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

Abstract

The application discloses a positive electrode active material, a preparation method of the positive electrode active material, a positive electrode sheet, a sodium ion battery and an electric device. x (Ni a Fe b Mn c ) y A m B s C n O2, A includes at least one of Ti, Zr, Sn, Mo, W, Ta and Si, B includes at least one of Cu, Mg, Ca, Al, Ga, Co, V and Sr, and C includes at least one of Zn, Sm, Li, Sc and Cd; 0.85<=x<=1, (a+b+c) * y + m + s + n = 1, (a+b+c) * y >= 0.8, and a>0, b>0, c>0, m>=0, s>0 and n>0. The positive electrode active material is doped with B and C elements, so that the charge-discharge capacity, structural stability and cycle stability of the material below 4.1V are improved. Further, the material is doped with A elements, so that the residual sodium and impurity phase content of the material are reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery preparation, in particular to a positive electrode active material, a preparation method thereof, a positive electrode sheet, a sodium ion battery and an electric device. BACKGROUND

[0002] The sodium ion battery has a similar working principle to the lithium ion battery, and is also called a "rocking chair battery". Sodium resources are abundant, and the material cost is low, but the energy density is lower than that of the sodium ion battery, and the watt-hour cost has no advantage, so it is particularly important to improve the energy density of the sodium ion battery.

[0003] The sodium ion battery positive electrode material mainly includes layered oxides, polyanions and prussian materials. Among them, the energy density of the layered oxide sodium ion battery positive electrode material is relatively high, but it still has a large gap compared with the lithium battery ternary material and the lithium iron phosphate material of the sodium ion battery.

[0004] At present, the discharge capacity of the sodium ion battery positive electrode material can be improved by increasing the voltage, but the structure of the layered oxide positive electrode material will undergo irreversible phase transition under high voltage, and the electrolyte also cannot meet the requirements of high voltage. SUMMARY

[0005] The main purpose of the present application is to provide a positive electrode active material, a preparation method thereof, a positive electrode sheet, a sodium ion battery and an electric device, which aims to solve the problem of low charge and discharge capacity of the layered oxide positive electrode material of the sodium ion battery in the prior art.

[0006] To achieve the above-mentioned purpose, the present application provides a positive electrode active material, the chemical formula of the positive electrode active material is Na x (Ni a Fe b Mn c ) y A m B s C n O2, A includes at least one of Ti, Zr, Sn, Mo, W, Ta, Si, B includes at least one of Cu, Mg, Ca, Al, Ga, Co, V, Sr, and C includes at least one of Zn, Sm, Li, Sc, Cd;

[0007] Wherein, 0.85≤x≤1, (a+b+c)×y+m+s+n=1, (a+b+c)×y≥0.8, and a>0, b>0, c>0, m≥0, s>0, n>0.

[0008] In an embodiment, m=0, and the value of n is in the range of 0.02≤n≤0.08.

[0009] In one embodiment, the relationship between the values ​​of s and n is: 0.3n≤s≤0.7n.

[0010] In one embodiment, m > 0, the value range of n is 0.02 ≤ n ≤ 0.08, the relationship between the values ​​of s and n is: 0.3n ≤ s ≤ 0.7n, n + s = w, and the value range of m is 0 < m < 6.69w.

[0011] In one embodiment, the value of m is in the range of 0.3w ≤ m ≤ 0.7w.

[0012] In one embodiment, the positive electrode active material has two redox potentials, denoted as E1 and E2, existing between 3.8 and 4.15V:

[0013] Where E1 < E2.

[0014] In one embodiment, 3.85V≤E1<4.05V, 4.0V≤E2<4.15V.

[0015] The present invention also provides a method for preparing a positive electrode active material, the method comprising the following steps:

[0016] Precursor powders were prepared using sodium source, source A, source B, source C, and transition metal source.

[0017] The precursor powder is sintered and crushed to obtain the positive electrode active material;

[0018] The transition metal source includes Ni, Fe, and Mn elements.

[0019] In one embodiment, the step of preparing precursor powder using sodium source, source A, source B, source C, and transition metal source includes: mechanically mixing sodium source, source A, source B, source C, and transition metal source to obtain precursor powder, wherein the mechanical mixing method includes mechanical mixing using at least one of a three-dimensional mixer, a high-speed mixer, and a VC mixer; and / or,

[0020] The transition metal source includes one or more of transition metal-based carbonates, transition metal-based hydroxides, and transition metal-based oxides; and / or,

[0021] The sodium source includes one or more of anhydrous sodium carbonate, sodium carbonate monohydrate, sodium carbonate decahydrate, sodium bicarbonate, and sodium hydroxide; and / or,

[0022] The A source includes at least one of oxides, sulfides, and nitrides containing element A; and / or,

[0023] The B source comprises at least one of an oxide, a sulfide, and a nitride containing a B element; and / or,

[0024] The C source comprises at least one of an oxide, a sulfide, and a nitride containing a C element.

[0025] In an embodiment, the step of sintering, crushing the precursor powder to obtain the positive electrode active material comprises:

[0026] sintering, crushing the precursor powder to obtain a positive electrode active material intermediate;

[0027] sintering, crushing the positive electrode active material intermediate to obtain the positive electrode active material.

[0028] In an embodiment, the atmosphere of the first sintering and / or the second sintering comprises one or more of air, oxygen, nitrogen, and argon; and / or,

[0029] The temperature of the first sintering is 800-1100℃; and / or,

[0030] The time of the first sintering is 6-15h; and / or,

[0031] The pressure of the first sintering is 0.02-0.1MPa; and / or,

[0032] The temperature of the second sintering is 700-1000℃; and / or,

[0033] The time of the second sintering is 8-24h; and / or,

[0034] The pressure of the second sintering is 10-100Pa; and / or,

[0035] The method of the first crushing and / or the second crushing comprises crushing by at least one of a roll mill, a rotary wheel mill, and an air jet mill.

[0036] The application also provides a positive electrode sheet comprising the positive electrode active material or the positive electrode active material prepared by the preparation method.

[0037] The application also provides a sodium ion battery comprising the positive electrode sheet.

[0038] The application also provides an electric device comprising the sodium ion battery.

[0039] In the technical solution of the present application, B and C elements are doped into the layered oxide positive electrode active material at the same time, on the one hand, the oxidation peak of the positive electrode active material charged above 3.8V is split into two, so that the oxidation and reduction occurs in advance under the condition of lower original pressure, the utilization rate of active transition metals nickel, iron and manganese in the positive electrode active material is improved, thereby the charge and discharge capacity of the positive electrode active material is improved; on the other hand, the B element can improve the bond energy between the C element and the oxygen element, thereby stabilizing the C element in the transition metal layer, reducing the migration rate of the C element to the sodium layer, thereby improving the cycle stability of the positive electrode active material. Therefore, the positive electrode active material provided by the present application has higher charge and discharge capacity below 4.1V, and better structural stability and cycle stability. Further, doping A element into the layered oxide positive electrode active material can reduce the residual sodium and impurity phase content of the material. BRIEF DESCRIPTION OF DRAWINGS

[0040] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor based on the drawings shown.

[0041] Figure 1 The scanning electron microscope graph of the positive electrode active material in Example 15 of the present application;

[0042] Figure 2 The XRD graph of the positive electrode active material in Example 1, Example 2, Example 3, Example 4, Example 13, Comparative Example 1 and Comparative Example 2 of the present application;

[0043] Figure 3 The cyclic voltammetry test graph of the button cell corresponding to Example 15 and Comparative Example 1 of the present application;

[0044] Figure 4 The charge and discharge curve graph of the button cell corresponding to Example 15 and Comparative Example 1 of the present application.

[0045] The implementation, functional features and advantages of the present application will be further described with reference to the embodiments and the drawings. DETAILED DESCRIPTION

[0046] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below. If the specific conditions are not specified in the embodiments, the conventional conditions or the conditions suggested by the manufacturers are adopted. If the reagents or instruments are not specified by the manufacturers, they are all the conventional products that can be purchased in the market. In addition, the meaning of "and / or" appearing in the whole text includes three parallel solutions. Taking "A and / or B" as an example, it includes the A solution, or the B solution, or the solution of A and B satisfying at the same time. In addition, the technical solutions of each embodiment can be combined with each other, but it must be based on the fact that the person skilled in the art can realize it. When the combination of technical solutions appears contradictory or unachievable, it should be considered that the combination of technical solutions does not exist and is not within the protection scope of the present application. Based on the embodiments in the present application, all other embodiments obtained by the person skilled in the art without creative labor are within the protection scope of the present application.

[0047] Sodium ion batteries have similar working principles as lithium ion batteries, also known as "rocking chair batteries". Sodium resources are abundant and the material cost is low, but the energy density is lower than that of sodium ion batteries, and the watt-hour cost has no advantage, so it is particularly important to improve the energy density of sodium ion batteries.

[0048] The sodium ion battery positive electrode material mainly includes layered oxides, polyanions and prussian materials. Among them, the energy density of the layered oxide sodium ion battery positive electrode material is relatively high, but it still has a large gap with the lithium battery ternary material and the lithium iron phosphate material of the sodium ion battery.

[0049] At present, the discharge capacity of the sodium ion battery positive electrode material can be improved by increasing the voltage, but the structure of the layered oxide positive electrode material will undergo irreversible phase transition under high voltage, and the electrolyte also cannot meet the requirements of high voltage.

[0050] Therefore, the present application provides a positive electrode active material, the chemical formula of the positive electrode active material is Na x (Ni a Fe b Mn c ) y A m B s C n O2, A includes at least one of Ti, Zr and Sn, B includes at least one of Cu, Mg and Ca, and C includes at least one of Zn and Sm; wherein 0.85≤x≤1, (a+b+c)×y+m+s+n=1, (a+b+c)×y≥0.8, and a>0, b>0, c>0, m≥0, s>0, n>0.

[0051] In the present application, B and C elements are doped into the layered oxide type positive electrode active material at the same time, on the one hand, the oxidation peak of the positive electrode active material charged above 3.8V is split into two, so that the oxidation and reduction occurs in advance under the condition of lower original pressure, the utilization rate of active transition metals nickel, iron and manganese in the positive electrode active material is improved, thereby the charge and discharge capacity of the positive electrode active material is improved; on the other hand, the B element can improve the bond energy between the C element and the oxygen element, thereby the C element is stabilized in the transition metal layer, the migration rate of the C element to the sodium layer is reduced, thereby the cycle stability of the positive electrode active material is improved. Therefore, the positive electrode active material provided by the present application has higher charge and discharge capacity below 4.1V, and better structural stability and cycle stability. Further doping A element into the layered oxide type positive electrode active material can reduce the residual sodium and the content of impurities. The residual sodium refers to the sodium element which does not participate in the reaction completely or exists in the positive electrode material in other forms during the preparation of the sodium ion positive electrode material.

[0052] It should be noted that the core feature of the layered oxide type positive electrode active material is that the transition metal layer and the alkali metal (Na) layer are alternately stacked to form a three-dimensional layered structure. The transition metal layer refers to the transition metal-oxygen layer composed of transition metals Ni, Fe and Mn and oxygen elements, and the transition metal is usually located at the center of the coordination polyhedron formed by oxygen atoms, and the alkali metal ion will be filled between adjacent transition metal layers as a charge carrier to participate in the embedding / detaching reaction in the charge and discharge process.

[0053] The oxidation peak of the positive electrode active material without doping A, B and C elements is at 4.145V above 3.8V in the charging process. When only C element is doped, the oxidation peak of the positive electrode active material shifts to the left and splits into two peaks in the charging process, and the oxidation and reduction reaction of the positive electrode active material can occur at a lower voltage, that is, the oxidation and reduction reaction of Ni or Fe in the transition metal layer can occur at a lower voltage and more sodium is detached, thereby improving the capacity at a lower voltage.

[0054] The present application finds that after C doping, the coordination structure with oxygen elements is unstable and easy to migrate from the transition metal layer into the sodium layer, enter the electrolyte and migrate to the negative electrode surface in the charging and discharging process, and easily cause gas production in the whole process. In this process, on the one hand, the C doping will competitively occupy part of the active transition metal (Ni) sites, which will make the active transition metal migrate out to form impurity phase, thereby increasing the proportion of impurity phase; on the other hand, the C elution migrating to the sodium layer will occupy the storage site of sodium element, which will increase the content of residual sodium.

[0055] When B element and C element are doped at the same time, the B element can increase the bond energy between C element and oxygen element, weaken the difference of bond length when C element coordinates with multiple oxygen elements, thereby improving the stability of C element in the transition metal layer, improving the structural stability of the positive active material, and improving the migration rate of sodium ions and the cycle stability (i.e., the capacity retention rate is high). However, the present application finds that the doping of B can further increase the proportion of impurities in the positive active material and the residual sodium content.

[0056] When A element, B element and C element are doped at the same time, the A element can increase the interlayer spacing, so that sodium is more convenient to join the bulk phase, and the residual sodium is reduced. Generally, since A, B and C elements do not belong to active transition metal elements, after doping, they will competitively occupy the sites of active transition metal elements Ni and Fe, so that they migrate out to form impurities, so theoretically, the addition of A will also lead to an increase in impurities. However, the present application unexpectedly finds that the addition of A element can better solve the problem of impurities caused by the doping of B and C elements. The principle may be that the addition of A element can balance the charge and the elements in the raw materials are squeezed out and restructured after A element doping, but no new impurity peak is generated, so no impurities are generated.

[0057] It can be understood that A can be any one of Ti, Zr and Sn, or two or three of Ti, Zr and Sn; B can be any one of Cu, Mg and Ca, or two or three of Cu, Mg and Ca; C can be any one of Zn and Sm, or both Zn and Sm, which are all within the protection scope of the present application. There can be two schemes in the present application, a) only doping B element and C element; b) doping A element, B element and C element. In the present application, the molar ratio of each element is set to meet the following range: 0.85≤x≤1, (a+b+c)×y+m+s+n=1, (a+b+c)×y≥0.8, and a>0, b>0, c>0, m≥0, s>0, n>0. Controlling the ratio of Na element, active transition metal elements (Ni, Fe and Mn) and doped metal elements in the above range can improve the charge-discharge capacity, structural stability and cycle stability of the positive active material below 4.1V.

[0058] In some embodiments, m = 0, and n is in the range of 0.02≤n≤0.08. It can be understood that n can be 0.02, 0.03, 0.04, 0.05, 0.06, 0.07 or 0.08, and the range of n represents the doping amount of C element. When n > 0.08, the left shift of the charging oxidation peak is large, the capacity between 3.8-4.1V is increased, but the structure is distorted to a large extent, the stability is poor, and the structure may collapse in the long-term charging and discharging process; when n < 0.02, the left shift of the charging oxidation peak is small, and the capacity at 4.1V is slightly improved; when n is in the range of 0.02-0.08, the left shift of the charging oxidation peak is appropriate, which is beneficial to the capacity release, and the structure distortion is within a reversible range. It can be understood that at this time, s is in the range of 0 < s ≤ 0.18, and 0 < s / n ≤ 9.

[0059] In some embodiments, the relationship between s and n is 0.3n≤s≤0.7n. That is, the molar ratio of B element and C element is 0.3-0.7, and the molar ratio can be 0.3, 0.4, 0.5, 0.6 or 0.7. When the molar ratio is less than 0.3, the effect of inhibiting the migration of C element from the transition metal layer to the sodium layer is limited; when the molar ratio is greater than 0.7, the excessive doping of non-active elements B and C will affect the capacity and generate more impurities. When the above formula is satisfied, the value of s is in the range of 0.006≤s≤0.055.

[0060] The doping of C element will hinder the migration of sodium, thus increasing the residual sodium, and the increase of B element and C element content will introduce impurities. Specifically, the introduction of B element and C element will cause part of the transition metal to precipitate during the sintering process, resulting in impurities in the crystal structure. In some embodiments, when m > 0, and 0.02≤n≤0.08 and 0.3n≤s≤0.7n are satisfied, n + s = w, and the value of m is in the range of 0 < m ≤ 6.69w. That is, m can be 0.01w, 0.05w, 1w, 2w, 3w, 4w, 5w, 6w or 6.69w. When the value of m is in the range of 0 < m ≤ 6.69w, A element can inhibit the content of impurities and residual sodium while maintaining the capacity and cycle stability, and also can balance the charge and increase the interlayer distance.

[0061] Preferably, the value of m is in the range of 0.3w≤m≤0.7w. That is, m can be 0.3w, 0.4w, 0.5w, 0.6w or 0.7w. When the value of m is in the range of 0.3w≤m≤0.7w, A element can significantly inhibit the content of impurities and residual sodium while maintaining the capacity and cycle stability, and the inhibition of the content of impurities and residual sodium in the positive active material is better.

[0062] In some embodiments, the positive electrode active material has two redox potentials between 3.8-4.15V, denoted as E1 and E2: wherein E1<E2. It can be understood that the positive electrode active material provided by the present application incorporates C element, so that the original single oxidation peak of the positive electrode active material splits into two oxidation peaks, and the corresponding redox potential E also changes from one to two, both E1 and E2 are less than the original redox potential E. This change helps to improve the charge and discharge capacity of the positive electrode active material. It can be understood that E1 will move to the left as the doping amount of C element increases.

[0063] In some embodiments, 3.85V≤E1<4.05V, 4.0V≤E2<4.15V. When the doping amount n of C element is in the range of 0.02≤n≤0.08, the corresponding two redox potentials E1 and E2 are respectively in the range of 3.85V-4.05V and 4.0V-4.15V, and at this time the charge and discharge capacity of the positive electrode active material is relatively optimal.

[0064] In some embodiments, the positive electrode active material provided by the present application has a first-week charge capacity of P at 4.0-4.1V and a charge capacity of Q at 2.0-4.1V, and P / Q=5%-15%, which is higher than that of the positive electrode active material without doping A, B and C elements. In some embodiments, the residual sodium of the positive electrode active material provided by the present application during the charge and discharge process is ≤0.5%, which is similar to the residual sodium of the positive electrode active material without doping A, B and C elements. In addition, the specific surface area of the positive electrode active material provided by the present application is 0.2-0.6 m 2 / g.

[0065] The present application also provides a preparation method of a positive electrode active material, the preparation method of the positive electrode active material comprising the following steps: using a sodium source, an A source, a B source, a C source and a transition metal source to prepare a precursor powder; sintering and crushing the precursor powder to obtain the positive electrode active material; the transition metal source comprises Ni element, Fe element and Mn element.

[0066] In the technical scheme of the present application, when m=0, there is no A source in the raw material, and the precursor powder is prepared by using a sodium source, a B source, a C source and a transition metal source. The preparation method can be that the sodium source, the B source, the C source and the transition metal source are mixed in a solvent and then the precursor powder is prepared by co-precipitation or spray pyrolysis. The preparation method can also be that the sodium source, the B source, the C source and the transition metal source (the transition metal source is a transition metal-based carbonate, a transition metal-based hydroxide obtained by co-precipitation or a transition metal-based oxide obtained by spray pyrolysis) are further sintered to make the elements interact with each other to form a crystal phase with balanced charge properties, and then the positive electrode active material is obtained after crushing. When m>0, there is an A source in the raw material, and the precursor powder is prepared by using a sodium source, an A source, a B source, a C source and a transition metal source. The preparation method of the present application is simple and suitable for industrial production, and the prepared positive electrode active material has good charge and discharge capacity, structural stability and cycle stability.

[0067] In some embodiments, the step of preparing the precursor powder by using a sodium source, an A source, a B source, a C source and a transition metal source comprises: mechanically mixing the sodium source, the A source, the B source, the C source and the transition metal source to obtain the precursor powder. The mechanical mixing method comprises at least one of mechanical mixing by a three-dimensional mixer, a high-speed mixer and a VC mixer. The above mechanical mixing method is efficient and fast, and can quickly mix the sodium source, the A source, the B source, the C source and the transition metal source uniformly.

[0068] In some embodiments, the transition metal source comprises one or more of a transition metal-based carbonate, a transition metal-based hydroxide and a transition metal-based oxide. The above transition metal source can be prepared in advance and stored in a corresponding environment for easy access.

[0069] In some embodiments, the sodium source comprises one or more of anhydrous sodium carbonate, sodium carbonate monohydrate, sodium carbonate decahydrate, sodium bicarbonate and sodium hydroxide. The above sodium source is widely available and low in price. Preferably, the sodium source is one or more of anhydrous sodium carbonate, sodium bicarbonate and sodium hydroxide.

[0070] In some embodiments, the A source comprises at least one of an oxide, a sulfide, and a nitride containing the A element. That is, the A source can be any one of an oxide containing the A element, a sulfide containing the A element, and a nitride containing the A element, or can be any two or three of an oxide containing the A element, a sulfide containing the A element, and a nitride containing the A element, all within the protection scope of the present application. For example, when the A source is a Ti source, the oxide containing the Ti element can be TiO2, the sulfide containing the Ti element can be TiS2, and the nitride containing the Ti element can be TiN. When the A source is a Zr source, the Zr source can be ZrO2, ZrS2, or ZrN; when the A source is a Sn source, the Sn source can be SnO2, SnS2, or Sn3N4.

[0071] In some embodiments, the B source comprises at least one of an oxide, a sulfide, and a nitride containing the B element. In some embodiments, the C source comprises at least one of an oxide, a sulfide, and a nitride containing the C element.

[0072] The types of the B element and the C element are similar to the A element, and can be any one of an oxide, a sulfide, and a nitride containing the corresponding element, or can be any two or three of an oxide, a sulfide, and a nitride containing the corresponding element, all within the protection scope of the present application. Among them, the B source can be CuO, Cu2S, Cu3N, MgO, MgS, Mg3N2, CaO, CaS, or Ca3N2, and the C source can be ZnO, ZnS, Zn3N2, Sm2O3, SmS, or SmN.

[0073] In some embodiments, the step of sintering, crushing the precursor powder to obtain the positive electrode active material comprises: sintering, crushing the precursor powder for the first time to obtain a positive electrode active material intermediate; sintering, crushing the positive electrode active material intermediate for the second time to obtain the positive electrode active material. The two-step sintering method obtains a positive electrode active material with a relatively complete particle morphology and a relatively uniform particle size.

[0074] In some embodiments, the atmosphere of the first sintering and / or the second sintering comprises one or more of air, oxygen, nitrogen, argon; and / or, the temperature of the first sintering is 800-1100℃; and / or, the time of the first sintering is 6-15h; and / or, the pressure of the first sintering is 0.02-0.1MPa; and / or, the temperature of the second sintering is 700-1000℃; and / or, the time of the second sintering is 8-24h; and / or, the pressure of the second sintering is 10-100Pa. That is, the atmosphere, the temperature, the pressure and the time of the two sinterings are simultaneously controlled in the above ranges, which can ensure that the positive electrode active material is sufficiently crystallized, the complete crystal structure is formed, the material particle morphology is complete, and the material has good electrochemical performance. Preferably, the first sintering temperature is 850-1000℃, the first sintering time is 8-12h; the second sintering temperature is 850-950℃, and the second sintering time is 10-20h.

[0075] In some embodiments, the method for crushing comprises crushing by at least one of a pair of rollers, a rotary wheel mill, and an air flow crusher. The above method can quickly crush the sintered material to obtain the positive electrode active material. The particle size distribution of the prepared positive electrode active material can satisfy D10≥2μm, 10μm≥D50≥4μm, and D90≤16μm.

[0076] The application also provides a positive electrode sheet comprising the positive electrode active material or the positive electrode active material prepared by the method for preparing a positive electrode active material. Therefore, the positive electrode sheet has all the beneficial effects of the positive electrode active material or the method for preparing a positive electrode active material, which will not be repeated here.

[0077] The application also provides a sodium ion battery comprising the positive electrode sheet. Therefore, the positive electrode sheet has all the beneficial effects of the positive electrode sheet, which will not be repeated here.

[0078] The application also provides an electric device comprising the sodium ion battery. Therefore, the electric device has all the beneficial effects of the sodium ion battery, which will not be repeated here.

[0079] The technical solutions of the application will be further described in detail below in combination with specific embodiments and drawings. It should be understood that the following embodiments are only used to explain the application and do not limit the application.

[0080] Embodiment 1

[0081] A positive electrode active material Na x (Ni a Fe b Mn c ) y Zrm Cu s Zn n O2, wherein x = 0.98, a = 0.4, b = 0.2, c = 0.4, y = 0.96, m = 0, s = 0.03, n = 0.01;

[0082] The positive electrode active material Na 0.98 (Ni 0.4 Fe 0.2 Mn 0.4 ) 0.96 Zr0Cu 0.03 Zn 0.01 O2 is prepared by the following steps:

[0083] A spherical hydroxide precursor (NFM424 precursor) with a Ni:Fe:Mn molar ratio of 4:2:4 is prepared by a coprecipitation method; then NFM424 precursor:CuO:ZnO:Na2CO3 is weighed according to a molar ratio of 0.96:0.03:0.01:0.49 and mixed uniformly by a three-dimensional mixer, and then placed in a box furnace for sintering in an air atmosphere, heated to 950°C at a rate of 5°C / min, and then naturally cooled after being kept at this temperature for 10 h, and then crushed and sieved. Then placed in a box furnace and heated to 920°C at a rate of 5°C / min, and then naturally cooled to room temperature after being kept at this temperature for 12 h, to obtain a Na 0.98 (Ni 0.4 Fe 0.2 Mn 0.4 ) 0.96 Zr0Cu 0.03 Zn 0.01 O2 positive electrode active material.

[0084] Examples 2-16 and Comparative Examples 1-2

[0085] Examples 2-16 and Comparative Examples 1-2 differ from Example 1 as shown in Table 1, and the others are the same as Example 1. In the examples where m>0, Zr is provided by ZrO2.

[0086] Table 1 Differences between Examples 2-16 and Comparative Examples 1-2 and Example 1

[0087]

[0088] Performance test

[0089] The morphology of the positive electrode active material prepared in Example 15 was tested by scanning electron microscopy (accelerating voltage 4KV, magnification 1000x), and the results are shown in Figure 1 .

[0090] By Figure 1 It can be seen that the positive electrode active material in Example 15 is a large-particle single crystal material.

[0091] Test method of impurity phase content, residual sodium and gas production:

[0092] (1) The positive electrode active materials prepared in Examples 1-16 were subjected to X-ray diffraction analysis, and the X-ray diffraction analysis charts of Examples 1, 2, 3, 4, 13, Comparative Example 1 and Comparative Example 2 are shown in FIG. 1. The impurity phase content was calculated by X-ray diffraction analysis, and the results are shown in Table 2. Figure 2

[0093] (2) Residual sodium (ω Na ) test: 5 g of positive electrode active material was dispersed in 35 mL of ethylene glycol for 30 min, the supernatant was filtered and diluted to 50 mL, 5 mL was diluted to 40 mL, and the automatic potential titration was titrated with hydrochloric acid standard solution. The following formula was used for calculation:

[0094] ω NaOH =[c HCl ×(V2-V1)×V3×40.01×%] / (m×V4×10);

[0095] ω Na2CO3 =(c HCl ×V2×V3×105.99×%) / (m×V4×10);

[0096] ω Na =(22.99 / 40.01)×ω NaOH +(22.99×2 / 105.99)×ω Na2CO3 .

[0097] In the formula:

[0098] ω Na — Residual sodium content in the sample;

[0099] ω NaOH — Sodium hydroxide (NaOH) content in the sample, unit wt%;

[0100] ω Na2CO3 — Sodium carbonate (Na2CO3) content in the sample, unit wt%;

[0101] c HCl — The concentration of HCl obtained by calibration, unit mol / L;

[0102] V1— The titration volume from the start of titration to the first jump point, unit mL;

[0103] ​V2 — Titration volume from jump point 1 to jump point 2, in mL;

[0104] V3 – Fixed volume of 50 mL, unit: mL;

[0105] V4 – Take a volume of 5 mL, unit: mL;

[0106] m—sample mass, in grams;

[0107] 40.01 — Molar mass of sodium hydroxide (NaOH), in g / mol;

[0108] 105.99 — Molar mass of sodium carbonate (Na2CO3), in g / mol;

[0109] The test results are shown in Table 2.

[0110] (3) The positive electrode active materials of Examples 1-16 and Comparative Examples 1-2 were assembled into 1Ah soft pack batteries and stored at a high temperature of 60°C for 7 days. Then the amount of gas generated, i.e. the amount of gas produced, was tested by the oil discharge method. The test results are shown in Table 2.

[0111] The assembly steps of the soft-pack battery are as follows: The positive and negative active materials of this invention are prepared into positive and negative electrode sheets respectively through homogenization, coating, baking, and slitting. Then, the positive and negative electrode sheets and the separator are assembled into a 1Ah soft-pack cell by winding. After formation and capacity testing, the soft-pack cell is obtained. The negative electrode is made of hard carbon material, the separator is a composite ceramic separator, and the electrolyte is a NaPF6 solution. The solvent of the NaPF6 solution is EC:DEC:PC = 1:5:4, where EC is ethylene carbonate, DEC is diethyl carbonate, and PC is propylene carbonate.

[0112] Table 2. Positive electrode active materials prepared in Examples 1-16 and their corresponding pouch cell performance.

[0113]

[0114] Depend on Figure 2 It can be seen that the position of the impurity phase peak is between 42° and 44°. In Comparative Example 1, there is no peak at 43° in the XRD pattern without C element doping. In Comparative Example 2, after C element doping, there is an additional impurity phase peak of nickel oxide in the XRD pattern. When B element is added at the same time as C element doping, the impurity phase peak still exists. In Example 13, the impurity phase peak disappears after doping with A element, B element and C element.

[0115] The positive electrode active materials prepared in Examples 1-16 and Comparative Examples 1-2 were assembled into button cells and tested for performance. The button cells were assembled as follows: the positive electrode material, conductive agent, and PVDF (polyvinylidene fluoride) were used in a ratio of 8:1:1, dispersed using a high-speed disperser, coated, baked, sliced, and assembled into button cells in a glove box (water and oxygen content ≤0.01 PPM) using a 2032 battery shell, sodium sheet, glass fiber separator, and sodium hexafluorophosphate electrolyte.

[0116] The performance test method is shown below:

[0117] (1) Cyclic voltammetry test: using an electrochemical workstation, the test range was 2-4.2 V, and the scan rate was 1 MV / S. The cyclic voltammetry test results of the button cells corresponding to Example 15 and Comparative Example 1 are shown in Figure 3 . The redox potentials E1 and E2 of each group of materials were recorded in Table 3.

[0118] (2) Constant current charge and discharge test: using a charge and discharge test cabinet, the test range was 2.0-4.1 V, and the charge and discharge rate was 0.2C in the first week, 1C for 50 cycles. During the 0.2C charge and discharge process in the first week, the 0.2C charge capacity, 0.2C discharge capacity, first week charge capacity P in the voltage range of 4.0-4.1 V, and first week charge capacity Q in the entire voltage range of 2.0-4.1 V were recorded, and P / Q was calculated. During the 50 cycles at a rate of 1C, the reversible specific capacity and capacity retention rate after 50 cycles were recorded. The cyclic voltammetry test results of the button cells corresponding to Example 15 and Comparative Example 1 are shown in Figure 4 . The various data were recorded in Table 3.

[0119] Table 3 Performance of positive electrode active materials and button cells in Examples 1-16 and Comparative Examples 1-2

[0120]

[0121] As can be seen from Figure 3 , the doped modified material has two peak positions at 3.8-4.1 V, and can release more specific capacity at low voltage compared to the original material.

[0122] As can be seen from Figure 4 , the slope of charging above 3.8 V changes significantly, and more sodium is removed above 3.8 V.

[0123] According to Tables 2 and 3:

[0124] (1) By comparing Examples 1-16 and Comparative Example 1, it can be seen that when there is no C element doping, the 0.2C discharge capacity is only 139 mAh / g (Comparative Example 1); while after doping the C element, the discharge capacity can reach more than 140 mAh / g (Examples 1-15), because the addition of C element promotes the advance of redox reaction, more Na can be stripped from the material at low potential, so more capacity is released at the same voltage of 4.1 V.

[0125] (2) By comparing Examples 1-16 and Comparative Example 2, it can be seen that when the C element is doped but no B element is doped, the capacity retention is poor, that is, the cycle stability is poor (Comparative Example 2); while after doping the C element and synchronously doping the B element, the cycle stability is greatly improved. The reason is that the doping of C element is doping the Ni site, so it is easy to produce impurities, but the bond length between C and O changes constantly during the cycle process, causing C to gradually migrate to the Na layer, dissolve and diffuse to the separator and the negative electrode. B element is also mainly doped in Ni site, but B element can inhibit the bond length change between C and O during the cycle process, so that the structure of the positive active material is more stable, and the cycle stability is improved.

[0126] (3) By comparing Examples 1-2 and Examples 3-6, it can be seen that when C element and B element are doped at the same time, but the doping amount n of C element is less than 0.02, the 0.2C discharge capacity is low (Example 1), n is greater than 0.08, the discharge capacity is high, but the impurity content and gas production are high, and the cycle stability is poor (Example 2); while when C element and B element are doped at the same time, and the doping amount n of C element is controlled between 0.02-0.08, the capacity and cycle stability are both well played (Examples 3-6). The reason is that when the doping amount of C element is less than 0.02, the redox reaction below 4.1 V increases less, so the capacity improvement is not obvious. While the doping amount of C element is higher than 0.8, the phase change caused by excessive redox reaction leads to poor structure stability, and the irreversible phase change increases rapidly during the cycle process, leading to accelerated decay and poor cycle stability.

[0127] (4) By comparing examples 3~6 and examples 7~10, when C element and B element are doped at the same time, the doping amount n of C element is controlled between 0.02~0.08, but the ratio of the doping amount of B element and the doping amount of C element (i.e. s / n) is less than 0.3, the improvement effect on structural stability is general, so the cycle stability is less improved (example 4 and example 6). When s / n is greater than 0.7, the structural stability is improved, but the impurity phase content increases and the charge-discharge capacity decreases (example 3 and example 5); when C element and B element are doped at the same time, the doping amount n of C element is controlled between 0.02~0.08, and the ratio of the doping amount of B element and the doping amount of C element (i.e. s / n) is controlled between 0.3~0.7, the cycle stability is better (example 7~10), because too little B element cannot play the role of stabilizing C, and too much B element will cause the impurity phase to increase significantly and reduce the electronic conductivity and ion diffusion rate of the material, and the charge-discharge capacity and cycle stability decrease.

[0128] (5) By comparing examples 7~10 and examples 11~16, when there is no further doping of A element, the gas production and residual sodium are still relatively high (example 7~9); when A element is further doped, the gas production and residual sodium are significantly reduced (example 11~16), because the proportion of impurity phase is reduced, and sodium element can enter the material, thereby reducing the surface residual sodium and gas production, and improving the structural stability.

[0129] (6) By comparing examples 11~12 and examples 13~16, when the ratio of the doping amount m of A element and the sum w of the doping amount of B element and C element (m / w) is less than 0.3, the proportion of impurity phase is still high, and the gas production and residual sodium are still high (example 11); when m / w is greater than 0.7, the impurity phase is very small, and the residual sodium and gas production are small, but since A is a non-active element, too high content will cause the charge-discharge capacity of the material to decrease and the cycle stability to decrease (example 12). When the ratio of the doping amount of A element and the sum of the doping amount of B element and C element is controlled within the range of 0.3~0.7, the impurity phase, charge-discharge capacity and cycle stability are all within a relatively good range, because the addition of A element can balance the charge of the material, reduce the impurity phase, and because A element is a high-valence ion, it increases the sodium layer spacing, which is beneficial to the transmission of sodium ions, and can improve the charge-discharge capacity and cycle stability of the material.

[0130] The above is only the preferred embodiment of the present application, and does not limit the patent scope of the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the patent protection scope of the present application.

Claims

1. A positive electrode active material, characterized by, The positive active material has a chemical formula of Na x (Ni a Fe b Mn c ) y A m B s C n O2, A includes at least one of Ti, Zr, Sn, Mo, W, Ta, Si, B includes at least one of Cu, Mg, Ca, Al, Ga, Co, V, and C includes at least one of Zn, Sm, Li, Sc, Cd. Wherein, 0.85≤x≤1, (a+b+c)×y+m+s+n=1, (a+b+c)×y≥0.8, and a>0, b>0, c>0, m>0, s>0, n>0; The n is 0.02≤n≤0.08, the s and the n between the value of the relationship is: 0.3n≤s≤0.7n, n+s=w, the m is m=0.43w or 0.62w≤m≤0.7w; The positive electrode active material has two redox potentials between 3.8-4.15V, which are respectively denoted as E1 and E2; 3.85V≤E1<4.05V, 4.0V≤E2<4.15V.

2. A method for producing the positive electrode active material according to claim 1, characterized by, The preparation method of the positive electrode active material comprises the following steps: A precursor powder is prepared by using a sodium source, an A source, a B source, a C source and a transition metal source; The precursor powder is sintered and crushed to obtain the positive electrode active material; The transition metal source comprises Ni elements, Fe elements and Mn elements.

3. The preparation method of the positive electrode active material according to claim 2, wherein, The step of preparing the precursor powder by using the sodium source, the A source, the B source, the C source and the transition metal source comprises: mechanically mixing the sodium source, the A source, the B source, the C source and the transition metal source to obtain the precursor powder, and the mechanical mixing method comprises at least one of three-dimensional mixing machine, high-speed mixing machine and VC mixing machine; and / or, The transition metal source comprises one or more of transition metal-based carbonate, transition metal-based hydroxide and transition metal-based oxide; and / or, The sodium source comprises one or more of anhydrous sodium carbonate, sodium carbonate monohydrate, sodium carbonate decahydrate, sodium bicarbonate and sodium hydroxide; and / or, The A source comprises at least one of oxides, sulfides and nitrides containing A elements; and / or, The B source comprises at least one of oxides, sulfides and nitrides containing B elements; and / or, The C source comprises at least one of oxides, sulfides and nitrides containing C elements.

4. The method for producing a positive electrode active material according to claim 2, wherein The step of sintering and crushing the precursor powder to obtain the positive electrode active material comprises: The precursor powder is first sintered and first crushed to obtain a positive electrode active material intermediate; The positive electrode active material intermediate is second sintered and second crushed to obtain the positive electrode active material.

5. The method for producing a positive electrode active material according to claim 4, wherein The atmosphere of the first sintering and / or the second sintering comprises one or more of air, oxygen, nitrogen and argon; and / or, The temperature of the first sintering is 800-1100℃; and / or, The time of the first sintering is 6-15h; and / or, The pressure of the first sintering is 0.02-0.1MPa; and / or, The temperature of the second sintering is 700-1000℃; and / or, The time of the second sintering is 8-24h; and / or, The pressure of the second sintering is 10-100Pa; and / or, The method of the first crushing and / or the second crushing comprises at least one of crushing by a pair of rollers, a rotary wheel mill and an air flow crusher.

6. A positive electrode sheet characterized by comprising: The positive electrode sheet includes the positive electrode active material according to claim 1 or the positive electrode active material prepared by the method for preparing a positive electrode active material according to any one of claims 2 to 5.

7. A sodium-ion battery, characterized in that, The sodium-ion battery includes the positive electrode sheet according to claim 6.

8. An electrical device, characterized by The electric device includes the sodium-ion battery according to claim 7.

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