Sodium ion secondary battery and electric device

Abstract

The invention provides a sodium ion secondary battery and a power utilization device, the secondary battery comprises a positive pole piece, the positive pole piece comprises a current collector and a positive pole film layer located on at least one side surface of the current collector, the positive pole film layer comprises a binder, the binder comprises a first polymer, the first polymer comprises a butadiene-M1 copolymer, and the first polymer comprises a second polymer. Wherein the M1 monomer comprises at least one of styrene or acrylonitrile, and based on the total mass of the first polymer, the mass content of the structural unit derived from the M1 monomer is 20-30%. The positive pole piece in the sodium ion secondary battery provided by the invention has excellent adhesive property, and the secondary battery has good cycle performance.

Description

Technical Field

[0001] This application relates to the field of secondary batteries, and more particularly to a sodium-ion secondary battery and an electrical device. Background Technology

[0002] Polyvinylidene fluoride (PVDF) is typically used as a binder for the positive electrode of secondary batteries. However, PVDF has poor alkali resistance and is prone to elimination reactions in alkaline environments, forming double bonds that cross-link the PVDF molecular chains. This leads to gelation, precipitation, and stratification of the positive electrode slurry during homogenization, preventing the formation of a uniform and stable dispersion system and severely impacting the processing and electrical performance of the positive electrode sheet. Simultaneously, the free hydrogen fluoride generated during the elimination reaction can damage the solid electrolyte interphase (SEI) film inside the battery and consume active ions, affecting the cycle performance of the secondary battery.

[0003] Therefore, there is an urgent need for a fluorine-free binder that can be applied to the positive electrode system of secondary batteries. Summary of the Invention

[0004] This application is made in view of the above-mentioned problems, and its purpose is to provide a fluorine-free binder that can be applied to the positive electrode system of sodium-ion secondary batteries. This fluorine-free binder can improve the cycle performance of the secondary battery while giving the positive electrode sheet excellent bonding performance.

[0005] A first aspect of the present invention provides a sodium-ion secondary battery, comprising a positive electrode sheet, the positive electrode sheet comprising a current collector and a positive electrode film layer located on at least one side surface of the current collector, the positive electrode film layer comprising an adhesive, the adhesive comprising a first polymer, the first polymer comprising a butadiene-M1 copolymer, the M1 monomer comprising at least one of styrene or acrylonitrile, and the mass content of structural units derived from the M1 monomer being 20%-30% based on the total mass of the first polymer.

[0006] In this application, the M1 monomer contains phenyl or nitrile groups, and the mass content of the structural units derived from the M1 monomer is 20%-30%. This allows the first polymer to contain a large number of phenyl or nitrile groups, resulting in good intermolecular forces between the first polymers and between the first polymer and the current collector. This can effectively alleviate the volume deformation of the positive electrode of the sodium-ion battery during charging and discharging, inhibit the detachment of the active material, maintain the mechanical integrity of the positive electrode, and thus improve the cycle performance of the sodium-ion secondary battery.

[0007] In any embodiment, the M1 monomer is selected from styrene or acrylonitrile.

[0008] In any embodiment, the weight-average molecular weight of the first polymer is 40W-80W.

[0009] The first polymer with a weight-average molecular weight of 40W-80W has good viscosity, which can enhance the interaction between the polymer, the current collector and the positive electrode solid material, so that the positive electrode sheet has excellent bonding performance.

[0010] In any embodiment, the mass content of structural units derived from the M1 monomer is 20%-25% based on the total mass of the first polymer.

[0011] The structural units derived from the M1 monomer have a mass content of 20%-25% in the polymer, which can further enhance the intermolecular forces between polymers and between the first polymer and the current collector, giving the positive electrode sheet excellent adhesion properties.

[0012] In any embodiment, the first polymer comprises a hydrogenated copolymer of butadiene monomer and M1 monomer.

[0013] In any embodiment, the first polymer comprises hydrogenated styrene-butadiene rubber or hydrogenated nitrile rubber.

[0014] In any embodiment, the degree of hydrogenation of the first polymer is 85%-99%.

[0015] The hydrogenation process can reduce the number of unsaturated units in the first polymer, making the first polymer softer and able to flow better and adhere tightly together under pressure. Under high compaction conditions, it helps the positive electrode sheet to have both excellent adhesion and flexibility, which is beneficial to the subsequent electrode sheet winding process.

[0016] In any embodiment, the first polymer comprises a maleic anhydride-grafted butadiene-M1 copolymer, wherein the maleic anhydride grafting rate is 1%-2%.

[0017] Maleic anhydride groups can provide polar aldehyde groups and nonpolar olefin segments. Maleic anhydride grafting modification facilitates the combination and compatibility of polar and nonpolar groups in the polymer, enabling better bonding of various solid materials with different polarities in the positive electrode. A maleic anhydride grafting rate of 1%-2% can enhance the interaction between the first polymer and the solid material of the positive electrode, giving the positive electrode excellent bonding strength.

[0018] In any embodiment, the mass content of the first polymer is 1%-2.5% based on the total mass of the positive electrode film. Adding an appropriate amount of the first polymer can reduce the impact on the loading of the positive electrode active material, enabling the positive electrode sheet to have both excellent adhesion properties and high energy density.

[0019] In any embodiment, the adhesive comprises a second polymer, the second polymer comprising a styrene-M2-styrene copolymer, wherein the M2 monomer comprises at least one of isoprene or butadiene.

[0020] Introducing a fluorine-free second polymer into the binder helps to balance the bonding and dispersing properties of the positive electrode slurry, improves the processing performance of the positive electrode slurry, and gives the positive electrode sheet excellent bonding strength. This can effectively maintain the mechanical integrity of the positive electrode sheet during cycling, thereby improving the cycle performance of the secondary battery.

[0021] In any embodiment, the second polymer has a weight-average molecular weight of 10W-30W, exhibiting suitable viscosity and good dispersibility. The use of the second polymer in combination with a high molecular weight (e.g., 40W-80W) first polymer facilitates the full dispersion of the first polymer, enhances the interaction between the polymer, the current collector, and the positive electrode solid material, and gives the positive electrode sheet excellent bonding properties.

[0022] In any embodiment, based on the total mass of the second polymer, the mass content of structural units derived from the M2 monomer is 60%-70%. The M2 monomer includes weakly polar olefin segments, and the mass content of structural units derived from the M2 monomer within the above range can improve the dispersibility of the second polymer, help improve the stability of the cathode slurry, and improve processing performance.

[0023] In any embodiment, based on the total mass of the positive electrode film, the mass content of the second polymer is 0.5%-1.5%, and / or the mass ratio of the first polymer to the second polymer is 1:(0.25-1.5).

[0024] When the mass content of the second polymer is within a suitable range and / or the mass ratio of the first polymer to the second polymer is within a suitable range, it is beneficial for the second polymer and the first polymer to be rationally and effectively compounded, so that the positive electrode sheet has excellent bonding performance.

[0025] In any embodiment, the binder comprises a third polymer, which comprises an ethylene-N1 copolymer, wherein the N1 monomer comprises at least one of 1-octene or 1-hexene.

[0026] Introducing a fluorine-free third polymer into the binder helps to balance the bonding and dispersing properties of the positive electrode slurry, improves the processing performance of the positive electrode slurry, and gives the positive electrode sheet excellent bonding strength. This can effectively maintain the mechanical integrity of the positive electrode sheet during cycling, thereby improving the cycle performance of the secondary battery.

[0027] In any embodiment, the weight-average molecular weight of the third polymer is 10W-30W.

[0028] The third polymer, with a weight-average molecular weight within the aforementioned range, exhibits suitable bonding and good dispersibility. The third polymer, when compounded with a high molecular weight (e.g., 40W-80W) first polymer, results in a slurry with excellent dispersibility and bonding properties.

[0029] In any embodiment, based on the total mass of the third polymer, the mass content of the structural units derived from the N1 monomer is 20%-30%.

[0030] In the third polymer, the mass content of structural units derived from N1 monomers within a suitable range can improve the dispersibility of the polymer, which helps to improve the stability of the cathode slurry and enhance processing performance.

[0031] In any embodiment, the third polymer comprises a maleic anhydride-grafted ethylene-N1 copolymer, wherein the maleic anhydride grafting rate is 0.5%-5%.

[0032] Using maleic anhydride-modified third polymers is beneficial because the polymers have both polar and non-polar groups. A maleic anhydride grafting rate of 0.5%-5% can improve the interaction force between high molecular weight third polymers and positive electrode solid materials of different polarities, giving the positive electrode sheet excellent adhesion properties.

[0033] Maleic anhydride-modified polymers can further improve the dispersibility of slurry media, contributing to the formation of a uniform and dense electrode structure and reducing membrane resistance. A suitable polymer grafting rate helps avoid excessive entanglement between binder molecular chains, preventing side reactions.

[0034] In any embodiment, based on the total mass of the positive electrode film, the mass content of the third polymer is 0.3%-1.5%; and or, the mass ratio of the first polymer to the third polymer is 1:(0.25-1.5).

[0035] When the mass content of the third polymer is within a suitable range, the binder exhibits good bonding and dispersing properties. When the mass ratio of the first polymer to the third polymer is within a suitable range, it facilitates a rational and effective blending of the third polymer and the first polymer, resulting in suitable bonding properties for the positive electrode sheet.

[0036] In any embodiment, based on the total mass of the positive electrode film, the mass content of the first polymer is 1%-2%, the mass content of the second polymer is 0.1%-0.5%, and the mass content of the third polymer is 0.1%-1.5%.

[0037] The binder comprises a first polymer, a second polymer, and a third polymer. The second and third polymers primarily function as dispersants, dispersing the high molecular weight first polymer and the positive electrode solid material. The first polymer mainly acts as a binder, providing adhesion between the slurry and the electrode. By appropriately proportioning the second and third polymers with the first polymer, the positive electrode can maintain good mechanical integrity during the secondary battery cycle, thereby improving the cycle performance of the secondary battery.

[0038] In any implementation, the positive electrode sheet satisfies at least one of the following conditions:

[0039] (1) The bonding strength of the positive electrode sheet is 22 N / m-36 N / m;

[0040] (2) The cohesive strength of the positive electrode is 18 N / m-30 N / m;

[0041] (3) The film resistance of the positive electrode is 0.6Ω-1.15Ω.

[0042] The positive electrode sheet exhibits excellent adhesion and cohesion, enabling it to withstand stress caused by volume changes in the film layer during cycling. The positive electrode film is less prone to detachment from the current collector and electrode breakage, thus improving the safety and lifespan of the secondary battery. Simultaneously, the positive electrode sheet possesses suitable resistance, which helps reduce the battery's internal resistance and improves the cycle performance of the secondary battery.

[0043] A second aspect of this application provides an electrical device including the positive electrode plate described in the first aspect of this application. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of a secondary battery according to one embodiment of this application;

[0045] Figure 2 yes Figure 1 An exploded view of a secondary battery according to an embodiment of this application is shown.

[0046] Figure 3 This is a schematic diagram of a battery module according to one embodiment of this application;

[0047] Figure 4 This is a schematic diagram of a battery pack according to one embodiment of this application;

[0048] Figure 5 yes Figure 4 An exploded view of a battery pack according to one embodiment of this application is shown;

[0049] Figure 6 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0050] Figure label:

[0051] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Casing; 52 Electrode assembly; 53 Cover plate. Detailed Implementation

[0052] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the secondary battery, its preparation method, and its power-consuming device. However, unnecessary details may be omitted. For example, detailed descriptions of well-known facts and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0053] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0054] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0055] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0056] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0057] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0058] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0059] The positive electrode system of secondary batteries typically uses polyvinylidene fluoride (PVDF) as a binder. PVDF has poor alkali resistance and is prone to elimination reactions in alkaline environments, forming double bonds that cross-link the PVDF molecular chains. This leads to gelation, precipitation, and stratification of the positive electrode slurry during homogenization, preventing the formation of a uniform and stable dispersion system and severely affecting the processing and electrical performance of the positive electrode sheet. Simultaneously, the free hydrogen fluoride (HF) produced in the elimination reaction is a highly corrosive acid that can erode and destroy the existing SEI film. After the SEI film is destroyed, a new SEI film needs to be reformed. This continuous destruction and reconstruction of the SEI film consumes active ions, ultimately causing the battery capacity to rapidly decline and preventing it from reaching its expected lifespan.

[0060] Therefore, a first aspect of this application provides a sodium-ion secondary battery, including a positive electrode sheet, the positive electrode sheet including a current collector and a positive electrode film layer located on at least one side surface of the current collector, the positive electrode film layer including a binder, the binder including a first polymer, the first polymer including a butadiene-M1 copolymer, the M1 monomer including at least one of styrene or acrylonitrile, and the mass content of structural units derived from the M1 monomer being 20%-30% based on the total mass of the first polymer.

[0061] In this document, the term "binder" refers to a chemical compound, polymer, or mixture that forms a colloidal solution or colloidal dispersion in a dispersion medium and can be used to bond positive electrode solid materials together.

[0062] In this document, the term "polymer" includes, on the one hand, an aggregate of chemically homogeneous macromolecules prepared by polymerization reactions, but differing in degree of polymerization, molar mass, and chain length. On the other hand, the term also includes derivatives of such macromolecular aggregates formed by polymerization reactions, i.e., compounds that can be obtained through reactions of the functional groups in the aforementioned macromolecules, such as addition or substitution, and can be chemically homogeneous or chemically heterogeneous. In this application, butadiene-M1 copolymer refers to a polymer prepared by polymerization of butadiene monomers and M1 monomers.

[0063] The sodium-ion secondary battery containing a first polymer binder provided in this application has improved cycle performance.

[0064] The reason for not wanting to be bound by any theory is that the phenyl groups in the styrene monomer or the nitrile groups in the acrylonitrile monomer in the M1 monomer help to form stronger van der Waals forces between molecules. The mass content of the structural units derived from the M1 monomer is 20%-30%, which can make the first polymer contain a large number of phenyl or nitrile groups. The polymer molecules form strong physical cross-links, and the interaction forces between the polymer, the current collector and the positive electrode material are improved. This can effectively alleviate the volume deformation of the positive electrode sheet of the sodium-ion battery during charging and discharging, inhibit the detachment of the active material, maintain the mechanical integrity of the positive electrode sheet, and thus improve the cycle performance of the secondary battery.

[0065] In addition, the binder provided in this application can reduce or avoid the introduction of hydrogen fluoride by binder degradation during the homogenization process of the positive electrode slurry and the cycling process of the secondary battery. Compared with fluorinated binders (e.g., PVDF), it can reduce or avoid the deterioration of battery electrical performance by free hydrogen fluoride.

[0066] In some embodiments, based on the total mass of the first polymer, the mass content of structural units derived from the M1 monomer is 20%-25%, 22%-30%, 23%-30%, 25%-30%, or 28%-30%. In some embodiments, based on the total mass of the first polymer, the mass content of structural units derived from the M1 monomer is 20%, 22%, 24%, 26%, 28%, 30%, or any combination of the aforementioned values ​​or any value within that range.

[0067] In this document, methods known in the art can be used to determine the structure and content of relevant structural units in compounds (e.g., structural units derived from monomer M1 in the first polymer described above, structural units derived from monomer M2 in the second polymer described below, and structural units derived from monomer N1 in the third polymer), such as nuclear magnetic resonance spectroscopy. Nuclear magnetic resonance spectroscopy is one of the most powerful tools for qualitative analysis of the composition and structure of various organic and inorganic substances, and sometimes quantitative analysis can also be performed. It can also be combined with infrared spectroscopy. For example, the national standard GB / T6040-2002, General Rules for Infrared Spectroscopy Analysis, can be referenced. ICP testing can also be used, for example, referring to standards YS / T1006.2-2014, GB / T23367.2-2009, or YS / T1028.5-2015. Specifically, according to the embodiments of this application, an ICP inductively coupled plasma atomic emission spectrometer (device model iCAP740) can be used, and measurements can be performed according to the manufacturer's instructions. Further analysis can be performed by combining pyrolysis gravimetric analysis and gas chromatography-mass spectrometry.

[0068] Within a certain range, increasing the mass content of structural units derived from M1 monomers in the first polymer can improve the interaction forces between the polymer, current collector, and positive electrode solid material, resulting in excellent adhesion properties of the positive electrode sheet. However, excessive mass content of structural units derived from M1 monomers can lead to excessively high adhesion performance of the binder and poor dispersion performance, which is detrimental to the preparation of the positive electrode slurry. Furthermore, excessive mass content of structural units derived from M1 monomers results in a low mass content of flexible segments in the first polymer, which is detrimental to the flexibility of the first polymer and the bending resistance of the electrode sheet. Controlling the mass content of structural units derived from M1 monomers in the first polymer within a suitable range can enable the positive electrode sheet to have excellent adhesion properties and good bending resistance.

[0069] In some embodiments, the weight-average molecular weight of the first polymer is 40W-80W. In this document, "W" means "ten thousand" or "10,000".

[0070] In some embodiments, the weight-average molecular weight of the first polymer is 50W-80W, 60W-80W, 40W-70W, or 50W-70W. In some embodiments, the weight-average molecular weight of the first polymer is 40W, 42W, 44W, 46W, 48W, 50W, 52W, 54W, 56W, 58W, 60W, 62W, 64W, 68W, 70W, 72W, 74W, 76W, 78W, 80W, or any combination of the aforementioned values ​​or any value within the range.

[0071] In this paper, the term "weight-average molecular weight" refers to the sum of the products of the weight fraction of molecules of different molecular weights in a polymer and their corresponding molecular weights.

[0072] In this article, the weight-average molecular weight (MAW) does not refer to a strict numerical value, but rather to a range of values ​​within 5% of the molecular weight. For example, a MAW of 40W means 40W ± 2W.

[0073] In this paper, the weight-average molecular weight of the first polymer may be measured using any method known in the art. For example, the weight-average molecular weight of the polymer may be determined using gel permeation chromatography (GPC).

[0074] Within a certain range, the higher the weight-average molecular weight of the first polymer, the stronger the interaction force between the polymer, the current collector, and the positive electrode solid material, resulting in excellent bonding properties of the positive electrode sheet. However, if the weight-average molecular weight of the first polymer is too high, it will cause the first polymer to form large aggregates, affecting the dispersion uniformity and processability of the positive electrode slurry. By controlling the molecular weight of the first polymer within a suitable range, the first polymer can improve the processability of the positive electrode slurry while maintaining good bonding properties.

[0075] In some embodiments, the first polymer comprises a hydrogenated copolymer of butadiene monomer and M1 monomer.

[0076] In some embodiments, the first polymer comprises hydrogenated styrene-butadiene rubber or hydrogenated nitrile rubber.

[0077] In some embodiments, the degree of hydrogenation of the first polymer is 85%-99%.

[0078] In some embodiments, the degree of hydrogenation of the first polymer is 88%-99%, 90%-99%, 95%-99%, or 97%-99%. In some embodiments, the degree of hydrogenation of the first polymer is 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or any combination thereof or any value within such range.

[0079] In this paper, the term "degree of hydrogenation" refers to the extent to which unsaturated double bonds in a polymer are hydrogenated.

[0080] In this document, the degree of hydrogenation of the first polymer may be measured using any method known in the art. For example, the degree of hydrogenation of the polymer may be determined using infrared spectroscopy.

[0081] The hydrogenation process can reduce the number of unsaturated units in the first polymer, making the polymer molecular chains linear and smoother, more flexible, and able to flow better and adhere tightly together under pressure. This allows the positive electrode sheet to still have excellent adhesion and flexibility under high compaction density, which is beneficial for subsequent electrode winding processes.

[0082] In some embodiments, the first polymer includes a grafting group, which, as an example, may be a maleic anhydride group. In some embodiments, the first polymer includes a maleic anhydride-grafted butadiene-M1 copolymer.

[0083] Maleic anhydride can impart polar groups (e.g., aldehyde groups) and non-polar groups (e.g., non-polar segments of olefins) to the first polymer. The first polymer modified with maleic anhydride can enhance the formation of stronger chemical bonds or hydrogen bonds between the first polymer and the positive electrode solid material, thereby giving the positive electrode sheet excellent bonding strength.

[0084] In some implementations, the grafting rate of maleic anhydride is 1%-2%.

[0085] In some embodiments, the grafting rate of maleic anhydride is 1.2%-2%, 1.5%-2%, 1.7%-2%, or 1.9%-2%. In some embodiments, the grafting rate of maleic anhydride is 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, or any value within a range of the aforementioned values.

[0086] In this paper, the term "grafting rate" refers to the extent to which a modified monomer is grafted onto the polymer backbone.

[0087] In this paper, the grafting rate of the first polymer can be measured using any method known in the art. For example, the grafting rate of the polymer can be determined by titration, nuclear magnetic resonance spectroscopy, or infrared spectroscopy.

[0088] Grafting typically involves introducing more groups of different polarities into the polymer macromolecular chain. These groups can comprehensively regulate the interaction forces between the polymer and the positive electrode solid material, as well as the interaction forces between the positive electrode film and the current collector, giving the positive electrode sheet excellent bonding strength. A grafting rate within the aforementioned range provides sufficient bonding performance while reducing the risk of excessive entanglement between molecular chains and side reactions caused by an excessively high grafting rate.

[0089] In some embodiments, the mass content of the first polymer is 1%-2.5% based on the total mass of the positive electrode film.

[0090] In some embodiments, based on the total mass of the positive electrode film, the mass content of the first polymer is 1.2%-2.5%, 1.5%-2.5%, 1.7%-2.5%, 2.0%-2.5%, or 2.2%-2.5%. In some embodiments, based on the total mass of the positive electrode film, the mass content of the first polymer is 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, 2.2%, 2.4%, 2.5%, or any value within any range of the above values.

[0091] In the positive electrode film, the first polymer acts as a binder. If its mass content is too high, it may lead to an overly rigid electrode, affecting its flexibility during battery winding or stacking, increasing manufacturing difficulty, and reducing the proportion of other active particles and conductive agents in the positive electrode film, thus affecting the battery's energy density and cell resistance. Conversely, if its mass content is too low, it will result in insufficient adhesion between active material particles, causing a decrease in the mechanical strength of the positive electrode and potentially leading to damage or delamination during battery charging and discharging. Controlling the mass content of the first polymer within a suitable range is beneficial for achieving both sufficient binding performance and high energy density in the system.

[0092] In some embodiments, the first polymer includes at least one of hydrogenated styrene-butadiene rubber, maleic anhydride-modified hydrogenated styrene-butadiene rubber, hydrogenated nitrile butadiene rubber, and maleic anhydride-modified hydrogenated nitrile butadiene rubber.

[0093] In some embodiments, the adhesive comprises the first polymer and a second polymer, the second polymer comprising a styrene-M2-styrene copolymer, wherein the M2 monomer comprises at least one of isoprene monomer or butadiene.

[0094] In some embodiments, the second polymer includes at least one of hydrogenated styrene-butadiene-styrene polymer and hydrogenated styrene-isoprene-styrene polymer.

[0095] The second polymer primarily serves to disperse the positive electrode slurry. Introducing a fluorine-free second polymer into the binder can improve the processing performance of the positive electrode slurry. By compounding the second polymer with the first polymer, the positive electrode slurry acquires suitable bonding and dispersing properties, which is beneficial for the positive electrode sheet to achieve good bonding strength. This effectively maintains the mechanical integrity of the positive electrode sheet during cycling, thereby improving the cycle performance of the sodium-ion secondary battery.

[0096] In addition, during the homogenization process of the positive electrode slurry for the secondary battery and during the cycling process of the secondary battery, the second polymer can reduce or avoid the hydrogen fluoride introduced by its own degradation and the resulting deterioration of the cycling performance of the secondary battery.

[0097] In some embodiments, the weight-average molecular weight of the second polymer is 10W-30W.

[0098] In some embodiments, the weight-average molecular weight of the second polymer is 15W-30W, 20W-30W, 20W-25W, or 25W-30W. In some embodiments, the weight-average molecular weight of the second polymer is 10W, 12W, 14W, 16W, 18W, 20W, 22W, 24W, 26W, 28W, 30W, or any combination of the above values ​​or any value within that range.

[0099] The second polymer has a weight-average molecular weight within a suitable range, along with appropriate viscosity and good dispersibility, which is beneficial for improving the processing performance of the cathode slurry.

[0100] In some embodiments, the degree of hydrogenation of the second polymer is 85%-99%. In some embodiments, the degree of hydrogenation of the second polymer is 87%-99%, 90%-99%, or 95%-99%. In some embodiments, the degree of hydrogenation of the second polymer is 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or any combination of the above values ​​or any value within a range.

[0101] Within a certain range, a higher degree of hydrogenation increases the number of saturated segments in the polymer, improving its chemical stability and flexibility. Controlling the degree of hydrogenation of the second polymer within a suitable range is beneficial for balancing the chemical stability and dispersibility of the binder.

[0102] In some embodiments, the mass content of structural units derived from the M2 monomer is 60%-70% based on the total mass of the second polymer.

[0103] In some embodiments, based on the total mass of the second polymer, the mass content of structural units derived from the M2 monomer is 63%-67% or 65%-70%. In some embodiments, based on the total mass of the second polymer, the mass content of structural units derived from the M2 monomer is 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, or any combination of the above values ​​or any value within a range.

[0104] The second polymer contains olefin groups with relatively weak polarity in the structural units derived from the M2 monomer, which is beneficial for improving the dispersion performance of the polymer, enhancing the dispersion of the positive electrode solid material and the stability of the positive electrode slurry, and improving the processing performance.

[0105] In some embodiments, based on the total mass of the positive electrode film, the mass content of the second polymer is 0.5%-1.5%. In some embodiments, based on the mass of the positive electrode film, the mass content of the second polymer is 0.5%, 0.6%, 0.8%, 1.0%, 1.2%, 1.5%, or any value within a range of any of the above values.

[0106] When the mass content of the second polymer is within a suitable range, it facilitates a rational and effective blending of the second polymer with the first polymer, improving the dispersion and processing performance of the positive electrode slurry. The interaction forces between the binder, current collector, and solid positive electrode materials result in excellent adhesion and cohesion in the positive electrode sheet, contributing to the formation of a denser active material layer, reducing interparticle voids and the length of electron and ion transport paths within the electrode, thus giving the positive electrode sheet good film resistance. Furthermore, a suitable mass content of the second polymer can reduce its impact on the loading of the positive electrode active material, resulting in a higher capacity for the secondary battery and comprehensively improving the cycle performance of the sodium-ion secondary battery.

[0107] In some embodiments, the mass content of the second polymer is 1%-1.2% based on the total mass of the positive electrode film, which can further improve the cycle performance of the secondary battery.

[0108] In some embodiments, based on the total mass of the positive electrode film, the mass ratio of the first polymer to the second polymer is 1:(0.25-1.5).

[0109] In some embodiments, based on the total mass of the positive electrode film, the mass ratio of the first polymer to the second polymer is 1:0.25, 1:0.6, 1:1, or 1:1.5. The binder includes the first polymer and the second polymer, wherein the second polymer acts as a dispersant, mainly dispersing the high molecular weight polymer and the slurry; the first polymer mainly acts as a binder, providing adhesion between the slurry and the electrode; by appropriately proportioning the second polymer and the first polymer, the positive electrode can have good adhesion and cohesion.

[0110] In some embodiments, based on the total mass of the positive electrode film, the mass ratio of the first polymer to the second polymer is 1:(0.6-1).

[0111] When the mass ratio of the first polymer to the second polymer is within a suitable range, it is beneficial for the second polymer to be rationally and effectively compounded with the first polymer. This increases the interaction between the binder, the current collector, and the positive electrode solid material, resulting in the positive electrode sheet having excellent adhesion and cohesion. This helps the positive electrode to form a denser active material layer, reduces the gaps between particles and the length of the electron and ion transport paths in the electrode, and gives the positive electrode sheet good film resistance and secondary battery cycle performance.

[0112] In some embodiments, the binder comprises the first polymer and the third polymer, the third polymer comprising an ethylene-N1 copolymer, wherein the N1 monomer comprises at least one of 1-octene monomer or 1-hexene.

[0113] Introducing a fluorine-free third polymer into the binder helps to balance the bonding and dispersing properties of the cathode slurry, improving its processing performance. The blending of the first and third polymers contributes to excellent bonding strength in the cathode electrode, maintaining its mechanical integrity during cycling and thus improving the cycle performance of sodium-ion batteries. Furthermore, as a fluorine-free polymer, the third polymer can reduce or prevent the introduction of hydrogen fluoride during its own degradation and the resulting deterioration in cycle performance during the homogenization process of the cathode slurry and the cycling process of the secondary battery.

[0114] In some embodiments, the weight-average molecular weight of the third polymer is 10W-30W.

[0115] In some embodiments, the weight-average molecular weight of the third polymer is 16W-30W or 18W-30W. In some embodiments, the weight-average molecular weight of the third polymer is 10W, 12W, 15W, 16W, 17W, 18W, 19W, 20W, 22W, 24W, 26W, 28W, 30W, or any combination of the above values ​​or any value within the range.

[0116] In the binder, the low molecular weight third polymer mainly acts as a dispersant, primarily dispersing the high molecular weight polymer and slurry; the high molecular weight first polymer mainly acts as a binder, providing adhesion between the slurry and the electrode; through the compounding of the low molecular weight polymer and the high molecular weight polymer, the positive electrode can have excellent bonding and processing properties.

[0117] In some embodiments, the mass content of structural units derived from the N1 monomer is 20%-30% based on the total mass of the third polymer.

[0118] In some embodiments, based on the total mass of the third polymer, the mass content of structural units derived from the N1 monomer is 20%-25% or 25%-30%. In some embodiments, based on the total mass of the third polymer, the mass content of structural units derived from the N1 monomer is 20%, 22%, 24%, 26%, 28%, 30%, or any value within a range of any of the above values.

[0119] The third polymer contains hydrocarbon segments in its structural units derived from the N1 monomer, which can adjust the polarity of the polymer. Controlling the mass content of the structural units derived from the N1 monomer in the third polymer within a suitable range can improve the dispersibility of the third polymer, which helps to improve the stability and processing performance of the positive electrode slurry, and makes the positive electrode sheet have excellent adhesion and uniform distribution.

[0120] In some embodiments, the third polymer includes grafting groups, such as maleic anhydride groups. In some embodiments, the third polymer includes a maleic anhydride-grafted ethylene-N1 copolymer, wherein the grafting rate of the maleic anhydride is 0.5%-5%.

[0121] In some embodiments, the third polymer comprises a maleic anhydride-grafted polymer, wherein the grafting rate of the maleic anhydride is 1%-5%, 1%-4%, 1-3%, or 1-2%. In some embodiments, the grafting rate of the maleic anhydride is 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or any value within a range of any of the above values.

[0122] Grafting typically involves introducing more functional groups into the polymer macromolecular chain. Using maleic anhydride-modified third polymers allows the polymer to possess both polar and non-polar groups. These groups with different polarities enhance the interaction between the polymer and the cathode material, conductive agent, and current collector, resulting in excellent adhesion properties for the cathode electrode. Within the aforementioned grafting rate of maleic anhydride, sufficient adhesive strength is provided while minimizing side reactions caused by excessive entanglement between molecular chains.

[0123] In some embodiments, the mass content of the third polymer is 0.3%-1.5% based on the total mass of the positive electrode film.

[0124] In some embodiments, based on the total mass of the positive electrode film, the mass content of the third polymer is 0.3%, 0.5%, 0.7%, 0.9%, 1.1%, 1.3%, 1.5%, or any value within a range of any of the above values.

[0125] When the mass content of the third polymer is within a suitable range, it facilitates a rational and effective blending of the third polymer with the first polymer, improving the dispersion and processing performance of the positive electrode slurry. This increases the interaction forces between the polymer, the current collector, and the solid positive electrode material, resulting in excellent adhesion and cohesion of the positive electrode sheet. This helps form a denser active material layer, reduces interparticle voids and the length of electron and ion transport paths in the electrode, and gives the positive electrode sheet good film resistance. On the other hand, it can reduce the impact on the loading of the positive electrode active material, enabling the secondary battery to have a higher capacity and comprehensively improving the cycle performance of sodium-ion secondary batteries.

[0126] In some embodiments, the mass content of the third polymer is 1%-1.2% based on the total mass of the positive electrode film, which is beneficial to further improving the cycle performance of the secondary battery.

[0127] In some embodiments, based on the total mass of the positive electrode film, the mass ratio of the first polymer to the third polymer is 1:(0.25-1.5).

[0128] In some embodiments, based on the total mass of the positive electrode film, the mass ratio of the first polymer to the third polymer is 1:0.25, 1:0.6, 1:0.75, 1:1, 1:1.25, or 1:1.5.

[0129] In the above embodiments, the binder includes a first polymer and a third polymer, wherein the third polymer mainly acts as a dispersant, primarily dispersing high molecular weight polymers and slurry; the first polymer mainly acts as a binder, providing adhesion between the slurry and the electrode sheet; by rationally proportioning the third polymer and the first polymer, the positive electrode sheet as a whole can have excellent bonding strength.

[0130] In some embodiments, based on the total mass of the positive electrode film, the mass ratio of the first polymer to the third polymer is 1:(0.6-1).

[0131] When the mass ratio of the first polymer to the third polymer is within a suitable range, it is beneficial for the third polymer to be rationally and effectively compounded with the first polymer, further improving the interaction force between the binder, current collector, and positive electrode solid material. This results in the positive electrode sheet having excellent adhesion and cohesion. High adhesion and cohesion help to form a denser active material layer, reduce the gaps between particles, and reduce the length of the electron and ion transport path in the electrode, giving the positive electrode sheet good film resistance, thereby improving the cycle performance of the secondary battery.

[0132] In some embodiments, based on the total mass of the positive electrode film, the mass content of the first polymer is 1%-2%, the mass content of the second polymer is 0.1%-0.5%, and the mass content of the third polymer is 0.1%-1.5%.

[0133] In the above embodiments, the second and third polymers mainly act as dispersants, primarily dispersing high molecular weight polymers and slurries; the first polymer mainly acts as a binder, providing adhesion between positive electrode solid materials and between the positive electrode sheet and the current collector; by rationally proportioning the second and third polymers with the first polymer, the positive electrode sheet can have excellent bonding strength, and the cycle performance of the secondary battery can be improved.

[0134] In some embodiments, based on the total mass of the positive electrode film, the mass content of the first polymer is 1%-1.5%, the mass content of the second polymer is 0.2%-0.4%, and the mass content of the third polymer is 0.8%-1.3%.

[0135] By rationally proportioning the second and third polymers with the first polymer within the aforementioned mass content range, the interaction between the binder, current collector, and positive electrode solid material can be further enhanced. The positive electrode sheet exhibits excellent adhesion and cohesion, which helps to form a denser active material layer, maintain mechanical integrity during cycling, and reduce interparticle voids and the length of electron and ion transport paths in the electrode. The positive electrode sheet also has good adhesion properties and film resistance, comprehensively improving the cycle performance of the secondary battery.

[0136] In the above embodiments, during the production and use of the electrode, the polymer usually does not undergo volatilization, reaction, or other consumption behaviors, and it can be assumed that the mass content of the polymer in the electrode is consistent with the amount of raw materials used in the original preparation.

[0137] In some embodiments, the positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including a positive electrode active material and a binder, the binder including the polymer described in this application.

[0138] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0139] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0140] In some embodiments, the positive electrode film layer includes a positive electrode active material and a conductive agent, wherein the positive electrode active material includes at least one of sodium-containing transition metal oxides and their respective modified compounds, Prussian blue analogs, and polyanionic compounds.

[0141] In some embodiments, the sodium-containing transition metal oxide includes sodium transition metal oxide. The transition metal may be one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce.

[0142] In some embodiments, the sodium-containing transition metal oxide is, for example, Na. y AO2, where A is one or more of Ti, V, Mn, Co, Ni, Fe, Cr, and Cu, and 0 <y≤1。

[0143] In some embodiments, the polyanionic compound may be a sodium ion, a transition metal ion, or a tetrahedral (YO4) compound. n- A class of compounds with anionic units, wherein the transition metal may include one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce, Y may include one or more of P, S, and Si, and n represents (YO4). n- The price state.

[0144] In some embodiments, the polyanionic compound may be a sodium ion, a transition metal ion, or a tetrahedral (YO4) compound. n- A class of compounds containing anionic units and halide anions, wherein the transition metal may include one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce, Y may include one or more of P, S and Si, and n represents (YO4). n- The valence state of halogens can include one or more of F, Cl and Br.

[0145] In some embodiments, the polyanionic compound includes one or more of the following: sodium vanadium trifluorophosphate Na3V2(PO4)2F3, sodium vanadium fluorophosphate NaVPO4F, sodium vanadium phosphate Na3V2(PO4)3, Na4Fe3(PO4)2P2O7, NaFePO4, Na3V2(PO4)3, disodium pyrophosphate Na2Q1P2O7 (Q1 = Fe, Co, Mn), and mixed pyrophosphate Na4Q23(PO4)2P2O7 (Q2 = Fe, Co, Mn, Ni).

[0146] In some embodiments, Prussian blue compounds may contain sodium ions, transition metal ions, and cyanide ions (CN). - A class of compounds. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce.

[0147] In some embodiments, the Prussian blue compound is NazE. 1 E 2 (CN)6, where E 1 E 2 It is one or more of Fe, Mn, Co, Ni, Cu, Zn, Cr, Ti, V, Zr, and Ce, wherein 0 <z≤2。

[0148] As an example, the positive electrode active material may include, but is not limited to, NaFeO2, NaCoO2, NaCrO2, NaMnO2, NaNiO2, Na 0.67 MO2 (M includes at least two of Fe, Co, Cr, Mn, Ni, V, Ti, and Mo), NaMO2 (M includes at least two of Fe, Co, Ni, V, Ti, and Mo), NaFePO4, NaMnPO4, NaCoPO4, Na4Fe3(PO4)2O7, Na3V2(PO4)2F3, Na3V2(PO4)3, Prussian blue, Prussian white, and one or more of their respective modified compounds.

[0149] Sodium-ion batteries offer advantages such as abundant resources and low cost. However, the large radius of sodium ions leads to significant structural and volume changes in the positive electrode active material and film layer during secondary battery cycling, potentially causing structural collapse or cracks in the positive electrode film and deteriorating the battery's cycle performance. Furthermore, sodium-ion batteries have a lower theoretical energy density than lithium-ion batteries, typically requiring thicker positive electrode films or higher compaction densities, which increases the difficulty of the battery winding process. The binder provided in this application includes a first polymer comprising flexible structural units derived from butadiene monomers, which improves the bending resistance of the positive electrode sheet. Structural units derived from M1 monomers provide excellent adhesion and cohesion to the positive electrode sheet, enabling the sodium-ion secondary battery provided in this application to possess both excellent industrial fabrication and cycle performance.

[0150] In some embodiments, the conductive agent includes at least one selected from conductive carbon black, conductive graphite, conductive carbon nanotubes, and conductive graphene. In some embodiments, the conductive agent includes at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0151] The positive electrode active material is the main component of the positive electrode film, directly affecting the energy density and capacity of the battery. The conductive agent improves the electronic conductivity of the positive electrode active material, ensuring smooth electron transport between the particles. The binder bonds the positive electrode active material and the conductive agent together and fixes them to the current collector. Based on the total mass of the positive electrode film, a suitable mass ratio of the positive electrode active material, conductive agent, and binder is beneficial for balancing the adhesion performance of the positive electrode film and the flexibility of the electrode sheet. Furthermore, when a positive electrode sheet containing the above-mentioned positive electrode film is used to prepare a secondary battery, it can achieve a balance between the energy density and capacity of the secondary battery.

[0152] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector to form a positive electrode film layer; and obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0153] The solvent can be any known and applicable solvent in the prior art, including but not limited to solvent oil, dimethylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, acetone, dimethyl carbonate, ethyl cellulose, and polycarbonate.

[0154] In some embodiments, the solvent is selected from weakly polar solvents. In some embodiments, the solvent includes solvent oil. Solvent oil is a petroleum distillation product, which is low in toxicity and environmentally friendly.

[0155] In some embodiments, the solvent oil includes solvent oils containing aromatics or those not containing aromatics. In some embodiments, the solvent oil includes one or more of D20, D40, D60, D80, D100, D120, and D200 solvent oils.

[0156] D20, D40, D60, D80, D100, D120, and D200 are typical grades of solvent oils, which are related to the boiling point of the solvent oil and are common industry names.

[0157] In some embodiments, the solvent oil includes D80 solvent oil.

[0158] Compared to high-boiling-point solvents, D80 has a relatively lower boiling point, allowing it to evaporate at lower temperatures and in a shorter time, thus improving efficiency. Compared to low-boiling-point solvents, D80 has a more moderate evaporation rate, preventing electrode cracking due to excessively rapid evaporation. Therefore, when the solvent oil includes D80 solvent oil, the operating time can be shortened, efficiency improved, and the resulting positive electrode exhibits better performance with no significant cracking.

[0159] In some implementations, the bonding strength of the positive electrode sheet is 22 N / m to 36 N / m.

[0160] In some embodiments, the bonding strength of the positive electrode sheet is 24 N / m-36 N / m, 26 N / m-36 N / m, 28 N / m-36 N / m, 30 N / m-36 N / m, 32 N / m-36 N / m, or 34 N / m-36 N / m. In some embodiments, the bonding strength of the positive electrode sheet is 22 N / m, 23 N / m, 24 N / m, 25 N / m, 26 N / m, 27 N / m, 28 N / m, 29 N / m, 30 N / m, 31 N / m, 32 N / m, 33 N / m, 34 N / m, 35 N / m, or 36 N / m, or any value within any range of the above values.

[0161] In some implementations, the cohesive strength of the positive electrode is 18 N / m to 30 N / m.

[0162] In some embodiments, the cohesive strength of the positive electrode is 20 N / m-30 N / m, 22 N / m-30 N / m, 24 N / m-30 N / m, 26 N / m-30 N / m, or 28 N / m-30 N / m. In some embodiments, the cohesive strength of the positive electrode is 18 N / m, 19 N / m, 20 N / m, 21 N / m, 22 N / m, 23 N / m, 24 N / m, 25 N / m, 26 N / m, 27 N / m, 28 N / m, 29 N / m, or 30 N / m, or any value within a range of the above values.

[0163] In some implementations, the film resistance of the positive electrode is 0.6Ω-1.15Ω.

[0164] In some embodiments, the film resistance of the positive electrode is 0.8Ω-1.15Ω, 1.0Ω-1.15Ω, or 1.1Ω-1.15Ω. In some embodiments, the film resistance of the positive electrode is 0.6Ω, 0.7Ω, 0.8Ω, 0.9Ω, 1.0Ω, 1.1Ω, 1.15Ω, or any value within a range of the above values.

[0165] The positive electrode sheet exhibits excellent bonding and cohesive strength, enabling it to withstand stress caused by film volume changes during cycling. The positive electrode film is less prone to detachment from the current collector and electrode breakage, thus improving the safety and lifespan of the secondary battery. Simultaneously, the positive electrode sheet provides improved resistance, which helps reduce the battery's internal resistance and enhances its cycle performance.

[0166] [Negative electrode plate]

[0167] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0168] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0169] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0170] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0171] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0172] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0173] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0174] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0175] Electrolyte

[0176] The electrolyte acts as a conductor of ions between the positive and negative electrodes. In this embodiment, the electrolyte is liquid and comprises an electrolyte salt and a solvent.

[0177] In some embodiments, the electrolyte salt may be selected from NaNO3, NaPF6, NaBF4, or NaClO4. - At least one of NaBOB, NaDFOB, NaSbF6, NaAsF6, NaN(SO2CF3)2, NaN(SO2C2F5)2, NaC(SO2CF3)3, and NaN(SO2F)2.

[0178] The type of solvent can be selected according to actual needs. In some embodiments, the solvent is a non-aqueous solvent. In some embodiments, the solvent is an ether solvent. In some embodiments, the solvent is selected from one or more of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, diethylene glycol diethyl ether, 1,3-dioxolane, tetrahydrofuran, methyltetrahydrofuran, diphenyl ether, and crown ether.

[0179] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0180] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0181] [Isolation membrane]

[0182] In some embodiments, the secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0183] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0184] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0185] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0186] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0187] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0188] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0189] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0190] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 This is an example of a square-structured secondary battery 5.

[0191] In some implementations, refer to Figure 2 The outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 can be placed over the opening to close the receiving cavity. The positive electrode, negative electrode, and separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0192] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0193] Figure 3 This is battery module 4, used as an example. (See reference...) Figure 3 In battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0194] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0195] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0196] Figure 4 and Figure 5 This is battery pack 1 as an example. (See reference...) Figure 5 and Figure 6 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0197] In addition, this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in this application. The secondary battery, battery module, or battery pack can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0198] As an electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0199] Figure 6 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0200] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0201] Example

[0202] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0203] I. Polymer Preparation

[0204] Preparation of the first polymer

[0205] 1) Preparation of polymer 1-1

[0206] Preparation of styrene-butadiene latex: 23g of styrene and 77g of butadiene were placed in a reactor, and 0.5wt% (based on the total mass of styrene and butadiene monomers) of initiator benzoyl peroxide and 5wt% of emulsifier sodium dodecylbenzenesulfonate were added to water to prepare an emulsion; after polymerization at 5℃ for 10h, 0.05wt% (based on the total mass of styrene and butadiene monomers) of terminator sodium polysulfide was added to terminate the polymerization reaction; unreacted butadiene and styrene monomers were recovered by flash degassing to obtain styrene-butadiene latex;

[0207] Preparation of hydrogenated styrene-butadiene rubber: The above-mentioned styrene-butadiene latex and 3 wt% (based on the total mass of styrene-butadiene latex) of sodium dodecylbenzenesulfonate surfactant were added to a reaction vessel and stirred for 30 min to obtain a pre-emulsion. Then, 0.5 wt% (based on the total mass of styrene-butadiene latex) of olefin metathesis catalyst was added to the pre-emulsion. The hydrogenation reaction was carried out for 8 h under hydrogen pressure of 3.0 MPa, 90 °C and stirring. Samples were taken for infrared spectroscopy to analyze the degree of hydrogenation until the degree of hydrogenation of the sample reached 95%. Finally, after precipitation with anhydrous ethanol, washing and drying, polymer 1-1, i.e., hydrogenated styrene-butadiene rubber, was obtained.

[0208] 2) Preparation of polymer 1-2

[0209] Hydrogenated nitrile butadiene rubber: 75 parts by weight of poly1,3-butadiene (weight average molecular weight 40W), 25 parts by weight of acrylonitrile, 5 parts by weight of octacarbonyldicobalt, and 2000 parts by weight of tetrahydrofuran were added to a reactor. Synthesis gas (H2 to CO volume ratio 77:23) was introduced into the reactor to replace the air, and this process was repeated twice. Then, synthesis gas was introduced into the reactor to bring the pressure to 10 MPa. The reactor was continuously stirred and heated, maintaining the reaction temperature at 150°C for 12 hours. After cooling and depressurization, 80 parts by weight of hydroxylamine hydrochloride were added, and the temperature was raised to 120°C. The reaction was continued for 12 hours. Samples were taken for infrared spectroscopy analysis to determine the degree of hydrogenation until the hydrogenation degree reached 95%. After the reaction, methanol was added for precipitation and washing, resulting in a grayish-white flocculent precipitate. This precipitate was dried to constant weight to obtain polymer 1-2, i.e., hydrogenated nitrile butadiene rubber.

[0210] 3) Preparation of polymers 1-3

[0211] Hydrogenated styrene-butadiene rubber (polymer 1-1) was plasticized in a mixer for 1 min. Based on the total mass of the hydrogenated styrene-butadiene rubber, 3 wt% maleic anhydride and 0.3 wt% initiator benzoyl peroxide were added to react and obtain the grafted product. The grafted product was added to xylene and heated to dissolve at 80°C. After the grafted product was completely dissolved, it was poured into acetone while hot for precipitation and purification to remove unreacted grafted monomers and initiators. After purification, the product was dried to constant weight to obtain polymer 1-3. The grafting rate of polymer 1-3 was 1.5%.

[0212] 4) Preparation of polymers 1-4

[0213] Similar to the preparation method of polymer 1-1, the difference is that the amount of styrene and butadiene monomers is increased, so that the weight average molecular weight of the final polymer 1-4 is 60W and the mass content of styrene structural units is 23%.

[0214] 5) Preparation of polymers 1-5

[0215] Similar to the preparation method of polymer 1-1, the difference is that the amount of styrene and butadiene monomers is increased, so that the weight average molecular weight of the final polymer 1-5 is 70W, and the mass content of styrene structural units is 23%.

[0216] The structural parameters of polymers 1-1 to 1-5 are as follows:

[0217]

[0218] The mass content (%) of structural units derived from styrene / acrylonitrile monomers = the mass of styrene or acrylonitrile monomers / the sum of the masses of polymer monomers.

[0219] Preparation of the third polymer

[0220] 1) Preparation of polymer 3-1

[0221] Ethylene monomer and 1-octene monomer in a mass ratio of 8:2 were added to a mixture of metallocene catalyst (main catalyst, concentration in solvent of 1 μmol / L), co-catalyst methylaluminoxane compound (co-catalyst:main catalyst molar ratio of 100:1), and n-hexane for polymerization reaction for 1 h (reaction conditions: temperature 150℃, pressure 3MPa); the obtained polymerization product was flash-evaporated for 10 min (reaction conditions: pressure 3MPa, pressure difference controlled at 10MPa, temperature 140℃) to remove gas and some impurities, yielding polymer 3-1, i.e., POE, poly(ethylene-octene).

[0222] 2) Preparation of polymer 3-2

[0223] Similar to the preparation method of polymer 3-1, the difference is that the amount of 1-octene and ethylene monomers is reduced, so that the weight average molecular weight of the final POE (poly(ethylene-octene)) is 15W.

[0224] 3) Preparation of polymer 3-3

[0225] Based on the total mass of polymer 3-1, 3 wt% maleic anhydride and 0.3 wt% initiator benzoyl peroxide were dissolved in acetone to obtain a grafting solution; the grafting solution was poured into POE (polymer 3-1) and stirred evenly to obtain a mixture; after the acetone evaporated, the mixture was extruded through a twin-screw extruder, pelletized, and dried (extrusion temperature 170℃) to obtain the grafted product; the grafted product was dissolved in xylene, refluxed for 1 hour, and then poured into excess acetone for precipitation and washing, and dried to constant weight to obtain polymer 3-3, i.e., maleic anhydride grafted modified POE, with a maleic anhydride grafting rate of 1.5%.

[0226] 4) Preparation of polymer 3-4

[0227] Similar to the preparation method of polymer 3-1, the difference is that the 1-octene monomer is replaced with the 1-hexene monomer.

[0228] serial number type Weight-average molecular weight / W Monomer mass ratio Polymer 3-1 POE 20 Ethylene:octene = 8:2 Polymer 3-2 POE 15 Ethylene:octene = 8:2 Polymer 3-3 Maleic anhydride-grafted modified POE 20 Ethylene:octene = 8:2 Polymer 3-4 POE 20 Ethylene:hexene = 8:2

[0229] Second polymer

[0230] Polymer 2-1: Hydrogenated styrene-butadiene-styrene polymer with a weight-average molecular weight of 20W and a styrene to butadiene monomer mass ratio of 33:67, purchased from Baling Petrochemical-YH-503T.

[0231] Polymer 2-2: Hydrogenated styrene-butadiene-styrene polymer with a weight-average molecular weight of 30W and a styrene to butadiene monomer mass ratio of 33:67, purchased from Baling Petrochemical-YH-603T.

[0232] Polymer 2-3: Hydrogenated styrene-isoprene-styrene polymer, with a weight-average molecular weight of 20W and a styrene to isoprene monomer mass ratio of 37:63, purchased from Baling Petrochemical-YH-4053.

[0233] II. Implementation Methods

[0234] Example 1

[0235] 1) Preparation of positive electrode sheet

[0236] Sodium iron phosphate (SO4), a positive electrode active material, SuperP (SAP), and polymer 1-1 were mixed in a weight ratio of 95.5:2:2.5. Solvent oil D80 was added as a solvent, and the mixture was stirred until homogeneous, yielding a positive electrode slurry with a solid content of 60 wt%. The positive electrode slurry was then mixed at a concentration of 300 mg / 1540.25 mm. 2 The amount of material is evenly coated onto an aluminum foil current collector to obtain a positive electrode film layer. After drying and cold pressing, a positive electrode sheet is obtained, with a compaction density of 1.9 g / cm³. 3 .

[0237] 2) Preparation of the negative electrode sheet:

[0238] Hard carbon, conductive carbon black (conductive agent), styrene-butadiene rubber (SBR) (binder), and sodium carboxymethyl cellulose (CMC-Na) (thickener) were mixed evenly in a weight percentage ratio of 95:1.0:2.0:2.0. Deionized water was added, and the mixture was stirred and dispersed to obtain the negative electrode slurry. The negative electrode slurry was then mixed at a concentration of 211 mg / 1540 mm. 2 The negative electrode sheet is obtained by coating the copper foil substrate, drying, cold pressing, slitting, and sheet forming.

[0239] 3) Preparation of the separating membrane

[0240] The separator uses a porous polyethylene (PE) polymer film as the separator.

[0241] 4) Electrolyte preparation

[0242] The fully dried sodium salt NaPF6 was dissolved in diethylene glycol dimethyl ether to prepare an electrolyte with a NaPF6 concentration of 1 mol / L.

[0243] 5) Battery fabrication:

[0244] The positive electrode, separator, and negative electrode are stacked in sequence. The separator must be able to isolate the anode and cathode. The bare cell is obtained by winding. The bare cell is placed in the outer packaging, electrolyte is injected, and after processes such as encapsulation, formation, and degassing, a sodium-ion battery is finally obtained.

[0245] Example 2-21

[0246] The main difference between Examples 2-21 and Example 1 lies in the change of the type or content of the binder in the positive electrode film layer. Specific parameters are shown in Tables 1-4. When the binder content in the positive electrode film layer changes, the proportion of the positive electrode active material in the positive electrode film layer is adaptively adjusted so that the total mass of the positive electrode active material and the binder accounts for 98% of the total mass of the positive electrode film layer.

[0247] Example 22:

[0248] The main difference between Example 22 and Example 1 is that the compaction density of the positive electrode sheet is 2.1 g / cm³. 3 .

[0249] Comparative Example 1

[0250] The main difference between Comparative Example 1 and Example 1 is that PVDF (manufacturer and model: Solvay 5130) was used instead of the first polymer, and NMP solvent was used instead of solvent oil D80.

[0251] Comparative Example 2:

[0252] The main difference between Comparative Example 2 and Comparative Example 1 is that the compaction density of the positive electrode sheet is 2.1 g / cm³. 3 .

[0253] Table 1 Table 2 Table 3 Table 4

[0254]

[0255]

[0256] III. Testing Methods

[0257] 1) Testing of polymer weight-average molecular weight

[0258] The obtained polymer was subjected to gel permeation chromatography to determine its weight-average molecular weight.

[0259] 2) Polymer grafting rate test

[0260] Weigh approximately 4g of the grafted polymer and add it together with 200mL of xylene into a 250mL distillation flask. Heat and dissolve the polymer, reflux for 8–10 hours, cool, add acetone and shake well. Allow the precipitate to stand, filter, wash with acetone, and dry the filtrate in a 90℃ oven for 10 hours. Cool to obtain the purified grafted polymer.

[0261] Weigh approximately 2g of the purified grafted polymer and place it in a 250mL distillation flask. Add approximately 80mL of xylene and heat under reflux for approximately 20min until the purified grafted polymer dissolves. After cooling, add excess 0.1mol / L KOH-ethanol standard solution and heat under reflux for 6h. After cooling, use phenolphthalein as an indicator and back-titrate the excess KOH-ethanol standard solution with 0.1mol / L HCl-isopropanol standard solution. Record the amount of alkali consumed and the amount of acid neutralized, and calculate the grafting rate according to equation (3):

[0262] G MAH =9.806(V1C1-C2V2) / 2m

[0263] In the formula: G MAH --lg - MAH mass fraction on the graft, %; C1 - KOH-ethanol standard solution concentration, mol / L; C2 - HCl-isopropanol standard solution concentration, mol / L; V1 - volume of excess KOH-ethanol standard solution added, mL; V2 - volume of HCl-isopropanol standard solution consumed in the back titration to neutralize the base, mL; m - mass of the purified sample of the grafted polymer, g.

[0264] 3) Test of bonding strength of positive electrode sheet

[0265] The prepared positive electrode sheet was cut into test specimens of 20mm × 100mm size for later use. One side of the double-sided tape (3M) was pasted onto the surface of the steel plate, and the other side was pasted onto the electrode sheet to be tested. The tape was then pressed with a pressure roller to ensure complete adhesion to the electrode sheet. One end of the current collector was bent in the opposite direction at a bending angle of 180°. The test was conducted using a high-speed rail tensile testing machine. One end of the steel plate was fixed to the lower clamp of the tensile testing machine, and the bent end of the current collector was fixed to the upper clamp. The angle of the current collector was adjusted to ensure that the upper and lower ends were in a vertical position. The specimen was then stretched at a speed of 50mm / min until the electrode sheet peeled off 5cm from the surface of the double-sided tape. The displacement and force during the process were recorded. The force when the force was balanced was taken as the bonding force of the electrode sheet. The bonding strength was obtained by dividing this force by the adhesion length of the specimen.

[0266] 4) Cohesive strength test of positive electrode sheet

[0267] Cut the prepared positive electrode sheet into test specimens of 20mm×100mm size for later use; attach one side of double-sided tape (3M) to the surface of the steel plate, and the other side to the electrode sheet to be tested, and press it with a pressure roller to make it completely adhered to the electrode sheet; attach the special tape for cohesion testing to the other side of the electrode sheet and press it with a pressure roller; bend one end of the special tape for cohesion testing in the opposite direction with a bending angle of 180°; use a high-speed rail tensile testing machine to test, fix one end of the steel plate to the lower clamp of the tensile testing machine, fix the bent end of the current collector to the upper clamp, adjust the angle of the current collector to ensure that the upper and lower ends are in a vertical position, and then stretch the specimen at a speed of 50mm / min until the special tape peels off 5 cm from the surface of the electrode sheet, record the displacement and force during the process, take the force when the force is balanced as the cohesion of the electrode sheet, and divide the force by the adhesion length of the specimen as the cohesive strength.

[0268] 5) Positive electrode film resistance test

[0269] The prepared positive electrode sheet was cut into test specimens with dimensions of 1.5mm × 10mm; the test specimens were placed in a film resistance meter (Yuaneng Technology BER series electrode resistance meter) for testing, the film resistance was recorded, and the average value of three measurements was taken as the film resistance value of the positive electrode sheet.

[0270] 6) Lateral testing of positive electrode slurry

[0271] Observe the state of the prepared positive electrode slurry.

[0272] 7) Flexibility test of the positive electrode sheet

[0273] Take a 20mm×100mm sample of the prepared positive electrode sheet (100mm in length parallel to the rolling direction) and take the sample along the rolling direction of the electrode sheet; place the pre-folded experimental electrode sheet on the experimental table plane and roll it with a 2kg cylindrical roller. After each rolling, observe whether the electrode sheet is transparent. When the electrode sheet is transparent, record the corresponding number of rolling times. The number of rolling times represents the flexibility of the electrode sheet.

[0274] 8) Secondary battery cycle performance test

[0275] The secondary battery was discharged to 3.0V at 0.5C at 25℃ and allowed to stand for 5 minutes. It was then charged to 4.45V at 0.5C, and then charged at a constant voltage of 0.05C at 4.45V. After charging, it was discharged to 3.0V at 0.5C, completing one charge-discharge cycle (i.e., one full cycle). This charge-discharge process was repeated 500 times at 25℃. The discharge capacity of the first cycle is denoted as C1, and the discharge capacity of the 500th cycle is denoted as C500. The cycle performance of the lithium-ion battery is measured by the following formula: (C500 / C1)×100%, which calculates the cycle capacity retention rate at 25℃.

[0276] Table 5-7 shows the test results as follows:

[0277] Table 5. Performance Test Results of Secondary Batteries

[0278]

[0279]

[0280] Table 6. Lateral Test Results of Positive Electrode Slurry

[0281]

[0282]

[0283] Table 7. Flexibility of the positive electrode sheet

[0284] Serial Number Electrode toughness / time Electrode winding effect Example 22 2 It will only become slightly translucent after being bent twice. Comparative Example 2 1 It breaks easily when slightly bent.

[0285] Based on the above results, it can be seen that the positive electrode sheet of Examples 1-22 includes a first polymer. The first polymer can provide suitable bonding performance while reducing or avoiding the introduction of hydrogen fluoride, thereby reducing the impact of the binder on the cycle performance of the sodium-ion secondary battery and improving the cycle performance of the sodium-ion secondary battery.

[0286] A comparison of the data from Examples 1-2 and Comparative Example 1 shows that the presence of a first polymer in the positive electrode film layer can improve the bonding strength and cohesive strength of the positive electrode sheet, reduce the film resistance of the positive electrode sheet, and improve the battery cycle performance; at the same time, the first polymer reduces the gelation problem of the positive electrode slurry and improves the processing performance of the positive electrode sheet.

[0287] As can be seen from the comparison of the data of Example 3 and Comparative Example 1, the positive electrode film layer includes a first polymer grafted with maleic anhydride, which can further improve the bonding strength and cohesive strength of the positive electrode sheet, reduce the film resistance of the positive electrode sheet, and improve the cycle performance of the secondary battery; at the same time, the modified first polymer can also reduce the gelation problem of the positive electrode slurry and improve the processing performance of the positive electrode sheet.

[0288] As can be seen from the comparison of the data of Examples 4-9 and Comparative Example 1, the positive electrode film layer includes a first polymer and a second polymer. Based on the mass of the positive electrode film layer, the mass ratio of the first polymer and the second polymer is 1:(0.25-1.5), which gives the positive electrode sheet excellent adhesion performance and can improve the cycle performance of the secondary battery by reducing or avoiding the introduction of hydrogen fluoride.

[0289] A comparison of the data from Examples 4-5 and 8 with Examples 6-7 and 9 shows that a mass ratio of the first polymer to the second polymer in the positive electrode film layer of 1:(0.6-1) can further improve the bonding strength and cohesive strength of the positive electrode sheet, reduce the film resistance of the positive electrode sheet, and improve the cycle performance of the secondary battery by reducing or avoiding the introduction of hydrogen fluoride. At the same time, the compounding of the first polymer and the second polymer in a mass ratio of 1:(0.6-1) can reduce the gelation problem of the positive electrode slurry and improve the processing performance of the positive electrode sheet.

[0290] As can be seen from the comparison of the data of Examples 10-16 and Comparative Example 1, the positive electrode film layer includes a first polymer and a third polymer. When the mass ratio of the first polymer to the third polymer is 1:(0.25-1.5), the positive electrode sheet can have excellent bonding performance, and the cycle performance of the secondary battery can be improved by reducing or avoiding the introduction of hydrogen fluoride.

[0291] A comparison of the data from Examples 10, 12, and 15 with Examples 13-14 and 16 shows that when the mass ratio of the first polymer to the third polymer in the positive electrode film is 1:(0.6-1), the bonding strength and cohesive strength of the positive electrode sheet can be further improved, the film resistance of the positive electrode sheet can be reduced, and the cycle performance of the secondary battery can be improved by reducing or avoiding the introduction of hydrogen fluoride. At the same time, the first polymer and the third polymer are compounded in a mass ratio of 1:(0.6-1), which can reduce the gelation problem of the positive electrode slurry and improve the processing performance of the positive electrode sheet.

[0292] As can be seen from the comparison of the data of Examples 17-21 and Comparative Example 1, when the positive electrode film layer includes a first polymer, a second polymer and a third polymer, and the mass content of the first polymer is 1%-2%, the mass content of the second polymer is 0.1%-0.5%, and the mass content of the third polymer is 0.1%-1.5%, the positive electrode sheet can have excellent bonding performance, and the cycle performance of the secondary battery can be improved by reducing or avoiding the introduction of hydrogen fluoride.

[0293] A comparison of the data from Examples 17, 19, and 21 with those from Examples 18 and 20 shows that when the mass content of the first polymer in the positive electrode film is 1%-1.5%, the mass content of the second polymer is 0.2%-0.4%, and the mass content of the third polymer is 0.8%-1.3%, the bonding strength and cohesive strength of the positive electrode sheet can be further improved, the film resistance of the positive electrode sheet can be reduced, and the cycle performance of the secondary battery can be improved by reducing or avoiding the introduction of hydrogen fluoride. At the same time, the compounding of the first polymer, the third polymer, and the third polymer within the above-mentioned mass content range can reduce the gelation problem of the positive electrode slurry and improve the processing performance of the positive electrode sheet.

[0294] A comparison of the data from Example 22 and Example 1 shows that, under high pressure and density conditions, the positive electrode sheet containing the first polymer exhibits excellent bonding properties and good toughness, allowing for winding. Furthermore, by reducing or avoiding the introduction of hydrogen fluoride, it can improve the cycle performance of the secondary battery. A comparison of the data from Example 22 and Comparative Example 2 shows that, although the positive electrode sheet containing PVDF can maintain bonding properties under high pressure and density conditions, its toughness is poor, making it impossible to wind and thus impossible to fabricate into a wound pouch cell for cycle performance testing. This indicates that the binder in Example 22 is more suitable for high-energy-density sodium-ion secondary batteries.

[0295] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A sodium-ion secondary battery, characterized in that, The device includes a positive electrode sheet, the positive electrode sheet comprising a current collector and a positive electrode film layer located on at least one side surface of the current collector, the positive electrode film layer comprising a binder, the binder comprising a first polymer comprising a butadiene-M1 copolymer, wherein the M1 monomer comprises at least one of styrene or acrylonitrile, and the mass content of structural units derived from the M1 monomer is 20%-30% based on the total mass of the first polymer.

2. The secondary battery according to claim 1, characterized in that, The M1 monomer is selected from styrene or acrylonitrile; and / or The weight-average molecular weight of the first polymer is 40W-80W; and / or Based on the total mass of the first polymer, the mass content of the structural units derived from the M1 monomer is 20%-25%.

3. The secondary battery according to claim 1 or 2, characterized in that, The first polymer comprises a hydrogenated copolymer of butadiene monomer and M1 monomer; and / or The first polymer comprises hydrogenated styrene-butadiene rubber or hydrogenated nitrile-butadiene rubber; and / or The degree of hydrogenation of the first polymer is 85%-99%.

4. The secondary battery according to any one of claims 1 to 3, characterized in that, The first polymer comprises a maleic anhydride-grafted butadiene-M1 copolymer, wherein the maleic anhydride grafting rate is 1%-2%.

5. The secondary battery according to any one of claims 1 to 4, characterized in that, Based on the total mass of the positive electrode film, the mass content of the first polymer is 1%-2.5%.

6. The secondary battery according to any one of claims 1 to 5, characterized in that, The adhesive includes a second polymer, which includes a styrene-M2-styrene copolymer, wherein the M2 monomer includes at least one of isoprene or butadiene.

7. The secondary battery according to claim 6, characterized in that, The second polymer has a weight-average molecular weight of 10W-30W; and / or, Based on the total mass of the second polymer, the mass content of structural units derived from the M2 monomer is 60%-70%.

8. The secondary battery according to any one of claims 6 to 7, characterized in that, Based on the total mass of the positive electrode film, the mass content of the second polymer is 0.5%-1.5%, and / or The mass ratio of the first polymer to the second polymer is 1:(0.25-1.5).

9. The secondary battery according to any one of claims 1 to 8, characterized in that, The binder comprises a third polymer, which comprises an ethylene-N1 copolymer, wherein the N1 monomer comprises at least one of 1-octene or 1-hexene.

10. The secondary battery according to claim 9, characterized in that, The third polymer has a weight-average molecular weight of 10W-30W; and / or Based on the total mass of the third polymer, the mass content of structural units derived from the N1 monomer is 20%-30%.

11. The secondary battery according to claim 9 or 10, characterized in that, The third polymer comprises a maleic anhydride-grafted ethylene-N1 copolymer, wherein the grafting rate of the maleic anhydride is 0.5%-5%.

12. The secondary battery according to any one of claims 9 to 11, characterized in that, Based on the mass of the positive electrode film, the mass content of the third polymer is 0.3%-1.5%; and / or The mass ratio of the first polymer to the third polymer is 1:(0.25-1.5).

13. The secondary battery according to any one of claims 1 to 12, characterized in that, Based on the total mass of the positive electrode film, the mass content of the first polymer is 1%-2%, the mass content of the second polymer is 0.1%-0.5%, and the mass content of the third polymer is 0.1%-1.5%.

14. The secondary battery according to any one of claims 1 to 13, characterized in that, The positive electrode sheet satisfies at least one of the following conditions: (1) The bonding strength of the positive electrode sheet is 22 N / m-36 N / m; (2) The cohesive strength of the positive electrode is 18 N / m-30 N / m; (3) The film resistance of the positive electrode is 0.6Ω-1.15Ω.

15. An electrical appliance, characterized in that, The secondary battery includes any one of claims 1 to 14.

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