Positive electrode material, preparation method therefor, positive electrode sheet, battery and electrical apparatus

Abstract

The present application provides a positive electrode material, a preparation method thereof, a positive electrode sheet, a battery and an electrical apparatus, and relates to the field of batteries. The positive electrode material comprises sodium iron phosphate pyrophosphate and a carbon coating layer covering the surface of same; and the positive electrode material contains an impurity phase sodium ferric phosphate, the mass ratio of the impurity phase sodium ferric phosphate in sodium iron phosphate pyrophosphate being 14% or below. The positive electrode material has a low content of the impurity phase sodium ferric phosphate, such that the initial charging capacity can be effectively improved, and the electrochemical performance of the positive electrode material is effectively improved.

Description

Positive electrode materials and their preparation methods, positive electrode sheets, batteries and electrical devices Cross-references

[0001] This application claims priority to Chinese Patent Application No. 202410494819.1, filed on April 23, 2024, entitled “Positive electrode material and preparation method thereof, positive electrode sheet, battery and power device”, the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of batteries, and more specifically, to a positive electrode material and its preparation method, a positive electrode sheet, a battery, and an electrical device. Background Technology

[0003] The cathode material plays a crucial role in the electrochemical performance of sodium-ion batteries. Theoretically, sodium iron pyrophosphate / carbon composite materials possess excellent electrochemical and rate performance, making them suitable for large-scale industrial applications. However, existing sodium iron pyrophosphate / carbon composite materials suffer from low initial charge capacity. Summary of the Invention

[0004] In view of the above problems, this application provides a positive electrode material and its preparation method, a positive electrode sheet, a battery and an electrical device, which can improve the technical problem of low initial charging capacity of existing sodium iron pyrophosphate / carbon composite materials.

[0005] In a first aspect, embodiments of this application provide a cathode material comprising sodium iron pyrophosphate and a carbon coating layer on its surface; the cathode material contains a heterophase sodium iron phosphate, wherein the mass percentage of the heterophase sodium iron phosphate in the sodium iron pyrophosphate is 14% or less.

[0006] The cathode material provided in this application utilizes the fact that the mass percentage of the heterophase sodium iron phosphate in the carbon-coated sodium iron phosphate pyrophosphate is 14% or less, which effectively reduces the content of the heterophase sodium iron phosphate, thereby effectively improving the initial charge capacity and the electrochemical performance of the cathode material.

[0007] In some embodiments, the mass percentage of heterogeneous sodium iron phosphate in sodium iron pyrophosphate is 12% or less. Further reducing the mass percentage of heterogeneous sodium iron phosphate in sodium iron pyrophosphate is beneficial for further improving the initial charge capacity and effectively enhancing the electrochemical performance of the cathode material.

[0008] In some embodiments, the cathode material satisfies one or more of (a1)-(a4):

[0009] (a1) The surface NaHCO3 content of the positive electrode material is 0.05wt%-2.5wt%;

[0010] (a2) The Na2CO3 content on the surface of the cathode material is 0.05wt%-2.5wt%;

[0011] (a3) The initial charge capacity of the cathode material is greater than 105 mAh / g;

[0012] (a4) The volumetric particle size distribution of the cathode material is Dv10, which is 0.3μm-1.3μm, Dv50, which is 0.5μm-5.5μm, and Dv90, which is 1.34μm-12.351μm, where (Dv90-Dv10) / Dv50 = 1.6-2.1.

[0013] While maintaining a small particle size, the above-mentioned cathode material also takes into account low surface residual alkali content and high initial charge capacity, which is beneficial to improving the processing performance and electrochemical performance of the cathode material.

[0014] In some embodiments, the cathode material satisfies one or more of (b1)-(b3):

[0015] (b1) The surface NaHCO3 content of the positive electrode material is 0.05wt%-0.6wt%;

[0016] (b2) The Na2CO3 content on the surface of the positive electrode material is 0.05wt%-1wt%;

[0017] (b3) The initial charge capacity of the cathode material is 110mAh / g-120mAh / g.

[0018] The aforementioned cathode material balances low surface residual alkali content and high initial charge capacity, which is beneficial to improving the processing performance and electrochemical performance of the cathode material.

[0019] In some embodiments, the carbon content in the cathode material is 1wt%-3.1wt%; and / or, the thickness of the carbon coating layer is 2.5-30nm.

[0020] By controlling the carbon content and / or carbon coating in the cathode material within the aforementioned suitable thickness range, it is beneficial to improve the conductivity of the cathode material, while also ensuring that the cathode material has both a low residual alkali content and a high initial charge capacity.

[0021] In some embodiments, the chemical formula of sodium iron pyrophosphate is (c1) or (c2):

[0022] (c1)Na x Fe y (PO4)2P2O7, x=3.5-4.5, y=2.75-3.25;

[0023] (c2)Nax Fe y P m O n , 3.5≤x≤4.5, 2.5≤y≤3.5, 3.7<m<4, 14.5≤n≤15.5.

[0024] Secondly, this application provides a method for preparing the cathode material in the above embodiments, which includes: dry mixing sodium iron pyrophosphate powder and a first carbon source, and sintering it for a second time in a reducing atmosphere for at least 10 hours, wherein the temperature of the second sintering is 400℃-600℃, to prepare a second sintered material.

[0025] The method for preparing the cathode material provided in this application involves dry mixing followed by secondary sintering in a reducing atmosphere at 400℃-600℃ for at least 10 hours. This not only improves the conductivity of the cathode material and prevents grain growth and secondary agglomeration, but also restores the crystal morphology of sodium iron pyrophosphate, removing impurities and residual Na. + By reducing the embedding in the original crystal lattice, the impurity phase content and surface residual alkali content of the sodium iron pyrophosphate cathode material are effectively reduced, thereby effectively improving the capacity of the sodium iron pyrophosphate cathode material and thus effectively improving the processing performance and electrochemical performance of the cathode material.

[0026] In some embodiments, the secondary sintering temperature is 450℃-580℃. Controlling the secondary sintering temperature within the above range is beneficial for the sodium iron pyrophosphate cathode material to achieve both low impurity phase content, low surface residual alkali content, and high initial charge capacity, thereby improving the processing performance and electrochemical performance of the cathode material.

[0027] In some embodiments, the secondary sintering time is 10-20 hours. Controlling the secondary sintering time within the above range can improve the processing performance and electrochemical performance of the cathode material while maintaining reasonable cost and good preparation efficiency.

[0028] In some embodiments, sodium iron pyrophosphate powder is obtained by crushing, and the secondary sintered material is used as the cathode material. Since the sodium iron pyrophosphate powder is obtained by crushing, the secondary sintered material obtained after sintering does not need to be further crushed and can be directly used as the cathode material.

[0029] In some embodiments, the difference between the volumetric particle size distribution Dv50 of the sodium iron pyrophosphate powder and the volumetric particle size distribution Dv50 of the cathode material is ≤0.5 μm. By controlling the difference between the volumetric particle size distribution Dv50 of the sodium iron pyrophosphate powder and the cathode material within the above range, the particle size of the cathode material after secondary sintering does not significantly increase compared to the powder before secondary sintering, remaining within a relatively small particle size range, which is beneficial for improving battery performance.

[0030] In some embodiments, the volumetric particle size distribution Dv10 of the positive electrode material is 0.3 μm-1.3 μm, the volumetric particle size distribution Dv50 is 0.5 μm-5.5 μm, and the volumetric particle size distribution Dv90 is 1.34 μm-12.351 μm, with (Dv90-Dv10) / Dv50 = 1.6-2.1. Here, (Dv90-Dv10) / Dv50 represents the concentration of the positive electrode active material particle size. When it is within the above range, it can be inferred that the positive electrode material has a high concentration and no agglomeration has occurred. In some embodiments, the first carbon source includes an inorganic carbon source and / or an organic carbon source; wherein the inorganic carbon source includes at least one of natural graphite powder, artificial graphite powder, carbon black, carbon nanotubes, and graphene; and the organic carbon source includes at least one of sucrose, glucose, citric acid, starch, cyclodextrin, pitch, and polyethylene glycol. The aforementioned first carbon sources can all enable the powder to undergo a carbothermic reduction solid-phase reaction after carbon coating, thereby restoring the crystal morphology of sodium iron pyrophosphate and removing residual alkali and Na from impurity phases. + It is then embedded back into the original lattice.

[0031] In some embodiments, the first carbon source is an inorganic carbon source, and the amount of inorganic carbon source added is less than or equal to 2 wt% of the powder. By controlling the amount of inorganic carbon source added within a suitable range, it is beneficial to improve the conductivity and initial charge capacity of the cathode material.

[0032] In some embodiments, the first carbon source is an organic carbon source, and the amount of organic carbon source added is less than or equal to 5 wt% of the powder. By controlling the amount of organic carbon source added within a suitable range, it is beneficial to improve the conductivity and specific capacity of the cathode material.

[0033] In some embodiments, the first carbon source comprises a mixture of inorganic and organic carbon sources, and the amount of the mixture added is less than or equal to 2 wt% of the powder, based on the carbon obtained by pyrolysis of the mixture at the secondary sintering temperature. By controlling the amount of the mixed carbon source added within a suitable range, it is beneficial to improve the conductivity and specific capacity of the cathode material.

[0034] In some embodiments, the reducing atmosphere is provided by a first carbon source; or...

[0035] The reducing atmosphere comprises a first component and a second component, the first component being provided by a first carbon source, and the second component comprising hydrogen and / or carbon monoxide.

[0036] The reducing atmosphere provided by the primary carbon source means that there are no additional reducing components. It utilizes the primary carbon source to form a carbon layer on the surface of microscopic particles, creating an atmosphere conducive to the reduction reaction, which is beneficial to Na. + During the secondary sintering process, it is reinserted into the sodium iron pyrophosphate lattice.

[0037] In some embodiments, the sodium iron pyrophosphate powder is carbon-coated sodium iron pyrophosphate, which is prepared by the following steps:

[0038] Sodium salt, iron source, phosphorus source, and a second carbon source are wet-mixed according to the chemical formula ratio, dried, and then sintered once under an inert atmosphere. The resulting sintered product is mechanically or by air-jet crushing to obtain carbon-coated sodium iron pyrophosphate powder. The powder prepared using the above method exhibits excellent electrochemical performance.

[0039] In some embodiments, the chemical formula of sodium iron pyrophosphate powder is (d1) or (d2):

[0040] (d1)Na x Fe y (PO4)2P2O7, x=3.5-4.5, y=2.75-3.25;

[0041] (d2)Na x Fe y P m O n , 3.5≤x≤4.5, 2.5≤y≤3.5, 3.7<m<4, 14.5≤n≤15.5.

[0042] Thirdly, this application provides a positive electrode sheet, which includes the positive electrode material in the above embodiments.

[0043] Fourthly, this application provides a battery that includes the positive electrode sheet in the above embodiments.

[0044] Fifthly, this application provides an electrical device that includes the battery described in the above embodiments, the battery being used to provide electrical energy.

[0045] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0046] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiments below. The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of this application. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings:

[0047] Figure 1 is a schematic diagram of the vehicle structure according to some embodiments of this application;

[0048] Figure 2 is an exploded structural diagram of a battery according to some embodiments of this application;

[0049] Figure 3 is an exploded structural diagram of a battery cell according to some embodiments of this application;

[0050] Figure 4 is a comparison of XRD patterns of sodium iron pyrophosphate before and after secondary sintering in Example 1 of this application;

[0051] Figure 5 is a SEM image of the powder used before secondary sintering in Example 1 of this application;

[0052] Figure 6 is a SEM image of the cathode material obtained after secondary sintering in Example 1 of this application;

[0053] Figure 7 is a charge-discharge curve of the positive electrode material prepared in Example 1 of this application.

[0054] The reference numerals in the detailed embodiments are as follows:

[0055] 1000 - Vehicles;

[0056] 100 - Battery; 200 - Controller; 300 - Motor;

[0057] 10-Box body; 11-First part; 12-Second part;

[0058] 20-Battery cell; 21-Casing; 22-Electrode assembly; 23-Electrode terminal; 24-Pressure relief structure;

[0059] 211-Shell; 212-Cover. Detailed Implementation

[0060] The following embodiments of the technical solution of the present application will be described in detail with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application and are therefore only examples and are not intended to limit the scope of protection of the present application.

[0061] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above-mentioned figure descriptions are intended to cover non-exclusive inclusions.

[0062] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0063] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0064] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0065] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces).

[0066] In the description of the embodiments of this application, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of this application and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0067] In the description of the embodiments of this application, unless otherwise expressly specified and limited, technical terms such as "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. For those skilled in the art, the specific meaning of the above terms in the embodiments of this application can be understood according to the specific circumstances.

[0068] The batteries disclosed in this application can be used, but are not limited to, in electrical devices such as vehicles, ships, or aircraft. A power system comprising such an electrical device can be used, incorporating batteries disclosed in this application, which is beneficial for increasing the initial charge capacity of the battery.

[0069] This application provides an electrical device that uses a battery as a power source. The electrical device can be, but is not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc. Electric toys can include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc. Spacecraft can include airplanes, rockets, space shuttles, and spacecraft, etc.

[0070] For ease of explanation, the following embodiments will be described using a vehicle 1000 as an example of an electrical device according to an embodiment of this application.

[0071] Please refer to Figure 1, which is a structural schematic diagram of a vehicle 1000 provided in some embodiments of this application. The vehicle 1000 can be a gasoline-powered vehicle, a natural gas-powered vehicle, or a new energy vehicle. New energy vehicles can be pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles, etc. A battery 100 is disposed inside the vehicle 1000, and the battery 100 can be located at the bottom, front, or rear of the vehicle 1000. The battery 100 can be used to power the vehicle 1000; for example, the battery 100 can serve as the operating power source for the vehicle 1000. The vehicle 1000 may also include a controller 200 and a motor 300. The controller 200 is used to control the battery 100 to supply power to the motor 300, for example, to meet the power needs of the vehicle 1000 during startup, navigation, and driving.

[0072] In some embodiments of this application, the battery 100 can not only serve as the operating power source for the vehicle 1000, but also as the driving power source for the vehicle 1000, replacing or partially replacing fuel or natural gas to provide driving power for the vehicle 1000.

[0073] In this application, battery 100 refers to a single physical module including one or more battery cells 20 to provide a certain voltage and capacity. It can be in the form of battery pack, battery module, etc. Battery 100 may also include a housing 10 for encapsulating one or more battery cells 20. The housing 10 can prevent liquids or other foreign objects from affecting the charging or discharging of the battery cells 20.

[0074] Please refer to Figure 2, which is an exploded view of a battery 100 provided in some embodiments of this application. The battery 100 includes a housing 10 and a battery cell 20, with the battery cell 20 housed within the housing 10. The housing 10 provides a space for the battery cell 20 and can have various structures. In some embodiments, the housing 10 may include a first portion 11 and a second portion 12, which overlap each other, jointly defining a space for accommodating the battery cell 20. The second portion 12 may be a hollow structure with one open end, and the first portion 11 may be a plate-like structure, covering the open side of the second portion 12 so that the first portion 11 and the second portion 12 jointly define the space. Alternatively, the first portion 11 and the second portion 12 may both be hollow structures with one open side, with the open side of the first portion 11 covering the open side of the second portion 12. Of course, the housing 10 formed by the first portion 11 and the second portion 12 can have various shapes, such as a cylinder, a cuboid, etc.

[0075] In battery 100, there can be multiple battery cells 20, which can be connected in series, parallel, or in a mixed manner. A mixed connection means that multiple battery cells 20 are connected in both series and parallel configurations. Multiple battery cells 20 can be directly connected in series, parallel, or in a mixed manner, and then the entire assembly of the multiple battery cells 20 is housed within the housing 10. Alternatively, battery 100 can also be composed of multiple battery cells 20 first connected in series, parallel, or in a mixed manner to form a battery module, and then multiple battery modules are connected in series, parallel, or in a mixed manner to form a whole, which is also housed within the housing 10. Battery 100 may also include other structures; for example, it may include a busbar component for electrical connection between the multiple battery cells 20.

[0076] Battery cell 20 refers to the smallest unit that makes up battery 100. Battery cell 20 can be a lithium-ion battery, lithium-sulfur battery, sodium-ion battery, or magnesium-ion battery, but is not limited to these.

[0077] Please refer to Figure 3, which is an exploded structural diagram of a battery cell 20 provided in some embodiments of this application. Referring to Figure 3, the battery cell 20 may include a housing 21, an electrode assembly 22, and an electrolyte, with the electrode assembly 22 and the electrolyte both housed within the housing 21.

[0078] The outer casing 21 may include a housing 211 and a cover 212. The housing 211 is an assembly that fits with the cover 212 to form an internal sealed space for the battery cell 20, wherein the formed sealed space can accommodate the electrode assembly 22, electrolyte, and other components. The cover 212 is a component that covers the opening of the housing 211 to isolate the internal environment of the battery cell 20 from the external environment. The shape of the cover 212 may be adapted to the shape of the housing 211 to fit the housing 211, and the cover 212 may also be provided with functional components such as electrode terminals 23 and pressure relief structures 24. A sealing ring may be provided between the opening of the housing 211 and the cover 212 to achieve a seal between the housing 211 and the cover 212.

[0079] The housing 211 and cover 212 can be of various shapes and sizes, such as cuboids, cylinders, and hexagonal prisms. Specifically, the shapes of the housing 211 and cover 212 can be determined according to the specific shape and size of the electrode assembly 22. The materials of the housing 211 and cover 212 can be various, such as, but not limited to, metals like copper, iron, aluminum, stainless steel, and aluminum alloys. The materials of the sealing ring can be various, such as, but not limited to, materials resistant to electrolyte corrosion, high toughness, and fatigue resistance, such as PP (polypropylene), PC (polycarbonate), and PET (polyethylene terephthalate). A plating layer can be formed on the outer surface of the housing 211, and the plating layer material can be various, such as, but not limited to, corrosion-resistant materials like Ni and Cr.

[0080] The battery cell 20 can also be in a pouch form, such as a bag-type pouch. The material of the pouch can be plastic, and examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0081] The electrode assembly 22 includes a negative electrode, a separator, and a positive electrode. The battery cell 20 primarily functions by the movement of metal ions between the positive and negative electrodes. During charging and discharging, active ions repeatedly insert and extract between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through. The electrode assembly 22 can be a wound structure or a stacked structure; this embodiment is not limited to either.

[0082] In some embodiments of this application, the negative electrode sheet includes a negative electrode current collector, a negative electrode tab, and a negative electrode active material layer. The negative electrode active material layer is disposed on at least one side of the negative electrode current collector, and a base coating layer may also be disposed between the negative electrode current collector and the negative electrode active material layer. The negative electrode tab protrudes from the negative electrode current collector and is located, for example, at one end or opposite ends of the negative electrode current collector. In some embodiments of this application, the negative electrode current collector may be a metal foil or a composite current collector. For example, the materials of the negative electrode current collector and the negative electrode tab may be copper. The composite current collector may include a polymer material base layer and a metal layer formed on at least one side of the polymer material powder. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material powder (such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc. powder).

[0083] The specific type of negative electrode active material in the negative electrode active material layer is not limited. Any active material known in the art that can be used as a negative electrode in sodium-ion batteries can be used, and those skilled in the art can select according to actual needs. As an example, the negative electrode active material may include, but is not limited to, carbon materials. Carbon materials include, but are not limited to, at least one of hard carbon, soft carbon, amorphous carbon, and nanostructured carbon materials, all of which are commercially available.

[0084] For example, the negative electrode active material may include one or more of hard carbon, soft carbon, artificial graphite, and natural graphite.

[0085] In some embodiments, the current collector of the negative electrode sheet may also include a current collector body and a base coating. The base coating may be disposed on at least one side of the current collector body. The base coating basically does not contain negative electrode active material, but may contain a small amount of carbon material. However, the carbon material forms a thin coating and cannot function as a negative electrode active material. In this embodiment, the negative electrode sheet can be an electrode sheet without a negative electrode active material layer. For a negative electrode sheet without a negative electrode active material layer, when the current collector of the negative electrode sheet does not contain a base coating, the film layer may be disposed on the surface of at least one side of the current collector; when the current collector of the negative electrode sheet includes a base coating, the film layer may be disposed on the surface of the base coating away from the current collector.

[0086] In some embodiments, the negative electrode active material layer may optionally include a binder. The binder may be selected from 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), and carboxymethyl chitosan (CMCS). The type of binder is not particularly limited, and those skilled in the art can choose flexibly according to actual needs.

[0087] In some embodiments, the negative electrode active material layer may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0088] In some embodiments, the negative electrode active material layer may also optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0089] The separator is located between the positive electrode and the negative electrode, and plays a role in isolation. The embodiments of this application do not have any particular restrictions on the type of separator, and any well-known porous structure separator with good chemical and mechanical stability can be selected.

[0090] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0091] In some embodiments, the positive electrode sheet includes a positive current collector, a positive electrode tab, and a positive active material layer. The positive active material layer is disposed on at least one side of the positive current collector, and an undercoating layer or the like may also be disposed between the positive active material layer and the positive current collector. The positive electrode tab protrudes from the positive current collector and is located, for example, at one end or at opposite ends of the positive current collector.

[0092] The positive current collector can be a metal foil or a composite current collector. For example, the materials of the positive current collector and the positive electrode tab can be aluminum. The composite current collector may include a polymer material base layer and a metal layer formed on at least one side of the polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material powder (such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc. powder).

[0093] The positive electrode active material layer includes a positive electrode material, which provides Na to travel between the positive and negative electrode plates during reversible charge and discharge processes. + .

[0094] In some embodiments, the positive electrode active material layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin. The type of binder is not particularly limited, and those skilled in the art can choose flexibly according to actual needs.

[0095] In some embodiments, the positive electrode active material layer may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0096] Sodium iron pyrophosphate / carbon composite materials are usually used as positive electrode active materials in sodium-ion batteries in the form of carbon-coated sodium iron pyrophosphate. However, existing carbon-coated sodium iron pyrophosphate has the problem of low initial charge capacity.

[0097] The main reason for the above problems is that, in actual mass production, carbon-coated sodium iron pyrophosphate typically shrinks into brittle or hard lumps after sintering. Therefore, the material needs to be mechanically or air-jet pulverized before it can be used in battery manufacturing. However, pulverization damages the microstructure of the sodium iron pyrophosphate material, resulting in: 1. detachment of the surface carbon layer, Na... + 1. Contact with carbon dioxide and moisture in the air leads to an increase in residual alkali on the material surface, causing gas generation and volume expansion during battery production, which is detrimental to processing. It also increases the content of impurity phase sodium iron phosphate. 2. Due to the destruction of the microstructure after pulverization, the carbon layer detaches, resulting in poor material conductivity. 3. Increased surface residual alkali and impurity phase content lead to a decrease in the content of active material (Na+). + Loss leads to a reduction in the material's bulk density.

[0098] To address the issue of low initial charge capacity in existing carbon-coated iron pyrophosphate sodium phosphate materials, according to some embodiments of this application, this application provides a cathode material comprising iron pyrophosphate sodium phosphate and a carbon coating layer on its surface; the cathode material contains a heterophase iron phosphate sodium phosphate, wherein the mass percentage of the heterophase iron phosphate sodium phosphate in the iron pyrophosphate sodium phosphate is 14% or less.

[0099] It is understandable that impurities such as sodium iron phosphate (SOF) are unavoidable in sodium iron pyrophosphate (SFP), and their content is greater than 0. The higher the content, the lower the capacity of the cathode material. Typically, the content of impurities in existing SOF is above 15%. Therefore, reducing the content of impurities in the cathode material, controlling its mass percentage to 14% or less, is beneficial for improving the initial charge-discharge capacity of the cathode material. For example, the mass percentage of impurities in SOF can be any value from 14%, 12%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, and 0.5%, or between any two values.

[0100] The content of heterophase sodium iron phosphate can be quantitatively obtained by performing XRD phase analysis on the product, refining the XRD data using Rietveld, and comparing the refined XRD with the standard card of sodium iron phosphate pyrophosphate corresponding to the product ratio.

[0101] The cathode material provided in this application utilizes the fact that the mass percentage of the impure phase sodium iron phosphate in the carbon-coated sodium iron phosphate pyrophosphate is 14% or less, which effectively reduces the content of the impure phase sodium iron phosphate, thereby effectively improving the first charge capacity and the electrochemical performance of the cathode material. At the same time, it is based on its low impure phase content and high first charge capacity.

[0102] According to some embodiments of this application, the mass percentage of heterophase sodium iron phosphate in sodium iron pyrophosphate is 12% or less.

[0103] Further reducing the mass proportion of heterogeneous sodium iron phosphate in sodium iron pyrophosphate is beneficial to further improve the initial charging capacity.

[0104] For example, the mass percentage of heterophase sodium iron phosphate in sodium iron phosphate pyrophosphate is any one of 11%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5% or between any two values.

[0105] Optionally, the mass percentage of heterogeneous sodium iron phosphate in sodium iron pyrophosphate is 9% or less.

[0106] Optionally, the mass percentage of heterogeneous sodium iron phosphate in sodium iron pyrophosphate is 8.5% or less.

[0107] Further reducing the mass proportion of heterogeneous sodium iron phosphate in sodium iron pyrophosphate is beneficial to further improve the initial charging capacity.

[0108] For example, the mass percentage of heterophase sodium iron phosphate in sodium iron phosphate pyrophosphate is any one of 8.5%, 8%, 7.5%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5% or between any two values.

[0109] According to some embodiments of this application, the cathode material satisfies one or more of (a1)-(a4):

[0110] (a1) The surface NaHCO3 content of the cathode material is 0.05-2.5 wt%.

[0111] Among them, Na + When in contact with carbon dioxide and moisture in the air, residual alkali will be produced. The residual alkali is mainly NaHCO3 and Na2CO3. Therefore, in this application, the content of NaHCO3 and Na2CO3 on the surface of the particles is used to characterize the amount of residual alkali on the surface of the cathode material.

[0112] Excessive residual alkali on the surface can lead to chemical gelation of the slurry, making coating impossible and increasing processing difficulty. Insufficient residual alkali content is practically difficult to achieve or has high preparation costs, and the Na content in the cathode material... + The formation of residual alkali after dissolution will also reduce the initial charge capacity of the product. Therefore, limiting the surface NaHCO3 content of the cathode material to the above-mentioned appropriate range can balance cost, processing performance, compaction density, film resistance and high initial charge capacity, thus achieving better electrochemical performance and application prospects.

[0113] For example, the NaHCO3 content on the surface of the cathode material is any one of 0.05wt%, 0.10wt%, 0.50wt%, 1.00wt%, 1.25wt%, 1.50wt%, 1.75wt%, 2.00wt%, 2.25wt%, 2.50wt%, or between any two values.

[0114] (a2) The Na2CO3 content on the surface of the cathode material is 0.05-2.5 wt%;

[0115] By limiting the surface Na2CO3 content of the cathode material to the above-mentioned suitable range, cost, processing performance, compaction density, and film resistance can be balanced, thereby achieving relatively excellent electrochemical performance and application prospects.

[0116] For example, the Na2CO3 content on the surface of the cathode material is any one of 0.05wt%, 0.10wt%, 0.50wt%, 1.00wt%, 1.25wt%, 1.50wt%, 1.75wt%, 2.00wt%, 2.25wt%, 2.50wt%, or between any two of these values.

[0117] (a3) The initial charge capacity of the cathode material is greater than 105 mAh / g.

[0118] The aforementioned cathode material has a high initial charge capacity while having a low impurity phase content.

[0119] For example, the initial charge capacity of the cathode material is any one of 106mAh / g, 110mAh / g, 112mAh / g, 115mAh / g, 117mAh / g, 120mAh / g, or between any two of these values.

[0120] (a4) The volumetric particle size distribution of the cathode material is Dv10, which is 0.3μm-1.3μm, Dv50, which is 0.5μm-5.5μm, and Dv90, which is 1.34μm-12.351μm, where (Dv90-Dv10) / Dv50 = 1.6-2.1.

[0121] Volumetric particle size distributions Dv10, Dv50, and Dv90 refer to particle size distribution parameters determined by particle size distribution measurements. For example, volumetric particle size distribution Dv50 is determined using a particle size analyzer-laser diffraction method. Specifically, refer to standard GB / T 19077-2016, which uses a laser diffraction scattering particle size analyzer for measurement.

[0122] It is understood that the volumetric particle size distributions Dv10, Dv50, and Dv90 mentioned here refer to the volumetric particle size distributions Dv10, Dv50, and Dv90 of the cathode material. By limiting the particle size distribution of the cathode material in this application to the above ranges, the cathode material can be kept within a small particle size range, which can be directly applied to the battery and avoid affecting the battery's dynamic performance.

[0123] (Dv90-Dv10) / Dv50 represents the concentration of particle size of the positive electrode active material. When it is within the above range, it can be said that the concentration of the positive electrode material is high and no agglomeration has occurred.

[0124] For example, the volumetric particle size distribution Dv50 of the cathode material is any value among 0.5μm, 1μm, 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, and 5μm, or between any two values.

[0125] For example, (Dv90-Dv10) / Dv50 is any one of 1.6, 1.7, 1.8, 1.9, 2.0, 2.1 or between any two of these values.

[0126] According to some embodiments of this application, the cathode material satisfies one or more of (b1)-(b3):

[0127] (b1) The surface NaHCO3 content of the positive electrode material is 0.05wt%-0.6wt%;

[0128] Controlling the amount of residual alkali on the surface of the cathode material is beneficial to improving its processing performance and electrochemical performance.

[0129] For example, the surface NaHCO3 content of the cathode material is any one of 0.05wt%, 0.10wt%, 0.15wt%, 0.20wt%, 0.25wt%, 0.30wt%, 0.35wt%, 0.40wt%, 0.45wt%, 0.50wt%, or between any two values.

[0130] (b2) The Na2CO3 content on the surface of the positive electrode material is 0.05wt%-1wt%;

[0131] Controlling the residual alkali content on the surface of the cathode material is beneficial to improving its processing performance and electrochemical performance.

[0132] For example, the Na2CO3 content on the surface of the cathode material is any one of 0.05wt%, 0.10wt%, 0.15wt%, 0.20wt%, 0.25wt%, 0.30wt%, 0.35wt%, 0.40wt%, 0.45wt%, 0.50wt%, 0.60wt%, 1.00wt%, or between any two values.

[0133] (b3) The initial charge capacity of the cathode material is 110mAh / g-120mAh / g.

[0134] For example, the initial charge capacity of the cathode material is any one of 110mAh / g, 111mAh / g, 115mAh / g, 116mAh / g, 117mAh / g, 118mAh / g, 120mAh / g, or between any two of these values. In summary, existing sodium iron pyrophosphate carbon composite materials have an initial charge capacity of less than 105 mAh / g at a current density of 0.1C. This application, by controlling the mass percentage of the impurity phase to 14% or less, the surface NaHCO3 content to 0.05-2.5 wt%, and the surface Na2CO3 content to 0.05-2.5 wt%, enables the cathode material with a volume particle size distribution Dv50 of 0.5-5 μm to have an initial charge capacity greater than 105 mAh / g at a current density of 0.1C. By further controlling the NaHCO3 content to 0.05-0.6 wt%, the Na2CO3 content to 0.05-1.6 wt%, and the impurity sodium iron phosphate to 8.5% or less, the initial charge capacity of the cathode material at a current density of 0.1C can even reach 115 mAh / g or more.

[0135] In other words, the reduction in residual alkali and impure sodium iron phosphate, along with the increase in capacity, indicates that the sodium ion loss in the cathode material of this application is reduced compared to existing sodium iron phosphate pyrophosphate carbon composite materials. This also indirectly demonstrates that when using existing sodium iron phosphate pyrophosphate carbon composite materials as raw materials to prepare the cathode material of this application, the method employed can reduce the sodium ion loss in the existing sodium iron phosphate pyrophosphate carbon composite materials by removing residual alkali and decomposed sodium iron phosphate. + It is re-embedded into the original lattice, thereby increasing the capacity and reducing residual alkali and impure sodium iron phosphate.

[0136] While increasing carbon content results in limited increases in impurity phases and even a decrease in residual alkali, excessively high carbon content leads to reduced capacity in the cathode material because carbon does not contribute to capacity. Conversely, insufficient carbon content fails to effectively protect sodium iron pyrophosphate, easily reducing conductivity and causing Na+ degradation. + The loss of these substances generates residual alkali and impurities, affecting the processing performance and electrochemical performance of the cathode material.

[0137] It should be noted that, based on the low impurity phase content and high initial charge capacity of the aforementioned cathode material, it can be inferred that it has good crystallinity.

[0138] Therefore, according to some embodiments of this application, the carbon content in the cathode material is 1wt%-3.1wt%; and / or, the thickness of the carbon coating layer is 2.5nm-30nm.

[0139] By controlling the carbon content in the cathode material within the aforementioned suitable range and / or the carbon coating within the aforementioned suitable thickness range, it is beneficial to improve the conductivity of the cathode material, while also ensuring that the cathode material has both a low residual alkali content and a high initial charge capacity.

[0140] For example, the carbon content in the cathode material is any one of 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, or 3.1wt%, or between any two of these values.

[0141] For example, the thickness of the carbon coating is any value of 2.5nm, 3nm, 5nm, 10nm, 15nm, 20nm, 25nm, 30nm or between any two values.

[0142] Optionally, the carbon content in the cathode material is 1.5wt%-2.9wt%.

[0143] For example, the carbon content in the cathode material is any one of 1.5wt%, 1.7wt%, 2.0wt%, 2.2wt%, 2.5wt%, 2.9wt%, or between any two of these values.

[0144] According to some embodiments of this application, the chemical formula of sodium iron pyrophosphate is (b1) or (b2):

[0145] (c1)Na x Fe y (PO4)2P2O7, x=3.5-4.5, y=2.75-3.25;

[0146] The aforementioned sodium ferric pyrophosphate exhibits superior electrochemical performance and has a wide range of applications.

[0147] For example, the chemical formula of sodium iron pyrophosphate is: Na 4.1 Fe 2.95 (PO4)2P2O7 or Na4Fe3(PO4)2P2O7.

[0148] (c2)Na x Fe y P m O n , 3.5≤x≤4.5, 2.5≤y≤3.5, 3.7<m<4, 14.5≤n≤15.5.

[0149] Introducing phosphorus vacancies facilitates the growth of its crystal lattice along the (602) crystal plane, shortening the Na... + The transmission path improves Na + The diffusion rate is beneficial to the rate performance and initial coulombic efficiency of the battery.

[0150] It should be noted that during the charging and discharging process of the battery, Na undergoes insertion / extraction and consumption, resulting in different molar Na contents at different discharge states. In the chemical formula of sodium iron pyrophosphate in this application, the molar Na content represents the initial state of the material, i.e., the state before feeding. When the cathode material is applied to the battery system, the molar Na content changes after charge-discharge cycles. The molar O content is only a theoretical value; lattice oxygen release causes changes in the molar O content, resulting in fluctuations in the actual molar O content.

[0151] According to some embodiments of this application, this application also provides a method for preparing a positive electrode material, which includes:

[0152] The sodium iron pyrophosphate powder and the first carbon source are dry-mixed and sintered for at least 10 hours in a reducing atmosphere at a temperature of 400-600℃ to prepare the sintered material.

[0153] It is understandable that sodium iron pyrophosphate powder is not a pure phase of sodium iron pyrophosphate, resulting in a lower initial charging capacity than the theoretical charging capacity. Sodium iron pyrophosphate powder refers to a material where sodium iron pyrophosphate is the active ingredient, but the material contains impurities, including sodium iron phosphate, and the material surface has residual alkali NaHCO3 and Na2CO3. For example, the content of the impurity sodium iron phosphate in the sodium iron pyrophosphate powder is 25 wt% or less, and the content of the impurity sodium iron phosphate in the sodium iron pyrophosphate powder is greater than the content of the impurity sodium iron phosphate in the cathode material. For example, the content of the impurity sodium iron phosphate in the sodium iron pyrophosphate powder is 12 wt%-25 wt%.

[0154] Sodium iron pyrophosphate powder mainly refers to the powder obtained by sintering raw materials such as sodium salt, iron source, and phosphorus source in a reducing atmosphere during mass production, according to a chemical formula. The sintered product shrinks into a sintered form or directly becomes a hard lump. The powder is then formed by mechanically pulverizing or air-jet milling of the sintered product. Sodium iron pyrophosphate powder can be purchased directly or prepared at home; no specific method is specified here. Because the powder usually requires crushing, the microstructure of the primary particles constituting the powder is destroyed, and Na... + After dissolution, sodium iron phosphate forms a heterogeneous phase in the material, and non-electrochemically active residual alkalis such as NaHCO3 and Na2CO3 also form on the surface, resulting in a decrease in the initial charging capacity of the powder compared to the theoretical value. Since breakage will damage the microstructure of the powder, if the powder is wet-mixed with the first carbon source at this time, water or organic solvents can easily cause sodium to dissolve, further damaging the powder structure and reducing capacity. Therefore, this application adopts dry mixing of the powder and the first carbon source, which is beneficial to ensure uniform mixing and can suppress sodium dissolution during the mixing process, thus improving the initial charging capacity of the final cathode material.

[0155] The first carbon source not only provides a reducing atmosphere, but also forms a carbon layer after subsequent high-temperature sintering to coat the sodium iron pyrophosphate powder, improving the conductivity of the cathode material and effectively blocking the sodium iron pyrophosphate from contacting water and air, which is beneficial to improving the first charging capacity of the cathode material.

[0156] If the sintering temperature is too high during the sintering process, impurities are easily generated, reducing the initial charge capacity. It also leads to grain growth and intensified secondary agglomeration, resulting in lower compaction and reduced kinetic performance of the cathode material during charge and discharge. Conversely, if the sintering temperature is too low and the sintering time is too short, Na... + It cannot be effectively burned back into the crystal lattice, thus failing to improve the initial charge capacity. Therefore, for the chemical formula: sodium iron pyrophosphate, sintering at 400℃-600℃ for at least 10 hours in a reducing atmosphere, the powder causes Fe during the sintering process... 3+ Na in the residual alkali on the surface of the powder+ Re-reacting under carbothermic reduction, for example: Na 4-a Fe 3-b (PO4)2P2O7+bFe 3+ +aNa + +C→Na4Fe3(PO4)2P2O7, therefore it can be inferred that this sintering process can remove residual alkali and Na from impurity phases. + By restoring the embedded phase in the original lattice and repairing the lattice morphology of sodium iron pyrophosphate, the impurity phase content and surface residual alkali content of sodium iron pyrophosphate cathode material are effectively reduced, and the capacity of sodium iron pyrophosphate cathode material is effectively improved.

[0157] The method for preparing the cathode material provided in this application involves dry mixing followed by secondary sintering in a reducing atmosphere at 400℃-600℃ for at least 10 hours. This not only improves the conductivity of the cathode material and prevents grain growth and secondary agglomeration, but also restores the crystal morphology of sodium iron pyrophosphate, removing impurities and residual Na. + By reducing the embedding in the original crystal lattice, the impurity phase content and surface residual alkali content of the sodium iron pyrophosphate cathode material are effectively reduced, thereby effectively improving the capacity of the sodium iron pyrophosphate cathode material and thus effectively improving the processing performance and electrochemical performance of the cathode material.

[0158] It should be noted that the cathode material after secondary sintering under the above conditions does not need to be crushed and can be used directly.

[0159] According to some embodiments of this application, the temperature for secondary sintering is 450-580°C.

[0160] Controlling the secondary sintering temperature within the above range is beneficial for the cathode material to maintain a small particle size while taking into account low impurity phase content, low surface residual alkali content, and high initial charge capacity, which is conducive to improving the processing performance and electrochemical performance of the cathode material.

[0161] For example, the secondary sintering temperature is any value of 450°C, 460°C, 470°C, 480°C, 490°C, 500°C, 510°C, 520°C, 530°C, 540°C, 550°C, 560°C, 570°C, or 580°C, or between any two of these values.

[0162] Optionally, the secondary sintering temperature is 500℃-580℃.

[0163] According to some embodiments of this application, the secondary sintering time is 10h-20h.

[0164] The drawbacks of excessively long secondary sintering time, such as increased energy consumption and reduced preparation efficiency, outweigh the improvement in cathode material performance. Therefore, controlling the secondary sintering time to 10-20 hours can improve the processing and electrochemical performance of the cathode material while maintaining reasonable cost and good preparation efficiency.

[0165] For example, the secondary sintering time is any value of 10h, 11h, 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, or 20h, or between any two values.

[0166] Optionally, the secondary sintering time is 10-15 hours.

[0167] According to some embodiments of this application, sodium iron pyrophosphate powder is obtained by crushing, and the secondary sintered material is used as the positive electrode material.

[0168] Since the sodium iron pyrophosphate powder is obtained through crushing, the secondary sintered material obtained after sintering does not need to be further crushed and can be directly used as the cathode material.

[0169] It is understandable that, since the first carbon source can form a carbon layer to coat the sodium iron pyrophosphate powder after subsequent high-temperature sintering, the volumetric particle size distribution Dv50 of the powder is smaller than that of the cathode material.

[0170] According to some embodiments of this application, the difference between the volumetric particle size distribution Dv50 of sodium iron pyrophosphate powder and the volumetric particle size distribution Dv50 of the cathode material is ≤0.5μm.

[0171] Since carbon content affects the capacity of cathode materials and residual alkali content, and a significant increase in particle size will affect the kinetic performance of the battery, the difference between the volumetric particle size distribution Dv50 of the powder and the volumetric particle size distribution Dv50 of the cathode material is within the above range. This means that the particle size of the cathode material after secondary sintering is not significantly increased compared to the powder before secondary sintering, and remains within a small particle size range, which is beneficial to improving battery performance.

[0172] According to some embodiments of this application, the volumetric particle size distribution Dv10 of the cathode material is 0.3μm-1.3μm, the volumetric particle size distribution Dv50 is 0.5μm-5.5μm, and the volumetric particle size distribution Dv90 is 1.34μm-12.351μm; wherein Dv90-Dv10) / Dv50=1.6-2.1.

[0173] Volumetric particle size distributions Dv10, Dv50, and Dv90 refer to particle size distribution parameters determined by particle size distribution measurements. For example, volumetric particle size distribution Dv50 is determined using a particle size analyzer-laser diffraction method. Specifically, refer to standard GB / T 19077-2016, which uses a laser diffraction scattering particle size analyzer for measurement.

[0174] Where (Dv90-Dv10) / Dv50 represents the concentration of the particle size of the positive electrode active material. When it is within the above range, it can be said that the concentration of the positive electrode material is high and no agglomeration has occurred.

[0175] According to some embodiments of this application, the first carbon source includes an inorganic carbon source and / or an organic carbon source; wherein, the inorganic carbon source includes at least one of natural graphite powder, artificial graphite powder, carbon black, carbon nanotubes, and graphene; and the organic carbon source includes at least one of sucrose, glucose, citric acid, starch, cyclodextrin, pitch, and polyethylene glycol.

[0176] Whether using the aforementioned organic or inorganic carbon source, both methods can achieve carbon coating and enable the powder to undergo a carbothermic reduction solid-phase reaction, restoring the crystal morphology of sodium iron pyrophosphate and removing Na from the residual alkali. + It is then embedded back into the original lattice.

[0177] For example, the first carbon source is an inorganic carbon source, or the first carbon source is an organic carbon source, or the first carbon source is a mixture of an organic carbon source and an inorganic carbon source.

[0178] According to some embodiments of this application, the first carbon source is an inorganic carbon source, and the amount of inorganic carbon source added is less than or equal to 2 wt% of the powder.

[0179] Inorganic carbon sources are used to provide a reducing atmosphere for sintering, but after high-temperature sintering, a carbon layer remains on the surface of the powder. If the amount of the primary carbon source is insufficient, the provided reducing atmosphere will not be enough to completely carry out the reduction reaction; if the carbon layer is too thick, it will affect the reduction of Na. + Diffusion, hindering Na + Return to the crystal lattice after burning.

[0180] Therefore, by controlling the addition of inorganic carbon source to be less than or equal to 2 wt% of the powder, it is beneficial to create an atmosphere conducive to the reduction reaction, and can also ultimately form a carbon layer of suitable thickness on the surface of sodium iron pyrophosphate particles, which is beneficial to Na + The re-intercalation of sodium iron phosphate pyrophosphate lattice during the reheating process is beneficial to improving the conductivity and initial charge capacity of the cathode material.

[0181] Optionally, the amount of inorganic carbon source added is 0.1-1 wt% of the powder.

[0182] By controlling the amount of inorganic carbon source added within the above range, it is beneficial to improve the conductivity and specific capacity of the cathode material.

[0183] For example, the amount of inorganic carbon source added is any one of 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, or 1wt% of the powder, or between any two values.

[0184] According to some embodiments of this application, the first carbon source is an organic carbon source, and the amount of organic carbon source added is less than or equal to 5 wt% of the powder.

[0185] Organic carbon sources are used to provide a reducing atmosphere for sintering, but after high-temperature sintering, a carbon layer remains on the surface of the powder. If the amount of the primary carbon source is insufficient, the provided reducing atmosphere will not be enough to completely carry out the reduction reaction; if the carbon layer is too thick, it will affect the reduction of Na. + Diffusion, hindering Na + Return to the crystal lattice after burning.

[0186] Therefore, by controlling the amount of organic carbon source added to be less than or equal to 5 wt% of the powder, it is beneficial to create an atmosphere for the reduction reaction to occur, and it can also ultimately form a carbon layer of suitable thickness on the surface of sodium iron pyrophosphate particles, which is conducive to the formation of Na+. + The re-intercalation process, which reintegrates sodium iron pyrophosphate lattice, is beneficial for improving the conductivity and specific capacity of the cathode material.

[0187] Optionally, the amount of organic carbon source added is 0.5-2 wt% of the powder.

[0188] By controlling the amount of organic carbon source added within the above range, it is beneficial to improve the conductivity and specific capacity of the cathode material.

[0189] For example, the amount of organic carbon source added is any one of 0.5wt%, 0.7wt%, 0.9wt%, 1.0wt%, 1.2wt%, 1.5wt%, 1.7wt%, 1.8wt%, 1.9wt%, or 2.0wt% of the powder, or between any two values.

[0190] According to some embodiments of this application, the first carbon source comprises a mixture of inorganic and organic carbon sources, and the amount of the mixture added is less than or equal to 2 wt% of the powder, based on the carbon obtained by pyrolysis of the mixture at the secondary sintering temperature.

[0191] By controlling the amount of mixture added based on the carbon obtained from the cracking of the mixture at the secondary sintering temperature, it is beneficial to create an atmosphere conducive to the reduction reaction and to ultimately form a carbon layer of suitable thickness on the surface of the sodium iron pyrophosphate particles, which is beneficial to the Na+ pyrophosphate reaction. +The re-intercalation process, which reintegrates sodium iron pyrophosphate lattice, is beneficial for improving the conductivity and specific capacity of the cathode material.

[0192] Optionally, the amount of the mixture added is less than or equal to 0.1-1 wt% of the powder, based on the carbon obtained by pyrolysis of the mixture at the secondary sintering temperature.

[0193] By controlling the amount of inorganic carbon source added within the above range, it is beneficial to improve the conductivity and specific capacity of the cathode material.

[0194] For example, the amount of inorganic carbon source added is any one of 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, or 1wt% of the powder, or between any two values.

[0195] According to some embodiments of this application, the reducing atmosphere is provided by a first carbon source; or...

[0196] The reducing atmosphere comprises a first component and a second component, the first component being provided by a first carbon source, and the second component comprising hydrogen and / or carbon monoxide.

[0197] By controlling the reducing atmosphere to the above-mentioned components, it is beneficial to carry out the carbothermic reduction solid-phase reaction, thereby reducing the impurity phase and reducing the Na in the impurity phase and residual alkali. + Reduction and embedding into the original lattice, thereby effectively improving Na + This improves utilization and effectively increases the capacity of sodium iron pyrophosphate cathode materials.

[0198] It should be noted that the sodium iron pyrophosphate powder can be carbon-coated sodium iron pyrophosphate powder or uncoated sodium iron pyrophosphate powder, which can be selected according to actual needs.

[0199] Sodium iron pyrophosphate powder without a surface coating can be obtained by: wet mixing of sodium salt, iron source, and phosphorus source according to the chemical formula ratio, drying, and sintering once under a reducing atmosphere, and then mechanically crushing or air-jet crushing the obtained sintered product.

[0200] According to some embodiments of this application, the sodium iron pyrophosphate powder is carbon-coated sodium iron pyrophosphate, which is prepared by the following steps: wet mixing of sodium salt, iron source, phosphorus source and second carbon source according to chemical formula ratio, drying and sintering once under an inert atmosphere, and mechanically crushing or air-jet crushing the obtained sintered product to obtain carbon-coated sodium iron pyrophosphate powder.

[0201] The above preparation method yields a carbon-coated porous sodium ferric phosphate pyrophosphate with good electrochemical performance.

[0202] For example, in the preparation methods of the above-mentioned powders, the sodium source includes, but is not limited to, one or more of sodium acetate, sodium carbonate, sodium citrate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium carbonate, sodium nitrate, sodium oxalate, sodium acetate, sodium sulfate, sodium hydroxide, sodium formate, sodium citrate, sodium pyrophosphate, sodium dihydrogen pyrophosphate, and sodium chloride. The phosphorus source includes, but is not limited to, one or more of ammonium monohydrogen phosphate, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, sodium phosphate, disodium hydrogen phosphate, phosphoric acid, ferric phosphate, triammonium phosphate, pyrophosphate, ferric pyrophosphate, sodium pyrophosphate, and sodium dihydrogen pyrophosphate. The iron source includes, but is not limited to, one or more of ferric phosphate, iron(III) oxide, ferric oxide, ferric oxalate, and ferrous oxalate; the second carbon source is an organic carbon source, which includes, but is not limited to, one or more of sucrose, glucose, citric acid, starch, cyclodextrin, and polyethylene glycol.

[0203] For example, in the above-mentioned methods for preparing powders, wet mixing includes at least one of wet ball milling or sand milling, and the solvent that can be used for wet mixing includes water.

[0204] For example, in the above-mentioned methods for preparing powders, drying can be carried out by spray drying, or by first heating and evaporating the solvent to form a wet gel, and then drying to obtain a dry gel and grinding it into powder.

[0205] For example, in the preparation methods of the above-mentioned powders, a single sintering process can be carried out at 400-600℃ for 6-24 hours.

[0206] According to some embodiments of this application, the chemical formula of sodium iron pyrophosphate powder is (d1) or (d2):

[0207] (d1)Na x Fe y (PO4)2P2O7, x=3.5-4.5, y=2.75-3.25;

[0208] The aforementioned sodium ferric pyrophosphate exhibits superior electrochemical performance and has a wide range of applications.

[0209] For example, the chemical formula of sodium iron pyrophosphate is: Na 4.1 Fe 2.95 (PO4)2P2O7 or Na4Fe3(PO4)2P2O7.

[0210] (d2)Na x Fe y P m O n , 3.5≤x≤4.5, 2.5≤y≤3.5, 3.7<m<4, 14.5≤n≤15.5.

[0211] Introducing phosphorus vacancies facilitates the growth of its crystal lattice along the (602) crystal plane, shortening the Na... + The transmission path improves Na + The diffusion rate is beneficial to the rate performance and initial coulombic efficiency of the battery.

[0212] According to some embodiments of this application, this application also provides a positive electrode sheet, including the positive electrode material of any of the above schemes.

[0213] According to some embodiments of this application, this application also provides a battery including a positive electrode sheet of any of the above schemes.

[0214] According to some embodiments of this application, this application also provides an electrical device including a battery of any of the above-described schemes, and the battery is used to provide electrical energy to the electrical device.

[0215] The electrical device can be any of the aforementioned battery-powered devices or systems.

[0216] The following specific embodiments are provided to better illustrate this application.

[0217] Powder 1 is prepared by the following method:

[0218] 1. Dissolve sodium pyrophosphate (Na4P2O7), ferrous oxalate, and ferric phosphate in deionized water at a molar ratio of 1.02:0.5:2 and stir continuously at room temperature for 30 minutes to obtain an initial mixed slurry; dissolve Super P and glucose in deionized water and mix to obtain a carbon solution.

[0219] 2. Mix the carbon solution and the initial mixed solution, and stir to obtain a mixed solution.

[0220] 3. Spray dry the above mixed solution with an inlet air temperature of 220°C and an outlet air temperature of 109°C to obtain a powdered precursor.

[0221] 4. The powdered precursor is heated to 320°C in a N2 atmosphere at a heating rate of 2°C and held for 4 hours. Then, it is sintered at 550°C at a heating rate of 2°C for 10 hours to obtain carbon-coated porous iron sodium pyrophosphate sinter.

[0222] 5. The carbon-coated porous sodium iron pyrophosphate sinter was subjected to air jet milling at a pressure of 0.58 MPa and a grading frequency of 58 Hz to obtain carbon-coated sodium iron pyrophosphate powder, hereinafter referred to as powder 1. The chemical formula of sodium iron pyrophosphate is Na4Fe3(PO4)2P2O7. Powder 1 was used to perform the processing steps of Examples 1-10 and Comparative Examples 1-5. The differences in parameters between the various examples and comparative examples are detailed in Table 1.

[0223] The powders used in Examples 11-16 were prepared using the same method as above, but the chemical formula ratio of sodium iron pyrophosphate was different, resulting in the powders used in Examples 11-16 being different from the powder 1 mentioned above.

[0224] Example 1

[0225] I. Preparation of cathode materials

[0226] Place 3 kg of the above powder 1 and 50 g of glucose monohydrate into a high-speed mixer and mix thoroughly for 30 minutes at a linear speed of 18 m / s to obtain a uniform mixed powder.

[0227] The mixed powder was placed in an electric roller conveyor high-temperature kiln and heated to 500℃ at a heating rate of 10℃ / h for secondary sintering for 10h, followed by natural cooling to obtain the cathode material.

[0228] Figure 4 shows the XRD comparison of sodium iron pyrophosphate before and after secondary sintering. As can be seen from Figure 4, the peak of sodium iron phosphate (33°) in the impurity phase, as indicated by the arrow, becomes lower after secondary sintering, indicating that the content of sodium iron phosphate in the impurity phase decreases after secondary sintering. In conjunction with Table 2, it can be seen that since the capacity of the cathode material is increased compared to the powder, it indicates that after the above-mentioned secondary sintering, the active material Na in the sodium iron phosphate in the impurity phase is reduced and embedded into the original lattice.

[0229] Figure 5 shows the SEM image of the powder. It can be seen that the integrity of the carbon coating layer in the primary particles of the powder is destroyed, the surface carbon layer falls off, and the pores of the internal sodium iron pyrophosphate are exposed.

[0230] Figure 6 is a SEM image of the cathode material prepared in Example 1. It can be seen that its surface is covered by a carbon layer, and the carbon coating layer can completely cover the porous sodium iron pyrophosphate.

[0231] II. Preparation of coin cell batteries

[0232] Sodium metal is used as the negative electrode.

[0233] Positive electrode sheet: 10 wt% polyvinylidene fluoride binder is fully dissolved in N-methylpyrrolidone, and 10 wt% carbon black conductive agent and 80 wt% of the above positive electrode materials are added to prepare a uniformly dispersed slurry. The slurry is uniformly coated on the zirconium surface and then transferred to a vacuum drying oven for complete drying. The resulting electrode sheet is rolled and then punched to obtain the target disc.

[0234] The separator is made of polypropylene polymer film.

[0235] Preparation of electrolyte: Dimethyl ethylene glycol (DME) was used as an organic solvent, and then fully dried sodium salt NaPF6 was dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0236] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The electrolyte is then added to assemble a button cell.

[0237] Examples 2-16 and Comparative Examples 1-5

[0238] The differences between Examples 2-16 and Comparative Examples 1-5 and Example 1 are shown in Table 1.

[0239] Table 1. Distinguishing Parameters

[0240]

[0241] In Comparative Example 1, powder 1 was actually used directly as the positive electrode material.

[0242] The final cathode materials of each embodiment and comparative example were tested.

[0243] The testing methods for each parameter are as follows:

[0244] [Volume Particle Size Distribution Dv50 Test]:

[0245] The measurement was performed using a laser diffraction scattering particle size analyzer, in accordance with the standard GB / T 19077-2016.

[0246] [Carbon Content Test]:

[0247] The HCS-140 high-frequency infrared carbon and sulfur analyzer was used to directly weigh the sample and obtain the results through equipment testing.

[0248] [Surface Residual Alkali Test]:

[0249] Acid-base titration method: Weigh 3g of sample, add 100mL of deionized water, stir for 3 minutes, and then filter. Titrate the filtrate with hydrochloric acid to ensure acid-base neutralization with the residual alkali in the filtrate. Use a Metrohm 905 potentiometric titrator with the pH electrode as the indicator electrode, and determine the endpoint by the sudden change in potential. Calculate the residual alkali content based on the volume of HCl standard solution consumed.

[0250] Na2CO3%=V1*C*105.99*n*100 / 1000m;

[0251] NaHCO3%=[V2-2V1]*C*83.9898*n*100 / 1000m;

[0252] Na + %=V2*C*22.9898*n*100 / (m*1000);

[0253] In the calculation formula, C is the concentration of the hydrochloric acid standard solution (0.05 mol), m is the mass of the sample, n is 100 mL / pipette volume; 105.99 is the molecular weight of Na2CO3, and 83.9898 is the molecular weight of NaHCO3.

[0254] [Impurity content]:

[0255] XRD phase analysis was performed on the product, and the XRD data was refined using Rietveld. The refined XRD was then compared with the corresponding ratio of sodium iron pyrophosphate standard card to automatically determine the impurity content.

[0256] [Initial Discharge Efficiency Test]:

[0257] First charge and first efficiency test methods for specific capacity: Using LAND testing instruments, the specific test procedure is as follows: The batteries prepared in the examples and comparative examples are left to stand for 3 hours, charged at a constant current of 0.1C to 3.75V, and charged at a constant voltage of 50uA to obtain the first charge capacity. After standing for 5 minutes, they are discharged at a constant current of 0.1C to 1.5V and left to stand for 4 minutes to obtain the first discharge capacity. The first discharge efficiency = first discharge capacity / first charge capacity * 100%.

[0258] The results are shown in Table 2 and Figure 7. Figure 7 is a charge-discharge curve of the positive electrode material prepared in Example 1.

[0259] Table 2 Test Results

[0260] It should be noted that, in Tables 1 and 2, taking the NaHCO3 content in Example 1 in Table 2 as an example, 0.2802 in Table 2 indicates that the NaHCO3 content on the surface of the cathode material is 0.2802 wt%.

[0261] It should be noted that in Table 2, Examples 1-16 and Comparative Examples 3-5 were all powders after secondary sintering, and Comparative Example 1 was also a powder. Therefore, all of them can be directly tested. Due to wet mixing, Comparative Example 2 resulted in a caking material after secondary sintering. Therefore, it was impossible to directly obtain its volume particle size distribution and particle carbon content. After crushing it, the surface residual alkali content, sodium iron phosphate content, first charge capacity, and first discharge capacity were measured. The results are shown in Table 2.

[0262] As can be seen from Tables 1 and 2, both the embodiments and comparative examples of this application contain heterogeneous sodium iron phosphate and residual alkali. The content of sodium iron phosphate is negatively correlated with the initial charge capacity, and the content of residual alkali is also generally negatively correlated with the initial charge capacity. Under the premise that the residual alkali content is not significantly different, the mass percentage of heterogeneous sodium iron phosphate in sodium iron pyrophosphate in Examples 1 to 16 is all 14% or less, generally controlled within 9%, and in particularly excellent cases, it can be reduced to 4% to 6%, resulting in better initial charge capacity, even reaching 115 mAh / g or higher.

[0263] As can be seen from Example 1 and Comparative Example 2, dry mixing can effectively improve the capacity of the cathode material obtained after secondary sintering compared to wet mixing.

[0264] As can be seen from Examples 1-4 and Comparative Example 3, the secondary sintering temperature affects the first charging capacity of the cathode material. If the secondary sintering temperature is too low, the increase in the first charging capacity is limited. If the temperature is too high, the amount of impurity phase sodium iron phosphate generated is large, which leads to a decrease in the first charging capacity. To improve the first charging capacity, the impurity phase content is reduced. In Examples 1-4 above, the first charging capacity of the coin cell is greater than 105 mAh / g.

[0265] As can be seen from Examples 1, 5-6 and Comparative Examples 4 and 5, sintering time affects the first charging capacity of the cathode material. If the sintering time is too short, the reduction in the content of impurity phase sodium iron phosphate and residual alkali is limited, and the improvement in the first charging capacity is not obvious. In Examples 1 and 5-6 above, the first charging capacity of the coin cell is greater than 105 mAh / g.

[0266] As can be seen from Examples 7-10, the amount of organic carbon source added also affects the first charge capacity of the battery. The amount of organic carbon source added is less than or equal to 5 wt% of the powder. Optionally, when the amount of organic carbon source added is 0.5-2 wt% of the powder, the battery has a better first charge capacity.

[0267] As can be seen from Examples 11-14, inorganic carbon sources can also be selected as carbon sources. Although the chemical formula ratios of sodium iron pyrophosphate are different, they can all be modified by the above-mentioned secondary sintering method to improve the first charge capacity.

[0268] Table 3 shows the volumetric particle size distribution test results for Example 1 and Comparative Examples 1-4.

[0269] Table 3 Test Results

[0270] Comparative Example 2, due to wet mixing, forms a caking material after secondary sintering, making it impossible to directly test the volumetric particle size distribution.

[0271] As can be seen from Table 3, since the first carbon source was added for secondary calcination in Examples 1, 3, and 4, the volumetric particle size distributions Dv10, Dv50, and Dv90 were slightly increased compared to Comparative Example 1.

[0272] The volumetric particle size distribution Dv90 of Example 1 did not increase significantly compared to Comparative Example 1, and the (Dv90-Dv10) / Dv50 of Example 1 was slightly smaller than that of Comparative Examples 1-4. This indicates that the particle size distribution was more concentrated after the second sintering compared to before the second sintering. Considering the particle size changes and (Dv90-Dv10) / Dv50 of Example 1 and Comparative Example 1, it can be concluded that no agglomeration occurred in Example 1 after the second sintering.

[0273] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them. Although this application 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 therein. 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 this application, and they should all be covered within the scope of the claims and specification of this application. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A cathode material, wherein, Including sodium iron pyrophosphate and a carbon coating layer on its surface; The cathode material contains a heterophase sodium iron phosphate, and the heterophase sodium iron phosphate accounts for 14% or less of the mass percentage of the sodium iron phosphate pyrophosphate.

2. The cathode material according to claim 1, wherein, The heterophase sodium iron phosphate accounts for 12% or less of the mass percentage of the sodium iron pyrophosphate.

3. The cathode material according to claim 1 or 2, wherein, The cathode material satisfies one or more of (a1)-(a4): (a1) The surface NaHCO3 content of the positive electrode material is 0.05wt%-2.5wt%; (a2) The Na2CO3 content on the surface of the positive electrode material is 0.05wt%-2.5wt%; (a3) The initial charge capacity of the cathode material is greater than 105 mAh / g; (a4) The volumetric particle size distribution Dv10 of the cathode material is 0.3μm-1.3μm, the volumetric particle size distribution Dv50 is 0.5μm-5.5μm, and the volumetric particle size distribution Dv90 is 1.34μm-12.351μm, wherein (Dv90-Dv10) / Dv50=1.6-2.

1.

4. The cathode material according to claim 1 or 2, wherein, The cathode material satisfies one or more of (b1)-(b3): (b1) The surface NaHCO3 content of the positive electrode material is 0.05wt%-0.6wt%; (b2) The Na2CO3 content on the surface of the positive electrode material is 0.05wt%-1wt%; (b3) The initial charge capacity of the cathode material is 110mAh / g-120mAh / g.

5. The cathode material according to any one of claims 1-4, wherein, The carbon content in the cathode material is 1wt%-3.5wt%; and / or, The thickness of the carbon coating layer is 2.5-30 nm.

6. The cathode material according to any one of claims 1-5, wherein, The chemical formula of the sodium ferric pyrophosphate is (c1) or (c2): (c1)Na x Fe y (PO4)2P2O7,x=3.5-4.5,y=2.75-3.25; (c2)In x Fe y P m Oh n ,3.5≤x≤4.5,2.5≤y≤3.5,3.7<m<4,14.5≤n≤15.5。 7. A method for preparing a positive electrode material, wherein, include: Sodium iron pyrophosphate powder and a first carbon source are dry-mixed and then sintered for at least 10 hours in a reducing atmosphere. The secondary sintering temperature is 400℃-600℃ to prepare a secondary sintered material.

8. The preparation method according to claim 7, wherein, The secondary sintering temperature is 450℃-580℃.

9. The preparation method according to claim 7 or 8, wherein, The secondary sintering time is 10-20 hours.

10. The preparation method according to any one of claims 7-9, wherein, The sodium iron pyrophosphate powder is obtained by crushing, and the secondary sintered material is used as the positive electrode material.

11. The preparation method according to any one of claims 7-10, wherein, The difference between the volumetric particle size distribution Dv50 of the sodium iron pyrophosphate powder and the volumetric particle size distribution Dv50 of the cathode material is ≤0.5μm.

12. The preparation method according to any one of claims 7-11, wherein, The positive electrode material has a volumetric particle size distribution Dv10 of 0.3 μm-1.3 μm, a volumetric particle size distribution Dv50 of 0.5 μm-5.5 μm, and a volumetric particle size distribution Dv90 of 1.34 μm-12.351 μm. Where (Dv90-Dv10) / Dv50=1.6-2.

1.

13. The preparation method according to any one of claims 7-12, wherein, The first carbon source includes inorganic carbon sources and / or organic carbon sources; The inorganic carbon source includes at least one of natural graphite powder, artificial graphite powder, carbon black, carbon nanotubes, and graphene. The organic carbon source includes at least one of sucrose, glucose, citric acid, starch, cyclodextrin, pitch, and polyethylene glycol.

14. The preparation method according to any one of claims 7-12, wherein, The first carbon source is an inorganic carbon source, and the amount of the inorganic carbon source added is less than or equal to 2 wt% of the powder.

15. The preparation method according to any one of claims 7-12, wherein, The first carbon source is an organic carbon source, and the amount of the organic carbon source added is less than or equal to 5 wt% of the powder.

16. The preparation method according to any one of claims 7-12, wherein, The first carbon source comprises a mixture of inorganic and organic carbon sources, and the amount of the mixture added is less than or equal to 2 wt% of the powder, based on the carbon obtained by pyrolysis of the mixture at the secondary sintering temperature.

17. The preparation method according to any one of claims 7-12, wherein, The reducing atmosphere is provided by a first carbon source; or... The reducing atmosphere comprises a first component and a second component, wherein the first component is provided by the first carbon source and the second component comprises hydrogen and / or carbon monoxide.

18. The preparation method according to any one of claims 7-17, wherein, The sodium iron pyrophosphate powder is carbon-coated sodium iron pyrophosphate, which is prepared by the following steps: Sodium salt, iron source, phosphorus source and second carbon source are wet-mixed according to chemical formula ratio, dried and sintered once under inert atmosphere. The obtained sintered product is mechanically crushed or air-jet crushed to obtain the carbon-coated sodium iron pyrophosphate pyrophosphate powder.

19. The preparation method according to any one of claims 7-18, wherein, The chemical formula of the sodium iron pyrophosphate powder is (d1) or (d2): (d1)Na x Fe y (PO4)2P2O7,x=3.5-4.5,y=2.75-3.25; (d2) Na x Feb y Q m O n ,3.5≤x≤4.5,2.5≤y≤3.5,3.7<m<4,14.5≤n≤15.5。 20. A positive electrode plate, wherein, This includes the cathode material according to any one of claims 1-6 or the cathode material prepared by the preparation method according to any one of claims 7-19.

21. A battery, wherein, It includes the positive electrode as described in claim 20.

22. An electrical appliance, wherein, It includes the battery as described in claim 21.

Images