Electrolyte, sodium secondary battery and electrical apparatus

Abstract

An electrolyte, a sodium secondary battery and an electrical apparatus. The electrolyte comprises a first additive and a second additive, the first additive comprising difluoro(oxalato)borate, and the second additive comprising one or more of fluorosulfonate and difluorophosphate. The combined use of the first additive and the second additive facilitates improving the stability of an SEI membrane, such that the amount of gas produced by the sodium secondary battery during a cycle process and a storage process is reduced, thereby improving the fast charging performance and storage performance of a sodium secondary battery.

Description

Electrolyte, sodium secondary battery and power-consuming device

[0001] Cross-references

[0002] This application claims priority to Chinese Patent Application No. 202311485830.3, filed on November 9, 2023, entitled “Electrolyte, 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 sodium batteries, and in particular to an electrolyte, a sodium secondary battery, and an electrical device. Background Art

[0004] In recent years, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields.

[0005] Compared to lithium-ion batteries, sodium-ion batteries offer a significant competitive advantage due to their abundant and widespread sodium resources. However, the gassing problem in sodium-ion batteries severely impacts their electrical performance, making them unable to meet the application needs of the next generation of electrochemical systems.

[0006] Summary of the Invention

[0007] The present application is made in view of the above-mentioned problems, and its purpose is to provide an electrolyte to improve the stability of the solid electrolyte membrane (SEI membrane), thereby reducing the gas production during the cycle and storage process of the sodium secondary battery, improving the storage performance, fast charging performance and cycle performance of the sodium secondary battery, and comprehensively improving the performance of the sodium secondary battery.

[0008] In a first aspect of the present application, an electrolyte for a sodium secondary battery is provided, comprising a first additive and a second additive, wherein the first additive comprises difluorooxalatoborate, and the second additive comprises one or more of fluorosulfonate and difluorophosphate, wherein the fluorosulfonate comprises a compound represented by formula II.

[0009] (FSO3) y2 M2 y2+ Formula II

[0010] Among them, M2 y2+ Including Li + 、Na + , K + , Rb + 、Cs + Mg 2+ , Ca 2+ 、Ba 2+ 、Fe2+ 、Ni 2+ 、Al 3+ 、Fe 3+ 、Ni 3+ One or more of , y2=1, 2 or 3.

[0011] The first additive, comprising difluorooxalatoborate, and / or the second additive, comprising difluorophosphate, can form a film at the negative electrode before the solvent undergoes reduction. This generates oxalic acid-based components and other components containing fluorine, sulfur, or phosphorus in the SEI film, reducing the solubility of the SEI film in the electrolyte and significantly reducing gas production. The synergistic effect of the two improves the stability of the SEI film while also maintaining its flexibility, reducing the DC impedance of the sodium secondary battery, improving its storage performance and fast-charging performance, and comprehensively improving its cycling performance.

[0012] In any embodiment, the difluorooxalate borate comprises a compound of formula I,

[0013] (F2C2O4B) y1 M1 y1+ Formula I

[0014] Among them, M1 y1+ Including Li + 、Na + , K + , Rb + 、Cs + Mg 2+ , Ca 2+ 、Ba 2+ 、Fe 2+ 、Ni 2+ 、Al 3+ 、Fe 3+ 、Ni 3+ One or more of , y1 = 1, 2 or 3.

[0015] The above-mentioned difluorooxalatoborate can generate oxalic acid-based components in the SEI film, and the SEI film can cover the surface of the negative electrode to reduce the degree of exposure of the negative electrode to the electrolyte, reduce side reactions and gas production, and improve the performance of sodium secondary batteries.

[0016] In any embodiment, the difluorophosphate comprises a compound of formula III,

[0017] (PO2F2) y3 M3 y3+ Formula III

[0018] Among them, M3 y3+ Including Li + 、Na + , K+ , Rb + 、Cs + Mg 2+ , Ca 2+ 、Ba 2+ 、Fe 2+ 、Ni 2+ 、Al 3+ 、Fe 3+ 、Ni 3+ One or more of , y3 = 1, 2 or 3.

[0019] The above-mentioned fluorosulfonates or difluorophosphates can form other components containing fluorine, sulfur or phosphorus elements in the SEI film, which can improve the stability of the SEI film on the surface of the negative electrode, reduce the overall oxidative decomposition degree of the SEI film, and its solubility in the electrolyte solvent, thereby improving the storage performance of the sodium secondary battery.

[0020] In any embodiment, the mass ratio of the first additive to the second additive is 0.001 to 1000.

[0021] Controlling the mass ratio of the first additive to the second additive within an appropriate range can reduce the solubility of the SEI film in the electrolyte, thereby significantly reducing gas generation. Simultaneously, the synergistic effect of the two additives can balance the storage performance and DC impedance of the sodium secondary battery, comprehensively improving its performance.

[0022] In any embodiment, the mass ratio of the first additive to the second additive is 0.01-200.

[0023] Controlling the mass ratio of the first additive to the second additive to be 0.01 to 200 is beneficial to further improving the cycle performance of the sodium secondary battery.

[0024] In any embodiment, the mass content of difluorooxalatoborate is 0.001% to 5% based on the total mass of the electrolyte.

[0025] Controlling the mass content of difluorooxalatoborate within an appropriate range is beneficial to the generation of oxalic acid-based components in the SEI film, reducing the degree of overall rupture of the SEI film caused by the expansion of the sodium secondary battery during the recycling process of the sodium secondary battery, thereby improving the overall protection ability of the SEI film and the storage performance of the sodium secondary battery.

[0026] In any embodiment, the mass content of difluorooxalatoborate is 0.1% to 2% based on the total mass of the electrolyte.

[0027] Controlling the mass content of difluorooxalatoborate to 0.1% to 2% is beneficial to taking into account both the storage performance and fast charging performance of the sodium secondary battery, thereby comprehensively improving the performance of the sodium secondary battery.

[0028] In any embodiment, the mass content of the fluorosulfonate is 0.001% to 5% based on the total mass of the electrolyte.

[0029] Controlling the mass content of fluorosulfonate within an appropriate range is conducive to the formation of components containing fluorine and sulfur elements in the SEI film, thereby effectively improving the overall stability of the interfacial SEI film and reducing the solubility of the interfacial SEI film in the electrolyte, thereby greatly reducing the degree of gas production and improving the cycle performance and storage performance of sodium secondary batteries.

[0030] In any embodiment, the mass content of the fluorosulfonate is 0.1% to 2% based on the total mass of the electrolyte.

[0031] Controlling the mass content of fluorosulfonate to 0.1% to 2% is beneficial to taking into account both the storage performance and fast charging performance of the sodium secondary battery, thereby comprehensively improving the performance of the sodium secondary battery.

[0032] In any embodiment, the mass content of the difluorophosphate is 0.001% to 5% based on the total mass of the electrolyte.

[0033] Controlling the mass content of difluorophosphate within an appropriate range is conducive to the formation of components containing fluorine and phosphorus elements in the SEI film, thereby effectively improving the overall stability of the interfacial SEI film and reducing the solubility of the interfacial SEI film in the electrolyte, thereby greatly reducing the degree of gas production and improving the cycle performance and storage performance of sodium secondary batteries.

[0034] In any embodiment, the mass content of the difluorophosphate is 0.1% to 2% based on the total mass of the electrolyte.

[0035] Controlling the mass content of difluorophosphate to 0.1% to 2% is beneficial to taking into account both the storage performance and fast charging performance of the sodium secondary battery, thereby comprehensively improving the performance of the sodium secondary battery.

[0036] In any embodiment, the electrolyte further includes a third additive, and the third additive includes one or more of fluoroethylene carbonate, bisfluoroethylene carbonate, vinylene carbonate, vinylethylene carbonate, maleic anhydride, succinic anhydride, and triallyl phosphate.

[0037] The introduction of the third additive containing unsaturated functional groups is conducive to forming a more soluble SEI film on the surface of the negative electrode plate in preference to the solvent in the electrolyte, reducing the overall solubility of the SEI film in the electrolyte, reducing the degree of exposure of the negative electrode plate to the electrolyte, and thereby reducing the degree of gas production, thereby comprehensively improving the performance of the battery.

[0038] In any embodiment, the mass content of the third additive is 0.01% to 10% based on the total mass of the electrolyte.

[0039] Controlling the mass content of the third additive within an appropriate range can provide enough third additive to form a SEI film with higher solubility, reduce the overall solubility of the SEI film in the electrolyte, reduce the degree of exposure of the negative electrode to the electrolyte, and thus reduce the degree of gas production, thereby comprehensively improving the performance of the battery.

[0040] In any embodiment, the mass content of the third additive is 0.1% to 5% based on the total mass of the electrolyte.

[0041] Controlling the mass content of the third additive to 0.1% to 5% is beneficial to taking into account both the storage performance and fast charging performance of the sodium secondary battery, thereby comprehensively improving the performance of the sodium secondary battery.

[0042] In any embodiment, the electrolyte includes a sodium salt, and the sodium salt includes one or more of NaPF6, NaFSI, NaBF4, NaN(SO2F)2, NaClO4, NaAsF6, NaB(C2O4)2, and NaBF2(C2O4).

[0043] In any embodiment, the electrolyte includes a solvent, and the solvent includes one or more of a chain carbonate solvent, a chain carboxylate solvent, a cyclic carbonate solvent, and an ether solvent.

[0044] In any embodiment, the linear carbonate solvent includes one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, methyl butyl carbonate, ethyl propyl carbonate, dipropyl carbonate, and dibutyl carbonate;

[0045] The chain carboxylate solvent includes one or more of methyl formate, ethyl formate, methyl propionate, ethyl propionate, propyl propionate, ethyl butyrate, methyl acetate, ethyl acetate, and propyl acetate;

[0046] The cyclic carbonate includes one or more of ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, ethylene sulfite, propylene sulfite, vinylethylene carbonate, 4-ethynyl-1,3-dioxolane-2-one, cis-4,5-difluoro-1,3-dioxolane-2-one, and trans-4,5-difluoro-1,3-dioxolane-2-one;

[0047] The ether solvent includes one or more of dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, 1,2-dimethoxyethane, diethylene glycol dimethyl ether, 1,2-diethoxyethane, and 1,2-dibutoxyethane.

[0048] A second aspect of the present application provides a sodium secondary battery, comprising a positive electrode sheet, a negative electrode sheet, and the electrolyte of the first aspect of the present application.

[0049] In any embodiment, the positive electrode sheet includes a positive electrode current collector and a positive electrode material layer located on at least one side of the positive electrode current collector, the positive electrode material layer includes a positive electrode active material, and the positive electrode active material includes at least one of a layered transition metal oxide, a Prussian blue compound, and a polyanion compound.

[0050] In any embodiment, the layered transition metal oxide has the general formula Na x Mn a Fe b Ni c M d N e O 2-δ Q f , wherein M includes at least one of Ti, Li, V, Cr, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, Mg, and Al, N includes at least one of Si, P, B, S, and Se, Q includes at least one of F, Cl, and N, 0.66≤x≤1, 0 <a≤0.7,0<b≤0.7,0<c≤0.23,0≤d<0.3,0≤e≤0.3,0≤f≤0.3,0≤δ≤0.3,a+b+c+d+e=1,0<e+f≤0.3,0<(e+f) / a≤0.3,0.2≤d+e+f≤0.3,(b+c) / a≤1.5。

[0051] In any embodiment, the positive electrode active material includes a layered transition metal oxide containing Cu element;

[0052] The ratio of the mass content of difluorooxalatoborate in the electrolyte to the mass content of the Cu element in the positive electrode active material is greater than or equal to 0.002.

[0053] Introducing Cu into the positive electrode active material contributes to the structural stability of the positive electrode active material. Furthermore, the Cu in the positive electrode active material can oxidize with the decomposition products of difluorooxalatoborate at the positive electrode plate interface to form an insoluble Cu salt, which helps reduce the extent of increased gas production caused by oxidative decomposition of the electrolyte on the positive electrode plate side. Controlling the ratio of the mass content of difluorooxalatoborate in the electrolyte to the mass content of Cu in the positive electrode active material within an appropriate range can effectively reduce the extent of increased gas production caused by oxidative decomposition of the electrolyte on the positive electrode plate side.

[0054] In any embodiment, the mass content of Cu element in the positive electrode active material is less than or equal to 23%.

[0055] Controlling the mass content of the Cu element in the positive electrode active material within an appropriate range is beneficial for providing sufficient Cu elements to increase the stability of the positive electrode active material structure, and can also reduce the degree of deterioration of the performance of the sodium secondary battery caused by the oxidative decomposition of the added electrolyte due to excessive Cu element mass content in the positive electrode active material.

[0056] In any embodiment, the mass content of Cu element in the positive electrode active material is 6.5% to 18%.

[0057] Controlling the mass content of the Cu element in the positive electrode active material to 6.5% to 18% is beneficial to further balancing the storage performance and fast charging performance of the sodium secondary battery.

[0058] In any embodiment, the negative electrode sheet includes a negative electrode current collector and a negative electrode material layer located on at least one side of the negative electrode current collector, and the negative electrode material layer includes Ca element;

[0059] The ratio of the mass content of difluorooxalatoborate in the electrolyte to the mass content of the Ca element in the negative electrode material layer is greater than or equal to 1.

[0060] Introducing Ca into the negative electrode active material helps reduce the formation of sodium dendrites. It also facilitates the formation of a calcium salt-containing SEI film with the reduction product of difluorooxalatoborate, thereby improving the toughness of the SEI film and reducing the DC impedance of the sodium secondary battery. By controlling the ratio of the mass content of difluorooxalatoborate in the electrolyte to the mass content of Ca in the negative electrode material layer within an appropriate range, both the storage performance and fast charging performance of the sodium secondary battery can be balanced.

[0061] In any embodiment, the mass content of Ca element in the negative electrode material layer is 0.1 ppm to 3000 ppm.

[0062] Controlling the mass content of the Ca element in the negative electrode material layer within an appropriate range can not only form a SEI film containing calcium salt components, improve the toughness of the SEI film, and reduce the DC impedance of the sodium secondary battery, but also reduce the degree of deterioration of the sodium secondary battery performance caused by the increase in the DC impedance of the sodium secondary battery, while taking into account the storage performance and fast charging performance of the sodium secondary battery.

[0063] In any embodiment, the mass content of Ca element in the negative electrode material layer is 2 ppm to 1000 ppm.

[0064] In any embodiment, the mass content of Ca element in the negative electrode material layer is 100 ppm to 1000 ppm.

[0065] Controlling the mass content of the Ca element in the negative electrode material layer to 2ppm to 1000ppm or 100ppm to 1000ppm is beneficial to further comprehensively improve the storage performance and fast charging performance of the sodium secondary battery.

[0066] In any embodiment, the negative electrode material layer further includes a negative electrode active material, and the negative electrode active material includes one or more of hard carbon, metallic sodium, tin alloy, and metal oxide.

[0067] In any embodiment, at a charge rate of 0.05C, the capacity of the negative electrode sheet in the charge range of 0.5V to 1V is 9mAh / g to 140mAh / g.

[0068] In the charging range of 0.5V to 1V, controlling the capacity of the negative electrode plate within an appropriate range can not only provide sufficient negative electrode plate capacity to meet the energy density requirements of the sodium secondary battery, but also reduce the degree of deterioration of the sodium secondary battery performance caused by increased gas production due to excessive negative electrode plate capacity.

[0069] In any embodiment, at a charge rate of 0.05C, the capacity of the negative electrode sheet in the charge range of 0.5V to 1V is 18mAh / g to 70mAh / g.

[0070] In the charging range of 0.5V to 1V, in some embodiments, the capacity of the negative electrode is controlled to be 18mAh / g to 70mAh / g, which is beneficial to further balance the energy density and storage performance of the sodium secondary battery.

[0071] A third aspect of the present application provides an electrical device comprising the sodium secondary battery according to the second aspect of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments of the present application. 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 the drawings without creative work.

[0073] FIG1 is a schematic diagram of a sodium secondary battery according to an embodiment of the present application;

[0074] FIG2 is an exploded view of the sodium secondary battery according to one embodiment of the present application shown in FIG1 ;

[0075] FIG3 is a schematic diagram of a battery module according to an embodiment of the present application;

[0076] FIG4 is a schematic diagram of a battery pack according to an embodiment of the present application;

[0077] FIG5 is an exploded view of the battery pack according to an embodiment of the present application shown in FIG4 ;

[0078] FIG6 is a schematic diagram of an electric device using a sodium secondary battery as a power source according to an embodiment of the present application.

[0079] Description of reference numerals:

[0080] 1. Battery pack; 2. Upper box; 3. Lower box; 4. Battery module; 5. Sodium secondary battery; 5. Casing; 5. Electrode assembly; 5. Cover plate. DETAILED DESCRIPTION

[0081] Below, the embodiments of the electrolyte, sodium secondary battery and 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 are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of 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.

[0082] " 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.

[0083] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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).

[0088] The gas production problem in sodium secondary batteries seriously affects their electrical performance, especially the gas production problem on the negative electrode side of sodium secondary batteries at low voltage. Usually, a film-forming additive is introduced into the electrolyte to form an SEI (Solid Electrolyte Interphase) film on the surface of the negative electrode. The formation of the SEI film can prevent the electrolyte from further decomposing and producing gas to a certain extent, thereby improving the performance of the sodium secondary battery. However, sodium secondary batteries still have gas production problems during the recycling process and storage process. Therefore, it is necessary to design an electrolyte to meet the application needs of the new generation of electrochemical systems.

[0089] [Electrolyte]

[0090] Based on this, the present application proposes an electrolyte for a sodium secondary battery, which includes a first additive and a second additive, wherein the first additive includes difluorooxalatoborate, and the second additive includes one or more of fluorosulfonate and difluorophosphate, wherein the fluorosulfonate includes a compound represented by formula II.

[0091] (FSO3) y2 M2 y2+ Formula II

[0092] Among them, M2y2+ Including Li + 、Na + , K + , Rb + 、Cs + Mg 2+ , Ca 2+ 、Ba 2+ 、Fe 2+ 、Ni 2+ 、Al 3+ 、Fe 3+ 、Ni 3+ One or more of , y2=1, 2 or 3.

[0093] In this context, "difluorooxalatoborate" refers to a salt with an anion of Cations include but are not limited to Li + 、Na + , K + , Rb + 、Cs + Mg 2+ , Ca 2+ 、Ba 2+ 、Fe 2+ 、Ni 2+ 、Al 3+ 、Fe 3+ 、Ni 3+ One or more compounds.

[0094] In this context, "fluorosulfonate" refers to a salt with an anion of Cations include but are not limited to Li + 、Na + , K + , Rb + 、Cs + Mg 2+ , Ca 2+ 、Ba 2+ 、Fe 2+ 、Ni 2+ 、Al 3+ 、Fe 3+ 、Ni 3+ One or more compounds.

[0095] In this context, "difluorophosphate" refers to a salt with an anion of Cations include but are not limited to Li + 、Na + , K + , Rb + 、Cs + Mg 2+ , Ca 2+ 、Ba 2+、Fe 2+ 、Ni 2+ 、Al 3+ 、Fe 3+ 、Ni 3+ One or more compounds.

[0096] In some embodiments, the second additive includes a fluorosulfonate.

[0097] In some embodiments, the second additive includes a difluorophosphate.

[0098] In some embodiments, the second additive includes a fluorosulfonate and a difluorophosphate.

[0099] In some embodiments, M2 y2+ Including Li + 、Na + One or more of , y2=1.

[0100] In some embodiments, the fluorosulfonate comprises one or more of sodium fluorosulfonate, lithium fluorosulfonate, potassium fluorosulfonate, magnesium fluorosulfonate, and iron fluorosulfonate. In some embodiments, the fluorosulfonate comprises sodium fluorosulfonate. In some embodiments, the fluorosulfonate comprises lithium fluorosulfonate.

[0101] The study found that under low voltage, sodium secondary batteries still produce serious gas during the recycling process and storage process for two reasons: On the one hand, the negative electrode potential of sodium secondary batteries is 0.3V higher than that of lithium secondary batteries. Because the higher the potential, the more it can drive the film-forming additives to form a SEI film mainly composed of organic components, but the organic components are unstable and can begin to oxidize and decompose at 0.5V, producing a large amount of gas. On the other hand, the solubility of the SEI film in the electrolyte of sodium secondary batteries is higher than that of the SEI film in the electrolyte of lithium secondary batteries. The dissolution of the SEI film will cause the negative electrode to be exposed to the electrolyte, causing a side reaction between the negative electrode and the electrolyte to produce a large amount of gas. In addition, the transition metal in the positive electrode active material containing layered metal oxides will catalyze the oxidation of the solvent in the electrolyte to form RH + , due to RH + Not resistant to reduction, exposure of the negative electrode will also cause RH to migrate to the surface of the negative electrode + It is reduced to form components that are not resistant to oxidation and produces a large amount of gas.

[0102] The first additive, comprising difluorooxalatoborate, and the second additive, comprising fluorosulfonate and / or difluorophosphate, can form a film at the negative electrode before the solvent is reduced. This generates oxalic acid-based components and other components containing fluorine, sulfur, or phosphorus in the SEI film, reducing the solubility of the SEI film in the electrolyte and significantly reducing gas production. The synergistic effect of the two enhances the stability of the SEI film while also maintaining its flexibility, reducing the DC impedance of the sodium secondary battery, improving its storage performance and fast-charging performance, and comprehensively improving its cycling performance.

[0103] In some embodiments, the difluorooxalate borate comprises a compound of Formula I,

[0104] (F2C2O4B) y1 M1 y1+ Formula I

[0105] Among them, M1 y1+ Including Li + 、Na + , K + , Rb + 、Cs + Mg 2+ , Ca 2+ 、Ba 2+ 、Fe 2+ 、 Ni 2+ 、Al 3+ 、Fe 3+ 、Ni 3+ One or more of , y1 = 1, 2 or 3.

[0106] In some embodiments, M1 y1+ Including Li + 、Na + One or more of , y1=1.

[0107] In some embodiments, the difluorooxalatoborate comprises one or more of sodium difluorooxalatoborate, lithium difluorooxalatoborate, potassium difluorooxalatoborate, magnesium difluorooxalatoborate, and iron difluorooxalatoborate. In some embodiments, the difluorooxalatoborate comprises sodium difluorooxalatoborate. In some embodiments, the difluorooxalatoborate comprises lithium difluorooxalatoborate.

[0108] The above-mentioned difluorooxalatoborate can generate oxalic acid-based components in the SEI film, and the SEI film can cover the surface of the negative electrode to reduce the degree of exposure of the negative electrode to the electrolyte, reduce side reactions and gas production, and improve the performance of the sodium secondary battery.

[0109] In some embodiments, the difluorophosphate comprises a compound of formula III,

[0110] (PO2F2) y3 M3 y3+ Formula III

[0111] Among them, M3 y3+ Including Li + 、Na + , K + , Rb + 、Cs + Mg 2+ , Ca 2+ 、Ba 2+ 、Fe 2+ 、Ni 2+ 、Al 3+ 、Fe 3+ 、Ni 3+ One or more of , y3 = 1, 2 or 3.

[0112] In some embodiments, M3 y3+ Including Li + 、Na + One or more of , y3=1.

[0113] In some embodiments, the difluorophosphate comprises one or more of sodium difluorophosphate, lithium difluorophosphate, potassium difluorophosphate, magnesium difluorophosphate, and iron difluorophosphate. In some embodiments, the difluorophosphate comprises sodium difluorophosphate. In some embodiments, the difluorophosphate comprises lithium difluorophosphate.

[0114] The above-mentioned fluorosulfonates or difluorophosphates can generate other components containing fluorine, sulfur or phosphorus elements in the SEI film, which can improve the stability of the SEI film on the surface of the negative electrode, reduce the overall oxidative decomposition degree of the SEI film, and its solubility in the electrolyte solvent, thereby reducing the gas production of the sodium secondary battery and improving its storage performance.

[0115] In some embodiments, the mass ratio of the first additive to the second additive is 0.001 to 1000. In some embodiments, the mass ratio of the first additive to the second additive can be 0.001, 0.005, 0.01, 0.05, 0.1, 0.5, 1, 5, 10, 50, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, or a value in a range consisting of any two of the above points.

[0116] Controlling the mass ratio of the first additive to the second additive within an appropriate range can reduce the solubility of the SEI film in the electrolyte, thereby significantly reducing gas generation. Simultaneously, the synergistic effect of the two additives can balance the storage performance and DC impedance of the sodium secondary battery, comprehensively improving its performance.

[0117] In some embodiments, the mass ratio of the first additive to the second additive is 0.01-200.

[0118] Further controlling the mass ratio of the first additive to the second additive to be 0.01 to 200 is beneficial to further improving the cycle performance of the sodium secondary battery.

[0119] In some embodiments, the mass content of difluorooxalatoborate is 0.001% to 5% based on the total mass of the electrolyte. In some embodiments, the mass content of difluorooxalatoborate can be 0.001%, 0.005%, 0.01%, 0.05%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, or a value in a range consisting of any two of the above values, based on the total mass of the electrolyte.

[0120] Controlling the mass content of difluorooxalatoborate within an appropriate range is beneficial to the generation of oxalic acid-based components in the SEI film, reducing the degree of overall rupture of the SEI film caused by the expansion of the sodium secondary battery during the recycling process of the sodium secondary battery, thereby improving the overall protection ability of the SEI film and the storage performance of the sodium secondary battery.

[0121] In some embodiments, the mass content of difluorooxalatoborate is 0.1% to 2% based on the total mass of the electrolyte.

[0122] Further controlling the mass content of difluorooxalatoborate to 0.1% to 2% is beneficial to taking into account both the storage performance and fast charging performance of the sodium secondary battery, thereby comprehensively improving the performance of the sodium secondary battery.

[0123] In some embodiments, the mass content of the fluorosulfonate is 0.001% to 5% based on the total mass of the electrolyte. In some embodiments, the mass content of the fluorosulfonate can be 0.001%, 0.005%, 0.01%, 0.05%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, or a value in a range consisting of any two of the foregoing values, based on the total mass of the electrolyte.

[0124] Controlling the mass content of fluorosulfonate within an appropriate range is conducive to the formation of components containing fluorine and sulfur elements in the SEI film, thereby effectively improving the overall stability of the interfacial SEI film and reducing the solubility of the interfacial SEI film in the electrolyte, thereby greatly reducing the degree of gas production and improving the cycle performance and storage performance of sodium secondary batteries.

[0125] In some embodiments, the mass content of the fluorosulfonate is 0.1% to 2% based on the total mass of the electrolyte. In some embodiments, the mass content of the fluorosulfonate is 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, or a value in a range consisting of any two of the foregoing values, based on the total mass of the electrolyte.

[0126] Further controlling the mass content of fluorosulfonate to 0.1% to 2% is beneficial to taking into account both the storage performance and fast charging performance of the sodium secondary battery, and comprehensively improving the performance of the sodium secondary battery.

[0127] In some embodiments, the mass content of the difluorophosphate is 0.001% to 5% based on the total mass of the electrolyte. In some embodiments, the mass content of the difluorophosphate can be 0.001%, 0.005%, 0.01%, 0.05%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, or a value in a range consisting of any two of the above values, based on the total mass of the electrolyte.

[0128] Controlling the mass content of difluorophosphate within an appropriate range is conducive to the formation of components containing fluorine and phosphorus elements in the SEI film, thereby effectively improving the overall stability of the interfacial SEI film and reducing the solubility of the interfacial SEI film in the electrolyte, thereby greatly reducing the degree of gas production and improving the cycle performance and storage performance of sodium secondary batteries.

[0129] In some embodiments, the mass content of the difluorophosphate is 0.1% to 2% based on the total mass of the electrolyte.

[0130] Further controlling the mass content of difluorophosphate to 0.1% to 2% is beneficial to taking into account both the storage performance and fast charging performance of the sodium secondary battery, thereby comprehensively improving the performance of the sodium secondary battery.

[0131] In some embodiments, the electrolyte further includes a third additive, the third additive including one or more of fluoroethylene carbonate (FEC), bisfluoroethylene carbonate (DFEC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), maleic anhydride, succinic anhydride, and triallyl phosphate.

[0132] In some embodiments, the third additive comprises fluoroethylene carbonate. In some embodiments, the third additive comprises vinylene carbonate. In some embodiments, the third additive comprises maleic anhydride. In some embodiments, the third additive comprises succinic anhydride. In some embodiments, the third additive comprises triallyl phosphate.

[0133] The introduction of the third additive containing an unsaturated functional group is conducive to the third solvent forming a lower solubility SEI film on the surface of the negative electrode plate in preference to the solvent in the electrolyte, thereby reducing the overall solubility of the SEI film in the electrolyte, reducing the degree of exposure of the negative electrode plate to the electrolyte, and thereby reducing the degree of gas production, thereby comprehensively improving the performance of the battery.

[0134] In some embodiments, the mass content of the third additive is 0.01% to 10% based on the total mass of the electrolyte. In some embodiments, the mass content of the third additive can be 0.01%, 0.05%, 0.1%, 0.5%, 1%, 2%, 4%, 5%, 6%, 8%, 10%, or a value in a range consisting of any two of the above values, based on the total mass of the electrolyte.

[0135] Controlling the mass content of the third additive within an appropriate range can provide enough third additive to form a SEI film with higher solubility, reduce the overall solubility of the SEI film in the electrolyte, reduce the degree of exposure of the negative electrode to the electrolyte, and thus reduce the degree of gas production, thereby comprehensively improving the performance of the battery.

[0136] In some embodiments, the mass content of the third additive is 0.1% to 5% based on the total mass of the electrolyte.

[0137] Controlling the mass content of the third additive to 0.1% to 5% is beneficial to taking into account both the storage performance and fast charging performance of the sodium secondary battery, thereby comprehensively improving the performance of the sodium secondary battery.

[0138] In some embodiments, the electrolyte includes a sodium salt, and the sodium salt includes one or more of NaPF6, NaFSI, NaBF4, NaN(SO2F)2, NaClO4, NaAsF6, NaB(C2O4)2, and NaBF2(C2O4).

[0139] In some embodiments, the sodium salt comprises NaPF6. In some embodiments, the sodium salt comprises NaBF4. In some embodiments, the sodium salt comprises NaFSI. In some embodiments, the sodium salt comprises NaPF6 and NaFSI. In some embodiments, the sodium salt comprises NaPF6 and NaClO4.

[0140] In some embodiments, the electrolyte includes a solvent, and the solvent includes one or more of a chain carbonate solvent, a chain carboxylate solvent, a cyclic carbonate solvent, and an ether solvent.

[0141] In some embodiments, the linear carbonate solvent includes one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, methyl butyl carbonate, ethyl propyl carbonate, dipropyl carbonate, and dibutyl carbonate;

[0142] The chain carboxylate solvent includes one or more of methyl formate, ethyl formate, methyl propionate, ethyl propionate, propyl propionate, ethyl butyrate, methyl acetate, ethyl acetate, and propyl acetate;

[0143] The cyclic carbonate includes one or more of ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, ethylene sulfite, propylene sulfite, vinylethylene carbonate, 4-ethynyl-1,3-dioxolane-2-one, cis-4,5-difluoro-1,3-dioxolane-2-one, and trans-4,5-difluoro-1,3-dioxolane-2-one;

[0144] The ether solvent includes one or more of dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, 1,2-dimethoxyethane, diethylene glycol dimethyl ether, 1,2-diethoxyethane, and 1,2-dibutoxyethane.

[0145] [Positive electrode]

[0146] The positive electrode sheet includes a positive electrode current collector and a positive electrode material layer located on at least one side of the positive electrode current collector.

[0147] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode material layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0148] 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.).

[0149] In some embodiments, the positive electrode material layer includes a positive electrode active material, and the positive electrode active material may be a positive electrode active material for batteries known in the art. As an example, the positive electrode active material may include at least one of the following materials: layered transition metal oxides, polyanion compounds, or Prussian blue 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 may also be used. These positive electrode active materials may be used alone or in combination of two or more. Among them, the Prussian blue compound includes Na x P[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 polyanion compound includes Na b Me c (PO4) d O2X, where A includes one or more of H, Li, Na, K, and NH4, Me includes one or more of Ti, Cr, Mn, Fe, Co, Ni, V, Cu, and Zn, X includes one or more of F, Cl, and Br, 0 < b ≤ 4, 0 < c ≤ 2, 1 ≤ d ≤ 3; the layered transition metal oxide includes Na a M b Fe c O2, M includes transition metal ions, 0.67 < a < 1.1, 0.5 < b < 1, 0 < c < 0.5.

[0150] In some embodiments, the positive electrode active material includes a layered transition metal oxide.

[0151] The layered transition metal oxide positive electrode active material has the advantage of high voltage, but the transition metal therein will catalyze the oxidation of the solvent in the electrolyte to form RH + , since RH + is not resistant to reduction, the exposure of the negative electrode sheet will also cause RH + migrating to the surface of the negative electrode sheet to be reduced to form components that are not resistant to oxidation, exacerbating the gas generation on the negative electrode sheet side. By using the first additive and the second additive in combination, so that the various components in the formed SEI film cooperate with each other, it is beneficial to improve the stability of the interfacial SEI film, and at the same time reduce the dissolution degree of the interfacial SEI film in the electrolyte, thereby greatly reducing the degree of gas generation and comprehensively improving the cycle performance and storage performance of the sodium secondary battery.

[0152] In some embodiments, the general formula of the layered transition metal oxide is Na x Mn a Fe b Ni c M d N e O2-δ Q f , wherein M includes at least one of Ti, Li, V, Cr, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, Mg, and Al, N includes at least one of Si, P, B, S, and Se, Q includes at least one of F, Cl, and N, 0.66≤x≤1, 0 <a≤0.7,0<b≤0.7,0<c≤0.23,0≤d<0.3,0≤e≤0.3,0≤f≤0.3,0≤δ≤0.3,a+b+c+d+e=1,0<e+f≤0.3,0<(e+f) / a≤0.3,0.2≤d+e+f≤0.3,(b+c) / a≤1.5。

[0153] In some embodiments, the positive electrode active material 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、NaNi 0.7 Co 0.15 Mn 0.15 At least one of O2.

[0154] In some embodiments, the positive electrode active material includes a layered transition metal oxide containing Cu; and the ratio of the mass content of difluorooxalatoborate in the electrolyte to the mass content of Cu in the positive electrode active material is greater than or equal to 0.002. In some embodiments, the ratio of the mass content of difluorooxalatoborate in the electrolyte to the mass content of Cu in the positive electrode active material can be 0.002, 0.004, 0.005, 0.01, 0.05, 0.1, 0.5, 1, 2, 3, 4, 5, 10, 50, 100, 500, 1000, 2000, 5000, or a value in a range consisting of any two of the foregoing values.

[0155] It is understandable that the introduction of Cu into the positive electrode active material is beneficial to the stability of the positive electrode active material structure and improves the cycle performance of sodium secondary batteries. However, at the same time, the Cu element in the positive electrode active material will undergo a valence change under high pressure, producing Cu 3+ .Cu 3+It has high oxidative activity, which can accelerate the decomposition of the electrolyte and thus deteriorate the performance of sodium secondary batteries. The decomposition products of difluorooxalatoborate in the electrolyte can oxidize with the Cu element in the positive electrode active material at the positive electrode plate interface to form insoluble Cu salts, which helps reduce the extent of increased gas production caused by electrolyte decomposition on the positive electrode plate side. Controlling the ratio of the mass content of difluorooxalatoborate in the electrolyte to the mass content of Cu in the positive electrode active material within an appropriate range can effectively reduce the extent of increased gas production caused by electrolyte decomposition on the positive electrode plate side.

[0156] In some embodiments, the mass content of Cu in the positive electrode active material is less than or equal to 23%. In some embodiments, the mass content of Cu in the positive electrode active material can be 1%, 2%, 5%, 7%, 10%, 12%, 15%, 18%, 20%, 23%, or a value in a range consisting of any two of the above.

[0157] Controlling the mass content of the Cu element in the positive electrode active material within an appropriate range is beneficial for providing sufficient Cu elements to increase the stability of the positive electrode active material structure, and can also reduce the degree of deterioration of the performance of the sodium secondary battery caused by the oxidative decomposition of the added electrolyte due to excessive Cu element mass content in the positive electrode active material.

[0158] In some embodiments, the mass content of Cu element in the positive electrode active material is 6.5% to 18%.

[0159] Further controlling the mass content of the Cu element in the positive electrode active material layer to 6.5% to 18% is beneficial to further taking into account the storage performance and fast charging performance of the sodium secondary battery.

[0160] In some embodiments, the positive electrode active material may not include the Cu element.

[0161] In some embodiments, the positive electrode material layer may further optionally include 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.

[0162] In some embodiments, the positive electrode material 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.

[0163] 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.

[0164] [Negative electrode]

[0165] The negative electrode plate includes a negative electrode current collector and a negative electrode material layer located on at least one side of the negative electrode current collector.

[0166] As an example, the negative electrode current collector has two surfaces facing each other in its thickness direction, and the negative electrode material layer is disposed on either or both of the two facing surfaces of the negative electrode current collector.

[0167] 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, 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.).

[0168] The negative electrode material layer further includes a negative electrode active material, which includes one or more of hard carbon, metallic sodium, sodium-tin alloy, and metal oxide.

[0169] The above-mentioned negative electrode active materials all have excellent sodium storage capacity, which can make the sodium secondary battery have high energy density.

[0170] In some embodiments, the negative electrode material layer further includes a Ca element; and the ratio of the mass content of difluorooxalatoborate in the electrolyte to the mass content of the Ca element in the negative electrode material layer is greater than or equal to 1. In some embodiments, the ratio of the mass content of difluorooxalatoborate in the electrolyte to the mass content of the Ca element in the negative electrode material layer can be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, or a value in a range consisting of any two of the foregoing values.

[0171] Introducing Ca into the negative electrode active material helps reduce the formation of sodium dendrites. It also facilitates the formation of a Ca-containing SEI film with the reduction product of difluorooxalatoborate, thereby improving the toughness of the SEI film and reducing the DC impedance of the sodium secondary battery. By controlling the ratio of the mass content of difluorooxalatoborate in the electrolyte to the mass content of Ca in the negative electrode material layer within an appropriate range, both the storage performance and fast-charging performance of the sodium secondary battery can be balanced.

[0172] In some embodiments, the mass content of Ca element in the negative electrode material layer is 0.1ppm to 3000ppm. In some embodiments, the mass content of Ca element in the negative electrode material layer can be 0.1ppm, 0.5ppm, 1ppm, 5ppm, 10ppm, 50ppm, 100ppm, 200ppm, 400ppm, 500ppm, 600ppm, 800ppm, 1000ppm, 1500ppm, 2000ppm, 2500ppm, 3000ppm, or a value in a range consisting of any two of the above.

[0173] In some embodiments, the mass content of difluorooxalatoborate in the electrolyte is 0.001% to 5%, and the mass content of Ca in the negative electrode material layer is 0.1 ppm to 3000 ppm. As an example, the ratio of the mass content of difluorooxalatoborate in the electrolyte to the mass content of Ca in the negative electrode material layer can be 0.1% / 10 ppm = 100, and 1% = 10000 ppm.

[0174] It can be understood that the Ca element in the negative electrode material layer can participate in the formation of the SEI film and consume part of the Ca element, resulting in a decrease in the mass content of the Ca element in the negative electrode material layer. For example, the mass content of the Ca element in the negative electrode material layer can be reduced to 1 ppm. Therefore, based on the total mass of the negative electrode material layer, the mass content of the Ca element is 0.1 ppm to 3000 ppm, which is the range protected in the embodiments of the present application.

[0175] Controlling the mass content of the Ca element in the negative electrode material layer within an appropriate range can not only form a SEI film containing calcium salt components, improve the overall toughness of the SEI film, and reduce the DC impedance of the sodium secondary battery, but also reduce the deterioration of the performance of the sodium secondary battery caused by the increase in the DC impedance of the sodium secondary battery, while taking into account the storage performance and fast charging performance of the sodium secondary battery.

[0176] In some embodiments, the mass content of Ca element in the negative electrode material layer is 2 ppm to 1000 ppm.

[0177] In some embodiments, the mass content of Ca element in the negative electrode material layer is 100 ppm to 1000 ppm.

[0178] Further controlling the mass content of the Ca element in the negative electrode material layer to 2ppm to 1000ppm or 100ppm to 1000ppm is conducive to further comprehensively improving the storage performance and fast charging performance of the sodium secondary battery.

[0179] In some embodiments, at a charge rate of 0.05C, the capacity of the negative electrode plate in the charge range of 0.5V to 1V is 9mAh / g to 140mAh / g.

[0180] In the charging range of 0.5V to 1V, controlling the capacity of the negative electrode plate within an appropriate range can not only provide sufficient negative electrode plate capacity to meet the energy density requirements of the sodium secondary battery, but also reduce the degree of deterioration of the sodium secondary battery performance caused by increased gas production due to excessive negative electrode plate capacity.

[0181] In this article, the capacity of the negative electrode sheet can be tested by any known method in the charging range of 0.5V to 1V. As an example, the negative electrode sheet is punched into small discs with a diameter of 14mm, and a metal sodium sheet is used as the negative electrode, a polypropylene film is used as the separator, and the electrolyte of Comparative Example 1 is used as the test electrolyte to assemble a button battery. Constant current charge and discharge tests are performed in the voltage range of 0.005-2V. The discharge process is discharged to 0.005V at a constant current rate of 0.05C, 40μA and 10μA in sequence. The charging process is charged to 2V at a constant current rate of 0.05C. The specific capacity (mAh / g) during the charging process at 0.5-1V is recorded as the capacity of the negative electrode at 0.5-1V.

[0182] As used herein, the term "specific capacity" refers to the actual capacity exerted by a unit mass of negative electrode active material.

[0183] In some embodiments, at a charge rate of 0.05C, the capacity of the negative electrode plate in the charge range of 0.5V to 1V is 18mAh / g to 70mAh / g.

[0184] In the charging range of 0.5V to 1V, further controlling the capacity of the negative electrode sheet to 18mAh / g to 70mAh / g is conducive to further balancing the energy density and storage performance of the sodium secondary battery.

[0185] In some embodiments, the negative electrode film layer may further optionally include a binder. The binder may include 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).

[0186] In some embodiments, the negative electrode film layer may further include a conductive agent, which may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0187] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0188] 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.

[0189] [Isolation film]

[0190] In some embodiments, the sodium secondary battery further includes a separator, which can be any known porous separator with good chemical and mechanical stability.

[0191] 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. When the separator is a multi-layer composite film, the materials of each layer can be the same or different.

[0192] [Sodium secondary battery]

[0193] 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.

[0194] In some embodiments, the sodium secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0195] In some embodiments, the outer packaging of the sodium 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 sodium 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.

[0196] The shape of the sodium secondary battery in this application can be cylindrical, square or any other shape. For example, FIG1 shows a sodium secondary battery 5 with a square structure as an example.

[0197] 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 infiltrated 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.

[0198] 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.

[0199] 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.

[0200] Optionally, the battery module 4 may further include a housing having a housing space, and the plurality of sodium secondary batteries 5 are housed in the housing space.

[0201] 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.

[0202] 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.

[0203] 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.

[0204] As the electrical device, a sodium secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0205] 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.

[0206] 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.

[0207] Example

[0208] 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.

[0209] 1. Preparation method

[0210] Example 1

[0211] 1) Electrolyte

[0212] In an argon atmosphere glove box (H2O content <10ppm, O2 content <1ppm), propylene carbonate (PC) and ethyl methyl carbonate (EMC) were mixed in a mass ratio of 30 / 70, and 1M NaPF6 sodium salt was dissolved. Then, the first additive, sodium difluorooxalatoborate, and the second additive, sodium difluorophosphate, were added and stirred evenly to prepare an electrolyte. Based on the total mass of the electrolyte, the mass content of the first additive, sodium difluorooxalatoborate, was 0.5%, and the mass content of the second additive, sodium difluorophosphate, was 0.5%.

[0213] 2) Preparation of positive electrode sheet

[0214] Na 7 / 9 Cu 2 / 9 Fe 1 / 9 Mn 2 / 3 Preparation of O2 (13% Cu): 0.39M Na2CO3, 0.22M CuO, 0.06M Fe2O3, and 0.67M MnO2 precursors were ball-milled in ethanol as a dispersant for 12 hours. After drying, the mixed powder was pressed into pellets at 20 MPa and sintered at 900°C for 12 hours. The sintered powder needed to be quickly transferred to a glove box for storage.

[0215] The positive electrode active material Na 7 / 9 Cu 2 / 9 Fe 1 / 9 Mn 2 / 3 O2 (13% Cu), conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) were fully stirred and mixed in an N-methylpyrrolidone solvent system in 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 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.

[0216] 3) Preparation of negative electrode sheet

[0217] Negative electrode active material H2 (negative electrode sheet capacity in the 0.5V-1V charging range is 40mAh / g): The biomass material was calcined at 800°C in a tube furnace containing an argon atmosphere for 2 hours, then washed with hydrochloric acid and deionized water and dried. After grinding for 2 hours, it was calcined at 1550°C in a tube furnace under an argon atmosphere for 4 hours to obtain the target material with a particle size of 10μm.

[0218] The negative electrode active material H2, conductive agent acetylene black, binder styrene butadiene rubber (SBR), thickener sodium carboxymethyl cellulose (CMC-Na) and CaO were fully stirred and mixed in a deionized water solvent system in a weight ratio of 90:4:4:2:0.014 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 8 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.

[0219] 4) Isolation film

[0220] A 12 μm polyethylene (PE) porous polymer film was used as the separator.

[0221] 5) Preparation of batteries

[0222] 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.

[0223] The secondary batteries of Examples 2 to 48 and the secondary batteries of Comparative Examples 1 to 4 were prepared in a similar manner to the secondary battery of Example 1, except that the composition of the battery electrodes and product parameters were adjusted. The different product parameters are detailed in Tables 1 and 2.

[0224] Among them, in Examples 2 to 14 and 19 to 23, the mass contents of the first additive, the second additive, and the third additive can be adjusted by adjusting the content of the solvent in the electrolyte;

[0225] The negative electrode material layers containing different mass contents of Ca elements in Examples 31 to 36 can be controlled by adding different mass contents of CaO during the preparation of the negative electrode sheets;

[0226] The capacity of the negative electrode plates in Examples 37 to 40 can be effectively controlled by changing the carbonization temperature of the negative electrode material during the preparation process and compounding negative electrode materials of different capacities, as follows:

[0227] Preparation of negative electrode active material H1 (negative electrode sheet capacity of 140 mAh / g in the 0.5V-1V charging range): The biomass material was calcined at 800°C in a tube furnace containing an argon atmosphere for 2 hours, then washed with hydrochloric acid and deionized water and dried. It was then ground for 2 hours and calcined at 1150°C in a tube furnace under an argon atmosphere for 2 hours to obtain the target material with a particle size of 2 μm.

[0228] Negative electrode active material H3 (the capacity of the negative electrode sheet in the charging range of 0.5V to 1V is 9mAh / g): the biomass material was calcined at 800℃ in a tubular furnace containing an argon atmosphere for 2h, then washed with hydrochloric acid and deionized water and dried respectively. After grinding it for 2h, it was calcined at 1650℃ in a tubular furnace in an argon atmosphere for 6h to obtain the target material with a particle size of 20μm. Increasing the temperature of the pyrolysis process further reduced defects and induced the formation of a large number of ordered micropores. The specific capacity of 0.5V to 1V is about 9mAh / g.

[0229] Example 37: The negative electrode active material H3, the conductive agent acetylene black, the binder styrene butadiene rubber (SBR), the thickener sodium carboxymethyl cellulose (CMC-Na) and CaO were fully stirred and mixed in a deionized water solvent system in a weight ratio of 90:4:4:2:0.014 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 8 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;

[0230] Example 38: The negative electrode active material 70% H3 and 30% H2, the conductive agent acetylene black, the binder styrene butadiene rubber (SBR), the thickener sodium carboxymethyl cellulose (CMC-Na) and CaO were fully stirred and mixed in a deionized water solvent system in a weight ratio of 90:4:4:2:0.014 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 8 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;

[0231] Example 39: The negative electrode active material 30% H1 and 70% H2, the conductive agent acetylene black, the binder styrene butadiene rubber (SBR), the thickener sodium carboxymethyl cellulose (CMC-Na) and CaO were fully stirred and mixed in a deionized water solvent system in a weight ratio of 90:4:4:2:0.014 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 8 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;

[0232] Example 40: The negative electrode active material H1, the conductive agent acetylene black, the binder styrene butadiene rubber (SBR), the thickener sodium carboxymethyl cellulose (CMC-Na) and CaO were fully stirred and mixed in a deionized water solvent system in a weight ratio of 90:4:4:2:0.014 to obtain a negative electrode slurry; the negative electrode slurry was mixed at a weight ratio of 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 8 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.

[0233] Examples 41 to 44 The positive electrode active materials containing different mass contents of Cu element can be obtained by regulating the stoichiometric ratio of the precursors Fe2O3, CuO, MnO2, and Na2CO3 during the sintering process and compounding the positive electrode materials with different contents of Cu element during the preparation of the positive electrode sheet, as follows:

[0234] Na 1 / 2 Fe 1 / 2 Mn 1 / 2 Preparation of O2 (0% Cu): 0.25M Na2CO3, 0.25M Fe2O3, and 0.5M MnO2 precursors were ball-milled in ethanol as a dispersant for 12 h. After drying, the mixed powder was pressed into pellets at 20 MPa and sintered at 900 °C for 12 h. The sintered powder needed to be quickly transferred to a glove box for storage.

[0235] Na 9 / 10 Cu 2 / 5 Fe 1 / 10 Mn 1 / 2 Preparation of O2 (23% Cu): 0.45M Na2CO3, 0.4M CuO, 0.05M Fe2O3, and 0.5M MnO2 precursors were ball-milled in ethanol as a dispersant for 12 hours. After drying, the mixed powder was pressed into a pellet at 20 MPa and sintered at 900°C for 12 hours. The sintered powder needed to be quickly transferred to a glove box for storage.

[0236] Example 41: 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 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 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;

[0237] Example 42: Positive electrode active material 50% Na 1 / 2 Fe 1 / 2 Mn 1 / 2 O2, 50% Na 7 / 9 Cu 2 / 9 Fe 1 / 9 Mn 2 / 3O2, 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 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;

[0238] Example 43: Positive electrode active material 50% Na 7 / 9 Cu 2 / 9 Fe 1 / 9 Mn 2 / 3 O2, 50% 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 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 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;

[0239] Example 44: 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 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 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.

[0240] 2. Performance Testing

[0241] 1. Electrolyte

[0242] 1) Test of the content of the first additive / second additive / third additive in the electrolyte

[0243] The content of the first additive and the second additive in the electrolyte of the sodium secondary battery can be tested by referring to the general rules of ion chromatography analysis methods in the general rules of analysis methods of modern analytical instruments (JY / T 020-1996, issued on January 23, 1997, and implemented on April 1, 1997), and the cations and anions in the electrolyte are tested and analyzed using the ion chromatography analysis method.

[0244] The content of the third additive in the electrolyte of the sodium secondary battery can be tested by qualitative and quantitative testing through organic component gas chromatography in accordance with GB / T 6041-2002 and GB / T 9722-2006.

[0245] 2. Positive electrode / negative electrode

[0246] 1) Determination of Cu content in positive electrode active materials

[0247] The content of Cu in the positive electrode active material can be tested by inductively coupled plasma atomic emission spectrometry according to the general rule EPA 6010D-2014.

[0248] 2) Determination of Ca content in the negative electrode material layer

[0249] The content of Ca in the negative electrode material can be tested by inductively coupled plasma atomic emission spectrometry according to the general rule EPA 6010D-2014.

[0250] 3) Negative electrode capacity test in the range of 0.5V to 1V

[0251] The negative electrode sheet was punched into small discs with a diameter of 14 mm, and a metal sodium sheet was used as the negative electrode, a polypropylene film was used as the isolation membrane, and a 1.3 mol / L sodium hexafluorophosphate solution was used as the electrolyte. The solvent in the electrolyte included ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate, and the mass ratio of the three solvents was 1:2:2. The test electrolyte was used to assemble a button cell, and a constant current charge and discharge test was performed in the voltage range of 0.005 to 2 V. The discharge process was discharged to 0.005 V at a constant current rate of 0.05 C, 40 μA and 10 μA in sequence. The charging process was charged to 2 V at a constant current rate of 0.05 C. The specific capacity (mAh / g) during the charging process at 0.5 to 1 V was recorded as the capacity of the negative electrode sheet in the range of 0.5 to 1 V.

[0252] 3. Battery

[0253] 1) High temperature storage volume change rate

[0254] At 25°C, the new 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, and then charged at a constant voltage to a current less than or equal to 0.05C. After that, they were left for 5 minutes, and then discharged to 1.5V at a constant current rate of 1C. The volume V1 of the battery was tested by the drainage 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%.

[0255] 2) Charging performance at -10℃

[0256] Preparation of a three-electrode battery: First, a sodium vanadium phosphate reference electrode is prepared. The active material sodium vanadium phosphate, the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) are thoroughly stirred and mixed in an N-methylpyrrolidone solvent system at a weight ratio of 90:5:5 to form a slurry. The slurry is evenly coated on a 10 cm long aluminum wire (the coated area occupies 1 cm) and dried at 100°C to obtain the target reference electrode. In the preparation process of the sodium secondary battery, the reference electrode is placed between the negative electrode sheet and the separator to obtain a three-electrode battery.

[0257] -10℃ charging performance test: The above-mentioned three-electrode battery containing a reference was charged at 25℃ at a constant current of 0.1C to a voltage of 4.0V, then charged at a constant voltage to a current less than or equal to 0.05C, then left for 5 minutes, and then discharged at a constant current of 0.1C to 1.5V, and the discharge capacity was recorded as C1; then the battery was placed in a -10℃ environment and left 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 obtained as C2. The battery's charging performance at -10℃ = C2 / C1×100%.

[0258] 3) DC impedance

[0259] At -25°C, adjust the single cell state of charge to 50% SOC, let it rest for 30 minutes, and record the battery voltage at this time as U1 (V). Discharge it at 0.36C for 10 seconds, and record the battery voltage at this time as U2 (V). The corresponding battery discharge current I (mA) is 0.36 × the battery design capacity (mAh). DC resistance DCR (mΩ) = (U1 - U2) / I.

[0260] 4) Cycle performance

[0261] At 25°C, the prepared battery was charged at a constant current of 1C to 4.0V, then charged at a constant voltage of 4.0V until the current dropped to 0.05C. After standing for 5 minutes, the battery was discharged at a constant current of 1C to 1.5V. This was the first charge / discharge cycle of the battery, and the discharge capacity at 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 300th cycle was recorded as (C1). The capacity retention rate after 300 cycles = C1 / C0 × 100%. The testing procedures for the comparative example and other examples were the same as above.

[0262] 3. Analysis of test results of various embodiments and comparative examples

[0263] 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 Tables 1 and 2 below.

[0264] Table 1

[0265] Table 2

[0266] The electrolytes in Examples 1 to 44 all include a first additive and a second additive, the first additive includes sodium difluorooxalatoborate, lithium difluorooxalatoborate or magnesium difluorooxalatoborate, and the second additive includes sodium difluorophosphate, lithium difluorophosphate, sodium fluorosulfonate or aluminum fluorosulfonate. The sodium secondary batteries containing the electrolytes all have excellent storage performance, cycle performance and fast charging performance.

[0267] From the comparison of Examples 1 to 18 with Comparative Example 1, it can be seen that the electrolyte including the first additive and the second additive can greatly reduce the gas production of the sodium secondary battery, which is beneficial to reducing the volume expansion rate of the sodium secondary battery and the DC impedance of the sodium secondary battery after high-temperature storage, and improving the charging performance and cycle capacity retention rate of the sodium secondary battery.

[0268] From the comparison of Examples 1 to 6 with Comparative Example 3, and Examples 13 to 14 with Comparative Example 4, it can be seen that compared with the electrolyte containing only the second additive, the electrolyte in the present application contains both the first additive and the second additive, which reduces the gas production level of the sodium secondary battery, is beneficial to reducing the volume expansion rate of the sodium secondary battery after high-temperature storage and the DC impedance of the sodium secondary battery, and improving the charging performance and cycle capacity retention rate of the sodium secondary battery.

[0269] From the comparison of Examples 7 to 10 and Comparative Example 2, it can be seen that compared with the electrolyte containing only the first additive, the electrolyte in the present application contains both the first additive and the second additive, which can greatly reduce the gas production of the sodium secondary battery, which is beneficial to reducing the volume expansion rate of the sodium secondary battery after high-temperature storage and improving the safety of the sodium secondary battery.

[0270] From the comparison of Examples 1 to 10 with Comparative Examples 2 to 3, and Examples 13 to 14 with Comparative Examples 2 and 4, it can be seen that compared with the electrolyte containing only the first additive or the second additive, the electrolyte in the present application contains both the first additive and the second additive, which can take into account the storage performance and charging performance of the sodium secondary battery and comprehensively improve the cycle performance of the sodium secondary battery.

[0271] As can be seen from Examples 1 to 12, controlling the mass ratio of the first additive, sodium difluorooxalatoborate, to the second additive, sodium difluorophosphate, within a range of 0.005 to 500 allows the sodium secondary battery to have excellent storage performance, cycle performance, and charging performance. A comparison of Examples 1 to 10 with Examples 11 to 12 shows that further controlling the mass ratio of the first additive, sodium difluorooxalatoborate, to the second additive, sodium difluorophosphate, within a range of 0.01 to 200 can balance the storage performance and charging performance of the sodium secondary battery, thereby comprehensively improving the cycle performance of the sodium secondary battery.

[0272] As can be seen from Examples 1 to 6, based on the total mass of the electrolyte, the mass content of the first additive, sodium difluorooxalatoborate, is controlled to be 0.005% to 5%, so that the sodium secondary battery has excellent storage performance, cycle performance, and charging performance. As can be seen from the comparison of Examples 1 to 2, 4 to 5 with Examples 3 and 6, based on the total mass of the electrolyte, further controlling the mass content of the first additive, sodium difluorooxalatoborate, to be 0.1% to 2%, further helps to reduce the DC impedance of the sodium secondary battery and the volume expansion rate of the sodium secondary battery after high-temperature storage, improve the charging performance and storage performance of the sodium secondary battery, and further comprehensively improve the cycle performance of the sodium secondary battery.

[0273] As can be seen from Examples 4, 7 to 10, the mass content of the second additive, sodium difluorophosphate, is controlled to be 0.005% to 5% based on the total mass of the electrolyte, so that the sodium secondary battery has excellent storage performance, cycle performance, and charging performance. As can be seen from the comparison of Examples 4, 7, and 9 with Examples 8 and 10, the mass content of the first additive, sodium difluorophosphate, is further controlled to be 0.1% to 2% based on the total mass of the electrolyte, which further helps to reduce the volume expansion rate of the sodium secondary battery after high-temperature storage and the DC impedance of the sodium secondary battery, improve the storage performance and charging performance of the sodium secondary battery, and further comprehensively improve the cycle performance of the sodium secondary battery.

[0274] It can be seen from Examples 4 and 13 to 18 that the first additive is sodium difluorooxalatoborate, lithium difluorooxalatoborate or magnesium difluorooxalatoborate, and the second additive is sodium difluorophosphate, lithium difluorophosphate, sodium fluorosulfonate or aluminum fluorosulfonate, all of which can make the sodium secondary battery have excellent storage performance, cycle performance and charging performance.

[0275] From the comparison of Examples 19 to 26 with Example 4, it can be seen that the introduction of a third additive into the electrolyte, and the third additive is a fluorocarbonate, vinylene carbonate, vinyl ethylene carbonate or maleic anhydride, is beneficial to reducing the volume expansion rate of the sodium secondary battery after high-temperature storage and improving the safety of the sodium secondary battery. From Examples 19 to 23, it can be seen that based on the total mass of the electrolyte, the mass content of the third additive is controlled to be 0.05% to 10%. All of them can make the sodium secondary battery have excellent storage performance, cycle performance and charging performance. From the comparison of Examples 19, 21 to 22 with Examples 20 and 23, it can be seen that based on the total mass of the electrolyte, the mass content of the third additive is further controlled to be 0.1% to 5%, which is further beneficial to reducing the DC impedance of the sodium secondary battery and the volume expansion rate of the sodium secondary battery after high-temperature storage, and improving the charging performance and cycle capacity retention rate of the sodium secondary battery.

[0276] It can be seen from Examples 4 and 27 to 28 that the lithium salt in the electrolyte is NaFSI and / or NaPF6, which, when used in combination with the first additive and the second additive, can enable the sodium secondary battery to have excellent storage performance, cycle performance and charging performance.

[0277] It can be seen from Examples 4 and 29 to 30 that the solvent in the electrolyte is two or more of PC, EMC, DG, and EP, and when used in combination with the first additive and the second additive, the sodium secondary battery can have excellent storage performance, cycle performance, and charging performance.

[0278] It can be seen from Examples 4 and 31 to 36 that by controlling the mass content of the Ca element in the negative electrode material layer to 50 ppm to 3000 ppm and using it in combination with the first additive and the second additive, the sodium secondary battery can have excellent storage performance, cycle performance and charging performance.

[0279] It can be seen from Examples 4 and 31 to 36 that by controlling the ratio of the mass content of sodium difluorooxalatoborate in the electrolyte to the mass content of the Ca element in the negative electrode material layer to be greater than 1, the sodium secondary battery can have excellent storage performance, cycle performance and charging performance.

[0280] As can be seen from Examples 4, 37, and 40, at a charge rate of 0.05C, the capacity of the negative electrode plate in the 0.5V to 1V charge range is controlled to 9 mAh / g to 140 mAh / g, enabling the sodium secondary batteries to exhibit excellent storage, cycling, and charging performance. A comparison of Examples 4, 38, and 39 with Examples 37 and 39 shows that at a charge rate of 0.05C, the capacity of the negative electrode plate in the 0.5V to 1V charge range is further controlled to 40 mAh / g to 70 mAh / g, further reducing the DC impedance of the sodium secondary battery and the volume expansion rate of the sodium secondary battery after high-temperature storage, thereby improving the charging performance and cycle capacity retention of the sodium secondary battery.

[0281] From the comparison between Examples 4, 42 to 44 and Example 41, it can be seen that the introduction of Cu element into the positive electrode active material can improve the cycle capacity retention rate of the sodium secondary battery.

[0282] It can be seen from Examples 4 and 42 to 44 that by controlling the ratio of the mass content of difluorooxalatoborate in the electrolyte to the mass content of the Cu element in the positive electrode active material to be greater than 0.002, the sodium secondary battery can have excellent storage performance, cycle performance and charging performance.

[0283] As can be seen from Examples 4, 42-44, controlling the mass content of the Cu element in the positive electrode active material to be less than or equal to 23% can all achieve excellent storage performance, cycle performance, and charging performance in sodium secondary batteries. As can be seen from the comparison of Examples 4, 42-43 with Example 44, controlling the mass content of the Cu element in the positive electrode active material to be 6.5%-18% can simultaneously improve storage performance, cycle performance, and charging performance.

[0284] 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. An electrolyte for a sodium secondary battery, wherein: The method comprises a first additive and a second additive, wherein the first additive comprises difluorooxalate borate, and the second additive comprises one or more of fluorosulfonate and difluorophosphate. The fluorosulfonate salt includes a compound shown in formula II, (FSO3) y2 M2 y2+ Formula II Among them, M2 y2+ Including Li + 、Na + , K + , Rb + , Cs + Mg 2+ , Ca 2+ , Ba 2+ , Fe 2+ 、Ni 2+ 、Al 3+ , Fe 3+ 、Ni 3+ One or more of , y2=1, 2 or 3.

2. The electrolyte according to claim 1, wherein The difluorooxalate borate comprises a compound shown in formula I, (F2C2O4B) y1 M1 y1+ Formula I Among them, M1 y1+ Including Li + 、Na + , K + , Rb + , Cs + Mg 2+ , Ca 2+ , Ba 2+ , Fe 2+ 、Ni 2+ 、Al 3+ , Fe 3+ 、Ni 3+ One or more of y1=1, 2 or 3.

3. The electrolyte according to claim 1 or 2, wherein The difluorophosphate includes a compound represented by formula III, (PO2F2) y3 M3 y3+ Formula III Among them, M3 y3+ Including Li + 、Na + , K + , Rb + , Cs + Mg 2+ , Ca 2+ , Ba 2+ , Fe 2+ 、Ni 2+ 、Al 3+ , Fe 3+ 、Ni 3+ One or more of y3=1, 2 or 3.

4. The electrolyte according to any one of claims 1 to 3, wherein The mass ratio of the first additive to the second additive is 0.001-1000.

5. The electrolyte according to any one of claims 1 to 4, wherein The mass ratio of the first additive to the second additive is 0.01-200.

6. The electrolyte according to any one of claims 1 to 5, wherein Based on the total mass of the electrolyte, the mass content of the difluorooxalatoborate is 0.001% to 5%.

7. The electrolyte according to any one of claims 1 to 6, wherein Based on the total mass of the electrolyte, the mass content of the difluorooxalatoborate is 0.1% to 2%.

8. The electrolyte according to any one of claims 1 to 7, wherein Based on the total mass of the electrolyte, the mass content of the fluorosulfonate is 0.001% to 5%.

9. The electrolyte according to any one of claims 1 to 8, wherein Based on the total mass of the electrolyte, the mass content of the fluorosulfonate is 0.1% to 2%.

10. The electrolyte according to any one of claims 1 to 9, wherein Based on the total mass of the electrolyte, the mass content of the difluorophosphate is 0.001% to 5%.

11. The electrolyte according to any one of claims 1 to 10, wherein Based on the total mass of the electrolyte, the mass content of the difluorophosphate is 0.1% to 2%.

12. The electrolyte according to any one of claims 1 to 11, wherein The electrolyte further includes a third additive, and the third additive includes one or more of fluoroethylene carbonate, difluoroethylene carbonate, vinylene carbonate, vinylethylene carbonate, maleic anhydride, succinic anhydride, and triallyl phosphate.

13. The electrolyte according to claim 12, wherein Based on the total mass of the electrolyte, the mass content of the third additive is 0.01% to 10%.

14. The electrolyte according to claim 12 or 13, wherein Based on the total mass of the electrolyte, the mass content of the third additive is 0.1% to 5%.

15. The electrolyte according to any one of claims 1 to 14, wherein The electrolyte includes sodium salt, and the sodium salt includes one or more of NaPF6, NaFSI, NaBF4, NaN(SO2F)2, NaClO4, NaAsF6, NaB(C2O4)2, and NaBF2(C2O4).

16. The electrolyte according to any one of claims 1 to 15, wherein The electrolyte includes a solvent, and the solvent includes one or more of a chain carbonate solvent, a chain carboxylate solvent, a cyclic carbonate solvent, and an ether solvent.

17. The electrolyte according to claim 16, wherein The chain carbonate solvent includes one or more of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, methyl butyl carbonate, ethyl propyl carbonate, dipropyl carbonate, and dibutyl carbonate; The chain carboxylic acid ester solvent includes one or more of methyl formate, ethyl formate, methyl propionate, ethyl propionate, propyl propionate, ethyl butyrate, methyl acetate, ethyl acetate, and propyl acetate; The cyclic carbonate includes one or more of ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, ethylene sulfite, propylene sulfite, vinylethylene carbonate, 4-ethynyl-1,3-dioxolane-2-one, cis-4,5-difluoro-1,3-dioxolane-2-one, and trans-4,5-difluoro-1,3-dioxolane-2-one; The ether solvent includes one or more of dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, 1,2-dimethoxyethane, diethylene glycol dimethyl ether, 1,2-diethoxyethane, and 1,2-dibutoxyethane.

18. A sodium secondary battery, wherein: The invention comprises a positive electrode sheet, a negative electrode sheet and the electrolyte according to any one of claims 1 to 17.

19. The sodium secondary battery according to claim 18, wherein The positive electrode plate includes a positive electrode current collector and a positive electrode material layer located on at least one side of the positive electrode current collector, the positive electrode material layer includes a positive electrode active material, and the positive electrode active material includes at least one of a layered transition metal oxide, a Prussian blue compound, and a polyanion compound.

20. The sodium secondary battery according to claim 18 or 19, wherein The general formula of the layered transition metal oxide is Na x Mn a Fe b Ni c M d N e O 2-δ Q f , wherein M includes at least one of Ti, Li, V, Cr, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, Mg, and Al, N includes at least one of Si, P, B, S, and Se, Q includes at least one of F, Cl, and N, 0.66≤x≤1, 0 <a≤0.7,0<b≤0.7,0<c≤0.23,0≤d<0.3,0≤e≤0.3,0≤f≤0.3,0≤δ≤0.3,a+b+c+d+e=1,0<e+f≤0.3,0<(e+f) / a≤0.3,0.2≤d+e+f≤0.3,(b+c) / a≤1.5。 21. The sodium secondary battery according to any one of claims 19 to 21, wherein The positive electrode active material includes a layered transition metal oxide containing Cu element; The ratio of the mass content of the difluorooxalatoborate in the electrolyte to the mass content of the Cu element in the positive electrode active material is greater than or equal to 0.

002.

22. The sodium secondary battery according to claim 21, wherein The mass content of Cu element in the positive electrode active material is less than or equal to 23%.

23. The sodium secondary battery according to claim 21 or 22, wherein: The mass content of Cu element in the positive electrode active material is 6.5% to 18%.

24. The sodium secondary battery according to any one of claims 18 to 23, wherein The negative electrode plate comprises a negative electrode current collector and a negative electrode material layer located on at least one side of the negative electrode current collector, wherein the negative electrode material layer comprises a Ca element; The ratio of the mass content of the difluorooxalato borate in the electrolyte to the mass content of the Ca element in the negative electrode material layer is greater than or equal to 1.

25. The sodium secondary battery according to claim 24, wherein The mass content of Ca element in the negative electrode material layer is 0.1ppm to 3000ppm.

26. The sodium secondary battery according to claim 24 or 25, wherein The mass content of Ca element in the negative electrode material layer is 2ppm to 1000ppm.

27. The sodium secondary battery according to any one of claims 24 to 26, wherein The mass content of Ca element in the negative electrode material layer is 100ppm to 1000ppm.

28. The sodium secondary battery according to any one of claims 24 to 27, wherein The negative electrode material layer further includes a negative electrode active material, and the negative electrode active material includes one or more of hard carbon, metallic sodium, tin alloy, and metal oxide.

29. The sodium secondary battery according to any one of claims 18 to 28, wherein At a charging rate of 0.05C, the capacity of the negative electrode plate in the charging range of 0.5V to 1V is 9mAh / g to 140mAh / g.

30. The sodium secondary battery according to any one of claims 18 to 29, wherein At a charging rate of 0.05C, the capacity of the negative electrode plate in the charging range of 0.5V to 1V is 18mAh / g to 70mAh / g.

31. An electrical device, wherein: A sodium secondary battery comprising the sodium secondary battery according to any one of claims 18 to 30.

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