WO2025175709A1 - Sodium secondary battery and electric apparatus - Google Patents

Abstract

A sodium secondary battery and an electric apparatus. During a discharge process, the ratio of the discharge capacity of the sodium secondary battery within at least one 2 V discharge interval to the total discharge capacity of the sodium secondary battery is greater than or equal to 95%; and the test condition of the discharge process of the sodium secondary battery is: at 25ºC, charging to 4.2 V at a constant rate of 0.33C, and then discharging to 1.5 V at a constant rate of 0.33C.

Description

Sodium secondary battery and electrical device

Cross-references

This application refers to Chinese patent application No. 202410194424.X, filed on February 21, 2024, entitled “Sodium Secondary Battery and Electrical Device,” which is incorporated herein by reference in its entirety.

Technical Field

The present application relates to the technical field of secondary batteries, and in particular to a sodium secondary battery and an electrical device.

Background Art

In recent years, secondary batteries have been widely used in energy storage systems such as hydropower, thermal, wind, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and other fields. With the widespread use of secondary batteries, higher requirements have been placed on the balance between cost and performance.

Sodium secondary batteries (NSBs) utilize the intercalation and deintercalation of sodium ions between the positive and negative electrodes to achieve charge and discharge. Compared to lithium secondary batteries, sodium ions in these batteries are more abundant, more widely distributed, and less expensive, offering the potential to replace them. However, existing sodium secondary batteries often struggle to adapt to existing motors, hindering their market adoption and application.

Summary of the Invention

The present application is made in view of the above-mentioned problems, and its purpose is to provide a sodium secondary battery that can be effectively adapted to existing motors and facilitate the promotion and application of sodium secondary batteries.

A first aspect of the present application provides a sodium secondary battery, wherein during a discharge process, a ratio of a discharge capacity of the sodium secondary battery within at least a 2V discharge range to a total discharge capacity of the sodium secondary battery is greater than or equal to 95%. The discharge process of the sodium secondary battery is tested under the following conditions: 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.

The sodium secondary battery has a high discharge specific capacity within the voltage range that can be used by the motor, which can fully utilize the capacity level of the sodium secondary battery and reduce resource waste.

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%, and the test conditions of 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.

The lower voltage limit for lithium secondary batteries is generally 2.5V. Existing motors often have difficulty starting when the battery cell voltage is below 2V. To accommodate motors, the lowest usable voltage for sodium secondary batteries is generally set at 2V. Sodium secondary batteries with a discharge capacity within the 2V-1.5V voltage range of 5% or less of their total discharge capacity have high usable capacity, maximizing their capacity and improving their power efficiency.

In any embodiment, the sodium secondary battery includes a positive electrode plate, and the ratio of the discharge capacity of the positive electrode plate in at least a 2.2V discharge range during the discharge process to the total discharge capacity of the positive electrode plate is greater than or equal to 90%; the test of the discharge process of the positive electrode plate is a buckle test, and the conditions are: after charging to 4.2V at a constant rate of 0.1C at 25°C, it is discharged to 1.5V at a constant rate of 0.1C.

The positive electrode plate has a high discharge capacity ratio within a relatively narrow voltage range, so that the sodium secondary battery has a high discharge capacity within the voltage range that can be used by the motor, which can fully utilize the capacity level of the positive electrode plate and reduce resource waste.

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

In any embodiment, the composition of the polyanionic compound is as 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, and 2≤n≤7, 1≤p≤4, 1≤a≤2, 1≤b≤7, 1≤c≤5, and 0≤w≤3.

In any embodiment, the positive electrode active material includes one or more of sodium ferric pyrophosphate, sodium vanadium phosphate, sodium fluorovanadium phosphate, sodium ferric sulfate, and modified materials thereof.

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 in a relatively narrow voltage range, which is beneficial for the sodium secondary battery to release a high discharge capacity in the voltage range that can be used by the motor and improve the discharge efficiency of the battery.

In any embodiment, the capacity per unit area M of the positive electrode sheet is less than or equal to 20 mAh/cm ² .

The positive electrode sheet with a unit area capacity within the above range is beneficial to reducing the deposition of sodium elements at the negative electrode, especially in a negative electrode-free sodium battery, and can effectively control the thickness of the sodium metal layer deposited at the negative electrode, so that the sodium secondary battery can always maintain high cycle stability during long-term cycling.

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.

The negative electrode plate with an average potential within the above range enables the sodium secondary battery to have a relatively high lower limit voltage, thereby meeting the requirements of existing motors for the lower limit voltage of battery cells.

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

In any embodiment, when the sodium secondary battery is fully charged, the film layer h located on the negative electrode current collector in the negative electrode electrode sheet is less than or equal to 250 μm, and can be optionally 10 μm-200 μm; 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.

When the sodium secondary battery is fully charged, the thickness h of the film layer located on the negative electrode current collector in the negative electrode plate is within the above range, indicating that the sodium secondary battery has high cycle stability and the phenomenon of large-scale deposition of sodium in the positive electrode at the negative electrode will not occur during the cycle.

In any embodiment, the sodium secondary battery includes an electrolyte, the electrolyte includes a short-chain ether and a long-chain ether; 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)n-O-R2 formula II;

Wherein, R1 and R2 are each independently selected from a straight-chain or branched alkyl group with 1 to 6 carbon atoms, R3 is selected from a straight-chain or branched alkylene group with 1 to 5 carbon atoms, and n is an integer of 2 to 5; or, R1 and R2 are each independently selected from a straight-chain or branched alkyl group with 2 to 6 carbon atoms, R3 is selected from a straight-chain or branched alkylene group with 1 to 5 carbon atoms, and n is 1.

Ether solvents have good reduction stability and are not easily reduced by the sodium metal in the negative electrode, so they will not be continuously decomposed, which makes the battery have good cycle stability; at the same time, ether solvents have a stable solvation structure, which helps to form a thin and stable solid electrolyte membrane (SEI), which is beneficial to the battery. Improved cycling stability; furthermore, ether solvents have low freezing points and viscosities, making secondary batteries suitable for low-temperature environments. Ethylene glycol dimethyl ether has good solvating properties, providing a foundation for a certain number of sodium electrolyte salt solvents; while long-chain ethers can enhance the oxidation resistance of electrolytes, improving the interfacial stability between the electrode and the interface in sodium secondary batteries, especially sodium metal batteries and batteries without negative electrodes.

In any embodiment, the long-chain ether comprises one or a combination of two or more 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.

In any embodiment, the mass proportion of the short-chain ether is 4% to 50% based on the total mass of the electrolyte.

In any embodiment, the sodium secondary battery includes a negative electrode-less battery or a sodium metal battery.

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

Sodium metal batteries are secondary batteries made by pre-depositing sodium metal or its alloys on the negative electrode during the battery manufacturing process. Using sodium metal as the negative electrode active material creates a low potential, which helps increase the usable capacity of sodium secondary batteries within their usable voltage range.

A second aspect of the present application further provides an electrical device comprising the sodium secondary battery of the first aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of a discharge curve of a sodium secondary battery according to an embodiment of the present application;

FIG2 is a schematic diagram of a secondary battery according to an embodiment of the present application;

FIG3 is an exploded view of the secondary battery according to one embodiment of the present application shown in FIG2 ;

FIG4 is a schematic diagram of a battery module according to an embodiment of the present application;

FIG5 is a schematic diagram of a battery pack according to an embodiment of the present application;

FIG6 is an exploded view of the battery pack shown in FIG5 according to an embodiment of the present application;

FIG. 7 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

Description of reference numerals:

1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 secondary battery; 51 shell; 52 electrode assembly; 53 cover plate.

DETAILED DESCRIPTION

Below, the embodiments of the sodium secondary battery and the electrical device of the present application are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there may be cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

" range " disclosed in the application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " a-b " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

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

Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

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

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

The common design in the prior art is to arrange more than 100 battery cells in series and in parallel to meet the motor's drive voltage requirements. The voltage difference between the motor's drive voltage and the minimum starting voltage is often 200V. In order to meet the requirements of both the motor's drive voltage and the minimum starting voltage, the battery's charge and discharge voltage range is generally required to be within 2V. The voltage difference between the fully charged and fully discharged voltages of each battery cell in a lithium secondary battery is between 1.1V and 1.8V, which can meet both the motor's drive voltage and the minimum starting voltage requirements. However, the voltage difference between the fully charged and fully discharged voltages of sodium secondary batteries in the prior art is generally greater than 2V, making it impossible for sodium secondary batteries to fully utilize their capacity within the 2V range required by the motor. Taking the layered oxide-hard carbon sodium secondary battery system as an example, only 88% of the capacity can be utilized within the 2V voltage range, resulting in a waste of resources.

[Sodium secondary battery]

Based on this, the present application proposes a sodium secondary battery, wherein the ratio of the discharge capacity of the sodium secondary battery in at least a 2V discharge range during the discharge process to the total discharge capacity of the sodium secondary battery is greater than or equal to 95%. The test conditions of the discharge process of the sodium secondary battery are: after charging 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.

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

The fully discharged state refers to a battery (including full batteries and button batteries) discharged to 1.5V, which corresponds to 0% SOC.

In the present application, the ratio of the discharge capacity of the sodium secondary battery within at least a 2V discharge interval during the discharge process to the total discharge capacity of the sodium secondary battery can be tested using methods known in the art. As an example, the test is performed using the discharge curve of the sodium secondary battery. For example, a VMP3 electrochemical workstation is used to measure the discharge curve of the sodium secondary battery. After the sodium secondary battery is 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; the state of charge (SOC) of the sodium secondary battery at different voltages during the discharge process is measured. The capacity of the sodium secondary battery at different voltages is divided by the total capacity released by the sodium secondary battery from a fully charged state to a fully discharged state as the state of charge of the sodium secondary battery. Figure 1 is a discharge curve of a sodium secondary battery according to an embodiment of the present application. When the voltage is aV, the state of charge of the sodium secondary battery is S1. When the voltage is (a-2)V or 1.5V, whichever is higher, the state of charge of the sodium secondary battery is S2. S1-S2 is used as the ratio of the discharge capacity of the sodium secondary battery within the 2V discharge range to the total discharge capacity of the sodium secondary battery.

In some embodiments, the ratio of the discharge capacity of the sodium secondary battery in at least one 2V discharge interval during the discharge process to the total discharge capacity of the sodium secondary battery is 95%, 96%, 97%, 98%, 99%, 100% or any value therebetween.

The sodium secondary battery has a high discharge specific capacity within the voltage range that can be used by the motor, which can fully utilize the capacity level of the sodium secondary battery and reduce resource waste.

In some embodiments, a ratio of the discharge capacity of the sodium secondary battery in a voltage range of 2V-1.5V during discharge to the total discharge capacity of the sodium secondary battery is less than or equal to 5%, and the test conditions of 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.

The ratio of the discharge capacity of a sodium secondary battery within the voltage range of 2V-1.5V during discharge to the total discharge capacity of the sodium secondary battery can be measured by referring to the discharge curve described above. The ratio of the discharge capacity of the sodium secondary battery within the voltage range of 2V-1.5V during discharge to the total discharge capacity of the sodium secondary battery can be calculated by subtracting the state of charge of the sodium secondary battery at 1.5V from the state of charge of the sodium secondary battery at 2V in the discharge curve.

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 therebetween.

The lower voltage limit for lithium secondary batteries is generally 2.5V. Existing motors often have difficulty starting when the battery cell voltage is below 2V. To accommodate motors, the lowest usable voltage for sodium secondary batteries is generally set at 2V. Sodium secondary batteries with a discharge capacity within the 2V-1.5V voltage range of 5% or less of their total discharge capacity have high usable capacity, effectively utilizing their full capacity and reducing resource waste.

In some embodiments, the sodium secondary battery includes a positive electrode plate, and the discharge capacity of the positive electrode plate in at least one 2.2V discharge range during the discharge process accounts for a ratio of greater than or equal to 90% of the total discharge capacity of the positive electrode plate; the discharge process test of the positive electrode plate is a buckle test, and the conditions are: after charging to 4.2V at a constant rate of 0.1C at 25°C, it is discharged to 1.5V at a constant rate of 0.1C.

In the present application, the ratio of the discharge capacity of the positive electrode sheet within at least a 2.2V discharge range during the discharge process to the total discharge capacity of the positive electrode sheet can be tested using methods known in the art. As an example, the positive electrode sheet of the sodium secondary battery is assembled with sodium metal into a button battery, and the discharge curve of the button battery is used for testing. The positive electrode sheet in the sodium secondary battery is pressed and made into a circular electrode sheet, and then a small circular sodium sheet is used as the counter electrode, a polypropylene isolation membrane is used, and an electrolyte is injected. The electrolyte includes 1 mol/L sodium hexafluorophosphate, and the solvent in the electrolyte is ethylene glycol dimethyl ether (DME) and diethylene glycol diethyl ether (DEE), and a button battery is assembled. At 25°C and normal pressure, the button battery 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 to obtain the discharge curve of the button battery. The state of charge of the button battery is determined by dividing the capacity of the button battery at different voltages by the total discharge capacity from the fully charged state to the fully discharged state. The state of charge of the button battery at different voltages during the discharge process is measured, and the difference in the state of charge 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.

In some embodiments, the ratio of the discharge capacity of the positive electrode sheet in at least one section of the 2.2V range during the discharge process to the total discharge capacity of the positive electrode sheet is 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, 100% or any value therebetween.

The positive electrode plate has a high discharge capacity ratio within a relatively narrow voltage range, so that the sodium secondary battery has a high discharge capacity within the voltage range that can be used by the motor, which can fully utilize the capacity level of the positive electrode plate and reduce resource waste.

In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one side of the positive electrode current collector, wherein the positive electrode film layer includes a positive electrode active material.

In some embodiments, the positive electrode active material may be a positive electrode active material for a battery known in the art. As an example, the positive electrode active material may include at least one of the following materials: a Prussian blue analogue, a polyanionic compound, a sodium-containing transition metal oxide, and modified compounds thereof. However, the present application is not limited to these materials; other conventional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. The Prussian blue analogue is Na _x P[R(CN) ₆ ] _δ ·zH ₂ O, wherein P and R are each independently selected from at least one transition metal element, 0<x≤2, 0<δ≤1, and 0≤z≤10.

In some embodiments, the positive electrode active material includes a polyanionic compound, a sodium-containing transition metal oxide, and modified compounds thereof.

In some embodiments, the transition metal in the sodium transition metal oxide may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. For example, the sodium transition metal oxide is Na _x MO ₂ , where M is one or more of Ti, V, Mn, Co, Ni, Fe, Cr, and Cu, and 0<x≤1.

In some embodiments, the composition of the polyanionic compound is as 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, and 2≤n≤7, 1≤p≤4, 1≤a≤2, 1≤b≤7, 1≤c≤5, and 0≤w≤3.

In some embodiments, the positive electrode active material includes one or more of sodium ferric pyrophosphate, sodium vanadium phosphate, sodium fluorovanadium phosphate, sodium ferric sulfate, and modified materials thereof.

In some embodiments, the modified material includes a doped modified material and/or a coated modified material.

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 in a relatively narrow voltage range, which is beneficial for the sodium secondary battery to release a high discharge capacity in the voltage range that can be used by the motor and reduce resource waste.

In some embodiments, the capacity per unit area of the positive electrode sheet is less than or equal to 20 mAh/cm ² .

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 is punched into small discs, assembled with sodium metal into a button cell, and a discharge curve test is performed on the button cell at a discharge rate of 0.1C. The specific preparation and testing methods of the button cell can be referred to as described above. The capacity per unit area of the positive electrode sheet is calculated as the discharge current of the button cell from a fully charged state to a fully discharged state multiplied by the discharge time divided 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 , 4 mAh/ ^cm2 , 6 mAh/ ^cm2 , 8 mAh/ ^cm2 , 10 mAh/ ^cm2 , 12 mAh/ ^cm2 , 14 mAh/ ^cm2 , 16 mAh/ ^cm2 , 18 mAh/ ^cm2 , 20 mAh/ ^cm2 , or any value therebetween.

The positive electrode sheet with a unit area capacity within the above range is beneficial to reducing the deposition of sodium elements at the negative electrode, especially in a negative electrode-free sodium battery, and can effectively control the thickness of the sodium metal layer deposited at the negative electrode, so that the sodium secondary battery can always maintain high cycle stability during long-term cycling.

In some embodiments, the sodium secondary battery includes a negative electrode plate, and when the sodium secondary battery is fully charged, the average potential of the negative electrode plate is less than or equal to 0.2V, and 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.

In this article, the average potential, upper limit potential, and lower limit potential of the negative electrode sheet can be obtained by combining the negative electrode sheet of a fully charged sodium secondary battery with sodium metal to form a button cell and performing a discharge curve test on the button cell. The preparation and testing methods of the button cell can be referred to as described above. The test range is 2-0V, and the voltage corresponding to 50% SOC, 99% SOC, and 1% SOC on the button cell discharge curve are used as the average potential, upper limit potential, and lower limit potential of the negative electrode sheet, respectively.

The negative electrode plate with an average potential within the above range enables the sodium secondary battery to have a relatively high lower limit voltage, thereby meeting the requirements of existing motors for the lower limit voltage of battery cells.

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

In some embodiments, the battery capacity loss value CB of the sodium secondary battery during discharge satisfies: CB≤1.5; wherein 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 in a fully discharged state, in units of mAh/cm ² ; J refers to the charging current density, which may be 0.5 mA/cm ² ; t is the time required for the voltage of the negative electrode sheet when the sodium secondary battery is in a fully discharged state to rise to 1 V at a current density of J, in units of hours; C2 is the maximum capacity per unit area of the positive electrode sheet, in units of mAh/cm ² .

Specifically, the above parameters can be tested through the following experiment. The positive electrode of a fully discharged sodium secondary battery is assembled with a sodium metal disc to form a button cell. The button cell preparation method is as described above. The button cell is discharged, and the discharge capacity of the button cell is divided by the area of the positive electrode in the button cell as C1. C1 can represent the actual discharge capacity of the positive electrode per unit area, which is the remaining capacity of the secondary battery in the fully discharged state. The fully discharged button cell is then charged, and the charge capacity of the button cell is divided by the area of the positive electrode in the button cell as C2. C2 can represent the maximum capacity that the positive electrode can reach. The negative electrode of a fully discharged sodium secondary battery is assembled with a sodium metal disc to form a button cell. The button cell preparation method is as described above. The button cell is charged at a current density of 0.5 mA/ ^cm2. The time corresponding to the voltage rising to 1 V is recorded as t hours. 0.5×t can represent the actual discharge capacity of the negative electrode.

In some embodiments, the battery capacity loss value CB of the sodium secondary battery during discharge satisfies 0.1, 0.3, 0.5, 0.7, 0.9, 1.1, 1.3, 1.5, or any value therebetween.

The negative electrode of a sodium secondary battery with a CB in the above range generally has a lower capacity, for example, a battery without a negative electrode. Compared with other sodium secondary batteries, the sodium secondary battery with a CB in the above range has a simpler process flow and a higher energy density.

In some embodiments, when the sodium secondary battery is fully charged, the thickness h of the film layer located on the negative electrode current collector in the negative electrode electrode sheet is less than or equal to 250 μm. In some embodiments, h is between 10 μm and 200 μm. A fully charged sodium secondary battery refers to a sodium secondary battery charged to 4.2 V at a constant rate of 0.33 C at 25°C.

In this article, the thickness h of the film layer located on the negative electrode current collector in the alkali metal layer of the negative electrode sheet of the sodium secondary battery in the fully charged state can be tested by methods known in the art. For example, at 25°C, the sodium secondary battery is charged to the upper limit voltage at a rate of 0.33C, the battery cell is disassembled to obtain the negative electrode sheet, and the thickness of the negative electrode sheet is measured using a micrometer (such as Mitutoyo293-100, with an accuracy of 0.1μm) in μm. In order to prevent metal adhesion on the test surface, the negative electrode sheet is fully charged before the test. Two layers of 0.1 μm thick acrylic sheet are evenly laid on the electrode sheet and fixed on all sides with dovetail clamps. The total thickness obtained by testing, minus the thickness of the negative electrode current collector and its surface functional coating, and then minus the thickness of the surface acrylic sheet, is the thickness h of the film layer located 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 electrode 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 therebetween.

When the sodium secondary battery is fully charged, the thickness h of the film layer located on the negative electrode current collector in the negative electrode plate is within the above range, indicating that the sodium secondary battery has high cycle stability and the phenomenon of large-scale deposition of sodium in the positive electrode at the negative electrode will not occur during the cycle.

In some embodiments, a sodium secondary battery includes an electrolyte, wherein the electrolyte includes a short-chain ether and a long-chain ether; 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)n-O-R2 formula II;

Wherein, R1 and R2 are each independently selected from a straight-chain or branched alkyl group with 1 to 6 carbon atoms, R3 is selected from a straight-chain or branched alkylene group with 1 to 5 carbon atoms, and n is an integer of 2 to 5; or, R1 and R2 are each independently selected from a straight-chain or branched alkyl group with 2 to 6 carbon atoms, R3 is selected from a straight-chain or branched alkylene group with 1 to 5 carbon atoms, and n is 1.

The term "alkyl" refers to a monovalent group having the general formula _{CnH2n +1} , which is derived from a saturated, unbranched or branched aliphatic hydrocarbon by removing one 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.

The term "alkylene" refers to an unbranched or branched divalent hydrocarbon group having 1 to 5 carbon atoms, including, for example, 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.

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.

In some embodiments, R1 and R2 each independently include one or more of ethyl, propyl, butyl, and pentyl, R3 includes 1,2-ethylene, and n is 1.

Ether solvents have good reduction stability and are not easily reduced by the sodium metal in the negative electrode, thus preventing continuous decomposition, which gives the battery good cycling stability. Ether solvents also have a stable solvation structure, which helps form a thin and stable solid electrolyte interface (SEI), which is beneficial for improving battery cycling stability. Furthermore, ether solvents have a low freezing point and viscosity, making secondary batteries also suitable for low-temperature environments. Ethylene glycol dimethyl ether has good solvation ability, providing a basis for a certain number of sodium electrolyte salt solvents. Long-chain ethers can enhance 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 negative electrodes.

In some embodiments, the long-chain ether comprises one or a combination of two or more 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.

The polymer may refer to a homopolymer or a copolymer, and is obtained by polymerizing monomers including the above-mentioned ethers and their derivatives.

In some embodiments, based on the total mass of the electrolyte, the mass proportion of the short-chain ether is 4% to 50%.

In some embodiments, based on the total mass of the electrolyte, the mass proportion of the short-chain ether is 4%, 8%, 12%, 16%, 20%, 24%, 28%, 32%, 36%, 40%, 45%, 50% or any value therebetween.

In some embodiments, the electrolyte includes an electrolyte salt, and the electrolyte salt is 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 is expressed as _{CbF2b +1} , and b is an integer within the range of 1 to 10, for example, an integer within the range of 1 to 3.

In some embodiments, the electrolyte salt is selected from one or more of NaPF ₆ , NaN(SO ₂ F) ₂ , NaN(CF ₃ SO ₂ ) ₂ , NaB(C ₂ O ₄ ) ₂ , and NaBF ₂ (C ₂ O ₄ ). In some embodiments, the electrolyte salt is selected from one or more of NaPF ₆ , NaN(SO ₂ RF) ₂ , and NaBF ₂ (C ₂ O ₄ ). In some embodiments, RF is -CF ₃ , -C ₂ F ₅ , or -CF ₂ CF ₂ CF ₃ .

In some embodiments, the sodium secondary battery comprises a negative electrode-less battery or a sodium metal battery.

Sodium metal batteries are secondary batteries made by pre-depositing sodium metal or its alloys on the negative electrode during the battery manufacturing process. Using sodium metal as the negative electrode active material creates a low potential, which helps increase the usable capacity of sodium secondary batteries within their usable voltage range.

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

In some embodiments, the negative electrode sheet includes a negative electrode current collector.

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 base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

In some embodiments, to improve battery performance, the negative electrode side of the negative electrode-free sodium secondary battery may be provided with some conventional substances that can be used as negative electrode active materials, such as carbonaceous materials, metal oxides, alloys, etc. Although these materials have a certain capacity, due to the small amount of these materials, they are not used as the main negative electrode active materials in the battery and are therefore not considered to form a negative electrode active material layer that plays a sodium intercalation role. The sodium secondary battery thus constructed can still be considered a negative electrode-free sodium secondary battery.

[Positive electrode]

The positive electrode sheet generally includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes a positive electrode active material.

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

In some embodiments, the positive electrode 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 material base and a metal layer formed on at least one surface of the polymer material base. 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 material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

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

In some embodiments, the positive electrode film layer may further include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[Isolation film]

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

In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven 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.

In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

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

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

In the present application, the shape of the sodium secondary battery includes but is not limited to cylindrical, square or any other shape. For example, FIG2 shows a sodium secondary battery 5 with a square structure as an example.

In some embodiments, referring to FIG3 , the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the separator can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the sodium secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

In some embodiments, sodium secondary batteries can be assembled into a battery module. The number of sodium secondary batteries contained in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

Figure 4 illustrates an exemplary battery module 4. Referring to Figure 4 , within battery module 4, multiple sodium secondary batteries 5 may be arranged sequentially along the length of battery module 4. Of course, any other arrangement is also possible. Furthermore, these multiple sodium secondary batteries 5 may be secured using fasteners.

In some embodiments, the battery module 4 may further include a housing having a housing space, and the plurality of sodium secondary batteries 5 may be housed in the housing space.

In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

Figures 5 and 6 illustrate an example battery pack 1. Referring to Figures 5 and 6 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be positioned over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

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

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

Figure 7 shows an example of an electric device. This device can be a pure electric vehicle, hybrid electric vehicle, or plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of sodium secondary batteries, a battery pack or battery module can be used.

Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a sodium secondary battery as a power source.

Example

Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

1. Preparation method

Example 1:

1) Preparation of positive electrode sheet

A uniformly dispersed positive electrode slurry is prepared by fully dissolving 90 wt% polyvinylidene fluoride binder in N-methylpyrrolidone, adding 5 wt% carbon black conductive agent and 5 wt% sodium ferric pyrophosphate, the positive electrode active material. The slurry is evenly coated on the surface of aluminum foil and then transferred to a vacuum drying oven for complete drying. The resulting electrode sheet is roll-pressed and then punched to produce the positive electrode sheet. The areal capacity of the positive electrode sheet is 2 mAh/ ^cm² , and the percentage of the positive electrode active material's discharge capacity in the 4.2V-2V range to the discharge capacity in the 4.2V-1.5V range is 95%.

2) Preparation of negative electrode sheet

A slurry containing conductive CNTs was applied to the surface of copper foil, which was then transferred to a vacuum drying oven for complete drying. The resulting negative electrode was then punched out. After full charge, the average potential of the negative electrode was 0.005V, with a potential difference of 0.1V between the upper and lower limits.

3) Preparation of electrolyte

A mixed solvent of ethylene glycol dimethyl ether (DME) and diethylene glycol diethyl ether (DEE) was prepared in a 1:1 volume ratio. In an argon atmosphere glove box ( _H₂O content <10 ppm, _O₂ content <1 ppm), sodium hexafluorophosphate (NaPF₆ ₎ was dissolved in the mixed solvent and stirred to obtain an electrolyte solution with a sodium salt concentration of 1 mol/L.

4) Isolation film

Polypropylene film is used as the isolation film.

5) Battery Preparation

The positive electrode sheet, the separator, and the negative electrode sheet are stacked in order, so that the separator is placed between the positive and negative electrode sheets to isolate the positive and negative electrode sheets. The bare battery cell is wound, the tabs are welded, and the bare battery cell is placed in an outer package. The above-prepared electrolyte is injected into the dried battery cell, and then the sodium secondary battery product of Example 1 is obtained after packaging, standing, formation, shaping, and capacity testing.

The preparation method of the sodium secondary battery of Example 2-3 is basically the same as the preparation method of Example 1, except that the type of the positive electrode active material is adjusted. Specific parameters are shown in Table 1.

Example 4

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:

In a glove box, a sodium metal block is rolled into a thin sheet, cut into the required size for the negative electrode, and placed snugly on the negative electrode current collector. The battery is packaged and tightly bonded together using force. The average potential of the negative electrode sheet is 0V, and the upper and lower potential differences are 0V.

Example 5

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.

The preparation methods of the sodium secondary batteries of Examples 6-8 are basically the same as the preparation method of Example 1, except that the capacity per unit area of the positive electrode sheet is adjusted. Specific parameters are shown in Table 1.

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

In an argon atmosphere glove box (H2O content <10ppm, O2 content <1ppm), sodium hexafluorophosphate NaPF6 was dissolved in an organic solvent, ethylene glycol dimethyl ether (DME), and stirred evenly to obtain an electrolyte with a sodium salt concentration of 1 mol/L.

Comparative Example 1

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:

The negative electrode active material, hard carbon, the conductive agent Super-P, and the binder sodium carboxymethyl cellulose (CMC-Na), were thoroughly stirred and 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 evenly coated on the negative electrode current collector copper foil. The copper foil was air-dried at room temperature and then transferred to a 120°C oven for 1 hour. The negative electrode sheet was then cold-pressed and slit. The hard carbon was purchased from Kuraray. The average potential of the negative electrode sheet was 0.28V, and the upper and lower limit potential difference was 1V.

Table 1

2. Battery performance test

1. Cycle capacity retention rate

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

2. Compatibility with lithium-ion motors

The starting voltage of the existing motor is 200V, and the number of battery cells it carries is 100. If the motor cannot start, it means it cannot be adapted, which is indicated by N; if the motor can start, it means it can be adapted, which is indicated by Y.

3. Analysis of test results of various embodiments and comparative examples

Batteries of various examples and comparative examples were prepared according to the above methods, and various performance parameters were measured. The results are shown in Tables 2 and 3.

Table 2

Table 3

As can be seen from Tables 2 and 3, the sodium secondary battery provided in the embodiments of the present application has a discharge capacity within at least a 2V discharge range during the discharge process, with a ratio of the discharge capacity to the total discharge capacity of the sodium secondary battery being greater than or equal to 95%. This enables the battery to be adapted to existing motors, thus facilitating the popularization and application of sodium secondary batteries.

From the comparison between Example 1 and Example 4, it can be seen that the secondary battery with a CB value less than or equal to 1.5 can further improve the cycle capacity retention rate of the secondary battery.

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

From the comparison between Example 9 and Example 1, it can be seen that the inclusion of both short-chain ether and long-chain ether in the electrolyte can further improve the cycle stability of the battery.

It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims (14)

A sodium secondary battery, wherein during a discharge process, a 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 is greater than or equal to 95%, and 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. The sodium secondary battery according to claim 1, wherein a 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 less than or equal to 5%, and the test conditions of 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. The sodium secondary battery according to claim 1 or 2, wherein the sodium secondary battery comprises a positive electrode plate, and the ratio of the discharge capacity of the positive electrode plate in at least a 2.2V discharge range during the discharge process to the total discharge capacity of the positive electrode plate is greater than or equal to 90%; the test of the discharge process of the positive electrode plate is a buckle test, under the conditions of: charging to 4.2V at a constant rate of 0.1C at 25°C, and then discharging to 1.5V at a constant rate of 0.1C. The sodium secondary battery according to claim 3, wherein the positive electrode plate comprises a positive electrode current collector and a positive electrode film layer provided on at least one side of the positive electrode current collector, the positive electrode film layer comprises a positive electrode active material, and the positive electrode active material comprises a polyanionic compound, a sodium-containing transition metal oxide, and modified compounds of each of these. The sodium secondary battery according to claim 4, wherein the composition of the polyanionic compound is as 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, and 2≤n≤7, 1≤p≤4, 1≤a≤2, 1≤b≤7, 1≤c≤5, and 0≤w≤3. The sodium secondary battery according to claim 4 or 5, wherein the positive electrode active material comprises one or more of sodium ferric pyrophosphate, sodium vanadium phosphate, sodium fluorovanadium phosphate, sodium ferric sulfate, and their respective modified materials. The sodium secondary battery according to any one of claims 1 to 6, wherein the capacity per unit area M of the positive electrode sheet is less than or equal to 20 mAh/cm ² . The sodium secondary battery according to any one of claims 1 to 7, wherein the sodium secondary battery comprises a negative electrode plate, and when the sodium secondary battery is fully charged, the flatness of the negative electrode plate is The average potential is less than or equal to 0.2V, and 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. The sodium secondary battery according to claim 7, wherein, when the sodium secondary battery is fully charged, the thickness h of the film layer located on the negative electrode current collector in the negative electrode plate is less than or equal to 250 μm; optionally, it is 10 μm-200 μm; 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. The sodium secondary battery according to any one of claims 1 to 9, wherein the sodium secondary battery comprises an electrolyte, the electrolyte comprises a short-chain ether and a long-chain ether; the short-chain ether comprises ethylene glycol dimethyl ether, and the long-chain ether comprises at least one component represented by formula II;
R1-(O-R3)nO-R2 formula II; wherein R1 and R2 are each independently selected from a linear or branched alkyl group having 1 to 6 carbon atoms, R3 is selected from a linear or branched alkylene group having 1 to 5 carbon atoms, and n is an integer of 2 to 5; or R1 and R2 are each independently selected from a linear or branched alkyl group having 2 to 6 carbon atoms, R3 is selected from a linear or branched alkylene group having 1 to 5 carbon atoms, and n is 1. The sodium secondary battery according to claim 10, wherein the long-chain ether comprises one or a combination of two or more 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. The sodium secondary battery according to claim 10 or 11, wherein the mass proportion of the short-chain ether is 4% to 50% based on the total mass of the electrolyte. The sodium secondary battery according to any one of claims 1 to 12, wherein the sodium secondary battery comprises a negative electrode-less battery or a sodium metal battery. An electrical device comprising the sodium secondary battery according to any one of claims 1 to 13.