Electrolytes for sodium secondary batteries, sodium secondary batteries, and electrical devices

Abstract

The present invention relates to an electrolyte for sodium secondary batteries, a sodium secondary battery, and an electrical device. The electrolyte for sodium secondary batteries contains a metallic sodium salt and a solvent, and the desolvation energy of the sodium ion-solvent complex formed by the sodium ions of the metallic sodium salt and the solvent is 100 kJ / mol or less. By the above method, the reaction kinetics of the sodium secondary battery at low temperatures can be improved, and the low-temperature performance of the sodium secondary battery can be enhanced. Figure 1

Description

Cross-reference of related applications

[0001] This application claims priority and benefits of the Chinese patent application filed with the China National Intellectual Property Administration on February 6, 2023, with application number 202310065507.4, titled "Electrolyte for Sodium Secondary Batteries, Sodium Secondary Batteries and Electrical Devices," which is incorporated into this application by reference in its entirety. [Technical Field]

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

[0003] Energy conservation and emission reduction are key to sustainable development, requiring adjustments to the energy structure and accelerating the development and application of battery technology. Electrolytes, as a crucial component of secondary batteries, play a vital role as ion carriers in electrochemical reactions. However, currently available sodium-based electrolytes for secondary batteries increase in viscosity and tend to solidify at low temperatures, leading to problems such as low ionic conductivity, high electrochemical impedance, and slowed dynamic processes. These issues seriously impact battery life and cycle stability, and further limit the practical application of sodium-based secondary batteries at low temperatures. [Overview of the Initiative]

[0004] In view of the above issues, the present invention provides an electrolyte for sodium secondary batteries, a sodium secondary battery, and an electrical device that can improve the reaction kinetics of sodium secondary batteries at low temperatures and improve the low-temperature performance of sodium secondary batteries.

[0005] In a first aspect, the present invention provides an electrolyte for a sodium secondary battery comprising a metallic sodium salt and a solvent, wherein the desolvation energy of the sodium ion-solvent complex formed by the sodium ions of the metallic sodium salt and the solvent is 100 kJ / mol or less.

[0006] In the technical method of the embodiment of the present application, by adjusting the type of solvent in the electrolyte, a complex (Na) formed by sodium ions and the solvent in the electrolyte is achieved. + -(solvent) x The desolvation energy of the complex can be adjusted, and with such a design, the desolvation process of the sodium ion-solvent complex has a low energy barrier at low temperatures, the affinity between sodium ions and solvent molecules is weakened, thereby improving the problem of difficulty in desolvating solvated sodium ions at low temperatures, enhancing the reaction kinetics of sodium secondary batteries at low temperatures, and improving the low-temperature performance of sodium secondary batteries.

[0007] In several examples, the molar concentration of the metallic sodium salt was 0.1 to 1.2 mol / L, and selectively, the molar concentration of the metallic sodium salt was 0.2 to 0.8 mol / L.

[0008] By controlling the concentration of metallic sodium salt to a low level, the affinity between sodium ions and solvent molecules can be weakened, thereby more effectively improving the reaction kinetics of the battery at low temperatures.

[0009] In some examples, the solvent comprises a first solvent and a second solvent, the first solvent comprising a linear ether system solvent, and the second solvent comprising one or more diethyl ether and cyclic ether systems.

[0010] Of these, the first solvent can increase the solubility of the metallic sodium salt and improve the conductivity of the electrolyte, while the second solvent can lower the desolvation barrier of the sodium ion-solvent complex and improve the desolvation process of the sodium ion-solvent complex. Through the cooperation of the first and second solvents, the conductivity and reaction kinetics of the electrolyte can be improved, and the low-temperature electrochemical performance of the battery can be more effectively enhanced.

[0011] In some embodiments, the first solvent includes one or more of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethyl glycol dimethyl ether, dipropyl ether, and dibutyl ether, and / or the second solvent includes one or more of diethyl ether, 1,3-dioxolane, tetrahydrofuran, and methyltetrahydrofuran.

[0012] In some embodiments, the volume ratio of the first solvent to the second solvent is 1:1 or more and 10:1 or less, and optionally, the volume ratio of the first solvent to the second solvent is 1.5:1 or more and 5:1 or less. By controlling the volume ratio of the first solvent to the second solvent within the above range, the solubility of the solvent in the sodium metal salt can be increased, thereby adjusting the concentration of the sodium metal salt, reducing the desolvation energy of the sodium ion-solvent complex, and more effectively improving the low-temperature electrochemical performance of the battery.

[0013] In some embodiments, the sodium metal salt includes one or more of sodium hexafluorophosphate, disodium difluoro(oxalato)borate, sodium tetrafluoroborate, sodium bis(oxalato)borate, sodium perchlorate, sodium hexafluoroarsenate, sodium bis(fluorosulfonyl)imide, sodium trifluoromethanesulfonate, and sodium bis(trifluoromethanesulfonyl)imide. Since the above sodium metal salt has high ionic conductivity and high electrochemical stability, the electrochemical performance of the battery at low temperatures can be easily improved.

[0014] In a second aspect, the present application provides a sodium secondary battery including the electrolyte for a sodium secondary battery in the above embodiments.

[0015] In the technical solution of the embodiment of the present application, the desolvation energy of the sodium ion-solvent complex in the electrolyte of the sodium secondary battery is 100 kJ / mol or less. With such a design, the sodium secondary battery has a low energy barrier for the sodium ion-solvent complex at low temperature during the desolvation process, and the affinity between sodium ions and solvent molecules becomes weak. Thereby, the problem that it is difficult to desolvate the solvated sodium ions at low temperature is improved, the reaction kinetics of the sodium secondary battery at low temperature is enhanced, and the low-temperature performance of the sodium secondary battery is improved.

[0016] In some embodiments, the sodium secondary battery is a sodium-free anode secondary battery. The anode structure of the sodium-free anode battery only includes a current collector in the initial state, and no anode active material is installed on the surface of the current collector. After the first charge of the sodium-free anode secondary battery, the metallic sodium in the cathode material moves to the anode side and deposits on the surface of the anode current collector. A part of the metallic sodium remains on the surface of the anode current collector. When charging and discharging are performed again, the metallic sodium deposits and peels off on the surface of the anode current collector to realize the cycle. In this embodiment, the desolvation energy barrier of the sodium ion-solvent complex in the electrolyte is low at low temperature, the deposition and peeling of sodium are easier, and the low-temperature reaction activity is relatively high.

[0017] In some embodiments, the battery includes an anode containing an anode current collector and an anode active material, and the anode active material includes one or more of a silicon-based material, a silicon-carbon material, a carbon material, and a selenium-based material. The desolvation energy barrier of the sodium ion-solvent complex in the electrolyte is low, the insertion of sodium ions into the anode active material and the desorption from the anode active material are easy, and it has excellent low-temperature reaction activity. Furthermore, when the battery performs charge and discharge cycles, the structural change of the active material is small, which contributes to extending the service life of the battery.

[0018] In several embodiments, the battery includes a positive electrode comprising a positive electrode current collector and a positive electrode active material, the positive electrode active material comprising one or more of transition metal oxides, polyanionic compounds, and Prussian blue analogs. Sodium ions are easily inserted into and removed from the positive electrode active material and exhibit excellent low-temperature reaction activity. Furthermore, during charge-discharge cycles, the structural change of the active material is small, contributing to extending the battery's lifespan.

[0019] In several embodiments, the diaphragm resistance of the positive electrode is 0.1Ω to 50Ω, and selectively, it is 0.1Ω to 10Ω. The magnitude of the diaphragm resistance has some influence on the desolvation process of the sodium ion-solvent complex. Lower diaphragm resistance results in lower electrode polarization, lower concentration polarization at low temperatures, and can promote desolvation. Setting the diaphragm resistance of the positive electrode within this resistance range further optimizes the effect of the desolvation process and contributes to improving the low-temperature performance of the battery.

[0020] In a third aspect, the present invention provides an electrical device including the aforementioned sodium secondary battery, which exhibits excellent low-temperature performance.

[0021] The above description is merely an outline of the proposed technology. In order to better understand the technical means of this application and to implement them in accordance with the specifications, and to better understand the above and other objectives, features, and advantages of this application, specific embodiments of this application are listed below. [Brief explanation of the drawing]

[0022] Various other advantages and benefits will become apparent to those skilled in the art by reading the detailed description of the preferred embodiments below. The drawings are for illustrative purposes only and should not be considered limiting to the present application. In addition, the same reference numerals are used for the same components in all drawings. In the figures, [Figure 1] These are schematic diagrams of the structures of several embodiments of the sodium secondary battery of the present invention. [Figure 2] This is a schematic diagram of the exploded structure of several embodiments of the sodium secondary battery of the present invention. [Figure 3] This is a schematic diagram of the disassembled structure of a battery cell according to several embodiments of the present invention. [Figure 4] This is a schematic diagram of the structure of a vehicle according to several embodiments of the present invention. [Modes for carrying out the invention]

[0023] The following describes in detail embodiments of the present invention. The following embodiments are merely for the purpose of more clearly illustrating the present invention and are therefore just examples, and do not limit the scope of the claims of this invention.

[0024] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art. Terms used herein are solely for the purpose of describing specific embodiments and are not intended to limit this application. The terms “includes” and “compose,” and any variations thereof, in the description of this application, claims, and drawings are intended to cover non-exclusive inclusion.

[0025] In the description of the embodiments of this application, technical terms such as “first,” “second,” etc., are merely used to distinguish different subjects, and should not be understood as indicating or implying relative importance, or implicitly indicating the quantity, specific order, and priority of the technical features being referred to. In the description of the embodiments of this application, “multiple” means two or more unless otherwise clearly and specifically limited.

[0026] As used herein, “Examples” means that the specific features, structures, or characteristics described in conjunction with the Examples are included in at least one Example of the Application. Each occurrence of the word in the Specification does not necessarily refer to the same Example, nor do they represent mutually exclusive, independent, or substitutable Examples. Those skilled in the art will understand, both explicitly and implicitly, that the Examples described herein may be combined with other Examples.

[0027] In the description of the embodiments of this application, the term "and / or" simply describes a related relationship that describes related objects, indicating that three types of relationships are possible. For example, A and / or B can represent three cases: A existing only, A and B existing simultaneously, and B existing only. In this specification, the letter " / " generally indicates that the related objects before and after it are in an "or" relationship.

[0028] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more sets (including two sets), and "multiple sheets" refers to two or more sheets (including two).

[0029] Judging from the current market developments, the applications of power batteries are becoming increasingly broad. Power batteries are not only used in energy storage systems such as hydroelectric, thermal, wind, and solar power generation, but also in electric transportation such as electric bicycles, electric motorcycles, and electric vehicles, as well as in multiple fields such as military equipment and aerospace. As the application fields of power batteries continue to expand, the market demand for them is also constantly growing, and performance stability in various operating environments, especially in low-temperature environments (below 0°C), has become a hot topic of research.

[0030] The inventors noticed that as ambient temperature decreases, the viscosity of conventional carbonate-based electrolytes gradually increases, slowing down the reaction rate of sodium secondary batteries and being highly detrimental to the stability of their low-temperature performance. Although lower viscosity linear carboxylic acid esters can be added to the electrolyte system as cosolvents to reduce the system's viscosity, carboxylic acid esters are easily oxidized during the sodium secondary battery's cycle, thus conversely reducing the battery's capacity.

[0031] To address the problem of poor low-temperature performance in sodium secondary batteries, the applicants conducted research and found that by lowering the desolvation energy of the sodium ion-solvent complex in the electrolyte, the barrier to the desolvation process of the sodium ion-solvent complex in the electrolyte is reduced, thereby improving the chemical reaction activity at low temperatures in sodium secondary batteries and enhancing their low-temperature performance.

[0032] Considering the above, in order to solve the problem of the low low-temperature performance of sodium secondary batteries, the inventors have conducted in-depth research and have designed an electrolyte for sodium secondary batteries that contains a metallic sodium salt and a solvent, wherein the desolvation energy of the sodium ion-solvent complex formed by the sodium ions of the metallic sodium salt and the solvent is 100 kJ / mol or less. By using the above method, the barrier to the desolvation process of the sodium ion-solvent complex can be reduced, and the low-temperature performance of sodium secondary batteries can be improved.

[0033] This electrolyte is used in sodium-based secondary batteries, which are batteries that can be used continuously by recharging after discharge to activate the active material. Due to the reversibility of the chemical reaction, the mutual conversion of chemical energy and electrical energy is achieved, enabling multiple charge-discharge cycles.

[0034] Generally, a secondary battery includes a positive electrode, a negative electrode, a separator film, and an electrolyte. The separator film is placed between the positive and negative electrodes and serves to isolate them. The electrolyte facilitates the conduction of ions between the positive and negative electrodes.

[0035] Specifically, the electrolyte contains an electrolyte salt and a solvent. The electrolyte salt dissolves to form electrolyte ions, and conductivity occurs through the movement of these electrolyte ions within the electrolyte salt. During battery discharge, the negative electrode absorbs metal cations in the electrolyte, releasing electrical energy. When the battery is charged, the negative electrode re-releases metal cations into the electrolyte. During the charge-discharge process, desolvation of electrolyte ions is necessary. Desolvation refers to the phenomenon in which metal electrolyte ions in the electrolyte detach from their surrounding solvent molecules, and is a process of interfacial charge transfer. The electrolyte of a sodium secondary battery contains sodium ions, and it operates mainly through the movement of sodium ions between the positive and negative electrodes.

[0036] In the case of a sodium secondary battery, the electroplating process of sodium ions on the negative electrode piece includes the following steps.

[0037] (1) Sodium ions dissolved in the electrolyte along with solvent molecules Na + -(solvent) × A complex is formed, and the solvated sodium ions move from the electrolyte to the surface of the negative electrode piece.

[0038] (2) The solvated sodium ions are desolvated on the surface of the negative electrode piece, and desolvation causes Na + -(solvent) × It forms sodium ions that are released from the complex.

[0039] (3) The desolvated sodium ions penetrate the SEI interface (Solid Electrolyte Interface).

[0040] (4) The sodium ions combine with electrons in the negative electrode piece and are reduced to sodium atoms.

[0041] Among them, the desolvation process usually exhibits a higher barrier, and since its kinetic process is very sensitive to temperature, the desolvation process is generally considered to be the rate-determining step at low temperatures, especially in high-rate cycles.

[0042] In this example, by controlling the desolvation energy of the sodium-ion - solvent complex to 100 kJ / mol or less, the energy barrier in the desolvation of the sodium-ion - solvent complex can be reduced, and the desolvation of the complex of sodium ions and the solvent can be easily realized. Also, with the decrease in desolvation energy, sodium ions can more easily penetrate the SEI interface and combine with electrons, the reaction activity inside the sodium secondary battery increases, the chemical reaction rate inside the battery at low temperatures rises, thereby improving the low-temperature performance of the sodium secondary battery.

[0043] Among them, the desolvation energy is 100 kJ / mol or less. In the test method for desolvation energy, taking the Na + -(solvent) × complex as an example, using multiple types of solvents and utilizing a Vmp3 electrochemical workstation to conduct an alternating current impedance test on electrolyte systems of various solvents, the impedance spectrum of the desolvation process of Na + can be obtained, and the desolvation energy of the Na + -(solvent) × complex in the electrolyte can be obtained according to the Arrhenius equation.

[0044] According to several embodiments of the present application, the molar concentration of the metallic sodium salt is 0.1 to 1.2 mol / L (e.g., 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1.0 mol / L, 1.1 mol / L, and 1.2 mol / L, etc.), and selectively, the molar concentration of the metallic sodium salt is 0.2 to 0.8 mol / L (e.g., 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, and 0.8 mol / L, etc.). Alternatively, it is a range consisting of any two of the above values; for example, the molar concentration of a metal salt is 0.2-0.5 mol / L, 0.5-0.8 mol / L, 0.3-0.7 mol / L, 0.4-0.6 mol / L, 0.5-1.0 mol / L, etc.

[0045] The molar concentration of a metallic sodium salt refers to the total molar concentration of the metallic sodium salt. For example, if the metallic sodium salt is sodium hexafluoride phosphate, the molar concentration range for sodium hexafluoride phosphate is 0.1 to 1.2 mol / L, and selectively, the molar concentration range for sodium hexafluoride phosphate is 0.2 to 0.8 mol / L.

[0046] As the concentration of the metallic sodium salt decreases, the affinity between sodium ions and solvent molecules weakens, allowing for easier separation of sodium ions and solvent molecules, thereby increasing the rate of electrochemical reactions. By setting the molar concentration range of the metallic sodium salt within the above range, sufficient battery capacity can be ensured while making it easier for sodium ions to separate from the solvent at low temperatures. This increases the rate of sodium ion deposition or insertion at the negative electrode, thereby contributing to improved low-temperature performance of sodium secondary batteries.

[0047] According to some embodiments of the present application, the solvent comprises a first solvent and a second solvent, the first solvent comprising a linear ether system solvent, and the second solvent comprising one or more diethyl ether and cyclic ether systems.

[0048] Chain-like ether solvents have lower viscosity, generally less than 1.2 mPa·s, which helps transport and move sodium ions at low temperatures. They also have a strong complexing effect with sodium ions, increasing the solubility of metallic sodium salts and raising the concentration of sodium ions. Using them as solvents in electrolytes significantly improves the conductivity of the electrolyte and can increase the reaction rate during the charging and discharging processes of batteries.

[0049] Both diethyl ether and cyclic ether can weaken the solvation action between sodium ions and solvents, thereby facilitating the desolvation process of solvated sodium ions, lowering the desolvation energy, and contributing to further improving the low-temperature performance of sodium secondary batteries.

[0050] The cooperation of the first and second solvents results in the electrolyte having a high transport rate for sodium ions, improving the conductivity of the electrolyte, while simultaneously forming a weakly solvated electrolyte that promotes the desolvation process of solvated sodium ions, thereby improving the low-temperature performance of the sodium secondary battery.

[0051] According to some embodiments of the present application, the first solvent comprises one or more of DME (ethylene glycol dimethyl ether), DEE (ethylene glycol diethyl ether), DEGDME (diethylene glycol dimethyl ether), TRGDME (triethylene glycol dimethyl ether), TEGDME (tetraethyl glycol dimethyl ether), dipropyl ether, and dibutyl ether, and / or the second solvent comprises one or more of diethyl ether, DOL (1,3-dioxolane), THF (tetrahydrofuran), and Me-THF (methyltetrahydrofuran).

[0052] The above-mentioned ether-based solvent molecules can construct a stable electrode / electrolyte interface on the surface of the negative electrode piece, form a stable SEI interface, and reduce electrochemical polarization.

[0053] Furthermore, the above-mentioned ether-based solvent has good compatibility with the metal anode piece and can effectively passivate the metal anode piece, forming a thin, uniform, and dense SEI interface on the surface of the metal anode piece, further preventing dendrite formation, preventing further thickening of the SEI interface due to dendrite growth and evolution, and contributing to promoting the smooth conduction of sodium ions.

[0054] According to some embodiments of the present application, the volume ratio of the first solvent to the second solvent is 1:1 or greater and 10:1 or less (e.g., 1:1, 1.5:1, 2:1, 3:1, 4:1, 4.5:1, 5:1, 6:1, 7:1, 8:1, 9:1, 9.5:1, 10:1, etc.), and selectively, the volume ratio of the first solvent to the second solvent is 1.5:1 or greater and 5:1 or less (e.g., 1.5:1, 2:1, 3:1, 4:1, 4.5:1, 5:1, etc.). Or, it is within a range consisting of any two of the above values. For example, the volume ratio of the first solvent to the second solvent may be 2:1 to 4:1, 1.5:1 to 4.5:1, 3:1 to 5:1, 4:1 to 9:1, etc.

[0055] By designing the volume ratio of the first solvent to the second solvent within the above range, the conductivity of the electrolyte can be increased, the solvation between sodium ions and the solvent can be effectively weakened, and this contributes to accelerating the electrochemical reaction rate of the electrolyte.

[0056] According to some embodiments of the present application, the metal sodium salt includes one or more of the following: NaPF6 (sodium hexafluoride phosphate), NaDFOB (sodium difluoro(oxalato)borate), NaBF4 (sodium tetrafluoroborate), NaBOB (sodium difluoro(oxalato)borate), NaClO4 (sodium perchlorate), NaAsF6 (sodium hexafluoride arsenate), NaFSI (sodium bis(fluorosulfonyl)imide), NaOTf (sodium trifluoromethanesulfonate), and NaTFSI (sodium bis(trifluoromethanesulfonyl)imide).

[0057] Because the above sodium salt has higher ionic conductivity and electrochemical stability, it can easily improve the electrochemical performance of batteries at low temperatures.

[0058] In some embodiments, the electrolyte further comprises additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve specific characteristics of the battery, such as additives that improve the overcharge performance of the battery, additives that improve the high-temperature performance of the battery, and additives that improve the low-temperature performance of the battery.

[0059] The present invention also provides a sodium secondary battery containing the electrolyte for the sodium secondary battery described in the above-described embodiment.

[0060] In one embodiment, the sodium secondary battery is a negative electrode-less sodium secondary battery. "Negative electrode-less" means a negative electrode structure that initially contains only a current collector and no active material. Upon completion of the initial charge, metals in the positive electrode material migrate to the surface of the negative electrode current collector, forming a metal layer on the negative electrode current collector. During the charging process, metal electrolyte ions in the electrolyte combine with electrons on the surface and / or within the pores of the negative electrode current collector and deposit. During the discharge process, the deposited layer on the surface of the negative electrode current collector is peeled and dissolved, returning as metal electrolyte ions and electrons to the positive electrode, and this process is repeated. Compared to conventional secondary batteries, the negative electrode-less secondary battery offers improved energy density, greater safety and stability, and smaller weight and volume. Because the sodium secondary battery contains the electrolyte described in the above-mentioned example, the desolvation energy barrier of solvated sodium ions in the electrolyte at low temperatures is low, allowing them to easily penetrate the SEI interface and deposit on the surface of the negative electrode current collector. At the same time, the electrolyte has high conductivity and high transport capacity to sodium ions, resulting in excellent low-temperature reaction activity.

[0061] According to some embodiments of the present invention, the negative electrode of a sodium secondary battery may be a conventional negative electrode structure that specifically includes a negative electrode current collector and a negative electrode film layer installed on the negative electrode current collector and containing a negative electrode active material.

[0062] Conventional metal foil pieces or composite current collectors may be used as the negative electrode current collector (for example, a composite current collector may be formed by placing metal materials on a polymer substrate). As an example, copper foil may be used as the negative electrode current collector.

[0063] The specific type of negative electrode active material is not limited, and any active material known in the art that can be used in the negative electrode of a sodium secondary battery can be used, and those skilled in the art can select it according to their actual needs. For example, the negative electrode active material may include, but is not limited to, one or more of silicon-based materials, silicon-carbon materials, carbon materials, and selenium-based materials. Specifically, it includes one or more of artificial graphite, natural graphite, hard carbon, soft carbon, silicon-based materials, and tin-based materials. Silicon-based materials can be selected from one or more of elemental silicon, silicon-oxygen compounds (such as silicon monoxide), silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Selenium-based materials can be selected from one or more of elemental selenium, selenium-oxygen compounds, and selenium alloys. All of these materials are commercially available.

[0064] The operating principle of this sodium secondary battery is the insertion and removal of metal cations from a metallic sodium salt. When the battery is charged, sodium ions are removed from the positive electrode, penetrate the separator film in the electrolyte, and are inserted into the negative electrode active material, while at the same time, electrons flow from the positive electrode to the negative electrode in the external circuit. When the battery is discharged, sodium ions are removed from the negative electrode active material, penetrate the separator film in the electrolyte, and are inserted into the positive electrode active material, while at the same time, electrons flow from the negative electrode to the positive electrode in the external circuit.

[0065] The negative electrode active material can reduce the electrode impedance of the negative electrode, increase the electrode capacity, and reduce the abnormal deposition of metallic sodium. Since metallic sodium is easily inserted into and easily removed from the negative electrode active material, the structural change of the active material is small during the battery's charge-discharge cycle, contributing to extending the battery's lifespan.

[0066] Because the sodium secondary battery contains the electrolyte described in the above-mentioned example, the desolvation energy barrier of solvated sodium ions in the electrolyte at low temperatures is low, making it easy to insert into the negative electrode active material. At the same time, the electrolyte has high conductivity and high transport capacity to sodium ions, resulting in excellent low-temperature reaction activity.

[0067] In some embodiments, the negative electrode active material may include a silicon-based material in order to further increase the energy density of the battery.

[0068] The negative electrode film layer typically contains, more selectively, an adhesive, a conductive agent, and any other optional additives.

[0069] For example, the conductive agent may be one or more of the following: superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0070] For example, the adhesive may be one or more of the following: styrene-butadiene rubber (SBR), water-based acrylic resin, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene vinyl acetate copolymer (EVA), polyvinyl alcohol (PVA), and polyvinyl butyral (PVB).

[0071] As an example, other optional additives may include thickeners and dispersants (e.g., sodium carboxymethylcellulose CMC-Na) and PTC thermistor materials.

[0072] A sodium-based secondary battery typically includes a positive electrode current collector and a positive electrode piece that is placed on the positive electrode current collector and contains a positive electrode film layer containing a positive electrode active material.

[0073] The positive electrode current collector may be a conventional metal foil piece or a composite current collector (a composite current collector may be formed by placing metal materials on a polymer substrate). As an example, aluminum foil may be used for the positive electrode current collector.

[0074] The specific type of the positive electrode active material is not limited, and active materials known in the art that can be used for the positive electrode of a sodium secondary battery can be used, and those skilled in the art can select according to actual needs.

[0075] As an example, the positive electrode active material may include, but is not limited to, at least one of sodium transition metal oxides, polyanion-based compounds, and Prussian blue-based compounds. In the sodium transition metal oxide, the transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. The sodium transition metal oxide is Na x MO2, where M may be one or more of Ti, V, Mn, Co, Ni, Fe, Cr, and Cu, and 0 < x ≦ 1. The polyanion-based compound may be a group of compounds having sodium ions, transition metal ions, and tetrahedral (YO4) n- anion units. The transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce, Y may be at least one of P, S, and Si, and n represents the valence state of (YO4) n- The polyanion-based compound may also be a compound having sodium ions, transition metal ions, tetrahedral (YO4) n- anion units and halogen anions. The transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce, Y may be at least one of P, S, and Si, and n represents the valence state of (YO4)<​​​​​​​​represents the valence state of [atom], Z represents a transition metal, which may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce, and m is (ZO y ) m+ represents the valence state of [atom], and the halogen may be at least one of F, Cl, and Br. The polyanion-based compound may be, for example, NaFePO4, Na3V2(PO4)3, NaM’PO4F (M’ is one or more of V, Fe, Mn, and Ni), and Na3(VO y )2(PO4)2F 3-2y (0≦y≦1). The Prussian blue-based compound may be a group of compounds containing sodium ions, transition metal ions, and cyanide ions (CN - ). The transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. The Prussian blue-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. All of these materials are commercially available.

[0076] The modified compound of each of the above materials may be modified by doping into the material and / or by surface coating.

[0077] The positive electrode film layer usually contains, more selectively, an adhesive, a conductive agent, and any other optional auxiliary agent.

[0078] As an example, the conductive agent can be one or more of superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, Super P (SP), graphene, and carbon nanofibers.

[0079] For example, the adhesive may be one or more of the following: styrene-butadiene rubber (SBR), water-based acrylic resin, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene vinyl acetate copolymer (EVA), polyacrylic acid (PAA), carboxymethylcellulose (CMC), polyvinyl alcohol (PVA), and polyvinyl butyral (PVB).

[0080] As an example, other optional additives may include thickeners and dispersants (e.g., sodium carboxymethylcellulose CMC-Na) and PTC thermistor materials.

[0081] Sodium metal, like the negative electrode active material, is easily inserted into or deposited on the positive electrode active material, and at the same time easily detached or peeled off from the positive electrode active material. This minimizes structural changes in the active material during charge-discharge cycles, contributing to extending the battery's lifespan. Furthermore, transition metal oxides are easy to synthesize, have high energy density, and both Prussian blue analogs and polyanions exhibit excellent positive electrode stability.

[0082] According to some embodiments of the present application, the positive electrode diaphragm resistance is 0.1Ω or more and 50Ω or less, and selectively, the positive electrode diaphragm resistance is 0.1Ω or more and 10Ω or less.

[0083] The diaphragm resistance can be measured by any known method, such as the single-probe method, the four-probe method, or the DC two-probe method, and the contact area between the probe and the positive electrode piece is 49π mm². 2 It is installed in [location]. In this embodiment, the diaphragm resistance R of the positive electrode piece is measured using a Hioki BT23562 internal resistance tester. The upper and lower sides of the positive electrode piece are sandwiched between the two conductive terminals of the tester, and pressure is applied to fix it in place. The diameter of the conductive terminals is 14 mm, and the applied pressure is 15 MPa to 27 MPa. As a result, the diaphragm resistance of the positive electrode piece is measured.

[0084] The magnitude of the diaphragm resistance affects the desolvation process of the sodium ion-solvent complex to some extent. When the diaphragm resistance is low, the electrode polarization is small, the concentration polarization at low temperatures is small, and desolvation can be promoted. By setting the diaphragm resistance of the positive electrode piece within this resistance range, the effect of the desolvation process can be further optimized, further contributing to the improvement of the low-temperature performance of sodium secondary batteries.

[0085] A sodium secondary battery further includes a separator film placed between the positive and negative electrodes. The separator film acts as an insulating layer, effectively preventing internal short circuits caused by contact between the positive and negative electrodes, while simultaneously allowing sodium ions to pass through. The performance of the separator film determines the interface structure and internal resistance of the sodium secondary battery, directly affecting its mechanical strength and safety performance.

[0086] The specific type of separator film is not limited, and materials known in the art that can be used for sodium secondary battery separator films can be used, and those skilled in the art can select them according to their needs. For example, the material of the separator film may include, but is not limited to, one or more of polyolefins, fluoropolymers, cellulose, and glass fibers. The polyolefin may include, but is not limited to, one or more of polypropylene and polyethylene. All of these materials are commercially available.

[0087] In some embodiments, the separator film comprises a base film and a coating located on one / both sides of the base film, the coating comprising a filler. The filler may include an inorganic material, a polymer adhesive, and a dispersant. The inorganic material may include one or more of boehmite and silica; the polymer adhesive material may include one or more of PVDF (polyvinylidene fluoride) and polystyrene acrylate; and the dispersant material may include polyvinyl alcohol, etc. The performance of the separator film can be improved and adjusted by providing a coating on one / both sides of the separator film. The performance of the separator film can be adjusted by adjusting the type of filler. For example, the heat resistance of the separator film can be improved by adding a filler having thermal and heat-resistant properties. The specific type of material of the base film is not limited and may include, but is not limited to, one or more of polyethylene, polypropylene, and glass fiber. All of these materials are commercially available.

[0088] In the embodiments of the present application, the shape of the sodium secondary battery is not particularly limited and may be cylindrical, prismatic, or any other shape. As shown in Figure 1, an example is a prismatic sodium secondary battery 100, and Figure 1 is a schematic diagram of the structure of several embodiments of the sodium secondary battery 100 of the present application.

[0089] In some embodiments, the sodium secondary battery 100 of the present invention includes a case 10 and a battery cell 20, the battery cell 20 being housed within the case 10; see Figure 2, which is a schematic diagram of the exploded structure of some embodiments of the sodium secondary battery 100 of the present invention. The case 10 is used to provide a housing space for the battery cell 20, and the case 10 can employ various structures. In some embodiments, the case 10 may include a first portion 11 and a second portion 12, the first portion 11 and the second portion 12 covering each other, and together the first portion 11 and the second portion 12 define a housing space for housing the battery cell 20. The second part 12 is a hollow structure with one end open, and the first part 11 may be a plate-like structure, and the first part 11 covers the open side of the second part 12 so that the first part 11 and the second part 12 together define a housing space, and both the first part 11 and the second part 12 may be hollow structures with one end open, and the open side of the first part 11 is covered by the open side of the second part 12. Of course, the case 10 formed by the first part 11 and the second part 12 can be in various shapes such as cylindrical or rectangular.

[0090] In the sodium secondary battery 100, there may be multiple battery cells 20, and the multiple battery cells 20 may be connected in series, parallel, or series-parallel, where series-parallel connection means that the multiple battery cells 20 are connected in both series and parallel. The multiple battery cells 20 may be directly connected in series, parallel, or series-parallel, and then the entire assembly composed of the multiple battery cells 20 is housed in the case 10. Of course, the sodium secondary battery 100 may first form a battery module by connecting the multiple battery cells 20 in series, parallel, or series-parallel, and then form the whole by connecting the multiple battery modules in series, parallel, or series-parallel, and then house it in the case 10. The sodium secondary battery 100 may further include other structures, for example, the sodium secondary battery 100 may further include bus members for realizing electrical connections between the multiple battery cells 20.

[0091] Among these, each battery cell 20 may be a sodium secondary battery. The shape of the battery cell 20 may be cylindrical, flattened, rectangular, or other shapes.

[0092] Battery manufacturing methods include stacked and wound types; in other words, batteries can be divided into two types: stacked batteries and wound batteries. Stacked batteries have a uniform current collection effect, lower internal resistance, and higher specific power output, but they require very high precision in the molds to guarantee accuracy, have high capital investment, and the process is relatively complex, resulting in low production efficiency. Wound batteries are easier to manufacture, and while the requirements for equipment precision in the sheet manufacturing and assembly processes are generally high, production efficiency is high, and costs are lower. In terms of performance, wound batteries have excellent high and low temperature performance, very fast charging, an ultra-long lifespan, a stable high output voltage, a robust structure, and strong shock resistance.

[0093] Please refer to Figure 3, which is a schematic diagram of the exploded structure of a battery cell 20 in several embodiments of the present invention. A battery cell 20 refers to the smallest unit that constitutes a battery. As shown in Figure 3, a battery cell 20 includes an end cap 21, a casing 22, an electrode assembly 23, and other functional components.

[0094] The end cap 21 refers to a component that covers the opening of the casing 22 to isolate the internal environment of the battery cell 20 from the external environment. The shape of the end cap 21 may, but is not limited to, conform to the shape of the casing 22 for fitting into the casing 22. Selectively, the end cap 21 may be made of a material having a certain hardness and strength (e.g., an aluminum alloy), thus making the end cap 21 less prone to deformation when subjected to pressure and impact, allowing the battery cell 20 to have higher structural strength and improving safety performance. Functional components such as electrode terminals 21a may be installed on the end cap 21. Electrode terminals 21a may be used to electrically connect to the electrode assembly 23 to output or input electrical energy from the battery cell 20. In some embodiments, the end cap 21 may further be equipped with a pressure release mechanism to release internal pressure when the internal pressure or temperature of the battery cell 20 reaches a threshold. The end cap 21 may be made of a variety of materials, such as copper, iron, aluminum, stainless steel, aluminum alloy, and plastic, and is not particularly limited in the embodiments of this application. In some embodiments, an insulating member may be further installed inside the end cap 21, which may be used to isolate electrical connection members within the casing 22 from the end cap 21 to reduce the risk of short circuits. Exemplarily, the insulating member may be made of plastic, rubber, or the like.

[0095] The casing 22 is an assembly that fits with the end cap 21 to form the internal environment of the battery cell 20, which can be used to house the electrode assembly 23, electrolyte, and other components. The casing 22 and the end cap 21 may be separate components, and the casing 22 may have an opening, which the end cap 21 covers to form the internal environment of the battery cell 20. The end cap 21 and the casing 22 may be integrated, but are not limited to this, and specifically, the end cap 21 and the casing 22 may first form a common connection surface before other components are placed in the casing, and the end cap 21 covers the casing 22 when it is necessary to seal the inside of the casing 22. The casing 22 may be of various shapes and dimensions, such as a rectangular parallelepiped, cylindrical shape, or hexagonal prism shape. Specifically, the shape of the casing 22 may be determined according to the specific shape and dimensions of the electrode assembly 23. The casing 22 may be made of various materials such as copper, iron, aluminum, stainless steel, aluminum alloy, or plastic, and is not particularly limited to the embodiments of this application.

[0096] The electrode assembly 23 is a component that undergoes an electrochemical reaction in the battery cell 20. The casing 22 may house one or more electrode assemblies 23. The electrode assembly 23 is mainly formed by winding or laminating positive electrode pieces and negative electrode pieces, and a separator film is usually provided between the positive electrode pieces and negative electrode pieces. The portions of the positive electrode pieces and negative electrode pieces containing active material constitute the main body of the electrode assembly, and the portions of the positive electrode pieces and negative electrode pieces not containing active material each constitute a tab 23a. The positive electrode tab and negative electrode tab may be located together at one end of the main body, or they may be located at both ends of the main body, respectively. During the charging and discharging process of the battery, the positive electrode active material and negative electrode active material react with the electrolyte, and the tabs 23a are connected to the electrode terminals to form a current loop.

[0097] The electrolytes and sodium secondary batteries disclosed in the embodiments of this application can be used in electrical devices that use sodium secondary batteries as a power source, or in various energy storage systems that use sodium secondary batteries as energy storage elements. Electrical devices include, but are not limited to, mobile phones, tablets, laptop computers, electric toys, power tools, electric scooters, electric vehicles, ships, and aerospace vehicles. Electric toys may include stationary or mobile electric toys, such as game consoles, electric toy cars, electric toy boats, and electric toy airplanes, while aerospace vehicles include airplanes, rockets, space shuttles, and spacecraft.

[0098] The energy storage system may also be an energy storage power source system such as a hydroelectric, thermal, wind, or solar power plant.

[0099] In the following embodiments, for the sake of explanation, an electrical device according to one embodiment of the present application will be described using vehicle 1000 as an example.

[0100] Please refer to Figure 4, which is a schematic diagram of the structure of vehicle 1000 according to several embodiments of the present invention. Vehicle 1000 may be a fuel cell vehicle, a gas vehicle, or a new energy vehicle, and the new energy vehicle may be a pure electric vehicle, a hybrid vehicle, or a range extender vehicle. A sodium secondary battery 100 is installed inside vehicle 1000, and the sodium secondary battery 100 may be installed in the bottom, head, or tail of vehicle 1000. The sodium secondary battery 100 is used to supply power to vehicle 1000, and for example, the sodium secondary battery 100 may be used as the operating power source for vehicle 1000. Vehicle 1000 may further include a controller 200 and a motor 300, the controller 200 is used to control the sodium secondary battery 100 to supply power to the motor 300 for, for example, the operating power needs of starting, navigating, and driving vehicle 1000.

[0101] In some embodiments of the present invention, the sodium secondary battery 100 can be used not only as an operating power source for the vehicle 1000, but also as a driving power source for the vehicle 1000, providing driving force to the vehicle 1000 in place of or in part of fuel or natural gas.

[0102] The desolvation energy of the sodium ion-solvent complex formed by the sodium ions of the metallic sodium salt in the electrolyte of the sodium secondary battery of this invention and the solvent is 100 kJ / mol or less. As a result, the barrier to the desolvation process of the metallic sodium salt at low temperatures is lowered, improving the problem of insufficient battery capacity due to the difficulty of desolvating solvated sodium ions at low temperatures, which is caused by the weak affinity between sodium ions and solvent molecules. Consequently, the sodium secondary battery has excellent low-temperature performance, and the electrical device and energy storage system also have excellent low-temperature performance.

[0103] The following describes the electrolyte for sodium secondary batteries, the manufacturing method for sodium secondary batteries, and the performance test results, along with specific examples. However, those skilled in the art will understand that the electrolyte for sodium secondary batteries and the manufacturing method for sodium secondary batteries described in this application are merely examples, and that other suitable manufacturing methods are also within the scope of this application.

[0104] Example 1 1. Manufacturing of positive electrode pieces: A slurry was prepared by thoroughly dissolving 10 wt% polyvinylidene fluoride adhesive in N-methylpyrrolidone, adding 10 wt% carbon black conductive agent and 80 wt% Na4Fe3(PO4)2P2O7, a positive electrode active material, to create a uniformly dispersed slurry. The slurry was uniformly applied to the surface of aluminum foil and then transferred to a vacuum drying oven for complete drying. The resulting electrode sheet was roll-rolled and then punched out to obtain positive electrode pieces.

[0105] 2. Manufacturing of the negative electrode piece: A negative electrode piece was obtained by adding 4 wt% carbon nanotube material and 1.6 wt% sodium carboxymethylcellulose, a polymer adhesive, to water and stirring until a uniform slurry was formed. The slurry was then applied to the surface of copper foil, which was then transferred to a vacuum drying oven for complete drying before being punched out.

[0106] 3. Manufacturing of the electrolyte: In an argon-atmosphered globe case (H2O < 0.1 ppm, O2 < 0.1 ppm), sodium hexafluoride phosphate NaPF6, a sodium salt, was dissolved in diethylene glycol dimethyl ether DEGDME (first solvent) and tetrahydrofuran THF (second solvent), and the mixture was uniformly stirred to obtain an electrolyte with a sodium salt concentration of 0.5 mol / L, i.e., the electrolyte of Example 1.

[0107] 4. Separator film: A polypropylene film was used as the separator film.

[0108] 5. Manufacturing of sodium-based secondary batteries: The positive electrode piece, separator film, and negative electrode piece were sequentially stacked so that the separator film was positioned between the positive and negative electrode pieces to act as a separator, and the electrolyte was added to assemble the stacked battery.

[0109] Aside from the difference in the electrolyte formulation, the other steps in Examples 2-18 and Comparative Examples 1-3 were the same as in Example 1.

[0110] Examples 19-20 Aside from the manufacturing of the negative electrode piece, the other steps were the same as in Example 1.

[0111] Example 19: Manufacturing of a negative electrode piece A negative electrode piece was obtained by adding 90 wt% hard carbon, 5 wt% acetylene black, 4% styrene-butadiene rubber (an adhesive), and 1 wt% sodium carboxymethylcellulose (an adhesive) to water, stirring until a uniform slurry was formed, applying the slurry to the surface of copper foil, then transferring it to a vacuum drying oven for complete drying, and finally punching it out.

[0112] Example 20: Manufacturing of a negative electrode piece The hard carbon in Example 19 was replaced with soft carbon, and the other steps were the same as in Example 19.

[0113] Performance test methods 1. Coulomb efficiency: The manufactured sodium secondary battery was charged to 3.7V at 0°C with a constant current of 1 / 3C, then charged at a constant voltage of 3.7V until the current dropped to 0.05C to obtain the initial charge capacity (Cc1). Subsequently, it was discharged to 1.5V at a constant voltage of 1 / 3C to obtain the initial discharge capacity (Cd1), and the Coulomb efficiency of the sodium secondary battery was calculated according to the following formula.

[0114] The Coulomb efficiency of a sodium-ion secondary battery is calculated as: initial discharge capacity (Cd1) / initial charge capacity (Cc1).

[0115] 2.Capacity retention rate: A sodium secondary battery was charged to 3.7V at 0°C with a constant current of 1C, then charged again at a constant voltage of 3.7V until the current dropped to 0.05C, and then discharged to 1.5V with a constant current of 1C to obtain the discharge capacity (Cd1) for the first cycle. This charging and discharging process was repeated for n cycles, and the discharge capacity of the sodium secondary battery after n cycles was obtained and recorded as Cdn. The capacity retention rate of the sodium secondary battery was then calculated according to the following formula.

[0116] Capacity retention rate = Discharge capacity after n cycles (Cdn) / Discharge capacity in the first cycle (Cd1).

[0117] 3.DCR (Directive Current Resistance): DC impedance refers to the resistance experienced by the current within the battery core. After the discharge process is complete, the battery voltage bounces back due to the presence of polarization. DC impedance technology is a technique that calculates the internal resistance of a battery by utilizing the difference between the instantaneous voltage just before the end of discharge and the voltage after the discharge has stabilized during the intermittent discharge process.

[0118] A sodium secondary battery was charged to 3.7V at 0°C with a constant current of 1C, then charged at a constant voltage of 3.7V until the current dropped to 0.05C, and then discharged to 1.5V with a constant current of 1C. After leaving it for 5 minutes (stabilization time), the next cycle was repeated. In each cycle, the battery voltage before discharge stopped and the battery voltage after stabilization were recorded, and the DC impedance R = ΔU / I (where ΔU is the voltage difference, R is the DC resistance, and I is the discharge current) was calculated using the following formula.

[0119] 4. Sodium dendrites: The sodium secondary battery, after 100 cycles, was disassembled in an argon atmosphere globe case (H2O < 0.1 ppm, O2 < 0.1 ppm), and the surface morphology of the negative electrode piece was visually observed to check for the presence or absence of sodium dendrites. If there were no white spots on the negative electrode piece, it was determined that there were no sodium dendrites. If there were scattered white spots on the negative electrode piece, it was determined that the sodium dendrites were minor. If there were densely packed white spots on the negative electrode piece, it was determined that the sodium dendrites were serious.

[0120] Table 1 shows the compositions and performance test results for each example and comparative example.

[0121] 5. Desolvation energy AC impedance tests were performed on the sodium secondary batteries of each example and comparative example using the Vmp3 electrochemical workstation. + - (solvent) × Impedance spectra of the desolvation process of the complex were obtained. According to the Arrhenius equation, the Na of each example and each comparative example was obtained. + -(solvent) ×We were able to obtain the desolvation energy of the complex.

[0122] [Table 1]

[0123] Note: V1:V2 is the volume ratio of the first solvent to the second solvent, Coulomb efficiency / % is the value after running the battery for 100 cycles, DCR is the value after running the battery for 100 cycles, and desolvation energy is Na + -(solvent) × This was the desolvation energy of the complex.

[0124] Analysis of results 1. Comparing Examples 1-20 with Comparative Examples 1-3, the Na content of Examples 1-20 is + (solvent) × The desolvation energy of all the complexes was less than 100 kJ / mol, and their Coulomb efficiency and capacity retention after 100 cycles were all higher than those of Comparative Examples 1-3. Furthermore, the DCR of the batteries after 100 cycles was lower than that of Comparative Examples 1-3, and they were found to have no sodium dendrites and excellent low-temperature performance. The desolvation energy of Comparative Examples 1-3 exceeded 100 kJ / mol, and their low-temperature performance was significantly lower than that of Examples 1-20. This indicates that a desolvation energy of 100 kJ / mol or less can effectively improve the low-temperature performance of sodium secondary batteries.

[0125] 2. Comparing Examples 1-20, Comparative Example 1, and Comparative Examples 2-3, it was found that the solvents in Examples 1-20 and Comparative Example 1, being combinations of the first and second solvents, had higher Coulombic efficiency and capacity retention rates after 100 cycles than Comparative Examples 2-3. The DCR of the batteries after 100 cycles was lower than that of Comparative Examples 2-3. Except for Comparative Example 1, where slight dendrite formation occurred, no dendrites were formed in Examples 1-20. Furthermore, from Comparative Examples 2, 3, and Example 1, it was found that the Na in Comparative Example 2, which used only DEGDME, and Comparative Example 3, which used only THF, was lower. + (solvent)× Although the desolvation energy of all complexes exceeded 100 kJ / mol, in Example 1, which used DEGDME and THF in combination, the desolvation energy decreased to 100 kJ / mol or less, and it was found that the low-temperature performance of the battery in Example 1 was significantly better than that of Comparative Examples 2 and 3. This demonstrated that the low-temperature performance of the battery can be improved by combining the first and second solvents.

[0126] 3. Comparing Examples 1-5 with Comparative Example 1, when the sodium salt concentration is 0.1-1.2 mol / L, It was found that the Coulomb efficiency and capacity retention rate after 100 cycles were higher, and there were no sodium dendrites, resulting in superior battery performance. In particular, when the sodium salt concentration was 0.2 to 0.8 mol / L, the battery performance improved even further. However, in Comparative Example 1, where the sodium salt concentration exceeded 0.1 to 1.2 mol / L, the sodium salt concentration was too high, even though the solvent contained both the first and second solvents. + (solvent) × The desolvation energy of the complex exceeded 100 kJ / mol, resulting in slight dendrite formation. The battery performance was not as good as in Examples 1-5, and it was demonstrated that using an electrolyte with a sodium salt concentration of 0.1-1.2 mol / L can improve the low-temperature performance of the battery.

[0127] 4. Comparing Example 1 with Examples 6-9, it was found that when the volume ratio of the first solvent to the second solvent was 1:1 to 10:1, both cases showed high Coulomb efficiency and capacity retention after 100 cycles, low battery DCR, absence of sodium dendrites, and excellent low-temperature performance. In particular, when the volume ratio of the first solvent to the second solvent was 1.5:1 to 5:1, the battery DCR after 100 cycles was 3000 mΩ or less in all cases.

[0128] 5. Comparing Example 1 with Examples 10-13, it was found that different types of sodium salts resulted in different low-temperature performance of the batteries. NaPF6 in Example 1 exhibited the best performance, followed by NaOTf, NaFSI, NaTFSI, and NaDFOB in that order.

[0129] 6. Comparing Example 1 with Examples 14-18, it was found that the types of the first and second solvents also affect the low-temperature performance of the battery. In the examples in Table 1, for the first solvent, Example 1, which used DEGDME, performed best, followed by TEGDME and DME. For the second solvent, Example 1, which used THF, performed better than the sample using DOL.

[0130] 7. Comparing Examples 18-20, it was found that both the "negative electrode-free sodium battery" (where the negative electrode does not contain an active material) and the conventional sodium battery (where the negative electrode contains an active material) exhibited excellent low-temperature performance. Furthermore, the performance of Example 18, which did not contain a negative electrode active material, was superior to that of Examples 19 and 20, which contained a negative electrode active material, and the performance of Example 19, where the negative electrode active material was hard carbon, was superior to that of Example 20, where the negative electrode active material was soft carbon.

[0131] In summary, this application provides an electrolyte for a sodium secondary battery, a sodium secondary battery, and an electrical device. The electrolyte for the sodium secondary battery comprises a metallic sodium salt and a solvent, wherein the desolvation energy of the sodium ion-solvent complex formed by the sodium ions of the metallic sodium salt and the solvent is 100 kJ / mol or less. By this method, the reaction kinetics of the sodium secondary battery at low temperatures can be improved, and the low-temperature performance of the sodium secondary battery can be enhanced.

[0132] Finally, it should be noted that the above embodiments are used solely to illustrate the technical concepts of the present application and are not limiting. While the present application has been described in detail with reference to the embodiments described above, those skilled in the art will understand that it is still possible to modify the technical concepts described in the embodiments described above, or to replace some or all of their technical features equally, and that such modifications or substitutions should not cause the essence of the corresponding technical concepts to deviate from the scope of the technical concepts of the embodiments of the present application, and should all be included within the scope of the claims and specification of the present application. In particular, the technical features mentioned in each embodiment can be combined in any way, as long as they are not structurally contradictory. The present application is not limited to the specific embodiments disclosed herein, but includes all technical concepts included in the claims.

Claims

1. An electrolyte for sodium secondary batteries, It contains a metallic sodium salt and a solvent. Among these, the desolvation energy of the sodium ion-solvent complex formed by the sodium ions of the metal sodium salt and the solvent is 100 kJ / mol or less. Electrolyte for sodium-based secondary batteries.

2. The molar concentration of the aforementioned sodium metal salt is characterized by being 0.1 to 1.2 mol / L. The electrolyte for a sodium secondary battery according to claim 1.

3. The molar concentration of the aforementioned sodium metal salt is 0.2 to 0.8 mol / L. The electrolyte for a sodium secondary battery according to claim 2.

4. The solvent comprises a first solvent and a second solvent. The first solvent includes a linear ether solvent, The second solvent is characterized by comprising one or more diethyl ethers and cyclic ether systems. The electrolyte for a sodium secondary battery according to claim 1.

5. The first solvent comprises one or more of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethyl glycol dimethyl ether, dipropyl ether, and / or dibutyl ether. The second solvent is characterized by comprising one or more of diethyl ether, 1,3-dioxolane, tetrahydrofuran, and methyltetrahydrofuran. The electrolyte for a sodium secondary battery according to claim 4.

6. The volume ratio of the first solvent to the second solvent is characterized by being 1:1 or more and 10:1 or less. The electrolyte for a sodium secondary battery according to claim 4.

7. The volume ratio of the first solvent to the second solvent is characterized by being 1.5:1 or more and 5:1 or less. The electrolyte for a sodium secondary battery according to claim 6.

8. The aforementioned sodium metal salt is characterized by comprising one or more of the following: sodium hexafluoride phosphate, sodium difluoro(oxalate)borate, sodium tetrafluoroborate, sodium bis(oxalate)borate, sodium perchlorate, sodium hexafluoride arsenate, sodium bis(fluorosulfonyl)imide, sodium trifluoromethanesulfonate, and sodium bis(trifluoromethanesulfonyl)imide. The electrolyte for a sodium secondary battery according to any one of claims 1 to 7.

9. The present invention is characterized by containing the electrolyte for sodium secondary batteries described in any one of claims 1 to 8. Sodium-based rechargeable battery.

10. The aforementioned battery is characterized by being a negative electrode-less sodium secondary battery. The sodium secondary battery according to claim 9.

11. The battery includes a negative electrode comprising a negative electrode current collector and a negative electrode active material. The negative electrode active material is characterized by comprising one or more of the following: silicon-based material, silicon-carbon material, carbon material, and selenium-based material. The sodium secondary battery according to claim 9.

12. The battery includes a positive electrode comprising a positive electrode current collector and a positive electrode active material, The positive electrode active material is characterized by comprising one or more of the following: transition metal oxides, polyanionic compounds, and Prussian blue analogs. A sodium secondary battery according to any one of claims 9 to 11.

13. The diaphragm resistance of the positive electrode is characterized by being between 0.1Ω and 50Ω. The sodium secondary battery according to claim 12.

14. The diaphragm resistance of the positive electrode is characterized by being 0.1Ω to 10Ω. The sodium secondary battery according to claim 13.

15. A sodium secondary battery as described in any one of claims 9 to 14, Electrical device.

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