A positive electrode active material, a preparation method thereof, a sodium ion battery, and an electrical device

Abstract

The present application provides a positive electrode active material, a preparation method thereof, a sodium ion battery, and an electrical device, belonging to the technical field of manufacturing positive electrode active materials. The positive electrode active material includes a core, a first shell layer, and a second shell layer. The material of the core includes: Naa1Nib1Fec1Mnd1O2, where 0.75 ≤ a1 ≤ 0.85; B element is also doped in the first shell layer, and the material of the first shell layer includes: Naa2Nib2Fec2Mnd2BfO2, where 0.85 ≤ a2 ≤ 0.95, and a2 is not equal to a1; the second shell layer is an inorganic metal oxide layer. The sodium ion battery prepared from this positive electrode active material has both a relatively high first discharge capacity, relatively excellent storage performance, cycling performance, and rate performance.

Description

Technical Field

[0001] The present application relates to the technical field of manufacturing cathode active materials, and in particular, to a cathode active material, a preparation method thereof, a sodium ion battery, and an electrical device. Background Art

[0002] In the prior art, sodium-based layered cathode materials have received increasing attention due to their advantages of low cost and excellent high and low temperature performance. Among them, the P2-phase layered cathode material is a sodium-deficient phase, and the corresponding battery has relatively excellent storage performance. However, the first discharge capacity and rate performance of the prepared sodium ion battery are relatively low. In order to improve the first discharge capacity and rate performance of the corresponding battery, an O3-phase cathode active material (with a sodium content higher than that of the P2-phase, which is a sodium-rich phase material) is selected to coat the P2-phase. However, due to the poor material stability of the O3-phase material, the corresponding battery has a problem of poor cycle performance. Therefore, there is an urgent need to develop a new sodium-based layered cathode material so that the corresponding battery can have relatively high first discharge capacity, relatively excellent storage performance, cycle performance, and rate performance. Summary of the Invention

[0003] The purpose of the present application is to provide a cathode active material, a preparation method thereof, a sodium ion battery, and an electrical device. The sodium ion battery prepared from the cathode active material has relatively high first discharge capacity, relatively excellent storage performance, cycle performance, and rate performance.

[0004] The embodiments of the present application are implemented as follows:

[0005] In a first aspect, an embodiment of the present application provides a cathode active material, including a core, a first shell layer, and a second shell layer. The material of the core includes: Na a1 Ni b1 Fe c1 Mn d1 O2, where 0.75 ≤ a1 ≤ 0.85; the first shell layer coats the core, and B element is also doped in the first shell layer. The material of the first shell layer includes: Na a2 Ni b2 Fe c2 Mn d2 B f O2, where 0.85 ≤ a2 ≤ 0.95, and a2 is not equal to a1; the second shell layer coats the first shell layer, and the second shell layer is an inorganic metal oxide layer.

[0006] In this application, the positive electrode active material has the above specific structure. Specifically, the inner core includes a sodium-deficient phase material with a sodium content of 0.75 to 0.85. The inner core is sequentially coated with a first shell layer (including a sodium-rich phase material with a sodium content of 0.85 to 0.95) and a second shell layer. Moreover, the first shell layer is also doped with element B. Among them, the sodium-deficient inner core material enables the corresponding battery to have relatively excellent storage performance. The coating of the first shell layer helps to improve the first discharge capacity and rate performance of the corresponding battery. The doping of element B and the coating of the second shell layer help to improve the material stability of the first shell layer, thereby enabling the corresponding battery to have relatively excellent cycle performance. Therefore, the sodium-ion battery prepared from the positive electrode active material provided in the embodiments of this application has both a relatively high first discharge capacity, relatively excellent storage performance, cycle performance, and rate performance.

[0007] In some optional embodiments, the material of the inner core further satisfies: 0.20 ≤ b1 ≤ 0.35, 0.20 ≤ c1 ≤ 0.40, 0.20 ≤ d1 ≤ 0.40, and b1 + c1 + d1 = 1; the material of the first shell layer further satisfies: 0.20 ≤ c2 ≤ 0.40, 0.20 ≤ d2 ≤ 0.40, 0.0001 ≤ f ≤ 0.01, and b2 + c2 + d2 = 1; the material of the second shell layer includes: Na a3 XO b3 , a3 is equal to a2, 1 ≤ b3 ≤ 4, and X is selected from at least one of Mg, Al, Sr, Zr, Ti, and Y.

[0008] In the above technical solution, the materials of the inner core and the first shell layer are limited within a more suitable range so that the corresponding battery can have both a higher first discharge capacity, relatively excellent storage performance, cycle performance, and rate performance. In addition, the metal elements other than sodium in the second shell layer are selected as the above elements, and these metal elements contribute to the formation of the interfacial film during the charge and discharge process of the corresponding battery, thereby helping to improve the storage performance and cycle performance of the corresponding battery.

[0009] In some optional embodiments, X is selected from at least one of Zr and Y.

[0010] In the above technical solution, selecting element X as Zr or / and Y is more conducive to the formation of the interfacial film during the charge and discharge process of the corresponding battery, and thus is more beneficial to improving the storage performance and cycle performance of the corresponding battery.

[0011] In some optional embodiments, the BET of the positive electrode active material is 0.34 to 0.44 m 2 / g, and the Dv3 of the positive electrode active material is 2.3 to 3.2 μm.

[0012] In the above technical solution, both the BET and Dv3 of the positive electrode active material are within a relatively appropriate range, indicating that the material has a relatively appropriate particle size and less fine powder in the material, thereby improving the cycle performance of the battery.

[0013] In some alternative embodiments, in the positive electrode active material, the doping depth > 0.3 μm and the mass percentage of element B < 0.005%; the doping depth is equal to 0.1 μm and the mass percentage of element B > 0.02%.

[0014] In the above technical solution, limiting the content of element B at different doping depths within a specific range helps to further improve the material stability of the material, so that the corresponding battery has more excellent cycle performance.

[0015] In a second aspect, an embodiment of the present application provides a method for preparing a positive electrode active material, including the following steps: mixing raw materials containing Ni, Fe, Mn, and a first Na source to obtain a first mixture; wherein, in the first mixture, the ratio of the total molar amount of Ni, Fe, and Mn to the molar amount of Na is 1:(0.75 - 0.85); performing a first-stage sintering on the first mixture to obtain a core; mixing the core with a second Na source to obtain a second mixture; wherein, in the second mixture, the ratio of the total molar amount of Ni, Fe, and Mn to the molar amount of Na is 1:(0.85 - 0.95); performing a second-stage sintering on the second mixture to obtain a positive electrode material intermediate; mixing the positive electrode material intermediate, boric acid, and a metal source to obtain a third mixture; performing a third-stage sintering and a fourth-stage sintering on the third mixture in sequence, wherein the treatment temperature of the fourth-stage sintering is higher than that of the third-stage sintering, to obtain the positive electrode active material.

[0016] In the above technical solution, on the one hand, during the preparation of the positive electrode active material, the ratio of the total molar amount of Ni, Fe, and Mn in the first mixture to the molar amount of Na is limited to the range of 1:(0.75 - 0.85), and the ratio of the total molar amount of Ni, Fe, and Mn in the second mixture to the molar amount of Na is limited to the range of 1:(0.85 - 0.95), so as to prepare a positive electrode active material intermediate with a core-shell structure having a sodium-poor phase inside and a sodium-rich phase outside, thereby enabling the corresponding battery to have relatively excellent storage performance, first discharge capacity, and rate performance; at the same time, there is a first shell layer of sodium-rich phase outside the sodium-poor phase core (the sodium-rich phase is more active than the sodium-poor phase), which helps the formation of the second shell layer during the subsequent sintering process (it is easier to form a coating layer and the formed coating layer is more uniform, so as to increase the structural strength of the material and make it not easy to form small particles during the subsequent crushing process), and thus the prepared positive electrode active material has the advantage of fewer fine powders. On the other hand, in addition to the positive electrode material intermediate and the metal source, the third mixture also contains boric acid. By virtue of the fluidity of boric acid, a relatively uniform inorganic metal oxide layer can be formed during the third-stage sintering process, thereby improving the coating uniformity of the inorganic metal oxide layer on the positive electrode material intermediate, and further reducing the alkali amount on the material surface and also helping to improve the stability of the material, so that the corresponding battery has relatively excellent cycle performance. At the same time, the relatively uniform coating layer also enables the prepared positive electrode active material to have the advantage of fewer fine powders; in addition, through the fourth-stage sintering at a higher temperature, the B element can be further doped from the material surface into the material, and the further doping of the B element also helps to improve the stability of the material, so that the corresponding battery has relatively excellent cycle performance. Through the combined action of the two aspects, the prepared positive electrode active material has the advantages of fewer fine powders, a lower alkali amount on the material surface, and higher material stability, and thus the prepared sodium-ion battery has a relatively high first discharge capacity, relatively excellent storage performance, cycle performance, and rate performance.

[0017] In some alternative embodiments, in the first mixture, the molar ratio of Ni, Fe, and Mn is successively (0.2 - 0.35):(0.2 - 0.4):(0.2 - 0.4); and / or, in the third mixture, the ratio of the total molar amount of Ni, Fe, and Mn to the molar amount of B is 1:(0.0001 - 0.01).

[0018] In the above technical solution, the molar ratio of Ni, Fe, and Mn in the first mixture is limited to a specific range of (0.2 to 0.35):(0.2 to 0.4):(0.2 to 0.4), and the molar ratio of the total amount of Ni, Fe, and Mn in the third mixture to the molar amount of B is limited to a specific range of 1:(0.0001 to 0.01), so that each element has a more appropriate mass percentage in the finally prepared cathode active material, and thus the corresponding battery can have both a higher initial discharge capacity, more excellent storage performance, cycling performance, and rate performance.

[0019] In some alternative embodiments, the metal element in the metal source is selected from at least one of Mg, Al, Sr, Zr, Ti, and Y.

[0020] In the above technical solution, there are many types of metals used to form the inorganic metal oxide layer, which can provide more alternative implementation solutions, thus facilitating the popularization and application of the technical solution provided by the embodiments of the present application.

[0021] In some alternative embodiments, the metal element in the metal source is selected from at least one of Zr and Y.

[0022] In the above technical solution, using the above specific types of metal elements in the metal source helps to form an interfacial film during the charge and discharge process of the corresponding battery, thus helping to improve the storage performance and cycling performance of the corresponding battery.

[0023] In some alternative embodiments, in the steps of the first-stage sintering and / or the second-stage sintering, the treatment temperature is 820 to 1000 °C, and the treatment time is 7 to 12 h.

[0024] In the above technical solution, limiting the treatment temperature and time during the first-stage sintering and / or the second-stage sintering to specific ranges can provide more appropriate sintering conditions for the raw materials to fully react.

[0025] In some alternative embodiments, in the step of the third-stage sintering, the treatment temperature is 250 to 500 °C, and the treatment time is 6 to 10 h; and / or, in the step of the fourth-stage sintering, the treatment temperature is 700 to 900 °C, and the treatment time is 8 to 12 h.

[0026] In the above technical solution, limiting the treatment temperature and time of the third-stage sintering to specific ranges helps to form an inorganic metal oxide layer with a uniform thickness and good coating integrity; limiting the treatment temperature and time of the fourth-stage sintering to specific ranges helps the B element to be doped from the material surface into the material interior.

[0027] In a third aspect, an embodiment of the present application provides a sodium-ion battery, including the positive electrode active material provided in the embodiment of the first aspect or the positive electrode active material prepared by the preparation method provided in the embodiment of the second aspect.

[0028] In the above technical solution, the sodium-ion battery includes a positive electrode active material with a specific structure. Specifically, the inner core includes a sodium-deficient phase material with a sodium content of 0.75 to 0.85. A first shell layer (including a sodium-rich phase material with a sodium content of 0.85 to 0.95) and a second shell layer are sequentially coated outside the inner core. Moreover, element B is doped in the first shell layer. Among them, the sodium-deficient inner core material enables the corresponding battery to have relatively excellent storage performance. The coating of the first shell layer helps to improve the first discharge capacity and rate performance of the corresponding battery. The doping of element B and the coating of the second shell layer help to improve the material stability of the first shell layer, so that the corresponding battery has relatively excellent cycle performance. Therefore, the sodium-ion battery prepared from this positive electrode active material can have both a high first discharge capacity, relatively excellent storage performance, cycle performance, and rate performance.

[0029] In a fourth aspect, an embodiment of the present application provides an electrical device, including the sodium-ion battery provided in the embodiment of the third aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings required for use in the embodiments. It should be understood that the following drawings only show some embodiments of the present application, so they should not be regarded as limiting the scope. For those of ordinary skill in the art, other relevant drawings can also be obtained based on these drawings without creative efforts.

[0031] Figure 1 It is a schematic structural diagram of a positive electrode active material provided in an embodiment of the present application;

[0032] Figure 2 It is a process flow diagram of a preparation method of a positive electrode active material provided in an embodiment of the present application;

[0033] Figure 3 It is an electron microscope image of the positive electrode active material in Example 1 of the present application;

[0034] Figure 4 It is an electron microscope image of the positive electrode active material in Example 2 of the present application;

[0035] Figure 5 It is an electron microscope image of the positive electrode active material in Comparative Example 4 of the present application;

[0036] Figure 6 It is a Zr element distribution diagram of the positive electrode active material in Example 1 of the present application;

[0037] Figure 7 Zr element distribution map of the positive electrode active material in Comparative Example 6 of this application;

[0038] Figure 8 Cross-sectional view of the positive electrode active material in Example 1 of this application;

[0039] Figure 9 is Figure 8 Result map of the content distribution of element B in

[0040] Figure 10 is Figure 8 Result map of the content distribution of element Na in

[0041] Figure 11 Surface micrograph of the positive electrode active material in Example 1 of this application;

[0042] Figure 12 is Figure 11 Result map of the content distribution of element B in

[0043] Figure 13 is Figure 11 Result map of the content distribution of element Na in

[0044] Icon: 10 - positive electrode active material; 100 - core; 200 - first shell layer; 300 - second shell layer. Detailed implementation manners

[0045] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be described clearly and completely below. For those not specified in the embodiments, the conventional conditions or the conditions recommended by the manufacturer are followed. Those reagents or instruments not specified by the manufacturer can be obtained as conventional products available for purchase in the market.

[0046] It should be noted that "and / or" in this application, such as "feature 1 and / or feature 2", refers to the three cases where it can be "feature 1" alone, "feature 2" alone, or "feature 1" plus "feature 2".

[0047] In addition, in the description of this application, unless otherwise specified, "multiple" in "one or more" means two or more; the range of "numerical value a ~ numerical value b" includes the two end values "a" and "b", and the "measurement unit" in "numerical value a ~ numerical value b + measurement unit" represents the "measurement unit" of both "numerical value a" and "numerical value b".

[0048] A positive electrode active material, its preparation method, a sodium ion battery, and an electrical device according to the embodiments of this application will be specifically described below.

[0049] In a first aspect, an embodiment of the present application provides a positive electrode active material, which includes a core, a first shell layer, and a second shell layer. The material of the core includes: Na a1 Ni b1 Fe c1 Mn d1 O2, where 0.75 ≤ a1 ≤ 0.85; the first shell layer is coated outside the core, and B element is also doped in the first shell layer. The material of the first shell layer includes: Na a2 Ni b2 Fe c2 Mn d2 B f O2, where 0.85 ≤ a2 ≤ 0.95, and a2 is not equal to a1; the second shell layer is coated outside the first shell layer, and the second shell layer is an inorganic metal oxide layer.

[0050] In the present application, the positive electrode active material has the above specific structure. Specifically, the core includes a sodium-poor phase material with a sodium content of 0.75 to 0.85. The first shell layer (including a sodium-rich phase material with a sodium content of 0.85 to 0.95) and the second shell layer are sequentially coated outside the core, and B element is also doped in the first shell layer. Among them, the sodium-poor core material enables the corresponding battery to have relatively excellent storage performance. The coating of the first shell layer helps to improve the first discharge capacity and rate performance of the corresponding battery. The doping of B element and the coating of the second shell layer help to improve the material stability of the first shell layer, so that the corresponding battery has relatively excellent cycle performance. Therefore, the sodium-ion battery prepared from the positive electrode active material provided by the embodiment of the present application has both a high first discharge capacity, relatively excellent storage performance, cycle performance, and rate performance.

[0051] It should be noted that for the core material, if the content of Na is too low or too high, the first discharge capacity of the corresponding battery will be affected. Therefore, the Na content in the core needs to be limited within a specific range.

[0052] Similarly, for the first shell layer, if the content of Na is too low or too high, the cycle performance of the corresponding battery will be affected. Therefore, the Na content in the first shell layer needs to be limited within a specific range.

[0053] As an example, the material of the core further satisfies: 0.20 ≤ b1 ≤ 0.35, 0.20 ≤ c1 ≤ 0.40, 0.20 ≤ d1 ≤ 0.40, and b1 + c1 + d1 = 1; the material of the first shell layer further satisfies: 0.20 ≤ c2 ≤ 0.40, 0.20 ≤ d2 ≤ 0.40, 0.0001 ≤ f ≤ 0.01, and b2 + c2 + d2 = 1; the material of the second shell layer includes: Na a3 XO b3 , a3 is equal to a2, 1 ≤ b3 ≤ 4, and X is selected from at least one of Mg, Al, Sr, Zr, Ti, and Y.

[0054] In this embodiment, the materials of the core and the first shell are limited within a more suitable range, so that the corresponding battery can have both a higher initial discharge capacity, more excellent storage performance, cycling performance, and rate performance. In addition, the metal elements other than sodium in the second shell are selected as the above elements, and these metal elements contribute to the formation of the interface film during the charge and discharge process of the corresponding battery, thereby contributing to the improvement of the storage performance and cycling performance of the corresponding battery.

[0055] It should be noted that in the second shell, due to the need to balance the positive and negative valence states, the actual value of b3 is (a3 + the positive valence value of X) / 2, that is, the elemental molar amount of O is the sum of the positive valences divided by 2.

[0056] As an example, X is selected from at least one of Zr and Y.

[0057] In this embodiment, selecting the element X as Zr or / and Y is more conducive to the formation of the interface film during the charge and discharge process of the corresponding battery, and thus more beneficial to improving the storage performance and cycling performance of the corresponding battery.

[0058] As an example, the BET of the positive electrode active material is 0.34 - 0.44 m 2 / g, and the Dv3 of the positive electrode active material is 2.3 - 3.2 μm.

[0059] In this embodiment, both the BET and Dv3 of the positive electrode active material are within a more suitable range, indicating that the material has a more suitable particle size and less fine powder, thus improving the cycling performance of the battery.

[0060] As an example, in the positive electrode active material, the doping depth > 0.3 μm, and the mass percentage of element B < 0.005%; the doping depth is equal to 0.1 μm, and the mass percentage of element B > 0.02%.

[0061] In this embodiment, limiting the content of element B at different doping depths within a specific range helps to further improve the material stability of the material, so that the corresponding battery has more excellent cycling performance.

[0062] As an example, the schematic structural diagram of the positive electrode active material 10 can be referred to Figure 1 , and it includes a core 100, a first shell 200 coated outside the core 100, and a second shell 300 coated outside the first shell 200.

[0063] Second aspect, an embodiment of the present application provides a method for preparing a positive electrode active material, comprising the following steps: mixing raw materials containing Ni, Fe, Mn and a first Na source to obtain a first mixture; wherein, in the first mixture, the ratio of the total molar amount of Ni, Fe and Mn to the molar amount of Na is 1:(0.75-0.85); performing a first-stage sintering on the first mixture to obtain a core; mixing the core with a second Na source to obtain a second mixture; wherein, in the second mixture, the ratio of the total molar amount of Ni, Fe and Mn to the molar amount of Na is 1:(0.85-0.95); performing a second-stage sintering on the second mixture to obtain a positive electrode material intermediate; mixing the positive electrode material intermediate, boric acid and a metal source to obtain a third mixture; performing a third-stage sintering (to achieve uniform coating of the inorganic metal oxide layer) and a fourth-stage sintering (to achieve further doping of B element) on the third mixture in sequence, wherein the treatment temperature of the fourth-stage sintering is higher than that of the third-stage sintering, to obtain the positive electrode active material.

[0064] In this application, on the one hand, during the preparation process of the cathode active material, the ratio of the total molar amount of Ni, Fe, and Mn in the first mixture to the molar amount of Na is limited to the range of 1:(0.75 - 0.85), and the ratio of the total molar amount of Ni, Fe, and Mn in the second mixture to the molar amount of Na is limited to the range of 1:(0.85 - 0.95), so as to prepare a cathode active material intermediate with a core-shell structure that is sodium-poor inside and sodium-rich outside, thereby enabling the corresponding battery to have relatively excellent storage performance, first discharge capacity, and rate performance; at the same time, there is a first sodium-rich shell layer outside the sodium-poor core (the sodium-rich phase is more active than the sodium-poor phase), which helps the formation of the second shell layer during the subsequent sintering process (it is easier to form a coating layer and the formed coating layer is more uniform, thus increasing the structural strength of the material and making it not easy to form small particles during the subsequent crushing process), and further enabling the prepared cathode active material to have the advantage of fewer fine powders. On the other hand, in addition to the cathode material intermediate and the metal source, the third mixture also contains boric acid. By virtue of the fluidity of boric acid, a relatively uniform inorganic metal oxide layer can be formed during the third-stage sintering process, thereby improving the coating uniformity of the inorganic metal oxide layer on the cathode material intermediate, and further reducing the alkali amount on the material surface and helping to improve the stability of the material, so that the corresponding battery has relatively excellent cycle performance, and at the same time, it can also make the prepared cathode active material have the advantage of fewer fine powders; in addition, through the fourth-stage sintering at a higher temperature, the B element can be further doped from the material surface into the material, and the further doping of the B element also helps to improve the stability of the material. Through the combined action of the two aspects, the prepared cathode active material has the advantages of fewer fine powders, a lower alkali amount on the material surface, and higher material stability, thereby enabling the prepared sodium-ion battery to have a higher first discharge capacity, relatively excellent storage performance, cycle performance, and rate performance.

[0065] It should be noted that the forms of the raw materials of Ni, Fe, and Mn are not limited. For example, it can be in the form of a mixture of three raw materials separately containing the three metal elements, or in the form of one raw material containing the three metal elements at the same time. In the embodiments of this application, the form of one raw material containing the three metal elements at the same time is taken as an example. For example, a nickel-iron-manganese hydroxide precursor is used as the raw material. Similarly, the type of the Na source is not limited and can be set according to the conventional selection in the art. For example, it can be Na2CO3.

[0066] As an example, in the first mixture, the molar ratio of Ni, Fe, and Mn is successively (0.2 - 0.35):(0.2 - 0.4):(0.2 - 0.4); and / or, in the third mixture, the ratio of the total molar amount of Ni, Fe, and Mn to the molar amount of B is 1:(0.0001 - 0.01).

[0067] In this embodiment, the molar ratio of Ni, Fe, and Mn in the first mixture is limited to a specific range of (0.2~0.35):(0.2~0.4):(0.2~0.4), and the ratio of the total molar amount of Ni, Fe, and Mn in the third mixture to the molar amount of B is limited to a specific range of 1:(0.0001~0.01), so that each element has a more appropriate mass percentage in the finally prepared cathode active material, and thus the corresponding battery can have both a higher initial discharge capacity, more excellent storage performance, cycle performance, and rate performance.

[0068] As an example, the metal element in the metal source is selected from at least one of Mg, Al, Sr, Zr, Ti, and Y; optionally, the metal element in the metal source is selected from at least one of Zr and Y.

[0069] In this embodiment, there are many types of metals used to form the inorganic metal oxide layer, which can provide more implementation schemes, thus facilitating the promotion and application of the technical solutions provided by the embodiments of the present application; in addition, using the above specific types of metal elements in the metal source helps to form an interfacial film during the charge and discharge process of the corresponding battery, thus helping to improve the storage performance and cycle performance of the corresponding battery.

[0070] As an example, in the steps of the first-stage sintering and / or the second-stage sintering, the treatment temperature is 820~1000 °C, such as but not limited to any point value of 820 °C, 850 °C, 900 °C, 950 °C, and 1000 °C or the range value between any two of them; the treatment time is 7~12 h, such as but not limited to any point value of 7 h, 8 h, 9 h, 10 h, 11 h, and 12 h or the range value between any two of them.

[0071] In this embodiment, the treatment temperature and time in the first-stage sintering and / or the second-stage sintering process are respectively limited to specific ranges, which can provide more appropriate sintering conditions to enable the raw materials to fully react.

[0072] As an example, in the steps of the third-stage sintering, the treatment temperature is 250 to 500 °C, such as but not limited to any one of the point values of 250 °C, 300 °C, 350 °C, 400 °C, 450 °C, and 500 °C or the range value between any two of them; the treatment time is 6 to 10 h, such as but not limited to any one of the point values of 6 h, 7 h, 8 h, 9 h, and 10 h or the range value between any two of them; and / or, in the steps of the fourth-stage sintering, the treatment temperature is 700 to 900 °C, such as but not limited to any one of the point values of 700 °C, 750 °C, 800 °C, 850 °C, and 900 °C or the range value between any two of them; the treatment time is 8 to 12 h, such as but not limited to any one of the point values of 8 h, 9 h, 10 h, 11 h, and 12 h or the range value between any two of them.

[0073] In this embodiment, limiting the treatment temperature and time of the third-stage sintering within specific ranges respectively helps to form an inorganic metal oxide layer with uniform thickness and good coating integrity; limiting the treatment temperature and time of the fourth-stage sintering within specific ranges respectively helps the doping of element B from the material surface into the material interior.

[0074] It should be noted that for the processes or steps not specifically described or limited in the preparation process, they can be set according to the conventional selection in the art.

[0075] As an example, after the first-stage sintering and before mixing with the second sodium source, it further includes the steps of pulverizing and sieving the core material.

[0076] As an example, after the second-stage sintering and before mixing with the metal source, it further includes the steps of pulverizing and sieving the material intermediate.

[0077] As an example, after the fourth-stage sintering, it further includes the steps of sieving, iron removal, and packaging of the positive electrode active material.

[0078] As an example, the process flow chart of the preparation method of the positive electrode active material is exemplarily referred to Figure 2 .

[0079] In a third aspect, an embodiment of the present application provides a sodium-ion battery, including the positive electrode active material provided in the embodiment of the first aspect or the positive electrode active material prepared by the preparation method provided in the embodiment of the second aspect.

[0080] In this application, the sodium-ion battery includes a cathode active material with a specific structure. Specifically, the core includes a sodium-deficient phase material with a sodium content of 0.75 to 0.85. A first shell layer (including a sodium-rich phase material with a sodium content of 0.85 to 0.95) and a second shell layer are sequentially coated outside the core. Moreover, element B is doped in the first shell layer. Among them, the sodium-deficient core material enables the corresponding battery to have relatively excellent storage performance. The coating of the first shell layer helps to improve the first discharge capacity and rate performance of the corresponding battery. The doping of element B and the coating of the second shell layer help to improve the material stability of the first shell layer, thereby enabling the corresponding battery to have relatively excellent cycle performance. Therefore, the sodium-ion battery prepared from this cathode active material can have both a high first discharge capacity, relatively excellent storage performance, cycle performance, and rate performance.

[0081] In a fourth aspect, an embodiment of this application provides an electrical device, including the sodium-ion battery provided in the embodiment of the third aspect.

[0082] It should be noted that the specific type of the electrical device is not limited and can be adaptively adjusted according to actual needs.

[0083] The features and performance of this application will be further described in detail below in conjunction with embodiments.

[0084] Example 1

[0085] An embodiment of this application provides a method for preparing a cathode active material, including the following steps:

[0086] Add 10 kg of Ni 0.33 Fe 0.33 Mn 0.33 (OH)2 (nickel-iron-manganese hydroxide precursor) and 4.83 Kg of Na2CO3 (the first Na source) into a 50 L high-speed mixer and stir and mix at 800 rpm for 12 min to obtain a first mixture. Among them, the ratio of the total molar amount of Ni, Fe, and Mn to the molar amount of Na is 1:0.8 (abbreviated as the primary Na ratio); transfer the first mixture to a crucible, then transfer the crucible to a sintering furnace, introduce an air atmosphere and heat up to 950°C at a rate of 3°C / min and hold for sintering for 10 h, and then cool with the furnace to obtain the core.

[0087] The core material is crushed and sieved, and then the crushed material is stirred and mixed with 0.60 kg of Na2CO3 (the second Na source) in a 30 L high-speed mixer at 800 rpm for 12 min to obtain a second mixture. Among them, the ratio of the total molar amount of Ni, Fe, and Mn to the molar amount of Na is 1:0.9 (abbreviated as the secondary Na ratio); the second mixture is transferred to a crucible, and then the crucible is transferred to a sintering furnace. Air atmosphere is introduced and the temperature is raised to 900 °C at 3 °C / min and sintered for 10 h, and then cooled with the furnace to obtain a cathode material intermediate.

[0088] The cathode material intermediate is crushed and sieved, and then the crushed material is stirred and mixed with 67.7 g of ZrO2 and 57.4 g of H3BO3 in a 30 L high-speed mixer at 800 rpm for 12 min to obtain a third mixture. Among them, the ratio of the total molar amount of Ni, Fe, and Mn to the molar amount of B is 1:0.001; the third mixture is transferred to a crucible, and then the crucible is transferred to a sintering furnace. Air atmosphere is introduced and the temperature is raised to 300 °C at 3 °C / min and sintered for 6 h; then the temperature is raised to 800 °C at 3 °C / min again and sintered for 10 h, and then cooled with the furnace. After the material is taken out of the furnace, it is sieved, iron-removed, and packaged in sequence to obtain the cathode active material; in the cathode active material, the core is Na 0.8 Ni 0.33 Fe 0.33 Mn 0.33 O2, the first shell is Na 0.9 Ni 0.33 Fe 0.33 Mn 0.33 B 0.001 O2, the second shell is Na 0.9 ZrO 2.45 。

[0089] Example 2

[0090] The embodiment of the present application provides a method for preparing a cathode active material, which is only different from Example 1 in that: 10 kg of Ni 0.33 Fe 0.33 Mn 0.33 (OH)2 (nickel-iron-manganese hydroxide precursor) and 4.53 Kg of Na2CO3 (the first Na source) are added to a 50 L high-speed mixer and stirred and mixed at 800 rpm for 12 min to obtain a first mixture. Among them, the ratio of the total molar amount of Ni, Fe, and Mn to the molar amount of Na is 1:0.75 (abbreviated as the primary Na ratio); in the cathode active material, the core is Na 0.75 Ni 0.33 Fe 0.33 Mn 0.33 O2, the first shell is Na0.85 Ni 0.33 Fe 0.33 Mn 0.33 B 0.001 O2, the second shell is Na 0.85 ZrO 2.425 。

[0091] Example 3

[0092] The embodiment of the present application provides a preparation method of a positive electrode active material, which is only different from that of Example 1 in that: 10 kg of Ni 0.33 Fe 0.33 Mn 0.33 (OH)2 (nickel-iron-manganese hydroxide precursor) and 5.13 Kg of Na2CO3 (the first Na source) are added to a 50 L high-speed mixer and stirred and mixed at 800 rpm for 12 min to obtain a first mixture. Among them, the ratio of the total molar amount of Ni, Fe, and Mn to the molar amount of Na is 1:0.85 (abbreviated as the primary Na ratio); in the positive electrode active material, the inner core is Na 0.85 Ni 0.33 Fe 0.33 Mn 0.33 O2, the first shell is Na 0.95 Ni 0.33 Fe 0.33 Mn 0.33 B 0.001 O2, the second shell is Na 0.95 ZrO 2.475 。

[0093] Example 4

[0094] The embodiment of the present application provides a preparation method of a positive electrode active material, which is only different from that of Example 1 in that: the inner core material is crushed and sieved, and then the crushed material is stirred and mixed with 0.30 kg of Na2CO3 (the second Na source) in a 30 L high-speed mixer at 800 rpm for 12 min to obtain a second mixture. Among them, the ratio of the total molar amount of Ni, Fe, and Mn to the molar amount of Na is 1:0.85 (abbreviated as the secondary Na ratio); in the positive electrode active material, the inner core is Na 0.8 Ni 0.33 Fe 0.33 Mn 0.33 O2, the first shell is Na 0.85 Ni 0.33 Fe 0.33 Mn 0.33 B 0.001 O2, the second shell is Na 0.85 ZrO 2.425 。

[0095] Example 5

[0096] The present application provides a method for preparing a positive electrode active material, which is only different from Example 1 in that: the core material is pulverized and sieved, and then the pulverized material is stirred and mixed with 0.91 kg of Na2CO3 (the second Na source) in a 30 L high-speed mixer at 800 rpm for 12 min to obtain a second mixture, wherein the ratio of the total molar amount of Ni, Fe and Mn to the molar amount of Na is 1:0.95 (abbreviated as the secondary Na ratio); in the positive electrode active material, the core is Na 0.8 Ni 0.33 Fe 0.33 Mn 0.33 O2, the first shell layer is Na 0.95 Ni 0.33 Fe 0.33 Mn 0.33 B 0.001 O2, the second shell layer is Na 0.95 ZrO 2.475 .

[0097] Example 6

[0098] The present application provides a method for preparing a positive electrode active material, which is only different from Example 1 in that: the positive electrode material intermediate is pulverized and sieved, and then the pulverized material is stirred and mixed with 63.4 g of Y2O3 and 57.4 g of H3BO3 in a 30 L high-speed mixer at 800 rpm for 12 min to obtain a third mixture; in the positive electrode active material, the core is Na 0.8 Ni 0.33 Fe 0.33 Mn 0.33 O2, the first shell layer is Na 0.9 Ni 0.33 Fe 0.33 Mn 0.33 B 0.001 O2, the second shell layer is Na 0.9 YO 1.95 .

[0099] Comparative Example 1

[0100] The comparative example of the present application provides a method for preparing a positive electrode active material, which is only different from Example 1 in that: 10 kg of Ni is added to a 50 L high-speed mixer 0.33 Fe 0.33 Mn 0.33(OH)2 (nickel-iron-manganese hydroxide precursor) and 4.23 Kg of Na2CO3 (the first Na source) are stirred and mixed at 800 rpm for 12 min to obtain a first mixture. Among them, the ratio of the total molar amount of Ni, Fe, and Mn to the molar amount of Na is 1:0.70 (abbreviated as the primary Na ratio); in the cathode active material, the core is Na 0.7 Ni 0.33 Fe 0.33 Mn 0.33 O2, the first shell is Na 0.8 Ni 0.33 Fe 0.33 Mn 0.33 B 0.001 O2, the second shell is Na 0.8 ZrO 2.4 。

[0101] Comparative Example 2

[0102] The comparative example of this application provides a method for preparing a cathode active material, which is only different from Example 1 in that: 10 kg of Ni 0.33 Fe 0.33 Mn 0.33 (OH)2 (nickel-iron-manganese hydroxide precursor) and 5.43 Kg of Na2CO3 (the first Na source) are stirred and mixed at 800 rpm for 12 min to obtain a first mixture. Among them, the ratio of the total molar amount of Ni, Fe, and Mn to the molar amount of Na is 1:0.90 (abbreviated as the primary Na ratio); in the cathode active material, the core is Na 0.9 Ni 0.33 Fe 0.33 Mn 0.33 O2, the first shell is Na 1.0 Ni 0.33 Fe 0.33 Mn 0.33 B 0.001 O2, the second shell is Na 1.0 ZrO 2.5 。

[0103] Comparative Example 3

[0104] The comparative example of this application provides a method for preparing a cathode active material, which is only different from Example 1 in that: the core material is pulverized and sieved, and then the pulverized material is stirred and mixed with 1.21 kg of Na2CO3 (the second Na source) in a 30 L high-speed mixer at 800 rpm for 12 min to obtain a second mixture. Among them, the ratio of the total molar amount of Ni, Fe, and Mn to the molar amount of Na is 1:1 (abbreviated as the secondary Na ratio); in the cathode active material, the core is Na 0.8Ni 0.33 Fe 0.33 Mn 0.33 O2, the first shell is Na 1.0 Ni 0.33 Fe 0.33 Mn 0.33 B 0.001 O2, the second shell is Na 1.0 ZrO 2. 5 。

[0105] Comparative Example 4

[0106] The comparative example of this application provides a method for preparing a positive electrode active material, which is only different from Example 1 in that: except for not adding the second Na source, the process steps are the same as those in Example 1; in the positive electrode active material, the inner core is Na 0.8 Ni 0.33 Fe 0.33 Mn 0.33 O2, the first shell is Na 0.8 Ni 0.33 Fe 0.33 Mn 0.33 B 0.001 O2 (i.e., the amount of Na in the first shell is the same as that in the inner core), the second shell is Na 0.8 ZrO 2.4 。

[0107] Comparative Example 5

[0108] The comparative example of this application provides a method for preparing a positive electrode active material, which is only different from Example 1 in that: except for not adding ZrO2, the process steps are the same as those in Example 1, that is, there is no second shell outside the first shell; in the positive electrode active material, the inner core is Na 0.8 Ni 0.33 Fe 0.33 Mn 0.33 O2, the first shell is Na 0.9 Ni 0.33 Fe 0.33 Mn 0.33 B 0.001 O2, there is no second shell.

[0109] Comparative Example 6

[0110] The comparative example of this application provides a method for preparing a positive electrode active material, which is only different from Example 1 in that: except for not adding boric acid, the process steps are the same as those in Example 1, that is, there is no B doping in the first shell and the coating uniformity of the second shell cannot be improved; in the positive electrode active material, the inner core is Na 0.8 Ni 0.33 Fe 0.33 Mn 0.33O2, the first shell is Na 0.9 Ni 0.33 Fe 0.33 Mn 0.33 O2, the second shell is Na 0.9 ZrO 2.45 。

[0111] Comparative Example 7

[0112] The comparative example of this application provides a method for preparing a cathode active material, the difference from Example 1 is only that: the fourth-stage sintering is not carried out, that is, element B is not further doped; in the cathode active material, the core is Na 0.8 Ni 0.33 Fe 0.33 Mn 0.33 O2, the first shell is Na 0.9 Ni 0.33 Fe 0.33 Mn 0.33 O2, the second shell is Na 0.95 Zr 0.75 B 0.25 O 2.35 。

[0113] To better understand the differences between each example and comparative example, the following is a summary description in the form of a table, which can be specifically referred to Table 1

[0114] Table 1

[0115]

[0116] Test Example 1

[0117] Performance test of materials

[0118] Test method:

[0119] The cathode active materials prepared in Examples 1 to 6 and Comparative Examples 1 to 7 were numbered respectively, and then the BET, Dv3, water-measured carbonate concentration, and ethanol-measured hydroxide concentration of each sample were tested; then each cathode active material was assembled into a battery respectively, and then the first discharge capacity, 21-day storage retention rate at 45°C, 21-day storage recovery rate at 45°C, capacity retention rate after 200 cycles at 45°C, and rate performance of the battery were tested. Finally, each data result obtained from the test was statistically tabulated in Table 2

[0120] Among them, the test steps of BET are as follows:

[0121] Use a sample tube with a bulb and a long tube of 3 / 8 inch and a length of 12 mm, and the sample loading amount is 2 / 3 of the bulb. Before measurement, perform pretreatment first. The degassing temperature for pretreatment is 200 °C, and the degassing time is 2 h. Then perform cooling and weighing; among them, cool to room temperature, and control the weighing accuracy within 0.0001 g. The adsorbate uses high-purity nitrogen of 99.999%, the adsorption pressure deviation is 0.05 mmHg, the equilibrium time is 5 s, and the relative pressure points (P / P0) are selected as 0.05, 0.1, 0.15, 0.2, 0.25, 0.30, the numerical linearity is >99.9%, and the read numerical value is reserved to three significant figures.

[0122] The test steps for Dv3 are as follows:

[0123] Use the Malvern 3000 device, the detection angle is 0~144°. First, ultrasonically disperse the sample in water. The dispersion method is to add the sample (0.15 g of the sample amount) + 1 dropper of 10% sodium hexametaphosphate + water to 20 mL in a 50 mL beaker (ultrasonic externally for 5 min, intensity 60%), the external ultrasonic intensity is 53 KHz. Add the dispersed solution to water for particle size testing, the rotation speed of the stirrer / pump is 3000 rpm, the analysis mode is the general mode, the obscuration is 8~12%, the test time is 10 s, and then read the particle size value with a volume fraction of 3% in the device.

[0124] The test steps for measuring the carbonate concentration in water are as follows:

[0125] Weigh 2 g of the sample and add it to 100 ml of pure water and stir for 30 min. Filter 50 ml with filter paper, then pipette 10 ml, and use 0.05 mol / L HCl solution to determine the end point by potentiometric titration to obtain the CO3 2- content, that is, the Na2CO3 content.

[0126] The test steps for measuring the hydroxide concentration in ethanol are as follows:

[0127] Weigh 2 g of the sample and add it to 100 ml of ethanol and stir for 60 min. Filter 50 ml with filter paper, then pipette 10 ml, and use 0.05 mol / L HCl solution to determine the end point by potentiometric titration to obtain the OH - content, which is the NaOH content.

[0128] The battery assembly steps are as follows:

[0129] Using each sample as the cathode material, the cathode material, acetylene black, and binder PVDF were added to NMP in a mass ratio of 90%:5%:5% to prepare the cathode slurry; the cathode slurry was evenly coated on aluminum foil, vacuum baked at 80 °C, pressed, and cut to obtain a cathode sheet with a diameter of 14 mm. A pure sodium sheet with a diameter of 16 mm was used as the anode sheet, ENA-18 from Tianci was used as the electrolyte, and a PP / PE / PP composite separator was used to assemble a button cell in a glove box filled with argon.

[0130] Among them, the test steps for the first discharge capacity of the battery are as follows:

[0131] At 25 °C, the battery was charged at a constant current of 0.5 C to 4.0 V, then charged at a constant voltage until the current was less than or equal to 0.01 mA, then left standing for 5 minutes, and then discharged at a constant current of 0.5 C to 2.0 V, and then left standing for 5 minutes. This is one charge-discharge cycle, and the discharge capacity this time is recorded as the discharge specific capacity of the battery in the 1st cycle.

[0132] The test steps for the storage retention rate at 45 °C for 21 days are as follows:

[0133] First, the battery was fully charged and then the discharge capacity T1 of the battery was tested. Then, the fully charged battery was stored at 45 °C for 21 days, and then the discharge capacity T2 of the battery was tested. The retention rate was calculated using the formula = T2 / T1 * 100%.

[0134] The test steps for the storage recovery rate at 45 °C for 21 days are as follows:

[0135] First, the battery was fully charged and then the discharge capacity T1 of the battery was tested. Then, the fully charged battery was stored at 45 °C for 21 days. After the battery was discharged, it was charged again and then discharged, and the discharge capacity T3 was tested. The recovery rate was calculated using the formula = T3 / T1 * 100%.

[0136] The test steps for the capacity retention rate after 200 cycles at 45 °C are as follows:

[0137] At 45 °C, first, the battery cell was charged to 4.0 V at a constant current of 1 C, further charged at a constant voltage of 4.0 V until the current was 0.05 C, and then discharged to 2.0 V at a constant current of 1 / 3 C. This is one charge-discharge cycle process, and the discharge capacity this time is the discharge capacity of the 1st cycle. The battery cell was subjected to multiple cycle charge-discharge tests in the above manner, the discharge capacity of the 200th cycle was detected, and the capacity retention rate of the battery cell after cycling was calculated through the following formula. The capacity retention rate of the battery after 200 cycles (%) = [discharge capacity of the 200th cycle / discharge capacity of the 1st cycle] × 100%.

[0138] The test steps for the rate performance are as follows:

[0139] The above button battery was tested at 25 °C using a Lan Dian battery test system CT2001A, and the constant voltage cut-off current was 0.05C. The specific steps are as follows:

[0140] Step 1: Charge at 0.1C to the cut-off voltage, stand for 2 min, discharge at 0.1C to the termination voltage, and stand for 2 min.

[0141] Step 2: Charge at 0.2C to the cut-off voltage and discharge at 0.2C to the termination voltage.

[0142] Step 3: Charge at 0.5C to the cut-off voltage and discharge at 0.5C to the termination voltage.

[0143] Step 4: Charge at 0.5C to the cut-off voltage and discharge at 1C to the termination voltage.

[0144] Step 5: Charge at 0.5C to the cut-off voltage and discharge at 2C to the termination voltage.

[0145] Step 6: Charge at 0.5C to the cut-off voltage and discharge at 3C to the termination voltage.

[0146] The rate performance data is the ratio of the 3C discharge capacity to the 0.1C discharge capacity.

[0147] Table 2

[0148]

[0149] Referring to Table 2, from the test results of Examples 1 to 6 and Comparative Examples 1 to 7, it can be seen that by preparing the positive active material according to the preparation process provided in the embodiments of the present application, the prepared positive active material has the advantages of less fine powder, lower alkali content on the material surface, and higher material stability, so that the corresponding battery can have both a high first discharge capacity, excellent storage performance, cycle performance, and rate performance.

[0150] From the test results of Examples 1 to 3 and Comparative Examples 1 to 2, it can be seen that by limiting the Na content in the core to the range of 0.75 to 0.85, the corresponding battery can have both a higher first discharge capacity, more excellent storage performance, cycle performance, and rate performance.

[0151] From the test results of Examples 1, 4, and 5 and Comparative Example 3, it can be seen that by limiting the Na content in the first shell layer to the range of 0.85 to 0.95, the corresponding battery can have both a higher first discharge capacity, more excellent storage performance, cycle performance, and rate performance.

[0152] From the test results of Examples 1, 4, and 5 and Comparative Example 4, it can be seen that the core of the sodium-poor phase has a first shell of sodium-rich phase, so that the corresponding battery has both a higher initial discharge capacity, more excellent storage performance, cycling performance, and rate performance; at the same time, it is also helpful for the formation of the second shell during the subsequent sintering process (it is easier to form a coating layer and the formed coating layer is more uniform, which can increase the structural strength of the material and make it not easy to form small particles during the subsequent crushing process), and then the prepared cathode active material has the advantage of less fine powder.

[0153] From the test results of Example 1 and Comparative Example 5, it can be seen that coating an inorganic metal oxide layer (i.e., the second shell) outside the first shell can reduce the amount of alkali on the material surface and also helps to improve the stability of the material, so that the corresponding battery can have both a relatively high initial discharge capacity, more excellent storage performance, cycling performance, and rate performance.

[0154] From the test results of Example 1 and Comparative Example 6, it can be seen that adding boric acid during the formation of the inorganic metal oxide layer helps to improve the coating uniformity of the inorganic metal oxide layer, thereby reducing the alkalinity on the material surface, and then the corresponding battery can have both a relatively high initial discharge capacity, more excellent storage performance, cycling performance, and rate performance; at the same time, it can also make the prepared cathode active material have the advantage of less fine powder.

[0155] From the test results of Example 1 and Comparative Example 7, it can be seen that doping with B element in the first shell helps to improve the problems of more fine powder and more alkali amount on the material surface, and also helps to improve the stability of the material, so that the corresponding battery can have both a relatively high initial discharge capacity, more excellent storage performance, cycling performance, and rate performance.

[0156] Test Example 2

[0157] Microstructural test of materials

[0158] Test method:

[0159] The cathode active materials prepared in Examples 1 to 2 and Comparative Example 4 were numbered respectively, and then the microstructures of each sample were characterized by using electron microscopy equipment.

[0160] Refer to Figures 3 - 5 According to the test results, secondary Na supplementation was carried out during the preparation process to form a core-shell structure with different Na contents (corresponding to the core and the first shell outside the core), and the prepared material had less fine powder, that is, the problem of fine powder of the material could be effectively improved.

[0161] Test Example 3

[0162] Coating uniformity test of materials

[0163] Testing method:

[0164] Number the cathode active materials prepared in Example 1 and Comparative Example 6 respectively, and then use electron microscopy equipment to characterize the distribution of Zr element in each sample.

[0165] Refer to Figure 6 and Figure 7 It can be seen that adding boric acid during the formation of the inorganic metal oxide layer helps to improve the coating uniformity of the inorganic metal oxide layer, and from Figure 6 and Figure 7 (the scale bar is 20 nm), it can be seen that the Zr element is distributed in the region of about 0 - 50 nm, indicating that the thickness of the second shell layer in the cathode active material is approximately 50 nm.

[0166] Test Example 4

[0167] Distribution of B element and Na element in the cathode active material

[0168] Testing method:

[0169] Take the material of Example 1 as the test object, perform ion milling using Gatan 697 Ilion II equipment, control the outlet air pressure of high-purity argon at 0.18 MPa, the air pressure in the sample chamber < 5×10 -6 Torr, the angles of the left and right guns are controlled at 0 degrees, and the ion beam energy is 5 keV. After milling, perform EDS measurement using Hitachi Regulus 8100 / SU 8010, control the test voltage at 1 KV, the test current at 10 μA, and perform point scanning at a magnification of 18 K. Select Ni, Fe, Mn, Zr, B, and Na as the main elements respectively, and automatically calculate the element ratios according to the selected points at different positions.

[0170] Refer to Figure 8 (wherein, Figure 8 the scale bar in is 3 μm, each small grid represents 0.3 μm. Specifically, the depths of spectra Figure 1 , 2 , 3, 4, and 5 from the surface layer are greater than 0.3 μm, the depth of spectrum Figure 6 from the surface layer is within 0.1 - 0.3 μm, and among them, the depths of spectra Figure 7 , 8 , 11, and 12 from the surface layer are within 0.1 μm but above 50 nm), Figure 9 , Figure 11 (test the B content on the surface of the cathode active material in Example 1) and Figure 12It can be known that in the positive electrode active material, the doping depth > 0.3 μm, the mass percentage of element B < 0.005%, and it is defaulted to contain no B, that is, the area with a depth dimension greater than 0.3 μm from the material surface corresponds to the core of the positive electrode active material; the doping depth is equal to 0.1 μm, the mass percentage of element B > 0.02%, corresponding to a part of the first shell outside the core; the mass percentage of element B on the material surface < 0.005%, defaulted to contain no B, corresponding to a part of the second shell outside the first shell, indicating that B enters the first shell from the second shell after high-temperature roasting.

[0171] Refer to Figure 8 (where Figure 8 the scale bar in is 3 μm, and each small grid represents 0.3 μm. Specifically, the spectra Figure 1 , 2 , 3, 4, 5 are at a depth greater than 0.3 μm from the surface layer, and the spectrum Figure 6 is at a depth within 0.1 - 0.3 μm from the surface layer. Among them, the spectra Figure 7 , 8 , 11, 12 are at a depth within 0.1 μm but above 50 nm from the surface layer), Figure 10 , Figure 11 and Figure 13 It can be known that in the positive electrode active material, in the area with a depth dimension greater than 0.3 μm from the material surface, the molar ratio of element Na is 0.75 - 0.85, corresponding to the core of the positive electrode active material; in the area with a depth dimension of 50 nm - 0.3 μm from the material surface, the molar ratio of element Na is 0.85 - 0.95, corresponding to the first shell of the positive electrode active material; the molar ratio of element Na on the material surface is 0.85 - 0.95, corresponding to the second shell of the positive electrode active material.

[0172] At the same time, combined with Figures 6 - 13 the results, it can be known that from the perspective of the B content and the molar ratio of Na, the area with a depth of more than 0.3 μm from the material surface layer is the core of the positive electrode active material, corresponding to the structural formula: Na a1 Ni b1 Fe c1 Mn d1 O2, 0.75 ≤ a1 ≤ 0.85; the area with a depth of 50 nm - 0.3 μm from the material surface layer is the first shell of the positive electrode active material, corresponding to the structural formula: Na a2 Ni b2 Fe c2 Mn d2 B f O2, 0.85 ≤ a2 ≤ 0.95, and a2 is not equal to a1; the area with a depth within 50 nm from the surface layer is the second shell of the positive electrode active material, corresponding to the structural formula: Na a3 XOb3 , a3 is equal to a2, and 1 ≤ b3 ≤ 4.

[0173] The embodiments described above are some, but not all, of the embodiments of the present application. The detailed description of the embodiments of the present application is not intended to limit the scope of the present application claimed, but merely represents selected embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present application without creative efforts shall fall within the scope of protection of the present application.

Claims

1. A positive electrode active material, characterized in that, Comprising: Core, the material of the core includes: Na a1 Ni b1 Fe c1 Mn d1 O2, 0.75 ≤ a1 ≤ 0.85, 0.20 ≤ b1 ≤ 0.35, 0.20 ≤ c1 ≤ 0.40, 0.20 ≤ d1 ≤ 0.40, and b1 + c1 + d1 = 1; The first shell layer, which is coated outside the inner core, and the first shell layer is also doped with element B. The material of the first shell layer includes: Na a2 Ni b2 Fe c2 Mn d2 B f O2, 0.85 ≤ a2 ≤ 0.95, and a2 is not equal to a1, 0.20 ≤ c2 ≤ 0.40, 0.20 ≤ d2 ≤ 0.40, 0.0001 ≤ f ≤ 0.01, and b2 + c2 + d2 = 1; A second shell layer, which is coated outside the first shell layer, and the second shell layer is an inorganic metal oxide layer; In the positive electrode active material, the doping depth > 0.3 μm, and the mass percentage of element B < 0.005%; the doping depth is equal to 0.1 μm, and the mass percentage of element B > 0.02%.

2. The positive electrode active material according to claim 1, characterized in that, The material of the second shell layer includes: Na a3 XO b3 , a3 is equal to a2, 1 ≤ b3 ≤ 4, and X is selected from at least one of Mg, Al, Sr, Zr, Ti, and Y.

3. The cathode active material according to claim 2, wherein The X is selected from at least one of Zr and Y.

4. The positive electrode active material according to claim 1, characterized in that, The BET of the positive electrode active material is 0.34 to 0.44 m 2 / g, and the Dv3 of the positive electrode active material is 2.3 to 3.2 μm.

5. A method for preparing a positive electrode active material, characterized in that, Comprising the following steps: Mix raw materials containing Ni, Fe, Mn and a first Na source to obtain a first mixture; wherein, in the first mixture, the ratio of the total molar amount of Ni, Fe and Mn to the molar amount of Na is 1:(0.75 - 0.85); perform a first-stage sintering on the first mixture to obtain a core; Mix the core with a second Na source to obtain a second mixture; wherein, in the second mixture, the ratio of the total molar amount of Ni, Fe and Mn to the molar amount of Na is 1:(0.85 - 0.95); perform a second-stage sintering on the second mixture to obtain a positive electrode material intermediate with a core-shell structure having a sodium-deficient phase inside and a sodium-rich phase outside; Mix the positive electrode material intermediate, boric acid and a metal source to obtain a third mixture; perform a third-stage sintering and a fourth-stage sintering on the third mixture in sequence, wherein the treatment temperature of the fourth-stage sintering is higher than that of the third-stage sintering, to obtain a positive electrode active material; In the step of the first-stage sintering and / or the second-stage sintering, the treatment temperature is 820 - 1000°C, and the treatment time is 7 - 12 h; In the step of the third-stage sintering, the treatment temperature is 250 - 500°C, and the treatment time is 6 - 10 h; and / or, in the step of the fourth-stage sintering, the treatment temperature is 700 - 900°C, and the treatment time is 8 - 12 h; In the first mixture, the molar ratio of Ni, Fe and Mn is (0.2 - 0.35):(0.2 - 0.4):(0.2 - 0.4) in sequence; and / or, in the third mixture, the ratio of the total molar amount of Ni, Fe and Mn to the molar amount of B is 1:(0.0001 - 0.01); The metal element in the metal source is selected from at least one of Mg, Al, Sr, Zr, Ti and Y.

6. The preparation method according to claim 5, characterized in that, The metal element in the metal source is selected from at least one of Zr and Y.

7. A sodium-ion battery, characterized in that, Comprising the positive electrode active material according to any one of claims 1 - 4 or the positive electrode active material prepared by the preparation method according to any one of claims 5 - 6.

8. An electrical device, characterized in that, Comprising the sodium ion battery according to claim 7.

Images