Electrolyte, sodium secondary battery and electric device

Abstract

An electrolyte, a sodium secondary battery and an electric device. The electrolyte comprises a first sodium salt and a first solvent, wherein the first sodium salt comprises sodium trifluoromethanesulfonate, and the first solvent has a structure as shown in formula I, with the mass ratio of the first sodium salt to the first solvent being 0.01-0.4. The electrolyte has a good low-temperature ionic conductivity, which is beneficial for improving the low-temperature cycling performance and low-temperature coulombic efficiency of the battery.

Description

Electrolyte, sodium secondary battery and electrical device

[0001] Cross-references

[0002] This application refers to Chinese patent application No. 202310799078.3, filed on June 30, 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 secondary 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] The performance of electrolytes has a key impact on the performance of secondary batteries. Currently, electrolytes have many defects and cannot meet the application needs of the new generation of electrochemical systems.

[0006] Summary of the Invention

[0007] 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.

[0008] 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.

[0009] Wherein, R1 and R2 each independently include CH2 or CH2CH2, and n=1, 2, 3 or 4;

[0010] Wherein, the mass ratio of the first sodium salt to the first solvent is 0.01 to 0.4.

[0011] The ethyl oxygen in the first solvent can coordinate with the sodium ions in the sodium trifluoromethanesulfonate. On the one hand, controlling the number of carbon atoms between two adjacent oxygen atoms and the number of repeating structural units n in the first solvent 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 coulombic 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 coulombic 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 temperature drops from 0°C to -30°C.

[0012] 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.

[0013] The introduction of a second sodium salt into the electrolyte is beneficial to improving the room temperature cycle performance of the battery.

[0014] 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

[0015] 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%.

[0016] Controlling the mass content of the first sodium salt within an appropriate range not only allows sufficient first sodium salt to dissolve in the first solvent, resulting in excellent low-temperature ionic conductivity for the electrolyte and excellent low-temperature cycling performance and low-temperature coulombic efficiency for the resulting battery, but also controls the extent of interfacial side reactions between the first sodium salt and the negative electrode. Controlling the mass content of the second sodium salt within an appropriate range allows sufficient second sodium salt to provide excellent room-temperature cycling performance for the battery and also improves the battery's low-temperature cycling performance to a certain extent.

[0017] 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.

[0018] The first sodium salt has relatively high low-temperature solubility in the first solvent, so that the electrolyte has excellent ion conductivity.

[0019] In any embodiment, the mass ratio of the first sodium salt to the first solvent is 0.05 to 0.2.

[0020] Further controlling the mass ratio of the first sodium salt to the first solvent to be 0.05-0.2 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.

[0021] 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%.

[0022] By controlling the mass content of the first solvent within a suitable range, sufficient first solvent can be provided to completely dissolve the first sodium salt, so that the electrolyte has excellent low-temperature ionic conductivity and improves the low-temperature cycle performance and low-temperature coulombic efficiency of the battery.

[0023] 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.

[0024] Introducing a second solvent with a lower freezing point than 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.

[0025] In any embodiment, 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, and methyltetrahydrofuran, and optionally comprises 1,3-dioxolane; and / or

[0026] 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%.

[0027] By controlling the mass content of the second solvent within an appropriate range, the ionic conductivity of the electrolyte at low temperatures can be effectively improved, thereby enhancing the low-temperature cycle performance of the battery.

[0028] 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.

[0029] 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, 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 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 is reduced, so the degree of oxidation reaction of the first solvent molecules on the surface of the positive electrode can be reduced, and the third solvent has stronger antioxidant properties, thereby effectively suppressing battery overcharge and improving the coulombic efficiency of the battery at low temperatures.

[0030] 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

[0031] 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%.

[0032] The above-mentioned 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 the 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 sodium ions, reducing the degree of oxidation reaction of the first solvent molecules on the surface of the positive electrode. The third solvent has a stronger antioxidant effect, thereby effectively suppressing battery overcharge and improving the coulombic efficiency of the battery at low temperatures. Controlling the mass content of the third solvent within an appropriate range can effectively improve the low-temperature coulombic efficiency of the battery.

[0033] A second aspect of the present application provides a sodium secondary battery, comprising the electrolyte of the first aspect of the present application.

[0034] In any embodiment, the sodium secondary battery is a negative electrode-less sodium secondary battery.

[0035] Anode-free sodium secondary batteries have high energy density.

[0036] In any embodiment, the sodium secondary battery includes a negative electrode sheet, wherein the negative electrode sheet includes a negative electrode current collector and a primer layer formed on at least a portion of a surface of the negative electrode current collector.

[0037] 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.

[0038] The above-mentioned primer layer 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.

[0039] In any embodiment, the surface density of the conductive carbon coating is 5g / m 2 ~50g / m 2 .

[0040] 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 metal without affecting the transport behavior of electrons.

[0041] 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

[0042] FIG1 is a schematic diagram of a sodium secondary battery according to a specific embodiment of the present application;

[0043] FIG2 is an exploded view of a sodium secondary battery according to a specific embodiment of the present application shown in FIG1 ;

[0044] FIG3 is a schematic diagram of a battery module according to a specific embodiment of the present application;

[0045] FIG4 is a schematic diagram of a battery pack according to a specific embodiment of the present application;

[0046] FIG5 is an exploded view of the battery pack shown in FIG4 according to a specific embodiment of the present application;

[0047] FIG6 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.

[0048] Description of reference numerals:

[0049] 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

[0050] Below, the embodiments of the electrolyte, 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 may be 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.

[0051] " 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.

[0052] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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).

[0057] 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 easily crystallizes, resulting in poor battery cycle performance and limiting its application scenarios. Therefore, it is necessary to design an electrolyte system that can enable batteries with excellent low-temperature performance and broaden their applications.

[0058] [Electrolyte]

[0059] 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.

[0060] Wherein, R1 and R2 each independently include CH2 or CH2CH2, and n=1, 2, 3 or 4;

[0061] The mass ratio of the first sodium salt to the first solvent is 0.01 to 0.4.

[0062] 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 a range consisting of any two of the above points.

[0063] 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 carbon atoms between two adjacent oxygen atoms in the first solvent and the number of repeating structural units n is beneficial 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 comprising 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 a suitable 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 temperature drops from 0°C to -30°C.

[0064] As used herein, the term "low temperature" refers to a temperature between 0°C and -30°C, inclusive.

[0065] 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.

[0066] 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.

[0067] Understandably, the first sodium salt has high low-temperature solubility in the first solvent, resulting in excellent low-temperature ionic conductivity in the electrolyte. However, at room temperature, the first sodium salt can undergo intense interfacial side reactions with the negative electrode, deteriorating the battery's room-temperature cycling performance. The second sodium salt, on the other hand, has relatively stable room-temperature stability. Its introduction into the electrolyte can improve the battery's room-temperature cycling performance.

[0068] In some embodiments, the mass content of the first sodium salt is 0.6% to 24%, optionally 2% to 12%, based on the total mass of the electrolyte. In some embodiments, the mass content of the first sodium salt is 0.6%, 1%, 2%, 4%, 5%, 6%, 8%, 10%, 12%, 14%, 15%, 16%, 18%, 20%, 22%, 24%, or a value in a range consisting of any two of the above values, based on the total mass of the electrolyte.

[0069] In some embodiments, the mass content of the second sodium salt is 3% to 40%, optionally 8% to 20%, based on the total mass of the electrolyte. In some embodiments, the mass content of the second sodium salt is 3%, 5%, 8%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, or a value in a range consisting of any two of the above values, based on the total mass of the electrolyte.

[0070] Controlling the mass content of the first sodium salt within an appropriate range not only allows sufficient first sodium salt to dissolve in the first solvent, resulting in excellent low-temperature ionic conductivity for the electrolyte and excellent low-temperature cycling performance and low-temperature coulombic efficiency for the resulting battery, but also controls the extent of interfacial side reactions between the first sodium salt and the negative electrode. Controlling the mass content of the second sodium salt within an appropriate range allows sufficient second sodium salt to provide excellent room-temperature cycling performance for the battery and also improves the battery's low-temperature cycling performance to a certain extent.

[0071] In some embodiments, the first solvent comprises one or more of diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and optionally comprises diethylene glycol dimethyl ether. In some embodiments, the first solvent comprises diethylene glycol dimethyl ether. In some embodiments, the first solvent comprises triethylene glycol dimethyl ether. In some embodiments, the first solvent comprises tetraethylene glycol dimethyl ether.

[0072] 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.

[0073] The first sodium salt has relatively high low-temperature solubility in the first solvent, so that the electrolyte has excellent ion conductivity.

[0074] In some embodiments, the mass ratio of the first sodium salt to the first solvent is 0.05 to 0.2.

[0075] Further controlling the mass ratio of the first sodium salt to the first solvent to be 0.05-0.2 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.

[0076] In some embodiments, the mass content of the first solvent is not less than 20% based on the total mass of the electrolyte, and can be 40% to 80%. In some embodiments, 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 a range consisting of any two of the above values, based on the total mass of the electrolyte.

[0077] By controlling the mass content of the first solvent within a suitable range, sufficient first solvent can be provided to completely dissolve the first sodium salt, so that the electrolyte has excellent low-temperature ionic conductivity and improves the low-temperature cycle performance and low-temperature coulombic efficiency of the battery.

[0078] In some embodiments, the electrolyte further includes a second solvent, and the freezing point of the second solvent is lower than the freezing point of the first solvent. Optionally, the freezing point of the second solvent is lower than -65°C.

[0079] Herein, the term "freezing point" refers to the temperature at which the first solvent or the second solvent freezes under normal pressure (0.1 MPa).

[0080] Introducing a second solvent with a lower freezing point than 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.

[0081] In some embodiments, the second solvent comprises a cyclic ether solvent, and the cyclic ether solvent comprises one or more of 1,3-dioxolane (DOL), 1,4-dioxane, tetrahydrofuran (THF), and methyltetrahydrofuran, and optionally comprises 1,3-dioxolane. In some embodiments, the cyclic ether solvent comprises 1,4-dioxane. In some embodiments, the cyclic ether solvent comprises tetrahydrofuran. In some embodiments, the cyclic ether solvent comprises methyltetrahydrofuran.

[0082] In some embodiments, the mass content of the second solvent is not less than 10% based on the total mass of the electrolyte, and can be 20% to 70%. In some embodiments, 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 a range consisting of any two of the above values, based on the total mass of the electrolyte.

[0083] By controlling the mass content of the second solvent within an appropriate range, the ionic conductivity of the electrolyte at low temperatures can be effectively improved, thereby enhancing the low-temperature cycle performance of the battery.

[0084] In some embodiments, the electrolyte further comprises 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.

[0085] It can be understood that 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, 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 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 is reduced, so the degree of oxidation reaction of the first solvent molecules on the surface of the positive electrode can be reduced, and the third solvent has stronger antioxidant properties, thereby effectively suppressing battery overcharge and improving the coulombic efficiency of the battery at low temperatures.

[0086] In this article, the term "battery overcharge" refers to the deviation between the actual charging capacity and the designed 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.

[0087] 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.

[0088] In some embodiments, the weight content of the third solvent is not less than 10% based on the total weight of the electrolyte, and can be 15% to 40%. In some embodiments, the weight content of the third solvent can be 10%, 14%, 15%, 16%, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 37%, 40%, or a value in a range consisting of any two of the above values, based on the total weight of the electrolyte.

[0089] The above-mentioned 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 the 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 sodium ions, reducing the degree of oxidation reaction of the first solvent molecules on the surface of the positive electrode. The third solvent has a stronger antioxidant effect, thereby effectively suppressing battery overcharge and improving the coulombic efficiency of the battery at low temperatures. Controlling the mass content of the third solvent within an appropriate range can effectively improve the low-temperature coulombic efficiency of the battery.

[0090] [Positive electrode]

[0091] 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, which may include at least one of a layered transition metal oxide, a polyanionic compound, and a Prussian blue compound.

[0092] The transition metal in the layered transition metal oxide can 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.

[0093] Polyanionic compounds can be 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, and 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.

[0094] Prussian blue compounds can be compounds containing sodium ions, transition metal ions, and cyanide ions (CN-). The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. Prussian blue compounds are, 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.

[0095] 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 Super P, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphite, graphene, and carbon nanofibers.

[0096] The positive electrode active material layer may further include a binder to firmly bond the positive electrode active material and optional conductive agent to the positive electrode current collector. The binder may be 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).

[0097] The positive electrode current collector can be a conductive carbon sheet, metal foil, carbon-coated metal foil, porous metal plate, or a composite current collector. The conductive carbon material of the conductive carbon sheet can 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, carbon-coated metal foil, and porous metal plate can each be independently selected from at least one of copper, aluminum, nickel, and stainless steel. The composite current collector can be a composite of metal foil and a polymer-based film.

[0098] 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.

[0099] [Negative electrode]

[0100] The negative electrode plate may only include a negative electrode current collector without including a negative electrode active material. The negative electrode plate may also be formed by pre-depositing a metal phase on the negative electrode current collector.

[0101] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil or copper 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.).

[0102] In some embodiments, the negative electrode sheet includes a negative electrode current collector and a primer layer formed on at least a portion of a surface of the negative electrode current collector.

[0103] In some embodiments, the primer layer 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.

[0104] The above-mentioned primer layer 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.

[0105] In some embodiments, the basecoat layer has an areal density of 5 g / m 2 ~50g / m 2 .

[0106] In some embodiments, the surface density of the base coating can 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.

[0107] 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 metal without affecting the transport behavior of electrons.

[0108] [Isolation film]

[0109] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0110] In some embodiments, the material of the separator can be selected from at least one of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0111] 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.

[0112] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0113] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0114] [Sodium secondary battery]

[0115] Sodium secondary batteries may include various forms, including but not limited to battery cells, battery modules, and battery packs, as examples.

[0116] A battery cell, including an electrolyte in some embodiments.

[0117] The present application has no particular limitation on the shape of the battery cell, which can be cylindrical, square, or any other shape. For example, FIG1 is an example of a square-shaped battery cell 5 , and FIG2 is an exploded view of the battery cell 5 .

[0118] In some embodiments, the sodium secondary battery cell further includes a positive electrode sheet, a negative electrode sheet, and a separator.

[0119] In some embodiments, the sodium secondary battery cell is a negative electrode-less sodium secondary battery.

[0120] A negative electrode-free sodium secondary battery refers to a battery in which a negative electrode active material layer is not actively provided on the negative electrode side during the battery manufacturing process. For example, a sodium metal or carbonaceous active material layer is not provided on the negative electrode through a coating or deposition process to form a negative electrode active material layer during the battery manufacturing process. During the first charge, sodium ions gain electrons on the anode side, and metallic sodium is deposited on the current collector surface to form a sodium metal phase. During discharge, the metallic sodium can be converted into sodium ions and return to the positive electrode, achieving cyclic charge and discharge. Compared to 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.

[0121] In some embodiments, 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 are therefore not considered to form a negative electrode active material layer that plays a sodium intercalation role. The sodium secondary battery thus constructed can still be considered a negative electrode-free sodium secondary battery.

[0122] In some embodiments, the CB value of the negative electrode-free sodium secondary battery is less than or equal to 0.1.

[0123] The CB value is the capacity per unit area of ​​the negative electrode in a secondary battery divided by the capacity per unit area of ​​the positive electrode. Since a battery without a negative electrode contains no negative active material, the capacity per unit area of ​​the negative electrode is relatively small, and the CB value of a secondary battery is less than or equal to 0.1.

[0124] In some embodiments, referring to FIG2 , the outer packaging may include a shell 51 and a cover plate 53. Specifically, the shell 51 may include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates 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 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.

[0125] [Battery Module]

[0126] In some embodiments, battery cells may be assembled into a battery module. The battery module may contain one or more battery cells. The specific number may be selected by those skilled in the art based on the application and capacity of the battery module.

[0127] FIG3 shows an example battery module 4. Referring to FIG3 , within the battery module 4, multiple battery cells 5 may be arranged sequentially along the length of the battery module 4. Of course, they may also be arranged in any other manner. Furthermore, the multiple battery cells 5 may be secured together using fasteners.

[0128] Optionally, the battery module 4 may further include a housing having an accommodation space, and the plurality of battery cells 5 are accommodated in the accommodation space.

[0129] [Battery Pack]

[0130] 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.

[0131] 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.

[0132] [Electrical devices]

[0133] In one embodiment of the present application, an electric device is provided, 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.

[0134] 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 can 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.

[0135] As an electrical device, a sodium secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0136] 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.

[0137] 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.

[0138] Example

[0139] 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.

[0140] 1. Preparation method

[0141] Example 1

[0142] 1) Electrolyte

[0143] 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.

[0144] 2) Preparation of positive electrode sheet

[0145] 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 using an extrusion coater according to the positive electrode active material unit area mass requirement and dried. The coated electrode was then pressed by a cold press at a pressure of 2.5 g / cm 3 The designed compaction is cold pressed to prepare the final positive electrode sheet.

[0146] 3) Preparation of negative electrode sheet

[0147] 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. The surface density of the base coating was 25 g / m 2 .

[0148] 4) Isolation film

[0149] Polypropylene film is used as the isolation film.

[0150] 5) Preparation of batteries

[0151] The positive electrode sheet, separator, and negative electrode sheet are wound or stacked in sequence, with the separator positioned between the positive and negative electrode sheets to provide insulation. Tabs are welded to the bare cell, which is then placed in an aluminum casing and baked at 80°C to remove moisture. The electrolyte is then injected and sealed to produce an uncharged battery. The uncharged battery then undergoes a series of steps, including resting, hot and cold pressing, formation, shaping, and capacity testing, to obtain the sodium secondary battery product of Example 1.

[0152] Examples 2 to 29

[0153] 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.

[0154] Comparative Example 1

[0155] 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.

[0156] Comparative Example 2

[0157] 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). The specific parameters are shown in Table 1.

[0158] Comparative Example 3

[0159] 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). The specific parameters are shown in Table 1.

[0160] Comparative Example 4

[0161] 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). The specific parameters are shown in Table 1.

[0162] Comparative Example 5

[0163] 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.

[0164] Comparative Example 6

[0165] 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.

[0166] Comparative Example 7

[0167] 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.

[0168] 2. Performance Testing

[0169] 1. Electrolyte performance test

[0170] 1) Low temperature ionic conductivity

[0171] Ionic conductivity was tested using a Leici DDSJ-318 conductivity meter. The electrolyte was placed in a centrifuge tube and maintained at -30°C and atmospheric pressure (0.1 MPa) for 30 minutes. The conductivity electrode was cleaned with deionized water and rinsed with anhydrous ethanol to remove any remaining moisture. After the electrode was dry, it was inserted vertically into a centrifuge tube containing the electrolyte to be tested, ensuring that the platinum electrode was submerged below the surface of the electrolyte to be tested. After the instrument reading stabilized, the results were recorded. The measurement was repeated three times and the average value was calculated.

[0172] 2. Battery performance test

[0173] 1) Low temperature Coulombic efficiency

[0174] At -30°C and normal pressure (0.1MPa), the battery was charged to 3.5V at a constant current rate 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, and then discharged at a constant current rate of 0.1C to 3.2V. It was 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%.

[0175] 2) Low temperature cycle performance

[0176] The test was conducted at two low-temperature environments, -30°C and 0°C (abbreviated as -30°C / 0°C). At -30°C / 0°C and atmospheric pressure (0.1MPa), the battery was 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 constituted one charge-discharge cycle. The charge-discharge cycle was repeated 500 times, with the initial discharge capacity set as 100%. The test was then stopped and the cycle capacity retention rate was recorded. The capacity retention rate at -30°C / 0°C was used as an indicator to evaluate the battery's low-temperature cycle performance.

[0177] 3) Normal temperature cycle performance

[0178] 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, then discharged at a constant current of 1C to a voltage of 3.2V. This is considered one charge-discharge cycle. With the initial discharge capacity as 100%, the charge-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 cycling performance.

[0179] 3. Analysis of test results of various embodiments and comparative examples

[0180] 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.

[0181] Table 1

[0182] Table 2

[0183] 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.

[0184] 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 coulombic efficiency at -30°C, and the combination of sodium trifluoromethanesulfonate and the first solvent can also reduce the temperature of the battery from 0°C to -30°C, and the loss of the cycle capacity retention rate of the battery is in a lower range, which broadens the temperature window of the battery.

[0185] Comparison of Examples 2-13 with Comparative Examples 6-7 shows that controlling the mass ratio of the first sodium salt to the first solvent to 0.01-0.4 is beneficial for further improving the -30°C / 0°C cycle performance of the battery, significantly improving the low-temperature cycle performance of the battery. Comparison of Examples 2, 5-8, 11-13 with Examples 3-4, 9-10 shows that further controlling the mass ratio of the first sodium salt to the first solvent to 0.05-0.2 can balance the ionic conductivity of the electrolyte at -30°C, the -30°C / 0°C cycle performance of the battery, and the coulombic efficiency at -30°C.

[0186] 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.

[0187] 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 thereof has excellent -30°C / 0°C cycle performance, cycle performance at 25°C, and coulombic efficiency at -30°C. As can be seen from the comparison of Examples 2, 4 to 8 with Examples 3 and 9, 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 improving the -30°C / 0°C cycle performance and coulombic efficiency at -30°C of the battery thereof.

[0188] As can be seen from Examples 2 to 13, 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.

[0189] As can be seen from Examples 2 and 10 to 12, based on the total mass of the electrolyte, the mass content of the first solvent is controlled to be no less than 20%, so that the electrolyte has excellent ionic conductivity at -30°C, and the battery thereof has excellent -30°C / 0°C cycling performance, 25°C cycling performance, and -30°C coulombic efficiency. As can be seen from the comparison of Examples 2 and 11 to 12 with Example 10, 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 -30°C ionic conductivity of the electrolyte, the -30°C / 0°C cycling performance, and the 25°C cycling performance of the battery.

[0190] 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.

[0191] As can be seen from Examples 14 to 18, the second sodium salt includes sodium hexafluorophosphate, sodium tetrafluoroborate, sodium hexafluoroarsenate, sodium tetraphenylborate or sodium bis(fluorosulfonyl)imide, which can make the electrolyte have excellent ionic conductivity at -30°C, and its battery has excellent -30°C / 0°C cycle performance, 25°C cycle performance and -30°C coulombic efficiency.

[0192] It can be seen from Examples 14, 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 -30°C / 0°C cycle performance, 25°C cycle performance and -30°C coulombic efficiency.

[0193] 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 -30°C / 0°C cycle performance and the -30°C coulombic efficiency of the battery.

[0194] 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 -30°C / 0°C cycle performance, 25°C cycle performance and -30°C coulombic efficiency.

[0195] From the comparison between Example 14 and Example 2, it can be seen that the electrolyte including the third solvent is beneficial for improving the battery's cycling performance at -30°C / 0°C, cycling performance at 25°C, and coulombic efficiency at -30°C. From Examples 14 and 23-24, it can be seen that the third solvent including ethylene glycol diethyl ether, ethylene glycol dipropyl ether, or propylene glycol dimethyl ether can all enable the electrolyte to have excellent ionic conductivity at -30°C, and the battery thereof to have excellent cycling performance at -30°C / 0°C, cycling performance at 25°C, and coulombic efficiency at -30°C.

[0196] 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.

[0197] 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.

[0198] 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, 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, Wherein, 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 optionally includes 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%, 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 includes a cyclic ether solvent, and the cyclic ether solvent includes one or more of 1,3-dioxolane, 1,4-dioxane, tetrahydrofuran, and methyltetrahydrofuran, and optionally includes 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 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 current collector and a primer layer formed on at least a portion of a surface of the negative electrode current collector.

14. The secondary battery according to claim 13, wherein: 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 5g / 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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