A positive electrode active material, a method for manufacturing the same, a secondary battery, and an electric device

Abstract

The application relates to a positive electrode active material and a preparation method thereof, a secondary battery and an electric device, and belongs to the technical field of batteries. The secondary battery comprises a positive electrode sheet, the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer attached to the positive electrode current collector, the positive electrode active material layer comprises a positive electrode active material, and the positive electrode active material comprises a sodium iron phosphate pyrophosphate material, wherein the sodium iron phosphate pyrophosphate material comprises a first doping element, and the first doping element comprises S. By doping the sodium iron phosphate pyrophosphate material with the first doping element such as the S element, the cycle performance of the secondary battery taking the sodium iron phosphate pyrophosphate material as the positive electrode active material can be improved.

Description

Technical Field

[0001] This application relates to the field of battery technology, and more specifically, to a positive electrode active material and its preparation method, a secondary battery, and an electrical device. Background Technology

[0002] Sodium iron pyrophosphate materials (e.g., Na₄Fe₃P₄O₂) 15 Sodium iron pyrophosphate (SAP) is considered the most promising cathode material for sodium-ion batteries due to its low cost, environmental friendliness, high theoretical capacity, high average operating voltage, and low volume expansion. However, the cycle performance of secondary batteries using SAP as the cathode active material still needs improvement. Summary of the Invention

[0003] In view of the above problems, this application provides a positive electrode active material and its preparation method, a secondary battery and an electrical device, which can improve the cycle performance of secondary batteries that use sodium iron pyrophosphate as the positive electrode active material.

[0004] In a first aspect, this application provides a secondary battery, the secondary battery including a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive active material layer attached to the positive current collector, the positive active material layer including a positive active material, the positive active material including a sodium iron pyrophosphate material, the sodium iron pyrophosphate material including a first doping element, the first doping element including S.

[0005] In the technical solution of this application embodiment, by doping sodium iron pyrophosphate phosphate materials with sulfur (S) as a first doping element, the cycle performance of the secondary battery as a positive electrode active material can be improved. The reason for this is presumably that the sulfur formed by the sulfur element... 2- With O 2- The formation of stable chemical bonds can stabilize the position of O in the crystal structure of sodium iron pyrophosphate materials, significantly improving the crystal structure stability of sodium iron pyrophosphate materials, obtaining better electrochemical stability, and thus benefiting the cycle performance of secondary batteries.

[0006] In some embodiments, the first doping element is doped at the oxygen sites of the sodium iron pyrophosphate-based material.

[0007] In some embodiments, the first doping element is doped into the oxygen site of the phosphate group in the sodium iron pyrophosphate material.

[0008] In some embodiments, in the sodium iron pyrophosphate material, the molar ratio of the first dopant element to all elements at the oxygen site is (0.02-0.04):(14.5-15.5).

[0009] In the above implementation process, the larger the doping amount of the first dopant element, the better the cycle performance of the secondary battery using sodium iron pyrophosphate as the positive electrode active material; the smaller the doping amount of the first dopant element, the better the sodium iron pyrophosphate as the positive electrode active material maintains a good specific capacity. By controlling the molar ratio of the first dopant element to all elements at the oxygen site to (0.02~0.04):(14.5~15.5), the sodium iron pyrophosphate as the positive electrode active material can maintain a good specific capacity while also taking into account the cycle performance of the secondary battery using it as the positive electrode active material.

[0010] In some embodiments, the sodium iron pyrophosphate material includes a second doping element, which includes at least one of Mg, Al, Ti, Nb, or Co.

[0011] In the above implementation process, by doping sodium iron pyrophosphate materials with second doping elements such as Mg, Al, Ti, Nb, or Co, their intrinsic conductivity can be improved, thereby enhancing the rate performance of secondary batteries using sodium iron pyrophosphate materials as the positive electrode active material. Simultaneously, the cations formed by the second doping element, such as Mg... 2+ It can occupy some of the iron sites in sodium iron pyrophosphate materials, thereby improving the crystal structure stability of sodium iron pyrophosphate materials and thus benefiting the cycle performance of secondary batteries.

[0012] In some embodiments, the second doping element is doped at the iron sites of the sodium iron pyrophosphate-based material.

[0013] In some embodiments, in the sodium iron pyrophosphate material, the molar ratio of the second dopant element to all elements at the iron site is (0-0.02):(2.5-3.5).

[0014] In the above implementation process, the larger the doping amount of the second dopant, the better the rate performance of the secondary battery using sodium iron pyrophosphate as the positive electrode active material; conversely, the smaller the doping amount of the second dopant, the better the sodium iron pyrophosphate material maintains its specific capacity. By controlling the molar ratio of the second dopant to all elements on the iron site to (0–0.02):(2.5–3.5), the sodium iron pyrophosphate material can maintain a good specific capacity while also ensuring good rate performance of the secondary battery using it as the positive electrode active material.

[0015] In some embodiments, the sodium iron pyrophosphate material includes Na x R y Mg z P m O n S aWherein, 3.5≤x≤4.5, 2.5≤y≤3.5, 0≤z≤0.02, 3.5≤m≤4.5, 14.5≤n≤15.5, 0.02≤a≤0.04, and R includes at least one of Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, or Pb.

[0016] Secondly, this application provides an electrical device that includes the secondary battery provided in the first aspect.

[0017] Thirdly, this application provides a positive electrode active material, the positive electrode active material comprising a sodium iron pyrophosphate material, the sodium iron pyrophosphate material comprising a first doping element, the first doping element comprising S.

[0018] In the technical solution of this application embodiment, by doping sodium iron pyrophosphate phosphate materials with sulfur (S) as a first doping element, the cycle performance of the secondary battery as a positive electrode active material can be improved. The reason for this is presumably that the sulfur formed by the sulfur element... 2- With O 2- The formation of stable chemical bonds can stabilize the position of O in the crystal structure of sodium iron pyrophosphate materials, significantly improving the crystal structure stability of sodium iron pyrophosphate materials, obtaining better electrochemical stability, and thus benefiting the cycle performance of secondary batteries.

[0019] In some embodiments, the first doping element is doped at the oxygen sites of the sodium iron pyrophosphate-based material.

[0020] In some embodiments, the first doping element is doped into the oxygen site of the phosphate group in the sodium iron pyrophosphate material.

[0021] In some embodiments, in the sodium iron pyrophosphate material, the molar ratio of the first dopant element to all elements at the oxygen site is (0.02-0.04):(14.5-15.5).

[0022] In the above implementation process, the larger the doping amount of the first dopant element, the better the cycle performance of the secondary battery using sodium iron pyrophosphate as the positive electrode active material; the smaller the doping amount of the first dopant element, the better the sodium iron pyrophosphate as the positive electrode active material maintains a good specific capacity. By controlling the molar ratio of the first dopant element to all elements at the oxygen site to (0.02~0.04):(14.5~15.5), the sodium iron pyrophosphate as the positive electrode active material can maintain a good specific capacity while also taking into account the cycle performance of the secondary battery using it as the positive electrode active material.

[0023] In some embodiments, the sodium iron pyrophosphate material includes a second doping element, which includes at least one of Mg, Al, Ti, Nb, or Co.

[0024] In the above implementation process, by doping sodium iron pyrophosphate materials with second doping elements such as Mg, Al, Ti, Nb, or Co, their intrinsic conductivity can be improved, thereby enhancing the rate performance of secondary batteries using sodium iron pyrophosphate materials as the positive electrode active material. Simultaneously, the cations formed by the second doping element, such as Mg... 2+ It can occupy some of the iron sites in sodium iron pyrophosphate materials, thereby improving the crystal structure stability of sodium iron pyrophosphate materials and thus benefiting the cycle performance of secondary batteries.

[0025] In some embodiments, the second doping element is doped at the iron sites of the sodium iron pyrophosphate-based material.

[0026] In some embodiments, in the sodium iron pyrophosphate material, the molar ratio of the second dopant element to all elements at the iron site is (0-0.02):(2.5-3.5).

[0027] In the above implementation process, the larger the doping amount of the second dopant, the better the rate performance of the secondary battery using sodium iron pyrophosphate as the positive electrode active material; conversely, the smaller the doping amount of the second dopant, the better the sodium iron pyrophosphate material maintains its specific capacity. By controlling the molar ratio of the second dopant to all elements on the iron site to (0–0.02):(2.5–3.5), the sodium iron pyrophosphate material can maintain a good specific capacity while also ensuring good rate performance of the secondary battery using it as the positive electrode active material.

[0028] In some embodiments, the sodium iron pyrophosphate material includes Na x R y Mg z P m O n S a Wherein, 3.5≤x≤4.5, 2.5≤y≤3.5, 0≤z≤0.02, 3.5≤m≤4.5, 14.5≤n≤15.5, 0.02≤a≤0.04, and R includes at least one of Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, or Pb.

[0029] Fourthly, this application provides a method for preparing a positive electrode active material, the method comprising:

[0030] A mixture of raw materials is obtained, wherein the raw materials include a first doping element source, and the first doping element source includes a sulfur-containing compound;

[0031] The mixture is sintered to obtain a positive electrode active material, which includes a sodium iron pyrophosphate material, and the sodium iron pyrophosphate material includes a first doping element, which includes S.

[0032] In the technical solution of this application embodiment, by adding an S source as the first dopant element source during preparation, it is possible to dope the sodium iron pyrophosphate phosphate material, thereby improving the cycle performance of the secondary battery where the sodium iron pyrophosphate phosphate material is used as the positive electrode active material. The reason for this is presumably that the S element forms S... 2- With O 2- The formation of stable chemical bonds can stabilize the position of O in the crystal structure of sodium iron pyrophosphate materials, significantly improving the crystal structure stability of sodium iron pyrophosphate materials, obtaining better electrochemical stability, and thus benefiting the cycle performance of secondary batteries.

[0033] In some embodiments, the mixture further includes a carbon source; the carbon source includes at least one selected from graphite, carbon black, carbon nanotubes, graphene, sucrose, glucose, citric acid, starch, cyclodextrin, pitch, or PEG.

[0034] In the above-described process, by adding a carbon source to the mixture, the sodium iron pyrophosphate material can be coated, improving the conductivity of the positive electrode active material and thus benefiting the rate performance of the secondary battery. Simultaneously, the coating layer formed by the carbon source reduces the possibility of the sodium iron pyrophosphate material reacting with water in the environment, which is beneficial to the cycle stability of the positive electrode active material and consequently improves the cycle performance of the secondary battery. Attached Figure Description

[0035] 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:

[0036] Figure 1 This application provides structural schematic diagrams of vehicles for some embodiments;

[0037] Figure 2 This is an exploded structural diagram of a secondary battery provided in some embodiments of this application;

[0038] Figure 3 This is a schematic diagram of the structure of a battery cell provided in some embodiments of this application;

[0039] Figure 4 Exploded views of a single battery cell provided in some embodiments of this application;

[0040] Figure 5 Flowcharts illustrating methods for preparing positive electrode active materials provided in some embodiments of this application;

[0041] Figure 6 A flowchart illustrating a method for preparing a battery cell according to some embodiments of this application.

[0042] The reference numerals in the detailed embodiments are as follows:

[0043] 1000 - Vehicle; 100 - Secondary battery; 200 - Motor; 300 - Controller; 10 - Housing; 11 - Accommodation space; 12 - First part; 13 - Second part; 20 - Battery cell; 21 - Housing; 211 - Opening; 22 - End cap assembly; 221 - End cap; 222 - Electrode terminal; 23 - Electrode assembly; 24 - Current collector; 25 - Insulation protection component. Detailed Implementation

[0044] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0045] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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).

[0050] 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.

[0051] 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.

[0052] Currently, judging from market trends, the application of power batteries is becoming increasingly widespread. Power batteries are not only used in energy storage systems such as hydropower, thermal power, wind power, and solar power plants, but also extensively used in electric vehicles such as electric bicycles, electric motorcycles, and electric cars, as well as in military equipment and aerospace. With the continuous expansion of power battery applications, market demand is also constantly increasing.

[0053] Sodium-ion batteries can be used as power batteries. Sodium is an abundant element, which can significantly reduce battery costs. Therefore, sodium-ion batteries have broad application prospects, such as in portable electronic devices and electric vehicles. Other materials include sodium iron pyrophosphate (e.g., Na₄Fe₃P₄O₃). 15Sodium iron pyrophosphate (SAP) is considered the most promising cathode material for sodium-ion batteries due to its low cost, environmental friendliness, high theoretical capacity, high average operating voltage, and low volume expansion. However, the cycle performance of secondary batteries using SAP as the cathode active material still needs improvement.

[0054] Based on the above considerations, in order to improve the cycle performance of secondary batteries using sodium iron pyrophosphate as the positive electrode active material, this application proposes a secondary battery, the secondary battery including a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive active material layer attached to the positive current collector, the positive active material layer including a positive active material, the positive active material including a sodium iron pyrophosphate material, the sodium iron pyrophosphate material including a first doping element, the first doping element including S.

[0055] In such secondary batteries, doping materials like sodium iron pyrophosphate (SOP) with sulfur (S) as the primary dopant can improve their cycle performance as positive electrode active materials. The reason for this is presumably that the sulfur forms S... 2- With O 2- The formation of stable chemical bonds can stabilize the position of O in the crystal structure of sodium iron pyrophosphate materials, significantly improving the crystal structure stability of sodium iron pyrophosphate materials, obtaining better electrochemical stability, and thus benefiting the cycle performance of secondary batteries.

[0056] This secondary battery can be used, but is not limited to, in electrical devices such as vehicles, ships, or aircraft. A power system for such an electrical device can be constructed using a secondary battery disclosed in this application.

[0057] 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.

[0058] 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.

[0059] Please refer to Figure 1 , Figure 1This is a schematic diagram of the structure 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 secondary battery 100 is installed inside the vehicle 1000, and the secondary battery 100 can be located at the bottom, front, or rear of the vehicle 1000. The secondary battery 100 can be used to power the vehicle 1000; for example, the secondary battery 100 can serve as the operating power source for the vehicle 1000. The vehicle 1000 may also include a controller 300 and a motor 200. The controller 300 is used to control the secondary battery 100 to supply power to the motor 200, for example, to meet the power needs of the vehicle 1000 during startup, navigation, and driving.

[0060] In some embodiments of this application, the secondary battery 100 can not only serve as the operating power source for the vehicle 1000, but also as the driving power source for the vehicle 1000, replacing or partially replacing fuel or natural gas to provide driving power for the vehicle 1000.

[0061] In this application, the secondary battery 100 can refer to a single battery cell 20, or it can refer to a single physical module comprising multiple battery cells 20 to provide higher voltage and capacity, which can be in the form of a battery pack, battery module, etc. The secondary battery 100 may include a housing 10 for encapsulating multiple battery cells 20, and the housing 10 can prevent liquids or other foreign objects from affecting the charging or discharging of the battery cells 20.

[0062] Figure 2 This is an exploded structural diagram of a secondary battery 100 provided in some embodiments of this application. Please refer to... Figure 2 The secondary battery 100 includes a housing 10 and a battery cell 20, with the battery cell 20 housed within the housing 10.

[0063] The housing 10 provides a receiving space 11 for the battery cell 20. In some embodiments, the housing 10 may include a first portion 12 and a second portion 13, which overlap each other to define the receiving space 11 for accommodating the battery cell 20. Of course, the connection between the first portion 12 and the second portion 13 may be sealed by a sealant (not shown), such as a sealing ring, sealant, etc.

[0064] The first part 12 and the second part 13 can be of various shapes, such as cuboids, cylinders, etc. The first part 12 can be a hollow structure with an opening on one side to form a cavity for accommodating the battery cell 20, and the second part 13 can also be a hollow structure with an opening on one side to form a cavity for accommodating the battery cell 20. When the opening side of the second part 13 covers the opening side of the first part 12, a housing 10 with an accommodating space 11 is formed. Of course, as... Figure 2As shown, the first part 12 can also be a hollow structure with an opening on one side, and the second part 13 can be a plate-like structure. The second part 13 covers the opening side of the first part 12, thus forming a box 10 with a accommodating space 11.

[0065] In the secondary battery 100, there are multiple battery cells 20. These multiple battery cells 20 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. 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, multiple battery cells 20 can first be connected in series, parallel, or in a mixed manner to form a battery module, and then multiple battery modules can be connected in series, parallel, or in a mixed manner to form a whole, which is also housed within the housing 10. The battery cells 20 can be cylindrical, flat, cuboid, or other shapes. Figure 2 An example is shown where the battery cell 20 is square.

[0066] In some embodiments, the secondary battery 100 may further include a busbar (not shown), through which multiple battery cells 20 can be electrically connected to each other to achieve series, parallel, or mixed connection of multiple battery cells 20.

[0067] Figure 3 This is a schematic diagram of the structure of a battery cell 20 provided in some embodiments of this application. Figure 4 Exploded views of a battery cell 20 provided for some embodiments of this application. Please refer to... Figure 3 and Figure 4 The battery cell 20 may include a housing 21, an end cap assembly 22, and an electrode assembly 23. The housing 21 has an opening 211, the electrode assembly 23 is housed within the housing 21, and the end cap assembly 22 is used to seal the opening 211.

[0068] The shape of the outer casing 21 can be determined according to the specific shape of the electrode assembly 23. For example, if the electrode assembly 23 is a cuboid structure, the outer casing 21 can be a cuboid structure. Figure 3 and Figure 4 An example is shown where the housing 21 and electrode assembly 23 are square.

[0069] The outer shell 21 can also be made of various materials, such as copper, iron, aluminum, stainless steel, aluminum alloy, etc. This application embodiment does not impose any special restrictions on this.

[0070] The end cap assembly 22 includes an end cap 221 and electrode terminals 222. The end cap assembly 22 is used to seal the opening 211 of the housing 21 to form a sealed mounting space (not shown) for accommodating the electrode assembly 23. The mounting space also accommodates an electrolyte, such as an electrolyte solution. As a component that outputs electrical energy to the electrode assembly 23, the end cap assembly 22 has electrode terminals 222 for electrical connection to the electrode assembly 23, specifically, the electrode terminals 222 are electrically connected to the tabs of the electrode assembly 23. For example, the electrode terminals 222 and the tabs are connected via a current collector 24 to achieve the electrical connection between the electrode terminals 222 and the tabs.

[0071] It should be noted that the opening 211 of the outer casing 21 can be one or two. If the outer casing 21 has one opening 211, the end cap assembly 22 can also be one, and two electrode terminals 222 can be provided in the end cap assembly 22. The two electrode terminals 222 are used to electrically connect to the positive electrode tab and the negative electrode tab of the electrode assembly 23, respectively. If the outer casing 21 has two openings 211, for example, the two openings 211 are located on opposite sides of the outer casing 21, the end cap assembly 22 can also be two, and the two end cap assemblies 22 respectively cover the two openings 211 of the outer casing 21. In this case, the electrode terminal 222 in one end cap assembly 22 can be a positive electrode terminal 222, used to electrically connect to the positive electrode tab of the electrode assembly 23; the electrode terminal 222 in the other end cap assembly 22 can be a negative electrode terminal 222, used to electrically connect to the negative electrode plate of the electrode assembly 23.

[0072] In some embodiments, such as Figure 4 As shown, the battery cell 20 may further include an insulating protective member 25 fixed to the outer periphery of the electrode assembly 23. The insulating protective member 25 is used to insulate and isolate the electrode assembly 23 from the housing 21. Exemplarily, the insulating protective member 25 is adhesive tape bonded to the outer periphery of the electrode assembly 23. In some embodiments, there are multiple electrode assemblies 23, and the insulating protective member 25 surrounds the outer periphery of multiple electrode assemblies 23, forming a single integral structure to maintain the structural stability of the electrode assembly 23.

[0073] The electrode assembly 23 includes a positive electrode, a negative electrode, and a separator. The separator is located between the positive and negative electrode and serves as a separator. The electrode assembly 23 can be a wound structure or a stacked structure, and the embodiments of this application are not limited to these.

[0074] This application does not impose any special restrictions on the positive electrode, negative electrode, and separator.

[0075] In some embodiments, the separator can be a PP (polypropylene) porous membrane, a PE (polyethylene) porous membrane, a polyimide porous membrane, or a porous membrane formed by a composite of various polymers.

[0076] In some embodiments, the positive electrode sheet includes a positive current collector and a positive active material layer covering at least one surface of the positive current collector in the thickness direction; the positive current collector without the positive active material layer protrudes from the positive current collector with the positive active material layer, and the positive current collector without the positive active material layer serves as the positive electrode tab. The material of the positive current collector may include aluminum foil, aluminum foam, aluminum composite current collector (with a polymer support layer in the middle, and both surfaces of the support layer having aluminum metal layers), nickel foil, nickel foam, etc.; the positive active material in the positive active material layer includes one or a mixture of several of lithium cobalt oxide, sodium cobalt oxide, lithium nickel oxide, sodium nickel oxide, lithium manganese oxide, sodium manganese oxide, lithium nickel manganese oxide, sodium nickel manganese oxide, lithium nickel cobalt manganese oxide, sodium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, sodium nickel cobalt aluminum oxide, and lithium / sodium phosphate with an olivine structure, such as lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium cobalt oxide, phosphorus Lithium iron oxide, lithium manganese oxide, sodium nickel cobalt manganate, sodium nickel cobalt aluminum oxide, sodium cobalt oxide, sodium iron phosphate, sodium manganate, etc.; the binder in the positive electrode active material layer is selected from at least one of vinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyacrylate, sodium carboxymethyl cellulose, styrene-butadiene rubber, polyurethane, ethylene-vinyl acetate copolymer, and ethylene-acrylic acid copolymer; the dispersant in the positive electrode active material layer is selected from polyvinylpyrrolidone, etc.; the conductive particles in the positive electrode active material layer are selected from at least one of conductive carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanotubes, graphene, activated carbon, graphite sheets, graphite particles, and mesophase carbon microspheres.

[0077] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0078] In some embodiments, the negative electrode includes a negative current collector and a negative active material layer covering at least one surface of the negative current collector in the thickness direction; the negative current collector without the negative active material layer protrudes from the negative current collector with the negative active material layer, and the negative current collector without the negative active material layer serves as a negative electrode tab. The material of the negative current collector may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foamed nickel, foamed copper, or a polymer substrate coated with a conductive metal, wherein the conductive metal includes, but is not limited to, copper, nickel, or titanium, and the polymer substrate material includes, but is not limited to, at least one of polyethylene, polypropylene, ethylene propylene copolymer, polyethylene terephthalate, polyethylene terephthalate, and poly(p-phenylene terephthalate); the negative active material in the negative active material layer includes carbon materials, elemental sodium, and alloys formed by sodium and other metallic or non-metallic elements, wherein the 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. Metallic elements include tin (Sn), zinc (Zn), aluminum (Al), magnesium (Mg), silver (Ag), gold (Au), gallium (Ga), indium (In), and tin foil (Pt), while non-metallic elements include boron (B), carbon (C), and silicon (Si).

[0079] In other 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 of the negative electrode sheet can be disposed on the surface of at least one side of the current collector body; when the current collector of the negative electrode sheet includes a base coating, the film layer of the negative electrode sheet can be disposed on the surface of the base coating away from the current collector body.

[0080] In some embodiments, the electrolyte acts as a conductor of ions between the positive and negative electrode plates. This application does not impose specific limitations on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel-like, or entirely solid.

[0081] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0082] In some embodiments, the electrolyte salt may be selected from at least one of sodium hexafluorophosphate, sodium tetrafluoroborate, sodium perchlorate, sodium hexafluoroarsenate, sodium difluorosulfonamide, sodium ditrifluoromethanesulfonamide, sodium trifluoromethanesulfonate, sodium difluorooxalate borate, sodium dioxalate borate, sodium difluorodioxalate phosphate, and sodium tetrafluorooxalate phosphate.

[0083] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0084] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0085] This application provides a secondary battery, which includes a positive electrode sheet, a positive current collector, and a positive active material layer attached to the positive current collector. The positive active material layer includes a positive active material, which includes a sodium iron pyrophosphate material. The sodium iron pyrophosphate material includes a first doping element, which includes sulfur (S).

[0086] Sodium iron pyrophosphate esters refer to materials containing sodium iron pyrophosphate (Na₄Fe₃P₄O₂). 15 Materials with the same or nearly identical structure do not necessarily contain iron at the iron sites. The elements contained in sodium iron pyrophosphate pyrophosphate materials can be determined by inductively coupled plasma atomic emission spectrometry (ICP).

[0087] By doping sodium iron pyrophosphate phosphate materials with sulfur (S) as the primary dopant, the cycle performance of these materials in secondary batteries as positive electrode active materials can be improved. The reason for this is presumably that the sulfur forms S... 2- With O 2- The formation of stable chemical bonds can stabilize the position of O in the crystal structure of sodium iron pyrophosphate materials, significantly improving the crystal structure stability of sodium iron pyrophosphate materials, obtaining better electrochemical stability, and thus benefiting the cycle performance of secondary batteries.

[0088] In some embodiments of this application, the first doping element is doped at the oxygen site of the sodium iron pyrophosphate-based material. Since S and O belong to the same group, it is presumed that S... 2- It replaces the O in the crystal lattice of sodium iron pyrophosphate-based materials. 2- The position of S. Of course, in other embodiments, the doping of S can also be accomplished in the form of sulfate, in which case it replaces the phosphate in sodium iron pyrophosphate-based materials.

[0089] In some embodiments of this application, the first doping element is doped into the oxygen site of the phosphate group in the sodium iron pyrophosphate material. Based on the analysis of the stability of phosphate and pyrophosphate in the sodium iron pyrophosphate material, it is speculated that when S is doped, it is S 2- It replaces the O in the crystal lattice of sodium iron pyrophosphate-based materials. 2- When the position of O is reached, 2- It should belong to the phosphate group O. 2- .

[0090] In some embodiments of this application, the molar ratio of the first dopant element to all elements at the oxygen site in the sodium iron pyrophosphate material is (0.02-0.04):(14.5-15.5).

[0091] The content of each element in sodium iron pyrophosphate materials can be obtained by inductively coupled plasma atomic emission spectrometry (ICP).

[0092] The higher the doping amount of the first dopant element, the better the cycle performance of secondary batteries using sodium iron pyrophosphate as the positive electrode active material. Conversely, the lower the doping amount of the first dopant element, the better the sodium iron pyrophosphate material maintains its specific capacity. By controlling the molar ratio of the first dopant element to all elements at the oxygen site to (0.02–0.04):(14.5–15.5), sodium iron pyrophosphate material can maintain good specific capacity while also ensuring good cycle performance of secondary batteries using it as the positive electrode active material.

[0093] For example, the molar ratio of the first dopant element to all elements at the oxygen site can be 0.02:14.5, 0.03:14.5, 0.04:14.5, 0.02:15, 0.03:15, 0.04:15, 0.02:15.5, 0.03:15.5, 0.04:15.5, etc., or any value within the range of (0.02 to 0.04): (14.5 to 15.5).

[0094] In some embodiments of this application, the sodium iron pyrophosphate material includes a second doping element, which includes at least one of Mg, Al, Ti, Nb, or Co.

[0095] The elements contained in sodium iron pyrophosphate materials can be determined by inductively coupled plasma atomic emission spectrometry (ICP).

[0096] By doping sodium iron pyrophosphate materials with secondary doping elements such as Mg, Al, Ti, Nb, or Co, their intrinsic conductivity can be improved, thereby enhancing the rate performance of secondary batteries using sodium iron pyrophosphate materials as the positive electrode active material. Simultaneously, the cations formed by the secondary doping elements, such as Mg... 2+ It can occupy some of the iron sites in sodium iron pyrophosphate materials, thereby improving the crystal structure stability of sodium iron pyrophosphate materials and thus benefiting the cycle performance of secondary batteries.

[0097] In some embodiments of this application, the second doping element is doped at the iron sites of the sodium iron pyrophosphate-based material.

[0098] The ionic radii of the second doping elements, such as Mg, Al, Ti, Nb, or Co, are not much different from the ionic radii of the iron sites in sodium iron pyrophosphate materials. Therefore, it is speculated that the second doping element is located at an iron site in the crystal lattice of sodium iron pyrophosphate materials.

[0099] In some embodiments of this application, the molar ratio of the second dopant element to all elements on the iron site in the sodium iron pyrophosphate material is (0-0.02):(2.5-3.5).

[0100] The content of each element in sodium iron pyrophosphate materials can be obtained by inductively coupled plasma atomic emission spectrometry (ICP).

[0101] The higher the doping amount of the second dopant, the better the rate performance of secondary batteries using sodium iron pyrophosphate as the positive electrode active material. Conversely, the lower the doping amount of the second dopant, the better the sodium iron pyrophosphate material maintains its specific capacity. By controlling the molar ratio of the second dopant to all elements on the iron site to (0–0.02):(2.5–3.5), sodium iron pyrophosphate can maintain good specific capacity while also ensuring good rate performance of secondary batteries using it as the positive electrode active material.

[0102] For example, the molar ratio of the second dopant element to all elements on the iron site can be 0:3, 0.01:2.5, 0.01:3, 0.01:3.5, 0.02:2.5, 0.02:3, 0.02:3.5, etc., or it can be any value in the range of (0 to 0.02):(2.5 to 3.5).

[0103] In some embodiments of this application, the sodium iron pyrophosphate material includes Na. x R y Mg z P m O n S aWherein, 3.5≤x≤4.5, 2.5≤y≤3.5, 0≤z≤0.02, 3.5≤m≤4.5, 14.5≤n≤15.5, 0.02≤a≤0.04, and R includes at least one of Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, or Pb.

[0104] It should be noted that the above limitation on x includes the molar content of Na under different charge and discharge states of the battery (typically the battery voltage is between 2-5V).

[0105] Understandably, sodium (Na) is intercalated and deintercalated during the charging and discharging process of a battery. The Na content in the positive electrode varies depending on the state of discharge. The Na content can be measured using molar content, but is not limited to this. Simultaneously, when a positive electrode material is applied to the positive electrode in a battery system, the Na content in the positive electrode material typically changes after charge-discharge cycles. In the examples of positive electrode materials listed in this application, unless otherwise specified, the Na content refers to the initial state of the material. Regarding "Na content refers to the initial state of the material," the initial state of the material refers to its state before being added to the positive electrode slurry. It is understood that new materials obtained by appropriately modifying the listed positive electrode materials are also within the scope of positive electrode materials. The aforementioned appropriate modification refers to acceptable modification methods for the positive electrode material; non-limiting examples include coating modification.

[0106] It is understandable that the molar content of R, y, and the molar content of Na are similar, so we will not elaborate further here.

[0107] For example, Na x R y Mg z P m O n S a In this case, x can be 3.5, 3.55, 3.6, 3.65, 3.7, 3.75, 3.8, 3.85, 3.9, 3.95, 4, 4.05, 4.1, 4.15, 4.2, 4.25, 4.3, 4.35, 4.4, 4.45, or 4.5, or any value within the range of 3.5 to 4.5. Na x R y Mg z P m O n S a In this case, y can be 2.5, 2.6, 2.7, 2.8, 2.9, 3, 3.1, 3.2, 3.3, 3.4, or 3.5, or any value within the range of 2.5 to 3.5. Na x R yMg z P m O n S a In this case, z can be 0, 0.01, 0.015, 0.02, 0.025, 0.03, etc., or any value within the range of 0 to 0.03. Na x R y Mg z P m O n S a In this case, m can be 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.1, 4.2, 4.3, 4.4, 4.5, etc., or any value within the range of 3.5 to 4.5. Na x R y Mg z P m O n S a In this case, n can be 14.5, 14.6, 14.7, 14.8, 14.9, 15, 15.1, 15.2, 15.3, 15.4, or 15.5, or any value within the range of 14.5 to 15.5. x R y Mg z P m O n S a In this case, 'a' can be 0.02, 0.025, 0.03, 0.035, 0.04, etc., or any value within the range of 0.02 to 0.04.

[0108] In some embodiments of this application, the chemical formula of the positive electrode active material is Na. x R y Mg z P m O n S a / C, where 3.5≤x≤4.5, 2.5≤y≤3.5, 0≤z≤0.02, 3.5≤m≤4.5, 14.5≤n≤15.5, 0.02≤a≤0.04, and R includes at least one of Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, or Pb.

[0109] Na x R y Mg z P m O n S a / C refers to Na x R y Mgz P m O n S a A complex with C.

[0110] By using Na, the positive electrode active material x R y Mg z P m O n S a Combining with C can reduce the Na content of the positive electrode active material. x R y Mg z P m O n S a During preparation and use, it comes into contact with water and other external environmental elements, thereby reducing the generation of residual alkali during preparation and use, and maintaining good cycle performance.

[0111] Figure 5 For flowcharts illustrating the preparation methods of the positive electrode active materials provided in some embodiments of this application, please refer to [link / reference]. Figure 5 This application also provides a method for preparing a positive electrode active material, the method comprising:

[0112] S110. Obtain a mixture of raw materials, wherein the raw materials include a first doping element source, and the first doping element source includes a sulfur-containing compound;

[0113] The sulfur-containing compound can be specifically selected from thiourea. It is understood that this does not limit the sulfur-containing compound. In other embodiments, other sulfur-containing compounds that can provide sulfur during the preparation process can be selected. Generally speaking, the better the solubility of the sulfur-containing compound, the more beneficial it is to the preparation of sodium iron pyrophosphate materials.

[0114] In some embodiments of this application, the raw materials include sodium source, R (i.e., the element at the iron site in sodium iron pyrophosphate materials), phosphorus source, and second doping element source, etc.

[0115] The sodium source can be at least one of sodium salt, sodium hydroxide, or sodium oxide. The phosphorus source includes at least one of phosphate or phosphoric acid. The R source includes at least one of a salt, oxide, or hydroxide of element R; an exemplary R source could be an iron source. The second dopant source can be at least one of a salt, oxide, or hydroxide of the second dopant element; an exemplary second dopant source could be a magnesium source, such as magnesium sulfate.

[0116] In some embodiments of this application, the mixture further includes a carbon source.

[0117] In some embodiments of this application, the mixture further includes a carbon source; the carbon source includes at least one of graphite, carbon black, carbon nanotubes, graphene, sucrose, glucose, citric acid, starch, cyclodextrin, pitch, or PEG. By adding a carbon source to the mixture, the sodium iron pyrophosphate material can be coated, improving the conductivity of the positive electrode active material, thereby benefiting the rate performance of the secondary battery. Simultaneously, the coating layer formed by the carbon source can reduce the possibility of the sodium iron pyrophosphate material reacting with water in the environment, which is beneficial to the cycle stability of the positive electrode active material, and thus to the cycle performance of the secondary battery.

[0118] S120. The mixture is sintered to obtain a positive electrode active material, wherein the positive electrode active material comprises a sodium iron pyrophosphate material, wherein the sodium iron pyrophosphate material comprises a first doping element, wherein the first doping element comprises S.

[0119] This method involves adding sulfur (S) as the first dopant element during preparation, enabling it to be incorporated into sodium iron pyrophosphate (SOP)-based materials. This improves the cycle performance of SOP-based materials in secondary batteries used as positive electrode active materials. The reason for this is presumably that the sulfur forms S... 2- With O 2- The formation of stable chemical bonds can stabilize the position of O in the crystal structure of sodium iron pyrophosphate materials, significantly improving the crystal structure stability of sodium iron pyrophosphate materials, obtaining better electrochemical stability, and thus benefiting the cycle performance of secondary batteries.

[0120] Having introduced the materials and structure of the battery cell 20, the preparation method of the battery cell 20 will be described in detail below.

[0121] The preparation method of the battery cell 20 includes the following steps: mixing positive electrode active material, conductive agent and binder, and then mixing with solvent to prepare positive electrode active slurry; coating the positive electrode active slurry onto positive electrode current collector to obtain positive electrode sheet; after preparing positive electrode sheet, stacking positive electrode sheet, separator, negative electrode sheet, separator, and so on in sequence to form stacked electrode assembly 23; then placing electrode assembly 23 in housing and injecting electrolyte to form battery cell 20.

[0122] Figure 6 Here is a flowchart illustrating the fabrication process of the battery cell 20 provided in some embodiments of this application, such as... Figure 6 As shown, the specific process for preparing battery cell 20 is as follows:

[0123] S210, Preparation of positive electrode active slurry: The positive electrode active material, binder, and conductive agent are dispersed in a solvent to form a positive electrode active slurry. The positive electrode active material used is the one provided above. Optionally, a small amount of other positive electrode active materials may be added.

[0124] The binder can be one or more of styrene-butadiene rubber, waterborne acrylic resin, carboxymethyl cellulose, polyvinylidene fluoride, polytetrafluoroethylene, ethylene-vinyl acetate copolymer, polyvinyl alcohol, and polyvinyl butyral. The conductive agent can be at least one of conductive carbon black, carbon fiber, carbon nanotubes, Ketjen black, graphene, or acetylene black. The solvent can be one or more of dimethyl glutarate and N-methylpyrrolidone. Leveling agents, dispersants, etc., can also be added to the positive electrode active slurry.

[0125] S220, Preparation of the positive electrode active material layer: The positive electrode active slurry is coated onto the surface of the positive electrode current collector, and then dried to form the positive electrode active material layer. The coating can be applied to one or both surfaces of the positive electrode current collector, depending on the requirements.

[0126] The coating method can be: scraping, roller coating, slot coating, etc., and this application does not limit it.

[0127] The positive electrode current collector can be a metal foil or a composite current collector. For example, aluminum foil can be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. 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 substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0128] S230, the positive electrode active material layer is rolled to obtain the positive electrode sheet.

[0129] S240, positive electrode, separator, negative electrode, separator, and so on are stacked in sequence to form a stacked electrode assembly 23.

[0130] S250, the stacked electrode assembly 23 is assembled into a battery cell 20. This battery cell 20 can be used to prepare a secondary battery 100 and provide electrical energy to an electrical device.

[0131] The following examples will describe one or more embodiments in more detail. Of course, these examples do not limit the scope of the one or more embodiments.

[0132] Examples and Comparative Examples

[0133] Preparation of positive electrode active materials

[0134] Sodium, iron, phosphorus, first dopant, and second dopant sources were dissolved in deionized water at a specific molar ratio and stirred continuously at room temperature for 30 minutes to obtain an initial mixed slurry. A carbon source was dissolved in deionized water and mixed to obtain a carbon solution. The carbon solution was then mixed with the initial mixed slurry and stirred to obtain a mixed solution. This mixed solution was spray-dried at an inlet air temperature of 220°C and an outlet air temperature of 109°C to obtain a powdered precursor. The powdered precursor was heated to 320°C in a N2 atmosphere at a heating rate of 2°C / min and held for 4 hours, followed by a further heating rate of 2°C / min to 550°C and held for 10 hours. The sintered product was then subjected to air jet milling and sieved to obtain a uniformly sized positive electrode active material.

[0135] Preparation of the positive electrode sheet

[0136] 2.5 wt% of the binder polyvinylidene fluoride was fully dissolved in the solvent N-methylpyrrolidone. 2.0 wt% of super p and 1.0 wt% of CNT were added as conductive agents, and 94.5 wt% of the above-mentioned positive electrode active material were mixed to form a uniformly dispersed slurry. The slurry was uniformly coated onto the surface of the positive electrode current collector aluminum foil, and after drying, cold pressing, and slitting, the positive electrode sheet was obtained.

[0137] Preparation of the negative electrode sheet

[0138] Sodium metal sheet is used as the negative electrode.

[0139] Preparation of Electrolyte

[0140] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), the organic solvents ethylene carbonate (EC) / diethyl carbonate (DEC) / ethyl methyl carbonate (EMC) are mixed evenly in a volume ratio of 1 / 1 / 1. 1 mol / L NaPF6 sodium salt is added and dispersed evenly. Then, 5% fluoroethylene carbonate is dissolved in the above organic solvent and stirred evenly to obtain the electrolyte.

[0141]

Isolation Film

[0142] Polyethylene film is used as the separation membrane.

[0143] [Preparation of button cells]

[0144] The prepared positive electrode, negative electrode, separator and electrolyte are assembled into CR2430 button cell in an argon-protected glove box.

[0145] The main parameter controls for Examples 1 to 12 and Comparative Examples 1 to 3 are shown in the table below:

[0146]

[0147] In the table, " / " indicates that the element is not doped.

[0148] The performance of the positive electrode active materials and batteries composed thereof provided in each embodiment and comparative example was tested. The performance testing specifically included:

[0149] Specific capacity test: After the battery was left to stand for 12 hours, it was discharged at a constant current of 0.05C until it reached 1.5V, and then left to stand for 10 minutes. It was then discharged at a constant current of 50μA until it reached 1.5V, and left to stand for 10 minutes. It was then discharged at a constant current of 10μA until it reached 1.5V. Finally, it was charged at a constant current of 0.1C until it reached 3.8V. The discharge capacity and charge capacity were recorded. The ratio of the discharge capacity and charge capacity to the weight of the positive electrode active material is the charge / discharge specific capacity of the prepared positive electrode active material.

[0150] Voltage plateau test: The positive electrode active material is assembled into a coin cell and its specific capacity is tested. The discharge voltage plateau can be calculated using the formula: Discharge voltage plateau = Discharge energy / Discharge capacity.

[0151] Battery cycle life test: Follow these steps: a) Charge the battery at a constant current of 1C to 3.85V, then switch to constant voltage charging until the charging current drops to 0.05C; b) Discharge the battery at a constant current of 1C to 1.5V; c) Repeat the charge-discharge cycle until the battery capacity drops to 90% of the initial capacity, then stop the test and record the number of cycles.

[0152] Rate performance test (3C discharge retention rate): At 25℃, charge at 0.33C constant current and constant voltage to 3.85V, cut off at 0.02C, let stand for 5 minutes, and then discharge at 3C constant current to 1.5V. Record the capacity discharged at different rates. Using the capacity discharged at 0.33C as the benchmark, calculate the discharge capacity ratio at different rates.

[0153] The test results are shown in the table below:

[0154]

[0155] As can be seen from the table above, the positive electrode active material prepared by the method provided in the embodiments of this application has a good cycle life, and the number of cycles in which the capacity is maintained at more than 90% is more than 2300.

[0156] By comparing the data from Examples 1 to 5 and Comparative Example 2, it can be seen that when the content of the first dopant element in the positive electrode active material gradually increases, the cycle life shows a trend of first increasing and then decreasing, and the discharge capacity also shows a trend of first increasing and then decreasing. When the content of the first dopant element is controlled at 0.01 to 0.02, the cycle life can be maintained at more than 2700 cycles, and the discharge capacity can be maintained at more than 105 mAh / g.

[0157] By comparing the data from Examples 3 and 6 to 8, it can be seen that when the content of the second dopant element in the positive electrode active material gradually increases, the discharge retention rate shows a trend of first increasing and then decreasing, and the discharge capacity gradually decreases. By controlling the content of the second dopant element to be below 0.02, the discharge specific capacity can be maintained above 105 mAh / g, and the rate performance is above 82%.

[0158] A comparison of the data from Examples 3 and 9 to 12 shows that selecting different second doping elements has essentially the same effect. Among them, selecting Mg, Ti, and V as second doping elements results in better cycle lifetimes.

[0159] The above are merely specific embodiments of this application and are not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A secondary battery, characterized in that, The secondary battery includes a positive electrode sheet, the positive electrode sheet includes a positive current collector and a positive active material layer attached to the positive current collector, the positive active material layer includes a positive active material, the positive active material includes a sodium iron pyrophosphate material, the sodium iron pyrophosphate material includes a first doping element, the first doping element includes S.

2. The secondary battery according to claim 1, characterized in that, The first doping element is doped into the oxygen sites of the sodium iron pyrophosphate-based material.

3. The secondary battery according to any one of claims 1 to 2, characterized in that, The first doping element is doped into the oxygen site of the phosphate group in the sodium iron pyrophosphate material.

4. The secondary battery according to any one of claims 1 to 3, characterized in that, In the sodium iron pyrophosphate material, the molar ratio of the first dopant element to all elements at the oxygen site is (0.02-0.04):(14.5-15.5).

5. The secondary battery according to any one of claims 1 to 4, characterized in that, The sodium iron pyrophosphate material includes a second doping element, which includes at least one of Mg, Al, Ti, Nb, or Co.

6. The secondary battery according to claim 5, characterized in that, The second doping element is doped into the iron sites of the sodium iron pyrophosphate-based material.

7. The secondary battery according to any one of claims 5 to 6, characterized in that, In the sodium iron pyrophosphate material, the molar ratio of the second dopant element to all elements on the iron site is (0-0.02):(2.5-3.5).

8. The secondary battery according to any one of claims 1 to 7, characterized in that, The sodium ferric pyrophosphate material includes Na. x R y Mg z P m O n S a Wherein, 3.5≤x≤4.5, 2.5≤y≤3.5, 0≤z≤0.02, 3.5≤m≤4.5, 14.5≤n≤15.5, 0.02≤a≤0.04, and R includes at least one of Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, or Pb.

9. An electrical device, characterized in that, The electrical device includes the secondary battery as described in any one of claims 1 to 8.

10. A positive electrode active material, characterized in that, The positive electrode active material includes sodium iron pyrophosphate, and the sodium iron pyrophosphate includes a first doping element, which includes S.

11. The positive electrode active material according to claim 10, characterized in that, The first doping element is doped into the oxygen site of the phosphate group in the sodium iron pyrophosphate material.

12. The positive electrode active material according to any one of claims 10 to 11, characterized in that, In the sodium iron pyrophosphate material, the molar ratio of the first dopant element to all elements at the oxygen site is (0.02-0.04):(14.5-15.5).

13. The positive electrode active material according to any one of claims 10 to 12, characterized in that, The sodium iron pyrophosphate material includes a second doping element, which includes at least one of Mg, Al, Ti, Nb, or Co.

14. The positive electrode active material according to claim 13, characterized in that, The second doping element is doped into the iron sites of the sodium iron pyrophosphate-based material.

15. The positive electrode active material according to any one of claims 13 to 14, characterized in that, In the sodium iron pyrophosphate material, the molar ratio of the second dopant element to all elements on the iron site is (0-0.02):(2.5-3.5).

16. The positive electrode active material according to any one of claims 10 to 15, characterized in that, The sodium ferric pyrophosphate material includes Na. x R y Mg z P m O n S a Wherein, 3.5≤x≤4.5, 2.5≤y≤3.5, 0≤z≤0.02, 3.5≤m≤4.5, 14.5≤n≤15.5, 0.02≤a≤0.04, and R includes at least one of Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, or Pb.

17. A method for preparing a positive electrode active material, characterized in that, The method includes: A mixture of raw materials is obtained, wherein the raw materials include a first doping element source, and the first doping element source includes a sulfur-containing compound; The mixture is sintered to obtain a positive electrode active material, which includes a sodium iron pyrophosphate material, and the sodium iron pyrophosphate material includes a first doping element, which includes S.

18. The method for preparing the positive electrode active material according to claim 17, characterized in that, The mixture also includes a carbon source; the carbon source includes at least one of graphite, carbon black, carbon nanotubes, graphene, sucrose, glucose, citric acid, starch, cyclodextrin, pitch or PEG.

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