A positive electrode active material, a sodium-ion secondary battery, and an electric device

Abstract

The application discloses a positive electrode active material, a sodium-ion secondary battery and an electric device, and belongs to the technical field of batteries; by adopting a first sodium iron pyrophosphate material and a second sodium iron pyrophosphate material with different effective phase contents and different particle sizes as a positive electrode active material, and by controlling Dv50 of the whole positive electrode active material to meet a specific range, the positive electrode active material layer can simultaneously have a high effective phase content and a high compaction density, and thus the energy density of the secondary battery 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, a sodium-ion 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, it is difficult to simultaneously achieve the desired compaction density and effective phase content in SAP materials, which means that the energy density of secondary batteries currently using SAP materials as cathode active materials still needs improvement. Summary of the Invention

[0003] In view of the above problems, this application provides a positive electrode active material, a sodium-ion secondary battery and an electrical device, which can improve the energy density of secondary batteries that use sodium iron pyrophosphate as the positive electrode active material.

[0004] In a first aspect, this application provides a sodium-ion secondary battery, the sodium-ion secondary battery comprising a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive active material layer attached to the positive current collector, the positive active material layer comprising a positive active material; the positive active material comprising a first sodium iron pyrophosphate material and a second sodium iron pyrophosphate material, the effective phase mass content of the first sodium iron pyrophosphate material being not less than 90%, the effective phase mass content of the second sodium iron pyrophosphate material being not less than 90%, the Dv50 of the second sodium iron pyrophosphate material being greater than the Dv50 of the first sodium iron pyrophosphate material, and the Dv50 of the positive active material being 5μm to 9.5μm.

[0005] It is difficult to simultaneously achieve high compaction density and high effective phase content in sodium iron pyrophosphate materials. Generally, sodium iron pyrophosphate materials with high effective phase content have low compaction density, while sodium iron pyrophosphate materials with high compaction density have low effective phase content. In the technical solution of this application embodiment, by using a combination of first sodium iron pyrophosphate materials and second sodium iron pyrophosphate materials with different effective phase contents and particle sizes as positive electrode active materials, and controlling the Dv50 of the entire positive electrode active material to meet a specific range, the positive electrode active material layer can simultaneously have high effective phase content and high compaction density, thereby improving the energy density of the secondary battery.

[0006] In some embodiments, the Dv50 of the positive electrode active material is 7 μm to 9.5 μm; and / or

[0007] The Dv100 of the positive electrode active material is 27 μm to 31 μm.

[0008] In the above implementation process, by controlling the Dv50 of the positive electrode active material to be 7μm to 9.5μm, a basis is provided for achieving a higher compaction density of the positive electrode active material layer, thereby enabling the secondary battery to have a higher energy density. By controlling the Dv100 of the positive electrode active material to be 27μm to 31μm, the compaction density of the positive electrode active material layer can be increased, thereby enabling the secondary battery to have a higher energy density.

[0009] In some embodiments, the Dv100 of the positive electrode active material is 29 μm to 31 μm.

[0010] In the above implementation process, by controlling the Dv100 of the positive electrode active material to be 29μm to 31μm, the compaction density of the positive electrode active material layer can be further increased, thereby enabling the secondary battery to have a higher energy density.

[0011] In some embodiments, the Dv50 of the first sodium iron pyrophosphate material is 3 μm to 6 μm; and / or

[0012] The Dv50 of the second phosphate iron sodium pyrophosphate material is 6.5 μm to 13 μm.

[0013] In the above implementation process, by controlling the median particle size Dv50 of the first phosphate iron sodium pyrophosphate material and / or the second phosphate iron sodium pyrophosphate material to be 3μm~6μm and 6.5μm~13μm respectively, it is beneficial to have a higher compaction density of the positive electrode active material layer, thereby improving the energy density of the secondary battery.

[0014] In some embodiments, the compaction density of the first sodium iron pyrophosphate material at 1 ton is 1.45 g / cm³. 3 ~1.7g / cm 3 ; and / or

[0015] The second type of phosphate pyrophosphate sodium material has a compacted density of 1.7 g / cm³ at 1 ton. 3 ~1.95g / cm 3 .

[0016] In the above implementation process, the higher the compaction density of the positive electrode active material, the better it is for the compaction density of the positive electrode active material layer, and thus the better it is for the battery capacity. By controlling the compaction density of the first phosphate iron sodium pyrophosphate material and / or the second phosphate iron sodium pyrophosphate material at 1t to be 1.45 g / cm³, the battery capacity is improved. 3 ~1.7g / cm 3 1.7g / cm 3 ~1.95g / cm3 It can enable the positive electrode active material layer to have a better compaction density, thereby enabling the secondary battery to have a higher energy density.

[0017] In some embodiments, the compaction density of the positive electrode active material layer is not less than 1.8 g / cm³. 3 .

[0018] In some embodiments, the first sodium iron pyrophosphate material and / or the second sodium iron pyrophosphate material includes Na. x R y P m O n Wherein, 3.5≤x≤4.5, 2.5≤y≤3.5, 3.5≤m≤4.5, 14.5≤n≤15.5, and R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, or Pb.

[0019] Secondly, this application provides an electrical device, which includes the sodium-ion secondary battery provided in the first aspect.

[0020] Thirdly, this application provides a positive electrode active material, which includes a first sodium iron pyrophosphate material and a second sodium iron pyrophosphate material. The effective phase mass content of the first sodium iron pyrophosphate material is not less than 90%, the effective phase mass content of the second sodium iron pyrophosphate material is not less than 90%, the Dv50 of the second sodium iron pyrophosphate material is greater than the Dv50 of the first sodium iron pyrophosphate material, and the Dv50 of the positive electrode active material is 5μm to 9.5μm.

[0021] It is difficult to simultaneously achieve high compaction density and high effective phase content in sodium iron pyrophosphate materials. Generally, sodium iron pyrophosphate materials with high effective phase content have low compaction density, while sodium iron pyrophosphate materials with high compaction density have low effective phase content. In the technical solution of this application embodiment, by using a combination of first sodium iron pyrophosphate materials and second sodium iron pyrophosphate materials with different effective phase contents and particle sizes as positive electrode active materials, and controlling the Dv50 of the entire positive electrode active material to meet a specific range, the positive electrode active material layer using this positive electrode active material can simultaneously have high effective phase content and high compaction density, thereby improving the energy density of the corresponding secondary battery.

[0022] In some embodiments, the Dv50 of the positive electrode active material is 7 μm to 9.5 μm; and / or

[0023] The Dv100 of the positive electrode active material is 27 μm to 31 μm.

[0024] In the above implementation process, by controlling the Dv50 of the positive electrode active material to be 7μm to 9.5μm, a basis is provided for achieving a higher compaction density of the positive electrode active material layer, thereby enabling the secondary battery to have a higher energy density. By controlling the Dv100 of the positive electrode active material to be 27μm to 31μm, the compaction density of the positive electrode active material layer can be increased, thereby enabling the secondary battery to have a higher energy density.

[0025] In some embodiments, the Dv100 of the positive electrode active material is 29 μm to 31 μm.

[0026] In the above implementation process, by controlling the Dv100 of the positive electrode active material to be 29μm to 31μm, the compaction density of the positive electrode active material layer can be further increased, thereby enabling the secondary battery to have a higher energy density.

[0027] In some embodiments, the Dv50 of the first sodium iron pyrophosphate material is 3 μm to 6 μm; and / or

[0028] The Dv50 of the second phosphate iron sodium pyrophosphate material is 6.5 μm to 13 μm.

[0029] In the above implementation process, by controlling the median particle size Dv50 of the first phosphate iron sodium pyrophosphate material and / or the second phosphate iron sodium pyrophosphate material to be 3μm~6μm and 6.5μm~13μm respectively, it is beneficial to have a higher compaction density of the positive electrode active material layer, thereby improving the energy density of the secondary battery.

[0030] In some embodiments, the compaction density of the first sodium iron pyrophosphate material at 1 ton is 1.45 g / cm³. 3 ~1.7g / cm 3 ; and / or

[0031] The second type of phosphate pyrophosphate sodium material has a compacted density of 1.7 g / cm³ at 1 ton. 3 ~1.95g / cm 3 .

[0032] In the above implementation process, the higher the compaction density of the positive electrode active material, the better it is for the compaction density of the positive electrode active material layer, and thus the better it is for the battery capacity. By controlling the compaction density of the first phosphate iron sodium pyrophosphate material and / or the second phosphate iron sodium pyrophosphate material at 1t to be 1.45 g / cm³, the battery capacity is improved. 3 ~1.7g / cm 3 1.7g / cm 3 ~1.95g / cm 3It can enable the positive electrode active material layer to have a better compaction density, thereby enabling the secondary battery to have a higher energy density.

[0033] In some embodiments, the first sodium iron pyrophosphate material and / or the second sodium iron pyrophosphate material includes Na. x R y P m O n Wherein, 3.5≤x≤4.5, 2.5≤y≤3.5, 3.5≤m≤4.5, 14.5≤n≤15.5, and R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, or Pb. Attached Figure Description

[0034] 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:

[0035] Figure 1 This application provides structural schematic diagrams of vehicles for some embodiments;

[0036] Figure 2 This is an exploded structural diagram of a secondary battery provided in some embodiments of this application;

[0037] Figure 3 This is a schematic diagram of the structure of a battery cell provided in some embodiments of this application;

[0038] Figure 4 Exploded views of a single battery cell provided in some embodiments of this application;

[0039] Figure 5 A flowchart illustrating a method for preparing a battery cell according to some embodiments of this application.

[0040] The reference numerals in the detailed embodiments are as follows:

[0041] 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

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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).

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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). 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, it is difficult to simultaneously achieve the desired compaction density and effective phase content in SAP materials. Typically, SAP materials with high effective phase content have lower compaction density, while SAP materials with high compaction density have lower effective phase content. As a result, the energy density of secondary batteries currently using SAP as the cathode active material still needs improvement.

[0052] Without any theoretical limitations, sodium iron pyrophosphate materials with low effective phase content usually have high compaction density and cycle life. Therefore, this application intends to use sodium iron pyrophosphate materials with low effective phase content as the base material, and combine them with sodium iron pyrophosphate materials with better effective phase content as the positive electrode active material, so that the entire positive electrode active material layer can have better performance in terms of compaction density, effective phase content and cycle life.

[0053] Based on the above considerations, in order to improve the compaction density of sodium iron pyrophosphate materials, this application proposes a sodium-ion secondary battery. The sodium-ion secondary battery includes a positive electrode sheet, which 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 first sodium iron pyrophosphate material and a second sodium iron pyrophosphate material. The effective phase mass content of the first sodium iron pyrophosphate material is not less than 90%, and the effective phase mass content of the second sodium iron pyrophosphate material is not less than 90%. The Dv50 of the second sodium iron pyrophosphate material is greater than that of the first sodium iron pyrophosphate material. The Dv50 of the positive active material is 5 μm to 9.5 μm, and the Dv100 of the positive active material is 27 μm to 31 μm.

[0054] In such a secondary battery, by using a combination of sodium iron pyrophosphate and sodium iron pyrophosphate materials with different effective phase contents and particle sizes as positive electrode active materials, and controlling the Dv50 of the entire positive electrode active material to meet a specific range, the positive electrode active material layer can simultaneously have a high effective phase content and compaction density, thereby improving the energy density of the secondary battery.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] Please refer to Figure 1 , Figure 1 This 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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 2 As 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] This application does not impose any special restrictions on the positive electrode, negative electrode, and separator.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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).

[0078] 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.

[0079] 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.

[0080] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0081] 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 difluorophosphate, sodium difluorooxalate borate, sodium dioxalate borate, sodium difluorodioxalate phosphate, and sodium tetrafluorooxalate phosphate.

[0082] 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.

[0083] 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.

[0084] This application provides a sodium-ion 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. The positive active material includes a first sodium iron pyrophosphate material and a second sodium iron pyrophosphate material. The effective phase mass content of the first sodium iron pyrophosphate material is not less than 90%, the effective phase mass content of the second sodium iron pyrophosphate material is not less than 90%, the Dv50 of the second sodium iron pyrophosphate material is greater than the Dv50 of the first sodium iron pyrophosphate material, and the Dv50 of the positive active material is 5 μm to 9.5 μm.

[0085] 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 effective phase refers to the portion that functions as a positive electrode active material; therefore, the higher the content of the effective phase, the greater the specific capacity of the material. The impurity phase refers to the portion that cannot function as a positive electrode active material, such as NFP, NNFP, and NaFeP2O7. The testing method for the effective and impurity phases is as follows: Weigh 10g of the material to be tested and perform diffraction analysis at 10-60° using an XRD diffractometer at a scan rate of 0.2° / min. After obtaining the data, consult the phase composition and CIF card. The main phase of the sample is NFPP, and the impurity phases are NFP, NNFP, and NaFeP2O7. Input the sample analysis data into the pre-set machine and instrument parameters (Pro file); import the found standard card (CIF) into the file for refinement; obtain the proportion of each phase; through refinement, the proportion of the effective phase NFPP can be identified.

[0086] The preparation of the first phosphate ferric phosphate sodium-based material can be as follows: S1, add 532.5g of high-purity ferric oxide, 1772.5g of sodium ferric phosphate, and 2010±100g of ferric phosphate to 8kg of deionized water, stir for 15min, mix evenly, then add 300g of glucose monohydrate, stir for 10min to completely dissolve, and then add the first mixture to a sand mill for grinding, controlling the grinding particle size D50=200nm, D90=3μm, to obtain the mixture; S2, pressurize the mixture obtained in step S1 under pressure. Spray drying is performed on a spray drying equipment with a feed pump speed of 30Hz, a spray inlet air temperature of 200℃, a spray outlet air temperature of 90℃, and a spray particle size of D50 = 4~7μm and D90 = 10μm to obtain a gray-black precursor powder; S3, under a nitrogen protective atmosphere, the gray-black precursor powder obtained in step S2 is sintered at a temperature of 500~550℃ for 8h. After natural cooling, it is taken out to obtain sodium iron pyrophosphate pyrophosphate material (this material has the characteristics of small particle size, high capacity, low compaction, and good circulation).

[0087] The preparation of sodium iron pyrophosphate pyrophosphate materials can be as follows: S1, add 532.5g of high-purity ferric oxide, 1772.5g of sodium iron pyrophosphate, and 1800±200g of ferric phosphate to 8kg of deionized water, stir for 15min, mix evenly, then add 300g of glucose monohydrate, stir for 10min to completely dissolve, and obtain the first mixture. Then add it to a sand mill for grinding, controlling the grinding particle size D50=400nm, D90=5μm, to obtain the mixture; S2, press the mixture obtained in step S1 under pressure. Spray drying is performed on a force-type spray equipment, wherein the feed pump speed is 30Hz, the spray inlet air temperature is 220℃, the spray outlet air temperature is 100℃, the spray particle size is D50=7~12μm, D90=12μm, and a gray-black precursor powder is obtained; S3, under a nitrogen protective atmosphere, the gray-black precursor powder obtained in step S2 is sintered at a temperature of 530~570℃ for 8h, and after natural cooling, it is taken out to obtain sodium iron pyrophosphate pyrophosphate material (this material has the characteristics of large particle size, low capacity, and high compaction).

[0088] Those skilled in the art will understand that, in the preparation of the first and second sodium iron pyrophosphate pyrophosphate materials, the particle size, effective phase content, and compaction density can be adjusted by modifying the amount of iron phosphate, the spray particle size D50, and the sintering temperature. For example, the spray particle size D50 is positively correlated with the particle size of the obtained sodium iron pyrophosphate pyrophosphate material; therefore, the particle size of the sodium iron pyrophosphate pyrophosphate pyrophosphate material can be adjusted by modifying the spray particle size D50.

[0089] Dv50 represents the particle size corresponding to 50% of the cumulative volumetric particle size distribution. The volumetric particle size distribution, also known as the differential particle size distribution, is a curve plotted with particle size on the x-axis and the differential distribution of particle size at different dimensions on the y-axis. It accurately reflects the particle size distribution characteristics of a material. A laser particle size analyzer can be used to determine the volumetric particle size distribution and plot interval particle size distribution curves. When measuring the particle size of the positive electrode active material layer in a positive electrode sheet, the positive electrode active material layer can be removed, immersed in the solvent NMP, and the binder in the positive electrode active material layer can be washed out, obtaining the powder material of the positive electrode active material layer. After drying the powder material, a Mastersizer 3000 laser particle size analyzer is used to detect the volumetric particle size distribution, and the Dv50 of the positive electrode active material can be obtained from the peaks in the volumetric particle size distribution.

[0090] It is difficult to achieve both high compaction density and high effective phase content in sodium iron pyrophosphate materials. Generally, sodium iron pyrophosphate materials with high effective phase content have low compaction density, while sodium iron pyrophosphate materials with high compaction density have low effective phase content. This solution uses a combination of first sodium iron pyrophosphate materials and second sodium iron pyrophosphate materials with different effective phase contents and particle sizes as positive electrode active materials, and controls the Dv50 of the entire positive electrode active material to meet a specific range. This allows the positive electrode active material layer to have both high effective phase content and high compaction density, thereby improving the energy density of the secondary battery.

[0091] For example, the effective phase mass content of the first phosphate (sodium iron pyrophosphate) material can be 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, etc., or any value within the range not less than 90%. Similarly, the effective phase mass content of the second phosphate (sodium iron pyrophosphate) material can be 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, etc., or any value within the range not less than 90%.

[0092] Those skilled in the art will understand that, apart from the difference in median particle size, the first phosphate iron sodium pyrophosphate material and the second phosphate iron sodium pyrophosphate material may have the same or different contents (such as effective phase content, doping status, structural formula, etc.).

[0093] In some embodiments of this application, the Dv50 of the positive electrode active material is 7 μm to 9.5 μm. By controlling the Dv50 of the positive electrode active material to be 7 μm to 9.5 μm, a basis is provided for the positive electrode active material layer to have a higher compaction density, thereby enabling the secondary battery to have a higher energy density.

[0094] In some embodiments of this application, the Dv100 of the positive electrode active material is 27 μm to 31 μm. By controlling the Dv100 of the positive electrode active material to be 27 μm to 31 μm, the compaction density of the positive electrode active material layer can be increased, thereby enabling the secondary battery to have a higher energy density.

[0095] Dv50 and Dv100 are the particle sizes corresponding to 50% and 100% of the cumulative amount in the volumetric particle size distribution map. The volumetric particle size distribution map, also known as the differential particle size distribution map, is a curve plotted with particle size on the x-axis and the differential distribution of particle size at different dimensions on the y-axis. It can accurately reflect the particle size distribution characteristics of a material. A laser particle size analyzer can be used to determine the volumetric particle size distribution of the material and plot the interval particle size distribution curve. When measuring the particle size of the positive electrode active material layer in the positive electrode sheet, the positive electrode active material layer can be removed, immersed in the solvent NMP, and the binder in the positive electrode active material layer can be washed out to obtain the powder material of the positive electrode active material layer. After drying the powder material, it is detected using a Mastersizer3000 laser particle size analyzer to obtain the volumetric particle size distribution map. The Dv50 and Dv100 of the positive electrode active material can be obtained from the peaks in the volumetric particle size distribution map.

[0096] For example, the Dv50 of the positive electrode active material can be 5μm, 5.5μm, 6μm, 6.5μm, 7μm, 7.5μm, 8μm, 8.5μm, 9μm, 9.5μm, etc., or it can be any value in the range of 5μm to 9.5μm.

[0097] In some embodiments of this application, the Dv100 of the positive electrode active material is 29 μm to 31 μm. By controlling the Dv100 of the positive electrode active material to be 29 μm to 31 μm, the compaction density of the positive electrode active material layer can be further increased, thereby enabling the secondary battery to have a higher energy density.

[0098] For example, the Dv100 of the positive electrode active material can be 27μm, 27.5μm, 28μm, 28.5μm, 29μm, 29.5μm, 30μm, 30.5μm, 31μm, etc., or it can be any value in the range of 27μm to 31μm.

[0099] In some embodiments of this application, the Dv50 of the first sodium iron pyrophosphate material is 3 μm to 6 μm. By controlling the median particle size Dv50 of the first sodium iron pyrophosphate material to be 3 μm to 6 μm, it is beneficial to have a higher compaction density of the positive electrode active material layer, thereby improving the energy density of the secondary battery.

[0100] For example, the Dv50 of the first phosphate iron sodium pyrophosphate material can be 3μm, 3.5μm, 4μm, 4.5μm, 5μm, 5.5μm, 6μm, etc., or it can be any value in the range of 3μm to 6μm.

[0101] In some embodiments of this application, the Dv50 of the second sodium iron pyrophosphate material is 6.5 μm to 13 μm. By controlling the median particle size Dv50 of the second sodium iron pyrophosphate material to be 6.5 μm to 13 μm, it is beneficial to have a higher compaction density of the positive electrode active material layer, thereby improving the energy density of the secondary battery.

[0102] For example, the Dv50 of sodium iron pyrophosphate pyrophosphate materials can be 6.5μm, 7μm, 7.5μm, 8μm, 8.5μm, 9μm, 9.5μm, 10μm, 10.5μm, 11μm, 11.5μm, 12μm, 12.5μm, 13μm, etc., or it can be any value in the range of 6.5μm to 13μm.

[0103] In some embodiments of this application, the compaction density of the first sodium iron pyrophosphate material at 1 ton is 1.45 g / cm³. 3 ~1.7g / cm 3 The higher the compaction density of the positive electrode active material, the better it is for the compaction density of the positive electrode active material layer, and thus the better it is for the battery capacity. By controlling the compaction density of the sodium iron pyrophosphate pyrophosphate material at 1 ton, it was found to be 1.45 g / cm³. 3 ~1.7g / cm 3 It can enable the positive electrode active material layer to have a better compaction density, thereby enabling the secondary battery to have a higher energy density.

[0104] For example, the compaction density of sodium iron pyrophosphate-based materials at 1 ton can be 1.45 g / cm³. 3 1.5g / cm 3 1.55g / cm 3 1.6g / cm 3 1.65g / cm 3 1.7g / cm 3 It can also be 1.45 g / cm³. 3 ~1.7g / cm 3 Any value within the range.

[0105] In some embodiments of this application, the compaction density of the second sodium iron pyrophosphate material at 1 ton is 1.7 g / cm³. 3 ~1.95g / cm 3A higher compaction density of the positive electrode active material is more beneficial to the compaction density of the positive electrode active material layer, and thus more beneficial to the battery capacity. By controlling the compaction density of the sodium iron pyrophosphate (Fe2PO4) material at 1 ton, it was found to be 1.7 g / cm³. 3 ~1.95g / cm 3 It can enable the positive electrode active material layer to have a better compaction density, thereby enabling the secondary battery to have a higher energy density.

[0106] For example, the compaction density of sodium iron pyrophosphate pyrophosphate-based materials at 1 ton can be 1.7 g / cm³. 3 1.75g / cm 3 1.8g / cm 3 1.85g / cm 3 1.9g / cm 3 1.95g / cm 3 It can also be 1.7 g / cm³. 3 ~1.95g / cm 3 Any value within the range.

[0107] In some embodiments of this application, the compaction density of the positive electrode active material layer is not less than 1.8 g / cm³. 3 For example, the compaction density of the positive electrode active material layer can be 1.8 g / cm³. 3 1.85g / cm 3 1.9g / cm 3 1.95g / cm 3 2g / cm 3 2.05g / cm 3 2.10 g / cm 3 2.15g / cm 3 2.2g / cm 3 etc., which can also be not less than 1.8 g / cm³. 3 Any value within the range.

[0108] In some embodiments of this application, the sodium iron pyrophosphate material includes Na. x R y P m O n Wherein, 3.5≤x≤4.5, 2.5≤y≤3.5, 3.5≤m≤4.5, 14.5≤n≤15.5, and R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, or Pb.

[0109] 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).

[0110] 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.

[0111] 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.

[0112] For example, Na x R y P m O n In this context, 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, etc., or any value within the range of 3.5 to 4.5. Na x R y P m O n In this context, 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 y P m O n In this context, m can be 3.8, 3.85, 3.9, 3.95, 4, 4.05, 4.1, 4.15, or 4.2, or any value within the range of 3.8 to 4.2. Na x R y P m O nThe value of 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.

[0113] In some embodiments of this application, the chemical formula of the positive electrode active material is Na. x R y P m O n / C, where 3.5≤x≤4.5, 2.5≤y≤3.5, 3.5≤m≤4.5, 14.5≤n≤15.5, and R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, or Pb.

[0114] Na x R y P m O n / C refers to Na x R y P m O n A complex with C.

[0115] By using Na, the positive electrode active material x R y P m O n Combining with C can reduce the Na content of the positive electrode active material. x R y P m O n 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.

[0116] Having introduced the materials and structure of secondary batteries, the following section will provide a detailed description of their preparation methods.

[0117] The preparation method of the secondary battery 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; the secondary battery can be composed of battery cell 20.

[0118] Figure 5Here is a flowchart illustrating the fabrication process of the battery cell 20 provided in some embodiments of this application, such as... Figure 5 As shown, the specific process for preparing battery cell 20 is as follows:

[0119] 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.

[0120] 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.

[0121] 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.

[0122] The coating method can be: scraping, roller coating, slot coating, etc., and this application does not limit it.

[0123] 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.).

[0124] S230, the positive electrode active material layer is rolled to obtain the positive electrode sheet.

[0125] S240, positive electrode, separator, negative electrode, separator, and so on are stacked in sequence to form a stacked electrode assembly 23.

[0126] 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.

[0127] 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.

[0128] Examples and Comparative Examples

[0129] Preparation of the positive electrode sheet

[0130] 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 positive electrode active material were used to prepare 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.

[0131] Preparation of the negative electrode sheet

[0132] Sodium metal sheet is used as the negative electrode.

[0133] Preparation of Electrolyte

[0134] 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.

[0135]

Isolation Film

[0136] Polyethylene film is used as the separation membrane.

[0137] [Preparation of button cells]

[0138] The prepared positive electrode, negative electrode, separator and electrolyte are assembled into CR2430 button cell in an argon-protected glove box.

[0139] The main parameter controls for Examples 1 to 12 and Comparative Examples 1 to 4 are shown in the table below:

[0140]

[0141]

[0142] In the table, " / " indicates that the substance is not present.

[0143] The performance of the positive electrode sheets and the batteries composed thereof provided in each embodiment and comparative example was tested. The performance testing specifically included:

[0144] Compacted density test: The compacted density of the positive electrode active material layer of the positive electrode sheet is obtained by calculation. Take a positive electrode sheet with a unit area of ​​S and weigh it as M1. The mass of the aluminum foil under a unit area of ​​S is M2. Measure the thickness of the positive electrode sheet H1 and the thickness of the aluminum foil H2. Then the compacted density of the positive electrode active material layer of the positive electrode sheet = (M1-M2) / ((H1-H2)×S).

[0145] Energy density test: After the secondary battery is left to stand for 12 hours, it is discharged at a constant current of 0.05C until it reaches 0.005V, and then left to stand for 10 minutes. It is then discharged at a constant current of 50μA until it reaches 0.005V, and left to stand for 10 minutes. Finally, it is discharged at a constant current of 10μA until it reaches 0.005V. Then, it is charged at a constant current of 0.1C until it reaches 2V. The charging capacity is recorded. The ratio of the charging capacity to the weight of the secondary battery is the energy density of the secondary battery.

[0146] Battery cycle life test: Follow these steps: a) Charge the battery at a constant current of 1C to 4.2V, 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 2.8V; c) Repeat the charge-discharge cycle until the battery capacity drops to 80% of the initial capacity, then stop the test and record the number of cycles.

[0147] The test results are shown in the table below:

[0148]

[0149]

[0150] As can be seen from the table above, the positive electrode active material layer prepared by the positive electrode active material formed by combining the first phosphate iron sodium pyrophosphate material and the second phosphate iron sodium pyrophosphate material provided in the embodiments of this application has a high compaction density and a high effective phase content, thereby having a higher energy density of over 80Wh / kg, and a high cycle life of over 4000 cycles to reach 80% capacity.

[0151] Comparison of data from Examples 1 to 3 and Comparative Examples 1 to 2 shows that as the Dv50 of the positive electrode active material gradually increases, the compaction density of the positive electrode active material layer first increases and then decreases. When the Dv50 of the positive electrode active material is controlled within the range of 5 μm to 9.5 μm, the compaction density of the positive electrode active material layer is 1.81 g / cm³. 3 above.

[0152] A comparison of data from Examples 4 to 6 shows that as the Dv50 of the first sodium iron pyrophosphate material gradually increases, the compaction density, energy density, and cycle life of the positive electrode active material layer all exhibit a trend of first improving and then deteriorating. Controlling the Dv50 of the first sodium iron pyrophosphate material within the range of 3 μm to 6 μm and the compaction density of the positive electrode active material layer within the range of 1.75 g / cm³ is optimal. 3 The above requirements include an energy density of 80Wh / kg or higher and a number of revolutions of 4000 or higher to reach 80% capacity.

[0153] A comparison of the data from Examples 7 to 9 shows that as the Dv50 of the second phosphate iron sodium pyrophosphate material gradually increases within the range of 6.5 μm to 11 μm, the compaction density, energy density, and cycle life of the positive electrode active material layer all show a gradual improvement trend. Controlling the Dv50 of the second phosphate iron sodium pyrophosphate material to above 6.5 μm allows the compaction density of the positive electrode active material layer to be around 1.81 g / cm³. 3 The above requirements include an energy density of 82Wh / kg or higher and a number of revolutions of 4000 or higher to reach 80% capacity.

[0154] Comparison of data from Examples 10 and 11 shows that as the effective phase content in the first phosphate iron sodium pyrophosphate material gradually increases, the energy density of the secondary battery gradually increases. By controlling the effective phase content in the first phosphate iron sodium pyrophosphate material to be above 95%, the energy density of the secondary battery can be above 94Wh / kg.

[0155] 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 sodium-ion secondary battery, characterized in that, The sodium-ion secondary battery includes a positive electrode sheet, which 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 first sodium iron pyrophosphate material and a second sodium iron pyrophosphate material. The effective phase mass content of the first sodium iron pyrophosphate material is not less than 95%, and the effective phase mass content of the second sodium iron pyrophosphate material is 85% to 93%. The Dv50 of the second sodium iron pyrophosphate material is greater than that of the first sodium iron pyrophosphate material, and the Dv50 of the positive active material is 5 μm to 9.5 μm.

2. The sodium-ion secondary battery according to claim 1, characterized in that, The Dv50 of the positive electrode active material is 7 μm to 9.5 μm; and / or The Dv100 of the positive electrode active material is 27 μm to 31 μm.

3. The sodium-ion secondary battery according to any one of claims 1 to 2, characterized in that, The Dv100 of the positive electrode active material is 29 μm to 31 μm.

4. The sodium-ion secondary battery according to any one of claims 1 to 3, characterized in that, The first sodium iron pyrophosphate material has a Dv50 of 3 μm to 6 μm; and / or The Dv50 of the second phosphate iron sodium pyrophosphate material is 6.5 μm to 13 μm.

5. The sodium-ion secondary battery according to any one of claims 1 to 4, characterized in that, The first sodium iron pyrophosphate material has a compacted density of 1.45 g / cm³ at 1 ton. 3 ~1.7g / cm 3 ; and / or The second type of phosphate pyrophosphate sodium material has a compacted density of 1.7 g / cm³ at 1 ton. 3 ~1.95g / cm 3 .

6. The sodium-ion secondary battery according to any one of claims 1 to 5, characterized in that, The compaction density of the positive electrode active material layer is not less than 1.8 g / cm³. 3 .

7. The sodium-ion secondary battery according to any one of claims 1 to 6, characterized in that, The first sodium iron pyrophosphate material and / or the second sodium iron pyrophosphate material include Na x R y P m O n Wherein, 3.5≤x≤4.5, 2.5≤y≤3.5, 3.5≤m≤4.5, 14.5≤n≤15.5, and R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, or Pb.

8. An electrical device, characterized in that, The electrical device includes a sodium-ion secondary battery as described in any one of claims 1 to 7.

9. A positive electrode active material, characterized in that, The positive electrode active material includes a first sodium iron pyrophosphate material and a second sodium iron pyrophosphate material. The effective phase mass content of the first sodium iron pyrophosphate material is not less than 95%, and the effective phase mass content of the second sodium iron pyrophosphate material is not less than 90%. The Dv50 of the second sodium iron pyrophosphate material is greater than that of the first sodium iron pyrophosphate material, and the Dv50 of the positive electrode active material is 5 μm to 9.5 μm.

10. The positive electrode active material according to claim 9, characterized in that, The Dv50 of the positive electrode active material is 7 μm to 9.5 μm; and / or The positive electrode active material has a Dv100 of 27 μm to 31 μm; and / or The first sodium iron pyrophosphate material has a Dv50 of 3 μm to 6 μm; and / or The Dv50 of the second sodium iron pyrophosphate material is 6.5 μm to 13 μm; and / or The first sodium iron pyrophosphate material has a compacted density of 1.45 g / cm³ at 1 ton. 3 ~1.7g / cm 3 ; and / or The second type of phosphate pyrophosphate sodium material has a compacted density of 1.7 g / cm³ at 1 ton. 3 ~1.95g / cm 3 ; and / or The first sodium iron pyrophosphate material includes Na x R y P m O n Wherein, 3.5≤x≤4.5, 2.5≤y≤3.5, 3.5≤m≤4.5, 14.5≤n≤15.5, and R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, or Pb; and / or The second type of phosphate pyrophosphate sodium material includes Na x R y P m O n Wherein, 3.5≤x≤4.5, 2.5≤y≤3.5, 3.5≤m≤4.5, 14.5≤n≤15.5, and R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, or Pb.

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