043] The list of items connected by the term "one or more of" or other similar terms can mean any combination of the listed items. For example, if items A and B are listed, then the phrase "one or more of A and B" means only A; only B; or A and B. In another instance, if items A, B, and C are listed, then the phrase "one or more of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single component or multiple components. Item B may contain a single component or multiple components. Item C may contain a single component or multiple components.

[0044] As described in the background section of this application, existing technologies suffer from the problem of batteries failing to simultaneously achieve high-temperature performance, low-temperature performance, fast charging, and cycle performance. To address this issue, in a typical embodiment of this application, a secondary battery is provided, comprising a positive electrode, a negative electrode, and an electrolyte. The electrolyte includes an ethoxylated additive and a nitrogen-containing additive. The negative electrode comprises a negative electrode active material layer and a solid electrolyte interface film located on the surface of the negative electrode active material layer. The solid electrolyte interface film contains an ethoxylated compound and lithium nitride, wherein the ethoxylated compound is derived from the ethoxylated additive, and the lithium nitride is derived from the nitrogen-containing additive. Using X-ray photoelectron spectroscopy, the weight percentage of oxygen from the ethoxylated compound in the solid electrolyte interface film is defined as W. O The weight percentage of nitrogen from lithium nitride in the solid electrolyte interface film is defined as W. N %, of which 3≤2W O +W N ≤10. In some implementations, 4≤2O+N≤8.

[0045] In the solid electrolyte interphase (SEI) film on the surface of the negative electrode, nitrogen from nitrogen-containing additives can form lithium nitride to improve the lithium-ion transport and structural stability of the SEI film at high temperatures, thereby improving the fast-charging performance and high-temperature cycle storage performance of the negative electrode. However, due to the high activity of nitrogen, a high content of nitrogen can affect the first-time efficiency and capacity of the electrode. Furthermore, nitrogen usually exists in the form of Li3N, which has low adhesion to the electrode. During fast charging, the change in electrode volume can easily lead to the rupture of the SEI, thus affecting the electrochemical performance of the battery.

[0046] In the solid electrolyte interphase (SEI) film on the negative electrode surface, oxygen elements from ethoxylated additives exist in the form of -(CH2-CH2O)-, which can enhance lithium-ion transport and improve the flexibility of the SEI film, thereby increasing the structural stability of the interfacial film during fast charging. However, when its content is too high, the swelling ratio of the SEI film on the surface of the negative electrode active material layer is too high, resulting in an unstable interfacial film structure, which in turn increases the side reactions of the electrolyte on the negative electrode side, ultimately degrading the cycle life of the battery.

[0047] The inventors unexpectedly discovered during their research that the content of nitrogen and oxygen elements conforms to the rule "3≤2W". O +W N When the SEI film is ≤10", it is dense and stable, which can effectively suppress the continuous decomposition of electrolyte caused by the contact between electrolyte and negative electrode active material, as well as suppress gas production, thereby improving the cycle performance and safety performance of secondary batteries, especially the cycle performance and storage performance at high temperatures.

[0048] Furthermore, lithium nitride and ethoxy compounds in the SEI film exhibit a good synergistic effect. Lithium nitride improves ion transport, while ethoxy compounds improve the flexibility of the SEI film, enhance the structural stability of the interfacial film during fast charging, improve the kinetic characteristics of lithium ion transport at the interface, reduce impedance, and improve the fast charging and low-temperature performance of the battery. They can also further improve the density and flexibility of the SEI film, thereby effectively suppressing SEI film rupture during high-temperature cycling. This helps to reduce the negative impacts of continuous side reactions between the electrolyte and the negative electrode, and extend the cycle performance of the secondary battery, especially the cycle life and storage performance at high temperatures.

[0049] Based on the above improvements, the secondary battery of this application has excellent fast charging performance, and at the same time, it has good cycle performance, storage performance and safety performance at high and low temperatures.

[0050] Typical but not limiting, 2W O +W N The value is 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, or a range of any two values.

[0051] The inventors further optimized W. O and W N The range. In a preferred embodiment, 0.05 < W O <5, preferably, 0.5≤W O ≤4, more preferably, 1≤W O ≤3. Typical but not limiting, W O The range is 0.1, 0.2, 0.5, 0.8, 1, 1.1, 1.3, 1.4, 1.5, 1.6, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3, 3.2, 3.5, 3.8, 4, 4.2, 4.5, 4.8, 4.9, or any two values. Under these conditions, the formation of a stable and flexible SEI film can be more effectively promoted, which is more conducive to improving the fast-charging performance and cycle stability of lithium-ion batteries. However, WO Too low or too high a value can lead to a poor SEI film structure, which in turn affects lithium-ion transport and battery performance. This is especially true under fast charging and high-temperature cycling conditions, which may accelerate battery degradation.

[0052] In a preferred embodiment, 1 < W N <10, preferably, 2≤W N ≤8. In a preferred embodiment, 2≤W N ≤6. In a preferred embodiment, 3≤W N ≤6. Typical but not limiting, W N The range is 1.6, 1.7, 2, 2.3, 2.5, 2.7, 3, 3.3, 3.5, 3.7, 4, 4.3, 4.5, 4.8, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, or any two values. Under the above conditions, the lithium-ion conductivity and high-temperature stability of the SEI film can be further enhanced, which is more conducive to improving the battery's fast charging capability and cycle life. However, if W N If the W value is too low, it may not be sufficient to optimize the SEI film. N Excessive levels may trigger additional side reactions or even damage battery performance, especially its electrochemical stability at first efficiency and high temperatures.

[0053] Tortuousness is a parameter reflecting the degree of tortuosity of the path from one end to the other in a porous medium, relative to a straight line; that is, the ratio of the actual flow path length to the straight-line distance between the ends of the flow path. In electrode structures, the electrolyte fully wets the pores, and the transport of lithium ions requires migration along the pores through the electrolyte. Therefore, electrode tortuousness has a significant impact on the lithium-ion conductivity and electrolyte diffusion in the electrode, and is also important for establishing the relationship between battery performance, electrode interface composition, and electrode structural characteristics. In this application, tortuousness can be adjusted according to the characteristics of the selected negative electrode active material using conventional techniques in the art, such as controlling the electrode rolling pressure, rolling temperature, rolling speed, and number of rolling cycles.

[0054] In a preferred embodiment, the tortuosity of the negative electrode is defined as τ, where 0 < 2(W N ×W O )-τ<1; Preferably, 0.2≤2(W N ×W O )-τ≤0.8; more preferably, 0.3≤2(W N ×W O)-τ≤0.6. Under the above conditions, the lithium-ion conductivity in the electrode is faster, the electrolyte can more fully wet the pores, and there are fewer side reactions between the electrolyte and the electrode. This allows for a more effective balance between high-temperature performance, low-temperature performance, fast charging, and cycle performance, resulting in better overall battery performance. When 2(W N ×W O When the τ value is too large, the nitrogen and oxygen content in the SEI film will be too high, or the tortuosity of the negative electrode will be too small. This may lead to an increase in the reaction between the electrolyte and the electrode, resulting in greater electrolyte consumption, reduced battery cycle life, and potentially increased battery gas production. N ×W O When the τ value is too small, the nitrogen and oxygen content in the SEI film is correspondingly too low, or the tortuosity of the negative electrode sheet is too large. This may lead to reduced wettability of the negative electrode electrolyte, resulting in poorer kinetics, increased polarization, and reduced capacity during cycling of the secondary battery, thus affecting its electrochemical performance. A typical but not limiting example is 2(W N ×W O The value of τ is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or any two values ​​in the range.

[0055] In a preferred embodiment, 2.5 < τ < 9; preferably, 3 < τ < 8; more preferably, 4 < τ < 7. Typically, but not limitingly, τ values ​​are 2.6, 2.8, 3, 3.2, 3.4, 3.6, 3.8, 4, 4.2, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.8, or any range of two values. If the tortuosity of the negative electrode is too large, it may lead to uneven heat dissipation inside the battery, exacerbating the expansion of the electrode material, affecting the stability and safety of the battery under high-temperature conditions. It may also lead to low ion mobility, long ion transport paths, and increased resistance, resulting in poor charge-discharge performance at low temperatures. Furthermore, it may lead to uneven current distribution, making it difficult to quickly dissipate heat generated during fast charging, thus affecting the battery's fast-charging performance, safety, and potentially the stability of the SEI film on the negative electrode surface, thereby affecting the battery's cycle life. If the tortuosity of the negative electrode is too small, it will also affect the battery's lifespan, electrochemical performance, and safety. Under the above conditions, the secondary battery has a shorter ion transport path, faster transport efficiency, more uniform current distribution, more stable SEI film, faster heat dissipation, better mechanical and chemical stability, and better high-temperature performance, low-temperature performance, fast charging and cycle performance.

[0056] In a preferred embodiment, the ethoxylated additive includes one or more of ethoxyacrylate, ethoxyethylene carbonate, and ethoxyethylene sulfite. These oxygen-containing additives can more effectively improve the flexibility and lithium-ion conductivity of the SEI film, thereby enhancing the battery's fast-charging performance and cycle stability. Furthermore, they can synergistically work with nitrogen-containing additives to optimize the SEI film structure, thereby significantly suppressing side reactions and extending battery life.

[0057] Preferably, the ethoxyacrylate has the structure shown in general formula (I-1):

[0058] In general formula (I-1), n ​​is an integer from 1 to 4, and R 11 Selected from hydrogen, C1-C3 alkyl, vinyl carboxyl ester or propenyl carboxyl ester, such as H, C2H5 or vinyl propionate, R 12 It is selected from hydrogen, C1 to C3 alkyl groups, such as H, CH3 or C2H5.

[0059] Preferably, ethoxyethylene carbonate has the structure shown in general formula (I-2):

[0060] In general formula (I-2), m is an integer from 1 to 4, and R 13 Selected from hydrogen, C1-C3 alkyl, vinyl carboxylic acid esters or propenyl carboxylic acid esters, such as H, C2H5 or vinyl propionate;

[0061] Preferably, ethoxyethylene sulfite has the structure shown in general formula (I-3):

[0062] In general formula (I-3), p is an integer from 1 to 4, and R 14 It is selected from hydrogen, C1-C3 alkyl, vinyl carboxylic acid esters or propylene carboxylic acid esters.

[0063] Ethoxylated additives with the above structure can significantly improve lithium-ion transport and further improve the flexibility of the SEI film, thereby enhancing the structural stability of the interface film during fast charging.

[0064] In a preferred embodiment, the nitrogen-containing additive includes one or more of nitrile compounds, phosphazenes, amides, organic nitrogen-containing lithium salts, inorganic nitrogen-containing alkali metal salts, nitrate esters, and nitro esters. These nitrogen-containing additives can further improve the ionic conductivity of the electrolyte, improve the composition and structure of the interfacial film, and enable it to more effectively exert the aforementioned effects, thereby further improving the fast-charging performance and cycle storage performance of the secondary battery.

[0065] Preferably, the nitrile compound has the structure shown in general formula (II-1): N≡C——R21 ——R 22 General formula (II-1);

[0066] In general formula (II-1), R 21 Selected from C2-C10 alkylene groups or nitrile-substituted C2-C10 alkylene groups, more preferably, R 21 Selected from C1-C6 alkylene groups or nitrile-substituted C2-C6 alkylene groups, such as ethylene, propylene, butylene, pentylene, hexylene, or nitrile-substituted ethylene, propylene, butylene, pentylene, hexylene; R 22 Selected from hydrogen, nitrile, C1-C6 alkyl or carboxylic ester groups, more preferably, R 22 It is selected from hydrogen, C1-C6 alkyl, or nitrile groups.

[0067] Preferably, the phosphazene has the structure shown in general formula (II-2):

[0068] In general formula (II-2), R 23 Selected from C1-C6 alkyl or fluorinated C1-C6 alkyl, R 24 R 25 R 26 R 27 and R 28 Each is independently selected from hydrogen, fluorine, C1-C6 alkyl, or fluorinated C1-C6 alkyl, and R 24 R 25 R 26 R 27 and R 28 At least one of them is fluorine or a fluorinated C1-C6 alkyl group; more preferably, R 23 Selected from C1-C4 alkyl or fluorinated C1-C4 alkyl, R 24 R 25 R 26 R 27 and R 28 Each is independently selected from fluorine, C1-C4 alkyl or fluorinated C1-C4 alkyl, or R 28 Selected from methyl, ethyl, n-propyl, isopropyl, trifluoromethyl, or 2,2,2-trifluoroethyl, R 23 R 24 R 25 R 26 R 27 Each is independently selected from fluorine or fluorinated C1-C4 alkyl groups, and R 23 R 24 R 25 R 26 and R 27 At least one of them is fluorine.

[0069] Preferably, the amide has the structure shown in general formula (II-3):

[0070] In general formula (II-3), R 29 R 210 R 211 Each is independently selected from hydrogen, C1-C6 alkyl, or fluorinated C1-C6 alkyl, and R 29 R 210 R 211 At least one of them is a fluorinated C1-C6 alkyl group; more preferably, R 29 R 210 R 211 Each is independently selected from hydrogen, C1-C4 alkyl, or fluorinated C1-C4 alkyl, and R 29 R 210 R 211 At least one of them is a fluorinated C1-C4 alkyl group.

[0071] Preferably, the organic nitrogen-containing lithium salt has the structure shown in general formula (II-4) or general formula (II-5):

[0072] In general formula (II-4), R 212 R 213 R 214 Each group is independently selected from hydrogen, fluorine, C1-C6 alkyl, fluorinated C1-C6 alkyl, or nitrile groups, and R 212 R 213 R 214 At least one of them is selected from fluorine, fluorinated C1-C6 alkyl or nitrile; more preferably, R 212 R 213 R 214 Each is independently selected from fluorine or fluorinated C1-C4 alkyl groups, and R 212 R 213 R 214 At least one of them is a nitrile group;

[0073] In general formula (II-5), R 215 R 216 Each is independently selected from fluorine, C1-C6 alkyl, or fluorinated C1-C6 alkyl, and R 215 and R 216 At least one of them is selected from fluorine or fluorinated C1-C6 alkyl; more preferably, R 215 R 216 Each is independently selected from fluorine, C1-C4 alkyl, or fluorinated C1-C4 alkyl, and R 215 and R 216 At least one of them is selected from fluorine or fluorinated C1-C4 alkyl groups.

[0074] Preferably, the inorganic nitrogen-containing alkali metal salt has the structure shown in general formula (II-6): R 217 NO t General formula (II-6);

[0075] In general formula (II-6), t is selected from 2 or 3; R 217 Selected from lithium, sodium, or potassium.

[0076] Preferably, the nitrate ester has the structure shown in general formula (II-7): R 218 NO r General formula (II-7);

[0077] In general formula (II-7), r is 3; R 218 Selected from C1 to C4 alkyl groups.

[0078] Preferably, the nitro ester has the structure shown in general formula (II-8): R 219 NO s General formula (II-8);

[0079] In general formula (II-8), s is 2; R 219 Selected from C1 to C4 alkyl groups.

[0080] Nitrogen-containing additives with the above structure can further improve the lithium-ion transport properties and structural stability of the SEI film at high temperatures, thus making it more beneficial to improve the fast-charging performance and high-temperature cycle storage performance of the negative electrode.

[0081] The inventors further optimized the composition and types of the ethoxy-containing and nitrogen-containing additives. In a preferred embodiment, the ethoxyacrylate includes one or more of ethoxyethoxyethyl acrylate (EOEA), 2-methoxyethyl acrylate (MOEA), and triethylene glycol diacrylate (TGD); and / or, the ethoxyvinyl carbonate includes methyl ethoxyvinyl carbonate and / or methoxyethoxymethylvinyl carbonate; and / or, the ethoxysulfite includes ethoxymethylsulfite, wherein the above ethoxy compounds are rich in -(CH2O-CH2O). n - The structure can further improve the flexibility and lithium-ion conductivity of the SEI film, thereby more effectively inhibiting electrolyte decomposition and extending battery life.

[0082] In a preferred embodiment, the nitrile compound includes one or more of butadionitrile SN, adiponitrile ADN, glutaronitrile GLN, hexanetrionitrile HTN, ethylene glycol (bis)propionitrile ether DENE, methoxypropionitrile MOPN, 2,3-dimethoxypropionitrile DMOPN, and methyl cyanoacetate MOCA; and / or, the phosphazene includes one or more of methoxypentafluorocyclotriphosphazene, trifluoromethoxypentafluorocyclotriphosphazene, ethoxypentafluorocyclotriphosphazene PFPN, and trifluoroethoxypentafluorocyclotriphosphazene TFPN; and / or, the amide includes trifluoroformamide or trifluoroacetamide; and / or, the organic nitrogen-containing lithium salt includes 4,5 One or more of the following: lithium (dicyano-2-(trifluoromethyl)imidazolium)LiTDI, lithium (fluorosulfonyl)(perfluorobutylsulfonyl)imideLiFNFSI, lithium (bis(fluorosulfonyl)imideLiFSI), lithium (bistrifluoromethylsulfonyl)imideLiTFSI, and lithium (bis(pentafluoroethylsulfonic acid)imideLiBETI); and / or, inorganic nitrogen-containing alkali metal salts including one or more of lithium nitrate LiNO3, lithium nitrite LiNO2, sodium nitrate NaNO3, and sodium nitrite NaNO2; and / or, nitrate esters including ethyl nitrate and / or propyl nitrate; and / or, nitro esters including nitromethane and / or nitroethane. The above nitrogen-containing compounds are more conducive to the formation of compounds such as Li3N, thereby further enhancing the thermal stability of the film and the lithium-ion transport rate.

[0083] The aforementioned specific types of ethoxylated and nitrogen-containing additives can work synergistically to build a dense, flexible, and highly ionicly conductive SEI film, thereby significantly reducing side reactions during fast charging and improving battery performance under high and low temperature environments.

[0084] The inventors further optimized the content range of the ethoxy-containing additive and the nitrogen-containing additive. In a preferred embodiment, the weight percentage of the ethoxy-containing additive in the electrolyte is 0.05–4%, preferably 0.2–3%; and / or, the weight percentage of the nitrogen-containing additive in the electrolyte is 0.5–10%, preferably 1–5%. In a preferred embodiment, the weight ratio of the ethoxy-containing additive to the nitrogen-containing additive is (0.05–4):1. Under the above conditions, the ethoxy-containing additive and the nitrogen-containing additive work synergistically to further optimize the SEI film structure, which is more conducive to improving the battery's fast-charging performance and cycle stability, while maintaining good safety and electrochemical efficiency. If the ethoxy-containing additive content is too high, it may lead to an excessively high swelling rate of the SEI film on the surface of the negative electrode active material layer, resulting in an unstable interfacial film structure, thereby increasing the side reactions of the electrolyte on the negative electrode side and ultimately degrading the battery's cycle life. If the content of ethoxylated additives is too low, the improvement effect on the SEI film may be insignificant; if the content of nitrogen-containing additives is too high, the first efficiency and capacity of the electrode may decrease. In addition, nitrogen is usually in the form of Li3N, which has low adhesion to the electrode. During fast charging, the change in electrode volume can easily lead to the rupture of the SEI, thereby affecting the electrochemical performance of the battery. If the content of nitrogen-containing additives is too low, the improvement effect on the SEI film may also be poor.

[0085] Typical, but not limiting, the electrolyte contains ethoxylated additives at a weight percentage of 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, or any combination of two values, and nitrogen-containing additives at a weight percentage of 0.5%. The weight ratio of ethoxylated additives to nitrogen-containing additives is 0.6%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.5%, 5%, 6%, 7%, 8%, 9%, 10%, or any two of these values, and the weight ratio of ethoxylated additives to nitrogen-containing additives is 0.05:1, 0.1:1, 0.5:1, 1:1, 2:1, 3:1, 4:1, or any two of these values.

[0086] In a preferred embodiment, the electrolyte comprises a mixture of ethoxyethoxyethyl acrylate and lithium bis(fluorosulfonyl)imide in a weight ratio of (0.05–4):1; or, the electrolyte comprises a mixture of ethoxyethoxyethyl acrylate and ethoxypentafluorocyclotriphosphazene in a weight ratio of (2–4):1; or, the electrolyte comprises a mixture of ethoxyethoxyethyl acrylate and ethyl nitrate in a weight ratio of (1–4):1. The synergistic effect of the electrolyte additives in the above combinations is more conducive to constructing an SEI film with both high lithium-ion transport and structural stability, thereby significantly improving the fast-charging capability and high-temperature cycling performance of the secondary battery, reducing gas generation, and enhancing battery safety.

[0087] The inventors further optimized the types and composition of electrolyte additives. In a preferred embodiment, the electrolyte further includes other additives, including one or more of vinylene carbonate (VC), ethylene ethylene carbonate (VEC), lithium difluorophosphate (LiDFP), tris(trimethylsilane) phosphate, tris(trimethylsilane) borate, fluoroethylene carbonate, difluoroethylene carbonate, trifluoropropylene carbonate, methyl ethyl 2,2,2-trifluorocarbonate, diethyl 2,2,2-trifluorocarbonate, tris(trifluoroethyl) phosphate, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, preferably one or more of vinylene carbonate, ethylene ethylene carbonate, and lithium difluorophosphate. In a preferred embodiment, the weight percentage of other additives in the electrolyte is 0.1% to 10%, preferably 0.1% to 5%. Other additives mentioned above can participate in the formation of the SEI film. When combined with ethoxylated and nitrogen-containing additives, they can form a composite SEI film, which is more conducive to maintaining the structural integrity of the SEI film during fast charging, thereby further reducing electrolyte consumption and improving battery cycle life and safety.

[0088] In a preferred embodiment, the electrolyte further includes other lithium salts, including one or more selected from lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium trifluoromethanesulfonate (LiOTF), lithium bis(fluoromalonic acid)borate (LiBOB), lithium bis(fluoromalonic acid)borate (LiBFMB), and lithium difluorooxalateborate (LiDFOB). These lithium salts can synergistically work with the ethoxylated and nitrogen-containing additives of this application to regulate the SEI film composition, further enhance film stability and lithium-ion conduction, suppress side reactions, and jointly improve the battery's fast-charging capability, cycle life, and safety.

[0089] Typically, but not limitingly, the other additives in the electrolyte are present in weight percentages of 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 6%, 7%, 8%, 9%, 10%, or any combination of two values.

[0090] In a preferred embodiment, the electrolyte further includes a solvent comprising one or more of linear carbonates, cyclic carbonates, and carboxylic acid esters. Linear carbonates include one or more of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, and fluorinated linear carbonates; and / or cyclic carbonates include one or more of ethylene carbonate, propylene carbonate, and butenyl carbonate; and / or carboxylic acid esters include one or more of methyl formate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, and fluorinated carboxylic acid esters. These solvents can synergistically work with the additives of this application, stabilizing and uniformly dispersing the additives, enhancing their effectiveness, forming a more stable SEI layer, further reducing side reactions during battery cycling, improving battery safety, and preventing thermal runaway caused by overcharging.

[0091] The inventors further optimized the type and composition of the cathode material. In a preferred embodiment, the cathode includes a cathode active material, which comprises a lithium-nickel transition metal oxide with the chemical formula LiNi. x Co y A (1-x-y) O2, wherein A includes one or more of manganese, aluminum, magnesium, chromium, calcium, zirconium, molybdenum, silver, and niobium, 0.5≤x≤1, 0≤y≤0.5, x+y≤1, preferably, the lithium nickel transition metal oxide includes one or more of NCA, NCM333, NCM523, NCM622, NCM811, Ni90, Ni92, and Ni95. The stable and highly conductive interface layer formed by the additives in this application can more effectively slow down the structural degradation of the above-mentioned cathode material during charge and discharge processes, thereby further extending the cycle life of the battery.

[0092] In a preferred embodiment, the positive electrode active material further includes a phosphate compound with the chemical formula LiMn. k E (1-k) PO4, wherein 0 ≤ k ≤ 1, and E includes one or more of iron, cobalt, magnesium, calcium, zinc, chromium, and lead, preferably, the phosphate compound includes lithium iron phosphate and LiMn. 0.6 Fe 0.4 PO4 or LiMn 0.8 Fe 0.2At least one of PO4. Typically, but not limitingly, k is 0, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1, or any combination of two values. The above-mentioned positive electrode active material can synergistically work with the additives of this application to optimize the SEI film structure, further enhance lithium-ion diffusion, and improve the battery's fast-charging performance, cycle stability, and safety. Especially under high-rate charge and discharge conditions, it can more effectively suppress structural changes in the positive electrode material and extend battery life.

[0093] In a preferred embodiment, the negative electrode active material in the negative electrode active material layer includes a silicon-based material, which includes one or more of silicon, silicon alloys, silicon oxides, and silicon-carbon compounds; preferably, the weight percentage of the silicon-based material in the negative electrode active material is 10-100%. The additives of this application can interact with the above-mentioned negative electrode active material to form a more stable SEI film, protecting the negative electrode material from electrolyte corrosion, more effectively buffering the volume change of the silicon-based material during charge and discharge, and improving the conductivity of the negative electrode, thereby further improving the charge and discharge efficiency, cycle stability, and safety of the battery.

[0094] The inventors further optimized the type and composition of the negative electrode material. In a preferred embodiment, the negative electrode further includes one or more of carbon-based materials, metallic materials, and conductive polymers. Preferably, the carbon-based material includes one or more of natural graphite, artificial graphite, silicon-carbon composite materials (silicon-carbon or silicon-oxygen), lithium titanate, carbon black, acetylene black, Ketjen black, and carbon fiber; and / or, the metallic material includes one or more of copper, nickel, aluminum, and silver, in a granular or fibrous form; and / or, the conductive polymer includes polyphenylene derivatives. The aforementioned types of carbon-based materials are more conducive to the stable existence of nitrogen and oxygen elements in the SEI film, thereby further enhancing lithium-ion conduction and suppressing side reactions; the high conductivity of the aforementioned types of metallic materials, combined with nitrogen-containing lithium salts, can further accelerate lithium-ion migration and improve fast-charging efficiency; the aforementioned types of conductive polymers can synergistically optimize the SEI film structure with additives, further increasing film toughness and reducing interfacial impedance during cycling, thereby further improving the cycle stability and energy density of the battery.

[0095] In a preferred embodiment, the positive and / or negative electrodes further include a binder and / or a conductive agent. In a preferred embodiment, the binder includes one or more of the following: polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride (PVDF), polyethylene, polypropylene, styrene-butadiene rubber (SBR), acrylated SBR, epoxy resin, and nylon. These types of binders can further improve the bonding force between the active material particles and also facilitate better bonding between the active material and the current collector.

[0096] In a preferred embodiment, the positive electrode further includes a positive current collector, which comprises a metal foil and / or a composite current collector. Preferably, the metal foil comprises aluminum foil; and / or, the composite current collector is prepared by a process of forming a metallic material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer substrate. In a preferred embodiment, the negative electrode further includes a negative current collector, which comprises one or more of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, and a polymer substrate coated with a conductive metal.

[0097] In a preferred embodiment, a separator is provided between the positive and negative electrodes to prevent short circuits. The material and shape of the separator used in the embodiments of this application are not particularly limited, and can be any technology disclosed in the prior art. In some embodiments, the separator comprises a polymer or inorganic material formed from a material stable to the electrolyte of this application. The inorganic layer comprising inorganic material includes inorganic particles and a binder. The inorganic particles include one or more 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, and barium sulfate; and / or, the binder includes one or more of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer comprising a polymer includes one or more of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride, and poly(vinylidene fluoride-hexafluoropropylene).

[0098] In a preferred embodiment, the separator may include a substrate layer and a surface treatment layer. The substrate layer is a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer includes one or more of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Specifically, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be selected. At least one surface of the substrate layer is provided with a surface treatment layer, which may be a polymer layer or an inorganic layer, or a layer formed by mixing polymers and inorganic materials.

[0099] In typical but not limiting lithium-nickel transition metal oxides, x is 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1 or any two of these values, and y is 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5 or any two of these values.

[0100] In a preferred embodiment, the method for preparing a secondary battery includes providing electrode assemblies, electrolyte injection, encapsulation, and formation. In some embodiments, the formation temperature is 40°C to 50°C, for example, 41°C, 42°C, 43°C, 44°C, 45°C, 46°C, 47°C, 48°C, or 49°C. In a preferred embodiment, the formation pressure is 150–250 kgf, for example, 160 kgf, 170 kgf, 180 kgf, 190 kgf, 200 kgf, 210 kgf, 220 kgf, 230 kgf, or 240 kgf. In a preferred embodiment, the formation charging current is 0.05–0.1 C, and the discharge current is 0.1–0.3 C. In a preferred embodiment, the formation includes: charging to 4.2V with a 0.05C current and letting stand for 60 minutes at a temperature of 40-50°C (preferably 45°C) and a pressure of 150-250 kgf (preferably 200 kgf), followed by charging to 4.2V with a 0.1C current and then discharging to 3.0V with a 0.2C current.

[0101] Excessive charging or discharging current during formation can cause a rapid increase in battery internal temperature, increasing the risk of overheating. This can damage battery performance and lifespan, and may also lead to instability in internal chemical reactions, affecting charge / discharge efficiency and energy density. Conversely, insufficient charging or discharging current during formation results in lower production efficiency and reduced electrochemical performance, cycle life, and safety. Excessive formation pressure can deform or damage the battery's electrode structure, hindering internal chemical reactions and reducing electrochemical performance, potentially even posing safety risks. Insufficient pressure can lead to poor contact between the positive and negative electrodes, causing localized overheating and increased resistance, all of which negatively impact battery performance, lifespan, and safety.

[0102] In a preferred embodiment, the secondary battery is a lithium secondary battery or a sodium secondary battery. In a preferred embodiment, the lithium secondary battery includes one or more of lithium metal secondary batteries, lithium-ion secondary batteries, lithium polymer secondary batteries, and lithium-ion polymer secondary batteries.

[0103] In a preferred embodiment, the secondary battery includes an outer packaging, which can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc.; and / or, the outer packaging can be a soft pack, such as a pouch-type soft pack, the material of which can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS). In a preferred embodiment, the shape of the secondary battery is not particularly limited, and it can be cylindrical, square, or any other arbitrary shape.

[0104] In a preferred embodiment, this application also provides a battery module including the aforementioned secondary battery. The battery module of this application uses the aforementioned secondary battery, and therefore has at least the same advantages as a secondary battery. The battery module of this application can contain multiple secondary batteries, and the specific number can be adjusted according to the application and capacity of the battery module.

[0105] In a preferred embodiment, this application also provides a battery pack including the aforementioned battery modules. The number of battery modules included in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0106] In another typical embodiment of this application, an electrical device is also provided, including the aforementioned secondary battery. The electrical device includes, but is not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and energy storage systems. The electrical device employing the secondary battery of this application can enhance its fast charging and cycle performance through an optimized SEI film, significantly improving overall energy density, power output, and durability, making it suitable for applications requiring high energy, fast charging, and long-term stable operation. To meet the device's demand for high power and high energy density from the secondary battery, a battery pack or battery module can be used. The electrical device can also be a mobile phone, tablet computer, laptop computer, etc. This electrical device is lightweight and thin, and can use a secondary battery as its power source.

[0107] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0108] Example 1

[0109] The preparation steps for the positive electrode sheet are as follows: Weigh out the positive electrode active material LiNi according to the weight ratio of positive electrode active material: conductive agent: binder = 95:3:2. 0.9 Co 0.05 Mn 0.05 A mixture of O2, conductive carbon nanotubes, and acetylene black (in a weight ratio of 2:1), and a binder of polyvinylidene fluoride (PVDF) is thoroughly homogenized in an N-methylpyrrolidone (NMP) solvent system, then coated onto a 12μm thick aluminum-coated current collector, dried, and rolled to obtain the positive electrode sheet.

[0110] The negative electrode preparation steps are as follows: The negative electrode active material, conductive agent, binder, thickener, and polyacrylic acid are prepared in a weight ratio of 95:2:1.5:1:0.5. The negative electrode active material is then mixed with silicon oxide (SiO₂). x The negative electrode sheet was obtained by thoroughly homogenizing a graphite composite (with a silicon-to-graphite weight ratio of 14:86), conductive agent acetylene black, binder styrene-butadiene rubber (SBR), thickener sodium carboxymethyl cellulose (CMCNa), and polyacrylic acid (PAA) in deionized water, coating it onto the surface of an 8μm thick copper current collector, drying, rolling, and slitting. The negative electrode sheet was obtained by a two-stage rolling process with a rolling temperature of 120℃ and a rolling speed of 2min / m, and the resulting negative electrode sheet had a tortuosity of 6.2.

[0111] The diaphragm is a three-layer composite PP / PE / PP membrane.

[0112] Electrolyte preparation: In an argon-filled glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), lithium salt LiPF6 and solvent EC / DMC / EMC = 25 / 20 / 55 were mixed evenly to prepare a 1 M solution. Finally, ethoxylated compound additive (EOEA, 1% by weight in the electrolyte) and nitrogen-containing compound additive (LiFSI, 3.5% by weight in the electrolyte) were added and stirred evenly to obtain the electrolyte.

[0113] Preparation of lithium-ion batteries: The prepared positive electrode, separator, and negative electrode are stacked in sequence, with the separator in the middle of the positive and negative electrode, and wound to obtain a bare cell. The bare cell is placed in an aluminum-plastic film outer packaging, and after being fully dried, it is injected with the prepared lithium-ion battery electrolyte. The battery is then subjected to a process of being placed at 45°C for 48 hours, high-temperature fixture formation (formation conditions are: temperature 45°C, pressure 200 kgf, 0.05C current charging to 4.2V and standing for 60 minutes, then 0.1C charging to 4.2V, then 0.2C discharging to 3.0V, and repeating this process twice), and secondary sealing. After this process, conventional capacity testing is performed to obtain a secondary battery - a lithium-ion battery.

[0114] Examples 2 to 11

[0115] The only difference from Example 1 is:

[0116] The composition of the electrolyte varies, as detailed in Table 1.

[0117] Examples 12 to 18

[0118] The only difference from Example 1 is:

[0119] The composition of the electrolyte and the manufacturing process parameters of the battery vary, as detailed in Table 1.

[0120] Comparative Example 1

[0121] The only difference from Example 8 is that it does not include ethoxylated additives.

[0122] Comparative Example 2

[0123] The only difference from Example 8 is that it does not include nitrogen-containing additives.

[0124] Comparative Example 3

[0125] The only difference from Example 8 is that the content of nitrogen-containing additives is too high, as detailed in Table 1.

[0126] Comparative Example 4

[0127] The only difference from Example 8 is that it contains too much ethoxylated additive, as detailed in Table 1.

[0128] Comparative examples 5 to 6

[0129] The only difference from Example 8 is that the rolling load on the negative electrode sheet is different in the preparation steps. The tortuosity of the negative electrode sheet is also different, as detailed in Table 1.

[0130] Comparative examples 7 to 13

[0131] The only difference from Example 8 is that the process parameters in the formation step of preparing the secondary battery are different, as detailed in Table 1.

[0132] The components and parameters of the secondary batteries prepared in the above embodiments and comparative examples, as well as the preparation process parameters, are shown in Table 1. The test results of the performance parameters of the secondary batteries are shown in Table 2.

[0133] Test method:

[0134] 1. Determination of electrode tortuosity:

[0135] The tortuosity of the electrode can be obtained using image recognition analysis. Specifically, the morphology image of the electrode surface is first captured using a scanning electron microscope (SEM). This image is then imported into Wolfram Mathmatica software. The tortuosity index estimation code file (CDF) is run to calibrate the active material particle outlines in the surface image. The Fit button is clicked to calculate the a, b, and c-axis features and particle orientation angles of the calibrated particles. The Calculate button is then clicked to calculate the tortuosity indices aX, aY, and aZ in the X, Y, and Z directions. The aZ value is the tortuosity index of the electrode, where tortuosity τ = ε. -aZ .

[0136] Wherein, ε represents the porosity of the electrode, which is determined using a mercury porosimeter. Specifically, the dried electrode sample is cut into thin strips of a certain size, and the apparent volume of the electrode coating is measured using a micrometer. Apparent volume = sample coating thickness × sample length × sample width. The electrode is then degassed under vacuum, wound up, and placed in the sample cell, ensuring that the sample volume is 40-70% of the effective volume of the sample tube to ensure measurement accuracy. Then, the pore volume of the sample, i.e., the volume of mercury pressed into the sample, is measured using a mercury porosimeter. Therefore, porosity ε = pore volume / apparent volume.

[0137] 2. Battery internal resistance test:

[0138] The lithium-ion battery was discharged at a constant current of 1C to the cutoff voltage of 3.0V, and then left to rest at 20±2℃ for 1 hour. It was then charged at a current of 1C for 18 minutes, the SOC was adjusted to 30%, and left to rest for 1 hour. Next, it was charged at a current of 3C for 1.5 minutes and left to rest for 1 hour. Then, it was discharged at a current of 9C for 0.5 minutes and left to rest for 1 hour. Finally, it was charged at a constant current of 1C for 6 minutes, the SOC was adjusted to 40%, and left to rest for 1 hour. This cycle was repeated until the SOC reached 70%. The DC resistance (DCR) of the battery was then calculated using the formula R = ΔU / ΔI.

[0139] 3. Battery cycle capacity retention test:

[0140] At 25°C, the lithium-ion battery was charged at a constant current of 2C to 4.25V, then charged at a constant voltage of 4.25V to 0.05C, and finally discharged at a constant current of 2C to 2.5V. After 500 charge-discharge cycles, the capacity retention rate after the 500th cycle at 25°C was calculated using the following formula: Discharge capacity after 500 cycles / Discharge capacity of the first cycle × 100%.

[0141] At 45°C, the lithium-ion battery was charged at a constant current of 2C to 4.25V, then charged at a constant voltage of 4.25V to 0.05C, and then discharged at a constant current of 2C to 2.5V. After 400 charge-discharge cycles, the capacity retention rate after the 400th cycle at 45°C was calculated using the following formula: Discharge capacity after 400 cycles / Discharge capacity of the first cycle × 100%.

[0142] 4. Battery thickness change rate test at 45℃:

[0143] The battery was discharged to 3.0V at a constant current of 0.5C at 25℃, then charged to 4.45V at a constant current of 0.5C, and then charged to 0.05C at a constant voltage of 4.45V. The thickness of the battery at this point was measured using a PPG soft-pack battery thickness gauge and recorded as 'a'. The battery was then placed in an oven and stored at a constant voltage of 4.45V at 45℃ for 15 days. The thickness after 15 days was recorded as 'b'. The formula for calculating the thickness expansion rate is: (ba) / a × 100%.

[0144] 5. Testing of oxygen and nitrogen content in the SEI membrane:

[0145] The lithium-ion battery was discharged to 2.5V at a current of 0.1C. The lithium-ion battery was then disassembled in an argon-filled glove box to obtain the electrode sheets. The obtained positive electrode sheets were cut into 8mm × 8mm test samples and immersed in a low-boiling-point dimethyl carbonate (DMC) solvent for half an hour. After being completely dried, they were pasted onto the XPS sample stage with the surface of the positive electrode active material layer facing upwards, and measurements were taken without exposure to the atmosphere.

[0146] The specific test conditions and steps are as follows: AlKα spectroscopy of single crystals was used. For X-ray points, an elliptical shape of 1000×1750μm with an output of 10KV and 22mA was used. Data was selected when the sputtering etching time was 0 seconds. 284.8eV was used for neutral carbon C1s. For data processing, such as peak differentiation, 3-point smoothing, peak area measurement, background subtraction, and peak synthesis were used to calculate the weight percentage of oxygen in the ethoxy compounds contained in the SEI film and nitrogen in the lithium nitride contained in the SEI film.

[0147] Table 1

[0148] Table 2

[0149] As can be seen from the above, compared with the comparative example, the embodiments of this application control the type of electrolyte in the secondary battery and control the content of nitrogen and oxygen elements in the solid electrolyte interphase (SEI) film formed on the surface of the negative electrode active material layer within a specific range, so that the SEI film has a fast lithium-ion transport speed, while maintaining density and stability. This effectively suppresses the phenomenon of continuous side reactions and electrolyte consumption caused by the contact between the electrolyte and the negative electrode active material, thereby extending the cycle life of the secondary battery.

[0150] Specifically, nitrogen-containing and ethoxylated compounds from the electrolyte in the SEI can rapidly transport lithium ions and reduce impedance, thereby improving the battery's fast-charging and low-temperature performance. Furthermore, the nitrogen-containing and ethoxylated compounds exhibit a good synergistic effect in their structure, further improving the density and flexibility of the SEI film. This effectively suppresses SEI film rupture during high-temperature cycling, thereby reducing negative impacts such as continuous side reactions between the electrolyte and the negative electrode, and extending the cycle performance of the secondary battery, especially its cycle life and storage performance at high temperatures. Based on these improvements, the secondary battery of this application exhibits excellent fast-charging performance, as well as good cycle performance, storage performance, and safety performance at both high and low temperatures.

[0151] Furthermore, it can be seen that when all process parameters are within the preferred range of this application, the overall performance of the secondary battery is better.

[0152] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A secondary battery, characterized in that, Including positive electrode, negative electrode, and electrolyte, The electrolyte includes ethoxylated additives and nitrogen-containing additives; The negative electrode includes a negative electrode active material layer and a solid electrolyte interface film located on the surface of the negative electrode active material layer. The solid electrolyte interface film contains an ethoxy compound and lithium nitride. The ethoxy compound is derived from the ethoxy-containing additive, and the lithium nitride is derived from the nitrogen-containing additive. The weight percentage of oxygen from the ethoxy compound in the solid electrolyte interface film, as determined by X-ray photoelectron spectroscopy, is defined as W. O The weight percentage of nitrogen element from the lithium nitride in the solid electrolyte interface film is defined as W. N %, of which 3≤2W O +W N ≤10.

2. The secondary battery according to claim 1, characterized in that, 0.05 < W O <5, preferably, 1≤W O ≤3; and / or, 1 < W N <10, preferably, 2≤W N ≤8.

3. The secondary battery according to claim 1 or 2, characterized in that, The tortuosity of the negative electrode is defined as τ, where 0 < 2(W N ×W O )-τ<1.

4. The secondary battery according to claim 3, characterized in that, 2.5 < τ < 9.

5. The secondary battery according to any one of claims 1 to 4, characterized in that, The ethoxylated additive includes one or more of ethoxyacrylate, ethoxyethylene carbonate and ethoxyethylene sulfite; Preferably, the ethoxyacrylate has the structure shown in general formula (I-1): In the general formula (I-1), n ​​is an integer from 1 to 4, and R 11 Selected from hydrogen, C1-C3 alkyl, vinyl carboxylate, or propenyl carboxylate, R 12 Selected from hydrogen and C1-C3 alkyl groups; Preferably, the ethoxyethylene carbonate has the structure shown in general formula (I-2): In the general formula (I-2), m is an integer from 1 to 4, and R 13 Selected from hydrogen, C1-C3 alkyl, vinyl carboxylic acid esters or propenyl carboxylic acid esters; Preferably, the ethoxyethylene sulfite has the structure shown in general formula (I-3): In the general formula (I-3), p is an integer from 1 to 4, and R 14 Selected from hydrogen, C1-C3 alkyl, vinyl carboxylic acid esters or propenyl carboxylic acid esters; And / or, The nitrogen-containing additives include one or more of the following: nitrile compounds, phosphazenes, amides, organic nitrogen-containing lithium salts, inorganic nitrogen-containing alkali metal salts, nitrate esters, and nitro esters; Preferably, the nitrile compound has the structure shown in general formula (II-1): In the general formula (II-1), R 21 Selected from C2-C10 alkylene groups or nitrile-substituted C2-C10 alkylene groups, R 22 Selected from hydrogen, nitrile, C1-C6 alkyl, or carboxylic ester groups; Preferably, the phosphazene has the structure shown in general formula (II-2): In the general formula (II-2), R 23 Selected from C1-C6 alkyl or fluorinated C1-C6 alkyl, R 24 R 25 R 26 R 27 and R 28 Each is independently selected from hydrogen, fluorine, C1-C6 alkyl, or fluorinated C1-C6 alkyl, and R 24 R 25 R 26 R 27 and R 28 At least one of them is fluorine or a fluorinated C1-C6 alkyl group; Preferably, the amide has the structure shown in general formula (II-3): In the general formula (II-3), R 29 R 210 R 211 Each is independently selected from hydrogen, C1-C6 alkyl, or fluorinated C1-C6 alkyl, and R 29 R 210 R 211 At least one of them is a fluorinated C1-C6 alkyl group; Preferably, the organic nitrogen-containing lithium salt has the structure shown in general formula (II-4) or general formula (II-5): In the general formula (II-4), R 212 R 213 R 214 Each group is independently selected from hydrogen, fluorine, C1-C6 alkyl, fluorinated C1-C6 alkyl, or nitrile groups, and R 212 R 213 R 214 At least one of them is selected from fluorine, fluorinated C1-C6 alkyl or nitrile groups; In the general formula (II-5), R 215 R 216 Each is independently selected from fluorine, C1-C6 alkyl, or fluorinated C1-C6 alkyl, and R 215 and R 216 At least one of them is selected from fluorine or fluorinated C1-C6 alkyl groups; Preferably, the inorganic nitrogen-containing alkali metal salt has the structure shown in general formula (II-6): R 217 NO t General formula (II-6); In the general formula (II-6), t is selected from 2 or 3; R 217 Selected from lithium, sodium, or potassium; Preferably, the nitrate ester has the structure shown in general formula (II-7): R 218 NO r General formula (II-7); In the general formula (II-7), r is 3; R 218 Selected from C1 to C4 alkyl groups; Preferably, the nitro ester has the structure shown in general formula (II-8): R 219 NO s General formula (II-8); In the general formula (II-8), s is 2; R 219 Selected from C1 to C4 alkyl groups.

6. The secondary battery according to claim 5, characterized in that, The ethoxyacrylate includes one or more selected from ethoxyethoxyethyl acrylate, 2-methoxyethyl acrylate, and triethylene glycol diacrylate; and / or, The ethoxyethylene carbonate includes methyl ethoxyethylene carbonate and / or methoxyethoxymethyl ethoxyethylene carbonate; and / or, The ethoxyethylene sulfite includes ethoxymethylethylene sulfite; and / or, The nitrile compound includes one or more of succinic anhydride, adiponitrile, glutaronitrile, hexanetrionitrile, ethylene glycol (bis)propionitrile ether, methoxypropionitrile, 2,3-dimethoxypropionitrile, and methyl cyanoacetate; and / or, The phosphazene includes one or more of methoxypentafluorocyclotriphosphazene, trifluoromethoxypentafluorocyclotriphosphazene, ethoxypentafluorocyclotriphosphazene, and trifluoroethoxypentafluorocyclotriphosphazene; and / or, The amide includes trifluoroformamide or trifluoroacetamide; and / or, The organic nitrogen-containing lithium salt includes one or more of 4,5-dicyano-2-(trifluoromethyl)imidazolium lithium, (fluorosulfonyl)(perfluorobutylsulfonyl)imide lithium, bis(fluorosulfonyl)imide lithium, bistrifluoromethylsulfonylimide lithium, and bis(pentafluoroethylsulfonic acid)imide lithium; and / or, The inorganic nitrogen-containing alkali metal salt includes one or more of lithium nitrate, lithium nitrite, sodium nitrate, and sodium nitrite; and / or, The nitrate esters include ethyl nitrate and / or propyl nitrate; and / or, The nitro esters include nitromethane and / or nitromethane.

7. The secondary battery according to any one of claims 1 to 6, characterized in that, In the electrolyte, the weight percentage of the ethoxylated additive is 0.05–4%; and / or, In the electrolyte, the nitrogen-containing additive has a weight percentage of 0.5% to 10%. Preferably, the weight ratio of the ethoxylated additive to the nitrogen-containing additive is (0.05-4):

1.

8. The secondary battery according to any one of claims 1 to 7, characterized in that, The electrolyte comprises a mixture of ethoxyethoxyethyl acrylate and lithium bis(fluorosulfonyl)imide in a weight ratio of (0.05–4):1; or, The electrolyte comprises a mixture of ethoxyethoxyethyl acrylate and ethoxypentafluorocyclotriphosphazene in a weight ratio of (2-4):1; or, The electrolyte comprises a mixture of ethoxyethoxyethyl acrylate and ethyl nitrate in a weight ratio of (1-4):

1.

9. The secondary battery according to any one of claims 1 to 8, characterized in that, The electrolyte further includes other additives, including one or more selected from vinylene carbonate, ethylene carbonate, lithium difluorophosphate, tris(trimethylsilane) phosphate, tris(trimethylsilane) borate, fluoroethylene carbonate, difluoroethylene carbonate, propylene trifluorocarbonate, methyl ethyl 2,2,2-trifluorocarbonate, diethyl 2,2,2-trifluorocarbonate, tris(trifluoroethyl) phosphate, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether; the weight percentage of the other additives in the electrolyte is 0.1-10%; and / or, The positive electrode includes a positive electrode active material, which comprises a lithium-nickel transition metal oxide with the chemical formula LiNi. x Co y A (1-x-y) O2, wherein A includes one or more of manganese, aluminum, magnesium, chromium, calcium, zirconium, molybdenum, silver, and niobium, 0.5≤x≤1, 0≤y≤0.5, x+y≤1; and / or, The negative electrode active material in the negative electrode active material layer includes a silicon-based material, which includes one or more of silicon, silicon alloys, silicon oxides, and silicon carbide compounds; preferably, the weight percentage of the silicon-based material in the negative electrode active material is 10-100%.

10. An electrical device, characterized in that, The secondary battery includes any one of claims 1 to 9.

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