Secondary battery and electric device

Abstract

The application discloses a secondary battery and a power consumption device, and belongs to the technical field of batteries. An active material layer on a negative electrode sheet is constructed into a composite structure, the silicon content of the active material in different layers is regulated, and an electrolyte with a specific composition is compounded, so that the secondary battery can not only maintain high cycle stability at normal temperature, but also can maintain high capacity and high stability in fast charging cycles, and has excellent comprehensive performance.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to a secondary battery and an electrical device. Background Technology

[0002] Silicon-based anode materials exhibit more ideal specific capacity compared to traditional graphite-based electrodes. However, due to the low lithium-ion electron conductivity and lithium-ion diffusion coefficient of silicon-based materials, their kinetic performance is poor. Furthermore, silicon-based materials experience significant volume effects during lithium insertion / extraction, making it difficult to achieve good fast-charging performance. Therefore, researchers have attempted to modify the anode materials by introducing modifying materials, but this approach may lead to a reduction in the capacity of the secondary battery. Summary of the Invention

[0003] The purpose of this application is to overcome the shortcomings of the existing technology and provide a secondary battery. By constructing the active material layer on the negative electrode as a composite structure, controlling the silicon content of the active material in different layers, and compounding an electrolyte with a specific composition, the resulting secondary battery can not only maintain high cycle stability at room temperature, but also maintain high capacity and high stability in fast charging cycles, resulting in excellent overall performance.

[0004] To achieve the above objectives, in a first aspect of this application, a secondary battery is provided, comprising a negative electrode and an electrolyte; The negative electrode sheet includes a current collector and an active material layer, wherein the active material layer includes a first active material layer on the side away from the current collector and a second active material layer on the side close to the current collector; The first active material layer contains a first active material, the second active material layer contains a second active material, and at least one of the first active material and the second active material contains a silicon-containing material; The mass of silicon in the first active material layer is greater than the mass of silicon in the second active material layer; The electrolyte contains acetate solvents and fluorosulfonic acid derivative additives; The secondary battery satisfies: 0.067≤1 / (a / 0.05+100b+1)≤0.2, and 30≤10000b×k≤150; Where a is the mass percentage of acetate solvent in the electrolyte solvent, b is the mass percentage of fluorosulfonic acid derivative additive in the electrolyte, and k is the total mass percentage of silicon in the first active material layer and the second active material layer.

[0005] As an embodiment of this application, a = 5~25%.

[0006] As an embodiment of this application, b = 0.5~10%.

[0007] As an embodiment of this application, c = 1~60%, where c is the mass percentage of silicon in the first active material.

[0008] As an embodiment of this application, d = 0~60%, where d is the mass percentage of silicon in the second active material.

[0009] As an embodiment of this application, the secondary battery satisfies: e = 0.5~2, where e is the mass ratio of the first active material to the second active material.

[0010] As an embodiment of this application, the secondary battery satisfies: 0% < k ≤ 30%.

[0011] As an embodiment of this application, the structure of the acetate solvent is as follows: ; R1 and R2 are each independently at least one of alkyl and fluoroalkyl groups.

[0012] As an embodiment of this application, the structure of the fluorosulfonic acid derivative additive is as follows: ; R3 and R4 are independently F, C1-C8 alkanes, and C1-C8 fluoroalkanes. or And at least one of R3 and R4 is a structural group with the following structure: or ; R5, R6 and R7 are each independently at least one of F, C1-C8 alkanes, C1-C8 fluoroalkanes, and benzene ring groups, or R6 and R7 form a cyclic structure with N, and at least one of R3, R4, R5, R6 and R7 contains fluorine.

[0013] In a second aspect of this application, an electrical device is provided, including the secondary battery, which serves as the power supply for the electrical device.

[0014] The beneficial effects of this application are as follows: This application provides a secondary battery that constructs a composite structure by adding active material layers to the negative electrode, controlling the silicon content of the active materials in different layers, and compounding an electrolyte with a specific composition. This allows the secondary battery to maintain high cycle stability not only at room temperature but also in fast charging cycles, resulting in excellent overall performance. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0016] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0017] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0018] The present application is further illustrated below with specific embodiments: A secondary battery includes a negative electrode and an electrolyte; The negative electrode sheet includes a current collector and an active material layer, wherein the active material layer includes a first active material layer on the side away from the current collector and a second active material layer on the side close to the current collector; The first active material layer contains a first active material, the second active material layer contains a second active material, and at least one of the first active material and the second active material contains a silicon-containing material; The mass of silicon in the first active material layer is greater than the mass of silicon in the second active material layer; The electrolyte contains acetate solvents and fluorosulfonic acid derivative additives; The secondary battery satisfies: 0.067≤1 / (a / 0.05+100b+1)≤0.2, and 30≤10000b×k≤150; Where a is the mass percentage of acetate solvent in the electrolyte solvent, and b is the mass percentage of fluorosulfonic acid derivative additive in the electrolyte.

[0019] To balance the energy density and kinetic performance of the secondary battery, achieving superior performance in both cycle and fast-charging tests, this application's technical solution constructs a composite structure for the active material layer of the negative electrode. Simultaneously, a silicon element mass relationship is established for the active material layers at different locations, creating a gradient distribution of silicon. The first active material layer, located at the top and with a larger contact area with the electrolyte, has a relatively higher silicon content, effectively shortening the lithium-ion transport path for the silicon material and providing more transport sites for lithium-ion silicon-lithium alloying. Furthermore, to match the kinetic performance requirements of the silicon-containing negative electrode, the electrolyte is compounded with a specific amount of acetate solvent and a fluorosulfonic acid derivative additive, ensuring that the amounts of both satisfy the aforementioned relationship. The limitations are twofold: firstly, acetate solvents can effectively ensure superior kinetic performance, reduce the steric hindrance of lithium ion conduction in the electrolyte, and decrease polarization effects; secondly, fluorosulfonic acid derivative additives can improve the stability of the SEI film formed on the surface of the negative electrode, while reducing interfacial charge transfer impedance, thus synergistically improving lithium ion conduction efficiency. By controlling the content of both within a specific range, and further synergistically limiting the content of fluorosulfonic acid derivative additives based on the silicon content in the active material layer of the negative electrode, the increased side reactions caused by acetate solvents in the later stages of secondary battery cycling, and the excessively thick SEI film caused by excessive fluorosulfonic acid derivative additives, can be avoided, reducing the negative impact on lithium ion transport kinetic performance and ensuring optimal electrolyte matching.

[0020] More preferably, the secondary battery satisfies: 0.07≤1 / (a / 0.05+100b+1)≤0.15.

[0021] The introduction of acetate solvents and fluorosulfonic acid derivative additives can improve the kinetic performance and deintercalation stability of lithium ions in the secondary battery, thereby improving the cycle stability and rate performance of the secondary battery. Even at higher rates, a large capacity can be achieved. When the secondary battery meets the above-mentioned preferred range, the cycle performance and rate performance of the secondary battery can be further improved.

[0022] In some embodiments, the structure of the acetate solvent is as follows: R1 and R2 are each independently at least one of alkyl and fluoroalkyl groups.

[0023] Preferably, R1 is methyl or fluoromethyl.

[0024] More preferably, the acetate solvent includes at least one of methyl acetate (A1), ethyl acetate (A2), and ethyl 2,2-difluoroacetate (A3).

[0025] Compared to common carbonate or propionate solvents, introducing acetate solvents into the electrolyte, especially those with short molecular chains, good wettability, and low viscosity, can effectively increase the rate of lithium-ion conduction in the electrolyte, and also improve the charge transfer kinetics at the electrode interface. On the other hand, these solvents also participate in the lithium-ion solvation structure, further reducing the energy barrier of lithium-ion desolvation, accelerating charge transfer, and further improving the fast-charging performance of the secondary battery.

[0026] In some implementations, a = 5~25%.

[0027] More preferably, 'a' is a range of one or any two of the following: 5%, 6%, 8%, 10%, 12%, 15%, 17%, 18%, 20%, and 25%.

[0028] As mentioned above, although the introduction of acetate solvents can improve the kinetic performance of lithium ions in secondary batteries, when the content of the acetate solvent in the total solvent of the electrolyte is preferably within the above-mentioned ratio range, the probability of the solvent reacting with the electrode side after multiple cycles can also be controlled at a low level, ensuring that the expansion rate of the electrode is at a low level.

[0029] In some embodiments, the structure of the fluorosulfonic acid derivative additive is as follows: R3 and R4 are independently F, C1-C8 alkanes, and C1-C8 fluoroalkanes. or And at least one of R3 and R4 is a structural group with the following structure: or ; R5, R6 and R7 are each independently at least one of F, C1-C8 alkanes, C1-C8 fluoroalkanes, and benzene ring groups, or R6 and R7 form a cyclic structure with N, and at least one of R3, R4, R5, R6 and R7 contains fluorine.

[0030] More preferably, the fluorosulfonic acid derivative additive comprises at least one of the following structures (B1) to (B4): (B1); (B2); (B3); (B4).

[0031] In some implementations, b = 0.5~10%.

[0032] More preferably, b is a range of one or any two of 0.5%, 1%, 3%, 5%, 8%, and 10%.

[0033] In some implementations, b = 2.5~8%.

[0034] Compared to other types of additives, fluorosulfonic acid derivative additives preferentially undergo reduction reactions when the electrolyte contacts the negative electrode, forming a high-mechanical-strength SEI film. This improves the negative electrode interface effect in the electrolyte. Simultaneously, since fluorine-containing inorganic substances are generated after the reaction, these substances can further enhance the ion conductivity of the SEI film, assisting in improving the charge-discharge performance of the negative electrode. When the content of the aforementioned substances in the electrolyte is preferably within the above-mentioned range, the total amount and thickness of the SEI film can be further coordinated within a moderate range, allowing the efficiency of lithium ion conduction at the interface between the negative electrode and the electrolyte to be maintained at a higher level.

[0035] In some embodiments, the electrolyte further includes carbonate solvents.

[0036] More preferably, the carbonate solvent includes at least one of ethylene carbonate (EC), propylene carbonate (PC), and dimethyl carbonate (DEC).

[0037] In some embodiments, the electrolyte also includes other types of carboxylic acid ester solvents, including but not limited to propionate ester solvents.

[0038] More preferably, the propionate solvent includes at least one of ethyl propionate (EP) and propyl propionate (PP).

[0039] In some embodiments, the electrolyte also includes auxiliary additives.

[0040] More preferably, the auxiliary additive includes at least one of fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), 1,3-propanesulfonate lactone (PS), vinyl sulfate (DTD), vinylene carbonate (VC), succinate (SN), adiponitrile (ADN), and 1,3,6-hexanetrionitrile (HTCN).

[0041] More preferably, the total mass content of the fluorosulfonic acid derivative additive and auxiliary additives in the electrolyte is 0.5-30%.

[0042] In some embodiments, the electrolyte also contains lithium salt.

[0043] More preferably, the lithium salt includes at least one of lithium hexafluorophosphate (LiPF6), lithium difluorophosphate (LiDFP), lithium difluorobis(oxalate) phosphate (LiODFP), lithium tetrafluoro(oxalate) phosphate (LiOTFP), lithium bis(oxalate) borate (LiBOB), lithium difluoro(oxalate) borate (LiODFB), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium bis(fluorosulfonyl)imide (LiFSI).

[0044] More preferably, the lithium salt in the electrolyte has a mass percentage content of 8-20%.

[0045] It should be noted that the mass percentage content of each component in the electrolyte described in this application can be confirmed by, but is not limited to, GC-MS testing. The specific method is as follows: After discharging the secondary battery to 3V at a rate of 0.33C, the secondary battery was disassembled. If free electrolyte was present, it was collected directly into a 5mL sample tube using a pipette and sealed with adhesive tape. If no free electrolyte was present, the disassembled secondary battery was pressurized using a hydraulic press FY-30 until free electrolyte appeared. 5mL of this electrolyte was collected into a sample tube using a pipette and sealed with adhesive tape. Subsequently, the electrolyte sample was injected into an Agilent Intuvo 9000 gas chromatograph to test the electrolyte composition. The mass content percentage of each component in the electrolyte (including acetate solvents, carbonate solvents, propionate solvents, fluorosulfonic acid derivative additives, auxiliary additives, and lithium salts) was obtained by comparing with a standard database. 'a' is calculated based on the total mass content of all solvents after confirming the solvent composition and mass content of each solvent in the electrolyte.

[0046] In some embodiments, the first active material and / or the second active material further contain graphite.

[0047] More preferably, the graphite includes at least one of artificial graphite and natural graphite.

[0048] More preferably, the silicon-containing material includes a silicon-carbon composite material.

[0049] In some embodiments, the silicon-carbon composite material has a silicon content of 1-60% by mass.

[0050] More preferably, the average particle size of the silicon-carbon composite material is 5~15μm.

[0051] When graphite and silicon-carbon composite materials are combined to construct an active material layer, the composite active layer and gradient distribution of silicon-carbon composite materials described in this application not only ensure that the surface silicon-carbon composite material can effectively exert its capacity, but also shorten the ion transport path. At the same time, the graphite material can provide a complete three-dimensional electron transport network for the silicon-carbon composite material, improve the overall conductivity of the material, thereby realizing rapid ion / electron conduction and optimizing the overall material dynamics.

[0052] In some embodiments, c = 1~60%, where c is the mass percentage of silicon in the first active material.

[0053] More preferably, c is a range of one or any two of the following: 1%, 5%, 10%, 15%, 20%, 25%, 30%, 40%, 45%, 50%, and 60%.

[0054] More preferably, c = 25~60% In some embodiments, d = 0~60%, where d is the mass percentage of silicon in the second active material.

[0055] More preferably, d is a range of one or any two of the following: 0%, 1%, 5%, 10%, 15%, 20%, 25%, 30%, 40%, 45%, 50%, and 60%.

[0056] More preferably, d = 0~15% It should be noted that the second active material layer, which is located near the current collector, can be pure graphite, i.e., d=0%, and is still within the protection scope of this application.

[0057] In some embodiments, the secondary battery satisfies: 0.5≤e≤2, where e is the mass ratio of the first active material to the second active material.

[0058] In the secondary battery, the two active material layers are set with different silicon content, and the lithium-ion conduction paths and conductivity of the two are different. At this time, maintaining the preferred ratio of the two active materials can indirectly control the thickness of the material layer to keep it within a moderate range, thereby avoiding the first active material layer from having an excessively long longitudinal lithium-ion transport path, which would lead to an excessive difference in the conduction rate of lithium ions in the entire active material layer, thereby weakening the dynamic performance. It can also maintain the dimensional stability of the entire active material layer.

[0059] In some embodiments, the secondary battery satisfies: 0% < k ≤ 30%.

[0060] More preferably, the secondary battery satisfies: 6% < k ≤ 30%.

[0061] In the technical solution of this application, the amount of silicon-containing material introduced is set through the composite gradient, resulting in higher ion conduction efficiency and dimensional stability of the negative electrode sheet. Compared with the existing traditional silicon-based negative electrode system, the overall silicon content of the electrode sheet can be increased to a maximum of 30%, thereby effectively improving the charge and discharge capacity of the secondary battery and achieving better electrochemical performance.

[0062] In some embodiments, the first active material layer and / or the second active material layer further contain a conductive agent, a binder, and a thickener.

[0063] More preferably, the conductive agent comprises carbon nanotubes.

[0064] Further preferably, the binder and thickener in the first and second active material layers of this application are used to improve the adhesion between active material particles and the adhesion between the active material and the current collector. Any binder can be used without particular limitation, as long as it has suitable adhesive properties and does not significantly cause adverse chemical changes in the battery. For example, the thickener includes, but is not limited to, carboxymethyl cellulose or its salts, and the binder includes, but is not limited to, styrene-butadiene rubber (SBR) and polyacrylic acid (PAA).

[0065] More preferably, in the first active material layer, the mass ratio of the first active material, the conductive agent, the binder, and the thickener is (95~99):(0.5~1.5):(1~2):(0.5~1.5).

[0066] More preferably, in the second active material layer, the mass ratio of the second active material, the conductive agent, the binder, and the thickener is (95~99):(0.5~1.5):(1~2):(0.5~1.5).

[0067] It should be noted that, in the technical solution of this application, the c / d / e / k ratio of the negative electrode sheet can be confirmed by, but is not limited to, the following methods: The secondary battery is discharged at a rate of 0.33C to 3V, the battery is disassembled to obtain the negative electrode sheet, which is then soaked in dimethyl carbonate (DMC) for 2 hours, dried, and the active material layer is scraped with powder of different thicknesses using a scraper. After scraping to a certain thickness, it is cut, and the thickness of the first and second active material layers is confirmed by observation using a scanning electron microscope. The content and distribution of silicon in the first and second active material layers are determined using EDS surface scanning, and the obtained powder is only for the first active material layer. For the batches of scraped powder containing only the first active material layer and the second active material layer, the scraped powder samples containing only the first active material layer and the second active material layer were soaked in water for 2 hours to remove the binder and thickener. After drying, the mixtures of the first active material and the conductive agent and the second active material and the conductive agent were obtained. The mixtures were weighed and then XRD tests were performed on the two materials respectively. Subsequently, TOPAS combined with the Rietveld method was used to quantitatively analyze the silicon-carbon composite material, graphite and carbon nanotubes in the first and second active material layers to confirm the mass of the silicon-carbon composite material and graphite. Finally, the c / d / e / k range of the secondary battery was calculated and confirmed.

[0068] In some embodiments, the secondary battery further includes a positive electrode and a separator.

[0069] In some embodiments, the positive electrode includes a current collector and a positive electrode material layer disposed on the current collector. The positive electrode material layer includes a positive electrode active material, and the mass content of the positive electrode active material in the positive electrode material layer is 93-97%.

[0070] In some embodiments, the positive electrode active material includes at least one of lithium cobalt oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, and lithium iron phosphate.

[0071] In some embodiments, the positive electrode material layer further includes at least one of a conductive agent and a binder.

[0072] More preferably, the adhesive comprises at least one of polyvinylidene fluoride, polyvinyl butyral, polytetrafluoroethylene, ethylene-vinyl acetate copolymer, and polyvinyl alcohol.

[0073] More preferably, the conductive agent includes at least one of graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0074] In some embodiments, the diaphragm includes at least one of a polypropylene diaphragm, a polyethylene diaphragm, and a polyvinylidene fluoride diaphragm.

[0075] In some embodiments, this application provides an electrical device including the secondary battery, which serves as the power supply for the electrical device.

[0076] The present invention is further illustrated below with specific embodiments, which should not be construed as limiting the scope of protection claimed by the present invention: Example 1 A secondary battery, the method for preparing the secondary battery includes the following steps: (1) Preparation of negative electrode sheet: Silicon-carbon composite material with an average particle size of 9 μm (silicon content of 60% by mass) and artificial graphite were compounded at a mass ratio of 40:60 as the first active material. Then, it was mixed with conductive agent (carbon nanotubes), binder (PAA) and thickener (sodium carboxymethyl cellulose) in water at a mass ratio of 97:1:1.2:0.8 to prepare the first active material layer slurry. Then, artificial graphite was used as the second active material, and it was mixed with conductive agent (carbon nanotubes), binder (PAA) and thickener (sodium carboxymethyl cellulose) in water. Sodium cellulose was mixed in water at a mass ratio of 97:1:1.2:0.8 to prepare a second active material layer slurry. This slurry was pre-coated onto a current collector (copper foil), dried, and the mass ratio of the first to second active material layers in the slurry was controlled to be 1. The first active material layer slurry was then coated again, dried at 85°C, cold-pressed, trimmed, and vacuum-dried at 85°C for 12 hours. The tabs were then welded to obtain the negative electrode sheet, which has a compacted density of 1.7 g / cm³. 3 ; (2) Preparation of the positive electrode sheet: Using commercially available lithium cobalt oxide with an average particle size of 5 μm as the positive electrode active material, a slurry was prepared by mixing it with a conductive agent (SP) and a binder (PVDF) in N-methylpyrrolidone at a mass ratio of 97:2:1. The slurry was coated onto a current collector (aluminum foil), dried at 85°C, cold-pressed, trimmed, and vacuum-dried at 85°C for 6 hours. The electrode tabs were then welded to obtain the positive electrode sheet. The compacted density of the positive electrode sheet was 4.25 g / cm³. 3 ; (3) Preparation of electrolyte: EC, PC, DEC, EP, PP and methyl acetate (Al) were mixed in an inert atmosphere at a mass ratio of 10:20:30:20:0:20 as a solvent. Then, 15% lithium hexafluorophosphate and the following additives were added according to the total electrolyte mass percentage: 20% FEC, 3% DFEC, 1% DTD, 1% ADN, 1.5% HTCN and 1% of fluorosulfonic acid derivatives with the following structures: ; Mix thoroughly to obtain the electrolyte; (4) Preparation of secondary battery: The positive electrode, commercially available polyethylene separator and negative electrode are stacked and wound together. The cell capacity is set to 5Ah and the voltage range is 3~4.55V. Then the cell is encapsulated with aluminum-plastic film, vacuum dried at 85℃ for 48h, liquid is injected, the liquid injection coefficient is controlled to be 1.5g / Ah, encapsulated, formed, capacity tested, and sealed again (liquid retention coefficient 1.3g / Ah) to obtain the secondary battery.

[0077] Examples 2-17 A secondary battery differs from Example 1 only in the parameters used in its preparation, as shown in Table 1.

[0078] In each embodiment, c and / or d are adjusted by adjusting the mass ratio of silicon-carbon composite material (silicon content 60% in both) to graphite material in the first active material and / or the second active material.

[0079] In Example 2, the electrolyte solvent was formulated as follows: EC, PC, DEC, EP, PP and methyl acetate were mixed in a mass ratio of 10:20:30:20:10:10 as the solvent. The electrolyte solvent in Example 3 was formulated as follows: EC, PC, DEC, EP, PP and methyl acetate were mixed in a mass ratio of 10:20:30:20:15:5 as the solvent. In Examples 9 and 11, the acetate solvent was replaced with ethyl acetate (A2) at an equal mass percentage, and the fluorosulfonic acid derivative additive was replaced with the following structural substance at an equal mass percentage in the electrolyte: ; In Examples 10 and 12, the acetate solvent was replaced with ethyl 2,2-difluoroacetate (A3) at an equal mass percentage, and the fluorosulfonic acid derivative additive was replaced with the following structural substance at an equal mass percentage in the electrolyte: ; Meanwhile, the difference between Example 10 and Example 1 is that the composition ratio of the solvents in the electrolyte is different. The content of acetate solvents in the solvent is reduced from 20% to 5%, while the total amount of the remaining carbonate solvents and propionate solvents is increased by 15% to keep the total solvent mass unchanged, and the mass ratio between the remaining solvents remains unchanged.

[0080] The difference between Example 13 and Example 1 is that the fluorosulfonic acid derivative additive is replaced with the following structural substance in an equal mass percentage of the electrolyte: Examples 14, 15, and 16 use the same electrolyte as Example 1, except that the proportion of silicon-carbon composite material in the first active material is increased, resulting in differences in c and k.

[0081] Comparative Examples 1-8 A secondary battery differs from Example 1 only in the parameters used in its preparation, as shown in Table 1.

[0082] Each comparison was made by adjusting the mass ratio of silicon-carbon composite material (silicon content 60% in both) to graphite material in the first and / or second active materials, thereby adjusting c and / or d.

[0083] The electrolyte solvent formulations in Comparative Examples 2 and 3 were as follows: EC, PC, DEC, EP, and PP were mixed in a mass ratio of 10:20:30:20:20 as solvents. In Comparative Example 2, no fluorosulfonic acid derivative additives were added, and the solvent was used to make up the mass of its electrolyte.

[0084] In Comparative Example 4, no fluorosulfonic acid derivative additives were added; the mass of the electrolyte was made up by solvent.

[0085] The proportions of solvent components in the electrolytes of Comparative Examples 5-7 and Example 1 are different. The content of acetate solvents in the solvents are 2%, 5% and 18% respectively. The total amount of the remaining carbonate and propionate solvents is made up to 100% of the total amount, and the mass ratio between the remaining solvents remains unchanged. The additives containing fluorosulfonic acid derivatives are different, and the total amount of electrolyte is kept constant by making up the amount of solvent.

[0086] Wherein, under the premise of limiting k, the total mass of the first active material and the second active material remains unchanged when adjusting e in each embodiment and comparative example.

[0087] Table 1 Example of effect To verify the performance of the secondary battery described in this application, the secondary batteries obtained in each embodiment and comparative example were subjected to the following tests: (1) Room temperature cycle test: Before testing each battery, the thickness H0 of each battery is measured in advance. Then, the secondary battery is charged to 4.55V at a constant current and constant voltage at a rate of 0.7C at 25℃, and the cut-off current is 0.05C. Then, it is discharged to 3V at a constant current at a rate of 0.5C. The first cycle discharge capacity A0 is recorded. Then, the above steps are repeated 400 times. The discharge capacity A1 at the 400th cycle is recorded. The thickness H1 of the secondary battery is measured. The cycle capacity retention rate of the secondary battery is calculated according to 100%×A1 / A0. The thickness growth rate of the battery is calculated according to 100%×(H1-H0) / H0.

[0088] (2) Rate performance test: At 25°C, the secondary battery was charged to 4.55V at a constant current and constant voltage of 0.7C, with a cutoff current of 0.05C. Then, it was discharged to 3V at a constant current of 0.5C. The above process was repeated 10 times. Then, the charge and discharge cycles were repeated 10 times at 1.2C, 2C, 3C, and 4C rates. During charging, the battery was charged to 4.55V at a constant current and constant voltage of 0.05C, with a cutoff current of 0.05C. During discharging, the battery was discharged to 3V at a constant current of 0.5C. The discharge capacity after 10 cycles at 4C was recorded. Then, the battery was charged to 4.55V at a constant current and constant voltage of 0.7C, with a cutoff current of 0.05C. Then, it was discharged to 3V at a constant current of 0.5C. The above process was repeated 10 times. The discharge capacity of the secondary battery obtained in the last cycle was recorded and recorded as the rate recovery capacity of the secondary battery.

[0089] (3) Electrolyte conductivity test: Before assembling each secondary battery, the electrolyte was injected into the coin cell with steel plates for both positive and negative electrodes in a 25°C environment. The impedance value was measured using an electrochemical workstation, and the ionic conductivity of the electrolyte was calculated using the obtained impedance value. The calculation formula is as follows: Electrolyte conductivity = impedance value / (steel sheet area × diaphragm thickness) The test results are shown in Table 2.

[0090] Table 2 The test results show that the secondary battery described in this application, based on the specific settings of the electrolyte and the negative electrode, not only achieves good cycle stability and battery stability during long cycles, but also maintains a capacity retention rate of over 88% after 400 cycles, while keeping the battery thickness growth rate within 16%, demonstrating excellent cycle performance. On the other hand, when considering rate performance, even at 4C, due to the high capacity of the silicon-containing material and the rapid insertion / extraction of lithium ions between the positive and negative electrodes, the secondary battery can still achieve a capacity of over 2400mAh. Furthermore, due to the structural stability of the active material layer in the negative electrode, the capacity of the secondary battery can reach over 4700mAh after high-rate recovery, demonstrating excellent rate performance. This is mainly due to the double-layer coating structure and settings in the negative electrode, the setting of the gradient silicon content in the coating, and the synergistic regulation of the corresponding electrolyte composition.

[0091] In contrast, Comparative Examples 1 and 2 did not follow the limitations of this application by setting the silicon-containing material content in a specific order through gradient. The silicon-carbon composite material and graphite in the negative electrode could not exert the expected synergistic effect, resulting in poor electrochemical performance of the secondary battery. In Comparative Examples 3 and 4, the electrolyte did not introduce acetate solvents or fluorosulfonic acid derivative additives, making it incompatible with the negative electrode of the specific composite active material layer in this application. This resulted in low ion conduction efficiency and significant loss of active lithium, failing to achieve the same cycle performance or rate performance as the product in the examples. In Comparative Examples 5-8, although two active material layers were set on the negative electrode, the improper combination of the mass ratio of acetate solvent in the solvent (a) and the mass content of fluorosulfonic acid derivative additive in the electrolyte (b) also failed to effectively improve the lithium-ion transport effect of the negative electrode. Instead, due to electrolyte side reactions or an excessively thick SEI film, the cycle performance or rate performance of the secondary battery decreased significantly. The negative electrode setting and electrolyte setting of the secondary battery will affect the cycle performance and rate performance of the product. Only when the electrolyte contains two key components at the same time, and the secondary battery is set to meet 1 / (a / 0.05+100b+1) and 10000b×k, can the secondary battery achieve the expected effect.

[0092] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this article and are not intended to limit the scope of protection of this article. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this article without departing from the essence and scope of the technical solutions of this article.

Claims

1. A secondary battery, characterized in that, Including the negative electrode and the electrolyte; The negative electrode sheet includes a current collector and an active material layer, wherein the active material layer includes a first active material layer on the side away from the current collector and a second active material layer on the side close to the current collector; The first active material layer contains a first active material, the second active material layer contains a second active material, and at least one of the first active material and the second active material contains a silicon-containing material; The mass of silicon in the first active material layer is greater than the mass of silicon in the second active material layer; The electrolyte contains acetate solvents and fluorosulfonic acid derivative additives; The secondary battery satisfies: 0.067≤1 / (a / 0.05+100b+1)≤0.2, and 30≤10000b×k≤150; Where a is the mass percentage of acetate solvent in the electrolyte solvent, b is the mass percentage of fluorosulfonic acid derivative additive in the electrolyte, and k is the total mass percentage of silicon in the first active material layer and the second active material layer.

2. The secondary battery as described in claim 1, characterized in that, The percentage 'a' is greater than or equal to 5% and less than or equal to 25%.

3. The secondary battery as described in claim 1, characterized in that, The value of b is greater than or equal to 0.5% and less than or equal to 10%.

4. The secondary battery as described in claim 1, characterized in that, 1%≤c≤60%, and / or, 0≤d≤60%; Where c is the mass percentage of silicon in the first active material, and d is the mass percentage of silicon in the second active material.

5. The secondary battery as described in claim 1, characterized in that, The secondary battery satisfies: 0.5≤e≤2, where e is the mass ratio of the first active material to the second active material.

6. The secondary battery as described in claim 1, characterized in that, The secondary battery satisfies the following condition: 0% < k ≤ 30%.

7. The secondary battery as described in claim 1, characterized in that, The structure of the acetate solvent is as follows: ； R1 and R2 are each independently at least one of alkyl and fluoroalkyl groups.

8. The secondary battery as described in claim 1, characterized in that, The acetate solvents include at least one of methyl acetate, ethyl acetate, and ethyl 2,2-difluoroacetate.

9. The secondary battery as described in claim 1, characterized in that, The structure of the fluorosulfonic acid derivative additive is as follows: ； R3 and R4 are independently F, C1-C8 alkanes, and C1-C8 fluoroalkanes. or And at least one of R3 and R4 is a structural group with the following structure: or ; R5, R6 and R7 are each independently at least one of F, C1-C8 alkanes, C1-C8 fluoroalkanes, and benzene ring groups, or R6 and R7 form a cyclic structure with N, and at least one of R3, R4, R5, R6 and R7 contains fluorine.

10. The secondary battery as described in claim 1, characterized in that, The fluorosulfonic acid derivative additive includes at least one of the following structures: ； ； ； 。 11. The secondary battery as described in claim 1, characterized in that, The silicon-containing material is a silicon-carbon composite material; and / or, the electrolyte further includes carbonate solvents.

12. The secondary battery as described in claim 1, characterized in that, The electrolyte also includes auxiliary additives; the auxiliary additives include at least one selected from fluoroethylene carbonate, difluoroethylene carbonate, 1,3-propanesulfonate lactone, ethylene sulfate, succinate, adiponitrile, and 1,3,6-hexanetrionitrile.

13. The secondary battery as described in claim 12, characterized in that, The total mass content of the electrolyte containing fluorosulfonic acid derivative additives and auxiliary additives is greater than or equal to 0.5% and less than or equal to 40%.

14. The secondary battery as described in claim 1, characterized in that, The electrolyte also contains lithium salt; the mass percentage of the lithium salt in the electrolyte is greater than or equal to 8% and less than or equal to 20%.

15. The secondary battery as described in claim 14, characterized in that, The lithium salt includes at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluorobis(oxalate) phosphate, lithium tetrafluoro(oxalate) phosphate, lithium bis(oxalate) borate, lithium difluoro(oxalate) borate, lithium tetrafluoroborate, lithium bis(trifluoromethanesulfonyl)imide, and lithium difluorosulfonylimide.

16. An electrical appliance, characterized in that, Includes the secondary battery as described in any one of claims 1 to 15, wherein the secondary battery serves as the power supply for the electrical device.

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