Sodium secondary battery and electric device

Abstract

The application provides a sodium secondary battery and an electric device. A ratio of a discharge capacity of the sodium secondary battery in at least one 2V discharge interval range in a discharge process to a total discharge capacity of the sodium secondary battery is greater than or equal to 95%, and a test condition of the discharge process of the sodium secondary battery is as follows: after being charged to 4.2V at a constant rate of 0.33C at 25 DEG C, the sodium secondary battery is discharged to 1.5V at a constant rate of 0.33C.

Description

Technical Field

[0001] This application relates to the field of secondary battery technology, and in particular to a sodium secondary battery and an electrical device thereof. Background Technology

[0002] In recent years, rechargeable batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric cars, military equipment, aerospace, and many other fields. With the increasing prevalence of rechargeable battery applications, higher demands are being placed on balancing cost and performance.

[0003] Sodium-ion batteries utilize the intercalation and deintercalation of sodium ions between the positive and negative electrodes for charging and discharging. Compared to lithium-ion batteries, sodium-ion batteries have abundant and widely distributed sodium reserves and are less expensive, making them a potential replacement for lithium-ion batteries. However, existing sodium-ion batteries often lack compatibility with existing motors, hindering their market promotion and application. Summary of the Invention

[0004] This application is made in view of the above-mentioned issues, and its purpose is to provide a sodium secondary battery that can be effectively adapted to existing motors, thereby facilitating the widespread application of sodium secondary batteries.

[0005] The first aspect of this application provides a sodium secondary battery, wherein the discharge capacity of the sodium secondary battery during the discharge process is greater than or equal to 95% of the total discharge capacity of the sodium secondary battery within at least a 2V discharge range. The test conditions for the discharge process of the sodium secondary battery are: charging to 4.2V at a constant rate of 0.33C at 25°C, and then discharging to 1.5V at a constant rate of 0.33C.

[0006] The aforementioned sodium secondary battery has a high discharge specific capacity within the voltage range that the motor can use, which can fully utilize the capacity of the sodium secondary battery and reduce resource waste.

[0007] In any embodiment, the ratio of the discharge capacity of the sodium secondary battery in the voltage range of 2V-1.5V during the discharge process to the total discharge capacity of the sodium secondary battery is less than or equal to 5%. The test conditions for the discharge process of the sodium secondary battery are: charging to 4.2V at a constant rate of 0.33C at 25°C, and then discharging to 1.5V at a constant rate of 0.33C.

[0008] The lower limit voltage of lithium secondary batteries is generally 2.5V. Existing motors often struggle to start when the voltage of a single battery cell is below 2V. To adapt to motors, the minimum usable voltage of sodium secondary batteries is generally set to 2V. Sodium secondary batteries with a discharge capacity of less than or equal to 5% of their total discharge capacity within the 2V-1.5V voltage range during discharge have high usable capacity, which helps to fully utilize the capacity of the sodium secondary battery and improve its power supply efficiency.

[0009] In any embodiment, the sodium secondary battery includes a positive electrode, wherein the discharge capacity of the positive electrode during the discharge process is greater than or equal to 90% of the total discharge capacity of the positive electrode within at least a 2.2V discharge range; the discharge process of the positive electrode is tested as a button charge test, under the following conditions: charged to 4.2V at a constant rate of 0.1C at 25°C, and then discharged to 1.5V at a constant rate of 0.1C.

[0010] This positive electrode has a high discharge capacity ratio within a relatively narrow voltage range, enabling the sodium secondary battery to have a high discharge capacity within the voltage range usable by the motor, thus fully utilizing the capacity of the positive electrode and reducing resource waste.

[0011] In any embodiment, the positive electrode sheet includes a positive current collector and a positive electrode film layer disposed on at least one side of the positive current collector. The positive electrode film layer includes a positive electrode active material, which includes a polyanionic compound, a sodium-containing transition metal oxide, and their respective modified compounds.

[0012] In any embodiment, the composition of the polyanionic compound is as shown in Formula I.

[0013] Na n M p (X a O b ) c Z w Formula I

[0014] Wherein, M includes one or more of Ti, V, Cr, Mn, Fe, Co, Ni, Ca, Mg, Al, Nb, and Zr; X includes one or more of Si, S, P, As, B, Mo, W, and Ge; Z includes one or more of F, O, Cl, and OH; 2≤n≤7, 1≤p≤4, 1≤a≤2, 1≤b≤7, 1≤c≤5, and 0≤w≤3.

[0015] In any embodiment, the positive electrode active material includes one or more of sodium iron pyrophosphate, sodium vanadium phosphate, sodium fluorovanadium phosphate, sodium iron sulfate, and their respective modified materials.

[0016] The positive electrode active material with the above composition enables the secondary battery to have a lower upper limit voltage and a high capacity ratio within a relatively narrow voltage range. This is beneficial for the sodium secondary battery to release a high discharge capacity within the voltage range that the motor can use, thereby improving the battery's discharge efficiency.

[0017] In any embodiment, the capacity M per unit area of ​​the positive electrode is less than or equal to 20 mAh / cm². 2 .

[0018] Positive electrode sheets with a unit area capacity within the above range are beneficial for reducing sodium deposition on the negative electrode. Especially in sodium batteries without a negative electrode, they can effectively control the thickness of the sodium metal layer deposited on the negative electrode, enabling the sodium secondary battery to maintain high cycle stability during long-term cycling.

[0019] In any embodiment, the sodium secondary battery includes a negative electrode plate. When the sodium secondary battery is fully charged, the average potential of the negative electrode plate is less than or equal to 0.2V. The fully charged state of the sodium secondary battery refers to a sodium secondary battery charged to 4.2V at a constant rate of 0.33C at 25°C, and / or the difference between the upper limit potential and the lower limit potential of the negative electrode plate is less than or equal to 0.5V.

[0020] The negative electrode with an average potential within the above range enables the sodium secondary battery to have a relatively high lower limit voltage, in order to meet the requirements of existing motors for the lower limit voltage of individual battery cells.

[0021] The negative electrode with a potential difference between the upper and lower limits within the above range enables the sodium secondary battery to have a narrower charge and discharge voltage range, thus meeting the requirements of existing motors for the voltage window of secondary battery cells.

[0022] In any embodiment, when the sodium secondary battery is fully charged, the film layer h located on the negative current collector in the negative electrode sheet is less than or equal to 250 μm, and can be selected as 10 μm-200 μm; the sodium secondary battery being fully charged refers to a sodium secondary battery charged to 4.2V at a constant rate of 0.33C at 25°C.

[0023] When the sodium secondary battery is fully charged, the thickness h of the film layer on the negative current collector in the negative electrode sheet is within the above range, indicating that the sodium secondary battery has high cycle stability and that there is no phenomenon of large-scale sodium deposition in the negative electrode during the cycle.

[0024] In any embodiment, the sodium secondary battery includes an electrolyte, which includes short-chain ethers and long-chain ethers; the short-chain ether includes ethylene glycol dimethyl ether, and the long-chain ether includes at least one component represented by Formula II.

[0025] Formula II: R1-(O-R3)nO-R2;

[0026] Wherein, R1 and R2 are each independently selected from alkyl groups having 1 to 6 carbon atoms in a straight or branched chain, and R3 is selected from alkylene groups having 1 to 5 carbon atoms in a straight or branched chain, and n is an integer from 2 to 5; or, R1 and R2 are each independently selected from alkyl groups having 2 to 6 carbon atoms in a straight or branched chain, and R3 is selected from alkylene groups having 1 to 5 carbon atoms in a straight or branched chain, and n is 1.

[0027] Ether solvents exhibit good reduction stability, are not easily reduced by sodium metal at the negative electrode, and are therefore not continuously decomposed, resulting in good cycle stability of the battery. Simultaneously, ether solvents possess a stable solvation structure, which helps form a thin and stable solid electrolyte interphase (SEI), further improving battery cycle stability. Furthermore, ether solvents have low freezing points and viscosities, making secondary batteries suitable for low-temperature environments. Ethylene glycol dimethyl ether has good solvation capabilities, providing a basis for a certain amount of sodium electrolyte salt solvent; while long-chain ethers can improve the oxidation resistance of the electrolyte, improving the interfacial stability between the electrode and the interface in sodium secondary batteries, especially sodium metal batteries and batteries without a negative electrode.

[0028] In any embodiment, the long-chain ether comprises one or more of the following groups: diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, pentaethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol diethyl ether, triethylene glycol diethyl ether, tetraethylene glycol diethyl ether, pentaethylene glycol diethyl ether, ethylene glycol dibutyl ether, and polymers thereof.

[0029] In any embodiment, the short-chain ether accounts for 4% to 50% of the total mass of the electrolyte.

[0030] In any embodiment, the sodium secondary battery includes a negative electrodeless battery or a sodium metal battery.

[0031] A negative electrode-less sodium secondary battery refers to a battery in which no negative electrode active material layer is actively placed on the negative electrode side during the battery manufacturing process. For example, a sodium metal or carbonaceous active material layer is not formed at the negative electrode through coating or deposition processes. During the first charge, sodium ions gain electrons on the anode side and deposit metallic sodium on the current collector surface to form a sodium metal phase. During discharge, the metallic sodium can be converted back into sodium ions and return to the positive electrode, achieving cyclic charging and discharging. Compared to other sodium secondary batteries, a negative electrode-less sodium secondary battery can achieve a higher energy density due to the absence of a negative electrode active material layer.

[0032] Sodium metal batteries are secondary batteries manufactured by pre-depositing sodium metal or its alloys onto the negative electrode during the battery manufacturing process. Using sodium metal as the negative electrode active material results in a low potential for the negative electrode, which is beneficial for improving the usable capacity of sodium secondary batteries within their usable voltage range.

[0033] A second aspect of this application also provides an electrical device, including the sodium secondary battery of the first aspect. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the discharge curve of a sodium secondary battery according to one embodiment of this application;

[0035] Figure 2 This is a schematic diagram of a secondary battery according to one embodiment of this application;

[0036] Figure 3 yes Figure 2 An exploded view of a secondary battery according to an embodiment of this application is shown.

[0037] Figure 4 This is a schematic diagram of a battery module according to one embodiment of this application;

[0038] Figure 5 This is a schematic diagram of a battery pack according to one embodiment of this application;

[0039] Figure 6 yes Figure 5 An exploded view of a battery pack according to one embodiment of this application is shown;

[0040] Figure 7 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application.

[0041] Explanation of reference numerals in the attached figures:

[0042] 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Casing; 52 Electrode assembly; 53 Cover plate. Detailed Implementation

[0043] The embodiments of the sodium secondary battery and power-consuming device of this application are hereby disclosed in detail with appropriate reference to the accompanying drawings. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of actually identical structures may be omitted. This is to avoid making the following description unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0044] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0045] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0046] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0047] 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.

[0048] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0049] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0050] Common designs in existing technologies involve arranging over 100 battery cells in series and parallel to meet the motor's drive voltage requirements. The voltage difference between the motor's drive voltage and its minimum starting voltage is often 200V. To simultaneously meet both the motor's drive voltage and minimum starting voltage requirements, the charge / discharge voltage range of the battery cells is generally required to be within 2V. The voltage difference between a lithium-ion battery cell in its fully charged and fully discharged states is between 1.1V and 1.8V, which can simultaneously meet the motor's drive voltage and minimum starting voltage requirements. However, the voltage difference between the fully charged and fully discharged states of existing sodium-ion batteries is generally greater than 2V, preventing them from reaching their full capacity within the 2V range required by the motor. Taking the layered oxide-hard carbon sodium-ion battery system as an example, it can only utilize up to 88% of its capacity within the 2V voltage range, resulting in resource waste.

[0051] Sodium secondary battery

[0052] Based on this, this application proposes a sodium secondary battery, wherein the discharge capacity of the sodium secondary battery in at least a 2V discharge range during the discharge process accounts for more than or equal to 95% of the total discharge capacity of the sodium secondary battery. The test conditions for the discharge process of the sodium secondary battery are: charging to 4.2V at a constant rate of 0.33C at 25°C, and then discharging to 1.5V at a constant rate of 0.33C.

[0053] A fully charged state refers to a battery (including full cells and button cells) charged to 4.2V, which corresponds to 100% SOC.

[0054] A fully discharged state refers to a battery (including full cells and button cells) that has been discharged to 1.5V, which corresponds to 0% SOC.

[0055] In this application, the ratio of the discharge capacity of the sodium secondary battery within at least a 2V discharge range to the total discharge capacity of the sodium secondary battery can be tested using methods known in the art. As an example, this is tested using the discharge curve of the sodium secondary battery. For instance, a VMP3 electrochemical workstation is used to measure the discharge curve of the sodium secondary battery. The sodium secondary battery is charged to 4.2V at a constant rate of 0.33C at 25°C, and then discharged to 1.5V at a constant rate of 0.33C. The state of charge (SOC) of the sodium secondary battery at different voltages during the discharge process is measured. The SOC of the sodium secondary battery is calculated by dividing the capacity of the sodium secondary battery at different voltages by the total capacity released from a fully charged state to a fully discharged state. Figure 1 This is the discharge curve of a sodium secondary battery according to an embodiment of this application. The state of charge of the sodium secondary battery corresponding to a voltage of aV is S1, and the state of charge of the sodium secondary battery corresponding to the higher of (a-2)V and 1.5V is S2. S1-S2 is used as the ratio of the discharge capacity of the sodium secondary battery in the 2V discharge range to the total discharge capacity of the sodium secondary battery.

[0056] In some embodiments, the ratio of the discharge capacity of the sodium secondary battery during at least a 2V discharge range to the total discharge capacity of the sodium secondary battery is 95%, 96%, 97%, 98%, 99%, 100%, or any value between them.

[0057] The aforementioned sodium secondary battery has a high discharge specific capacity within the voltage range that the motor can use, which can fully utilize the capacity of the sodium secondary battery and reduce resource waste.

[0058] In some embodiments, the discharge capacity of the sodium secondary battery in the voltage range of 2V-1.5V during discharge is less than or equal to 5% of the total discharge capacity of the sodium secondary battery. The test conditions for the discharge process of the sodium secondary battery are: charging to 4.2V at a constant rate of 0.33C at 25°C, and then discharging to 1.5V at a constant rate of 0.33C.

[0059] The ratio of the discharge capacity of a sodium secondary battery within the 2V-1.5V voltage range to its total discharge capacity can be tested using the discharge curve described above. The ratio of the sodium secondary battery's state of charge (SOC) at 2V to its SOC at 1.5V is taken as the ratio of the sodium secondary battery's discharge capacity within the 2V-1.5V voltage range to its total discharge capacity.

[0060] In some embodiments, the ratio of the discharge capacity of the sodium secondary battery in the voltage range of 2V-1.5V during discharge to the total discharge capacity of the sodium secondary battery is 0%, 1%, 2%, 3%, 4%, 5%, or any value between them.

[0061] The lower limit voltage of lithium-ion batteries is generally 2.5V. Existing motors often struggle to start when the voltage of a single battery cell is below 2V. To adapt to motors, the minimum usable voltage of sodium-ion batteries is generally set to 2V. Sodium-ion batteries with a discharge capacity of less than or equal to 5% of their total discharge capacity within the 2V-1.5V voltage range during discharge have high usable capacity, facilitating full utilization of their capacity and reducing resource waste.

[0062] In some embodiments, the sodium secondary battery includes a positive electrode, wherein the discharge capacity of the positive electrode during the discharge process is greater than or equal to 90% of the total discharge capacity of the positive electrode within at least a 2.2V discharge range; the discharge process of the positive electrode is tested as a button charge test, under the following conditions: charged to 4.2V at a constant rate of 0.1C at 25°C, and then discharged to 1.5V at a constant rate of 0.1C.

[0063] In this application, the ratio of the discharge capacity of the positive electrode within at least a 2.2V discharge range during discharge to the total discharge capacity of the positive electrode can be tested using methods known in the art. As an example, a coin cell is assembled from the positive electrode of a sodium secondary battery and sodium metal, and the discharge curve of the coin cell is used for testing. The positive electrode from the sodium secondary battery is pressed into a circular electrode sheet, then a small circular sodium sheet is used as the counter electrode. A polypropylene separator is used, and an electrolyte is injected. The electrolyte includes 1 mol / L sodium hexafluorophosphate, and the solvents in the electrolyte are dimethyl ethylene glycol (DME) and diethylene glycol diethyl ether (DEE). A coin cell is then assembled. At 25°C and normal pressure, the coin cell is charged at a constant current rate of 0.1C to a voltage of 4.2V, and then discharged at a constant current rate of 0.1C to 1.5V, obtaining the discharge curve of the coin cell. The state of charge (SOC) of a coin cell is determined by dividing its capacity at different voltages by the total discharge capacity from full charge to full discharge. The SOC of the coin cell is measured at different voltages during discharge, and the difference in SOC within the 2.2V discharge range is calculated as the ratio of the discharge capacity of the positive electrode within the 2.2V discharge range to the total discharge capacity of the positive electrode.

[0064] In some embodiments, the ratio of the discharge capacity of the positive electrode in at least a 2.2V range during the discharge process to the total discharge capacity of the positive electrode is 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, 100%, or any value between these values.

[0065] The positive electrode plate has a high proportion of discharge capacity within a relatively narrow voltage range, which enables the sodium secondary battery to have high discharge capacity within the voltage range usable for the motor, can fully exert the capacity level of the positive electrode plate, and reduces resource waste.

[0066] In some embodiments, the positive electrode plate comprises a positive current collector and a positive film layer disposed on at least one side of the positive current collector, and the positive film layer comprises a positive active material.

[0067] In some embodiments, the positive active material can be a positive active material well known in the art for use in batteries. By way of example, the positive active material can include at least one of the following materials: Prussian blue analogs, polyanionic compounds, sodium-containing transition metal oxides, and their respective modified compounds. However, the present application is not limited to these materials, and other conventional materials that can be used as positive active materials for batteries can also be used. These positive active materials can be used alone alone in only one type, or two or more types can be used in combination. Wherein, the Prussian blue analog is Na x P[R(CN)6] δ ·zH2O, wherein P and R are each independently selected from at least one of transition metal elements, 0<x≤2, 0<δ≤1 and 0≤z≤10.

[0068] In some embodiments, the positive active material comprises polyanionic compounds, sodium-containing transition metal oxides and their respective modified compounds.

[0069] In some embodiments, in the sodium transition metal oxide, the transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce. The sodium transition metal oxide is, for example, Na x MO2, wherein M comprises one or more of Ti, V, Mn, Co, Ni, Fe, Cr and Cu, and 0<x≤1.

[0070] In some embodiments, the composition of the polyanionic compound is shown in Formula I,

[0071] Na n M p (X a O b ) c Z w Formula I

[0072] Wherein, M includes one or more of Ti, V, Cr, Mn, Fe, Co, Ni, Ca, Mg, Al, Nb, and Zr; X includes one or more of Si, S, P, As, B, Mo, W, and Ge; Z includes one or more of F, O, Cl, and OH; 2≤n≤7, 1≤p≤4, 1≤a≤2, 1≤b≤7, 1≤c≤5, and 0≤w≤3.

[0073] In some embodiments, the positive electrode active material includes one or more of sodium iron pyrophosphate, sodium vanadium phosphate, sodium fluorovanadium phosphate, sodium iron sulfate, and their respective modified materials.

[0074] In some embodiments, the modified material includes doped modified materials and / or coated modified materials.

[0075] The positive electrode active material with the above composition enables the secondary battery to have a lower upper limit voltage and a high capacity ratio within a relatively narrow voltage range. This is beneficial for sodium secondary batteries to release high discharge capacity within the voltage range that the motor can use, thus reducing resource waste.

[0076] In some implementations, the capacity per unit area of ​​the positive electrode is less than or equal to 20 mAh / cm². 2 .

[0077] In this application, the capacity per unit area of ​​the positive electrode sheet can be tested using methods known in the art. For example, the positive electrode sheet of a sodium secondary battery can be cut into small discs and assembled with sodium metal to form a coin cell. The coin cell can then be tested for discharge curves using a discharge rate of 0.1C. Specific preparation and testing methods for the coin cell can be found above. The capacity per unit area of ​​the positive electrode sheet is calculated by multiplying the discharge current from a fully charged state to a fully discharged state of the coin cell by the discharge time and dividing by the area of ​​the positive electrode sheet. In some embodiments, the capacity per unit area of ​​the positive electrode sheet is 2 mAh / cm². 2 4mAh / cm 2 6mAh / cm 2 8mAh / cm 2 10mAh / cm 2 12mAh / cm 2 14mAh / cm 2 16mAh / cm 2 18mAh / cm 2 20mAh / cm 2 Or any value in between.

[0078] Positive electrode sheets with a unit area capacity within the above range are beneficial for reducing sodium deposition on the negative electrode. Especially in sodium batteries without a negative electrode, they can effectively control the thickness of the sodium metal layer deposited on the negative electrode, enabling the sodium secondary battery to maintain high cycle stability during long-term cycling.

[0079] In some embodiments, the sodium secondary battery includes a negative electrode plate, wherein the average potential of the negative electrode plate is less than or equal to 0.2V when the sodium secondary battery is fully charged. The fully charged state of the sodium secondary battery refers to a sodium secondary battery charged to 4.2V at a constant rate of 0.33C at 25°C, and / or the difference between the upper limit potential and the lower limit potential of the negative electrode plate is less than or equal to 0.5V.

[0080] In this paper, the average potential, upper limit potential, and lower limit potential of the negative electrode are obtained by combining the negative electrode of a fully charged sodium secondary battery with sodium metal to form a coin cell, and then testing the discharge curve of the coin cell. The preparation and testing methods of the coin cell can be referred to above. The test range is 2-0V. The voltages corresponding to 50% SOC, 99% SOC, and 1% SOC in the coin cell discharge curve are used as the average potential, upper limit potential, and lower limit potential of the negative electrode, respectively.

[0081] The negative electrode with an average potential within the above range enables the sodium secondary battery to have a relatively high lower limit voltage, in order to meet the requirements of existing motors for the lower limit voltage of individual battery cells.

[0082] The negative electrode with a potential difference between the upper and lower limits within the above range enables the sodium secondary battery to have a narrower charge and discharge voltage range, thus meeting the requirements of existing motors for the voltage window of secondary battery cells.

[0083] In some embodiments, the capacity loss CB of the sodium secondary battery during discharge satisfies: CB≤1.5; where CB is tested by the following formula: CB=(C1+J×t) / C2; C1 is the residual capacity per unit area of ​​the positive electrode sheet when the sodium secondary battery is fully discharged, in mAh / cm². 2 J refers to the charging current density, which can be 0.5 mA / cm². 2 t represents the time required for the voltage of the negative electrode in a fully discharged sodium secondary battery to rise to 1V at a current density of J, expressed in hours; C2 represents the maximum capacity of the positive electrode per unit area, expressed in mAh / cm². 2 .

[0084] Specifically, the above parameters can be tested through the following experiments. A fully discharged sodium secondary battery's positive electrode and a sodium metal disc are assembled into a coin cell, as described above. The coin cell preparation method is as described previously. A discharge test is performed on the coin cell. The discharge capacity of the coin cell is divided by the area of ​​the positive electrode in the coin cell as C1. C1 characterizes the residual capacity of the positive electrode per unit area in the fully discharged state of the secondary battery. A charging test is then performed on the fully discharged coin cell. The charging capacity of the coin cell is divided by the area of ​​the positive electrode in the coin cell as C2. C2 characterizes the maximum capacity achievable by the positive electrode. A fully discharged sodium secondary battery's negative electrode and a sodium metal disc are assembled into a coin cell, as described above. The coin cell preparation method is as described previously. The coin cell is then charged at 0.5 mA / cm². 2 The current density is used for charging, and the time corresponding to the voltage rising to 1V is recorded as t hours. 0.5×t can characterize the actual discharge capacity of the negative electrode.

[0085] In some embodiments, the capacity loss CB of the sodium secondary battery during discharge is satisfied to be 0.1, 0.3, 0.5, 0.7, 0.9, 1.1, 1.3, 1.5 or any value between them.

[0086] Sodium secondary batteries with CB in the aforementioned range generally have low negative electrode capacity, such as being electrodeless batteries. Compared to other sodium secondary batteries, sodium secondary batteries with CB in the aforementioned range have a simpler manufacturing process and higher energy density.

[0087] In some embodiments, when the sodium secondary battery is fully charged, the thickness h of the film layer on the negative current collector in the negative electrode sheet is less than or equal to 250 μm, and can be selected as 10 μm-200 μm; the sodium secondary battery being fully charged refers to a sodium secondary battery charged to 4.2V at a constant rate of 0.33C at 25°C.

[0088] In this paper, the thickness h of the film layer on the negative electrode current collector in the alkali metal layer of the negative electrode sheet under full charge of a sodium secondary battery can be tested using methods known in the art. For example, at 25°C, the sodium secondary battery is charged to the upper voltage limit at a rate of 0.33C. The cell is then disassembled to obtain the negative electrode sheet, and the thickness of the negative electrode sheet is measured in μm using a micrometer (e.g., Mitutoyo 293-100, with an accuracy of 0.1 μm). To prevent metal adhesion on the test surface, two 0.1 μm thick acrylic sheets are laid flat on the negative electrode sheet under full charge before testing, and the four sides are fixed with dovetail clips. The total thickness obtained from the test, minus the thickness of the negative electrode current collector and its surface functional coating, and then minus the thickness of the surface acrylic sheet, gives the thickness h of the film layer on the negative electrode current collector in the negative electrode sheet. In some embodiments, when the sodium secondary battery is fully charged, the thickness h of the film layer located on the negative current collector in the negative electrode sheet is 5 μm, 10 μm, 30 μm, 50 μm, 70 μm, 90 μm, 110 μm, 130 μm, 150 μm, 170 μm, 200 μm, 220 μm, 250 μm or any value between these values.

[0089] When the sodium secondary battery is fully charged, the thickness h of the film layer on the negative current collector in the negative electrode sheet is within the above range, indicating that the sodium secondary battery has high cycle stability and that there is no phenomenon of large-scale deposition of sodium from the positive electrode to the negative electrode during the cycle.

[0090] In some embodiments, the sodium secondary battery includes an electrolyte comprising short-chain ethers and long-chain ethers; the short-chain ethers include ethylene glycol dimethyl ether, and the long-chain ethers include at least one component represented by Formula II;

[0091] Formula II: R1-(O-R3)nO-R2;

[0092] Wherein, R1 and R2 are each independently selected from alkyl groups having 1 to 6 carbon atoms in a straight or branched chain, and R3 is selected from alkylene groups having 1 to 5 carbon atoms in a straight or branched chain, and n is an integer from 2 to 5; or, R1 and R2 are each independently selected from alkyl groups having 2 to 6 carbon atoms in a straight or branched chain, and R3 is selected from alkylene groups having 1 to 5 carbon atoms in a straight or branched chain, and n is 1.

[0093] The term "alkyl" refers to an alkyl group having the general formula C1. n H 2n+1 A monovalent group derived from a saturated, unbranched, or branched aliphatic hydrocarbon by removing a hydrogen atom. Examples of alkyl groups include, but are not limited to, (C1–C6) alkyl groups, such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, isopentyl, neopentyl, and hexyl.

[0094] The term "alkylene" refers to an unbranched or branched divalent hydrocarbon group having 1 to 5 carbon atoms. Examples include, for instance, methylene, 1,2-ethylene, 1,2-propylene, 1,3-propylene, 1,3-butylene, 1,4-butylene, 2-methyl-1,3-propylene, 1,1-dimethyl-1,2-ethylene, 1,4-pentylene, and 1,5-pentylene.

[0095] In some embodiments, R1 and R2 each independently include one or more of methyl, ethyl, propyl, butyl, and pentyl, R3 includes 1,2-ethylene, and n is any one of 2, 3, 4, and 5.

[0096] In some embodiments, R1 and R2 each independently comprise one or more of ethyl, propyl, butyl, and pentyl, R3 comprises 1,2-ethylene, and n is 1.

[0097] Ether solvents exhibit good reduction stability, are not easily reduced by sodium metal at the negative electrode, and are therefore not continuously decomposed, resulting in good cycle stability of the battery. Simultaneously, ether solvents possess a stable solvation structure, which helps form a thin and stable solid electrolyte interphase (SEI), further improving battery cycle stability. Furthermore, ether solvents have low freezing points and viscosities, making secondary batteries suitable for low-temperature environments. Ethylene glycol dimethyl ether has good solvation capabilities, providing a basis for a certain amount of sodium electrolyte salt solvent; while long-chain ethers can improve the oxidation resistance of the electrolyte, improving the interfacial stability between the electrode and the interface in sodium secondary batteries, especially sodium metal batteries and batteries without a negative electrode.

[0098] In some embodiments, the long-chain ether comprises one or more combinations of the following: diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, pentaethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol diethyl ether, triethylene glycol diethyl ether, tetraethylene glycol diethyl ether, pentaethylene glycol diethyl ether, ethylene glycol dibutyl ether, and polymers thereof.

[0099] The polymer can refer to either a homopolymer or a copolymer, obtained by polymerizing monomers including the aforementioned ethers and their derivatives.

[0100] In some embodiments, the short-chain ether accounts for 4% to 50% of the total mass of the electrolyte.

[0101] In some embodiments, the mass percentage of the short-chain ether, based on the total mass of the electrolyte, is 4%, 8%, 12%, 16%, 20%, 24%, 28%, 32%, 36%, 40%, 45%, 50%, or any value between these values.

[0102] In some embodiments, the electrolyte comprises an electrolyte salt selected from at least one of NaPF6, NaBF4, NaN(SO2F)2 (NaFSI), NaClO4, NaAsF6, NaB(C2O4)2 (NaBOB), NaBF2(C2O4) (NaDFOB), NaN(SO2RF)2, and NaN(SO2F) (SO2RF), wherein RF represents C b F 2b+1 b is an integer between 1 and 10, and can be an integer between 1 and 3.

[0103] In some embodiments, the electrolyte salt is selected from one or more of NaPF6, NaN(SO2F)2, NaN(CF3SO2)2, NaB(C2O4)2, and NaBF2(C2O4). In some embodiments, the electrolyte salt is selected from one or more of NaPF6, NaN(SO2RF)2, and NaBF2(C2O4). In some embodiments, RF is -CF3, -C2F5, or -CF2CF2CF3.

[0104] In some implementations, sodium secondary batteries include negative electrodeless batteries or sodium metal batteries.

[0105] Sodium metal batteries are secondary batteries manufactured by pre-depositing sodium metal or its alloys onto the negative electrode during the battery manufacturing process. Using sodium metal as the negative electrode active material results in a low potential for the negative electrode, which is beneficial for improving the usable capacity of sodium secondary batteries within their usable voltage range.

[0106] A negative electrode-less sodium secondary battery refers to a battery in which no negative electrode active material layer is actively placed on the negative electrode side during the battery manufacturing process. For example, a sodium metal or carbonaceous active material layer is not formed at the negative electrode through coating or deposition processes. During the first charge, sodium ions gain electrons on the anode side and deposit metallic sodium on the current collector surface to form a sodium metal phase. During discharge, the metallic sodium can be converted back into sodium ions and return to the positive electrode, achieving cyclic charging and discharging. Compared to other sodium secondary batteries, a negative electrode-less sodium secondary battery can achieve a higher energy density due to the absence of a negative electrode active material layer.

[0107] In some implementations, the negative electrode includes a negative current collector.

[0108] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil or aluminum foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0109] In some implementations, to improve battery performance, the negative electrode side of a sodium-free secondary battery can be provided with conventional materials that can be used as negative electrode active materials, such as carbonaceous materials, metal oxides, and alloys. Although these materials have a certain capacity, because the amount of these materials is small, they are not used as the main negative electrode active materials in the battery, and therefore are not considered to form a negative electrode active material layer that intercalates sodium. In this way, the sodium secondary battery can still be regarded as a sodium-free secondary battery.

[0110] [Positive electrode plate]

[0111] A positive electrode typically includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including a positive electrode active material.

[0112] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0113] 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.).

[0114] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0115] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0116] 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.

[0117] [Isolation membrane]

[0118] In some embodiments, the 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.

[0119] In some embodiments, the material of the separator can be selected from at least one 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.

[0120] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0121] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the aforementioned electrode assembly and electrolyte.

[0122] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0123] In this application, the shape of the sodium secondary battery includes, but is not limited to, cylindrical, square, or other arbitrary shapes. For example, Figure 2 This is an example of a square-structured sodium secondary battery.

[0124] In some implementations, refer to Figure 3The outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 can be placed over the opening to close the receiving cavity. The positive electrode, negative electrode, and separator may 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.

[0125] In some embodiments, sodium secondary batteries can be assembled into battery modules, and the number of sodium secondary 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.

[0126] Figure 4 This is battery module 4, used as an example. (See reference...) Figure 4 In battery module 4, multiple sodium secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple sodium secondary batteries 5 can be fixed in place using fasteners.

[0127] 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.

[0128] 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.

[0129] Figure 5 and Figure 6 This is battery pack 1 as an example. (See reference...) Figure 5 and Figure 6 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 body 2 and a lower body 3, with the upper body 2 covering the lower body 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.

[0130] In addition, this application also provides an electrical device, which includes at least one of the sodium secondary battery, battery module, or battery pack provided in this application. The sodium secondary battery, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0131] As an electrical device, sodium secondary batteries, battery modules, or battery packs can be selected according to their usage requirements.

[0132] Figure 7 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of sodium-ion batteries for this device, a battery pack or battery module can be used.

[0133] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use sodium-ion batteries as their power source.

[0134] Example

[0135] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0136] I. Preparation Method

[0137] Example 1:

[0138] 1) Preparation of positive electrode sheet

[0139] 90 wt% of polyvinylidene fluoride binder was fully dissolved in N-methylpyrrolidone, and 5 wt% of carbon black conductive agent and 5 wt% of sodium iron pyrophosphate, the positive electrode active material, were added to prepare a uniformly dispersed positive electrode slurry. The positive electrode slurry was uniformly coated onto the surface of aluminum foil, then transferred to a vacuum drying oven for complete drying. The resulting electrode sheet was rolled and then punched to obtain the positive electrode sheet. The areal capacity of the positive electrode sheet was 2 mAh / cm². 2 The discharge capacity of the positive electrode active material in the 4.2V-2V range accounts for 95% of the discharge capacity in the 4.2V-1.5V range.

[0140] 2) Preparation of negative electrode sheet

[0141] A paste containing dispersed conductive CNTs was coated onto the surface of a copper foil, then transferred to a vacuum drying oven for complete drying, followed by punching to obtain the negative electrode sheet. After full charging, the average potential of the negative electrode sheet was measured to be 0.005V, with a potential difference of 0.1V between the upper and lower limits.

[0142] 3) Preparation of electrolyte

[0143] A mixed solvent was prepared by mixing ethylene glycol dimethyl ether (DME) and diethylene glycol diethyl ether (DEE) in a 1:1 volume ratio. Sodium hexafluorophosphate (NaPF6) was dissolved in the mixed solvent in an argon-atmospheric glove box (H₂O content <10 ppm, O₂ content <1 ppm), and stirred until homogeneous to obtain an electrolyte with a sodium salt concentration of 1 mol / L.

[0144] 4) Separating membrane

[0145] Polypropylene film is used as the separator.

[0146] 5) Battery manufacturing

[0147] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to isolate them. The cells are then wound to obtain a bare cell. The tabs are welded on, and the bare cell is placed in an outer package. The electrolyte prepared above is injected into the dried cell. After processes such as encapsulation, settling, formation, shaping, and capacity testing, the sodium secondary battery product of Example 1 is obtained.

[0148] The preparation methods of sodium secondary batteries in Examples 2-3 are basically the same as those in Example 1, except that the types of positive electrode active materials are adjusted. Specific parameters are shown in Table 1.

[0149] Example 4

[0150] The preparation method in Example 4 is basically the same as that in Example 1, except that the negative electrode in Example 4 is a sodium metal strip, and the preparation process of the negative electrode sheet is as follows:

[0151] The sodium metal block is rolled into a thin sheet in a glove box, cut to the required size for the negative electrode, and tightly attached to the negative electrode current collector. This is achieved through battery encapsulation and force to ensure a tight fit. The average potential of the negative electrode is 0V, and the upper and lower potential differences are 0V.

[0152] Example 5

[0153] The preparation method of Example 5 is basically the same as that of Example 1, except that the positive electrode active material is replaced with sodium transition metal oxide.

[0154] The preparation methods of sodium secondary batteries in Examples 6-8 are basically the same as those in Example 1, except that the unit area capacity of the positive electrode is adjusted. The specific parameters are shown in Table 1.

[0155] The preparation method of the sodium secondary battery in Example 9 is basically the same as that in Example 1, except that the composition of the electrolyte is adjusted. The specific parameters are shown in Table 1, and the preparation process is as follows:

[0156] In an argon-atmospheric glove box (H2O content <10ppm, O2 content <1ppm), sodium hexafluorophosphate (NaPF6) was dissolved in the organic solvent dimethyl ethylene glycol (DME) and stirred until homogeneous to obtain an electrolyte with a sodium salt concentration of 1 mol / L. Comparative Example 1

[0157] The preparation method of Comparative Example 1 is basically the same as that of Example 1, except that the negative electrode of the sodium secondary battery in Comparative Example 1 is hard carbon, and the preparation process of the negative electrode sheet is as follows:

[0158] Hard carbon (the negative electrode active material), Super-P (a conductive agent), and sodium carboxymethyl cellulose (CMC-Na) (a binder) were thoroughly mixed in a deionized water solvent system at a mass ratio of 90:5:5 to obtain a negative electrode slurry. The negative electrode slurry was then uniformly coated onto a copper foil (the negative electrode current collector). After drying the copper foil at room temperature, it was transferred to a 120°C oven for 1 hour. The resulting material was then cold-pressed and slit to obtain the negative electrode sheet. The hard carbon was purchased from Kuraray. The average potential of the negative electrode sheet was 0.28V, and the upper and lower potential difference was 1V.

[0159] Table 1

[0160]

[0161] II. Battery Performance Testing

[0162] 1. Cyclic capacity retention

[0163] At 25°C and ambient pressure (0.1MPa), the battery was charged at a constant current of 0.5C to a voltage of 3.5V, and then discharged at a constant current of 1C to a voltage of 3.2V. This constitutes one charge-discharge cycle. The initial discharge capacity was taken as 100%, and the charge-discharge cycle was repeated 200 times. The test was then stopped, and the ratio of the discharge capacity of the 200th cycle to the initial discharge capacity was taken as the cycle capacity retention rate.

[0164] 2. Compatibility with lithium-ion motors

[0165] The existing motor has a starting voltage of 200V and can carry 100 battery cells. If the motor cannot start, it means it is not compatible, which is represented by N; if the motor can start, it means it is compatible, which is represented by Y.

[0166] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0167] Batteries for each embodiment and comparative example were prepared according to the above method, and various performance parameters were measured. The results are shown in Tables 2 and 3.

[0168] Table 2

[0169]

[0170] Table 3

[0171]

[0172] As can be seen from Tables 2 and 3, the sodium secondary battery provided in this application has a discharge capacity within at least a 2V discharge range during the discharge process that accounts for more than 95% of the total discharge capacity of the sodium secondary battery, which makes it compatible with existing motors and facilitates the promotion and application of sodium secondary batteries.

[0173] As can be seen from the comparison between Example 1 and Example 4, secondary batteries with a CB value of less than or equal to 1.5 can further improve the cycle capacity retention rate of secondary batteries.

[0174] As can be seen from the comparison between Example 8 and Examples 1 and 6-7, when the sodium secondary battery is fully charged, the thickness h of the film layer on the negative current collector in the negative electrode sheet is less than or equal to 250 μm, which is beneficial to improving the cycle capacity retention rate of the secondary battery; when h is 10 μm-200 μm, the cycle capacity retention rate of the secondary battery is further optimized.

[0175] As can be seen from the comparison between Example 9 and Example 1, the simultaneous inclusion of short-chain ethers and long-chain ethers in the electrolyte can further improve the cycle stability of the battery.

[0176] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A sodium secondary battery, characterized in that, The sodium secondary battery includes a negative electrodeless battery or a sodium metal battery; The sodium secondary battery includes a positive electrode sheet, which includes a positive current collector and a positive electrode film layer disposed on at least one side of the positive current collector. The positive electrode film layer includes a positive electrode active material, which includes a polyanionic compound, a sodium-containing transition metal oxide, and their respective modified compounds. The unit area capacity M of the positive electrode sheet is less than or equal to 20 mAh / cm². 2 ; The sodium secondary battery includes an electrolyte, which comprises short-chain ethers and long-chain ethers. The short-chain ether includes ethylene glycol dimethyl ether, and the long-chain ether includes at least one component represented by Formula II. R1-(O-R3)nO-R2 Equation II; Wherein, R1 and R2 are each independently selected from alkyl groups having 1 to 6 carbon atoms in a straight or branched chain, R3 is selected from alkylene groups having 1 to 5 carbon atoms in a straight or branched chain, and n is an integer from 2 to 5; or, R1 and R2 are each independently selected from alkyl groups with 2 to 6 carbon atoms in a straight or branched chain, and R3 is selected from alkylene groups with 1 to 5 carbon atoms in a straight or branched chain, with n being 1. Based on the total mass of the electrolyte, the short-chain ether accounts for 4% to 50% of the mass. The sodium secondary battery has a discharge capacity within at least a 2V discharge range during the discharge process that accounts for more than 95% of the total discharge capacity of the sodium secondary battery. The test conditions for the discharge process of the sodium secondary battery are as follows: after being charged to 4.2V at a constant rate of 0.33C at 25°C, it is discharged to 1.5V at a constant rate of 0.33C.

2. The sodium secondary battery according to claim 1, characterized in that, The sodium secondary battery's discharge capacity within the 2V-1.5V voltage range during discharge is less than or equal to 5% of the total discharge capacity of the sodium secondary battery. The test conditions for the discharge process of the sodium secondary battery are: charging to 4.2V at a constant rate of 0.33C at 25℃, and then discharging to 1.5V at a constant rate of 0.33C.

3. The sodium secondary battery according to claim 1, characterized in that, The discharge capacity of the positive electrode sheet during the discharge process is greater than or equal to 90% of the total discharge capacity of the positive electrode sheet within at least a 2.2V discharge range. The discharge process of the positive electrode sheet is tested by a button charge test, under the following conditions: charged to 4.2V at a constant rate of 0.1C at 25℃, and then discharged to 1.5V at a constant rate of 0.1C.

4. The sodium secondary battery according to claim 1, characterized in that, The composition of the polyanionic compound is shown in Formula I. Na n M p (X a O b ) c Z w Formula I Wherein, M includes one or more of Ti, V, Cr, Mn, Fe, Co, Ni, Ca, Mg, Al, Nb, and Zr; X includes one or more of Si, S, P, As, B, Mo, W, and Ge; Z includes one or more of F, O, Cl, and OH; 2≤n≤7, 1≤p≤4, 1≤a≤2, 1≤b≤7, 1≤c≤5, and 0≤w≤3.

5. The sodium secondary battery according to claim 4, characterized in that, The positive electrode active material includes one or more of sodium iron pyrophosphate, sodium vanadium phosphate, sodium fluorovanadium phosphate, sodium iron sulfate, and their respective modified materials.

6. The sodium secondary battery according to claim 1, characterized in that, The sodium secondary battery includes a negative electrode plate. When the sodium secondary battery is fully charged, the average potential of the negative electrode plate is less than or equal to 0.2V. The fully charged state of the sodium secondary battery refers to a sodium secondary battery charged to 4.2V at a constant rate of 0.33C at 25°C, and / or; The difference between the upper and lower potential limits of the negative electrode is less than or equal to 0.5V.

7. The sodium secondary battery according to any one of claims 1 to 6, characterized in that, In the fully charged state of the sodium secondary battery, the thickness h of the film layer located on the negative electrode current collector in the negative electrode sheet is less than or equal to 250 μm; The sodium secondary battery in its fully charged state refers to a sodium secondary battery that has been charged to 4.2V at a constant rate of 0.33C at 25°C.

8. The sodium secondary battery according to claim 7, characterized in that, In the fully charged state of the sodium secondary battery, the thickness h of the film layer located on the negative electrode current collector in the negative electrode sheet is 10μm-200μm.

9. The sodium secondary battery according to any one of claims 1 to 6, characterized in that, The long-chain ethers include one or more combinations of the following: diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, pentaethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol diethyl ether, triethylene glycol diethyl ether, tetraethylene glycol diethyl ether, pentaethylene glycol diethyl ether, ethylene glycol dibutyl ether, and their polymers.

10. An electrical appliance, characterized in that, The sodium secondary battery includes any one of claims 1 to 9.

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