Negative electrode additive, negative electrode pole piece, secondary battery and electric device

Abstract

The invention relates to the technical field of secondary batteries, and provides a negative electrode additive, a negative electrode plate, a secondary battery and an electric device. The chemical general formula of the negative electrode additive provided by the invention is # imgabs0 #. The negative electrode additive provided by the invention can promote formation of an SEI film with a high-activity ion diffusion coefficient, high thermal stability and low impedance on the surface of a negative electrode active material, and a boron compound contained in the SEI film can also enhance and improve the desolvation capability of active ions; therefore, the problem of active ion precipitation on the surface of the negative electrode active material is not easy to occur, so that the secondary battery shows high electrochemical properties such as first coulombic efficiency and cycle performance.

Description

Technical Field

[0001] This application belongs to the technical field of secondary batteries, and particularly relates to a negative electrode additive, a negative electrode plate, a secondary battery, and an electrical device. Background Art

[0002] A secondary battery, also known as a rechargeable battery or a storage battery, refers to a battery that can activate active materials through charging after discharging. Currently, secondary batteries are widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power stations, as well as in multiple fields such as power tools, electric bicycles, electric motorcycles, and electric vehicles. With the application and popularization of secondary batteries, their energy density and cycling performance have received increasing attention. Therefore, effective technical means are needed to improve the energy density and cycling performance of secondary batteries.

[0003] The above statements are only used to provide background technical information related to this application, and do not necessarily constitute prior art. Summary of the Invention

[0004] The purpose of this application is to provide a negative electrode additive, a negative electrode plate, a secondary battery, and an electrical device, aiming to solve the problem that the cycling performance and energy density of secondary batteries are not ideal.

[0005] To achieve the above application purpose, the technical solution adopted in this application is as follows:

[0006] In a first aspect, this application provides a negative electrode additive, and the chemical general formula of the negative electrode additive is shown in Formula I:

[0007]

[0008] Among them, R1 is an atom containing a lone pair of electrons, R2 includes any one of a hydrogen atom and a nitrile group, R3 includes any one of a hydrogen atom, a hydroxyl group, and an alkoxy group with 1 to 3 carbon atoms, and R4 includes any one of a lithium ion and a sodium ion;

[0009] n is a natural number greater than or equal to 50.

[0010] The negative electrode additive provided in the embodiment of this application contains SP 3Hybridized boron atoms are covalently bonded to rigid aromatic groups, and flexible chain segments with carbonate structures are grafted onto the aromatic groups. This additive is a water-soluble polymer, and due to the presence of the flexible chain segments, the negative electrode additive can coat the surface of the negative electrode active material; the rigid aromatic groups contained in the negative electrode additive form a conjugate with the double bonds in the carbonate structure, improving the electron transport ability of the additive. In this way, the negative electrode additive exhibits strong reducibility and reactivity. Therefore, it can be preferentially reduced over the electrolyte during formation, and the reduction product of the negative electrode additive becomes a part of the SEI film, meaning that the SEI film formed on the surface of the negative electrode active material contains boron compounds. Boron compounds can not only enhance the ability of solvated lithium / sodium ions to desolvate, but also enhance the active ion diffusion ability in the SEI film, reduce the interfacial impedance of the SEI film, and endow the SEI film with high thermal stability, making it not easy for active ions to precipitate on the surface of the negative electrode active material; at the same time, the boron atoms are in SP 3 hybridization, meaning that the boron atoms are in a negatively charged full octet electron structure state. In this way, the boron atoms exhibit the properties of electron donors or Lewis bases. At this time, the boron atoms are anionic four-coordination centers, carrying negative charges, and lithium ions / sodium ions are the positively charged parts. Therefore, the positively charged active ions can provide inherent active ion diffusion sites, thereby effectively improving the diffusion ability of active ions in the negative electrode active material. Based on this, the secondary battery exhibits high first Coulomb efficiency and cycling performance and other electrochemical performances.

[0011] Therefore, the negative electrode additive provided by the embodiments of the present application can promote the formation of an SEI film with high active ion diffusion coefficient, high thermal stability, and low impedance on the surface of the negative electrode active material, and the boron compounds contained in the SEI film can also improve the desolvation process of active ions, so that it is not easy for active ions to precipitate on the surface of the negative electrode active material. In this way, the secondary battery exhibits high first Coulomb efficiency and cycling performance and other electrochemical performances.

[0012] In some embodiments, R1 includes at least one of an oxygen atom and a nitrogen atom.

[0013] In the embodiments of the present application, the oxygen atom and the nitrogen atom contain lone pairs of electrons, making the negative electrode additive have appropriate electronegativity, and thus stronger hydrogen bonds can be formed, which is more conducive to improving the hydrophilicity of the negative electrode additive.

[0014] In some embodiments, R3 includes at least one of a hydrogen atom, a hydroxyl group, and a methoxy group.

[0015] In the embodiments of the present application, the hydroxyl group and the methoxy group can improve the flexibility and polarity of the negative electrode additive, thereby increasing the coating property of the negative electrode additive on the negative electrode active material.

[0016] In some embodiments, the value range of n is 50 ≤ n ≤ 600.

[0017] In the embodiments of the present application, the value of n is controlled within the above range, so that the negative electrode additive has an appropriate molecular weight, thereby improving the bonding effect of the negative electrode additive and the coating effect on the negative electrode active material, which helps to improve the bonding force and stability of the negative electrode sheet.

[0018] In some embodiments, the weight-average molecular weight of the negative electrode additive is 10,000 to 100,000.

[0019] In the embodiments of the present application, by controlling the weight-average molecular weight of the negative electrode additive within the above range, the negative electrode additive can form an effective coating on the surface of the negative electrode active material, so that the negative electrode slurry has excellent processing performance to form a negative electrode sheet with a high bonding force.

[0020] In some embodiments, the negative electrode additive includes at least one of the compounds shown in Formula I-1 to Formula I-4:

[0021]

[0022]

[0023] Among them, in Formula I-1, Formula I-2, Formula I-3 and Formula I-4, the value range of n is 50 ≤ n ≤ 200.

[0024] In the embodiments of the present application, the negative electrode additives shown in Formula I-1 to Formula I-4 can be coated on the surface of the negative electrode active material, and preferentially reduce to form a SEI film containing boron element during formation. This SEI film has the characteristics of high active ion diffusion coefficient, high thermal stability and low impedance, and can also enhance the ability of active ions to desolvate. Therefore, the problem of active ion precipitation is not likely to occur on the surface of the negative electrode active material. In this way, the secondary battery exhibits high electrochemical performances such as first Coulomb efficiency, rate performance and cycle performance.

[0025] In a second aspect, the present application provides a method for preparing a negative electrode additive, including the following steps:

[0026] React a caffeic acid compound with sodium borohydride or lithium borohydride to obtain the negative electrode additive of the present application;

[0027] The general chemical formula of the caffeic acid compound is shown in Formula II:

[0028]

[0029] Among them, R1 is an atom with a lone pair of electrons, R2 includes any one of a hydrogen atom and a nitrile group, and R3 includes any one of a hydrogen atom, a hydroxyl group and an alkoxy group with 1 to 3 carbon atoms.

[0030] In the preparation method of the negative electrode additive provided by the embodiment of the present application, sodium borohydride or lithium borohydride can be used as an initiator to cause the polymerization reaction of caffeic acid compounds, and at the same time, it can also react with the hydroxyl groups in the caffeic acid compounds as a reactant, thereby effectively preparing a negative electrode additive with the performance as described above in the present application. In addition, the preparation method of the negative electrode additive of the present application can ensure the stability of the structure and electrochemical performance of the prepared negative electrode additive, and has high efficiency and saves production costs.

[0031] In some embodiments, the step of reacting the caffeic acid compound with sodium borohydride or lithium borohydride includes:

[0032] Under a protective atmosphere, the caffeic acid compound is added to a first organic solvent under ice bath conditions to obtain a first solution;

[0033] Sodium borohydride or lithium borohydride is added to a second organic solvent to obtain a second solution;

[0034] The second solution is added to the first solution for reaction to obtain a negative electrode additive.

[0035] In the preparation method of the negative electrode additive provided by the embodiment of the present application, first, the caffeic acid compound is added to a first organic solvent under specific conditions such as ice bath conditions for dissolution to form a first solution, and then the second solution containing sodium borohydride or lithium borohydride is added to the first solution, so that the caffeic acid compound reacts fully with sodium borohydride or lithium borohydride to form a polymer containing SP 3 hybrid boron atoms, thereby obtaining the negative electrode additive shown in Formula I.

[0036] In some embodiments, the first organic solvent and the second organic solvent are mutually soluble, and the first organic solvent and the second organic solvent independently include at least one of tetrahydrofuran, acetonitrile, dimethyl sulfoxide, cyclohexanone, N-methylpyrrolidone, toluene, chlorobenzene, benzene, benzotrichloride, and benzotetrachloride.

[0037] The solvent provided by the embodiment of the present application has appropriate polarity, which is conducive to the full dispersion of each reaction raw material such as caffeic acid compounds and sodium borohydride or lithium borohydride in the solvent, so as to fully carry out the reaction.

[0038] In some embodiments, the second solution is added dropwise, and the dropping rate is 4-6 mL / min.

[0039] The reaction between the caffeic acid compound and sodium borohydride or lithium borohydride is an exothermic reaction. The dropping method makes the reaction more uniform and is not prone to the phenomenon of explosive polymerization.

[0040] In some embodiments, the reaction conditions are as follows: the temperature is 0°C to 30°C, and the time is 12 h to 24 h.

[0041] The reaction conditions provided by the embodiments of the present application can enable the nucleic acid caffeic acid compound to react fully with sodium borohydride or lithium borohydride, so as to obtain the negative electrode additive shown in Formula I.

[0042] In a third aspect, the present application provides a negative electrode plate, which includes a negative electrode current collector and a negative electrode film layer located on at least one side of the negative electrode current collector. The negative electrode film layer includes the negative electrode additive of the present application and / or the negative electrode additive prepared by the preparation method of the negative electrode additive of the present application.

[0043] Since the negative electrode plate provided by the embodiments of the present application contains the negative electrode additive of the present application, the problem that active ions are not likely to precipitate at the interface of the negative electrode active material is avoided, thereby improving the electrochemical properties such as the capacity, first Coulomb efficiency, rate performance, and cycle performance of the battery cell.

[0044] In some embodiments, the negative electrode film layer further includes a negative electrode active material, and at least part of the negative electrode additive is coated on the surface of the negative electrode active material.

[0045] In the embodiments of the present application, the negative electrode additive can be coated on the surface of the negative electrode active material to form a coating layer. In this way, after formation, an SEI film coated on the surface of the negative electrode active material can be formed, and the SEI contains reduction products of the negative electrode additive, such as boron compounds.

[0046] In some embodiments, the mass ratio of the negative electrode additive to the negative electrode active material is 0.1 to 0.6:100.

[0047] By controlling the content of the negative electrode additive within the above range in the embodiments of the present application, the function of the negative electrode additive can be fully exerted, and an SEI film containing boron elements can be formed on the surface of the negative electrode active material. The SEI film has the characteristics of high active ion diffusion coefficient, high thermal stability, and low impedance, and can also enhance the ability of active ions to desolvate. Therefore, the problem that active ions are not likely to precipitate on the surface of the negative electrode active material is avoided. In this way, the secondary battery exhibits high electrochemical properties such as first Coulomb efficiency, rate performance, and cycle performance.

[0048] In some embodiments, the negative electrode film layer further includes a thickening agent, and the mass ratio of the thickening agent to the negative electrode active material is 0.7 to 1.5:100.

[0049] By regulating the content of the thickening agent in the embodiments of the present application, the negative electrode plate has a high adhesive force.

[0050] In some embodiments, the negative electrode film layer further includes a binder, and the mass ratio of the binder to the negative electrode active material is 1.3 to 2.2:100.

[0051] In the embodiments of the present application, by regulating the binder content, the negative electrode sheet has a high adhesive force.

[0052] In some embodiments, the mass ratio of the negative electrode additive, the thickener and the binder is (0.3 - 0.5):(0.7 - 0.9):(1.6 - 1.8).

[0053] In some embodiments, the mass ratio of the negative electrode additive, the thickener and the binder is 0.5:0.8:1.6.

[0054] In the embodiments of the present application, by controlling the mass ratio of the negative electrode additive, the thickener and the binder, the negative electrode additive, the thickener and the binder can give full play to the synergistic effect, not only making the negative electrode sheet have a high adhesive force, but also forming a SEI film containing boron compounds on the surface of the negative electrode active material after formation, thereby endowing the negative electrode sheet with high...

[0055] Fourthly, the present application provides a secondary battery, including the negative electrode sheet of the present application.

[0056] By arranging the negative electrode sheet, the secondary battery improves its electrochemical performances such as capacity, Coulomb efficiency and cycle performance.

[0057] In some embodiments, at least part of the surface of the negative electrode film layer is covered with a SEI film, and the SEI film contains boron compounds.

[0058] Since the SEI film formed in the secondary battery of the present application contains boron compounds, it can enhance the ability of active ions to desolvate, and can also enhance the diffusion ability of active ions in the SEI film, reduce the interfacial impedance of the SEI film, and endow the SEI film with high thermal stability. In this way, the secondary battery exhibits high electrochemical performances such as cycle performance and rate performance.

[0059] Fifthly, the present application provides an electrical device, including the secondary battery of the present application.

[0060] By arranging the secondary battery, the electrical device has a long standby or battery life, good safety and a relatively low replacement frequency. Description of the Drawings

[0061] By reading the following detailed description of the preferred embodiments, various other advantages and benefits will become clear to those of ordinary skill in the art. The drawings are only for the purpose of showing the preferred embodiments and are not considered as a limitation of the present application. Moreover, in all the drawings, the same reference numerals are used to represent the same components. In the drawings:

[0062] Figure 1 is an exploded structural schematic diagram of a battery cell in some embodiments of the present application;

[0063] Figure 2 Schematic structural diagram of a battery module in some embodiments of the present application;

[0064] Figure 3 Exploded structural diagram of a battery pack in some embodiments of the present application;

[0065] Figure 4 Schematic diagram of an embodiment of an electrical device including a secondary battery of an embodiment of the present application as a power source;

[0066] Figure 5 Infrared spectrum diagram of the negative electrode additive provided in Embodiment 14 of the present application.

[0067] Among them, each reference numeral in the figure:

[0068] 3. Battery cell, 31. Housing, 32. Electrode assembly, 33. Cover plate;

[0069] 4. Battery module;

[0070] 5. Battery pack, 51. Box body, 52. Lower box body. Specific embodiments

[0071] The embodiments of the technical solution of the present application will be described in detail below with reference to the accompanying drawings. The following embodiments are only used to illustrate the technical solution of the present application more clearly, so they are only examples and cannot be used to limit the protection scope of the present application.

[0072] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above drawings are intended to cover non-exclusive inclusion.

[0073] In the description of the embodiments of the present application, technical terms such as "first" and "second" are only used to distinguish different objects and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity, specific order or primary-secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "a plurality" is more than two, unless otherwise specifically defined.

[0074] Reference to "embodiment" in this document means that the specific features, structures, or characteristics described in connection with the embodiment can be included in at least one embodiment of this application. The phrase appears in various places in the specification and does not necessarily refer to the same embodiment each time, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0075] In the description of the embodiments of this application, the term "and / or" is merely a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in this document generally represents an "or" relationship between the associated objects before and after.

[0076] In the description of the embodiments of this application, the term "plurality" refers to two or more (including two). Similarly, "multiple groups" refers to two or more groups (including two groups), and "multiple pieces" refers to two or more pieces (including two pieces).

[0077] In the description of the embodiments of this application, the orientation or positional relationship indicated by technical terms such as "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc. is based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the embodiments of this application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the embodiments of this application.

[0078] In the description of the embodiments of this application, unless otherwise clearly specified and limited, technical terms such as "installation", "connection", "connection", "fixation", etc. should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or integrated; it can also be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two elements or the interaction relationship between two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the embodiments of this application can be understood according to specific circumstances.

[0079] In this application, "at least one" means one or more, and "a plurality of" means two or more. "At least one of the following" or a similar expression means any combination of these items, including any combination of single item or plural items. For example, "at least one of a, b, or c", or "at least one of a, b, and c" can both represent: a, b, c, a - b (i.e., a and b), a - c, b - c, or a - b - c, where a, b, and c can be single or multiple respectively.

[0080] It should be understood that in various embodiments of this application, the magnitude of the sequence numbers of the above processes does not mean the order of execution. Some or all of the steps can be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation to the implementation process of the embodiments of this application.

[0081] The weight of the relevant components mentioned in the specification of the embodiments of this application not only can refer to the specific content of each component, but also can represent the proportional relationship of the weights between each component. Therefore, as long as it is scaled up or down in proportion according to the content of the relevant components in the specification of the embodiments of this application, it is within the scope disclosed in the specification of the embodiments of this application. Specifically, the mass described in the specification of the embodiments of this application can be mass units well - known in the chemical industry such as μg, mg, g, kg, etc.

[0082] In the description of the embodiments of this application, the SEI film is the abbreviation of "solid electrolyte interface", referring to a solid electrolyte interface film with the characteristics of a solid electrolyte, that is, during the first charge - discharge process of a liquid lithium - ion battery, a passivation layer formed by the reaction between the electrode material and the electrolyte at the solid - liquid phase interface forms a film covering the surface of the negative electrode material.

[0083] As an energy storage carrier, lithium - ion batteries have received extensive attention and applications in the fields of portable 3C products, hybrid electric vehicles, energy storage devices, etc. due to their small size, portability, high energy density, and high specific capacity. However, there are still many problems to be solved in lithium - ion batteries, such as how to improve the charge transfer rate in the anode. Especially for the high - capacity graphite battery cells, when the charging speed exceeds the intercalation speed, lithium deposition will occur at the graphite interface. Therefore, improving the kinetics of charging / lithiation of graphite anodes has been a hot issue in the field of battery research in recent years.

[0084] Charge transfer in the anode primarily involves two processes: the diffusion of desolvated lithium ions within the solid electrolyte interface (SEI), and the diffusion of lithium ions within the anode active material. Currently, numerous strategies exist to improve the diffusion of lithium ions within the anode active material. However, due to the importance of the SEI membrane, the SEI membrane formed within the battery's basic structure must be continuously modified during charge / discharge / cycling at different rates, resulting in irreversible capacity loss. Therefore, improving the diffusion of desolvated lithium ions within the solid electrolyte interface (SEI) can be achieved by improving the interfacial properties of the SEI membrane to enhance battery performance. For example, these efforts can reduce the irreversible capacity loss caused by SEI formation, lower the interfacial resistance of the SEI membrane, and enhance the stability of the SEI membrane by creating a robust SEI composition.

[0085] Of course, some lithium-ion batteries currently use built-in additives to improve the properties of the SEI film. However, these publicly available additives are primarily used in the electrolyte and are unable to form a specific SEI film on the surface of the negative electrode active material. Furthermore, they face issues with their own redox reactions, interfacial reaction compatibility, and compatibility with battery cell materials. In particular, current additives are primarily oil-soluble and cannot form a uniform system with the various raw materials in the negative electrode sheet, let alone coat the surface of the negative electrode active material. Consequently, even if these additives are used in the negative electrode sheet, they cannot form a specific SEI film on the surface of the negative electrode active material.

[0086] Based on the above background, the first aspect of the embodiment of the present application provides a negative electrode additive, the chemical formula of the negative electrode additive is shown in Formula I:

[0087]

[0088] wherein R1 is an atom containing a lone pair of electrons, R2 includes any one of a hydrogen atom and a nitrile group, R3 includes any one of a hydrogen atom, a hydroxyl group, and a C1-C3 alkoxy group, and R4 includes any one of a lithium ion and a sodium ion;

[0089] n is a natural number ≥50.

[0090] The negative electrode additive provided in the embodiment of the present application includes SP 3Hybridized boron atoms are covalently bonded to rigid aromatic groups, and flexible segments with carbonate ester structures are grafted onto the aromatic groups. The flexible carbonate-like segments and rigid aromatic conjugated groups on the negative electrode additive segments can promote the transport and migration of lithium ions and provide a certain adhesive effect. The incorporation of the latter aromatic conjugated groups can lead to the delocalization of the negative charge of the anion part, thus overall improving the ionic conductivity of the negative electrode additive. At the same time, the active double bonds between the carbonate-like segments and the rigid aromatic conjugated groups are easily attacked by electrons and then polymerize. In addition, the negative electrode additive has a low LUMO energy level, so that the negative electrode additive can be preferentially reduced over the electrolyte during formation. Since the negative electrode additive shown in Formula I is a water-soluble polymer and can coat the surface of the negative electrode active material, it means that the reduction product of the negative electrode additive becomes a part of the SEI film coating the negative electrode active material. For example, the SEI film contains boron compounds. On the one hand, boron compounds can reduce the activation energy for the desolvation of active ions from the solvent sheath at the SEI film interface, thus promoting the desolvation process of active ions, reducing the interfacial impedance and the energy barrier for the desolvation of active ions. Therefore, the boron-containing SEI film can effectively promote the desolvation of active ions in the electrolyte and enhance the diffusion ability of active ions in the SEI film; on the other hand, SP 3 The delocalization of electrons caused by the conjugation of the hybridized boron atoms through the benzene ring promotes the dissociation of lithium ions and improves the ductility of ions. In addition, the boron atoms exhibit the properties of electron donors. By introducing electron-withdrawing groups, the negative charge on the boron atoms is highly delocalized. In this way, the negative electrode additive exhibits high ionic conductivity, and the SEI containing boron elements also has high ionic conductivity. At the same time, the boron atoms are in the SP 3 hybridization state, which means that the boron atoms are in a negatively charged full octet electron structure state. In this way, the boron atoms exhibit the properties of electron donors or Lewis bases. At this time, the boron atoms are the anion four-coordination centers and carry negative charges, while lithium ions / sodium ions are the positively charged parts. Therefore, positively charged active ions such as lithium ions or sodium ions can provide inherent active ion diffusion sites, effectively improving the diffusion ability of active ions in the negative electrode active material; in addition, the inherent lithium ions / sodium ions in the negative electrode additive are active ions and can also supplement the irreversible active ions consumed during the formation of the SEI film on the negative electrode, thus maintaining the abundance of active ions in the battery system and improving the first efficiency and overall electrochemical performance of the battery.

[0091] Therefore, the negative electrode additive provided by the embodiments of the present application can coat the surface of the negative electrode active material and can react with the electrolyte to form a SEI film containing boron element, which has the characteristics of high active ion diffusion coefficient, high thermal stability and low impedance, and can also improve the desolvation rate of solvated active ions, so that the problem of active ion precipitation is not likely to occur on the surface of the negative electrode active material. In this way, the secondary battery exhibits high initial Coulomb efficiency and cycling performance and other electrochemical performances.

[0092] In some embodiments, R1 includes at least one of an oxygen atom and a nitrogen atom.

[0093] In the embodiments of the present application, the oxygen atom and the nitrogen atom contain lone pairs of electrons. Thus, active ions can jump through the oxygen atom and the nitrogen atom, thereby improving the active ion transport rate; in addition, the lone pair of electrons gives the negative electrode additive an appropriate electronegativity, and thus a stronger hydrogen bond can be formed, which is more conducive to improving the hydrophilicity of the negative electrode additive.

[0094] When R1 is an oxygen atom, the negative electrode additive shown in Formula I contains a flexible carbonate chain segment, which can not only promote the transport and migration of active ions, but also provide a certain binding effect, making the negative electrode sheet have high stability.

[0095] In some embodiments, R3 includes at least one of a hydrogen atom, a hydroxyl group, and a methoxy group.

[0096] In the embodiments of the present application, the hydroxyl group and the methoxy group can improve the flexibility and polarity of the negative electrode additive, thereby increasing the coating property of the negative electrode additive on the negative electrode active material.

[0097] In some embodiments, the value range of n is 50 ≤ n ≤ 600. Exemplarily, the value of n can be 50, 60, 70, 80, 90, 100, 120, 150, 180, 200, 300, 400, 500, 600 or within the range composed of any of the above point values.

[0098] The embodiments of the present application control the value of n within the above range, so that the negative electrode additive has an appropriate molecular weight, thereby improving the binding effect of the negative electrode additive and the coating effect on the negative electrode active material, which is beneficial to forming a SEI film containing boron element on the surface of the negative electrode active material during formation, thereby improving the desolvation rate of active ions and the transport rate of active ions in the SEI film. In this way, the battery cell exhibits high capacity, initial Coulomb efficiency, rate performance, cycling performance and other electrochemical performances.

[0099] In some embodiments, the weight-average molecular weight of the negative electrode additive is 10,000 to 100,000. Exemplarily, the weight-average molecular weight of the negative electrode additive can be typical but non-limiting values such as 10,000, 20,000, 30,000, 40,000, 50,000, 60,000, 70,000, 80,000, 90,000, 100,000, etc.

[0100] In the embodiments of the present application, by controlling the weight-average molecular weight of the negative electrode additive within the above range, the negative electrode additive can form an effective coating on the surface of the negative electrode active material, so that the negative electrode slurry has excellent processing performance and certain bonding ability, thereby facilitating the formation of a negative electrode sheet with high bonding strength.

[0101] The higher the mass of the negative electrode active material contained per unit volume in the negative electrode sheet, the higher the volume capacity exhibited by the battery. Therefore, in the embodiments of the present application, the weight-average molecular weight of the negative electrode additive is controlled within the above range, so that each raw material component contained in the negative electrode sheet can fully play a synergistic effect, which is beneficial to obtaining an SEI film with high active ion diffusion ability and low interfacial impedance, and at the same time, the mass of the negative electrode active material contained in the negative electrode sheet is relatively high, and the stability of the negative electrode sheet is good.

[0102] In some embodiments, the negative electrode additive includes at least one of the compounds represented by Formula I-1 to Formula I-4:

[0103]

[0104]

[0105] Among them, in Formula I-1, Formula I-2, Formula I-3 and Formula I-4, the value range of n is 50 ≤ n ≤ 200.

[0106] In the embodiments of the present application, the negative electrode additives represented by Formula I-1 to Formula I-4 can coat the surface of the negative electrode active material and preferentially reduce to form an SEI film containing boron element during formation. This SEI film has the characteristics of high active ion diffusion coefficient, high thermal stability and low impedance, and can also improve the desolvation rate of solvated active ions, so that the problem of active ion precipitation on the surface of the negative electrode active material is not likely to occur. In this way, the secondary battery exhibits high electrochemical performances such as first Coulomb efficiency, rate performance and cycle performance.

[0107] In a second aspect, the embodiments of the present application provide a preparation method of a negative electrode additive, including the following steps:

[0108] React a caffeic acid compound with sodium borohydride or lithium borohydride to obtain the negative electrode additive of the present application;

[0109] The chemical general formula of caffeic acid compounds is shown in Formula II:

[0110]

[0111] Among them, R1 is an atom with a lone pair of electrons, R2 includes any one of a hydrogen atom and a nitrile group, and R3 includes any one of a hydrogen atom, a hydroxyl group, and an alkoxy group with 1 to 3 carbon atoms.

[0112] In the preparation method of the negative electrode additive provided by the embodiments of the present application, sodium borohydride or lithium borohydride can be used as an initiator to cause the polymerization reaction of caffeic acid compounds, and at the same time, it can also be used as a reactant to react with the hydroxyl group in the caffeic acid compounds, thereby effectively preparing a negative electrode additive with the performance as described above in the present application. In addition, the preparation method of the negative electrode additive of the present application can ensure the stability of the structure and electrochemical performance of the prepared negative electrode additive, and has high efficiency and saves production costs.

[0113] In Formula II, when R1 is an oxygen atom and R2 is a hydrogen atom, the compound corresponding to Formula II is caffeic acid, and other cases correspond to caffeic acid derivatives.

[0114] In some embodiments, the step of reacting the caffeic acid compound with sodium borohydride or lithium borohydride includes:

[0115] Under a protective atmosphere, the caffeic acid compound is added to a first organic solvent under ice bath conditions to obtain a first solution;

[0116] Sodium borohydride or lithium borohydride is added to a second organic solvent to obtain a second solution;

[0117] The second solution is added to the first solution for reaction to obtain a negative electrode additive.

[0118] In the preparation method of the negative electrode additive provided by the embodiments of the present application, first, the caffeic acid compound is dissolved under specific conditions such as ice bath conditions to form a first solution, and then the second solution containing sodium borohydride or lithium borohydride is added to the first solution, so that the caffeic acid compound reacts fully with sodium borohydride or lithium borohydride to form a polymer containing SP 3 hybridized boron atoms, thereby obtaining the negative electrode additive shown in Formula I.

[0119] In this reaction process, the addition amount of sodium borohydride or lithium borohydride is excessive relative to the caffeic acid compound, so that while the polymerization reaction occurs fully, the structural unit of the polymer can contain SP 3 hybridized boron atoms with negative charges.

[0120] In some embodiments, the first organic solvent is miscible with the second organic solvent, and the first organic solvent and the second organic solvent each independently include at least one of tetrahydrofuran, acetonitrile, dimethyl sulfoxide, cyclohexanone, N-methylpyrrolidone, toluene, chlorobenzene, benzene, benzotrichloride, and benzotetrachloride.

[0121] The solvent provided by the embodiments of the present application has appropriate polarity, which is conducive to the sufficient dispersion of each reaction raw material such as caffeic acid compounds and sodium borohydride or lithium borohydride in the solvent, so that the reaction can proceed fully.

[0122] It can be understood that the first organic solvent should be miscible with the second organic solvent. In this way, when the second solution containing sodium borohydride or lithium borohydride is added dropwise to the first solution, sodium borohydride or lithium borohydride can react with caffeic acid compounds.

[0123] In some specific embodiments, the first organic solvent is the same as the second organic solvent, and the first organic solvent includes at least one of tetrahydrofuran, acetonitrile, dimethyl sulfoxide, cyclohexanone, N-methylpyrrolidone, toluene, chlorobenzene, benzene, benzotrichloride, and benzotetrachloride.

[0124] When the first organic solvent is the same as the second organic solvent, it is more conducive to the smooth reaction of sodium borohydride or lithium borohydride with caffeic acid compounds.

[0125] In some embodiments, the second solution is added dropwise, and the dropping rate is 4 - 6 mL / min. Exemplarily, the dropping rate can be typical but non-limiting values such as 4 mL / min, 4.5 mL / min, 5 mL / min, 5.5 mL / min, 6 mL / min, etc.

[0126] The reaction between caffeic acid compounds and sodium borohydride or lithium borohydride is an exothermic reaction. By using the dropwise addition method, the conditions such as temperature and pressure during the reaction can be controlled, making the reaction more uniform and not prone to the phenomenon of explosive polymerization; the reaction rate can also be controlled, and it is not easy to obtain polymers with a wide molecular weight distribution.

[0127] In some embodiments, the reaction conditions are: the temperature is 0°C - 30°C, and the time is 12 h - 24 h. Exemplarily, the reaction temperature can be typical but non-limiting values such as 0°C, 5°C, 10°C, 15°C, 20°C, 21°C, 22°C, 23°C, 24°C, 25°C, 26°C, 27°C, 28°C, 29°C, 30°C, etc., and the reaction time can be typical but non-limiting values such as 12 h, 11 h, 12 h, 13 h, 14 h, 15 h, 18 h, 20 h, 22 h, 24 h, etc.

[0128] The reaction conditions provided in the embodiments of the present application can enable the nucleic acid caffeic acid compounds to react fully with sodium borohydride or lithium borohydride, thereby obtaining a negative electrode additive as shown in Formula I.

[0129] In a third aspect, the embodiments of the present application provide a negative electrode plate, including a negative electrode current collector and a negative electrode film layer located on at least one side of the negative electrode current collector. The negative electrode film layer includes the negative electrode additive of the embodiments of the present application and / or the negative electrode additive prepared by the preparation method of the negative electrode additive of the embodiments of the present application.

[0130] The negative electrode plate provided by the embodiments of the present application contains the negative electrode additive of the present application, which makes it not easy for the interface of the negative electrode active material to have the problem of active ion precipitation, thereby improving the electrochemical performance of the battery cell, such as capacity, initial Coulomb efficiency, rate performance, and cycle performance.

[0131] The current collector is a structure or part used to collect current in the battery. The negative electrode current collector refers to the structure or part used to collect current at the negative electrode in the battery. Exemplarily, the negative electrode current collector can be a metal foil or a composite current collector. The metal foil can be a copper foil. The composite current collector can be a polymer matrix material and a metal layer formed on at least one surface of the polymer matrix material. The composite current collector can be formed on the surface of the polymer matrix material by copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy. The polymer matrix material can be polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, polyethylene, etc.

[0132] The negative electrode film layer refers to the film layer provided on the negative electrode current collector and containing the negative electrode active material. The negative electrode active layer can be provided on one side surface of the negative electrode current collector, or can be provided on both side surfaces of the negative electrode current collector. Among them, the negative electrode film layer includes the negative electrode additive of the present application.

[0133] In some embodiments, the negative electrode film layer further includes a negative electrode active material, and at least part of the negative electrode additive is coated on the surface of the negative electrode active material.

[0134] In the embodiments of the present application, the negative electrode additive is a water-soluble polymer and has appropriate flexibility, so that it can be coated on the surface of the negative electrode active material to form a coating layer. In this way, the reduction product of the negative electrode additive, such as a boron compound, is contained in the SEI film formed during formation, and the distribution density of the boron compound in the SEI film shows a tendency to be uniform. Therefore, the performance of the SEI film can be significantly improved, such as high active ion diffusion ability, low impedance, and improved desolvation ability of solvated active ions.

[0135] In some embodiments, the mass ratio of the negative electrode additive to the negative electrode active material is 0.1 to 0.6:100. Exemplarily, the mass percentage of the negative electrode additive can be typical but non-limiting values such as 0.1, 0.3, 0.35, 0.4, 0.45, 0.5, 0.6, etc.

[0136] In the embodiments of the present application, controlling the content of the negative electrode additive within the above range can fully exert the role of the negative electrode additive, form a boron-containing SEI film on the surface of the negative electrode active material. This SEI film has the characteristics of high active ion diffusion coefficient, high thermal stability and low impedance, and can also improve the desolvation process of solvated active ions, so that the problem of active ion precipitation is not likely to occur on the surface of the negative electrode active material. In this way, the secondary battery exhibits high electrochemical performances such as high first Coulomb efficiency, rate performance and cycle performance.

[0137] In some embodiments, the negative electrode film layer further includes a thickening agent, and the mass ratio of the thickening agent to the negative electrode active material is 0.7 to 1.5:100. Exemplarily, the mass percentage of the thickening agent can be typical but non-limiting values such as 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, etc.

[0138] In the embodiments of the present application, by regulating the content of the thickening agent, the negative electrode sheet has a high bonding strength.

[0139] In some embodiments, the negative electrode film layer further includes a binder, and the mass ratio of the binder to the negative electrode active material is 1.3 to 2.2:100. Exemplarily, the mass percentage of the binder can be typical but non-limiting values such as 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, etc.

[0140] In the embodiments of the present application, by regulating the content of the binder, the negative electrode sheet has a high bonding strength.

[0141] In some embodiments, the mass ratio of the negative electrode additive, the thickening agent and the binder is (0.3 to 0.5):(0.7 to 0.9):(1.6 to 1.8). Exemplarily, the mass percentage of the negative electrode additive can be typical but non-limiting values such as 0.3, 0.35, 0.4, 0.45, 0.5, etc.; the mass percentage of the thickening agent can be typical but non-limiting values such as 0.7, 0.75, 0.8, 0.85, 0.9, etc.; the mass percentage of the binder can be typical but non-limiting values such as 1.6, 1.65, 1.7, 1.75, 1.8, etc.

[0142] In the embodiments of the present application, by controlling the mass ratio of the negative electrode additive, thickening agent, and binder, the negative electrode additive, thickening agent, and binder can fully play a synergistic effect, not only making the negative electrode sheet have a high adhesive force, but also forming a SEI film containing boron compounds on the surface of the negative electrode active material after formation, thereby improving the electrochemical properties such as the capacity, first Coulomb efficiency, rate performance, and cycle performance of the battery cell.

[0143] In some embodiments, the mass ratio of the negative electrode additive, thickening agent, and binder is 0.5:0.8:1.6.

[0144] In the embodiments of the present application, by controlling the content ratio of each component, the negative electrode additive and other components play a synergistic effect. For example, the main function of the thickening agent is to achieve the coating of the negative electrode active material and provide surface adhesion, and the negative electrode additive can also coat the surface of the negative electrode active material and provide surface adhesion, which means that part of the function of the negative electrode additive is the same as that of the thickening agent. In this way, there is a competitive relationship between the negative electrode additive and the thickening agent. Therefore, controlling within the above range is not easy to form an overly thick SEI film on the surface of the negative electrode active material, and makes the negative electrode sheet have a high adhesive force, thereby improving the electrochemical properties such as the capacity, first Coulomb efficiency, rate performance, and cycle performance of the battery cell.

[0145] In some embodiments, the negative electrode active material can be the negative electrode active material for batteries well-known in the art. As an example, the negative electrode active material can include one or more of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials can also be used. These negative electrode active materials can be used alone or in combination of two or more.

[0146] In some embodiments, the binder can include but is not limited to 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).

[0147] In some embodiments, the thickening agent can include but is not limited to sodium carboxymethyl cellulose (CMC-Na), etc.

[0148] In some embodiments, the negative electrode film layer further includes a conductive agent, and the conductive agent can include but is not limited to at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0149] In some embodiments, the negative electrode sheet can be prepared in the following manner: the components for preparing the negative electrode sheet described above, such as the negative electrode active material, the conductive agent, the binder, the negative electrode additive, and the thickening agent, are dispersed in deionized water to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after processes such as drying and cold pressing, the negative electrode sheet can be obtained, wherein the negative electrode additive includes the negative electrode additive of the present application.

[0150] Fourthly, an embodiment of the present application provides a secondary battery, which includes the negative electrode sheet in the above embodiment, thereby improving the electrochemical performance of the secondary battery, such as capacity, Coulomb efficiency, and cycle performance.

[0151] In some embodiments, at least part of the surface of the negative electrode film layer is covered with a SEI film, and the SEI film contains a boron compound.

[0152] The negative electrode additive provided by the present application is a water-soluble polymer and has appropriate flexibility, so that it can coat the surface of the negative electrode active material, and after formation, a SEI film for coating the negative electrode active material can be formed. Since the SEI film formed in the secondary battery of the present application contains a boron compound, it can enhance the ability of solvated active ions to desolvate, and can also enhance the diffusion ability of active ions in the SEI film, reduce the interfacial impedance of the SEI film, and endow the SEI film with high thermal stability. In this way, the secondary battery exhibits high electrochemical performance such as cycle performance and rate performance.

[0153] In some embodiments, the electrode assembly and the electrolyte can be assembled to form a battery cell. The electrode assembly includes a negative electrode sheet, a separator, and a positive electrode sheet, wherein the negative electrode sheet is the negative electrode sheet in the embodiment of the present application.

[0154] Exemplarily, the negative electrode sheet, the separator, and the positive electrode sheet can be formed into an electrode assembly by a winding process or a stacking process. The electrode assembly is placed in an outer package, dried, and then injected with an electrolyte, and after processes such as vacuum packaging, standing, formation, and shaping, a battery cell is obtained. The shape of the battery cell is not particularly limited, and it can be cylindrical, square, or any other shape.

[0155] It should be noted that the positive electrode sheet includes a positive electrode current collector and a positive electrode active layer provided on at least one surface of the positive electrode current collector. The positive electrode current collector can adopt a metal foil or a porous metal plate with good electrical conductivity and mechanical properties, such as aluminum foil.

[0156] The positive electrode active layer refers to a film layer provided on the positive electrode current collector and containing a positive electrode active material. The positive electrode active layer can be provided on one side surface of the positive electrode current collector, or on both side surfaces of the positive electrode current collector. Optionally, the positive electrode active layer includes a conductive agent and a binder.

[0157] In some embodiments, the positive electrode active material may adopt the commonly used positive electrode active materials in the present application. As an example, the positive electrode active material may include at least one of the following materials: lithium-containing phosphates, lithium transition metal oxides, and their respective modified compounds. However, the present application is not limited to these materials, and other conventional materials that can be used as the positive electrode active material of the battery can also be used. These positive electrode active materials can be used alone or in combination of two or more. Among them, the lithium transition metal oxides include, but are not limited to, at least one of lithium cobaltate, nickel cobalt manganese ternary material, nickel cobalt aluminum ternary material, nickel cobalt manganese aluminum quaternary material, lithium iron phosphate, lithium manganese phosphate, lithium vanadium phosphate, and lithium manganate.

[0158] In some embodiments, the positive electrode sheet can be prepared in the following manner: Disperse the above components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; coat the positive electrode slurry on the positive electrode current collector, and after processes such as drying and cold pressing, the positive electrode sheet can be obtained.

[0159] It should be noted that the electrolyte plays a role in conducting ions between the positive electrode sheet and the negative electrode sheet. The present application has no specific limitation on the type of the electrolyte, and it can be selected according to requirements. For example, the electrolyte can be liquid, gel-like, or all-solid-state.

[0160] In some embodiments, the electrolyte includes an electrolyte and a solvent. Exemplarily, the electrolyte includes a lithium salt or a sodium salt. The lithium salts include, but are not limited to, at least one of lithium bis(fluorosulfonyl)imide, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium bis(trifluoromethylsulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium difluoro bis(oxalato)phosphate, and lithium tetrafluoro(oxalato)phosphate.

[0161] It should be noted that the material of the separator may include, but is not limited to, at least one of glass fiber, non-woven 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.

[0162] In some embodiments, the secondary battery includes at least one battery cell, generally including a plurality of battery cells. The plurality of battery cells are connected in series, parallel, or in a hybrid connection to increase the power supply capacity of the battery. In some cases, the secondary battery further includes a box body, and the battery cells are accommodated in the box body.

[0163] Therefore, in the embodiments, the secondary battery may include any one of a battery cell, a battery module, and a battery pack.

[0164] In some embodiments, when the secondary battery in the embodiments of the present application is a battery module, the battery module contains a plurality of battery cells, and the plurality of battery cells may be arranged in sequence along the length direction of the battery module. Of course, they may also be arranged in any other manner. Further, the plurality of battery cells may be fixed by fasteners.

[0165] In some embodiments, the battery module may further include a housing having an accommodation space, and the plurality of battery cells are accommodated in the accommodation space.

[0166] In some embodiments, when the secondary battery in the embodiments of the present application is a battery pack, the battery pack may contain a plurality of battery cells, and the plurality of battery cells may be assembled into the above-mentioned battery module. Therefore, the specific number of battery cells or battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0167] As in the embodiments, the battery pack may include a battery box and a plurality of battery modules disposed in the battery box. The battery box includes an upper box body and a lower box body. The upper box body is used to cover the lower box body and form a closed space for accommodating the battery modules. The plurality of battery modules may be arranged in the battery box in any manner.

[0168] Please refer to Figure 1 , Figure 1 FIG. is a schematic exploded view of a battery cell 3 provided in some embodiments of the present application. The housing of the battery cell 3 may include a cell case 31 and a cover plate 33. The cell case 31 may include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates enclose an accommodation cavity. The cell case 31 has an opening communicating with the accommodation cavity, and the cover plate 33 is used to cover the opening to close the accommodation cavity. The positive electrode plate, the negative electrode plate, and the separator may be formed into an electrode assembly 32 through a winding process and / or a stacking process. The electrode assembly 32 is encapsulated in the accommodation cavity. The electrolyte is infiltrated in the electrode assembly 32. The number of electrode assemblies 32 contained in the battery cell 3 may be one or more, and can be adjusted according to actual needs. Among them, the negative electrode plate contains the negative electrode additive in the above embodiments.

[0169] Please refer to Figure 2 , Figure 2 FIG. is a schematic diagram of a battery module 4 provided in an embodiment of the present application. In the battery module 4, a plurality of battery cells 3 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. Further, the plurality of battery cells 3 may be fixed by fasteners.

[0170] Please refer to Figure 3 , Figure 3Schematic exploded view of the battery pack 5 provided by the embodiments of the present application. The battery pack 5 may include a battery box and a plurality of battery modules 4 disposed in the battery box. The battery box includes an upper box body 51 and a lower box body 52. The upper box body 51 is used to cover the lower box body 52 and form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 may be arranged in the battery box in any manner.

[0171] In a fifth aspect, the embodiments of the present application provide an electrical device including the secondary battery of the embodiments of the present application. In the embodiments of the present application, the secondary battery may be used as the power source of the electrical device or as the energy storage unit of the electrical device. Therefore, the electrical device of the embodiments of the present application has a long standby or endurance time and good safety performance.

[0172] Some embodiments of the present application provide an electrical device using a battery as a power source. The electrical device may be, but is not limited to, a vehicle, a mobile phone, a portable device, a laptop computer, a ship, a spacecraft, an electric toy, and an electric tool, etc. The vehicle may be, but is not limited to, a fuel vehicle, a gas vehicle, or a new energy vehicle. The new energy vehicle may be, but is not limited to, a pure electric vehicle, a hybrid electric vehicle, or an extended-range electric vehicle, etc.; the spacecraft includes an airplane, a rocket, a space shuttle, and a spaceship, etc.; the electric toy includes a fixed or mobile electric toy, for example, a game console, an electric vehicle toy, an electric ship toy, and an electric airplane toy, etc.; the electric tool includes a metal cutting electric tool, a grinding electric tool, an assembly electric tool, and a railway electric tool, for example, an electric drill, an electric grinding wheel, an electric wrench, an electric screwdriver, a hammer drill, an impact electric drill, a concrete vibrator, and a planer, etc.

[0173] Please refer to Figure 4 , Figure 4 Schematic diagram of the electrical device provided by the embodiments of the present application. The electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the requirements of the electrical device for high power and high energy density, a battery pack or a battery module may be adopted.

[0174] Embodiment

[0175] Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application and should not be construed as a limitation to the present application. For those not specified in the embodiments regarding specific technologies or conditions, the technologies or conditions described in the literature in the art or according to the product specifications are followed. For the reagents or instruments not specified regarding the manufacturer, they are all conventional products that can be obtained through commercial procurement.

[0176] Embodiment 1

[0177] This embodiment provides a negative electrode additive, a negative electrode plate, and a secondary battery.

[0178] Negative electrode additive:

[0179] A negative electrode additive, whose structural formula is shown in Formula I-2, and the weight average molecular weight is 3.74×10 4 .

[0180]

[0181] A preparation method of a negative electrode additive, comprising the following steps:

[0182] Step S1: Add 3.0 mol of caffeic acid compounds (chemical formula shown in Formula II-2) and 20000 mL of dry tetrahydrofuran into a 5000 mL three-necked round-bottom flask in sequence, and then stir under nitrogen and ice bath conditions until completely dissolved to obtain a first solution;

[0183]

[0184] Step S2: Dissolve lithium borohydride in tetrahydrofuran to form a second solution with a concentration of 2.0 M, and drop 1.5 L of the second solution into the first solution under stirring. The first solution changes from light brown to fluorescent yellow. After complete dropping, continue to stir and react at 25 °C for 12 hours.

[0185] Step S3: Filter the reaction solution obtained in Step S2 and wash it thoroughly with tetrahydrofuran to remove unreacted starting materials, and dissolve the product in ether for repeated recrystallization. Finally, a light yellow powder, namely the additive shown in Formula I-2, is obtained.

[0186] Negative electrode plate:

[0187] Mix the negative electrode additive, graphite active material, conductive agent carbon black, thickening agent (sodium carboxymethyl cellulose), and binder (styrene-butadiene rubber) in Example 1 above according to a weight ratio of 0.5:100:0.8:1.2:1.8 and stir evenly to obtain a negative electrode slurry; then evenly coat the negative electrode slurry on two opposite surfaces of the negative electrode current collector copper foil, and after drying, cold pressing, slitting, and cutting (specification: 5 cm * 5 cm), a negative electrode plate is obtained.

[0188] Button cell:

[0189] Assemble a half-button battery: Use the negative electrode plate of Example 1 of this application as the positive electrode, a lithium metal sheet as the negative electrode, nickel foam as the filler, a polypropylene film as the separator, 1 mol / l LiPF6, and a combined solvent EC / DEC / DMC = 1:1:1 (volume ratio) as the electrolyte, and assemble it using a CR2430 button cell case. Assemble it according to the negative electrode case - nickel foam - lithium sheet - electrolyte (30 μL) - separator - electrolyte (30 μL) - electrode plate - positive electrode case, and seal it with a pressure of 650 PSI.

[0190] Secondary battery:

[0191] [Preparation of negative electrode sheet]

[0192] That is, the negative electrode sheet of Example 1 of the present application.

[0193] [Preparation of positive electrode sheet]

[0194] Mix the positive active material lithium iron phosphate, the conductive agent acetylene black, and the binder PVDF (polyvinylidene fluoride) in a mass ratio of 98:1:1, add the solvent NMP (N-methylpyrrolidone), and stir until the system becomes homogeneous to obtain a positive electrode slurry (solid content: 70%); coat the positive electrode slurry on the positive electrode current collector aluminum foil uniformly on both sides with a loading amount of 25 mg / cm 2 , air-dry at room temperature, transfer to an oven for further drying, and then cut into positive electrode sheets with a specification of 40 mm × 50 mm.

[0195] [Separator]

[0196] Separator: Polyethylene porous film, with a specification of 45 mm × 55 mm.

[0197] [Electrolyte]

[0198] Mix ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 1:1:1, and then uniformly dissolve LiPF6 in the above mixed solvent to obtain an electrolyte. In this electrolyte, the concentration of the lithium salt is 1 mol / L.

[0199] Preparation of secondary battery: Stack the above positive electrode sheet, separator, and negative electrode sheet in sequence, place the separator between the positive and negative electrode sheets, and wrap them in an aluminum-plastic film bag to form a stacked dry battery cell. Inject 0.3 g of the above electrolyte, and perform vacuum hot pressing and encapsulation on the aluminum-plastic film bag to assemble a soft-pack battery.

[0200] Example 2

[0201] This example provides a negative electrode additive, a negative electrode sheet, and a secondary battery. The difference from Example 1 is that the ratio of each raw material in the negative electrode slurry is different. Specifically, the weight ratio of the negative electrode additive, graphite active material, conductive agent carbon black, thickening agent (sodium carboxymethyl cellulose), and binder (styrene-butadiene rubber) prepared in Example 1 is 0.3:100:0.8:1.2:1.8.

[0202] Example 3

[0203] This embodiment provides a negative electrode additive, a negative electrode sheet and a secondary battery. The difference from Embodiment 1 is that the ratio of each raw material in the negative electrode paste is different. Specifically, the weight ratio of the negative electrode additive, graphite active material, conductive agent carbon black, thickener (sodium carboxymethylcellulose), and binder (styrene-butadiene rubber) prepared in Embodiment 1 is 0.1:100:0.8:1.2:1.8.

[0204] Embodiment 4

[0205] This embodiment provides a negative electrode additive, a negative electrode sheet and a secondary battery. The difference from Embodiment 1 is that the ratio of each raw material in the negative electrode paste is different. Specifically, the weight ratio of the negative electrode additive, graphite active material, conductive agent carbon black, thickener (sodium carboxymethylcellulose), and binder (styrene-butadiene rubber) prepared in Embodiment 1 is 0.6:100:0.8:1.2:1.8.

[0206] Embodiment 5

[0207] This embodiment provides a negative electrode additive, a negative electrode sheet and a secondary battery. The difference from Embodiment 1 is that the ratio of each raw material in the negative electrode paste is different. Specifically, the weight ratio of the negative electrode additive, graphite active material, conductive agent carbon black, thickener (sodium carboxymethylcellulose), and binder (styrene-butadiene rubber) prepared in Embodiment 1 is 0.5:100:0.8:1.5:1.8.

[0208] Embodiment 6

[0209] This embodiment provides a negative electrode additive, a negative electrode sheet and a secondary battery. The difference from Embodiment 1 is that the ratio of each raw material in the negative electrode paste is different. Specifically, the weight ratio of the negative electrode additive, graphite active material, conductive agent carbon black, thickener (sodium carboxymethylcellulose), and binder (styrene-butadiene rubber) prepared in Embodiment 1 is 0.5:100:0.8:1.1:1.8.

[0210] Embodiment 7

[0211] This embodiment provides a negative electrode additive, a negative electrode sheet and a secondary battery. The difference from Embodiment 1 is that the ratio of each raw material in the negative electrode paste is different. Specifically, the weight ratio of the negative electrode additive, graphite active material, conductive agent carbon black, thickener (sodium carboxymethylcellulose), and binder (styrene-butadiene rubber) prepared in Embodiment 1 is 0.5:100:0.8:0.7:1.8.

[0212] Embodiment 8

[0213] This embodiment provides a negative electrode additive, a negative electrode sheet and a secondary battery. The difference from Embodiment 1 is that the proportion of each raw material in the negative electrode slurry is different. Specifically, the weight ratio of the negative electrode additive, graphite active material, conductive agent carbon black, thickener (sodium carboxymethylcellulose), and binder (styrene-butadiene rubber) prepared in Embodiment 1 is 0.5:100:0.8:1.2:2.2.

[0214] Embodiment 9

[0215] This embodiment provides a negative electrode additive, a negative electrode sheet and a secondary battery. The difference from Embodiment 1 is that the proportion of each raw material in the negative electrode slurry is different. Specifically, the weight ratio of the negative electrode additive, graphite active material, conductive agent carbon black, thickener (sodium carboxymethylcellulose), and binder (styrene-butadiene rubber) prepared in Embodiment 1 is 0.5:100:0.8:1.2:1.6.

[0216] Embodiment 10

[0217] This embodiment provides a negative electrode additive, a negative electrode sheet and a secondary battery. The difference from Embodiment 1 is that the proportion of each raw material in the negative electrode slurry is different. Specifically, the weight ratio of the negative electrode additive, graphite active material, conductive agent carbon black, thickener (sodium carboxymethylcellulose), and binder (styrene-butadiene rubber) prepared in Embodiment 1 is 0.5:100:0.8:1.2:1.3.

[0218] Embodiment 11

[0219] This embodiment provides a negative electrode additive, a negative electrode sheet and a secondary battery. The difference from Embodiment 1 is that the proportion of each raw material in the negative electrode slurry is different. Specifically, the weight ratio of the negative electrode additive, graphite active material, conductive agent carbon black, thickener (sodium carboxymethylcellulose), and binder (styrene-butadiene rubber) prepared in Embodiment 1 is 0.5:100:0.8:0.8:1.6.

[0220] Embodiment 12

[0221] This embodiment provides a negative electrode additive, a negative electrode sheet and a secondary battery. The difference from Embodiment 11 is that the weight average molecular weight of the negative electrode additive is different. Specifically, the structural formula of the negative electrode additive is shown in Formula I-2, and the weight average molecular weight is 1.82×10 4 。

[0222]

[0223] Embodiment 13

[0224] This embodiment provides a negative electrode additive, a negative electrode sheet, and a secondary battery. The difference from Embodiment 11 is that the weight average molecular weight of the negative electrode additive is different. Specifically, the structural formula of the negative electrode additive is as shown in Formula I-2, and the weight average molecular weight is 6.42×10 4 .

[0225]

[0226] Embodiment 14

[0227] This embodiment provides a negative electrode additive, a negative electrode sheet, and a secondary battery. The difference from Embodiment 11 is that the structural formula of the negative electrode additive is different. Specifically, the structural formula of the negative electrode additive is as shown in Formula I-1, and the weight average molecular weight is 3.77×10 4 .

[0228]

[0229] A method for preparing a negative electrode additive includes the following steps:

[0230] Step S1: Add 540 g (3.0 mol) of caffeic acid and 20000 mL of dry tetrahydrofuran to a 5000 mL three-necked round-bottom flask in sequence, and then stir under nitrogen and ice bath conditions until completely dissolved to obtain a first solution.

[0231] Step S2: Dissolve lithium borohydride in tetrahydrofuran to form a second solution with a concentration of 2.0 M. Add 1.5 L of the second solution dropwise to the first solution under stirring. The first solution changes from light brown to fluorescent yellow. After complete dropwise addition, continue to stir and react at 25 °C for 12 hours.

[0232] Step S3: Filter the reaction solution obtained in Step S2 and wash it thoroughly with tetrahydrofuran to remove unreacted starting materials. Dissolve the product in ether and recrystallize it repeatedly to finally obtain a pale yellow powder, which is the additive shown in Formula I-1.

[0233] Embodiment 15

[0234] This embodiment provides a negative electrode additive, a negative electrode sheet, and a secondary battery. The difference from Embodiment 11 is that the structural formula of the negative electrode additive is different. Specifically, the structural formula of the negative electrode additive is as shown in Formula I-3, and the weight average molecular weight is 3.68×10 4 .

[0235]

[0236] A method for preparing a negative electrode additive includes the following steps:

[0237] Step S1: Add 3.0 mol of caffeic acid compounds (chemical formula as shown in Formula II-3) and 20000 mL of dry tetrahydrofuran into a 5000 mL three-necked round-bottom flask in sequence, and then stir under nitrogen and ice bath conditions until completely dissolved to obtain a first solution;

[0238]

[0239] Step S2: Dissolve lithium borohydride in tetrahydrofuran to form a second solution with a concentration of 2.0 M. Add 1.5 L of the second solution dropwise to the first solution under stirring. The first solution changes from light brown to fluorescent yellow. After complete dropwise addition, continue to stir and react at 25 °C for 12 hours.

[0240] Step S3: Filter the reaction solution obtained in Step S2 and wash it thoroughly with tetrahydrofuran to remove unreacted starting materials, and dissolve the product in ether and recrystallize repeatedly to finally obtain a light yellow powder, namely the additive shown in Formula I-3.

[0241] Example 16

[0242] This example provides a negative electrode additive, a negative electrode plate and a secondary battery. The difference from Example 11 is that the structural formula of the negative electrode additive is different. Specifically, the structural formula of the negative electrode additive is as shown in Formula I-4, and the weight average molecular weight is 3.82×10 4 。

[0243]

[0244] A preparation method of a negative electrode additive includes the following steps:

[0245] Step S1: Add 3.0 mol of caffeic acid compounds (chemical formula as shown in Formula II-4) and 20000 mL of dry tetrahydrofuran into a 5000 mL three-necked round-bottom flask in sequence, and then stir under nitrogen and ice bath conditions until completely dissolved to obtain a first solution;

[0246]

[0247] Step S2: Dissolve lithium borohydride in tetrahydrofuran to form a second solution with a concentration of 2.0 M. Add 1.5 L of the second solution dropwise to the first solution under stirring. The first solution changes from light brown to fluorescent yellow. After complete dropwise addition, continue to stir and react at 25 °C for 12 hours.

[0248] Step S3: Filter the reaction solution obtained in Step S2 and wash it thoroughly with tetrahydrofuran to remove unreacted starting materials, and dissolve the product in ether and recrystallize repeatedly to finally obtain a light yellow powder, namely the additive shown in Formula I-4.

[0249] Comparative Example 1

[0250] This comparative example provides a negative electrode additive, a negative electrode sheet, and a secondary battery. The difference from Example 1 is that: the negative electrode additive is not added to the negative electrode sheet. Specifically:

[0251] Negative electrode sheet:

[0252] Graphite active material, conductive agent carbon black, thickening agent (sodium carboxymethyl cellulose), and binder (styrene-butadiene rubber) are mixed evenly according to a weight ratio of 100:0.8:1.2:1.8 to obtain a negative electrode slurry; then the negative electrode slurry is evenly coated on two opposite surfaces of the negative electrode current collector copper foil, and after drying, cold pressing, slitting, and cutting (specification: 5 cm * 5 cm), a negative electrode sheet is obtained.

[0253] Performance testing

[0254] (1) Infrared spectroscopy analysis

[0255] The negative electrode additive prepared in Example 14 was tested by infrared spectroscopy, and the test results are as Figure 5 shown.

[0256] From Figure 5 it can be observed that there is a stretching vibration peak of intermolecular hydrogen bond at 3400 cm -1 , a characteristic absorption peak of carbonyl at 1600 cm -1 , a vibration absorption peak of double bond at 1500 cm -1 , and characteristic absorption peaks of O-B-O at 1250 cm -1 and 1063 cm -1 . Thus, it can be proved that the structural formula of the negative electrode sheet is as shown in Formula I-1.

[0257] (2) Adhesion

[0258] The adhesion test method of the negative electrode sheet is as follows:

[0259] The negative electrode sheet is cut into a spline of 300 mm * 20 mm, and the spline is fixed on the test steel plate with double-sided tape, leaving 50 mm of length for the traction of the tensile machine. First, the test steel plate with the fixed spline is fixed on the 90° test fixture of the tensile machine, and the reserved end is fixed on the tensile sensor of the tensile machine. Then, after selecting the corresponding test mode of the electrode sheet, the test is started. Each group of samples is tested 3 times, and finally the average value is taken as the adhesion of the negative electrode sheet.

[0260] (3) Coin cell test

[0261] First Coulombic efficiency test: The first efficiency test was carried out using a 5V / 1mA Blue Energy test machine. For the prepared button cell, it was left standing for 5 hours in a constant temperature environment of 25°C, then discharged at a constant current of 0.05C to 0.005V, discharged at a constant current of 50μA to 0.005V, then left standing for 5 minutes, discharged at a constant current of 10μA to 0.005V, left standing for 5 minutes, charged at a constant voltage of 0.1C to 2V, left standing for 5 minutes, and the test was completed.

[0262] First Coulombic efficiency = first charging gram capacity / first discharging gram capacity, gram capacity = capacity / mass of active material × 100%.

[0263] (4) Full cell detection

[0264] First Coulombic efficiency test:

[0265] At 25°C, for the formed secondary battery, first discharge at a constant current of 1 / 3C (DC) to 2.5V, leave standing for 10 min; then charge at a constant current of 1 / 3C (CC) to 3.65V, then charge at a constant voltage of 3.65V (CV) until the current is 0.05C, leave standing for 10 min, and record the charging capacity; then discharge at a constant current of 1 / 3C (DC) to 2.5V and record the discharging capacity; First Coulombic efficiency = discharging capacity / charging capacity × 100%.

[0266] Capacity retention rate test:

[0267] (1) At 45°C, charge the lithium-ion battery at a constant current of 1 / 3C to 3.65V, then charge at a constant voltage of 3.65V until the current is 0.05C, leave standing for 5 min, and then discharge at a constant current of 1 / 3C to 2.5V and record the discharging capacity C0.

[0268] (2) Then charge the lithium-ion battery at a constant current of 1.0C to 3.65V, leave standing for 5 min, and then discharge at a constant current of 1 / 3C to 2.5V and record the discharging capacity C1.

[0269] Repeat the above step (2) 1000 times, record the discharging capacity C1000 of the lithium-ion battery after the 1000th cycle, and the capacity retention rate P1000 = C1000 / C0 × 100%.

[0270] The test results of the adhesion force between the negative electrode sheets prepared in Examples 1 to 16 and the negative electrode sheet provided in Comparative Example 1 are shown in Table 1 below.

[0271] Table 1

[0272]

[0273]

[0274] In the negative electrode sheet, the main function of the thickening agent is to achieve the coating of the negative active material and provide surface adhesion, while the main function of the binder is to provide point adhesion. Thus, the adhesion of the negative electrode sheet is relatively high. Since the negative electrode additive is included in the embodiments of the present application, the negative electrode additive can also be coated on the surface of the graphite and has the same surface adhesion mechanism as the thickening agent. In this way, when the mass ratio of the negative electrode additive in the negative electrode sheet changes, such as in Examples 1 to 4, with the addition amounts of the thickening agent and the binder remaining unchanged, the adhesion of the negative electrode sheet decreases as the content of the negative electrode additive decreases, indicating that the negative electrode additive has a certain adhesive ability; in Examples 5 to 7, with the contents of the negative electrode additive and the binder fixed, at this time, the adhesion of the negative electrode sheet decreases as the content of the thickening agent decreases and is higher than that of the comparative example; in Examples 8 to 10, with the contents of the negative electrode additive and the thickening agent unchanged, the adhesion of the negative electrode sheet decreases as the content of the binder decreases, which is because the negative electrode additive cannot provide the ability of point adhesion, indicating that the adhesiveness of the negative electrode sheet is positively correlated with the content of the binder. In Examples 12 to 13, as the molecular weight of the negative electrode additive increases, the adhesion of the negative electrode sheet at the same content shows a certain increase, which is because the larger the molecular weight, the better the ductility of the negative electrode additive and the easier it is to achieve the coating of the negative active material. In Examples 14 to 16, for different additive systems, the presence of polar groups on the main chain can slightly improve the adhesion of the negative electrode sheet.

[0275] The performance test results of the button cells prepared in Examples 1 to 16 and the button cell provided in Comparative Example 1 are listed in Table 2 below.

[0276] Table 2

[0277]

[0278]

[0279] As can be seen from Table 2, for the half-cell of the button cell, since the adhesions of the negative electrode sheets prepared in Examples 1 to 16 are all higher than that of the comparative example, the negative electrode sheets can ensure the integrity of the overall morphology. Their initial coulombic efficiency and charge-discharge specific capacity do not differ much. However, when the content of the negative electrode additive increases, the initial coulombic efficiency still shows a gradually increasing trend, indicating that the negative electrode additive provided in the embodiments of the present application can exhibit a better lithium intercalation / deintercalation channel. This is mainly because after the negative electrode additive coats the graphite, its inherent lithium ions exhibit a better lithium ion channel, thereby increasing the charge capacity and the initial coulombic efficiency.

[0280] However, it should be noted that the button cell does not have the process of forming the SEI film during full battery formation. Therefore, although the negative electrode additives provided in the embodiments of the present application are included in Embodiments 1 to 16, the charge-discharge capacity and Coulomb efficiency of Embodiments 1 to 16 do not show a significant improvement compared to the comparative example. This shows that the function of the negative electrode additives provided in the embodiments of the present application can be fully demonstrated only when the SEI film is formed during the formation stage.

[0281] The performance test results of the secondary batteries prepared in Embodiments 1 to 16 and the secondary battery provided in Comparative Example 1 are listed in Table 3 below.

[0282] Table 3

[0283]

[0284]

[0285] As can be seen from Table 3, after formation, the first efficiency and cycle capacity retention rate of the battery cells containing the negative electrode additive of the embodiments of the present application are both improved to a certain extent compared with those of the comparative examples. Specifically, in Embodiments 1 to 4, when the contents of other substances remain unchanged, the first efficiency shows a trend of first increasing and then decreasing with the increase of the negative electrode additive content. There are mainly two factors affecting the first efficiency of the battery. On the one hand, during the formation stage, the reduction products of the negative electrode additive, such as boron compounds, become part of the SEI film, thus affecting the battery performance such as the first efficiency. On the other hand, it lies in the thickness of the SEI film. The thickness of the SEI film affects the consumption of active lithium and the intercalation / deintercalation channels of active lithium, and the total content of the negative electrode additive and the thickening agent has a certain influence on the thickness of the SEI film. Considering both factors, in Embodiments 1 to 4, when the content of the negative electrode additive is 0.5%, the first efficiency and cycle performance of the battery are better. Embodiments 5 to 7 investigate the influence of the change in the thickening agent content on the battery performance. At this time, the content of the negative electrode additive remains unchanged, and the first efficiency of the full battery shows a gradually increasing trend as the thickening agent content gradually decreases. Embodiments 8 to 10 investigate the influence of the change in the binder content on the battery performance. At this time, the amounts of other substances remain unchanged. As the binder content continuously decreases, the adhesion of the negative electrode sheet decreases, and the first efficiency also shows a gradually decreasing trend. This may be because the adhesion of the negative electrode sheet affects the integrity of the conductive network and the SEI film during the cycling process. Therefore, in Embodiments 8 to 10, the adhesion of Embodiment 8 is better, and its electrochemical performance such as the cycle performance is also better. Among Embodiments 1 to 11, Embodiment 11 shows the most excellent battery cell performance. This is because the negative electrode additive and the thickening agent form an SEI layer with an appropriate thickness during the formation stage, and an appropriate amount of binder ensures the integrity of the electrode sheet and the coherence of the conductive network, guaranteeing the subsequent cycle stability. In Embodiments 11 to 13, as the molecular weight of the negative electrode additive increases, the cycle performance of the battery also increases. Embodiments 11, 14 to 16 investigate the influence of different negative electrode additives on the battery performance. It can be found that the presence of polar functional groups such as -CN on the polymer chain makes the negative electrode additive have better wettability to the electrolyte and also improves the adhesion to the negative electrode sheet to a certain extent, making its electrochemical performance better, such as better cycle performance.

[0286] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present application, and they should all be covered within the scope of the claims and the specification of the present application. In particular, as long as there is no structural conflict, the technical features mentioned in each embodiment can be combined in any way. The present application is not limited to the specific embodiments disclosed in the text, but includes all technical solutions falling within the scope of the claims.

Claims

1. A negative electrode additive, characterized in that, The chemical general formula of the negative electrode additive is shown in Formula I: Wherein, R1 is an atom with a lone pair of electrons, R2 includes any one of a hydrogen atom and a nitrile group, R3 includes any one of a hydrogen atom, a hydroxyl group, and an alkoxy group with 1 to 3 carbon atoms, and R4 includes any one of a lithium ion and a sodium ion; n is a natural number greater than or equal to 50.

2. The negative electrode additive according to claim 1, wherein The R1 includes at least one of an oxygen atom and a nitrogen atom.

3. The negative electrode additive according to claim 1 or 2, characterized in that, The R3 includes at least one of a hydrogen atom, a hydroxyl group, and a methoxy group.

4. The negative electrode additive according to any one of claims 1 to 3, wherein The value range of n is 50 ≤ n ≤ 600.

5. The negative electrode additive according to claim 4, wherein, The weight-average molecular weight of the negative electrode additive is 10,000 to 100,000.

6. The negative electrode additive according to any one of claims 1 to 5, characterized in that, The negative electrode additive includes at least one of the compounds shown in Formula I-1 to Formula I-4: Wherein, in Formula I-1, Formula I-2, Formula I-3, and Formula I-4, the value range of n is 50 ≤ n ≤ 200.

7. A preparation method of a negative electrode additive, characterized in that, Comprising the following steps: Reacting a caffeic acid compound with sodium borohydride or lithium borohydride to obtain the negative electrode additive according to any one of claims 1 to 6; The chemical general formula of the caffeic acid compound is shown in Formula II: Wherein, R1 is an atom with a lone pair of electrons, R2 includes any one of a hydrogen atom and a nitrile group, and R3 includes any one of a hydrogen atom, a hydroxyl group, and an alkoxy group with 1 to 3 carbon atoms.

8. The preparation method of the negative electrode additive according to claim 7, characterized in that, The step of reacting the caffeic acid compound with sodium borohydride or lithium borohydride includes: Under a protective atmosphere, adding the caffeic acid compound to a first organic solvent under ice bath conditions to obtain a first solution; Adding the sodium borohydride or the lithium borohydride to a second organic solvent to obtain a second solution; Adding the second solution to the first solution for reaction to obtain the negative electrode additive.

9. The preparation method of the negative electrode additive according to claim 8, characterized in that, The first organic solvent and the second organic solvent are mutually soluble, and the first organic solvent and the second organic solvent independently include at least one of tetrahydrofuran, acetonitrile, dimethyl sulfoxide, cyclohexanone, N-methylpyrrolidone, toluene, chlorobenzene, benzene, benzotrichloride, and benzotetrachloride.

10. The method for preparing the negative electrode additive according to claim 8 or 9, characterized in that, The adding method of the second solution is dropwise addition, and the dropping rate is 4 to 6 mL / min.

11. The preparation method of the negative electrode additive according to any one of claims 7 to 10, characterized in that, The reaction conditions are: temperature is 0°C to 30°C, and time is 12 h to 24 h.

12. A negative electrode sheet, characterized in that, Comprising a negative electrode current collector and a negative electrode film layer located on at least one side of the negative electrode current collector, and the negative electrode film layer includes the negative electrode additive according to any one of claims 1 to 6 and / or the negative electrode additive prepared by the preparation method of the negative electrode additive according to any one of claims 7 to 11.

13. The negative electrode sheet according to claim 12, wherein, The negative electrode film layer further includes a negative electrode active material, and at least part of the negative electrode additive is coated on the surface of the negative electrode active material.

14. The negative electrode sheet according to claim 12 or 13, characterized in that, The mass ratio of the negative electrode additive to the negative electrode active material is 0.1 to 0.6:

100.

15. The negative electrode sheet according to any one of claims 12 to 14, characterized in that, The negative electrode film layer further includes a thickening agent, and the mass ratio of the thickening agent to the negative electrode active material is 0.7 to 1.5:

100.

16. The negative electrode sheet according to any one of claims 12 to 15, characterized in that, The negative electrode film layer further includes a binder, and the mass ratio of the binder to the negative electrode active material is 1.3 to 2.2:

100.

17. The negative electrode sheet according to any one of claims 14 to 16, characterized in that, The mass ratio of the negative electrode additive, the thickening agent, and the binder is (0.3 to 0.5):(0.7 to 0.9):(1.6 to 1.8).

18. The negative electrode sheet according to claim 17, characterized in that, The mass ratio of the negative electrode additive, the thickening agent, and the binder is 0.5:0.8:1.

6.

19. A secondary battery, characterized in that, It includes the negative electrode sheet according to any one of claims 12 to 18.

20. The secondary battery according to claim 19, characterized in that, At least part of the surface of the negative electrode film layer is covered with a SEI film, and the SEI film contains a boron compound.

21. An electrical device, characterized in that, It includes the secondary battery according to claim 19 or 20.

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