Sodium ion positive electrode material and preparation method thereof and secondary battery

Abstract

This application relates to the technical field of secondary batteries, and particularly to a sodium ion cathode material, a preparation method thereof, and a secondary battery. The sodium ion cathode material includes a matrix material, a first coating layer, and a second coating layer. The general formula of the matrix material is Na a1 Ni b Fe c Mn d M e N f O 2‑g P g , the first coating layer coats the matrix material, and the general formula is Na a2 Ni b Fe c Mn d M e N f O 2‑g P g , the second coating layer coats the first coating layer, and the general formula is Na a3 Cu i Fe j Mn k O2. The sodium ion cathode material of this application is coated and modified, which can effectively reduce the residual alkali on the surface, improve the ion migration rate, reduce the interfacial side reaction, and improve the cycle stability.

Description

Technical Field

[0001] The present application relates to the technical field of secondary batteries, and in particular to a sodium ion positive electrode material, a preparation method thereof, and a secondary battery. Background Art

[0002] The promotion of electric vehicles has rapidly expanded the market for lithium-ion batteries, but lithium resources are expensive and extremely unevenly distributed. Most lithium mines are distributed in South American countries, and more than 80% of my country's lithium resources are imported. Sodium and lithium are in the same main group and have similar physical and chemical properties to lithium. In addition, sodium-ion batteries have the advantages of abundant reserves, wide distribution, and low cost, and have broad application prospects in the fields of power and energy storage.

[0003] The cathode material of sodium-ion batteries is a key factor in determining battery performance. There are many types of cathode materials, and the three main types with application prospects are oxides, Prussian blue analogs, and polyanions. Among them, O3 oxides have the fastest commercialization progress due to their advantages such as high energy density, good rate and cycle performance. However, O3 oxides have problems such as high surface residual alkali, poor cycle stability, low ion migration rate, and interface reaction. Summary of the invention

[0004] Based on this, the first aspect of the present application provides a sodium ion positive electrode material, and its technical solution is as follows:

[0005] A sodium ion positive electrode material, comprising:

[0006] Matrix material, general formula is Na a1 Ni b Fe c Mn d M e N f O 2-g P g ;

[0007] The first coating layer, which covers the base material, has the general formula of Na a2 Ni b Fe c Mn d M e N f O 2-g P g ;

[0008] The second coating layer, covering the first coating layer, has the general formula of Na a3 Cu i Fe j Mn k O2;

[0009] Among them, M is a 3d transition metal element, N is a non-3d transition metal element, P is a VII main group element, 0.8≤a1≤1.2, 0.8≤a2≤1.2, a1>a2, 0.1≤b≤0.6, 0.1≤c≤0.4, 0.1≤d≤0.5, 0.01≤e≤0.2, 0.001≤f≤0.05, 0.01≤g≤0.1, 0.44≤a3≤1.0, 0.05≤i≤0.3, 0.1≤j≤0.5, 0.1≤k≤0.6.

[0010] The second aspect of the present application provides a method for preparing a sodium ion positive electrode material, and its technical solution is as follows:

[0011] A method for preparing a sodium ion positive electrode material comprises the following steps:

[0012] Mixing a raw material base material, a weakly acidic substance and a solvent to obtain a premix, wherein the raw material base material has a general formula of Na a Ni b Fe c Mn d M e N f O 2-g P g , where M is a 3d transition metal element, N is a non-3d transition metal element, P is a VII main group element, 0.8≤a≤1.2, 0.1≤b≤0.6, 0.1≤c≤0.4, 0.1≤d≤0.5, 0.01≤e≤0.2, 0.001≤f≤0.05, 0.01≤g≤0.1;

[0013] According to Cu i Fe j Mn k O2 ratio to prepare a copper source, an iron source and a manganese source, mix the premix, the copper source, the iron source and the manganese source, remove the solvent, and obtain a composite powder, wherein 0.05≤i≤0.3, 0.1≤j≤0.5, 0.1≤k≤0.6;

[0014] The composite powder is heat-treated to prepare a sodium ion positive electrode material including a base material, a first coating layer and a second coating layer, wherein the first coating layer coats the base material, the second coating layer coats the first coating layer, and the amount of the weakly acidic substance added is controlled so that the general formula of the base material is Na a1 Ni b Fe c Mn d M e N f O 2-g P g , the general formula of the first coating layer is Na a2 Ni b Fec Mn d M e N f O 2-g P g , the general formula of the second coating layer is Na a3 Cu i Fe j Mn k O2, 0.8≤a1≤1.2, 0.8≤a2≤1.2, 0.44≤a3≤1.0, a1>a2.

[0015] The third aspect of the present application provides a secondary battery, and its technical solution is as follows:

[0016] A secondary battery comprises the sodium ion positive electrode material as described above.

[0017] Compared with the traditional solution, this application has the following beneficial effects:

[0018] The base material of the present application is an O3-type oxide, which is coated and modified by a first coating layer and a second coating layer, and the sodium ion positive electrode material presents a "sandwich structure". After coating and modification, the surface residual alkali can be effectively reduced, the ion migration rate can be increased, the interface side reaction can be reduced, and the cycle stability can be improved. Specifically, the general formula is Na a1 Ni b Fe c Mn d M e N f O 2-g P g The matrix material can provide a higher specific capacity, the general formula is Na a2 Ni b Fe c Mn d M e N f O 2-g P g The first coating layer is a sodium-deficient layer, which has a higher sodium ion migration rate, which is beneficial to improve the rate and cycle stability. a3 Cu i Fe j Mn k The second coating layer of O2 itself has good stability. As a coating layer, it can effectively inhibit the escape of lattice oxygen from the material, inhibit interface side reactions and the dissolution of transition metals, thereby improving the material's cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present application and to more completely understand the present application and its beneficial effects, the following is a brief introduction to the drawings required for the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application, and those skilled in the art can obtain other drawings based on these drawings without creative work.

[0020] Figure 1 Schematic diagram of the structure of a sodium ion positive electrode material according to one embodiment;

[0021] Figure 2 is the XRD pattern of the sodium ion positive electrode material of Example 1;

[0022] Figure 3 is the XRD pattern of the sodium ion positive electrode material of Example 2;

[0023] Figure 4 This is the XRD pattern of the sodium ion positive electrode material of Comparative Example 1;

[0024] Figure 5 This is the XRD pattern of the sodium ion positive electrode material of Comparative Example 2;

[0025] Figure 6 is a SEM image of the sodium ion positive electrode material of Example 1;

[0026] Figure 7 is a SEM image of the sodium ion positive electrode material of Example 2;

[0027] Figure 8 This is a SEM image of the sodium ion positive electrode material of Comparative Example 1;

[0028] Fig. 9 This is the SEM image of the sodium ion positive electrode material of Comparative Example 2. DETAILED DESCRIPTION

[0029] The present application will be further described in detail below in conjunction with specific embodiments. The present application can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present application more thoroughly understood.

[0030] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0031] Unless otherwise specified or incompatible herewith, the terms and phrases used herein shall have the following meanings:

[0032] In the present application, "plurality", "multiple", "multiple times", "plurality", etc., unless otherwise specified, refer to a number greater than 2 or equal to 2. For example, "one or more" means one or greater than or equal to two.

[0033] In this application, "optionally", "optional" and "optional" refer to optional, that is, to any one of the two parallel schemes of "yes" or "no". If multiple "options" appear in a technical solution, unless otherwise specified and there is no contradiction or mutual restriction, each "optional" is independent.

[0034] In this application, in the "first aspect", "second aspect", "third aspect", "fourth aspect", etc., the terms "first", "second", "third", "fourth", etc. are used only for descriptive purposes and cannot be understood as indicating or implying relative importance or quantity, nor can they be understood as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first", "second", "third", "fourth", etc. only serve the purpose of non-exhaustive enumeration and description, and it should be understood that they do not constitute a closed limitation on quantity.

[0035] The first aspect of the present application provides a sodium ion positive electrode material, see Figure 1 In one embodiment, the sodium ion positive electrode material 100 includes a base material 10, a first coating layer 11 and a second coating layer 12. The general formula of the base material 10 is Na a1 Ni b Fe c Mn d M e N f O 2-g P g The first coating layer 11 covers the base material 10, and the general formula of the first coating layer 11 is Na a2 Ni b Fe c Mn d M e N f O 2-g P g The second coating layer 12 covers the first coating layer 11, and the general formula of the second coating layer 12 is Na a3 Cu i Fe j Mn kO2. Wherein, M is a 3d transition metal element, N is a non-3d transition metal element, P is a VII main group element, 0.8≤a1≤1.2, 0.8≤a2≤1.2, a1>a2, 0.1≤b≤0.6, 0.1≤c≤0.4, 0.1≤d≤0.5, 0.01≤e≤0.2, 0.001≤f≤0.05, 0.01≤g≤0.1, 0.44≤a3≤1.0, 0.05≤i≤0.3, 0.1≤j≤0.5, 0.1≤k≤0.6.

[0036] The base material of the above embodiment is an O3-type oxide, which is coated and modified by the first coating layer and the second coating layer, and the sodium ion positive electrode material presents a "sandwich structure". After coating and modification, the surface residual alkali can be effectively reduced, the ion migration rate can be increased, the interface side reaction can be reduced, and the cycle stability can be improved. Specifically, the general formula is Na a1 Ni b Fe c Mn d M e N f O 2-g P g The matrix material can provide a higher specific capacity, the general formula is Na a2 Ni b Fe c Mn d M e N f O 2-g P g The first coating layer is a sodium-deficient layer, which has a higher sodium ion migration rate, which is beneficial to improving the rate and cycle stability. Among them, the M element is doped as the main element, and its Fermi level is quite different from that of nickel-iron-manganese, which can inhibit phase change, inhibit Fe migration and support the material structure. The N element is doped in trace amounts, which stabilizes the lattice oxygen during deep sodium removal. The P element can also stabilize the lattice oxygen and inhibit the formation of oxygen. At the same time, the introduction of -1 valent anions P can increase the average valence state of metals M and N, which is beneficial to increase air stability and cycle stability, and is also beneficial to the formation of oxygen vacancies. Oxygen vacancies are beneficial to improving the cycle stability of the material. At the same time, the general formula is Na a3 Cu i Fe j Mn k The second coating layer of O2 itself has good stability. As a coating layer, it can effectively inhibit the escape of lattice oxygen from the material, inhibit interface side reactions and the dissolution of transition metals, thereby improving the material's cycle stability.

[0037] Optionally, M includes at least one of Ti, V, Cr, Co, Cu and Zn; N element includes at least one of Ca, Mg, Al, Li, B, K, Y, Zr, Nb, Mo, Cd, Ta, W, Sn and Te; P element includes at least one of F, Cl, Br and I.

[0038] Optionally, the thickness of the first coating layer is 5 nm to 100 nm. For example, the thickness of the first coating layer is 5 nm, 20 nm, 50 nm, 80 nm, or 100 nm.

[0039] Optionally, the thickness of the second coating layer is 5nm~100nm, and the mass ratio of the second coating layer in the sodium ion positive electrode material is 0.1%~5%. For example, the thickness of the second coating layer is 5nm, 20nm, 50nm, 80nm, 100nm. The mass ratio of the second coating layer in the sodium ion positive electrode material is 0.1%, 1%, 3%, 5%.

[0040] It can be understood that the thickness of the first coating layer and the second coating layer can be measured by HRTEM (high resolution transmission electron microscopy) and SAED (selected area electron diffraction).

[0041] Optionally, the XRD spectrum of the sodium ion positive electrode material has no impure phase diffraction peaks at 35°~36°, 38.2°~39.2° and 42.5°~43.5°, or the sum of the impure phase masses corresponding to the impure phase diffraction peaks at 35°~36°, 38.2°~39.2° and 42.5°~43.5° accounts for less than 5wt% of the mass of the sodium ion positive electrode material.

[0042] Among them, the diffraction peaks at 35°~36°, 38.2°~39.2° and 42.5°~43.5° are characteristic diffraction peaks of impurity phases such as NiO and CuO.

[0043] Optionally, the crystal structure of the sodium ion positive electrode material satisfies: O-Na-O interlayer spacing is 3.20Å-3.50Å, crystal plane spacing d(003) is 5.20Å-5.50Å, unit cell parameter a is 2.90-3.05Å, and unit cell parameter c is 15.90-16.30Å. It can be understood that the above crystal structure can be obtained through XRD spectrum, and the XRD spectrum can be an XRD spectrum refined by full spectrum analysis software (TopaS).

[0044] Optionally, the particle size D50 of the sodium ion positive electrode material is 2 μm to 15 μm.

[0045] Optionally, the specific surface area of ​​the sodium ion positive electrode material is 0.2 m 2 / g~2m 2 / g.

[0046] Optionally, the tap density of the sodium ion positive electrode material is ≥1.6 g / cm 3 .

[0047] A second aspect of the present application provides a method for preparing a sodium ion positive electrode material. In one embodiment, the method for preparing a sodium ion positive electrode material comprises the following steps:

[0048] S10, mixing a raw material substrate, a weakly acidic substance and a solvent to obtain a premix, wherein the raw material substrate has a general formula of Na a Ni b Fe c Mn d M e N f O 2-g P g , wherein M is a 3d transition metal element, N is a non-3d transition metal element, P is a VII main group element, 0.8≤a≤1.2, 0.1≤b≤0.6, 0.1≤c≤0.4, 0.1≤d≤0.5, 0.01≤e≤0.2, 0.001≤f≤0.05, 0.01≤g≤0.1.

[0049] The method for preparing the raw material substrate comprises the following steps:

[0050] S101, according to Na a Ni b Fe c Mn d M e N f O 2-g P g A nickel source, a manganese source, an iron source, a sodium source, an M source, an N source and a P source are mixed in a certain proportion to obtain a precursor.

[0051] Among them, according to Na a Ni b Fe c Mn d M e N f O 2-g P g Mixing the nickel source, manganese source, iron source, sodium source, M source, N source and P source in a ratio of 1:1 to 1:1 may include the following steps:

[0052] S1011, according to Na a Ni b Fe c Mn d M e N f O 2-g P gA nickel source, a manganese source, an iron source and an alkaline substance are mixed in a ratio of , a co-precipitation reaction is carried out in water, and the obtained precipitate is dried to obtain a pre-precursor.

[0053] Optionally, the nickel source is nickel sulfate.

[0054] Optionally, the manganese source is manganese sulfate.

[0055] Optionally, the iron source is ferric sulfate.

[0056] Optionally, the alkaline substance is selected from one or more of sodium hydroxide and ammonia water.

[0057] S1012, according to Na a Ni b Fe c Mn d M e N f O 2-g P g The pre-precursor, the sodium source, the M source, the N source and the P source are mixed in a ratio of , to obtain a precursor.

[0058] Optionally, the sodium source is sodium carbonate.

[0059] Optionally, the amount of carbon source added is 2 wt % higher than the theoretical value.

[0060] When M is copper, the source of M may be copper oxide. When N is niobium, the source of N may be niobium pentoxide. When P is F, the source of P may be sodium fluoride.

[0061] In addition, according to Na a Ni b Fe c Mn d M e N f O 2-g P g A nickel source, a manganese source, an iron source, a sodium source, an M source, a N source and a P source are mixed in a solvent in a ratio of , and then spray-dried to prepare a precursor.

[0062] In addition, according to Na a Ni b Fe c Mn d M e N f O 2-g P g The nickel source, manganese source, iron source, sodium source, M source, N source and P source are directly mixed in a ratio of 1:1 to prepare a precursor.

[0063] Optionally, the nickel source is nickelous oxide.

[0064] Optionally, the manganese source is trimanganese tetraoxide.

[0065] Optionally, the iron source is ferric oxide.

[0066] Optionally, the carbon source is sodium carbonate.

[0067] Optionally, the amount of carbon source added is 2 wt % higher than the theoretical value.

[0068] When M is titanium, the source of M can be titanium dioxide. When N is aluminum, the source of N can be aluminum oxide. When P is F, the source of P can be sodium fluoride. When P is Cl, the source of P can be sodium chloride.

[0069] S102, calcining the precursor.

[0070] Optionally, the calcination procedure includes: heating to 400°C~600°C at a heating rate of 1°C / min~5°C / min and calcining for 4h~8h; heating to 800°C~1000°C at a heating rate of 1°C / min~5°C / min and calcining for 10h~14h.

[0071] It is understood that the calcination atmosphere can be an air atmosphere. After calcination, the furnace is cooled to obtain a raw material substrate.

[0072] It is understandable that the raw material substrate can be placed in a solvent, and then a weak acidic substance is added and stirred to obtain a premix. The weak acidic substance can slightly corrode the surface of the raw material substrate, and because the acidity of the weak acidic substance is relatively weak, it will not cause excessive corrosion to damage the structure of the matrix material. The surface exposed after corrosion by the weak acidic substance is more active than the uncorroded surface, and has more nucleation sites, which is conducive to the smooth chemical reaction between the coating material and the surface.

[0073] Optionally, the concentration of the weak acidic substance is 0.04 mol / L to 2 mol / L. Preferably, the concentration of the weak acidic substance is 0.05 mol / L to 2 mol / L.

[0074] Optionally, the pH of the weakly acidic substance is 2-7.

[0075] Optionally, the weakly acidic substance is selected from one or more of carbonic acid, formic acid, acetic acid, propionic acid, boric acid, sodium dihydrogen phosphate, citric acid, maleic acid, tartaric acid, vitamin C, benzoic acid, salicylic acid and benzenesulfonic acid.

[0076] Optionally, the solvent is selected from one or more of ethanol, ethylene glycol, ethyl ether, methanol, propanol and glycerol.

[0077] S20, according to Cu i Fe j Mn kPrepare a copper source, an iron source and a manganese source in a ratio of O2, mix the premix, the copper source, the iron source and the manganese source, remove the solvent, and obtain a composite powder, wherein 0.05≤i≤0.3, 0.1≤j≤0.5, and 0.1≤k≤0.6.

[0078] Optionally, the copper source is copper oxide.

[0079] Optionally, the iron source is ferric oxide.

[0080] Optionally, the manganese source is manganese dioxide.

[0081] The copper source, iron source and manganese source are used as coating materials to form a second coating layer. The content of the copper source, iron source and manganese source can be controlled so that the thickness of the second coating layer is 5nm~100nm, and the mass ratio of the copper source, iron source and manganese source in the sodium ion positive electrode material is 0.1%~5%.

[0082] The solvent can be removed by evaporation or drying.

[0083] S20, heat-treating the composite powder to prepare a sodium ion positive electrode material including a base material, a first coating layer and a second coating layer, wherein the first coating layer coats the base material, the second coating layer coats the first coating layer, and the amount of the weakly acidic substance added is controlled so that the general formula of the base material is Na a1 Ni b Fe c Mn d M e N f O 2-g P g , the general formula of the first coating layer is Na a2 Ni b Fe c Mn d M e N f O 2-g P g , the general formula of the second coating layer is Na a3 Cu i Fe j Mn k O2, 0.8≤a1≤1.2, 0.8≤a2≤1.2, 0.44≤a3≤1.0, a1>a2.

[0084] Optionally, 0.05 mol to 0.2 mol of weak acidic substance is added per 100 g of matrix material. Optionally, the concentration of the weak acidic substance is 0.04 mol / L to 2 mol / L. Preferably, the concentration of the weak acidic substance is 0.05 mol / L to 2 mol / L. Optionally, the pH of the weak acidic substance is 2 to 7.

[0085] Among them, due to the diffusion of Na during the reaction, the molar proportion of Na in the matrix material is slightly smaller than the molar proportion of Na in the raw material substrate, but the two are very close, and in the sodium ion positive electrode material, the mass fraction of the matrix material is larger, and the mass fractions of the first coating layer and the second coating layer are smaller.

[0086] The purpose of heat treatment is to allow the residual alkali on the surface to fully react with the coating material, to form a uniform second coating layer in situ, and to form a sodium-deficient layer (i.e., the first coating layer) on the subsurface. If the holding temperature of the heat treatment is too low, the reaction will be insufficient and the sodium-deficient layer will not be formed. If the holding temperature is too high, the material structure will collapse. Optionally, the holding temperature of the heat treatment is 450°C~900°C. Optionally, the heat treatment procedure includes: heating from 15°C~35°C to 450°C~900°C at a heating rate of 1°C / min~5°C / min, and keeping warm for 1h~15h.

[0087] During the heat treatment process, the residual alkali on the surface of the raw material substrate and the coating material of the second coating layer nucleate and grow on the surface treated with weak acidic substances, and finally form a uniform and stable second coating layer in situ. At the same time, because the raw material substrate has been micro-corroded by weak acidic substances, the sodium on the surface is partially reacted. During the heat treatment process, part of the sodium in the inner layer will migrate to the second coating layer on the surface under the combined action of concentration gradient and temperature field, thereby forming a sodium-deficient layer (i.e., the first coating layer) of a certain thickness covering the innermost layer of the substrate material on the subsurface between the innermost layer and the second coating layer on the surface. The sodium-deficient layer has lower residual alkali, larger lattice parameters, and smaller strain during the cycle process, which can not only increase the structural stability of the material, but also reduce the diffusion energy barrier of sodium ions and improve the migration rate of sodium ions.

[0088] After being treated by the above method, a uniform and stable second coating layer can be generated in situ on the surface of the base material, and a sodium-deficient layer (i.e., the first coating layer) can be formed on the sub-surface, thereby forming a "sandwich structure". The resulting sodium ion positive electrode material has good rate and cycle performance.

[0089] A third aspect of the present application provides a secondary battery. In one embodiment, the secondary battery includes the sodium ion positive electrode material as described above.

[0090] The following is further described in conjunction with specific embodiments and comparative examples. The raw materials involved in the following specific embodiments and comparative examples, unless otherwise specified, can all be commercially available, the instruments used, unless otherwise specified, can all be commercially available, and the processes involved, unless otherwise specified, are all routinely selected by those skilled in the art.

[0091] Example 1

[0092] This embodiment provides a sodium ion positive electrode material and a preparation method thereof, the steps are as follows:

[0093] Step 1: Follow the Na 0.97 Ni 0.2 Fe 0.3 Mn 0.42 Cu 0.075 Nb 0.005 O 1.98 F 0.02 Nickel sulfate, manganese sulfate, iron sulfate, sodium hydroxide and ammonia water are mixed in a ratio of , the mixture is subjected to a coprecipitation reaction in water, and the obtained precipitate is dried to obtain precursor particles; the precursor, sodium carbonate, copper oxide, niobium pentoxide and sodium fluoride are mixed in the above ratio, wherein the sodium carbonate is 2wt% higher than the theoretical value, the temperature is increased to 500°C at a heating rate of 3°C / min in an air atmosphere, and calcined for 6h, and then the temperature is increased to 920°C at a heating rate of 2°C / min and calcined for 12h, and the raw material substrate is obtained after cooling in the furnace.

[0094] Step 2: Take 100 g of the raw material substrate and add it to ethanol, then add citric acid to form a 0.1 mol / L citric acid solution, continue stirring, and then 0.2 Fe 0.4 Mn 0.4 O2 ratio to add copper oxide, ferric oxide, and manganese dioxide as coating materials, and control the Na a3 Cu 0.2 Fe 0.4 Mn 0.4 The mass ratio of O2 to the entire sodium ion positive electrode material is 0.5%. After sufficient stirring, the solvent is evaporated and dried to obtain a composite powder.

[0095] Step 3, heating to 800°C at a heating rate of 2°C / min for heat treatment for 8 hours to obtain a sodium ion positive electrode material, which includes a base material, a first coating layer (sodium-deficient layer) coating the base material, and a second coating layer coating the first coating layer.

[0096] Example 2

[0097] This embodiment provides a sodium ion positive electrode material and a preparation method thereof, the steps are as follows:

[0098] Step 1: Follow the Na 0.96 Ni 0.23 Fe 0.28 Mn 0.39 Ti 0.1 Ca 0.01 O 1.98 F 0.02Nickelous oxide, manganese tetraoxide, ferric oxide, sodium carbonate, titanium dioxide, calcium hydroxide and sodium fluoride are mixed in a ratio of , wherein sodium carbonate is 2.5 wt % higher than the theoretical value, ethylene glycol is added, ball milled and spray dried, and the temperature is increased to 600°C at a temperature rate of 2°C / min in an air atmosphere and calcined for 5 h, and then increased to 950°C at a temperature rate of 2°C / min and calcined for 12 h. After cooling in the furnace, the raw material substrate is obtained.

[0099] Step 2: Take 100 g of the raw material substrate and add it to ethylene glycol, then add acetic acid to form a 0.05 mol / L acetic acid solution, continue stirring, and then 0.15 Fe 0.3 Mn 0.55 O2 ratio to add copper oxide, ferric oxide, and manganese dioxide as coating materials, and control the Na a3 Cu 0.15 Fe 0.3 Mn 0.55 The mass ratio of O2 to the entire sodium ion positive electrode material is 0.8%. After sufficient stirring, the solvent is evaporated and dried to obtain a composite powder.

[0100] Step 3, heating to 800°C at a heating rate of 2°C / min and heat treating for 10 hours to obtain a sodium ion positive electrode material, which includes a base material, a first coating layer (sodium-deficient layer) coating the base material, and a second coating layer coating the first coating layer.

[0101] Example 3

[0102] This embodiment provides a sodium ion positive electrode material and a preparation method thereof, the steps are as follows:

[0103] Step 1: Follow the Na 0.94 Ni 0.3 Fe 0.3 Mn 0.37 Ti 0.05 Al 0.05 O 1.97 Cl 0.03 Nickelous oxide, manganese tetraoxide, ferric oxide, sodium carbonate, titanium dioxide, aluminum oxide and sodium chloride are fully mixed in a high mixer in a ratio of , wherein sodium carbonate is 3wt% higher than the theoretical value. The mixture is heated to 700°C at a heating rate of 2°C / min in an air atmosphere and calcined for 5h, then heated to 960°C at a heating rate of 2°C / min and calcined for 15h. After cooling with the furnace, the obtained material is crushed, sieved and then second calcined. During the second calcination, the temperature is increased to 850°C at a heating rate of 2°C / min and calcined for 8h to obtain a raw material substrate.

[0104] Step 2: Take 100 g of the raw material substrate and add it to ethylene glycol, then add acetic acid dropwise to form a 0.06 mol / L acetic acid solution, continue stirring, and then0.25 Fe 0.2 Mn 0.55 O2 ratio to add copper oxide, ferric oxide, and manganese dioxide as coating materials, and control the Na a3 Cu 0.25 Fe 0.2 Mn 0.55 The mass ratio of O2 to the entire sodium ion positive electrode material is 1.0%. After sufficient stirring, the solvent is evaporated and dried to obtain a composite powder.

[0105] Step 3: Heat the mixture to 825°C at a heating rate of 2°C / min for 10 hours to obtain a sodium ion positive electrode material, which includes a base material, a first coating layer (sodium-deficient layer) coating the base material, and a second coating layer coating the first coating layer.

[0106] Example 4

[0107] This embodiment provides a sodium ion positive electrode material and a preparation method thereof, which is basically the same as that of embodiment 3. The main difference is that the second coating layer Na a3 Cu 0.25 Fe 0.2 Mn 0.55 The mass ratio of O2 to the entire sodium ion positive electrode material is 2.0%.

[0108] Example 5

[0109] This embodiment provides a sodium ion positive electrode material and a preparation method thereof, which is basically the same as that of embodiment 3. The main difference is that the second coating layer Na a3 Cu 0.25 Fe 0.2 Mn 0.55 The mass ratio of O2 to the entire sodium ion positive electrode material is 0.5%.

[0110] Example 6

[0111] This embodiment provides a sodium ion positive electrode material and a preparation method thereof, which is basically the same as Example 3, except that 100 g of the raw material substrate is added to ethylene glycol, and acetic acid is then added dropwise to form a 0.04 mol / L acetic acid solution.

[0112] Comparative Example 1

[0113] In this comparative example, the raw material substrate prepared in step 1 of Example 1 is used as the sodium ion positive electrode material.

[0114] Comparative Example 2

[0115] In this comparative example, the raw material substrate prepared in step 1 of Example 2 is used as the sodium ion positive electrode material.

[0116] Comparative Example 3

[0117] In this comparative example, the raw material substrate prepared in step 1 of Example 3 is used as the sodium ion positive electrode material.

[0118] Comparative Example 4

[0119] This comparative example provides a sodium ion positive electrode material and a preparation method thereof, which is basically the same as Example 3, except that acetic acid is not added in step 2. The specific steps are as follows:

[0120] Step 2: Take 100g of the raw material substrate and add it to ethylene glycol, then 0.25 Fe 0.2 Mn 0.55 O2 ratio to add copper oxide, ferric oxide, and manganese dioxide as coating materials, and control the second coating layer Cu 0.25 Fe 0.2 Mn 0.55 The mass ratio of O2 to the entire sodium ion positive electrode material is 1.0%. After sufficient stirring, the solvent is evaporated and dried to obtain a composite powder.

[0121] Table 1

[0122]

[0123] Performance characterization:

[0124] 1) Figure 2 , Figure 3 , Figure 4 and Figure 5 The XRD patterns of Example 1, Example 2, Comparative Example 1 and Comparative Example 2 are shown respectively;

[0125] By comparing the XRD patterns, it can be found that the coating does not change the crystal structure of the material, and the R-3m space group structure is still maintained. The XRD patterns of each embodiment have no impurity diffraction peaks at 35°~36°, 38.2°~39.2° and 42.5°~43.5°, and the O-Na-O interlayer spacing after TopaS refinement is 3.20Å-3.50Å, the crystal plane spacing d(003) is 5.20Å-5.50Å, the unit cell parameter a is 2.90-3.05Å, and the unit cell parameter c is 15.90-16.30Å.

[0126] 2) Figure 6 , Figure 7 , Figure 8 and Fig. 9 They are SEM images of Example 1, Example 2, Comparative Example 1 and Comparative Example 2 respectively;

[0127] By comparing the SEM images, it can be found that the surface of the coated material is smoother and the particles are rounder, which is beneficial to improving the material processing performance and electrochemical properties.

[0128] 3) The sodium ion positive electrode materials of the above embodiments and comparative examples were assembled into button cells, and their electrochemical properties were tested. The assembly steps and testing methods were as follows:

[0129] Button battery production: the negative electrode is metallic sodium, the electrolyte is a common commercial model, the separator is glass fiber, the positive electrode sheet formula is 90wt% positive electrode active material, 5wt% PVDF and 5wt% SP, and its electrochemical performance is tested in the range of 1.5V~4.2V.

[0130] The test results are shown in Table 1.

[0131] From Comparative Example 1 and Example 1, Comparative Example 2 and Example 2, Comparative Example 3 and Example 3, it can be seen that after treatment with weak acidic substances and coating with coating materials, the battery capacity is slightly reduced, but the battery's cycle capacity retention rate and ion diffusion coefficient are significantly improved. From Example 3, Example 4 and Example 5, it can be seen that different coating amounts of the second coating layer achieve different coating effects, which have different effects on the specific capacity, cycle performance and ion diffusion coefficient of the battery. From Example 3, Example 6 and Comparative Example 4, it can be seen that the coating of the first coating layer can effectively improve the ion diffusion coefficient and capacity retention rate, and the concentration of the weak acid solution affects the thickness of the first coating layer, and different coating thicknesses produce different effects.

[0132] 4) The particle size D50, specific surface area and tap density of the sodium ion positive electrode materials of the above embodiments and comparative examples were tested. The results are shown in Table 2.

[0133] Table 2

[0134]

[0135] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0136] The above-mentioned embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the attached claims.

Claims

1. A method for preparing a sodium ion positive electrode material, characterized in that: The following steps are involved: Mixing a raw material base material, a weakly acidic substance and a solvent to obtain a premix, wherein the raw material base material has a general formula of Na a Ni b Fe c Mn d M e N f O 2-g P g , where M is a 3d transition metal element, N is a non-3d transition metal element, P is a VII main group element, 0.8≤a≤1.2, 0.1≤b≤0.6, 0.1≤c≤0.4, 0.1≤d≤0.5, 0.01≤e≤0.2, 0.001≤f≤0.05, 0.01≤g≤0.1; According to Cu i Fe j Mn k O2 ratio to prepare a copper source, an iron source and a manganese source, mix the premix, the copper source, the iron source and the manganese source, remove the solvent, and obtain a composite powder, wherein 0.05≤i≤0.3, 0.1≤j≤0.5, 0.1≤k≤0.6; Heat treating the composite powder, wherein the heat treatment procedure includes: heating from 15°C to 35°C to 450°C to 900°C at a heating rate of 1°C / min to 5°C / min, and keeping the temperature for 1h to 15h; The sodium ion positive electrode material comprises a base material, a first coating layer and a second coating layer, wherein the first coating layer coats the base material, the second coating layer coats the first coating layer, and the amount of the weakly acidic substance added is controlled so that the general formula of the base material is Na a1 Ni b Fe c Mn d M e N f O 2-g P g , the general formula of the first coating layer is Na a2 Ni b Fe c Mn d M e N f O 2-g P g , the general formula of the second coating layer is Na a3 Cu i Fe j Mn k O2, 0.8≤a1≤1.2, 0.8≤a2≤1.2, 0.44≤a3≤1.0, a1>a2.

2. The method for preparing a sodium ion positive electrode material according to claim 1, characterized in that: The weakly acidic substance is selected from one or more of acetic acid, citric acid, maleic acid, tartaric acid, vitamin C, benzoic acid, salicylic acid and benzenesulfonic acid.

3. The method for preparing the sodium ion positive electrode material according to claim 1, characterized in that: The solvent is selected from one or more of ethanol, ethylene glycol, ethyl ether, methanol, propanol and glycerol.

4. A sodium ion positive electrode material, prepared by the preparation method according to any one of claims 1 to 3, characterized in that: include: Matrix material, general formula is Na a1 Ni b Fe c Mn d M e N f O 2-g P g ; The first coating layer, which covers the base material, has the general formula of Na a2 Ni b Fe c Mn d M e N f O 2-g P g ; The second coating layer, covering the first coating layer, has the general formula of Na a3 Cu i Fe j Mn k O2; Among them, M is a 3d transition metal element, N is a non-3d transition metal element, P is a VII main group element, 0.8≤a1≤1.2, 0.8≤a2≤1.2, a1>a2, 0.1≤b≤0.6, 0.1≤c≤0.4, 0.1≤d≤0.5, 0.01≤e≤0.2, 0.001≤f≤0.05, 0.01≤g≤0.1, 0.44≤a3≤1.0, 0.05≤i≤0.3, 0.1≤j≤0.5, 0.1≤k≤0.

6.

5. The sodium ion positive electrode material according to claim 4, characterized in that The M element includes at least one of Ti, V, Cr, Co, Cu and Zn; the N element includes at least one of Ca, Mg, Al, Li, B, K, Y, Zr, Nb, Mo, Cd, Ta, W, Sn and Te; the P element includes at least one of F, Cl, Br and I.

6. The sodium ion positive electrode material according to claim 5, characterized in that The thickness of the first coating layer is 5nm~100nm.

7. The sodium ion positive electrode material according to claim 4, characterized in that The thickness of the second coating layer is 5 nm to 100 nm, and the mass ratio of the second coating layer in the sodium ion positive electrode material is 0.1% to 5%.

8. The sodium ion positive electrode material according to any one of claims 4 to 7, characterized in that The crystal structure of the sodium ion positive electrode material satisfies at least one of the following conditions: (1) There are no impure phase diffraction peaks at 35°~36°, 38.2°~39.2° and 42.5°~43.5°, or the sum of the impure phase masses corresponding to the impure phase diffraction peaks at 35°~36°, 38.2°~39.2° and 42.5°~43.5° accounts for less than 5wt% of the mass of the sodium ion positive electrode material; (2) The O-Na-O interlayer spacing is 3.20Å-3.50Å; (3) The interplanar spacing d(003) is 5.20Å-5.50Å; (4) The unit cell parameter a is 2.90-3.05 Å; (5) The unit cell parameter c is 15.90-16.30 Å.

9. The sodium ion positive electrode material according to any one of claims 4 to 7, characterized in that The sodium ion positive electrode material satisfies at least one of the following characteristics: (1) Particle size D50 is 2μm~15μm; (2) Specific surface area is 0.2m 2 / g~2m 2 / g; (3) Tap density ≥ 1.6 g / cm 3 .

10. A secondary battery, characterized in that: The method comprises the sodium ion positive electrode material according to any one of claims 4 to 9.

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