Positive electrode active material, method for manufacturing the same, positive electrode sheet, battery, and electric device

Abstract

The application discloses a positive electrode active material and a preparation method thereof, a positive electrode sheet, a battery and an electric device. The positive electrode active material comprises a core and a first coating layer. The core comprises Na x R y (PO4) z (P2O7) k , wherein 1≤x≤7, 1≤y≤4, 1≤z≤4, 1≤k≤4, R comprises at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W or Pb; the first coating layer is formed on at least part of the core, and the first coating layer comprises M a O b , M comprises at least one of Ca, Bi, Ba, Ti, Al, Nb, Mg, Fe, Cu, Zn, Mn, Ni or Co, 1≤a≤7, 1≤b≤12. Thus, the positive electrode active material has excellent environmental stability and moisture resistance, thereby improving the cycle performance of the battery, and the positive electrode active material has excellent first-cycle coulomb efficiency, thereby making the battery maintain a high first-cycle coulomb efficiency.

Description

Technical Field

[0001] This application belongs to the field of batteries, specifically relating to a positive electrode active material and its preparation method, a positive electrode sheet, a battery, and an electrical device. Background Technology

[0002] The increasing scarcity of lithium resources, the continuous rise in upstream material prices, the lagging development of recycling technologies, and the low recycling rate of old batteries pose significant challenges to lithium-ion batteries. Sodium-ion batteries, on the other hand, utilize the intercalation and deintercalation of sodium ions between the positive and negative electrodes for charging and discharging. Furthermore, sodium resources are far more abundant and widely distributed than lithium, and their cost is much lower. Therefore, sodium-ion batteries have become a promising next-generation electrochemical system to replace lithium-ion rechargeable batteries. However, due to the poor stability of existing positive electrode active materials, sodium-ion batteries suffer from low cycle performance. Summary of the Invention

[0003] In view of the technical problems existing in the background art, this application provides a positive electrode active material, which aims to improve the cycle performance of batteries containing it.

[0004] To achieve the above objectives, one aspect of this application provides a positive electrode active material, said positive electrode active material comprising:

[0005] The kernel, the kernel comprising Na x R y (PO4) z (P2O7) k Wherein, 1≤x≤7, 1≤y≤4, 1≤z≤4, 1≤k≤4, and R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W, or Pb;

[0006] A first coating layer is formed on at least a portion of the core, the first coating layer comprising M a O b M includes at least one of Ca, Bi, Ba, Ti, Al, Nb, Mg, Fe, Cu, Zn, Mn, Ni or Co, 1≤a≤7, 1≤b≤12.

[0007] This application includes at least the following beneficial effects: This application, through the inclusion of Na x R y (PO4) z (P2O7) k The kernel is formed at least partially by M a O bThe first coating layer, M, includes at least one of Ca, Bi, Ba, Ti, Al, Mg, Fe, Cu, Zn, Mn, Ni, or Co. This first coating layer reduces the contact between the core and atmospheric substances such as H2O and CO2, thereby improving the material's environmental stability and moisture resistance, and ultimately enhancing the battery's cycle performance. Simultaneously, this first coating layer reduces side reactions between the core surface and the electrolyte, stabilizes the interface between the positive electrode active material and the electrolyte, improves the material's stability, and further enhances the cycle performance of batteries containing it. Furthermore, this first coating layer does not reduce the first-cycle coulombic efficiency of the positive electrode active material, thus maintaining a high first-cycle coulombic efficiency for the battery.

[0008] In some embodiments of this application, the core includes at least one of Na4R3(PO4)2P2O7 or Na7V4(PO4)4P2O7, wherein R includes at least one of Fe, Ni, Co or Mn.

[0009] In some embodiments of this application, M a O b Including TiO2, Al2O3, Nb2O5, MgO, Co3O4, CaTiO3, Bi4Ti3O 12 Or at least one of BaTiO3. This can improve the cycle performance of batteries containing it and maintain a high first-cycle coulombic efficiency.

[0010] In some embodiments of this application, the content of the first coating layer is w% based on the total mass of the positive electrode active material.

[0011] Based on the total mass of the core, the residual alkali content of the core is z%, 0.06≤z / w≤3, and can be optionally 0.06≤z / w≤1. This improves the cycle performance of batteries containing the core and allows the battery to maintain a high first-cycle coulombic efficiency.

[0012] In some embodiments of this application, 1% ≤ w% ≤ 3%, and optionally 1.5% ≤ w% ≤ 2.5%. This improves the cycle performance of the battery containing it and allows the battery to maintain a high first-cycle coulombic efficiency.

[0013] In some embodiments of this application, 0.2% ≤ z% ≤ 3%, and optionally 0.2% ≤ z% ≤ 0.6%.

[0014] In some embodiments of this application, the thickness of the first coating layer is 1 nm-10 nm, optionally 2 nm-8 nm. This improves the cycle performance of the battery containing it and allows the battery to maintain a high first-cycle coulombic efficiency.

[0015] In some embodiments of this application, the positive electrode active material further includes a second coating layer formed on at least a portion of the core, the first coating layer being formed on at least a portion of the second coating layer, and the second coating layer comprising carbon. This improves the environmental stability, moisture resistance, and conductivity of the positive electrode active material, thereby enhancing the kinetic and cycle performance of batteries containing it.

[0016] In some embodiments of this application, based on the total mass of the positive electrode active material, the carbon content in the second coating layer is n%, 0.16 ≤ n / w ≤ 1.5, and optionally 0.32 ≤ n / w ≤ 0.8. This improves the environmental stability, moisture resistance, and conductivity of the positive electrode active material, thereby enhancing the kinetic and cycle performance of batteries containing it.

[0017] In some embodiments of this application, 0.5% ≤ n% ≤ 1.5%, and optionally 0.8% ≤ n% ≤ 1.2%. This improves the environmental stability, moisture resistance, and conductivity of the positive electrode active material, thereby enhancing the kinetic and cycle performance of batteries containing it.

[0018] In some embodiments of this application, the positive electrode active material satisfies at least one of the following conditions:

[0019] The volume average particle size Dv50 of the positive electrode active material is 2μm-8μm, and can be selected as 2μm-5μm;

[0020] The Dv99 of the positive electrode active material is 10μm-40μm, and can be selected as 15μm-30μm;

[0021] The BET specific surface area of ​​the positive electrode active material is 3m². 2 / g-11m 2 / g, optional 7m 2 / g-10m 2 / g;

[0022] The compaction density of the positive electrode active material under 100 MPa pressure is 1.3 g / cm³. 3 -2.1g / cm 3 1.7g / cm³ is an optional value. 3 -1.9g / cm 3 .

[0023] The second aspect of this application discloses a method for preparing a positive electrode active material, comprising:

[0024] Provide a core material, the core material comprising Na x R y (PO4) z (P2O7)k Wherein, 1≤x≤7, 1≤y≤4, 1≤z≤2, 1≤k≤4, and R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W, or Pb;

[0025] The core material and M a O b Mixed sintering, where M includes at least one of Ti, Al, Mg, Fe, Cu, Zn, Mn, Ni or Co, 1≤a≤7, 1≤b≤12, yields a positive electrode active material.

[0026] Therefore, the method of this application can be used in areas including Na x R y (PO4) z (P2O7) k The kernel forms including M a O b The first coating layer (M includes at least one of Ti, Al, Mg, Fe, Cu, Zn, Mn, Ni or Co, 1≤a≤7, 1≤b≤12) can reduce the contact between the core and H2O, CO2 and other substances in the air, thereby improving the environmental stability and moisture resistance of the material, and thus improving the cycle performance of the battery containing it. Furthermore, the first coating layer does not degrade the first-cycle coulombic efficiency of the positive electrode active material, thereby enabling the battery to maintain a high first-cycle coulombic efficiency.

[0027] In some embodiments of this application, the sintering temperature is 250℃-550℃, and the holding time is 2h-8h.

[0028] In some embodiments of this application, when the core material is combined with M a O b Before sintering, the core material is pre-mixed and sintered with a carbon source. This forms a carbon-containing second coating layer on the surface of the core material, improving the environmental stability, moisture resistance, and conductivity of the positive electrode active material, thereby enhancing the cycle performance and kinetic performance of the battery containing it.

[0029] A third aspect of this application provides a positive electrode sheet, which includes the positive active material described in the first aspect of this application or the positive active material obtained by the method described in the second aspect of this application.

[0030] A fourth aspect of this application discloses a battery comprising the positive electrode sheet described in the third aspect of this application.

[0031] In some embodiments of this application, the battery includes a sodium-ion battery.

[0032] The fifth aspect of this application discloses an electrical device comprising the battery described in the fourth aspect of this application.

[0033] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. 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 is a schematic diagram of the positive electrode active material according to one embodiment of this application.

[0036] Figure 2 This is a schematic diagram of the positive electrode active material according to another embodiment of this application.

[0037] Figure 3 This is a schematic diagram of a battery according to one embodiment of this application.

[0038] Figure 4 yes Figure 3 An exploded view of a battery according to one embodiment of this application is shown.

[0039] Figure 5 This is a schematic diagram of a battery module according to one embodiment of this application.

[0040] Figure 6 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0041] Figure 7 yes Figure 6 An exploded view of a battery pack according to one embodiment of this application is shown.

[0042] Figure 8 This is a schematic diagram of an electrical device in which a battery is used as a power source according to one embodiment of this application.

[0043] Figure 9 These are XRD comparison images of the positive electrode active materials of Example 1 and Comparative Example 1 of this application.

[0044] Figure 10 This is the distribution diagram of Ti element in the EDS energy spectrum of the positive electrode active material of Example 1 of this application.

[0045] Explanation of reference numerals in the attached figures:

[0046] 1 Battery; 11 Casing; 12 Electrode assembly; 13 Cover plate; 2 Battery module; 3 Battery pack; 31 Upper casing; 32 Lower casing; 20 Positive electrode active material; 21 Core; 22 First coating layer; 23 Second coating layer. Detailed Implementation

[0047] The embodiments of the technical solution of this application are described in detail below. The following embodiments are only used to illustrate the technical solution of this application more clearly, and are therefore only examples, and should not be used to limit the scope of protection of this application.

[0048] 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.

[0049] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0050] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0051] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0052] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0053] With the technological advancements and increasing demands for electric vehicles and rechargeable mobile devices, secondary batteries, as a representative of the new energy field, have seen rapid development in related research. Sodium-ion batteries offer a significant price advantage over traditional lithium-ion batteries and have broad application prospects in large-scale energy storage systems.

[0054] Polyanionic phosphate cathode materials have become a popular choice for sodium-ion batteries due to their abundant resources, environmental friendliness, ease of large-scale production, open sodium-ion diffusion channels, and good thermal stability. However, due to factors such as processing and formulation, residual alkali (including sodium bicarbonate and sodium carbonate) often remains on the surface of polyanionic phosphate cathode materials. This residual alkali readily reacts with H2O and CO2 in the air, resulting in poor environmental stability and moisture resistance, which in turn reduces battery cycle performance.

[0055] This application, through including Na x R y (PO4) z (P2O7) k The kernel is formed at least partially by M a O b The first coating layer, M, includes at least one of Ca, Bi, Ba, Ti, Al, Nb, Mg, Fe, Cu, Zn, Mn, Ni, or Co. This first coating layer reduces the contact between the core and atmospheric substances such as H2O and CO2, thereby improving the material's environmental stability and moisture resistance, and ultimately enhancing the battery's cycle performance. Simultaneously, this first coating layer reduces side reactions between the core surface and the electrolyte, stabilizes the interface between the positive electrode active material and the electrolyte, improves the material's stability, and further enhances the cycle performance of batteries containing it. Furthermore, this first coating layer does not reduce the first-cycle coulombic efficiency of the positive electrode active material, thus maintaining a high first-cycle coulombic efficiency for the battery.

[0056] The positive electrode active material disclosed in this application is suitable for sodium-ion batteries, and the battery disclosed in this application can be used in electrical devices that use batteries as a power source or in various energy storage systems that use batteries as energy storage elements. Electrical devices may include, but are not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc. Electric toys may include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc. Spacecraft may include airplanes, rockets, space shuttles, and spacecraft, etc.

[0057] The first aspect of this application discloses a positive electrode active material, referring to... Figure 1 The positive electrode active material 20 includes a core 21 and a first coating layer 22, wherein the core includes Na x R y (PO4) z (P2O7) k Wherein, 1≤x≤7, 1≤y≤4, 1≤z≤4, 1≤k≤4, and R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W, or Pb. The first coating layer 22 is formed on at least a portion of the core 21, and the first coating layer 22 includes M a O b M includes at least one of Ca, Bi, Ba, Ti, Al, Nb, Mg, Fe, Cu, Zn, Mn, Ni or Co, 1≤a≤7, 1≤b≤12.

[0058] This application includes at least the following beneficial effects: This application, through the inclusion of Na x R y (PO4) z (P2O7) k The kernel 21 is at least partially formed with M a O b The first coating layer 22, M, includes at least one of Ca, Bi, Ba, Ti, Al, Nb, Mg, Fe, Cu, Zn, Mn, Ni, or Co. This first coating layer 22 reduces the contact between the core 21 and H2O, CO2, etc., in the air, thereby improving the environmental stability and moisture resistance of the material, and thus improving the cycle performance of the battery. Simultaneously, the first coating layer 22 reduces side reactions between the surface of the core 21 and the electrolyte, stabilizes the interface between the positive electrode active material 20 and the electrolyte, improves the stability of the material, and thus improves the cycle performance of the battery containing it. Furthermore, this first coating layer 22 does not reduce the first-cycle coulombic efficiency of the positive electrode active material 20, thus maintaining a high first-cycle coulombic efficiency for the battery.

[0059] In some embodiments of this application, the Na x R y (PO4) z (P2O7) k In this context, 1 ≤ x ≤ 7, for example, x can take values ​​of 2 ≤ x ≤ 6, 3 ≤ x ≤ 5, 3 ≤ x ≤ 4, etc. Therefore, including this amount of sodium ions in the core 21 can increase the specific capacity of the battery, thus giving the battery a higher capacity.

[0060] In some embodiments of this application, the Na x R y (PO4) z (P2O7) k R in this composition includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W, or Pb, and 1 ≤ y ≤ 4, for example, 2 ≤ x ≤ 4, 3 ≤ x ≤ 4, etc. Therefore, the positive electrode active material composed in this application has advantages in chemical stability, thermal stability, electrochemical stability, and high specific capacity.

[0061] In some embodiments of this application, the Na x R y (PO4) z (P2O7) k In the above, 1≤z≤4, 1≤k≤4, for example 2≤z≤4, 3≤z≤4, etc., 2≤k≤4, 3≤k≤4, etc.

[0062] As an example, the Na x R y (PO4) z (P2O7) k It can include Na4Fe3(PO4)2P2O7, Na4Ni3(PO4)2P2O7, Na4Co3(PO4)2P2O7, Na4Mn3(PO4)2P2O7, Na7V4(PO4)4P2O7, Na 4.02 Fe 2.85 (PO4)2P2O7, Na4Fe 2.9 Mg 0.1 (PO4)2P2O7, Na4Fe 2.95 Al 0.05 (PO4)2P2O7 or Na4Fe 2.99 Cu 0.01 At least one of (PO4)2P2O7.

[0063] In other embodiments of this application, the core 21 comprises at least one of Na4R3(PO4)2P2O7 or Na7V4(PO4)4P2O7, wherein R comprises at least one of Fe, Ni, Co, or Mn, such as Na4Fe3(PO4)2P2O7, Na4Ni3(PO4)2P2O7, Na4Co3(PO4)2P2O7, and Na4Mn3(PO4)2P2O7. Therefore, using a core 21 with this composition can increase the specific capacity of the positive electrode active material 20, thereby increasing the capacity of the battery.

[0064] It should be noted that during the charging and discharging process of the battery, Na is involved in both insertion / extraction and consumption, and the molar content of Na varies when the battery is discharged to different states. In the enumeration of core 21 in this application, the molar content of Na refers to the initial state of the material, i.e., the state before feeding. When the positive electrode active material is applied to the battery system, the molar content of Na will change after charge-discharge cycles.

[0065] In the enumeration of kernel 21 in this application, the molar content of O is only a theoretical state value. The release of oxygen from the crystal lattice will cause the molar content of oxygen to change, and the actual molar content of O will fluctuate.

[0066] In some embodiments of this application, the first covering layer 22 is formed on at least a portion of the core 21, and the first covering layer 22 includes M a O b M includes at least one of Ca, Bi, Ba, Ti, Al, Nb, Mg, Fe, Cu, Zn, Mn, Ni, or Co, where 1 ≤ a ≤ 7, 1 ≤ b ≤ 12, for example, a can take values ​​of 2 ≤ a ≤ 6, 3 ≤ a ≤ 5, 3 ≤ a ≤ 4, etc. Therefore, this application includes Na... x R y (PO4) z (P2O7) k At least a portion of the kernel 21 is formed with an M comprising the above-described components. a O b The first coating layer 22 reduces the contact between the core 21 and atmospheric substances such as H2O and CO2, thereby improving the material's environmental stability and moisture resistance, and ultimately enhancing the battery's cycle performance. Simultaneously, the first coating layer 22 reduces side reactions between the core 21 surface and the electrolyte, stabilizing the interface between the positive electrode active material 20 and the electrolyte, improving material stability, and further enhancing the cycle performance of batteries containing it. Furthermore, the first coating layer 22 does not reduce the first-cycle coulombic efficiency of the positive electrode active material 20, thus maintaining a high first-cycle coulombic efficiency for the battery.

[0067] As an example, M a O bIt may include at least one of CaO, Bi2O3, BaO, TiO2, Al2O3, Nb2O5, MgO, FeO, CuO, ZnO, MnO2, NiO, or Co3O4.

[0068] In some embodiments of this application, the M a O b It can include TiO2, Al2O3, Nb2O5, MgO, Co3O4, CaTiO3, Bi4Ti3O 12 Or at least one of BaTiO3. Therefore, by forming a first coating layer 22 with the above-described composition on the core 21, this application can reduce the contact between the core 21 and H2O, CO2, etc. in the air, thereby improving the environmental stability and moisture resistance of the material, and thus improving the cycle performance of the battery. Simultaneously, the first coating layer 22 can reduce side reactions between the surface of the core 21 and the electrolyte, stabilize the interface between the positive electrode active material 20 and the electrolyte, improve the stability of the material, and further improve the cycle performance of the battery containing it. Furthermore, the first coating layer 22 does not reduce the first-cycle coulombic efficiency of the positive electrode active material 20, thus maintaining a high first-cycle coulombic efficiency of the battery.

[0069] In some embodiments of this application, the content of the first coating layer 22 is w% based on the total mass of the positive electrode active material 20; the residual alkali content of the core 21 is z% based on the total mass of the core 21, and is 0.06≤z / w≤3, for example 0.08≤z / w≤3, 0.1≤z / w≤3, 0.2≤z / w≤2.8, 0.5≤z / w≤2.5, 0.7≤z / w≤2.2, 1≤z / w≤2, 1.2≤z / w≤1.8, 1.5≤z / w≤1.7, etc. Specifically, due to process and formulation reasons, residual alkali (including sodium bicarbonate and sodium carbonate, etc.) is present on the surface of the core 21. This residual alkali readily reacts with H2O, CO2, etc., in the air, resulting in poor environmental stability and moisture resistance of the material. This application controls the content w% of the first coating layer 22 and the residual alkali amount z% of the core 21 to meet the aforementioned ratio, which can significantly improve the environmental stability and moisture resistance of the material, thereby improving the cycle performance of the battery. In other embodiments of this application, the content w% of the first coating layer 22 and the residual alkali amount z% of the core 21 satisfy 0.06 ≤ z / w ≤ 1.

[0070] In this application, the residual alkali content z% of the core 21 can be determined using instruments and methods known in the art. For example, the free sodium in the core of the positive electrode active material of sodium-ion batteries can be tested according to the test of free lithium in GB / T9725-2007 "General Rules for Potentiometric Titration of Chemical Reagents". The test result is calculated based on the molecular weight ratio of residual sodium and residual lithium. For example, if the molecular weight of NaHCO3 is 84 and the molecular weight of LiHCO3 is 68, the test result shows a free lithium content of 0.1%. According to the molecular weight ratio of NaHCO3 to LiHCO3, the free sodium content of the positive electrode material is calculated to be 0.124%. The method for testing the free lithium content of the positive electrode material according to GB / T9725-2007 "General Rules for Potentiometric Titration of Chemical Reagents" is as follows: sample and prepare a test solution according to the product standard. Insert the specified electrode, start the electromagnetic stirrer, and titrate with the specified standard titration solution. Add approximately 90% of the required titration volume of standard titrant to the burette and measure the potential or pH of the solution. Measure the potential or pH after each 1 mL or additional volume of standard titrant added. Before and after the stoichiometric point, measure the potential or pH after each 0.1 mL addition of standard titrant. Continue titrating until the potential or pH changes minimally. Record the burette reading and the measured potential or pH after each addition of standard titrant. Use a graphical method or second-order derivative method to determine the titration endpoint.

[0071] In some embodiments of this application, based on the total mass of the positive electrode active material 20, the content w% of the first coating layer 22 satisfies 1% ≤ w% ≤ 3%, for example, 1.2% ≤ w% ≤ 2.8%, 1.5% ≤ w% ≤ 2.5%, 1.7% ≤ w% ≤ 2.2%, 1.8% ≤ w% ≤ 2%, etc. Therefore, the content w% of the first coating layer 22 in this application satisfies the above conditions, which can improve the environmental stability and moisture resistance of the material, thereby improving the cycle performance of the battery. Simultaneously, this content of the first coating layer 22 can reduce side reactions between the core 21 surface and the electrolyte, stabilize the interface between the positive electrode active material 20 and the electrolyte, improve the stability of the material, and thus improve the cycle performance of the battery containing it. Furthermore, this content of the first coating layer 22 does not reduce the first-cycle coulombic efficiency of the positive electrode active material 20, thereby allowing the battery to maintain a high first-cycle coulombic efficiency. In other embodiments of this application, the content w% of the first coating layer 22, based on the total mass of the positive electrode active material 20, satisfies 1.5% ≤ w% ≤ 2.5%.

[0072] In some embodiments of this application, based on the total mass of the core 21, the residual alkali content z% of the core 21 satisfies 0.2% ≤ z% ≤ 3%, for example, 0.5% ≤ z% ≤ 2.8%, 0.8% ≤ z% ≤ 2.5%, 1% ≤ z% ≤ 2.2%, 1.2% ≤ z% ≤ 2%, 1.5% ≤ z% ≤ 1.8%, etc. Therefore, the residual alkali content z% of the core 21 in this application satisfies the above conditions, which can reduce the decomposition and gas generation of residual alkali during battery cycling, reduce battery gas expansion, and thus improve the cycle performance of the battery. In some embodiments of this application, based on the total mass of the positive electrode active material 20, the residual alkali content z% of the core 21 satisfies 0.2% ≤ z% ≤ 0.6%.

[0073] In some embodiments of this application, the thickness of the first coating layer 22 can be 1nm-10nm, such as 2nm-9nm, 3nm-8nm, 4nm-7nm, 5nm-6nm, etc. Therefore, the thickness of the first coating layer 22 in this application satisfies the above conditions, which can improve the environmental stability and moisture resistance of the material, thereby improving the cycle performance of the battery. Simultaneously, this thickness of the first coating layer 22 can reduce side reactions between the surface of the core 21 and the electrolyte, stabilize the interface between the positive electrode active material 20 and the electrolyte, improve the stability of the material, and thus improve the cycle performance of the battery containing it. Furthermore, this thickness of the first coating layer 22 does not reduce the first-cycle coulombic efficiency of the positive electrode active material, thereby maintaining a high first-cycle coulombic efficiency of the battery. In other embodiments of this application, the thickness of the first coating layer 22 is 2nm-8nm.

[0074] In some embodiments of this application, reference is made to Figure 2 The positive electrode active material 20 further includes a second coating layer 23, which is formed on at least a portion of the core 21. A first coating layer 22 is formed on at least a portion of the second coating layer 23, and the second coating layer 23 comprises carbon. Thus, by coating the outer surface of the core 21 with a second coating layer 23 comprising carbon, the second coating layer 23 not only improves the environmental stability and moisture resistance of the core 21 but also enhances its conductivity, thereby improving the kinetic and cycle performance of the battery containing it.

[0075] In some embodiments of this application, based on the total mass of the positive electrode active material 20, the carbon content in the second coating layer 23 is n%, 0.16≤n / w≤1.5, for example 0.18≤n / w≤1.5, 0.2≤n / w≤1.5, 0.25≤n / w≤1.5, 0.3≤n / w≤1.5, 0.35≤n / w≤1.45, 0.4≤n / w≤1.4, 0.45≤n / w≤1.35, 0.5≤n / w≤1.3, 0.55≤n / w≤1.25, 0.6≤n / w≤1.2, 0.65≤n / w≤1.1, 0.7≤n / w≤1, 0.8≤n / w≤0.9, 0.85≤n / w≤0.9, etc. Therefore, by controlling the carbon content n% in the second coating layer 23 and the content w% in the first coating layer 22 within the aforementioned range, the environmental stability, moisture resistance, and conductivity of the positive electrode active material 20 can be significantly improved, thereby enhancing the kinetic and cycle performance of the battery containing it. In other embodiments of this application, based on the total mass of the positive electrode active material 20, the carbon content n% in the second coating layer 23 satisfies 0.32 ≤ n / w ≤ 0.8.

[0076] In some embodiments of this application, based on the total mass of the positive electrode active material 20, the carbon content in the second coating layer 23 is n%, which is 0.5% ≤ n% ≤ 1.5%, for example, 0.6% ≤ n% ≤ 1.4%, 0.7% ≤ n% ≤ 1.3%, 0.8% ≤ n% ≤ 1.3%, 0.9% ≤ n% ≤ 1.2%, 1% ≤ n% ≤ 1.1%, etc. Therefore, controlling the carbon content n% in the second coating layer 23 within the above range can significantly improve the environmental stability, moisture resistance, and conductivity of the positive electrode active material 20, thereby improving the kinetic performance and cycle performance of the battery containing it. In other embodiments of this application, based on the total mass of the positive electrode active material 20, the carbon content n% in the second coating layer 23 satisfies 0.8% ≤ n% ≤ 1.2%.

[0077] In some embodiments of this application, the volume average particle size Dv50 of the positive electrode active material 20 is 2μm-8μm. For example, the Dv50 of the positive electrode active material can be 2.5μm-7.5μm, 3μm-7μm, 3.5μm-6.5μm, 4μm-6μm, 4.5μm-5.5μm, 4.8μm-5μm, etc. In other embodiments of this application, the volume average particle size Dv50 of the positive electrode active material 20 is 2μm-5μm.

[0078] In some embodiments of this application, the Dv99 of the positive electrode active material 20 is 10μm-40μm. For example, the Dv99 of the positive electrode active material can be 12μm-37μm, 15μm-35μm, 18μm-32μm, 20μm-30μm, 22μm-28μm, 23μm-25μm, etc. In other embodiments of this application, the Dv99 of the positive electrode active material 20 is 15μm-30μm.

[0079] In this application, the volume average particle size Dv50 refers to the particle size corresponding to a cumulative volume distribution percentage of 50%, and Dv99 refers to the particle size corresponding to a cumulative volume distribution percentage of 99%. For example, the test methods for volume average particle sizes Dv50 and Dv99 can refer to the standard GB / T 19077-2016 and be determined using a laser particle size analyzer (e.g., MalvernMaster Size3000).

[0080] In some embodiments of this application, the BET specific surface area of ​​the positive electrode active material 20 is 3m². 2 / g-11m 2 / g, for example, the BET specific surface area of ​​positive electrode active material 20 can be 4m². 2 / g-10m 2 / g, 5m 2 / g-9m 2 / g, 6m 2 / g-8m 2 / g, 7m 2 / g-7.5m 2 / g, etc. In other embodiments of this application, the BET specific surface area of ​​the positive electrode active material 20 is 7m². 2 / g-10m 2 / g.

[0081] In this application, the BET specific surface area of ​​the positive electrode active material 20 has a meaning known in the art and can be measured using instruments and methods known in the art. For example, it can be tested using the following method: using a US-made Gemini VII2390 multi-station fully automated specific surface area and porosity analyzer, take about 7g of sample and put it into a 9cc long tube with a bulb, degas at 200°C for 2 hours, and then put it into the main unit to test and obtain the BET specific surface area data of the positive electrode active material 20.

[0082] In some embodiments of this application, the compaction density of the positive electrode active material 20 under a pressure of 100 MPa is 1.3 g / cm³. 3 -2.1g / cm 3 For example, the compaction density of the positive electrode active material 20 under a pressure of 100 MPa can be 1.4 g / cm³.3 -2g / cm 3 1.5g / cm 3 -1.9g / cm 3 1.6g / cm 3 -1.8g / cm 3 1.7g / cm 3 -1.8g / cm 3 In some embodiments of this application, the compaction density of the positive electrode active material 20 under a pressure of 100 MPa is 1.7 g / cm³. 3 -1.9g / cm 3 .

[0083] In this application, "compacted density" has a well-known meaning in the art and can be determined using instruments and methods well-known in the art. For example, the following test method can be used: a quantitative amount of powder m is placed in a compaction mold, the mold is placed on a compaction density instrument, a pressure of 100 MPa is set, the thickness and volume v of the powder under a pressure of 300 MPa are measured on the instrument, and the compacted density is calculated by using density = mass m / volume v; (refer to GB / T24533-2009 for details).

[0084] Specifically, when at least one of the volume average particle size Dv50, volume average particle size Dv99, specific surface area, and compaction density under 100MPa pressure of the positive electrode active material 20 of this application is within the above range, the conduction distance within the positive electrode active material 20 is small and the surface side reactions are few, which promotes the positive electrode active material 20 to exert its specific capacity and improves the capacity retention rate of the battery containing it.

[0085] The second aspect of this application provides a method for preparing the positive electrode active material described in the first aspect, comprising:

[0086] S100: Provides core materials

[0087] In some embodiments of this application, the core material includes Na. x R y (PO4) z (P2O7) k Wherein, 1≤x≤7, 1≤y≤4, 1≤z≤2, 1≤k≤4, and R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W, or Pb.

[0088] S200: Combine the core material with M a O b Hybrid Sintering

[0089] In some embodiments of this application, the core material described in step S100 is combined with M a O b Hybrid sintering, coating the surface of the core material with M a O b The first coating layer, M, includes at least one of Ti, Al, Mg, Fe, Cu, Zn, Mn, Ni, or Co, where 1 ≤ a ≤ 7 and 1 ≤ b ≤ 12, thus obtaining the positive electrode active material. Specifically, during the sintering process, free sodium ions (residual alkali) on the surface of the core material enter the crystal structure of the core material, thereby increasing the sodium ion content in the core material. Therefore, the sintering process forms a coating layer including M... a O b The first coating layer will not degrade the first-cycle coulombic efficiency of the positive electrode active material.

[0090] As an example, the core material is uniformly mixed with a solvent such as anhydrous ethanol, and then M is added. a O b After thorough mixing, the solvent is removed by vacuum evaporation using a rotary evaporator (70℃-100℃, e.g., 70℃, 80℃, 90℃, or 100℃). The filter cake is then washed several times using a vacuum filtration device and dried in a vacuum oven. Subsequently, the powder material is calcined at 250℃-550℃ for 2-8 hours under a nitrogen atmosphere to form a core material surface containing M... a O b The first coating layer is formed. For example, the sintering temperature can be 300℃-500℃, 350℃-450℃, 350℃-400℃, etc. The calcination time can be 3h-7h, 4h-6h, 5h-6h, etc. At this sintering temperature, a dense first coating layer can be formed on the surface of the core, thereby improving the environmental stability and moisture resistance of the material, and thus improving the cycle performance of the battery containing it.

[0091] In some embodiments of this application, when the core material is combined with M a O b Before sintering, the core material is pre-mixed and sintered with a carbon source, thereby forming a carbon-containing second coating layer on the surface of the core material. This improves the environmental stability, moisture resistance, and conductivity of the positive electrode active material, thereby enhancing the cycle performance and kinetic performance of the battery containing it.

[0092] In some embodiments of this application, the core material is mixed with a carbon source and sintered under nitrogen as a protective gas, and the temperature is raised to 500℃-600℃, for example 520℃-580℃, 540℃-560℃, and held for 6h-14h, for example 7h-13h, 8h-12h, 9h-11h, 10h-11h. As non-carbon elements in the carbon source are released, a second carbon-containing coating layer is formed on the surface of the core material.

[0093] As an example, the carbon source may include at least one of sucrose, tannic acid, polyethylene glycol, polyvinylpyrrolidone, glucose, ascorbic acid, or citric acid.

[0094] A third aspect of this application provides a positive electrode sheet, which includes the positive active material described in the first aspect of this application or the positive active material obtained by the method described in the second aspect of this application.

[0095] The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, wherein the positive active material layer includes the positive active material described above or a positive active material obtained by the method described above.

[0096] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive active material layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0097] In some embodiments of this application, the positive electrode current collector may be a metal foil or a composite negative electrode current collector. For example, aluminum foil may be used as the metal foil. The composite negative electrode 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 negative electrode current collector may 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.).

[0098] In some embodiments of this application, the positive electrode active material layer may optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, or fluorinated acrylate resin.

[0099] In some embodiments of this application, the positive electrode active material layer may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers.

[0100] 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.

[0101] A fourth aspect of this application provides a battery comprising the positive electrode sheet described in the third aspect of this application.

[0102] In some embodiments of this application, the battery includes a sodium-ion battery.

[0103] Typically, a battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0104] The negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, the negative active material layer including the polymer described in the first aspect of this application.

[0105] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode active material layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0106] In some embodiments of this application, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may 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 may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0107] In some embodiments of this application, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material may include at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, or silicon alloys. The tin-based material may include at least one of elemental tin, tin oxide compounds, or tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0108] In some embodiments of this application, the negative electrode active material layer may optionally include a conductive agent. The conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers.

[0109] In some embodiments of this application, the negative electrode active material layer may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0110] In some embodiments of this application, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative electrode active material, conductive agent and binder, in a solvent (e.g. deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0111] This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or completely solid.

[0112] In some embodiments of this application, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0113] In some embodiments of this application, the electrolyte sodium salt may include at least one of sodium hexafluorophosphate, sodium difluorooxalate borate, sodium tetrafluoroborate, sodium dioxalate borate, sodium perchlorate, sodium hexafluoroarsenate, sodium bis(fluorosulfonyl)imide, sodium trifluoromethanesulfonate, or sodium bis(trifluoromethanesulfonyl)imide.

[0114] In some embodiments of this application, the solvent may include at least one selected from 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, ethylene glycol dimethyl ether, methyl ethyl sulfone, or diethyl sulfone.

[0115] In some embodiments of this application, 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.

[0116] This application does not impose any particular restrictions on the type of separator membrane; any known porous separator membrane with good chemical and mechanical stability can be selected.

[0117] In some embodiments, the material of the separator may include at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, or polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0118] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0119] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0120] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0121] This application does not impose any particular limitation on the shape of the battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 3 Here is a square-structured battery cell 1 as an example.

[0122] In some implementations, refer to Figure 4 The outer packaging may include a housing 11 and a cover plate 13. The housing 11 may include a bottom plate and side plates connected to the bottom plate, the bottom plate and side plates forming a receiving cavity. The housing 11 has an opening communicating with the receiving cavity, and the cover plate 13 can be placed over the opening to close the receiving cavity. The positive electrode sheet, negative electrode sheet, and separator can be formed into an electrode assembly 12 by a winding process or a stacking process. The electrode assembly 12 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 12. The number of electrode assemblies 12 contained in a single battery cell 1 can be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0123] In some implementations, the batteries can be assembled into battery modules, and the number of batteries contained in a battery module can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery module.

[0124] Figure 5 This is battery module 2 as an example. (See reference...) Figure 5 In battery module 2, multiple battery cells 1 can be arranged sequentially along the length of battery module 2. Of course, they can also be arranged in any other way. Furthermore, these multiple battery cells 1 can be fixed in place using fasteners.

[0125] Optionally, the battery module 2 may also include a housing with a receiving space in which multiple battery cells 1 are received.

[0126] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0127] Figure 6 and Figure 7 This is battery pack 3 as an example. (See reference...) Figure 6 and Figure 7 The battery pack 1 may include a battery box and multiple battery modules 2 disposed within the battery box. The battery box includes an upper box 31 and a lower box 32, with the upper box 31 covering the lower box 32 to form a closed space for accommodating the battery modules 2. The multiple battery modules 2 can be arranged in any manner within the battery box.

[0128] In addition, this application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in this application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0129] As the electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0130] Figure 8 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery for this device, a battery pack or battery module can be used.

[0131] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can be powered by a battery.

[0132] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0133] Example 1

[0134] 1. Preparation of positive electrode sheet

[0135] (1) Preparation of positive electrode active material

[0136] Na4Fe3(PO4)2P2O7 material was uniformly mixed in anhydrous ethanol solvent, and titanium dioxide sol was added. The mass ratio of Na4Fe3(PO4)2P2O7 material to solvent was 80:200. The solvent was removed by vacuum evaporation (80℃) using a rotary evaporator. The material was then washed using a suction filter, and the filter cake was dried in a vacuum oven. Subsequently, the powder material was calcined at 300℃ for 6 hours under a nitrogen atmosphere. After natural cooling, a first coating layer including titanium dioxide was formed on the surface of Na4Fe3(PO4)2P2O7 material, which is the positive electrode active material.

[0137] (2) Preparation of positive electrode sheet

[0138] Polyvinylidene fluoride binder is fully dissolved in N-methylpyrrolidone, and carbon black conductive agent and positive electrode active material are added to form a uniformly dispersed slurry. The mass ratio of polyvinylidene fluoride binder to carbon black conductive agent and positive electrode active material is 10:20:70. The slurry is uniformly coated on both sides of aluminum foil, and then transferred to a vacuum drying oven for complete drying. The resulting electrode sheet is rolled and then punched to obtain the positive electrode sheet.

[0139] 2. Preparation of negative electrode sheet

[0140] The negative electrode active material hard carbon, conductive agent carbon black (Super P), binder styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC-Na) are mixed evenly in an appropriate amount of deionized water at a mass ratio of 96:1.5:1.5:1 to obtain a negative electrode slurry. The negative electrode slurry is coated on both sides of the negative electrode current collector copper foil. After drying and cold pressing, a negative electrode active material layer is formed on both sides of the negative electrode current collector. Finally, the negative electrode sheet is obtained through slitting and cutting processes.

[0141] 3. Preparation of electrolyte

[0142] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), fully dried electrolyte salt NaPF6 was dissolved in ethylene glycol dimethyl ether and mixed thoroughly to obtain an electrolyte with a concentration of 0.5 mol / L.

[0143] 4. Separating membrane

[0144] Polypropylene film is used as the separator.

[0145] 5. Preparation of secondary batteries

[0146] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. The electrode assembly is then wound up. The electrode assembly is placed in an outer package, and the prepared electrolyte is injected into the dried sodium-ion battery. After vacuum sealing, settling, formation, and shaping, the sodium-ion battery is obtained.

[0147] The preparation methods of sodium-ion batteries in Examples 2-35 and Comparative Example 1 are the same as those in Example 1, except that the composition of the positive electrode active material is different, as shown in Table 1.

[0148] Example 36

[0149] Preparation of negative electrode sheet

[0150] Conductive agent carbon black (Super P) and binder styrene-butadiene rubber (SBR) are mixed evenly in an appropriate amount of deionized water at a mass ratio of 1.5:1.5 to obtain a negative electrode slurry. The negative electrode slurry is coated on both sides of the negative electrode current collector copper foil. By drying and cold pressing, a negative electrode active material layer is formed on both sides of the negative electrode current collector. Finally, the negative electrode sheet is obtained by slitting and cutting. The remaining steps, such as the preparation of the positive electrode sheet, electrolyte, separator and sodium-ion battery, are the same as in Example 1.

[0151] The XRD patterns of the positive electrode active materials of Example 1 and Comparative Example 1 are shown in the figure. Figure 9 As shown, by Figure 9 It can be seen that the XRD patterns of the positive electrode active materials of Example 1 and Comparative Example 1 have the same peak positions as the XRD pattern of the positive electrode active material of Comparative Example 1, indicating that coating the outer surface of the core with a first coating layer in Example 1 will not destroy the crystal structure of the core material, and because the amount of the first coating is small, the characteristic peaks of the coating layer material do not appear in the XRD diffraction peaks.

[0152] The distribution of Ti element in the EDS energy spectrum of the positive electrode active material in Example 1 (referencing the test method in GB / T 17359-2012) is shown below. Figure 10 As shown, the Ti element is uniformly distributed in the first coating layer on the surface of the positive electrode active material.

[0153]

[0154]

[0155] Example 37

[0156] Preparation of positive electrode active materials

[0157] Na4R3(PO4)2P2O7 material was mixed with glucose and sintered under nitrogen as a protective gas, heated to 550℃ and held for 4 hours. As non-carbon elements in the glucose were released, a second carbon-containing coating layer was formed on the surface of the core material. The core with the second coating layer was then uniformly mixed in anhydrous ethanol solvent, and titanium dioxide sol was added. The mass ratio of Na4R3(PO4)2P2O7 material to solvent was 80:200. The solvent was removed by vacuum evaporation (80℃) using a rotary evaporator. The material was then washed using a filtration device, and the filter cake was dried in a vacuum oven. The powder material was then calcined at 300℃ for 6 hours under a nitrogen atmosphere. After natural cooling, a first coating layer including titanium dioxide was formed on the surface of the second coating layer, which is the positive electrode active material.

[0158] The remaining steps, such as the preparation of the positive electrode, negative electrode, electrolyte, separator, and sodium-ion battery, are the same as in Example 1.

[0159] The preparation methods of sodium-ion batteries in Examples 38-44 and Comparative Example 2 are the same as those in Example 37, except that the composition of the positive electrode active material is different, as shown in Table 2.

[0160] Example 45

[0161] Preparation of negative electrode sheet

[0162] Conductive agent carbon black (Super P) and binder styrene-butadiene rubber (SBR) are mixed evenly in an appropriate amount of deionized water at a mass ratio of 1.5:1.5 to obtain a negative electrode slurry. The negative electrode slurry is coated on both sides of the negative electrode current collector copper foil. By drying and cold pressing, a negative electrode active material layer is formed on both sides of the negative electrode current collector. Finally, the negative electrode sheet is obtained by slitting and cutting. The remaining steps, such as the preparation of the positive electrode sheet, electrolyte, separator and sodium-ion battery, are the same as in Example 37.

[0163]

[0164] The environmental stability of the positive electrode active materials of Examples 1-45 and Comparative Examples 1-2, as well as the first-cycle coulombic efficiency and cycle performance of the obtained sodium-ion batteries, were characterized. The characterization results are shown in Table 3.

[0165] (1) Environmental stability of positive electrode active materials

[0166] Expose the positive electrode active material to room temperature (humidity approximately 10%) and test the water content of the positive electrode active material at 0 min and 60 min respectively. For specific test methods, please refer to the test method in GB / T 11133-2015.

[0167] (2) Coulombic efficiency of sodium-ion battery in the first cycle

[0168] The sodium-ion secondary battery was charged at 0°C with a constant current of 1 / 3C to 3.75V, and then charged at a constant voltage of 3.75V until the current dropped to 0.05C, yielding the initial charge capacity (C). c1 Then discharge at a constant current of 1 / 3C to 1.5V to obtain the initial discharge capacity (C). d1 The first-cycle coulombic efficiency of the sodium-ion battery is calculated as: first discharge capacity (Cd1) / first charge capacity (Cc1) * 100%.

[0169] (3) Cyclic performance test:

[0170] The ambient temperature was controlled at 25℃. The battery was charged at 1C to 3.65V, then charged at constant voltage to 0.05C, left to stand for 10 minutes, and then discharged at 1C to 1.5V. The discharge capacity was recorded as C0. The above charge and discharge process was repeated for 300 cycles. The discharge capacity of the 300th cycle was C1. The cycle capacity retention rate of the cell = C1 / C0*100%.

[0171] Table 3

[0172]

[0173]

[0174] Conclusion: Comparing Examples 1-36 and Comparative Example 1, the positive electrode active material of Examples 1-36 had significantly lower water content after 60 minutes compared to the positive electrode active material of Comparative Example 1, indicating that the positive electrode active material of Examples 1-36 has higher moisture resistance. Furthermore, the first-cycle coulombic efficiency and capacity retention of the batteries corresponding to Examples 1-36 were higher than those of Comparative Example 1. Comparing Examples 37-45 and Comparative Example 2, the positive electrode active material of Examples 37-45 had significantly lower water content after 60 minutes compared to the positive electrode active material of Comparative Example 1, indicating that the positive electrode active material of Examples 37-45 had higher moisture resistance. Furthermore, the first-cycle coulombic efficiency and capacity retention of the batteries corresponding to Examples 37-45 were higher than those of Comparative Example 2. This demonstrates that the positive electrode active material of this application has excellent moisture resistance and improves the first-cycle coulombic efficiency and cycle performance of batteries containing it.

[0175] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A positive electrode active material, characterized in that, include: The kernel, the kernel comprising Na x R y (PO4) z (P2O7) k Wherein, 1≤x≤7, 1≤y≤4, 1≤z≤4, 1≤k≤4, and R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W, or Pb; A first coating layer is formed on at least a portion of the core, the first coating layer comprising M a O b M includes at least one of Ca, Bi, Ba, Ti, Al, Nb, Mg, Fe, Cu, Zn, Mn, Ni or Co, 1≤a≤7, 1≤b≤12; The positive electrode active material further includes a second coating layer formed on at least a portion of the core, the first coating layer being formed on at least a portion of the second coating layer, and the second coating layer comprising carbon.

2. The positive electrode active material according to claim 1, characterized in that, The core comprises at least one of Na4R3(PO4)2P2O7 or Na7V4(PO4)4P2O7, wherein R comprises at least one of Fe, Ni, Co or Mn.

3. The positive electrode active material according to claim 1 or 2, characterized in that, M a O b Including TiO2, Al2O3, Nb2O5, MgO, Co3O4, CaTiO3, Bi4Ti3O 12 Or at least one of BaTiO3.

4. The positive electrode active material according to any one of claims 1-3, characterized in that, Based on the total mass of the positive electrode active material, the content of the first coating layer is w%. Based on the total mass of the kernel, the residual alkali content of the kernel is z%, and 0.06≤z / w≤3.

5. The positive electrode active material according to any one of claims 1-3, characterized in that, Based on the total mass of the positive electrode active material, the content of the first coating layer is w%. Based on the total mass of the kernel, the residual alkali content of the kernel is z%, and 0.06≤z / w≤1.

6. The positive electrode active material according to claim 4 or 5, characterized in that, 1%≤w%≤3%。 7. The positive electrode active material according to claim 4 or 5, characterized in that, 1.5%≤w%≤2.5%。 8. The positive electrode active material according to claim 4 or 5, characterized in that, 0.2%≤z%≤3%。 9. The positive electrode active material according to claim 4 or 5, characterized in that, 0.2%≤z%≤0.6%。 10. The positive electrode active material according to any one of claims 1-9, characterized in that, The thickness of the first coating layer is 1nm-10nm.

11. The positive electrode active material according to any one of claims 1-9, characterized in that, The thickness of the first coating layer is 2nm-8nm.

12. The positive electrode active material according to any one of claims 1 to 11, characterized in that, Based on the total mass of the positive electrode active material, the carbon content in the second coating layer is n%, and 0.16≤n / w≤1.

5.

13. The positive electrode active material according to any one of claims 1 to 11, characterized in that, Based on the total mass of the positive electrode active material, the carbon content in the second coating layer is n%, and 0.32≤n / w≤0.

8.

14. The positive electrode active material according to claim 12 or 13, characterized in that, 0.5%≤n%≤1.5%。 15. The positive electrode active material according to claim 12 or 13, characterized in that, 0.8%≤n%≤1.2%。 16. The positive electrode active material according to any one of claims 1-15, characterized in that, The positive electrode active material satisfies at least one of the following conditions: The volume average particle size Dv50 of the positive electrode active material is 2μm-8μm; The Dv99 of the positive electrode active material is 10μm-40μm; The BET specific surface area of ​​the positive electrode active material is 3m². 2 / g-11m 2 / g; The compaction density of the positive electrode active material under 100 MPa pressure is 1.3 g / cm³. 3 -2.1g / cm 3 .

17. The positive electrode active material according to any one of claims 1-16, characterized in that, The positive electrode active material satisfies at least one of the following conditions: The volume average particle size Dv50 of the positive electrode active material is 2μm-5μm; The Dv99 of the positive electrode active material is 15μm-30μm; The BET specific surface area of ​​the positive electrode active material is 7m². 2 / g-10m 2 / g; The compaction density of the positive electrode active material under 100 MPa pressure is 1.7 g / cm³. 3 -1.9g / cm 3 .

18. A method for preparing a positive electrode active material, characterized in that, include: Provide a core material, the core material comprising Na x R y (PO4) z (P2O7) k Wherein, 1≤x≤7, 1≤y≤4, 1≤z≤2, 1≤k≤4, and R includes at least one of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Hf, Ta, W, or Pb; The core material and M a O b Mixed sintering, where M includes at least one of Ti, Al, Mg, Fe, Cu, Zn, Mn, Ni or Co, 1≤a≤7, 1≤b≤12, yields a positive electrode active material.

19. The method according to claim 18, characterized in that, The sintering temperature is 250℃-550℃, and the holding time is 2h-8h.

20. The method according to claim 18 or 19, characterized in that, When the core material is combined with M a O b Before sintering, the core material is pre-mixed and sintered with a carbon source.

21. A positive electrode plate, characterized in that, It includes the positive electrode active material according to any one of claims 1-17 or the positive electrode active material obtained by the method according to any one of claims 18-20.

22. A battery, characterized in that, Includes the positive electrode sheet as described in claim 21.

23. The battery according to claim 22, characterized in that, The battery includes a sodium-ion battery.

24. An electrical appliance, characterized in that, Includes the battery as described in claim 22 or 23.

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