Preparation method of sodium ion secondary battery

Abstract

The embodiment of the invention relates to the technical field of energy storage, and provides a preparation method of a sodium ion secondary battery, which comprises the following steps: providing a battery cell and placing the battery cell in a shell; a first electrolyte is adopted to carry out first liquid injection treatment on the battery cell, and the first electrolyte comprises 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 sodium salt; carrying out infiltration treatment and pre-charging treatment; a second electrolyte is adopted to carry out second liquid injection treatment on the battery cell, and the second electrolyte comprises 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 salt; and standing. According to the embodiment of the invention, the overall performance of the sodium-ion battery is improved.

Description

[0001] This application is a divisional application of the patent application with the application date of November 7, 2024, application number 2024115877236, and invention title "Preparation Method of Sodium-Ion Secondary Battery". Technical Field

[0002] The embodiments of this application relate to the technical field of energy storage, and particularly to a preparation method of a sodium-ion secondary battery. Background Art

[0003] 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.

[0004] 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 is abundant in the earth's crust, far exceeding lithium, so the resource supply of sodium-ion batteries is more sufficient. Second, the production cost of sodium-ion batteries is relatively low, and the raw material price fluctuates less, 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 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.

[0005] 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

[0006] The embodiments of this application provide a preparation method of a sodium-ion secondary battery, which is at least beneficial to improving the problems of easy gas generation, large impedance, and poor cycle performance of sodium-ion batteries under high-temperature conditions.

[0007] 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, the electrode assembly including a positive electrode plate, a separator, and a negative electrode plate, the separator being located between the positive electrode plate and the negative electrode plate, and the positive electrode plate, the separator, and the negative electrode plate being stacked to form the electrode assembly, or, the positive electrode plate, the separator, and the negative electrode plate being stacked and wound to form the electrode assembly, wherein, the positive electrode plate includes a positive current collector and a positive active material layer disposed on the positive current collector, the negative electrode plate includes a negative current collector and a negative active material layer disposed 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, wherein, by mass percentage, the first electrolyte includes: 0.5 wt% to 2 wt% of 1,3-propane sultone, 0.1 wt% to 2 wt% of tris(trimethylsilyl) phosphate, 0.1 wt% to 2 wt% of fluoroethylene carbonate, 0.01 wt% to 5 wt% 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, wherein, by mass percentage, the second electrolyte includes: 5 wt% to 20 wt% of 1,3-propane sultone, 3 wt% to 30 wt% of a nitrile compound, 0.005 wt% to 30 wt% of a water and acid removing additive, and a sodium salt; after injecting the second electrolyte, performing a formation and standing treatment.

[0008] 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 to 20 and satisfies: 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.

[0009] In some embodiments, the nitrile compound accounts for 10 wt% to 25 wt% of the second electrolyte, and the water and acid removing additive accounts for 5 wt% to 10 wt% of the second electrolyte.

[0010] In some embodiments, the nitrile compound is a dinitrile compound, a trinitrile compound, an unsaturated nitrile compound, an aromatic ring-containing nitrile compound, an alkoxy nitrile compound, a sulfonyl group-containing nitrile compound, or a trimethylsilyl group-containing nitrile compound.

[0011] 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, crotononitrile, fumarodinitrile, or trans-hexenedinitrile; the aromatic ring-containing nitrile compounds 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 group-containing nitrile compounds include bis(cyanoethyl) sulfone; and the trimethylsilyl group-containing nitrile compounds include 3-(trimethylsilyloxy)propionitrile.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] In some embodiments, the infiltration treatment includes a first infiltration stage and a second infiltration stage that are carried out in sequence, wherein 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 10 h to 15 h; the temperature of the second infiltration stage is 15°C to 25°C, and the duration of the second infiltration stage is 20 h to 30 h.

[0016] 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.

[0017] The technical solutions provided by the embodiments of the present application have at least the following advantages:

[0018] In the technical solution of the preparation method of the sodium-ion secondary battery provided by the embodiments of the present application, after placing the battery cell in the casing, 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% to 2 wt% of TMSP (i.e., tris(trimethylsilyl) phosphate), 0.1 wt% to 2 wt% of FEC (i.e., fluoroethylene carbonate), and 0.5 wt% to 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. PS can provide protection for the SEI film formed based on FEC first, which is beneficial to forming a solid electrolyte interface film (i.e., SEI film) with low impedance, dense and uniform on the negative electrode, and helps to improve the battery performance and life during high-temperature storage and cycling. 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, the second injection treatment is carried out. The second electrolyte provided by the second injection treatment contains 5 wt% to 20 wt% of 1,3-propane sultone, 3 wt% to 30 wt% of nitrile compounds, 0.005 wt% to 30 wt% of water-removing and acid-removing 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. The water-removing and acid-removing additives can remove the moisture and acid in the electrolyte, reduce 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 can also reduce the internal resistance growth rate during long-term cycling. All in all, the technical solution provided by the embodiments of the present application is beneficial 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

[0019] 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 scale limitation. To more clearly illustrate the technical solutions in the embodiments of the present application or in the prior art, 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.

[0020] Figure 1Schematic flow chart corresponding to the preparation method of the sodium ion secondary battery provided by the embodiment of the present application;

[0021] Figure 2 Schematic diagram of the principle structure of the sodium ion secondary battery provided by the embodiment of the present application after the second liquid injection treatment. Detailed implementation manners

[0022] As can be seen from the background art, lithium iron phosphate-based lithium ion batteries have been widely used in the energy storage field due to their low price 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.

[0023] The embodiment of the present application provides 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 liquid injection treatment (i.e., the first liquid injection). The 1,3-propane sultone provided in the second liquid 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 the water and acid removal additives provided in the second liquid 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 thus effectively inhibiting the gas generation phenomenon during long-term cycling or under high temperature conditions.

[0024] In the description of the embodiment 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 embodiment of the present application, "a plurality" means more than two, unless otherwise specifically defined.

[0025] Referring to "embodiment" in this article means that the specific features, structures or characteristics described in connection with the embodiment can be included in at least one embodiment of the present application. The appearance of this phrase in various positions in the specification 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.

[0026] In the description of the embodiments of the present application, the term "and / or" is merely an association relationship describing associated objects, indicating that there can be three relationships. For example, A and / or B can represent three situations: the existence of A, the simultaneous existence of A and B, and the existence of B. Additionally, in this text, the character " / " generally represents an "or" relationship between the associated objects before and after.

[0027] In the description of the embodiments of the present application, the term "multiple" 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).

[0028] In the description of the embodiments of the present application, when a certain component "includes" another component, unless otherwise specified, other components are not excluded, and other components may further be included. In addition, when components such as a layer, film, region, or plate are referred to as "on / located on" another component, it can be "directly on" another component (i.e., located on the surface of another component with no other components in between), or there can be other components in between. In addition, when components such as a layer, film, region, or plate are "directly located on" another component, or when components such as a layer, film, region, or plate are located on the surface of another component, it means that no other components are located in between.

[0029] The terms used in the description of various embodiments herein are only used to describe 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.

[0030] The following will elaborate on each embodiment of the present application in conjunction with the accompanying drawings. However, those of ordinary skill in the art can understand that in each embodiment of the present application, many technical details are presented for the reader to 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.

[0031] Figure 1 It is a schematic flow diagram corresponding to the preparation method of the sodium-ion secondary battery provided for the embodiments of the present application.

[0032] It should be noted that the sodium-ion secondary battery mentioned in the embodiments of the present application is the sodium-ion battery.

[0033] Refer to Figure 1 , the preparation method of the sodium-ion secondary battery includes the following steps:

[0034] Provide an electrode core and place the electrode core in a housing. The electrode core 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 core. Alternatively, the positive electrode plate, the separator, and the negative electrode plate are stacked and wound to form the electrode core. Wherein, the positive electrode plate includes a positive current collector and a positive active material layer provided on the positive current collector, and the negative electrode plate includes a negative current collector and a negative active material layer provided on the negative current collector.

[0035] The electrode core can be a stacked structure formed by sequentially stacking the negative electrode plate, the separator, and the positive electrode plate, or a wound structure formed by winding the negative electrode plate, the separator, and the positive electrode plate after sequential stacking.

[0036] The positive electrode plate includes a positive current collector and a positive active material layer provided on at least one surface of the positive current collector and containing a positive active material. For example, the positive current collector includes two opposite surfaces, and the positive active material layer is provided on either or both of the two opposite surfaces of the positive current collector in its own thickness direction.

[0037] The positive 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, the material of the positive active material layer includes an oxide positive electrode material, a polyanion-based positive electrode material, or a Prussian blue-based positive electrode material.

[0038] Among them, the chemical formula of the layered oxide positive electrode material is Na x M1O2, 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 NaFeO2, NaCrO2, Na 0.5 CoO2, or NaNiO2, etc.

[0039] The general formula of the polyanion-based positive electrode material is Na x M2 y (X a O b ) z Z w , and the general formula is electrochemically neutral. Wherein 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 several of Si, S, P, As, B, Mo, W, Ge, and Z is F or OH. For example, the polyanion-based positive electrode material can be sodium phosphate salt, sodium sulfate salt, or sodium pyrosulfate salt. More specifically, the polyanion-based positive electrode material can be sodium iron phosphate (NaFePO4) or sodium iron sulfate (Na2Fe(SO4)2), etc.

[0040] Prussian blue-based cathode materials have a structural general formula A x M1[M2(CN)6] 1-y ·V y ·nH2O, where 0 ≤ x ≤ 2, 0 ≤ y ≤ 1. Here, V represents the vacancy of [M2(CN)6], A is an alkali metal ion, such as a Na ion or a K ion, etc. In a sodium-ion secondary battery, A is a sodium ion; both M1 and M2 are transition metal ions, such as Mn, Fe, Co, Ni, Cu, Zn, or Cr, etc.; H2O represents interstitial water (or zeolitic water) and coordinated water.

[0041] For example, a Prussian blue-based cathode material can be represented as Na x MFe(CN)6, where M can be Mn, Co, Ni, Cu, Zn, or Cr, etc. Prussian blue-based cathode materials include 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.

[0042] The cathode active material layer can also include a conductive agent and a binder. The conductive agent is used to improve the conductivity of the cathode active material layer, and the binder is used to firmly adhere the cathode active material and the binder to the cathode current collector.

[0043] 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).

[0044] The cathode current collector can be made of a metal foil, a carbon-coated metal foil, or a porous metal plate. In this embodiment, the cathode current collector is an aluminum foil.

[0045] In some examples, the process steps for preparing the cathode electrode can include: dispersing the cathode active material, the conductive agent, and the binder in a solvent to form a cathode slurry; coating the cathode slurry on the surface of the cathode current collector, and after drying and cold pressing, obtaining the cathode electrode.

[0046] The negative electrode plate 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 a 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.

[0047] The negative electrode active material can be a metal compound, a carbon-based material, an alloy material, or a non-metal simple substance. That is, the material of the negative electrode active material layer includes a metal compound, a carbon-based material, an alloy material, or a non-metal simple substance.

[0048] 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 a large specific surface area, which helps to improve the battery performance; expanded graphite and graphene are widely used in negative electrode plates due to their excellent conductivity and mechanical properties.

[0049] The metal compound can be lithium titanate, and lithium titanate can provide good cycle life and high safety.

[0050] 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 paste the negative electrode active material and the binder on the negative electrode current collector.

[0051] The negative electrode current collector can be made of a metal foil, a carbon-coated metal foil, or a porous metal plate. In this embodiment, the negative electrode current collector is a copper foil. In other embodiments, the negative electrode current collector can also be an aluminum foil.

[0052] In some examples, the process steps for preparing the negative electrode plate can include: dispersing the negative electrode active material, the conductive agent, and the 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 plate.

[0053] The separator can be made of one or more materials, including polypropylene (PP), polyethylene (PE), ceramic-coated separator, and non-woven separator. The PP and PE porous separators have good mechanical strength and chemical stability; the ceramic-coated separator is coated with a ceramic material on the basis of the PP or PE separator, which improves the high-temperature resistance and safety of the separator; the non-woven separator is composed of polymer fibers and has excellent electrolyte wettability and ion conductivity.

[0054] In addition, the separator can be a single-layer film or a multi-layer film.

[0055] In this embodiment, after preparing the positive electrode sheet, the negative electrode sheet, and the separator, the positive electrode sheet, the separator, and the negative electrode sheet are stacked and wound to form an electric core. In other embodiments, the electric core may also have the stacked structure as described above.

[0056] In this embodiment, the sodium-ion secondary battery is a pouch cell, and correspondingly, the housing is a soft outer shell. In some examples, the housing may be an aluminum-plastic film.

[0057] In a specific example, the capacity of the sodium-ion secondary battery prepared in this embodiment may be 3 Ah, where Ah represents ampere-hour.

[0058] It should be noted that the embodiments of the present application do not limit the type of the sodium-ion secondary battery. In other embodiments, the sodium-ion secondary battery may also be a prismatic and cylindrical cell, which may 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 rigid materials.

[0059] Hereinafter, a detailed description will be given taking the sodium-ion secondary battery as a pouch cell and the housing as an aluminum-plastic film as an example.

[0060] 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.

[0061] Continue to refer to Figure 1 , and perform a first liquid injection process on the electric core placed in the housing with a 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 a film-forming additive, and a sodium salt.

[0062] The electrolyte injected in the first liquid injection process is used to form a stable SEI film, and this SEI film has the characteristics of low impedance, good compactness, and uniformity.

[0063] By mass percentage, the content of the sodium salt in the first electrolyte may be 5 wt% - 15 wt%.

[0064] In addition to PS, FEC, TMSP, the sodium salt, and the film-forming additive in the first electrolyte, the balance is a non-aqueous organic solvent.

[0065] In some examples, the first electrolyte may contain 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1.0 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt% or 2.0 wt% of 1,3 - propane sultone.

[0066] In some examples, the first electrolyte may contain 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%, 1.0 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt% or 2.0 wt% of tris(trimethylsilyl) phosphate.

[0067] In some examples, the first electrolyte may contain 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%, 1.0 wt%, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt% or 2.0 wt% of fluoroethylene carbonate.

[0068] In some examples, the first electrolyte may contain 5 wt%, 5.5 wt%, 6 wt%, 6.5 wt%, 7 wt%, 7.5 wt%, 8 wt%, 8.5 wt%, 9 wt%, 9.5 wt%, 10 wt%, 10.5 wt%, 11 wt%, 11.5 wt%, 12 wt%, 12. wt%, 13 wt%, 13.5 wt%, 14 wt%, 14.5 wt% or 15 wt% of sodium salt.

[0069] Among them, the sodium salt can be one or several of sodium hexafluorophosphate (NaPF6), sodium perchlorate (NaClO4), sodium tetrafluoroborate (NaBF4), sodium hexafluoroarsenate (NaAsF6), sodium tetrachloroaluminate (NaAlCl4), sodium trifluoroacetate (NaTFA), sodium tetraphenylborate (NaBPh4), sodium difluorophosphate (NaDFP), sodium bis(oxalato)borate (NaBOB), sodium difluorooxalatoborate (NaDFOB), sodium trifluoromethanesulfonate (NaOTf), sodium bis(fluorosulfonyl)imide (NaFSI), and sodium bis(trifluoromethanesulfonyl)imide (NaTf2N).

[0070] In some examples, the first electrolyte can 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 a film-forming additive.

[0071] The film-forming additive may include, but is not limited to, one or more combinations of the following: sodium difluorophosphate (NaDFP), methylene methanedisulfonate (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).

[0072] In a specific example, by mass percentage, the first electrolyte may include 0.5 wt% to 3 wt% of the film-forming additive. 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.

[0073] 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), methylpropyl carbonate (MPC), ethyl propyl carbonate (EPC), 1,4-butyrolactone (GBL), methyl propionate (MP), methyl butyrate (MB), ethyl acetate (EA), ethyl propionate (EP), and propyl propionate (PP).

[0074] In a specific example, the non-aqueous organic solvent may be selected from a mixture of two or more of dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), propylene carbonate (PC), and ethylene carbonate (EC) in any proportion.

[0075] 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.

[0076] Continue to refer to Figure 1 , after injecting the first electrolyte, an infiltration treatment and a pre-charging treatment are sequentially performed.

[0077] 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.

[0078] After the infiltration treatment and 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).

[0079] The infiltration treatment may include a first infiltration stage and a second infiltration stage 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] Before 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, components in the first electrolyte react with each other to produce by - products such as gas, and the gas will escape to the sealed part with the space margin, making the sealed part filled with gas to form an air bag.

[0084] 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.

[0085] In some examples, the pre - charge treatment can perform constant - current charging at 0.045C, 0.05C or 0.055C.

[0086] After the pre - charge treatment, the preparation method can also include: standing for 10h to 14h at the third temperature, where 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 time can be 11h, 12h or 13h. This standing can enable the battery cell to discharge to a certain extent.

[0087] 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 and avoid the gas remaining inside the housing, thereby further reducing the gas content remaining in the housing.

[0088] In some examples, as described above, before 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, components in the first electrolyte react with each other to produce by - products such as gas, and the gas will escape to the sealed part with the space margin, making the sealed part filled with gas to form an air bag. Before 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.

[0089] In other examples, the evacuation treatment can also be: before the pre - charge treatment, the electrolyte injection port on the housing can be sealed, and an additional evacuation pipe and an evacuation pump are provided. The evacuation pipe enters the inside of the housing through the sealed part; under the drive of the evacuation pump, the evacuation pipe extracts the gas inside the housing and discharges the gas from the inside of the housing.

[0090] Continue to refer to Figure 1 , the 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.

[0091] Specifically, during the aforementioned pre - charge treatment 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.

[0092] The concentration of PS in the second electrolyte provided by the second liquid injection treatment is greater than the concentration of PS 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 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.

[0093] 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 the pre - charge treatment (i.e., formation). If the amount of the second electrolyte provided by the second liquid injection treatment is too large, the relative amount of additives in the corresponding second electrolyte is relatively large, which may lead to uneven concentration distribution of additives in the housing.

[0094] Therefore, the mass of the first electrolyte can account for 65wt% - 90wt% of the total electrolyte mass. For example, it can be 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 be caused. After the second liquid injection treatment, the concentration distribution of 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.

[0095] In a specific example, the mass ratio of the first electrolyte to the second electrolyte can be 85:15.

[0096] 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 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.

[0097] 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, where M = m1 + m2. Here, 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 effective repair of the SEI film during long - term cycling.

[0098] 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.

[0099] In terms of mass percentage, the mass percentage of sodium salt in the second electrolyte can be 5wt% - 15wt%.

[0100] 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.

[0101] 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.

[0102] 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.

[0103] 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 a water and acid removal additive.

[0104] 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 a nitrile compound.

[0105] 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 production rate during long-term cycling, thereby further improving the long-term cycling performance of the sodium-ion secondary battery.

[0106] Regarding the materials of sodium salts and non-aqueous organic solvents, reference can be made to the corresponding descriptions of the aforementioned first electrolyte, which will not be elaborated here.

[0107] Nitrile compounds are nitrile substances containing a C≡N bond. For example, 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.

[0108] 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 (1,4-dicyanobutene, DCB); nitrile compounds containing an aromatic ring include 4-fluorobenzonitrile (4-FBN), 4-methylbenzonitrile (4-MBN), or tricyanobenzene (TCB); alkoxy nitrile compounds include ethylene glycol bis(propionitrile) ether (1,2-di(cyanoethoxy)ethane, DENE) or 1,2,3-tris(cyanoethoxy)propane (TCEP); nitrile compounds containing a sulfonyl group include bis(cyanoethyl) sulfone (Sulfolane dinitrile, SDPN); nitrile compounds containing a trimethylsilyl group include 3-(trimethylsilyloxy)propionitrile (TMSOPN).

[0109] The water and acid removal additives can be carbodiimide compounds, silazane compounds, amine compounds, methanesulfonyl chloride (MSC), tetrahydrofuran (THF), or trifluoroacetic anhydride (TFAA).

[0110] 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).

[0111] In addition, by mass percentage, the second electrolyte may 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 may include 1.5 wt%, 1 wt%, 0.8 wt%, or 0.4 wt% of functional additives.

[0112] The functional additives include one or more of the following materials: sodium difluorooxalate, sodium difluorophosphate, sodium dioxalate, methylene methanedisulfonate, vinylene sulfate, triallyl phosphate, tripropynyl phosphate, 1,3 - propylene sultone, fluorobenzene, trifluoroethoxy ethylene carbonate, 2 - methylmaleic anhydride, difluoroethylene carbonate, succinic anhydride, tris(trimethylsilyl) phosphate, vinylene sulfite, or hexamethylene diisocyanate.

[0113] 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.

[0114] 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.

[0115] 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.

[0116] 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.

[0117] 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 a comprehensive improvement in 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.

[0118] 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.

[0119] 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 solve the long-term gas generation problem. When the second electrolyte exists in the sodium-ion secondary battery in a free state in the case of an already formed effective SEI film / CEI film, 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.

[0120] In addition, due to the high bond energy of its cyano group and good oxidation resistance, nitrile compounds can coordinate with high-valence 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 generate a polymer film containing cyano groups on the surface of the negative electrode plate, further improving the cycle stability and high-temperature performance of the sodium-ion secondary battery.

[0121] 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.

[0122] 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.

[0123] Continue to refer to Figure 1 , after injecting the second electrolyte, a standing treatment is carried out.

[0124] After injecting the second electrolyte, the electrolyte injection hole of the housing is sealed.

[0125] In some examples, the process parameters of the standing treatment may include: standing at a temperature of 40°C to 50°C for 10h to 15h. For example, the process temperature of the standing treatment can be 42°C, 45°C or 48°C.

[0126] 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 selected through capacity testing, and this process is called grading.

[0127] From the foregoing analysis, it can be seen that in this embodiment, through the two-step liquid injection technology, the advantages of FEC, PS, TMSP, nitrile substances and water and acid removal additives are comprehensively utilized, which can significantly improve the overall stability of the SEI film and the performance of the sodium-ion secondary battery. 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 denseness 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.

[0128] 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.

[0129] 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 interface impedance, low gas generation amount, 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.

[0130] After injecting the first electrolyte, during the pre-charging process, a gas extraction treatment can also be performed, 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 treatment can also discharge the by-products generated by the redox reaction in the electrolyte, further reducing the interface impedance of the SEI film, and the redox reaction occurs during the pre-charging process.

[0131] 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 actual structure of the sodium-ion secondary battery product. 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 together.

[0132] 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 shown by a square frame, the nitrile compound provided by the second liquid injection treatment is shown by a triangular frame, and the water and acid removal additive provided by the second liquid injection treatment is shown by a black-filled dot.

[0133] It should be noted that Figure 2 the content and positions of each substance (such as PS, sodium salt, nitrile compound, and water and acid removal additive) in are only for illustration 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 for illustration and do not limit the present application.

[0134] Reference Figure 2 After the first liquid injection process, a pre-charging process is carried out 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 prior 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 compactness and stability of the SEI film 105.

[0135] 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 the PS for repairing the SEI film 105 is schematically shown by the arrowed line in

[0136] 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 intercalated into and deintercalated from the positive electrode plate 101 and the negative electrode plate 103 during long-term cycling, avoiding the problem of a decrease in the content of mobile sodium ions during long-term cycling due to the inability of some sodium ions to deintercalate 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.

[0137] 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, the sodium ions in the positive electrode plate 101 are deintercalated from the positive electrode plate 101. Under the action of an electric field, the deintercalated sodium ions pass through the separator 102 in the electrolyte 104 and are intercalated into the negative electrode plate 103, making the positive electrode plate 101 in a state of less sodium with a high electric potential and the negative electrode plate 103 in a state of rich sodium with a low electric potential.

[0138] The discharging process is the opposite of the charging process. Sodium ions detach from the negative electrode plate 103, pass through the electrolyte 104 and the separator 102, and then 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 charge and discharge 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 that loses electrons undergoes a reduction reaction, and the one that gains electrons undergoes an oxidation reaction.

[0139] 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 changes within the scope of the present application 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.

[0140] Table 1

[0141]

[0142] 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 (NaPF6); 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.

[0143] 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 to 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 to E4.

[0144] 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 for the actual proportion 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 volumetric 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 consists of EC, PC, DEC, and EMC.

[0145] Taking the mass of the first electrolyte as 100 g as an example, the calculation method for the mass of EC in the first electrolyte in Comparative Example 1 is: (100 g - mass of sodium salt - mass of additive) × 15%, where the additive is PS, and the mass of the additive, i.e., 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 for each component in each comparative example and each experimental example will not be elaborated here.

[0146] The preparation methods of the sodium-ion secondary batteries corresponding to each comparative example and each experimental example include the following steps:

[0147] I. Prepare the positive electrode sheet.

[0148] Dissolve the positive electrode active material NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2, the conductive agent Super-P, the conductive agent CNT, and the 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 steps such as drying, cold pressing, trimming, slicing, and strip cutting, weld the tab to make the positive electrode sheet.

[0149] II. Prepare the negative electrode sheet.

[0150] Dissolve the negative electrode active material hard carbon, the conductive agent Super-P, the thickening agent CMC, and the 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 steps such as drying, cold pressing, trimming, slicing, and strip cutting, make the negative electrode sheet.

[0151] III. Prepare an electrode assembly and place the electrode assembly in a housing.

[0152] Assemble the positive electrode sheet, negative electrode sheet and separator into an electrode assembly by winding process. Package the electrode assembly with an aluminum-plastic composite film and perform vacuum drying.

[0153] IV. Perform a first electrolyte injection treatment on the electrode assembly placed in the housing. The first electrolyte used in the first electrolyte injection treatment has the corresponding formulations for each comparative ratio and each example in Table 1 respectively.

[0154] V. Soak for 10 - 15 h at 40°C - 50°C and soak for 20 - 30 h at room temperature, then perform pre-charging. The pre-charging is carried out with a current of 0.05C. 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.

[0155] In the pre-charging stage, add a gas extraction process so that the gas generated by the pre-charging treatment can be fully discharged. The vacuum degree of the housing in the gas extraction process is maintained within the range of -90 Kpa to -80 Kpa.

[0156] VI. Perform a second electrolyte injection treatment on the electrode assembly placed in the housing. The second electrolyte used in the second electrolyte injection treatment has the corresponding formulations for each comparative ratio and each experimental example in Table 1 respectively.

[0157] 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 3 Ah.

[0158] Test the performance of the sodium-ion secondary batteries prepared in each example and each comparative ratio. The test process is as follows:

[0159] 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-discharge tests and record the discharge capacity of the 1000th cycle. The capacity retention rate = (the discharge capacity of the 1000th cycle / the first discharge capacity) × 100%, and record the cycle retention rate of the 500th cycle.

[0160] 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-discharge tests and record the discharge capacity of the 1000th cycle. The capacity retention rate = (the discharge capacity of the 1000th cycle / the first discharge capacity) × 100%, and record the cycle retention rate of the 500th cycle.

[0161] 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 is 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 is 0.05C. Store the fully charged battery at 45°C for 15 days. After that, place it 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.

[0162] These test methods are designed to comprehensively evaluate the performance of sodium-ion secondary batteries prepared in each example and comparative example.

[0163] Table 2

[0164]

[0165] Table 2 shows the performance test results of sodium-ion secondary batteries prepared in each comparative example and each example.

[0166] Referring to Table 2, the following comparative analysis is as follows:

[0167] 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 production. 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 formed SEI film.

[0168] 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 compactness and stability of the SEI film, thereby improving the cycling performance of sodium-ion secondary batteries.

[0169] 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.

[0170] 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.

[0171] 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, form a stable complex, and reduce side reactions and gas generation phenomena.

[0172] The comparison between Experimental Example 1, Experimental Example 2, Experimental Example 3 and Experimental Example 4 and the above comparative examples shows that the first electrolyte contains FEC, TMSP and PS at a first concentration, the second electrolyte contains PS at 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.

[0173] 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.

[0174] 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, Comprising: Providing an electrode core and placing the electrode core in a housing. The electrode core 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 core. Alternatively, the positive electrode plate, the separator, and the negative electrode plate are stacked and wound to form the electrode core. Wherein, 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-type positive electrode material, or a Prussian blue-type positive electrode material, and the material of the negative active material layer includes a metal compound, a carbon-based material, an alloy-type material, or a non-metal simple substance; Performing a first liquid injection treatment on the electrode core placed in the housing with a first electrolyte. Wherein, by mass percentage, 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; 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 core placed in the housing with a second electrolyte. Wherein, by mass percentage, 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, a sodium salt, and a functional additive not exceeding 2 wt%. The functional additive includes a film-forming additive or a gas generation inhibitor; After injecting the second electrolyte, performing a standing treatment.

2. The preparation method of the sodium ion secondary battery according to claim 1, wherein 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, 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.

3. The preparation method of the sodium-ion secondary battery according to claim 1, wherein 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.

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 preparation method of the sodium ion secondary battery according to claim 4, wherein 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, crotononitrile, 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; the nitrile compounds containing a trimethylsilyl group 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 and acid removal additive is a carbodiimide compound, a silazane compound, an amine compound, methylsulfonyl chloride, tetrahydrofuran or trifluoroacetic anhydride.

7. The preparation method of the sodium-ion secondary battery according to claim 6, wherein The carbodiimide compounds include dicyclohexylcarbodiimide or diisopropylcarbodiimide; the silazane compounds include hexamethyldisilazane, heptamethyldisilazane or trimethylchlorosilane; the amine compounds include triethylamine or diethylamine.

8. The preparation method of the sodium ion secondary battery according to claim 1, wherein, 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.

9. The preparation method of the sodium-ion secondary battery according to claim 1, wherein, The infiltration treatment includes a first infiltration stage and a second infiltration stage carried out in sequence, wherein 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; Among them, 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.

10. The preparation method of the sodium-ion secondary battery according to claim 1, wherein, 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.

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