Preparation Method of Sodium Ion Secondary Battery

Abstract

Embodiments of the present application relate to the field of energy storage technologies, and provide a method for preparing a sodium-ion secondary battery, including: providing an electrode assembly and placing the electrode assembly in a housing; performing a first liquid injection treatment on the electrode assembly using a first electrolyte, where the first electrolyte includes: 0.5 wt% - 2 wt% of 1,3-propane sultone, 0.1 wt% - 2 wt% of tris(trimethylsilyl) phosphate, 0.1 wt% - 2 wt% of fluoroethylene carbonate, 0.01 wt% - 5 wt% of a film-forming additive, and a sodium salt; performing an infiltration treatment and a pre-charging treatment; performing a second liquid injection treatment on the electrode assembly using a second electrolyte, where the second electrolyte includes: 5 wt% - 20 wt% of 1,3-propane sultone, 3 wt% - 30 wt% of a nitrile compound, 0.005 wt% - 30 wt% of a water and acid removal additive, and a sodium salt; and performing a standing treatment. Embodiments of the present application are beneficial to improving the overall performance of sodium-ion batteries.

Description

Technical Field

[0001] The embodiments of the present application relate to the technical field of energy storage, and particularly to a preparation method of a sodium-ion secondary battery. Background Art

[0002] Currently, lithium-ion batteries occupy the core position in power batteries. At the same time, lithium-ion batteries also face great challenges, such as the increasingly scarce lithium resources, the continuous rise of material prices, and the low recycling rate of old batteries.

[0003] Sodium-ion batteries are similar to lithium-ion batteries in principle and structure. However, compared with lithium-ion batteries, sodium-ion batteries have many unique advantages. First, sodium elements are abundant in the earth's crust, far exceeding lithium elements. Therefore, the resource supply of sodium-ion batteries is more sufficient. Second, the production cost of sodium-ion batteries is relatively low, and the price fluctuations of raw materials are small, making them more stable in market competition. In addition, sodium-ion batteries can work in a wider temperature range and have higher safety, which makes them perform well in various application scenarios. These advantages make sodium-ion batteries a powerful supplement to lithium-ion batteries in specific application scenarios and can even replace lithium-ion batteries in some aspects. For example, in large-scale energy storage systems, sodium-ion batteries are particularly suitable due to their cost-effectiveness and safety advantages.

[0004] Therefore, promoting the research and development of high-performance and low-cost sodium-ion batteries has become a key link in the large-scale industrialization of sodium-ion batteries. Only by improving the energy density, cycle life, and safety of sodium-ion batteries can sodium-ion batteries truly become the mainstream choice in the market. Summary of the Invention

[0005] The embodiments of the present application provide a preparation method of a sodium-ion secondary battery, which is at least beneficial to improving the problems that sodium-ion batteries are prone to generate gas, have large impedance, and poor cycle performance under high-temperature conditions.

[0006] According to some embodiments of the present application, on the one hand, an embodiment of the present application provides a method for preparing a sodium-ion secondary battery, including: providing an electrode assembly and placing the electrode assembly in a housing, where the electrode assembly includes a positive electrode plate, a separator, and a negative electrode plate, the separator is located between the positive electrode plate and the negative electrode plate, and the positive electrode plate, the separator, and the negative electrode plate are stacked to form the electrode assembly, or the positive electrode plate, the separator, and the negative electrode plate are stacked and wound to form the electrode assembly. Among them, the positive electrode plate includes a positive current collector and a positive active material layer provided on the positive current collector, the negative electrode plate includes a negative current collector and a negative active material layer provided on the negative current collector, the material of the positive active material layer includes a layered oxide positive electrode material, a polyanion-based positive electrode material, or a Prussian blue-based positive electrode material, and the material of the negative active material layer includes a metal compound, a carbon-based material, an alloy-based material, or a non-metal simple substance; performing a first liquid injection treatment on the electrode assembly placed in the housing with a first electrolyte. Among them, calculated by mass percentage, the first electrolyte includes: 0.5wt% - 2wt% of 1, 3 - propane sultone, 0.1wt% - 2wt% of tris(trimethylsilyl) phosphate, 0.1wt% - 2wt% of fluoroethylene carbonate, 0.01wt% - 5wt% of a film-forming additive, and a sodium salt; after injecting the first electrolyte, performing an infiltration treatment and a pre-charging treatment in sequence; performing a second liquid injection treatment on the electrode assembly placed in the housing with a second electrolyte. Among them, calculated by mass percentage, the second electrolyte includes: 5wt% - 20wt% of 1, 3 - propane sultone, 3wt% - 30wt% of a nitrile compound, 0.005wt% - 30wt% of a water and acid removal additive, and a sodium salt; after injecting the second electrolyte, performing a formation and standing treatment.

[0007] In some embodiments, the ratio range of the mass percentage of 1, 3 - propane sultone in the second electrolyte to the mass percentage of 1, 3 - propane sultone in the first electrolyte is 5 - 20, satisfying: 40%×M ≤ m1 ≤ 50%×M, 50%×M ≤ m2 ≤ 60%×M, M = m1 + m2, where m1 is the mass of 1, 3 - propane sultone in the first electrolyte and m2 is the mass of 1, 3 - propane sultone in the second electrolyte.

[0008] In some embodiments, the nitrile compound accounts for 10wt% - 25wt% of the second electrolyte, and the water and acid removal additive accounts for 5wt% - 10wt% of the second electrolyte.

[0009] In some embodiments, the nitrile compound is a dinitrile compound, a trinitrile compound, an unsaturated nitrile compound, a nitrile compound containing an aromatic ring, an alkoxy nitrile compound, a nitrile compound containing a sulfonyl group, or a nitrile compound containing a trimethylsilyl group.

[0010] In some embodiments, the dinitrile compounds include succinonitrile, adiponitrile, glutaronitrile, suberonitrile, or sebaconitrile; the trinitrile compounds include 1,3,6-hexanetricarbonitrile or 1,3,5-pentanetricarbonitrile; the unsaturated nitrile compounds include acrylonitrile, crotonitrile, fumarodinitrile, or trans-hexenedinitrile; the nitrile compounds containing an aromatic ring include p-fluorobenzonitrile, p-methylbenzonitrile, or tricyanobenzene; the alkoxy nitrile compounds include ethylene glycol bis(propionitrile) ether or 1,2,3-tris(cyanoethoxy)propane; the nitrile compounds containing a sulfonyl group include bis(cyanoethyl) sulfone; and the nitrile compounds containing a trimethylsilyl group include 3-(trimethylsilyloxy)propionitrile.

[0011] In some embodiments, the water and acid removal additive is a carbodiimide compound, a silazane compound, an amine compound, methylsulfonyl chloride, tetrahydrofuran, or trifluoroacetic anhydride.

[0012] In some embodiments, the carbodiimide compounds include dicyclohexylcarbodiimide or diisopropylcarbodiimide; the silazane compounds include hexamethyldisilazane, heptamethyldisilazane, or trimethylchlorosilane; and the amine compounds include triethylamine or diethylamine.

[0013] In some embodiments, the mass of the first electrolyte accounts for 65% to 90% of the sum of the masses of the first electrolyte and the second electrolyte.

[0014] In some embodiments, the infiltration treatment includes a first infiltration stage and a second infiltration stage that are carried out in sequence. Among them, the temperature of the first infiltration stage is higher than that of the second infiltration stage, and the duration of the first infiltration stage is shorter than that of the second infiltration stage; wherein, the temperature of the first infiltration stage is 40°C to 50°C, and the duration of the first infiltration stage is 10h to 15h; the temperature of the second infiltration stage is 15°C to 25°C, and the duration of the second infiltration stage is 20h to 30h.

[0015] In some embodiments, the process parameters of the pre-charging treatment include: constant current charging at a charging rate of 0.04C to 0.06C, and the pre-charging cut-off voltage is 3.4V.

[0016] The technical solutions provided by the embodiments of the present application have at least the following advantages:

[0017] In the technical solution of the preparation method of the sodium-ion secondary battery provided by the embodiment of the present application, after placing the battery cell in the housing, a two-injection solution scheme including a first injection treatment and a second injection treatment is adopted. In the electrolyte provided by the first injection treatment, in addition to sodium salts and film-forming additives, it also contains 0.1 wt% - 2 wt% of TMSP (i.e., tris(trimethylsilyl) phosphate), 0.1 wt% - 2 wt% of FEC (i.e., fluoroethylene carbonate), and 0.5 wt% - 2 wt% of PS (i.e., 1,3-propane sultone). After the first injection treatment, a pre-charging treatment is carried out. The standard decomposition potential of FEC is lower than that of PS, and a stable SEI film is preferentially formed during the pre-charging treatment. The decomposition potential of PS is between that of FEC and TMSP, and PS can provide protection for the SEI film formed based on FEC first, thus facilitating the formation of a solid electrolyte interface film (i.e., SEI film) with low impedance, dense and uniform on the negative electrode, which helps to improve the battery performance and life during high-temperature storage and cycling. And TMSP, as a typical positive electrode film-forming additive, is mainly used to protect the positive electrode and forms a low-impedance and stable positive electrode interface during the pre-charging treatment, which is beneficial to the improvement of the cycling performance. After the pre-charging treatment, a second injection treatment is carried out. The second electrolyte provided by the second injection treatment contains 5 wt% - 20 wt% of 1,3-propane sultone, 3 wt% - 30 wt% of nitrile compounds, 0.005 wt% - 30 wt% of water and acid removal additives, and sodium salts. Since the second electrolyte contains a relatively high concentration of PS, the high concentration of PS can effectively repair the SEI film and improve the high-temperature performance and cycling life of the sodium-ion secondary battery; and nitrile substances can withstand high temperatures and inhibit gas generation, and the water and acid removal additives can remove the moisture and acid in the electrolyte, reducing the amount of gas generated by the reaction of these impurities with sodium salts, thus effectively inhibiting the gas generation phenomenon, and at the same time, it can also reduce the internal resistance growth rate during long-term cycling. All in all, the technical solution provided by the embodiment of the present application is conducive to achieving an effective balance of low SEI film interface impedance, low gas generation amount, and long cycling stability of the sodium-ion secondary battery. Brief Description of the Drawings

[0018] One or more embodiments are exemplarily illustrated by the pictures in the corresponding drawings. These exemplary illustrations do not constitute a limitation on the embodiments. Unless otherwise stated, the figures in the drawings do not constitute a proportional limitation; in order to more clearly illustrate the technical solutions in the embodiments of the present application or the traditional technologies, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the following described drawings are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0019] Figure 1Schematic flowchart corresponding to the preparation method of the sodium-ion secondary battery provided by the embodiments of the present application;

[0020] Figure 2 Schematic diagram of the principle structure of the sodium-ion secondary battery provided by the embodiments of the present application after the second electrolyte injection treatment. Detailed implementation manners

[0021] As known from the background art, lithium iron phosphate-based lithium-ion batteries have been widely used in the energy storage field due to their low cost and long cycle life. In the actual development process, sodium-ion batteries still face many problems, the most prominent of which include the easy generation of gas under high-temperature conditions and poor cycle performance.

[0022] The embodiments of the present application provide a preparation method of a sodium-ion secondary battery, providing a solution based on injecting electrolytes with different formulations twice. Using 1,3-propane sultone and fluoroethylene carbonate as basic additives, by adjusting the dosage of the basic additives in the first-injected electrolyte (i.e., the first electrolyte) and adding film-forming additives, an SEI film (Solid Electrolyte Interface) with small polarization, small internal resistance, and uniform distribution can be formed after the first electrolyte injection treatment. The 1,3-propane sultone provided in the second electrolyte injection treatment in the second time can be used as an additive required for subsequent long-term cycling to accurately repair the SEI film. The nitrile solvent and water- and acid-removing additives provided in the second electrolyte injection treatment are beneficial to inhibiting gas generation, removing moisture and acid in the electrolyte, thereby reducing the reaction of these impurities with other components to generate gas, and effectively inhibiting the gas generation phenomenon during long-term cycling or under high-temperature conditions.

[0023] 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, "a plurality" means more than two, unless otherwise specifically defined.

[0024] Referring to "embodiments" herein means that specific features, structures, or characteristics described in connection with the embodiments can be included in at least one embodiment of the present application. The phrase appears in various places in the specification and does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art will explicitly and implicitly understand that the embodiments described herein can be combined with other embodiments.

[0025] In the description of the embodiments of the present application, the term "and / or" is merely a relational term describing associated objects, indicating that there can be three relationships. For example, A and / or B can mean: there is A, there is both A and B, and there is B, these three situations. Additionally, in this text, the character " / " generally indicates that the associated objects before and after are in an "or" relationship.

[0026] In the description of the embodiments of the present application, the term "plural" 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).

[0027] In the description of the embodiments of the present application, when a certain component "includes" another component, unless otherwise stated, other components are not excluded, and other components may further be included. In addition, when a component such as a layer, film, region, or plate is referred to as "on / located on" another component, it can be "directly on" another component (i.e., on the surface of another component with no other components in between), or there can be another component in between. Further, when a layer, film, region, plate, etc. component is "directly located on" another component, or when a layer, film, region, plate, etc. component is located on the surface of another component, it means that no other components are located in between.

[0028] The terms used in the description of the various embodiments herein are only for describing specific embodiments and are not intended to be limiting. As used in the descriptions of the various embodiments and the appended claims, "component" is also intended to include the plural form, unless the context clearly indicates otherwise. Among them, components include components such as layers, films, regions, or plates.

[0029] The following will elaborate on the various embodiments of the present application in conjunction with the accompanying drawings. However, those of ordinary skill in the art can understand that in the various embodiments of the present application, many technical details are provided to help readers better understand the present application. However, even without these technical details and various changes and modifications based on the following embodiments, the technical solutions claimed in the present application can still be implemented.

[0030] Figure 1 It is a schematic flowchart corresponding to the preparation method of the sodium-ion secondary battery provided by the embodiments of the present application.

[0031] It should be noted that the sodium-ion secondary battery mentioned in the embodiments of the present application is the sodium-ion battery.

[0032] Refer to Figure 1 , the preparation method of the sodium-ion secondary battery includes the following steps:

[0033] Provide an electrode core and place the electrode core in a housing. The electrode core includes a positive electrode tab, a separator, and a negative electrode tab. The separator is located between the positive electrode tab and the negative electrode tab, and the positive electrode tab, the separator, and the negative electrode tab are stacked to form the electrode core. Alternatively, the positive electrode tab, the separator, and the negative electrode tab are stacked and wound to form the electrode core. Among them, the positive electrode tab includes a positive electrode current collector and a positive electrode active material layer provided on the positive electrode current collector, and the negative electrode tab includes a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector.

[0034] The electrode core can be a stacked structure formed by sequentially stacking the negative electrode tab, the separator, and the positive electrode tab, or a wound structure formed by sequentially stacking the negative electrode tab, the separator, and the positive electrode tab and then winding them.

[0035] The positive electrode tab includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector and containing the positive electrode active material. For example, the positive electrode current collector includes two opposite surfaces, and the positive electrode active material layer is provided on either one or both of the two opposite surfaces of the positive electrode current collector in its own thickness direction.

[0036] The positive electrode active material can include a layered oxide positive electrode material, a polyanion-based positive electrode material, a Prussian blue-based positive electrode material, an organic positive electrode material, or a conversion positive electrode material. That is to say, the material of the positive electrode active material layer includes an oxide positive electrode material, a polyanion-based positive electrode material, or a Prussian blue-based positive electrode material.

[0037] Among them, the chemical formula of the layered oxide positive electrode material is Na x M1O 2 , where M1 is one or more transition metal elements such as Fe, Mn, Ni, Co, Cr, etc., and 0 < x ≤ 1. For example, the layered oxide positive electrode material can be NaFeO 2 , NaCrO 2 , Na 0.5 CoO 2 , or NaNiO 2 , etc.

[0038] The general formula of the polyanion-based positive electrode material is Na x M2 y (X a O b ) z Z w, the general formula is electrochemically neutral, where M2 can be selected from transition metals such as Ti, V, Cr, Mn, Fe, Co, Ni, Ca, Mg, Al, or Nb, X is one or more of Si, S, P, As, B, Mo, W, Ge, and Z is F or OH. For example, the polyanion cathode material can be sodium phosphate, sodium sulfate, or sodium pyrosulfate. More specifically, the polyanion cathode material can be sodium iron phosphate (NaFePO 4 ), or sodium iron sulfate (Na 2 Fe(SO 4 ) 2 ), etc.

[0039] The Prussian blue cathode material has a structural general formula A x M1[M2(CN) 6 1-y ·V y ·nH 2 O, 0 ≤ x ≤ 2, 0 ≤ y ≤ 1, where V represents the [M2(CN) 6 vacancy, A is an alkali metal ion, such as Na ion or K ion, etc. In a sodium-ion secondary battery, A is a sodium ion; M1 and M2 are both transition metal ions, such as Mn, Fe, Co, Ni, Cu, Zn, or Cr, etc.; H 2 O represents interstitial water (or zeolite water) and coordinated water.

[0040] For example, a Prussian blue cathode material can be represented as Na x MFe(CN) 6 , M can be Mn, Co, Ni, Cu, Zn, or Cr, etc. The Prussian blue cathode material includes iron-manganese-based Prussian blue, iron-iron-based Prussian blue, iron-nickel-based Prussian blue, iron-cobalt-based Prussian blue, iron-copper-based Prussian blue, iron-zinc-based Prussian blue, and Prussian blue derivatives formed by doping different transition metal ions, etc.

[0041] The positive electrode active material layer can also include a conductive agent and a binder. The conductive agent is used to improve the conductivity of the positive electrode active material layer, and the binder is used to firmly paste the positive electrode active material and the binder onto the positive electrode current collector.

[0042] As an example, the conductive agent can be one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers; the binder can be one or more of styrene-butadiene rubber (SBR), water-based acrylic resin, carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA), and polyvinyl alcohol (PVA).

[0043] ​The positive electrode current collector can be made of metal foil, carbon-coated metal foil, or porous metal plate. In this embodiment, the positive electrode current collector is aluminum foil.

[0044] In some examples, the process steps for preparing the positive electrode sheet may include: dispersing the positive electrode active material, conductive agent, and binder in a solvent to form a positive electrode slurry; coating the positive electrode slurry on the surface of the positive electrode current collector, and after drying and cold pressing, obtaining the positive electrode sheet.

[0045] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector and containing the negative electrode active material. For example, the negative electrode current collector includes two opposite surfaces, and the negative electrode active material layer is provided on either or both of the two opposite surfaces of the negative electrode current collector in its own thickness direction.

[0046] The negative electrode active material can be a metal compound, carbon-based material, alloy material, or non-metallic element. That is, the material of the negative electrode active material layer includes a metal compound, carbon-based material, alloy material, or non-metallic element.

[0047] Among them, the carbon-based material can be hard carbon, soft carbon, carbon nanotubes, expanded graphite, or graphene. Among them, hard carbon and soft carbon have excellent electrochemical performance and cycle stability; carbon nanotubes have high conductivity and large specific surface area, which helps to improve the battery performance; expanded graphite and graphene are widely used in negative electrode sheets due to their excellent conductivity and mechanical properties.

[0048] The metal compound can be lithium titanate, and lithium titanate can provide good cycle life and high safety.

[0049] The negative electrode active material layer can also include a conductive agent and a binder. The conductive agent is used to improve the conductivity of the negative electrode active material layer, and the binder is used to firmly adhere the negative electrode active material and the binder to the negative electrode current collector.

[0050] The negative electrode current collector can be made of metal foil, carbon-coated metal foil, or porous metal plate. In this embodiment, the negative electrode current collector is copper foil. In other embodiments, the negative electrode current collector can also be aluminum foil.

[0051] In some examples, the process steps for preparing the negative electrode sheet may include: dispersing the negative electrode active material, conductive agent, and binder in a solvent to form a negative electrode slurry; coating the negative electrode slurry on the surface of the negative electrode current collector, and after drying and cold pressing, obtaining the negative electrode sheet.

[0052] The separator membrane can be made of one or more materials, including polypropylene (PP), polyethylene (PE), ceramic-coated separator membranes, and non-woven separator membranes. PP and PE porous separator membranes have good mechanical strength and chemical stability; the ceramic-coated separator membrane is coated with ceramic materials on the basis of the PP or PE separator membrane, improving the high-temperature resistance and safety of the separator membrane; the non-woven separator membrane is composed of polymer fibers and has excellent electrolyte wettability and ion conductivity.

[0053] In addition, the separator membrane can be a single-layer film or a multi-layer film.

[0054] In this embodiment, after preparing the positive electrode sheet, the negative electrode sheet, and the separator membrane, the positive electrode sheet, the separator membrane, and the negative electrode sheet are stacked and wound to form an electric core. In other embodiments, the electric core can also be a stacked structure as described above.

[0055] In this embodiment, the sodium-ion secondary battery is a soft-pack battery (Pouch Cell), and correspondingly, the housing is a soft outer shell. In some examples, the housing can be an aluminum-plastic film.

[0056] In a specific example, the capacity of the sodium-ion secondary battery prepared in this embodiment can be 3 Ah, where Ah represents ampere-hour.

[0057] It should be noted that the embodiments of this application do not limit the type of the sodium-ion secondary battery. In other embodiments, the sodium-ion secondary battery can also be a hard-pack battery (Prismatic and Cylindrical Cell), and can be a square sodium-ion battery or a cylindrical sodium-ion battery. Correspondingly, the housing is a metal housing or a housing made of other hard materials.

[0058] The following will take the sodium-ion secondary battery as a soft-pack battery and the housing as an aluminum-plastic film as an example for detailed description.

[0059] Placing the electric core in the housing includes: after preparing the aforementioned electric core, completely wrapping the electric core with an aluminum-plastic film, and then, sealing the aluminum-plastic film and leaving an opening as the electrolyte injection port for the subsequent first liquid injection process.

[0060] Continue to refer to Figure 1 , and perform the first liquid injection process on the electric core placed in the housing with the first electrolyte. Among them, by mass percentage, the first electrolyte includes: 0.5 wt% - 2 wt% of 1, 3-propane sultone (abbreviation: PS), 0.1 wt% - 2 wt% of tris(trimethylsilyl) phosphate (abbreviation: TMSP), 0.1 wt% - 2 wt% of fluoroethylene carbonate (abbreviation: FEC), 0.01 wt% - 5 wt% of film-forming additive, and sodium salt.

[0061] The electrolyte injected during the first liquid injection process is used to form a stable SEI film, which has the characteristics of low impedance, good compactness and uniformity.

[0062] In terms of mass percentage, the content of sodium salt in the first electrolyte can be 5wt% - 15wt%.

[0063] In the first electrolyte, except for PS, FEC, TMSP, sodium salt and film-forming additives, the balance is a non-aqueous organic solvent.

[0064] In some examples, the first electrolyte may contain 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1.0wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt%, 1.5wt%, 1.6wt%, 1.7wt%, 1.8wt%, 1.9wt% or 2.0wt% of 1, 3 - propane sultone.

[0065] In some examples, the first electrolyte may contain 0.1 wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1.0wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt%, 1.5wt%, 1.6wt%, 1.7wt%, 1.8wt%, 1.9wt% or 2.0wt% of tris(trimethylsilyl) phosphate.

[0066] In some examples, the first electrolyte may contain 0.1 wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1.0wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt%, 1.5wt%, 1.6wt%, 1.7wt%, 1.8wt%, 1.9wt% or 2.0wt% of fluoroethylene carbonate.

[0067] In some examples, the first electrolyte may contain 5wt%, 5.5wt%, 6wt%, 6.5wt%, 7wt%, 7.5wt%, 8wt%, 8.5wt%, 9wt%, 9.5wt%, 10wt%, 10.5wt%, 11wt%, 11.5wt%, 12wt%, 12.wt%, 13wt%, 13.5wt%, 14wt%, 14.5wt% or 15wt% of sodium salt.

[0068] Among them, the sodium salt can be sodium hexafluorophosphate (NaPF 6), sodium perchlorate (Sodium perchlorate, NaClO 4 ), sodium tetrafluoroborate (Sodium tetrafluoroborate, NaBF 4 ), sodium hexafluoroarsenate (Sodium hexafluoroarsenate, NaAsF 6 ), sodium tetrachloroaluminate (Sodiumtetrachloroaluminate, NaAlCl 4 ), sodium trifluoroacetate (Sodium trifluoroacetate, NaTFA), sodium tetraphenylborate (Sodium tetraphenylborate, NaBPh 4 ), sodium difluorophosphate (Sodiumdifluorophosphate, NaDFP), sodium bis(oxalato)borate (Sodium bis(oxalato)borate, NaBOB), sodium difluorooxalatoborate (Sodium difluorooxalatoborate, NaDFOB), sodium trifluoromethanesulfonate (Sodiumtrifluoromethanesulfonate, NaOTf), sodium bis(fluorosulfonyl)imide (Sodium bis(fluorosulfonyl)imide, NaFSI) and sodium bis(trifluoromethanesulfonyl)imide (Sodium bis(trifluoromethanesulfonyl)imide, NaTf 2 N), or one or more of them.

[0069] In some examples, the first electrolyte may contain 0.01 wt%, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, 4.5 wt% or 5 wt% of the film-forming additive.

[0070] The film-forming additives may include, but are not limited to, one or more combinations of the following: sodium difluorophosphate (NaDFP), methylene methane disulfonate (MMDS), ethylene sulfate (DTD), tris(trimethylsilyl)borate (TMSB), 2-propyn-1-yl 1H-imidazole-1-carboxylate, triallyl isocyanurate (TAIC), toluene diisocyanate (TDI), 2-phenyl-1-yl 1H-imidazole-1-sulfonate, hexamethylene diisocyanate (HDI), sodium difluorooxalatophosphate (NaDFOP), 2-fluoropyridine (2-FP), sodium bis(oxalato)borate (NaBOB), sodium difluorooxalatoborate (NaDFOB), allyl methanesulfonate (AMS), and methylene propane disulfonate (DTD).

[0071] In a specific example, by mass percentage, the first electrolyte may include 0.5 wt% to 3 wt% of the film-forming additives. The selection and dosage adjustment of these film-forming additives are aimed at optimizing the overall performance of the sodium-ion secondary battery, especially improving the quality of the SEI film, thereby enhancing the cycle stability and capacity retention rate of the sodium-ion secondary battery.

[0072] The non-aqueous organic solvent may include at least one selected from ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), 1,4-butyrolactone (GBL), methyl propionate (MP), methylbutyrate (MB), ethyl acetate (EA), ethyl propionate (EP), and propyl propionate (PP).

[0073] In a specific example, the non-aqueous organic solvent may be a mixture of two or more selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), propylene carbonate (PC), and ethylene carbonate (EC) in any proportion.

[0074] In addition, cyclic esters, linear esters, ether compounds, or sulfone compounds may also be added to the first electrolyte to optimize the performance and stability of the first electrolyte.

[0075] Continue to refer to Figure 1 , after injecting the first electrolyte, a wetting treatment and a pre-charging treatment are sequentially performed.

[0076] The standard decomposition potential of FEC is lower than that of PS. FEC has more advantages under low-temperature and high-power discharge conditions and preferentially forms a stable SEI film. The decomposition potential of PS is between that of FEC and TMSP, and it can provide moderate interfacial protection for the SEI film, enhance the compactness and stability of the SEI film, and contribute to improving the performance and lifespan of sodium-ion secondary batteries during high-temperature storage and cycling. As a typical cathode film-forming additive, TMSP is mainly used to protect the cathode and form a low-impedance and stable cathode protection layer, namely the CEI (Cathode electrolyte interphase) film, which is beneficial to the improvement of cycling performance.

[0077] After the infiltration treatment and the pre-charging treatment, the combined action of FEC and PS generates a stable SEI film. Since the concentration of PS in the first electrolyte is relatively low, the problem of a sharp increase in the impedance of the SEI film caused by the use of a large amount of PS is avoided, and the formed SEI film has a relatively small internal resistance (i.e., impedance).

[0078] The infiltration treatment may include a first infiltration stage and a second infiltration stage carried out in sequence, where the temperature of the first infiltration stage is higher than that of the second infiltration stage, and the duration of the first infiltration stage is shorter than that of the second infiltration stage.

[0079] In some examples, the temperature of the first infiltration stage can be 40°C to 50°C, such as 42°C, 44°C, 45°C, 47°C, or 49°C, and the duration of the first infiltration stage can be 10h to 15h, such as 11h, 12h, 13h, or 14h; the temperature of the second infiltration stage can be 15°C to 25°C, such as 16°C, 18°C, 20°C, 21°C, or 24°C, and the duration of the second infiltration stage can be 20h to 30h, such as 22h, 24h, 26h, or 28h.

[0080] In the above infiltration treatment process, the wettability of the first electrolyte on the battery cell can be improved, enabling the battery cell to effectively and fully contact the first electrolyte, which helps the first electrolyte to better penetrate into the pores of the positive electrode active material layer and the negative electrode active material layer, especially better penetrate into the tiny gaps between the active material particles of the positive electrode active material layer and the negative electrode active material layer, increasing the contact area and reaction activity between the first electrolyte and the positive electrode plate or the negative electrode plate, and being beneficial to further promoting the formation of a stable SEI film on the surface of the negative electrode plate subsequently.

[0081] The pre-charging treatment is the formation treatment. That is to say, in this embodiment, before the subsequent second liquid injection treatment, the SEI film and the CEI film in the sodium-ion secondary battery have been formed.

[0082] Before performing the pre - charge treatment, the electrolyte injection port on the housing can also be sealed, and there is a space margin at the sealed part. During the pre - charge treatment, reactions occur among the components in the first electrolyte to generate by - products such as gas, and the gas will escape to the sealed part with the space margin, causing the sealed part to be filled with gas to form an air bag.

[0083] The process parameters of the pre - charge treatment can be: constant - current charging at a charging rate of 0.04C to 0.06C, and the pre - charge cut - off voltage is 3.4V. The charging rate is a measure of the charging speed, which refers to the current value required for the battery to be charged to its rated capacity within a specified time. Generally, the charging rate is represented by C, and the charging rate C = charging current (A) / battery rated capacity (Ah). The larger the charging rate, the faster the charge - discharge speed of the battery.

[0084] In some examples, the pre - charge treatment can perform constant - current charging at 0.045C, 0.05C or 0.055C.

[0085] After the pre - charge treatment, the preparation method can also include: standing still for 10h to 14h at a third temperature, and the third temperature is 40°C to 50°C. For example, the third temperature can be 42°C, 44°C, 45°C, 47°C or 49°C, and the standing - still duration can be 11h, 12h or 13h. This standing - still can enable the battery cell to discharge to a certain extent.

[0086] During the pre - charge treatment, the inside of the housing can also be evacuated to discharge the gas by - products generated during the pre - charge treatment, so as to facilitate the full discharge of the gas, avoid the gas remaining inside the housing, and thus further reduce the gas content remaining in the housing.

[0087] In some examples, as described above, before performing the pre - charge treatment, the electrolyte injection port on the housing can be sealed, and there is a space margin at the sealed part. During the pre - charge treatment, reactions occur among the components in the first electrolyte to generate by - products such as gas, and the gas will escape to the sealed part with the space margin, causing the sealed part to be filled with gas to form an air bag. Before performing the second liquid injection treatment, the housing can be sealed again to completely separate the formed air bag from the battery cell and all the electrolytes. After re - sealing, the air bag is cut off and removed from the housing. This process is the evacuation treatment.

[0088] In other examples, the evacuation treatment can also be: before performing the pre - charge treatment, the electrolyte injection port on the housing can be sealed, and an additional evacuation pipeline and an evacuation pump are provided. The evacuation pipeline enters the inside of the housing through the sealed part; under the drive of the evacuation pump, the evacuation pipeline extracts the gas inside the housing and discharges the gas from the inside of the housing.

[0089] Continue to refer to Figure 1 A second electrolyte is used to perform a second liquid injection treatment on the battery cell placed in the housing. Among them, by mass percentage, the second electrolyte includes: 5wt% - 20wt% of 1,3 - propane sultone, 3wt% - 30wt% of nitrile compounds, 0.005wt% - 30wt% of water - and acid - removing additives, and sodium salts.

[0090] Specifically, during the aforementioned pre - charging process, the housing is sealed. Before performing the second liquid injection treatment, the housing is opened to form an electrolyte injection port for the second liquid injection treatment again.

[0091] The concentration of PS in the second electrolyte provided by the second liquid injection treatment is greater than that in the first electrolyte provided by the first liquid injection treatment. The second electrolyte is used to further repair and stabilize the SEI film and the CEI film. For example, it provides the PS and sodium salts required for long - term cycling. PS can repair the SEI film during long - term cycling, and the sodium salts provided in the second electrolyte can be used as a supplement to the sodium salts in the electrolyte.

[0092] If the amount of the first electrolyte provided by the first liquid injection treatment is insufficient, the battery cell may be difficult to be fully wetted, and it is easy to cause the problem of sodium ion precipitation after pre - charging, that is, formation. If the amount of the second electrolyte provided by the second liquid injection treatment is too large, the corresponding additives in the second electrolyte are relatively more, which may lead to uneven concentration distribution of the additives in the housing.

[0093] Therefore, the mass of the first electrolyte can account for 65wt% - 90wt% of the total electrolyte mass, for example, 70wt%, 75wt%, 80wt%, 82wt%, 85wt% or 88wt%. In this way, the best performance of the battery cell can be ensured. In the first liquid injection treatment, the battery cell can be fully wetted, and the problem of sodium ion precipitation will not occur. After the second liquid injection treatment, the concentration distribution of the additives in the housing is uniform. The total electrolyte is the electrolyte composed of the first electrolyte and the second electrolyte together. That is to say, the mass of the first electrolyte accounts for 65% - 90% of the sum of the masses of the first electrolyte and the second electrolyte.

[0094] In a specific example, the mass ratio of the first electrolyte to the second electrolyte can be 85:15.

[0095] The nitrile compounds in the second electrolyte can withstand high temperatures and inhibit gas generation. At the same time, a high concentration of PS can effectively repair the formed SEI film, improving the high - temperature performance and cycle life of the sodium - ion secondary battery. The water - and acid - removing additives can remove the moisture and acid in the electrolyte, reducing the gas content generated by the reaction of these impurities with sodium salts, thereby effectively inhibiting the gas - generation phenomenon.

[0096] The mass percentage of 1,3 - propane sultone in the second electrolyte and the mass of 1,3 - propane sultone in the first electrolyte can satisfy: 40%×M ≤ m1 ≤ 50%×M, 50%×M ≤ m2 ≤ 60%×M, M = m1 + m2, where m1 is the mass of 1,3 - propane sultone in the first electrolyte and m2 is the mass of 1,3 - propane sultone in the second electrolyte. In this way, the total amount of 1,3 - propane sultone in the electrolyte after secondary liquid injection is moderate. Moreover, the amount of 1,3 - propane sultone provided in the first liquid injection process can significantly enhance the compactness and stability of the SEI film, while the amount of 1,3 - propane sultone provided in the second liquid injection process can ensure the effective repair of the SEI film during long - term cycling.

[0097] In some specific examples, the mass of 1,3 - propane sultone in the first electrolyte accounts for 42%, 44%, 45%, 47% or 49% of the total mass. Correspondingly, the mass of 1,3 - propane sultone in the second electrolyte accounts for 58%, 56%, 55%, 53% or 51% of the total mass. The total mass refers to the sum of the mass of 1,3 - propane sultone in the first electrolyte and the mass of 1,3 - propane sultone in the second electrolyte.

[0098] In terms of mass percentage, the mass percentage of sodium salt in the second electrolyte can be 5wt% - 15wt%.

[0099] In addition, the second electrolyte also contains a non - aqueous organic solvent. The materials of the sodium salts in the first electrolyte and the second electrolyte can be the same, and the materials of the non - aqueous organic solvents in the first electrolyte and the second electrolyte can also be the same.

[0100] In some examples, the second electrolyte can contain 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, 15wt%, 16wt%, 17wt%, 18wt%, 19wt% or 20wt% of 1,3 - propane sultone.

[0101] In some examples, the second electrolyte can contain 5wt%, 5.5wt%, 6wt%, 6.5wt%, 7wt%, 7.5wt%, 8wt%, 8.5wt%, 9wt%, 9.5wt%, 10wt%, 10.5wt%, 11wt%, 11.5wt%, 12wt%, 12.5wt%, 13wt%, 13.5wt%, 14wt%, 14.5wt% or 15wt% of sodium salt.

[0102] In some examples, the second electrolyte may contain 0.005 wt%, 0.01 wt%, 0.02 wt%, 0.03 wt%, 0.04 wt%, 0.05 wt%, 0.06 wt%, 0.07 wt%, 0.08 wt%, 0.09 wt%, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt%, 15 wt%, 16 wt%, 17 wt%, 18 wt%, 19 wt%, 20 wt%, 21 wt%, 22 wt%, 23 wt%, 24 wt%, 25 wt%, 26 wt%, 27 wt%, 28 wt%, 29 wt% or 30 wt% of the water and acid removal additive.

[0103] In some examples, the second electrolyte may contain 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt%, 15 wt%, 16 wt%, 17 wt%, 18 wt%, 19 wt%, 20 wt%, 21 wt%, 22 wt%, 23 wt%, 24 wt%, 25 wt%, 26 wt%, 27 wt%, 28 wt%, 29 wt%, 30 wt% of the nitrile compound.

[0104] In some specific examples, the nitrile compound accounts for 10 wt% - 25 wt% of the second electrolyte, and the water and acid removal additive accounts for 5 wt% - 10 wt% of the second electrolyte. The mass percentages of the nitrile compound and the water and acid removal additive within the corresponding ranges are beneficial to further reducing the initial internal resistance of the sodium-ion secondary battery, and further reducing the internal resistance growth rate and gas generation rate during long-term cycling, thereby further improving the long-term cycling performance of the sodium-ion secondary battery.

[0105] Regarding the materials of the sodium salt and the non-aqueous organic solvent, reference can be made to the corresponding description of the aforementioned first electrolyte, which will not be elaborated here.

[0106] Nitrile compounds are nitrile substances containing a C≡N bond. For example, the nitrile compounds can be dinitrile compounds, trinitrile compounds, unsaturated nitrile compounds, nitrile compounds containing an aromatic ring, alkoxy nitrile compounds, nitrile compounds containing a sulfonyl group, or nitrile compounds containing a trimethylsilyl group. Through their unique chemical structures and properties, these nitrile compounds can play an important role in the electrolyte of sodium-ion secondary batteries, improving high-temperature storage and cycling performance, reducing gas generation, and extending the life of sodium-ion secondary batteries.

[0107] Dinitrile compounds include succinonitrile (SN), adiponitrile (ADN), glutaronitrile (GN), suberonitrile (SNB), or sebaconitrile (SCN); trinitrile compounds include 1,3,6-hexanetricarbonitrile (HTCN) or 1,3,5-pentanetetricarbonitrile (PTN); unsaturated nitrile compounds include acrylonitrile (AN), crotononitrile (CN), trans-butenedinitrile (TBN), or trans-hexenedinitrile (DCB); nitrile compounds containing an aromatic ring include 4-fluorobenzonitrile (4-FBN), 4-methylbenzonitrile (4-MBN), or tricyanobenzene (TCB); alkoxy nitrile compounds include 1,2-di(cyanoethoxy)ethane (DENE) or 1,2,3-tris(cyanoethoxy)propane (TCEP); nitrile compounds containing a sulfonyl group include sulfolanedinitrile (SDPN); nitrile compounds containing a trimethylsilyl group include 3-(trimethylsilyloxy)propionitrile (TMSOPN).

[0108] The water and acid removal additives can be carbodiimide compounds, silazane compounds, amine compounds, methanesulfonyl chloride (MSC), tetrahydrofuran (THF), or trifluoroacetic anhydride (TFAA).

[0109] The carbodiimide compounds include dicyclohexylcarbodiimide (DCC) or diisopropylcarbodiimide (DIC); the silazane compounds include hexamethyldisilazane (HMDS), heptamethyldisilazane (H7DMS), or trimethylchlorosilane (TMCS); the amine compounds include triethylamine (TEA) or diethylamine (DEA).

[0110] In addition, by mass percentage, the second electrolyte can further include: no more than 2 wt% of functional additives, and the functional additives include film-forming additives or gas-generation inhibiting additives. For example, the second electrolyte can include 1.5 wt%, 1 wt%, 0.8 wt%, or 0.4 wt% of functional additives.

[0111] The functional additives include one or more of the following materials: sodium difluoroborate oxalate, sodium difluorophosphate, sodium bis(oxalato)borate, methylene methanedisulfonate, vinylene sulfate, triallyl phosphate, tripropynyl phosphate, 1,3-propane sultone, fluorobenzene, vinylene carbonate trifluoroethoxy, 2-methylmaleic anhydride, vinylene carbonate difluoro, succinic anhydride, tris(trimethylsilyl) phosphate, vinylene sulfite, or hexamethylene diisocyanate.

[0112] It can be understood that the selection and dosage of the above functional additives in the embodiments of the present application can be flexibly adjusted according to needs.

[0113] Due to the high bond energy of the cyano group and good oxidation resistance of nitrile compounds, they can coordinate with high-valent transition metal ions on the surface of the positive electrode plate in a high-temperature environment to form stable complexes, reduce the catalytic decomposition of the electrolyte by the electrode, inhibit side reactions, and reduce the possibility of gas generation. Nitrile compounds can also form a cyano-containing polymer film on the surface of the negative electrode plate, further improving the cycle stability and high-temperature performance of the sodium-ion secondary battery.

[0114] In addition, the second electrolyte has a high concentration of PS. The high concentration of PS can effectively repair the formed SEI film, enhancing the high-temperature performance and cycle life of the sodium-ion secondary battery.

[0115] Water and acid removal additives such as dicyclohexylcarbodiimide (DCC) and diisopropylcarbodiimide (DIC) react with water and acidic substances to generate amide and neutral by-products, reducing the amount of gas generated by the reaction of these impurities with sodium salts, thereby effectively suppressing the gas generation phenomenon and improving the stability of the sodium-ion secondary battery during high-temperature storage and cycling.

[0116] An electrolyte combination with different formulations for two-step liquid injection is adopted, that is, the first electrolyte is used for the first liquid injection treatment first, and then the second electrolyte is used for the second liquid injection treatment. This preparation method can effectively take into account the low impedance, long cycle life, and high and low-temperature adaptability of sodium-ion batteries, and is compatible with existing production processes, thereby achieving comprehensive improvement of the electrochemical performance of sodium-ion batteries. Among them, the high and low-temperature adaptability mainly includes the improvement of the gas generation problem under high or low-temperature conditions and having a low impedance.

[0117] The first electrolyte has a relatively low content of 0.5 wt% - 2 wt% of 1,3 - propane sultone. The relatively low content of 0.5 wt% - 2 wt% of 1,3 - propane sultone, together with tris(trimethylsilyl) phosphate, fluoroethylene carbonate, and film-forming additives, forms a dense and low-impedance effective SEI film and CEI film during the pre-charging process. This avoids the problems of incomplete reaction and too high impedance caused by adding a large amount of additives at one time, and overcomes the defect that the formation sequence cannot be artificially controlled due to different film-forming potentials of different additives, so that a relatively ideal SEI film / CEI film can be selectively designed.

[0118] The second electrolyte has a nitrile solvent and an effective functional additive for the moisture problem (i.e., water and acid removal additive), which jointly solves the long-term gas generation problem. When the second electrolyte is in the case of an already formed effective SEI film / CEI film, it generally exists in the sodium-ion secondary battery in a free state. By adding a nitrile solvent and an effective functional additive for the moisture problem, the long-term gas generation problem of the sodium-ion secondary battery can be targeted for directional repair and improvement, and the 1,3 - propane sultone in the second electrolyte can also directionally repair the SEI film, thereby improving the long-cycle performance of the sodium-ion secondary battery.

[0119] In addition, due to the high bond energy of the cyano group and good oxidation resistance of nitrile compounds, they can coordinate with high-valent transition metal ions on the surface of the positive electrode plate in a high-temperature environment to form stable complexes, reduce the catalytic decomposition of the electrolyte by the electrode, inhibit side reactions, and reduce the possibility of gas generation. In addition, nitrile compounds form a cyano-containing polymer film on the surface of the negative electrode plate, further improving the cycle stability and high-temperature performance of the sodium-ion secondary battery.

[0120] The second electrolyte provides a high concentration of PS. The high concentration of PS effectively repairs the SEI film formed after the first liquid injection treatment, enhancing the high-temperature performance and cycle life of the sodium-ion secondary battery.

[0121] Water and acid removal additives such as dicyclohexylcarbodiimide (DCC) and diisopropylcarbodiimide (DIC) react with water and acidic substances to generate amide and neutral by-products, reducing the amount of gas generated by the reaction of these impurities with sodium salts (such as sodium hexafluorophosphate), thereby effectively inhibiting gas generation and enhancing the stability of the sodium-ion secondary battery during high-temperature storage and cycling.

[0122] Continue to refer to Figure 1 , after injecting the second electrolyte, a standing treatment is carried out.

[0123] After injecting the second electrolyte, the electrolyte injection hole of the housing is sealed.

[0124] In some examples, the process parameters of the standing treatment may include: standing at a temperature of 40 °C to 50 °C for 10 h to 15 h. For example, the process temperature of the standing treatment can be 42 °C, 45 °C or 48 °C.

[0125] After the standing treatment is completed, grading can be carried out. The full name of grading is capacity sorting. The capacities of sodium-ion secondary batteries produced on the same production line will vary. Qualified batteries are screened through capacity testing, and this process is called grading.

[0126] From the foregoing analysis, it can be seen that in this embodiment, through the two-step liquid injection technology, by comprehensively utilizing the advantages of FEC, PS, TMSP, nitrile substances and water and acid removal additives, the overall stability of the SEI film and the performance of the sodium-ion secondary battery can be significantly improved. The FEC provided by the first liquid injection treatment is used to provide the initial formation of the SEI film to ensure low-temperature and energy efficiency performance; the PS provided by the first liquid injection treatment is used to enhance the compactness and stability of the SEI film, especially to provide additional protection under high-temperature conditions; the TMSP provided by the first liquid injection treatment forms a stable protective layer on the positive electrode plate, reducing impedance and enhancing cycle performance.

[0127] The synergistic effect of the nitrile substances and the water and acid removal additives provided by the second liquid injection treatment effectively inhibits the gas generation phenomenon of the sodium-ion secondary battery and improves the high-temperature cycling performance. The PS and sodium salts provided by the second liquid injection treatment are used to effectively repair the formed SEI film during long-term cycling, improving the high-temperature performance and cycling life of the battery cell.

[0128] That is to say, the embodiments of the present application not only improve the initial performance of the sodium-ion secondary battery, but also maintain excellent cycling stability during long-term use. The two-step liquid injection technology successfully achieves an effective balance of low interfacial impedance, low gas generation, and long cycling stability of the sodium-ion secondary battery. This synergistic effect not only improves the initial performance of the battery by optimizing the functions and application timing of each additive, but also maintains excellent cycling stability during long-term use.

[0129] After injecting the first electrolyte, during the pre-charging process, a gas extraction process can also be carried out, which is beneficial to fully discharge the gas generated during formation (i.e., pre-charging), avoiding the retention of these gases in the electrolyte, thereby further improving the common problems of excessive gas generation and poor cycling performance of sodium-ion batteries. In addition, the gas extraction process can also discharge the by-products generated by the redox reaction in the electrolyte, further reducing the interfacial impedance of the SEI film. This redox reaction occurs during the pre-charging process.

[0130] Figure 2 It is a schematic diagram of the principle structure of the sodium-ion secondary battery provided by the embodiments of the present application after the second liquid injection treatment. It should be noted that Figure 2 the power supply shown in is the power supply provided for pre-charging before the second liquid injection treatment, and Figure 2 it is not a schematic diagram of the structure of the actual product of the sodium-ion secondary battery. Figure 2 The housing 10, the positive electrode plate 101, the separator 102, the negative electrode plate 103, and the electrolyte 104 are schematically shown in, where the electrolyte 104 is composed of the first electrolyte and the second electrolyte.

[0131] Figure 2 The PS provided by the second liquid injection treatment is also schematically shown by a white-filled circular frame, the sodium salt provided by the second liquid injection treatment is schematically shown by a square frame, the nitrile compound provided by the second liquid injection treatment is schematically shown by a triangular frame, and the water and acid removal additive provided by the second liquid injection treatment is schematically shown by a black-filled dot.

[0132] It should be noted that Figure 2 the content and location of each substance (such as PS, sodium salt, nitrile compound, and water and acid removal additive) in are only schematic and do not limit the present application. In addition, Figure 2 the shapes of the SEI film 105 and the CEI film 106 in are only schematic and do not limit the present application.

[0133] Reference Figure 2 , after the first liquid injection process, a pre-charging process is performed using a power source. During the pre-charging process, the negative electrode plate 103 is polarized, and the first electrolyte components react to form new chemical products. Then, part of the chemical products precipitate on the surface of the negative electrode plate 103 to form the SEI film 105, and part of the chemical products also precipitate on the surface of the positive electrode plate 101 to form the CEI film 106. Specifically, FEC in the first electrolyte reacts preferentially to PS to form an initial SEI film on the surface of the negative electrode plate 103. Then, PS in the first electrolyte reacts to generate chemical products, and these chemical products continue to form the SEI film 105 on the basis of the initial SEI film, thereby enhancing the density and stability of the SEI film 105.

[0134] Continue to refer to Figure 2 , after the second liquid injection process, PS, sodium salt, nitrile compounds, and water and acid removal additives are present in the electrolyte 104. Among them, during long-term cycling, PS can undergo reduction decomposition to form new chemical products, and these new chemical products can repair the SEI film 105, such as continuing to deposit and form a film on the surface of the SEI film 105. Figure 2 The path of the chemical products corresponding to part of PS to repair the SEI film 105 is schematically shown by the line with an arrow in

[0135] In addition, the sodium ions in the sodium salt provided by the second liquid injection process can be used to supplement the amount of sodium ions that can be embedded and detached in the positive electrode plate 101 and the negative electrode plate 103 during long-term cycling, avoiding the problem of the reduction in the content of mobile sodium ions during long-term cycling due to the inability of some sodium ions to detach from the positive electrode plate 101 or the negative electrode plate 103. The functions of the nitrile compounds and the water and acid removal additives can refer to the foregoing description and will not be elaborated here.

[0136] In addition, refer to Figure 2 , the working principle of the sodium-ion secondary battery is as follows: during the charging of the sodium-ion secondary battery, sodium ions in the positive electrode plate 101 detach from the positive electrode plate 101, and the de-embedded sodium ions, under the action of the electric field, pass through the separator 102 in the electrolyte 104 and are embedded in the negative electrode plate 103, making the positive electrode plate 101 in a state of less sodium with a high potential and the negative electrode plate 103 in a state of rich sodium with a low potential.

[0137] The discharging process is the opposite of the charging process. Sodium ions detach from the negative electrode plate 103, pass through the electrolyte 104 via the separator 102, and re-embed into the positive electrode plate 101, restoring the positive electrode plate 101 to a sodium-rich state. To maintain charge balance, the same number of electrons are transferred through the external circuit during the charging and discharging processes, migrating between the positive electrode plate 101 and the negative electrode plate 103 together with the sodium ions, causing continuous oxidation or reduction reactions on the positive electrode plate 101 and the negative electrode plate 103. Among them, the one losing electrons undergoes a reduction reaction, and the one gaining electrons undergoes an oxidation reaction.

[0138] For ease of understanding, the following embodiments more specifically describe the content disclosed in the present application. These embodiments are only for illustrative purposes, as various modifications and variations within the scope of the present application disclosure are obvious to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios mentioned in the following embodiments are based on weight.

[0139] Table 1

[0140]

[0141] Table 1 shows the formulations of the first electrolyte and the second electrolyte corresponding to the comparative examples and the examples provided in the embodiments of the present application. Among them, the non-aqueous organic solvent is ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC); the sodium salt is sodium hexafluorophosphate (NaPF 6 );the nitrile compound is ethylene glycol bis(propionitrile) ether (DENE) or 1,3,6-hexanetricarbonitrile (HTCN); the water and acid removal additive is dicyclohexylcarbodiimide (DCC) or diisopropylcarbodiimide (DIC); TMSP is tris(trimethylsilyl) phosphate; PS is 1,3-propane sultone; FEC is fluoroethylene carbonate. In Table 1, "first" refers to the formulation of the first electrolyte used in the first liquid injection treatment, and "second" refers to the formulation of the second electrolyte used in the second liquid injection treatment.

[0142] Referring to Table 1, a total of 7 comparative examples are provided, namely Comparative Example 1 to Comparative Example 7, and the battery numbers prepared from Comparative Example 1 to Comparative Example 7 are respectively denoted as D1~D7. A total of 4 experimental examples are provided, namely Experimental Example 1 to Experimental Example 4, and the battery numbers prepared from Experimental Example 1 to Experimental Example 4 are respectively denoted as E1~E4.

[0143] Among them, the formulations of the first electrolyte and the second electrolyte in Comparative Example 1 are the same, while the formulations of the first electrolyte and the second electrolyte in the remaining comparative examples and experimental examples are different. It should be noted that all the electrolyte formulations in Table 1 are comprehensive formulations. The calculation method of the actual ratio of additives in the electrolyte is as follows: for non-aqueous organic solvents, the displayed value is the mass percentage of the solvent; for the remaining sodium salts and additives, the value is the mass percentage of the overall formulation (i.e., the first electrolyte or the second electrolyte). That is, in actual preparation, the amount of non-aqueous organic solvent is calculated as follows: after subtracting the amount of sodium salt and additives from the corresponding electrolyte amount, multiply by the volume mass percentage of the non-aqueous organic solvent. When calculating the non-aqueous organic solvent in the first electrolyte, the corresponding electrolyte refers to the first electrolyte; when calculating the non-aqueous organic solvent in the second electrolyte, the corresponding electrolyte refers to the second electrolyte. In addition, additives refer to other components except sodium salts and non-aqueous organic solvents. In Table 1, the non-aqueous organic solvent is composed of EC, PC, DEC, and EMC.

[0144] Taking the mass of the first electrolyte as 100 g as an example, the calculation method of the mass of EC in the first electrolyte in Comparative Example 1 is: (100 g - mass of sodium salt - mass of additive) × 15%. Among them, the additive is PS, and the mass of the additive, that is, PS, is 3.5% × 100 = 3.5 g, and the mass of the sodium salt is 12.5% × 100 = 12.5 g. The mass of EC in Comparative Example 1 is (100 - 3.5 - 12.5) × 15% = 12.6 g. The calculation methods of the components in each comparative example and each experimental example will not be elaborated here.

[0145] The preparation methods of the sodium-ion secondary batteries corresponding to each comparative example and each experimental example include the following steps:

[0146] I. Prepare the positive electrode sheet.

[0147] Dissolve the positive electrode active material NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O 2 , conductive agent Super-P, conductive agent CNT, and binder PVDF in a ratio of 95.5:2.0:0.5:2.0 in N-methylpyrrolidone to make a positive electrode slurry, and uniformly coat it on aluminum foil. After going through steps such as drying, cold pressing, edge trimming, slicing, and strip cutting, weld the tab to make the positive electrode sheet.

[0148] II. Prepare the negative electrode sheet.

[0149] Dissolve the negative electrode active material hard carbon, conductive agent Super-P, thickening agent CMC, and binder SBR in a ratio of 96.5:1.0:1.0:1.5 in deionized water to make a negative electrode slurry, and uniformly coat it on copper foil. After going through steps such as drying, cold pressing, edge trimming, slicing, and strip cutting, make the negative electrode sheet.

[0150] III. Prepare the battery cell and place it in the housing.

[0151] Assemble the positive electrode plate, negative electrode plate and separator into a battery cell through a winding process. Package the battery cell with an aluminum-plastic composite film and perform vacuum drying.

[0152] IV. Perform a first liquid injection treatment on the battery cell placed in the housing. The first electrolyte used in the first liquid injection treatment has the corresponding formulations for each comparative ratio and each example in Table 1 respectively.

[0153] V. Soak for 10 - 15 h at 40°C - 50°C and soak for 20 - 30 h at room temperature, then perform pre-charging. Use a current of 0.05C for pre-charging. The charging control condition is constant current charging at 0.05C and set the cut-off voltage to 3.4V. After the charging is completed, let it stand at 45°C for 12 h.

[0154] During the pre-charging stage, add an air extraction process so that the gas generated by the pre-charging treatment can be fully discharged. The vacuum degree of the housing in the air extraction process is maintained within the range of -90Kpa to -80Kpa.

[0155] VI. Perform a second liquid injection treatment on the battery cell placed in the housing. The second electrolyte used in the second liquid injection treatment has the corresponding formulations for each comparative ratio and each experimental example in Table 1 respectively.

[0156] After injecting the second electrolyte into the housing, let it stand for 12 h and then perform grading to obtain a soft-pack sodium-ion secondary battery with a capacity of 3Ah.

[0157] Test the performance of the sodium-ion secondary batteries prepared in each example and each comparative ratio. The test process is as follows:

[0158] 1. 25°C cycle capacity retention rate test: At 25°C, charge the sodium-ion secondary battery at a constant current of 1C to the upper limit voltage, then charge at a constant voltage until the current is 0.05C, and then discharge at a constant current of 1C to the lower limit voltage. Perform 1000 cycle charge and discharge tests, and record the discharge capacity of the 1000th cycle. Capacity retention rate = (discharge capacity of the 1000th cycle / first discharge capacity) × 100%, and record the cycle retention rate of the 500th cycle.

[0159] 2. 45°C cycle capacity retention rate test: At 45°C, charge the sodium-ion secondary battery at a constant current of 1C to the upper limit voltage, then charge at a constant voltage until the current is 0.05C, and then discharge at a constant current of 1C to the lower limit voltage. Perform 1000 cycle charge and discharge tests, and record the discharge capacity of the 1000th cycle. Capacity retention rate = (discharge capacity of the 1000th cycle / first discharge capacity) × 100%, and record the cycle retention rate of the 500th cycle.

[0160] 3. 45°C High Temperature Storage Performance Test: At 45°C, charge the sodium-ion secondary battery at a constant current of 1C to the upper limit voltage, then charge at a constant voltage until the current reaches 0.05C, and then discharge at a constant current of 1C to the lower limit voltage. Record the discharge capacity at this time as the initial capacity C1, the initial battery volume V1, and the initial impedance F1. Then charge at a constant current of 1C to the upper limit voltage and then charge at a constant voltage until the current reaches 0.05C. Store the fully charged battery at 45°C for 15 days. After that, let it stand at room temperature for at least 2 hours and discharge at a constant current of 1C to the lower limit voltage. Record the discharge capacity C2, the impedance F2 after storage, and the battery volume V2 after storage. Calculate the capacity retention rate (%) = (C2 / C1) × 100%. Then charge at a constant current and constant voltage of 1C to the upper limit voltage at room temperature, cut off at 0.05C, and then discharge at a constant current of 1C to the lower limit voltage. Record the recovered capacity C3 and calculate the capacity recovery rate (%) = (C3 / C1) × 100%. The internal resistance growth rate (%) = impedance F2 after storage / initial impedance F1 × 100%. The gas growth rate (%) = (battery volume V2 after storage - battery volume V1 before storage) / battery volume V1 before storage.

[0161] These test methods are designed to comprehensively evaluate the performance of sodium-ion secondary batteries prepared in each example and comparative example.

[0162] Table 2

[0163]

[0164] Table 2 shows the performance test results of sodium-ion secondary batteries prepared in each comparative example and each example.

[0165] Referring to Table 2, the following comparative analysis is as follows:

[0166] In the electrical performance test results of normal temperature cycling, high temperature cycling, and high temperature storage, the comparison between Comparative Example 1 and Comparative Example 2 shows that the secondary injection method has a positive effect on improving the capacity retention rate of normal temperature and high temperature cycling, the capacity retention rate of high temperature storage, the capacity recovery rate of high temperature storage, and suppressing the growth of internal resistance and gas generation. Especially in the later stage of cycling, the improvement of the capacity retention rate is more significant. This is related to the directional repair of the high-concentration PS in the second electrolyte on the already formed SEI film.

[0167] The comparison between Comparative Example 3, Comparative Example 4, Comparative Example 5 and Comparative Example 2 shows that adding TMSP, FEC or a mixture of FEC + TMSP additives alone to the first electrolyte formulation helps to improve the cycling retention rate at normal temperature (the temperature of 20°C to 25°C is defined as normal temperature) and high temperature. Among them, FEC has a more significant improvement in the cycling capacity retention rate. This may be because FEC can promote the formation of the SEI film in the electrolyte, increase the density and stability of the SEI film, thereby improving the cycling performance of sodium-ion secondary batteries.

[0168] However, in the high-temperature storage test project, the improvement effect of these additives is not significant. This is consistent with the characteristics of TMSP and FEC. TMSP can form a SEI film with low impedance, effectively reducing the initial internal resistance, while FEC enhances the compactness and stability of the SEI film, contributing to the improvement of the cycling performance. The scheme of using TMSP and FEC comprehensively can take into account the advantages of both and show better performance.

[0169] The comparison between Comparative Example 6, Comparative Example 7 and Comparative Example 5 shows that adding nitrile compounds DENE or HTCN can significantly improve the capacity retention rate of normal-temperature and high-temperature cycling, the capacity retention rate of high-temperature storage, the capacity recovery rate of high-temperature storage, and can inhibit the growth of internal resistance and the growth of gas generation.

[0170] However, both of these nitrile additives increase the internal resistance, especially HTCN. DENE has a more significant improvement effect on normal-temperature cycling, while HTCN is more prominent in high-temperature cycling and high-temperature storage. This may be related to its stronger ability to coordinate with high-valence transition metal ions on the surface of the positive electrode plate in a high-temperature environment to form stable complexes, reducing side reactions and gas generation phenomena.

[0171] The comparison between Experimental Example 1, Experimental Example 2, Experimental Example 3, Experimental Example 4 and the above comparative examples shows that the first electrolyte contains FEC, TMSP and PS with a first concentration, the second electrolyte contains PS with a second concentration, the second concentration is greater than the first concentration, and a nitrile compound and a water and acid removal additive (DCC or DIC) are added to the second electrolyte at the same time. The test performance of the prepared sodium-ion secondary battery is comprehensively improved. It can not only improve the capacity retention rate of normal-temperature and high-temperature cycling, the capacity retention rate of high-temperature storage, the capacity recovery rate of high-temperature storage, but also further effectively inhibit the growth of internal resistance and the growth of gas generation, and the sodium-ion secondary battery has a lower initial internal resistance.

[0172] From the above experimental data, it can be seen that the preparation method of the sodium-ion secondary battery provided by the embodiments of the present application is beneficial to improving the overall performance of the sodium-ion secondary battery, improving the gas generation problem commonly existing in sodium-ion batteries, and taking into account the long cycling performance at high and low temperatures, so as to achieve comprehensive improvement of electrochemical performance.

[0173] Those of ordinary skill in the art can understand that the above embodiments are specific embodiments for implementing the present application. In actual applications, various changes can be made in form and details without departing from the spirit and scope of the present application. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present application. Therefore, the protection scope of the present application should be subject to the scope defined by the claims.

Claims

1. A method for preparing a sodium ion secondary battery, characterized in that: include: Providing a battery cell and placing the battery cell in a shell, the battery cell comprising a positive electrode sheet, a separator and a negative electrode sheet, the separator being located between the positive electrode sheet and the negative electrode sheet, and the positive electrode sheet, the separator and the negative electrode sheet being stacked to form the battery cell, or the positive electrode sheet, the separator and the negative electrode sheet being stacked and wound to form the battery cell, wherein the positive electrode sheet comprises a positive electrode collector and a positive electrode active material layer disposed on the positive electrode collector, the negative electrode sheet comprises a negative electrode collector and a negative electrode active material layer disposed on the negative electrode collector, the material of the positive electrode active material layer comprises a layered oxide positive electrode material, a polyanion positive electrode material or a Prussian blue positive electrode material, and the material of the negative electrode active material layer comprises a metal compound, a carbon-based material, an alloy material or a non-metallic element; The battery cell placed in the shell is subjected to a first liquid injection treatment using a first electrolyte, wherein the first electrolyte is composed of the following components in terms of mass percentage: 0.5wt% to 2wt% of 1,3-propane sultone, 0.1wt% to 2wt% of tris(trimethylsilyl)phosphate, 0.1wt% to 2wt% of fluoroethylene carbonate, 0.01wt% to 5wt% of a film-forming additive and a sodium salt, and the remainder is a non-aqueous organic solvent; After injecting the first electrolyte, performing an immersion treatment and a pre-charging treatment in sequence; The battery cell placed in the shell is subjected to a second liquid injection treatment using a second electrolyte, wherein the second electrolyte is composed of the following components by mass percentage: 5wt% to 20wt% of 1,3-propane sultone, 3wt% to 30wt% of nitrile compounds, 0.005wt% to 30wt% of water-removing and acid-removing additives and sodium salts, and the remainder is a non-aqueous organic solvent; After the second electrolyte is injected, a standing treatment is performed.

2. The method for preparing a sodium ion secondary battery according to claim 1, characterized in that: The mass of 1,3-propane sultone in the second electrolyte and the mass of 1,3-propane sultone in the first electrolyte satisfy: 40%×M≤m1≤50%×M, 50%×M≤m2≤60%×M, M=m1+m2, wherein m1 is the mass of 1,3-propane sultone in the first electrolyte, and m2 is the mass of 1,3-propane sultone in the second electrolyte.

3. The method for preparing a sodium ion secondary battery according to claim 1, characterized in that: The nitrile compound accounts for 10 wt % to 25 wt % of the second electrolyte, and the water-removing and acid-removing additive accounts for 5 wt % to 10 wt % of the second electrolyte.

4. The method for preparing a sodium ion secondary battery according to claim 1 or 3, characterized in that: The nitrile compound is a dinitrile compound, a trinitrile compound, an unsaturated nitrile compound, a nitrile compound containing an aromatic ring, an alkoxy nitrile compound, a nitrile compound containing a sulfonyl group, or a nitrile compound containing a trimethylsilyl group.

5. The method for preparing a sodium ion secondary battery according to claim 4, characterized in that: The dinitrile compounds include succinonitrile, adiponitrile, glutaronitrile, suberonitrile or sebacate; the trinitrile compounds include 1, 3, 6-hexanetricarbonitrile or 1, 3, 5-pentanetricarbonitrile; the unsaturated nitrile compounds include acrylonitrile, crotononitrile, trans-butenedinitrile or trans-hexenedinitrile; the nitrile compounds containing aromatic rings include p-fluorobenzonitrile, p-methylbenzonitrile or tricyanobenzene; the alkoxy nitrile compounds include ethylene glycol bis(propionitrile) ether or 1, 2, 3-tris(cyanoethoxy)propane; the sulfonyl-containing nitrile compounds include bis(cyanoethyl)sulfone; the trimethylsilyl-containing nitrile compounds include 3-(trimethylsilyloxy)propionitrile.

6. The method for preparing a sodium ion secondary battery according to claim 1 or 3, characterized in that: The water-removing and acid-removing additive is a carbodiimide compound, a silazane compound, an amine compound, methylsulfonyl chloride, tetrahydrofuran or trifluoroacetic anhydride.

7. The method for preparing a sodium ion secondary battery according to claim 6, characterized in that: The carbodiimide compound includes dicyclohexylcarbodiimide or diisopropylcarbodiimide; the silazane compound includes hexamethyldisilazane, heptamethyldisilazane or trimethylchlorosilane; the amine compound includes triethylamine or diethylamine.

8. The method for preparing a sodium ion secondary battery according to claim 1, characterized in that: The mass of the first electrolyte accounts for 65% to 90% of the sum of the mass of the first electrolyte and the second electrolyte.

9. The method for preparing a sodium ion secondary battery according to claim 1, characterized in that: The infiltration treatment includes a first infiltration stage and a second infiltration stage which are performed sequentially, wherein the temperature of the first infiltration stage is greater than the temperature of the second infiltration stage, and the duration of the first infiltration stage is less than the duration of the second infiltration stage; Among them, the temperature of the first infiltration stage is 40℃~50℃, and the duration of the first infiltration stage is 10h~15h; the temperature of the second infiltration stage is 15℃~25℃, and the duration of the second infiltration stage is 20h~30h.

10. The method for preparing a sodium ion secondary battery according to claim 1, characterized in that: The process parameters of the pre-charging process include: constant current charging at a charging rate of 0.04C-0.06C, and a pre-charging cut-off voltage of 3.4V.

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