Sodium secondary battery, electrolyte and electric device

Abstract

A sodium secondary battery, an electrolyte and an electric device. The sodium secondary battery comprises: a positive electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector, at least one side of the positive electrode current collector is provided with a positive electrode active material layer, and the positive electrode active material layer comprises a positive electrode active material; a negative electrode sheet, wherein the negative electrode sheet comprises a negative electrode current collector, and when the sodium secondary battery is charged, metal sodium is deposited in situ on the negative electrode current collector; and an electrolyte, wherein the electrolyte comprises an inorganic salt and / or an organic salt, with the inorganic salt comprising one or more of sodium nitrate, sodium trifluoroacetate and sodium tetrafluoroborate, and the organic salt comprising sodium tetraphenoxyborate.

Description

Sodium secondary batteries, electrolytes, and electrical equipment

[0001] Cross-referencing

[0002] This application incorporates Chinese Patent Application No. 202411479682.9, filed on October 22, 2024, entitled “Sodium Secondary Battery, Electrolyte and Electrical Equipment”, which is incorporated herein by reference in its entirety. Technical Field

[0003] This application relates to the field of batteries, specifically to sodium secondary batteries, electrolytes, and electrical equipment. Background Technology

[0004] Batteries are not only used in energy storage systems for hydropower, thermal power, wind power, and solar power plants, but also widely used in electric vehicles such as electric bicycles, electric motorcycles, and electric cars, as well as in military equipment and aerospace. However, sodium-ion batteries without a negative electrode exhibit a low solvent oxidation potential window and poor stability under high voltage conditions during cycling, thus reducing the battery's cycle stability. Summary of the Invention

[0005] The first aspect of this application provides a sodium secondary battery, comprising a positive electrode plate including a positive current collector, at least one side of which has a positive active material layer, the positive active material layer comprising a positive active material; a negative electrode plate including a negative current collector, wherein metallic sodium is deposited in situ on the negative current collector during charging; and an electrolyte comprising an inorganic salt and / or an organic salt, wherein the inorganic salt comprises one or more of sodium nitrate, sodium trifluoroacetate, and sodium tetrafluoroborate, and the organic salt comprises sodium tetraphenoxyborate. Thus, these inorganic salts all possess anions with high donor numbers, which can preferentially occupy the inner solvation sheath layer in the electrolyte, thereby altering the solvation structure of the solution, reducing the coordination number of the solvent, and forming coordination of contact ion pairs and ion aggregates similar to locally high-concentration electrolytes, thereby significantly increasing the potential window of the electrolyte. The sodium tetraphenoxyborate contains B(OPh)4. - Anions in the electrolyte and Na + It has better binding ability, shorter distance, and is easier to enter the sodium ion solvation inner layer in the electrolyte.

[0006] Meanwhile, the decomposition of these anions helps to form a stable inorganic CEI film, stabilize the interface, reduce the continuous occurrence of side reactions, reduce the structural degradation of the positive electrode active material and the dissolution of transition metals during cycling, improve the high-voltage cycling stability of the battery, and reduce the gas production of the battery.

[0007] Meanwhile, organic salts can form CEI and SEI films containing boron-oxygen components, which can induce Na+ oxidation at the negative electrode. + More uniform deposition on the negative electrode reduces the formation of dead sodium and sodium dendrites, and improves the reversibility of the sodium deposition and stripping process. For the positive electrode, it can improve the integrity of the positive electrode structure and reduce the dissolution of transition metals, thereby improving the cycle stability of sodium secondary batteries.

[0008] According to some embodiments of this application, the molar concentration of the inorganic salt in the electrolyte is 0.1 mol / L-0.3 mol / L. This increases the voltage window of the electrolyte, forms a stable CEI film on the positive electrode surface, improves the high-voltage cycle stability of the sodium secondary battery, and reduces gas production in the battery.

[0009] According to some embodiments of this application, the molar concentration of sodium tetraphenoxyborate in the electrolyte is 0.1 mol / L to 0.4 mol / L. This reduces gas production in the sodium secondary battery and improves its cycle performance.

[0010] According to some embodiments of this application, the electrolyte further includes an electrolyte salt, namely sodium hexafluorophosphate, wherein the molar concentration of sodium hexafluorophosphate in the electrolyte is 1.5 mol / L-2 mol / L. This improves the conductivity of the electrolyte and increases its voltage window.

[0011] According to some embodiments of this application, the electrolyte further includes a co-solvent, the co-solvent comprising 3%-6% of the volume of the electrolyte. This improves the solubility of the inorganic salt in the electrolyte.

[0012] According to some embodiments of this application, the co-solvent includes fluorinated nitrile compounds.

[0013] According to some embodiments of this application, the fluorinated nitrile compounds include one or more of fluoroacetonitrile, difluoroacetonitrile, fluoropropionitrile, difluoropropionitrile, fluorobutyronitrile, difluorobutyronitrile, and trifluorobutyronitrile.

[0014] Therefore, the above-mentioned types of co-solvents can improve the solubility of inorganic salts in electrolytes.

[0015] According to some embodiments of this application, the electrolyte further includes a solvent, which comprises one or both of linear ethers and cyclic ethers. This improves the compatibility between the electrolyte solvent and the sodium metal anode.

[0016] According to some embodiments of this application, the linear ether comprises one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol diethyl ether, ethylene glycol dibutyl ether, and diethylene glycol dibutyl ether; and / or the cyclic ether comprises one or more of tetrahydrofuran, methyltetrahydrofuran, and 1,3-dioxane. This improves the compatibility between the electrolyte solvent and the sodium metal anode.

[0017] According to some embodiments of this application, the positive electrode active material includes NaMO2 and Na x R y P m O n One or two of the following, wherein M includes one or more of Ti, V, Cr, Mn, Fe, Co, Ni, and Cu, 2.5≤x≤4.5, 1.5≤y≤3.5, 2.5<m<4.5, and 11.5≤n≤15.5, and R includes one or more of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, and Pb.

[0018] A second aspect of this application provides an electrolyte comprising an inorganic salt and an organic salt. The inorganic salt includes one or more of sodium nitrate, sodium trifluoroacetate, and sodium tetrafluoroborate, and the organic salt includes sodium tetraphenoxyborate. These inorganic salts all possess anions with high donor numbers, which can preferentially occupy the inner solvation sheath in the electrolyte, thereby altering the solvation structure of the solution, reducing the coordination number of the solvent, and forming coordination of contact ion pairs and ion aggregates similar to locally high-concentration electrolytes, thus significantly increasing the potential window of the electrolyte. The sodium tetraphenoxyborate contains B(OPh)4. - Anions in the electrolyte and Na + It has better binding ability, shorter distance, and is easier to enter the sodium ion solvation inner layer in the electrolyte.

[0019] Meanwhile, the decomposition of these anions helps to form a stable inorganic CEI film, stabilize the interface, reduce the continuous occurrence of side reactions, reduce the structural degradation of the positive electrode active material and the dissolution of transition metals during cycling, improve the high-voltage cycling stability of the battery, and reduce the gas production of the battery.

[0020] Organic salts can form CEI and SEI films containing boron-oxygen components, which can induce Na+ oxidation at the negative electrode. + More uniform deposition on the negative electrode reduces the formation of dead sodium and sodium dendrites, and improves the reversibility of the sodium deposition and stripping process. For the positive electrode, it can improve the integrity of the positive electrode structure and reduce the dissolution of transition metals, thereby improving the cycle stability of sodium secondary batteries.

[0021] According to some embodiments of this application, the molar concentration of the inorganic salt in the electrolyte is 0.1 mol / L-0.3 mol / L. This increases the voltage window of the electrolyte, forms a stable CEI film on the positive electrode surface, improves the high-voltage cycle stability of the sodium secondary battery, and reduces gas production in the battery.

[0022] According to some embodiments of this application, the molar concentration of sodium tetraphenoxyborate in the electrolyte is 0.1 mol / L to 0.4 mol / L. This reduces gas production in the sodium secondary battery and improves its cycle performance.

[0023] According to some embodiments of this application, the electrolyte further includes an electrolyte salt, namely sodium hexafluorophosphate, wherein the molar concentration of sodium hexafluorophosphate in the electrolyte is 1.5 mol / L-2 mol / L. This improves the conductivity of the electrolyte and increases its voltage window.

[0024] According to some embodiments of this application, the electrolyte further includes a co-solvent, the co-solvent comprising 3%-6% of the volume of the electrolyte. This improves the solubility of the inorganic salt in the electrolyte.

[0025] According to some embodiments of this application, the co-solvent includes fluorinated nitrile compounds.

[0026] According to some embodiments of this application, the fluorinated nitrile compounds include one or more of fluoroacetonitrile, difluoroacetonitrile, fluoropropionitrile, difluoropropionitrile, fluorobutyronitrile, difluorobutyronitrile, and trifluorobutyronitrile.

[0027] Therefore, the above-mentioned types of co-solvents can improve the solubility of inorganic salts in electrolytes.

[0028] According to some embodiments of this application, the electrolyte further includes a solvent, which comprises one or both of linear ethers and cyclic ethers. This improves the compatibility between the electrolyte solvent and the sodium metal anode.

[0029] According to some embodiments of this application, the linear ether comprises one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol diethyl ether, ethylene glycol dibutyl ether, and diethylene glycol dibutyl ether; and / or the cyclic ether comprises one or more of tetrahydrofuran, methyltetrahydrofuran, and 1,3-dioxane. This improves the compatibility between the electrolyte solvent and the sodium metal anode.

[0030] A third aspect of this application provides an electrical device, including the sodium secondary battery provided in the first aspect of this application.

[0031] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0032] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiments below. The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of this application. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0033] Figure 1 is a schematic diagram of a sodium secondary battery according to an embodiment of this application.

[0034] Figure 2 is an exploded view of a sodium secondary battery according to an embodiment of this application shown in Figure 1.

[0035] Figure 3 is a schematic diagram of a battery module according to one embodiment of this application.

[0036] Figure 4 is a schematic diagram of a battery pack according to one embodiment of this application.

[0037] Figure 5 is an exploded view of the battery pack of one embodiment of this application shown in Figure 4.

[0038] Figure 6 is a schematic diagram of an electrical device using a sodium secondary battery as a power source according to an embodiment of this application.

[0039] Explanation of reference numerals in the attached drawings: 1 Battery pack; 2 Upper casing; 3 Lower casing; 4 Battery module; 5 Sodium secondary battery; 51 Housing; 52 Electrode assembly; 53 Cover plate. Detailed Implementation

[0040] The embodiments of the technical solution of this application are described in detail below. The following embodiments are only used to illustrate the technical solution of this application more clearly, and are therefore only examples, and should not be used to limit the scope of protection of this application.

[0041] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0042] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0043] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0044] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0045] Currently, judging from market trends, battery applications are becoming increasingly widespread. Batteries are not only used in energy storage systems such as hydropower, thermal power, wind power, and solar power plants, but also extensively in electric vehicles such as electric bicycles, electric motorcycles, and electric cars, as well as in military equipment and aerospace. With the continuous expansion of battery applications, market demand is also constantly increasing.

[0046] Currently, ether-based solvent systems are used as electrolytes in sodium-based secondary batteries without negative electrodes to ensure compatibility with sodium metal. However, ether-based solvents have a low oxidation potential window. In oxide-based sodium-based battery systems, the positive electrode active material exhibits strong oxidizing properties under full charge conditions, which can easily lead to side reactions in the electrolyte during cycling, thereby reducing the cycle stability of the sodium secondary battery.

[0047] The sodium secondary battery proposed in this application incorporates one or more inorganic and / or organic salts selected from sodium nitrate, sodium trifluoroacetate, and sodium tetrafluoroborate in the electrolyte. The anions (nitrate, trifluoroacetate, and tetrafluoroborate) in these inorganic salts have a higher donor number than hexafluorophosphate. Anions with a high donor number can preferentially coordinate with sodium ions in the electrolyte, preferentially occupying the solvation sheath, thereby reducing the coordination number of the solvent. The sodium tetraphenoxyborate contains B(OPh)4... - Anions in the electrolyte and Na + It has better binding ability and a shorter distance, making it easier for sodium ions to enter the solvation inner layer in the electrolyte and participate in subsequent reactions. At the same time, it reduces the solvent coordination number and reduces the Na+ concentration. + Desolvation energy.

[0048] In other words, by adding the aforementioned inorganic and / or organic salts to the electrolyte, the coordination number of the solvent can be reduced, forming contact ion pairs and ion aggregates similar to those in locally high-concentration electrolytes. The number of active solvent molecules in a free state capable of reacting on the electrode surface is also reduced accordingly, thereby improving the electrolyte's high-voltage resistance and increasing its voltage window. Simultaneously, the decomposition of anions in the solvation sheath can form an inorganic-rich CEI film on the positive electrode surface, improving the stability of the CEI film. Under high-voltage conditions, this reduces side reactions between the electrolyte and the positive electrode active material, decreases structural degradation and transition metal dissolution of the positive electrode active material during cycling, improves the battery's high-voltage cycle stability, and reduces gas generation.

[0049] Sodium tetraphenoxyborate in the electrolyte can form CEI and SEI films containing boron-oxygen components. For the negative electrode, it can induce Na... + More uniform deposition on the negative electrode reduces the formation of dead sodium and sodium dendrites, and improves the reversibility of the sodium deposition and stripping process. For the positive electrode, it can improve the integrity of the positive electrode structure and reduce the dissolution of transition metals, thereby improving the cycle stability of sodium secondary batteries.

[0050] When the concentration of electrolyte salt in the electrolyte is low, the number of solvent molecules is much greater than that of Na. + Quantity, Na + Preferentially interacting with solvent molecules, the solvent coordination structure in the electrolyte is mainly composed of solvent-separated ion pairs (SSIPs), where one anion is on average associated with less than one Na+ ion. + Interaction); when the concentration of electrolyte salts in the electrolyte is high, the number of solvent molecules decreases, Na + As the number increases, anions react with Na+. + The interactions between them are enhanced, and the solvent coordination structure in the electrolyte gradually transforms into contact ion pairs (CIP, one anion and one Na+). + Interactions) and aggregates (AGG, one anion with two or more Na+ ions) + (Interactions). In high-concentration electrolytes, the electrolyte mainly consists of contact ion pairs and aggregates.

[0051] The sodium secondary battery proposed in 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 batteries as energy storage elements. Electrical devices can include, but are not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc. Electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc., while spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc.

[0052] The first aspect of this application provides a sodium secondary battery, the sodium secondary battery comprising a positive electrode sheet, the positive electrode sheet comprising a positive current collector, at least one side of the positive current collector having a positive active material layer, the positive active material layer comprising a positive active material; a negative electrode sheet, the negative electrode sheet comprising a negative current collector, wherein metallic sodium is deposited in situ on the negative current collector during charging of the sodium secondary battery; and an electrolyte comprising an inorganic salt and / or an organic salt, the inorganic salt comprising one or more of sodium nitrate (NaNO3), sodium trifluoroacetate (NaTFA), and sodium tetrafluoroborate (NaBF4), and the organic salt comprising sodium tetraphenoxyborate.

[0053] The sodium secondary battery proposed in this application incorporates one or more of sodium nitrate, sodium trifluoroacetate, and sodium tetrafluoroborate in its electrolyte. The nitrate, trifluoroacetate, and tetrafluoroborate ions have high donor numbers and can preferentially coordinate with sodium ions, reducing the coordination number of the solvent and thus increasing the voltage window of the electrolyte. Simultaneously, these anions can form a stable CEI film at the positive electrode, improving the cycle stability of the sodium secondary battery under high-voltage conditions and reducing gas production. Furthermore, the organic salts can form CEI and SEI films containing boron-oxygen components, which, for the negative electrode, can induce Na+ oxidation. + More uniform deposition on the negative electrode reduces the formation of dead sodium and sodium dendrites, and improves the reversibility of the sodium deposition and stripping process. For the positive electrode, it can improve the integrity of the positive electrode structure and reduce the dissolution of transition metals, thereby improving the cycle stability of sodium secondary batteries.

[0054] In this application, the identification method for sodium nitrate, sodium trifluoroacetate, and sodium tetraphenoxyborate is ion chromatography. Ion chromatography is a method that utilizes the principle of ion exchange to continuously separate, qualitatively identify, and quantitatively analyze multiple coexisting anions or cations. Before testing, the instrument needs to be calibrated using a standard solution. By comparing the measurement results between the electrolyte to be tested and the standard solution, the analyte ions in the sample are identified and quantified.

[0055] According to some embodiments of this application, the molar concentration of the inorganic salt in the electrolyte can be 0.1 mol / L-0.3 mol / L, for example, it can be 0.1 mol / L, 0.15 mol / L, 0.2 mol / L, 0.25 mol / L, 0.3 mol / L, etc., or it can be any range of the above values. Therefore, by adding the above-mentioned amount of inorganic salt to the electrolyte, the coordination number of anions in the sodium ion solvation layer is increased, the high-voltage resistance of the electrolyte is improved, and the voltage window of the electrolyte is increased. Simultaneously, the increased anion content in the solvation layer can increase the inorganic content in the CEI membrane, improve the stability of the CEI membrane, and thus improve the high-voltage cycle stability of the sodium secondary battery and reduce gas production in the sodium secondary battery.

[0056] By keeping the content of inorganic salts within the above-mentioned range, this application can also reduce the impact on the conductivity and viscosity of the electrolyte, and reduce the impact on the cycle performance and rate performance of sodium secondary batteries.

[0057] In this application, ion chromatography is used to detect the content of inorganic salts in the electrolyte.

[0058] According to some embodiments of this application, the molar concentration of sodium tetraphenoxyborate in the electrolyte is 0.1 mol / L to 0.4 mol / L. This reduces gas production in the sodium secondary battery and improves its cycle performance.

[0059] As an example, the molar concentration of the sodium tetraphenoxyborate can be 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, or a range of any of the above values.

[0060] In this application, ion chromatography can be used to detect the content of sodium tetraphenoxyborate in the electrolyte.

[0061] According to some embodiments of this application, the electrolyte further includes an electrolyte salt, which includes sodium hexafluorophosphate (NaPF6). The molar concentration of sodium hexafluorophosphate in the electrolyte can be between 1.5 mol / L and 2 mol / L, for example, 1.5 mol / L, 1.6 mol / L, 1.7 mol / L, 1.8 mol / L, 1.9 mol / L, 2 mol / L, or any range of the above values. Therefore, a higher concentration of the electrolyte salt in the electrolyte can, on the one hand, increase the ionic conductivity of the electrolyte, and on the other hand, reduce the coordination number of the solvent in the solvated sodium ions, thereby improving the high-voltage resistance of the electrolyte, widening the voltage window of the electrolyte, and improving the cycle stability of the sodium secondary battery.

[0062] In this application, ion chromatography can be used to detect the content of electrolyte salts in the electrolyte.

[0063] According to some embodiments of this application, the electrolyte further includes a co-solvent, the co-solvent comprising 3%-6% of the volume of the electrolyte, for example, 3%, 4%, 5%, 6%, or any range of the above values. Specifically, the content of the co-solvent can be adjusted according to the amount of inorganic salt added; when the content of the added inorganic salt is large, the content of the co-solvent can be increased accordingly. This improves the solubility of the inorganic salt in the electrolyte and increases the coordination number of anions in the sodium ion solvation sheath.

[0064] According to some embodiments of this application, the co-solvent may include fluorinated nitrile compounds, including one or more of fluoroacetonitrile, difluoroacetonitrile, fluoropropionitrile, difluoropropionitrile, fluorobutyronitrile, difluorobutyronitrile, and trifluorobutyronitrile. This improves the solubility of the inorganic salt in the electrolyte.

[0065] According to some embodiments of this application, the electrolyte further includes a solvent, which includes one or both of linear ethers and cyclic ethers.

[0066] As an example, solvents include linear ethers and cyclic ethers. First, ether solvents have good compatibility with sodium metal at the negative electrode. Second, cyclic ethers themselves have a weaker solvation effect with sodium ions, which can further reduce the coordination number of the solvent in the solvation sheath, reduce the number of active solvent molecules in the free state that can react on the electrode surface, improve the high voltage resistance of the electrolyte, increase the voltage window of the electrolyte, and improve the high voltage cycle stability of the sodium secondary battery.

[0067] According to some embodiments of this application, the linear ether may include one or more of ethylene glycol dimethyl ether (DME), diethylene glycol dimethyl ether (DEGDME), triethylene glycol dimethyl ether (TRGDME), tetraethylene glycol dimethyl ether (TEGDME), ethylene glycol diethyl ether (DEE), diethylene glycol diethyl ether (DEGDEE), ethylene glycol dibutyl ether (DBE), and diethylene glycol dibutyl ether (DEGDBE).

[0068] According to some embodiments of this application, the cyclic ether may include one or more of tetrahydrofuran (THF), methyltetrahydrofuran (MeTHF), and 1,3-dioxane. This improves the compatibility between the electrolyte solvent and the sodium metal anode.

[0069] According to some embodiments of this application, the positive electrode active material includes NaMO2 and Na x R y P m O nOne or two of the above-mentioned positive electrode active materials are used, wherein M includes one or more of Ti, V, Cr, Mn, Fe, Co, Ni, and Cu, with 2.5≤x≤4.5, 1.5≤y≤3.5, 2.5<m<4.5, and 11.5≤n≤15.5; and R includes one or more of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, and Pb. When the positive electrode uses the above-mentioned positive electrode active materials, the inorganic salts in the electrolyte can improve the high-voltage resistance of the electrolyte and form a stable CEI film on the positive electrode surface, which can reduce the side reactions between the electrolyte and the positive electrode active material and improve the high-voltage cycle stability of the sodium secondary battery.

[0070] As an example, x can be 2.5, 3, 3.5, 4, 4.5, etc., or a range of any of the above values.

[0071] As an example, y can be 1.5, 2, 2.5, 3, 3.5, etc., or a range of any of the above values.

[0072] As an example, m can be 2.7, 3, 3.3, 3.6, 3.9, 4.1, 4.5, etc., or a range of any of the above values.

[0073] As an example, n can be 11.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, etc., or a range of any of the above values.

[0074] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0075] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resins.

[0076] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0077] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0078] According to some embodiments of this application, when the positive electrode active material includes NaMO2, the compaction density of the positive electrode sheet can be 2.5 g / cm³. 3 -3.5g / cm 3 For example, it could be 2.5 g / cm³. 3 2.7g / cm 3 2.9g / cm 3 3.1g / cm 3 3.3g / cm 3 3.5g / cm 3 The range can be any of the values ​​mentioned above. This increases the energy density of the sodium-based secondary battery without a negative electrode.

[0079] According to some embodiments of this application, when the positive electrode active material includes Na x R y P m O n At that time, the compaction density of the positive electrode sheet can be 1.7 g / cm³. 3 -2.5g / cm 3 For example, it could be 1.7 g / cm³. 3 1.9g / cm 3 2.1g / cm 3 2.3g / cm 3 2.5g / cm 3 The range can be any of the values ​​mentioned above. This increases the energy density of the sodium-based secondary battery without a negative electrode.

[0080] In this application, the compaction density of the positive electrode sheet is determined by measuring the mass of the positive electrode sheet per unit area (g / cm³). 2 The positive electrode thickness (cm) was determined by the number of sampling points > 14. The compaction density PD of the positive electrode is calculated as: PD = (mass of the positive electrode per unit area, g / cm³) 2 ) / Thickness of positive electrode (cm).

[0081] According to some embodiments of this application, the coating weight of the positive electrode sheet can be 5 mg / cm³. 2 -30mg / cm 2 For example, it could be 5 mg / cm³. 2 10mg / cm 2 15mg / cm 2 20mg / cm 2 25mg / cm 2 Or 30mg / cm 2 The range can be any of the values ​​mentioned above. This increases the energy density of the sodium-based secondary battery without a negative electrode.

[0082] It should be noted that the coating weight of the positive electrode film referred to here refers to the coating weight of the positive electrode film on one side of the positive electrode current collector.

[0083] In this application, a fixed-size mold is used to punch and coat the middle area and the tab area. The mass of the positive active material layer on both sides of the positive electrode sheet in the middle area is m1, and the mass of the positive film layer on both sides of the positive electrode sheet in the tab area is m2. The coating mass of one side of the positive film layer is ((m1-m2) / 2) / S, where S is the mold area.

[0084] According to some embodiments of this application, at least one side of the negative electrode current collector has an interface modification layer, which includes a binder and a conductive agent. This improves the electronic conductivity of the negative electrode.

[0085] According to some embodiments of this application, the thickness of the interface modification layer can be 0.5 μm-2 μm. This improves the electronic conductivity of the negative electrode.

[0086] As an example, the thickness of the interface modification layer can be 0.5μm, 0.7μm, 0.9μm, 1.1μm, 1.3μm, 1.5μm, 1.7μm, 1.9μm or 2μm, or can be a range of any of the above values.

[0087] In this application, the longitudinal section of the negative electrode sheet along its thickness direction can be obtained by plasma, and the thickness of the interface modification layer can be obtained by scanning electron microscopy (SEM).

[0088] According to some embodiments of this application, the compaction density of the negative electrode sheet can be 1 g / cm³. 3 -2g / cm 3 For example, it could be 1g / cm³ 3 1.2g / cm 3 1.4g / cm 3 1.6g / cm 3 1.8g / cm 3 or 2g / cm3 The range can be any of the values ​​mentioned above. This increases the energy density of the sodium-based secondary battery without a negative electrode.

[0089] In this application, the compaction density of the negative electrode sheet is determined by measuring the mass of the negative electrode sheet per unit area (g / cm³). 2 The density of the negative electrode sheet (PD) is determined by the negative electrode sheet thickness (cm) (number of sampling points > 14). The compaction density PD of the negative electrode sheet is calculated as: PD = (mass of the negative electrode sheet per unit area) g / cm³. 2 ) / Thickness of negative electrode sheet (cm).

[0090] According to some embodiments of this application, the coating weight of the negative electrode sheet can be 0.1 mg / cm³. 2 -0.2mg / cm 2 For example, it could be 0.1 mg / cm³. 2 0.12 mg / cm 2 0.14 mg / cm 2 0.16 mg / cm 2 0.18 mg / cm 2 Or 0.2 mg / cm 2 The range can be any of the values ​​mentioned above. This increases the energy density of sodium secondary batteries.

[0091] It should be noted that the coating weight of the negative electrode film referred to here refers to the coating weight of the negative electrode film on one side of the negative electrode current collector.

[0092] In this application, a fixed-size die is used to punch and coat the middle area and the tab area. The mass of the negative active material layer on both sides of the negative electrode sheet in the middle area is m1, and the mass of the negative film layer on both sides of the negative electrode sheet in the tab area is m2. The coating mass of one side of the negative film layer is ((m1-m2) / 2) / S, where S is the die area.

[0093] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0094] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0095] In some embodiments, the sodium secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0096] In some embodiments, the material of the separator can be selected from one or more of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0097] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0098] In some embodiments, the battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0099] In some implementations, the battery's outer packaging can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The battery's outer packaging can also be a soft pack, such as a pouch. The soft pack can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0100] This application does not impose any particular limitation on the shape of the battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 shows a square-structured sodium secondary battery 5 as an example.

[0101] In some embodiments, referring to FIG2, the outer packaging may include a housing 51 and a cover plate 53. The housing 51 may include a bottom plate and side plates connected to the bottom plate, the bottom plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be placed over the opening to close the receiving cavity. A positive electrode, a negative electrode, and a separator can be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The sodium secondary battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0102] In some implementations, the batteries can be assembled into battery modules, and the number of batteries contained in a battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0103] Figure 3 shows a battery module 4 as an example. Referring to Figure 3, in the battery module 4, multiple sodium secondary batteries 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other arbitrary manner. Furthermore, the multiple sodium secondary batteries 5 can be fixed in place using fasteners.

[0104] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of sodium secondary batteries 5 are received.

[0105] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0106] Figures 4 and 5 show a battery pack 1 as an example. Referring to Figures 4 and 5, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper box 2 and a lower box 3, with the upper box 2 covering the lower box 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0107] A second aspect of this application provides an electrolyte comprising an inorganic salt and / or an organic salt, wherein the inorganic salt comprises one or more of sodium nitrate, sodium trifluoroacetate, and sodium tetrafluoroborate, and the organic salt comprises sodium tetraphenoxyborate. This improves the high-voltage resistance of the electrolyte and increases its potential window; simultaneously, it forms a stable CEI film on the positive electrode surface, reducing side reactions in the electrolyte, reducing structural degradation and transition metal dissolution of the positive electrode active material during cycling, improving the high-voltage cycling stability of the battery, and reducing gas production. Furthermore, sodium tetraphenoxyborate can form CEI and SEI films containing boron-oxygen components, which, for the negative electrode, can induce Na+... + More uniform deposition on the negative electrode reduces the formation of dead sodium and sodium dendrites, and improves the reversibility of the sodium deposition and stripping process. For the positive electrode, it can improve the integrity of the positive electrode structure and reduce the dissolution of transition metals, thereby improving the cycle stability of sodium secondary batteries.

[0108] According to some embodiments of this application, the molar concentration of the inorganic salt in the electrolyte is 0.1 mol / L-0.3 mol / L. For example, it can be 0.1 mol / L, 0.15 mol / L, 0.2 mol / L, 0.25 mol / L, 0.3 mol / L, etc., or any range of the above values. Therefore, by adding the above-mentioned amount of inorganic salt to the electrolyte, the coordination number of anions in the sodium ion solvation layer is increased, the high-voltage resistance of the electrolyte is improved, and the voltage window of the electrolyte is increased. Simultaneously, the increased anion content in the solvation layer can increase the inorganic content in the CEI membrane, improve the stability of the CEI membrane, and thus improve the high-voltage cycle stability of the sodium secondary battery and reduce gas production in the sodium secondary battery.

[0109] According to some embodiments of this application, the molar concentration of sodium tetraphenoxyborate in the electrolyte is 0.1 mol / L to 0.4 mol / L. This reduces gas production in the sodium secondary battery and improves its cycle performance.

[0110] According to some embodiments of this application, the electrolyte further includes an electrolyte salt, which includes sodium hexafluorophosphate (NaPF6). The molar concentration of sodium hexafluorophosphate in the electrolyte can be between 1.5 mol / L and 2 mol / L, for example, 1.5 mol / L, 1.6 mol / L, 1.7 mol / L, 1.8 mol / L, 1.9 mol / L, 2 mol / L, or any range of the above values. Therefore, a higher concentration of the electrolyte salt in the electrolyte can, on the one hand, increase the ionic conductivity of the electrolyte, and on the other hand, reduce the coordination number of the solvent in the solvated sodium ions, thereby improving the high-voltage resistance of the electrolyte, widening the voltage window of the electrolyte, and improving the cycle stability of the sodium secondary battery.

[0111] According to some embodiments of this application, the electrolyte further includes a co-solvent, the co-solvent comprising 3%-6% of the volume of the electrolyte, for example, 3%, 4%, 5%, 6%, or any range of the above values. Specifically, the content of the co-solvent can be adjusted according to the amount of inorganic salt added; when the content of the added inorganic salt is large, the content of the co-solvent can be increased accordingly. This improves the solubility of the inorganic salt in the electrolyte and increases the coordination number of anions in the sodium ion solvation sheath.

[0112] According to some embodiments of this application, the co-solvent includes fluorinated nitrile compounds, which include one or more of fluoroacetonitrile, difluoroacetonitrile, fluoropropionitrile, difluoropropionitrile, fluorobutyronitrile, difluorobutyronitrile, and trifluorobutyronitrile. This improves the solubility of the inorganic salt in the electrolyte.

[0113] According to some embodiments of this application, the electrolyte further includes a solvent, which includes one or both of linear ethers and cyclic ethers.

[0114] As an example, solvents include linear ethers and cyclic ethers. First, ether solvents have good compatibility with sodium metal at the negative electrode. Second, cyclic ethers themselves have a weaker solvation effect with sodium ions, which can further reduce the coordination number of the solvent in the solvation sheath, reduce the number of active solvent molecules in the free state that can react on the electrode surface, improve the high voltage resistance of the electrolyte, increase the voltage window of the electrolyte, and improve the high voltage cycle stability of the sodium secondary battery.

[0115] According to some embodiments of this application, the linear ether comprises one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol diethyl ether, ethylene glycol dibutyl ether, and diethylene glycol dibutyl ether; and / or the cyclic ether comprises one or more of tetrahydrofuran, methyltetrahydrofuran, and 1,3-dioxane. This improves the compatibility between the electrolyte solvent and the sodium metal anode.

[0116] A third aspect of this application provides an electrical device, including the sodium secondary battery provided in the first aspect of this application.

[0117] Figure 6 shows an example of an electrical device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of this device, a battery pack or battery module can be used.

[0118] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can be powered by a battery.

[0119] To make the technical problems, technical solutions, and beneficial effects solved by the embodiments of this application clearer, the following will provide a more detailed description in conjunction with the embodiments and accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit this application or its applications. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0120] Example 1

[0121] 1. Preparation of positive electrode sheet

[0122] NaNi, the positive electrode active material 0.20 Fe 0.22 Cu 0.13 Mn0.45 O2, polyvinylidene fluoride (PVDF) binder, and carbon black conductive agent are dissolved in N-methylpyrrolidone at a mass ratio of 80:10:10 to form a uniformly dispersed slurry. This slurry is then evenly coated onto the surface of aluminum foil and transferred to a vacuum drying oven for complete drying. The resulting electrode is then rolled and punched to obtain the positive electrode sheet, which has a compacted density of 2.8 g / cm³. 3 The coating weight is 10 mg / cm³. 2 .

[0123] 2. Preparation of negative electrode sheet

[0124] Sodium carboxymethyl cellulose and single-walled carbon nanotubes were dissolved in water at a mass ratio of 50:50, and then ultrasonically dispersed to prepare a slurry. The slurry was then coated onto the surface of copper foil, transferred to a vacuum drying oven for complete drying, and subsequently slit and die-cut to prepare a negative electrode sheet without a negative electrode structure. The compacted density of the negative electrode sheet was 1.2 g / cm³. 3 The coating weight is 0.1 mg / cm³. 2 .

[0125] 3. Preparation of electrolyte

[0126] In an argon-atmospheric glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), DME and THF were mixed at a volume ratio of 50:50 and stirred until homogeneous. A certain amount of NaPF6 was dissolved in the above mixed solvent and stirred until homogeneous, wherein the molar concentration of NaPF6 was 1.8 mol / L. NaNO3 was added, and the molar concentration of NaNO3 in the electrolyte was 0.3 mol / L. Fluoroacetonitrile, a co-solvent, was added, and the volume percentage of fluoroacetonitrile in the electrolyte was 5%. The mixture was stirred until homogeneous to form the final electrolyte.

[0127] 4. Separating membrane

[0128] Polypropylene film is used as the separator.

[0129] 5. Preparation of sodium-based secondary batteries without negative electrodes

[0130] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a barrier between the positive and negative electrodes. Tabs are welded onto the bare cell, which is then placed in an aluminum casing and baked at 80°C to remove moisture. The electrolyte is then injected and the casing is sealed, resulting in a non-charged battery. This non-charged battery then undergoes a series of processes including settling, hot and cold pressing, formation, shaping, and capacity testing to obtain a sodium-free negative electrode secondary battery product.

[0131] The preparation methods of the sodium-free secondary batteries in Examples 2-18 and Comparative Examples 1-4 are the same as those in the Examples, with the differences detailed in Table 1.

[0132] Table 1

[0133] Performance testing

[0134] 1. Storage capacity retention rate

[0135] A sodium-ion-free secondary battery was charged at 25°C with a constant current of 0.2C to 4.0V, then charged at a constant voltage of 4.0V until the current dropped to 0.05C, and then discharged at a constant current of 0.2C to 1.5V, yielding the initial discharge capacity (Cd1). This battery was then charged again with a constant current of 0.2C to 4.0V, followed by a constant voltage charge of 4.0V until the current dropped to 0.05C. The battery was then stored in a 60°C incubator for 30 days. After removal, the battery was charged at 25°C with a constant current of 0.2C to 4.0V, then charged at a constant voltage of 4.0V until the current dropped to 0.05C, and then discharged at a constant current of 0.2C to 1.5V, yielding the final discharge capacity (Cd2). The capacity retention rate of the sodium-ion-free secondary battery was calculated using the following formula:

[0136] Storage capacity retention rate = Cd2 / Cd1 × 100%.

[0137] 2. Cyclic performance test

[0138] At 25°C, the prepared battery was left to stand for 30 minutes, then charged to 4.0V with a constant current of 0.33C, and then charged to 0.05C with a constant voltage of 4.0V. After standing for 1 hour, it was discharged to 1.5V with a constant current of 0.33C to obtain the initial capacity (C0). After standing for 1 hour, it was charged to 4.0V with a constant current of 0.33C, and then charged to 0.05C with a constant voltage of 4.0V. After standing for 1 hour, it was discharged to 1.5V with a constant current of 0.33C to obtain the process capacity (C1). The above steps were repeated for the same battery, and the number of cycles N when the cycle capacity decayed to 80% was recorded.

[0139] At 25°C, the prepared battery was left to stand for 30 minutes, then charged to 4.2V with a constant current of 0.33C, and then charged to 0.05C with a constant voltage of 4.2V. After standing for 1 hour, it was discharged to 1.5V with a constant current of 0.33C to obtain the initial capacity (C0). After standing for 1 hour, it was charged to 4.2V with a constant current of 0.33C, and then charged to 0.05C with a constant voltage of 4.2V. After standing for 1 hour, it was discharged to 1.5V with a constant current of 0.33C to obtain the process capacity (C1). The above steps were repeated for the same battery, and the number of cycles N when the cycle capacity decayed to 80% was recorded.

[0140] 3. Gas production volume test during storage

[0141] Before capacity testing, the cell volume (V1) was measured at 25°C using the water displacement method. The sodium-ion secondary battery without a negative electrode was charged to 4.0V at a constant current of 0.2C at 25°C, then charged at a constant voltage of 4.0V until the current dropped to 0.05C, and finally discharged to 1.5V at a constant current of 0.2C. The discharge capacity before storage (C) was obtained. d1 Then, the battery is charged again at a constant current of 0.2C to 4.0V, followed by constant voltage charging at 4.0V until the current drops to 0.05C. The battery is then stored in a 60℃ constant temperature chamber for 30 days. After removal, the battery is placed at 25℃ to test the cell volume (V2) after storage, and the gas volume of the sodium-free secondary battery is calculated using the following formula:

[0142] Gas production = [Cell volume after storage (V2) - Cell volume before storage (V1)] / Cell capacity C d1 .

[0143] The test results of the sodium-free secondary batteries in Examples 1-18 and Comparative Examples 1-4 are shown in Table 2.

[0144] Table 2

[0145] As can be seen from the comparison between Examples 1-18 and Comparative Examples 1-4, the sodium-free secondary battery proposed in this application exhibits less gas production, higher capacity retention, and better high-voltage cycle stability after high-temperature storage. This indicates that by adding one or more of NaNO3, NaTFA, and NaBF4 to the electrolyte, this application can improve the electrolyte's high-voltage resistance, reduce side reactions between the electrolyte and the positive electrode active material, improve the battery's high-voltage cycle stability, and reduce gas production.

[0146] As can be seen from the comparison between Examples 1-3 and Comparative Examples 2-4, the inorganic salts in Comparative Examples 2-4 react with the sodium metal at the negative electrode in the sodium-free secondary battery system, increasing gas production and reducing the battery's high-voltage cycle stability and storage performance. This application, by adding specific types of inorganic salts, can improve the electrolyte's high-voltage resistance, reduce side reactions between the electrolyte and the positive electrode active material, improve the battery's high-voltage cycle stability, and reduce gas production, all without reacting with the sodium metal at the negative electrode.

[0147] As can be seen from Examples 1 and 4-7, by adding characteristic types of inorganic salts and adjusting the content of inorganic salts in the electrolyte, the gas production of the battery can be further reduced, the high-voltage cycle stability of the battery can be improved, and a sodium-free secondary battery with both excellent high-temperature storage performance and excellent high-voltage cycle stability can be obtained.

[0148] As can be seen from Examples 8-10, two inorganic salts can also be used in combination in the electrolyte, and the ratio of the two inorganic salts can be further adjusted to obtain a sodium-free secondary battery with both excellent high-temperature storage performance and excellent high-voltage cycle stability.

[0149] As can be seen from Examples 1, 11, and 12, the electrolyte salt concentration in the sodium-free secondary battery electrolyte proposed in this application is relatively high. On the one hand, this can improve the ionic conductivity of the electrolyte. On the other hand, the presence of a high concentration of electrolyte salt and inorganic salt in the electrolyte can reduce the coordination number of the solvent in the solvated sodium ions, improve the high voltage resistance of the electrolyte, and improve the high voltage cycle stability of the battery.

[0150] As can be seen from the comparison between Examples 1, 13-16, 18 and Example 17, the use of a combination of linear ether and cyclic ether as electrolyte solvent can further improve the high voltage resistance of the electrolyte, and further improve the storage performance and high voltage cycle stability of the negative electrode-free sodium secondary battery.

[0151] Example 19

[0152] The preparation method of the sodium-free secondary battery is the same as in Example 1, except that the composition of the electrolyte is different, as detailed below:

[0153] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), DME and 1,3-dioxapentane were used as solvents with a volume ratio of 8:2. Then, a certain amount of NaPF6 and sodium tetraphenoxyborate NaB(OPh)4 were dissolved in the above solvents and stirred evenly to form the final electrolyte. The molar concentrations of NaPF6 and NaB(OPh)4 in the electrolyte and the test results of the sodium-free secondary battery are shown in Table 3.

[0154] Table 3

[0155] This shows that by adding an appropriate amount of NaB(OPh)4 to the electrolyte, the gas production of the battery cells can be reduced and the cycle performance of the battery cells can be improved.

[0156] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application, and they should all be covered within the scope of the claims and specification of this application. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A sodium secondary battery, comprising: A positive electrode sheet, the positive electrode sheet including a positive current collector, the positive current collector having a positive active material layer on at least one side, the positive active material layer including a positive active material; The negative electrode includes a negative current collector, and metallic sodium is deposited in situ on the negative current collector during charging of the sodium secondary battery. An electrolyte comprising inorganic and / or organic salts, wherein the inorganic salts comprise one or more of sodium nitrate, sodium trifluoroacetate, and sodium tetrafluoroborate, and the organic salts comprise sodium tetraphenoxyborate.

2. The sodium secondary battery according to claim 1, wherein, The molar concentration of the inorganic salt in the electrolyte is 0.1 mol / L to 0.3 mol / L.

3. The sodium secondary battery according to claim 1 or 2, wherein, The molar concentration of sodium tetraphenoxyborate in the electrolyte is 0.1 mol / L to 0.4 mol / L.

4. The sodium secondary battery according to any one of claims 1-3, wherein, The electrolyte also includes an electrolyte salt, which includes sodium hexafluorophosphate, and the molar concentration of sodium hexafluorophosphate in the electrolyte is 1.5 mol / L-2 mol / L.

5. The sodium secondary battery according to any one of claims 1-4, wherein, The electrolyte also includes a co-solvent, which accounts for 3%-6% of the volume of the electrolyte.

6. The sodium secondary battery according to claim 5, wherein, The co-solvent includes fluorinated nitrile compounds.

7. The sodium secondary battery according to claim 6, wherein, The fluorinated nitrile compounds include one or more of fluoroacetonitrile, difluoroacetonitrile, fluoropropionitrile, difluoropropionitrile, fluorobutyronitrile, difluorobutyronitrile, and trifluorobutyronitrile.

8. The sodium secondary battery according to any one of claims 1-7, wherein, The electrolyte also includes a solvent, which includes one or both of linear ethers and cyclic ethers.

9. The sodium secondary battery according to claim 8, wherein, The linear ether comprises one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol diethyl ether, ethylene glycol dibutyl ether, and diethylene glycol dibutyl ether; and / or The cyclic ethers include one or more of tetrahydrofuran, methyltetrahydrofuran, and 1,3-dioxane.

10. The sodium secondary battery according to any one of claims 1-9, wherein, The positive electrode active material includes NaMO2 and Na x R y P m O n One or two of the following, wherein M includes one or more of Ti, V, Cr, Mn, Fe, Co, Ni, and Cu, 2.5≤x≤4.5, 1.5≤y≤3.5, 2.5<m<4.5, and 11.5≤n≤15.5, and R includes one or more of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, and Pb.

11. An electrolyte, wherein, It includes inorganic salts and organic salts, wherein the inorganic salts include one or more of sodium nitrate, sodium trifluoroacetate, and sodium tetrafluoroborate, and the organic salts include sodium tetraphenoxyborate.

12. The electrolyte according to claim 11, wherein, The molar concentration of the inorganic salt in the electrolyte is 0.1 mol / L to 0.3 mol / L.

13. The electrolyte according to claim 11 or 12, wherein, The molar concentration of sodium tetraphenoxyborate in the electrolyte is 0.1 mol / L to 0.4 mol / L.

14. The electrolyte according to any one of claims 11-13, wherein, The electrolyte also includes an electrolyte salt, which includes sodium hexafluorophosphate, and the molar concentration of sodium hexafluorophosphate in the electrolyte is 1.5 mol / L-2 mol / L.

15. The electrolyte according to any one of claims 11-14, wherein, The electrolyte also includes a co-solvent, which accounts for 3%-6% of the volume of the electrolyte.

16. The electrolyte according to claim 15, wherein, The co-solvent includes fluorinated nitrile solvents.

17. The electrolyte according to claim 16, wherein, The fluorinated nitrile solvents include one or more of fluoroacetonitrile, difluoroacetonitrile, fluoropropionitrile, difluoropropionitrile, fluorobutyronitrile, difluorobutyronitrile, and trifluorobutyronitrile.

18. The electrolyte according to any one of claims 11-17, wherein, The electrolyte also includes a solvent, which includes one or both of linear ethers and cyclic ethers.

19. The electrolyte according to claim 18, wherein, The linear ether comprises one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol diethyl ether, ethylene glycol dibutyl ether, and diethylene glycol dibutyl ether; and / or The cyclic ethers include one or more of tetrahydrofuran, methyltetrahydrofuran, and 1,3-dioxane.

20. An electrical appliance, wherein, Includes the sodium secondary battery as described in any one of claims 1-10.

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