Secondary battery, preparation method thereof and electric equipment

Abstract

The invention discloses a secondary battery, a preparation method thereof and electric equipment, the secondary battery comprises a sodium ion positive electrode active material and free sodium, and the free sodium comprises one or more of Na2CO3, NaHCO3 and Na < + >; in the free sodium, the content of Na2CO3 is 0.2 ppm to 0.38 ppm; and / or the content of NaHCO3 is 0.38 ppm to 0.53 ppm, and / or the content of NaHCO3 is 0.38 ppm to 0.53 ppm; and / or, the content of Na < + > is 0.20 ppm to 0.35 ppm. By regulating and controlling the content of free sodium in the positive electrode active material of the secondary battery, the alkalinity and hygroscopicity of the sodium ion positive electrode active material are reduced, the consistency of the product performance of the positive electrode plate is improved, and the flatulence condition of the secondary battery is reduced; the capacity attenuation of the secondary battery is slowed down; and the device performance of the secondary battery, such as the charge-discharge efficiency, the cycle service life and the safety, is improved.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to Chinese patent application 2023108705334 filed on July 14, 2023, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to the field of batteries, and in particular to a secondary battery and a preparation method thereof, and electrical equipment. Background Art

[0004] Sodium-ion secondary batteries have shown great development potential due to the rich sodium resources in the earth's crust and their widespread distribution around the world, as well as their excellent high and low temperature performance, high safety, and the availability of low-salt concentration electrolytes. The performance quality of the positive electrode active materials of sodium-ion secondary batteries can easily affect the device performance of sodium-ion secondary batteries.

[0005] The above statements are only used to provide background information related to the present application and do not necessarily constitute prior art. Summary of the invention

[0006] In view of the technical problems existing in the background technology, the present application provides a secondary battery and a preparation method thereof, and an electrical device, aiming to improve the device performance of the secondary battery.

[0007] In a first aspect, an embodiment of the present application provides a secondary battery, the secondary battery comprising a positive electrode sheet, a separator and a negative electrode sheet; the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector; the positive electrode active material layer comprises a sodium ion positive electrode active material and free sodium, and the free sodium comprises Na2CO3, NaHCO3, Na + One or more of; free sodium: Na2CO3 content of 0.2ppm ~ 0.38ppm; and / or, NaHCO3 content of 0.38ppm ~ 0.53ppm; and / or, Na + The content is 0.20ppm~0.35ppm.

[0008] In any embodiment of the present application, by regulating the content of free sodium in the positive electrode active material of the secondary battery, the residual alkali content on the surface of the sodium ion positive electrode active material is reduced, thereby reducing the alkalinity and hygroscopicity of the sodium ion positive electrode active material, thereby improving the product performance consistency of the positive electrode plate; reducing the side reactions between free sodium and the electrolyte and even the generation of gas, and reducing the flatulence of the secondary battery cells; slowing down the capacity decay of the secondary battery; and improving the device performance of the secondary battery, such as charge and discharge efficiency, cycle life, and safety.

[0009] In some embodiments, the total free sodium content is 0.5 ppm to 2.0 ppm.

[0010] In any embodiment of the present application, by regulating the total content of free sodium, the side reaction between free sodium and electrolyte and even the generation of gas can be reduced, thereby reducing the bloating of the secondary battery.

[0011] In some embodiments, the particle size of the sodium ion positive electrode active material is: DV 50 =2μm~5μm,DV 99 =7μm~20μm.

[0012] In any embodiment of the present application, when the particle size is controlled within the above range, parameters such as oxygen defects, specific surface area, and tap density of the sodium ion positive electrode active material can be better regulated.

[0013] In some embodiments, the sodium ion positive electrode active material has a chemical formula comprising Na x R y (PO4) z (P2O7) k , wherein 1≤x≤7, 1≤y≤4, 1≤z≤2, 1≤k≤4; and R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W and Pb.

[0014] In any embodiment of the present application, the device performance of the secondary battery, such as charge and discharge efficiency, cycle life and safety, is improved by regulating the content of free sodium in the sodium ion positive electrode active material of the provided chemical formula.

[0015] In a second aspect, an embodiment of the present application provides a method for preparing any secondary battery provided in the first aspect, comprising:

[0016] Mixing the sodium ion positive electrode active material to be treated and an organic solvent to obtain a premix;

[0017] Stirring the premix to obtain a mixed material;

[0018] The mixed material is subjected to solid-liquid separation treatment to obtain a treated sodium ion positive electrode active material, wherein the treated sodium ion positive electrode active material comprises free sodium, and the free sodium comprises Na2CO3, NaHCO3, Na + One or more of; free sodium: Na2CO3 content of 0.2ppm ~ 0.38ppm; and / or, NaHCO3 content of 0.38ppm ~ 0.53ppm; and / or, Na + The content is 0.20ppm~0.35ppm;

[0019] The treated sodium ion positive electrode active material forms a positive electrode active material layer on at least one surface of a positive electrode current collector, and the positive electrode current collector and the positive electrode active material layer disposed on at least one surface of the positive electrode current collector form a positive electrode sheet;

[0020] Provide separator and negative electrode;

[0021] The positive electrode sheet, the separator and the negative electrode sheet are assembled to form a secondary battery.

[0022] In any embodiment of the present application, the sodium ion positive electrode active material to be treated is washed with an organic solvent to reduce the free sodium on the surface of the sodium ion positive electrode active material, reduce the alkalinity and hygroscopicity of the sodium ion positive electrode active material, improve the uniformity of its dispersion in the slurry, and improve the consistency of the product performance of the positive electrode sheet; reduce the side reaction of free sodium with the electrolyte to produce gas, reduce the flatulence of the secondary battery, and slow down the capacity decay of the secondary battery; improve the device performance of the secondary battery, such as charge and discharge efficiency, cycle life and safety.

[0023] In some embodiments, the mass ratio of the sodium ion positive electrode active material to be treated to the organic solvent is (10-200):200.

[0024] In any embodiment of the present application, the organic solvent needs to wet the sodium ion positive electrode active material to be treated and form a stirring state on the one hand, and on the other hand, it is necessary to control the amount of organic solvent added. Too much organic solvent is added, but it is wasted. Within the above range, a smaller mass ratio allows the sodium ion positive electrode active material to be more fully in contact with the organic solvent, which can increase the degree of reduction of free sodium on the surface of the sodium ion positive electrode active material. A larger mass ratio can improve the utilization rate of the organic solvent, reduce the amount of organic solvent used, and reduce the impact of the organic solvent on the environment.

[0025] In some embodiments, the mass ratio of the sodium ion positive electrode active material to be treated to the organic solvent is (50-110):200.

[0026] In any embodiment of the present application, within the above range, it is beneficial to further make the sodium ion positive electrode active material more fully contact with the organic solvent, which can increase the degree of reduction of free sodium on the surface of the sodium ion positive electrode active material, and improve the utilization rate of the organic solvent, reduce the amount of organic solvent used, and reduce the impact of the organic solvent on the environment.

[0027] In some embodiments, the organic solvent includes: one or more of N,N-dimethylformamide, anhydrous ethanol, N-methylpyrrolidone, acetonitrile, dimethyl carbonate, diethyl carbonate, diisopropyl carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, and 1,3-dioxolane.

[0028] In any embodiment of the present application, the organic solvent may be any one of the above-mentioned solvents, or may be a combination of the above-mentioned solvents. The selection of the above-mentioned solvents can make it difficult for the organic solvent to react with the sodium ion positive electrode active material during the washing process of the sodium ion positive electrode active material, and it is not easy to destroy the sodium ion positive electrode active material. Among them, the multiple combinations of different solvents refer to only physical mixing between different solvents. Within the above range, the use of a single organic solvent is conducive to the recovery of the organic solvent after use, simplifies the recovery procedure, and reduces the difficulty of recovery. The use of multiple organic solvents is conducive to utilizing the characteristics of multiple organic solvents, learning from each other's strengths and weaknesses, promoting the dissolution of free alkali, and improving the washing effect.

[0029] In some embodiments, the water content in the organic solvent is less than 0.3%.

[0030] Since the sodium ion positive electrode active material is prone to hydrolysis, any embodiment of the present application controls the water content of the organic solvent to reduce possible side reactions of the sodium ion positive electrode active material.

[0031] In some embodiments, there are at least two organic solvents, and the mass ratio / volume ratio of the two organic solvents is (0-10):(0-10).

[0032] In any embodiment of the present application, a plurality of organic solvents are used to wash the sodium ion positive electrode active material. The characteristics of the plurality of organic solvents can be utilized to complement each other, promote the dissolution of free sodium, and improve the washing effect.

[0033] In some embodiments, the stirring process includes: stirring the premix at a rotation speed of 10 r / min to 100 r / min for 0.25 h to 5 h.

[0034] In any embodiment of the present application, stirring treatment is carried out within the above range, the sodium ion positive electrode active material and the organic solvent can be mixed more evenly, the contact probability between the surface of the sodium ion positive electrode active material and the organic solvent is greatly improved, and a good washing effect is achieved, so that the free sodium on the surface of the sodium ion positive electrode active material is reduced to a lower level.

[0035] In some embodiments, the stirring process includes: stirring the premix for 0.5 h to 3 h at a rotation speed of 15 r / min to 50 r / min.

[0036] In any embodiment of the present application, the stirring treatment is performed within the above range, so that the free sodium on the surface of the sodium ion positive electrode active material is further reduced to a lower level.

[0037] In some embodiments, the stirring treatment is performed under normal temperature and pressure and an ambient humidity of ≤10%.

[0038] Since sodium ion positive electrode active materials are prone to hydrolysis, it is necessary to control the environmental humidity. When the environmental humidity is ≤10%, the moisture in the environment is not easy to affect the sodium ion positive electrode active materials, reducing the impact of the external environment on the performance stability of the sodium ion positive electrode active materials. The stirring process can be achieved at room temperature and pressure, which is conducive to reducing the difficulty of the stirring process.

[0039] In some embodiments, the solid-liquid separation process includes a filtration process; the filtration process includes:

[0040] Perform a suction filtration treatment on the mixed material;

[0041] Adding new organic solvent to the filtration product after the first filtration treatment, and performing a second filtration treatment;

[0042] The mass ratio of the new organic solvent to the sodium ion positive electrode active material to be treated is (1-2):1.

[0043] In any embodiment of the present application, two filtration treatments are performed within the above range to control the amount of washing solvent, and the contact probability between the surface of the sodium ion positive electrode active material and the organic solvent is greatly improved, which has a good washing effect, so that the free sodium on the surface of the sodium ion positive electrode active material is reduced to a lower level.

[0044] In some embodiments, the solid-liquid separation process comprises a spray drying process.

[0045] In any embodiment of the present application, spray drying is another solid-liquid separation method, in which the mixed material liquid is atomized, and the atomized material is in a dry or semi-dry state after air drying, and a dry material is formed through the discharge end.

[0046] In some embodiments, during the spray drying process, the inlet air temperature is 190°C to 230°C, and the outlet air temperature is 85°C to 95°C.

[0047] When the air inlet temperature is within the above range, the atomized material is in a dry or semi-dry state, the solid separation effect is good, and it is not easy to melt or decompose. When the outlet temperature is within the above range, it is beneficial for the material at the discharge end to use the residual heat to form and dry the material at a lower temperature, and it is not easy to affect the performance of the material. In some embodiments, the solid-liquid separation treatment of the mixture also includes drying the material after the solid-liquid separation treatment; the drying treatment is: drying at a temperature of 80°C to 200°C for 4h to 24h.

[0048] In any embodiment of the present application, by drying the sodium ion positive electrode active material gently at a suitable temperature, the occurrence of internal defects or peeling of the sodium ion positive electrode active material caused by excessively rapid temperature increase is reduced, thereby improving the quality of the obtained sodium ion positive electrode active material.

[0049] In some embodiments, the drying process is: performing a vacuum drying process at a temperature of 100° C. to 120° C. for 6 h to 12 h.

[0050] In any embodiment of the present application, by drying the sodium ion positive electrode active material under the above conditions, the quality of the obtained sodium ion positive electrode active material is further improved.

[0051] In some embodiments, the method further includes collecting used organic solvents and recycling the collected organic solvents.

[0052] In any embodiment of the present application, by collecting used organic solvents and recycling the collected organic solvents, it is beneficial to fully utilize solvent resources, save resources, protect the environment, and meet the large-scale production of enterprises.

[0053] In some embodiments, the used organic solvent is collected by a condensation process.

[0054] In any embodiment of the present application, the used organic solvent is collected through condensation treatment, which is beneficial to fully utilize solvent resources, save resources, protect the environment, and meet the large-scale production of enterprises.

[0055] In some embodiments, the collected organic solvent is subjected to suction filtration or distillation purification.

[0056] In any embodiment of the present application, the used organic solvent is collected by suction filtration or distillation purification, which is conducive to making full use of solvent resources, saving resources, protecting the environment, and meeting the large-scale production of enterprises. In the third aspect, the embodiments of the present application provide an electrical device, including a secondary battery as provided in the first aspect or a secondary battery prepared by the preparation method of the secondary battery provided in the second aspect. The electrical device adopts the provided secondary battery, which has at least the same advantages as the secondary battery and can improve the reliability of the electrical device. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0058] Figure 1 is a schematic diagram of an exploded structure of a secondary battery according to one or more embodiments;

[0059] Figure 2 is a schematic diagram of an exploded structure of a battery cell according to one or more embodiments;

[0060] Figure 3 is a schematic diagram of the structure of a vehicle according to one or more embodiments.

[0061] Description of Figure Numbers:

[0062] 1000 - vehicle, 100 - secondary battery, 200 - controller, 300 - motor, 10 - housing, 20 - battery cell, 11 - first part, 12 - second part, 21 - end cover, 22 - housing, 23 - electrode assembly, 21a - electrode terminal, 23a - pole ear. DETAILED DESCRIPTION

[0063] In order to make the purpose, technical solution and effect of the present application clearer and more specific, the following will describe the embodiments of the technical solution of the present application in detail with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application, and are therefore only used as examples, and cannot be used to limit the scope of protection of the present application.

[0064] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by technicians in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" in the specification and claims of this application and the above-mentioned figure descriptions and any variations thereof are intended to cover non-exclusive inclusions.

[0065] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two), and similarly, "multiple groups" refers to more than two (including two groups), and "multiple pieces" refers to more than two (including two pieces), unless otherwise clearly and specifically defined.

[0066] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0067] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of the associated objects, indicating that there may be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0068] Amounts, ratios and other numerical values ​​are presented herein in a range format. It should be understood that such a range format is for convenience and brevity and should be flexibly understood to include not only the values ​​explicitly specified as range limits, but also all individual values ​​or sub-ranges encompassed within the range, as if each value and sub-range were explicitly specified.

[0069] Lithium-ion secondary batteries have the advantages of high voltage, light weight, long cycle life, no memory effect, and good safety, and have been widely used. With the rapid growth of demand for lithium-ion secondary batteries, the supply of lithium resources has become increasingly tight. In this context, sodium-ion secondary batteries, which have a similar working principle to lithium-ion secondary batteries, have shown great development potential due to the rich reserves of sodium resources in the earth's crust and their widespread distribution around the world, as well as excellent high and low temperature performance, high safety, and the availability of low-salt concentration electrolytes.

[0070] Sodium ion secondary batteries include positive electrode sheets, negative electrode sheets, separators, electrolytes, and the like. During the charge and discharge process of sodium ion secondary batteries, sodium ions are embedded and removed back and forth between the positive electrode sheets and the negative electrode sheets. The separator is arranged between the positive electrode sheets and the negative electrode sheets to play an isolating role. The electrolyte plays the role of conducting ions between the positive electrode sheets and the negative electrode sheets. Among them, the sodium ion positive electrode active material is the key to restricting the overall performance of sodium ion secondary batteries. During the production process of sodium ion positive electrode active materials, it is easy for impurity sodium to remain in the materials due to the process, formula ratio, and other reasons. In addition, sodium in the bulk phase of sodium ion positive electrode active materials is relatively easy to be removed, which is also easy to cause residual free sodium on the surface of sodium ion positive electrode active materials. The free sodium in the sodium ion positive electrode active material is relatively large, which increases its alkalinity and increases the hygroscopicity of the sodium ion positive electrode active material, causing the sodium ion positive electrode active material to be easily affected by moisture and deteriorate, affecting the consistency of the function of the positive electrode active material layer formed by the slurry coating, and thus affecting the charge and discharge efficiency and cycle life of the secondary battery. In addition, at least part of the free sodium is prone to react with the electrolyte to produce gas, which is one of the reasons why secondary batteries produce flatulence and cause safety hazards. In addition, free sodium is prone to cause the sodium ion positive electrode active material to have a higher alkalinity, and it is easy to react with the electrolyte, which increases the risk of battery swelling and accelerates the battery's capacity decay. For the above reasons, it is necessary to reduce the content of residual sodium in the sodium ion positive electrode active material to improve the device performance of the battery, such as charge and discharge efficiency, cycle life and safety.

[0071] To solve the above technical problems, an embodiment of the present application provides a secondary battery, which improves the charge and discharge specific capacity, cycle life and safety of the secondary battery by reducing the residual free sodium on the surface of the sodium ion positive electrode active material.

[0072] The technical solution described in the embodiments of this application is applicable to secondary batteries and their preparation methods, and electrical equipment. The secondary batteries and their preparation methods, and electrical equipment disclosed in this application can be used in the field of sodium ion secondary batteries, and can also be used in sodium metal secondary batteries, sodium polymer secondary batteries, or sodium ion polymer secondary batteries, and can be specifically configured as needed.

[0073] The present application is described in detail below with reference to the accompanying drawings and embodiments.

[0074] The embodiment of the present application provides a secondary battery, which includes a positive electrode sheet, a separator and a negative electrode sheet; the positive electrode sheet includes a positive electrode collector and a positive electrode active material layer arranged on at least one surface of the positive electrode collector; the positive electrode active material layer includes a sodium ion positive electrode active material and free sodium, and the free sodium includes Na2CO3, NaHCO3, Na +One or more of; free sodium: Na2CO3 content of 0.2ppm ~ 0.38ppm; and / or, NaHCO3 content of 0.38ppm ~ 0.53ppm; and / or, Na + The content is 0.20ppm~0.35ppm.

[0075] In the embodiments of the present application, the sodium ion positive electrode active material is generally composed of active compounds of sodium. In some embodiments, the sodium ion positive electrode active material can be one or more of sodium-containing transition metal oxides, sulfate compounds, fluorides, Prussian blue analogs, and phosphate compounds. During the charging process, sodium ions are released from the crystal lattice of the sodium ion positive electrode active material, and are embedded in the crystal lattice of the negative electrode active material through the electrolyte, making the negative electrode rich in sodium and the positive electrode poor in sodium. At the same time, electrons flow from the negative electrode to the positive electrode through an external circuit; the discharge process is the opposite of the charging process. Free sodium refers to sodium compounds and / or ionization products attached to the surface of the sodium ion positive electrode active material. In some embodiments, free sodium includes Na2CO3, NaHCO3, Na + One or more of the following. PPM refers to parts per million by mass, 1 ppm = 0.0001%.

[0076] The test method for the content of free sodium in the sodium ion positive electrode active material is: refer to GB / T9725-2007 "General Rules for Potentiometric Titration of Chemical Reagents" for the test of free lithium, test the free sodium of the sodium ion positive electrode active material, and the test results are converted according to the ratio of the molecular weight of free sodium and free lithium. For example, the molecular weight of NaHCO3 is 84, the molecular weight of LiHCO3 is 68, and the free lithium content obtained by the test result is 0.1ppm. According to the ratio of the molecular weight of NaHCO3 to LiHCO3 is 1.235, the free sodium content of the sodium ion positive electrode active material is converted to 0.124ppm. The method for testing the free lithium content of lithium ion positive electrode active materials in GB / T9725-2007 "General Rules for Potentiometric Titration of Chemical Reagents" is: take samples and prepare test solutions according to the provisions of the product standard. Insert the specified electrode, start the electromagnetic stirrer, and titrate with the specified standard titration solution. Add about 90% of the required titration volume of standard titration solution from the burette and measure the potential or pH value of the solution. Measure the potential or pH value every time 1 mL or an appropriate amount of standard titration solution is added. Before and after the stoichiometric point, measure once every time 0.1 mL of standard titration solution is added. Continue titrating until the potential or pH value does not change much. Record the reading of the burette and the measured potential or pH value after each addition of the standard titration solution, and use the graphical method or the second-order derivative method to determine the titration endpoint.

[0077] In some embodiments, the content of Na2CO3 in free sodium is 0.2ppm to 0.38ppm. The content of Na2CO3 in free sodium can be 0.2ppm, 0.21ppm, 0.22ppm, 0.23ppm, 0.24ppm, 0.25ppm, 0.26ppm, 0.27ppm, 0.28ppm, 0.29ppm, 0.30ppm, 0.31ppm, 0.32ppm, 0.33ppm, 0.34ppm, 0.35ppm, 0.36ppm, 0.37ppm, 0.38ppm, etc., or a range consisting of any two of the above values, for example, 0.2ppm to 0.25ppm, 0.23ppm to 0.28ppm, 0.25ppm to 0.30ppm, 0.28ppm to 0.33ppm, 0.30ppm to 0.35ppm, 0.33ppm to 0.38ppm, etc.

[0078] In some embodiments, the content of NaHCO3 is 0.38ppm to 0.53ppm. The content of NaHCO3 can be 0.38ppm, 0.39ppm, 0.40ppm, 0.41ppm, 0.42ppm, 0.43ppm, 0.44ppm, 0.45ppm, 0.46ppm, 0.47ppm, 0.48ppm, 0.49ppm, 0.50ppm, 0.51ppm, 0.52ppm, 0.53ppm, etc., or a range consisting of any two of the above values, for example, 0.38ppm to 0.43ppm, 0.40ppm to 0.45ppm, 0.43ppm to 0.48ppm, 0.45ppm to 0.50ppm, 0.48ppm to 0.53ppm, etc.

[0079] In some embodiments, Na + The content of Na is 0.20ppm~0.35ppm. + The content can be 0.2ppm, 0.21ppm, 0.22ppm, 0.23ppm, 0.24ppm, 0.25ppm, 0.26ppm, 0.27ppm, 0.28ppm, 0.29ppm, 0.30ppm, 0.31ppm, 0.32ppm, 0.33ppm, 0.34ppm, 0.35ppm, etc., or a range consisting of any two of the above values, for example, it can be 0.2ppm~0.25ppm, 0.23ppm~0.28ppm, 0.25ppm~0.30ppm, 0.28ppm~0.33ppm, 0.30ppm~0.35ppm, etc.

[0080] In any embodiment of the present application, by regulating the content of free sodium in the sodium ion positive electrode active material, the content of residual alkali on the surface of the sodium ion positive electrode active material is reduced, thereby reducing the alkalinity and hygroscopicity of the sodium ion positive electrode active material, and improving the consistency of the product performance of the positive electrode plate; reducing the side reaction of free sodium with the electrolyte and even the generation of gas, and reducing the flatulence of the secondary battery; slowing down the capacity decay of the secondary battery; and improving the device performance of the secondary battery, such as charge and discharge efficiency, cycle life, and safety.

[0081] In some embodiments, the total free sodium content is 0.5 ppm to 2.0 ppm.

[0082] In any embodiment, the total content of free sodium in the sodium ion positive electrode active material can be 0.5ppm, 0.6ppm, 0.7ppm, 0.8ppm, 0.9ppm, 1.0ppm, 1.1ppm, 1.2ppm, 1.3ppm, 1.4ppm, 1.5ppm, 1.6ppm, 1.7ppm, 1.8ppm, 1.9ppm, 2.0ppm, or a range consisting of any two of the above values, for example, the content of free sodium in the sodium ion positive electrode active material can be 0.5ppm to 0.8ppm, 0.8ppm to 1.5ppm or 1.5ppm to 2.0ppm or the like. In any embodiment of the present application, by regulating the total content of free sodium, the side reaction of free sodium with the electrolyte or even the generation of gas can be reduced, thereby reducing the bloating of the secondary battery.

[0083] In some embodiments, the particle size of the sodium ion positive electrode active material is: DV 50 =2μm~5μm,DV 99 =7μm~20μm.

[0084] In the examples of the present application, particle size refers to the average diameter of the sodium ion positive electrode active material particles. 50 DV refers to the particle size at which the volume accumulation reaches 50% in the volume-based particle size distribution, starting from the smallest particle size. 99 It refers to the particle size that reaches 99% of the volume accumulation from the smallest particle size in the volume-based particle size distribution. The particle size test refers to the laser particle size analyzer diffraction method of GB / T19077-2016. Take a clean beaker, add an appropriate amount of the sample to be tested, add a surfactant and then a dispersant. After fully dispersed, use a laser particle size analyzer to measure the particle size distribution characteristics (shading degree: 8-12%).

[0085] In any embodiment of the present application, when the particle size is controlled within this range, parameters such as oxygen defects, specific surface area, and tap density of the sodium ion positive electrode active material can be better regulated.

[0086] In some embodiments, the sodium ion positive electrode active material has a chemical formula comprising Na x R y (PO4) z (P2O7) k , wherein 1≤x≤7, 1≤y≤4, 1≤z≤2, 1≤k≤4; R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W and Pb. For example, the sodium ion positive electrode active material may be NFPP (Na4Fe3(PO4)2P2O7).

[0087] In any embodiment of the present application, the device performance of the secondary battery, such as charge and discharge efficiency, cycle life and safety, is improved by regulating the content of free sodium in the sodium ion positive electrode active material of the provided chemical formula.

[0088] An embodiment of the present application provides a method for preparing any secondary battery provided in the first aspect, comprising:

[0089] Mixing the sodium ion positive electrode active material to be treated and an organic solvent to obtain a premix;

[0090] Stirring the premix to obtain a mixed material;

[0091] The mixed material is subjected to solid-liquid separation treatment to obtain a treated sodium ion positive electrode active material, wherein the treated sodium ion positive electrode active material comprises free sodium, and the free sodium comprises Na2CO3, NaHCO3, Na + One or more of; free sodium: Na2CO3 content of 0.2ppm ~ 0.38ppm; and / or, NaHCO3 content of 0.38ppm ~ 0.53ppm; and / or, Na + The content is 0.20ppm~0.35ppm;

[0092] The treated sodium ion positive electrode active material forms a positive electrode active material layer on at least one surface of a positive electrode current collector, and the positive electrode current collector and the positive electrode active material layer disposed on at least one surface of the positive electrode current collector form a positive electrode sheet;

[0093] Provide separator and negative electrode;

[0094] The positive electrode sheet, the separator and the negative electrode sheet are assembled to form a secondary battery.

[0095] In the embodiments of the present application, the mixing method of the sodium ion positive electrode active material to be treated and the organic solvent can be to add the sodium ion positive electrode active material to be treated to the organic solvent at one time, or to add the sodium ion positive electrode active material to be treated to the organic solvent in multiple times, or to add the sodium ion positive electrode active material to be treated to a part of the organic solvent once or multiple times, and then add the remaining organic solvent to mix after mixing for a period of time. Stirring treatment refers to rotating the premix at a certain range of speeds for a certain period of time to improve the uniformity of the mixing of the sodium ion positive electrode active material and the organic solvent. During the stirring treatment, the contact probability between the surface of the sodium ion positive electrode active material and the organic solvent is increased, which is beneficial to reduce the content of free sodium on the surface of the sodium ion positive electrode active material. Solid-liquid separation treatment refers to the process of separating the solid and liquid in the mixture. In the embodiments of the present application, the sodium ion positive electrode active material in the mixture is separated from the organic solvent in which free sodium is dissolved by solid-liquid separation treatment.

[0096] In any embodiment of the present application, the sodium ion positive electrode active material to be treated is washed with an organic solvent to reduce the free sodium on the surface of the sodium ion positive electrode active material, reduce the alkalinity and hygroscopicity of the sodium ion positive electrode active material, improve the uniformity of its dispersion in the slurry, and improve the consistency of the product performance of the positive electrode sheet; reduce the side reaction of free sodium with the electrolyte to produce gas, reduce the flatulence of the secondary battery, and slow down the capacity decay of the secondary battery; improve the device performance of the secondary battery, such as charge and discharge efficiency, cycle life and safety.

[0097] In some embodiments, the mass ratio of the sodium ion positive electrode active material to be treated to the organic solvent is (10-200):200.

[0098] In any embodiment of the present application, the organic solvent needs to wet the sodium ion positive electrode active material to be treated and can form a stirring state, and on the other hand, the amount of organic solvent added needs to be controlled. Too much organic solvent is added, but it is wasted. For example, in some embodiments, the mass ratio of the sodium ion positive electrode active material to be treated to the organic solvent can be 10:200, 20:200, 30:200, 40:200, 50:200, 60:200, 70:200, 80:200, 90:200, 100:200, 110:200, 120:200, 130:200, 140:200, 150:200, 160:200, 170:200, 180:200, 190:200, 200:200, etc. Within the above range, a smaller mass ratio allows the sodium ion positive electrode active material to be more fully in contact with the organic solvent, which can increase the degree of reduction of free sodium on the surface of the sodium ion positive electrode active material. A larger mass ratio can increase the utilization rate of the organic solvent, reduce the amount of organic solvent used, and reduce the impact of the organic solvent on the environment.

[0099] In some embodiments, the mass ratio of the sodium ion positive electrode active material to be treated to the organic solvent is (50-110):200.

[0100] In any embodiment of the present application, within the above range, it is beneficial to further make the sodium ion positive electrode active material more fully contact with the organic solvent, which can increase the degree of reduction of free sodium on the surface of the sodium ion positive electrode active material, and improve the utilization rate of the organic solvent, reduce the amount of organic solvent used, and reduce the impact of the organic solvent on the environment.

[0101] In some embodiments, the organic solvent includes: one or more of N,N-dimethylformamide, anhydrous ethanol, N-methylpyrrolidone, acetonitrile, dimethyl carbonate, diethyl carbonate, diisopropyl carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, and 1,3-dioxolane.

[0102] In any embodiment of the present application, the organic solvent may be any one of the above-mentioned solvents, or a combination of the above-mentioned solvents. The selection of the above-mentioned solvents can make it difficult for the organic solvent to react with the active ingredients in the sodium ion positive electrode active material during the washing process of the sodium ion positive electrode active material, and it is not easy to destroy the sodium ion positive electrode active material. Among them, the multiple combinations of different solvents refer to only physical mixing between different solvents. Within the above range, the use of a single organic solvent is conducive to the recovery of the organic solvent after use, simplifies the recovery procedure, and reduces the difficulty of recovery. The use of multiple organic solvents is conducive to utilizing the characteristics of multiple organic solvents, taking advantage of each other's strengths and weaknesses, promoting the dissolution of free alkali, and improving the washing effect.

[0103] In some embodiments, the water content in the organic solvent is less than 0.3%.

[0104] Since sodium ion positive electrode active materials are prone to hydrolysis, any embodiment of the present application needs to control the water content of the organic solvent to reduce possible side reactions of the sodium ion positive electrode active materials. The lower the water content in the organic solvent, the better, and anhydrous solvents should be used as much as possible. In any embodiment, the water content in the organic solvent can be 0, 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.11%, 0.12%, 0.13%, 0.14%, 0.15%, 0.16%, 0.17%, 0.18%, 0.19%, 0.2%, 0.21%, 0.22%, 0.23%, 0.24%, 0.25%, 0.26%, 0.27%, 0.28%, 0.29%, 0.3%, or a range consisting of any two of the above values. For example, the water content in the organic solvent can be 0-0.05%, 0.05%-0.15% or 0.15%-0.3%, etc.

[0105] In some embodiments, there are at least two organic solvents, and the mass ratio of the two organic solvents is (0-10):(0-10). Alternatively, it can be 0.1:10-10:0.1, 1:10-1:1, etc.

[0106] In any embodiment, the mass ratio of the two organic solvents can be 0:10, 0.5:10, 1:10, 1.5:10, 2:10, 2.5:10, 3:10, 3.5:10, 4:10, 4.5:10, 5:10, 5.5:10, 6:10, 6.5:10, 7:10, 7.5:10, 8:10, 8.5:10, 9:10, 9.5:10, 10:10, 10:0, 10:0.5, 10:1, 10:1.5 , 10:2, 10:2.5, 10:3, 10:3.5, 10:4, 10:4.5, 10:5, 10:5.5, 10:6, 10:6.5, 10:7, 10:7.5, 10:8, 10:8.5, 10:9, 10:9.5, 10:10, or a range consisting of any two of the above values. For example, the mass ratio of the two organic solvents can be (0-3):(0-3), (2-7):(2-7) or (4-8):(4-8), etc.

[0107] In any embodiment of the present application, a plurality of organic solvents are used to wash the sodium ion positive electrode active material. The characteristics of the plurality of organic solvents can be utilized to complement each other, promote the dissolution of free sodium, and improve the washing effect.

[0108] In some embodiments, the stirring process includes: stirring the premix at a rotation speed of 10 r / min to 100 r / min for 0.25 h to 5 h.

[0109] For example, the speed of the stirring treatment can be 10r / min, 15r / min, 20r / min, 25r / min, 30r / min, 35r / min, 40r / min, 45r / min, 50r / min, 55r / min, 60r / min, 65r / min, 70r / min, 75r / min, 80r / min, 85r / min, 90r / min, 95r / min, 100r / min. For example, the time of the stirring treatment can be 0.25h, 0.5h, 1h, 1.5h, 2h, 2.5h, 3h. Within the above range, a smaller stirring speed is conducive to reducing the damage to the sodium ion positive electrode active material. A larger stirring speed is conducive to completing the washing of the sodium ion positive electrode active material in a shorter time. Within the above range, a shorter stirring time can reduce the power consumption of the equipment. A longer stirring time is conducive to increasing the contact degree between the sodium ion positive electrode active material and the organic solvent, which is conducive to improving the washing effect.

[0110] In any embodiment of the present application, stirring treatment is carried out within the above range, the sodium ion positive electrode active material and the organic solvent can be mixed more evenly, the contact probability between the surface of the sodium ion positive electrode active material and the organic solvent is greatly improved, and a good washing effect is achieved, so that the free sodium on the surface of the sodium ion positive electrode active material is reduced to a lower level.

[0111] In some embodiments, the stirring process includes: stirring the premix for 0.5 h to 3 h at a rotation speed of 15 r / min to 50 r / min.

[0112] In any embodiment of the present application, stirring treatment is performed within the above range to further reduce the free sodium on the surface of the sodium ion positive electrode active material to a lower level. In some embodiments, the stirring treatment is performed at room temperature and pressure and the ambient humidity is ≤10%.

[0113] Since sodium ion positive electrode active materials are prone to hydrolysis, the environmental humidity needs to be regulated. Under the condition of environmental humidity ≤10%, the moisture in the environment is not easy to affect the sodium ion positive electrode active materials, reducing the impact of the external environment on the performance stability of the sodium ion positive electrode active materials. The stirring process can be achieved at normal temperature and pressure, which helps to reduce the difficulty of the stirring process. For example, the stirring process is carried out at a temperature of 25°C and a pressure of one standard atmospheric pressure.

[0114] In some embodiments, the solid-liquid separation process includes a filtration process; the filtration process includes:

[0115] Perform a suction filtration treatment on the mixed material;

[0116] Adding new organic solvent to the filtration product after the first filtration treatment, and performing a second filtration treatment;

[0117] The mass ratio of the new organic solvent to the sodium ion positive electrode active material to be treated is (1-2):1.

[0118] In the embodiments of the present application, suction filtration refers to a method of using a vacuum pump to reduce the pressure in the suction filtration device to achieve the purpose of solid-liquid separation. The embodiments of the present application separate the washed sodium ion positive electrode active material from the used solvent by suction filtration. Some solvent may still remain in the product after a suction filtration treatment, the washing effect may not be good, and the surface of some sodium ion positive electrode active materials may not be fully contacted with the organic solvent. The content of free sodium in the sodium ion positive electrode active material may still be at a high level. By adding a new organic solvent for a secondary suction filtration treatment, the contact degree between the surface of the sodium ion positive electrode active material and the organic solvent is increased, and the content of free sodium in the sodium ion positive electrode active material is further reduced.

[0119] For example, the mass ratio of the new organic solvent to the sodium ion positive electrode active material to be treated can be 1:1, 1.15:1, 1.2:1, 1.25:1, 1.3:1, 1.35:1, 1.4:1, 1.45:1, 1.5:1, 1.55:1, 1.6:1, 1.65:1, 1.7:1, 1.75:1, 1.8:1, 1.85:1, 1.9:1, 1.95:1, 2:1. Or a range consisting of any two of the above values, for example, the mass ratio of the new organic solvent to the sodium ion positive electrode active material to be treated can be (1-1.2):1, (1.2-1.6):1 or (1.6-2):1, etc. Within the above range, the mass of the new organic solvent and the sodium ion positive electrode active material to be treated is close, which is conducive to improving the contact degree between the surface of the sodium ion positive electrode active material and the organic solvent, and further reducing the content of free sodium in the sodium ion positive electrode active material.

[0120] In any embodiment of the present application, two suction filtration treatments are performed within the above range, the sodium ion positive electrode active material is further washed, and the amount of organic solvent used for washing is controlled. The contact probability between the surface of the sodium ion positive electrode active material and the organic solvent is greatly improved, and a good washing effect is achieved, so that the free sodium on the surface of the sodium ion positive electrode active material is reduced to a lower level.

[0121] In some embodiments, the solid-liquid separation process comprises a spray drying process.

[0122] In any embodiment of the present application, spray drying is another solid-liquid separation method, in which the mixed material liquid is atomized, and the atomized material is in a dry or semi-dry state after air drying, and a dry material is formed through the discharge end. When the air inlet temperature is within the above range, the atomized material is in a dry or semi-dry state, the solid separation effect is good, and melting or decomposition is not likely to occur. When the outlet temperature is within the above range, it is beneficial for the material at the discharge end to use the residual heat at a lower temperature to form and dry the material, and it is not easy to affect the performance of the material.

[0123] In some embodiments, during the spray drying process, the inlet air temperature is 190°C to 230°C, and the outlet air temperature is 85°C to 95°C.

[0124] For example, the inlet air temperature may be 190° C., 195° C., 200° C., 205° C., 210° C., 215° C., 220° C., 225° C., 230° C. For example, the outlet temperature may be 85° C., 86° C., 87° C., 88° C., 89° C., 90° C., 91° C., 92° C., 93° C., 94° C., 95° C.

[0125] When the inlet air temperature is within the above range, the atomized material is in a dry or semi-dry state, the solid separation effect is good, and it is not easy to melt or decompose. When the outlet temperature is within the above range, it is beneficial for the material at the discharge end to be formed and dried at a lower temperature with the residual heat, and it is not easy to affect the performance of the material.

[0126] In some embodiments, the solid-liquid separation treatment of the mixture also includes drying the material after the solid-liquid separation treatment; the drying treatment is: drying the material at a temperature of 80°C to 200°C for 4h to 24h.

[0127] For example, the temperature of the drying process can be 80° C., 90° C., 100° C., 110° C., 120° C., 130° C., 140° C., 150° C., 160° C., 170° C., 180° C., 190° C., 200° C. The time of the drying process can be 4h, 6h, 8h, 10h, 12h, 14h, 16h, 18h, 20h, 22h, 24h.

[0128] In any embodiment of the present application, by drying the sodium ion positive electrode active material gently at a suitable temperature, the occurrence of internal defects or peeling of the sodium ion positive electrode active material caused by excessively rapid temperature increase is reduced, thereby improving the quality of the obtained sodium ion positive electrode active material.

[0129] In some embodiments, the drying process is: performing a vacuum drying process at a temperature of 100° C. to 120° C. for 6 h to 12 h.

[0130] In any embodiment of the present application, the quality of the obtained sodium ion positive electrode active material is further improved by drying the sodium ion positive electrode active material under the above conditions. In some embodiments, the process further includes collecting the used organic solvent and recycling the collected organic solvent.

[0131] In any embodiment of the present application, by collecting used organic solvents and recycling the collected organic solvents, it is beneficial to fully utilize solvent resources, save resources, protect the environment, and meet the large-scale production of enterprises.

[0132] In some embodiments, the used organic solvent is collected by condensation. In some embodiments of the present application, the solvent after spray drying is collected by condensation and flows into a collection container for standby use.

[0133] In any embodiment of the present application, the used organic solvent is collected through condensation treatment, which is beneficial to fully utilize solvent resources, save resources, protect the environment, and meet the large-scale production of enterprises.

[0134] In some embodiments, the collected organic solvent is subjected to suction filtration or distillation purification. In some embodiments of the present application, the collected organic solvent is subjected to suction filtration or distillation purification to remove some impurities introduced during use and enter a recycling collection container.

[0135] In any embodiment of the present application, the used organic solvent is collected by suction filtration or distillation purification, which is conducive to making full use of solvent resources, saving resources, protecting the environment, and meeting the large-scale production of enterprises.

[0136] Please refer to Figure 1 , Figure 1 Schematic diagram of the exploded structure of a secondary battery according to one or more embodiments. The secondary battery 100 includes a case 10 and a battery cell 20, and the battery cell 20 is contained in the case 10. Among them, the case 10 is used to provide a storage space for the battery cell 20, and the case 10 can adopt a variety of structures. In some embodiments, the case 10 may include a first part 11 and a second part 12, the first part 11 and the second part 12 cover each other, and the first part 11 and the second part 12 jointly define a storage space for accommodating the battery cell 20. The second part 12 may be a hollow structure with one end open, the first part 11 may be a plate-like structure, and the first part 11 covers the open side of the second part 12, so that the first part 11 and the second part 12 jointly define a storage space; the first part 11 and the second part 12 may also be hollow structures with one side open, and the open side of the first part 11 covers the open side of the second part 12. Of course, the case 10 formed by the first part 11 and the second part 12 may be in a variety of shapes, such as a cylinder, a cuboid, etc.

[0137] In the secondary battery 100, there may be multiple battery cells 20, and the multiple battery cells 20 may be connected in series, in parallel, or in a mixed connection. A mixed connection means that the multiple battery cells 20 are both connected in series and in parallel. Multiple battery cells 20 may be directly connected in series, in parallel, or in a mixed connection, and then the whole formed by the multiple battery cells 20 is accommodated in the box 10; of course, the secondary battery 100 may also be a battery module in which multiple battery cells 20 are first connected in series, in parallel, or in a mixed connection, and then the multiple battery modules are connected in series, in parallel, or in a mixed connection to form a whole, and accommodated in the box 10. The secondary battery 100 may also include other structures. For example, the secondary battery 100 may also include a confluence component for realizing electrical connection between multiple battery cells 20. Each battery cell 20 may be a secondary battery or a primary battery; it may be a sodium ion battery, but is not limited thereto. The battery cell 20 may be cylindrical, flat, rectangular, or in other shapes.

[0138] Please refer to Figure 2 , Figure 2 2 is a schematic diagram of the exploded structure of a battery cell according to one or more embodiments. A battery cell 20 refers to the smallest unit that constitutes a battery. Figure 2 The battery cell 20 includes an end cover 21, a shell 22, an electrode assembly 23 and other functional components.

[0139] The end cap 21 refers to a component that covers the opening of the shell 22 to isolate the internal environment of the battery cell 20 from the external environment. Without limitation, the shape of the end cap 21 can be adapted to the shape of the shell 22 to match the shell 22. Optionally, the end cap 21 can be made of a material with a certain hardness and strength (such as aluminum alloy), so that the end cap 21 is not easily deformed when squeezed and collided, so that the battery cell 20 can have a higher structural strength and the safety performance can also be improved. Functional components such as electrode terminals 21a can be provided on the end cap 21. The electrode terminal 21a can be used to electrically connect to the electrode assembly 23 for outputting or inputting electrical energy of the battery cell 20. In some embodiments, the end cap 21 can also be provided with a pressure relief mechanism for releasing the internal pressure when the internal pressure or temperature of the battery cell 20 reaches a threshold. The material of the end cap 21 can also be a variety of materials, such as copper, iron, aluminum, stainless steel, aluminum alloy, plastic, etc., and the embodiments of the present application do not impose special restrictions on this. In some embodiments, an insulating member may be provided inside the end cap 21, and the insulating member may be used to isolate the electrical connection components in the housing 22 from the end cap 21 to reduce the risk of short circuit. For example, the insulating member may be plastic, rubber, or the like.

[0140] The shell 22 is a component used to cooperate with the end cap 21 to form the internal environment of the battery cell 20, wherein the formed internal environment can be used to accommodate the electrode assembly 23, the electrolyte and other components. The shell 22 and the end cap 21 can be independent components, and an opening can be set on the shell 22, and the internal environment of the battery cell 20 is formed by covering the opening with the end cap 21 at the opening. Without limitation, the end cap 21 and the shell 22 can also be integrated. Specifically, the end cap 21 and the shell 22 can form a common connection surface before other components are put into the shell, and when the interior of the shell 22 needs to be encapsulated, the end cap 21 covers the shell 22. The shell 22 can be of various shapes and sizes, such as a rectangular parallelepiped, a cylindrical shape, a hexagonal prism, etc. Specifically, the shape of the shell 22 can be determined according to the specific shape and size of the electrode assembly 23. The material of the shell 22 can be various, such as copper, iron, aluminum, stainless steel, aluminum alloy, plastic, etc., and the embodiment of the present application does not impose any special restrictions on this.

[0141] The electrode assembly 23 is a component in the battery cell 20 where electrochemical reactions occur. One or more electrode assemblies 23 may be included in the housing 22. The electrode assembly 23 is mainly formed by winding or stacking the positive electrode sheet and the negative electrode sheet, and a separator is usually provided between the positive electrode sheet and the negative electrode sheet. The parts of the positive electrode sheet and the negative electrode sheet with active materials constitute the main body of the electrode assembly, and the parts of the positive electrode sheet and the negative electrode sheet without active materials each constitute a tab 23a. The positive tab and the negative tab may be located together at one end of the main body or respectively at both ends of the main body. During the charge and discharge process of the secondary battery 100, the positive electrode active material and the negative electrode active material react with the electrolyte, and the tab 23a connects the electrode terminal to form a current loop.

[0142] In some embodiments, the positive electrode sheet includes a current collector and a positive electrode active material layer disposed on the current collector. The positive electrode active material can be the positive electrode active material treated with an organic solvent in any of the above embodiments.

[0143] In some embodiments, the negative electrode plate includes a current collector and a negative electrode active material layer disposed on the current collector. The specific types of negative electrode active materials are not subject to specific restrictions and can be selected according to needs. For example, the negative electrode active material is selected from one or more of hard carbon, soft carbon, silicon, silicon-carbon composite, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2.

[0144] The negative electrode plate may also include a binder, a conductive agent and other optional additives. As an example, the conductive agent may be one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, Super P (SP), graphene and carbon nanofibers. As an example, the binder may be one or more of styrene-butadiene rubber (SBR), water-based acrylic resin, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA), polyacrylic acid (PAA), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA) and polyvinyl butyral (PVB). As an example, other optional additives may be thickeners and dispersants (e.g., sodium carboxymethyl cellulose CMC-Na), PTC thermistor materials.

[0145] In some embodiments, the isolation membrane is selected from one or more of polyethylene, polypropylene, non-woven fabric, and polyvinylidene fluoride.

[0146] In some embodiments, the solvent of the electrolyte includes one or more of fluorinated carbonates, fluorinated carboxylates, sulfones, and fluorinated ethers, and fluorinated ethers may be selected. The selected electrolyte is a high-voltage resistant electrolyte, which has reduced acidity under high voltage, can significantly reduce surface side reactions, and improve battery stability.

[0147] In some embodiments, the purpose of the electrochemical device of the present application is not particularly limited, and it can be used for any electronic device known in the prior art. The battery disclosed in the embodiment of the present application can be used for various energy storage systems using batteries as power supplies or using batteries as energy storage elements. That is, a kind of electrical equipment is provided. In some embodiments, the electrochemical device of the present application can be used for, but not limited to, notebook computers, pen-input computers, mobile computers, e-book players, portable phones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini-discs, transceivers, electronic notepads, calculators, memory cards, portable recorders, radios, backup power supplies, motors, cars, motorcycles, power-assisted bicycles, bicycles, ships, spacecraft, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries and sodium ion capacitors, etc.

[0148] The device can select battery cells, battery modules or battery packs according to its usage requirements.

[0149] Please refer to Figure 3 , Figure 31 is a schematic diagram of the structure of a vehicle according to one or more embodiments. The vehicle 1000 may be a fuel vehicle, a gas vehicle or a new energy vehicle, and the new energy vehicle may be a pure electric vehicle, a hybrid vehicle or an extended-range vehicle, etc. A secondary battery 100 is provided inside the vehicle 1000, and the secondary battery 100 may be provided at the bottom, head or tail of the vehicle 1000. The secondary battery 100 may be used to power the vehicle 1000, for example, the secondary battery 100 may be used as an operating power source for the vehicle 1000. The vehicle 1000 may also include a controller 200 and a motor 300, and the controller 200 is used to control the secondary battery 100 to power the motor 300, for example, for starting, navigating and driving the vehicle 1000.

[0150] In some embodiments of the present application, the secondary battery 100 can be used not only as an operating power source for the vehicle 1000 , but also as a driving power source for the vehicle 1000 , replacing or partially replacing fuel or natural gas to provide driving power for the vehicle 1000 .

[0151] The beneficial effects of the present application are further illustrated below in conjunction with embodiments.

[0152] In order to make the technical problems, technical solutions and beneficial effects solved by the embodiments of the present application clearer, the following will be further described in detail with reference to the embodiments and the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present application and its applications. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present application.

[0153] 1. Preparation of sodium ion positive electrode active materials:

[0154] Stirring and washing in a dry room environment (humidity <10%): Take 10 grams of NFPP (Na4Fe3(PO4)2P2O7) material and 200 grams of DMF (N,N dimethylformamide) solvent in a beaker, place it on a magnetic stirrer and stir at a speed of 200 rpm, stir for 2 hours, filter it with a suction filtration device, rinse the filter cake with 10 grams of DMF again, and separate the solid and liquid to obtain the washed sodium ion positive electrode active material. The washed sodium ion positive electrode active material is placed in a vacuum oven and dried at 105°C for 10 hours. After crushing and sieving, the powdered treated sodium ion positive electrode active material is obtained. The separated filtrate enters a circulation storage tank for recovery. The specific experimental conditions of each embodiment and comparative example are detailed in Table 1.

[0155] 2. The relevant parameter testing process of the sodium ion positive electrode active materials of the embodiments and comparative examples of the present application is as follows:

[0156] (1) Free sodium test

[0157] The test method for the content of free sodium in the sodium ion positive electrode active material is as follows: the embodiment of the present application refers to the test of free lithium in GB / T9725-2007 "General Rules for Potentiometric Titration of Chemical Reagents", and the free sodium of the sodium ion positive electrode active material is tested, and the test results are obtained by converting the molecular weight of free sodium to free lithium. For example, the molecular weight of NaHCO3 is 84, the molecular weight of LiHCO3 is 68, and the content of free lithium (LiHCO3) obtained by the test result is 0.1ppm. According to the ratio of the molecular weight of NaHCO3 to LiHCO3 is 1.235, the content of free sodium (NaHCO3) of the sodium ion positive electrode active material is converted to 0.124ppm (i.e., 0.1ppm multiplied by 1.235 rounded to 0.124ppm). For example, the molecular weight of Na2CO3 is 106, the molecular weight of Li2CO3 is 74, and the free lithium (Li2CO3) content obtained by the test result is 0.1ppm. According to the molecular weight ratio of Na2CO3 to Li2CO3 is 1.432, the free sodium (Na2CO3) content of the sodium ion positive electrode active material is converted to 0.143ppm. + The molecular weight is 23, Li + The molecular weight is 7, and the free lithium (Li + ) content is 0.1ppm, according to Na + With Li + The molecular weight ratio of the sodium ion positive electrode active material is 3.286, which is converted to the free sodium (Na + ) content is 0.329ppm.

[0158] GB / T9725-2007 "General Rules for Potentiometric Titration of Chemical Reagents" The method for testing the free lithium content of lithium-ion positive electrode active materials is as follows: The standard titration solution used in this standard is prepared in accordance with the provisions of GB / T601, and the experimental water meets the specifications of grade 3 water in GB / T6682. Take samples and prepare test solutions according to the provisions of the product standard. Insert the specified electrode, start the electromagnetic stirrer, and titrate with the specified standard titration solution. Drop about 90% of the required titration volume into the burette, and measure the potential or pH value of the solution. After that, measure the potential or pH value once for each drop of 1mL or an appropriate amount of standard titration solution. Before and after the chemical measurement point, measure once for each drop of 0.1mL standard titration solution. Continue titration until the potential or pH value does not change much. Record the reading of the burette after each drop of the standard titration solution and the measured potential or pH value, and use the graphical method or the second-order differential method to determine the titration end point.

[0159] Among them, the method of determining the titration end point by graphical method is: draw the titration curve with the potential (mV) or pH value of the indicator electrode as the ordinate and the reading (mL) of the burette as the abscissa. Make two tangent lines of the titration curve at 45° to the abscissa, and make a parallel line between the two tangent lines with the same distance as the two tangent lines. The intersection of this line and the titration curve is the titration end point. The abscissa of the intersection is the amount of standard titration solution at the titration end point, and the ordinate of the intersection is the potential or pH value at the titration end point.

[0160] The method for determining the titration endpoint by the second-order derivative method is to list the burette reading V (mL) and the corresponding potential E (mV) or pH value in a table and calculate the following values:

[0161] The volume of standard titration solution added each time (ΔV).

[0162] The change in potential or pH (ΔE or ΔpH) caused by each addition of a standard titration solution.

[0163] First derivative value. That is, the change in potential or pH value caused by a unit volume of standard titration solution, which is numerically equal to ΔE / ΔV or ΔpH / ΔV.

[0164] Second-order derivative value. Numerically equal to the difference between adjacent first-order derivatives.

[0165] The titration endpoint is when the first derivative is maximum and the second derivative is zero.

[0166] The volume of the standard titration solution at the titration endpoint is V0, and the value is expressed in milliliters (mL) and is calculated according to formula (1):

[0167]

[0168] Where:

[0169] V-the volume of the standard titration solution when the second-order derivative is a, in milliliters (mL);

[0170] a-the value of the secondary derivative before the secondary derivative reaches zero;

[0171] b-the value of the secondary derivative after the secondary derivative is zero;

[0172] ΔV-The volume of standard titration solution added from the second derivative a to the second derivative b, in milliliters (mL).

[0173] Table 1: Battery parameters and performance of each embodiment and comparative example

[0174]

[0175]

[0176] Note: The positive electrode amount is the mass of the sodium ion positive electrode active material; the solvent amount is the mass of the organic solvent added for the first time.

[0177] The results show that compared with the examples, the sodium ion positive electrode active material of Comparative Example 1 is not washed with an organic solvent, and the free sodium (Na2CO3, NaHCO3, Na + Compared with Comparative Example 1, the sodium ion positive electrode active materials of each embodiment treated with solvent have a higher content of free sodium (Na2CO3, NaHCO3, Na + ) content is low, which is beneficial to reduce the side reaction between free sodium and electrolyte and even the generation of gas, slowing down the capacity decay of the secondary battery, and thus helping to improve the device performance of the secondary battery, such as charge and discharge efficiency, cycle life and safety.

[0178] From Examples 1 to 3 and 18, when the mass ratio of the solvent decreases, the free sodium content of the sodium ion positive electrode active material first increases, then decreases, and then increases again. Therefore, when the sodium ion positive electrode active material is treated, the amount of solvent used has a certain influence on the treatment effect, and the amount of solvent used should be adjusted to reduce the free sodium content of the sodium ion positive electrode active material to a greater extent.

[0179] From Examples 4 to 6, when different single solvents are used, the free sodium content of the sodium ion positive electrode active material is not much different, and the sodium ion positive electrode active materials all have a lower free sodium content.

[0180] From Examples 2, 7 to 12, it can be seen that when a variety of solvents are mixed, the content of free sodium in the sodium ion positive electrode active material fluctuates to a certain extent. Therefore, when the sodium ion positive electrode active material is treated, different solvent mixing systems have a certain influence on the treatment effect, and the mixing system should be adjusted to reduce the free sodium content of the sodium ion positive electrode active material to a greater extent.

[0181] From Examples 2 and 13 to 16, when the stirring time increases to a certain extent, the free sodium content of the sodium ion positive electrode active material stops decreasing after decreasing to a certain extent. Therefore, on the basis of achieving the purpose of reducing the free sodium content of the sodium ion positive electrode active material, the stirring time does not need to be too long.

[0182] From Examples 2 and 17, it can be seen that when the stirring rate is increased to a certain extent, it is beneficial to further reduce the free sodium content of the sodium ion positive electrode active material. Therefore, the stirring rate can be appropriately increased to further reduce the free sodium content of the sodium ion positive electrode active material.

[0183] The above description is only an implementation method of the present application, and does not limit the patent scope of the present application. Any equivalent structure or equivalent process transformation made using the contents of the present application specification and drawings, or directly or indirectly used in other related technical fields, are also included in the patent protection scope of the present application.

Claims

1. A secondary battery, characterized in that: The secondary battery comprises a positive electrode sheet, a separator and a negative electrode sheet; the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer arranged on at least one surface of the positive electrode current collector; the positive electrode active material layer comprises a sodium ion positive electrode active material and free sodium, and the free sodium comprises Na2CO3, NaHCO3, Na + One or more of the free sodium: the content of Na2CO3 is 0.2ppm to 0.38ppm; and / or, the content of NaHCO3 is 0.38ppm to 0.53ppm; and / or, Na + The content is 0.20ppm~0.35ppm.

2. The secondary battery according to claim 1, characterized in that: The total content of free sodium is 0.5ppm to 2.0ppm.

3. The secondary battery according to claim 1 or 2, characterized in that: The particle size of the sodium ion positive electrode active material is: DV 50 =2μm~5μm,DV 99 =7μm~20μm.

4. The secondary battery according to any one of claims 1 to 3, characterized in that The chemical formula of the sodium ion positive electrode active material includes Na x R y (PO4) z (P2O7) k , wherein 1≤x≤7, 1≤y≤4, 1≤z≤2, 1≤k≤4; and R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W and Pb.

5. A method for preparing a secondary battery according to any one of claims 1 to 4, characterized in that: include: Mixing the sodium ion positive electrode active material to be treated and an organic solvent to obtain a premix; Stirring the premix to obtain a mixture; The mixed material is subjected to solid-liquid separation treatment to obtain a treated sodium ion positive electrode active material, wherein the treated sodium ion positive electrode active material comprises free sodium, and the free sodium comprises Na2CO3, NaHCO3, Na + One or more of the free sodium: the content of Na2CO3 is 0.2ppm to 0.38ppm; and / or, the content of NaHCO3 is 0.38ppm to 0.53ppm; and / or, Na + The content is 0.20ppm~0.35ppm; Forming a positive electrode active material layer on at least one surface of a positive electrode current collector using the treated sodium ion positive electrode active material, wherein the positive electrode current collector and the positive electrode active material layer disposed on at least one surface of the positive electrode current collector form a positive electrode sheet; Provide separator and negative electrode; The positive electrode sheet, the separator and the negative electrode sheet are assembled to form a secondary battery.

6. The method for preparing a secondary battery according to claim 5, characterized in that: The mass ratio of the sodium ion positive electrode active material to be treated to the organic solvent is (10-200):

200.

7. The method for preparing a secondary battery according to claim 6, characterized in that: The mass ratio of the sodium ion positive electrode active material to be treated to the organic solvent is (50-110):

200.

8. The method for preparing a secondary battery according to any one of claims 5 to 7, characterized in that: The organic solvent includes: one or more of N,N-dimethylformamide, anhydrous ethanol, N-methylpyrrolidone, acetonitrile, dimethyl carbonate, diethyl carbonate, diisopropyl carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, and 1,3-dioxolane.

9. The method for preparing a secondary battery according to any one of claims 5 to 8, characterized in that: The water content in the organic solvent is less than 0.3%.

10. The method for preparing a secondary battery according to any one of claims 5 to 9, characterized in that: The organic solvents are at least two, and the mass ratio of the two organic solvents is (0-10):(0-10).

11. The method for preparing a secondary battery according to any one of claims 5 to 10, characterized in that: The stirring process comprises: stirring the premix for 0.25h to 5h at a rotation speed of 10r / min to 100r / min.

12. The method for preparing a secondary battery according to claim 11, characterized in that: The stirring process comprises: stirring the premix for 0.5h to 3h at a rotation speed of 15r / min to 50r / min.

13. The method for preparing a secondary battery according to any one of claims 5 to 12, characterized in that: The stirring treatment is carried out under the conditions of normal temperature and pressure and an ambient humidity of ≤10%.

14. The method for preparing a secondary battery according to any one of claims 5 to 13, characterized in that: The solid-liquid separation process includes a suction filtration process; the suction filtration process includes: Performing a suction filtration treatment on the mixed material; Adding new organic solvent to the filtration product after the first filtration treatment, and performing a second filtration treatment; Wherein, the mass ratio of the new organic solvent to the sodium ion positive electrode active material to be treated is (1-2):

1.

15. The method for preparing a secondary battery according to any one of claims 5 to 14, characterized in that: The solid-liquid separation process includes a spray drying process.

16. The method for preparing a secondary battery according to claim 15, characterized in that: During the spray drying process, the air inlet temperature is 190°C to 230°C, and the air outlet temperature is 85°C to 95°C.

17. The method for preparing a secondary battery according to any one of claims 5 to 16, characterized in that: The solid-liquid separation treatment of the mixed material also includes drying the material after the solid-liquid separation treatment; the drying treatment is: drying the material at a temperature of 80°C to 200°C for 4h to 24h.

18. The method for preparing a secondary battery according to claim 17, characterized in that: The drying process is: vacuum drying for 6 hours to 12 hours at a temperature of 100° C. to 120° C.

19. The method for preparing a secondary battery according to any one of claims 5 to 18, characterized in that: The method also includes collecting used organic solvents and recycling the collected organic solvents.

20. The method for preparing a secondary battery according to claim 19, characterized in that: The used organic solvent was collected by condensation treatment.

21. The method for preparing a secondary battery according to claim 20, characterized in that: The collected organic solvent is subjected to suction filtration or distillation purification.

22. An electrical equipment, characterized in that: A secondary battery comprising the secondary battery according to any one of claims 1 to 4 or a secondary battery prepared by the method for preparing a secondary battery according to any one of claims 5 to 21.

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