Positive electrode materials and their preparation, positive electrode sheets, batteries, battery packs, and electrical equipment.

Abstract

This invention provides a positive electrode material and its preparation, a positive electrode sheet, a battery, a battery pack, and electrical devices. The positive electrode material includes a material body and a hollow structure formed by the material body. The material body includes a sodium ferrous sulfate matrix and a conductive carbon material. This invention can simultaneously improve the conductivity and mechanical strength of the positive electrode material, and enhance the energy density, rate performance, low-temperature adaptability, and cycle life of the battery.

Description

Technical Field

[0001] This invention relates to the field of battery technology, specifically to a positive electrode material and its preparation method, a positive electrode sheet, a battery, a battery pack, and electrical equipment. Background Technology

[0002] Sodium-ion batteries have shown great promise in recent years in energy storage systems, low-speed electric vehicles, and power tools due to their abundant resources, low cost, and high safety. As the core component of sodium-ion batteries, the performance of the cathode material directly determines the battery's energy density, cycle life, rate performance, and low-temperature adaptability. Sodium ferrous sulfate is considered a highly promising cathode material due to its high theoretical capacity, low raw material cost, and environmental friendliness. However, existing sodium ferrous sulfate materials generally suffer from poor conductivity, low sodium ion diffusion rate, and poor structural stability, affecting the discharge performance, rate performance, low-temperature performance, and cycle life of sodium-ion batteries. Summary of the Invention

[0003] This invention provides a cathode material and its preparation method, a cathode sheet, a battery, a battery pack, and an electrical device, which can improve the conductivity, ion transport efficiency, and structural stability of the cathode material, and enhance the battery's discharge performance, rate performance, low-temperature performance, and cycle life.

[0004] One aspect of the present invention provides a positive electrode material, the positive electrode material comprising a material body and a hollow structure formed by the material body, the material body comprising a sodium ferrous sulfate matrix and a conductive carbon material.

[0005] According to one embodiment of the present invention, the wall thickness of the hollow structure is 0.5μm-2.5μm, preferably 0.7μm-1.8μm.

[0006] According to one embodiment of the present invention, the conductive carbon material accounts for 0.95%-6% of the mass fraction of the positive electrode material, preferably 0.95%-5%.

[0007] According to one embodiment of the present invention, the conductive carbon material includes an aqueous conductive carbon material.

[0008] According to one embodiment of the present invention, the conductive carbon material includes at least one of carbon nanotubes, graphene, and conductive carbon black.

[0009] According to one embodiment of the present invention, the particle size D of the positive electrode material 50 The surface area is 7.5 μm-25 μm, preferably 9 μm-20 μm; and / or, the specific surface area of ​​the positive electrode material is 3 m². 2 / g-17 m 2 / g, preferably 5 m 2 / g-12 m 2 / g.

[0010] In another aspect, the present invention provides a method for preparing the above-mentioned cathode material, comprising the following steps: drying a mixture comprising an iron source, a sodium source, a sulfur source and the conductive carbon material to obtain a precursor; and annealing the precursor to obtain the cathode material.

[0011] According to one embodiment of the present invention, the mixture includes water; and / or, the mass ratio of the iron source to the conductive carbon material is 100:(1-7), preferably 100:(1-5.5); and / or, the mixture further includes an antioxidant.

[0012] According to one embodiment of the present invention, the drying process includes: spray drying the mixture to obtain the precursor; preferably, the mixture is fed into a spray dryer for spray drying, the inlet temperature of the spray dryer is 180℃-240℃, and the feed rate of the mixture is 8mm / s-14mm / s.

[0013] According to one embodiment of the present invention, the annealing treatment is performed under a protective atmosphere; and / or, the annealing treatment temperature is 300℃-450℃ and the time is 4h-24h; and / or, the precursor is calcined before the annealing treatment is performed, the calcination temperature is 180℃-220℃ and the calcination time is 3h-5h.

[0014] In another aspect, the present invention provides a positive electrode sheet comprising the above-described positive electrode material or a positive electrode material prepared according to the above-described method for preparing the positive electrode material.

[0015] In another aspect, the present invention provides a battery comprising the above-described positive electrode.

[0016] In another aspect, the present invention provides a battery pack comprising at least two batteries connected to each other.

[0017] In another aspect, the present invention provides an electrical device comprising the aforementioned battery or battery pack.

[0018] The cathode material, its preparation method, cathode sheet, battery, battery pack, and electrical device provided in this invention, by introducing conductive carbon material into the cathode material and forming a hollow structure from the material body containing a sodium ferrous sulfate matrix and conductive carbon material, helps to increase the specific surface area of ​​the cathode material, providing more active sites for the adsorption and desorption of nanoparticles, thereby reducing sodium ion transport resistance and improving the sodium ion transport efficiency of the cathode material. Simultaneously, the introduction of conductive carbon material synergistically optimizes the ion transport path with the sodium ferrous sulfate matrix, improving the sodium ion diffusion rate, and the conductive carbon material in the material body further enhances the overall efficiency. A three-dimensional conductive network is constructed within the cathode material to enhance electron transport efficiency. This simultaneously improves both the ion-conducting and electron-conducting capabilities of the cathode material, thereby improving the battery's charge-discharge performance, discharge specific capacity, discharge efficiency, and rate and low-temperature performance. Simultaneously, the bulk material forms a hollow structure, which provides mechanical support for the cathode material, improving its structural stability and effectively preventing material pulverization caused by volume changes during charge-discharge, thus enhancing the battery's cycle life. Therefore, this invention can simultaneously improve the conductivity, ion transport efficiency, and structural stability of the cathode material, enhancing the battery's discharge performance, rate performance, low-temperature performance, and cycle life. Attached Figure Description

[0019] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0020] Figure 1 This is a schematic diagram of the structure of the positive electrode material in Embodiment 3 of the present invention;

[0021] Figure 2 This is a scanning electron microscope (SEM) image of the cathode material in Comparative Example 1 of the present invention.

[0022] Figure 3 This is a scanning electron microscope (SEM) image of the cathode material in Embodiment 3 of the present invention.

[0023] Figure 4 This is a cross-sectional view (CP diagram) of the positive electrode material in Embodiment 3 of the present invention.

[0024] Figure 5 The X-ray diffraction (XRD) patterns of the cathode materials in Embodiment 3 and Comparative Example 1 of the present invention are shown.

[0025] The accompanying drawings have illustrated specific embodiments of the invention, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the invention in any way, but rather to illustrate the concept of the invention to those skilled in the art through reference to particular embodiments. Detailed Implementation

[0026] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below. The specific embodiments listed below are merely descriptions of the principles and features of the present invention, and the examples are only for explaining the present invention and are not intended to limit the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] As mentioned above, traditional sodium ferrous sulfate materials generally suffer from problems such as poor conductivity, low sodium ion diffusion rate, and poor structural stability, resulting in poor discharge performance, rate performance, low-temperature performance, and cycle life of sodium-ion batteries, which urgently need to be addressed.

[0028] According to the inventors' research, conductive carbon materials (such as carbon nanotubes, CNTs) have good conductivity. Combining conductive carbon materials with sodium ferrous sulfate helps to improve the conductivity of the resulting composite cathode material. However, the way conductive carbon materials are combined with sodium ferrous sulfate affects the structural stability and ion conductivity of the resulting composite cathode material, which in turn affects the battery's charge-discharge performance, rate performance, low-temperature performance, and other electrochemical performance, as well as the battery's cycle life.

[0029] For example, sodium ferrous sulfate materials with a double-core-shell structure, where the core consists of sodium ferrous sulfate and CNTs, and the outer shell consists of conductive carbon materials such as amorphous carbon, graphene, and conductive carbon black, can improve the conductivity and structural stability of sodium ferrous sulfate materials to some extent. However, on the one hand, this method of doping CNTs in the core and coating the outer shell with conductive carbon materials has limited effect on improving the stability and other properties of sodium ferrous sulfate materials. This core-shell structure of sodium ferrous sulfate materials relies on the CNTs in the core and the conductive carbon materials in the outer shell to jointly construct a conductive network to meet its high conductivity requirements. However, this solid structure lacks stress release space. During battery charging and discharging, sodium ions are inserted and extracted in the cathode material, causing volume changes in the cathode material. The resulting stress can easily lead to problems such as pulverization of the cathode material, which not only affects the electrochemical performance of the cathode material (such as discharge capacity, discharge efficiency, rate performance, and low-temperature performance) but also deteriorates the cycle life of the battery. On the other hand, this solid core-shell structure is not conducive to electrolyte wetting, affecting the ion transport efficiency of the cathode material, and thus affecting the electrochemical performance of the battery.

[0030] In view of this, embodiments of the present invention provide a positive electrode material, the positive electrode material comprising a material body and a hollow structure formed by the material body, the material body comprising a sodium ferrous sulfate matrix and a conductive carbon material.

[0031] According to the inventors' research, in the embodiments of the present invention, at least the following synergistic effects can be achieved to improve the electronic conductivity, ionic conductivity, and structural stability of the cathode material, thereby enhancing the battery's discharge performance, rate performance, low-temperature performance, and cycle life:

[0032] (1) The cathode material has a hollow structure, which increases the specific surface area of ​​the cathode material, providing more active sites for the adsorption and desorption of sodium ions, helping to reduce ion transport resistance and enhance the ion diffusion and transport efficiency of the cathode material. Simultaneously, the hollow structure improves the wettability of the electrolyte on the cathode material, facilitating electrolyte wetting of the cathode material and increasing the electrolyte retention capacity of the cathode material, thereby enhancing the ion transport efficiency within the cathode material. Therefore, based on the hollow structure formed in the material body, the conductive carbon material introduced into the material body synergistically constructs an ion transport path with the sodium ferrous sulfate matrix, helping to achieve rapid and efficient ion conduction, thereby improving the battery's charge-discharge performance and low-temperature performance, and other electrochemical properties.

[0033] (2) Conductive carbon materials have good conductivity. Introducing conductive carbon materials into the material body helps to build an efficient electron transport network, improve the electronic conductivity of the cathode material, reduce the internal resistance of the battery, improve the power density and energy density of the battery, and improve the discharge capacity and discharge efficiency of the battery. At the same time, the improvement of the electronic conductivity of the cathode material also helps to reduce the polarization phenomenon during the charging and discharging process of the battery, and improve the cycle life and rate performance of the battery.

[0034] (3) The introduction of conductive carbon material and the presence of hollow structure in the material body can provide strong mechanical support for the positive electrode material. Specifically, on the one hand, the conductive carbon material can intertwine with sodium ferrous sulfate to form a stable material body structure, which provides mechanical support for the positive electrode material through the material body and maintains the stability of the hollow structure, thereby improving the structural stability of the positive electrode material. On the other hand, the hollow structure can provide stress buffer space for the positive electrode material. During the charging and discharging process of the battery, sodium ions are inserted and removed from the positive electrode material, causing changes in the volume of the positive electrode material. The hollow structure provides buffer space for the volume changes of the positive electrode material, effectively preventing stress concentration caused by volume changes during the charging and discharging process and the resulting pulverization of the positive electrode material, thereby ensuring the structural stability and capacity of the positive electrode material, extending the battery's service life, and improving the battery's discharge capacity and discharge efficiency and other electrochemical performance.

[0035] (4) Conductive carbon materials (such as CNT and graphite) have good flexibility. By introducing conductive carbon materials and sodium ferrous sulfate matrix into the material body at the same time, and making the material body form a hollow structure, under such a positive electrode material composition and structure system, the good flexibility of conductive carbon materials can alleviate the stress generated by the positive electrode material during battery charging and discharging, and enhance the structural stability and other properties of the positive electrode material.

[0036] (5) Conductive carbon materials (such as CNT and graphite) also have good thermal conductivity. By introducing conductive carbon materials and sodium ferrous sulfate matrix into the material body at the same time, and making the material body form a hollow structure, under such a positive electrode material composition and structure system, the heat generated by sodium ferrous sulfate during battery charging and discharging can be effectively dispersed by conductive carbon materials, avoiding local overheating of the positive electrode material, thereby improving the thermal stability of the battery, reducing the risk of battery thermal runaway, and improving the cycle life and other performance of the battery.

[0037] Therefore, in this embodiment of the invention, by introducing sodium ferrous sulfate matrix and conductive carbon material into the cathode material body, as well as the hollow structure formed by the material body, based on the synergistic effect of these aspects, it is possible to improve the sodium ion transport efficiency, electron transport efficiency and structural stability of the cathode material, thereby improving the discharge performance of the battery, such as discharge capacity and discharge efficiency, as well as the battery's rate performance, low temperature performance and cycle life.

[0038] In some embodiments, the wall thickness of the hollow structure can be 0.5 μm-2.5 μm, for example, it can be a range of 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm or any combination thereof, preferably 0.7 μm-1.8 μm. A wall thickness of not less than 0.5 μm is more conducive to enhancing the structural stability of the cathode material and preventing structural collapse during cycling; at the same time, a wall thickness of not more than 2.5 μm is more conducive to improving the sodium ion diffusion efficiency in the cathode material. Therefore, by controlling the wall thickness of the hollow structure within this range, it is more beneficial to enhance the sodium ion transport efficiency in the cathode material while ensuring the structural stability of the cathode material, thereby improving the battery's charge-discharge performance, low-temperature performance, and cycle life.

[0039] In this invention, the wall thickness of the hollow structure of the cathode material can be tested using conventional methods, such as SEM scanning electron microscopy and cross-sectional testing.

[0040] In some embodiments, the conductive carbon material accounts for 0.95% to 6% of the mass fraction of the cathode material, for example, a range of 0.95%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, or any combination thereof, preferably 0.95%-5%. A mass fraction of conductive carbon material of not less than 0.95% in the cathode material is more conducive to enhancing the electron transport capability of the cathode material, thereby effectively improving the power density and energy density of the battery. At the same time, this mass fraction of no more than 6% helps to maintain a relatively high amount of sodium ferrous sulfate in the cathode material, further improving the capacity of the cathode material. It also helps to optimize the sodium ion diffusion path in the cathode material, thus improving the electronic conductivity, ionic conductivity, and structural stability of the cathode material, and enhancing the charge-discharge performance, low-temperature performance, and cycle life of the battery.

[0041] In this invention, the mass fraction of conductive carbon material in the cathode material can be measured by conventional methods, such as by using a carbon-sulfur analyzer.

[0042] In some embodiments, the conductive carbon material may include an aqueous conductive carbon material. Aqueous conductive carbon materials have better hydrophilicity, making them more compatible with the sodium ferrous sulfate matrix. This improves the dispersibility and stability of the aqueous conductive carbon material in the sodium ferrous sulfate matrix, and further enhances the interfacial bonding strength between the conductive carbon material and the sodium ferrous sulfate matrix. This allows for more thorough mixing and entanglement of the conductive carbon material and the sodium ferrous sulfate matrix, forming the aforementioned hollow structure (e.g., Figure 1 As shown in the figure, it improves the stability of the hollow structure and further improves the ion and electron transport efficiency of the cathode material, thereby improving the battery's discharge performance, rate performance, low temperature performance and cycle life.

[0043] Furthermore, using aqueous conductive carbon materials is more conducive to the preparation of cathode materials. Specifically, aqueous conductive carbon materials can be mixed with raw materials used to synthesize sodium ferrous sulfate (such as sodium, iron, and sulfur sources) in an aqueous phase, and then dried and heat-treated (such as annealing) to obtain the cathode material. By dispersing and mixing the aqueous conductive carbon materials with the raw materials used to synthesize sodium ferrous sulfate in an aqueous phase, it is easier to achieve uniform dispersion of the aqueous conductive carbon materials and other raw materials in the aqueous phase. This allows the aqueous conductive carbon materials to be fully mixed and entangled with the in-situ synthesized sodium ferrous sulfate matrix, forming a hollow structure and improving the preparation efficiency of the cathode material.

[0044] In some specific embodiments, the aqueous carbon material may include aqueous carbon nanotubes.

[0045] Specifically, aqueous carbon nanotubes can include aqueous single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). In practice, CNT solutions with different mass fractions and different aspect ratios can be used. In this invention, the aspect ratio of CNTs refers to the ratio of the length to the diameter of the CNT.

[0046] The mass fraction of the CNT solution can be 4%-6%, for example, it can be a range of 4%, 4.3%, 5%, 6% or any two of them.

[0047] In some embodiments, the conductive carbon material may include at least one of carbon nanotubes, graphene, and conductive carbon black. Furthermore, by simultaneously introducing multiple conductive carbon materials, a multi-level conductive network can be constructed in the cathode material, which is beneficial for further improving the electronic conductivity of the cathode material.

[0048] In some embodiments, the particle size D of the positive electrode material 50 The particle size can range from 7.5 μm to 25 μm, for example, it can be within the range of 7.5 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, or any combination thereof, preferably 9 μm to 20 μm. The particle size D of the cathode material... 50 A particle size of not less than 7.5 μm is more conducive to avoiding interfacial side reactions between the cathode material and the electrolyte, and also to preventing the agglomeration of the cathode material, thereby improving the dispersion uniformity of the cathode material; at the same time, the particle size D of the cathode material 50 A diameter of no more than 25 μm is more conducive to reducing the diffusion path of sodium ions, improving the transport efficiency of sodium ions in the cathode material, and also more conducive to reducing the porosity of the cathode material, thereby increasing the energy density of the battery.

[0049] In this invention, the particle size D of the positive electrode material 50 This refers to the particle size Dn 50 That is, the particle size of 50% of the material particles in this Dn 50 Values ​​below.

[0050] In this invention, the particle size D of the cathode material can be measured using conventional methods. 50 For example, it can be measured by a laser particle size analyzer.

[0051] In some embodiments, the specific surface area of ​​the cathode material is 3 m². 2 / g-17 m 2 / g, for example, can be 3 m 2 / g、4m 2 / g、5 m 2 / g、6 m 2 / g、7 m 2 / g、8 m 2 / g、9 m 2 / g、10 m 2 / g、11 m 2 / g、12 m 2 / g、13 m 2 / g、14 m 2 / g、15 m 2 / g、16 m 2 / g、17 m 2 The range of / g or any two thereof, preferably 5 m 2 / g-12 m 2 / g. The specific surface area of ​​the cathode material is not less than 3 m². 2 / g, which is more conducive to improving the wettability of the cathode material in the electrolyte, thereby further optimizing the electrochemical performance of the battery; at the same time, the specific surface area of ​​the cathode material does not exceed 17 m². 2 / g, which is more conducive to reducing the viscosity of the positive electrode slurry used when coating the positive electrode with this positive electrode material to form the positive electrode sheet, which is beneficial to construction and improves the preparation efficiency and performance of the positive electrode sheet.

[0052] In this invention, the specific surface area of ​​the cathode material can be measured by conventional methods, such as by using a BST-BET400 instrument.

[0053] The present invention also provides a method for preparing the above-mentioned cathode material, comprising the following steps: drying a mixture comprising an iron source, a sodium source, a sulfur source and a conductive carbon material to obtain a precursor; and annealing the precursor to obtain the cathode material.

[0054] In the above preparation process, iron, sodium, and sulfur sources are used as raw materials for synthesizing sodium ferrous sulfate matrix. After drying the mixture, a precursor containing iron, sodium, sulfur, and conductive carbon material is formed. During annealing, the moisture inside the precursor is evaporated to form a hollow structure. The conductive carbon material and the in-situ formed sodium ferrous sulfate matrix are externally bonded and intertwined to form a positive electrode material with a hollow structure. This hollow structure allows the sodium ferrous sulfate material to be more uniformly distributed in the electrode, reducing the non-uniformity of the sodium ferrous sulfate material and thus improving the utilization rate of the sodium ferrous sulfate material.

[0055] In this embodiment of the invention, the iron source may include a conventional divalent iron source. For example, the iron source may include at least one of ferrous sulfate heptahydrate, battery-grade ferrous sulfate purified from titanium dioxide by-products, battery-grade ferrous sulfate self-made from iron oxide, ferrous ammonium sulfate, ferrous nitrate and their hydrates. The iron source used can be obtained by conventional methods, such as commercial purchase or self-made by conventional methods, and there are no particular limitations on this.

[0056] In practice, the amount of iron source material can account for 36.56%-40% of the total raw material mass.

[0057] In some specific embodiments, the sodium source may include at least one of sodium sulfate, sodium bisulfate, sodium nitrate, sodium carbonate, and sodium bicarbonate.

[0058] In practice, the amount of sodium source added can be 13.34%-20% of the total raw material mass.

[0059] In some specific embodiments, the sulfur source may include at least one of ferrous sulfate, ferrous ammonium sulfate, ferrous nitrate and their hydrates.

[0060] In practice, the amount of sulfur source fed into the total raw materials can be 15%-25% by mass.

[0061] It should be noted that sodium sulfate and sodium bisulfate can be used as both sodium and sulfur sources; ferrous sulfate and ferrous ammonium sulfate can be used as both iron and sulfur sources.

[0062] In some embodiments, the amounts of iron source, sodium source and sulfur source satisfy the following: in the mixed solution, the molar ratio of iron element, sodium element and sulfur element is (1.6-2):(2-2.4):(2-3).

[0063] In some embodiments, the mixture may include water as a solvent in the mixture, used to disperse or dissolve materials such as iron source, sodium source, sulfur source and conductive carbon material.

[0064] In some specific embodiments, the preparation process of the mixture may include the following steps: mixing a first liquid containing an iron source, a sodium source, a sulfur source and water with a second liquid containing a conductive carbon material and water to obtain a mixture.

[0065] In practice, before the second liquid containing conductive carbon material and water is mixed with the first liquid, it can be pre-dispersed by ultrasonic treatment. This is more conducive to avoiding the agglomeration of conductive carbon material, improving the dispersion uniformity of conductive carbon material in the cathode material, and thus enhancing the electron transport efficiency and other properties of the cathode material.

[0066] In some embodiments, the mass ratio of iron source to conductive carbon material can be 100:(1-7), for example, it can be a range of 100:1, 100:2, 100:3, 100:4, 100:5, 100:6, 100:7 or any two of them, preferably 100:(1-5.5).

[0067] In some embodiments, the mixture may also include antioxidants, which are more conducive to inhibiting the oxidation of conductive carbon materials during high-temperature treatment (such as annealing), maintaining their structural integrity, thereby improving the structural stability and other properties of the cathode material and enhancing the cycle performance of the battery.

[0068] In some specific embodiments, the antioxidant may include at least one of ascorbic acid, citric acid, and oxalic acid.

[0069] In practice, the antioxidant can be 5% to 10% of the total feed amount.

[0070] In some embodiments, the drying process may include: spray drying the mixture to obtain a precursor; preferably, the mixture is fed into a spray dryer for spray drying, wherein the inlet temperature of the spray dryer is 180℃-240℃, for example, a range of 180℃, 190℃, 200℃, 210℃, 220℃, 230℃, 240℃ or any two thereof; the feed rate of the mixture is 8mm / s-14mm / s, for example, a range of 8mm / s, 9mm / s, 10mm / s, 11mm / s, 12mm / s, 13mm / s, 14mm / s or any two thereof; and the rotational speed of the spray dryer is 180 rpm / min-220 rpm / min, for example, 180 rpm / min, 190 rpm / min, 200 rpm / min, 210 rpm / min, 220 rpm / min or any two thereof. The range of rpm / min or any two thereof; the outlet temperature of the spray dryer is 95℃-115℃, for example, it can be a range of 95℃, 100℃, 105℃, 110℃, 115℃ or any two thereof.

[0071] During spray drying, the morphology and particle size distribution of the precursor can be controlled by adjusting parameters such as the inlet temperature of the spray dryer and the feed rate of the mixture, thereby affecting the morphology and electrochemical performance of the cathode material. Specifically, a higher inlet temperature and a slower feed rate are more conducive to the formation of a uniform hollow structure in the precursor.

[0072] In this invention, a conventional spray dryer can be used to spray dry the mixture, such as the Buqi spray dryer (S390-S395).

[0073] In some embodiments, annealing can be performed under a protective atmosphere, which is more conducive to avoiding the oxidation of ferrous iron, thereby improving the preparation efficiency of the cathode material and optimizing the performance of the cathode material.

[0074] Specifically, the protective atmosphere may include an inert gas atmosphere, such as at least one of nitrogen, helium, neon, and argon atmospheres.

[0075] In some embodiments, the annealing temperature can be between 300°C and 450°C, for example, a range of 300°C, 350°C, 400°C, 450°C, or any combination thereof. An annealing temperature not lower than 300°C is more conducive to the uniform dispersion of the cathode material, thereby improving the electronic conductivity of the cathode material and enhancing the rate performance of the battery; an annealing temperature not exceeding 450°C is more conducive to avoiding the decomposition of the cathode material and the generation of byproducts, thereby improving the electrochemical performance of the cathode material.

[0076] In some embodiments, the annealing time can be 4h-24h, for example, a range of 4h, 8h, 12h, 16h, 18h, 20h, 24h or any two of these.

[0077] In some embodiments, the precursor is calcined and then annealed. The calcination temperature can be 180℃-220℃, for example, a range of 180℃, 190℃, 200℃, 210℃, 220℃ or any two of these. The calcination time can be 3h-5h, for example, a range of 3h, 3.5h, 4h, 4.5h, 5h or any two of these.

[0078] After calcining the precursor, annealing it not only facilitates the evaporation of internal moisture and the formation of an internal hollow structure, but also improves the purity of the cathode material and reduces the impact of impurities on its electrochemical performance. At the same time, during the calcination process, the Na, Fe, and S elements in the precursor will further diffuse and mix, making the components more uniform at the microscopic scale. This is more conducive to forming a cathode material with stable structure and consistent performance, thereby improving the electrochemical performance of the battery.

[0079] In some specific embodiments, the precursor can be laid flat in a crucible for calcination. It should be noted that the thickness of the precursor layer in the crucible generally does not exceed 10 cm. This is because if the precursor layer is too thick, it is not conducive to the uniform mixing of Na and Fe in the precursor. Therefore, by setting an appropriate thickness of the precursor layer in the crucible, it is more beneficial to the uniform formation of the pure sodium ferrous sulfate phase, thereby improving the electrochemical performance and stability of the battery.

[0080] The present invention also provides a positive electrode sheet, comprising the above-described positive electrode material or a positive electrode material prepared according to the above-described method for preparing the positive electrode material. This positive electrode sheet has advantages corresponding to the above-described positive electrode material, which will not be elaborated further.

[0081] Specifically, the positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector. The positive active layer includes the aforementioned positive electrode material, which serves as the positive active material of the positive electrode sheet.

[0082] In one specific embodiment, the positive electrode active layer comprises, by weight percentage, 70-99 wt% of positive electrode material, 0.5-15 wt% of conductive agent, and 0.5-15 wt% of binder; further, the positive electrode active layer comprises 80-98 wt% of positive electrode material, 1-10 wt% of conductive agent, and 1-10 wt% of binder.

[0083] The positive electrode current collector material may include at least one of aluminum foil and nickel foil; the conductive agent may be selected from at least one of carbon black, acetylene black, graphene, Ketjen black, and carbon fiber; the binder may be selected from at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, ethylene oxide-containing polymers, polyvinylpyrrolidone, and polyurethane.

[0084] In this embodiment of the invention, the positive electrode sheet can be prepared by conventional coating method. For example, the positive electrode material of the present invention and the raw materials such as conductive agent and binder used to form the positive electrode active layer can be dispersed in N-methylpyrrolidone (NMP) solvent, and thoroughly stirred and mixed to form a uniform positive electrode slurry. The positive electrode slurry is uniformly coated on the positive electrode current collector, and after drying, rolling and cutting, the positive electrode sheet is obtained.

[0085] The present invention also provides a battery comprising the above-described positive electrode. This battery has advantages corresponding to the above-described positive electrode, which will not be elaborated further.

[0086] It is conceivable that the battery provided by the present invention, in addition to the aforementioned positive electrode, also includes a negative electrode, an electrolyte, and a separator.

[0087] Specifically, a battery may include a cell and a casing for encapsulating the cell. The cell includes a positive electrode, a negative electrode, and a separator between the positive and negative electrodes. Electrolyte is injected into the casing to wet the cell.

[0088] This invention does not strictly limit the negative electrode active material in the negative electrode sheet. It can be a negative electrode active material commonly used in batteries, such as at least one of graphite, hard carbon, soft carbon, mesophase carbon microspheres, silicon-based negative electrode materials (mainly including silicon suboxide and silicon-carbon negative electrode), and tin-based negative electrode materials (mainly including tin and tin alloy).

[0089] This invention does not strictly limit the choice of electrolyte. Conventional electrolytes in the art can be used, such as non-aqueous electrolytes. The electrolyte includes an organic solvent and an electrolyte salt. The organic solvent may include one or more solvents commonly used in current battery electrolytes. The electrolyte salt may include lithium salts, specifically lithium salts commonly used in current lithium-ion electrolytes. For example, the solvent may include one or more of ethylene carbonate, propylene carbonate, butene carbonate, fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), diethyl carbonate (DEC), difluoroethylene carbonate (DFEC), dipropyl carbonate, methyl ethyl carbonate (EMC), ethyl acetate, ethyl propionate, propyl acetate, propyl propionate, sulfolane, γ-butyrolactone, etc. The lithium salt may include one or more of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0090] This invention does not strictly limit the choice of separator material. It can be one of the separator materials commonly used in batteries, such as polypropylene separator (PP), polyethylene separator (PE), polypropylene / polyethylene double-layer composite membrane (PP / PE), polyimide electrospun separator (PI), polypropylene / polyethylene / polypropylene triple-layer composite membrane (PP / PE / PP), cellulose nonwoven separator, and separator with ceramic coating.

[0091] The battery of the present invention can be manufactured according to conventional methods in the art. For example, when manufacturing the battery, the positive electrode sheet, separator and negative electrode sheet are wound or stacked to obtain a bare cell, and the bare cell is packaged into a pre-stamped shell (such as an aluminum-plastic film bag). After the packaged battery precursor is dried at 85°C, the electrolyte is injected into the dried battery precursor. After the process of resting, formation and secondary sealing, the battery manufacturing is completed.

[0092] The present invention also provides a battery pack comprising at least two interconnected batteries as described above. This battery pack has advantages corresponding to the batteries described above, which will not be elaborated further.

[0093] Generally, a battery pack includes multiple batteries as individual cells, which are connected to form the battery pack. These batteries can be electrically connected using methods conventional in the art, such as series connection, parallel connection, or a combination of these connection methods, without any particular limitation.

[0094] The present invention also provides an electrical device, including the battery or battery pack described above, which has advantages corresponding to the battery or battery pack described above, and will not be described in detail here.

[0095] The electrical equipment used in the embodiments of the present invention can be conventional electrical equipment in the art, such as power equipment (e.g., electric vehicles, electric cars), electronic equipment (e.g., mobile phones, tablets, laptops, digital cameras, etc.), wearable devices (e.g., watches, bracelets, VR glasses, etc.), energy storage power stations, etc., and there are no particular limitations on this.

[0096] The present invention will be further described below through specific embodiments. In the following embodiments and comparative examples, unless otherwise specified, the separator used is a Celgard 2400 polypropylene porous membrane, the negative electrode is a 15mm diameter sodium sheet from KELOD Company, the conductive agent is TC-35 from Greenpowder Company, the binders PVDF, NMP, carbonized aluminum foil and propylene carbonate solution used are all purchased from KELOD Company, the ferrous sulfate and sodium sulfate used are industrial grade materials from McLean Company, and the conductive carbon materials (carbon nanotubes, graphene, conductive carbon black) used are aqueous electrocarbon materials purchased from Jiangsu Tiannai.

[0097] Example 1:

[0098] The cathode material provided in this embodiment 1 includes a material body and a hollow structure formed by the material body. The preparation process of this cathode material is as follows.

[0099] 1. Preparation of cathode materials

[0100] (1) Preparation of mixture

[0101] 0.56 mol of anhydrous sodium sulfate (serving as both a sodium and sulfur source) was dissolved in 600 g of pure water and ultrasonically dispersed at 100 Hz for 30 min to obtain anhydrous sodium sulfate solution. Then, 0.78 mol of ferrous sulfate heptahydrate (serving as both an iron and sulfur source) and 0.01 mol of antioxidant (citric acid) were added to the anhydrous sodium sulfate solution, and stirring was continued for 30 min to obtain the first solution.

[0102] The conductive carbon material (4.3% CNT solution) was added to 100 mL of pure water and ultrasonically dispersed at 100 Hz for 30 min to obtain the second solution, wherein the mass ratio of ferrous sulfate heptahydrate to CNT (from 4.3% CNT solution) was 100:1.1.

[0103] Then the second liquid was added to the first liquid and ultrasonically dispersed at 100 Hz for 30 min to obtain a mixture.

[0104] (2) Preparation of precursor

[0105] The above mixture was dried in a spray dryer (Buqi S390-S395) to obtain a precursor; wherein the spray dryer had a rotation speed of 200 rpm / min, an inlet temperature of 220℃, a spray gas volume flow rate of 1800 L / min, a feed rate of 11 mm / s, and an outlet temperature of 105℃.

[0106] (3) Preparation of cathode materials

[0107] 20g of precursor (powder) was evenly spread in a 500ml corundum crucible (crucible dimensions: 120mm × 120mm × 50mm), and then the corundum crucible was placed in the middle of a three-section tube furnace. First, calcination was performed, i.e., the temperature was increased from an initial temperature of 30℃ to 200℃ at a heating rate of 5℃ / min, and calcined at 200℃ for 4 hours. Then, annealing was performed by increasing the temperature to 450℃ at a heating rate of 3℃ / min for 8 hours, yielding a cathode material with a hollow structure. This cathode material is 1% CNT-Na. 2.5 Fe 1.75 (SO4)3, where "1%" indicates that the conductive carbon material (CNT) and Na in this cathode material... 2.5 Fe 1.75 The mass ratio of (SO4)3 is 1:100 (1%).

[0108] 2. Preparation of the positive electrode sheet

[0109] The above-mentioned positive electrode material, conductive agent (acetylene black), binder (polyvinylidene fluoride, PVDF), and dispersant (N-methylpyrrolidone, NMP) are mixed evenly in a mass ratio of 95:3:2:50 to obtain a positive electrode slurry. The positive electrode slurry is then coated on both sides of the positive electrode current collector (carbon-coated aluminum foil), and then placed in a 120℃ oven for vacuum drying for 24 hours. After pressing and rolling, a positive electrode sheet is obtained.

[0110] 3. Battery manufacturing

[0111] The above-mentioned positive electrode, separator and negative electrode are stacked to obtain a bare cell, and the bare cell is packaged into a pre-stamped aluminum-plastic film bag. After the packaged battery precursor is dried at 85°C, the electrolyte (1 mol / L propylene carbonate solution) is injected into the dried battery precursor. After the process of resting, formation and secondary sealing, the battery is obtained.

[0112] Example 2:

[0113] Compared to Example 1, the difference lies in the preparation of the mixture, where the mass ratio of ferrous sulfate heptahydrate to CNTs (from a 4.3% CNT solution) is 100:2.2. The resulting cathode material with a hollow structure is 2% CNT-Na. 2.5 Fe 1.75 (SO4)3.

[0114] The remaining conditions are the same as in Example 1.

[0115] Example 3:

[0116] Compared to Example 1, the difference lies in the preparation of the mixture, where the mass ratio of ferrous sulfate heptahydrate to CNTs (from a 4.3% CNT solution) is 100:3.3. The resulting cathode material with a hollow structure is 3% CNT-Na. 2.5 Fe 1.75 (SO4)3.

[0117] The remaining conditions are the same as in Example 1.

[0118] Example 4:

[0119] Compared to Example 1, the difference lies in the preparation of the mixture, where the mass ratio of ferrous sulfate heptahydrate to CNTs (from a 4.3% CNT solution) is 100:4.4. The resulting cathode material with a hollow structure is 4% CNT-Na. 2.5 Fe 1.75 (SO4)3.

[0120] The remaining conditions are the same as in Example 1.

[0121] Example 5:

[0122] Compared to Example 1, the difference lies in the preparation of the mixture, where the mass ratio of ferrous sulfate heptahydrate to CNTs (from a 4.3% CNT solution) is 100:5.5. The resulting cathode material with a hollow structure is 5% CNT-Na. 2.5 Fe 1.75 (SO4)3.

[0123] The remaining conditions are the same as in Example 1.

[0124] Example 6:

[0125] Compared to Example 1, the difference lies in the preparation of the mixture, where the mass ratio of ferrous sulfate heptahydrate to CNTs (from a 4.3% CNT solution) is 100:6.6. The resulting cathode material with a hollow structure is 6% CNT-Na. 2.5 Fe 1.75 (SO4)3.

[0126] The remaining conditions are the same as in Example 1.

[0127] Example 7:

[0128] Compared to Example 1, the difference lies in the preparation of the mixture, where the mass ratio of ferrous sulfate heptahydrate to CNTs (from a 4.3% CNT solution) is 100:0.8. The resulting cathode material with a hollow structure is 0.8% CNT-Na. 2.5 Fe 1.75 (SO4)3.

[0129] The remaining conditions are the same as in Example 1.

[0130] Example 8:

[0131] Compared to Example 1, the difference lies in the preparation of the mixture, where the mass ratio of ferrous sulfate heptahydrate to CNTs (from a 4.3% CNT solution) is 100:0.5. The resulting cathode material with a hollow structure is 0.5% CNT-Na. 2.5 Fe 1.75 (SO4)3.

[0132] The remaining conditions are the same as in Example 1.

[0133] Example 9:

[0134] Compared to Example 1, the difference lies in the preparation process of the second solution, which includes: adding conductive carbon material (4.3% CNT solution and graphene) to 100 mL of pure water and ultrasonically dispersing it at 100 Hz for 30 min to obtain the second solution. The mass ratio of ferrous sulfate heptahydrate to the total mass of CNTs (from the 4.3% CNT solution) and graphene (GO) is 100:3, and the mass ratio of CNTs to graphene is 1:1. The resulting cathode material with a hollow structure is 3% (CNT / GO)-Na. 2.5 Fe 1.75 (SO4)3.

[0135] The remaining conditions are the same as in Example 1.

[0136] Example 10:

[0137] Compared to Example 1, the difference lies in the preparation process of the second solution, which includes: adding conductive carbon material (4.3% CNT solution and conductive carbon black) to 100 mL of pure water and ultrasonically dispersing it at 100 Hz for 30 min to obtain the second solution. The mass ratio of ferrous sulfate heptahydrate to the total mass of CNTs (from the 4.3% CNT solution) and conductive carbon black (CB) is 100:3, and the mass ratio of CNTs to conductive carbon black is 1:1. The resulting cathode material with a hollow structure is 3% (CNT / CB)-Na. 2.5 Fe 1.75 (SO4)3.

[0138] The remaining conditions are the same as in Example 1.

[0139] Example 11:

[0140] Compared to Example 1, the difference lies in the preparation process of the second solution, which includes: adding conductive carbon material (4.3% CNT solution, graphene, and conductive carbon black) to 100 mL of pure water and ultrasonically dispersing it at 100 Hz for 30 min to obtain the second solution. The mass ratio of ferrous sulfate heptahydrate to the total mass of CNTs (from the 4.3% CNT solution), graphene (GO), and conductive carbon black (CB) is 100:3, and the mass ratio of CNTs, graphene, and conductive carbon black is 1:1:1. The resulting cathode material with a hollow structure is 3% (CNT / GO / CB)-Na. 2.5 Fe 1.75 (SO4)3.

[0141] The remaining conditions are the same as in Example 1.

[0142] Comparative Example 1:

[0143] Compared to Example 1, the difference lies in the absence of conductive carbon material. The chemical formula of the obtained pure sodium ferrous sulfate cathode material is Na. 2.5 Fe 1.75 (SO4)3.

[0144] The remaining conditions are the same as in Example 1.

[0145] In addition, the following physical property tests were performed on the cathode materials in the examples and comparative examples, and the test results are shown in Table 1.

[0146] (1) Test of conductive carbon material content

[0147] Take 0.5g of positive electrode material, crush it, and measure the carbon content in the positive electrode material using an SES-906 high-frequency infrared carbon-sulfur analyzer. The carbon content, i.e., the mass fraction of conductive carbon material in the positive electrode material, is recorded as conductive carbon material / positive electrode material. The specific results are shown in Table 1.

[0148] (2) BET test

[0149] Take 0.5g of positive electrode material, crush it, and then use a BST-BET400 instrument to test the specific surface area of ​​the positive electrode material. Record the specific surface area as the specific results. See Table 1 for details.

[0150] (3) Particle size test

[0151] 0.1 g of the cathode material was dissolved in 20 mL of anhydrous ethanol and sonicated for 20 min to ensure uniform dispersion. The particle size of the cathode material was then measured using an organic Malvern laser particle size analyzer, and the results were denoted as D. 50 D 100 The specific results are shown in Table 1.

[0152] (4) SEM test

[0153] The cathode material was subjected to SEM testing using a Hitachi SU70 instrument from Japan, and SEM images of the cathode material were obtained.

[0154] The cathode material and binder (PVDF) were then mixed at a mass ratio of 95:5 and coated onto the surface of aluminum foil. The mixture was then ion-milled to obtain the observation sample, which had a thickness of 500 μm-600 μm. SEM analysis was performed on the observation sample to obtain a cross-sectional image (CP image) of the cathode material. The upper and lower surface boundaries of the cross-section were observed and located using the CP image. Using the instrument's built-in length measuring tool, linear distance measurements were taken at different locations along the cross-section. The values ​​at each measurement point were recorded, and the average value was taken as the average wall thickness of the hollow structure of the cathode material, denoted as the average wall thickness.

[0155] (5) XRD test

[0156] The cathode material was subjected to XRD testing using a BruKer D8 instrument from Bruker GmbH, Germany, and the XRD pattern was obtained.

[0157] In addition, the following performance tests were performed on the batteries in the embodiments and comparative examples, and the test results are shown in Table 2.

[0158] (6) Charge and discharge test

[0159] The battery was installed in the Xinwei BTS-51 battery test cabinet. Within the charge / discharge cutoff voltage range of 2.0V-4.0V, the discharge specific capacity and discharge efficiency of the positive electrode material were tested at a charge / discharge rate of 0.1C, and recorded as discharge specific capacity and discharge efficiency, respectively.

[0160] (7) Ratio performance test

[0161] The battery was installed in the Xinwei BTS-51 battery test cabinet. At 25℃, the discharge cutoff voltage was within the range of 2.0V-4.0V. The discharge ratio of the positive electrode material was tested at a charge / discharge rate of 30C, and recorded as the 30C discharge ratio.

[0162] (8) Low temperature performance test

[0163] The battery was installed in the Xinwei BTS-51 battery test cabinet. Under conditions of -20℃ and within the range of charge and discharge cutoff voltage of 2.0V-4.0V, the discharge specific capacity of the positive electrode material at a charge and discharge rate of 1C was tested and recorded as the -20℃ discharge specific capacity.

[0164] (9) Long-cycle performance test

[0165] The battery was installed in the Xinwei BTS-51 battery test cabinet. Under high temperature conditions of 45℃, and within the range of charge and discharge cutoff voltage of 2.0V-4.0V, the capacity retention rate after 100 cycles of constant current and constant voltage discharge at 0.5C and 1.0C was recorded as the capacity retention rate.

[0166] The average wall thickness of the hollow structure of the cathode material in each embodiment and comparative example, the mass fraction of conductive carbon material in the cathode material (denoted as conductive carbon material / cathode material), the composition of conductive carbon material in the material body (denoted as the type of conductive carbon material), and the particle size D of the cathode material are defined as follows: 50 and D 100 The specific surface area and other conditions of the cathode material are summarized in Table 1.

[0167] Table 1:

[0168]

[0169] Table 2:

[0170]

[0171] According to the data in Tables 1 and 2, compared with Comparative Example 1, Examples 1-11, by introducing conductive carbon material into the positive electrode material, formed a material body with a hollow structure by the sodium ferrous sulfate matrix and the conductive carbon material, which increased the specific surface area of ​​the positive electrode material and improved the energy density, rate performance and low temperature adaptability of the battery. This shows that the uniquely designed hollow structure can effectively improve the electrochemical performance of the battery.

[0172] Specifically, the SEM test results show that, compared to Comparative Example 1 ( Figure 2 The cathode material prepared in Example 3 exhibits a distinct hollow structure formed by the winding of conductive carbon material (CNT) and sodium ferrous sulfate. Figure 3 , Figure 4 The structural schematic diagram of the positive electrode material in Example 3 is shown below. Figure 1 As shown.

[0173] In addition, XRD test results show (such as) Figure 5 As shown), the cathode material phases in Example 3 and Comparative Example 1 are both related to Na. 2.5 Fe 1.75 This is consistent with the standard card (PDF#97-025-2379) for (SO4)3, indicating that the present invention has successfully prepared a pure-phase sodium ferrous sulfate cathode material and that the presence of conductive carbon material (CNT) does not affect the generation of the pure-phase sodium ferrous sulfate.

[0174] According to Table 2, compared to Comparative Example 1, even under a rate of 30.0C, the batteries prepared with the cathode materials in Examples 1-11 still have good discharge ratios. This indicates that the conductive carbon material and sodium ferrous sulfate in the cathode material are fully mixed and entangled to form an efficient electron transport network, which effectively reduces the internal resistance of the battery, thereby reducing polarization during the charging and discharging process and further improving the cycle stability and rate performance of the battery.

[0175] Furthermore, compared to Comparative Example 1, the sodium-ion battery prepared from the cathode material in Examples 1-11 still exhibits good discharge specific capacity at an ultra-low temperature of -20℃ and a current of 1C, demonstrating excellent low-temperature performance. After 100 cycles at a high temperature of 45℃, the capacity retention rate is over 90%. This indicates that the unique hollow structure in the cathode material can effectively reduce the ion transport resistance of the sodium-ion battery, increase the diffusion rate of sodium ions, and enhance the long-cycle performance of the sodium-ion battery, which is beneficial to the multifaceted practicality of sodium-ion batteries.

[0176] Furthermore, compared to Examples 6-8, Examples 1-5, by controlling the mass ratio of iron source to conductive carbon material within the preferred range of 100:(1-5.5) during the preparation of the cathode material, make the mass fraction of conductive carbon material in the cathode material within the preferred range of 0.95%-5%. This is more conducive to controlling the average wall thickness of the hollow structure in the cathode material, the particle size of the cathode material, and the specific surface area of ​​the cathode material within a suitable range, thereby optimizing the electrochemical performance of the battery.

[0177] In addition, Examples 9-11, by simultaneously introducing multiple conductive carbon materials into the cathode material, can optimize both the cycle stability and rate performance of the battery.

[0178] In summary, this invention designs a cathode material with a hollow structure, which helps to improve the structural stability and conductivity of the cathode material, thereby improving the energy density, rate performance, low-temperature adaptability and cycle life of the battery. It is expected to promote the practical application of sodium-ion batteries with sodium ferrous sulfate as the cathode material.

[0179] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A positive electrode material, characterized in that, The positive electrode material includes a material body and a hollow structure formed by the material body, wherein the material body includes a sodium ferrous sulfate matrix and a conductive carbon material.

2. The cathode material according to claim 1, characterized in that, The average wall thickness of the hollow structure is 0.5μm-2.5μm, preferably 0.7μm-1.8μm.

3. The cathode material according to claim 1 or 2, characterized in that, The conductive carbon material accounts for 0.95%-6% of the mass fraction of the positive electrode material, preferably 0.95%-5%.

4. The cathode material according to any one of claims 1-3, characterized in that, The conductive carbon material includes aqueous conductive carbon material.

5. The cathode material according to any one of claims 1-4, characterized in that, The conductive carbon material includes at least one of carbon nanotubes, graphene, and conductive carbon black.

6. The cathode material according to any one of claims 1-5, characterized in that, The particle size D of the positive electrode material 50 The thickness ranges from 7.5 μm to 25 μm, preferably from 9 μm to 20 μm; And / or, the specific surface area of ​​the positive electrode material is 3 m². 2 / g-17 m 2 / g, preferably 5 m 2 / g-12 m 2 / g.

7. A method for preparing the cathode material according to any one of claims 1-6, characterized in that, Includes the following steps: The mixture comprising an iron source, a sodium source, a sulfur source, and the conductive carbon material is dried to obtain a precursor. The precursor is annealed to obtain the cathode material.

8. The method for preparing the cathode material according to claim 7, characterized in that, The mixture includes water; And / or, the mass ratio of the iron source to the conductive carbon material is 100:(1-7), preferably 100:(1-5.5). And / or, the mixture may also include an antioxidant.

9. The method for preparing the cathode material according to claim 7 or 8, characterized in that, The drying process includes: spray drying the mixture to obtain the precursor; Preferably, the mixture is fed into a spray dryer for spray drying, the inlet temperature of the spray dryer is 180℃-240℃, and the feed rate of the mixture is 8mm / s-14mm / s.

10. The method for preparing the cathode material according to any one of claims 7-9, characterized in that, The annealing process is performed under a protective atmosphere; And / or, the annealing treatment is performed at a temperature of 300℃-450℃ for a time of 4h-24h; And / or, after calcining the precursor, the annealing treatment is performed, wherein the calcination temperature is 180℃-220℃ and the calcination time is 3h-5h.

11. A positive electrode plate, characterized in that, The cathode material includes the cathode material according to any one of claims 1-6 or the cathode material prepared according to the method of preparing the cathode material according to any one of claims 7-10.

12. A battery, characterized in that, Includes the positive electrode sheet as described in claim 11.

13. A battery pack, characterized in that, It includes at least two interconnected batteries as described in claim 12.

14. An electrical appliance, characterized in that, Includes the battery of claim 12 or the battery pack of claim 13.

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