SECONDARY SODIUM BATTERIES AND ELECTRICAL EQUIPMENT

Abstract

The invention relates to a sodium secondary battery and an electrical device. The sodium secondary battery comprises a cathode plate and an electrolyte. The cathode plate comprises a cathode film, and this cathode film contains calcium. The electrolyte comprises a primary component, and that primary component is a fluorinated carbonate compound. The sodium secondary battery can reduce the rate of volume expansion of the battery after storage at high temperatures, while improving the low-temperature charging performance, the initial Coulomb performance, and the charge / discharge cycle stability of the battery.

Description

Sodium secondary battery and electrical device

[0001] Cross-references

[0002] This application refers to Chinese Patent Application No. 202311486636.7, filed on November 9, 2023, entitled “Sodium Secondary Battery and Electrical Device,” which is incorporated herein by reference in its entirety. Technical Field

[0003] The present application relates to the technical field of secondary batteries, and in particular to a sodium secondary battery and an electrical device. Background Art

[0004] In recent years, secondary batteries have been widely used in energy storage systems such as hydropower, thermal, wind, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and other fields. With the widespread use of secondary batteries, higher requirements have been placed on their cycle performance and service life.

[0005] In terms of resources and cost, sodium secondary batteries have greater advantages than lithium secondary batteries, but sodium secondary batteries produce more serious gas during circulation and storage, which limits their further application.

[0006] Summary of the Invention

[0007] The present application is made in view of the above-mentioned problems, and its purpose is to provide a sodium secondary battery for reducing gas production of the sodium secondary battery and improving the cycle stability of the battery.

[0008] A first aspect of the present application provides a sodium secondary battery, comprising a negative electrode plate, the negative electrode plate comprising a negative electrode film layer, the negative electrode film layer containing calcium; and an electrolyte, the electrolyte comprising a first component, the first component being a fluorinated carbonate compound.

[0009] The introduction of calcium into the negative electrode film layer can induce the deposition of sodium ions, help inhibit the formation of sodium dendrites, reduce the oxidation gas production of unstable components generated by sodium dendrites at the negative electrode, thereby reducing the gas production of the negative electrode during discharge and reducing the volume expansion rate of the battery after high-temperature storage. However, calcium easily forms calcium hydroxide during the negative electrode slurrying process, resulting in high impedance and low first efficiency of the battery, thereby reducing the battery's kinetic performance, especially the battery's charging capacity at low temperatures. Fluorinated carbonate compounds are introduced into the electrolyte, which can react with calcium hydroxide during the cycle to form stable CaF2, reduce the calcium hydroxide content, and weaken the negative impact of calcium hydroxide on the negative electrode. A stable solid electrolyte interface (SEI film) component is formed on the negative electrode surface, which reduces the unstable components from producing gas at the negative electrode while improving the battery's kinetic performance and first efficiency, thereby comprehensively improving the battery's cycle stability.

[0010] In any embodiment, the mass fraction of the fluorinated carbonate compound based on the total mass of the electrolyte is a, and the mass fraction of the calcium element in the negative electrode film layer based on the total mass of the negative electrode film layer is b, and a and b satisfy: a / b ≥ 1. In some embodiments, a and b satisfy: 10000 ≥ a / b ≥ 10.

[0011] When the value of a / b is within an appropriate range, through the synergistic effect of calcium elements and fluorocarbonate compounds in the negative electrode, the sodium secondary battery can effectively inhibit the gas production of the negative electrode, reduce the gas production during high-temperature storage of the battery, and improve the battery's kinetic performance, first efficiency and cycle stability.

[0012] In any embodiment, the mass proportion a of the fluorinated carbonate compound is 0.01% to 10% based on the total mass of the electrolyte. In some embodiments, the mass proportion a of the fluorinated carbonate compound is 0.2% to 10% based on the total mass of the electrolyte.

[0013] When the mass proportion a of the fluorocarbonate compound is within an appropriate range, the fluorocarbonate compound can form a complex with calcium hydroxide, which can not only reduce the calcium hydroxide content in the negative electrode and improve the battery kinetic performance and first efficiency; but also form stable organic and inorganic components in the negative electrode SEI film together with calcium hydroxide, reduce the gas production of unstable components in the negative electrode, reduce the gas production rate of the battery after high-temperature storage, and comprehensively improve the cycle stability of the battery.

[0014] In any embodiment, based on the total mass of the negative electrode film layer, the mass fraction b of the calcium element in the negative electrode film layer is 1 ppm to 3000 ppm. In any embodiment, based on the total mass of the negative electrode film layer, the mass fraction b of the calcium element in the negative electrode film layer is 50 ppm to 1000 ppm.

[0015] When the mass fraction b of the calcium element in the negative electrode film is within the appropriate range, it can not only reduce the negative impact of excessive calcium content on the secondary battery's impedance and initial efficiency, but also fully utilize the calcium element's role in inhibiting dendrites and reducing gas production. This reduces battery gas production while improving the battery's low-temperature charging performance, initial efficiency, and room-temperature cycling capacity retention. When the mass fraction b of the calcium element in the negative electrode film is 50ppm to 1000ppm, the battery's kinetic performance, initial efficiency, and cycling stability are further improved while reducing battery gas production.

[0016] In any embodiment, the fluorinated carbonate compound includes a compound represented by formula I,

[0017] Wherein, R1, R2, R3, and R4 are independently hydrogen atoms, halogen atoms, C 1-6 Hydrocarbon, C 1-3Halogenated alkyl, C 1-3 Alkoxy, C 1-3 At least one of a haloalkoxy group, an ester group, a cyano group, a sulfonic acid group, and an isocyanate group; and at least one of R1, R2, R3, and R4 is a fluorine atom.

[0018] The cyclic fluorinated carbonate in which at least one of R1, R2, R3, and R4 is a fluorine atom is easy to open the ring and form an SEI film on the surface of the negative electrode, thereby reducing battery gas production and improving battery kinetic performance, first efficiency, and cycle stability.

[0019] In any embodiment, the fluorinated carbonate compound comprises at least one of the following compounds,

[0020] In any embodiment, the electrolyte further comprises a second component, wherein the second component is difluorooxalatoborate.

[0021] The SEI membrane formed at the interface of the negative electrode film in sodium secondary batteries is primarily composed of sodium alkyl carbonate (ROCO2Na) and sodium carbonate. However, sodium alkyl carbonate has a greater solubility in solvents than lithium alkyl carbonate, making the SEI membrane of sodium secondary batteries highly unstable. This makes it prone to continuous side reactions between the electrolyte and the negative electrode film, a key factor in the short service life of sodium secondary batteries.

[0022] Difluorooxalatoborate has a high reduction potential and can be reduced to form a film before the solvent. On the one hand, it effectively inhibits the formation of easily soluble substances such as sodium alkyl carbonate. On the other hand, difluorooxalatoborate can form a thin, dense, low-impedance SEI film containing a large number of inorganic components such as sodium fluoride and sodium borate. It cooperates with the organic components generated by the fluorocarbonate compound, taking into account the flexibility and stability of the SEI film, thereby further inhibiting gas production and improving the kinetic performance, first efficiency and cycle life of sodium secondary batteries.

[0023] In any embodiment, based on the total mass of the electrolyte, the mass proportion of the difluorooxalatoborate is c, the mass proportion of the fluorocarbonate compound is a, and c and a satisfy: c / a ≥ 0.001. In some embodiments, c and a satisfy: 2 ≥ c / a ≥ 0.1.

[0024] When the c / a value is within an appropriate range, the synergistic effect of difluorooxalatoborate and fluorocarbonate in the electrolyte can improve the stability, flexibility and sodium conductivity of the SEI film, reduce gas production during high-temperature storage of the battery, improve the battery's kinetic performance and first efficiency, and comprehensively improve the battery's cycle stability.

[0025] In any embodiment, based on the total mass of the electrolyte, the mass percentage c of the difluorooxalatoborate is 0.01%-5%. In some embodiments, based on the total mass of the electrolyte, the mass percentage c of the difluorooxalatoborate is 0.1%-2%.

[0026] When the mass proportion c of difluorooxalatoborate is within an appropriate range, difluorooxalatoborate can form a complex with the fluorocarbonate compound, which can not only improve the stability and flexibility of the SEI film, reduce the gas production of unstable components at the negative electrode, and reduce the gas production rate after high-temperature storage of the battery; but also improve the sodium conductivity characteristics of the SEI film, improve the battery kinetic performance and first efficiency, and comprehensively improve the cycle stability of the battery.

[0027] In any embodiment, the chemical formula of the difluorooxalatoborate is as shown in Formula III, (F2C2O4B) y M Type III

[0028] Wherein, M includes one or more of Li, Na, K, Rb, Cs, Mg, Ca, Ba, Fe, Ni, and Al, and y is 1, 2, or 3.

[0029] In any embodiment, the negative electrode film layer includes a negative electrode active material, and the negative electrode active material includes one or more of hard carbon, sodium metal, sodium-tin alloy, and metal oxide.

[0030] In any embodiment, the sodium secondary battery further includes a positive electrode sheet, the positive electrode sheet includes a positive electrode active material, and the positive electrode active material includes a sodium transition metal oxide.

[0031] In any embodiment, the positive electrode active material further contains copper, and the mass fraction (d) of the copper element is 0.01% to 23% based on the total mass of the positive electrode active material. In some embodiments, the mass fraction (d) of the copper element is 6.5% to 18% based on the total mass of the positive electrode active material.

[0032] The positive electrode active material containing copper has a more stable structure, which can further improve the cycle stability of the battery. The mass ratio of copper is within the appropriate range, and the battery cycle stability is improved without the copper element being converted into Cu under high voltage. 3+ , causing the electrolyte to decompose faster due to its high oxidizing properties, sharply worsening the battery's gas production. A copper content of 6.5% to 18% by mass can further balance low gas production, high kinetic performance, high initial efficiency, and high cycle stability for secondary batteries.

[0033] In any embodiment, the sodium transition metal oxide comprises Na m Cu n Xo Fe p Mn q O 2-s， where X includes one or more of Li, Be, B, Mg, Al, K, Ca, Ti, Co, Ni, Zn, Ga, Sr, Y, Nb, Mo, In, Sn, Fe, Ba, 0 ≤ m ≤ 0.5, 0 ≤ n ≤ 0.5, 0 ≤ o < 0.5, 0 ≤ p ≤ 0.5, 0 < q ≤ 0.68, n + o + p + q = 1, 0 ≤ s < . In some embodiments, the sodium transition metal oxide includes Na[Cu 1 / 9 Ni 2 / 9 Fe 1 / 3 Mn 1 / 3 O2, Na 7 / 9 [Cu 2 / 9 Fe 1 / 9 Mn 2 / 3 O2, Na 9 / 10 [Cu 2 / 5 Fe 1 / 10 Mn 1 / 2 O2, or at least one of them.

[0034] The cathode active material of the sodium transition metal oxide has a high voltage. The anionic oxygen therein generates a large amount of proton hydrogen while contributing to the capacity, accelerating the oxidation gas generation of the unstable components on the anode side, resulting in serious gas generation on the anode side. Through the combined action of calcium in the anode film layer of the anode and the fluorinated carbonate compound in the electrolyte provided by the embodiments of the present application, the gas generation of the battery can be effectively reduced while improving the battery capacity and energy density, enhancing the kinetic performance, initial efficiency and cycle stability of the battery.

[0035] In any embodiment, the electrolyte further includes a third component, and the third component is one or more of vinylene carbonate, ethylene vinylene carbonate, 1,3 - propanesultone, 1,3 - propenesulfonic acid lactone, ethylene sulfate, maleic anhydride, succinic anhydride, triallyl phosphate, sodium tetrafluorophosphate oxalate, sodium difluorobis(oxalato)phosphate, sodium difluorophosphate, sodium fluorosulfonate.

[0036] The components in the SEI film formed at the anode interface of the sodium secondary battery mainly include alkyl sodium carbonate and sodium carbonate. However, alkyl sodium carbonate has a greater solubility in the electrolyte solvent than alkyl lithium carbonate, which makes the SEI film of the sodium secondary battery very unstable, and the electrolyte continuously reacts with the anode side, resulting in poor cycle performance of the secondary battery. The second component containing unsaturated functional groups can preferentially reduce and form a film on the anode prior to the solvent, cooperate with the fluorinated carbonate compound and the calcium element on the anode to jointly inhibit the formation of easily soluble substances such as alkyl sodium carbonate, reduce battery gas generation, and improve the kinetic performance, initial efficiency and cycle stability of the battery.

[0037] In any embodiment, the mass proportion of the third component is 0.01% to 10% based on the total mass of the electrolyte. In some embodiments, the mass proportion of the third component is 0.1% to 5% based on the total mass of the electrolyte.

[0038] The mass proportion of the third component is within the above range, which can improve the gas production of the battery while controlling the thickness of the SEI film, thereby achieving low impedance and low gas production of the battery at the same time, and improving the battery's kinetic performance, first efficiency and cycle stability.

[0039] A second aspect of the present application provides an electrical device comprising the sodium secondary battery of the first aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] FIG1 is a schematic diagram of a secondary battery according to an embodiment of the present application;

[0041] FIG2 is an exploded view of the secondary battery according to one embodiment of the present application shown in FIG1 ;

[0042] FIG3 is a schematic diagram of a battery module according to an embodiment of the present application;

[0043] FIG4 is a schematic diagram of a battery pack according to an embodiment of the present application;

[0044] FIG5 is an exploded view of the battery pack according to an embodiment of the present application shown in FIG4 ;

[0045] FIG6 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0046] Explanation of reference numerals: 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 secondary battery; 51 housing; 52 electrode assembly; 53 cover plate. DETAILED DESCRIPTION

[0047] Below, the embodiments of the sodium secondary battery and the electrical device of the present application are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there may be cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0048] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0049] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0050] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0051] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0052] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0053] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0054] It is generally believed in the prior art that the gas production of sodium secondary batteries mainly comes from the oxidation of electrolyte by the positive electrode active material under high voltage. Therefore, the prior art often adopts the scheme of coating the positive electrode active material and forming a film on the positive electrode surface to reduce the gas production of secondary batteries. During the research process, the applicant found that another important factor for the gas production of sodium secondary batteries lies in the negative electrode. Unlike lithium secondary batteries, sodium secondary batteries often use hard carbon as their negative electrode active material. The capacity of hard carbon mainly includes two stages. The first stage is between 1.5V and 0.1V (vs Na / Na + ) capacity comes from Na + Adsorption process at the surface defects of hard carbon, the second stage at 0.1V (vs Na / Na + )The following capacity contributions come from Na + Filling process in hard carbon micropores. In order to improve the negative electrode capacity, the porosity of hard carbon is often increased in the prior art. However, Na + The potential during the hard carbon micropore filling process is close to the potential of metallic sodium deposition (0V), which can easily cause sodium precipitation problems during the charging process. The precipitated sodium dendrites are extremely reactive and react rapidly with the electrolyte to produce a large amount of gas and unstable by-products. These unstable by-products will further oxidize and decompose as the negative electrode potential increases during discharge due to insufficient film-forming driving force. In addition, unstable organic by-products are easily soluble in the electrolyte, causing the solid electrolyte interface (SEI film) to be in a cyclical process of dissolution and repair, exacerbating the deterioration of battery gas production and cycle stability.

[0055] [Sodium secondary battery]

[0056] Based on this, the present application proposes a sodium secondary battery, which includes a negative electrode plate, the negative electrode plate includes a negative electrode film layer, the negative electrode film layer contains calcium; and an electrolyte, the electrolyte includes a first component, and the first component is a fluorinated carbonate compound.

[0057] Sodium secondary battery is a secondary battery that mainly relies on the movement of sodium ions between the positive and negative electrodes to work.

[0058] It is understood that calcium can be introduced into the negative electrode membrane in any form. In some embodiments, calcium is introduced into the negative electrode membrane in the form of calcium oxide or calcium salt. In some embodiments, calcium is introduced into the negative electrode membrane in the form of CaO.

[0059] The fluorinated carbonate compound refers to a compound containing a carbonate group (-OC(O)O-) in which at least one hydrogen atom is replaced by a fluorine atom. It can be a linear compound or a cyclic compound.

[0060] The introduction of calcium into the negative electrode film layer can induce the deposition of sodium ions, help inhibit the formation of sodium dendrites, reduce the oxidation gas production of unstable components generated by sodium dendrites at the negative electrode, thereby reducing the gas production of the negative electrode during discharge and reducing the volume expansion rate of the battery after high-temperature storage. However, calcium easily forms calcium hydroxide during the negative electrode slurrying process, resulting in high impedance and low first efficiency of the battery, thereby reducing the battery's kinetic performance, especially the battery's charging capacity at low temperatures. Fluorinated carbonate compounds are introduced into the electrolyte, which can react with calcium hydroxide during the cycle to form stable CaF2, reduce the calcium hydroxide content, and weaken the negative impact of calcium hydroxide on the negative electrode. A stable solid electrolyte interface (SEI film) component is formed on the negative electrode surface, which reduces the unstable components from producing gas at the negative electrode while improving the battery's kinetic performance and first efficiency, thereby comprehensively improving the battery's cycle stability.

[0061] In some embodiments, the mass fraction of the fluorinated carbonate compound based on the total mass of the electrolyte is a, and the mass fraction of the calcium element in the negative electrode film layer based on the total mass of the negative electrode film layer is b, and a and b satisfy: a / b ≥ 1. In some embodiments, a and b satisfy: 10000 ≥ a / b ≥ 10.

[0062] In some embodiments, the value of a / b can be 1, 3.3, 5, 10, 20, 50, 100, 200, 500, 1000, 5000, 10000, or any range therebetween.

[0063] When the value of a / b is within an appropriate range, through the synergistic effect of calcium elements and fluorocarbonate compounds in the negative electrode, the sodium secondary battery can effectively inhibit the gas production of the negative electrode, reduce the gas production during high-temperature storage of the battery, and improve the battery's kinetic performance, first efficiency and cycle stability.

[0064] In some embodiments, the mass proportion a of the fluorinated carbonate compound is 0.01% to 10% based on the total mass of the electrolyte. In some embodiments, the mass proportion a of the fluorinated carbonate compound is 0.2% to 10% based on the total mass of the electrolyte.

[0065] In some embodiments, based on the total mass of the electrolyte, the mass proportion a of the fluorinated carbonate compound is 0.01%, 0.05%, 0.1%, 0.2%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or any value therebetween.

[0066] When the mass proportion a of the fluorocarbonate compound is within an appropriate range, the fluorocarbonate compound can form a complex with calcium hydroxide, which can not only reduce the calcium hydroxide content in the negative electrode and improve the battery kinetic performance and first efficiency; but also form stable organic and inorganic components in the negative electrode SEI film together with calcium hydroxide, reduce the gas production of unstable components in the negative electrode, reduce the gas production rate of the battery after high-temperature storage, and comprehensively improve the cycle stability of the battery.

[0067] In some embodiments, based on the total mass of the negative electrode film layer, the mass fraction b of the calcium element in the negative electrode film layer is 1 ppm to 3000 ppm. In some embodiments, based on the total mass of the negative electrode film layer, the mass fraction b of the calcium element in the negative electrode film layer is 50 ppm to 1000 ppm.

[0068] In some embodiments, based on the total mass of the negative electrode film layer, the mass proportion b of the calcium element in the negative electrode film layer is 1ppm, 50ppm, 100ppm, 200ppm, 300ppm, 400ppm, 500ppm, 600ppm, 700ppm, 800ppm, 900ppm, 1000ppm, 1500ppm, 2000ppm, 2500ppm, 3000ppm or any value therebetween.

[0069] In this article, ppm means parts per million.

[0070] When the mass fraction (b) of calcium in the negative electrode film is within an appropriate range, it can not only reduce the negative impact of excessive calcium content on the secondary battery's impedance and initial efficiency, but also fully utilize calcium's role in inhibiting dendrites and reducing gas production. This reduces battery gas production while improving the battery's low-temperature charging performance, initial efficiency, and room-temperature cycling capacity retention. When the mass fraction (b) of calcium in the negative electrode film is 50ppm to 1000ppm, the battery's kinetic performance, initial efficiency, and cycling stability are further improved while reducing battery gas production.

[0071] In some embodiments, the fluorinated carbonate compound includes a compound represented by Formula I,

[0072] Wherein, R1, R2, R3, and R4 are independently hydrogen atoms, halogen atoms, C 1-6 Hydrocarbon, C 1-3 Halogenated alkyl, C 1-3Alkoxy, C 1-3 At least one of a haloalkoxy group, an ester group, a cyano group, a sulfonic acid group, and an isocyanate group; and at least one of R1, R2, R3, and R4 is a fluorine atom.

[0073] As used herein, the term "halogen atom" refers to elements of Group VIIA of the periodic system, including but not limited to: F, Cl, Br, and I.

[0074] In this article, the term “C 1-6 "Hydrocarbon" refers to a group containing carbon and hydrogen atoms including 1-6 carbon atoms. In some embodiments, C 1-6 The hydrocarbon group does not include an unsaturated bond and is an alkyl group, and as examples, includes but is not limited to methyl, ethyl, propyl, isopropyl, 2-methyl-1-propyl, 2-methyl-2-propyl, 2-methyl-1-butyl, 3-methyl-1-butyl, 2-methyl-3-butyl, 2,2-dimethyl-1-propyl, 2-methyl-1-pentyl, 3-methyl-1-pentyl, 4-methyl-1-pentyl, 2-methyl-2-pentyl, 3-methyl-2-pentyl, 4-methyl-2-pentyl, 2,2-dimethyl-1-butyl, 3,3-dimethyl-1-butyl, 2-ethyl-1-butyl, butyl, isobutyl, tert-butyl, pentyl, isopentyl, neopentyl, and hexyl. In some embodiments, C 1-6 The hydrocarbon group includes an unsaturated bond and is a cycloalkyl group, an alkenyl group, an alkynyl group, or an aryl group, and includes, by way of example, but not limited to, a cyclopropyl group, a cyclobutyl group, a vinyl group, a 1-propenyl group or a 2-propenyl group, a phenyl group, and a naphthyl group.

[0075] In this article, the term “C 1-3 "Haloalkyl" refers to a C 1-3 Alkyl groups include, but are not limited to, -CF3, -CF2CH2, and -CF2CH2CH3.

[0076] In this article, the term “C 1-3 "Alkoxy" refers to a C-type group connected to the main carbon chain through an oxygen atom. 1-3 Alkyl groups include, by way of example, but are not limited to, methoxy (CH3O-), ethoxy (C2H5O-), and propoxy (C3H7O-).

[0077] In this article, the term “C 1-3 "Haloalkoxy" refers to a C 1-3 Alkoxy.

[0078] As used herein, the term "ester group" refers to a -COO- group.

[0079] As used herein, the term "cyano" refers to a -CN group.

[0080] As used herein, the term "sulfonic acid" refers to a -SO3H group.

[0081] As used herein, the term "isocyanate" refers to an -NCO group.

[0082] The cyclic fluorinated carbonate in which at least one of R1, R2, R3, and R4 is a fluorine atom is easy to open the ring and form an SEI film on the surface of the negative electrode, thereby reducing battery gas production and improving battery kinetic performance, first efficiency, and cycle stability.

[0083] In some embodiments, the fluorinated carbonate compound comprises at least one of the following compounds,

[0084] In some embodiments, the electrolyte further includes a second component, wherein the second component is difluorooxalatoborate.

[0085] As used herein, "difluorooxalatoborate" is a borate salt whose anion is Difluorooxalatoborate salts include, but are not limited to, lithium difluorooxalatoborate, sodium difluorooxalatoborate, magnesium difluorooxalatoborate, calcium difluorooxalatoborate, barium difluorooxalatoborate, aluminum difluorooxalatoborate, and iron difluorooxalatoborate.

[0086] In some embodiments, the difluorooxalatoborate salt comprises sodium difluorooxalatoborate.

[0087] In some embodiments, the difluorooxalatoborate comprises calcium difluorooxalatoborate.

[0088] The SEI membrane formed at the interface of the negative electrode film in sodium secondary batteries is primarily composed of sodium alkyl carbonate (ROCO2Na) and sodium carbonate. However, sodium alkyl carbonate has a greater solubility in solvents than lithium alkyl carbonate, making the SEI membrane of sodium secondary batteries highly unstable. This makes it prone to continuous side reactions between the electrolyte and the negative electrode film, a key factor in the short service life of sodium secondary batteries.

[0089] Difluorooxalatoborate has a high reduction potential and can be reduced to form a film before the solvent. On the one hand, it effectively inhibits the formation of easily soluble substances such as sodium alkyl carbonate. On the other hand, difluorooxalatoborate can form a thin, dense, low-impedance SEI film containing a large number of inorganic components such as sodium fluoride and sodium borate. It cooperates with the organic components generated by the fluorocarbonate compound, taking into account the flexibility and stability of the SEI film, thereby further inhibiting gas production and improving the kinetic performance, first efficiency and cycle life of sodium secondary batteries.

[0090] In some embodiments, based on the total mass of the electrolyte, the mass proportion of the difluorooxalatoborate is c, the mass proportion of the fluorocarbonate compound is a, and c and a satisfy: c / a ≥ 0.001. In some embodiments, c and a satisfy: 2 ≥ c / a ≥ 0.1.

[0091] In some embodiments, the value of c / a can be 0.001, 0.1, 0.8, 1, 2, 3, 4, 5, or any value therebetween.

[0092] When the c / a value is within an appropriate range, the synergistic effect of difluorooxalatoborate and fluorocarbonate in the electrolyte can improve the stability and sodium conductivity of the SEI film, reduce gas production during high-temperature storage of the battery, improve the battery's kinetic performance and first efficiency, and comprehensively improve the battery's cycle stability.

[0093] In some embodiments, the mass percentage c of the difluorooxalatoborate is 0.01%-5% based on the total mass of the electrolyte. In some embodiments, the mass percentage c of the difluorooxalatoborate is 0.1%-2% based on the total mass of the electrolyte.

[0094] In some embodiments, based on the total mass of the electrolyte, the mass proportion c of the difluorooxalatoborate is 0.01%, 0.05%, 0.1%, 0.5%, 0.8%, 1%, 2%, 3%, 4%, 5% or any value therebetween.

[0095] When the mass proportion c of difluorooxalatoborate is within an appropriate range, difluorooxalatoborate can form a complex with a fluorocarbonate compound, which can not only improve the stability of the SEI film, reduce the gas production of unstable components at the negative electrode, and reduce the gas production rate after high-temperature storage of the battery; but also improve the sodium conductivity characteristics of the SEI film, improve the battery kinetic performance and first efficiency, and comprehensively improve the cycle stability of the battery.

[0096] In some embodiments, the difluorooxalatoborate has a general chemical formula as shown in Formula III, (F2C2O4B) y M Type III

[0097] Wherein, M includes one or more of Li, Na, K, Rb, Cs, Mg, Ca, Ba, Fe, Ni, and Al, and y is 1, 2, or 3.

[0098] In some embodiments, the negative electrode film layer includes a negative electrode active material, and the negative electrode active material includes one or more of hard carbon, sodium metal, sodium-tin alloy, and metal oxide.

[0099] In some embodiments, the negative electrode active material includes hard carbon.

[0100] In some embodiments, the sodium secondary battery further includes a positive electrode plate, and the positive electrode plate includes a positive electrode active material.

[0101] In some embodiments, the positive electrode active material further contains copper element. Based on the total mass of the positive electrode active material, the mass ratio d of the copper element is 0.01% to 23%. In some embodiments, based on the total mass of the positive electrode active material, the mass ratio d of the copper element is 6.5% to 18%. In some embodiments, based on the total mass of the positive electrode active material, the mass ratio d of the copper element is 0.01%, 4%, 6.5%, 10%, 13%, 15%, 18%, 20%, 23% or the numerical range between any two of them.

[0102] The positive electrode active material containing copper element has a more stable structure, which can further improve the cycle stability of the battery. When the mass ratio of the copper element is within a suitable range, while the cycle stability of the battery is improved, it will not cause the electrolyte to decompose rapidly due to the conversion of copper element into Cu 3+ at high voltage, resulting in the rapid deterioration of the gas generation phenomenon of the battery. When the mass ratio of the copper element is within the range of 6.5% to 18%, it can further balance the low gas generation amount, high kinetic performance, high initial efficiency and high cycle stability of the secondary battery.

[0103] In some embodiments, the positive electrode active material can be a positive electrode active material for batteries well-known in the art. As an example, the positive electrode active material can include at least one of the following materials: Prussian blue analogs, sodium-containing phosphates, sodium-containing transition metal oxides and their respective modified compounds. However, the present application is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials can also be used. These positive electrode active materials can be used alone or in combination of two or more. Among them, the Prussian blue analog is NaxP[R(CN)6] δ ·zH2O, where P and R are each independently selected from at least one of transition metal elements, 0 < x ≤ 2, 0 < δ ≤ 1 and 0 ≤ z ≤ 10; the sodium-containing phosphate is Na b Me c (PO4) d O2X, where A is one or more of H, Li, Na, K and NH4, Me is one or more of Ti, Cr, Mn, Fe, Co, Ni, V, Cu and Zn, X is one or more of F, Cl and Br, 0 < b ≤ 4, 0 < c ≤ 2, 1 ≤ d ≤ 3; the sodium-containing transition metal oxide is Na a M b N c Fe d Mn eO2, M and N include at least one of Sc, Ti, V, Cr, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W and Pb, 0.05 ≤ b ≤ 0.2, 0.2 ≤ c ≤ 0.3, 0.2 ≤ d ≤ 0.3, 0.3 ≤ e ≤ 0.4, 0.75 ≤ a / (b + c + d + e) ≤ 1.

[0104] In some embodiments, the positive electrode active material includes sodium transition metal oxide.

[0105] In some embodiments, the sodium transition metal oxide includes Na m Cu n X o Fe p Mn q O 2-s， where X includes one or more of Li, Be, B, Mg, Al, K, Ca, Ti, Co, Ni, Zn, Ga, Sr, Y, Nb, Mo, In, Sn, Fe, Ba, 0 ≤ m ≤ 0.5, 0 ≤ n ≤ 0.5, 0 ≤ o < 0.5, 0 ≤ p ≤ 0.5, 0 < q ≤ 0.68, n + o + p + q = 1, 0 ≤ s < 0.2. In some embodiments, the sodium transition metal oxide includes Na[Cu 1 / 9 Ni 2 / 9 Fe 1 / 3 Mn 1 / 3 O2, Na 7 / 9 [Cu 2 / 9 Fe 1 / 9 Mn 2 / 3 O2, Na 9 / 10 [Cu 2 / 5 Fe 1 / 10 Mn 1 / 2 O2 or at least one of them.

[0106] The sodium transition metal oxide positive electrode active material has a high voltage. The anionic oxygen therein will generate a large amount of proton hydrogen while contributing to the capacity, accelerating the oxidation gas generation of the unstable components on the negative electrode side, resulting in serious gas generation on the negative electrode side. Through the combined action of calcium in the negative electrode film layer of the negative electrode plate and the fluorinated carbonate compound in the electrolyte provided by the embodiments of the present application, the gas generation of the battery can be effectively reduced while improving the battery capacity and energy density, enhancing the kinetic performance, initial efficiency and cycle stability of the battery.

[0107] In some embodiments, the electrolyte further includes a third component, and the third component is one or more of vinylene carbonate, ethylene vinylene carbonate, 1,3 - propanesultone, ......

[0108] The SEI film formed at the negative electrode interface of sodium secondary batteries mainly consists of sodium alkyl carbonate and sodium carbonate. However, sodium alkyl carbonate has a greater solubility in electrolyte solvents than lithium alkyl carbonate, which makes the SEI film of sodium secondary batteries very unstable. Side reactions between the electrolyte and the negative electrode continue, resulting in poor secondary battery cycle performance. The second component containing unsaturated functional groups can be reduced to form a film at the negative electrode before the solvent, and cooperates with the fluorocarbonate compound and the negative electrode calcium element to inhibit the formation of easily soluble substances such as sodium alkyl carbonate, reduce battery gas production, and improve the battery's kinetic performance, initial efficiency, and cycle stability.

[0109] In some embodiments, the weight percentage of the third component is 0.01% to 10% based on the total weight of the electrolyte. In some embodiments, the weight percentage of the third component is 0.1% to 5% based on the total weight of the electrolyte.

[0110] In some embodiments, based on the total mass of the electrolyte, the mass proportion of the third component is 0.01%, 0.1%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or any value therebetween.

[0111] The mass proportion of the third component is within the above range, which can improve the gas production of the battery while controlling the thickness of the SEI film, thereby achieving low impedance and low gas production of the battery at the same time, and improving the battery's kinetic performance, first efficiency and cycle stability.

[0112] In some embodiments, the electrolyte includes an electrolyte salt selected from at least one of NaPF6, NaBF4, NaN(SO2F)2(NaFSI), NaClO4, NaAsF6, NaB(C2O4)2(NaBOB), NaBF2(C2O4)(NaDFOB), NaN(SO2RF)2, and NaN(SO2F)(SO2RF), wherein RF is represented by C b F 2b+1 , b is an integer between 1 and 10. In some embodiments, b can be an integer between 1 and 3. In some embodiments, the electrolyte salt is selected from one or more of NaPF6, NaClO4, NaN(SO2F)2, NaN(CF3SO2)2, NaB(C2O4)2, and NaBF2(C2O4). In some embodiments, the electrolyte salt is selected from one or more of NaPF6, NaN(SO2RF)2, and NaBF2(C2O4). In some embodiments, RF is -CF3, -C2F5, or -CF2CF2CF3.

[0113] In some embodiments, the electrolyte includes a solvent, and the solvent includes at least one of a chain carbonate, a chain carboxylate, a cyclic carbonic acid, an ether solvent, a sulfone solvent, and a nitrile solvent. In some embodiments, the chain carbonate includes at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), methyl isopropyl carbonate (MIPC), methyl butyl carbonate, ethyl propyl carbonate, dipropyl carbonate, and dibutyl carbonate. In some embodiments, the chain carbonate includes at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and methyl propyl carbonate (MPC). In some embodiments, the chain carboxylate includes at least one of methyl formate (MF), ethyl formate (EF), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), ethyl butyrate (EB), methyl acetate (MA), ethyl acetate (EA), and propyl acetate (PA). In some embodiments, the linear carboxylic acid ester includes at least one of methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl acetate (MA), ethyl acetate (EA), and propyl acetate (PA). In some embodiments, the ether solvent includes at least one of dioxolane (DOL), tetrahydrofuran (THF), 2-methyltetrahydrofuran (2Me-THF), tetrahydropyran (THP), 1,2-dimethoxyethane (DME), diethylene glycol dimethyl ether (DG), 1,2-diethoxyethane, and 1,2-dibutoxyethane.

[0114] [Positive electrode]

[0115] The positive electrode sheet generally includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes a positive electrode active material.

[0116] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0117] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0118] In some embodiments, the positive electrode film layer includes a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0119] In some embodiments, the positive electrode film layer may further include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0120] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0121] [Negative electrode]

[0122] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material.

[0123] As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0124] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil or aluminum foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0125] In some embodiments, the negative electrode film layer includes a binder. As an example, the binder can be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0126] In some embodiments, the negative electrode film layer includes a conductive agent. As an example, the conductive agent can be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0127] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0128] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0129] [Isolation film]

[0130] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0131] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0132] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0133] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0134] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0135] In the present application, the shape of the sodium secondary battery includes but is not limited to cylindrical, square or any other shape. For example, FIG1 shows a sodium secondary battery 5 with a square structure as an example.

[0136] In some embodiments, referring to FIG2 , the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the separator can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the sodium secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0137] In some embodiments, sodium secondary batteries can be assembled into a battery module. The number of sodium secondary batteries contained in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0138] Figure 3 shows an example battery module 4. Referring to Figure 3 , within battery module 4, multiple sodium secondary batteries 5 may be arranged sequentially along the length of battery module 4. Of course, any other arrangement is also possible. Furthermore, these multiple sodium secondary batteries 5 may be secured using fasteners.

[0139] Optionally, the battery module 4 may further include a housing having an accommodation space, and the plurality of sodium secondary batteries 5 are accommodated in the accommodation space.

[0140] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0141] Figures 4 and 5 illustrate an example battery pack 1. Referring to Figures 4 and 5 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be placed over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0142] In addition, the present application also provides an electrical device, which includes at least one of the sodium secondary battery, battery module, or battery pack provided in the present application. The sodium secondary battery, battery module, or battery pack can be used as a power source for the electrical device, and can also be used as an energy storage unit for the electrical device. The electrical device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto.

[0143] As an electrical device, a sodium secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0144] Figure 6 shows an example of an electric device. This device can be a pure electric vehicle, hybrid electric vehicle, or plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of sodium secondary batteries, a battery pack or battery module can be used.

[0145] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a sodium secondary battery as a power source.

[0146] Example

[0147] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0148] 1. Preparation method

[0149] Example 1:

[0150] electrolyte

[0151] In an argon atmosphere glove box (H2O content <10ppm, O2 content <1ppm), ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a 30 / 70 mass ratio, 1M NaPF6 sodium salt was dissolved, and then fluoroethylene carbonate was added and stirred to prepare an electrolyte. The mass proportion of fluoroethylene carbonate was 1% based on the total mass of the electrolyte.

[0152] 2) Preparation of positive electrode active materials

[0153] Na 7 / 9 Cu 2 / 9 Fe 1 / 9 Mn 2 / 3 Preparation of O2: 0.39 mol Na2CO3, 0.22 mol CuO, 0.06 mol Fe2O3, and 0.67 mol MnO2 precursors are ball-milled in a ball mill using ethanol as a dispersant for 12 hours. After drying, the evenly mixed powder is pressed into a tablet at 20 MPa and sintered at 900°C for 12 hours. The sintered powder needs to be quickly transferred to a glove box for storage.

[0154] 3) Preparation of positive electrode sheet

[0155] The positive electrode active material Na 7 / 9 Cu 2 / 9 Fe 1 / 9 Mn 2 / 3 O2, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) were fully stirred and mixed in an N-methylpyrrolidone solvent system at a weight ratio of 90:5:5 to obtain a positive electrode slurry; the positive electrode slurry was added at 0.28 g (dry weight) / 1540.25 mm 2 The density is evenly coated on the positive electrode current collector aluminum foil with a thickness of 13 μm; the aluminum foil is dried at room temperature and then transferred to a 120°C oven for drying for 1 hour, and then cold pressed and cut to obtain the positive electrode sheet.

[0156] 4) Preparation of negative electrode sheet

[0157] The negative electrode active material hard carbon, the conductive agent acetylene black, the binder styrene butadiene rubber (SBR), and the thickener sodium carboxymethyl cellulose (CMC-Na) were fully stirred and mixed in a deionized water solvent system in a weight ratio of 90:4:4:2, and a certain amount of CaO was added so that the mass proportion of calcium in the slurry dry material (i.e., the total mass of the negative electrode active material hard carbon, the conductive agent acetylene black, the binder styrene butadiene rubber (SBR), the thickener sodium carboxymethyl cellulose (CMC-Na), and CaO) was 100 ppm to obtain a negative electrode slurry; the negative electrode slurry was mixed at 0.14 g (dry weight) / 1540.25 mm 2 The amount of the coating was evenly coated on the negative electrode current collector copper foil with a thickness of 13 μm; the copper foil was dried at room temperature and then transferred to a 120°C oven for drying for 1 hour, and then cold pressed and cut to obtain the negative electrode sheet.

[0158] 5) Isolation film

[0159] A 9 μm polyethylene (PE) porous polymer film was used as the separator.

[0160] 6) Preparation of batteries

[0161] The positive electrode sheet, the separator, and the negative electrode sheet are stacked in order, so that the separator is placed between the positive and negative electrode sheets to isolate the positive and negative electrode sheets. The bare battery cell is wound, the tabs are welded, and the bare battery cell is placed in an outer package. The above-prepared electrolyte is injected into the dried battery cell, and then the sodium secondary battery product of Example 1 is obtained after packaging, standing, formation, shaping, and capacity testing.

[0162] The preparation methods of the sodium secondary batteries of Examples 2-4 are substantially the same as the preparation method of Example 1, except that the type of the fluorinated carbonate compound is adjusted. Specific parameters are shown in Table 1.

[0163] The preparation methods of the sodium secondary batteries of Examples 5-8 are basically the same as the preparation method of Example 1, except that the mass ratio of the fluorinated carbonate compound based on the electrolyte is adjusted. Specific parameters are shown in Table 1.

[0164] The preparation methods of the sodium secondary batteries of Examples 9-12 are basically the same as the preparation method of Example 1, except that the mass ratio of calcium element in the negative electrode film layer is adjusted by adding different amounts of CaO to the negative electrode slurry. Specific parameters are shown in Table 2.

[0165] The preparation methods of the sodium secondary batteries of Examples 13-18 are basically the same as the preparation method of Example 1, except that a second component, sodium difluorooxalatoborate, is added to the electrolyte, and the mass ratio of sodium difluorooxalatoborate and / or the fluorocarbonate compound is adjusted. Specific parameters are shown in Table 1.

[0166] The preparation method of the sodium secondary battery of Example 19 is basically the same as the preparation method of Example 14, except that the type of the second component is adjusted. Specific parameters are shown in Table 1.

[0167] The preparation methods of the sodium secondary batteries of Examples 20-23 are substantially the same as those of Example 1, except that the preparation processes of the positive electrode active material and the positive electrode sheet are adjusted to adjust the mass ratio of the copper element in the positive electrode active material. Specific parameters are shown in Table 2. The preparation process is as follows:

[0168] In Example 20, the copper element mass percentage of the positive electrode active material in the positive electrode plate is 0%, and the preparation method is:

[0169] Na 1 / 2 Fe 1 / 2 Mn 1 / 2 Preparation of O2: 0.25 mol Na2CO3, 0.25 mol Fe2O3, and 0.5 mol MnO2 precursors are ball-milled in a ball mill with ethanol as a dispersant for 12 hours. After drying, the evenly mixed powder is pressed into a tablet at 20 MPa and sintered at 900°C for 12 hours. The sintered powder needs to be quickly transferred to a glove box for storage.

[0170] The positive electrode active material Na 1 / 2 Fe 1 / 2 Mn 1 / 2 O2, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) were fully stirred and mixed in N-methylpyrrolidone solvent system at a weight ratio of 90:5:5 to obtain positive electrode slurry; the positive electrode slurry was added at 0.28 g (dry weight) / 1540.25 mm 2 The amount of the coating was evenly coated on the positive electrode current collector aluminum foil with a thickness of 13 μm; the aluminum foil was dried at room temperature and then transferred to a 120°C oven for drying for 1 hour, and then cold pressed and cut to obtain the positive electrode sheet.

[0171] In Example 21, the copper element in the positive electrode active material of the positive electrode plate accounts for 6.5% by mass, and the preparation method is as follows:

[0172] The positive electrode active material (50 wt% Na 1 / 2 Fe 1 / 2 Mn 1 / 2 O2 and 50wt% Na 7 / 9 Cu 2 / 9 Fe 1 / 9 Mn 2 / 3 O2), conductive agent acetylene black, binder polyvinylidene fluoride (PVDF) in a weight ratio of 90:5:5 in N-methylpyrrolidone solvent system and fully stirred and mixed to obtain positive electrode slurry; the positive electrode slurry was added at 0.28g (dry weight) / 1540.25mm 2 The amount of the coating was evenly coated on the positive electrode current collector aluminum foil with a thickness of 13 μm; the aluminum foil was dried at room temperature and then transferred to a 120°C oven for drying for 1 hour, and then cold pressed and cut to obtain the positive electrode sheet.

[0173] In Example 22, the copper element in the positive electrode active material of the positive electrode plate accounts for 23% by weight, and the preparation method is as follows:

[0174] Na 9 / 10 Cu 2 / 5 Fe 1 / 10 Mn 1 / 2 Preparation of O2: 0.45 mol Na2CO3, 0.4 mol CuO, 0.05 mol Fe2O3, and 0.5 mol MnO2 precursors are ball-milled in a ball mill with ethanol as a dispersant for 12 hours. After drying, the evenly mixed powder is pressed into a tablet at 20 MPa and sintered at 900°C for 12 hours. The sintered powder needs to be quickly transferred to a glove box for storage.

[0175] The positive electrode active material Na 9 / 10 Cu 2 / 5 Fe 1 / 10 Mn 1 / 2 O2, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) were fully stirred and mixed in N-methylpyrrolidone solvent system at a weight ratio of 90:5:5 to obtain positive electrode slurry; the positive electrode slurry was added at 0.28 g (dry weight) / 1540.25 mm 2 The amount of the coating was evenly coated on the positive electrode current collector aluminum foil with a thickness of 13 μm; the aluminum foil was dried at room temperature and then transferred to a 120°C oven for drying for 1 hour, and then cold pressed and cut to obtain the positive electrode sheet.

[0176] In Example 23, the copper element in the positive electrode active material of the positive electrode plate accounts for 18% by weight, and the preparation method is:

[0177] The positive electrode active material (50 wt% Na 7 / 9 Cu 2 / 9 Fe 1 / 9 Mn 2 / 3 O2 and 50wt% Na 9 / 10 Cu 2 / 5 Fe 1 / 10 Mn 1 / 2 O2), conductive agent acetylene black, binder polyvinylidene fluoride (PVDF) in a weight ratio of 90:5:5 in N-methylpyrrolidone solvent system and fully stirred and mixed to obtain positive electrode slurry; the positive electrode slurry was added at 0.28g (dry weight) / 1540.25mm 2 The amount of the coating was evenly coated on the positive electrode current collector aluminum foil with a thickness of 13 μm; the aluminum foil was dried at room temperature and then transferred to a 120°C oven for drying for 1 hour, and then cold pressed and cut to obtain the positive electrode sheet.

[0178] The preparation methods of the sodium secondary batteries of Examples 24-26 are substantially the same as the preparation method of Example 1, except that a third component is further added to the electrolyte. Specific parameters are shown in Table 1.

[0179] Comparative Example 1

[0180] Compared with Example 1, the electrolyte of Comparative Example 1 does not contain a fluorinated carbonate compound. For specific parameters, see Table 1.

[0181] Comparative Example 2

[0182] Compared with Example 5, the negative electrode film layer of Comparative Example 2 does not contain calcium element. For specific parameters, see Table 1.

[0183] Comparative Example 3

[0184] Compared with Example 1, the electrolyte of Comparative Example 3 does not contain a fluorinated carbonate compound, and the negative electrode film layer does not contain calcium. For specific parameters, see Table 1.

[0185] 2. Battery performance test

[0186] High temperature storage volume change rate

[0187] At 25°C, the sodium secondary batteries prepared in the examples and comparative examples were left for 5 minutes, charged to 4.0V at a constant current rate of 1C, then charged at a constant voltage until the current was less than or equal to 0.05C, and then left for 5 minutes. They were then discharged to 1.5V at a constant current rate of 1C, and the volume V1 of the battery was tested by the water displacement method. The battery was then placed in a 60°C oven and stored for 2 months. The battery was taken out and the test volume was V2. The volume change rate of the battery was =(V2-V1) / V1×100%.

[0188] 2. Charging performance at -10℃

[0189] A three-electrode battery containing a reference was prepared, where the reference electrode was sodium vanadium phosphate. The battery was charged at 25°C at a constant current of 0.1C to a voltage of 4.0V, then charged at a constant voltage to a current of less than or equal to 0.05C, then left for 5 minutes, and then discharged at a constant current of 1C to 1.5V, and the discharge capacity was recorded as C1. The battery was then placed in a -10°C environment and allowed to rest for 2 hours, and charged at a constant current of 0.1C to a voltage of 4.0V. The charging capacity before the negative electrode potential was compared to the reference potential of -3.377V was C2, and the -10°C charging capacity of the battery was = C2 / C1×100%.

[0190] 3. Determination of the mass ratio of calcium in the negative electrode film

[0191] The mass percentage of calcium in the negative electrode film can be determined by referring to the general principle EPA 6010D-2014 and using inductively coupled plasma atomic emission spectrometry. The mass percentage of calcium in the negative electrode film is calculated by dividing the mass of the negative electrode film sample by the mass of the negative electrode film sample.

[0192] 4. Determination of the mass ratio of copper element in positive electrode active material

[0193] The mass percentage of Cu in the positive electrode active material can be determined by referring to the general principle EPA 6010D-2014 and using inductively coupled plasma atomic emission spectrometry. The mass percentage of Cu in the positive electrode active material is calculated by dividing the mass of the Cu in the positive electrode active material sample by the mass of the positive electrode active material sample.

[0194] 5. First Coulomb efficiency (first efficiency) test

[0195] After the battery is filled with liquid, at 25°C, the prepared battery is charged to 4.0V at a constant current of 0.1C, and then charged at a constant voltage of 4.0V until the current drops to 0.05C, which is recorded as the battery's first charge capacity C1. After standing for 5 minutes, it is discharged to 1.5V at a constant current of 0.1C, which is the battery's first discharge capacity (C2). The battery's first efficiency is = C2 / C1×100%.

[0196] 6. Battery cycle capacity retention rate

[0197] At 25°C, the prepared battery was charged to 4.0V at a constant current of 1C, then charged at a constant voltage of 4.0V until the current dropped to 0.05C. After standing for 5 minutes, it was discharged to 1.5V at a constant current of 1C. This was the first charge / discharge cycle of the battery, and the discharge capacity of this time was recorded as the discharge capacity of the battery in the first cycle (C0). The above steps were repeated for the same battery, and the discharge capacity of the battery after the 400th cycle was (C1). The capacity retention rate after 400 cycles = C1 / C0×100%.

[0198] 3. Analysis of test results of various embodiments and comparative examples

[0199] Batteries of various examples and comparative examples were prepared according to the above methods, and various performance parameters were measured. The results are shown in the table below.

[0200] Table 1

[0201] Table 2

[0202] Table 3

[0203] According to the above results, the sodium secondary batteries in Examples 1-26 all include a negative electrode plate and an electrolyte, the negative electrode plate includes a negative electrode film layer, and the negative electrode film layer contains calcium; the electrolyte includes a first component, and the first component is a fluorinated carbonate compound.

[0204] From the comparison of Examples 1-26 and Comparative Examples 1-3, it can be seen that the sodium secondary battery in which the negative electrode film layer contains calcium and the electrolyte includes a fluorinated carbonate compound has excellent cycle stability, low-temperature charging performance and first efficiency, and has a low volume expansion rate after high-temperature storage.

[0205] As shown in Examples 1 and 5-8, when the mass percentage a of the fluorinated carbonate compound is 0.01%-10% based on the total mass of the electrolyte, the sodium secondary battery has a low volume expansion rate after high-temperature storage and excellent low-temperature charging performance, initial efficiency, and cycling stability. When the mass percentage a of the fluorinated carbonate compound is 0.2%-10%, the volume expansion rate after high-temperature storage, low-temperature charging performance, initial efficiency, and cycling stability of the sodium secondary battery can be further improved.

[0206] It can be seen from Example 1 and Examples 9-12 that, based on the total mass of the negative electrode film layer, when the mass proportion b of the calcium element in the negative electrode film layer is 1ppm-3000ppm, the sodium secondary battery has a low battery volume expansion rate after high-temperature storage, and excellent low-temperature charging performance, first efficiency and cycle stability. When the mass proportion b of the calcium element in the negative electrode film layer is 50ppm-1000ppm, the sodium secondary battery takes into account a low battery volume expansion rate after high-temperature storage, and more excellent low-temperature charging performance, first efficiency and cycle stability. It can be seen from Example 1 and Examples 5-12 and Example 17 that when a / b satisfies a / b≥1, the sodium secondary battery has a low battery volume expansion rate after high-temperature storage, and excellent low-temperature charging performance, first efficiency and cycle stability. When a / b satisfies 10000≥a / b≥10, the sodium secondary battery takes into account a low battery volume expansion rate after high-temperature storage, excellent low-temperature charging performance, first efficiency and cycle stability.

[0207] Comparison of Examples 1, 5, and 13-19 shows that, based on the total mass of the electrolyte, when the mass percentage c of difluorooxalatoborate is 0.01%-5%, the volume expansion rate of the battery after high-temperature storage can be reduced, and the low-temperature charging performance, initial efficiency, and cycle stability of the battery can be improved. When the mass percentage c of difluorooxalatoborate is 0.1%-2%, the volume expansion rate of the battery after high-temperature storage can be further reduced, and the low-temperature charging performance, initial efficiency, and cycle stability of the battery can be improved.

[0208] It can be seen from Examples 13-19 that when c / a satisfies ≥0.001, the sodium secondary battery has a low battery volume expansion rate after high-temperature storage, excellent low-temperature charging performance, first efficiency and cycle stability. When c / a satisfies 2≥c / a≥0.1, the sodium secondary battery takes into account a low battery volume expansion rate after high-temperature storage, and more excellent low-temperature charging performance, first efficiency and cycle stability. It can be seen from Examples 1 and 20-23 that based on the total mass of the positive electrode active material, when the mass proportion d of the copper element is 0% to 23%, the sodium secondary battery has a low battery volume expansion rate after high-temperature storage, excellent low-temperature charging performance, first efficiency and cycle stability. When the mass proportion d of the copper element is 6.5% to 18%, the sodium secondary battery takes into account a low battery volume expansion rate after high-temperature storage, excellent low-temperature charging performance, first efficiency and cycle stability.

[0209] From the comparison between Example 1 and Examples 24-26, it can be seen that when the third component is included in the electrolyte, the battery volume expansion rate after high-temperature storage, low-temperature charging performance, first efficiency and cycle stability of the sodium secondary battery can be further optimized.

[0210] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A sodium secondary battery comprising: A negative electrode sheet, wherein the negative electrode sheet comprises a negative electrode film layer, and the negative electrode film layer contains calcium; and The electrolyte comprises a first component, wherein the first component is a fluorinated carbonate compound.

2. The sodium secondary battery according to claim 1, wherein Based on the total mass of the electrolyte, the mass proportion of the fluorinated carbonate compound is a, based on the total mass of the negative electrode film layer, the mass proportion of the calcium element in the negative electrode film layer is b, and a and b satisfy: a / b≥1; optionally, a and b satisfy: 10000≥a / b≥10.

3. The sodium secondary battery according to claim 1 or 2, wherein: Based on the total mass of the electrolyte, the mass proportion a of the fluorinated carbonate compound is 0.01% to 10%; optionally, a is 0.2% to 10%.

4. The sodium secondary battery according to claim 1 or 2, wherein Based on the total mass of the negative electrode film layer, the mass proportion b of the calcium element in the negative electrode film layer is 1 ppm to 3000 ppm; optionally, b is 50 ppm to 1000 ppm.

5. The sodium secondary battery according to claim 1 or 2, wherein The fluorocarbonate compound includes a compound represented by formula I, Wherein, R1, R2, R3, and R4 each independently include a hydrogen atom, a halogen atom, a C 1-6 Hydrocarbon, C 1-3 Haloalkyl, C 1-3 Alkoxy, C 1-3 At least one of a halogenated alkoxy group, an ester group, a cyano group, a sulfonic acid group, and an isocyanate group; and at least one of R1, R2, R3, and R4 is a fluorine atom.

6. The sodium secondary battery according to claim 1 or 2, wherein: The fluorinated carbonate compound comprises at least one of the following compounds, 7. The sodium secondary battery according to claim 1, wherein The electrolyte further includes a second component, wherein the second component is difluorooxalate borate, and the difluorooxalate borate satisfies at least one of the following conditions: (1) Based on the total mass of the electrolyte, the mass proportion of the difluorooxalatoborate is c, the mass proportion of the fluorocarbonate compound is a, and c and a satisfy: c / a≥0.001; optionally 2≥c / a≥0.1; (2) Based on the total mass of the electrolyte, the mass proportion c of the difluorooxalate borate is 0.01%-5%; The optional range is 0.1%-2%.

8. The sodium secondary battery according to claim 7, wherein The chemical formula of the difluorooxalate borate is shown in Formula III, (F2C2O4B) y M Type III Wherein, M includes one or more of Li, Na, K, Rb, Cs, Mg, Ca, Ba, Fe, Ni, and Al, and y is 1, 2, or 3.

9. The sodium secondary battery according to claim 1, wherein The negative electrode film layer includes a negative electrode active material, and the negative electrode active material includes one or more of hard carbon, sodium metal, sodium-tin alloy, and metal oxide.

10. The sodium secondary battery according to claim 1, wherein The sodium secondary battery also includes a positive electrode sheet, the positive electrode sheet includes a positive electrode active material, and the positive electrode active material includes a sodium transition metal oxide; 11. The sodium secondary battery according to claim 10, wherein The sodium transition metal oxide includes Na m Cu n X o Fe p Mn q O 2-s， Wherein X includes one or more of Li, Be, B, Mg, Al, K, Ca, Ti, Co, Ni, Zn, Ga, Sr, Y, Nb, Mo, In, Sn, Fe, Ba, 0≤m≤0.5, 0≤n≤0.5, 0≤o<0.5, 0≤p≤0.5, 0 <q≤0.68,n+o+p+q=1,0≤s<0.2; Optionally, the sodium transition metal oxide comprises Na[Cu 1 / 9 Ni 2 / 9 Fe 1 / 3 Mn 1 / 3 ]O2、Na 7 / 9 [Cu 2 / 9 Fe 1 / 9 Mn 2 / 3 ]O2、Na 9 / 10 [Cu 2 / 5 Fe 1 / 10 Mn 1 / 2 ]At least one of O2.

12. The sodium secondary battery according to claim 10, wherein The positive electrode active material further contains copper element. Based on the total mass of the positive electrode active material, the mass proportion d of the copper element is 0.01% to 23%; optionally 6.5% to 18%.

13. The sodium secondary battery according to claim 1, wherein The electrolyte also includes a third component, which is one or more of vinylene carbonate, ethylene carbonate, 1,3-propane sultone, 1,3-propylene sultone, vinyl sulfate, maleic anhydride, succinic anhydride, triallyl phosphate, sodium tetrafluorooxalate phosphate, sodium difluorobisoxalate phosphate, sodium difluorophosphate, and sodium fluorosulfonate.

14. The sodium secondary battery according to claim 13, wherein Based on the total mass of the electrolyte, the mass proportion of the third component is 0.01% to 10%; optionally 0.1% to 5%.

15. An electrical device comprising the sodium secondary battery according to any one of claims 1 to 14.

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