Secondary battery, method for manufacturing the same, and electric device

Abstract

The application discloses a secondary battery and a preparation method and an electric device thereof. The secondary battery comprises a sodium ion positive electrode active material and free sodium. The free sodium comprises one or more of Na2CO3, NaHCO3, Na + The content of Na2CO3 in the free sodium is 0.2 ppm to 0.38 ppm; and / or the content of NaHCO3 in the free sodium is 0.38 ppm to 0.53 ppm; and / or the content of Na + The content of Na2CO3 in the free sodium is 0.2 ppm to 0.38 ppm; and / or the content of NaHCO3 in the free sodium is 0.38 ppm to 0.53 ppm; and / or the content of Na The application controls the content of free sodium in the positive electrode active material of the secondary battery, reduces the alkalinity and hygroscopicity of the sodium ion positive electrode active material, improves the consistency of the product performance of the positive electrode sheet, reduces the swelling of the secondary battery, slows down the capacity attenuation of the secondary battery, and improves the device performance of the secondary battery, such as the charge-discharge efficiency, the cycle service life and the safety.

Description

[0001] Cross-references to related applications

[0002] This application claims priority to Chinese Patent Application No. 2023108705334, filed on July 14, 2023, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This invention relates to the field of batteries, and more particularly to a secondary battery, its preparation method, and electrical equipment. Background Technology

[0004] Sodium-ion secondary batteries have shown great development potential due to the abundant and widely distributed sodium resources in the Earth's crust, as well as their excellent high and low temperature performance, high safety, and the ability to use low-salt concentration electrolytes. However, the performance of the positive electrode active material in sodium-ion secondary batteries can significantly affect the device performance.

[0005] The above statements are for the purpose of providing background information in relation to this application only and do not necessarily constitute prior art. Summary of the Invention

[0006] In view of the technical problems existing in the background art, this application provides a secondary battery and its preparation method and electrical equipment, aiming to improve the device performance of the secondary battery.

[0007] In a first aspect, embodiments of this application provide a secondary battery, the secondary battery comprising a positive electrode, a separator, and a negative electrode; the positive electrode comprises a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector; the positive active material layer comprises a sodium ion positive active material and free sodium, the free sodium comprising Na2CO3, NaHCO3, Na2 ... + One or more of the following; in 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 to 0.35ppm.

[0008] In any embodiment of this application, by controlling 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 and improving the product performance consistency of the positive electrode sheet; reducing the occurrence of side reactions or even gas generation between free sodium and electrolyte, reducing the occurrence of gas swelling in 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 content of free sodium is 0.5 ppm to 2.0 ppm.

[0010] In any embodiment of this application, by adjusting the total content of free sodium, the side reactions between free sodium and electrolyte, or even the generation of gas, can be reduced, thereby reducing the occurrence of gas swelling in 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 this application, when the particle size is controlled within the above-mentioned range, parameters such as oxygen defects, specific surface area, and tap density of the sodium ion cathode active material can be better controlled.

[0013] In some embodiments, the general chemical formula of 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; 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 this application, by regulating the content of free sodium in the sodium-ion positive electrode active material of the provided chemical formula, the device performance of the secondary battery, such as charge-discharge efficiency, cycle life, and safety, can be improved.

[0015] Secondly, embodiments of this application provide a method for preparing any of the secondary batteries provided in the first aspect, comprising:

[0016] The sodium ion positive electrode active material to be treated is mixed with an organic solvent to obtain a premix;

[0017] The premixed material is stirred to obtain a mixture;

[0018] The mixture is subjected to solid-liquid separation treatment to obtain the treated sodium ion positive electrode active material. The treated sodium ion positive electrode active material includes free sodium, which includes Na2CO3, NaHCO3, and Na2CO3. + One or more of the following; in 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 to 0.35ppm;

[0019] The treated sodium ion positive electrode active material is used to form a positive electrode active material layer on at least one surface of the 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] Provides separator membrane and negative electrode sheet;

[0021] A secondary battery is formed by assembling a positive electrode, a separator, and a negative electrode.

[0022] In any embodiment of this application, the sodium-ion positive electrode active material to be treated is washed with an organic solvent, which reduces the free sodium on the surface of the sodium-ion positive electrode active material, reduces the alkalinity and hygroscopicity of the sodium-ion positive electrode active material, improves its uniformity of dispersion in the slurry, and enhances the consistency of the product performance of the positive electrode sheet; it also reduces the occurrence of gas generation from side reactions between free sodium and electrolyte, reduces the occurrence of gas expansion in the secondary battery, and slows down the capacity decay of the secondary battery; and improves 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 this application, the organic solvent needs to both wet the sodium-ion cathode active material to be treated and create a stirred state, while also controlling the amount of organic solvent added, as excessive addition is wasteful. Within the aforementioned range, a smaller mass ratio allows for more thorough contact between the sodium-ion cathode active material and the organic solvent, thereby increasing the reduction of free sodium on the surface of the sodium-ion cathode 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 environmental impact of the organic solvent.

[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 this application, within the above-mentioned scope, it is advantageous to further enable the sodium ion positive electrode active material to come into more sufficient contact with the organic solvent, which can improve the reduction of free sodium on the surface of the sodium ion positive electrode active material, improve the utilization rate of the organic solvent, reduce the amount of organic solvent used, and reduce the environmental impact of the organic solvent.

[0027] In some embodiments, the organic solvent includes one or more of the following: 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 this application, the organic solvent can be any one of the solvents mentioned above, or a combination of several solvents. The selection of the solvents ensures that the organic solvent does not easily react with or damage the sodium ion cathode active material during the washing process. Here, "combinations of different solvents" refers to a mere physical mixing of different solvents. Within the aforementioned scope, using a single organic solvent facilitates the recovery of the used organic solvent, simplifying the recovery process and reducing its difficulty. Using multiple organic solvents allows for the utilization of their respective properties, compensating for their 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 sodium ion positive electrode active materials are prone to hydrolysis, any embodiment of this application reduces the possible side reactions of sodium ion positive electrode active materials by controlling the water content of the organic solvent.

[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 this application, a variety of organic solvents are used to wash the sodium ion positive electrode active material. This can take advantage of the characteristics of various organic solvents, combine their strengths and compensate for their weaknesses, 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 this application, stirring is performed within the above-mentioned range, which allows the sodium ion positive electrode active material and the organic solvent to be mixed more uniformly. This significantly increases the contact probability between the surface of the sodium ion positive electrode active material and the organic solvent, resulting in a good washing effect and reducing the free sodium on the surface of the sodium ion positive electrode active material to a low level.

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

[0036] In any embodiment of this application, stirring is performed within the above-mentioned range to further reduce the free sodium on the surface of the sodium ion positive electrode active material to a lower level.

[0037] In some embodiments, the stirring process is carried out at normal temperature and pressure and with an ambient humidity of ≤10%.

[0038] Because sodium-ion cathode active materials are prone to hydrolysis, environmental humidity needs to be controlled. Under conditions where the ambient humidity is ≤10%, moisture in the environment is less likely to affect the sodium-ion cathode active material, reducing the impact of the external environment on its performance stability. The stirring process can be performed at room temperature and pressure, which helps to reduce the difficulty of the stirring process.

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

[0040] The mixture is subjected to a single filtration process.

[0041] Add a new organic solvent to the filtration product after the first filtration process and perform a second filtration process.

[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 this application, two filtration processes are performed within the above-mentioned range, and the amount of solvent used for washing is controlled. The probability of contact between the surface of the sodium ion positive electrode active material and the organic solvent is greatly increased, resulting in a good washing effect and reducing the free sodium on the surface of the sodium ion positive electrode active material to a low level.

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

[0045] In any embodiment of this application, spray drying is another solid-liquid separation process. By atomizing the liquid mixture, the atomized material is dried or semi-dry after being dried by air intake, and then dried material is formed at 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 temperature is 85°C to 95°C.

[0047] When the inlet air temperature is within the above range, the atomized material is in a dry or semi-dry state, resulting in good solid-state separation and minimizing the risk of melting or decomposition. When the outlet temperature is within the above range, the material at the outlet end can be dried using residual heat at a lower temperature, minimizing the impact on material properties. In some embodiments, the solid-liquid separation treatment of the mixture further includes drying the material after solid-liquid separation; the drying process involves drying at a temperature of 80°C to 200°C for 4 to 24 hours.

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

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

[0050] In any embodiment of this 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.

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

[0052] In any embodiment of this application, by collecting used organic solvents and recycling them, it is beneficial to make full use of solvent resources, save resources, protect the environment, and meet the needs of large-scale production of enterprises.

[0053] In some embodiments, used organic solvents are collected by condensation.

[0054] In any embodiment of this application, the collection of used organic solvents through condensation treatment is beneficial for making full use of solvent resources, saving resources, protecting the environment, and meeting the needs of large-scale production in enterprises.

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

[0056] In any embodiment of this application, the used organic solvent is collected through filtration or distillation purification, which is beneficial for making full use of solvent resources, saving resources, and protecting the environment, and can meet the needs of large-scale production in enterprises. Thirdly, embodiments of this application provide an electrical device, including a secondary battery prepared by the method described in the first aspect or the second aspect. The electrical device using the provided secondary battery has at least the same advantages as the secondary battery, and can improve the reliability of the electrical device. Attached Figure Description

[0057] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

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

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

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

[0061] Explanation of icon numbers:

[0062] 1000-Vehicle, 100-Secondary battery, 200-Controller, 300-Motor, 10-Box, 20-Battery cell, 11-First part, 12-Second part, 21-End cap, 22-Housing shell, 23-Electrode assembly, 21a-Electrode terminal, 23a-Taper. Detailed Implementation

[0063] To make the objectives, technical solutions, and effects of this application clearer and more explicit, the embodiments of the technical solutions of this application will be described in detail below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of this application, and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0064] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0065] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces), unless otherwise explicitly specified.

[0066] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0067] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0068] Quantities, ratios, and other numerical values ​​are presented in range format in this document. It should be understood that this range format is for convenience and brevity and should be interpreted flexibly to include not only numerical values ​​explicitly specified as range limits, but also all individual numerical values ​​or subranges covered within the range, as if each numerical value and subrange were explicitly specified.

[0069] Lithium-ion batteries, with their advantages of high voltage, light weight, long cycle life, no memory effect, and good safety, have been widely used. However, with the rapid growth in demand for lithium-ion batteries, the supply of lithium resources is becoming increasingly strained. Against this backdrop, sodium-ion batteries, which operate on a similar principle to lithium-ion batteries, have shown great development potential due to the abundance and widespread global distribution of sodium resources in the Earth's crust, as well as their excellent high and low temperature performance, high safety, and ability to use low-salt concentration electrolytes.

[0070] Sodium-ion secondary batteries consist of a positive electrode, a negative electrode, a separator, and an electrolyte. During the charge and discharge process, sodium ions repeatedly insert and extract between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, serves as a barrier. The electrolyte, acting as a conductor between the electrodes, facilitates ion exchange. The sodium-ion positive electrode active material is crucial to the overall performance of the sodium-ion secondary battery. During production, factors such as process and formulation ratios can lead to residual sodium in the material. Furthermore, sodium readily leaches from the bulk phase, resulting in residual free sodium on the surface. Excessive free sodium increases the alkalinity of the material, enhancing its hygroscopicity and making it susceptible to moisture absorption and deterioration. This affects the consistency of the positive active material layer formed by the slurry coating, ultimately impacting the battery's charge / discharge efficiency and cycle life. Furthermore, at least some free sodium readily reacts with the electrolyte to produce gas, which is one of the reasons for gas buildup in secondary batteries and the resulting safety hazards. In addition, free sodium tends to result in higher alkalinity in the sodium-ion cathode active material, making it more susceptible to side reactions with the electrolyte, increasing the risk of battery swelling and accelerating capacity decay. For these reasons, it is necessary to reduce the residual sodium content in the sodium-ion cathode active material to improve battery performance, such as charge / discharge efficiency, cycle life, and safety.

[0071] To address the aforementioned technical problems, embodiments of this application provide a secondary battery that improves the charge / 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 solutions described in the embodiments of this application are applicable to secondary batteries, their preparation methods, and electrical devices. The secondary batteries, their preparation methods, and electrical devices disclosed in this application can be used in the field of sodium-ion secondary batteries, and also in sodium metal secondary batteries, sodium polymer secondary batteries, or sodium-ion polymer secondary batteries, depending on specific requirements.

[0073] The present application will now be described in detail with reference to the accompanying drawings and embodiments.

[0074] Embodiments of this application provide a secondary battery, which includes a positive electrode, a separator, and a negative electrode. The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer includes a sodium ion positive active material and free sodium, wherein the free sodium includes Na2CO3, NaHCO3, and Na2CO3. +One or more of the following; in 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 to 0.35ppm.

[0075] In the embodiments of this application, the sodium-ion positive electrode active material is typically composed of sodium active compounds. 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 charging, sodium ions are released from the crystal lattice of the sodium-ion positive electrode active material and intercalated into the crystal lattice of the negative electrode active material via the electrolyte, resulting in a sodium-rich negative electrode and a sodium-poor positive electrode. Simultaneously, electrons flow from the negative electrode to the positive electrode through an external circuit; the discharge process is the reverse of the charging process. Free sodium refers to sodium compounds and / or ionized products adhering to the surface of the sodium-ion positive electrode active material. In some embodiments, free sodium includes Na₂CO₃, NaHCO₃, Na + One or more of these. Here, ppm refers to parts per million (ppm), where 1 ppm = 0.0001%.

[0076] The test method for the free sodium content in sodium-ion cathode active materials is as follows: Referring to GB / T9725-2007 "General Rules for Potentiometric Titration of Chemical Reagents" for the test of free lithium, the free sodium content in the sodium-ion cathode active material is tested. The test result is calculated based on the molecular weight ratio of free sodium to free lithium. For example, if the molecular weight of NaHCO3 is 84 and the molecular weight of LiHCO3 is 68, the tested result shows a free lithium content of 0.1 ppm. Based on the molecular weight ratio of NaHCO3 to LiHCO3 of 1.235, the free sodium content of the sodium-ion cathode active material is calculated to be 0.124 ppm. The method for testing the free lithium content in lithium-ion cathode active materials according to GB / T9725-2007 "General Rules for Potentiometric Titration of Chemical Reagents" is as follows: Sample and prepare a test solution according to the product standard specifications. Insert the specified electrode, start the electromagnetic stirrer, and titrate with the specified standard titration solution. Add approximately 90% of the required titration volume of standard titrant to the burette and measure the potential or pH of the solution. Measure the potential or pH after each 1 mL or additional volume of standard titrant added. Before and after the stoichiometric point, measure the potential or pH after each 0.1 mL addition of standard titrant. Continue titrating until the potential or pH changes minimally. Record the burette reading and the measured potential or pH after each addition of standard titrant. Use a graphical method or second-order derivative method to determine the titration endpoint.

[0077] In some embodiments, the Na₂CO₃ content in the free sodium is 0.2 ppm to 0.38 ppm. The Na₂CO₃ content in the free sodium can be 0.2 ppm, 0.21 ppm, 0.22 ppm, 0.23 ppm, 0.24 ppm, 0.25 ppm, 0.26 ppm, 0.27 ppm, 0.28 ppm, 0.29 ppm, 0.30 ppm, 0.31 ppm, 0.32 ppm, 0.33 ppm, 0.34 ppm, 0.35 ppm, 0.36 ppm, 0.37 ppm, 0.38 ppm, or a range of any two of the above values. For example, it can be 0.2 ppm to 0.25 ppm, 0.23 ppm to 0.28 ppm, 0.25 ppm to 0.30 ppm, 0.28 ppm to 0.33 ppm, 0.30 ppm to 0.35 ppm, 0.33 ppm to 0.38 ppm, 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 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 is 0.20ppm to 0.35ppm. Na + 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 of any two of the above values, for example, 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 this application, by controlling 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 sheet; it reduces the occurrence of side reactions or even gas generation between free sodium and electrolyte, and reduces the occurrence of gas swelling in the secondary battery; it slows down the capacity decay of the secondary battery; and it improves the device performance of the secondary battery, such as charge and discharge efficiency, cycle life and safety.

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

[0082] In any embodiment, the total free sodium content 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, or 2.0ppm, or a range of any two of the above values. For example, the free sodium content 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 equivalent. In any embodiment of this application, by controlling the total free sodium content, the occurrence of side reactions or even gas generation between free sodium and the electrolyte can be reduced, thereby reducing the occurrence of gas expansion in 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 embodiments of this application, particle size refers to the average diameter of the sodium ion positive electrode active material particles. DV 50 DV refers to the particle size that, in a volumetric particle size distribution, reaches 50% of the cumulative volume from the smallest particle size. 99 This refers to the particle size that, in a volumetric particle size distribution, reaches 99% of the total volumetric size, measured from the smallest particle size. Particle size testing is performed according to GB / T19077-2016, using a laser particle size analyzer diffraction method. A clean beaker is taken, an appropriate amount of the sample to be tested is added, a surfactant is added, followed by a dispersant. After thorough dispersion, the particle size distribution characteristics are determined using a laser particle size analyzer (opause: 8–12%).

[0085] In any embodiment of this 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 cathode active material can be better controlled.

[0086] In some embodiments, the general chemical formula of 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; 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 can be NFPP(Na4Fe3(PO4)2P2O7).

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

[0088] Embodiments of this application provide a method for preparing any of the secondary batteries provided in the first aspect, comprising:

[0089] The sodium ion positive electrode active material to be treated is mixed with an organic solvent to obtain a premix;

[0090] The premixed material is stirred to obtain a mixture;

[0091] The mixture is subjected to solid-liquid separation treatment to obtain the treated sodium ion positive electrode active material. The treated sodium ion positive electrode active material includes free sodium, which includes Na2CO3, NaHCO3, and Na2CO3. + One or more of the following; in 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 to 0.35ppm;

[0092] The treated sodium ion positive electrode active material is used to form a positive electrode active material layer on at least one surface of the 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] Provides separator membrane and negative electrode sheet;

[0094] A secondary battery is formed by assembling a positive electrode, a separator, and a negative electrode.

[0095] In the embodiments of this application, the mixing method of the sodium ion positive electrode active material to be treated and the organic solvent can be as follows: the sodium ion positive electrode active material to be treated can be added to the organic solvent all at once; the sodium ion positive electrode active material to be treated can be added to the organic solvent in multiple portions; or the sodium ion positive electrode active material to be treated can be added to a portion of the organic solvent in one or multiple portions, mixed for a period of time, and then the remaining organic solvent can be added for further mixing. Stirring treatment refers to rotating the premix at a certain speed for a certain period of time to improve the uniformity of the mixing between 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 reducing 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 this application, solid-liquid separation treatment is used to separate the sodium ion positive electrode active material in the mixture from the organic solvent containing dissolved free sodium.

[0096] In any embodiment of this application, the sodium-ion positive electrode active material to be treated is washed with an organic solvent, which reduces the free sodium on the surface of the sodium-ion positive electrode active material, reduces the alkalinity and hygroscopicity of the sodium-ion positive electrode active material, improves its uniformity of dispersion in the slurry, and enhances the consistency of the product performance of the positive electrode sheet; it also reduces the occurrence of gas generation from side reactions between free sodium and electrolyte, reduces the occurrence of gas expansion in the secondary battery, and slows down the capacity decay of the secondary battery; and improves 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 this application, the organic solvent needs to both wet the sodium ion positive electrode active material to be treated and create a stirred state, while also controlling the amount of organic solvent added, as excessive addition would be wasteful. For example, in some embodiments, the mass ratio of the sodium ion positive electrode active material 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 aforementioned range, a smaller mass ratio allows for more thorough contact between the sodium-ion cathode active material and the organic solvent, thereby increasing the reduction of free sodium on the surface of the sodium-ion cathode 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 environmental impact of the organic solvent.

[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 this application, within the above-mentioned scope, it is advantageous to further enable the sodium ion positive electrode active material to come into more sufficient contact with the organic solvent, which can improve the reduction of free sodium on the surface of the sodium ion positive electrode active material, improve the utilization rate of the organic solvent, reduce the amount of organic solvent used, and reduce the environmental impact of the organic solvent.

[0101] In some embodiments, the organic solvent includes one or more of the following: 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 this application, the organic solvent can be any one of the solvents mentioned above, or a combination of the solvents mentioned above. The selection of the solvents ensures that the organic solvent is less likely to react with the active components in the sodium ion cathode active material during the washing process, and is less likely to damage the sodium ion cathode active material. Here, "combinations of different solvents" refers to the physical mixing of different solvents. Within the above scope, using a single organic solvent is beneficial for the recovery of the used organic solvent, simplifying the recovery process and reducing the difficulty of recovery. Using multiple organic solvents allows for the utilization of the characteristics of various organic solvents, compensating for their 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] Because sodium ion cathode active materials are prone to hydrolysis, any embodiment of this application requires controlling the water content of the organic solvent to reduce potential side reactions of the sodium ion cathode active material. The lower the water content in the organic solvent, the better; anhydrous solvents should be used whenever 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%, or 0.3%, or a range 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, at least two organic solvents are used, and the mass ratio of the two organic solvents is (0-10):(0-10). Optionally, 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, or 10:1.5. The ratios can be 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 any range of 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 this application, a variety of organic solvents are used to wash the sodium ion positive electrode active material. This can take advantage of the characteristics of various organic solvents, combine their strengths and compensate for their weaknesses, 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 stirring speed can be 10 r / min, 15 r / min, 20 r / min, 25 r / min, 30 r / min, 35 r / min, 40 r / min, 45 r / min, 50 r / min, 55 r / min, 60 r / min, 65 r / min, 70 r / min, 75 r / min, 80 r / min, 85 r / min, 90 r / min, 95 r / min, or 100 r / min. For example, the stirring time can be 0.25 h, 0.5 h, 1 h, 1.5 h, 2 h, 2.5 h, or 3 h. Within the above ranges, a lower stirring speed helps reduce damage to the sodium ion positive electrode active material. A higher stirring speed helps complete the washing of the sodium ion positive electrode active material in a shorter time. Within the above ranges, a shorter stirring time can reduce the power consumption of the equipment. A longer stirring time helps increase the contact between the sodium ion positive electrode active material and the organic solvent, which is beneficial for improving the washing effect.

[0110] In any embodiment of this application, stirring is performed within the above-mentioned range, which allows the sodium ion positive electrode active material and the organic solvent to be mixed more uniformly. This significantly increases the contact probability between the surface of the sodium ion positive electrode active material and the organic solvent, resulting in a good washing effect and reducing the free sodium on the surface of the sodium ion positive electrode active material to a low level.

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

[0112] In any embodiment of this application, a stirring treatment is performed within the aforementioned range to further reduce the free sodium on the surface of the sodium ion positive electrode active material to a low level. In some embodiments, the stirring treatment is performed under normal temperature and pressure conditions and an ambient humidity of ≤10%.

[0113] Because sodium-ion cathode active materials are prone to hydrolysis, environmental humidity needs to be controlled. Under conditions where the ambient humidity is ≤10%, moisture in the environment is less likely to affect the sodium-ion cathode active material, reducing the impact of the external environment on its performance stability. Stirring can be performed at room temperature and pressure, which helps reduce the difficulty of the stirring process. For example, stirring is carried out at a temperature of 25°C and a pressure of one standard atmosphere.

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

[0115] The mixture is subjected to a single filtration process.

[0116] Add a new organic solvent to the filtration product after the first filtration process and perform a second filtration process.

[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 this application, vacuum filtration refers to a method that uses a vacuum pump to reduce the pressure in the vacuum filtration device to achieve solid-liquid separation. The embodiments of this application use vacuum filtration to separate the washed sodium-ion positive electrode active material from the used solvent. After one vacuum filtration process, some solvent may remain in the product, the washing effect may be poor, and the surface of some sodium-ion positive electrode active material may not have sufficient contact with the organic solvent, resulting in a potentially high level of free sodium content. By adding new organic solvent for a second vacuum filtration process, the contact degree between the surface of the sodium-ion positive electrode active material and the organic solvent is improved, further reducing the free sodium content in the sodium-ion positive electrode active material.

[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, or 2:1. Alternatively, it can be 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 ranges, the mass of the new organic solvent and the sodium ion positive electrode active material to be treated are close, which is beneficial to improving the surface contact between the sodium ion positive electrode active material and the organic solvent, further reducing the free sodium content in the sodium ion positive electrode active material.

[0120] In any embodiment of this application, two filtration processes are performed within the above-mentioned range to further wash the sodium ion positive electrode active material. The amount of organic solvent used for washing is controlled, which greatly increases the contact probability between the surface of the sodium ion positive electrode active material and the organic solvent, resulting in a good washing effect and reducing the free sodium on the surface of the sodium ion positive electrode active material to a low level.

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

[0122] In any embodiment of this application, spray drying is another solid-liquid separation method. The mixture is atomized, and the atomized material is dried or semi-dry after being dried by inlet air. The dried material is then discharged through the outlet. When the inlet air temperature is within the aforementioned range, the atomized material is in a dry or semi-dry state, resulting in good solid-liquid separation and minimizing the risk of melting or decomposition. When the outlet temperature is within the aforementioned range, the material at the outlet is dried using residual heat at a lower temperature, minimizing the impact on material properties.

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

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

[0125] When the inlet air temperature is within the above range, the atomized material is in a dry or semi-dry state, resulting in good solid-state separation and minimizing the risk of melting or decomposition. When the outlet temperature is within the above range, the material at the outlet end can be dried and shaped using residual heat at a lower temperature, minimizing the impact on material properties.

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

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

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

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

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

[0131] In any embodiment of this application, by collecting used organic solvents and recycling them, it is beneficial to make full use of solvent resources, save resources, protect the environment, and meet the needs of large-scale production of enterprises.

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

[0133] In any embodiment of this application, the collection of used organic solvents through condensation treatment is beneficial for making full use of solvent resources, saving resources, protecting the environment, and meeting the needs of large-scale production in enterprises.

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

[0135] In any embodiment of this application, the used organic solvents are collected by filtration or distillation purification, which helps to make full use of solvent resources, save resources, protect the environment, and meet the needs of large-scale production of enterprises.

[0136] Please refer to Figure 1 , Figure 1 This is an exploded structural diagram of a secondary battery according to one or more embodiments. The secondary battery 100 includes a housing 10 and a battery cell 20, with the battery cell 20 housed within the housing 10. The housing 10 provides a space for the battery cell 20 and can have various structures. In some embodiments, the housing 10 may include a first portion 11 and a second portion 12, which overlap each other, jointly defining a space for accommodating the battery cell 20. The second portion 12 may be a hollow structure with one open end, and the first portion 11 may be a plate-like structure, covering the open side of the second portion 12 so that the first portion 11 and the second portion 12 jointly define the space; alternatively, the first portion 11 and the second portion 12 may both be hollow structures with one open side, with the open side of the first portion 11 covering the open side of the second portion 12. Of course, the housing 10 formed by the first portion 11 and the second portion 12 can have various shapes, such as a cylinder, a cuboid, etc.

[0137] In the secondary battery 100, there can be multiple battery cells 20, which can be connected in series, parallel, or in a mixed configuration. A mixed configuration means that multiple battery cells 20 are connected in both series and parallel connections. Multiple battery cells 20 can be directly connected in series, parallel, or in a mixed configuration, and then the entire assembly of the multiple battery cells 20 is housed within the housing 10. Alternatively, the secondary battery 100 can also consist of multiple battery cells 20 first connected in series, parallel, or in a mixed configuration to form a battery module, and then these battery modules are connected in series, parallel, or in a mixed configuration to form a whole, which is then housed within the housing 10. The secondary battery 100 may also include other structures; for example, it may include a busbar component for electrical connection between the multiple battery cells 20. Each battery cell 20 can be a secondary battery or a primary battery; it can be a sodium-ion battery, but is not limited to this. The battery cell 20 can be cylindrical, flat, cuboid, or other shapes.

[0138] Please refer to Figure 2 , Figure 2 This is an exploded structural diagram of a battery cell according to one or more embodiments. Battery cell 20 refers to the smallest unit that makes up the battery. Figure 2 The battery cell 20 includes an end cap 21, a housing 22, an electrode assembly 23, and other functional components.

[0139] End cap 21 refers to a component that covers the opening of housing 22 to isolate the internal environment of battery cell 20 from the external environment. The shape of end cap 21 can be adapted to the shape of housing 22 to fit it. Optionally, end cap 21 can be made of a material with certain hardness and strength (such as aluminum alloy), so that end cap 21 is not easily deformed under pressure and impact, giving battery cell 20 higher structural strength and improved safety performance. Functional components such as electrode terminals 21a can be provided on end cap 21. Electrode terminals 21a can be used for electrical connection with electrode assembly 23 to output or input electrical energy to battery cell 20. In some embodiments, end cap 21 can also be provided with a pressure relief mechanism for releasing internal pressure when the internal pressure or temperature of battery cell 20 reaches a threshold. The material of end cap 21 can also be various, such as copper, iron, aluminum, stainless steel, aluminum alloy, plastic, etc., and this application embodiment does not impose any special limitations on this. In some embodiments, an insulating element may be provided on the inner side of the end cap 21. The insulating element can be used to isolate the electrical connection components within the housing 22 from the end cap 21 to reduce the risk of short circuits. For example, the insulating element may be made of plastic, rubber, etc.

[0140] The housing 22 is a component used to cooperate with the end cap 21 to form the internal environment of the battery cell 20. This internal environment can accommodate the electrode assembly 23, electrolyte, and other components. The housing 22 and the end cap 21 can be independent components. An opening can be provided on the housing 22, and the end cap 21 can be used to close the opening to form the internal environment of the battery cell 20. Alternatively, the end cap 21 and the housing 22 can be integrated. Specifically, the end cap 21 and the housing 22 can form a common connecting surface before other components are inserted into the housing. When it is necessary to encapsulate the interior of the housing 22, the end cap 21 closes the housing 22. The housing 22 can be of various shapes and sizes, such as cuboid, cylindrical, hexagonal prism, etc. Specifically, the shape of the housing 22 can be determined according to the specific shape and size of the electrode assembly 23. The material of the housing 22 can be various, such as copper, iron, aluminum, stainless steel, aluminum alloy, plastic, etc. This application embodiment does not impose any special limitations on this.

[0141] Electrode assembly 23 is the component in the battery cell 20 where electrochemical reactions occur. The casing 22 may contain one or more electrode assemblies 23. The electrode assembly 23 is mainly formed by winding or stacking positive and negative electrode sheets, and typically a separator is provided between the positive and negative electrode sheets. The portions of the positive and negative electrode sheets containing active material constitute the main body of the electrode assembly, while the portions of the positive and negative electrode sheets without active material each constitute a tab 23a. The positive and negative tabs may be located together at one end of the main body or separately at both ends of the main body. During the charging and discharging process of the secondary battery 100, the positive and negative active materials react with the electrolyte, and the tabs 23a connect to the electrode terminals to form a current loop.

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

[0143] In some embodiments, the negative electrode includes a current collector and a layer of negative electrode active material disposed on the current collector. The specific type of negative electrode active material is not limited and can be selected according to requirements. For example, the negative electrode active material can be selected from one or more of hard carbon, soft carbon, silicon, silicon-carbon composites, Li-Sn alloys, Li-Sn-O alloys, Sn, SnO, and SnO2.

[0144] The negative electrode may also include a binder, a conductive agent, and other optional additives. For 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. For 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). For example, other optional additives may be thickeners and dispersants (e.g., sodium carboxymethyl cellulose CMC-Na) and PTC thermistor materials.

[0145] In some embodiments, the separator is selected from one or more materials selected from polyethylene, polypropylene, nonwoven fabric, and polyvinylidene fluoride.

[0146] In some embodiments, the solvent of the electrolyte includes one or more of fluorocarbonates, fluorocarboxylic esters, sulfones, and fluoroethers, with fluoroethers being optional. The selected electrolyte is a high-voltage resistant electrolyte, which reduces acidity under high voltage, significantly reducing surface side reactions and improving battery stability.

[0147] In some embodiments, the application of the electrochemical device of this application is not particularly limited, and it can be used in any electronic device known in the prior art. The battery disclosed in the embodiments of this application can be used in electrical devices that use batteries as a power source or in various energy storage systems that use batteries as energy storage elements. That is, an electrical device is provided. In some embodiments, the electrochemical device of this application can be used in, but is not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, ships, spacecraft, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, household large-capacity batteries, and sodium-ion capacitors, etc.

[0148] The device can be configured to use individual battery cells, battery modules, or battery packs depending on its application requirements.

[0149] Please refer to Figure 3 , Figure 3This is a schematic diagram of a vehicle according to one or more embodiments. The vehicle 1000 can be a gasoline-powered vehicle, a natural gas-powered vehicle, or a new energy vehicle. The new energy vehicle can be a pure electric vehicle, a hybrid electric vehicle, or a range-extended electric vehicle, etc. A secondary battery 100 is disposed inside the vehicle 1000, and the secondary battery 100 can be located at the bottom, front, or rear of the vehicle 1000. The secondary battery 100 can be used to power the vehicle 1000; for example, the secondary battery 100 can serve as the operating power source for the vehicle 1000. The vehicle 1000 may also include a controller 200 and a motor 300. The controller 200 is used to control the secondary battery 100 to supply power to the motor 300, for example, to meet the power needs of the vehicle 1000 during startup, navigation, and driving.

[0150] In some embodiments of this application, the secondary battery 100 can not only serve as the operating power source for the vehicle 1000, but also as the 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 this application are further illustrated below with reference to the embodiments.

[0152] To make the technical problems, technical solutions, and beneficial effects solved by the embodiments of this application clearer, the following will provide a more detailed description in conjunction with the embodiments and accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit this application or its applications. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

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

[0154] Drying chamber environment (humidity <10%) stirring and washing: Take 10 g of NFPP (Na4Fe3(PO4)2P2O7) material and 200 g of DMF (N,N-dimethylformamide) solvent in a beaker, place it on a magnetic stirrer and stir at 200 rpm for 2 h. Filter using a vacuum filter, and wash the filter cake again with 10 g of DMF. Solid-liquid separation yields the washed sodium ion positive electrode active material. Place the washed sodium ion positive electrode active material in a vacuum oven and dry at 105℃ for 10 h. After crushing and sieving, obtain powdered processed sodium ion positive electrode active material. The separated filtrate is recycled into a circulating storage tank. Specific experimental conditions for each example and comparative example are detailed in Table 1.

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

[0156] (1) Free sodium test

[0157] The method for testing the free sodium content in sodium-ion cathode active materials is as follows: In this application, the free sodium content of the sodium-ion cathode active material is tested according to GB / T9725-2007 "General Rules for Potentiometric Titration of Chemical Reagents" for the test of free lithium. The test results are calculated based on the molecular weight ratio of free sodium to free lithium. For example, if the molecular weight of NaHCO3 is 84 and the molecular weight of LiHCO3 is 68, the tested result shows a free lithium (LiHCO3) content of 0.1 ppm. Based on the molecular weight ratio of NaHCO3 to LiHCO3 of 1.235, the free sodium (NaHCO3) content of the sodium-ion cathode active material is calculated to be 0.124 ppm (i.e., 0.1 ppm multiplied by 1.235 rounded to 0.124 ppm). For example, Na₂CO₃ has a molecular weight of 10⁶, and Li₂CO₃ has a molecular weight of 74. The test result shows that the content of free lithium (Li₂CO₃) is 0.1 ppm. Based on the molecular weight ratio of Na₂CO₃ to Li₂CO₃ of 1.432, the content of free sodium (Na₂CO₃) in the sodium ion cathode active material is calculated to be 0.143 ppm. For example, Na₂CO₃ has a molecular weight of 10⁶, and Li₂CO₃ has a molecular weight of 74. The test result shows that the content of free lithium (Li₂CO₃) in the sodium ion cathode active material is 0.143 ppm. + With a molecular weight of 23, Li + With a molecular weight of 7, the test results yielded free lithium (Li). + The content of ) is 0.1 ppm, according to Na + With Li + The molecular weight ratio is 3.286, which translates to the free sodium (Na) in the sodium ion positive electrode active material. + The content of ) was 0.329 ppm.

[0158] GB / T9725-2007, "General Rules for Potentiometric Titration of Chemical Reagents," specifies the method for testing the free lithium content in lithium-ion cathode active materials as follows: The standard titrant used in this standard is prepared according to GB / T601, and the experimental water meets the specifications for Grade III water in GB / T6682. Samples are taken and test solutions are prepared according to the product standard. The specified electrodes are inserted, the electromagnetic stirrer is turned on, and titration is performed using the specified standard titrant. Approximately 90% of the required titration volume of standard titrant is added to the burette, and the potential or pH value of the solution is measured. The potential or pH value is measured once after each addition of 1 mL or an appropriate amount of standard titrant. Before and after the stoichiometric point, a measurement is taken once after each addition of 0.1 mL of standard titrant. Titration continues until the change in potential or pH value is minimal. The burette reading and the measured potential or pH value are recorded after each addition of standard titrant. The titration endpoint is determined using a graphical method or the second derivative method.

[0159] The method for determining the titration endpoint using a graphical approach is as follows: Plot a titration curve with the indicator electrode potential (mV) or pH value on the ordinate and the burette reading (mL) on the abscissa. Draw two tangents to the titration curve at 45° to the abscissa, and then draw a parallel line equidistant from the two tangents between them. The intersection of this parallel line with the titration curve is the titration endpoint. The abscissa of the intersection point represents the volume of standard titrant solution used at the titration endpoint, and the ordinate represents the potential or pH value at the titration endpoint.

[0160] The method for determining the titration endpoint using the second derivative method is as follows: 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 (ΔV) of standard titrant solution added each time.

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

[0163] The first-order derivative value. This refers to the change in potential or pH caused by a unit volume of standard titration solution, numerically equal to ΔE / ΔV or ΔpH / ΔV.

[0164] Second-level micro-business value. Numerically, it is equal to the difference between the adjacent first-level micro-business value.

[0165] The titration endpoint is when the number of first-level micro-businesses reaches its maximum and the number of second-level micro-businesses reaches zero.

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

[0167]

[0168] In the formula:

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

[0170] a- Level 2 WeChat merchant value before zero;

[0171] b - The value of a Level 2 WeChat merchant after the Level 2 WeChat merchant is zero;

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

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

[0174]

[0175]

[0176] Note: Positive electrode amount refers to the mass of sodium ion positive electrode active material; solvent amount refers to the initial mass of organic solvent added.

[0177] The results showed that, compared to the other examples, the sodium ion positive electrode active material in Comparative Example 1, without being washed with organic solvents, contained less free sodium (Na₂CO₃, NaHCO₃, Na₂CO₃). + The content of sodium ions is relatively high. Compared to Comparative Example 1, the sodium ion positive electrode active materials of the various embodiments treated with solvent have a higher content of free sodium (Na2CO3, NaHCO3, Na2CO3, Na2CO3). + The lower sodium content helps reduce the occurrence of side reactions or even gas generation between free sodium and the electrolyte, slowing down the capacity decay of the secondary battery. This, in turn, helps improve the device performance of the secondary battery, such as charge and discharge efficiency, cycle life, and safety.

[0178] As seen in Examples 1-3 and 18, when the solvent mass ratio decreases, the free sodium content of the sodium ion positive electrode active material first increases, then decreases, and then increases again. Therefore, when treating the sodium ion positive electrode active material, the amount of solvent used has a certain impact on the treatment effect. 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] As seen in Examples 4-6, when different single solvents are used, the free sodium content of the sodium ion positive electrode active material does not differ much, and all sodium ion positive electrode active materials have a low free sodium content.

[0180] As seen in Examples 2 and 7-12, when multiple solvents are mixed, the content of free sodium in the sodium ion positive electrode active material fluctuates to some extent. Therefore, when treating the sodium ion positive electrode active material, different solvent mixing systems have a certain impact on the treatment effect. The mixing system should be adjusted to reduce the content of free sodium in the sodium ion positive electrode active material to a greater extent.

[0181] As seen in Examples 2, 13-16, when the stirring time increases to a certain extent, the free sodium content of the sodium ion positive electrode active material decreases to a certain level and then stops decreasing. Therefore, while achieving the goal 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] As seen in Examples 2 and 17, 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 merely an embodiment of this application and does not limit the patent scope of this application. Any equivalent structural or procedural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A secondary battery characterized by comprising: 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, the free sodium comprises one or more of Na2CO3, NaHCO3, Na + 2CO3, NaHCO3, and Na2CO3, wherein 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 Na2CO3 is 0.20 ppm to 0.35 ppm. + 2CO3, NaHCO3, and Na2CO3, wherein 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 Na2CO3 is 0.20 ppm to 0.35 ppm.

2. The secondary battery according to claim 1, characterized by The total content of the free sodium is 0.5ppm-2.0ppm.

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

4. The secondary battery according to any one of claims 1 to 3, characterized by The chemical general 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; 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 of producing the secondary battery according to any one of claims 1 to 4, characterized by, The method comprises the following steps: Mixing the sodium ion positive electrode active material to be treated and the organic solvent to obtain a premix; Stirring the premix to obtain a mixture; The mixture is subjected to solid-liquid separation treatment to obtain a treated sodium-ion positive electrode active material, the treated sodium-ion positive electrode active material comprising free sodium, the free sodium comprising one or more of Na2CO3, NaHCO3, Na + The content of Na2CO3 in the free sodium is 0.2 ppm to 0.38 ppm; and / or the content of NaHCO3 in the free sodium is 0.38 ppm to 0.53 ppm; and / or the content of Na + The content of Na2CO3 in the free sodium is 0.2 ppm to 0.38 ppm; and / or the content of NaHCO3 in the free sodium is 0.38 ppm to 0.53 ppm; and / or the content of Na Forming a positive electrode active material layer on at least one surface of a positive electrode current collector by using the treated sodium ion positive electrode active material, and forming a positive electrode sheet by using the positive electrode current collector and the positive electrode active material layer arranged on at least one surface of the positive electrode current collector; Providing a separator and a negative electrode sheet; Assembling the positive electrode sheet, the separator and the negative electrode sheet to form a secondary battery.

6. The method of claim 5, wherein the step of forming the second electrode is performed by a method selected from the group consisting of a sputtering method, a vapor deposition method, a plating method, and a printing method. 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 of claim 6, wherein the step of forming the second electrode is performed by a method selected from the group consisting of a sputtering method, a vapor deposition method, a plating method, and a printing method. 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 of producing a secondary battery according to any one of claims 5 to 7, characterized by, The organic solvent comprises one or more of N,N dimethylformamide, anhydrous ethanol, N-methyl pyrrolidone, acetonitrile, dimethyl carbonate, diethyl carbonate, diisopropyl carbonate, tetrahydrofuran, 2-methyl tetrahydrofuran, 1,3-dioxolane and 1,3-dioxolane.

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

10. The method of producing a secondary battery according to any one of claims 5 to 9, characterized by, The organic solvent comprises at least two kinds, and the mass ratio of the two kinds of organic solvent is (0-10):(0-10).

11. The method of producing a secondary battery according to any one of claims 5 to 10, characterized by, The stirring treatment comprises stirring the premix at a stirring speed of 10r / min-100r / min for 0.25h-5h.

12. The method of claim 11, wherein the method further comprises: The stirring treatment comprises stirring the premix at a stirring speed of 15r / min-50r / min for 0.5h-3h.

13. The method of producing a secondary battery according to any one of claims 5 to 12, characterized by, The stirring treatment is performed at normal temperature, normal pressure and an environmental humidity of ≤10%.

14. The method of producing a secondary battery according to any one of claims 5 to 13, characterized by, The solid-liquid separation treatment comprises suction filtration treatment, and the suction filtration treatment comprises the following steps: Performing primary suction filtration treatment on the mixture; Adding new organic solvent to the suction filtration product after the primary suction filtration treatment to perform secondary suction filtration treatment; 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 of producing a secondary battery according to any one of claims 5 to 14, characterized by, The solid-liquid separation treatment comprises spray drying treatment.

16. The method of claim 15, wherein the method further comprises: During the spray drying treatment, the inlet air temperature is 190℃-230℃, and the outlet temperature is 85℃-95℃.

17. The method of producing a secondary battery according to any one of claims 5 to 16, characterized by, The solid-liquid separation treatment on the mixture further comprises drying treatment on the material after the solid-liquid separation treatment; the drying treatment is performed at a temperature of 80℃-200℃ for 4h-24h.

18. The method of claim 17, wherein the method further comprises: The drying treatment is performed at a temperature of 100℃-120℃ for 6h-12h.

19. The method of producing a secondary battery according to any one of claims 5 to 18, characterized by, The method further comprises collecting used organic solvent and recycling the collected organic solvent.

20. The method of claim 19, wherein the method further comprises: The used organic solvent is collected by condensation treatment.

21. The method of claim 20, wherein the method further comprises: The collected organic solvent is subjected to suction filtration treatment or distillation purification treatment.

22. An electrical device, comprising: The secondary battery is prepared by the method of any one of claims 1-4 or the method of any one of claims 5-21.

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