Positive electrode active material and preparation method therefor, positive electrode sheet, battery and electric device

Abstract

The present invention belongs to the technical field of batteries. Provided are a positive electrode active material and a preparation method therefor, a positive electrode sheet, a battery and an electric device. The positive electrode active material comprises an active material body and a boride, wherein the active material body comprises a plurality of agglomerated active particles; the inner core of each active particle comprises NaaRb(PO4)2P2O7, where a is 3.5-4.5, b is 2.5-3.5, and R comprises at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W and Pb; and the shell layer of each active particle comprises a carbon coating layer. By converting residual alkali on the surface of the positive electrode active material into a boride, a reduction in residual alkali on the surface of the positive electrode active material is achieved. The boride has a more stable structure, and is not prone to reacting during the production and use of a battery, thereby ameliorating the problems of gas production, etc., of the battery.

Description

Positive electrode active material and preparation method thereof, positive electrode sheet, battery and electrical device

[0001] Cross-references

[0002] This application claims priority to the Chinese invention patent application with application number 2024103709051 filed on March 28, 2024, and invention name “A positive electrode active material and its preparation method, positive electrode sheet, battery and electrical device”, the contents of which are incorporated herein by reference. Technical Field

[0003] The present application relates to the field of battery technology, and in particular to a positive electrode active material and a preparation method thereof, a positive electrode sheet, a battery, and an electrical device. Background Art

[0004] Polyanionic sodium cathode active materials, such as sodium ferric pyrophosphate (Na4Fe3(PO4)2P2O7), are considered the most promising cathode materials for sodium-ion batteries due to their low cost, environmental friendliness, high theoretical capacity (129 mAh / g), high average operating voltage (3.1 VS. Na+ / Na), and low volume expansion (less than 4%). However, they are prone to gassing during use, leading to battery safety and quality issues.

[0005] Summary of the Invention

[0006] In view of the above problems, the present application provides a positive electrode active material and a preparation method thereof, a positive electrode plate, a battery and an electrical device, which can improve the material flatulence of polyanion sodium positive electrode active materials.

[0007] In the first aspect, the present application provides a positive electrode active material, the positive electrode active material includes an active material body and a boride attached to the active material body; the active material body includes a plurality of agglomerated active particles, the active particles have a core-shell structure, and the core of the active particles includes Na a R b (PO4)2P2O7, wherein a is 3.5-4.5, b is 2.5-3.5, R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W and Pb, and the shell layer of the active particles includes a carbon coating layer.

[0008] In the technical solution of the present embodiment, residual alkali on the surface of the positive electrode active material is converted into a boride, thereby reducing the residual alkali on the surface of the positive electrode active material. Furthermore, the boride has a more stable structure and is less likely to react during battery production and use, thereby improving battery gassing and other issues.

[0009] In some embodiments, the mass percentage of the boride in the positive electrode active material is 0.1% to 2.5%.

[0010] In the above implementation process, boride is formed by the reaction of residual alkali on the surface of the positive electrode active material. The higher the mass percentage of boride in the positive electrode active material, the less residual alkali on the surface of the positive electrode active material, which is beneficial to improving battery gas production. Controlling the mass percentage of boride in the positive electrode active material to 0.1% to 2.5% can fully react with most residual alkali on the surface of the positive electrode active material, thereby improving battery gas production.

[0011] In some embodiments, the mass percentage of the boride in the positive electrode active material is 0.1% to 1%.

[0012] In the above implementation process, by controlling the mass proportion of boride in the positive electrode active material to 0.1% to 1%, it is possible to better adapt to the complete reaction of residual alkali on the surface of most positive electrode active materials.

[0013] In some embodiments, the boride comprises Na3BO3, Na4B2O5, NaBO2, Na2B4O7, Na 12 B 26 O 45 、NaB3O5、Na2B8O 13 and at least one of B2O3.

[0014] In some embodiments, the gram capacity of the positive electrode active material is 100 to 129 mAh / g; and / or

[0015] The mass content of residual alkali in the positive electrode active material is less than 1.5%.

[0016] In a second aspect, the present application provides a method for preparing a positive electrode active material, the method comprising:

[0017] Get the intermediate powder Na 4+x R 3-y (PO4)2P2O7 / C, wherein 0<x<0.5, 0≤y≤0.5, R comprises at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W and Pb; intermediate powder Na 4+x R 3-y (PO4)2P2O7 / C includes multiple agglomerated active particles with a core-shell structure. The core of the active particles includes Na a R b(PO4)2P2O7, wherein a is 3.5-4.5, b is 2.5-3.5, R comprises at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W and Pb, and the shell layer of the active particles comprises a carbon coating layer;

[0018] The intermediate powder Na 4+x R 3-y (PO4)2P2O7 / C and solid boric acid are mixed and sintered twice to make the intermediate powder Na 4+x R 3-y At least part of the residual alkali of (PO4)2P2O7 / C reacts to form a boride to obtain a positive electrode active material.

[0019] In the technical solution of the embodiment of the present application, solid boric acid and intermediate powder Na 4+x R 3-y (PO4)2P2O7 / C is sintered twice to make the intermediate powder Na 4+x R 3-y The residual alkali on the surface of (PO4)2P2O7 / C is converted into boride, which reduces the residual alkali on the surface of the positive electrode active material. The structure of the boride is more stable and is not easy to react during the production and use of the battery, thus improving the problem of battery gas production. At the same time, the acid is in solid form and the reaction process only reacts with the intermediate powder Na 4+x R 3-y (PO4)2P2O7 / C contacts the surface and does not react with the effective components inside that can provide capacity; moreover, the secondary sintering will cause the intermediate powder Na 4+x R3- y Na in the residual alkali on the surface of (PO4)2P2O7 / C + Re-entering the internal lattice of the effective components that can provide capacity is beneficial to the gram capacity of the positive electrode active material.

[0020] In some embodiments, the secondary sintering temperature is 400-600° C.; and / or

[0021] The secondary sintering time is 2 to 20 hours.

[0022] In the above implementation process, the higher the sintering temperature and the longer the sintering time, the more favorable it is for the intermediate powder Na 4+x R 3- y The residual alkali on the surface of (PO4)2P2O7 / C is converted into boride, thereby reducing the gas production of the battery; the lower the sintering temperature and the shorter the sintering time, the more it can reduce the Na 4+x R 3-y The release of sodium from (PO4)2P2O7 / C reduces the Na4+x R 3-y Na in (PO4)2P2O7 / C a R b The structure of (PO4)2P2O7 is likely to be destroyed, which is beneficial to maintaining the specific capacity of the positive electrode active material. By controlling the secondary sintering temperature to 400-600°C and the time to 2-20 hours, it is possible to balance the battery's gas production and the maintenance of the specific capacity of the positive electrode active material.

[0023] In some embodiments, the secondary sintering temperature is 450-550° C.; and / or

[0024] The secondary sintering time is 6 to 15 hours.

[0025] In the above implementation process, by controlling the secondary sintering temperature to 450-550° C. and the time to 6-15 hours, it is possible to better balance the gas production of the battery and the maintenance of the gram capacity of the positive electrode active material.

[0026] In some embodiments, the solid boric acid has a median particle size Dv50 of 0.2 to 10 μm.

[0027] In the above implementation process, the smaller the particle size of solid boric acid, the more favorable it is for the reaction with Na 4+x R 3-y Thorough mixing of (PO4)2P2O7 / C facilitates the complete conversion of residual alkali on its surface into boride, reducing battery gas production. Larger solid boric acid particle sizes facilitate preparation, leading to cost savings. Controlling the solid boric acid's median particle size (Dv50) to 0.2-10 μm effectively balances battery gas production with reduced production costs.

[0028] In some embodiments, the intermediate powder Na is obtained 4+x R 3-y (PO4)2P2O7 / C includes:

[0029] A sodium source, an R source, a phosphorus source, and a carbon source are mixed to obtain a mixture, wherein the molar ratio of Na to R in the mixture is (4+x):(3-y), wherein 0<x<0.5, 0≤y≤0.5, and R comprises at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, and Pb;

[0030] The mixed material is sintered once to obtain the intermediate powder Na 4+x R 3-y (PO4)2P2O7 / C.

[0031] In the above implementation process, by reducing the amount of R source added, the intermediate powder Na4+x R 3-y The reduction of NaFePO4 and other impurity contents in (PO4)2P2O7 / C, and the increase of Na a R b The content of (PO4)2P2O7 is beneficial to improving the gram capacity of the positive electrode active material.

[0032] In some embodiments, the carbon source comprises a first carbon source and a second carbon source, the first carbon source comprises an inorganic carbon source, and the second carbon source comprises an organic carbon source.

[0033] In the above implementation process, by using both inorganic carbon source and organic carbon source to achieve coating, it is helpful for the crystal growth of the positive electrode active material, making the lattice more regular. At the same time, it can also reduce the content of impurities in the positive electrode active material and increase the gram capacity of the positive electrode active material. In addition, it can also achieve better coating of the positive electrode active material and reduce the Na a R b The possibility of (PO4)2P2O7 reacting with water in the environment is beneficial to the cycle stability of the positive electrode active material.

[0034] In some embodiments, the first carbon source comprises at least one of graphite, carbon black, carbon nanotubes, and graphene; and / or

[0035] The second carbon source includes at least one of sucrose, glucose, citric acid, starch, cyclodextrin, asphalt and PEG.

[0036] In a third aspect, the present application provides a positive electrode plate, which includes a positive electrode active material layer, and the positive electrode active material layer includes the positive electrode active material provided in the first aspect or the positive electrode active material prepared by the method provided in the second aspect.

[0037] In a fourth aspect, the present application provides a battery, which includes the positive electrode plate provided in the third aspect.

[0038] In a fifth aspect, the present application provides an electrical device, which includes the battery provided in the fourth aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiment below. The accompanying drawings are for illustration purposes only and are not to be considered as limiting the present application. The same reference numerals are used throughout the drawings to represent the same components. In the drawings:

[0040] FIG1 is a schematic structural diagram of a vehicle provided in some embodiments of the present application;

[0041] FIG2 is a schematic diagram of the exploded structure of a secondary battery provided in some embodiments of the present application;

[0042] FIG3 is a schematic structural diagram of a battery cell provided in some embodiments of the present application;

[0043] FIG4 is an exploded view of a battery cell provided in some embodiments of the present application;

[0044] FIG5 is a scanning electron microscope image of a positive electrode active material provided in some embodiments of the present application;

[0045] FIG6 is a flow chart of a method for preparing a positive electrode active material according to some embodiments of the present application;

[0046] FIG7 is a schematic diagram of a first structure of a positive electrode sheet provided in some embodiments of the present application;

[0047] FIG8 is a second structural schematic diagram of a positive electrode sheet provided in some embodiments of the present application;

[0048] FIG9 is a flow chart of the preparation of battery cells provided in some embodiments of the present application.

[0049] The figure numbers in the specific implementation manner are as follows: 1000-vehicle; 100-secondary battery; 200-motor; 300-controller; 10-housing; 11-accommodating space; 12-first part; 13-second part; 20-battery cell; 21-housing; 211-opening; 22-end cover assembly; 221-end cover; 222-electrode terminal; 23-electrode assembly; 231-positive electrode sheet; 2311-positive current collector; 2312-positive active material layer; 24-current collecting component; 25-insulating protection part. DETAILED DESCRIPTION

[0050] The following embodiments of the technical solution of the present application will be described in detail with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application and are therefore only examples and are not intended to limit the scope of protection of the present application.

[0051] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above-mentioned figure descriptions are intended to cover non-exclusive inclusions.

[0052] In the description of the embodiments of this application, the technical terms "first" and "second" are used only to distinguish different objects and should not be understood to indicate or imply relative importance or implicitly specify the quantity, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, the meaning of "plurality" is more than two, unless otherwise clearly and specifically defined.

[0053] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0054] In the description of the embodiments of this application, the term "and / or" is simply a description of the association relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent the following three situations: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in this document generally indicates that the associated objects are in an "or" relationship.

[0055] In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0056] In the description of the embodiments of the present application, the technical terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the embodiments of the present application.

[0057] In the description of the embodiments of the present application, unless otherwise expressly specified or limited, technical terms such as "installed," "connected," "connected," and "fixed" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integration; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; internal connections between two components or interactions between two components. Those skilled in the art can understand the specific meanings of the above terms in the embodiments of the present application based on specific circumstances.

[0058] Currently, market developments indicate that power batteries are becoming increasingly widely used. They are not only used in energy storage systems such as hydropower, thermal, wind, and solar power plants, but are also widely used in electric vehicles like electric bicycles, electric motorcycles, and electric vehicles, as well as in military equipment and aerospace. As power battery applications continue to expand, market demand is also growing.

[0059] Power batteries can be sodium ion batteries. The reserves of sodium are relatively abundant, which can significantly reduce the cost of batteries. Therefore, sodium ion batteries have broad application prospects. For example, sodium ion batteries can be used in portable electronic devices, electric vehicles and other fields. Polyanion sodium positive electrode active materials such as sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) have the advantages of low cost, environmental friendliness, high theoretical capacity (129mAh g -1 ), high average operating voltage (3.1 VS. Na+ / Na), and low volume expansion (less than 4%), making it considered the most promising cathode material for sodium-ion batteries. However, during its production process, due to factors such as the process and the proportion of the material, residual sodium may remain in the material. At the same time, sodium in the bulk phase is easily released, resulting in residual alkali such as sodium carbonate, sodium bicarbonate, and sodium ions on the material surface, causing the material to swell, which in turn leads to battery safety risks and quality issues.

[0060] Based on the above considerations, in order to improve the material flatulence of the polyanion sodium positive electrode active material, the present application proposes a positive electrode active material, the positive electrode active material includes an active material body and a boride attached to the active material body; the active material body includes a plurality of agglomerated active particles, the active particles have a core-shell structure, and the core of the active particles includes Na a R b (PO4)2P2O7, wherein a is 3.5-4.5, b is 2.5-3.5, R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W and Pb, and the shell layer of the active particles includes a carbon coating layer.

[0061] In such positive electrode active materials, residual alkali on the surface of the positive electrode active material is reduced by converting it into a boride. Furthermore, the boride has a more stable structure and is less likely to react during battery production and use, thus improving battery gassing and other issues.

[0062] The positive electrode active material can be used to prepare the positive electrode active material layer of a positive electrode sheet, which can be used to prepare an electrode assembly. The electrode assembly can be used, but is not limited to, in electrical devices such as vehicles, ships, or aircraft. The power supply system of the electrical device can be composed of a battery or the like disclosed in this application.

[0063] The present invention provides an electric device that uses a battery as a power source. The electric device may be, but is not limited to, a mobile phone, a tablet, a laptop computer, an electric toy, an electric tool, a battery-powered vehicle, an electric car, a ship, a spacecraft, etc. The electric toy may include a fixed or mobile electric toy, such as a game console, an electric car toy, an electric ship toy, and an electric airplane toy, etc. The spacecraft may include an airplane, a rocket, a space shuttle, and a spacecraft, etc.

[0064] For the convenience of description, the following embodiments are described by taking a vehicle 1000 as an example of an electrical device according to an embodiment of the present application.

[0065] Please refer to Figure 1, which is a structural schematic diagram of a vehicle 1000 provided in some embodiments of the present application. The vehicle 1000 can be a fuel vehicle, a gas vehicle or a new energy vehicle. The new energy vehicle can be a pure electric vehicle, a hybrid vehicle or an extended-range vehicle, etc. A secondary battery 100 is provided inside the vehicle 1000, and the secondary battery 100 can be provided at the bottom, head or tail 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 an 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 power the motor 200, for example, for starting, navigating and driving the vehicle 1000.

[0066] In some embodiments of the present application, the secondary battery 100 can serve not only as an operating power source for the vehicle 1000 , but also as a driving power source for the vehicle 1000 , replacing or partially replacing fuel or natural gas to provide driving power for the vehicle 1000 .

[0067] In this application, a secondary battery 100 may refer to a single battery cell 20, or it may refer to a single physical module comprising multiple battery cells 20 to provide higher voltage and capacity, which may be in the form of a battery pack, a battery module, etc. The secondary battery 100 may include a housing 10 for enclosing the multiple battery cells 20. The housing 10 may prevent liquids or other foreign matter from affecting the charging or discharging of the battery cells 20.

[0068] FIG2 is a schematic diagram of the exploded structure of a secondary battery 100 provided in some embodiments of the present application. Referring to FIG2 , the secondary battery 100 includes a housing 10 and a battery cell 20 , wherein the battery cell 20 is housed in the housing 10 .

[0069] The housing 10 is used to provide a storage space 11 for the battery cells 20. In some embodiments, the housing 10 may include a first portion 12 and a second portion 13, which overlap to define the storage space 11 for accommodating the battery cells 20. Of course, the connection between the first portion 12 and the second portion 13 can be sealed by a seal (not shown), such as a sealing ring, sealant, or the like.

[0070] The first portion 12 and the second portion 13 can have various shapes, such as a rectangular parallelepiped, a cylinder, etc. The first portion 12 can be a hollow structure with an opening on one side to form a receiving cavity for accommodating the battery cell 20. The second portion 13 can also be a hollow structure with an opening on one side to form a receiving cavity for accommodating the battery cell 20. The open side of the second portion 13 covers the open side of the first portion 12, thereby forming the box 10 with the receiving space 11. Of course, as shown in Figure 2, the first portion 12 can also be a hollow structure with an opening on one side, and the second portion 13 can be a plate-like structure. The second portion 13 covers the open side of the first portion 12, thereby forming the box 10 with the receiving space 11.

[0071] In the secondary battery 100, there are multiple battery cells 20. These multiple battery cells 20 can be connected in series, in parallel, or in a hybrid connection. Hybrid connection refers to a combination of series and parallel connections among the multiple battery cells 20. Multiple battery cells 20 can be directly connected in series, in parallel, or in a hybrid connection, and then the entire battery cell 20 is housed within the housing 10. Alternatively, multiple battery cells 20 can be first connected in series, in parallel, or in a hybrid connection to form a battery module, and then the multiple battery modules are further connected in series, in parallel, or in a hybrid connection to form an entire battery module, which is then housed within the housing 10. The battery cells 20 can be cylindrical, flat, rectangular, or in other shapes. Figure 2 exemplifies a case where the battery cells 20 are square.

[0072] In some embodiments, the secondary battery 100 may further include a busbar component (not shown), and the multiple battery cells 20 may be electrically connected via the busbar component to achieve series connection, parallel connection, or hybrid connection of the multiple battery cells 20 .

[0073] Figure 3 is a schematic diagram of the structure of a battery cell 20 provided in some embodiments of the present application, and Figure 4 is an exploded view of a battery cell 20 provided in some embodiments of the present application. Referring to Figures 3 and 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, and the electrode assembly 23 is accommodated within the housing 21. The end cap assembly 22 is used to seal the opening 211.

[0074] The shape of the housing 21 can be determined according to the specific shape of the electrode assembly 23. For example, if the electrode assembly 23 is a rectangular parallelepiped structure, the housing 21 can be a rectangular parallelepiped structure. Figures 3 and 4 exemplarily show the case where the housing 21 and the electrode assembly 23 are square.

[0075] The shell 21 may be made of a variety of materials, such as copper, iron, aluminum, stainless steel, aluminum alloy, etc., and the embodiment of the present application does not impose any special restrictions on this.

[0076] The end cap assembly 22 includes an end cap 221 and an electrode terminal 222. The end cap assembly 22 is used to seal the opening 211 of the outer shell 21 to form a sealed installation space (not shown), which is used to accommodate the electrode assembly 23. The installation space is also used to accommodate an electrolyte, such as an electrolyte. The end cap assembly 22 serves as a component for outputting the electrical energy of the electrode assembly 23. The electrode terminal 222 in the end cap assembly 22 is used to electrically connect to the electrode assembly 23, that is, the electrode terminal 222 is electrically connected to the tab of the electrode assembly 23. For example, the electrode terminal 222 is connected to the tab through the current collecting member 24 to achieve electrical connection between the electrode terminal 222 and the tab.

[0077] It should be noted that the number of openings 211 of the outer shell 21 can be one or two. If the number of openings 211 of the outer shell 21 is one, the number of 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 respectively used to electrically connect to the positive electrode tab and the negative electrode tab of the electrode assembly 23. If the number of openings 211 of the outer shell 21 is two, for example, the two openings 211 are provided on opposite sides of the outer shell 21, the number of end cap assemblies 22 can also be two, and the two end cap assemblies 22 are respectively covered on the two openings 211 of the outer shell 21. In this case, the electrode terminal 222 in one end cap assembly 22 can be a positive electrode terminal 222, which is used to electrically connect to the positive electrode tab of the electrode assembly 23; and the electrode terminal 222 in the other end cap assembly 22 can be a negative electrode terminal 222, which is used to electrically connect to the negative electrode sheet of the electrode assembly 23.

[0078] In some embodiments, as shown in FIG4 , the battery cell 20 may further include an insulating protective member 25 secured to the 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 a tape adhered to the periphery of the electrode assembly 23. In some embodiments, there are multiple electrode assemblies 23, and the insulating protective member 25 is disposed around the periphery of the multiple electrode assemblies 23, forming the multiple electrode assemblies 23 into a single integrated structure to maintain structural stability.

[0079] The electrode assembly 23 includes a positive electrode sheet 231, a negative electrode sheet, and a separator. The positive electrode sheet 231 includes a positive current collector 2311 and a positive active material layer 2312. The positive active material layer 2312 is coated on the surface of the positive electrode collector 2311. The positive electrode collector 2311 not coated with the positive active material layer 2312 protrudes from the positive electrode collector 2311 coated with the positive active material layer 2312. The positive electrode collector 2311 not coated with the positive active material layer 2312 serves as a positive electrode tab.

[0080] The negative electrode plate includes a negative electrode current collector and a negative electrode active material layer. The negative electrode active material layer is coated on the surface of the negative electrode current collector. The negative electrode current collector not coated with the negative electrode active material layer protrudes from the negative electrode current collector coated with the negative electrode active material layer. The negative electrode current collector not coated with the negative electrode active material layer serves as a negative electrode tab. The material of the negative electrode current collector may be copper, and the negative electrode active material may be carbon or silicon, etc. In order to ensure that a large current passes without melting, the number of positive electrode tabs is multiple and stacked together, and the number of negative electrode tabs is multiple and stacked together. The material of the isolation membrane may be PP (polypropylene, polypropylene) or PE (polyethylene, polyethylene), etc. In addition, the electrode assembly 23 may be a laminated electrode assembly, but the embodiments of the present application are not limited to this.

[0081] FIG5 is a scanning electron microscope image of a positive electrode active material provided in some embodiments of the present application; Referring to FIG5, an embodiment of the present application provides a positive electrode active material, the positive electrode active material includes an active material body and a boride attached to the active material body; the active material body includes a plurality of agglomerated active particles, the active particles have a core-shell structure, and the core of the active particles includes Na a R b (PO4)2P2O7, wherein a is 3.5-4.5, b is 2.5-3.5, R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W and Pb, and the shell layer of the active particles includes a carbon coating layer.

[0082] The amount and location of boride attached to the active material are determined to a certain extent by the residual alkali on the surface during its preparation. The boride may be attached to all surfaces of the active material to form a coating layer, or it may be attached in the form of spaced islands on the active material.

[0083] The active material body includes a plurality of agglomerated active particles, which means that the active material body is formed by agglomerating a plurality of active particles. The active material body can be understood as a secondary particle to a certain extent, while the active particles can be understood as primary particles.

[0084] It should be noted that the above definition of a includes the molar content of Na under different charge and discharge states of the battery (usually the battery voltage is between 2-5V).

[0085] It is understandable that the battery will be accompanied by the deintercalation and consumption of sodium (Na) during the charge and discharge process, and the content of Na in the positive electrode plate is different when the battery is discharged to different states, wherein the content of Na can be measured by molar content, but is not limited to this. At the same time, the positive electrode material is applied to the positive electrode plate in the battery system, and after the charge and discharge cycle, the content of Na in the positive electrode material contained in the plate will usually change. In the enumeration of positive electrode materials in this application, unless otherwise specified, the content of Na is the initial state of the material. Regarding "the content of Na is the initial state of the material", the initial state of the material refers to the state before the material is added to the positive electrode slurry. It is understandable that new materials obtained by appropriate modification on the basis of the listed positive electrode materials are also within the scope of positive electrode materials. The aforementioned appropriate modification refers to acceptable modification methods for positive electrode materials, and non-limiting examples include coating modification. Illustratively, the value of a can be 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.1, 4.2, 4.3, 4.4 or 4.5, etc. It can also be any value within the range of 3.5 to 4.5.

[0086] It is understandable that the molar content b of R and the molar content of Na are similar and will not be further described here.

[0087] Exemplarily, b can be 2.5, 2.6, 2.7, 2.8, 2.9, 3, 3.1, 3.2, 3.3, 3.4 or 3.5, etc. It can also be any value within the range of 2.5 to 3.5.

[0088] This positive electrode active material reduces residual alkali on the surface of the positive electrode active material by converting it into a boride. Furthermore, the boride has a more stable structure and is less likely to react during battery production and use, thus improving battery gassing and other issues.

[0089] In the technical solutions of some embodiments of the present application, the mass proportion of boride in the positive electrode active material is 0.1% to 2.5%.

[0090] The mass proportion of boride in the positive electrode active material refers to the value obtained by dividing the mass of the boride by the mass of the entire positive electrode active material, wherein the mass of the boride can be tested by inductively coupled plasma emission spectrometry (ICP) to determine the B element content and then calculated by molecular formula.

[0091] Borides are formed by the reaction of residual alkali on the surface of the positive electrode active material. The higher the mass percentage of boride in the positive electrode active material, the less residual alkali on the surface of the positive electrode active material, which in turn helps improve battery gas production. Controlling the mass percentage of boride in the positive electrode active material to 0.1% to 2.5% can fully react with most residual alkali on the surface of the positive electrode active material, thereby improving battery gas production.

[0092] For example, the mass proportion of boride in the positive electrode active material can be 0.1%, 0.3%, 0.5%, 0.7%, 0.9%, 1.1%, 1.3%, 1.5%, 1.7%, 1.9%, 2.1%, 2.3% or 2.5%, etc., and it can also be any value in the range of 0.1% to 2.5%.

[0093] Furthermore, the mass proportion of the boride in the positive electrode active material is 0.1% to 1%. By controlling the mass proportion of the boride in the positive electrode active material to 0.1% to 1%, it is possible to better adapt to the complete reaction of the residual alkali on the surface of most positive electrode active materials.

[0094] In the technical solutions of some embodiments of the present application, borides include Na3BO3, Na4B2O5, NaBO2, Na2B4O7, Na 12 B 26 O 45 、NaB3O5、Na2B8O 13 and at least one of B2O3.

[0095] In the technical solutions of some embodiments of the present application, the mass proportion of the carbon coating layer in the positive electrode active material is 1% to 2%.

[0096] The mass proportion of the carbon coating layer in the positive electrode active material refers to the value of the mass of the carbon coating layer divided by the mass of the entire positive electrode active material, wherein the mass of the carbon coating layer can be measured by infrared absorption method after combustion in a high-frequency induction furnace, specifically: burning in an oxygen flow to convert carbon into CO and / or CO2, and measuring using the infrared absorption spectrum of CO2 and CO in the oxygen flow.

[0097] The higher the mass proportion of the carbon coating layer in the positive electrode active material, the more favorable it is for Na a R b (PO4)2P2O7 is isolated from the outside world, reducing the possibility of it reacting with external water to produce residual alkali, which is beneficial to improving the gas production of the battery, the gram capacity and electronic conductivity of the positive electrode active material; and the lower the mass proportion of the carbon coating layer in the positive electrode active material, the lower the Na a R bThe higher the proportion of (PO4)2P2O7, the more beneficial it is to the gram capacity of the positive electrode active material. Controlling the mass proportion of the carbon coating layer in the positive electrode active material to 1% to 2% can take into account the battery's gas production, the gram capacity of the positive electrode active material and electronic conductivity.

[0098] For example, the mass proportion of the carbon coating layer in the positive electrode active material can be 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9% or 2%, etc., and it can also be any value within the range of 1% to 2%.

[0099] Furthermore, the carbon coating layer accounts for 1.2% to 1.8% by mass of the positive electrode active material. By controlling the carbon coating layer to account for 1.2% to 1.8% by mass of the positive electrode active material, it is possible to better balance the gas production of the battery, the gram capacity of the positive electrode active material, and the electronic conductivity.

[0100] In the technical solutions of some embodiments of the present application, the median particle size Dv50 of the positive electrode active material is 2.5 to 3.5 μm.

[0101] The median particle size Dv50 is the particle size corresponding to 50% of the cumulative amount in the volume particle size cumulative distribution diagram. The volume particle size cumulative distribution diagram, also known as the differential distribution diagram of the particle size, is a curve drawn with the particle size as the horizontal coordinate and the differential distribution of the content at different particle sizes as the vertical coordinate. It can more accurately reflect the particle size distribution characteristics of the material particles. Among them, a laser particle size analyzer can be used to measure the volume particle size distribution of the material and draw an interval particle size distribution curve. When the median particle size of the positive electrode active material in the active material layer of the electrode is measured, the positive electrode active material layer can be removed and immersed in the solvent NMP to wash out the binder in the positive electrode active material layer to obtain the powder material of the positive electrode active material layer. After the powder material is dried, it is detected using a laser particle size analyzer with the model number Mastersizer3000 to obtain a volume particle size cumulative distribution diagram. The median particle size of polycrystalline particles and single crystal particles can be obtained based on the peaks in the volume particle size cumulative distribution diagram.

[0102] The median particle size Dv50 of the positive electrode active material refers to the median particle size Dv50 of the positive electrode active material when the positive electrode active material is accumulated from small to large and accounts for 50% of the total volume.

[0103] The smaller the particle size of the positive electrode active material, the more favorable it is for the compaction density of the positive electrode active material layer, and thus for the volumetric energy density of the battery. Conversely, the larger the particle size of the positive electrode active material, the easier it is to prepare, which is more conducive to cost control. By controlling the median particle size Dv50 of the positive electrode active material to 2.5-3.5μm, it is possible to achieve a balance between volumetric energy density and cost control.

[0104] For example, the median particle size Dv50 of the positive electrode active material can be 2.5 μm, 2.6 μm, 2.7 μm, 2.8 μm, 2.9 μm, 3 μm, 3.1 μm, 3.2 μm, 3.3 μm, 3.4 μm or 3.5 μm, etc., and can also be any value within the range of 2.5 to 3.5 μm.

[0105] In the technical solutions of some embodiments of the present application, the powder resistivity of the positive electrode active material is 50Ω·m~500MΩ·m; the gram capacity of the positive electrode active material is 100~129mAh / g; and the mass content of residual alkali in the positive electrode active material is less than 1.5%.

[0106] For example, the powder resistivity of the positive electrode active material may be 50Ω·m, 1MΩ·m, 5MΩ·m, 10MΩ·m, 50MΩ·m, 100MΩ·m, 150MΩ·m, 200MΩ·m, 250MΩ·m, 300MΩ·m, 350MΩ·m, 400MΩ·m, 450MΩ·m, 500MΩ·m, etc., and may be any value within the range of 50Ω·m to 500MΩ·m. The gram capacity of the positive electrode active material may be 100mAh / g, 105mAh / g, 110mAh / g, 115mAh / g, 120mAh / g, 125mAh / g, or 129mAh / g, etc., and may be any value within the range of 100 to 129mAh / g. The mass content of residual alkali in the positive electrode active material can be 0, 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, 0.85%, 0.9%, 0.95%, 1%, 1.05%, 1.1%, 1.15%, 1.2%, 1.25%, 1.3%, 1.35%, 1.4%, 1.45% or 1.5%, etc., and it can also be any value within the range of less than 1.5%.

[0107] FIG6 is a flow chart of a method for preparing a positive electrode active material according to some embodiments of the present application. Referring to FIG6 , an embodiment of the present application further provides a method for preparing a positive electrode active material, the method comprising:

[0108] S110. Obtaining intermediate powder Na 4+x R 3-y (PO4)2P2O7 / C, wherein 0<x<0.5, 0≤y≤0.5, R comprises at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W and Pb; intermediate powder Na 4+x R3-y (PO4)2P2O7 / C includes multiple agglomerated active particles with a core-shell structure. The core of the active particles includes Na a R b (PO4)2P2O7, wherein a is 3.5-4.5, b is 2.5-3.5, R comprises at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W and Pb, and the shell layer of the active particles comprises a carbon coating layer;

[0109] In the technical solutions of some embodiments of the present application, the intermediate powder Na is obtained. 4+x R 3-y (PO4)2P2O7 / C comprises: S111. mixing a sodium source, an R source, a phosphorus source and a carbon source to obtain a mixture, wherein the molar ratio of Na to R in the mixture is (4+x):(3-y), wherein 0<x<0.5, 0≤y≤0.5, and R comprises at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W and Pb; S112. sintering the mixture once to obtain an intermediate powder Na 4+x R 3-y (PO4)2P2O7 / C. By reducing the amount of R source added, the intermediate powder Na 4+x R 3-y The reduction of NaFePO4 and other impurity contents in (PO4)2P2O7 / C, and the increase of Na a R b The content of (PO4)2P2O7 is beneficial to improving the gram capacity of the positive electrode active material.

[0110] Furthermore, the carbon source includes a first carbon source and a second carbon source, the first carbon source includes an inorganic carbon source, and the second carbon source includes an organic carbon source. By simultaneously using an inorganic carbon source and an organic carbon source to achieve coating, it is helpful for the crystal growth of the positive electrode active material to make the lattice more regular. At the same time, it can also reduce the content of the impurity phase in the positive electrode active material and increase the gram capacity of the positive electrode active material. In addition, it can also achieve better coating of the positive electrode active material and reduce the Na a R b The possibility of (PO4)2P2O7 reacting with water in the environment is beneficial to the cycle stability of the positive electrode active material.

[0111] The first carbon source includes at least one of graphite, carbon black, carbon nanotubes and graphene; the second carbon source includes at least one of sucrose, glucose, citric acid, starch, cyclodextrin, asphalt and PEG.

[0112] S120. The intermediate powder Na 4+x R 3-y (PO4)2P2O7 / C and solid boric acid are mixed and sintered twice to make the intermediate powder Na 4+x R 3-y At least part of the residual alkali of (PO4)2P2O7 / C reacts to form a boride to obtain a positive electrode active material.

[0113] In the technical solutions of some embodiments of the present application, the secondary sintering temperature is 400-600°C; the secondary sintering time is 2-20 hours. The higher the sintering temperature and the longer the sintering time, the more conducive it is to the intermediate powder Na 4+x R 3-y The residual alkali on the surface of (PO4)2P2O7 / C is converted into boride, thereby reducing the gas production of the battery; the lower the sintering temperature and the shorter the sintering time, the more it can reduce the Na 4+x R 3-y The release of sodium from (PO4)2P2O7 / C reduces the Na 4+x R 3-y Na in (PO4)2P2O7 / C a R b The structure of (PO4)2P2O7 is likely to be destroyed, which is beneficial to maintaining the specific capacity of the positive electrode active material. By controlling the secondary sintering temperature to 400-600°C and the time to 2-20 hours, it is possible to balance the battery's gas production and the maintenance of the specific capacity of the positive electrode active material.

[0114] For example, the secondary sintering temperature may be 400° C., 420° C., 440° C., 460° C., 480° C., 500° C., 520° C., 540° C., 560° C., 580° C., or 600° C., or any value within the range of 400 to 600° C. The secondary sintering time may be 2 h, 4 h, 6 h, 8 h, 10 h, 12 h, 14 h, 16 h, 18 h, or 20 h, or any value within the range of 2 to 20 h.

[0115] Furthermore, the secondary sintering temperature is 450-550°C and the secondary sintering time is 6-15 hours. By controlling the secondary sintering temperature to 450-550°C and the time to 6-15 hours, it is possible to better balance the gas production of the battery and the maintenance of the gram capacity of the positive electrode active material.

[0116] In the technical solutions of some embodiments of the present application, the median particle size Dv50 of the solid boric acid is 0.2 to 10 μm. The smaller the particle size of the solid boric acid, the more favorable it is for the reaction with Na 4+x R 3-yThorough mixing of (PO4)2P2O7 / C facilitates the complete conversion of residual alkali on its surface into boride, reducing battery gas production. Larger solid boric acid particle sizes facilitate preparation, leading to cost savings. Controlling the solid boric acid's median particle size (Dv50) to 0.2-10 μm effectively balances battery gas production with reduced production costs.

[0117] Exemplarily, the median particle size Dv50 of solid boric acid can be 0.2μm, 0.5μm, 1μm, 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, 5.5μm, 6μm, 6.5μm, 7μm, 7.5μm, 8μm, 8.5μm, 9μm, 9.5μm or 10μm, etc., and it can also be any value in the range of 0.2 to 10μm.

[0118] This method uses solid boric acid and intermediate powder Na 4+x R 3-y (PO4)2P2O7 / C is sintered twice to make the intermediate powder Na 4+x R 3-y The residual alkali on the surface of (PO4)2P2O7 / C is converted into boride, which reduces the residual alkali on the surface of the positive electrode active material. The structure of the boride is more stable and is not easy to react during the production and use of the battery, thus improving the problem of battery gas production. At the same time, boric acid is in solid form and only reacts with the intermediate powder Na 4+x R 3-y (PO4)2P2O7 / C contacts the surface and does not react with the internal active ingredients that can provide capacity. In addition, secondary sintering will cause the intermediate powder Na 4+x R 3-y Na in the residual alkali on the surface of (PO4)2P2O7 / C + Re-entering the internal lattice of the effective components that can provide capacity is beneficial to the gram capacity of the positive electrode active material.

[0119] Figure 7 is a first structural schematic diagram of the positive electrode plate 231 provided in some embodiments of the present application, and Figure 8 is a second structural schematic diagram of the positive electrode plate 231 provided in some embodiments of the present application; please refer to Figures 7 and 8, the embodiments of the present application provide a positive electrode plate 231, the positive electrode plate 231 includes a positive electrode active material layer 2312, and the positive electrode active material layer 2312 includes the positive electrode active material provided above.

[0120] Continuing with Figure 7, the positive electrode active material layer 2312 can be disposed on one side of the positive electrode current collector. Continuing with Figure 8, the positive electrode active material layer 2312 can also be disposed on both sides of the positive electrode current collector. The material of the positive electrode current collector 2311 can be one or more of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy.

[0121] After the above introduction to the materials and structure of the battery cell 20 , the preparation method of the battery cell 20 will be specifically introduced below.

[0122] The preparation method of the battery cell 20 includes the following steps: mixing the positive electrode active material, the conductive agent and the binder, and then mixing them with a solvent to prepare a positive electrode active slurry, coating the positive electrode active slurry on the positive electrode collector 2311 to obtain the positive electrode plate 231, and after preparing the positive electrode plate 231, the positive electrode plate 231, the isolation membrane, the negative electrode plate, the isolation membrane, and so on, are stacked in sequence to form a laminated electrode assembly 23, and then the electrode assembly 23 is placed in a shell, and the electrolyte is injected to form a battery cell 20.

[0123] FIG9 is a flow chart of manufacturing a battery cell 20 according to some embodiments of the present application. As shown in FIG9 , the specific process of manufacturing the battery cell 20 is as follows:

[0124] S210, preparing a positive electrode active slurry: dispersing a positive electrode active material, a binder, and a conductive agent in a solvent to form a positive electrode active slurry. The positive electrode active material is the positive electrode active material provided above. A small amount of other positive electrode active materials may optionally be added.

[0125] The binder can be one or more of styrene-butadiene rubber, water-based 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, and the like can also be added to the positive electrode active slurry.

[0126] S220, preparing the positive electrode active material layer 2312: coating the positive electrode active slurry on the surface of the positive electrode current collector 2311, and then drying to form the positive electrode active material layer 2312. The coating can be applied to one or both surfaces of the positive electrode current collector 2311 as required.

[0127] The coating method may be: blade coating, roller coating, slit coating, etc., which is not limited in this application.

[0128] The positive electrode current collector 2311 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 can include a polymer base layer and a metal layer formed on at least one surface of the polymer base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy) on a polymer substrate (such as a substrate made of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0129] S230 , rolling the positive electrode active material layer 2312 to obtain the positive electrode sheet 231 .

[0130] S240 , stacking the positive electrode sheet 231 , the separator, the negative electrode sheet, the separator, and so on in sequence to form a laminated electrode assembly 23 .

[0131] S250: Assemble the laminated electrode assembly 23 into a battery cell 20. The battery cell 20 can be used to prepare a secondary battery 100 and provide power to an electrical device.

[0132] Next, one or more embodiments will be described in more detail with reference to the following examples. Of course, these examples do not limit the scope of one or more embodiments.

[0133] Examples and Comparative Examples

[0134] Preparation of positive electrode active materials

[0135] The sodium source, R source, and phosphorus source 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. The first and second carbon sources were dissolved in deionized water and mixed to obtain a carbon solution. The carbon solution and the initial mixed solution were mixed and stirred to obtain a mixed solution. The 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. Finally, in an N2 atmosphere, the temperature was increased at a rate of 2°C to 320°C and held for 4 hours. The temperature was then increased at a further rate of 2°C to 550°C and held for 10 hours. The sintered product was air-jet pulverized and sieved to obtain a powder with uniform particle size. The powder was mixed with an acid according to a specific weight. The mixed material was then placed in an electric roller high-temperature kiln and heated at a rate of 10°C / h to a specific temperature. The mixture was then sintered at this temperature for 10 hours before cooling to obtain the positive electrode active material.

[0136]

Preparation of positive electrode sheet

[0137] 2.5 wt% of the binder polyvinylidene fluoride was fully dissolved in N-methylpyrrolidone, and 2.0 wt% of super P and 1.0 wt% of CNT as a conductive agent were added to form a uniformly dispersed slurry with 94.5 wt% of the above-mentioned positive electrode active material. The slurry was evenly coated on the surface of the positive electrode current collector aluminum foil, and the positive electrode sheets were obtained after drying, cold pressing, and slitting.

[0138]

Preparation of negative electrode sheet

[0139] The active material artificial graphite, the conductive agent carbon nanotubes, the binder styrene-butadiene rubber (SBR), and the thickener sodium carboxymethyl cellulose (CMC) are dissolved in the solvent deionized water in a mass ratio of 90:5:3:2, and mixed evenly to prepare the negative electrode slurry; the negative electrode slurry is evenly coated on the negative electrode current collector copper foil once or multiple times, and the negative electrode sheet is obtained after drying, cold pressing, and slitting.

[0140] Preparation of electrolyte

[0141] In an argon atmosphere glove box (H2O<0.1ppm, O2<0.1ppm), the organic solvents ethylene carbonate (EC) / diethyl carbonate (DEC) / ethyl methyl carbonate (EMC) were mixed in a volume ratio of 1 / 1 / 1, 1 mol / L NaPF6 sodium salt was added and dispersed evenly, and then 5% fluoroethylene carbonate was dissolved in the above organic solvents and stirred evenly to obtain an electrolyte.

[0142]

Isolation film

[0143] Polyethylene film is used as the isolation film.

[0144] Preparation of battery cells

[0145] The prepared positive and negative electrode sheets and separator (polyethylene (PE) porous polymer film) are stacked in a Z-shaped structure to form the corresponding electrode assembly. The electrode assembly is vacuum-dried at 90°C for 12 hours, followed by ultrasonic welding of the positive and negative electrode tabs. The positive electrode uses aluminum tabs, and the negative electrode uses nickel tabs, with the positive and negative tabs located on the same side of the electrode assembly. The electrode assembly with the tabs welded is placed in an aluminum-plastic film of appropriate size and sealed top and side at a temperature of 145°C. The electrolyte is then injected and sealed to obtain an uncharged battery. The uncharged battery then undergoes a series of processes including static standing, hot and cold pressing, formation, shaping, and capacity testing to obtain a battery cell.

[0146] The main parameter controls of Examples 1 to 16 and Comparative Examples 1 to 4 are shown in the following table:

[0147] The performance test was performed on the positive electrode active materials provided in each embodiment and comparative example and the batteries composed thereof. The performance test specifically included:

[0148] Surface residual alkali mass test: Using the acid-base titration method, titrate the sodium bicarbonate and sodium carbonate in the positive electrode material with a standard hydrochloric acid solution. Using a pH electrode as the indicator electrode, determine the endpoint by the sudden jump caused by the potential change. Based on the titration endpoint, determine the titration volume of the standard titration solution. Divide the calculated mass of Na2CO3 and NaHCO3 by the mass of the positive electrode active material, and use the sodium ion mass content as the residual alkali content of the positive electrode active material. The calculation formula is as follows:

[0149] Na2CO3%=(V2-V1)*C*106*100n / 1000m

[0150] NaHCO3%=V2*C*84*n*100 / 1000m

[0151] Na + %=V2*C*23*n*100 / 1000m.

[0152] Gram Capacity Test: After the battery rests for 12 hours, discharge it at a constant current of 0.05C to 0.005V, then rest for 10 minutes. Discharge it again at a constant current of 50μA to 0.005V, then rest for 10 minutes. Discharge it again at a constant current of 10μA to 0.005V. Then, charge it at a constant current of 0.1C to 2V. Record the charge capacity. The ratio of the charge capacity to the positive electrode active material is the gram capacity of the prepared positive electrode active material.

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

[0154] As can be seen from the table above, the positive electrode active material prepared by the method provided in the embodiment of the present application has a low residual alkali content and a high gram capacity. The residual alkali content can reach 0.05%, and the gram capacity can reach 123.1 mA hg -1 .

[0155] By comparing the data of Example 1 and Example 2 and Examples 15 to 16, it can be seen that when iron defects exist in the positive electrode active material, the gram capacity of the entire positive electrode active material is significantly improved.

[0156] By comparing the data of Example 1 and Examples 3 to 8, it can be seen that as the sintering temperature increases, the residual alkali content of the positive electrode active material shows a trend of gradually decreasing, and its gram capacity shows a trend of first increasing and then decreasing. When the sintering temperature is 400-600°C, the residual alkali content of the positive electrode active material is below 0.16%, and the gram capacity is 118.6 mA h g -1 Furthermore, when the sintering temperature is 450-550°C, the residual alkali content of the positive electrode active material is below 0.11%, and the gram capacity is 121.2 mA hg -1 above.

[0157] By comparing the data of Example 1 and Examples 9 to 10, it can be seen that as the median particle size of solid boric acid increases, the residual alkali mass content of the positive electrode active material shows a trend of gradually increasing, while its gram capacity does not change much. It is believed that the smaller the median particle size of solid boric acid, the more beneficial it is to the comprehensive performance of the positive electrode active material.

[0158] Comparison of the data from Example 1 and Examples 11 to 14 shows that the use of different carbon-coated raw materials can effectively control the residual alkali content while maintaining a good gram capacity. In particular, the use of two carbon sources can better control the residual alkali content and better maintain the gram capacity.

[0159] The above are merely specific embodiments of the present application and are not intended to limit the present application. Those skilled in the art will readily appreciate that various modifications and variations are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A positive electrode active material, characterized in that The positive electrode active material includes an active material body and a boride attached to the active material body; the active material body includes a plurality of agglomerated active particles, the active particles have a core-shell structure, and the core of the active particles includes Na a R b (PO4)2P2O7, wherein a is 3.5-4.5, b is 2.5-3.5, R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W and Pb, and the shell layer of the active particles includes a carbon coating layer.

2. The positive electrode active material according to claim 1, characterized in that The mass proportion of the boride in the positive electrode active material is 0.1% to 2.5%.

3. The positive electrode active material according to claim 2, characterized in that The mass proportion of the boride in the positive electrode active material is 0.1% to 1%.

4. The positive electrode active material according to any one of claims 1 to 3, characterized in that The boride includes Na3BO3, Na4B2O5, NaBO2, Na2B4O7, Na 12 B 26 O 45 、NaB3O5、Na2B8O 13 and at least one of B2O3.

5. The positive electrode active material according to any one of claims 1 to 4, characterized in that The gram capacity of the positive electrode active material is 100 to 129 mAh / g; and / or The mass content of residual alkali in the positive electrode active material is less than 1.5%.

6. A method for preparing a positive electrode active material, characterized in that: The method comprises: Get the intermediate powder Na 4+x R 3-y (PO4)2P2O7 / C, wherein 0<x<0.5, 0≤y≤0.5, R comprises at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W and Pb; the intermediate powder Na 4+x R 3-y (PO4)2P2O7 / C comprises a plurality of agglomerated active particles having a core-shell structure, wherein the core of the active particle comprises Na a R b (PO4)2P2O7, wherein a is 3.5-4.5, b is 2.5-3.5, R comprises at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W and Pb, and the shell layer of the active particles comprises a carbon coating layer; The intermediate powder Na 4+x R 3-y (PO4)2P2O7 / C and solid boric acid are mixed and then sintered twice to make the intermediate powder Na 4+x R 3-y At least part of the residual alkali of (PO4)2P2O7 / C reacts to form a boride to obtain a positive electrode active material.

7. The method for preparing a positive electrode active material according to claim 6, wherein: The secondary sintering temperature is 400-600°C; and / or The secondary sintering time is 2 to 20 hours.

8. The method for preparing a positive electrode active material according to claim 7, wherein: The secondary sintering temperature is 450-550°C; and / or The secondary sintering time is 6 to 15 hours.

9. The method for preparing a positive electrode active material according to any one of claims 6 to 8, characterized in that: The median particle size Dv50 of the solid boric acid is 0.2 to 10 μm.

10. The method for preparing a positive electrode active material according to any one of claims 6 to 9, characterized in that: The intermediate powder Na 4+x R 3-y (PO4)2P2O7 / C includes: A sodium source, an R source, a phosphorus source, and a carbon source are mixed to obtain a mixture, wherein the molar ratio of Na to R in the mixture is (4+x):(3-y), wherein 0<x<0.5, 0≤y≤0.5, and R comprises at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Cr, Nb, Mo, In, Ga, Sn, Hf, Ta, W, and Pb; The mixed material is sintered once to obtain the intermediate powder Na 4+x R 3-y (PO4)2P2O7 / C.

11. The method for preparing a positive electrode active material according to claim 10, characterized in that: The carbon source includes a first carbon source and a second carbon source, the first carbon source includes an inorganic carbon source, and the second carbon source includes an organic carbon source.

12. The method for preparing a positive electrode active material according to claim 11, wherein: The first carbon source comprises at least one of graphite, carbon black, carbon nanotubes and graphene; and / or The second carbon source includes at least one of sucrose, glucose, citric acid, starch, cyclodextrin, asphalt and PEG.

13. A positive electrode plate, characterized in that: The positive electrode sheet includes a positive electrode active material layer, and the positive electrode active material layer includes the positive electrode active material according to any one of claims 1 to 5 or the positive electrode active material prepared by the method according to claims 6 to 12.

14. A battery, characterized in that: The battery comprises the positive electrode sheet according to claim 13.

15. An electrical device, characterized in that: The electric device comprises the battery according to claim 14.

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