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

Abstract

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 inner core and a first coating layer; the inner core comprises NaxRy(PO4)z(P2O7)k, wherein 1≤x≤7, 1≤y≤4, 1≤z≤4, 1≤k≤4, and 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 inner core, and the first coating layer comprises MaOb, wherein M comprises at least one of Ca, Bi, Ba, Ti, Al, Nb, Mg, Fe, Cu, Zn, Mn, Ni or Co, 1≤a≤7, and 1≤b≤12.

Description

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

[0001] The present application belongs to the field of batteries, and specifically relates to a positive electrode active material and a preparation method thereof, a positive electrode sheet, a battery and an electrical device. Background Art

[0002] Lithium-ion batteries face significant challenges due to the increasing scarcity of lithium resources, rising upstream material prices, lagging recycling technology development, and low recycling rates for older batteries. Sodium-ion batteries utilize the intercalation and deintercalation of sodium ions between the positive and negative electrodes to achieve charge and discharge. Furthermore, sodium resources are far more abundant and widely distributed than lithium, and their cost is significantly lower. Therefore, sodium-ion batteries have become a promising next-generation electrochemical system to replace lithium secondary batteries. However, the poor stability of existing cathode active materials has led to poor cycling performance in sodium-ion batteries.

[0003] Summary of the Invention

[0004] In view of the technical problems existing in the background technology, the present application provides a positive electrode active material, aiming to improve the cycle performance of a battery containing the positive electrode active material.

[0005] In order to achieve the above-mentioned objectives, the present application provides, in one aspect, a positive electrode active material, comprising:

[0006] The core comprises Na x R y (PO4) z (P2O7) k , wherein 1≤x≤7, 1≤y≤4, 1≤z≤4, 1≤k≤4, and 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;

[0007] A first cladding layer is formed on at least a portion of the core, wherein the first cladding layer 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.

[0008] The present application at least includes the following beneficial effects: x R y (PO4) z (P2O7) k At least a portion of the core is formed including M a Ob The first coating layer, wherein M comprises at least one of Ca, Bi, Ba, Ti, Al, Mg, Fe, Cu, Zn, Mn, Ni, or Co, can reduce the contact of the core with H2O, CO2, and the like in the air, thereby improving the environmental stability and moisture resistance of the material, and thus improving the cycle performance of the battery. At the same time, the first coating layer can reduce side reactions between the core surface and the electrolyte, stabilize the interface between the positive electrode active material and the electrolyte, improve the stability of the material, and thus improve the cycle performance of the battery containing the same. Furthermore, the first coating layer of this composition does not reduce the first-cycle coulombic efficiency of the positive electrode active material, thereby allowing the battery to maintain a high first-cycle coulombic efficiency.

[0009] In some embodiments of the present 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.

[0010] In some embodiments of the present application, M a O b Including TiO2, Al2O3, Nb2O5, MgO, Co3O4, CaTiO3, Bi4Ti3O 12 Or at least one of BaTiO3. Thus, the cycle performance of the battery containing it can be improved and the battery can maintain a high first-cycle coulombic efficiency.

[0011] In some embodiments of the present application, based on the total mass of the positive electrode active material, the content of the first coating layer is w%,

[0012] Based on the total mass of the core, the residual alkali content of the core is z%, 0.06≤z / w≤3, and optionally 0.06≤z / w≤1. Thus, the cycle performance of the battery containing the core can be improved and the battery can maintain a high first-cycle coulombic efficiency.

[0013] In some embodiments of the present application, 1%≤w%≤3%, and optionally 1.5%≤w%≤2.5%, thereby improving the cycle performance of the battery containing the same and maintaining a high first-cycle coulombic efficiency of the battery.

[0014] In some embodiments of the present application, 0.2%≤z%≤3%, optionally 0.2%≤z%≤0.6%.

[0015] In some embodiments of the present application, the thickness of the first coating layer is 1 nm to 10 nm, and may be 2 nm to 8 nm, thereby improving the cycle performance of a battery containing the first coating layer and maintaining a high first-cycle coulombic efficiency.

[0016] In some embodiments of the present application, the positive electrode active material further includes a second coating layer, the second coating layer being 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 can improve the environmental stability, moisture resistance, and electrical conductivity of the positive electrode active material, thereby improving the kinetic performance and cycle performance of a battery containing the same.

[0017] In some embodiments of the present application, the carbon content of the second coating layer is n%, based on the total mass of the positive electrode active material, 0.16≤n / w≤1.5, and optionally 0.32≤n / w≤0.8. This can improve the environmental stability, moisture resistance, and conductivity of the positive electrode active material, thereby improving the kinetic performance and cycle performance of batteries containing the same.

[0018] In some embodiments of the present application, 0.5%≤n%≤1.5%, optionally 0.8%≤n%≤1.2%. This can improve the environmental stability, moisture resistance, and conductivity of the positive electrode active material, thereby improving the kinetic performance and cycle performance of the battery containing the positive electrode active material.

[0019] In some embodiments of the present application, the positive electrode active material satisfies at least one of the following conditions:

[0020] The volume average particle size Dv50 of the positive electrode active material is 2 μm-8 μm, and can be optionally 2 μm-5 μm;

[0021] The Dv99 of the positive electrode active material is 10 μm-40 μm, and can be optionally 15 μm-30 μm;

[0022] The BET specific surface area of ​​the positive electrode active material is 3 m 2 / g-11m 2 / g, optional 7m 2 / g-10m 2 / g;

[0023] The compaction density of the positive electrode active material under a pressure of 100 MPa is 1.3 g / cm 3 -2.1g / cm 3 , optional 1.7g / cm 3 -1.9g / cm 3 .

[0024] The second aspect of the present application provides a method for preparing a positive electrode active material, comprising:

[0025] A core material is provided, 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 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;

[0026] The core material and M a O b Mixed sintering, M includes at least one of Ti, Al, Mg, Fe, Cu, Zn, Mn, Ni or Co, 1≤a≤7, 1≤b≤12, to obtain a positive electrode active material.

[0027] Therefore, the method of the present application can be used in the x R y (PO4) z (P2O7) k The core is formed 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) of the composition can reduce the contact of the core with H2O, CO2 and the like in the air, thereby improving the environmental stability and moisture resistance of the material, and then improving the cycle performance of the battery containing the same, and the first coating layer of the composition will not deteriorate the first-cycle coulomb efficiency of the positive electrode active material, so that the battery maintains a higher first-cycle coulomb efficiency.

[0028] In some embodiments of the present application, the sintering temperature is 250° C.-550° C., and the holding time is 2 h-8 h.

[0029] In some embodiments of the present application, when the core material is combined with M a O b Prior to mixed sintering, the core material is pre-mixed with a carbon source and sintered. This allows for the formation of a second carbon-containing coating layer on the surface of the core material, thereby improving the environmental stability, moisture resistance, and conductivity of the positive electrode active material, thereby enhancing the cycling and kinetic performance of the battery containing the material.

[0030] In a third aspect, the present application provides a positive electrode plate, which includes the positive electrode active material described in the first aspect of the present application or the positive electrode active material obtained by the method described in the second aspect of the present application.

[0031] The fourth aspect of the present application provides a battery, which includes the positive electrode sheet described in the third aspect of the present application.

[0032] In some embodiments of the present application, the battery comprises a sodium ion battery.

[0033] The fifth aspect of the present application provides an electrical device, which includes the battery described in the fourth aspect of the present application.

[0034] Additional aspects and advantages of the present application will be given in part in the description below, and in part will become obvious from the description below, or will be learned through practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] 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:

[0036] FIG1 is a schematic diagram of a positive electrode active material according to an embodiment of the present application.

[0037] FIG. 2 is a schematic diagram of a positive electrode active material according to another embodiment of the present application.

[0038] FIG3 is a schematic diagram of a battery according to an embodiment of the present application.

[0039] FIG. 4 is an exploded view of the battery shown in FIG. 3 according to an embodiment of the present application.

[0040] FIG5 is a schematic diagram of a battery module according to an embodiment of the present application.

[0041] FIG6 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0042] FIG. 7 is an exploded view of the battery pack shown in FIG. 6 according to an embodiment of the present application.

[0043] FIG8 is a schematic diagram of an electrical device using a battery as a power source according to an embodiment of the present application.

[0044] FIG9 is an XRD comparison diagram of the positive electrode active materials of Example 1 and Comparative Example 1 of the present application.

[0045] FIG10 is a distribution diagram of the Ti element in the EDS spectrum of the positive electrode active material of Example 1 of the present application.

[0046] Explanation of reference numerals: 1 battery; 11 housing; 12 electrode assembly; 13 cover plate; 2 battery module; 3 battery pack; 31 upper case; 32 lower case; 20 positive electrode active material; 21 core; 22 first covering layer; 23 second covering layer. DETAILED DESCRIPTION

[0047] The following embodiments of the technical solution of the present application are described in detail. The following embodiments are only used to more clearly illustrate the technical solution of the present application and are therefore only used as examples and are not intended to limit the scope of protection of the present application.

[0048] 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.

[0049] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0050] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0051] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0052] Unless otherwise specified, all steps of the present 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 may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0053] With the technological advancements and increasing demand for electric vehicles and rechargeable mobile devices, secondary batteries, as a representative of the new energy sector, are experiencing rapid growth in research. Sodium-ion batteries offer significant price advantages over traditional lithium-ion batteries and hold broad application prospects in large-scale energy storage systems.

[0054] Polyanionic phosphate cathode materials have become a popular choice for sodium-ion battery cathode active materials due to their abundant resources, environmental friendliness, ease of scalable production, open sodium ion diffusion channels, and excellent thermal stability. However, due to processing and formulation issues, these materials often contain residual alkali (such as sodium bicarbonate and sodium carbonate) on their surfaces. This residual alkali easily reacts with H2O and CO2 in the air, resulting in poor environmental stability and moisture resistance, which in turn reduces battery cycling performance.

[0055] This application is made by including Na x R y (PO4) z (P2O7) k At least a portion of the core is formed including M a O b The first coating layer, wherein M comprises at least one of Ca, Bi, Ba, Ti, Al, Nb, Mg, Fe, Cu, Zn, Mn, Ni, or Co, can reduce the contact of the core with H2O, CO2, and the like in the air, thereby improving the environmental stability and moisture resistance of the material, and thus improving the cycle performance of the battery. At the same time, the first coating layer can reduce side reactions between the core surface and the electrolyte, stabilize the interface between the positive electrode active material and the electrolyte, improve the stability of the material, and thus improve the cycle performance of the battery containing the same. Furthermore, the first coating layer of this composition does not reduce the first-cycle coulombic efficiency of the positive electrode active material, thereby allowing the battery to maintain a high first-cycle coulombic efficiency.

[0056] The positive electrode active material disclosed in the embodiments of the present application is suitable for sodium ion batteries, and the battery disclosed in the embodiments of the present application can be used in electrical equipment that uses the battery as a power source or various energy storage systems that use the battery as an energy storage element. Electrical equipment may include but is not limited to mobile phones, tablets, laptops, electric toys, power tools, battery cars, electric cars, ships, spacecraft, etc. Among them, electric toys may include fixed or mobile electric toys, for example, game consoles, electric car toys, electric ship toys and electric airplane toys, etc., and spacecraft may include airplanes, rockets, space shuttles and spacecraft, etc.

[0057] In the first aspect of the present application, a positive electrode active material is proposed. Referring to FIG1 , 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, 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] The present application at least includes the following beneficial effects: x R y (PO4) z (P2O7) k At least a portion of the core 21 is formed including 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. The first coating layer 22 of this composition can reduce the contact between the core 21 and H2O, CO2 and the like in the air, thereby improving the environmental stability and moisture resistance of the material, and thus improving the cycle performance of the battery. At the same time, the first coating layer 22 can reduce the 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. In addition, the first coating layer 22 of this composition will not reduce the first-cycle coulomb efficiency of the positive electrode active material 20, thereby allowing the battery to maintain a high first-cycle coulomb efficiency.

[0059] In some embodiments of the present application, the Na x R y (PO4) z (P2O7) k , 1≤x≤7, for example, x is 2≤x≤6, 3≤x≤5, 3≤x≤4, etc. Therefore, the core 21 includes this content of sodium ions, which can increase the gram capacity of the battery, thereby making the battery have a higher capacity.

[0060] In some embodiments of the present application, the Na x R y (PO4) z (P2O7) k 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, and 1≤y≤4, for example, 2≤x≤4, 3≤x≤4, etc. Therefore, the positive electrode active material composed of the present application has the advantages of chemical stability, thermal stability, electrochemical stability, and high specific capacity.

[0061] In some embodiments of the present application, the Na x R y (PO4) z (P2O7) k , 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 the present application, the core 21 includes at least one of Na4R3(PO4)2P2O7 or Na7V4(PO4)4P2O7, wherein R includes at least one of Fe, Ni, Co, or Mn, such as Na4Fe3(PO4)2P2O7, Na4Ni3(PO4)2P2O7, Na4Co3(PO4)2P2O7, and Na4Mn3(PO4)2P2O7. Thus, using the core 21 with this composition can increase the gram capacity of the positive electrode active material 20, thereby increasing the capacity of the battery containing the positive electrode active material 20.

[0064] It should be noted that Na is intercalated and consumed during the charge and discharge process of the battery, and the molar content of Na varies when the battery is discharged to different states. The molar content of Na in the examples of the core 21 in this application refers to the initial state of the material, i.e., the state before the material is added. When the positive electrode active material is used in a battery system, the molar content of Na will change after charge and discharge cycles.

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

[0066] In some embodiments of the present application, 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, for example, a is 2≤a≤6, 3≤a≤5, 3≤a≤4, etc. Therefore, the present application includes Na x R y (PO4) z (P2O7) k The core 21 is formed on at least a portion thereof with the M comprising the above composition. a O b The first coating layer 22 of this composition 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. At the same time, the first coating layer 22 can reduce the 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. In addition, the first coating layer 22 of this composition will 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.

[0067] As an example, M a Ob It 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 the present application, the M a O b Can include TiO2, Al2O3, Nb2O5, MgO, Co3O4, CaTiO3, Bi4Ti3O 12 Or at least one of BaTiO3. Therefore, the present application forms a first coating layer 22 of the above composition on the core 21. The first coating layer 22 of this composition 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 further improving the cycle performance of the battery. At the same time, the first coating layer 22 can reduce the 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. In addition, the first coating layer 22 of this composition will not reduce the first-cycle coulomb efficiency of the positive electrode active material 20, so that the battery maintains a high first-cycle coulomb efficiency.

[0069] In some embodiments of the present application, based on the total mass of the positive electrode active material 20, the content of the first coating layer 22 is w%; based on the total mass of the core 21, the residual alkali amount of the core 21 is z%, 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 formed on the surface of the core 21. The residual alkali on the surface easily reacts with H2O, CO2, etc. in the air, resulting in poor environmental stability and moisture resistance of the material. However, the present application controls the content w% of the first coating layer 22 and the residual alkali content z% of the core 21 to meet the above 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 the present application, the content w% of the first coating layer 22 and the residual alkali content z% of the core 21 meet 0.06≤z / w≤1.

[0070] In the present application, the residual alkali content z% of the core 21 can be measured using instruments and methods known in the art. For example, the free sodium content of the core in the sodium-ion battery positive electrode active material can be tested with reference to the test for free lithium in GB / T9725-2007 "General Rules for Potentiometric Titration of Chemical Reagents." The test result is calculated based on the ratio of the molecular weights of residual sodium to residual lithium. For example, the molecular weight of NaHCO3 is 84 and the molecular weight of LiHCO3 is 68. The free lithium content obtained from the test result is 0.1%. Based on the ratio of the molecular weights of NaHCO3 to LiHCO3, the free sodium content of the positive electrode material is converted to 0.124%. The method for testing the free lithium content of the positive electrode material in GB / T9725-2007 "General Rules for Potentiometric Titration of Chemical Reagents" is as follows: sample and prepare the test solution according to the product standard. Insert the specified electrodes, start the electromagnetic stirrer, and titrate with the specified standard titration solution. Add approximately 90 percent of the desired titration volume of standard titrant from the burette and measure the potential or pH of the solution. Measure the potential or pH every time 1 mL or an appropriate amount of standard titrant is added. Before and after the stoichiometric point, measure the potential or pH every time 0.1 mL of standard titrant is added. Continue titrating until the potential or pH changes little. Record the burette reading and the measured potential or pH after each addition of standard titrant. Use this method graphically or by the second derivative to determine the endpoint of the titration.

[0071] In some embodiments of the present 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. Thus, the content w% of the first coating layer 22 of the present 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. At the same time, the first coating layer 22 of this content can reduce the side reaction 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 thereby improve the cycle performance of the battery containing it. In addition, the first coating layer 22 of this content will not reduce the first cycle coulomb efficiency of the positive electrode active material 20, thereby allowing the battery to maintain a high first cycle coulomb efficiency. In some other embodiments of the present application, based on the total mass of the positive electrode active material 20 , the content w% of the first coating layer 22 satisfies 1.5%≤w%≤2.5%.

[0072] In some embodiments of the present 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. Thus, the residual alkali content z% of the core 21 of the present application satisfies the above conditions, which can reduce the decomposition and gas production of residual alkali during battery cycling, reduce battery flatulence, and thus improve the cycle performance of the battery. In some embodiments of the present 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 the present application, the thickness of the first coating layer 22 can be 1nm-10nm, for example, 2nm-9nm, 3nm-8nm, 4nm-7nm, 5nm-6nm, etc. Therefore, the thickness of the first coating layer 22 of the present application meets the above conditions, which can improve the environmental stability and moisture resistance of the material, thereby improving the cycle performance of the battery. At the same time, the first coating layer 22 of this thickness can reduce the 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 thereby improve the cycle performance of the battery containing it. In addition, the first coating layer 22 of this thickness will not reduce the first-cycle coulomb efficiency of the positive electrode active material, thereby allowing the battery to maintain a higher first-cycle coulomb efficiency. In other embodiments of the present application, the thickness of the first coating layer 22 is 2nm-8nm.

[0074] In some embodiments of the present application, referring to FIG2 , the positive electrode active material 20 further includes a second coating layer 23, wherein the second coating layer 23 is formed on at least a portion of the core 21, and the first coating layer 22 is formed on at least a portion of the second coating layer 23, wherein the second coating layer 23 includes carbon. Thus, by coating the outer surface of the core 21 with the second coating layer 23 including carbon, the second coating layer 23 can not only improve the environmental stability and moisture resistance of the core 21, but also improve the electrical conductivity of the core 21, thereby improving the kinetic performance and cycle performance of a battery containing the same.

[0075] In some embodiments of the present 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, controlling the carbon content n% in the second coating layer 23 and the carbon content w% in the first coating layer 22 of the present application within the above ranges 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 batteries containing the positive electrode active material 20. In other embodiments of the present 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 the present application, based on the total mass of the positive electrode active material 20, the carbon content in the second coating layer 23 is n%, 0.5% ≤ 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 of the present application 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 the same. In other embodiments of the present 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 the present application, the volume average particle size Dv50 of the positive electrode active material 20 is 2 μm-8 μm. For example, the Dv50 of the positive electrode active material can be 2.5 μm-7.5 μm, 3 μm-7 μm, 3.5 μm-6.5 μm, 4 μm-6 μm, 4.5 μm-5.5 μm, 4.8 μm-5 μm, etc. In other embodiments of the present application, the volume average particle size Dv50 of the positive electrode active material 20 is 2 μm-5 μm.

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

[0079] In this application, the volume average particle size Dv50 refers to the particle size corresponding to when the cumulative volume distribution percentage reaches 50%, and Dv99 refers to the particle size corresponding to when the cumulative volume distribution percentage reaches 99%. For example, the test method of the volume average particle size Dv50 and Dv99 can refer to the standard GB / T 19077-2016 and be measured using a laser particle size analyzer (e.g., Malvern Master Size 3000).

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

[0081] In this application, the BET specific surface area of ​​the positive electrode active material 20 has a meaning well known in the art and can be measured using instruments and methods well known in the art. For example, it can be tested by referring to the following method: using the American Micrometer multi-station fully automatic specific surface area and pore analyzer GeminiVII2390, take about 7g of sample and put it into a 9cc long tube with a bulb, degas at 200℃ for 2h, and then put it into the main machine for testing to obtain the BET specific surface area data of the positive electrode active material 20.

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

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

[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 100 MPa pressure of the positive electrode active material 20 of the present application is within the above range, the conduction distance within the positive electrode active material 20 is small and the surface side reactions are small, which promotes the positive electrode active material 20 to exert its gram capacity and improves the capacity retention rate of the battery containing it.

[0085] The second aspect of the present application provides a method for preparing the positive electrode active material according to the first aspect, comprising:

[0086] S100: Provide core materials

[0087] In some embodiments of the present 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: The core material and M a O b Mixed sintering

[0089] In some embodiments of the present application, the core material of step S100 is mixed with M a O b Mixed sintering, the surface of the core material is coated with M a O b The first coating layer is formed by the sintering process, wherein M comprises at least one of Ti, Al, Mg, Fe, Cu, Zn, Mn, Ni or Co, 1≤a≤7, 1≤b≤12, and the positive electrode active material is obtained. Specifically, during the sintering process, the free sodium ions (residual alkali) on the surface of the core material will enter the crystal structure of the core material, thereby increasing the sodium ions in the core material. Therefore, the sintering process forms the positive electrode active material including M a O b The first coating layer does not deteriorate 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 a O b After mixing evenly, the solvent is evaporated off in vacuum using a rotary evaporator (70°C-100°C, for example, 70°C, 80°C, 90°C or 100°C), and the filter cake is washed several times using a suction filtration device, and the filter cake is dried in a vacuum oven, and then the powder material is calcined at 250°C-550°C under a nitrogen atmosphere for 2h-8h to form a M-containing matrix on the surface of the core material. a O b For example, the sintering temperature can be 300°C-500°C, 350°C-450°C, 350°C-400°C, etc. The calcination time can be 3 hours-7 hours, 4 hours-6 hours, 5 hours-6 hours, 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 further improving the cycle performance of batteries containing the same.

[0091] In some embodiments of the present application, when the core material is combined with M a O b Before mixed sintering, the core material is mixed with a carbon source and sintered in advance, so that a second carbon-containing coating layer can be formed on the surface of the core material, thereby improving the environmental stability, moisture resistance and conductivity of the positive electrode active material, thereby improving the cycle performance and kinetic performance of the battery containing it.

[0092] In some embodiments of the present application, the core material is mixed with a carbon source and sintered, and nitrogen is used as a protective gas, and the temperature is raised to 500°C-600°C, for example, 520°C-580°C, 540°C-560°C, and the temperature is kept for 6h-14h, for example, 7h-13h, 8h-12h, 9h-11h, 10h-11h for sintering. As the non-carbon elements in the carbon source are removed, 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, polyvinyl pyrrolidone, glucose, ascorbic acid, or citric acid.

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

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

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

[0097] In some embodiments of the present 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 base layer and a metal layer formed on at least one surface of the polymer material base layer. 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 the present application, the positive electrode active material layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, or a fluorine-containing acrylate resin.

[0099] In some embodiments of the present application, the positive electrode active material layer may further optionally include a conductive agent. As an example, the conductive agent may include at least one of 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 the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

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

[0102] In some embodiments of the present application, the battery comprises a sodium ion battery.

[0103] Typically, a battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge processes, active ions are inserted and removed between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

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

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

[0106] In some embodiments of the present 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 base layer and a metal layer formed on at least one surface of the polymer base material. 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 base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0107] In some embodiments of the present application, the negative electrode active material may be a negative electrode active material for a battery that is well known in the art. 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. The silicon-based material may include at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, or silicon alloys. The tin-based material may include at least one of elemental tin, tin oxides, or tin alloys. However, the present application is not limited to these materials, and other traditional 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 the present application, the negative electrode active material layer may further include a conductive agent, which may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers.

[0109] In some embodiments of the present application, the negative electrode active material layer may optionally further include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0110] In some embodiments of the present application, the negative electrode sheet can be prepared by the following method: the above-mentioned components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent and the binder are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0111] The present application has no specific restrictions on the type of electrolyte, which can be selected according to needs. For example, the electrolyte can be liquid, gel or solid.

[0112] In some embodiments of the present application, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0113] In some embodiments of the present application, the electrolyte sodium salt may include at least one of sodium hexafluorophosphate, sodium difluorooxalatoborate, sodium tetrafluoroborate, sodium bisoxalatoborate, sodium perchlorate, sodium hexafluoroarsenate, sodium bis(fluorosulfonyl)imide, sodium trifluoromethylsulfonate or sodium bis(trifluoromethylsulfonyl)imide.

[0114] In some embodiments of the present application, the solvent may include at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, ethylene glycol dimethyl ether, methyl ethyl sulfone or diethyl sulfone.

[0115] In some embodiments of the present application, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0116] The present application has no particular limitation on the type of isolation membrane, and any known isolation membrane with a porous structure having good chemical stability and mechanical stability can be selected.

[0117] In some embodiments, the material of the separator may include at least one of glass fiber, non-woven 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 embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

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

[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. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0121] The present application has no particular limitation on the shape of the battery, which can be cylindrical, square, or any other shape. For example, FIG3 shows a square-structured battery cell 1 as an example.

[0122] In some embodiments, referring to Figure 4, the outer packaging may include a shell 11 and a cover plate 13. The shell 11 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 11 has an opening connected to the receiving cavity, and the cover plate 13 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can form an electrode assembly 12 through a winding process or a lamination process. The electrode assembly 12 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 12. The number of electrode assemblies 12 contained in the battery cell 1 can be one or more, and those skilled in the art can select according to specific actual needs.

[0123] In some embodiments, batteries may be assembled into a battery module. The number of batteries contained in the battery module may be one or more. The specific number may be selected by those skilled in the art based on the application and capacity of the battery module.

[0124] Figure 5 shows an example battery module 2. Referring to Figure 5 , within the battery module 2, multiple battery cells 1 may be arranged sequentially along the length of the battery module 2. Of course, they may also be arranged in any other manner. Furthermore, the multiple battery cells 1 may be secured together using fasteners.

[0125] Optionally, the battery module 2 may further include a housing having an accommodation space, and the plurality of battery cells 1 are accommodated in the accommodation space.

[0126] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0127] Figures 6 and 7 illustrate an example battery pack 3. Referring to Figures 6 and 7, a battery pack 1 may include a battery box and multiple battery modules 2 disposed within the battery box. The battery box comprises an upper case 31 and a lower case 32. The upper case 31 can be positioned over the lower case 32 to form an enclosed 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, the present application also provides an electric device, which includes at least one of the secondary battery, battery module, or battery pack provided in the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as 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., but is not limited thereto.

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

[0130] Figure 8 shows an example of an electric device. This device is 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, a battery pack or battery module can be used.

[0131] Another example device may be a mobile phone, a tablet computer, a laptop computer, etc. Such a device is usually required to be light and thin, and may use a battery as a power source.

[0132] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0133] Example 1

[0134] 1. Preparation of positive electrode sheet

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

[0136] Na4Fe3(PO4)2P2O7 material is evenly mixed in an anhydrous ethanol solvent, and titanium dioxide sol is added, wherein the mass ratio of Na4Fe3(PO4)2P2O7 material to solvent is 80:200. The solvent is evaporated out in vacuum (80°C) using a rotary evaporator, and then the material is washed with a suction filtration device. The filter cake is dried in a vacuum oven, and then the powder material is calcined at 300°C for 6 hours under a nitrogen atmosphere. After natural cooling, a first coating layer including titanium dioxide is formed on the surface of the Na4Fe3(PO4)2P2O7 material, i.e., the positive electrode active material.

[0137] (2) Preparation of positive electrode sheet

[0138] The polyvinylidene fluoride binder is fully dissolved in N-methylpyrrolidone, and a carbon black conductive agent and a positive electrode active material are added to prepare a uniformly dispersed slurry, wherein the mass ratio of the polyvinylidene fluoride binder to the carbon black conductive agent and the positive electrode active material is 10:20:70. The slurry is evenly coated on both sides of the aluminum foil, and then transferred to a vacuum drying oven for complete drying. The obtained 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 solvent 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. Through drying and cold pressing, a negative electrode active material layer is formed on both sides of the negative electrode current collector. Finally, after striping and cutting processes, a negative electrode sheet is obtained.

[0141] 3. Preparation of electrolyte

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

[0143] 4. Isolation film

[0144] Polypropylene film is used as the isolation film.

[0145] 5. Preparation of secondary batteries

[0146] The positive electrode sheet, the separator, and the negative electrode sheet are stacked in order, so that the separator is placed between the positive and negative electrode sheets to play an isolating role, and then wound to obtain an electrode assembly; the electrode assembly is placed in an outer package, and the prepared electrolyte is injected into the dried sodium ion battery. After vacuum packaging, standing, formation, and shaping processes, a sodium ion battery is obtained.

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

[0148] Example 36

[0149] Preparation of negative electrode sheet

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

[0151] The XRD comparison diagram of the positive electrode active materials of Example 1 and Comparative Example 1 is shown in Figure 9. As can be seen from Figure 9, the peak positions of the XRD diagrams of the positive electrode active materials of Example 1 and Comparative Example 1 are the same as those of the XRD diagram of the positive electrode active material of Comparative Example 1, indicating that coating the first coating layer on the outer surface of the core in Example 1 does not destroy the lattice structure of the core material, and because the amount of the first coating is small, the characteristic peak of the coating layer material does not appear in the XRD diffraction peak.

[0152] The distribution diagram of the Ti element in the EDS spectrum of the positive electrode active material of Example 1 (refer to the test method of GB / T 17359-2012) is shown in FIG10 , which shows that the Ti element is evenly distributed in the first coating layer on the surface of the positive electrode active material.

[0153] Example 37

[0154] Preparation of positive electrode active materials

[0155] Na4R3(PO4)2P2O7 material is mixed with glucose and sintered. The temperature is raised to 550℃ under nitrogen as a protective gas and kept warm for 4 hours. As the non-carbon elements in the glucose are removed, a carbon-containing second coating layer is formed on the surface of the core material. The core forming the second coating layer is placed in anhydrous ethanol solvent and evenly mixed. Titanium dioxide sol is added, wherein the mass ratio of Na4R3(PO4)2P2O7 material to the solvent is 80:200. The solvent is evaporated out in a vacuum (80℃) using a rotary evaporator, and then the material is washed with a suction filtration device. The filter cake is dried in a vacuum oven, and then the powder material is calcined at 300℃ under a nitrogen atmosphere for 6 hours. After natural cooling, a first coating layer including titanium dioxide is formed on the surface of the second coating layer, i.e., the positive electrode active material.

[0156] The remaining steps such as the preparation of the positive electrode sheet, the negative electrode sheet, the electrolyte, the isolation membrane and the sodium ion battery are the same as those in Example 1.

[0157] The preparation methods of the sodium ion batteries of Examples 38-44 and Comparative Example 2 are the same as those of Example 37, except that the compositions of the prepared positive electrode active materials are different, as shown in Table 2.

[0158] Example 45

[0159] Preparation of negative electrode sheet

[0160] The conductive agent carbon black (Super P) and the binder styrene-butadiene rubber (SBR) were mixed uniformly in a proper amount of deionized water as a solvent at a mass ratio of 1.5:1.5 to obtain a negative electrode slurry. The negative electrode slurry was coated on both sides of the negative electrode current collector copper foil. A negative electrode active material layer was formed on both sides of the negative electrode current collector through drying and cold pressing. Finally, the negative electrode was obtained through striping and cutting processes. The remaining steps, such as the preparation of the positive electrode, electrolyte, isolation membrane and sodium ion battery, were the same as those in Example 37.

[0161] 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.

[0162] (1) Environmental stability of positive electrode active materials

[0163] The positive electrode active material was exposed to an environment at room temperature (humidity of about 10%), and the water content of the positive electrode active material was tested at 0 min and 60 min, respectively. The specific test method can refer to the GB / T 11133-2015 test method.

[0164] (2) First-cycle Coulombic efficiency of sodium-ion batteries

[0165] The sodium ion secondary battery was charged at 0°C at 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. The initial charge capacity (C c1 ); then discharge to 1.5V at a constant current of 1 / 3C to obtain the first discharge capacity (C d1 ), and calculate the first cycle coulombic efficiency of the sodium ion battery = first discharge capacity (Cd1) / first charge capacity (Cc1) * 100%.

[0166] (3) Cyclic performance test:

[0167] The ambient temperature was controlled at 25°C. The battery was charged to 3.65V at 1C, then charged to 0.05C at a constant voltage, left to stand for 10 minutes, and then discharged to 1.5V at 1C. The discharge capacity was recorded as C0. 300 cycles were performed according to the above charge and discharge process. The discharge capacity at the 300th cycle was C1. The cycle capacity retention rate of the battery cell = C1 / C0*100%.

[0168] Table 3

[0169] Conclusion: Comparing Examples 1-36 with Comparative Example 1, the water content of the positive electrode active materials of Examples 1-36 after 60 minutes is significantly lower than that of the positive electrode active materials of Comparative Example 1, indicating that the positive electrode active materials of Examples 1-36 have higher moisture resistance, and the first-cycle coulombic efficiency and capacity retention rate of the corresponding batteries of Examples 1-36 are higher than those of Comparative Example 1. Comparing Examples 37-45 with Comparative Example 2, the water content of the positive electrode active materials of Examples 37-45 is significantly lower than that of the positive electrode active materials of Comparative Example 1 after 60 minutes, indicating that the positive electrode active materials of Examples 37-45 have higher moisture resistance, and the first-cycle coulombic efficiency and capacity retention rate of the corresponding batteries of Examples 37-45 are higher than those of Comparative Example 2. This shows that the positive electrode active material of the present application has excellent moisture resistance and improves the first-cycle coulombic efficiency and cycle performance of the battery containing it.

[0170] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A positive electrode active material, wherein include: The kernel 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; A first cladding layer, the first cladding layer is formed on at least a portion of the core, the first cladding 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.

2. The positive electrode active material according to claim 1, wherein The inner 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.

3. The positive electrode active material according to claim 1 or 2, wherein 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 to 3, wherein 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 inner core, the residual alkali content of the inner core is z%, 0.06≤z / w≤3, and can be optionally 0.06≤z / w≤1.

5. The positive electrode active material according to claim 4, wherein 1%≤w%≤3%, optionally 1.5%≤w%≤2.5%.

6. The positive electrode active material according to claim 4 or 5, wherein 0.2%≤z%≤3%, optionally 0.2%≤z%≤0.6%.

7. The positive electrode active material according to any one of claims 1 to 6, wherein The thickness of the first coating layer is 1 nm-10 nm, and can be optionally 2 nm-8 nm.

8. The positive electrode active material according to any one of claims 4 to 6, wherein The positive electrode active material further includes a second coating layer formed on at least a portion of the inner core, the first coating layer formed on at least a portion of the second coating layer, and the second coating layer includes carbon.

9. The positive electrode active material according to claim 8, wherein 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.

10. The positive electrode active material according to claim 9, wherein 0.5%≤n%≤1.5%, optionally 0.8%≤n%≤1.2%.

11. The positive electrode active material according to any one of claims 1 to 10, wherein 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 2 μm-8 μm, and can be 2 μm-5 μm; The Dv99 of the positive electrode active material is 10 μm-40 μm, and can be optionally 15 μm-30 μm; The BET specific surface area of ​​the positive electrode active material is 3 m 2 / g-11m 2 / g, optional 7m 2 / g-10m 2 / g; The compaction density of the positive electrode active material under a pressure of 100 MPa is 1.3 g / cm 3 -2.1g / cm 3 , optional 1.7g / cm 3 -1.9g / cm 3 .

12. A method for preparing a positive electrode active material, wherein: include: A core material is provided, 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, M includes at least one of Ti, Al, Mg, Fe, Cu, Zn, Mn, Ni or Co, 1≤a≤7, 1≤b≤12, to obtain a positive electrode active material.

13. The method according to claim 12, wherein: The sintering temperature is 250° C.-550° C., and the heat preservation time is 2 h-8 h.

14. The method according to claim 12 or 13, wherein: In the case of mixing the core material with M a O b Before the mixed sintering, the core material and the carbon source are mixed and sintered in advance.

15. A positive electrode sheet, wherein: The invention comprises the positive electrode active material according to any one of claims 1 to 11 or the positive electrode active material obtained by the method according to any one of claims 12 to 14.

16. A battery, wherein: Including the positive electrode sheet as described in claim 15.

17. The battery according to claim 16, wherein: The battery comprises a sodium ion battery.

18. An electrical device, wherein: A battery comprising the battery of claim 16 or 17.

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