Electrolytes, sodium secondary batteries, and electrical devices

Abstract

This application provides an electrolyte, a sodium secondary battery, and an electrical device. The electrolyte comprises a first sodium salt and a first solvent, the first sodium salt comprising sodium trifluoromethanesulfonic acid, and the first solvent having a structure represented by formula I, wherein the mass ratio of the first sodium salt to the first solvent is 0.01 to 0.4. The electrolyte has excellent low-temperature ionic conductivity and is advantageous for improving the low-temperature cycle performance and low-temperature Coulomb efficiency of the battery.

Description

cross reference

[0001] This application incorporates, by reference, Chinese Patent Application No. 202310799078.3, filed on June 30, 2023, entitled “Electrolyte, Sodium Secondary Battery and Electrical Device,” in its entirety. [Technical Field]

[0002] This application relates to the technical field of secondary batteries, and more particularly to electrolytes, sodium secondary batteries, and electrical devices. [Background technology]

[0003] In recent years, secondary batteries have been widely applied in energy storage and power systems such as hydroelectric power plants, thermal power plants, wind power plants, and solar power plants, as well as in various fields such as power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, and aerospace.

[0004] The performance of the electrolyte has a decisive impact on the performance of secondary batteries. Currently, electrolytes have many defects and cannot meet the needs of the application of new generation electrochemical systems. [Overview of the Initiative]

[0005] In view of the problems of the background art, this application provides an electrolyte intended to improve the low-temperature ionic conductivity of the electrolyte, as well as the low-temperature cycle performance and low-temperature Coulomb efficiency of the battery.

[0006] A first aspect of the present application is an electrolyte for a sodium secondary battery comprising a first sodium salt and a first solvent, wherein the first sodium salt comprises sodium trifluoromethanesulfonic acid and the first solvent has a structure represented by formula I. [ka] In the formula, R1 and R2 each independently contain either CH2 or CH2CH2, and n = 1, 2, 3, or 4. The present invention provides an electrolyte in which the mass ratio of the first sodium salt to the first solvent is 0.01 to 0.4.

[0007] The ethyl oxide in the first solvent can coordinate with the sodium ions in sodium trifluoromethanesulfonic acid. On the other 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 advantageous for the first solvent to chelate with the sodium ions in sodium trifluoromethanesulfonic acid to form a relatively stable 5-membered or 6-membered ring solvation structure. As a result, sodium trifluoromethanesulfonic acid has relatively high solubility in the first solvent containing the structure represented by formula I at low temperatures, and the electrolyte can have excellent low-temperature ionic conductivity. By controlling the number of repeating structural units n, the electrolyte can have an appropriate viscosity, and by achieving both the low-temperature ionic conductivity of the electrolyte and the compatibility between the electrolyte and the positive and negative electrode pieces, the battery can have excellent low-temperature cycle performance and low-temperature Coulomb efficiency. On the other hand, by controlling the mass ratio of the first sodium salt to the first solvent within an appropriate range, a sufficient amount of the first solvent can be provided to completely dissolve the first sodium salt, and the concentration needs of the first sodium salt can be met, resulting in the electrolyte having excellent ionic conductivity and improving the low-temperature cycle performance and low-temperature Coulomb efficiency of the battery. Furthermore, at low temperatures, sodium trifluoromethanesulfonic acid has high solubility in the first solvent having the structure represented by formula I, so that the electrolyte system can maintain excellent cycle performance even when the temperature drops from 0°C to -30°C.

[0008] In any embodiment, the electrolyte further comprises a second sodium salt, the second sodium salt comprising one or more of sodium bis(fluorosulfonyl)imide, sodium hexafluoride phosphate, sodium tetrafluoroborate, sodium hexafluoride arsenate, and sodium tetraphenylborate, and selectively comprising one or more of sodium hexafluoride phosphate, sodium tetrafluoroborate, sodium hexafluoride arsenate, and sodium tetraphenylborate.

[0009] Introducing a second sodium salt into the electrolyte is advantageous in improving the battery's room-temperature cycle performance.

[0010] In any embodiment, 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 described above is 3% to 40% of the total mass of the electrolyte, and optionally 8% to 20%.

[0011] By controlling the mass content of the first sodium salt within an appropriate range, a sufficient amount of the primary sodium salt can be provided and dissolved 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, while also controlling the degree to which interfacial side reactions occur between the first sodium salt and the negative electrode piece. By controlling the mass content of the second sodium salt within an appropriate range, a sufficient amount of the second sodium salt can be provided, so that the battery has excellent room-temperature cycle performance, and the low-temperature cycle performance of the battery can be improved to some extent.

[0012] In any embodiment, the first solvent comprises one or more of diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether, and selectively comprises diethylene glycol dimethyl ether.

[0013] The first sodium salt has a relatively high solubility at low temperatures in all of the above-mentioned first solvents, so that the electrolyte has excellent ionic conductivity.

[0014] In any embodiment, the mass ratio of the first sodium salt to the first solvent is 0.05 to 0.2.

[0015] By further controlling the mass ratio of the first sodium salt to the first solvent to be 0.05 to 0.2, it is possible to achieve both the low-temperature ionic conductivity of the electrolyte and the low-temperature cycle performance and low-temperature Coulomb efficiency of the battery.

[0016] In any embodiment, the mass content of the first solvent is 20% or more, optionally 40% to 80%, based on the total mass of the electrolyte.

[0017] By controlling the mass content of the first solvent within an appropriate range, a sufficient amount of the first solvent is provided to completely dissolve the first sodium salt, so that the electrolyte has excellent low-temperature ionic conductivity, and the low-temperature cycle performance and low-temperature Coulomb efficiency of the battery can be improved.

[0018] In any embodiment, the electrolyte further contains 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.

[0019] Introducing a second solvent with a freezing point lower than that of the first solvent into the electrolyte is advantageous for 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.

[0020] In any embodiment, 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. Optionally, it includes 1,3-dioxolane, and / or The mass content of the second solvent described above is 10% or more of the total mass of the electrolyte, and can be optionally selected from 20% to 70%.

[0021] By controlling the mass content of the second solvent within an appropriate range, the ionic conductivity of the electrolyte at low temperatures is effectively improved, thereby enhancing the low-temperature cycle performance of the battery.

[0022] In any embodiment, the electrolyte further comprises a third solvent, and the bonding force between the third solvent and the anion in the first sodium salt is such that the first solvent and the The bonding force between the anion and the first sodium salt is greater than that between the anion and the first sodium salt.

[0023] The anions in sodium trifluoromethanesulfonic acid have a relatively strong polarization effect on the weak coordination structure of the outer layer of the solvation structure after coordination between the first solvent and sodium ions. This makes the first solvent molecules prone to oxidation on the positive electrode surface, causing the battery to overcharge at low temperatures. In contrast, the introduction of a third solvent can change the solvation structure between the first solvent and sodium trifluoromethanesulfonic acid. The bonding force between the third solvent and the anions in sodium trifluoromethanesulfonic acid is stronger than the bonding force between the first solvent and the anions in sodium trifluoromethanesulfonic acid. Therefore, the polarization effect of the anions in sodium trifluoromethanesulfonic acid on the weak coordination structure of the outer layer of the solvation structure after coordination between the first solvent and sodium ions is reduced. Consequently, the degree to which the first solvent molecules oxidize on the positive electrode surface can be reduced. Furthermore, the third solvent more effectively prevents oxidation, further suppresses battery overcharging, and improves the Coulomb efficiency of the battery at low temperatures.

[0024] In any embodiment, the third solvent comprises one or more of ethylene glycol diethyl ether, ethylene glycol dipropyl ether, propylene glycol dimethyl ether, and diethylene glycol diethyl ether, and / or comprises one or more of ethylene glycol diethyl ether and / or diethylene glycol diethyl ether. The mass content of the third solvent described above is 10% or more of the total mass of the electrolyte, and optionally between 15% and 40%.

[0025] Any of the above third solvents can effectively reduce the degree to which the first solvent molecules oxidize 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 at the terminal group C due to the presence of the ethyl or propyl group, or propylene glycol dimethyl ether can change the coordination structure by forming a 6-membered ring structure. In either case, the polarization effect of the anion in sodium trifluoromethanesulfonic acid on the weak coordination structure of the outer layer of the solvation structure after coordination between the first solvent and sodium ions is weakened, thereby reducing the degree to which the first solvent molecules oxidize on the surface of the positive electrode. Furthermore, the third solvent more strongly prevents oxidation, effectively suppresses battery overcharging, and improves the Coulomb efficiency of the battery at low temperatures. By controlling the mass content of the third solvent within an appropriate range, the low-temperature Coulomb efficiency of the battery can be effectively improved.

[0026] A second aspect of the present application provides a sodium secondary battery comprising the electrolyte of the first aspect of the present application.

[0027] In any embodiment, the sodium secondary battery is a non-negative sodium secondary battery.

[0028] A negative electrode sodium secondary battery has a high energy density.

[0029] In any embodiment, the sodium secondary battery includes a negative electrode piece, the negative electrode piece 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.

[0030] In any embodiment, the primer layer comprises a conductive carbon coating, the conductive coating comprising 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 selectively comprising one or more of single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0031] The above-mentioned primer layer not only has excellent conductivity but is also advantageous for the uniform deposition of metal ions on the current collector surface, thereby improving the battery's cycle performance and Coulomb efficiency.

[0032] In any embodiment, the surface density of the conductive carbon coating is 5 g / m². 2 ~50g / m 2 That is the case.

[0033] Surface density is 5 g / m 2 ~50g / m 2 The primer layer is favorable for the uniform distribution of nucleation sites and promotes uniform deposition of the metal, while not affecting electron transport behavior.

[0034] A third aspect of the present application provides an electrical device including a sodium secondary battery as described in the second aspect of the present application. [Brief explanation of the drawing]

[0035] [Figure 1] This is a schematic diagram of a sodium secondary battery according to one specific embodiment of the present invention. [Figure 2] Figure 1 is an exploded view of a sodium secondary battery according to one specific embodiment of the present invention. [Figure 3] This is a schematic diagram of a battery module according to one specific embodiment of the present invention. [Figure 4] This is a schematic diagram of a battery pack according to one specific embodiment of the present invention. [Figure 5] Figure 4 is an exploded view of a battery pack according to one specific embodiment of the present invention. [Figure 6] This is a schematic diagram of an electrical device that uses a sodium secondary battery as a power source according to one specific embodiment of the present invention. [Explanation of Symbols]

[0036] 1 Battery pack, 2 Upper case, 3 Lower case, 4 Battery module, 5 Sodium secondary battery, 51 Casing, 52 Electrode assembly, 53 Cover plate. [Modes for carrying out the invention]

[0037] Hereinafter, embodiments specifically disclosing the electrolyte, secondary battery, and electrical device relating to the present application will be described in detail, with reference to the drawings as appropriate. However, unnecessary detailed explanations may be omitted. For example, detailed explanations of matters already known or repeated explanations of substantially the same structures may be omitted. This is to avoid making the following explanation unnecessarily long and to facilitate understanding by those skilled in the art. Furthermore, the drawings and the following explanation are provided so that the present application can be fully understood by those skilled in the art and are not intended to limit the essence of what is stated in the claims.

[0038] The “range” disclosed herein is limited in the form of a lower limit and an upper limit, and a given range is limited by selecting one lower limit and one upper limit, and the boundaries of a special range are limited by the selected lower and upper limits. The range thus limited may include or exclude the values ​​at both ends, and may be in any combination, that is, any lower limit may be combined with any upper limit to form a single range. For example, if the ranges 60-120 and 80-110 are given for a particular parameter, it is understood that the ranges 60-110 and 80-120 are also expected. Similarly, if the minimum range values ​​1 and 2 are given, and the maximum range values ​​3, 4 and 5 are given, the ranges 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5 are all expected. In this application, unless otherwise stated, the numerical range “a-b” represents an abbreviated expression for any combination of real numbers between a and b, where a and b are both real numbers. For example, the numerical range "0 to 5" indicates that all real numbers between "0 to 5" have already been listed in this specification, and "0 to 5" is simply an abbreviation for combinations of those numbers. Also, when it is stated that a parameter is an integer ≥ 2, this is equivalent to disclosing that the parameter is an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0039] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0040] Unless otherwise specified, all technical features and optional technical features of this application can be combined to form new technical solutions.

[0041] Unless otherwise specified, all steps of this invention may be performed sequentially or randomly, but it is preferable that they be performed sequentially. For example, the fact that the above method includes steps (a) and (b) means that the above method may include steps (a) and (b) performed sequentially, or steps (b) and (a) performed sequentially. For example, the fact that the above method may further include step (c) means that step (c) can be added to the above method in any order, for example, the above method may include steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0042] Unless otherwise specified, the terms "include" and "incorporate" as used in this application may be open or closed. For example, the terms "include" and "incorporate" above may further include or incorporate other components not listed, or may include or incorporate only the listed components.

[0043] Unless otherwise specified, the term "or" in this application is 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 by any of the following: 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).

[0044] The low-temperature solubility of electrolyte salts in the solvent significantly impacts the low-temperature performance of a battery. For example, at -20°C, sodium hexafluoride phosphate has relatively low solubility in ether-based solvents and is prone to crystallization, resulting in relatively poor battery cycle performance and limiting battery applications. Therefore, it is necessary to design electrolyte systems to ensure excellent low-temperature performance and broaden the range of battery applications.

[0045] [Electrolyte] Based on this, the present application relates to an electrolyte for a sodium secondary battery comprising a first sodium salt and a first solvent, wherein the first sodium salt comprises sodium trifluoromethanesulfonic acid (NaOTf) and the first solvent has a structure represented by formula I. [ka] In the formula, R1 and R2 each independently contain either CH2 or CH2CH2, and n = 1, 2, 3, or 4. The present invention provides an electrolyte in which the mass ratio of the first sodium salt to the first solvent is 0.01 to 0.4.

[0046] In some embodiments, the mass ratio of the first sodium salt to the first solvent is optionally a value within the range of 0.01, 0.02, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, or any two of the above.

[0047] The ethyl oxide in the first solvent can coordinate with the sodium ions in sodium trifluoromethanesulfonic acid. On the other 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 advantageous for the first solvent to chelate with the sodium ions in sodium trifluoromethanesulfonic acid to form a relatively stable 5-membered or 6-membered ring solvation structure. This allows sodium trifluoromethanesulfonic acid to have relatively high solubility in the first solvent containing the structure represented by formula I at low temperatures, and the electrolyte to have excellent low-temperature ionic conductivity. It is understood that by controlling the number of return structural units n, the electrolyte can have an appropriate viscosity, and by achieving both the low-temperature ionic conductivity of the electrolyte and the compatibility between the electrolyte and the positive and negative electrode pieces, the battery can have excellent low-temperature cycle performance and low-temperature Coulomb efficiency. On the other hand, by controlling the mass ratio of the first sodium salt to the first solvent within an appropriate range, a sufficient amount of the first solvent can be provided to completely dissolve the first sodium salt, and the concentration needs of the first sodium salt can be met, resulting in the electrolyte having excellent ionic conductivity and improving the low-temperature cycle performance and low-temperature Coulomb efficiency of the battery. Furthermore, at low temperatures, sodium trifluoromethanesulfonic acid has high solubility in the first solvent having the structure represented by formula I, so that the electrolyte system can maintain excellent cycle performance even when the temperature drops from 0°C to -30°C.

[0048] In this specification, the term "low temperature" refers to temperatures between 0°C and -30°C, including 0°C and -30°C.

[0049] In some embodiments, the electrolyte further comprises a second sodium salt, the second sodium salt comprising one or more of sodium bis(fluorosulfonyl)imide (NaFSI), sodium hexafluoride phosphate (NaPF6), sodium tetraborate (NaBF4), sodium hexafluoride arsenate (NaAsF6), and sodium tetraphenylborate ((NaBPh4)), and selectively comprising one or more of sodium hexafluoride phosphate, sodium tetraborate, sodium hexafluoride arsenate, and sodium tetraphenylborate.

[0050] In some embodiments, the second sodium salt comprises sodium hexafluoride phosphate. In some embodiments, the second sodium salt comprises sodium tetrafluoroborate. In some embodiments, the second sodium salt comprises sodium hexafluoride arsenate. In some embodiments, the second sodium salt comprises sodium tetraphenylborate.

[0051] The first sodium salt has very high low-temperature solubility in the first solvent, which allows the electrolyte to have excellent low-temperature ionic conductivity. However, it is understood that the first sodium salt undergoes violent interfacial side reactions with the negative electrode at room temperature, degrading the room-temperature cycle performance of the battery. On the other hand, the second sodium salt has relatively stable room-temperature stability, and introducing the second sodium salt into the electrolyte is advantageous for improving the room-temperature cycle performance of the battery.

[0052] In some embodiments, the mass content of the first sodium salt is 0.6% to 24%, and optionally 2% to 12%, based on the total mass of the electrolyte. In some embodiments, the mass content of the first sodium salt is a value within the range of 0.6%, 1%, 2%, 4%, 5%, 6%, 8%, 10%, 12%, 14%, 15%, 16%, 18%, 20%, 22%, 24%, or any two of the above, based on the total mass of the electrolyte.

[0053] In some embodiments, the mass content of the second sodium salt is 3% to 40%, and optionally 8% to 20%, based on the total mass of the electrolyte. In some embodiments, the mass content of the second sodium salt is a value within the range of 3%, 5%, 8%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, or any two of the above, based on the total mass of the electrolyte.

[0054] By controlling the mass content of the first sodium salt within an appropriate range, a sufficient amount of the primary sodium salt can be provided and dissolved 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, while also controlling the degree to which interfacial side reactions occur between the first sodium salt and the negative electrode piece. By controlling the mass content of the second sodium salt within an appropriate range, a sufficient amount of the second sodium salt can be provided, so that the battery has excellent room-temperature cycle performance, and the low-temperature cycle performance of the battery can be improved to some extent.

[0055] In some embodiments, the first solvent comprises one or more of diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether, selectively comprising 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.

[0056] In some embodiments, the first sodium salt comprises sodium trifluoromethanesulfonic acid, the second sodium salt comprises sodium hexafluoride phosphate, and the first solvent comprises diethylene glycol dimethyl ether. In some embodiments, the first sodium salt comprises sodium trifluoromethanesulfonic acid, the second sodium salt comprises sodium tetraborate tetrafluoride, and the first solvent comprises diethylene glycol dimethyl ether. In some embodiments, the first sodium salt comprises sodium trifluoromethanesulfonic acid, the second sodium salt comprises sodium hexafluoride arsenate, and the first solvent comprises diethylene glycol dimethyl ether.

[0057] The first sodium salts all have relatively high low-temperature solubility in the first solvent, thereby ensuring that the electrolyte has excellent ionic conductivity.

[0058] In some embodiments, the mass ratio of the first sodium salt to the first solvent is 0.05 to 0.2.

[0059] By further controlling the mass ratio of the first sodium salt to the first solvent to 0.05-0.2, it is possible to achieve both the low-temperature ionic conductivity of the electrolyte and the low-temperature cycle performance and low-temperature Coulomb efficiency of the battery.

[0060] In some embodiments, the mass content of the first solvent is 20% or more, and optionally 40% to 80%, based on the total mass of the electrolyte. In some embodiments, the mass content of the first solvent is optionally 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or any two of the above values, based on the total mass of the electrolyte.

[0061] By controlling the mass content of the first solvent within an appropriate range, a sufficient amount of the first solvent can be provided to completely dissolve the first sodium salt, thereby ensuring that the electrolyte has excellent low-temperature ionic conductivity and improving the low-temperature cycle performance and low-temperature Coulomb efficiency of the battery.

[0062] In some embodiments, the electrolyte further comprises a second solvent, the freezing point of which is lower than that of the first solvent, and selectively, the freezing point of which is lower than -65°C.

[0063] In this specification, the term "freezing point" refers to the temperature at which the first solvent or the second solvent solidifies at atmospheric pressure (0.1 MPa).

[0064] Introducing a second solvent to the electrolyte, whose freezing point is lower than that of the first solvent, is advantageous in further lowering the freezing point of the electrolyte. This significantly improves the ionic conductivity of the electrolyte at low temperatures, thereby improving the low-temperature cycle performance of the battery.

[0065] In some embodiments, the second solvent comprises a cyclic ether solvent, which comprises one or more of 1,3-dioxolane (DOL), 1,4-dioxane, tetrahydrofuran (THF), and methyltetrahydrofuran, and selectively 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.

[0066] In some embodiments, the mass content of the second solvent is 10% or more, and optionally 20% to 70%, based on the total mass of the electrolyte. In some embodiments, the mass content of the second solvent is optionally 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, or any two of the above values, based on the total mass of the electrolyte.

[0067] By controlling the mass content of the second solvent within an appropriate range, the ionic conductivity of the electrolyte at low temperatures is effectively improved, thereby enhancing the low-temperature cycle performance of the battery.

[0068] In some embodiments, the electrolyte further comprises a third solvent, and the bonding force between the third solvent and the anion in the first sodium salt is greater than the bonding force between the first solvent and the anion in the first sodium salt.

[0069] The anions in sodium trifluoromethanesulfonic acid have a relatively strong polarization effect on the weak coordination structure of the outer layer of the solvation structure after coordination between the first solvent and sodium ions. This makes the first solvent molecules prone to oxidation on the positive electrode surface, causing the battery to overcharge at low temperatures. In contrast, the introduction of a third solvent can alter the solvation structure between the first solvent and sodium trifluoromethanesulfonic acid. The bonding force between the third solvent and the anions in sodium trifluoromethanesulfonic acid is stronger than that between the first solvent and the anions in sodium trifluoromethanesulfonic acid. Therefore, the polarization effect of the anions in sodium trifluoromethanesulfonic acid on the weak coordination structure of the outer layer of the solvation structure after coordination between the first solvent and sodium ions is reduced. Consequently, the degree to which the first solvent molecules oxidize on the positive electrode surface is reduced. Furthermore, the third solvent more effectively prevents oxidation, further suppresses battery overcharging, and improves the Coulomb efficiency of the battery at low temperatures.

[0070] In this specification, the term "battery overcharge" refers to a situation where the actual charging capacity deviates from the battery's design capacity due to the introduction of extra capacity through side reactions of the electrolyte, resulting in a decrease in the Coulomb efficiency of the secondary battery.

[0071] 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), and diethylene glycol diethyl ether, and selectively comprises one or more of ethylene glycol diethyl ether and 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.

[0072] In some embodiments, the mass content of the third solvent is 10% or more, and optionally 15% to 40%, based on the total mass of the electrolyte. In some embodiments, the mass content of the third solvent is optionally 10%, 14%, 15%, 16%, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 37%, 40%, or any two of the above values, based on the total mass of the electrolyte.

[0073] Any of the above third solvents can effectively reduce the degree to which the first solvent molecules oxidize 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 at the terminal group C due to the presence of the ethyl or propyl group, or propylene glycol dimethyl ether can change the coordination structure by forming a 6-membered ring structure. In either case, the polarization effect of the anion in sodium trifluoromethanesulfonic acid on the weak coordination structure of the outer layer of the solvation structure after coordination between the first solvent and sodium ions is weakened, thereby reducing the degree to which the first solvent molecules oxidize on the surface of the positive electrode. Furthermore, the third solvent more strongly prevents oxidation, effectively suppresses battery overcharging, and improves the Coulomb efficiency of the battery at low temperatures. By controlling the mass content of the third solvent within an appropriate range, the low-temperature Coulomb efficiency of the battery can be effectively improved.

[0074] [Positive electrode piece] The positive electrode piece 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 layered transition metal oxides, polyanionic compounds, and Prussian blue compounds.

[0075] 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. Selectively, the layered transition metal oxide is, for example, NaxMO2, in which M is one or more of Ti, V, Mn, Co, Ni, Fe, Cr, and Cu, and 0 <x≦1である。

[0076] Polyanionic compounds include metal ions, transition metal ions, and tetrahedral (YO4) ions. n-It may be one kind of compound having an anion unit. The metal ion is optionally one of sodium ion, lithium ion, potassium ion, and zinc ion, the transition metal is optionally at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce, Y is optionally at least one of P, S, and Si, and n represents the valence state of (YO4). n- represents the valence state of.

[0077] The Prussian blue-based compound may be one kind of compound having sodium ion, transition metal ion, and cyanide ion (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-based compound is, for example, Na a Me b Me’ c (CN)6, where Me and Me’ are each independently at least one of Ni, Cu, Fe, Mn, Co, and Zn, and 0 < a ≤ 2, 0 < b < 1, 0 < c < 1.

[0078] The positive electrode active material layer may contain a conductive agent to improve the conductive performance of the positive electrode. The conductive agent is optionally one or more of Super P, acetylene black, carbon black, ketjen black, carbon dot, carbon nanotube, graphite, graphene, and carbon nanofiber.

[0079] The positive electrode active material layer may further contain an adhesive to firmly adhere the positive electrode active material and the optional conductive agent to the positive electrode current collector. The adhesive is optionally 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).

[0080] The positive electrode current collector can 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 is optionally selected from one or more of Super P, carbon black, Ketjenblack, carbon dots, carbon nanotubes, graphite, graphene, and carbon nanofibers. The metal material of the metal foil, carbon-coated metal foil, and porous metal plate is independently selected from at least one of copper, aluminum, nickel, and stainless steel. The composite current collector may be a composite current collector formed by combining a metal foil and a polymer substrate film.

[0081] In some embodiments, positive electrode pieces can be manufactured by dispersing components such as positive electrode active material, conductive agent, adhesive, and any other components in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry, applying the positive electrode slurry to a positive electrode current collector, and obtaining a positive electrode piece after processes such as drying and cold pressing.

[0082] [Negative electrode piece] The negative electrode piece may consist only of a negative electrode current collector and may not contain any negative electrode active material. The negative electrode piece may also have a metal phase pre-deposited on the negative electrode current collector.

[0083] In some embodiments, the negative electrode current collector can be a metal foil or a composite current collector. For example, aluminum foil or copper foil can be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (such as copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver, and silver alloys) on a polymer material substrate (for example, a substrate such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), or polyethylene (PE)).

[0084] In some embodiments, the negative electrode piece 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.

[0085] In some embodiments, the primer layer comprises a conductive carbon coating, the conductive coating comprising one or more of the following: single-walled carbon nanotubes, multi-walled carbon nanotubes, hard carbon, super P, graphite, graphene, silver-composite carbon nanoparticles, and tin-composite carbon nanoparticles, and selectively comprising one or more of the single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0086] The above-mentioned primer layer not only has excellent conductivity but is also advantageous for the uniform deposition of metal ions on the current collector surface, thereby improving the battery's cycle performance and safety.

[0087] In some embodiments, the surface density of the primer layer is 5 g / m². 2 ~50g / m 2 That is the case.

[0088] In some embodiments, the surface density of the primer layer is optionally 5 g / m². 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 number within the range of any two of the above.

[0089] Surface density is 5 g / m 2 ~50g / m 2 The primer layer is favorable for the uniform distribution of nucleation sites and promotes uniform deposition of the metal, while not affecting electron transport behavior.

[0090] [Separator film] In some embodiments, the secondary battery further includes a separator film. The present application does not impose any particular restrictions on the type of separator film, and any known porous separator film having good chemical and mechanical stability can be selected.

[0091] In some embodiments, the material of the separator film may be at least one selected from polyethylene, polypropylene, polyvinylidene fluoride, aramid fibers, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers. The separator film may be a single-layer film or a multilayer composite film, and there are no particular restrictions. If the separator film is a multilayer composite film, the materials of each layer may be the same or different, and there are no particular restrictions.

[0092] In some embodiments, the positive electrode piece, negative electrode piece, and separator film can be fabricated into an electrode assembly by a winding process or a lamination process.

[0093] In some embodiments, the secondary battery may include an outer packaging. This outer packaging can be used to enclose the electrode assembly and electrolyte.

[0094] In some embodiments, the outer packaging of the secondary battery may be a rigid casing, such as a rigid plastic casing, an aluminum casing, or a steel casing. The outer packaging of the secondary battery may also be a pouch, such as a bag-shaped pouch. The material of the pouch may be plastic, and examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0095] [Sodium secondary battery] Sodium secondary batteries may include various forms, including, but are not limited to, battery cells, battery modules, and battery packs.

[0096] The battery cell contains an electrolyte according to several embodiments.

[0097] This application does not impose any special restrictions on the shape of the battery cell, which may be cylindrical, rectangular, or any other shape. For example, Figure 1 shows a rectangular battery cell 5 as an example, and Figure 2 is an exploded view of the battery cell 5.

[0098] In some embodiments, the sodium secondary battery cell further includes a positive electrode piece, a negative electrode piece, and a separator film.

[0099] In some embodiments, the sodium secondary battery cell is a non-negative sodium secondary battery.

[0100] A negative electrode-less sodium secondary battery is a battery constructed without actively installing a negative electrode active material layer on the negative electrode side during the battery manufacturing process. For example, in the battery manufacturing process, a sodium metal or carbonaceous active material layer is not installed on the negative electrode by processes such as coating or deposition. During the initial charge, sodium ions gain electrons on the anode side and deposit metallic sodium on the surface of the current collector to form a sodium metallic phase. During discharge, the metallic sodium can change back into sodium ions and return to the positive electrode, enabling cycle charging and discharging. Compared to other sodium secondary batteries, a negative electrode-less sodium secondary battery can achieve a higher energy density because it does not have a negative electrode active material layer.

[0101] In some embodiments, to improve battery performance, substances conventionally used as negative electrode active materials, such as carbonaceous materials, metal oxides, and alloys, can be placed on the negative electrode side of a non-negative electrode sodium secondary battery. Although these materials have a certain capacity, their quantity is relatively small and they are not used as the main negative electrode active material in the battery. Therefore, they are not considered to form a negative electrode active material layer that performs the sodium insertion action, and a sodium secondary battery configured in this way can still be considered a non-negative electrode sodium secondary battery.

[0102] In some embodiments, the CB value of a non-negative electrode sodium secondary battery is 0.1 or less.

[0103] The CB value in a secondary battery is calculated by dividing the unit area capacity of the negative electrode by the unit area capacity of the positive electrode. In a battery without a negative electrode, the unit area capacity of the negative electrode is relatively small because it does not contain negative electrode active material, and the CB value of the secondary battery is 0.1 or less.

[0104] In some embodiments, referring to Figure 2, the outer packaging may include a casing 51 and a cover plate 53. The casing 51 may include a base plate and side plates connected to the base plate, with the base plate and side plates surrounding each other to form a housing cavity. The casing 51 has an opening that communicates with the housing cavity, and the cover plate 53 can cover the opening and seal the housing cavity. The positive electrode piece, negative electrode piece and separator film can form an electrode assembly 52 by a winding process or a lamination process. The electrode assembly 52 is sealed within the housing cavity. The number of electrode assemblies 52 included in the sodium secondary battery 5 may be one or more, and those skilled in the art can select according to their specific practical needs.

[0105] [Battery Module] In some embodiments, battery cells can be assembled into a battery module, and the number of battery cells included in the battery module may be one or more, and those skilled in the art can select a specific number depending on the application and capacity of the battery module.

[0106] Figure 3 shows an example of a battery module 4. Referring to Figure 3, in the battery module 4, the multiple battery cells 5 may be arranged sequentially along the length of the battery module 4. Of course, they may be arranged according to any other method. Furthermore, the multiple battery cells 5 may be fixed with fastening members.

[0107] Selectively, the battery module 4 may further include an outer casing having a housing space, in which a plurality of battery cells 5 are housed.

[0108] [Battery pack] In some embodiments, the battery modules can also be assembled into a battery pack, and the number of battery modules included in the battery pack may be one or more, and those skilled in the art can select a specific number depending on the application and capacity of the battery pack.

[0109] Figures 4 and 5 show an example of a battery pack 1. Referring to Figures 4 and 5, the battery pack 1 may include a battery box and a plurality of battery modules 4 installed inside the battery box. The battery box includes an upper case 2 and a lower case 3, the upper case 2 being covered by the lower case 3 and forming a sealed space for housing the battery modules 4. The plurality of battery modules 4 may be arranged inside the battery box in any manner.

[0110] [Electrical equipment] In one embodiment of the present application, an electrical device is provided that includes at least one of a sodium secondary battery of any embodiment, a battery module of any embodiment, or a battery pack of any embodiment.

[0111] The electrical device includes at least one of the sodium secondary battery, battery module, or battery pack provided in this 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, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., 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.

[0112] Depending on the usage needs, the electrical device can be selected from a sodium secondary battery, a battery module, or a battery pack.

[0113] Figure 6 shows an example of an electrical device. This electrical device may be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density needs of the sodium secondary battery in this electrical device, a battery pack or battery module can be employed.

[0114] Other examples of such devices may include mobile phones, tablets, and laptop computers. Typically, these devices are required to be lightweight and thin, and can employ sodium-ion batteries as their power source.

[0115] Examples Examples of the present application are described below. The examples described below are illustrative and are used only to interpret the present application and should not be understood as limiting the present application. Unless otherwise specified in the examples, specific techniques or conditions are followed in accordance with the techniques or conditions described in the literature in the art or in accordance with the product instructions. Unless otherwise specified, the reagents or equipment used are all commercially available, common products.

[0116] 1. Manufacturing method Example 1 1) Electrolyte In a glove box under an argon gas atmosphere (H2O < 0.1 ppm, O2 < 0.1 ppm), 6% of the first sodium salt, sodium trifluoromethanesulfonic acid (NaTOf), and 34% of the second sodium salt, sodium hexafluoride phosphate (NaPF6), were dissolved in diethylene glycol dimethyl ether (G2), the first solvent, and the mixture was uniformly stirred and mixed to prepare an electrolyte.

[0117] 2) Manufacturing of positive electrode pieces A cathode slurry is prepared by uniformly mixing Na3V2(PO4)3, a cathode active material, polyvinylidene fluoride (PVDF), an adhesive, and conductive carbon black (Super-P), a conductive agent, in N-methylpyrrolidone (NMP) solvent in a mass ratio of 96%:2%:2%. The slurry is then coated onto the surface of aluminum foil using an extrusion coating machine according to the required mass per unit area of ​​the cathode active material, and after drying, the coated electrode pieces are applied using a cold press at a rate of 2.5 g / cm². 3 The final positive electrode piece was manufactured by cold pressurizing at the designed pressure density.

[0118] 3) Manufacturing of negative electrode pieces Single-walled carbon nanotubes and sodium alginate are added to deionized water and stirred to form a homogeneous slurry. The slurry is then applied to a negative electrode current collector, dried, and cut to obtain negative electrode pieces. The surface density of the primer layer is 25 g / m². 2 That is the case.

[0119] 4) Separator film A polypropylene film was used as the separator film.

[0120] 5) Battery manufacturing The positive electrode piece, separator film, and negative electrode piece were sequentially wound or laminated, with the separator film interposed between the positive and negative electrode pieces to provide isolation. Tabs were welded to the bare cell, the bare cell was placed in an aluminum casing, dried at 80°C to remove moisture, and then the electrolyte was injected and sealed to obtain an uncharged battery. The uncharged battery was further subjected to processes such as standing, hot and cold pressurization, chemical conversion, shaping, and capacity testing to obtain the sodium secondary battery product of Example 1.

[0121] Examples 2-29 The battery manufacturing methods for Examples 2 to 29 are basically the same as those for Example 1, but the electrolyte preparation parameters are adjusted, and the specific parameters are shown in Table 1.

[0122] Comparative Example 1 The method for manufacturing the battery in Comparative Example 1 is basically the same as in Example 1, except that the first and second solvents are not added, and ethylene glycol diethyl ether (DEE) is added as the third solvent. The specific parameters are as shown in Table 1.

[0123] Comparative Example 2 The manufacturing method for the battery in Comparative Example 2 is basically the same as that of Example 1, except that the first sodium salt is adjusted to sodium tetrafluoroborate (NaBF4). The specific parameters are as shown in Table 1.

[0124] Comparative Example 3 The manufacturing method for the battery in Comparative Example 3 is basically the same as that of Example 2, except that the first sodium salt is adjusted to sodium tetrafluoroborate (NaBF4). The specific parameters are as shown in Table 1.

[0125] Comparative Example 4 The manufacturing method for the battery in Comparative Example 4 is basically the same as that of Example 14, except that the first sodium salt is adjusted to sodium tetrafluoroborate (NaBF4). The specific parameters are as shown in Table 1.

[0126] Comparative Example 5 The manufacturing method for the battery in Comparative Example 5 is basically the same as that of Example 14, except that the first solvent is not added to the electrolyte, and the contents of the second and third solvents are adjusted accordingly. The specific parameters are shown in Table 1.

[0127] Comparative Example 6 The manufacturing method for the battery in Comparative Example 6 is basically the same as that of Example 2, except that the mass content of the first sodium salt, the first solvent, and the second solvent is 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.

[0128] Comparative Example 7 The manufacturing method for the battery in Comparative Example 7 is basically the same as that of Example 2, except that the mass content of the first sodium salt, the first solvent, and the second solvent is 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.

[0129] 2. Performance Test 1. Electrolyte performance test 1) Low-temperature ionic conductivity The ionic conductivity test instrument used was a Raiji DDSJ-318 conductivity meter. The electrolyte was placed in a centrifuge tube and left for 30 minutes under conditions of -30°C and atmospheric pressure (0.1 MPa). The conductive electrode was washed with deionized water, residual moisture was rinsed off with anhydrous ethanol, and the electrode was dried. It was then extended vertically into the centrifuge tube containing the electrolyte awaiting measurement, ensuring that the platinum electrode was submerged below the liquid surface of the electrolyte. After the reading displayed on the instrument stabilized, the result was recorded, and the measurement was repeated three times, with the average value taken.

[0130] 2. Battery performance test 1) Low-temperature Coulomb efficiency At -30℃ and normal pressure (0.1MPa), the battery is charged with a constant current at a rate of 0.1C up to 3.5V, and the charge capacity at this time is recorded as the battery's initial charge capacity. After standing for 5 minutes, it is discharged and charged again with a constant current at a rate of 0.1C up to 3.2V, and then left to stand for 5 minutes. The discharge capacity at this time is recorded as the battery's initial discharge capacity. The battery's initial Coulomb efficiency (%) = initial discharge capacity / initial charge capacity × 100%.

[0131] 2) Low-temperature cycle performance Tests were conducted in two low-temperature environments, -30°C and 0°C (abbreviated as -30°C / 0°C). One charge-discharge cycle was defined as charging the battery at 0.5C with a constant current until the voltage reached 3.5V, and then discharging it at 1C with a constant current until the voltage reached 3.2V, at -30°C / 0°C and normal pressure (0.1MPa). The capacity after the initial discharge was set to 100%, and the charge-discharge cycle was repeated 500 times. After stopping the test, the cycle capacity retention rate was recorded, and the capacity retention rate at -30°C / 0°C was used as an indicator to evaluate the battery's low-temperature cycle performance.

[0132] 3) Room temperature cycling performance At 25°C and normal pressure (0.1 MPa), one charge-discharge cycle is defined as charging the battery at 0.5C with a constant current until the voltage reaches 3.5V, and then discharging it at 1C with a constant current until the voltage reaches 3.2V. The capacity after the initial discharge is set to 100%, and the charge-discharge cycle is repeated 500 times. After stopping the test, the cycle capacity retention rate is recorded, and the capacity retention rate at 25°C is used as an indicator to evaluate the battery's room-temperature cycle performance.

[0133] III. Analysis of the test results of each example and comparative example. Batteries for each example and comparative example were manufactured according to the method described above, and each performance parameter was measured. The results are shown in Tables 1 and 2 below.

[0134] [Table 1-1] [Table 1-2] [Table 1-3] [Table 2-1] [Table 2-2]

[0135] As can be seen from the above results, the electrolytes of Examples 1 to 29 all contain a first sodium salt and a first solvent, the first sodium salt contains sodium trifluoromethanesulfonic acid, and the first solvent contains diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, or tetraethylene glycol dimethyl ether, with the mass ratio of the first sodium salt to the first solvent being 0.01 to 0.4.

[0136] As can be seen from the comparison between Example 1 and Comparative Examples 1-2, Example 2 and Comparative Example 3, and Examples 14-27 and Comparative Examples 4-5, the inclusion of sodium trifluoromethanesulfonic acid and the first solvent in the electrolyte of the present invention simultaneously is advantageous in improving the ionic conductivity of the electrolyte at -30°C, thereby improving the cycle performance of the battery at -30°C / 0°C and the Coulomb efficiency at -30°C. Furthermore, the combination of sodium trifluoromethanesulfonic acid and the first solvent allows the battery to be cooled from 0°C to -30°C, resulting in a relatively low loss in the battery's cycle capacity retention rate and a wider temperature window for the battery.

[0137] As can be seen from a comparison between Examples 2-13 and Comparative Examples 6-7, controlling the mass ratio of the first sodium salt to the first solvent to 0.01-0.4 is advantageous for further improving the battery's cycle performance at -30°C / 0°C and significantly improving the battery's low-temperature cycle performance. As can be seen from a comparison between Examples 2, 5-8, 11-13 and Examples 3-4, 9-10, further controlling the mass ratio of the first sodium salt to the first solvent to 0.05-0.2 makes it possible to achieve both the ionic conductivity of the electrolyte at -30°C, the battery's cycle performance at -30°C / 0°C, and the Coulomb efficiency at -30°C.

[0138] As can be seen from the comparison between Example 2 and Example 1, introducing a second solvent is advantageous in improving the ionic conductivity of the electrolyte at -30°C, the cycle performance of the battery at -30°C / 0°C, and the Coulomb efficiency at -30°C.

[0139] As can be seen from Examples 2 to 9, by controlling the mass content of the first sodium salt to 0.6% to 24% based on the total mass of the electrolyte, the electrolyte has excellent ionic conductivity at -30°C, and the battery has excellent cycle performance at -30°C / 0°C, cycle performance at 25°C, and Coulomb efficiency at -30°C. As can be seen from a comparison between Examples 2, 4 to 8 and Examples 3 and 9, further controlling the mass content of the first sodium salt to 2% to 12% based on the total mass of the electrolyte is advantageous for further improving the cycle performance at -30°C / 0°C and Coulomb efficiency at -30°C of the battery.

[0140] As can be seen from Examples 2 to 13, by controlling the mass content of the second sodium salt to 3% to 40% based on the total mass of the electrolyte, the electrolyte has excellent ionic conductivity at -30°C, and the battery has excellent cycle performance at -30°C / 0°C, cycle performance at 25°C, and Coulomb efficiency at -30°C.

[0141] As can be seen from Examples 2, 10-12, by controlling the mass content of the first solvent to 20% or more relative to the total mass of the electrolyte, the electrolyte has excellent ionic conductivity at -30°C, and the battery has excellent cycle performance at -30°C / 0°C, cycle performance at 25°C, and Coulomb efficiency at -30°C. As can be seen from a comparison between Examples 2, 11-12 and Example 10, further controlling the mass content of the first solvent to 40%-80% relative to the total mass of the electrolyte is advantageous for improving the ionic conductivity of the electrolyte at -30°C, cycle performance at -30°C / 0°C, and cycle performance of the battery at 25°C.

[0142] As can be seen from the comparison between Example 14 and Example 2, introducing a third solvent is advantageous in improving the battery's cycle performance at -30°C / 0°C, cycle performance at 25°C, and Coulomb efficiency at -30°C.

[0143] As can be seen from Examples 14-18, the second sodium salt, by including sodium hexafluoride phosphate, sodium tetrafluoroborate, sodium hexafluoride arsenate, sodium tetraphenylborate, or sodium bis(fluorosulfonyl)imide, can result in an electrolyte with excellent ionic conductivity at -30°C, and a battery with excellent cycle performance at -30°C / 0°C, cycle performance at 25°C, and Coulomb efficiency at -30°C.

[0144] As can be seen from Examples 14, 19-20, by including diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, or tetraethylene glycol dimethyl ether as the first solvent, the electrolyte can have excellent ionic conductivity at -30°C, and the battery can have excellent cycle performance at -30°C / 0°C, cycle performance at 25°C, and Coulomb efficiency at -30°C.

[0145] As can be seen from the comparison between Example 2 and Example 1, the electrolyte containing the second solvent is advantageous for improving the battery's cycle performance at -30°C / 0°C and its Coulomb efficiency at -30°C.

[0146] As can be seen from Examples 14, 21-22, the second solvent can contain 1,3-dioxolane, 1,4-dioxane, or tetrahydrofuran, all of which can result in an electrolyte with excellent ionic conductivity at -30°C, and the battery having excellent cycle performance at -30°C / 0°C, cycle performance at 25°C, and Coulomb efficiency at -30°C.

[0147] As can be seen from the comparison between Example 14 and Example 2, the electrolyte containing the third solvent is advantageous for improving the battery's cycle performance at -30°C / 0°C, cycle performance at 25°C, and Coulomb efficiency at -30°C. As can be seen from Examples 14, 23-24, by including ethylene glycol diethyl ether, ethylene glycol dipropyl ether, or propylene glycol dimethyl ether as the third solvent, the electrolyte can have excellent ionic conductivity at -30°C, and the battery can have excellent cycle performance at -30°C / 0°C, cycle performance at 25°C, and Coulomb efficiency at -30°C.

[0148] As can be seen from the comparison between Examples 14, 26-27 and Example 25, having a primer layer on the negative electrode piece is advantageous in improving the battery's cycle performance at -30°C / 0°C, cycle performance at 25°C, and Coulomb efficiency at -30°C.

[0149] As can be seen from Examples 14 and 28-29, by controlling the mass content of the third solvent to 10% or more based on the total mass of the electrolyte, the electrolyte has excellent low-temperature ionic conductivity, and the battery has excellent cycle performance at -30°C / 0°C, cycle performance at 25°C, and Coulomb efficiency at -30°C.

[0150] It should be noted that this application is not limited to the embodiments described above. The embodiments described above are merely illustrative, and any embodiment that has substantially the same configuration as the technical idea and produces the same functions and effects within the scope of the technical idea of ​​this application is included within the scope of this application. Furthermore, other forms constructed by applying various modifications to the embodiments or by combining some of the components of the embodiments, without departing from the gist of this application, are also included within the scope of this application.

Claims

1. An electrolyte for a sodium secondary battery, comprising a first sodium salt and a first solvent, wherein the first sodium salt comprises sodium trifluoromethanesulfonic acid and the first solvent has a structure represented by formula I. 【Chemistry 1】 In the formula, R １ , R ２ However, each is independent of CH ２ or CH ２ CH ２ It includes n = 1, 2, 3 or 4, Among these, the mass ratio of the first sodium salt to the first solvent is 0.01 to 0.4, Electrolyte.

2. The electrolyte according to claim 1, wherein the electrolyte further comprises a second sodium salt, the second sodium salt comprising one or more of sodium bis(fluorosulfonyl)imide, sodium hexafluoride phosphate, sodium tetraborate, sodium hexafluoride arsenate, and sodium tetraphenylborate, and selectively comprising one or more of sodium hexafluoride phosphate, sodium tetraborate, sodium hexafluoride arsenate, and sodium tetraphenylborate.

3. 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 characterized in that it is 3% to 40%, and optionally 8% to 20%, based on the total mass of the electrolyte. The electrolyte according to claim 2.

4. The electrolyte according to any one of claims 1 to 3, characterized in that the first solvent comprises one or more of diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether, and selectively comprises 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 the mass content of the first solvent is 20% or more, and optionally 40% to 80%, based on the total mass of the electrolyte.

7. The electrolyte according to any one of claims 1 to 6, wherein the electrolyte further comprises a second solvent, the freezing point of the second solvent is lower than the freezing point of the first solvent, and selectively, the freezing point of the second solvent is lower than -65°C.

8. 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 selectively comprises 1,3-dioxolane, and / or The mass content of the second solvent is characterized in that it is 10% or more, and optionally between 20% and 70%, based on the total mass of the electrolyte. The electrolyte according to claim 7.

9. The electrolyte according to any one of claims 1 to 8, wherein the electrolyte further comprises a third solvent, and the bonding force between the third solvent and the anion in the first sodium salt is greater than the bonding force between the first solvent and the anion in the first sodium salt.

10. The third solvent comprises one or more of ethylene glycol diethyl ether, ethylene glycol dipropyl ether, propylene glycol dimethyl ether, and diethylene glycol diethyl ether, and / or comprises one or more of ethylene glycol diethyl ether and diethylene glycol diethyl ether, and / or The mass content of the third solvent is 10% or more, and optionally 15% to 40%, based on the total mass of the electrolyte. The electrolyte according to claim 9.

11. A sodium secondary battery characterized by containing the electrolyte described in 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 sodium secondary battery without a negative electrode.

13. The sodium secondary battery according to claim 11 or 12, characterized in that the sodium secondary battery includes a negative electrode piece, and the negative electrode piece 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.

14. The secondary battery according to claim 13, wherein 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 selectively includes one or more of single-walled carbon nanotubes and multi-walled carbon nanotubes.

15. The surface density of the conductive carbon coating is 5 g / m². ２ ~50g / m ２ The secondary battery according to claim 14, characterized in that it is the same as the one described above.

16. An electrical device characterized by comprising a sodium secondary battery as described in any one of claims 11 to 15.

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