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

Technical Field

[0001] The embodiments of the present application relate to the field of energy storage technology, and in particular to a method for preparing a sodium ion secondary battery. Background Art

[0002] At present, lithium-ion batteries occupy the core position of power batteries. At the same time, lithium-ion batteries also face great challenges, such as the increasing shortage of lithium resources, rising material prices, and 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 several unique advantages. First, the reserves of sodium in the earth's crust are abundant, far exceeding lithium, so the resource supply of sodium-ion batteries is more sufficient. Secondly, 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 operate 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, 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 of the market. Summary of the invention

[0005] The embodiment of the present application provides a method for preparing a sodium ion secondary battery, which is at least beneficial to improving the problems of the sodium ion battery being prone to gas generation under high temperature conditions, having high impedance and poor cycle performance.

[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, comprising: 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 is located between the positive electrode sheet and the negative electrode sheet, and the positive electrode sheet, the separator and the negative electrode sheet are stacked to form the battery cell, or the positive electrode sheet, the separator and the negative electrode sheet are stacked and wound to form the battery cell, wherein the positive electrode sheet comprises a positive current collector and a positive electrode current collector arranged on the positive electrode sheet. The positive electrode active material layer on the negative electrode current collector, the negative electrode plate includes a negative electrode current collector and a negative electrode active material layer arranged on the negative electrode current collector, the material of the positive electrode active material layer includes 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 includes a metal compound, a carbon-based material, an alloy material or a non-metallic element; the first electrolyte is used to perform a first liquid injection treatment on the battery cell placed in the shell, wherein, in terms of 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 film-forming additives and sodium salt; after injecting the first electrolyte, performing an infiltration treatment and a pre-charging treatment in sequence; using a second electrolyte to perform a second liquid injection treatment on the battery cell placed in the shell, wherein, in terms of mass percentage, the second electrolyte comprises: 5wt%~20wt% of 1, 3-propane sultone, 3wt%~30wt% of nitrile compounds, 0.005wt%~30wt% of water-removing and acid-removing additives and sodium salt; after injecting the second electrolyte, performing a formation static treatment.

[0007] In some embodiments, the ratio 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 in the range of 5 to 20, satisfying: 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.

[0008] In some embodiments, 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.

[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 sulfonyl-containing nitrile compound, or a trimethylsilyl-containing nitrile compound.

[0010] In some embodiments, the dinitrile compound includes succinonitrile, adiponitrile, glutaronitrile, suberonitrile or sebacate; the trinitrile compound includes 1, 3, 6-hexanetricarbonitrile or 1, 3, 5-pentanetricarbonitrile; the unsaturated nitrile compound includes acrylonitrile, crotononitrile, trans-butene dinitrile or trans-hexene dinitrile; the nitrile compound containing an aromatic ring includes p-fluorobenzonitrile, p-methylbenzonitrile or tricyanobenzene; the alkoxy nitrile compound includes ethylene glycol bis(propionitrile) ether or 1, 2, 3-tris(cyanoethoxy)propane; the sulfonyl-containing nitrile compound includes bis(cyanoethyl)sulfone; the trimethylsilyl-containing nitrile compound includes 3-(trimethylsilyloxy)propionitrile.

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

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

[0013] In some embodiments, 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.

[0014] In some embodiments, the infiltration treatment includes a first infiltration stage and a second infiltration stage 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; wherein the temperature of the first infiltration stage is 40°C~50°C, and the duration of the first infiltration stage is 10h~15h; the temperature of the second infiltration stage is 15°C~25°C, and the duration of the second infiltration stage is 20h~30h.

[0015] In some embodiments, 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.

[0016] The technical solution provided by the embodiments of the present application has at least the following advantages: In the technical solution of the method for preparing a sodium ion secondary battery provided in the embodiment of the present application, after the battery cell is placed in the shell, a two-liquid injection scheme including a first liquid injection treatment and a second liquid injection treatment is adopted. The electrolyte provided by the first liquid injection treatment has 0.1wt%~2wt%TMSP (i.e., tris(trimethylsilyl)phosphate), 0.1wt%~2wt%FEC (i.e., fluoroethylene carbonate) and 0.5wt%~2wt%PS (i.e., 1,3-propane sultone) in addition to sodium salt and film-forming additives. After the first liquid injection treatment, a pre-charging treatment is performed. The standard decomposition potential of FEC is lower than that of PS, and a stable SEI film is preferentially formed in the pre-charging treatment, while the decomposition potential of PS is between FEC and TMSP. PS can provide protection for the SEI film first formed based on FEC, thereby facilitating the formation of a low-impedance, dense and uniform solid electrolyte interface film (i.e., SEI film) on the negative electrode, which helps to improve battery performance and life in high-temperature storage and cycling. As a typical positive electrode film-forming additive, TMSP is mainly used to protect the positive electrode, forming a low-impedance and stable positive electrode interface during the pre-charging process, which is conducive to the improvement of cycle performance. After the pre-charging process, the second liquid injection process is carried out, and the second electrolyte provided by the second liquid injection process has 5wt%~20wt% of 1, 3-propane sultone, 3wt%~30wt% of nitrile compounds, 0.005wt%~30wt% of water-removing and acid-removing additives and sodium salts. Since the second electrolyte has a relatively high concentration of PS, the high concentration of PS can effectively repair the SEI film and improve the high temperature performance and cycle life of the sodium ion secondary battery; and nitrile substances can withstand high temperatures and inhibit gas production, and water-removing and acid-removing additives can remove moisture and acid in the electrolyte, reduce the amount of gas generated by the reaction of these impurities with sodium salts, thereby effectively inhibiting gas production, and can also reduce the internal resistance growth rate during long-term cycles. In summary, the technical solution provided in the embodiments of the present application is conducive to achieving an effective balance between low SEI membrane interface impedance, low gas production and long cycle stability of sodium ion secondary batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] One or more embodiments are exemplarily illustrated by the pictures in the corresponding drawings. These exemplified descriptions do not constitute a limitation on the embodiments. Unless otherwise specified, the pictures in the drawings do not constitute a scale limitation. In order to more clearly illustrate the embodiments of the present application or the technical solutions in the traditional technology, the drawings required for use in the embodiments are briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0018] Figure 1A schematic diagram of a process flow corresponding to a method for preparing a sodium ion secondary battery provided in an embodiment of the present application; Figure 2 A schematic diagram of the principle structure of the sodium ion secondary battery provided in an embodiment of the present application after the second liquid injection treatment. DETAILED DESCRIPTION

[0019] As can be seen from the background technology, lithium iron phosphate lithium-ion batteries have been widely used in the field of energy storage 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 easy generation of gas under high temperature conditions and poor cycle performance.

[0020] The embodiment of the present application provides a method for preparing a sodium ion secondary battery, and provides a scheme based on twice injecting electrolytes with different formulations, using 1,3-propane sultone and fluoroethylene carbonate as basic additives, and by regulating the amount of the basic additives in the first injected electrolyte (i.e., the first electrolyte) and adding a film-forming additive, a SEI film (Solid Electrolyte Interface) with small polarization, small internal resistance and uniform distribution can be formed after the first injection treatment. The 1,3-propane sultone provided in the second injection treatment can be used as an additive required for subsequent long-term circulation, and is used to accurately repair the SEI film. The nitrile solvent and water-removing and acid-removing additive provided by the second injection treatment are conducive to inhibiting gas production, removing water and acid in the electrolyte, thereby reducing the reaction of these impurities with other components to generate gas, thereby effectively inhibiting the gas production phenomenon during long-term circulation or under high temperature conditions.

[0021] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is more than two, unless otherwise clearly and specifically defined.

[0022] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0023] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can represent: A exists, A and B exist at the same time, and B exists. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0024] In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0025] In the description of the embodiments of the present application, when a component "includes" another component, unless otherwise specified, other components are not excluded, and other components may be further 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" the other component (i.e., located on the surface of the other component with no other components between them), or another component may exist in between. In addition, when a component such as a layer, film, region, or plate is "directly located on" another component, or when a component such as a layer, film, region, or plate is located on the surface of another component, it means that no other components are located in between.

[0026] The terms used in the description of the various embodiments described herein are only used to describe specific embodiments and are not intended to be limiting. As used in the description of the various embodiments described and in 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.

[0027] The following will describe the various embodiments of the present application in detail with reference to the accompanying drawings. However, it will be appreciated by those skilled in the art that in the various embodiments of the present application, many technical details are provided in order to enable 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 solution claimed in the present application can be implemented.

[0028] Figure 1 A schematic diagram of the process flow corresponding to the method for preparing a sodium ion secondary battery provided in an embodiment of the present application.

[0029] It should be noted that the sodium ion secondary battery mentioned in the embodiments of the present application is a sodium ion battery.

[0030] refer to Figure 1 , the preparation method of the sodium ion secondary battery comprises the following steps: A battery cell is provided and placed in a shell, wherein the battery cell includes a positive electrode sheet, a separator and a negative electrode sheet, wherein the separator is located between the positive electrode sheet and the negative electrode sheet, and the positive electrode sheet, the separator and the negative electrode sheet are stacked to form a battery cell, or the positive electrode sheet, the separator and the negative electrode sheet are stacked and wound to form a battery cell, wherein the positive electrode sheet includes a positive current collector and a positive active material layer arranged on the positive current collector, and the negative electrode sheet includes a negative current collector and a negative active material layer arranged on the negative current collector.

[0031] The battery cell may be a stacked structure formed by stacking a negative electrode sheet, a separator and a positive electrode sheet in sequence, or a wound structure formed by stacking a negative electrode sheet, a separator and a positive electrode sheet in sequence and then winding them.

[0032] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector and containing a positive electrode active material. For example, the positive electrode current collector includes two opposite surfaces, and the positive electrode active material layer is disposed on any one or both of the two opposite surfaces of the positive electrode current collector in its thickness direction.

[0033] The positive electrode active material may include a layered oxide positive electrode material, a polyanion positive electrode material, a Prussian blue positive electrode material, an organic positive electrode material or a conversion positive electrode material. In other words, the material of the positive electrode active material layer includes an oxide positive electrode material, a polyanion positive electrode material or a Prussian blue positive electrode material.

[0034] Among them, the chemical formula of the layered oxide positive electrode material is Na x M1O2, wherein M1 is one or more transition metal elements such as Fe, Mn, Ni, Co, Cr, etc., wherein 0<x≤1. For example, the layered oxide positive electrode material can be NaFeO2, NaCrO2, Na 0.5 CoO2, or NaNiO2, etc.

[0035] The general formula of polyanion cathode materials is Na x M2 y (X a O b ) z Z w , 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 more of Si, S, P, As, B, Mo, W, Ge, and Z is F or OH. For example, the polyanion positive electrode material can be sodium phosphate, sodium sulfate or sodium pyrosulfate. More specifically, the polyanion positive electrode material can be sodium iron phosphate (NaFePO4) or sodium iron sulfate (Na2Fe(SO4)2).

[0036] Prussian blue cathode materials have a general structural formula A x M1[M2(CN)6] 1-y ·V y ·nH2O, 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. In sodium ion secondary batteries, A is a sodium ion; M1 and M2 are both transition metal ions, such as Mn, Fe, Co, Ni, Cu, Zn or Cr; H2O represents interstitial water (or zeolite water) and coordinated water.

[0037] 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. Prussian blue-based positive electrode 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 with different transition metal ions.

[0038] The positive electrode active material layer may further 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 adhere the positive electrode active material and the binder to the positive electrode current collector.

[0039] As an example, the conductive agent may be one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers; the binder may 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).

[0040] The positive electrode current collector may be made of metal foil, carbon-coated metal foil or porous metal plate. In this embodiment, the positive electrode current collector is aluminum foil.

[0041] In some examples, the process steps for preparing a positive electrode sheet may include: dispersing a positive electrode active material, a conductive agent, and a binder in a solvent to form a positive electrode slurry; coating the positive electrode slurry on the surface of a positive electrode collector, and obtaining a positive electrode sheet after drying and cold pressing.

[0042] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed 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 disposed on any one or both of the two opposite surfaces of the negative electrode current collector in its thickness direction.

[0043] The negative electrode active material may be a metal compound, a carbon-based material, an alloy material or a non-metallic single substance. In other words, the material of the negative electrode active material layer includes a metal compound, a carbon-based material, an alloy material or a non-metallic single substance.

[0044] 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 properties and cycle stability; carbon nanotubes have high conductivity and large specific surface area, which helps to improve battery performance; expanded graphite and graphene are widely used in negative electrode sheets due to their excellent conductivity and mechanical properties.

[0045] The metal compound may be lithium titanate, which may provide good cycle life and high safety.

[0046] The negative electrode active material layer may further 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.

[0047] The negative electrode current collector may 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 may also be aluminum foil.

[0048] In some examples, the process steps for preparing a negative electrode sheet may include: dispersing a negative electrode active material, a conductive agent, and a binder in a solvent to form a negative electrode slurry; coating the negative electrode slurry on the surface of a negative electrode collector, and obtaining a negative electrode sheet after drying and cold pressing.

[0049] The separator can be made of one or more materials, including polypropylene (PP), polyethylene (PE), ceramic coating separator and non-woven separator. PP and PE porous separators have good mechanical strength and chemical stability; ceramic coating separators are coated with ceramic materials on the basis of PP or PE separators, which improves the high temperature resistance and safety of the separator; non-woven separators are composed of polymer fibers and have excellent electrolyte wettability and ion conductivity.

[0050] Furthermore, the isolation film may be a single-layer film or a multi-layer film.

[0051] In this embodiment, after the positive electrode sheet, the negative electrode sheet and the separator are prepared, the positive electrode sheet, the separator and the negative electrode sheet are stacked and wound to form a battery cell. In other embodiments, the battery cell may also be a stacked structure as described above.

[0052] In this embodiment, the sodium ion secondary battery is a pouch cell, and accordingly, the shell is a soft shell. In some examples, the shell can be an aluminum-plastic film.

[0053] In a specific example, the capacity of the sodium ion secondary battery prepared in this embodiment can be 3Ah, where Ah represents ampere-hour.

[0054] It should be noted that the embodiments of the present application do not limit the type of sodium ion secondary batteries. In other embodiments, the sodium ion secondary batteries may also be hard-pack batteries (Prismatic and Cylindrical Cell), which may be square sodium ion batteries or cylindrical sodium ion batteries. Accordingly, the shell is a metal shell or a shell made of other hard materials.

[0055] The following is a detailed description of a sodium ion secondary battery that is a soft-pack battery and a shell that is an aluminum-plastic film.

[0056] Placing the battery cell in the shell includes: after preparing the aforementioned battery cell, completely wrapping the battery cell with an aluminum-plastic film, then sealing the aluminum-plastic film and retaining an opening as an electrolyte injection port for subsequent first injection treatment.

[0057] Continue to refer Figure 1 , a first electrolyte is used to perform a first liquid injection treatment on the battery cell placed in the shell, wherein the first electrolyte includes, by mass percentage: 0.5wt%~2wt% of 1, 3-propane sultone (PS for short), 0.1wt%~2wt% of tris(trimethylsilyl) phosphate (TMSP for short), 0.1wt%~2wt% of fluoroethylene carbonate (FEC for short), 0.01wt%~5wt% of film-forming additives and sodium salt.

[0058] The electrolyte injected in the first liquid injection process is used to form a stable SEI film, which has the characteristics of low impedance, good density and uniformity.

[0059] In terms of mass percentage, the content of sodium salt in the first electrolyte can be 5wt%~15wt%.

[0060] In the first electrolyte, in addition to PS, FEC, TMSP, sodium salt and film-forming additives, the balance is non-aqueous organic solvent.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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% sodium salt.

[0065] The sodium salt may be 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 (NaOTF), sodium bis(fluorosulfonyl)imide (NaBF4), sodium bis(oxalato)borate (NaBOB), sodium bis(oxalato)borate (NaDFOB), sodium trifluoromethanesulfonate (NaOTf), sodium bis(fluorosulfonyl)imide (NaBF4), sodium bis(fluorosulfonyl)imide (NaBF4), sodium bis(fluorosulfonyl)imide (NaBF4), sodium bis(fluorosulfonyl)imide (NaBF4), sodium bis(fluorosulfonyl)imide (NaBF4), sodium bis(oxalato)borate (NaBF4), sodium bis(oxalato)borate (NaBF4), sodium bis(oxalato)borate (NaBF4), sodium bis(fluorosulfonyl)imide ... One or more of NaFSI and sodium bis(trifluoromethanesulfonyl)imide (NaTf2N).

[0066] In some examples, the first electrolyte may contain 0.01wt%, 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt% or 5wt% of the film-forming additive.

[0067] The film-forming additives may include, but are not limited to, one or more of the following combinations: sodium difluorophosphate (NaDFP), methane disulfonate (MMDS), ethylene sulfate (DTD), tris (trimethylsilyl) borate (TMSB), 2-propynyl-1-yl 1H-imidazole-1-carboxylate, triallyl isocyanurate (TAIC), 2,4-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), Sodiumdifluorooxalatoborate (NaDFOB), Allyl methanesulfonate (AMS) and Propane disulfonate (DTD).

[0068] In a specific example, the first electrolyte may include 0.5 wt% to 3 wt% of a film-forming additive, measured by mass percentage. The selection and dosage of these film-forming additives are intended to optimize the overall performance of the sodium-ion secondary battery, especially to improve the quality of the SEI film, thereby improving the cycle stability and capacity retention of the sodium-ion secondary battery.

[0069] 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), methyl butyrate (MB), ethyl acetate (EA), ethyl propionate (EP), and propyl propionate (PP).

[0070] In a specific example, the non-aqueous organic solvent can be a mixture of two or more selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), propylene carbonate (PC), and ethylene carbonate (EC) in any proportion.

[0071] In addition, cyclic carboxylic acid esters, linear carboxylic acid esters, ether compounds or sulfone compounds may also be added to the first electrolyte to optimize the performance and stability of the first electrolyte.

[0072] Continue to refer Figure 1 After injecting the first electrolyte, the wetting treatment and the pre-charging treatment are carried out in sequence.

[0073] 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 FEC and TMSP, and it can provide moderate interface protection for the SEI film, enhance the compactness and stability of the SEI film, and help improve the performance and life of sodium-ion secondary batteries during high-temperature storage and cycling. As a typical positive electrode film-forming additive, TMSP is mainly used to protect the positive electrode, forming a low-impedance and stable positive electrode protection layer, namely the CEI (Cathode electrolyte interphase, positive electrode electrolyte interface layer) film, which is beneficial to the improvement of cycle performance.

[0074] After the infiltration treatment and pre-charging treatment, the combined action of FEC and PS generates a stable SEI film. Since the PS concentration in the first electrolyte is relatively low, the problem of a sharp increase in SEI film impedance caused by the use of a large amount of PS is avoided, so that the formed SEI film has a smaller internal resistance (i.e. impedance).

[0075] The infiltration process may include a first infiltration stage and a second infiltration stage performed sequentially, wherein the temperature of the first infiltration stage is higher than the temperature of the second infiltration stage, and the duration of the first infiltration stage is shorter than the duration of the second infiltration stage.

[0076] In some examples, the temperature of the first infiltration stage can be 40°C~50°C, for example, 42°C, 44°C, 45°C, 47°C or 49°C, and the duration of the first infiltration stage can be 10h~15h, for example, 11h, 12h, 13h or 14h; the temperature of the second infiltration stage can be 15°C~25°C, for example, 16°C, 18°C, 20°C, 21°C or 24°C, and the duration of the second infiltration stage can be 20h~30h, for example, 22h, 24h, 26h or 28h.

[0077] In the above-mentioned infiltration treatment process, the wettability of the first electrolyte to the battery cell can be improved, so that the battery cell can 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 pores of 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, thereby increasing the contact area and reaction activity of the first electrolyte with the positive electrode plate or the negative electrode plate, which is beneficial to further promote the subsequent formation of a stable SEI film on the surface of the negative electrode plate.

[0078] The pre-charging process is a formation process, that is, in this embodiment, before the subsequent second liquid injection process, the SEI film and the CEI film in the sodium ion secondary battery have been formed.

[0079] Before the pre-charging process, the electrolyte injection port on the shell can also be sealed, and the seal has a space margin. During the pre-charging process, the components in the first electrolyte react to produce by-products such as gas, and the gas will escape to the seal with space margin, so that the seal is filled with gas to form an air bag.

[0080] The process parameters of the pre-charge treatment can be: constant current charging at a charging rate of 0.04C~0.06C, and the pre-charge cut-off voltage is 3.4V. The charging rate is a measure of 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 expressed by C, and the charging rate C=charging current (A) / battery rated capacity (Ah). The larger the charging rate, the faster the battery charges and discharges.

[0081] In some examples, the pre-charge process may be performed at a constant current charge of 0.045C, 0.05C, or 0.055C.

[0082] After the pre-charging process, the preparation method may further include: standing at a third temperature for 10 hours to 14 hours, the third temperature being 40° C. to 50° C. For example, the third temperature may be 42° C., 44° C., 45° C., 47° C. or 49° C., and the standing time may be 11 hours, 12 hours or 13 hours. The standing may allow the battery cell to discharge to a certain extent.

[0083] During the pre-charging process, the inside of the shell can also be evacuated to discharge the gas by-products generated during the pre-charging process, so that the gas can be fully discharged to avoid gas remaining inside the shell, thereby further reducing the gas content remaining in the shell.

[0084] In some examples, as described above, before the pre-charging process, the electrolyte injection port on the shell can be sealed, and the seal has a space margin. During the pre-charging process, the components in the first electrolyte react with each other to produce byproducts such as gas, and the gas will escape to the seal with a space margin, so that the seal is filled with gas to form an air bag. Before the second injection process, the shell can be sealed again to completely separate the formed air bag from the battery cell and all electrolytes. After resealing, the air bag is cut and removed from the shell. This process is called air extraction.

[0085] In other examples, the exhaust process may also be as follows: before the pre-charging process, the electrolyte filling port on the shell may be sealed, and an exhaust pipe and an exhaust pump may be additionally provided, and the exhaust pipe enters the interior of the shell through the sealed portion; driven by the exhaust pump, the exhaust pipe extracts the gas inside the shell and discharges the gas from the inside of the shell.

[0086] Continue to refer Figure 1 The second electrolyte is used to perform a second liquid injection treatment on the battery cell placed in the shell, wherein the second electrolyte includes, by mass percentage: 5wt%~20wt% of 1,3-propane sultone, 3wt%~30wt% of nitrile compounds, 0.005wt%~30wt% of water-removing and acid-removing additives and sodium salt.

[0087] Specifically, during the aforementioned pre-charging process, the shell is sealed, and before the second liquid injection process is performed, the shell is opened to form an electrolyte injection port for the second liquid injection process again.

[0088] The PS concentration in the second electrolyte provided by the second injection treatment is greater than the PS concentration in the first electrolyte provided by the first injection treatment. The second electrolyte is used to further repair and stabilize the SEI film and the CEI film, for example, to provide the PS and sodium salt required for long-term circulation. The PS can repair the SEI film during the long-term circulation process, and the sodium salt provided in the second electrolyte can be used as a supplement to the sodium salt in the electrolyte.

[0089] If the amount of the first electrolyte provided by the first liquid injection treatment is insufficient, the battery cell may be difficult to fully soak, which may easily cause the problem of sodium ion precipitation during the pre-charging treatment, i.e. after formation; if the amount of the second electrolyte provided by the second liquid injection treatment is too large, the corresponding second electrolyte contains relatively more additives, which may cause uneven concentration distribution of the additives in the shell.

[0090] 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, the battery cell can be fully infiltrated in the first injection treatment, and the problem of sodium ion precipitation will not be caused, and after the second injection treatment, the concentration of the additive in the shell is evenly distributed. The total electrolyte is the electrolyte composed of the first electrolyte and the second electrolyte, that is, the mass of the first electrolyte accounts for 65%~90% of the sum of the mass of the first electrolyte and the second electrolyte.

[0091] In a specific example, the mass ratio of the first electrolyte to the second electrolyte may be 85:15.

[0092] The nitrile compounds in the second electrolyte can withstand high temperatures and inhibit gas production, while the high concentration of PS effectively repairs the formed SEI film, improving the high temperature performance and cycle life of the sodium ion secondary battery. The water and acid removal additives can remove water and acid from the electrolyte, reduce the gas content generated by the reaction of these impurities with sodium salts, and effectively inhibit gas production.

[0093] 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, 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. In this way, the total amount of 1, 3-propane sultone in the electrolyte after the second injection is moderate, and the amount of 1, 3-propane sultone provided in the first injection treatment can significantly enhance the compactness and stability of the SEI film, while the amount of 1, 3-propane sultone provided in the second injection treatment can ensure that the SEI film can be effectively repaired during a long cycle.

[0094] 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, and correspondingly, the mass of 1,3-propane sultone in the second electrolyte accounts for 58%, 56%, 55%, 53% or 51% of the total mass, and 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.

[0095] In terms of mass percentage, the mass percentage of the sodium salt in the second electrolyte may be 5wt% to 15wt%.

[0096] In addition, the second electrolyte further contains a non-aqueous organic solvent, the material of the sodium salt in the first electrolyte and the second electrolyte can be the same, and the material of the non-aqueous organic solvent in the first electrolyte and the second electrolyte can also be the same.

[0097] In some examples, the second electrolyte may contain 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, 15wt%, 16wt%, 17wt%, 18wt%, 19wt% or 20wt% of 1,3-propane sultone.

[0098] In some examples, the second 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%, 12.5wt%, 13wt%, 13.5wt%, 14wt%, 14.5wt% or 15wt% sodium salt.

[0099] In some examples, the second electrolyte may contain 0.005wt%, 0.01wt%, 0.02wt%, 0.03wt%, 0.04wt%, 0.05wt%, 0.06wt%, 0.07wt%, 0.08wt%, 0.09wt%, 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1wt%, 1.5wt%, 2wt%. %, 2.5wt%, 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, 15wt%, 16wt%, 17wt%, 18wt%, 19wt%, 20wt%, 21wt%, 22wt%, 23wt%, 24wt%, 25wt%, 26wt%, 27wt%, 28wt%, 29wt% or 30wt% of the water-removing and acid-removing additive.

[0100] In some examples, the second electrolyte may contain 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, 15wt%, 16wt%, 17wt%, 18wt%, 19wt%, 20wt%, 21wt%, 22wt%, 23wt%, 24wt%, 25wt%, 26wt%, 27wt%, 28wt%, 29wt%, or 30wt% of a nitrile compound.

[0101] In some specific examples, the nitrile compound accounts for 10wt% to 25wt% of the second electrolyte, and the water-removing and acid-removing additive accounts for 5wt% to 10wt% of the second electrolyte. The mass percentages of the nitrile compound and the water-removing and acid-removing additive are within the corresponding ranges, which is beneficial to further reduce the initial internal resistance of the sodium ion secondary battery, and further reduce 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.

[0102] Regarding the materials of the sodium salt and the non-aqueous organic solvent, reference may be made to the corresponding description of the aforementioned first electrolyte, which will not be elaborated here.

[0103] Nitrile compounds are nitrile substances containing C≡N bonds, for example, nitrile compounds can be dinitrile compounds, trinitrile compounds, unsaturated nitrile compounds, nitrile compounds containing aromatic rings, alkoxy nitrile compounds, nitrile compounds containing sulfonyl groups or nitrile compounds containing trimethylsilyl groups. These nitrile compounds can play an important role in the electrolyte of sodium ion secondary batteries through their unique chemical structure and properties, improve high temperature storage and cycle performance, reduce gas production and extend the life of sodium ion secondary batteries.

[0104] The dinitrile compounds include succinonitrile (SN), adiponitrile (ADN), glutaronitrile (GN), suberonitrile (SNB) or sebaconitrile (SCN); the trinitrile compounds include 1,3,6-hexanetricarbonitrile (HTCN) or 1,3,5-pentanetricarbonitrile (PTN); the unsaturated nitrile compounds include acrylonitrile (AN), crotononitrile (CN), trans-butenedinitrile (TBN) or trans-hexenedinitrile (1,4-dicyanobutene, DCB); the aromatic ring-containing nitrile compounds include 4-fluorobenzonitrile (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 (1,2,3-Tris(cyanoethoxy)propane, TCEP); sulfonyl-containing nitrile compounds include bis(cyanoethyl)sulfone (Sulfolanedinitrile, SDPN); trimethylsilyl-containing nitrile compounds include 3-(trimethylsilyloxy)propionitrile (3-(Trimethylsilyloxy)propionitrile, TMSOPN).

[0105] The water-removing and acid-removing additives may be carbodiimide compounds, silazane compounds, amine compounds, methylsulfonyl chloride (MSC), tetrahydrofuran (THF) or trifluoroacetic anhydride (TFAA).

[0106] Carbodiimide compounds include dicyclohexylcarbodiimide (DCC) or diisopropylcarbodiimide (DIC); silazane compounds include hexamethyldisilazane (HMDS), heptamethyldisilazane (H7DMS) or trimethylchlorosilane (TMCS); amine compounds include triethylamine (TEA) or diethylamine (DEA).

[0107] In addition, the second electrolyte may further include, by weight percentage, no more than 2 wt% of a functional additive, the functional additive including a film-forming additive or a gas generation inhibiting additive. For example, the second electrolyte may include 1.5 wt%, 1 wt%, 0.8 wt% or 0.4 wt% of a functional additive.

[0108] The functional additives include one or more of the following materials: sodium difluorooxalatoborate, sodium difluorophosphate, sodium dioxalatoborate, methylene disulfonate, vinyl sulfate, triallyl phosphate, tripropynyl phosphate, 1,3-propylene sultone, fluorobenzene, trifluoroethoxyethylene carbonate, 2-methylmaleic anhydride, difluoroethylene carbonate, succinic anhydride, tris(trimethylsilyl)phosphate, vinyl sulfite or hexamethylene diisocyanate.

[0109] It is understandable that the selection and dosage of the above-mentioned functional additives in the embodiments of the present application can be flexibly adjusted as needed.

[0110] Due to the high bond energy and good oxidation resistance of cyanide, nitrile compounds can coordinate with high-valent transition metal ions on the surface of the positive electrode under high temperature conditions to form stable complexes, reduce the catalytic decomposition of the electrode to the electrolyte, inhibit side reactions, and reduce the possibility of gas production. Nitrile compounds can also form a cyanide-containing polymer film on the surface of the negative electrode, further improving the cycle stability and high-temperature performance of sodium-ion secondary batteries.

[0111] In addition, the second electrolyte has a higher concentration of PS, which can effectively repair the formed SEI film and enhance the high temperature performance and cycle life of the sodium ion secondary battery.

[0112] Water-removing and acid-removing additives such as dicyclohexylcarbodiimide (DCC) and diisopropylcarbodiimide (DIC) react with water and acidic substances to generate amides and neutral by-products, reducing the amount of gas generated by the reaction of these impurities with sodium salts, thereby effectively inhibiting gas production and improving the stability of sodium-ion secondary batteries during high-temperature storage and cycling.

[0113] An electrolyte combination with different formulations is used for two injections, that is, the first electrolyte is used for the first injection treatment, and then the second electrolyte is used for the second 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 gas production problems under high or low temperature conditions, and has lower impedance.

[0114] The first electrolyte has a relatively low content of 0.5wt%~2wt% of 1, 3-propane sultone, and the relatively low content of 0.5wt%~2wt% of 1, 3-propane sultone forms a dense and low-impedance effective SEI film and CEI film during the pre-charge process with tris(trimethylsilyl)phosphate, fluoroethylene carbonate and film-forming additives. This avoids the problem of incomplete reaction and excessive impedance caused by adding a large amount of additives at one time, and overcomes the defect that the generation order cannot be artificially controlled due to the different film-forming potentials of different additives, so that a relatively ideal SEI film / CEI film can be selectively designed.

[0115] The second electrolyte contains nitrile solvents and effective functional additives for moisture problems (i.e., water-removing and acid-removing additives), which synergistically solve the long-term gas production problem. When the second electrolyte has generated an effective SEI film / CEI film, it generally exists in the sodium-ion secondary battery in a free state. By adding nitrile solvents and effective functional additives for moisture problems, the long-term gas production problem of the sodium-ion secondary battery can be repaired and improved in a targeted manner, and the 1, 3-propane sultone in the second electrolyte can also repair the SEI film in a targeted manner, thereby improving the long-cycle performance of the sodium-ion secondary battery.

[0116] In addition, due to the high bond energy and good oxidation resistance of the cyanide group, nitrile compounds can coordinate with high-valent transition metal ions on the surface of the positive electrode under high temperature conditions to form a stable complex, reduce the catalytic decomposition of the electrode to the electrolyte, inhibit side reactions, and reduce the possibility of gas production. In addition, nitrile compounds form a cyanide-containing polymer film on the surface of the negative electrode, further improving the cycle stability and high-temperature performance of the sodium-ion secondary battery.

[0117] The second electrolyte provides a higher concentration of PS, and the high concentration of PS effectively repairs the SEI film formed after the first liquid injection treatment, thereby enhancing the high temperature performance and cycle life of the sodium ion secondary battery.

[0118] Water-removing and acid-removing additives such as dicyclohexylcarbodiimide (DCC) and diisopropylcarbodiimide (DIC) react with water and acidic substances to generate amides 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 production and improving the stability of sodium-ion secondary batteries during high-temperature storage and cycling.

[0119] Continue to refer Figure 1 After injecting the second electrolyte, the mixture is left to stand.

[0120] After the second electrolyte is injected, the electrolyte injection hole of the shell is sealed.

[0121] In some examples, the process parameters of the static treatment may include: static treatment for 10 hours to 15 hours at a temperature of 40° C. to 50° C. For example, the process temperature of the static treatment may be 42° C., 45° C., or 48° C.

[0122] After the static treatment is completed, the capacity can be sorted. The full name of capacity sorting is capacity sorting. The capacity of sodium ion secondary batteries produced on the same production line will be different. The qualified batteries are screened out through capacity testing. This process is called capacity sorting.

[0123] From the above analysis, it can be seen that in this embodiment, the overall stability of the SEI film and the performance of the sodium ion secondary battery can be significantly improved by using the advantages of FEC, PS, TMSP, nitrile substances and water and acid removal additives through two-step injection technology. The FEC provided by the first injection treatment is used to provide the initial SEI film formation to ensure low temperature and energy efficiency performance; the PS provided by the first injection treatment is used to enhance the density and stability of the SEI film, especially to provide additional protection under high temperature conditions; the TMSP provided by the first injection treatment forms a stable protective layer on the positive electrode sheet, reducing impedance and enhancing cycle performance.

[0124] The synergistic effect of the nitrile substances and water and acid removal additives provided by the second injection treatment effectively inhibits the gas production of sodium ion secondary batteries and improves high-temperature cycle performance. The PS and sodium salt provided by the second injection treatment are used to effectively repair the formed SEI film in long-term cycles, improving the high-temperature performance and cycle life of the battery cell.

[0125] In other words, the embodiments of the present application not only improve the initial performance of the sodium-ion secondary battery, but also maintain excellent cycle stability during long-term use. The two-step injection technology successfully achieves an effective balance between low interfacial impedance and low gas production and long cycle stability of the sodium-ion secondary battery. This synergistic effect not only improves the initial performance of the battery, but also maintains excellent cycle stability during long-term use by optimizing the function and application timing of each additive.

[0126] After the first electrolyte is injected, a gas extraction process can also be performed during the pre-charging process, which is conducive to fully discharging the gas generated during the formation (i.e., pre-charging) process and avoiding these gases from being left in the electrolyte, thereby further improving the common problems of excessive gas production and poor cycle 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, which is a reaction that occurs during the pre-charging process, and further reduce the SEI membrane interface impedance.

[0127] Figure 2 This is a schematic diagram of the principle structure of the sodium ion secondary battery provided in the embodiment of the present application after the second liquid injection process. 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 process, and Figure 2 This is not a schematic diagram of the structure of an actual sodium-ion secondary battery product. Figure 2 Schematically shows a housing 10, a positive electrode sheet 101, a separator 102, a negative electrode sheet 103 and an electrolyte 104, wherein the electrolyte 104 is composed of a first electrolyte and a second electrolyte.

[0128] Figure 2 The PS provided by the second injection treatment is indicated by a white filled circular frame, the sodium salt provided by the second injection treatment is indicated by a square frame, the nitrile compound provided by the second injection treatment is indicated by a triangle frame, and the water-removing and acid-removing additive provided by the second injection treatment is indicated by a black filled circle.

[0129] It should be noted that Figure 2 The contents and positions of the substances (such as PS, sodium salt, nitrile compounds and water and acid removal additives) are for illustration only and do not constitute a limitation to the present application. Figure 2 The shapes of the SEI film 105 and the CEI film 106 are for illustration only and do not constitute a limitation to the present application.

[0130] refer to Figure 2 After the first liquid injection process, a pre-charging process is performed using a power supply. During the pre-charging process, the negative electrode plate 103 is polarized, and the components of the first electrolyte react to form new chemical products. Then, part of the chemical products are precipitated on the surface of the negative electrode plate 103 to form a SEI film 105, and part of the chemical products are precipitated on the surface of the positive electrode plate 101 to form a CEI film 106. Specifically, FEC in the first electrolyte reacts before 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, which continue to form a SEI film 105 on the basis of the initial SEI film, thereby enhancing the compactness and stability of the SEI film 105.

[0131] Continue to refer Figure 2 After the second injection treatment, PS, sodium salt, nitrile compounds and water and acid removal additives are present in the electrolyte 104. During the long-term cycle, PS can undergo reduction and decomposition to form new chemical products, which can repair the SEI film 105, such as continuing to deposit on the surface of the SEI film 105. Figure 2 The arrowed line in the figure illustrates the path for the chemical products corresponding to some PS to repair the SEI film 105 .

[0132] In addition, the sodium ions in the sodium salt provided by the second injection treatment can be used to supplement the amount of sodium ions that can be embedded and detached in the positive electrode sheet 101 and the negative electrode sheet 103 in the electrolyte 104 during the long-term circulation process, thereby avoiding the problem of a reduction in the content of mobile sodium ions during the long-term circulation process due to the inability of some sodium ions to detach from the positive electrode sheet 101 or the negative electrode sheet 103. The effects of nitrile compounds and water-removing and acid-removing additives can be referred to the above description and will not be repeated here.

[0133] In addition, reference 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 separated from the positive electrode plate 101, and the deintercalated sodium ions pass through the isolation membrane 102 in the electrolyte 104 under the action of the electric field and are embedded into the negative electrode plate 103, so that the positive electrode plate 101 is in a high potential sodium-poor state, and the negative electrode plate 103 is in a low potential sodium-rich state.

[0134] The discharge process is the opposite of the charging process. Sodium ions are separated from the negative electrode sheet 103, and are re-embedded into the positive electrode sheet 101 through the electrolyte 104 and the isolation membrane 102, so that the positive electrode sheet 101 is restored to a sodium-rich state. In order to maintain charge balance, the same number of electrons are transferred through the external circuit during the charge and discharge process, and migrate between the positive electrode sheet 101 and the negative electrode sheet 103 together with the sodium ions, so that the positive electrode sheet 101 and the negative electrode sheet 103 continuously undergo oxidation or reduction reactions, wherein the corresponding reduction reaction occurs when electrons are lost, and the corresponding oxidation reaction occurs when electrons are gained.

[0135] For ease of understanding, the following examples more specifically describe the contents disclosed in the present application, and these examples are only used for illustrative explanations, because it is obvious to those skilled in the art to make various modifications and changes within the scope of the present disclosure. Unless otherwise stated, all parts, percentages, and ratios mentioned in the following examples are based on weight.

[0136] Table 1

[0137] Table 1 is a comparative example provided in the embodiment of the present application and the formula of the first electrolyte and the second electrolyte corresponding to the embodiment, wherein 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-removing and acid-removing 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, the first time refers to the formula of the first electrolyte used in the first injection treatment, and the second time refers to the formula of the second electrolyte used in the second injection treatment.

[0138] Referring to Table 1, a total of 7 comparative examples are provided, namely, comparative examples 1 to comparative examples 7, and the battery numbers obtained from comparative examples 1 to comparative examples 7 are respectively indicated as D1 to D7. A total of 4 experimental examples are provided, namely, experimental examples 1 to experimental examples 4, and the battery numbers obtained from experimental examples 1 to experimental examples 4 are respectively indicated as E1 to E4.

[0139] Among them, the formula of the first electrolyte and the second electrolyte in comparative example 1 is the same, and the formula of the first electrolyte and the second electrolyte in the other comparative examples and experimental examples is different. It should be noted that all electrolyte formulas in Table 1 are comprehensive formulas, and the actual ratio calculation method of the additives in the electrolyte is that the non-aqueous organic solvent shows the mass percentage of the solvent, and the remaining sodium salts and additives are the mass percentages of the overall formula (i.e., the first electrolyte or the second electrolyte), that is, in the actual configuration, the amount of non-aqueous organic solvent is the corresponding electrolyte amount minus the amount of sodium salt and additives, multiplied by the volume mass percentage occupied by the non-aqueous organic solvent, wherein the non-aqueous organic solvent in the first electrolyte is calculated, and the corresponding electrolyte refers to the first electrolyte; the non-aqueous organic solvent in the second electrolyte is calculated, and the corresponding electrolyte refers to the second electrolyte. In addition, additives refer to other ingredients except sodium salts and non-aqueous organic solvents. In Table 1, the non-aqueous organic solvent is composed of EC, PC, DEC and EMC.

[0140] Taking the mass of the first electrolyte as 100g as an example, the mass of EC in the first electrolyte in Comparative Example 1 is calculated as follows: (100g-mass of sodium salt-mass of additive)×15%, wherein the additive is PS, the mass of the additive, i.e., PS, is 3.5%×100=3.5g, the mass of sodium salt is 12.5%×100=12.5g, and the mass of EC in Comparative Example 1 is (100-3.5-12.5)×15%=12.6g. The calculation methods of the components in the comparative examples and experimental examples are not described in detail here.

[0141] The preparation method of the sodium ion secondary battery corresponding to each comparative example and each experimental example comprises the following steps: 1. Prepare the positive electrode.

[0142] The positive electrode active material NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2, conductive agent Super-P, conductive agent CNT and adhesive PVDF are dissolved in N-methylpyrrolidone in a ratio of 95.5:2.0:0.5:2.0 to make positive electrode slurry, and evenly coated on aluminum foil, and then go through the steps of drying, cold pressing, trimming, cutting, striping, etc., and weld the pole ears to make positive electrode sheets.

[0143] 2. Prepare the negative electrode sheet.

[0144] The negative electrode active material hard carbon, conductive agent Super-P, thickener CMC and adhesive SBR are dissolved in deionized water in a ratio of 96.5:1.0:1.0:1.5 to make a negative electrode slurry, which is evenly coated on copper foil and then made into a negative electrode sheet through the steps of drying, cold pressing, trimming, cutting and striping.

[0145] 3. Prepare the battery cell and place it in the shell.

[0146] The positive electrode sheet, negative electrode sheet and separator are assembled into a battery cell through a winding process. The battery cell is packaged with an aluminum-plastic composite film and vacuum dried.

[0147] 4. Performing a first liquid injection treatment on the battery cell placed in the shell, the first electrolyte used in the first liquid injection treatment has a formula corresponding to each comparative example and each embodiment in Table 1.

[0148] 5. Soak at 40℃~50℃ for 10~15h, soak at room temperature for 20~30h, then pre-charge with a current of 0.05C. The charging control condition is to charge at a constant current of 0.05C and set the cut-off voltage to 3.4V. After charging is completed, let it stand at 45℃ for 12h.

[0149] During the pre-charging stage, an exhaust process is added so that the gas generated by the pre-charging process can be fully discharged. During the exhaust process, the vacuum degree of the shell is maintained in the range of -90Kpa~-80Kpa.

[0150] 6. Perform a second liquid injection treatment on the battery cell placed in the shell. The second electrolyte used in the second liquid injection treatment has a formula corresponding to each comparative example and each experimental example in Table 1.

[0151] After the second electrolyte was injected into the shell, the shell was left to stand for 12 hours before capacity division to obtain a soft-pack sodium-ion secondary battery with a capacity of 3Ah.

[0152] The performance of the sodium ion secondary batteries prepared in each embodiment and each comparative example was tested, and the testing process was as follows: 1. 25℃ Cycle Capacity Retention Test: At 25℃, charge the sodium ion secondary battery to the upper limit voltage at 1C constant current, then charge at constant voltage to a current of 0.05C, and then discharge at 1C constant current to the lower limit voltage. Perform 1000 cycles of charge and discharge test, and record the discharge capacity of the 1000th cycle. Capacity retention rate = (1000th discharge capacity / first discharge capacity) × 100%, and record the cycle retention rate of the 500th time.

[0153] 2. 45℃ Cycle Capacity Retention Test: At 45℃, charge the sodium ion battery to the upper limit voltage at 1C constant current, charge it to 0.05C at constant voltage, and then discharge it to the lower limit voltage at 1C constant current. Perform 1000 cycles of charge and discharge test, and record the discharge capacity of the 1000th cycle. Capacity retention rate = (1000th discharge capacity / first discharge capacity) × 100%, and record the cycle retention rate of the 500th cycle.

[0154] 3. 45℃ high temperature storage performance test: At 45℃, the sodium ion is recharged to the upper limit voltage at 1C constant current, and then charged to the current of 0.05C at constant voltage, and then discharged to the lower limit voltage at 1C constant current, and the discharge capacity is recorded as the initial capacity C1, the initial battery volume V1 and the initial impedance F1. Then charge to the upper limit voltage at 1C constant current, and then charge to the current of 0.05C at constant voltage. Store the fully charged battery at 45℃ for 15 days, leave it at room temperature for at least 2 hours after the end, discharge to the lower limit voltage at 1C constant current, 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 to the upper limit voltage at 1C constant current and constant voltage at room temperature, cut off at 0.05C, and then discharge to the lower limit voltage at 1C constant current, record the recovery capacity C3, and calculate the capacity recovery rate (%) = (C3 / C1) × 100%. Internal resistance growth rate (%) = impedance after storage F2 / initial impedance F1 × 100%. Gas growth rate (%) = (battery volume after storage V2 - battery volume before storage V1) / battery volume before storage V1.

[0155] These testing methods are intended to comprehensively evaluate the performance of the sodium ion secondary batteries prepared in each embodiment and comparative example.

[0156] Table 2

[0157] Table 2 shows the performance test results of the sodium ion secondary batteries prepared in the comparative examples and the embodiments.

[0158] Referring to Table 2, there is the following comparative analysis: In the electrical performance test results of normal temperature cycle, high temperature cycle and high temperature storage, the comparison between comparative examples 1 and 2 shows that the secondary injection method has a positive effect on improving the capacity retention rate of normal temperature and high temperature cycle, high temperature storage capacity retention rate, high temperature storage capacity recovery rate, and inhibiting the growth of internal resistance and gas production, especially in the later stage of the cycle. The improvement of capacity retention is more significant. This is related to the directional repair of the high concentration PS in the second electrolyte on the formed SEI film.

[0159] Comparison of Comparative Examples 3, 4, and 5 with Comparative Example 2 shows that adding TMSP, FEC, or FEC+TMSP mixed additives alone to the first electrolyte formula can help improve the cycle retention rate at room temperature (20°C~25°C, defined as room temperature) and high temperature, among which FEC has a more significant improvement on the cycle capacity retention rate. This may be because FEC can promote the formation of SEI film in the electrolyte, increase the density and stability of the SEI film, and thus improve the cycle performance of the sodium ion secondary battery.

[0160] However, in the high-temperature storage test items, the improvement effect of these additives is not significant. This is consistent with the characteristics of TMSP and FEC. TMSP can form a low-impedance SEI film, effectively reducing the initial internal resistance, while FEC enhances the density and stability of the SEI film, which helps to improve the cycle performance. The solution of using TMSP and FEC in combination can take into account the advantages of both and show better performance.

[0161] Comparison of Comparative Examples 6 and 7 with Comparative Example 5 shows that the addition of nitrile compounds DENE or HTCN can significantly improve the capacity retention rate, high-temperature storage capacity retention rate, and high-temperature storage capacity recovery rate of normal temperature and high-temperature cycles, and can inhibit the growth of internal resistance and gas production.

[0162] However, both nitrile additives increase internal resistance, especially HTCN. DENE has a more significant effect on room temperature cycling, while HTCN has a more prominent effect on high temperature cycling and high temperature storage, which may be related to its stronger ability to coordinate with high-valent transition metal ions on the surface of the positive electrode under high temperature environment to form a stable complex and reduce side reactions and gas production.

[0163] Comparison of Experimental Examples 1, 2, 3 and 4 with the above-mentioned comparative examples shows that the first electrolyte contains FEC, TMSP and a first concentration of PS, the second electrolyte contains a second concentration of PS, the second concentration is greater than the first concentration, and a nitrile compound and a water-removing and acid-removing 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, which can not only improve the capacity retention rate of room temperature and high temperature cycles, the high temperature storage capacity retention rate, and the high temperature storage capacity recovery rate, but also can further effectively inhibit the growth of internal resistance and gas production, and the sodium ion secondary battery has a lower initial internal resistance.

[0164] It can be seen from the above experimental data that the preparation method of the sodium ion secondary battery provided in the embodiment of the present application is conducive to improving the overall performance of the sodium ion secondary battery, improving the gas production problem commonly existing in the sodium ion battery, and taking into account the long cycle performance at high and low temperatures, thereby achieving a comprehensive improvement in electrochemical performance.

[0165] Those skilled in the art will appreciate that the above-mentioned embodiments are specific examples for implementing the present application, and in practical applications, various changes may be made to the embodiments in form and detail without departing from the spirit and scope of the present application. Any person skilled in the art may make various changes and modifications without departing from the spirit and scope of the present application, and therefore the scope of protection of the present application shall be subject to the scope defined in the claims.

Claims

1. A method for preparing a sodium ion secondary battery, It is 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 in the shell is subjected to a first liquid injection treatment using a first electrolyte, wherein the first electrolyte comprises, by weight percentage, 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 immersion treatment and a pre-charging treatment in sequence; The battery cell in the shell is subjected to a second liquid injection treatment using a second electrolyte, wherein the second electrolyte comprises, by weight percentage, 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-removing and acid-removing additive and a sodium salt; 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, It is 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, It is 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, It is 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, It is 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, It is 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, It is 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, It is 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, It is 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, It is 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.

Images