Negative pole piece for sodium ion battery and sodium ion battery

Abstract

The invention relates to the technical field of sodium ion batteries, in particular to a negative pole piece for a sodium ion battery. The negative electrode plate comprises a negative electrode current collector; the negative electrode active material layer is arranged on at least one side of the negative electrode current collector, the negative electrode active material layer comprises a carbon layer, a pre-sodium layer and a polymer layer which are stacked, the carbon layer is arranged on the surface of the negative electrode current collector, the pre-sodium layer is arranged between the carbon layer and the polymer layer, the pre-sodium layer comprises sodium powder, and the polymer layer is arranged between the carbon layer and the polymer layer. The polymer layer includes a first polymer monomer containing a cyano group and an ester group. The negative pole piece disclosed by the invention has synergistic pre-sodium and in-situ polymerization reaction, and a secondary battery formed by the negative pole piece has relatively high first cycle efficiency and capacity retention rate.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion batteries, and in particular to a negative electrode sheet for a sodium ion battery and a sodium ion battery. Background Art

[0002] As the global energy transition accelerates, demand for lithium-ion batteries in energy storage, electric vehicles, and other fields has surged. However, issues such as the uneven distribution of lithium resources and fluctuating costs have driven the exploration of alternative battery technologies. Sodium-ion batteries, due to their abundant sodium resources, low cost, and similar "rocking chair" energy storage principle, have become an ideal candidate technology for large-scale energy storage and low-range electric vehicles. However, the development of sodium-ion batteries is limited by key bottlenecks: the radius of sodium ions is larger than that of lithium ions, resulting in poor structural stability and significant volume expansion of electrode materials during the insertion and extraction process; more importantly, side reactions such as the formation of a solid electrolyte film at the negative electrode and the dissolution of transition metals at the positive electrode during the first cycle will cause irreversible "sodium loss", which directly reduces the battery's coulombic efficiency and energy density, limiting its commercialization process.

[0003] Therefore, improving the coulombic efficiency and energy density of batteries has become an important direction for the commercial development of sodium-ion batteries. Summary of the Invention

[0004] To overcome the shortcomings of the prior art, the present invention aims to provide a negative electrode sheet for a sodium ion battery and a sodium ion battery. By pre-sodiumizing the negative electrode sheet, the loss of sodium ions during the initial charge and discharge process is reduced, thereby improving the electrochemical performance of the battery.

[0005] To achieve the above objectives, the first aspect of the present application provides a negative electrode sheet for a sodium ion battery, comprising: negative electrode current collector; A negative electrode active material layer is arranged on at least one side of the negative electrode current collector, the negative electrode active material layer includes a stacked carbon layer, a pre-sodium layer and a polymer layer, the carbon layer is arranged on the surface of the negative electrode current collector, the pre-sodium layer is arranged between the carbon layer and the polymer layer, the pre-sodium layer includes sodium powder, and the polymer layer includes a first polymer monomer, which contains a cyano group and an ester group.

[0006] The negative electrode plate of the present application cooperates with pre-sodium and in-situ polymerization, which can not only reduce the loss of positive electrode sodium ions during the formation of the protective layer, but also form a protective layer with higher contact resistance, reduce the risk of internal short circuit of the battery, and significantly improve the first cycle efficiency and capacity retention rate of the battery.

[0007] In some embodiments, the active material layer includes only one carbon layer, one pre-sodium layer, and one polymer layer.

[0008] In some embodiments, the first polymer monomer is selected from one or more of ethyl 2-cyanoacrylate, n-butyl cyanoacrylate, n-octyl cyanoacrylate, methyl cyanoacrylate, propyl cyanoacrylate, allyl cyanoacrylate, cyanoacrylic acid-methoxyethyl ester.

[0009] In some embodiments, the polymer layer further comprises a second polymer, the second polymer being a high melting point polymer having a melting point or glass transition temperature higher than 150℃.

[0010] In some embodiments, the high melting point polymer is selected from one or more of polyimide, polyether ether ketone, polyphenylene sulfide, polysulfone, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polybenzimidazole, polyamide-imide, polyether sulfone.

[0011] In some embodiments, the mass ratio of the first polymer monomer and the second polymer satisfies: 1: (0.5-1).

[0012] In some embodiments, the polymer layer further comprises a first solvent, the first solvent having a boiling point less than or equal to 150℃.

[0013] In some embodiments, the first solvent is selected from one or more of tetrahydrofuran, dimethyl carbonate, acetonitrile, toluene.

[0014] In some embodiments, the pre-sodium layer further comprises a second solvent, the second solvent being an aprotic solvent.

[0015] In some embodiments, the molar surface density of sodium powder in the pre-sodium layer ranges from 0.07 to 0.20 mol / m².

[0016] In some embodiments, the thickness of the pre-sodium layer is 1-50um.

[0017] In some embodiments, the mass fraction of sodium powder in the pre-sodium layer is 3-8%.

[0018] The second aspect of the present application provides a sodium-ion battery comprising the negative electrode sheet provided by the first aspect of the present application.

[0019] In some embodiments, the sodium-ion battery further comprises a positive electrode sheet, the positive electrode sheet comprising a positive electrode current collector and a positive electrode active material layer, the positive electrode active material layer comprising a sodium-ion active material.

[0020] The third aspect of the present application provides an electronic device comprising the negative electrode sheet of the first aspect of the present application or the sodium-ion battery of the second aspect of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 A schematic view of a negative electrode tab according to the present application.

[0022] Figure 2 Another schematic view of a negative electrode tab according to the present application. DETAILED DESCRIPTION

[0023] The technical solutions in the embodiments of the present application will be clearly and completely described in connection with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative effort belong to the scope of protection of the present application.

[0024] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description of the application herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. As used in this description, the singular forms "a", "an" and "the" include plural references unless the context clearly dictates otherwise. The term "and / or" includes any and all combinations of one or more of the associated listed items.

[0025] In the description of the present application, the terms "(1)", "(2)", "first", "second", are used only for the purpose of description, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, the features defined with "(1)", "(2)", "first", "second" can include at least one of the features, explicitly or implicitly. In the description of the present application, the meaning of "multiple" is at least two, such as two, three, etc., unless otherwise specifically limited.

[0026] In the description of the present application, unless otherwise specified, the term "or" is inclusive. That is, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or A and B are both true (or exist).

[0027] The weight of the related components mentioned in the specification of the present application can not only refer to the specific content of each component, but also represent the proportional relationship between the weights of each component. Therefore, as long as the content of the related components in the specification of the present application is enlarged or reduced in proportion, it is within the scope disclosed in the specification of the present application. Specifically, the weight described in the specification of the present application can be μg, mg, g, kg, etc. mass units commonly known in the chemical industry.

[0028] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0029] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form new technical solutions.

[0030] The following describes the implementation of the present application in detail.

[0031] During the first cycle of sodium-ion batteries, the electrolyte will decompose on the surface of the negative electrode and form a solid electrolyte interface film (SEI film), also known as a protective layer; the main component of the SEI film is sodium-containing compounds, which will directly consume some of the sodium ions in the positive electrode and the electrolyte, resulting in the inability of sodium ions to be reversibly embedded after being released from the positive electrode, making the battery's first coulombic efficiency low, and thus reducing the energy density and cycle stability.

[0032] During the manufacturing process of sodium-ion batteries, additional sodium ions can be injected into the anode or cathode materials to compensate for irreversible sodium loss during battery cycling. This technique of pre-sodization is commonly referred to as pre-sodiumization. As can be understood, pre-sodiumization can replenish the sodium ions lost during SEI film formation and improve battery performance.

[0033] First aspect The first aspect of the present application provides a negative electrode sheet for a sodium ion battery, comprising: negative electrode current collector; A negative electrode active material layer is arranged on at least one side of the negative electrode current collector, the negative electrode active material layer includes a stacked carbon layer, a pre-sodium layer and a polymer layer, the carbon layer is arranged on the surface of the negative electrode current collector, the pre-sodium layer is arranged between the carbon layer and the polymer layer, the pre-sodium layer includes sodium powder, and the polymer layer includes a first polymer monomer, which contains a cyano group and an ester group.

[0034] In the negative electrode provided by this application, the sodium powder in the pre-sodium layer provides an additional sodium source for the formation of the secondary battery's protective layer (SEI film). Furthermore, the sodium powder can serve as a catalyst for the in-situ polymerization of the first polymer monomer. During the formation of the protective layer, the reaction heat generated by the reaction can also lead to higher catalytic activity, enabling the in-situ polymerization of the first polymer monomer. As a result, the prepared negative electrode synergizes the pre-sodium and in-situ polymerization reactions, exhibits high contact resistance, and reduces the risk of internal short circuits in the battery. This can significantly improve the initial cycle efficiency and capacity retention of the secondary battery.

[0035] In some embodiments, the first polymer monomer is selected from one or more of ethyl 2-cyanoacrylate, n-butyl cyanoacrylate, n-octyl cyanoacrylate, methyl cyanoacrylate, propyl cyanoacrylate, allyl cyanoacrylate, and methoxyethyl cyanoacrylate. The first polymer monomer contains a cyano group and an ester group, and can initiate an in-situ polymerization reaction in the presence of trace amounts of water in an electrolyte environment. Through in-situ polymerization, a three-dimensional cross-linked network can be formed. The protective layer formed by this cross-linked network can further inhibit dendrite growth and has a high contact resistance, thereby reducing the risk of internal short circuits in the battery and improving the cycle performance of the secondary battery.

[0036] In some embodiments, the mass fraction of the first polymer monomer in the polymer layer is 3-5%. It should be understood that the mass fraction of the first polymer monomer in the polymer layer refers to the solid content of the first polymer monomer in the polymer layer slurry. If the mass fraction of the first polymer monomer is too high, the resulting protective layer is too thick, resulting in excessively high internal resistance of the battery. If the mass fraction of the first polymer monomer is too low, an effective and uniform protective layer cannot be formed.

[0037] In some embodiments, the polymer layer further comprises a second polymer, wherein the second polymer is a high melting point polymer having a melting point or a glass transition temperature higher than 150°C.

[0038] In this application, high melting point polymer refers to a polymer material with a glass transition temperature (Tg) or melting point (Tm) greater than 150°C. It is understood that the above-mentioned high melting point polymers include crystalline polymers and amorphous polymers. Since crystalline polymers have a clear melting point, while amorphous polymers have no fixed melting point and only have a glass transition temperature (Tg). Therefore, for amorphous polymers, their melting point can also be the glass transition temperature. High melting point polymers have a higher melting point than traditional separators. The melting point of traditional separators is generally less than 150°C. For example, the melting point of PE separators is generally around 130°C. The use of high melting point polymers as a component of the negative electrode plate can have higher high temperature resistance in the protective layer formed, which helps to improve the high temperature safety and high temperature cycle performance of the battery (that is, improve the capacity retention rate of the battery at high temperature).

[0039] The glass transition temperature or melting point of the high-melting-point polymer is much higher than the melting point of the separator. The high-melting-point polymer also provides a physical support network, preventing shrinkage and wrinkling of the negative electrode sheet caused by polymerization shrinkage of the first polymer during in-situ polymerization. Furthermore, the high-melting-point polymer can be intertwined with the first polymer through intermolecular forces such as hydrogen bonds. This not only increases the contact resistance between the positive and negative electrodes of the battery, reduces the risk of internal short circuits, and thus improves the battery's high-temperature safety performance, but also effectively adsorbs carbon materials in the carbon layer during the battery's cycle, preventing the carbon layer from shedding and improving the battery's cycling performance.

[0040] Furthermore, the high melting point polymer is selected from one or more of polyimide (PI), polyetheretherketone (PEEK), polyphenylene sulfide (PPS), polysulfone (PSU), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polybenzimidazole, polyamide-imide, and polyethersulfone. The second polymer not only has a high melting point, but can also be entangled and combined with the first polymer monomer through intermolecular forces such as hydrogen bonds, which can further increase the contact resistance of the protective layer, reduce the risk of short circuits in the battery, and improve the high temperature safety of the battery. Polyetheretherketone is resistant to electrolyte corrosion, which helps to improve the safety of the battery; polyphenylene sulfide has good flame retardancy and can improve the high temperature safety of the battery.

[0041] Preferably, the high melting point polymer is selected from polyimide (PI) and / or polyethylene terephthalate (PET).

[0042] Preferably, the high melting point polymer is selected from polyethylene terephthalate (PET). Polyethylene terephthalate (PET) has good dimensional stability and mechanical strength, can prevent deformation of the protective layer caused by expansion of the electrode, and is resistant to corrosion by the electrolyte.

[0043] Further preferably, the high-melting-point polymer is selected from polyimide (PI). Polyimide has a high melting point and does not decompose to produce gases. It also has excellent tensile strength and elongation at break, combining rigidity and flexibility, and can effectively resist the mechanical stress caused by expansion and contraction of the electrode, preventing damage to the protective layer.

[0044] In some embodiments, the mass fraction of the second polymer in the polymer layer is 3-5%. It should be understood that the mass fraction of the second polymer in the polymer layer refers to the solids content of the second polymer in the polymer layer slurry. If the mass fraction of the second polymer is too high, the molecular weight of the copolymer produced by crosslinking with the first polymer will be too high, resulting in excessively high battery internal resistance. If the mass fraction of the second polymer is too low, the amount of high-temperature resistant material required for high-temperature performance of the battery will not be sufficient, and the improvement in high-temperature cycling performance will not be achieved.

[0045] In some embodiments, the mass fraction of the first polymer monomer is greater than or equal to the mass fraction of the second polymer.

[0046] In some embodiments, the mass ratio of the first polymer monomer to the second polymer monomer satisfies: 1:(0.5-1).

[0047] As the main material of in-situ polymerization, the mass ratio of the first polymer monomer should be equivalent to or higher than that of the second polymer, which is conducive to the polymerization reaction and avoids incomplete polymerization caused by too high a mass fraction of the second polymer.

[0048] In some embodiments, the polymer layer further includes a first solvent having a boiling point of 150°C or less. The first solvent has a relatively low boiling point and can evaporate and decompose during the formation of the protective layer. This reduces the first solvent's adverse effects on the protective layer (such as the formation of pores or inclusion of impurity particles in the formed protective layer), resulting in a denser and more uniform protective layer.

[0049] Furthermore, the first solvent is selected from one or more of tetrahydrofuran, dimethyl carbonate, acetonitrile, and toluene. The first solvent is a volatile small molecule solvent. The polymer layer using the first solvent can further reduce the effect of the solvent on the formation of the protective layer during the subsequent formation of the protective layer, thereby improving the density and uniformity of the protective layer.

[0050] Preferably, the first solvent is selected from tetrahydrofuran, dimethyl carbonate, and acetonitrile. Tetrahydrofuran, dimethyl carbonate, and acetonitrile are polar solvents that are compatible with the first polymer and / or the second polymer and facilitate the dissolution of the first polymer and / or the second polymer.

[0051] More preferably, the second solvent is selected from tetrahydrofuran, which has a relatively lower boiling point and evaporates faster, thereby minimizing the adverse effects of the solvent on the formation of the protective layer.

[0052] In some embodiments, the thickness D1 of the polymer layer is 1-50 μm.

[0053] In some embodiments, the coating weight of the polymer layer is 6-8 mg / cm². If the thickness or coating weight of the polymer layer is too high, the protective layer formed will be too thick and the interface impedance will be too high, which will cause the internal resistance of the secondary battery to be too high and lead to low first efficiency.

[0054] In some embodiments, the pre-sodium layer further includes a second solvent, which is an aprotic solvent. Aprotic solvents do not donate protons, thus preventing protons from participating in the reaction. Since sodium powder is a highly reactive substance and is sensitive to protons in a solution (e.g., an electrolyte environment), the use of an aprotic solvent can prevent the sodium powder from reacting with the second solvent.

[0055] Furthermore, the second solvent is a non-polar, aprotic solvent. The non-polar, aprotic solvent is selected from one or more of toluene, xylene, n-hexane, and cyclohexane. Toluene, xylene, n-hexane, and cyclohexane are aprotic solvents, and their molecular structures contain only C-H bonds, resulting in extremely weak polarity, no dissociable hydrogen atoms, and inability to donate protons. The carbon material (e.g., hard carbon) and sodium metal (e.g., sodium powder) form a non-polar system. Selecting a second solvent that is also a non-polar system prevents the second solution from reacting with the carbon material and sodium powder in the carbon layer. Therefore, the second solvent can disperse the sodium powder to form a slurry, allowing it to be evenly coated onto the carbon layer during the negative electrode manufacturing process. This forms a pre-sodium layer with a relatively uniform surface density, preventing the formation of sodium dendrites caused by localized high sodium powder concentrations. Furthermore, because traditional coating methods are used, existing electrode manufacturing equipment can be reused, reducing equipment development costs.

[0056] In some embodiments, the thickness D2 of the pre-sodium layer is 1-50 μm.

[0057] In some embodiments, the coating weight of the pre-sodium layer is 6-8 mg / cm². Preferably, the coating weight of the pre-sodium layer is less than that of the carbon layer. For example, the coating weight of the pre-sodium layer is 6 mg / cm². Selecting a relatively low coating weight for the pre-sodium layer facilitates sufficient contact and reaction between the sodium powder and the graphite during the formation of the protective layer, thereby avoiding sodium precipitation caused by excessive sodium powder content.

[0058] In some embodiments, the mass fraction of sodium powder in the pre-sodium layer is 3-8%. It can be understood that the mass fraction of sodium powder in the pre-sodium layer refers to the solid content of sodium powder in the pre-sodium layer slurry. The solid content of sodium powder is the ratio of the mass of sodium powder to the total mass of the pre-sodium layer slurry. For example, the total mass of the slurry is the sum of the mass of sodium powder and the mass of the second solvent. If the solid content of sodium powder is too low, the pre-sodium effect is not obvious, and if the content of sodium powder is too high, it will lead to sodium precipitation.

[0059] In some embodiments, the molar surface density of the sodium powder in the pre-sodium layer is 0.07-0.20 mol / m 2 . If the molar surface density of sodium powder is too large, sodium precipitation will occur in the negative electrode during the cycle, causing the battery capacity retention rate to decay rapidly. If the surface density of sodium powder is too small, it cannot provide sufficient sodium to embed into hard carbon, and cannot play an effective pre-sodium role. The higher the molar surface density of sodium powder, the higher the content of sodium powder in direct contact with graphite, which helps to efficiently utilize sodium powder during the formation of the protective layer. It should be noted that the molar surface density of sodium powder can be calculated based on the mass fraction of sodium powder and the coating amount of the pre-sodium layer.

[0060] In some embodiments, the active material layer only includes a carbon layer, a pre-sodium layer, and a polymer layer. The pre-sodium layer is between the carbon layer and the polymer layer. During the formation of the protective layer, the highly active sodium powder of the pre-sodium layer provides a sodium source, which directly contacts the carbon layer to react and generate a large amount of heat; the first polymer of the polymer layer triggers an in-situ polymerization reaction based on the electrolyte, and the sodium powder acts as a catalyst, and the heat generated by the reaction between the sodium powder and the carbon layer can promote the polymerization reaction. The above scheme not only reduces the sodium loss of the positive electrode material during the formation of the protective layer, but also the coordinated in-situ polymerization makes the protective layer more dense and uniform; so that the formed protective layer has a higher contact resistance.

[0061] It should be noted that the applicant has experimentally discovered that when the sodium source for the pre-sodium layer is sodium salt rather than sodium powder, the sodium salt cannot directly react with the carbon layer, preventing effective pre-sodiumization. Correspondingly, when the polymer layer is interposed between the carbon layer and the pre-sodium layer, the pre-sodium layer also cannot react with the carbon layer, and thus the first polymer monomer cannot undergo efficient in-situ polymerization.

[0062] In some embodiments, the carbon layer is selected from hard carbon and / or soft carbon.

[0063] Preferably, the carbon layer is selected from hard carbon. Since sodium ions need to be embedded in the active material of the negative electrode matrix, hard carbon possesses a large number of sodium storage active sites, which facilitates the embedding of sodium ions and helps form a stable and relatively thick protective film during the battery formation process. This reduces the risk of the protective film being too thin and cracking during subsequent charge and discharge, which could affect the battery's cycle life.

[0064] Preferably, the coating amount of the carbon layer is 8-14 mg / cm². It can be understood that the coating amount of the carbon layer refers to the mass of the carbon material in each square centimeter of the carbon layer in the negative electrode sheet.

[0065] Furthermore, the thickness D3 of the carbon layer is 20-200 nm. Preferably, the thickness of the carbon layer is 80-120 nm.

[0066] The carbon layer is the main coating in the negative electrode sheet. Carbon material is used as the negative electrode active material. Selecting a relatively high coating amount and appropriate thickness can help increase the proportion of negative electrode active material, thereby increasing the energy density of the material. It can also reduce the ion diffusion path, which helps to improve the battery's rate performance.

[0067] In some embodiments, the carbon layer further comprises a conductive agent. The conductive agent is selected from one or more of conductive carbon black, acetylene black, carbon nanotubes, carbon fibers, and graphene. Preferably, the conductive agent is selected from conductive carbon black.

[0068] In some embodiments, the carbon layer further includes a binder. The binder is selected from one or more of polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), and polytetrafluoroethylene (PTFE). Preferably, the binder is selected from styrene-butadiene rubber.

[0069] In some embodiments, the carbon layer further includes a thickener. It is understood that the slurry is composed of a negative electrode active material, a conductive agent, a binder, and a thickener. Optionally, the thickener is selected from one or more of sodium carboxymethyl cellulose, sodium alginate, and polyacrylic acid. Preferably, the thickener is selected from sodium carboxymethyl cellulose.

[0070] Further preferably, the negative electrode active material (such as hard carbon), the conductive agent (such as conductive carbon black), the thickener (such as sodium carboxymethyl cellulose), and the binder (such as styrene-butadiene rubber) are mixed in a mass ratio of 94:2.5:2.5:1, and water is used as the solvent, and the mixture is stirred to form a slurry.

[0071] In some embodiments, the negative electrode current collector is copper foil.

[0072] The negative electrode sheet of the present application can be coated on one side or on both sides. Figure 1 As shown, the negative electrode sheet is sequentially composed of polymer layer-pre-sodium layer-carbon layer-negative electrode current collector; when double-sided coating is used, as shown in FIG. Figure 2 As shown, the negative electrode sheet is sequentially composed of polymer layer-pre-sodium layer-carbon layer-negative electrode current collector-carbon layer-pre-sodium layer-polymer layer.

[0073] Second aspect The second aspect of the present application provides a sodium ion battery, which includes a positive electrode plate, wherein the positive electrode plate includes a positive electrode collector and a positive electrode active material layer, wherein the positive electrode active material layer includes a sodium ion active material; and the negative electrode plate provided by the first aspect of the present application.

[0074] Wherein, the positive electrode active material is a sodium ion active material. Optionally, the molecular formula of the positive electrode active material is Na4M 3- 3a N 6a / m (PO4)2P2O7, wherein the M element is selected from at least one of Mn, Fe, Ni, and Co, the N element is a doping metal element different from the M element, and the valence state of the N element is m, where m is a natural number greater than 0, and 0≤a≤0.3.

[0075] The positive electrode plate also includes a positive electrode current collector, which is aluminum foil.

[0076] The positive electrode active material layer further includes a conductive agent and a binder.

[0077] The sodium-ion battery of the present application further comprises an electrolyte. The electrolyte comprises a solvent, a sodium salt and an additive. For example, the solvent can be ethylene carbonate (EC) and diethyl carbonate (DEC), the sodium salt can be sodium hexafluorophosphate or sodium tetrafluoroborate, and the additive can be fluoroethylene carbonate (FEC).

[0078] The separator of the sodium-ion battery of the present application can be any material suitable for use in the separator of an electrochemical energy storage device in the art, for example, can be at least one of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester and natural fiber.

[0079] The third aspect The third aspect of the present application provides an electronic device comprising the sodium-ion battery of the second aspect. The sodium-ion battery is used to provide power supply for the electronic device.

[0080] As an optional technical solution of the present application, the electronic device includes, but is not limited to, a computer, a player, a telephone, a fax machine, a copier, a printer, a headset, a video recorder, a television, a calculator, a memory card, a record player, a backup power supply, a motor, a car, a motorcycle, a bicycle, a lighting appliance, a toy, a game machine, a clock, a power tool, a household large battery, an energy storage or lithium-ion capacitor, etc.

[0081] The beneficial effects of the present application will be further illustrated by the following examples. Examples

[0082] In order to make the technical problems, technical solutions and beneficial effects of the present application clearer, the present application will be further described in detail in combination with examples and drawings. Obviously, the described examples are only a part of the examples of the present application, rather than all the examples. The following description of at least one exemplary embodiment is merely illustrative in nature, and in no way should be taken as any limitation on the present application and its applications. Based on the examples in the present application, all other examples obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present application. Example 1

[0083] 1. Preparation method of negative electrode sheet: The negative active material (hard carbon), conductive agent (super P), thickening agent (sodium carboxymethyl cellulose) and binder (styrene butadiene rubber) are mixed in a mass ratio of 94:2.5:2.5:1, water is used as the solvent, and the slurry is formed after stirring. The slurry is coated on the negative current collector copper foil at a coating amount of 10 mg / cm² to form a carbon layer with a thickness of 100 nm, and then dried to obtain the carbon layer. Sodium powder was dispersed in toluene solvent to obtain a pre-sodium slurry, wherein the mass fraction of sodium powder was 3%. The obtained pre-sodiumized slurry was coated on the surface of the carbon layer at a coating amount of 6 mg / cm² (correspondingly, it can be calculated that the molar surface density of sodium powder under this condition is 0.0783 mol / m 2 ), dried under vacuum at 70°C to obtain a pre-sodium layer; The first polymer monomer (ethyl 2-cyanoacrylate) was dispersed in tetrahydrofuran solvent to obtain a polymer slurry, wherein the mass fraction of ethyl 2-cyanoacrylate was 3%. The obtained polymer slurry was coated on the surface of the above-mentioned pre-sodium layer at a coating amount of 8 mg / cm², about 50μm, and vacuum dried at 70°C to obtain a negative electrode sheet with a composite carbon layer, pre-sodium layer, and polymer layer.

[0084] 2. Preparation of Coin Cells 0.8g of Na4Fe3(PO4)2P2O7 material was mixed with 0.1g of conductive carbon black and 0.1g of PVDF, ground with NMP as a medium, and the resulting slurry was coated on an aluminum foil current collector. After drying, the positive electrode sheet was obtained. The positive electrode sheet and the negative electrode sheet were used as the negative electrode. A button cell (CR2032) was assembled under an inert atmosphere using 1M NaPF6 dissolved in EC:DEC (50:50 vol / vol) as the electrolyte.

[0085] 3. Performance Testing The prepared button battery (CR2032) was subjected to charge and discharge tests at 45°C with a charge and discharge current of 1C and 100 cycles.

[0086] The formula for calculating the first cycle coulombic efficiency (CE) is: CE = (discharge capacity / charge capacity) × 100%; The formula for calculating the capacity retention rate is: Capacity retention rate = (current capacity / initial capacity) × 100%. Example 2

[0087] The preparation method of the negative electrode plate is similar to that of Example 1, except that the first polymer monomer (ethyl 2-cyanoacrylate) and the second polymer (polyimide) are dispersed together in tetrahydrofuran solvent to obtain a polymer slurry, wherein the mass fraction of ethyl 2-cyanoacrylate is 3%, and the mass fraction of polyimide is 3%. The obtained polymer slurry is coated on the surface of the above-mentioned pre-sodium layer at a coating amount of 8 mg / cm², about 50 μm, and vacuum dried at 70°C to obtain a negative electrode plate consisting of a composite carbon layer, a pre-sodium layer, and a polymer layer. Example 3

[0088] The preparation method of the negative electrode plate is similar to that of Example 2, except that the first polymer monomer is n-butyl cyanoacrylate. Example 4

[0089] The preparation method of the negative electrode plate is similar to that of Example 2, except that the first polymer monomer is n-octyl cyanoacrylate. Example 5

[0090] The preparation method of the negative electrode plate is similar to that of Example 2, except that the mass fraction of the first polymer monomer is 1%. Example 6

[0091] The preparation method of the negative electrode plate is similar to that of Example 2, except that the mass fraction of the first polymer monomer is 2%. Example 7

[0092] The preparation method of the negative electrode plate is similar to that of Example 2, except that the mass fraction of the first polymer monomer is 4%. Example 8

[0093] The preparation method of the negative electrode plate is similar to that of Example 2, except that the mass fraction of the first polymer monomer is 5%. Example 9

[0094] The preparation method of the negative electrode plate is similar to that of Example 2, except that the mass fraction of the first polymer monomer is 6%. Example 10

[0095] The preparation method of the negative electrode plate is similar to that of Example 2, except that the second polymer is PET.

[0096] Examples 11-15 The preparation method of the negative electrode sheet is similar to that of Example 2, except that the mass fractions of the second polymer powder are 1%, 2%, 4%, 5%, and 6%, respectively. Example 16

[0097] The preparation method of the negative electrode plate is similar to that of Example 2, except that the first solvent is xylene. Example 17

[0098] The preparation method of the negative electrode plate is similar to that of Example 2, except that the second solvent is xylene.

[0099] Examples 18-27 The preparation method of the negative electrode sheet is similar to that of Example 2, except that the mass fractions of sodium powder are 1%, 2%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, and 15%, respectively.

[0100] Comparative Example 1 The negative electrode active material (hard carbon), conductive agent (super P), thickener (sodium carboxymethyl cellulose), and binder (styrene-butadiene rubber) were mixed in a mass ratio of 94:2.5:2.5:1, using water as the solvent. After stirring, a slurry was formed. The slurry was applied to the negative electrode current collector copper foil at a coating weight of 10 mg / cm² to a thickness of 100 nm to form a negative electrode sheet DN1. This negative electrode sheet DN1 was used as the negative electrode in a secondary battery.

[0101] Comparative Example 2 The negative electrode active material (hard carbon), conductive agent (super P), thickener (sodium carboxymethyl cellulose), and binder (styrene-butadiene rubber) were mixed in a mass ratio of 94:2.5:2.5:1. Water was used as the solvent. After stirring, a slurry was formed. The slurry was applied to the negative electrode current collector copper foil at a coating weight of 10 mg / cm² to a thickness of 100 nm and dried to obtain a carbon layer. Sodium powder was dispersed in toluene solvent to obtain a pre-sodium slurry, wherein the mass fraction of sodium powder was 3%. The pre-sodiumized slurry was applied at a coating weight of 8 mg / cm² to a thickness of approximately 50 μm on the surface of the carbon layer. The mixture was then vacuum dried at 70°C to obtain a negative electrode sheet DN2, which was a composite of the pre-sodiumized layer and the carbon layer. A secondary battery was constructed using the negative electrode sheet DN2 as the negative electrode.

[0102] Comparative Example 3 The negative electrode active material (hard carbon), conductive agent (super P), thickener (sodium carboxymethyl cellulose), and binder (styrene-butadiene rubber) were mixed in a mass ratio of 94:2.5:2.5:1. Water was used as the solvent. After stirring, a slurry was formed. The slurry was applied to the negative electrode current collector copper foil at a coating weight of 10 mg / cm² to a thickness of 100 nm and dried to obtain a carbon layer. Ethyl 2-cyanoacrylate was dispersed in tetrahydrofuran solvent to obtain a polymer slurry with a mass fraction of 3% ethyl 2-cyanoacrylate. The resulting polymer slurry was applied to a thickness of approximately 50 μm on the surface of the pre-sodium layer at a coating weight of 8 mg / cm². The slurry was then vacuum-dried at 70°C to obtain a negative electrode sheet DN3 comprising a composite carbon layer and polymer layer. A secondary battery was constructed using this negative electrode sheet DN3 as the negative electrode.

[0103] The secondary batteries prepared in Examples 1-27 and Comparative Examples 1-3 were subjected to charge and discharge tests. The test results are shown in Table 1: Table 1 Electrical performance test results of Examples 1-27 and Comparative Examples 1-3 First-cycle Coulomb efficiency / % Capacity retention rate at 45°C / % Example 1 90 94 Example 2 90 96 Example 3 90 92 Example 4 89 92 Example 5 72 78 Example 6 82 90 Example 7 90 92 Example 8 86 85 Example 9 78 82 Example 10 90 94 Example 11 80 78 Example 12 92 93 Example 13 93 96 Example 14 90 92 Example 15 82 84 Example 16 87 85 Example 17 91 94 Example 18 82 78 Example 19 86 86 Example 20 87 90 Example 21 92 94 Example 22 90 92 Example 23 86 90 Example 24 83 87 Example 25 83 72 Example 26 83 65 Example 27 83 62 Comparative Example 1 78 68 Comparative Example 2 91 78 Comparative Example 3 78 76 From the data in Table 1, it can be seen that the secondary battery prepared by synergistic pre-sodium and in-situ polymerization has good first coulombic efficiency and capacity retention in a high temperature environment of 45°C. More specifically: Comparing Example 1 and Comparative Example 2, it can be seen that a single pre-sodium can improve the first coulombic efficiency, but its long-term cycle performance is significantly weaker; comparing Comparative Example 3, it can be seen that the secondary battery without pre-sodium has poor first coulombic efficiency and capacity retention. Comparing Comparative Examples 1 and 2, it can be seen that the high melting point second polymer can significantly improve the high temperature cycle performance of the battery. Comparing Examples 5-9, it can be seen that the battery performance is better when the mass fraction of the first polymer monomer is between 2-5%; comparing Examples 11-15, it can be seen that the battery performance is better when the mass fraction of the second polymer is between 2-6%; comparing Examples 18-27, it can be seen that as the content of sodium powder increases, the first coulombic efficiency of the battery increases, but the cycle performance does not continue to increase. This is because excessive pre-sodium causes sodium precipitation during the cycle, resulting in poor cycle performance.

[0104] Comparing Examples 2 and 10, it can be seen that the second polymer selected from PI performs better than PEI, indicating that PI can form a better protective layer. Comparing Examples 2 and 16, it can be seen that the first solvent selected from tetrahydrofuran, which has a relatively lower boiling point, performs better.

[0105] It should be noted that although embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations may be made to these embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A negative electrode sheet for a sodium ion battery, comprising: negative electrode current collector; A negative electrode active material layer is arranged on at least one side of the negative electrode current collector, the negative electrode active material layer includes a stacked carbon layer, a pre-sodium layer and a polymer layer, the carbon layer is arranged on the surface of the negative electrode current collector, the pre-sodium layer is arranged between the carbon layer and the polymer layer, the pre-sodium layer includes sodium powder, and the polymer layer includes a first polymer monomer, which contains a cyano group and an ester group.

2. The negative electrode sheet according to claim 1, characterized in that: The active material layer only includes a carbon layer, a pre-sodium layer, and a polymer layer.

3. The negative electrode sheet according to claim 1, characterized in that: The first polymer monomer is selected from one or more of 2-ethyl cyanoacrylate, n-butyl cyanoacrylate, n-octyl cyanoacrylate, methyl cyanoacrylate, propyl cyanoacrylate, allyl cyanoacrylate, and methoxyethyl cyanoacrylate.

4. The negative electrode sheet according to claim 1, characterized in that: The polymer layer also includes a second polymer, which is a high-melting-point polymer with a melting point or glass transition temperature higher than 150°C. The high-melting-point polymer is selected from one or more of polyimide, polyetheretherketone, polyphenylene sulfide, polysulfone, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polybenzimidazole, polyamideimide, and polyethersulfone.

5. The negative electrode sheet according to claim 4, characterized in that: The mass ratio of the first polymer monomer to the second polymer meets the following conditions: 1:(0.5-1). 6 . The negative electrode plate according to claim 1 , wherein the polymer layer further comprises a first solvent, and the boiling point of the first solvent is less than or equal to 150° C. 7 . The negative electrode sheet according to claim 6 , wherein the first solvent is selected from one or more of tetrahydrofuran, dimethyl carbonate, acetonitrile, and toluene.

8. According to the negative electrode plate according to claim 1, the pre-sodium layer further includes a second solvent, and the second solvent is an aprotic solvent.

9. The negative electrode plate according to claim 1, characterized in that: The molar surface density of the sodium powder in the pre-sodium layer is in the range of 0.07-0.20 mol / m²; and / or The thickness of the pre-sodium layer is 1-50 μm; and / or The mass fraction of sodium powder in the pre-sodium layer is 3-8%.

10. A sodium ion battery comprising: A positive electrode sheet, the positive electrode sheet comprising a positive electrode current collector and a positive electrode active material layer, the positive electrode active material layer comprising a sodium ion active material; and The negative electrode sheet according to any one of claims 1 to 9.

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