Electrolyte solution, sodium secondary battery, and electric device

Abstract

The invention provides an electrolyte, a sodium secondary battery and an electric device. The electrolyte comprises a first sodium salt and a first solvent, the first sodium salt comprises sodium trifluoromethanesulfonate, the first solvent has a structure as shown in a formula I, and the mass ratio of the first sodium salt to the first solvent is 0.01-0.4. The electrolyte has excellent low-temperature ionic conductivity, and is beneficial to improving the low-temperature cycle performance and low-temperature coulombic efficiency of the battery.

Description

Technical Field

[0001] The present application relates to the technical field of secondary batteries, and in particular to an electrolyte, a sodium secondary battery and an electrical device. Background Art

[0002] In recent years, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal, wind and solar power stations, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields.

[0003] The performance of electrolyte has a key impact on the performance of secondary batteries. At present, electrolyte has many defects and cannot meet the application needs of the new generation of electrochemical systems. Summary of the invention

[0004] In view of the problems existing in the background technology, the present application provides an electrolyte, aiming to improve the low-temperature ionic conductivity of the electrolyte, as well as the low-temperature cycle performance and low-temperature coulombic efficiency of the battery thereof.

[0005] In a first aspect of the present application, an electrolyte for a sodium secondary battery is provided, comprising a first sodium salt and a first solvent, wherein the first sodium salt comprises sodium trifluoromethanesulfonate, and the first solvent has a structure shown in Formula I,

[0006]

[0007] In the formula, R1 and R2 each independently include CH2 or CH2CH2, and n=1, 2, 3 or 4;

[0008] Wherein, the mass ratio of the first sodium salt to the first solvent is 0.01 to 0.4.

[0009] The ethyl oxygen in the first solvent can coordinate with the sodium ion in the sodium trifluoromethanesulfonate. On the one hand, controlling the number of carbons between two adjacent oxygens in the first solvent and the number of repeating structural units n is conducive to the chelation coordination of the first solvent with the sodium ions in the sodium trifluoromethanesulfonate to form a relatively stable five-membered ring or six-membered ring solvation structure, so that the sodium trifluoromethanesulfonate has a higher solubility in the first solvent including the structure shown in Formula I at low temperature, and the electrolyte can have excellent low-temperature ionic conductivity. At the same time, the number of repeating structural units n is controlled so that the electrolyte has a suitable viscosity, thereby taking into account the low-temperature ionic conductivity of the electrolyte and the compatibility between the electrolyte and the positive and negative electrode sheets, so that the battery has excellent low-temperature cycle performance and low-temperature coulomb efficiency. On the other hand, controlling the mass ratio of the first sodium salt to the first solvent within an appropriate range can provide enough first solvent to completely dissolve the first sodium salt, and can meet the concentration requirement of the first sodium salt so that the electrolyte has excellent ionic conductivity, thereby improving the low-temperature cycle performance and low-temperature coulomb efficiency of the battery. In addition, at low temperatures, since sodium trifluoromethanesulfonate has a high solubility in the first solvent having the structure shown in Formula I, its electrolyte system can still maintain excellent cycle performance when the low temperature drops from 0°C to -30°C.

[0010] In any embodiment, the electrolyte further includes a second sodium salt, which includes one or more of sodium bis(fluorosulfonyl)imide, sodium hexafluorophosphate, sodium tetrafluoroborate, sodium hexafluoroarsenate, and sodium tetraphenylborate, and optionally includes one or more of sodium hexafluorophosphate, sodium tetrafluoroborate, sodium hexafluoroarsenate, and sodium tetraphenylborate.

[0011] The introduction of a second sodium salt into the electrolyte is beneficial to improving the room temperature cycle performance of the battery.

[0012] In any embodiment, based on the total mass of the electrolyte, the mass content of the first sodium salt is 0.6% to 24%, optionally 2% to 12%; and / or

[0013] Based on the total mass of the electrolyte, the mass content of the second sodium salt is 3% to 40%, and can be optionally 8% to 20%.

[0014] Controlling the mass content of the first sodium salt within a suitable range can not only provide enough first sodium salt to dissolve in the first solvent, so that the electrolyte has excellent low-temperature ionic conductivity, and the battery has excellent low-temperature cycle performance and low-temperature coulomb efficiency; but also control the degree of interfacial side reactions between the first sodium salt and the negative electrode. Controlling the mass content of the second sodium salt within a suitable range can not only provide enough second sodium salt to make the battery have excellent room-temperature cycle performance, but also improve the low-temperature cycle performance of the battery to a certain extent.

[0015] In any embodiment, the first solvent includes one or more of diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether, and optionally includes diethylene glycol dimethyl ether.

[0016] The first sodium salt has a relatively high low-temperature solubility in the first solvent, so that the electrolyte has excellent ion conductivity.

[0017] In any embodiment, the mass ratio of the first sodium salt to the first solvent is 0.05 to 0.2.

[0018] The mass ratio of the first sodium salt to the first solvent is further controlled to be 0.05-0.2, which can take into account the low-temperature ionic conductivity of the electrolyte, the low-temperature cycle performance of the battery and the low-temperature coulombic efficiency.

[0019] In any embodiment, based on the total mass of the electrolyte, the mass content of the first solvent is not less than 20%, and can be optionally 40% to 80%.

[0020] By controlling the mass content of the first solvent within a suitable range, a sufficient amount of the first solvent can be provided to completely dissolve the first sodium salt, so that the electrolyte has excellent low-temperature ionic conductivity, thereby improving the low-temperature cycle performance and low-temperature coulombic efficiency of the battery.

[0021] In any embodiment, the electrolyte further includes a second solvent, the freezing point of the second solvent is lower than the freezing point of the first solvent, and optionally, the freezing point of the second solvent is lower than -65°C.

[0022] Introducing a second solvent having a freezing point lower than that of the first solvent into the electrolyte is beneficial to further lowering the freezing point of the electrolyte, thereby greatly improving the ionic conductivity of the electrolyte at low temperatures and improving the low-temperature cycle performance of the battery.

[0023] In any embodiment, the second solvent comprises a cyclic ether solvent, and the cyclic ether solvent comprises one or more of 1,3-dioxolane, 1,4-dioxane, tetrahydrofuran, methyltetrahydrofuran, and optionally 1,3-dioxolane; and / or

[0024] Based on the total mass of the electrolyte, the mass content of the second solvent is not less than 10%, and can be optionally 20% to 70%.

[0025] The mass content of the second solvent is controlled within an appropriate range, which effectively improves the ionic conductivity of the electrolyte at low temperatures and enhances the low-temperature cycle performance of the battery.

[0026] In any embodiment, the electrolyte further includes a third solvent, and the binding force between the third solvent and the anions in the first sodium salt is greater than the binding force between the first solvent and the anions in the first sodium salt.

[0027] Since the anions in sodium trifluoromethanesulfonate have a strong polarizing effect on the outer weak coordination structure of the solvation structure after the first solvent and the sodium ions are coordinated, the first solvent molecules are easily oxidized on the surface of the positive electrode plate, causing the battery to be overcharged at low temperatures. The introduction of the third solvent can change the solvation structure between the first solvent and sodium trifluoromethanesulfonate. Since the binding force between the third solvent and the anions in sodium trifluoromethanesulfonate is stronger than the binding force between the first solvent and the anions in sodium trifluoromethanesulfonate, the polarizing effect of the anions in sodium trifluoromethanesulfonate on the outer weak coordination structure of the solvation structure after the first solvent and the sodium ions are coordinated is reduced, so the degree of oxidation reaction of the first solvent molecules on the surface of the positive electrode plate can be reduced, and the third solvent has a stronger antioxidant effect, thereby effectively inhibiting battery overcharge and improving the coulombic efficiency of the battery at low temperatures.

[0028] In any embodiment, the third solvent includes one or more of ethylene glycol diethyl ether, ethylene glycol dipropyl ether, propylene glycol dimethyl ether, and diethylene glycol diethyl ether, and optionally includes one or more of ethylene glycol diethyl ether and diethylene glycol diethyl ether; and / or

[0029] Based on the total mass of the electrolyte, the mass content of the third solvent is not less than 10%, and can be optionally 15% to 40%.

[0030] The above third solvents can effectively reduce the degree of oxidation reaction of the first solvent molecules on the surface of the positive electrode. For example, ethylene glycol diethyl ether, ethylene glycol dipropyl ether or diethylene glycol diethyl ether can increase the steric hindrance of α hydrogen on the terminal C due to the presence of ethyl or propyl groups, or propylene glycol dimethyl ether forms a six-membered ring structure to change the coordination structure, which can weaken the polarization effect of the anions in sodium trifluoromethanesulfonate on the outer weak coordination structure of the solvation structure after the coordination of the first solvent and the sodium ion, reduce the degree of oxidation reaction of the first solvent molecules on the surface of the positive electrode, and the third solvent has a stronger antioxidant effect, thereby effectively inhibiting battery overcharge and improving the coulomb efficiency of the battery at low temperature. Controlling the mass content of the third solvent within an appropriate range can effectively improve the low-temperature coulomb efficiency of the battery.

[0031] A second aspect of the present application provides a sodium secondary battery, comprising the electrolyte of the first aspect of the present application.

[0032] In any embodiment, the sodium secondary battery is a negative electrode-less sodium secondary battery.

[0033] Anode-free sodium secondary batteries have high energy density.

[0034] In any embodiment, the sodium secondary battery includes a negative electrode sheet, wherein the negative electrode sheet includes a negative electrode collector and a primer layer formed on at least a portion of a surface of the negative electrode collector.

[0035] In any embodiment, the base coating includes a conductive carbon coating, and the conductive coating includes one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes, hard carbon, super P, graphite, graphene, silver-composite carbon nanoparticles, and tin-composite carbon nanoparticles, and optionally includes one or more of single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0036] The above-mentioned base coating not only has excellent conductivity, but also facilitates the uniform deposition of metal ions on the surface of the current collector, thereby improving the cycle performance and coulombic efficiency of the battery.

[0037] In any embodiment, the surface density of the conductive carbon coating is 5 g / m 2 ~50g / m 2 .

[0038] Surface density is 5g / m 2 ~50g / m 2 The primer layer is beneficial to the uniform distribution of nucleation sites and promotes the uniform deposition of metals without affecting the transport behavior of electrons.

[0039] 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

[0040] Figure 1 is a schematic diagram of a sodium secondary battery according to a specific embodiment of the present application;

[0041] Figure 2 yes Figure 1 An exploded view of a sodium secondary battery according to a specific embodiment of the present application is shown;

[0042] Figure 3 is a schematic diagram of a battery module according to a specific embodiment of the present application;

[0043] Figure 4 is a schematic diagram of a battery pack according to a specific embodiment of the present application;

[0044] Figure 5 yes Figure 4 An exploded view of a battery pack according to a specific embodiment of the present application is shown;

[0045] Figure 6 It is a schematic diagram of an electrical device using a sodium secondary battery as a power source according to a specific embodiment of the present application.

[0046] Description of reference numerals:

[0047] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 sodium secondary battery; 51 shell; 52 electrode assembly; 53 cover plate. DETAILED DESCRIPTION

[0048] Hereinafter, the embodiments of the electrolyte, secondary battery and electrical device of the present application are specifically disclosed 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 descriptions 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.

[0049] "Scope" disclosed in the present application is limited in the form of lower limit and upper limit, and a 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 a special range. The scope limited in this way can be including end values ​​or not including end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a scope. For example, if the scope of 60-120 and 80-110 is listed for a specific parameter, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following scope can be all expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present 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 real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it 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.

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

[0051] 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.

[0052] If there is no special explanation, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means 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), which means 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.

[0053] If there is no special explanation, the "include" and "comprising" mentioned in this application are open-ended or closed-ended. For example, the "include" and "comprising" may mean that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

[0054] If not specifically stated, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": 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).

[0055] The low-temperature solubility of the electrolyte salt in the solvent significantly affects the low-temperature performance of the battery. For example, at -20°C, sodium hexafluorophosphate has a low solubility in ether solvents and is prone to crystallization, resulting in poor battery cycle performance and limiting the application scenarios of the battery. Therefore, it is necessary to design an electrolyte system to enable the battery to have excellent low-temperature performance and broaden the application of the battery.

[0056] [Electrolyte]

[0057] Based on this, the present application proposes an electrolyte for a sodium secondary battery, comprising a first sodium salt and a first solvent, wherein the first sodium salt comprises sodium trifluoromethanesulfonate (NaOTf), and the first solvent has a structure shown in Formula I,

[0058]

[0059] In the formula, R1 and R2 each independently include CH2 or CH2CH2, and n=1, 2, 3 or 4;

[0060] Wherein, the mass ratio of the first sodium salt to the first solvent is 0.01 to 0.4.

[0061] In some embodiments, the mass ratio of the first sodium salt to the first solvent can be selected as 0.01, 0.02, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, or a value in the range formed by any two of the above points.

[0062] It can be understood that the ethyl oxygen in the first solvent can be coordinated with the sodium ions in the sodium trifluoromethanesulfonate. On the one hand, controlling the number of carbons between two adjacent oxygens in the first solvent and the number of repeating structural units n is conducive to the chelation and coordination of the first solvent with the sodium ions in the sodium trifluoromethanesulfonate to form a relatively stable five-membered ring or six-membered ring solvation structure, so that the sodium trifluoromethanesulfonate has a high solubility in the first solvent including the structure shown in Formula I at low temperature, and the electrolyte can have excellent low-temperature ionic conductivity. At the same time, the number of repeating structural units n is controlled so that the electrolyte has a suitable viscosity, thereby taking into account the low-temperature ionic conductivity of the electrolyte and the compatibility between the electrolyte and the positive and negative electrode sheets, so that the battery has excellent low-temperature cycle performance and low-temperature coulomb efficiency. On the other hand, controlling the mass ratio of the first sodium salt and the first solvent within an appropriate range can provide enough first solvent to completely dissolve the first sodium salt, and can meet the concentration requirement of the first sodium salt so that the electrolyte has excellent ionic conductivity, thereby improving the low-temperature cycle performance and low-temperature coulomb efficiency of the battery. In addition, at low temperatures, since sodium trifluoromethanesulfonate has a high solubility in the first solvent having the structure shown in Formula I, its electrolyte system can still maintain excellent cycle performance when the low temperature drops from 0°C to -30°C.

[0063] As used herein, the term "low temperature" refers to a temperature between 0°C and -30°C, inclusive.

[0064] In some embodiments, the electrolyte further includes a second sodium salt, and the second sodium salt includes one or more of sodium bis(fluorosulfonyl)imide (NaFSI), sodium hexafluorophosphate (NaPF6), sodium tetrafluoroborate (NaBF4), sodium hexafluoroarsenate (NaAsF6), and sodium tetraphenylborate (NaBPh4), and optionally includes one or more of sodium hexafluorophosphate, sodium tetrafluoroborate, sodium hexafluoroarsenate, and sodium tetraphenylborate.

[0065] In some embodiments, the second sodium salt comprises sodium hexafluorophosphate. In some embodiments, the second sodium salt comprises sodium tetrafluoroborate. In some embodiments, the second sodium salt comprises sodium hexafluoroarsenate. In some embodiments, the second sodium salt comprises sodium tetraphenylborate.

[0066] It is understandable that the first sodium salt has a high low-temperature solubility in the first solvent, so that the electrolyte has excellent low-temperature ion conductivity. However, the first sodium salt will have a violent interface side reaction with the negative electrode at room temperature, which deteriorates the room-temperature cycle performance of the battery. The second sodium salt has relatively stable room-temperature stability. The introduction of the second sodium salt into the electrolyte is conducive to improving the room-temperature cycle performance of the battery.

[0067] In some embodiments, based on the total mass of the electrolyte, the mass content of the first sodium salt is 0.6% to 24%, and can be 2% to 12%. In some embodiments, based on the total mass of the electrolyte, the mass content of the first sodium salt can be 0.6%, 1%, 2%, 4%, 5%, 6%, 8%, 10%, 12%, 14%, 15%, 16%, 18%, 20%, 22%, 24%, or a value in the range consisting of any two of the above points.

[0068] In some embodiments, based on the total mass of the electrolyte, the mass content of the second sodium salt is 3% to 40%, optionally 8% to 20%. In some embodiments, based on the total mass of the electrolyte, the mass content of the second sodium salt can be 3%, 5%, 8%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, or a value in the range consisting of any two of the above points.

[0069] Controlling the mass content of the first sodium salt within a suitable range can not only provide enough first sodium salt to dissolve in the first solvent, so that the electrolyte has excellent low-temperature ionic conductivity, and the battery has excellent low-temperature cycle performance and low-temperature coulomb efficiency; but also control the degree of interfacial side reactions between the first sodium salt and the negative electrode. Controlling the mass content of the second sodium salt within a suitable range can not only provide enough second sodium salt to make the battery have excellent room-temperature cycle performance, but also improve the low-temperature cycle performance of the battery to a certain extent.

[0070] In some embodiments, the first solvent includes one or more of diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and optionally includes diethylene glycol dimethyl ether. In some embodiments, the first solvent includes diethylene glycol dimethyl ether. In some embodiments, the first solvent includes triethylene glycol dimethyl ether. In some embodiments, the first solvent includes tetraethylene glycol dimethyl ether.

[0071] In some embodiments, the first sodium salt comprises sodium trifluoromethanesulfonate, the second sodium salt comprises sodium hexafluorophosphate, and the first solvent comprises diethylene glycol dimethyl ether. In some embodiments, the first sodium salt comprises sodium trifluoromethanesulfonate, the second sodium salt comprises sodium tetrafluoroborate, and the first solvent comprises diethylene glycol dimethyl ether. In some embodiments, the first sodium salt comprises sodium trifluoromethanesulfonate, the second sodium salt comprises sodium hexafluoroarsenate, and the first solvent comprises diethylene glycol dimethyl ether.

[0072] The first sodium salt has a relatively high low-temperature solubility in the first solvent, so that the electrolyte has excellent ion conductivity.

[0073] In some embodiments, the mass ratio of the first sodium salt to the first solvent is 0.05 to 0.2.

[0074] The mass ratio of the first sodium salt to the first solvent is further controlled to be 0.05-0.2, which can take into account the low-temperature ionic conductivity of the electrolyte, the low-temperature cycle performance of the battery and the low-temperature coulombic efficiency.

[0075] In some embodiments, based on the total mass of the electrolyte, the mass content of the first solvent is not less than 20%, and can be 40% to 80%. In some embodiments, based on the total mass of the electrolyte, the mass content of the first solvent can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or a value in the range consisting of any two of the above points.

[0076] By controlling the mass content of the first solvent within a suitable range, a sufficient amount of the first solvent can be provided to completely dissolve the first sodium salt, so that the electrolyte has excellent low-temperature ionic conductivity, thereby improving the low-temperature cycle performance and low-temperature coulombic efficiency of the battery.

[0077] In some embodiments, the electrolyte further includes a second solvent, the freezing point of the second solvent is lower than the freezing point of the first solvent, and optionally, the freezing point of the second solvent is lower than -65°C.

[0078] Herein, the term "freezing point" refers to the temperature at which the first solvent or the second solvent solidifies under normal pressure (0.1 MPa).

[0079] Introducing a second solvent having a freezing point lower than that of the first solvent into the electrolyte is beneficial to further lowering the freezing point of the electrolyte, thereby greatly improving the ionic conductivity of the electrolyte at low temperatures and improving the low-temperature cycle performance of the battery.

[0080] In some embodiments, the second solvent includes a cyclic ether solvent, and the cyclic ether solvent includes one or more of 1,3-dioxolane (DOL), 1,4-dioxane, tetrahydrofuran (THF), and methyltetrahydrofuran, and optionally includes 1,3-dioxolane. In some embodiments, the cyclic ether solvent includes 1,4-dioxane. In some embodiments, the cyclic ether solvent includes tetrahydrofuran. In some embodiments, the cyclic ether solvent includes methyltetrahydrofuran.

[0081] In some embodiments, based on the total mass of the electrolyte, the mass content of the second solvent is not less than 10%, and can be 20% to 70%. In some embodiments, based on the total mass of the electrolyte, the mass content of the second solvent can be 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, or a value in the range consisting of any two of the above points.

[0082] The mass content of the second solvent is controlled within an appropriate range, which effectively improves the ionic conductivity of the electrolyte at low temperatures and enhances the low-temperature cycle performance of the battery.

[0083] In some embodiments, the electrolyte further includes a third solvent, and the binding force between the third solvent and the anions in the first sodium salt is greater than the binding force between the first solvent and the anions in the first sodium salt.

[0084] It can be understood that, due to the strong polarization effect of the anions in sodium trifluoromethanesulfonate on the outer weak coordination structure of the solvation structure after the coordination of the first solvent and the sodium ions, the first solvent molecules are easily oxidized on the surface of the positive electrode plate, causing the battery to overcharge at low temperatures. The introduction of the third solvent can change the solvation structure between the first solvent and sodium trifluoromethanesulfonate. Since the binding force between the third solvent and the anions in sodium trifluoromethanesulfonate is stronger than the binding force between the first solvent and the anions in sodium trifluoromethanesulfonate, the polarization effect of the anions in sodium trifluoromethanesulfonate on the outer weak coordination structure of the solvation structure after the coordination of the first solvent and the sodium ions is reduced, so the degree of oxidation reaction of the first solvent molecules on the surface of the positive electrode plate can be reduced, and the third solvent has a stronger antioxidant effect, thereby effectively inhibiting battery overcharge and improving the coulombic efficiency of the battery at low temperatures.

[0085] In this article, the term "battery overcharge" refers to the deviation between the actual charging capacity and the design capacity of the battery due to the introduction of additional capacity by side reactions of the electrolyte, which is reflected in the secondary battery as a decrease in the coulombic efficiency of the secondary battery.

[0086] In some embodiments, the third solvent comprises one or more of ethylene glycol diethyl ether (DEE), ethylene glycol dipropyl ether, propylene glycol dimethyl ether (DME), diethylene glycol diethyl ether, optionally comprises one or more of ethylene glycol diethyl ether, diethylene glycol diethyl ether. In some embodiments, the third solvent comprises ethylene glycol diethyl ether. In some embodiments, the third solvent comprises ethylene glycol diethyl ether. In some embodiments, the third solvent comprises ethylene glycol dipropyl ether. In some embodiments, the third solvent comprises propylene glycol dimethyl ether.

[0087] In some embodiments, based on the total mass of the electrolyte, the mass content of the third solvent is not less than 10%, and can be 15% to 40%. In some embodiments, based on the total mass of the electrolyte, the mass content of the third solvent can be 10%, 14%, 15%, 16%, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 37%, 40%, or a value in the range consisting of any two of the above points.

[0088] The above third solvents can effectively reduce the degree of oxidation reaction of the first solvent molecules on the surface of the positive electrode. For example, ethylene glycol diethyl ether, ethylene glycol dipropyl ether or diethylene glycol diethyl ether can increase the steric hindrance of α hydrogen on the terminal C due to the presence of ethyl or propyl groups, or propylene glycol dimethyl ether forms a six-membered ring structure to change the coordination structure, which can weaken the polarization effect of the anions in sodium trifluoromethanesulfonate on the outer weak coordination structure of the solvation structure after the coordination of the first solvent and the sodium ion, reduce the degree of oxidation reaction of the first solvent molecules on the surface of the positive electrode, and the third solvent has a stronger antioxidant effect, thereby effectively inhibiting battery overcharge and improving the coulomb efficiency of the battery at low temperature. Controlling the mass content of the third solvent within an appropriate range can effectively improve the low-temperature coulomb efficiency of the battery.

[0089] [Positive electrode]

[0090] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer formed on at least a portion of the surface of the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, and the positive electrode active material may include at least one of a layered transition metal oxide, a polyanionic compound and a Prussian blue compound.

[0091] The transition metal in the layered transition metal oxide may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce. Optionally, the layered transition metal oxide is, for example, NaxMO2, wherein M is one or more of Ti, V, Mn, Co, Ni, Fe, Cr and Cu, and 0<x≤1.

[0092] The polyanionic compound can be a compound having metal ions, transition metal ions and tetrahedral (YO4) n- A class of compounds with anionic units. The metal ion can be selected from sodium ion, lithium ion, potassium ion, zinc ion; the transition metal can be selected from at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce; Y can be selected from at least one of P, S and Si; n represents (YO4) n- valence state.

[0093] The Prussian blue compound may be a compound having sodium ions, transition metal ions and cyanide ions (CN-). The transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce. The Prussian blue compound may be, for example, Na a Me b Me' c (CN)6, wherein Me and Me' are each independently at least one of Ni, Cu, Fe, Mn, Co and Zn, 0<a≤2, 0<b<1, 0<c<1.

[0094] The positive electrode active material layer may further include a conductive agent to improve the conductivity of the positive electrode. The conductive agent may be selected from one or more of SuperP, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphite, graphene and carbon nanofibers.

[0095] The positive electrode active material layer may further include a binder to firmly bind the positive electrode active material and the optional conductive agent to the positive electrode current collector. The binder may be selected from at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), polyvinyl alcohol (PVA), ethylene-vinyl acetate copolymer (EVA), styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), sodium alginate (SA), polymethacrylic acid (PMA) and carboxymethyl chitosan (CMCS).

[0096] The positive electrode current collector may be a conductive carbon sheet, a metal foil, a carbon-coated metal foil, a porous metal plate or a composite current collector. The conductive carbon material of the conductive carbon sheet may be selected from one or more of Super P, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphite, graphene and carbon nanofibers, the metal material of the metal foil, the carbon-coated metal foil and the porous metal plate may be independently selected from at least one of copper, aluminum, nickel and stainless steel, and the composite current collector may be a composite current collector formed by a metal foil and a polymer base film.

[0097] In some embodiments, the positive electrode sheet can be prepared in the following manner: 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 collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0098] [Negative electrode]

[0099] The negative electrode sheet may only include a negative electrode current collector without including a negative electrode active material. The negative electrode sheet may also be formed by pre-depositing a metal phase on the negative electrode current collector.

[0100] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, as the metal foil, aluminum foil or copper foil may be used. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. 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 material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0101] In some embodiments, the negative electrode sheet includes a negative electrode current collector and a primer layer formed on at least a portion of the surface of the negative electrode current collector.

[0102] In some embodiments, the base coating includes a conductive carbon coating, and the conductive coating includes one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes, hard carbon, super P, graphite, graphene, silver-composite carbon nanoparticles, and tin-composite carbon nanoparticles, and optionally includes one or more of single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0103] The above-mentioned base coating not only has excellent conductivity, but also facilitates the uniform deposition of metal ions on the surface of the current collector, thereby improving the cycle performance and safety of the battery.

[0104] In some embodiments, the surface density of the base coating is 5 g / m 2 ~50g / m 2 .

[0105] In some embodiments, the surface density of the base coating may be 5 g / m 2 , 10g / m 2 , 15g / m 2 , 20g / m 2 , 25g / m 2 , 30g / m 2 , 35g / m 2 , 40g / m 2 , 45g / m 2 , 50g / m 2 , or a value in the range formed by any two of the above points.

[0106] Surface density is 5g / m 2 ~50g / m 2 The undercoat layer is beneficial to the uniform distribution of nucleation sites and promotes the uniform deposition of metals without affecting the transport behavior of electrons.

[0107] [Isolation film]

[0108] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical stability and mechanical stability can be selected.

[0109] In some embodiments, the material of the isolation membrane can be selected from at least one of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester and natural fiber. The isolation membrane can be a single-layer film or a multi-layer composite film, without particular limitation. When the isolation membrane is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0110] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly by a winding process or a lamination process.

[0111] In some embodiments, the secondary battery may include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte.

[0112] In some embodiments, the outer packaging of the secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0113] [Sodium secondary battery]

[0114] Sodium secondary batteries may include various forms, including but not limited to battery cells, battery modules, and battery packs as examples.

[0115] A battery cell, including an electrolyte in some embodiments.

[0116] The present application has no particular limitation on the shape of the battery cell, which may be cylindrical, square or any other shape. Figure 1 A square-shaped battery cell 5 is used as an example. Figure 2 It is an exploded view of the battery cell 5 .

[0117] In some embodiments, the sodium secondary battery cell further includes a positive electrode sheet, a negative electrode sheet, and a separator.

[0118] In some embodiments, the sodium secondary battery cell is a negative electrode-less sodium secondary battery.

[0119] A negative electrode-free sodium secondary battery refers to a battery that does not actively set a negative electrode active material layer on the negative electrode side during the manufacturing process of the battery. For example, a sodium metal or carbonaceous active material layer is not set at the negative electrode through coating or deposition to form a negative electrode active material layer during the manufacturing process of the battery. During the first charge, the sodium ions obtain electrons on the anode side and metallic sodium is deposited on the surface of the current collector to form a sodium metal phase. During discharge, the metallic sodium can be converted into sodium ions and return to the positive electrode to achieve cyclic charge and discharge. Compared with other sodium secondary batteries, negative electrode-free sodium secondary batteries can achieve higher energy density due to the lack of a negative electrode active material layer.

[0120] In some embodiments, in order to improve battery performance, the negative electrode side of the negative electrode-free sodium secondary battery may be provided with some conventional substances that can be used as negative electrode active materials, such as carbonaceous materials, metal oxides, alloys, etc. Although these materials have a certain capacity, due to the small amount of these materials, they are not used as the main negative electrode active materials in the battery, and therefore are not considered to form a negative electrode active material layer that plays a role in sodium insertion, and the sodium secondary battery thus constructed can still be considered as a negative electrode-free sodium secondary battery.

[0121] In some embodiments, the CB value of the negative electrode-free sodium secondary battery is less than or equal to 0.1.

[0122] The CB value is the unit area capacity of the negative electrode plate in the secondary battery divided by the unit area capacity of the positive electrode plate. Since the negative electrode-free battery does not contain negative electrode active materials, the unit area capacity of the negative electrode plate is small, and the CB value of the secondary battery is less than or equal to 0.1.

[0123] In some embodiments, reference Figure 2 , the outer packaging may include a shell 51 and a cover plate 53. Among them, 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 isolation film can form an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. 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.

[0124] [Battery module]

[0125] In some embodiments, battery cells may be assembled into a battery module. The number of battery cells contained in the battery module may be one or more, and the specific number may be selected by those skilled in the art according to the application and capacity of the battery module.

[0126] Figure 3 4 is an example of a battery module. Figure 3In the battery module 4, the plurality of battery cells 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. Further, the plurality of battery cells 5 may be fixed by fasteners.

[0127] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of battery cells 5 are received in the receiving space.

[0128] [Battery Pack]

[0129] 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 according to the application and capacity of the battery pack.

[0130] Figure 4 and Figure 5 1 is a battery pack 1 as an example. Figure 4 and Figure 5 The battery pack 1 may include a battery box and a plurality of battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0131] [Electrical devices]

[0132] In one embodiment of the present application, there is provided an electric device, comprising at least one of the sodium secondary battery of any embodiment, the battery module of any embodiment, or the battery pack of any embodiment.

[0133] The electrical device 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, or 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 are not limited thereto.

[0134] As an electrical device, a sodium secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0135] Figure 6 The power consumption device is taken as an example. The power consumption device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. In order to meet the power consumption device's requirements for high power and high energy density of sodium secondary batteries, a battery pack or a battery module can be used.

[0136] Another example of a device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be thin and light, and a sodium secondary battery may be used as a power source.

[0137] Example

[0138] Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. If no specific techniques or conditions are indicated in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. If the manufacturer is not indicated in the reagents or instruments used, they are all conventional products that can be obtained commercially.

[0139] 1. Preparation method

[0140] Example 1

[0141] 1) Electrolyte

[0142] In an argon atmosphere glove box (H2O<0.1ppm, O2<0.1ppm), 6% of the first sodium salt sodium trifluoromethanesulfonate (NaTOf) and 34% of the second sodium salt sodium hexafluorophosphate (NaPF6) were dissolved in the first solvent diethylene glycol dimethyl ether (G2), stirred evenly, and mixed to prepare an electrolyte.

[0143] 2) Preparation of positive electrode

[0144] The positive electrode active material Na3V2(PO4)3, the binder polyvinylidene fluoride (PVDF), and the conductive agent conductive carbon black (Super-P) were mixed in N-methylpyrrolidone (NMP) solvent at a mass ratio of 96%:2%:2% to prepare the positive electrode slurry. The positive electrode slurry was coated on the surface of aluminum foil according to the mass requirement of the positive electrode active material per unit area by an extrusion coater and dried. The coated electrode was then pressed by a cold press at a temperature of 2.5 g / cm 3 The designed compaction is then cold pressed to prepare the final positive electrode sheet.

[0145] 3) Preparation of negative electrode sheet

[0146] Single-walled carbon nanotubes and sodium alginate were added to deionized water and stirred into a uniform slurry. The slurry was coated on the negative electrode current collector, dried and cut to obtain the negative electrode sheet, wherein the surface density of the base coating was 25 g / m 2 .

[0147] 4) Isolation film

[0148] Polypropylene film is used as the isolation film.

[0149] 5) Preparation of batteries

[0150] The positive electrode sheet, the separator, and the negative electrode sheet are wound or stacked in order, the separator is placed between the positive and negative electrode sheets to play an isolating role, the bare cell is welded with a pole ear, and the bare cell is placed in an aluminum shell, and baked at 80°C to remove water, and then the above-mentioned electrolyte is injected and sealed to obtain an uncharged battery. The uncharged battery is then subjected to the processes of static, hot and cold pressing, formation, shaping, and capacity testing in sequence to obtain the sodium secondary battery product of Example 1.

[0151] Embodiments 2 to 29

[0152] The battery preparation methods in Examples 2 to 29 are basically similar to those in Example 1, but the preparation parameters of the electrolyte are adjusted. The specific parameters are shown in Table 1.

[0153] Comparative Example 1

[0154] The battery preparation method in Comparative Example 1 is basically similar to that in Example 1, except that: the first solvent and the second solvent are not added, and the third solvent ethylene glycol diethyl ether (DEE) is added. The specific parameters are shown in Table 1.

[0155] Comparative Example 2

[0156] The battery preparation method in Comparative Example 2 is basically similar to that in Example 1, except that the first sodium salt is adjusted to sodium tetrafluoroborate (NaBF4), and the specific parameters are shown in Table 1.

[0157] Comparative Example 3

[0158] The battery preparation method in Comparative Example 3 is basically similar to that in Example 2, except that the first sodium salt is adjusted to sodium tetrafluoroborate (NaBF4), and the specific parameters are shown in Table 1.

[0159] Comparative Example 4

[0160] The battery preparation method in Comparative Example 4 is basically similar to that in Example 14, except that the first sodium salt is adjusted to sodium tetrafluoroborate (NaBF4), and the specific parameters are shown in Table 1.

[0161] Comparative Example 5

[0162] The battery preparation method in Comparative Example 5 is basically similar to that in Example 14, except that the first solvent is not added to the electrolyte, and the contents of the second solvent and the third solvent are adjusted accordingly. The specific parameters are shown in Table 1.

[0163] Comparative Example 6

[0164] The battery preparation method in Comparative Example 6 is basically similar to that in Example 2, except that the mass contents of the first sodium salt, the first solvent, and the second solvent are adjusted so that the mass ratio of the first sodium salt to the first solvent is less than 0.01. The specific parameters are shown in Table 1.

[0165] Comparative Example 7

[0166] The battery preparation method in Comparative Example 7 is basically similar to that in Example 2, except that the mass contents of the first sodium salt, the first solvent, and the second solvent are adjusted so that the mass ratio of the first sodium salt to the first solvent is greater than 0.4. The specific parameters are shown in Table 1.

[0167] 2. Performance Test

[0168] 1. Electrolyte performance test

[0169] 1) Low temperature ionic conductivity

[0170] The test instrument for ionic conductivity is the Leici DDSJ-318 conductivity meter. The electrolyte is placed in a centrifuge tube and placed at -30°C and normal pressure (0.1MPa) for 30 minutes. Wash the conductivity electrode with deionized water and rinse off the residual water with anhydrous ethanol. After the electrode is dry, vertically insert it into the centrifuge tube containing the electrolyte to be tested, ensuring that the platinum electrode can be immersed below the surface of the electrolyte to be tested. After the instrument reading is stable, record the result, repeat the measurement three times, and take the average value.

[0171] 2. Battery performance test

[0172] 1) Low temperature Coulomb efficiency

[0173] At -30℃ and normal pressure (0.1MPa), the battery was charged to 3.5V at a constant current of 0.1C. The charging capacity at this time was recorded as the first charging capacity of the battery. After that, it was allowed to stand for 5 minutes. Then, it was discharged to 3.2V at a constant current of 0.1C and allowed to stand for 5 minutes. The discharge capacity at this time was recorded as the first discharge capacity of the battery. The first coulombic efficiency of the battery (%) = first discharge capacity / first charging capacity × 100%.

[0174] 2) Low temperature cycle performance

[0175] Tested in two low temperature environments of -30℃ and 0℃ (abbreviated as -30℃ / 0℃), at -30℃ / 0℃ and normal pressure (0.1MPa), the battery was charged at 0.5C constant current to a voltage of 3.5V, and then discharged at 1C constant current to a voltage of 3.2V, which is a charge and discharge cycle. With the capacity of the first discharge as 100%, the charge and discharge cycle was repeated 500 times, the test was stopped, and the cycle capacity retention rate was recorded. The capacity retention rate at -30℃ / 0℃ was used as an indicator to evaluate the low temperature cycle performance of the battery.

[0176] 3) Normal temperature cycle performance

[0177] At 25°C and normal pressure (0.1MPa), the battery is charged at a constant current of 0.5C to a voltage of 3.5V, and then discharged at a constant current of 1C to a voltage of 3.2V. This is a charge and discharge cycle. The capacity of the first discharge is 100%, and the charge and discharge cycle is repeated 500 times. The test is stopped and the cycle capacity retention rate is recorded. The capacity retention rate at 25°C is used as an indicator to evaluate the battery's room temperature cycle performance.

[0178] III. Analysis of test results of various embodiments and comparative examples

[0179] The batteries of the embodiments and comparative examples were prepared according to the above method, and various performance parameters were measured. The results are shown in Tables 1 and 2 below.

[0180] Table 1

[0181]

[0182]

[0183] Table 2

[0184]

[0185]

[0186] According to the above results, the electrolytes in Examples 1 to 29 all include a first sodium salt and a first solvent, the first sodium salt includes sodium trifluoromethanesulfonate, and the first solvent includes diethylene glycol dimethyl ether, triethylene glycol dimethyl ether or tetraethylene glycol dimethyl ether; wherein the mass ratio of the first sodium salt to the first solvent is 0.01 to 0.4.

[0187] From the comparison between Example 1 and Comparative Examples 1 to 2, Example 2 and Comparative Example 3, and Examples 14 to 27 and Comparative Examples 4 to 5, it can be seen that compared with the electrolyte not containing sodium trifluoromethanesulfonate or the first solvent, the electrolyte in the embodiment of the present application includes both sodium trifluoromethanesulfonate and the first solvent, which is beneficial to improving the ionic conductivity of the electrolyte at -30°C, improving the cycle performance of the battery at -30°C / 0°C and the coulomb efficiency at -30°C, and the combination of sodium trifluoromethanesulfonate and the first solvent can also reduce the battery temperature from 0°C to -30°C, and the loss of the battery's cycle capacity retention rate is in a lower range, which broadens the temperature window of the battery.

[0188] From the comparison between Examples 2 to 13 and Comparative Examples 6 to 7, it can be seen that controlling the mass ratio of the first sodium salt to the first solvent to be 0.01 to 0.4 is conducive to further improving the -30°C / 0°C cycle performance of the battery and significantly improving the low-temperature cycle performance of the battery. From the comparison between Examples 2, 5 to 8, 11 to 13 and Examples 3 to 4, 9 to 10, it can be seen that further controlling the mass ratio of the first sodium salt to the first solvent to be 0.05 to 0.2 can take into account the ionic conductivity of the electrolyte at -30°C, the cycle performance of the battery at -30°C / 0°C, and the coulombic efficiency at -30°C.

[0189] From the comparison between Example 2 and Example 1, it can be seen that the introduction of the second solvent is beneficial to improving the ionic conductivity of the electrolyte at -30°C, the -30°C / 0°C cycle performance of its battery and the coulombic efficiency at -30°C.

[0190] As can be seen from Examples 2 to 9, based on the total mass of the electrolyte, the mass content of the first sodium salt is controlled to be 0.6% to 24%, so that the electrolyte has excellent ionic conductivity at -30°C, and the battery has excellent -30°C / 0°C cycle performance, cycle performance at 25°C, and coulomb efficiency at -30°C. From the comparison of Examples 2, 4 to 8 with Examples 3 and 9, it can be seen that based on the total mass of the electrolyte, further controlling the mass content of the first sodium salt to be 2% to 12% is beneficial to further improve the cycle performance at -30°C / 0°C and coulomb efficiency at -30°C of the battery.

[0191] It can be seen from Examples 2 to 13 that, based on the total mass of the electrolyte, the mass content of the second sodium salt is controlled to be 3% to 40% so that the electrolyte has excellent ionic conductivity at -30°C, and its battery has excellent cycle performance at -30°C / 0°C, cycle performance at 25°C and coulombic efficiency at -30°C.

[0192] It can be seen from Examples 2, 10 to 12 that, based on the total mass of the electrolyte, the mass content of the first solvent is controlled to be not less than 20%, so that the electrolyte has excellent ionic conductivity at -30°C, and its battery has excellent cycle performance at -30°C / 0°C, cycle performance at 25°C, and coulombic efficiency at -30°C. From the comparison of Examples 2, 11 to 12 with Example 10, it can be seen that, based on the total mass of the electrolyte, further controlling the mass content of the first solvent to 40% to 80% is beneficial to improving the ionic conductivity at -30°C, the cycle performance at -30°C / 0°C of the electrolyte, and the cycle performance at 25°C of the battery.

[0193] From the comparison between Example 14 and Example 2, it can be seen that the introduction of the third solvent is beneficial to improving the -30°C / 0°C cycle performance, the cycle performance at 25°C and the coulombic efficiency at -30°C of the battery.

[0194] It can be seen from Examples 14 to 18 that the second sodium salt includes sodium hexafluorophosphate, sodium tetrafluoroborate, sodium hexafluoroarsenate, sodium tetraphenylborate or sodium bis(fluorosulfonyl)imide, all of which can make the electrolyte have excellent ionic conductivity at -30°C, and its battery have excellent cycle performance at -30°C / 0°C, cycle performance at 25°C and coulomb efficiency at -30°C.

[0195] It can be seen from Examples 14 and 19 to 20 that the first solvent includes diethylene glycol dimethyl ether, triethylene glycol dimethyl ether or tetraethylene glycol dimethyl ether, which can make the electrolyte have excellent ionic conductivity at -30°C, and its battery has excellent cycle performance at -30°C / 0°C, cycle performance at 25°C and coulombic efficiency at -30°C.

[0196] From the comparison between Example 2 and Example 1, it can be seen that the electrolyte including the second solvent is beneficial to improving the cycle performance of the battery at -30°C / 0°C and the coulombic efficiency at -30°C.

[0197] It can be seen from Examples 14 and 21 to 22 that the second solvent includes 1,3-dioxolane, 1,4-dioxane or tetrahydrofuran, which can make the electrolyte have excellent ionic conductivity at -30°C, and its battery has excellent cycle performance at -30°C / 0°C, cycle performance at 25°C and coulombic efficiency at -30°C.

[0198] From the comparison between Example 14 and Example 2, it can be seen that the electrolyte including the third solvent is beneficial to improving the cycle performance at -30°C / 0°C, the cycle performance at 25°C and the coulombic efficiency at -30°C of the battery. From Examples 14, 23 to 24, it can be seen that the third solvent includes ethylene glycol diethyl ether, ethylene glycol dipropyl ether or propylene glycol dimethyl ether, which can make the electrolyte have excellent ionic conductivity at -30°C, and the battery thereof has excellent cycle performance at -30°C / 0°C, cycle performance at 25°C and coulombic efficiency at -30°C.

[0199] From the comparison between Examples 14, 26 to 27 and Example 25, it can be seen that the negative electrode plate has a primer layer, which is beneficial to improving the cycle performance of the battery at -30°C / 0°C, the cycle performance at 25°C and the coulombic efficiency at -30°C.

[0200] It can be seen from Examples 14 and 28 to 29 that, based on the total mass of the electrolyte, the mass content of the third solvent is controlled to be not less than 10% so that the electrolyte has excellent low-temperature ionic conductivity and its battery has excellent cycle performance at -30°C / 0°C, cycle performance at 25°C and coulombic efficiency at -30°C.

[0201] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining 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, characterized in that: It includes a first sodium salt and a first solvent, wherein the first sodium salt includes sodium trifluoromethanesulfonate, and the first solvent has a structure shown in Formula I, In the formula, R1 and R2 each independently include CH2 or CH2CH2, and n=1, 2, 3 or 4; Wherein, the mass ratio of the first sodium salt to the first solvent is 0.01 to 0.

4.

2. The electrolyte according to claim 1, characterized in that The electrolyte also includes a second sodium salt, which includes one or more of sodium bis(fluorosulfonyl)imide, sodium hexafluorophosphate, sodium tetrafluoroborate, sodium hexafluoroarsenate, and sodium tetraphenylborate, and can optionally include one or more of sodium hexafluorophosphate, sodium tetrafluoroborate, sodium hexafluoroarsenate, and sodium tetraphenylborate.

3. The electrolyte according to claim 2, characterized in that Based on the total mass of the electrolyte, the mass content of the first sodium salt is 0.6% to 24%, and can be optionally 2% to 12%; and / or Based on the total mass of the electrolyte, the mass content of the second sodium salt is 3% to 40%, and can be optionally 8% to 20%.

4. The electrolyte according to any one of claims 1 to 3, characterized in that The first solvent includes one or more of diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether, and may optionally include diethylene glycol dimethyl ether.

5. The electrolyte according to any one of claims 1 to 4, characterized in that The mass ratio of the first sodium salt to the first solvent is 0.05 to 0.

2.

6. The electrolyte according to any one of claims 1 to 5, characterized in that Based on the total mass of the electrolyte, the mass content of the first solvent is not less than 20%, and can be optionally 40% to 80%.

7. The electrolyte according to any one of claims 1 to 6, characterized in that The electrolyte further includes a second solvent, the freezing point of the second solvent is lower than the freezing point of the first solvent, and optionally, the freezing point of the second solvent is lower than -65°C.

8. The electrolyte according to claim 7, characterized in that The second solvent comprises a cyclic ether solvent, wherein the cyclic ether solvent comprises one or more of 1,3-dioxolane, 1,4-dioxane, tetrahydrofuran, methyltetrahydrofuran, and optionally 1,3-dioxolane; and / or Based on the total mass of the electrolyte, the mass content of the second solvent is not less than 10%, and can be optionally 20% to 70%.

9. The electrolyte according to any one of claims 1 to 8, characterized in that The electrolyte further includes a third solvent, and the binding force between the third solvent and the anions in the first sodium salt is greater than the binding force between the first solvent and the anions in the first sodium salt.

10. The electrolyte according to claim 9, characterized in that The third solvent includes one or more of ethylene glycol diethyl ether, ethylene glycol dipropyl ether, propylene glycol dimethyl ether, and diethylene glycol diethyl ether, and optionally includes one or more of ethylene glycol diethyl ether and diethylene glycol diethyl ether; and / or Based on the total mass of the electrolyte, the mass content of the third solvent is not less than 10%, and can be optionally 15% to 40%.

11. A sodium secondary battery, characterized in that: The electrolyte comprising the electrolyte according to any one of claims 1 to 10.

12. The sodium secondary battery according to claim 11, characterized in that The sodium secondary battery is a negative electrode-free sodium secondary battery.

13. The sodium secondary battery according to claim 11 or 12, characterized in that: The sodium secondary battery includes a negative electrode sheet, wherein the negative electrode sheet includes a negative electrode collector and a primer layer formed on at least a portion of a surface of the negative electrode collector.

14. The secondary battery according to claim 13, characterized in that: The base coating includes a conductive carbon coating, and the conductive coating includes one or more of single-walled carbon nanotubes, multi-walled carbon nanotubes, hard carbon, super P, graphite, graphene, silver composite carbon nanoparticles, and tin composite carbon nanoparticles, and optionally includes one or more of single-walled carbon nanotubes and multi-walled carbon nanotubes.

15. The secondary battery according to claim 14, characterized in that: The surface density of the conductive carbon coating is 5 g / m 2 ~50g / m 2 .

16. An electrical device, characterized in that: A sodium secondary battery comprising the sodium secondary battery according to any one of claims 11 to 15.

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