Single-crystal positive electrode material, preparation method and application

Abstract

The invention discloses a single-crystal positive electrode material which comprises a single-crystal layered oxide and sodium-poor phase layered oxide nanocrystals discretely distributed on the surface of the single-crystal layered oxide, the chemical general formula of the single crystal layered oxide is NanMn1-x-yMxByO2, in the formula, x is more than or equal to 0.4 and less than or equal to 0.7, y is more than or equal to 0.001 and less than or equal to 0.01, and n is more than or equal to 0.85 and less than or equal to 1; m is selected from at least one active element and at least one inactive element; the active element is selected from one or more of Ni, Fe and Cu; and the inactive element is selected from one or more of Li, Mg, Al, Zn, Sn and Ti. According to the single-crystal positive electrode material disclosed by the invention, the formation of the single crystal is promoted through fine doping of crystal lattices, and the sodium-poor phase layered oxide nanocrystals which are discretely distributed are separated out on the surface of the single crystal; the structure is favorable for improving the stability of the positive electrode material in the air, reducing the surface alkalinity and inhibiting the battery gas expansion, thereby improving the cycle stability of the sodium ion battery.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and particularly to a single-crystal cathode material, a preparation method and an application thereof. Background Art

[0002] With the warming of the global temperature, using clean energy and reducing carbon dioxide emissions have become an inevitable trend. To overcome the disadvantages of clean energy such as intermittency and volatility, energy storage batteries need to be configured to effectively utilize clean energy. Currently, the energy storage batteries are mainly lithium-ion batteries represented by lithium iron phosphate batteries. With the large-scale use of lithium-ion batteries in energy storage, lithium resources are rapidly consumed, resulting in an imbalance between supply and demand. Therefore, developing resource-rich and sustainable non-lithium-ion batteries has become a global hot spot. Sodium-ion batteries have significant advantages such as rich resources, environmental friendliness, good safety, and excellent low-temperature performance, and are very suitable for large-scale energy storage.

[0003] To develop sodium-ion batteries, developing suitable cathode materials is the key, which is related to the cost and sustainable development of the batteries. Among many optional materials, manganese-based layered materials, especially O3-type materials, are suitable as cathode materials for sodium-ion batteries due to their high capacity and long cycle life. However, these materials are prone to phase transformation during cycling, which can cause lattice distortion and reduce the cycle life. In addition, sodium-based layered materials are unstable in air, prone to absorbing water and carbon dioxide, resulting in strong surface alkalinity, easy gelation of the slurry during homogenization, and easy gas generation during charge and discharge of the battery, affecting the cycle life. Using single crystals can solve the above problems to a certain extent, but single crystals often require high temperature and time, resulting in high energy consumption, and have high requirements for the morphology and composition of the precursors and high requirements for the processing conditions of the batteries.

[0004] Therefore, there is an urgent need for a simple and general technical method for preparing single crystals, and the single crystal materials need to have excellent electrochemical performance and processing performance at the same time. Summary of the Invention

[0005] Aiming at the deficiencies of the prior art, the present invention discloses a single-crystal cathode material. By fine doping of the lattice, the formation of single crystals is promoted, and sodium-deficient phase layered oxide nanocrystals are precipitated on the surface of the single crystals; this structure is beneficial to improving the stability of the cathode material in air, reducing the surface alkalinity, inhibiting gas generation in the battery, and thus improving the cycle stability of sodium-ion batteries.

[0006] The specific technical solutions are as follows:

[0007] A single-crystal cathode material, comprising a single-crystal layered oxide and sodium-deficient phase layered oxide nanocrystals discretely distributed on the surface of the single-crystal layered oxide;

[0008] The chemical general formula of the single-crystal layered oxide is Nan Mn 1-x-y M x B y O 2 , where 0.4 ≤ x ≤ 0.7, 0.001 ≤ y ≤ 0.01, 0.85 ≤ n ≤ 1;

[0009] M is selected from at least one active element and at least one inactive element;

[0010] The active element is selected from one or more of Ni, Fe, and Cu;

[0011] The inactive element is selected from one or more of Li, Mg, Al, Zn, Sn, and Ti.

[0012] The single-crystal cathode material disclosed in the present invention is a manganese-based layered oxide. By co-doping with a trace amount of B element and M element containing both active and inactive elements, and precisely controlling the doping content and the content of Na, the formation of single crystals can be promoted, and discrete sodium-deficient phase layered oxide nanocrystals can be precipitated on the surface of the single crystals, so as to achieve the purpose of reducing the surface alkalinity of the product, suppressing battery swelling, and improving the cycle stability of sodium-ion batteries.

[0013] A single crystal refers to a crystal in which atoms or ions are arranged orderly and continuously, having an independent structure, generally with a micron-scale size. In contrast, polycrystals generally refer to aggregates composed of small-sized grains of dozens to hundreds of nanometers. Single crystals have a small specific surface area and high structural stability, which is beneficial to maintaining the structural stability of the material during charge and discharge processes.

[0014] In the present invention, 0.85 ≤ n ≤ 1. A high n value can promote the formation of the O3 phase, and the O3 phase material has a high specific capacity; through experiments, the single-crystal cathode material prepared in the present invention is indeed the O3 phase.

[0015] Through experiments, it is found that trace B doping can promote the formation of disc-shaped single crystals and the generation of sodium-deficient phase layered oxide nanocrystals on the surface, thereby reducing the surface alkalinity, suppressing battery swelling, and improving the cycle stability of sodium-ion batteries; through further analysis, when trace B element doping is carried out, B atoms are located at the interstitial positions of the lattice and form BO 4 tetrahedral configurations, that is, polyanion configurations are formed, rather than layered structures.

[0016] Through comparative experiments, it is found that when trace B doping is replaced with trace P doping, although P atoms can also form tetrahedral configurations with O atoms, it cannot promote the formation of disc-shaped single crystals and the generation of sodium-deficient phase layered oxide nanocrystals on the surface in this system.

[0017] It has been found through experiments that the simultaneous presence of active elements and inactive elements can promote the precipitation of sodium-deficient layered oxide nanocrystals on the surface; if only active elements are doped without doping inactive elements, the prepared product is still in a single-crystal structure, but the precipitation of tetrahedral nanocrystals is not observed; this results in the pH value of the product being higher than 12.

[0018] It has also been found through experiments that the control of the contents of element B, doping element M and Na element is also particularly important. When the content of element B is too high, boron oxide will precipitate on the surface instead of tetrahedral nanocrystals, leading to too high a pH of the product; when the content of doping element M is too high, a single-crystal structure can be formed in the product, but there are no tetrahedral nanocrystals precipitating on the surface of the single crystal; when the content of Na element is too low, a single-crystal structure can be formed in the product, and there are also tetrahedral nanocrystals precipitating on the surface of the single crystal, but the nanocrystals are not sodium-deficient layered oxides; all of the above situations will lead to too high a pH value of the product, reducing the cycle stability of the sodium-ion battery.

[0019] Preferably:

[0020] The single-crystal layered oxide is in a disc shape, with a particle diameter of 1 - 6 μm and a thickness of 0.5 - 1.5 μm.

[0021] It has been found through experiments that, compared with other shapes, the exposed surface of the disc-shaped single crystal along the c-axis direction is smaller, which is beneficial to reducing side reactions in air or electrolyte. The experiment also found that the larger the ratio of the diameter to the thickness in the disc-shaped structure, the more beneficial it is to maintaining the stability of the material in air or electrolyte, but it is not conducive to the rapid transmission and activity of sodium ions. Within the above diameter and thickness ranges, the capacity, rate performance and cycle life of the product reach a balance.

[0022] Preferably:

[0023] The sodium-deficient layered oxide nanocrystals are in a tetrahedral shape, with a size of 50 - 200 nm; more preferably 100 nm.

[0024] For the sodium-deficient layered oxide nanocrystals, the molar ratio of element Na to the total molar number of element Mn and element M ≤ 0.5.

[0025] It has been found through experiments that the presence of sodium-deficient layered oxide nanocrystals can improve the hydrophobicity of the material surface, thereby reducing surface alkalinity, and can also improve the interfacial stability with the electrolyte.

[0026] Preferably:

[0027] The active elements are selected from two or three of Ni, Fe, and Cu;

[0028] The inactive elements are selected from Mg and / or Al.

[0029] Preferably:

[0030] Through the optimization of the above parameters, the pH value of the single-crystal cathode material prepared by the present invention is less than 12.

[0031] The present invention also discloses a method for preparing the above single-crystal cathode material, which includes the following steps:

[0032] Mix the raw materials including Na-containing compound, Mn-containing compound, M-containing compound, and B-containing compound evenly according to the stoichiometric ratio, and prepare it through high-temperature solid-state reaction;

[0033] For the above high-temperature solid-state reaction, the temperature is 900 - 1000 °C, and the time is 8 - 20 h.

[0034] It is found through experiments that the conditions of the above high-temperature solid-state reaction have a crucial influence on the microscopic morphology of the prepared single-crystal cathode material.

[0035] Preferably:

[0036] For the above high-temperature solid-state reaction, the temperature is 940 - 980 °C, and the time is 10 - 16 h.

[0037] Preferably:

[0038] The Na-containing compound is selected from one or more of sodium carbonate, sodium nitrate, sodium citrate, sodium acetate, and sodium oxalate;

[0039] The Mn-containing compound is selected from one or more of Mn-containing oxides, Mn-containing hydroxides, and Mn-containing hydroxyoxides;

[0040] The M-containing compound is selected from one or more of M-containing oxides, M-containing hydroxides, and M-containing hydroxyoxides;

[0041] The B-containing compound is selected from one or more of boron oxide, boric acid, and borax.

[0042] It should be noted that the above conditions are interdependent. Any deviation from the above conditions will not result in disc-shaped single crystals and there will be no sodium-deficient phase layered oxide nanocrystals on the surface.

[0043] The present invention also discloses a sodium-ion secondary battery, which includes the above single-crystal cathode material. This sodium-ion secondary battery has both high cycle stability, high rate performance, and high capacity.

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] The present invention discloses a single-crystal cathode material. Through simple lattice doping, a product with a specific structure including a single-crystal layered oxide and sodium-deficient phase layered oxide nanocrystals discretely distributed on the surface of the single-crystal layered oxide is obtained. The obtained single crystal has a disc-shaped structure with optimized diameter and thickness, which can maintain the lattice stability during charge and discharge, as well as the stability in air or electrolyte. At the same time, doping can promote the formation of sodium-deficient phase layered oxide nanocrystals discretely distributed on the surface, further improving the stability in air and electrolyte, thereby achieving an optimized balance of capacity, rate performance, and lifespan.

[0046] The preparation method of the single-crystal cathode material disclosed by the present invention is simple, effective, and highly versatile. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 It is the X-ray diffraction (XRD) pattern of the product prepared in Example 1 of the present invention;

[0048] Figure 2 It is the scanning electron microscope (SEM) photograph of the product prepared in Example 1 of the present invention;

[0049] Figure 3 It is the charge-discharge curve graph of the battery assembled with the product prepared in Example 1 of the present invention as the cathode;

[0050] Figure 4 It is the cycle life graph of the battery assembled with the product prepared in Example 1 of the present invention as the cathode;

[0051] Figure 5 It is the scanning electron microscope (SEM) photograph of the product prepared in Example 2 of the present invention;

[0052] Figure 6 It is the scanning electron microscope (SEM) photograph of the product prepared in Example 5 of the present invention;

[0053] Figure 7 It is the scanning electron microscope (SEM) photograph of the product prepared in Example 7 of the present invention;

[0054] Figure 8 It is the scanning electron microscope (SEM) photograph of the product prepared in Example 9 of the present invention;

[0055] Figure 9 It is the charge-discharge curve graph of the battery assembled with the product prepared in Example 9 of the present invention as the cathode;

[0056] Figure 10 It is the cycle life graph of the battery assembled with the product prepared in Example 9 of the present invention as the cathode;

[0057] Figure 11 It is the scanning electron microscope (SEM) photograph of the product prepared in Comparative Example 1 of the present invention;

[0058] Figure 12 The charge-discharge curve of the battery assembled with the product prepared in Comparative Example 1 of the present invention as the positive electrode;

[0059] Figure 13 The cycle life diagram of the battery assembled with the product prepared in Comparative Example 1 of the present invention as the positive electrode;

[0060] Figure 14 The scanning electron microscope (SEM) photograph of the product prepared in Comparative Example 2 of the present invention;

[0061] Figure 15 The scanning electron microscope (SEM) photograph of the product prepared in Comparative Example 3 of the present invention;

[0062] Figure 16 The scanning electron microscope (SEM) photograph of the product prepared in Comparative Example 4 of the present invention;

[0063] Figure 17 The scanning electron microscope (SEM) photograph of the product prepared in Comparative Example 5 of the present invention. Detailed Description of the Invention

[0064] The present invention will be further described in detail below in conjunction with examples and comparative examples, but the embodiments of the present invention are not limited thereto.

[0065] Example 1

[0066] According to the stoichiometric ratio of Na 0.98 [Mg 0.03 Ni 0.29 Fe 0.335 Mn 0.34 B 0.005 O 2 The material was prepared by a solid-state reaction method. First, according to the stoichiometric ratio, Na 2 CO 3 , MnO 2 , MgO, Fe 2 O 3 , NiO, H 3 BO 3 were mixed evenly, and the precursor was obtained after ball milling in an acetone medium, where the ball milling time was 10 h and the ball milling speed was 400 rpm. Then, the ball milled product was placed in a muffle furnace and calcined at 940 °C in an air atmosphere for 14 h to obtain the product.

[0067] The product was analyzed by XRD (Rigaku D / Max-2550pc powder diffractometer, Rigaku, Japan) and was found to be in the O3 phase, as shown in Figure 1 .

[0068] Analyzed by SEM (ZEISS Sigma 300, Carl Zeiss, Germany), the product is a disc-shaped single crystal material with a diameter of 1 - 6 μm and a thickness of 0.5 - 1.5 μm. The surface of the single crystal particles contains tetrahedral nanocrystals with a grain size of approximately 100 nm, as shown in Figure 2 .

[0069] By elemental analysis, the tetrahedral nanocrystals are sodium-deficient layered oxides, and the ratio of the molar number of Na to the total molar number of Mg, Ni, Fe, and Mn is 0.49. However, due to the low content, it does not appear in XRD.

[0070] After testing, the pH value of the product prepared in this example is 11.44 (testing instrument: PHS-3E pH meter, manufacturer: Shanghai Instrumentation Electric Scientific Instrument Co., Ltd.). The specific testing method for pH is as follows: Weigh 3 grams of the sample and place it in a beaker. Measure 30 milliliters of pure water and pour it into the beaker to mix with the sample. After stirring for one minute, filter the mixture to obtain a clear liquid, and finally use the pH meter to measure the pH value of the clear liquid, which is the pH value of the material.

[0071] Using the single crystal cathode material prepared in this example as the cathode, metallic sodium as the anode, glass fiber as the separator, and a 1 mol / L NaPF 6 propylene carbonate / ethyl methyl carbonate solution as the electrolyte, and adding 4% by weight of fluoroethylene carbonate to the above electrolyte, assemble a button battery and conduct charge-discharge tests (testing instrument: MIHW-200-160CH-B, Neware, Shenzhen). The current density is 0.1C (1C = 150 mA / g), the voltage range is 2 - 4V, and the charge-discharge curve is as shown in Figure 3 , with a specific capacity of 141.6 mAh / g. After 100 cycles of the material at 1C, the capacity retention rate is 86.2%, as shown in Figure 4 , and the capacity retention rate of 1C current relative to 0.1C current is 91.4%.

[0072] Example 2

[0073] The preparation process is basically the same as that in Example 1, except that the doping amount of B is replaced with 0.002, and the doping amount of Fe is increased, and the prepared product is Na 0.98 [Mg 0.03 Ni 0.29 Fe 0.338 Mn 0.34 B 0.002 O 2 .

[0074] Analyzed by SEM, the product prepared in this example is a disc-shaped single crystal structure with a diameter of 1 - 6 μm and a thickness of 0.5 - 1.5 μm. The surface of the single crystal particles contains tetrahedral nanocrystals with a grain size of approximately 100 nm, as shown in Figure 5。

[0075] After elemental analysis, the tetrahedral nanocrystals are sodium-deficient layered oxides.

[0076] The charge-discharge experiment was carried out in the same manner as in Example 1. The specific capacity was 142.1 mAh / g at 0.1C. After 100 cycles at 1C, the capacity retention rate of the material was 87.1%.

[0077] Example 3

[0078] The preparation process is basically the same as that in Example 1, except that the doping amount of B is replaced by 0.07, and the doping amount of Ni is reduced. The prepared product is Na 0.984 [Mg 0.03 Ni 0.288 Fe 0.335 Mn 0.34 B 0.007 O 2 。

[0079] After SEM analysis, the product prepared in this example is a round cake-shaped single crystal structure, with a diameter of 1-6 μm and a thickness of 0.5-1.5 μm. The surface of the single crystal particles contains tetrahedral nanocrystals, and the grain size is about 100 nm. After elemental analysis, the tetrahedral nanocrystals are sodium-deficient layered oxides.

[0080] Example 4

[0081] The preparation process is basically the same as that in Example 1, except that the doping amount of B is replaced by 0.01, and the doping amount of Fe is reduced. The prepared product is Na 0.98 [Mg 0.03 Ni 0.29 Fe 0.330 Mn 0.34 B 0.01 O 2 。

[0082] After SEM analysis, the product prepared in this example is a round cake-shaped single crystal structure, with a diameter of 1-6 μm and a thickness of 0.5-1.5 μm. The surface of the single crystal particles contains tetrahedral nanocrystals, and the grain size is about 100 nm. After elemental analysis, the tetrahedral nanocrystals are sodium-deficient layered oxides.

[0083] Example 5

[0084] The preparation process is basically the same as that in Example 1, except that the calcination temperature is replaced by 960 °C.

[0085] After SEM analysis, the product prepared in this example is a round cake-shaped single crystal structure, with a diameter of 1-6 μm and a thickness of 0.5-1.5 μm. The surface of the single crystal particles contains tetrahedral nanocrystals, and the grain size is about 100 nm, as shown inFigure 6 .

[0086] After elemental analysis, the tetrahedral nanocrystals are sodium-deficient layered oxides.

[0087] The charge-discharge experiment was carried out in the same manner as in Example 1. The specific capacity was 140.8 mAh / g at 0.1C. After 100 cycles at 1C, the capacity retention rate of the material was 86.4%.

[0088] Example 6

[0089] The preparation process is basically the same as that in Example 1, except that the calcination temperature is replaced with 980 °C.

[0090] After SEM analysis, the product prepared in this example has a disc-shaped single crystal structure with a diameter of 1-6 μm and a thickness of 0.5-1.5 μm. The surface of the single crystal particles contains tetrahedral nanocrystals with a grain size of about 100 nm. After elemental analysis, the tetrahedral nanocrystals are sodium-deficient layered oxides.

[0091] Example 7

[0092] The preparation process is basically the same as that in Example 1, except that the calcination time is replaced with 10 h.

[0093] After SEM analysis, the product prepared in this example has a disc-shaped single crystal structure with a diameter of 1-6 μm and a thickness of 0.5-1.5 μm. The surface of the single crystal particles contains tetrahedral nanocrystals with a grain size of about 100 nm, as shown in Figure 7 .

[0094] After elemental analysis, the tetrahedral nanocrystals are sodium-deficient layered oxides.

[0095] The charge-discharge experiment was carried out in the same manner as in Example 1. The specific capacity was 140.9 mAh / g at 0.1C. After 100 cycles at 1C, the capacity retention rate of the material was 88.0%.

[0096] Example 8

[0097] The preparation process is basically the same as that in Example 1, except that the calcination time is replaced with 16 h.

[0098] After SEM analysis, the product prepared in this example has a disc-shaped single crystal structure with a diameter of 1-6 μm and a thickness of 0.5-1.5 μm. The surface of the single crystal particles contains tetrahedral nanocrystals with a grain size of about 100 nm. After elemental analysis, the tetrahedral nanocrystals are sodium-deficient layered oxides.

[0099] Example 9

[0100] According to Na 0.98 [Al 0.03 Ni0.26 Cu 0.04 Fe 0.347 Mn 0.32 B 0.003 O 2 The stoichiometric ratio is used to prepare the material by the solid-phase reaction method. First, according to the stoichiometric ratio, Na 2 CO 3 , MnO 2 , Al 2 O 3 , CuO, Fe 2 O 3 , NiO, H 3 BO 3 are mixed evenly and a precursor is obtained after ball milling in an acetone medium. The ball milling time is 10 h, the ball milling speed is 400 rpm. Then the ball-milled product is placed in a muffle furnace and calcined at 970 °C in an air atmosphere for 16 h to obtain the product. After XRD analysis, the product is in the O3 phase.

[0101] The product is a disc-shaped single-crystal material with a diameter of 1 - 6 μm and a thickness of 0.5 - 1.5 μm. The surface of the single-crystal particles contains tetrahedral nanocrystals with a grain size of about 100 nm, as shown in Figure 8 ; after elemental analysis, the nanocrystals are sodium-deficient layered oxides; after detection, the pH value of the product is 11.51.

[0102] The charge-discharge experiment is carried out in the same way as in Example 1. The charge-discharge curve at 0.1C is as shown in Figure 9 , and the specific capacity is 140.1 mAh / g. After 100 cycles of the material at 1C, the capacity retention rate is 85.2%, as shown in Figure 10 .

[0103] Comparative Example 1

[0104] The preparation process is basically the same as that in Example 1, except that B is not doped and part of B is replaced by Fe to obtain Na 0.98 [Mg 0.03 Ni 0.29 Fe 0.34 Mn 0.34 O 2 .

[0105] After SEM analysis, the product is a polycrystalline structure and there are no tetrahedral nanocrystals on the surface, as shown in Figure 11 .

[0106] After detection, the pH value of the product is 12.34.

[0107] The charge-discharge is carried out in the same way as in Example 1. At 0.1C and a voltage range of 2 - 4V, the charge-discharge curve is as shown in Figure 12As shown, the specific capacity is 131.5 mAh / g. After the material undergoes cycles at 1C, the capacity retention rate is 71.7%, see Figure 13 .

[0108] Comparative Example 2

[0109] The preparation process is basically the same as that in Example 1, except that Mg is not doped and the Mg part is replaced by Ni, obtaining Na 0.98 [Ni 0.32 Fe 0.335 Mn 0.34 B 0.005 O 2 .

[0110] After SEM analysis, the product is a single crystal structure, but there are no tetrahedral nanocrystals on the surface, see Figure 14 .

[0111] After testing, the pH value of the product is 12.14.

[0112] Comparative Example 3

[0113] The preparation process is basically the same as that in Example 1, except that Mg is not doped and the Mg part is replaced by Ni, and B is not doped and the B part is replaced by Fe, obtaining Na 0.98 [Ni 0.32 Fe 0.34 Mn 0.34 O 2 .

[0114] After SEM analysis, the product is a polycrystalline structure and there are no tetrahedral nanocrystals on the surface either, see Figure 15 .

[0115] After testing, the pH value of the product is 12.58.

[0116] Comparative Example 4

[0117] The preparation process is basically the same as that in Example 1, except that the calcination temperature is replaced by 890 °C.

[0118] After SEM analysis, the single crystal structure in the product is not fully formed and there are no tetrahedral nanocrystals on the surface either, see Figure 16 .

[0119] After testing, the pH value of the product is 12.41.

[0120] Comparative Example 5

[0121] The preparation process is basically the same as that in Example 1, except that the calcination time is replaced by 6 h.

[0122] By SEM analysis, the single-crystal structure in the product was not fully formed, and there were no tetrahedral nanocrystals on the surface, as shown in Figure 17 .

[0123] After detection, the pH value of the product was 12.38.

[0124] Comparative Example 6

[0125] According to the stoichiometric ratio of Na 0.99 [Mg 0.03 Ni 0.30 Fe 0.375 Mn 0.29 B 0.005 O 2 This material was prepared by the solid-phase reaction method, and the preparation process was basically the same as that in Example 1.

[0126] By SEM analysis, a single-crystal structure could be formed in the product, but there were no tetrahedral nanocrystals on the surface of the single crystal.

[0127] After detection, the pH value of the product was 12.35.

[0128] Comparative Example 7

[0129] According to the stoichiometric ratio of Na 0.83 [Mg 0.03 Ni 0.15 Fe 0.205 Mn 0.61 B 0.005 O 2 This material was prepared by the solid-phase reaction method, and the preparation process was basically the same as that in Example 1.

[0130] By SEM analysis, a single-crystal structure could be formed in the product, there were tetrahedral nanocrystals on the surface of the single crystal, but the molar ratio of Na in the nanocrystals to the total molar amounts of Mg, Ni, Fe, and Mn was 0.57.

[0131] After detection, the pH value of the product was 12.24.

[0132] Comparative Example 8

[0133] The preparation process was basically the same as that in Example 1, except that the B element was replaced with an equimolar amount of the P element, and the synthesized product was Na 0.98 [Mg 0.03 Ni 0.29 Fe 0.335 Mn 0.34 P 0.005 O 2 .

[0134] By SEM analysis, the product was a polycrystalline structure, and there were no tetrahedral nanocrystals on the surface.

[0135] After detection, the pH value of the product is 12.42.

[0136] Comparative Example 9

[0137] The preparation process is basically the same as that in Example 1, except that the content of element B is increased to y = 0.02, and at the same time the content of Fe is reduced, that is, the molecular formula is Na 0.98 [Mg 0.03 Ni 0.29 Fe 0.32 Mn 0.34 B 0.02 O 2 .

[0138] After SEM analysis, the product is a single crystal structure, but there are no tetrahedral nanocrystals on the surface.

[0139] After detection, the pH value of the product is 12.33.

[0140] The above-disclosed are preferred embodiments, but the protection scope of the present invention is not limited thereto. Those of ordinary skill in the art can easily understand the spirit of the present invention based on the above embodiments and make different extensions and changes. However, as long as they do not depart from the spirit of the present invention, they are within the protection scope of the present invention.

Claims

1. A single crystal positive electrode material, characterized in that: It comprises a single crystal layered oxide and sodium-poor phase layered oxide nanocrystals discretely distributed on the surface of the single crystal layered oxide; The chemical formula of the single crystal layered oxide is Na n Mn 1-x-y M x B y O2, where 0.4≤x≤0.7, 0.001≤y≤0.01, 0.85≤n≤1; M is selected from at least one active element and at least one inactive element; The active element is selected from one or more of Ni, Fe, and Cu; The inactive element is selected from one or more of Li, Mg, Al, Zn, Sn and Ti.

2. The single crystal positive electrode material according to claim 1, characterized in that: The single crystal layered oxide is in the shape of a round cake, with a particle diameter of 1 to 6 μm and a thickness of 0.5 to 1.5 μm.

3. The single crystal positive electrode material according to claim 1, characterized in that: The sodium-poor phase layered oxide nanocrystals are tetrahedral in shape and have a size of 50 to 200 nm; In the sodium-poor layered oxide nanocrystals, the ratio of the molar number of element Na to the total molar number of element Mn and element M is ≤0.

5.

4. The single crystal positive electrode material according to claim 1, characterized in that: The active elements are selected from two or three of Ni, Fe and Cu; The inactive element is selected from Mg and / or Al.

5. The single crystal positive electrode material according to any one of claims 1 to 4, characterized in that: The pH value of the surface of the single crystal positive electrode material is lower than 12.

6. A method for preparing a single crystal positive electrode material according to any one of claims 1 to 5, characterized in that: The steps include: The raw materials including a Na compound, a Mn compound, an M compound and a B compound are uniformly mixed in a stoichiometric ratio and prepared by a high-temperature solid phase reaction; The high temperature solid phase reaction has a temperature of 900 to 1000° C. and a time of 8 to 20 hours.

7. The method for preparing a single crystal positive electrode material according to claim 6, characterized in that: The Na-containing compound is selected from one or more of sodium carbonate, sodium nitrate, sodium citrate, sodium acetate, and sodium oxalate; The Mn-containing compound is selected from one or more of a Mn-containing oxide, a Mn-containing hydroxide, and a Mn-containing oxyhydroxide; The M-containing compound is selected from one or more of an M-containing oxide, an M-containing hydroxide, and an M-containing oxyhydroxide; The B-containing compound is selected from one or more of boron oxide, boric acid and borax.

8. The method for preparing a single crystal positive electrode material according to claim 6, characterized in that: The high temperature solid phase reaction has a temperature of 940-980° C. and a time of 10-16 hours.

9. A sodium ion secondary battery, characterized in that: It comprises the single crystal positive electrode material according to any one of claims 1 to 5.

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