Positive electrode material of sodium secondary battery, preparation method of positive electrode material, positive electrode plate, sodium secondary battery and electric device

Abstract

The invention belongs to the technical field of batteries, and provides a sodium secondary battery positive electrode material and a preparation method thereof, a positive electrode plate, a sodium secondary battery and an electric device. The sodium secondary battery positive electrode material comprises a sodium-containing positive electrode material and a coating layer located on at least part of the surface of the sodium-containing positive electrode material, and the coating layer comprises a hydrophobic polymer. The sodium secondary battery positive electrode material provided by the invention can improve the conductivity and cycling stability of the battery.

Description

Technical Field

[0001] This application belongs to the technical field of batteries, and provides a positive electrode material for a sodium secondary battery, a preparation method thereof, a positive electrode plate, a sodium secondary battery, and an electrical device. Background Art

[0002] Secondary batteries represented by sodium batteries have been applied in energy storage power systems (such as hydraulic, thermal, wind, and solar power stations, etc.) and many fields such as electric vehicles and aerospace. Compared with lithium batteries, sodium batteries have a cost advantage in terms of raw materials. In particular, the reserves of sodium salts, which are the main components of sodium battery positive electrode materials, are more abundant, and the price of sodium salts is also much lower than that of lithium salts used in lithium battery positive electrode materials, which makes the cost of sodium battery positive electrode materials lower than that of lithium batteries. However, the electrochemical performance of sodium batteries in terms of rate performance and stability is not good, resulting in limitations in their practical applications. Summary of the Invention

[0003] Aiming at the above-mentioned problems, the purpose of this application is to provide a positive electrode material for a sodium secondary battery, a preparation method thereof, a positive electrode plate, a sodium secondary battery, and an electrical device. The positive electrode material for the sodium secondary battery has high environmental stability and can improve the electrochemical performance of the battery.

[0004] The first aspect of this application provides a positive electrode material for a sodium secondary battery, including a sodium-containing positive electrode material and a coating layer located on at least part of the surface of the sodium-containing positive electrode material. Among them, the sodium-containing positive electrode material includes at least one of a layered oxide, a Prussian blue compound, or a polyanion-type sodium-containing compound; the coating layer contains a hydrophobic polymer.

[0005] In the positive electrode material for the sodium secondary battery of this application, introducing a coating layer containing a hydrophobic polymer on the surface of the sodium-containing positive electrode material can effectively resist the reaction rate and ability of residual alkali (such as sodium bicarbonate, sodium carbonate, etc.) with air components (such as H2O, CO2), and improve the environmental stability of the material; in addition, the coating layer containing a hydrophobic polymer can be used as a buffer layer to relieve the stress generated during the insertion / extraction of sodium ions. In this way, the positive electrode material for the sodium secondary battery can improve the conductivity and cycle stability of the battery.

[0006] In some embodiments of this application, the hydrophobic polymer and the sodium-containing positive electrode material satisfy the following relationship: T m <T a where T m is the melting point of the hydrophobic polymer, and T a is the phase transition temperature of the sodium-containing positive electrode material. Thus, the hydrophobic polymer has high processability and can improve the coating efficiency on the surface of the sodium-containing positive electrode material.

[0007] In some embodiments of the present application, the phase transition temperature T of the sodium-containing cathode material a ≤600 °C.

[0008] In some embodiments of the present application, the mass ratio of the hydrophobic polymer to the sodium-containing cathode material is (0.2 to 10):100.

[0009] In some embodiments of the present application, the hydrophobic polymer includes a first hydrophobic polymer material, and the first hydrophobic polymer material includes at least one of polyethylene, polypropylene, polystyrene, polydimethylsiloxane, polyethylene terephthalate, polytetrafluoroethylene, polyamide, polycarbonate, or silicone wax.

[0010] In some embodiments of the present application, the hydrophobic polymer includes polydimethylsiloxane. Thus, the environmental stability of the cathode material can be further improved.

[0011] In some embodiments of the present application, the hydrophobic polymer includes the first hydrophobic polymer material, and the mass ratio of the first hydrophobic polymer material to the sodium-containing cathode material is (0.3 to 3):100, further optionally (0.5 to 2):100.

[0012] In some embodiments of the present application, the hydrophobic polymer includes a second hydrophobic polymer material and / or a third hydrophobic polymer material; the second hydrophobic polymer material includes polyacrylonitrile; the third hydrophobic polymer material includes a conjugated polymer and is a thermal cracking product of polyacrylonitrile.

[0013] Optionally, the hydrophobic polymer includes the conjugated polymer. Thus, the conductivity of the cathode material can be further improved.

[0014] In some embodiments of the present application, the hydrophobic polymer includes the second hydrophobic polymer material and / or the third hydrophobic polymer material, and the mass ratio of the hydrophobic polymer to the sodium-containing cathode material is (0.5 to 10):100, further optionally (1 to 5):100.

[0015] In some embodiments of the present application, the thermal cracking temperature T of the second hydrophobic polymer material satisfies the following relationship: T m1 <T<T a , where T m1 is the melting point of the second hydrophobic polymer material; T a is the phase transition temperature of the sodium-containing cathode material.

[0016] In some embodiments of the present application, the sodium-containing cathode material includes a sodium-containing polyanionic compound, and the chemical formula of the sodium-containing polyanionic compound is: Na x R y (PO4) Z (P2O7) k , where 1 ≤ x ≤ 7, 1 ≤ y ≤ 4, 1 ≤ z ≤ 2, 1 ≤ k ≤ 4, and R is at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W, or Pb. Thus, the sodium-containing cathode material has a low residual alkali content, which can further reduce the influence of residual alkali on battery performance.

[0017] In some embodiments of the present application, the sodium-containing cathode material includes the sodium-containing polyanionic compound and a carbon-based material modification layer on at least part of its surface. Thus, the conductivity of the cathode material for sodium secondary batteries can be improved.

[0018] In some embodiments of the present application, the carbon-based material includes a first carbon material and a second carbon material. The first carbon material includes at least one of natural graphite powder, artificial graphite powder, carbon black, carbon nanotubes, or graphene. The second carbon material includes a sintering product of an organic carbon source at 450 °C to 550 °C. Thus, the combined use of the first carbon material and the second carbon material can limit the grain growth of the sodium-containing polyanionic compound during sintering and improve the conductivity of the material.

[0019] Optionally, the organic carbon source includes at least one of sucrose, glucose, citric acid, starch, cyclodextrin, asphalt, or the hydrophobic polymer.

[0020] In some embodiments of the present application, in the sodium-containing cathode material, the mass ratio of the first carbon material to the sodium-containing polyanionic compound is (0.5 to 2) : 100. Thus, while improving the conductivity, the capacity of the positive electrode sheet is not affected.

[0021] In some embodiments of the present application, in the sodium-containing cathode material, the mass ratio of the sintering product to the sodium-containing polyanionic compound is (0.5 to 2) : 100. Thus, while improving the conductivity, the capacity of the positive electrode sheet is not affected.

[0022] In some embodiments of the present application, the median particle size Dv of the sodium-containing cathode material 50 is 2 μm to 8 μm.

[0023] In some embodiments of the present application, the BET specific surface area of the sodium-containing cathode material is 3 m 2 / g to 11 m 2 / g.

[0024] In some embodiments of the present application, the water content of the positive electrode material of the sodium secondary battery in a sealed environment is not higher than 2000 ppm.

[0025] Optionally, the water content of the positive electrode material of the sodium secondary battery in a sealed environment is 50 ppm to 1600 ppm.

[0026] The second aspect of the present application provides a method for preparing a positive electrode material of a sodium secondary battery, including: mixing a coating raw material with a sodium-containing positive electrode material and performing a first sintering to form a coating layer containing a hydrophobic polymer on at least a part of the surface of the sodium-containing positive electrode material, wherein the sodium-containing positive electrode material includes at least one of a layered oxide, a Prussian blue compound, or a polyanion-type sodium-containing compound.

[0027] In some embodiments of the present application, the first sintering temperature T1 satisfies the following relationship: T m <T1<T a ; wherein, T m is the melting point of the coating raw material, and T a is the phase transition temperature of the sodium-containing positive electrode material. Thus, while improving the processing fluidity of the coating raw material to improve the coating effect, the influence on the sodium-containing positive electrode material is minimized.

[0028] Optionally, the phase transition temperature T a of the sodium-containing positive electrode material ≤ 600 °C.

[0029] In some embodiments of the present application, the coating raw material includes a first hydrophobic polymer material and / or a second hydrophobic polymer material; wherein, the first hydrophobic polymer material includes at least one of polyethylene, polypropylene, polystyrene, polydimethylsiloxane, polyethylene terephthalate, polytetrafluoroethylene, polyamide, polycarbonate, or silicone wax; the second hydrophobic polymer material includes polyacrylonitrile.

[0030] In some embodiments of the present application, the coating raw material includes the first hydrophobic polymer material, and the first sintering temperature T1 and the initial thermal cracking temperature T' of the first hydrophobic polymer material satisfy the relationship: T1 < T'. Thus, the decomposition of the coating raw material can be minimized, ensuring that the formed coating layer has high hydrophobicity.

[0031] In some embodiments of the present application, the coating raw material includes polydimethylsiloxane.

[0032] Optionally, the first sintering temperature T1 is 180 °C to 260 °C, and the sintering time is 2 h to 8 h.

[0033] Further, the first sintering temperature T1 is 200°C to 250°C, and the sintering time is 4h to 7h. Thus, a cathode material uniformly coated with polydimethylsiloxane can be obtained through sintering, improving the environmental stability of the material.

[0034] In some embodiments of the present application, the coating raw material includes the second hydrophobic polymer material (polyacrylonitrile), and the first sintering temperature T1 and the initial thermal cracking temperature T” of the second hydrophobic polymer material satisfy the following relationship: T1 > T”. Thus, the second hydrophobic polymer material undergoes thermal cracking through sintering to form a hydrophobic polymer with a conjugated structure (i.e., conjugated polymer), improving the conductivity.

[0035] Optionally, the first sintering temperature T1 is 350°C to 550°C, and the sintering time is 4h to 12h.

[0036] Further, the first sintering temperature T1 is 400°C to 500°C, and the sintering time is 5h to 12h. Thus, it is possible to avoid as much as possible the high degree of carbonization of polyacrylonitrile caused by too high temperature, resulting in weakened hydrophobicity of the coating layer, and it is also possible to avoid too low conjugation degree that may be caused by too low temperature and affect the electrical properties of the material.

[0037] In some embodiments of the present application, the sodium-containing cathode material contains a polyanionic sodium-containing compound, and the chemical formula of the polyanionic sodium-containing compound is: Na x R y (PO4) Z (P2O7) k , where 1 ≤ x ≤ 7, 1 ≤ y ≤ 4, 1 ≤ z ≤ 2, 1 ≤ k ≤ 4, and R is at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W, or Pb.

[0038] In some embodiments of the present application, the sodium-containing cathode material includes the polyanionic sodium-containing compound and a carbon-based material modification layer on at least part of its surface. The specific description of the sodium-containing cathode material is as shown in the first aspect of the present application.

[0039] The third aspect of the present application provides a positive electrode sheet, including the sodium secondary battery cathode material described in the first aspect of the present application or the sodium secondary battery cathode material prepared by the method described in the second aspect of the present application.

[0040] The fourth aspect of the present application provides a sodium secondary battery, including the positive electrode sheet described in the third aspect of the present application.

[0041] In some embodiments of the present application, the sodium secondary battery includes a sodium ion secondary battery.

[0042] In some embodiments of the present application, the sodium secondary battery includes a sodium secondary battery without a negative electrode. Since no sodium metal is pre-set on the negative electrode side of the sodium secondary battery without a negative electrode, the problems of the decrease in Coulombic efficiency and cycle performance caused by the violent reaction between sodium metal and the electrolyte can be effectively alleviated.

[0043] Optionally, the sodium secondary battery includes a negative electrode plate, and the negative electrode plate includes a negative electrode current collector and an optional conductive layer.

[0044] Optionally, the negative electrode plate includes the conductive layer, and the conductive layer is disposed on at least one side of the negative electrode current collector. The setting of the conductive layer is more conducive to the reduction and deposition of sodium ions by electrons on the negative electrode current collector during charging to form a sodium metal negative electrode.

[0045] Optionally, the negative electrode current collector includes any one of bare copper, aluminum foil, aluminum alloy foil, or an aluminum-based composite current collector.

[0046] Optionally, the conductive layer includes a conductive agent and a binder.

[0047] Further, the conductive agent includes at least one of graphite, graphene, carbon fiber, carbon black, carbon dots, soft carbon, hard carbon, multi-walled carbon nanotubes, or single-walled carbon nanotubes.

[0048] A fifth aspect of the present application provides an electrical device including the sodium secondary battery described above.

[0049] Additional aspects and advantages of the present application will be given in part in the following description, become apparent in part from the following description, or be learned through the practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] By reading the following detailed description of the preferred embodiments, various other advantages and benefits will become clear to those of ordinary skill in the art. The drawings are only for the purpose of showing the preferred embodiments and are not considered to be a limitation of the present application. In the drawings:

[0051] Figure 1 is a scanning electron microscope image of the sodium-containing positive electrode material (NFPP@C) of Comparative Example 1;

[0052] Figure 2 is a scanning electron microscope image of the positive electrode material (NFPP@C@PAN) of the sodium secondary battery of Example 1;

[0053] Figure 3 is a transmission electron microscope image of the positive electrode material (NFPP@C@PAN) of the sodium secondary battery prepared in Example 2;

[0054] Figure 4It is the EDS diagram of the positive electrode material (NFPP@C@PDMS) of the sodium secondary battery prepared in Example 12;

[0055] Figure 5 It is a schematic diagram of a battery cell according to an embodiment of the present application;

[0056] Figure 6 It is an exploded view of a battery cell according to an embodiment of the present application;

[0057] Figure 7 It is a schematic diagram of a battery module according to an embodiment of the present application;

[0058] Figure 8 It is a schematic diagram of a battery pack according to an embodiment of the present application;

[0059] Figure 9 It is Figure 8 The exploded view of the battery pack according to an embodiment of the present application shown;

[0060] Figure 10 It is a schematic diagram of an electrical device using the battery according to an embodiment of the present application as a power source.

[0061] Explanation of reference numerals:

[0062] 1: Battery pack; 2: Upper box body; 3: Lower box body; 4: Battery module; 5: Battery cell; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Detailed embodiments

[0063] The present application will be further described below in conjunction with the detailed embodiments. It should be understood that these detailed embodiments are only used to illustrate the present application and not to limit the scope of the present application.

[0064] In the present application, the mention of "embodiment" means that the specific features, structures or characteristics described in connection with the embodiment may be included in at least one embodiment of the present application. The phrase appears in various positions in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described in the present application may be combined with other embodiments.

[0065] The "scope" disclosed in this application is defined in the form of a lower limit and / or an upper limit. A given scope is defined by selecting a lower limit and / or an upper limit, and the selected lower limit and / or upper limit define the boundaries of a particular scope. The scope defined in this way can include the end values or not include the end values, and can be combined arbitrarily, that is, any lower limit can be combined with any upper limit to form a scope not explicitly recited, and any lower limit can be combined with other lower limits to form a scope not explicitly recited. Similarly, any upper limit can be combined with any other upper limit to form a scope not explicitly recited. In addition, each individually disclosed point or single numerical value itself can be used as a lower limit or an upper limit and combined with any other point or single numerical value or combined with other lower limits or upper limits to form a scope not explicitly recited.

[0066] If there is no special instruction, all the technical features and optional technical features of this application can be combined with each other to form a new technical solution, and such a technical solution should be considered to be included in the disclosure of this application.

[0067] If there is no special instruction, the terms "comprising", "including", "containing", "having" mentioned in this application mean open-ended or can also be closed-ended. For example, the said "comprising", "including", "containing", "having" can mean that other components not listed can also be included or contained, or can only include or contain the listed components. In addition, in this application, the terms "a plurality of", "a variety of", "at least one" mean more than two. "Above" and "below" include the number itself. For example, "more than two" includes two itself, such as two, three, four or more.

[0068] If there is no special instruction, in this application, the term "and / or" is only a relational term describing the associated objects, indicating that there can be three relationships. For example, A and / or B can mean: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in this text generally means that the associated objects before and after are in an "or" relationship.

[0069] Unless otherwise defined, all the technical and scientific terms used in this application have the same meaning as those commonly understood by those skilled in the technical field to which this application belongs; the terms used in this application are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "comprising" and "having" in the specification and claims of this application and any variations thereof are intended to cover non-exclusive inclusion. Unless otherwise stated, the terms used in this application have the well-known meanings commonly understood by those skilled in the art. Unless otherwise stated, the numerical values of the various parameters mentioned in this application can be measured by various common measurement methods in the art (for example, they can be tested according to the methods given in the embodiments of this application).

[0070] Although the cathode materials for sodium secondary batteries are similar in performance to those for lithium secondary batteries, their interfaces are more sensitive to air and electrolytes during both storage and use. In the current sodium battery field, due to reasons such as process and formulation ratio, the surface of the cathode material has residual alkali (such as sodium bicarbonate, sodium carbonate, etc.), and the surface residual alkali is prone to react with water and carbon dioxide in the air, resulting in poor environmental stability of the sodium battery material, which is not conducive to the subsequent application of the material, easily causing the cell to swell and the stability to decrease, and further leading to safety hazards and poor quality of the battery. Therefore, it is necessary to modify the sodium battery cathode material.

[0071] Accordingly, in a first aspect, the present application provides a cathode material for a sodium secondary battery, including a sodium-containing cathode material and a coating layer located on at least a part of the surface of the sodium-containing cathode material. The sodium-containing cathode material includes at least one of a layered oxide, a Prussian blue compound, or a polyanion-type sodium-containing compound, and the coating layer contains a hydrophobic polymer.

[0072] In the present application, a hydrophobic polymer refers to a polymer material that is not easily adsorbed with moisture and has water-proof and moisture-proof properties. The structure of a hydrophobic polymer usually does not contain hydrophilic groups such as hydroxyl groups, carboxyl groups, and sulfonic acid groups. In addition, the hydrophobicity of a polymer can also be judged according to its surface energy. The lower the surface energy, the greater the hydrophobicity, and the surface energy of a hydrophobic polymer is lower than that of water. Generally, the surface energy of the hydrophobic polymer measured by the contact angle method is less than 71 mJ / m 2 。

[0073] In some embodiments, the hydrophobic polymer and the sodium-containing cathode material satisfy the following relationship:

[0074] T m <T a ,wherein,

[0075] T m is the melting point of the hydrophobic polymer,

[0076] T a is the phase transition temperature of the sodium-containing cathode material. Thus, the hydrophobic polymer has high processability and can improve the coating efficiency on the surface of the sodium-containing cathode material.

[0077] As an example, the phase transition temperature T a of the sodium-containing cathode material ≤ 600 °C.

[0078] As an example, the melting point T m of the hydrophobic polymer ≤ 500 °C.

[0079] In some embodiments, the hydrophobic polymer includes a first hydrophobic polymer material, and the first hydrophobic polymer material includes at least one of polyethylene, polypropylene, polystyrene, polydimethylsiloxane (PDMS), polyethylene terephthalate, polytetrafluoroethylene, polyamide, polycarbonate, or silicone wax.

[0080] Furthermore, the hydrophobic polymer includes polydimethylsiloxane. Forming a coating layer from polydimethylsiloxane can further improve the environmental stability of the cathode material.

[0081] In other embodiments, the hydrophobic polymer includes a second hydrophobic polymer material and / or a third hydrophobic polymer material; the second hydrophobic polymer material includes polyacrylonitrile, and the third hydrophobic polymer material includes a conjugated polymer, which is a thermal pyrolysis product of polyacrylonitrile.

[0082] Optionally, the hydrophobic polymer includes the conjugated polymer. The conjugated polymer has semiconductor properties, and thus, the conductivity of the cathode material can be further improved.

[0083] In some embodiments, the thermal pyrolysis temperature T (the temperature for forming the conjugated polymer) of the second hydrophobic polymer material satisfies the following relationship: T m1 <T<T a wherein, T m1 is the melting point of the second hydrophobic polymer material; T a is the phase transition temperature of the sodium-containing cathode material.

[0084] Furthermore, the hydrophobic polymer is the thermal pyrolysis product of polyacrylonitrile. It can be understood that polyacrylonitrile can undergo partial carbonization at the thermal pyrolysis temperature to form double bonds (including carbon-carbon double bonds and carbon-nitrogen double bonds), and a conjugated structure is formed between the double bonds, thereby converting polyacrylonitrile into a polymer with a conjugated structure and having conductivity.

[0085] In the present application, the sodium-containing cathode material includes one or several of layered oxides, Prussian blue compounds, or polyanion-type sodium-containing compounds.

[0086] As some examples, the chemical formula of the layered oxide can be Nax1MO2, 0 < x1 ≤ 1, and M includes at least one of transition metal elements. For example, M includes variable valence transition metals such as vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), etc. Examples of the layered oxide include, but are not limited to, NaCrO2, NaMnO2, NaMnO2, Na 0.61 Ti 0.48 Mn 0.52 O2, Na[Fe0.5 Co 0.5 O2, etc.

[0087] In some embodiments, from the perspective of reducing the residual alkali content, the sodium-containing cathode material includes a polyanionic sodium-containing compound. The chemical formula of the polyanionic sodium-containing compound is: Na x R y (PO4) Z (P2O7) k , where 1 ≤ x ≤ 7, 1 ≤ y ≤ 4, 1 ≤ z ≤ 2, 1 ≤ k ≤ 4, and R is at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W, or Pb.

[0088] As an example, the polyanionic sodium-containing compound is sodium phosphate-based salt Na4Fe3(PO4)2(P2O7) (abbreviated as NFPP).

[0089] In some embodiments, the sodium-containing cathode material includes the polyanionic sodium-containing compound and a carbon-based material modification layer located on at least part of its surface. The carbon-based material has high conductivity. By forming the modification layer, the problem of low conductivity of the polyanionic sodium-containing compound can be compensated for.

[0090] In some embodiments, the carbon-based material includes a first carbon material and a second carbon material. The combined use of the first carbon material and the second carbon material can limit the grain growth of the polyanionic sodium-containing compound during sintering (grain growth will cause a decrease in conductivity), and jointly improve the conductivity of the material. The first carbon material and the second carbon material will be described separately below.

[0091] The first carbon material is a conductive carbon material, including at least one of natural graphite powder, artificial graphite powder, carbon black, carbon nanotubes, or graphene.

[0092] Optionally, the mass ratio of the first carbon material to the polyanionic sodium-containing compound is (0.5 - 2)∶100, such as 0.5∶100, 1∶100, 1.5∶100, 2∶100, etc. Thus, it is beneficial to obtain a suitable surface carbon content, taking into account cost and reducing the film resistance.

[0093] The second carbon material includes the sintering product of an organic carbon source at 450°C - 550°C. Optionally, the organic carbon source may include at least one of sucrose, glucose, citric acid, starch, cyclodextrin, asphalt, or the hydrophobic polymer. As an example, the second carbon material is the sintering product of glucose at 500°C. After TGA testing, the residual carbon content of glucose after this sintering is about 20%.

[0094] Optionally, the mass ratio of the sintered product to the sodium-containing polyanionic compound is (0.5 to 2):100, such as 0.5:100, 1:100, 1.5:100, 2:100, etc. Thus, it is beneficial to obtain an appropriate surface carbon content, taking into account cost and reducing the sheet resistance.

[0095] In some embodiments, in the sodium-containing cathode material, the thickness of the carbon-based material modification layer is 1 nm to 5 nm.

[0096] In some embodiments, the median particle size (Dv 50 ) of the sodium-containing cathode material is 2 μm to 8 μm, such as 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, etc. In this application, the median particle size Dv50 can be measured by the particle size distribution - laser diffraction method with reference to the GB / T 19077-2016 standard.

[0097] In some embodiments, the BET specific surface area of the sodium-containing cathode material is 3 m 2 / g to 11 m 2 / g, such as 3 m 2 / g, 4 m 2 / g, 5 m 2 / g, 6 m 2 / g, 8 m 2 / g, 9 m 2 / g, 11 m 2 / g, etc. In this application, the BET specific surface area can be measured by a flow method gas adsorption type specific surface area measuring device with reference to the GB / T 19587-2017 standard.

[0098] In some embodiments, the tap density of the sodium-containing cathode material under a pressure of 100 MPa can be 1.3 g / cm 3 to 2.1 g / cm 3 .

[0099] In some embodiments, the coating amount of the hydrophobic polymer can be determined according to the material type and coating thickness. Specifically, the coating amount of the hydrophobic polymer satisfies the following relational expression:

[0100] M = A × (M0 / 100 g) × (H / 10 nm) × ρ

[0101] where M represents the coating thickness, in g;

[0102] A represents the BET specific surface area of the sodium-containing cathode material, in m 2 / g;

[0103] M0 represents the mass of the sodium-containing cathode material, in g;

[0104] H represents the thickness of the coating layer, with the unit of nm;

[0105] ρ represents the density of the hydrophobic polymer, with the unit of g / mL or g / cm 3 Thus, while ensuring the coating effect, the influence of the coating layer on the battery capacity density can be reduced. Additionally, the coating thickness in this formula can refer to the preset coating thickness, that is, the coating amount of the hydrophobic polymer can be determined according to the required coating thickness.

[0106] In some embodiments, the mass ratio of the hydrophobic polymer to the sodium-containing cathode material is (0.2 - 10):100, such as 0.2:100, 0.3:100, 0.5:100, 0.8:100, 1:100, 2:100, 2.5:100, 3:100, 5:100, 8:100, etc.

[0107] Optionally, the hydrophobic polymer includes the first hydrophobic polymer material (such as PDMS), and the mass ratio of the first hydrophobic polymer material to the sodium-containing cathode material is (0.3 - 3):100, such as 0.3:100, 0.5:100, 0.8:100, 1:100, 2:100, 3:100, etc. Thus, the environmental stability of the cathode material can be improved without affecting the electrochemical performance of the material. Further optionally, the mass ratio of the first hydrophobic polymer material to the sodium-containing cathode material is (0.5 - 2):100.

[0108] Optionally, the hydrophobic polymer includes the second hydrophobic polymer material and / or the third hydrophobic polymer material, and the mass ratio of the hydrophobic polymer to the sodium-containing cathode material is (0.5 - 10):100, such as 0.5:100, 1:100, 2:100, 3:100, 4:100, 5:100, 8:100, 10:100, etc. Further, the mass ratio of the hydrophobic polymer to the sodium-containing cathode material is (1 - 5):100. Thus, the cathode material can have both high environmental stability and conductivity.

[0109] In some embodiments, in the sodium secondary battery cathode material, the total thickness range of the coating layer and the carbon-based material modification layer is 2nm - 8nm, and this thickness can be measured by transmission electron microscopy.

[0110] In some embodiments, the water content of the sodium secondary battery cathode material in a sealed environment is not higher than 2000ppm, and can be optionally 50ppm - 1600ppm. In this application, the water content of the material can be measured by referring to the method of GB / T 11133 - 2015. Optionally, the temperature of the sealed environment is 0°C - 10°C.

[0111] In a second aspect, the present application provides a method for preparing a positive electrode material for a sodium secondary battery, including: mixing a coating raw material and a sodium-containing positive electrode material, and performing a first sintering to form a coating layer containing a hydrophobic polymer on at least a part of the surface of the sodium-containing positive electrode material, where the sodium-containing positive electrode material includes at least one of a layered oxide, a Prussian blue compound, or a polyanion-type sodium-containing compound.

[0112] In some embodiments, the coating raw material includes a first hydrophobic polymer material and / or a second hydrophobic polymer material. The first hydrophobic polymer material and the second hydrophobic polymer material are as described in the first aspect of the present application, and will not be elaborated here.

[0113] In the present application, the temperature of the first sintering can be selected according to the types of the sodium-containing positive electrode material and the coating raw material. As some examples, the first sintering temperature T1 can be 180°C to 550°C.

[0114] In some embodiments, the mass ratio of the coating raw material to the sodium-containing positive electrode material is (0.2 to 10):100, such as 0.2:100, 0.3:100, 0.5:100, 0.8:100, 1:100, 2:100, 2.5:100, 3:100, 5:100, 8:100, etc.

[0115] In some embodiments, the first sintering temperature T1 satisfies the relationship: T m <T1<T a ; where T m is the melting point of the coating raw material, and T a is the phase transition temperature of the sodium-containing positive electrode material. Thus, while improving the processing fluidity of the coating raw material to improve the coating effect, the influence on the sodium-containing positive electrode material is minimized.

[0116] Optionally, the phase transition temperature T a of the sodium-containing positive electrode material ≤ 600°C.

[0117] In some embodiments, the coating raw material includes the first hydrophobic polymer material, and the mass ratio of the first hydrophobic polymer material to the sodium-containing positive electrode material is (0.3 to 3):100, such as 0.3:100, 0.5:100, 0.8:100, 1:100, 2:100, 3:100, etc. Thus, the prepared positive electrode material has high environmental stability without affecting the electrochemical performance of the material. Further, the mass ratio of the coating raw material to the sodium-containing positive electrode material is (0.5 to 2):100.

[0118] In some embodiments, the coating raw material includes the first hydrophobic polymer material, and the relationship between the first sintering temperature T1 and the initial thermal cracking temperature T' of the first hydrophobic polymer material is: T1 < T'. Thus, the decomposition of the coating raw material can be minimized, ensuring that the formed coating layer has high hydrophobicity. It should be understood that the initial thermal cracking temperature refers to the temperature at which the polymer material begins to decompose. The initial thermal cracking temperature can be measured by thermogravimetric analysis (TGA).

[0119] In some specific embodiments, the coating raw material includes polydimethylsiloxane.

[0120] Optionally, the first sintering temperature T1 can be 180°C to 260°C, such as 180°C, 200°C, 225°C, 230°C, 240°C, 250°C, 260°C, etc.

[0121] Optionally, the time for the first sintering is 2 h to 8 h, such as 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h.

[0122] Further, the first sintering temperature T1 is 200°C to 250°C, and the sintering time is 4 h to 7 h. This sintering temperature is lower than the initial thermal decomposition temperature of PDMS (about 300°C), which can minimize the decomposition of polydimethylsiloxane and obtain a cathode material with uniformly coated polydimethylsiloxane, improving the environmental stability of the material.

[0123] In some embodiments, the coating raw material includes the second hydrophobic polymer material, and the mass ratio of the coating raw material to the sodium-containing cathode material is (0.5 - 10):100, such as 0.5:100, 1:100, 2:100, 3:100, 4:100, 5:100, 8:100, 10:100, etc. Further, the mass ratio of the coating raw material to the sodium-containing cathode material is (1 - 5):100. Thus, the prepared cathode material can have both high environmental stability and conductivity.

[0124] In some embodiments, the coating raw material includes the second hydrophobic polymer material, and the relationship between the first sintering temperature T1 and the initial thermal cracking temperature T” of the second hydrophobic polymer material is: T1 > T”. Thus, by sintering, the second hydrophobic polymer material (polyacrylonitrile) undergoes thermal cracking to form a polymer with a high degree of conjugation. The resulting conjugated polymer uniformly coats the surface of the sodium-containing cathode material as a hydrophobic polymer, which can improve the conductivity of the material.

[0125] In some specific embodiments, the coating raw material includes polyacrylonitrile (PAN).

[0126] Optionally, the first sintering temperature T1 is 350°C to 550°C, such as 350°C, 400°C, 450°C, 500°C, 520°C, 550°C, etc. Optionally, the time for the first sintering is 4 h to 12 h, such as 4 h, 5 h, 6 h, 8 h, 10 h, 12 h, etc. In some examples, PAN and the sintering products at 400°C, 450°C, and 500°C are characterized and analyzed by infrared spectroscopy. The polyacrylonitrile material has an obvious characteristic peak of cyano group (-C≡N) near 2200 cm -1 nearby, while this characteristic peak disappears in the sintering products at the three temperatures, and an obvious C=C-C=N characteristic peak appears near 1600 cm -1 nearby, indicating that a conjugated structure is formed in PAN after sintering.

[0127] Furthermore, the first sintering temperature T1 is 400°C to 500°C, and the sintering time is 5 h to 12 h. Thus, it is possible to avoid the high carbonization degree of polyacrylonitrile caused by too high temperature, resulting in the weakening of the hydrophobicity of the coating layer, and it is also possible to avoid the too low conjugation degree that may be caused by too low temperature and affect the electrical properties of the material.

[0128] In the present application, the mixing method of the coating raw material and the sodium-containing cathode material can be a dry mixing method or a wet method. Among them, the wet method can be carried out in the presence of a solvent, and the solvent is, for example, a polar organic solvent such as N,N-dimethylformamide (DMF), N-methylpyrrolidone, dimethyl sulfoxide, sulfolane, ethylene nitrate, etc. When the wet method is adopted, the mixing further includes drying (such as spray drying) to remove the solvent. Optionally, the dry mixing method is adopted for mixing the coating raw material and the sodium-containing cathode material, thereby simplifying the preparation process and reducing the cost.

[0129] In the preparation method of the present application, the specific description of the sodium-containing cathode material is as described in the first aspect of the present application, and will not be elaborated here.

[0130] In some embodiments, the sodium-containing cathode material includes the polyanion-type sodium-containing compound and a carbon-based material modification layer on at least part of its surface. The method further includes preparing the sodium-containing cathode material, specifically including:

[0131] Mixing and second sintering a sodium source, an R source, a phosphorus source, and a carbon source to obtain a sodium-containing cathode material; wherein,

[0132] The carbon source includes a first carbon material and an organic carbon source.

[0133] Optionally, the phase transition temperature of the polyanionic sodium-containing compound is < 600 °C. The temperature of the second sintering is 450 °C to 550 °C, such as 450 °C, 500 °C, 550 °C, etc. If the sintering temperature is too low, it may lead to a low degree of carbonization of the organic carbon source, affecting the conductivity of the material. If the sintering temperature is too high, it may cause the grains of the material to grow rapidly, incomplete solid-phase reaction, and generation of impurity phases.

[0134] Optionally, the time of the second sintering is 4 h to 10 h, such as 4 h, 5 h, 7 h, 8 h, 10 h, etc.

[0135] Optionally, the mixing in the preparation of the sodium-containing cathode material can be carried out in the presence of a solvent (such as water).

[0136] As some examples, the sodium source may include sodium pyrophosphate (Na4P2O7).

[0137] As some examples, the R source may include an iron source. The iron source may be selected, for example, from at least one of inorganic iron salts, organic iron salts (such as ferrous oxalate), metallic iron, or iron oxides.

[0138] As some examples, the phosphorus source may include phosphoric acid, phosphates (such as ammonium dihydrogen phosphate).

[0139] In addition, the explanations of the chemical formula Na x R y (PO4) Z (P2O7) k and the first carbon material and the organic carbon source are specifically as described in the first aspect of the present application, and will not be elaborated herein.

[0140] In a third aspect, the present application provides a positive electrode sheet, including the sodium secondary battery cathode material described in the first aspect of the present application or the sodium secondary battery cathode material prepared by the method described in the second aspect of the present application.

[0141] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one side of the positive electrode current collector, and the positive electrode film layer contains the sodium secondary battery cathode material.

[0142] In the present application, the positive electrode current collector may be, for example, a metal foil or a composite current collector. The metal foil is, for example, an aluminum foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one side of the polymer base layer. Among them, the material of the metal layer includes, but is not limited to, aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.; the polymer material of the polymer base layer may be, for example, polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0143] In some embodiments, the positive electrode material of the sodium secondary battery serves as the positive electrode active material in the positive electrode film layer. In addition to this positive electrode material, the positive electrode film layer may optionally include a conductive agent and / or a binder. As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers; the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, or fluorinated acrylate resin.

[0144] The present application does not particularly limit the preparation method of the positive electrode plate, and it can be prepared with reference to existing methods. For example, the positive electrode slurry is coated on the positive electrode current collector, and after drying and cold pressing, the positive electrode plate is formed. The positive electrode slurry can be formed by dispersing components such as the positive electrode material of the sodium secondary battery, optionally a conductive agent, and optionally a binder in a solvent (such as N-methylpyrrolidone) and stirring evenly.

[0145] In addition, the positive electrode plate of the present application does not exclude other additional functional layers in addition to the positive electrode film layer. For example, the positive electrode plate may further include a conductive bottom layer (for example, composed of a conductive agent and a binder) provided between the positive electrode current collector and the positive electrode film layer. For another example, the positive electrode plate further includes a protective layer covering the surface of the positive electrode film layer.

[0146] The fourth aspect of the present application provides a sodium secondary battery, including the positive electrode plate described in the third aspect of the present application. Using the positive electrode plate containing the positive electrode material of the sodium battery as the positive electrode side, the water content on the positive electrode side is low, which can slow down the growth of sodium dendrites on the negative electrode side and improve the quality and reliability of the battery.

[0147] In some embodiments, the sodium secondary battery includes a sodium ion secondary battery.

[0148] In some embodiments, the sodium secondary battery further includes a negative electrode plate, a separator, and an electrolyte. During the charge and discharge process of the battery, active ions are embedded and extracted back and forth between the positive electrode plate and the negative electrode plate. The electrolyte plays a role in conducting ions between the positive electrode plate and the negative electrode plate. The separator is disposed between the positive electrode plate and the negative electrode plate, which can prevent short circuit between the positive and negative electrodes and at the same time allow ions to pass through.

[0149] In some embodiments, the sodium secondary battery includes a sodium-free negative electrode sodium secondary battery. The negative electrode side of the sodium-free negative electrode sodium secondary battery does not pre-set sodium metal, so it effectively alleviates the problems of the decline in Coulomb efficiency and cycle performance caused by the violent reaction between sodium metal and the electrolyte (especially the water therein).

[0150] [Negative electrode plate]

[0151] In some embodiments, the sodium secondary battery is a sodium-ion secondary battery, and its negative electrode sheet may include a negative electrode current collector and a negative electrode film layer provided on at least one side of the negative electrode current collector. The negative electrode film layer may include a negative electrode active material.

[0152] In some embodiments, the negative electrode current collector may include a metal foil or a composite current collector. The metal foil is, for example, a copper foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one side of the polymer base layer. Among them, the materials of the metal layer include, but are not limited to, copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc., and the polymer materials of the polymer base layer include, but are not limited to, polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0153] In some embodiments, the negative electrode active material may include negative electrode materials well-known in the art for secondary batteries. For example, the negative electrode active material includes at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based materials may include at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based materials may include at least one of elemental tin, tin oxides, and tin alloys.

[0154] In some embodiments, the negative electrode film layer may also optionally include a binder. The binder includes, for example, at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), carboxymethyl cellulose (CMC), or carboxymethyl chitosan (CMCS).

[0155] In some embodiments, the negative electrode film layer may also optionally include a conductive agent. As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers.

[0156] In some embodiments, the negative electrode film layer may also optionally include other additive auxiliaries, such as a thickening agent. Specific examples of the thickening agent include, but are not limited to, sodium carboxymethyl cellulose (CMC-Na).

[0157] In some other embodiments, the sodium secondary battery is a sodium secondary battery without a negative electrode, and its negative electrode includes a negative electrode current collector and an optional conductive layer.

[0158] Optionally, the negative electrode sheet has a conductive layer disposed on at least one side of the negative electrode current collector. The provision of the conductive layer is more conducive to the electron reduction and deposition of sodium ions on the negative electrode current collector during charging to form a sodium metal negative electrode.

[0159] Optionally, the negative electrode current collector includes any one of bare copper, aluminum foil, aluminum alloy foil, and aluminum-based composite current collector.

[0160] Optionally, the conductive layer includes a conductive agent and a binder. The binder is as described above and will not be elaborated here.

[0161] Optionally, the conductive agent includes at least one of graphite, graphene, carbon fiber, carbon black, carbon dots, soft carbon, hard carbon, multi-walled carbon nanotubes, or single-walled carbon nanotubes.

[0162] The present application does not particularly limit the preparation method of the negative electrode sheet, and it can be obtained by referring to existing methods. For example: dispersing negative electrode components such as negative electrode materials, conductive agents, and binders in a solvent (such as deionized water) to form a negative electrode slurry; then coating the negative electrode slurry on the negative electrode current collector and drying, cold pressing, etc. to obtain the negative electrode sheet.

[0163] [Electrolyte]

[0164] In the present application, the electrolyte can be selected with reference to existing secondary batteries.

[0165] In some embodiments, the electrolyte uses an electrolytic solution.

[0166] In some embodiments, the electrolyte may include an organic solvent, a sodium salt, and an optional additive. The sodium salt includes, but is not limited to, at least one of NaPF6, NaClO4, NaBCl4, NaSO3CF3, or Na(CH3)C6H4SO3. The organic solvent includes, but is not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS), diethyl sulfone (ESE), or an ether solvent. The ether solvent may include cyclic ethers and / or chain ethers. Specific examples of cyclic ethers include, but are not limited to, tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), 1,3-dioxolane (DOL), etc. Specific examples of chain ethers include, but are not limited to, ethylene glycol dimethyl ether (DME), diethylene glycol dimethyl ether (DEGDME), etc.

[0167] In some embodiments, the additives in the electrolyte may include negative electrode film-forming additives, positive electrode film-forming additives; and may also include additives that can improve certain battery performance, such as additives that improve the overcharge performance of the battery, additives that improve the high-temperature or low-temperature performance of the battery, etc. As an example, the additive may include, but is not limited to, at least one of vinylene carbonate (VC), vinyl ethylene carbonate (VEC), ethylene sulfate (DTD), propylene sulfate, ethylene sulfite (ES), 1,3-propane sultone (PS), 1,3-propene sultone (PST), sulfonate cyclic quaternary ammonium salt, succinic anhydride, succinonitrile (SN), adiponitrile (AND), tris(trimethylsilyl) phosphate (TMSP), or tris(trimethylsilyl) borate (TMSB).

[0168] [Separator film]

[0169] In the present application, the separator is disposed between the positive electrode plate and the negative electrode plate, mainly serving to prevent short circuit between the positive and negative electrodes, and at the same time allowing active ions to pass through. The present application does not particularly limit the type of the separator, and various porous structure separators well-known in the art can be selected. In some embodiments, the material of the separator may include at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. Additionally, the separator can be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer can be the same or different.

[0170] In some embodiments, a ceramic coating and / or a metal oxide coating are also disposed on the separator.

[0171] In some embodiments, the positive electrode plate, the negative electrode plate, and the separator can be made into an electrode assembly through a winding process or a stacking process.

[0172] In some embodiments, the battery may include an outer package. The outer package can be used to encapsulate the above-mentioned electrode assembly and electrolyte.

[0173] In the present application, the outer package of the battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the battery can also be a soft package, such as a pouch soft package. The material of the soft package can be plastic, and as plastics, polypropylene, polybutylene terephthalate, and polybutylene succinate can be listed, etc.

[0174] The battery of the present application can include a battery cell form, a battery module form, and a battery pack form. The battery cell, battery module, and battery pack of the present application will be described below with appropriate reference to the drawings.

[0175] The present application does not particularly limit the shape of the battery, and it can be cylindrical, square, or any other arbitrary shape. For example, Figure 5 is a battery cell 5 in the shape of a square as an example.

[0176] In some embodiments, referring to Figure 6 , the outer package may include a housing 51 and a top cover assembly 53. Among them, the housing 51 may include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates enclose a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can be disposed on the opening to close the receiving cavity. The positive electrode plate, the negative electrode plate, and the separator can form an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte infiltrates in the electrode assembly 52. The number of the electrode assemblies 52 included in the battery cell can be one or more, and those skilled in the art can select according to specific actual requirements.

[0177] In some embodiments, battery cells can be assembled into a battery module. The number of battery cells included in the battery module can be one or more, and those skilled in the art can select the specific number according to the application and capacity of the battery module.

[0178] Figure 7 is a battery module 4 as an example. Refer to Figure 7 , in the battery module 4, a plurality of battery cells 5 can be arranged in sequence along the length direction of the battery module 4. Of course, they can also be arranged in any other manner. Further, the plurality of batteries can be fixed by fasteners.

[0179] Optionally, the battery module 4 can further include a housing having an accommodation space, and the plurality of battery cells 5 are accommodated in the accommodation space.

[0180] In some embodiments, the above battery module can be further assembled into a battery pack. The number of battery modules included in the battery pack can be one or more, and those skilled in the art can select the specific number according to the application and capacity of the battery pack.

[0181] Figure 8 and Figure 9 is a battery pack 1 as an example. Refer to Figure 8 and Figure 9 , the battery pack 1 can include a battery box and a plurality of battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 to form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0182] The fifth aspect of the present application provides an electrical device, including the battery described in the fourth aspect of the present application. The battery can be used as the power source of the electrical device or as the energy storage unit of the electrical device.

[0183] The electrical device can include mobile devices (such as mobile phones, laptop computers, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships, satellites, energy storage systems, etc., but not limited thereto.

[0184] As the electrical device, battery cells, battery modules or battery packs can be selected according to its usage requirements.

[0185] Figure 10 is an electrical device as an example. The electrical device is a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, etc. In order to meet the high power and high energy density requirements of the electrical device for the battery, a battery pack or a battery module can be adopted.

[0186] As another example, the electrical device can be a mobile phone, a tablet computer, a laptop computer, etc. The electrical device generally requires being thin and light, and a single battery can be used as the power source.

[0187] Hereinafter, embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as a limitation to the present application. For those without specific techniques or conditions noted in the embodiments, the techniques or conditions described in the literature in the field or according to the product specifications are followed. For the reagents or instruments without the manufacturer noted, they are all conventional products that can be obtained through commercial purchase.

[0188] The following embodiments are used to illustrate the positive electrode material of the sodium secondary battery and its preparation method of the present application.

[0189] Example 1

[0190] (1) Preparation of NFPP@C positive electrode material

[0191] Sodium pyrophosphate, ammonium dihydrogen phosphate, and ferrous oxalate are proportioned according to the stoichiometric ratio of Na4Fe3(PO4)2(P2O7) to obtain a composition. Conductive carbon black and glucose (the mass ratio of conductive carbon black, glucose to the composition is 0.5∶5∶100) are added, and then added to deionized water. After grinding, a positive electrode slurry is obtained. The positive electrode slurry is spray-dried and sintered at 500 °C for 10 h, and then pulverized by air flow to obtain NFPP@C (Dv50 is 3 μm, and the specific surface area BET is 7 m 2 / g).

[0192] (2) Preparation of NFPP@C@PAN

[0193] NFPP@C and PAN are mixed evenly according to the mass ratio of 100∶1 and sintered at 500 °C for 10 h to obtain the positive electrode material NFPP@C@PAN of the sodium secondary battery.

[0194] Examples 2 - 5

[0195] The positive electrode material of the sodium secondary battery is prepared according to the method of Example 1, except that the relative amount of PAN is adjusted to obtain NFPP@C@PAN with different coating amounts.

[0196] Examples 6 - 11

[0197] The positive electrode material of the sodium secondary battery is prepared according to the method of Example 1, except that the sintering conditions after mixing NFPP@C and PAN are adjusted to obtain different NFPP@C@PAN.

[0198] Comparative Example 1

[0199] The NFPP@C prepared in Example 1 was used as a comparative sample.

[0200] Example 12

[0201] (1) Preparation of the positive electrode material slurry

[0202] Same as Example 1.

[0203] (2) Preparation of NFPP@C

[0204] Same as Example 1.

[0205] (3) Preparation of NFPP@C@PDM

[0206] NFPP@C and PDMS were mixed evenly according to a mass ratio of 100∶0.5 and sintered at 230 °C for 6 h to obtain the positive electrode material NFPP@C@PDMS for sodium secondary batteries.

[0207] Examples 13 - 16

[0208] The positive electrode material for sodium secondary batteries was prepared according to the method of Example 12, except that the relative amount of PDMS was adjusted to obtain NFPP@C@PDMS with different coating amounts.

[0209] Examples 17 - 20

[0210] The positive electrode material for sodium secondary batteries was prepared according to the method of Example 12, except that the sintering conditions after mixing NFPP@C and PDMS were adjusted to obtain different NFPP@C@PDMS.

[0211] Testing section

[0212] 1. Characterization of the positive electrode material

[0213] (1) Median particle size (Dv 50 ) test

[0214] Particle size testing was carried out using a Malvern laser particle size analyzer (instrument model: Mastersizer - 3000), and the reference standard was GB / T19077 - 2016.

[0215] Pretreatment: An appropriate amount of the sample to be tested and water were added to a beaker, and a dispersant (sodium hexametaphosphate) was added and ultrasonically dispersed to ensure that the sample was completely dispersed in the dispersant.

[0216] (2) BET specific surface area test

[0217] The BET specific surface area was tested using a specific surface area tester F - Sorb 1400CES, referring to the GB / T 19587 - 2017 standard.

[0218] Pretreatment: Take a certain mass of powder and degas it for 12 h under a nitrogen atmosphere at 80 °C.

[0219] (3) Environmental stability test

[0220] After heating the positive electrode material to 170 °C for drying, store it in air with a relative humidity of 50% for 30 min, and test the water content before and after storage according to the standard of GB / T 11133-2015. Test steps: Blow the solid sample with dry gas into the titration cup of the Karl Fischer coulometer for titration, and convert the result into the water content of the solid sample;

[0221] Calculation formula: Water content = (sample water value - blank water value) / sample mass.

[0222] 2. Performance test

[0223] The following is used to illustrate the performance test of applying the positive electrode materials prepared in the examples and comparative examples to the battery.

[0224] Battery preparation

[0225] 1) Preparation of positive electrode sheet

[0226] Respectively mix the positive electrode materials of the examples and comparative examples with the conductive agent carbon black Super P and the binder PVDF in a mass ratio of 7:2:1 and mix them evenly with an appropriate amount of solvent NMP to obtain a positive electrode slurry; evenly coat the positive electrode slurry on the surface of the aluminum foil, and then after drying and punching, obtain a positive electrode sheet with a compaction density of 2 g / cm 3 .

[0227] 2) Preparation of negative electrode sheet

[0228] Mix the negative electrode material hard carbon, the conductive agent carbon black Super P, and the binder CMC in a mass ratio of 8:1:1 and mix them evenly with an appropriate amount of solvent water to obtain a negative electrode slurry; then evenly coat the negative electrode slurry on the surface of the copper foil; after drying, cold pressing, and slitting, obtain a negative electrode sheet with a compaction density of 1.8 g / cm 3 .

[0229] 3) Preparation of electrolyte

[0230] In a glove box under an argon atmosphere (H2O < 0.1 ppm, O2 < 0.1 ppm), dissolve sodium hexafluorophosphate (NaPF6) in ethylene glycol dimethyl ether (DME) to obtain a NaPF6 electrolyte with a concentration of 0.5 mol / L.

[0231] 4) Preparation of sodium-ion battery

[0232] Stack the above-mentioned positive electrode sheet, separator (polypropylene film), and negative electrode sheet in sequence, with the separator placed between the positive electrode sheet and the negative electrode sheet to play an isolation role. The separator is impregnated with the above-mentioned electrolyte to assemble a laminated battery.

[0233] Performance testing of the battery

[0234] 1) Coulombic efficiency testing

[0235] Charge the sodium-ion battery at a constant current of 1 / 3C to 3.75V at 25°C, then charge it at a constant voltage of 3.75V until the current drops to 0.05C to obtain the first charging capacity (Cc1); then discharge it at a constant current of 1 / 3C to 1.5V to obtain the first discharge capacity (Cd1), and calculate the Coulombic efficiency of the sodium-ion battery according to the following formula.

[0236] Coulombic efficiency of sodium-ion battery = First discharge capacity (Cd1) / First charging capacity (Cc1)

[0237] 2) Capacity retention rate testing

[0238] Charge the sodium-ion battery at a constant current of 1C to 3.65V at 25°C, then charge it at a constant voltage of 3.65V until the current drops to 0.05C, and then discharge it at a constant current of 1C to 1.5V to obtain the first-cycle discharge capacity (Cd1); repeat charging and discharging in this way until the nth cycle (n = 1000) to obtain the discharge capacity (Cdn) of the sodium battery after cycling n times, and calculate the capacity retention rate of the sodium battery according to the following formula:

[0239] Capacity retention rate = Discharge capacity after cycling n times (Cdn) / First-cycle discharge capacity (Cd1).

[0240] The test results of the battery performance are shown in Table 2.

[0241] Table 1

[0242]

[0243] Note: The coating amount refers to the mass percentage of the coating raw material in the sodium-containing positive electrode material.

[0244] Table 2

[0245]

[0246] As can be seen from Table 2, by comparing Examples 1-20 with Comparative Example 1, it can be known that after coating and modifying the sodium-containing positive electrode material with PAN and PDMS, the stability of the material in a high-humidity environment can be improved, and the improvement of the material environmental stability is beneficial to the stable performance of the material during subsequent use and processing, and the prepared battery also has relatively high electrochemical performance.

[0247] Figure 1 SEM image showing the apparent morphology of NFPP@C of Comparative Example 1 Figure 2 SEM image showing the apparent morphology of NFPP@C@PAN of Example 1. Figure 1 and Figure 2 By comparison, it can be seen that the surface of the material is rough before the coating of NFPP@C and becomes smooth after the coating.

[0248] Figure 3 TEM image of the cathode material prepared in Example 2. It can be seen from the figure that the total thickness of the carbon modification layer and the hydrophobic coating layer on the surface of NFPP is about between 2 nm and 6 nm.

[0249] Figure 4 EDS image of the cathode material prepared in Example 12. It can be seen from the figure that Si and O are distributed on the cathode material, indicating that PDMS is coated on NFPP@C.

[0250] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application and are not intended to limit them. Although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application, and they should all be covered within the scope of the claims and the description of the present application. In particular, as long as there is no structural conflict, the technical features mentioned in each embodiment can be combined in any way. The present application is not limited to the specific embodiments disclosed in the text, but includes all technical solutions falling within the scope of the claims.

Claims

1. A cathode material for a sodium secondary battery, characterized in that, The positive electrode material of the sodium secondary battery includes a sodium-containing positive electrode material and a coating layer located on at least a part of the surface of the sodium-containing positive electrode material; wherein, The sodium-containing positive electrode material includes at least one of a layered oxide, a Prussian blue compound, or a polyanion-type sodium-containing compound; The coating layer contains a hydrophobic polymer.

2. The positive electrode material for a sodium secondary battery according to claim 1, characterized in that, Satisfying one or more of the following characteristics: (a) The hydrophobic polymer and the sodium-containing cathode material satisfy the relationship: T m < T a , where T m is the melting point of the hydrophobic polymer, and T a is the phase transition temperature of the sodium-containing cathode material; (b) The phase transition temperature T of the sodium-containing cathode material a ≤ 600 °C; (c) The mass ratio of the hydrophobic polymer to the sodium-containing positive electrode material is (0.2 to 10):

100.

3. The cathode material for a sodium secondary battery according to claim 1 or 2, characterized in that, The hydrophobic polymer includes a first hydrophobic polymer material, and the first hydrophobic polymer material includes at least one of polyethylene, polypropylene, polystyrene, polydimethylsiloxane, polyethylene terephthalate, polytetrafluoroethylene, polyamide, polycarbonate, or silicone wax.

4. The positive electrode material for a sodium secondary battery according to claim 3, wherein Satisfying one of the following characteristics: (a) The mass ratio of the hydrophobic polymer to the sodium-containing positive electrode material is (0.3 to 3):100; or (b) The mass ratio of the hydrophobic polymer to the sodium-containing positive electrode material is (0.5 to 2):

100.

5. The cathode material for a sodium secondary battery according to any one of claims 1-4, characterized in that, The hydrophobic polymer includes a second hydrophobic polymer material and / or a third hydrophobic polymer material. Among them, the second hydrophobic polymer material includes polyacrylonitrile, the third hydrophobic polymer material includes a conjugated polymer, and the conjugated polymer is a thermal pyrolysis product of polyacrylonitrile.

6. The positive electrode material for a sodium secondary battery according to claim 5, characterized in that, Satisfying one of the following characteristics: (a) The mass ratio of the hydrophobic polymer to the sodium-containing positive electrode material is (0.5 to 10):100; or (b) The mass ratio of the hydrophobic polymer to the sodium-containing positive electrode material is (1 to 5):

100.

7. The positive electrode material for a sodium secondary battery according to claim 5 or 6, characterized in that, The thermal cracking temperature T of the second hydrophobic polymer material satisfies the following relationship: T m1 <T<T a , wherein, T m1 is the melting point of the second hydrophobic polymer material; T a is the phase transition temperature of the sodium-containing cathode material.

8. The cathode material for a sodium secondary battery according to any one of claims 1-7, characterized in that, The sodium-containing cathode material contains a sodium-containing polyanionic compound, and the chemical formula of the sodium-containing polyanionic compound is: Na x R y (PO4) Z (P2O7) k , where 1 ≤ x ≤ 7, 1 ≤ y ≤ 4, 1 ≤ z ≤ 2, 1 ≤ k ≤ 4, and R is at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W, or Pb.

9. The cathode material for a sodium secondary battery according to claim 8, characterized in that, The sodium-containing positive electrode material includes the polyanion-type sodium-containing compound and a carbon-based material modification layer located on at least a part of its surface; The carbon-based material modification layer includes a first carbon material and a second carbon material. The first carbon material includes at least one of natural graphite powder, artificial graphite powder, carbon black, carbon nanotubes, or graphene, and the second carbon material includes a sintering product of an organic carbon source at 450°C to 550°C.

10. The positive electrode material for a sodium secondary battery according to claim 9, wherein, Satisfying one or more of the following characteristics: (a) The organic carbon source includes at least one of sucrose, glucose, citric acid, starch, cyclodextrin, asphalt, or the hydrophobic polymer; (b) The mass ratio of the first carbon material to the polyanion-type sodium-containing compound is (0.5 to 2):100; (c) The mass ratio of the sintering product to the polyanion-type sodium-containing compound is (0.5 to 2):100; (d) The median particle size Dv of the sodium-containing cathode material 50 is 2 μm to 8 μm; (e) The BET specific surface area of the sodium-containing cathode material is 3 m 2 / g to 11 m 2 / g.

11. The cathode material for a sodium secondary battery according to any one of claims 1-10, characterized in that, Satisfying one of the following characteristics: (a) The water content in the sealed environment is not higher than 2000 ppm; or (b) The water content in the sealed environment is 50 ppm to 1600 ppm.

12. A method for preparing a cathode material for a sodium secondary battery, characterized in that, Including: Mixing the coating raw materials and the sodium-containing positive electrode material and performing a first sintering to form a coating layer containing a hydrophobic polymer on at least a part of the surface of the sodium-containing positive electrode material. The sodium-containing positive electrode material includes at least one of a layered oxide, a Prussian blue compound, or a polyanion-type sodium-containing compound.

13. The method according to claim 12, wherein Satisfying one or more of the following characteristics: (a) The first sintering temperature T1 satisfies the relation: T m <T1<T a , where T m is the melting point of the coating raw material, and T a is the phase transition temperature of the sodium-containing cathode material; (b) The phase transition temperature T of the sodium-containing cathode material a ≤ 600 °C; (c) The mass ratio of the coating raw material to the sodium-containing cathode material is (0.2 to 10):

100.

14. The method according to claim 12 or 13, characterized in that, Meet one of the following characteristics: (a) The coating raw material includes a first hydrophobic polymer material, and the first hydrophobic polymer material includes at least one of polyethylene, polypropylene, polystyrene, polydimethylsiloxane, polyethylene terephthalate, polytetrafluoroethylene, polyamide, polycarbonate, or silicone wax; or (b) The coating raw material includes polydimethylsiloxane.

15. The method according to claim 14, wherein Meet one of the following characteristics: (a) The mass ratio of the coating raw material to the sodium-containing cathode material is (0.3 to 3):100; or (b) The mass ratio of the coating raw material to the sodium-containing cathode material is (0.5 to 2):

100.

16. The method according to claim 14 or 15, characterized in that Meet one of the following characteristics: (a) The first sintering temperature T1 and the initial thermal cracking temperature T' of the first hydrophobic polymer material satisfy the following relationship: T1 < T'; or (b) The first sintering temperature T1 is 180°C to 260°C, and the sintering time is 2 h to 8 h; or (c) The first sintering temperature T1 is 200°C to 250°C, and the sintering time is 4 h to 7 h.

17. The method according to any one of claims 12 - 16, characterized in that, The coating raw material includes a second hydrophobic polymer material, and the second hydrophobic polymer material includes polyacrylonitrile.

18. The method according to claim 17, wherein Meet one of the following characteristics: (a) The mass ratio of the coating raw material to the sodium-containing cathode material is (0.5 to 10):100; or (b) The mass ratio of the coating raw material to the sodium-containing cathode material is (1 to 5):

100.

19. The method according to claim 17 or 18, characterized in that, Meet one of the following characteristics: (a) The first sintering temperature T1 and the initial thermal cracking temperature T” of the second hydrophobic polymer material satisfy the following relationship: T1 > T”; or (b) The first sintering temperature T1 is 350°C to 550°C, and the sintering time is 4 h to 12 h; or (c) The first sintering temperature T1 is 400°C to 500°C, and the sintering time is 5 h to 12 h.

20. The method according to any one of claims 12 - 19, characterized in that, The sodium-containing cathode material contains a sodium-containing polyanionic compound, and the chemical formula of the sodium-containing polyanionic compound is: Na x R y (PO4) Z (P2O7) k , where 1 ≤ x ≤ 7, 1 ≤ y ≤ 4, 1 ≤ z ≤ 2, 1 ≤ k ≤ 4, and R is at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W, or Pb.

21. The method according to claim 20, wherein The sodium-containing cathode material includes the polyanion-type sodium-containing compound and a carbon-based material modification layer on at least part of its surface; The carbon-based material modification layer includes a first carbon material and a second carbon material; the first carbon material includes at least one of natural graphite powder, artificial graphite powder, carbon black, carbon nanotubes, or graphene, and the second carbon material includes a sintering product of an organic carbon source at 450°C to 550°C.

22. The method according to claim 21, wherein Meet one or more of the following characteristics: (a) The organic carbon source includes at least one of sucrose, glucose, citric acid, starch, cyclodextrin, asphalt, or the hydrophobic polymer; (b) The mass ratio of the first carbon material to the polyanion-type sodium-containing compound is (0.5 to 2):100; (c) The mass ratio of the sintering product to the polyanion-type sodium-containing compound is (0.5 to 2):100; (d) The median particle size Dv of the sodium-containing cathode material 50 is 2 μm to 8 μm; (e) The specific surface area BET of the sodium-containing cathode material is 3 m 2 / g to 11 m 2 / g.

23. A positive electrode sheet, characterized in that, Include the sodium secondary battery cathode material according to any one of claims 1-11 or the sodium secondary battery cathode material prepared by the method according to any one of claims 12-22.

24. A sodium secondary battery, characterized in that, Include the positive electrode plate according to claim 23.

25. The sodium secondary battery according to claim 24, characterized in that, The sodium secondary battery includes a sodium ion secondary battery.

26. The sodium secondary battery according to claim 24, wherein The sodium secondary battery includes a sodium secondary battery without a negative electrode, the sodium secondary battery without a negative electrode includes a negative electrode tab, and the sodium secondary battery without a negative electrode satisfies one of the following characteristics: (a) The negative electrode tab includes a negative electrode current collector; or (b) The negative electrode tab includes a negative electrode current collector and a conductive layer.

27. The sodium secondary battery according to claim 26, wherein, The negative electrode tab includes the conductive layer, and the conductive layer satisfies one or more of the following characteristics: (a) The conductive layer is disposed on at least one side of the negative electrode current collector; (b) The conductive layer includes a conductive agent and a binder; (c) The conductive agent includes at least one of graphite, graphene, carbon fiber, carbon black, carbon dots, soft carbon, hard carbon, multi-walled carbon nanotubes or single-walled carbon nanotubes.

28. An electrical device, characterized in that, Including the sodium secondary battery according to any one of claims 24-27.

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