Double in-situ coating modified iron-rich na x Fe a Me1 b Me2 c O2 materials, methods of making and use in sodium-ion batteries

Abstract

The application belongs to the field of sodium-ion battery cathode materials, and particularly relates to a double-in-situ coated modified iron-rich Na x Fe a Me1 b Me2 c O2 material, which comprises a core, an intermediate layer in-situ coated on the surface of the core, and an outer layer in-situ coated on the intermediate layer; the material of the core has a chemical formula of Na x Fe a Me1 b Me2 c O2; the material of the intermediate layer has a chemical formula of Fe a Me1 b Me2 c O2; the material of the outer layer has a chemical formula of NaTO3; wherein Me1 is a +4 valence transition metal element, Me2 is a non +4 valence transition metal element with a chemical valence of k, a+b+c=1, wherein 0.8<=x<=1.04, a>=0.4, b and c are both greater than 0; and T is at least one of Nb, Bi, Ti, Ta, Mo, Zr, Sb, Ga, In, Y, W, Hf and La. The application also provides preparation and application of the material. The material has excellent performance, for example, can reduce iron migration at high voltage, and can improve long cycle performance at high voltage and high rate.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sodium-ion battery cathode materials, and particularly relates to a sodium-ion battery cathode material. BACKGROUND

[0002] Sodium-ion batteries have attracted extensive attention due to their similar working principle to lithium-ion batteries and lower cost. Among several kinds of cathode materials of sodium-ion batteries, layered oxide cathode materials have become the most promising sodium-ion battery cathode material due to their high capacity, high voltage platform, simple preparation and other advantages. In order to further reduce the cost, the research and development of iron-rich oxide cathode materials of sodium-ion batteries has become an important research direction.

[0003] In order to obtain high capacity while reducing cost, iron-rich oxide cathode materials need to work at a higher cut-off voltage. At a high cut-off voltage, Fe 4+ There is a serious Jahn-Teller effect, and a large amount of Na is removed, which will lead to a strong tendency of Fe to migrate to the Na layer and eventually dissolve into the electrolyte. At the surface, HF produced by the reaction of the electrolyte and the residual alkali at the surface at a high cut-off voltage will actively attack the surface of the cathode material, leading to further dissolution of Fe elements. Under the combined action of the problems in the bulk phase and at the surface, the structure of the material will be irreversibly destroyed, eventually leading to irreversible reduction of the electrochemical performance of the material. In addition, the air stability of the iron-rich system is also poor, and the presence of residual alkali will seriously affect the processing performance of the material, which will also

[0004] Existing technologies tend to solve this problem from the bulk phase, such as introducing Li to occupy the site in advance to prevent Fe from migrating to the Na layer, and introducing Ru to enhance the bonding between transition metal elements and O elements to inhibit the migration of Fe. However, these methods still have certain limitations: the former cannot avoid the migration of Fe because Li itself will be removed after long cycle; and the latter uses expensive Ru, which is contrary to the original intention of developing iron-rich oxide cathode materials, and both methods are difficult to inhibit the migration of Fe at the surface and to protect the material from air erosion.

[0005] In summary, for the problems of Fe migration and dissolution of iron-rich oxide cathode materials of sodium-ion batteries, the main idea of existing technologies is to stabilize Fe elements in the bulk phase of the material, but the current means still have certain limitations, and there is almost no method to modify the problem from the surface structure of the material. SUMMARY

[0006] In view of the existing problems of the iron-rich oxide cathode material of the sodium-ion battery, such as irreversible iron migration at a high voltage, high voltage, and unsatisfactory high-rate long cycle performance, the first purpose of the present application is to provide a double-in-situ coated modified iron-rich Na xFe a Me1 b Me2 c O2 material, aiming to provide a new sodium-ion battery positive active material capable of effectively inhibiting Fe migration and adapting to high-voltage high-rate long-cycle requirements.

[0007] The second object of the present application is to provide the double in-situ coating modified iron-rich Na x Fe a Me1 b Me2 c O2 material, and application thereof in sodium-ion batteries.

[0008] The third object of the present application is to provide a sodium-ion battery comprising the double in-situ coating modified iron-rich Na x Fe a Me1 b Me2 c O2 material, and a positive electrode and a positive electrode material thereof.

[0009] For iron-rich sodium-ion batteries, they are different from other manganese-rich, nickel-rich and other materials, and have more significant migration problems, and such significant migration problems are particularly prominent at high voltage and high rate, which seriously affects the long cycle performance of such materials at high voltage and high rate. In view of this problem, the present application provides the following improvement scheme:

[0010] A double in-situ coating modified iron-rich Na x Fe a Me1 b Me2 c O2 material, comprising a core, an intermediate layer in-situ coated on the surface of the core, and an outer layer in-situ coated on the intermediate layer; the material of the core has a chemical formula of Na x Fe a Me1 b Me2 c O2; the material of the intermediate layer has a chemical formula of Fe a Me1 b Me2 c O2;

[0011] The material of the outer layer has a chemical formula of NaTO3;

[0012] Wherein, Me1 is a +4 valence transition metal element; Me2 is a non +4 valence transition metal element with a valence of k, a+b+c=1, wherein 0.8≤x≤1.04, a≥0.4, and b, c are both greater than 0;

[0013] T is at least one of Nb, Bi, Ti, Ta, Mo, Zr, Sb, Ga, In, Y, W, Hf, and La.

[0014] To address the iron migration failure problem inherent in iron-rich oxide sodium electrochemical materials, this invention innovatively incorporates Fe of the aforementioned chemical formula in situ onto their surface. a Me1 b Me2 c An O2 intermediate layer is then formed in situ with NaTO3, which has the chemical formula described above. This allows for synergy between the various layers and phases of the material, effectively inhibiting the migration of iron-rich components in the matrix, improving its stability and oxygen resistance, and enabling it to unexpectedly meet the requirements for long-term application at high voltage and high rate, thus achieving long-cycle performance at high voltage and high rate.

[0015] In this invention, Me1 is at least one of Mn, Ti, Sn, Zr, V, Ir, and Hf, preferably at least one of Mn, Ti, Sn, and Zr. Preferably, Me2 is at least one of Ni, Cu, Co, Cr, V, Bi, Al, Mg, Ru, Sb, and Bi. This invention demonstrates that, based on the aforementioned material and hierarchical synergistic innovation, further coordination with the joint control of Me1 and Me2 can further enhance the high-voltage, high-rate, and long-cycle performance of the material.

[0016] In this invention, a is 0.4 to 0.5, and b is 0.2 to 0.4.

[0017] Preferably, T is at least one of Nb, Ti, Ta, and Sb. This invention also demonstrates that innovative joint control of the chemical formula of T can further synergize and effectively suppress iron migration at high voltage and high rate, thereby further enhancing the long-cycle performance of the material under high voltage and high rate conditions.

[0018] Preferably, the dual in-situ coated modified iron-rich Na x Fe a Me1 b Me2 c In O2 materials, the content of the intermediate layer is 1wt% to 5wt%, and the content of the outer layer is 1wt% to 10wt%.

[0019] The present invention also provides the aforementioned dual in-situ coated modified iron-rich Na x Fe a Me1 b Me2 c O2 material preparation method, Na x Fe a Me1 b Me2 c O2 undergoes surface ion exchange transformation in an organic solution containing Formula 1, forming Fe in situ on the surface. a Me1b Me2 c O2 intermediate layer, and the T source is calcined, thereby forming the outer layer in situ on the intermediate layer to obtain the double-layer in-situ coated modified Na x Fe a Me1 b Me2 c O2 material.

[0020]

[0021] The R is C1-C6 alkyl, C2-C6 alkenyl, aryl, carboxyl or carboxyalkyl.

[0022] In view of the iron migration failure of the iron oxide under high pressure and high rate, the Na x Fe a Me1 b Me2 c O2 surface is ion-exchanged and transformed, so that the Fe a Me1 b Me2 c O2 intermediate layer is constructed, so that the interface adaptability between the intermediate layer and the substrate is improved, the uniformity and stability are improved, in addition, the distribution of the surface Na is also regulated, so that the active sites are provided for the construction of the surface layer, and the outer layer with uniformity and good interface combination is in-situ formed. x Fe a Me1 b Me2 c O2 material, and the material prepared by the method can effectively inhibit the migration of iron under high pressure and high rate, and excellent long-term cycle effect can be obtained under the condition.

[0023] In the present application, the Na x Fe a Me1 b Me2 c O2 can be prepared based on known means, for example, mixed raw materials containing a Na source, a Fe source, a Me1 source and a Me2 source are sintered to obtain the Na x Fe a Me1 b Me2 cO2. The sintering process is carried out in an oxygen-containing atmosphere. The oxygen-containing atmosphere can be, for example, an atmosphere of oxygen, air, etc. The sintering temperature is 900-1200°C, and can further be 1000-1100°C. The sintering time is 8-20h, and can further be 10-15h. In the present application, a pre-sintering step can also be included before sintering, wherein the pre-sintering temperature can be 400-600°C, and can further be 450-500°C. The pre-sintering time can be, for example, 3-8h, and can further be 5-6h.

[0024] In the present application, the preparation method can at least include the following two embodiments: Embodiment A: Na x Fe a Me1 b Me2 c O2is subjected to surface ion exchange transformation treatment in advance in a solution containing Formula 1, and the intermediate layer is formed in situ, and then mixed with a T source and calcined, and the outer layer is formed in situ. Alternatively, Embodiment B: Na x Fe a Me1 b Me2 c O2and a T source are mixed in advance to obtain a mixture, and then the mixture is placed together in a solution containing Formula 1 for treatment, so that Na x Fe a Me1 b Me2 c O2in the mixture is subjected to surface ion exchange transformation treatment, and then calcination treatment is performed together. In the present application, Embodiment B is adopted, which can obtain better process synergy compared to Embodiment A, and can further improve the long cycle performance of the iron-rich material at high voltage and high rate.

[0025] In the present application, through liquid phase treatment of the organic solvent of Formula 1, the intermediate layer phase can be efficiently formed in situ based on ion exchange, which improves the interface adaptability of the intermediate layer and the bulk phase, and also optimizes the distribution of surface Na, thereby effectively constructing the outer layer with low impedance in situ.

[0026] In the present application, Formula 1 can be at least one of Formula 1A, Formula 1B, Formula 1C, and Formula 1D.

[0027]

[0028] R1and R2are independently methyl, ethyl, propyl, butyl, or phenyl.

[0029] Unexpectedly, the innovative use of Formula 1D can further optimize the in-situ coating structure, and further improve the long cycle stability of the prepared material at high voltage and high rate.

[0030] Preferably, the organic solvent in the organic solution containing Formula 1 includes but is not limited to at least one of acetone, ethanol, cyclohexane, ethyl acetate, tetrahydrofuran.

[0031] Preferably, the concentration of Formula 1 in the solution containing Formula 1 is 0.001 mol / L-0.005 mol / L; further can be 0.002-0.004M.

[0032] In the present application, during the exchange modification process, the components to be treated can be loaded into a filter bag and completely immersed in the organic solution containing Formula 1.

[0033] Preferably, the time for surface ion exchange transformation treatment is 2-20s, further can be 3-10s; further can be 4-6s.

[0034] In the present application, the T source is an oxide of T element. The present application research also shows that the preferred T source, combined with the process of the present application, can further synergize, help to further build a high-adaptability coating interface, and is beneficial to effectively inhibit the migration of iron under high rate and high voltage, and help to significantly improve the long cycle effect of the material under high voltage and high rate.

[0035] Preferably, the T source and the Na x Fe a Me1 b Me2 c The weight ratio of O2 is 0.01-0.2:1, further can be 0.05-0.15:1, further can be 0.08-0.12:1; research shows that the preferred ratio can further improve the long cycle stability of the prepared material under high voltage and high rate.

[0036] Preferably, the atmosphere for calcination is an oxygen-containing atmosphere;

[0037] Preferably, the calcination temperature is 600-1000℃, further can be 750-950℃; further can be 860-920℃; under the preferred temperature, the process can obtain better combined synergy, which is helpful to further improve the long cycle performance of the iron-rich material under high voltage and high rate.

[0038] Preferably, the calcination time is 8-15h; further can be 9-12h.

[0039] The present application also provides a positive electrode material of a sodium ion battery, which comprises the double-in-situ coating modified iron-rich Na x Fe a Me1 b Me2 c O2 material.

[0040] The application also provides a positive electrode of a sodium ion battery, comprising a current collector and a positive electrode material compounded on the surface of the current collector, wherein the positive electrode material is the double-in-situ-coated modified iron-rich Na x Fe a Me1 b Me2 c O2 material.

[0041] The application also provides a sodium ion battery comprising the double-in-situ-coated modified iron-rich Na x Fe a Me1 b Me2 c O2 material.

[0042] The sodium ion battery, the positive electrode and the positive electrode material of the application can be conventional in other components and structural relationships, except that the positive electrode comprises the double-in-situ-coated modified iron-rich Na x Fe a Me1 b Me2 c O2 material.

[0043] Advantages

[0044] The application provides a special material of double-in-situ-coated novel iron-rich oxide, which can synergistically inhibit the migration of iron-rich components in the matrix, improve the stability and oxygen resistance of the material, and unexpectedly achieve the requirements of long-term application of high voltage and high rate, and obtain long cycle performance under high voltage and high rate.

[0045] The application also provides a simple preparation method of the material, which forms the intermediate layer through the liquid phase exchange transformation mechanism of formula 1, optimizes the surface distribution of Na, and then forms the outer layer of high interface adaptation with the subsequent T source in-situ. The preparation method of the application can effectively improve the long cycle performance of the material under high voltage and high rate. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1 The XRD pattern of the material prepared in Example 1.

[0047] Figure 2 The SEM comparison chart of the material prepared in Example 1.

[0048] Figure 3 The 0.1C first cycle charge-discharge chart of the material prepared in Example 1.

[0049] Figure 4The long cycle performance test result graph of the material prepared for example 1 at high voltage (2.0-4.2V);

[0050] Figure 5 The cycle performance test result graph of the material prepared for example 1 at 5C rate. DETAILED DESCRIPTION

[0051] The sodium ion battery modified iron-rich oxide positive electrode material provided by the application has a matrix of Na x Fe a Me1 b Me2 c O2, and an intermediate layer on the surface of the matrix is Fe a Me1 b Me2 c O2; and an outer layer on the surface of the intermediate layer is NaTO3.

[0052] wherein Me1 is a +4 valence transition metal element, preferably one or more than two combinations of Mn, Ti, Sn, Zr, V, Ir, Hf, further preferably Mn and one or more than two combinations of (Ti / Sn / Zr), further preferably the molar ratio of Mn:(one or more than two combinations of Ti / Sn / Zr) is 1:4-4:1, Mn:Ti / Sn / Zr; Me2 is a non +4 valence transition metal element with a valence of k, preferably Ni, Cu, Co, Cr, V, Bi, Al, Mg, Ru, Sb, Bi, etc.; wherein x, a, b, c are the molar percentages of the corresponding elements, the relationship between them satisfies a+b+c=1, and x+3a+4b+kc=4; wherein 0.8≤x≤1.04, a≥0.4, b, c are greater than 0.

[0053] T is one or more than two combinations of, but not limited to, Nb, Bi, Ti, Ta, Mo, Zr, Sb, Ga, In, Y, W, Hf, La, preferably Nb / Ti / Ta / Sb, etc.

[0054] An alternative preparation method of the sodium ion battery modified iron-rich oxide positive electrode material provided by the application has the following steps, for example:

[0055] (1) mixing a sodium source, an iron source, a Me1 source and a Me2 source in a certain proportion, sintering, grinding and sieving to obtain a matrix material M0(Na x Fe a Me1 b Me2 c O2);

[0056] (2) pre-mixing M0 and T oxide and then performing secondary mixing to obtain a mixture M1;

[0057] (3) M1 is loaded into a filter bag, ion exchange transformation is carried out in solution A containing formula 1, and mixture M2 is obtained after taking out and drying;

[0058] (4) M2 is crushed and dried, sintering is carried out under a certain atmosphere according to a specific sintering schedule, and the sodium ion battery modified iron oxide positive electrode material is obtained after cooling, crushing and screening.

[0059] The sodium source, iron source, Me1 source and Me2 source include but are not limited to one or more than two combinations of oxides, hydroxides, carbonates, sulfates, nitrates, oxalates and acetates of corresponding metal elements;

[0060] The sodium salt is sodium oxide, the iron source is diiron trioxide, the Me1 source is Me1 oxide, and the Me2 source is Me2 oxide.

[0061] The mixing method includes but is not limited to one or more of dry ball milling, wet ball milling, sand milling, spray granulation, air flow crushing, liquid phase stirring mixing, ultrasonic mixing and hand grinding;

[0062] Wet ball milling is used for mixing, the solvent is ethanol, and the rotation speed is 300-800 r / min, and is further preferably 400-600 r / min.

[0063] In step 1, the sintering is sintering in an air or oxygen atmosphere, the sintering temperature is 900-1200℃, and the sintering time is 8-20h. Further, the sintering is sintering in an oxygen atmosphere, the sintering temperature is 1000-1100℃, and the sintering time is 12-16h.

[0064] In step 2, the Na x Fe a Me1 b Me2 c The addition ratio of O2 and T oxide is the mass ratio of the two, and the ratio is 80:20-99:1; further, it can be 85:15-95:5.

[0065] The premixing method includes but is not limited to one or more of hand grinding, ultrasonic mixing, stirring mixing and other methods. Further, hand grinding is used for premixing. The premixing time is 10-60min, and further can be 20-40min.

[0066] The secondary mixing includes but is not limited to one or more of dry ball milling, wet ball milling, sand milling, spray granulation, air flow crushing and other methods;

[0067] Dry ball milling is used, the rotation speed is 250-500 r / min, and the ball milling time is 6-12h.

[0068] The pore size of the filter bag is 0.2-0.5 μm; further, a 0.3 μm filter bag can be used;

[0069] In formula 1 of the solution A, it can be at least one of formula 1A-1D. The organic solvent in the solution A includes but is not limited to one or more than two combinations of ethanol, acetone, cyclohexane, ethyl acetate, tetrahydrofuran. The total concentration of formula 1 is 0.001-0.005 mol / L.

[0070] The surface ion exchange time can be 3-7 s; further, it can be 4-6 s.

[0071] The drying method is spray drying, air drying or vacuum drying, the drying temperature is 60-100 ℃, and the drying time is 8-12 h.

[0072] The drying temperature is 90 ℃, and the drying time is 10 h.

[0073] The sintering process has a heating rate of 1-5 ℃ / min, a sintering temperature of 600-1000 ℃, and a sintering time of 8-15 h; further, it can be 9-12 h, and the sintering atmosphere is oxygen.

[0074] Example 1

[0075] Step 1:

[0076] Anhydrous sodium carbonate, nickel oxide, diiron trioxide and manganese dioxide with a molar ratio of Na:Ni:Fe:Mn of 0.9:0.2:0.4:0.4 were added to a ball mill jar, and then ethanol was added and wet-milled at a speed of 400 rpm for 8 h; then the milled raw materials were dried in a 90 ℃ air oven for 10 h; the dried raw materials were dry-milled at a speed of 400 rpm for 4 h. The milled raw materials were pressed into a sheet shape by a powder tablet press at a pressure of 10 T. The sheet-shaped raw materials were placed in a corundum boat and transferred to a muffle furnace, heated to 500 ℃ (marked as T1) at a rate of 3 ℃ / min, kept at constant temperature for 5 h, then heated to 1080 ℃ (marked as T2) at a rate of 3 ℃ / min, kept at constant temperature for 12 h, cooled, ground and sieved, to obtain an iron-rich layered oxide positive electrode material Na 0.9 Ni 0.2 Fe 0.4 Mn 0.4 O2.

[0077] Step 2:

[0078] Na 0.9 Ni 0.2 Fe 0.4 Mn 0.4O2, T source (Nb2O5) is pre-mixed by hand grinding for 30 min, then added to the ball mill tank, dry grinding at 400 rpm for 10 h, then the material is taken out and loaded into a 0.3 μm filter bag, then immersed in a 0.003 mol / L acetone solution of formula 1A1 (a compound of formula 1A in which R1 is methyl) for ion exchange for 5 s, then taken out, and uniform and rapid Na + / H + exchange to form an intermediate coating phase, and then placed in a 90℃ vacuum oven for drying for 10 h;

[0079] Step 3:

[0080] The powder-coated after drying in step 2 is placed in a corundum boat and transferred to a tube furnace, heated to 900℃ at a rate of 3℃ / min in an oxygen atmosphere, and kept at constant temperature for 10 h. After cooling, it is ground and sieved to obtain a modified iron oxide-rich positive electrode material for sodium-ion batteries. The XRD and SEM of the prepared material are shown in Figure 1 and Figure 2 , and the electrochemical performance graphs of the material are shown in Figure 3 , Figure 4 and Figure 5 .

[0081] Example 2

[0082] Compared with Example 1, the only difference is that the base material in step 1 is changed, and the other operations and parameters are the same as in Example 1. The experimental groups are as follows:

[0083] 2-1 group: replace the nickel oxide in step 1 with an equal molar amount of copper oxide, and the other operations and parameters are the same as in Example 1.

[0084] 2-2 group: replace the manganese dioxide in step 1 with an equal molar amount of titanium dioxide, and the other operations and parameters are the same as in Example 1.

[0085] 2-3 group: the molar ratio of Na, Ni, Fe, Mn elements of anhydrous sodium carbonate, nickel oxide, diiron trioxide, manganese dioxide in step 1 is 1:0.25:0.5:0.25, and the temperature T1 is 450℃, the holding time is 6h; the temperature T2 is 1000℃, the holding time is 14h; the other operations and parameters are the same as in Example 1.

[0086] Example 3

[0087] Compared with Example 1, the only difference is that the process in step 2 is changed, and the other operations and parameters are the same as in Example 1. Specifically:

[0088] 3-1 group: replace formula 1A1 in step 2 with formula 1B, and the other operations and parameters are the same as in Example 1.

[0089] 3-2 group: replace acetone in step 2 with ethanol, the concentration of solute is 0.004M, other operations and parameters are the same as example 1.

[0090] 3-3 group: increase the soaking time in step 2 from 5s to 7s, other operations and parameters are the same as example 1.

[0091] 3-4 group: decrease the soaking time in step 2 from 5s to 3s, other operations and parameters are the same as example 1.

[0092] 3-5 group: replace 1A1 in step 2 with 1D1 (1D with R2 as methyl), other operations and parameters are the same as example 1.

[0093] Example 4

[0094] Compared with example 1, the only difference is that the conditions in step 2 are changed, other operations and parameters are the same as example 1, and the experimental groups are respectively:

[0095] 4-1 group: replace Nb2O5 in step 2 with Bi2O3, other operations and parameters are the same as example 1.

[0096] 4-2 group: Na 0.9 Ni 0.2 Fe 0.4 Mn 0.4 O2, T source mass ratio is adjusted to 95:5, other operations and parameters are the same as example 1.

[0097] 4-3 group: Na 0.9 Ni 0.2 Fe 0.4 Mn 0.4 O2, T source mass ratio is adjusted to 85:15, other operations and parameters are the same as example 1.

[0098] Example 5

[0099] Compared with example 1, the only difference is that the conditions in step 3 are changed, other operations and parameters are the same as example 1, and the experimental groups are respectively:

[0100] 5-1 group: the calcination temperature is controlled at 1000℃, other operations and parameters are the same as example 1.

[0101] 5-2 group: the calcination temperature is controlled at 800℃, the calcination time is 12h, other operations and parameters are the same as example 1.

[0102] Example 6

[0103] The difference between the example 1 and the example 2 is that, in the step 2, no T source is added, and after the step 2 is completed, the exchange product without the T source in the step 2 is mixed with the T source again, and then the step 3 is performed. The raw material usage and other operations and parameters are the same as those in the example 1.

[0104] Comparative example 1

[0105] The difference between the example 1 and the example 2 is that, in the step 2, no T source is added, and after the step 2 is completed, the exchange product without the T source in the step 2 is mixed with the T source again, and then the step 3 is performed. The raw material usage and other operations and parameters are the same as those in the example 1.

[0106] Comparative example 2

[0107] The difference between the example 1 and the example 2 is that, in the step 2, no T source is added, and after the step 2 is completed, the exchange product without the T source in the step 2 is mixed with the T source again, and then the step 3 is performed. The raw material usage and other operations and parameters are the same as those in the example 1.

[0108] Comparative example 3

[0109] The difference between the example 1 and the example 2 is that, in the step 2, no T source is added, and after the step 2 is completed, the exchange product without the T source in the step 2 is mixed with the T source again, and then the step 3 is performed. The raw material usage and other operations and parameters are the same as those in the example 1.

[0110] Comparative example 4

[0111] The difference between the example 1 and the example 2 is that, in the step 2, no T source is added, and after the step 2 is completed, the exchange product without the T source in the step 2 is mixed with the T source again, and then the step 3 is performed. The raw material usage and other operations and parameters are the same as those in the example 1.

[0112] Comparative example 5

[0113] The difference between the example 1 and the example 2 is that, in the step 2, no T source is added, and after the step 2 is completed, the exchange product without the T source in the step 2 is mixed with the T source again, and then the step 3 is performed. The raw material usage and other operations and parameters are the same as those in the example 1.

[0114] Comparative example 6

[0115] The difference between the example 1 and the example 2 is that, in the step 2, no T source is added, and after the step 2 is completed, the exchange product without the T source in the step 2 is mixed with the T source again, and then the step 3 is performed. The raw material usage and other operations and parameters are the same as those in the example 1.

[0116] Experimental example:

[0117] The positive electrode material prepared in the example 1 above is subjected to XRD test, electrochemical performance test and Fe elution test. The results are shown in Figure 1 and Tables 1-3.

[0118] Electrochemical performance test: the positive electrode material and acetylene black, polyvinylidene fluoride prepared in the above cases are uniformly mixed in N-methyl pyrrolidone in a mass ratio of 80:10:10 to prepare a slurry and coat on an aluminum current collector, and then dried and sheeted. The prepared sheet is used as a positive electrode, sodium sheet is used as a negative electrode, glass fiber is used as a separator, 1 mol / L NaClO4 is mixed in a mixed solution of ethylene carbonate / diethyl carbonate (volume ratio 1:1) and additional 5wt% FEC as an electrolyte, and a button type 2032 battery is assembled in a glove box with less than 0.01ppm of water and oxygen content. Constant current charge and discharge test is carried out on a blue cell test system, and the voltage interval is 2.0-4.2V. After three cycles of activation at 0.1C, 1C long cycle test and 5C high rate cycle test are carried out respectively.

[0119] Fe elution test: the battery after 1C cycle for 500 cycles is disassembled, the separator, nickel mesh and negative electrode are soaked in ethanol, after the metallic sodium is completely dissolved, dilute sulfuric acid is added for further dissolution. The obtained solution is subjected to ICP test to detect the concentration of Fe element in the solution, and the mass of Fe element elution is calculated.

[0120] Table 1 Long cycle performance test results (mAh / g) of the battery at 25℃ under high voltage (2.0-4.2V)

[0121]

[0122]

[0123] Table 2 5C cycle test results (mAh / g) of the battery at 25℃ (mAh / g)

[0124]

[0125]

[0126] Table 3 Fe elution test (mg) after 1C cycle for 500 cycles at 25℃ under 2.0-4.2V (mg)

[0127]

[0128]

[0129] In summary, the intermediate layer is formed by the liquid phase exchange transformation mechanism of formula 1, the surface distribution of Na is optimized, and then the outer layer with high interface adaptation is formed in situ with the subsequent T source. The preparation method can effectively improve the long cycle performance of the material under high voltage and high rate.

[0130] In addition, it is known from Examples 1 and 3 that the ion exchange using the compound of Formula 1 containing a hydroxyl group can achieve a more excellent synergistic effect. In addition, it is also known from Examples 1 and 6 that the simultaneous treatment of the substrate and the T source with the compound of Formula 1 can further enhance the synergism of the process, and can achieve more excellent high-voltage, high-rate, and long-cycle performance.

[0131] Obviously, the above examples are merely illustrative in nature and are not intended to limit the embodiments. Based upon the above description, those skilled in the art can make other different forms of changes or variations. Here, it is not necessary and impossible to exhaust all the embodiments. The obvious changes or variations derived therefrom are still within the protection scope of the present application.

Claims

1. A dual in-situ coated modified iron-rich Na x Fe a Me1 b Me2 c O2 material, characterized in that, It includes a core, an intermediate layer in situ coating the surface of the core, and an outer layer in situ coating the intermediate layer; the material of the core has the chemical formula Na. x Fe a Me1 b Me2 c O2; the chemical formula of the intermediate layer material is Fe. a Me1 b Me2 c O2; The chemical formula of the outer layer material is NaTO3; Me1 is a +4 valence transition metal element; Me2 is a non-+4 valence transition metal element with valence k, a+b+c=1, where 0.8≤x≤1.04, a≥0.4, and b and c are both greater than 0; T is at least one of the following: Nb, Bi, Ti, Ta, Mo, Zr, Sb, Ga, In, Y, W, Hf, and La.

2. The dual in-situ coated modified iron-rich Na as described in claim 1 x Fe a Me1 b Me2 c O2 material, characterized in that, The Me1 is at least one of Mn, Ti, Sn, Zr, V, Ir, and Hf.

3. The dual in-situ coated modified iron-rich Na as described in claim 2 x Fe a Me1 b Me2 c O2 material, characterized in that, The Me1 is at least one of Mn, Ti, Sn, and Zr.

4. The dual in-situ coated modified iron-rich Na as described in claim 1 x Fe a Me1 b Me2 c O2 material, characterized in that, The Me2 is at least one of Ni, Cu, Co, Cr, V, Bi, Al, Mg, Ru, Sb, and Bi.

5. The dual in-situ coated modified iron-rich Na as described in claim 1 x Fe a Me1 b Me2 c O2 material, characterized in that, The value of a is 0.4~0.5, and the value of b is 0.2~0.

4.

6. The dual in-situ coated modified iron-rich Na as described in claim 1 x Fe a Me1 b Me2 c O2 material, characterized in that, T is at least one of Nb, Ti, Ta, and Sb.

7. The dual in-situ coated modified iron-rich Na as described in claim 1 x Fe a Me1 b Me2 c O2 material, characterized in that, The dual in-situ coated modified iron-rich Na x Fe a Me1 b Me2 c In O2 materials, the content of the intermediate layer is 1wt%~5wt%, and the content of the outer layer is 1wt%~10wt%.

8. A dual in-situ coated modified iron-rich Na according to any one of claims 1 to 7 x Fe a Me1 b Me2 c The method for preparing O2 materials is characterized by, Will Na x Fe a Me1 b Me2 c O2 undergoes surface ion exchange transformation in an organic solution containing Formula 1, forming Fe in situ on the surface. a Me1 b Me2 c An O2 intermediate layer is then calcined with a T source to form the outer layer in situ on the intermediate layer, thus obtaining the double in-situ coated modified iron-rich Na. x Fe a Me1 b Me2 c O2 materials; Formula 1 The R is a C1-C6 alkyl group, a C2-C6 olefin group, an aromatic group, a carboxyl group, or a carboxylalkyl group.

9. The dual in-situ coated modified iron-rich Na as described in claim 8 x Fe a Me1 b Me2 c The method for preparing O2 materials is characterized by, The Na is prepared by sintering a mixed raw material containing Na source, Fe source, Me1 source and Me2 source. x Fe a Me1 b Me2 c O2.

10. The dual in-situ coated modified iron-rich Na as described in claim 9 x Fe a Me1 b Me2 c The method for preparing O2 materials is characterized by, The sintering process is carried out in an oxygen-containing atmosphere.

11. The dual in-situ coated modified iron-rich Na as described in claim 10 x Fe a Me1 b Me2 c The method for preparing O2 materials is characterized by, The sintering temperature is 900~1200℃, and the sintering time is 8h~20h.

12. The dual in-situ coated modified iron-rich Na as described in claim 8 x Fe a Me1 b Me2 c The method for preparing O2 materials is characterized by, Will Na x Fe a Me1 b Me2 c O2 is pre-treated with surface ion exchange in a solution containing Formula 1 to form the intermediate layer in situ, and then mixed with a T source and calcined to form the outer layer in situ. Or, Na x Fe a Me1 b Me2 c O2 and T source are premixed to obtain a mixture, which is then placed in a solution containing Formula 1 for treatment, so that the Na in the mixture... x Fe a Me1 b Me2 c O2 is used for surface ion exchange transformation treatment, followed by calcination treatment.

13. The dual in-situ coated modified iron-rich Na as described in claim 12 x Fe a Me1 b Me2 c The method for preparing O2 materials is characterized by, Formula 1 can specifically be at least one of Formula 1A, Formula 1B, Formula 1C, and Formula 1D; Formula 1A Formula 1B Formula 1C Formula 1D R1 and R2 are individually methyl, ethyl, propyl, butyl, or phenyl.

14. The dual in-situ coated modified iron-rich Na as described in claim 13 x Fe a Me1 b Me2 c The method for preparing O2 materials is characterized by, The organic solvent in the organic solution containing Formula 1 includes at least one of acetone, ethanol, cyclohexane, ethyl acetate, and tetrahydrofuran.

15. The dual in-situ coated modified iron-rich Na as described in claim 14 x Fe a Me1 b Me2 c The method for preparing O2 materials is characterized by, The concentration of Formula 1 in the solution containing Formula 1 is 0.001 mol / L to 0.005 mol / L.

16. The dual in-situ coated modified iron-rich Na as described in claim 12 x Fe a Me1 b Me2 c The method for preparing O2 materials is characterized by, The surface ion exchange transformation treatment time is 2~20s.

17. The dual in-situ coated modified iron-rich Na as described in claim 12 x Fe a Me1 b Me2 c The method for preparing O2 materials is characterized by, The T source is an oxide of the element T.

18. The dual in-situ coated modified iron-rich Na as described in claim 17 x Fe a Me1 b Me2 c The method for preparing O2 materials is characterized by, The T source and the Na x Fe a Me1 b Me2 c The weight ratio of O2 is 0.01~0.2:

1.

19. The dual in-situ coated modified iron-rich Na as described in claim 12 x Fe a Me1 b Me2 c The method for preparing O2 materials is characterized by, The roasting atmosphere is an oxygen-containing atmosphere.

20. The dual in-situ coated modified iron-rich Na as described in claim 12 x Fe a Me1 b Me2 c The method for preparing O2 materials is characterized by, The roasting temperature is 600℃~1000℃.

21. The dual in-situ coated modified iron-rich Na as described in claim 12 x Fe a Me1 b Me2 c The method for preparing O2 materials is characterized by, The roasting time is 8 to 15 hours.

22. The dual in-situ coated modified iron-rich Na as described in claim 21 x Fe a Me1 b Me2 c The method for preparing O2 materials is characterized by, The roasting time is 9-12 hours.

23. A positive electrode material for a sodium-ion battery, characterized in that, Contains the dual in-situ coated modified iron-rich Na as described in any one of claims 1 to 7 x Fe a Me1 b Me2 c O2 material or the preparation method described in any one of claims 8-22, double in-situ coated modified iron-rich Na x Fe a Me1 b Me2 c O2 material.

24. A positive electrode for a sodium-ion battery, comprising a current collector and a positive electrode material composited thereon, characterized in that, The cathode material is the cathode material as described in claim 23.

25. A sodium-ion battery, characterized in that, It includes the positive electrode as described in claim 24.

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