Double in-situ coating modified iron-rich NaxFeaMe1bMe2cO2 material as well as preparation method and application thereof in sodium ion battery

Abstract

The invention belongs to the field of sodium ion battery positive electrode materials, and particularly relates to a double in-situ coated modified iron-rich NaxFeaMe1bMe2cO2 material which comprises a core, a middle layer coated on the surface of the core in situ, and an outer layer coated on the middle layer in situ, the chemical formula of the material of the core is Na < x > Fe Me1 Me2 < c > O < 2 >; the chemical formula of the material of the middle layer is FeaMe1bMe2cO2; the chemical formula of the material of the outer layer is NaTO3; wherein Me1 is a transition metal element with valence of + 4; me2 is a non-+ 4-valence transition metal element with the valence of k, a + b + c = 1, 0.8 < = x < = 1.04, a > = 0.4, and 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 series. The invention also provides preparation and application of the material. The material provided by the invention has excellent performance, for example, iron migration under high voltage can be reduced, and long cycle performance under high voltage and high rate can be improved.

Description

Technical Field

[0001] The invention belongs to the technical field of positive electrode materials for sodium ion batteries, and in particular relates to positive electrode materials for sodium ion batteries. Background Art

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

[0003] In order to achieve higher capacity while reducing cost, iron-rich oxide cathode materials need to operate at higher cut-off voltages. 4+ There is a serious Jan-Taylor effect, and a large amount of Na is released at the same time, which will cause Fe to have a strong tendency to migrate to the Na layer and eventually dissolve into the electrolyte. On the surface, HF produced by the reaction of the electrolyte with the residual alkali on the surface under a high cut-off voltage will actively attack the surface of the positive electrode material, leading to further dissolution of the Fe element. Under the combined effect of problems in the bulk phase and on the surface, the structure of the material will be irreversibly destroyed, ultimately leading to an irreversible decrease in the electrochemical properties 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 Fe migration. However, these methods still have certain limitations: in the former, Li will be released after a long cycle, and the migration of Fe cannot be avoided; and the Ru used in the latter is expensive, which is contrary to the original intention of developing iron-rich oxide positive electrode materials. At the same time, both methods are difficult to inhibit the migration of Fe on the surface, and it is difficult to protect the material from air erosion.

[0005] In summary, for the problem of Fe migration and dissolution in iron-rich oxide cathode materials for sodium-ion batteries, the main idea of ​​existing technologies is to stabilize the Fe element in the bulk phase of the material. However, the current methods still have certain limitations, and there are almost no methods to modify this problem from the surface structure of the material. Summary of the invention

[0006] In view of the irreversible iron migration under high voltage, unsatisfactory high voltage and high rate long cycle performance of existing iron-rich oxide cathode materials for sodium ion batteries, the first object of the present invention is to provide a double in-situ coated modified iron-rich Na xFe a Me1 b Me2 c O 2 The material aims to provide a new sodium ion battery positive electrode active material that can effectively inhibit Fe migration and adapt it to the requirements of high voltage, high rate and long cycle.

[0007] The second object of the present invention is to provide the double in-situ coated modified iron-rich Na x Fe a Me1 b Me2 c O 2 Preparation method of materials and application in sodium ion batteries.

[0008] The third object of the present invention is to provide a double in-situ coated modified iron-rich Na x Fe a Me1 b Me2 c O 2 Sodium-ion batteries and their positive electrodes and positive electrode materials.

[0009] For iron-rich sodium ion batteries, unlike other manganese-rich and nickel-rich materials, they have more significant migration problems, and this significant migration problem is particularly prominent under high voltage and high rate, which seriously affects the long cycle performance of such materials under high voltage and high rate. To address this problem, the present invention provides the following improvement scheme:

[0010] A double in-situ coated modified iron-rich Na x Fe a Me1 b Me2 c O 2 The material 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 chemical formula of the core material is Na x Fe a Me1 b Me2 c O 2 ; The chemical formula of the material of the intermediate layer is Fe a Me1 b Me2 c O 2 ;

[0011] The chemical formula of the outer layer material is NaTO 3 ;

[0012] Wherein, Me1 is a +4 valent transition metal element; Me2 is a non-+4 valent transition metal element with a valence of k, a+b+c=1, wherein 0.8≤x≤1.04, a≥0.4, and b and 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] In view of the iron migration failure problem of iron-rich oxide sodium electrolytic materials, the present invention innovatively in-situ compounds the chemical formula Fe a Me1 b Me2 c O 2 The intermediate layer is then in situ compounded with the chemical formula NaTO 3 In this way, based on the combination of each layer of materials and phases, synergy can be achieved, which can effectively inhibit the migration of iron-rich components in the matrix, improve its stability and oxygen resistance, enable it to unexpectedly achieve high voltage, high rate long-term application requirements, and obtain long cycle performance under high voltage and high rate.

[0015] In the present invention, the 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, the Me2 is at least one of Ni, Cu, Co, Cr, V, Bi, Al, Mg, Ru, Sb, and Bi. The present invention shows that on the basis of the material and hierarchical collaborative innovation, further coordination with the joint control of Me1 and Me2 can further enhance the high voltage and high rate long cycle performance of the material.

[0016] In the present 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. The present invention also shows that innovative joint control of the chemical formula of T can further cooperate and further help inhibit the migration of iron at high voltage and high rate, and further enhance the high voltage and high rate long cycle performance of the material.

[0018] Preferably, the double in-situ coated modified iron-rich Na x Fe a Me1 b Me2 c O 2 In the material, the content of the middle layer is 1wt% to 5wt%, and the content of the outer layer is 1wt% to 10wt%.

[0019] The present invention also provides a double in-situ coated modified iron-rich Na x Fe a Me1 b Me2 c O 2 The preparation method of the material is to x Fe a Me1 b Me2 c O 2 In an organic solution containing Formula 1, a surface ion exchange transformation is performed to form Fe in situ on the surface. a Me1 b Me2 c O 2 The intermediate layer is then calcined with the T source to form the outer layer in situ on the intermediate layer to obtain the double in-situ coated modified iron-rich Na x Fe a Me1 b Me2 c O 2 Material;

[0020]

[0021] The R is C 1 ~C 6 Alkyl, C 2 ~C 6 an olefin group, an aromatic group, a carboxyl group or a carboxyalkyl group.

[0022] In view of the prominent iron migration failure problem of iron-rich oxides under high pressure and high rate, the present invention innovatively adopts formula 1 to pre-treat Na x Fe a Me1 b Me2 c O 2 The surface is transformed by ion exchange, so that Fe a Me1 b Me2 c O 2 The intermediate layer improves the interface adaptability between the intermediate layer and the substrate, improves its uniformity and stability, and is also conducive to regulating the distribution of surface Na, thereby providing active sites for the construction of the surface layer and facilitating the in-situ formation of a uniform outer layer with good interface bonding. The present invention shows that the double-layer coated modified Na can be in-situ constructed by the preparation method. x Fe a Me1 b Me2 c O 2The material prepared by the method can effectively and synergistically inhibit the migration of iron under high pressure and high rate, and can still obtain excellent long-term circulation effect under this condition.

[0023] In the present invention, Na x Fe a Me1 b Me2 c O 2 It can be prepared based on known means, for example, a mixed raw material containing a Na source, a Fe source, a Me1 source, and a Me2 source can be sintered to obtain the Na x Fe a Me1 b Me2 c O 2 . The sintering process is carried out in an oxygen-containing atmosphere. The oxygen-containing atmosphere may be, for example, an atmosphere of oxygen, air, or the like. The sintering temperature is 900 to 1200°C, and may further be 1000 to 1100°C, and the sintering time is 8h to 20h, and may further be 10 to 15h. In the present invention, a pre-sintering step may be included before sintering, wherein the pre-sintering temperature may be 400 to 600°C, and further may be 450 to 500°C. The pre-sintering time may be, for example, 3 to 8h, and may further be 5 to 6h.

[0024] In the present invention, the preparation method may include at least the following two implementation modes: Method A: Na x Fe a Me1 b Me2 c O 2 The surface ion exchange transformation treatment is carried out in advance in a solution containing Formula 1 to form the intermediate layer in situ, and then mixed with the T source and calcined to form the outer layer in situ. Alternatively, method B: Na x Fe a Me1 b Me2 c O 2 and T source to obtain a mixture, and then the mixture is placed in a solution containing formula 1 for treatment, so that the Na in the mixture x Fe a Me1 b Me2 c O 2 The surface ion exchange transformation treatment is carried out, and then the calcination treatment is carried out at the same time. In the present invention, the method B is adopted, which can obtain better process combination synergy compared with the method A, and can further improve the long cycle performance of the iron-rich material at high voltage and high rate.

[0025] In the present invention, by liquid phase treatment of the organic solvent of Formula 1, the intermediate layer phase can be efficiently formed in situ based on ion exchange, thereby improving its interface compatibility with the bulk phase. Not only that, it is also beneficial to optimize the distribution of surface Na, thereby facilitating the effective in-situ low-impedance construction of the outer layer.

[0026] In the present invention, the formula 1 may specifically be at least one of formula 1A, formula 1B, formula 1C, and formula 1D;

[0027]

[0028] The R 1 , R 2 is independently methyl, ethyl, propyl, butyl or phenyl.

[0029] The study unexpectedly showed that the innovative use of formula 1D can further optimize the in-situ coating structure and further improve the long-cycle stability of the prepared materials under high pressure 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 and tetrahydrofuran.

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

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

[0033] Preferably, the surface ion exchange transformation treatment time is 2 to 20 seconds, further can be 3 to 10 seconds, and further can be 4 to 6 seconds.

[0034] In the present invention, the T source is an oxide of the element T. The present invention also shows that the preferred T source, combined with the process of the present invention, can further cooperate and help to further construct a highly adaptable coating interface, which is beneficial to effectively inhibit the migration of iron at high rates and high voltages, and helps to significantly improve the long-term cycle effect of the material at high voltages and high rates.

[0035] Preferably, the T source and the Na x Fe a Me1 b Me2 c O 2The weight ratio is 0.01-0.2:1, further can be 0.05-0.15:1, and further can be 0.08-0.12:1; studies have shown that under the preferred ratio, the long-cycle stability of the prepared material under high pressure and high rate can be further improved.

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

[0037] Preferably, the calcination temperature is 600°C to 1000°C, and can further be 750°C to 950°C; further 860°C to 920°C; at the preferred temperature, better process synergy can be obtained, which helps to further improve the long cycle performance of iron-rich materials at high voltage and high rate.

[0038] Preferably, the calcination time is 8 to 15 hours; further, it can be 9 to 12 hours.

[0039] The present invention also provides a positive electrode material for a sodium ion battery, which comprises the double in-situ coated modified iron-rich Na x Fe a Me1 b Me2 c O 2 Material.

[0040] The present invention also provides a positive electrode for a sodium ion battery, comprising a current collector and a positive electrode material composited on the surface thereof, wherein the positive electrode material is the double in-situ coated modified iron-rich Na x Fe a Me1 b Me2 c O 2 Positive electrode material of the material.

[0041] The present invention also provides a sodium ion battery, which comprises the double in-situ coated modified iron-rich Na x Fe a Me1 b Me2 c O 2 Positive electrode material.

[0042] The sodium ion battery and the positive electrode and positive electrode material of the present invention, in addition to comprising the double in-situ coated modified iron-rich Na x Fe a Me1 b Me2 c O 2 The positive electrode of the material, other components and structural relationships can all be conventional.

[0043] Beneficial Effects

[0044] The present invention provides a new iron-rich oxide with double in-situ coating of special materials, which can synergistically inhibit the migration of iron-rich components in the matrix based on the material and hierarchical relationship, improve its stability and oxygen resistance, and enable it to unexpectedly achieve high voltage, high rate and long-term application requirements, and can obtain long cycle performance under high voltage and high rate.

[0045] The present invention 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 an outer layer with high interface adaptation in situ with the subsequent T source. The preparation method of the present invention can effectively improve the long cycle performance of the material under high voltage and high rate. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0047] Figure 2 This is a SEM comparison image of the material obtained in Example 1.

[0048] Figure 3 This is the 0.1C first cycle charge and discharge diagram of the material prepared in Example 1.

[0049] Figure 4 This is a graph showing the long cycle performance test results of the material prepared in Example 1 at high voltage (2.0-4.2V);

[0050] Figure 5 This is a graph showing the cycling performance test results of the material prepared in Example 1 at a 5C rate. DETAILED DESCRIPTION

[0051] The modified iron-rich oxide positive electrode material for sodium ion batteries of the present invention has a matrix of Na x Fe a Me1 b Me2 c O 2 , the intermediate layer on the substrate surface is Fe a Me1 b Me2 c O 2 ; The outer layer on the surface of the middle layer is NaTO 3 .

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

[0053] T is one or a combination of two or more selected from the group consisting of, but not limited to, Nb, Bi, Ti, Ta, Mo, Zr, Sb, Ga, In, Y, W, Hf, and La, preferably Nb / Ti / Ta / Sb, etc.;

[0054] An optional method for preparing the modified iron-rich oxide positive electrode material for a sodium ion battery of the present invention comprises, for example, the following steps:

[0055] (1) Mix the sodium source, iron source, Me1 source and Me2 source in proportion, sinter, grind and sieve to obtain the matrix material M0 (Na x Fe a Me1 b Me2 c O 2 );

[0056] (2) premixing M0 and T oxide and then remixing them to obtain a mixture M1;

[0057] (3) M1 is placed in a filter bag, subjected to ion exchange transformation in a solution A containing Formula 1, and then taken out and dried to obtain a mixture M2;

[0058] (4) M2 is crushed and dried, sintered under a certain atmosphere according to a specific sintering system, cooled, crushed, and sieved to obtain a modified iron-rich oxide positive electrode material for a sodium ion battery.

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

[0060] The sodium salt uses sodium oxide, the iron source uses ferric oxide, the Me1 source uses Me1 oxide, and the Me2 source uses Me2 oxide.

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

[0062] The wet ball milling is used for mixing, the solvent is ethanol, and the rotation speed is 300 to 800 r / min, and more preferably 400 to 600 r / min.

[0063] In step 1, the sintering is carried out in air or oxygen atmosphere, the sintering temperature is 900-1200° C., and the sintering time is 8-20 hours. Further, the sintering is carried out in oxygen atmosphere, the sintering temperature is 1000-1100° C., and the sintering time is 12-16 hours.

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

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

[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 the like;

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

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

[0069] The solution A in Formula 1 may be at least one of Formula 1A to Formula 1D. The organic solvent in Solution A includes, but is not limited to, one or a combination of two or more of ethanol, acetone, cyclohexane, ethyl acetate, and tetrahydrofuran. The total concentration of Formula 1 is 0.001 mol / L to 0.005 mol / L.

[0070] The time for surface ion exchange may be 3 to 7 seconds; further, it may be 4 to 6 seconds.

[0071] The drying method is spray drying, blast drying or vacuum drying, the drying temperature is 60-100° C., and the drying time is 8-12 hours.

[0072] The product was dried in a forced air oven at a temperature of 90°C and a drying time of 10 h.

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

[0074] Example 1

[0075] Step 1:

[0076] Anhydrous sodium carbonate, nickel oxide, ferric oxide and manganese dioxide in a molar ratio of 0.9:0.2:0.4:0.4 for Na, Ni, Fe and Mn elements were added to a ball mill, and after adding ethanol, wet milled at a speed of 400 rpm for 8 hours; then the ball-milled raw material was placed in a 90°C blast oven for drying for 10 hours; the dried raw material was ball-milled again and dry-milled at a speed of 400 rpm for 4 hours. The ball-milled raw material was pressed into sheets using a powder tablet press at a pressure of 10T. The sheet raw material was placed in a corundum ark and transferred to a muffle furnace, heated to 500°C (marked as T1) at a rate of 3°C / min, kept at a constant temperature for 5 hours, and then heated to 1080°C (marked as T2) at a rate of 3°C / min, kept at a constant temperature for 12 hours, and after cooling, ground and sieved to obtain the iron-rich layered oxide positive electrode material Na 0.9 Ni 0.2 Fe 0.4 Mn 0.4 O 2 .

[0077] Step 2:

[0078] The mass ratio of Na 0.9 Ni 0.2 Fe 0.4 Mn 0.4 O 2 、T source (Nb 2 O 5 ) was pre-mixed by hand grinding for 30 minutes, added to a ball mill, and dry-milled at a speed of 400 rpm for 10 hours. The material was then taken out and placed in a 0.3 μm filter bag, and then immersed in an acetone solution of 0.003 mol / L formula 1A1 (a compound of formula 1A in which R1 is methyl) for 5 seconds to perform ion exchange, and then taken out to achieve uniform and rapid Na + / H + The intermediate coating phase was exchanged and then dried in a vacuum oven at 90 °C for 10 h;

[0079] Step 3:

[0080] The coated powder dried in step 2 was placed in a corundum ark and transferred to a tube furnace. In an oxygen atmosphere, the temperature was raised to 900°C at a rate of 3°C / min and kept constant for 10 hours. After cooling, the powder was ground and sieved to obtain a modified iron-rich oxide positive electrode material for sodium ion batteries. The XRD and SEM of the prepared material showed Figure 1 and Figure 2 , and the electrochemical performance diagrams of the materials 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 Example 1. The experimental groups are:

[0083] Group 2-1: The nickel oxide in step 1 was replaced by an equal molar amount of copper oxide, and the other operations and parameters were the same as those in Example 1.

[0084] Group 2-2: The manganese dioxide in step 1 was replaced by an equal molar amount of titanium dioxide, and the other operations and parameters were the same as those in Example 1.

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

[0086] Example 3

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

[0088] Group 3-1: Formula 1A1 in step 2 was replaced by Formula 1B, and other operations and parameters were the same as those in Example 1.

[0089] Group 3-2: The acetone in step 2 was replaced by ethanol, wherein the concentration of the solute was 0.004 M, and the other operations and parameters were the same as those in Example 1.

[0090] Group 3-3: The immersion time in step 2 was increased from 5 s to 7 s, and other operations and parameters were the same as those in Example 1.

[0091] Group 3-4: The immersion time in step 2 was reduced from 5 s to 3 s, and other operations and parameters were the same as those in Example 1.

[0092] Group 3-5: Formula 1A1 in step 2 is replaced by Formula 1D1 (Formula 1D wherein R2 is methyl), and other operations and parameters are the same as those in Example 1.

[0093] Example 4

[0094] Compared with Example 1, the only difference is that the conditions of step 2 are changed, and the other operations and parameters are the same as those of Example 1. The experimental groups are:

[0095] Group 4-1: Replace Nb in step 2 2 O 5 Replace with Bi 2 O 3 , other operations and parameters are the same as in Example 1.

[0096] Group 4-2: Na 0.9 Ni 0.2 Fe 0.4 Mn 0.4 O 2 The mass ratio of the T source is adjusted to 95:5, and the other operations and parameters are the same as those in Example 1.

[0097] Group 4-3: Na 0.9 Ni 0.2 Fe 0.4 Mn 0.4 O 2 The mass ratio of the T source is adjusted to 85:15, and the other operations and parameters are the same as those in Example 1.

[0098] Example 5

[0099] Compared with Example 1, the only difference is that the conditions of step 3 are changed, and the other operations and parameters are the same as those of Example 1. The experimental groups are:

[0100] Group 5-1: The calcination temperature was controlled at 1000° C., and other operations and parameters were the same as those in Example 1.

[0101] Group 5-2: The calcination temperature was controlled at 800° C., the calcination time was 12 h, and the other operations and parameters were the same as those in Example 1.

[0102] Example 6

[0103] Compared with Example 1, the only difference is that in step 2, no T source is added. After step 2 is completed, the exchange product of step 2 without adding T source is mixed with T source again, and then step 3 is carried out. The amount of raw materials and other operations and parameters are the same as in Example 1.

[0104] Comparative Example 1

[0105] Compared with Example 1, the only difference is that the processing of steps 2 and 3 is not performed, and other operations and parameters are the same as Example 1.

[0106] Comparative Example 2

[0107] Compared with Example 1, the only difference is that the processing of step 3 is not performed, and other operations and parameters are the same as Example 1.

[0108] Comparative Example 3

[0109] Compared with Example 1, the only difference is that in step 2, no Nb is added. 2 O 5 , other operations and parameters are the same as in Example 1.

[0110] Comparative Example 4

[0111] Compared with Example 1, the only difference is that in step 2, Formula 1A1 is not added to the acetone solution, and other operations and parameters are the same as in Example 1.

[0112] Comparative Example 5

[0113] Compared with Example 1, the only difference is that in step 2, no liquid phase exchange treatment is performed, that is, the formula 1A1 is directly mixed with the matrix and the T source in the form of a solid, and then the treatment of step 3 is performed together. Other operations and parameters are the same as in Example 1.

[0114] Comparative Example 6

[0115] Compared with Example 1, the only difference is that in step 2, the aqueous solution of Formula 1A1 is used for the exchange treatment, that is, water is used to replace the acetone solvent therein, and other operations and parameters are the same as in Example 1.

[0116] Experimental example:

[0117] The positive electrode material prepared in Example 1 was subjected to XRD test, electrochemical performance test, and Fe dissolution test. The results are as follows Figure 1 As shown in Table 1-3.

[0118] Electrochemical performance test: The cathode materials finally prepared in the above cases were mixed with acetylene black and polyvinylidene fluoride in a mass ratio of 80:10:10 in N-methylpyrrolidone to form a slurry and coated on the aluminum current collector, and then dried and made into sheets. The prepared electrode sheet was used as the cathode, the sodium sheet as the anode, and the glass fiber as the separator. 1 mol / L NaClO 4A mixed solution of ethylene carbonate / diethyl carbonate (volume ratio 1:1) and additionally added 5wt% FEC was used as the electrolyte, and button-type 2032 batteries were assembled in a glove box with water and oxygen content less than 0.01ppm. Constant current charge and discharge tests were carried out on the Blue Electric Battery Test System with a voltage range of 2.0 to 4.2V. After three cycles of activation at 0.1C, 1C long cycle tests and 5C high rate cycle tests were carried out respectively;

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

[0120] Table 1 Long cycle performance test results of batteries at high voltage (2.0-4.2V) at 25°C (mAh / g)

[0121]

[0122]

[0123] Table 2: Battery high rate (5C) cycle test results at 25°C (mAh / g)

[0124]

[0125]

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

[0127]

[0128]

[0129] In summary, the present invention forms the intermediate layer through the liquid phase exchange transformation mechanism of Formula 1, optimizes the surface distribution of Na, and then forms an outer layer with high interface adaptation in situ with the subsequent T source. The preparation method of the present invention can effectively improve the long cycle performance of the material under high voltage and high rate.

[0130] In addition, it can be seen from Examples 1 and 3 that the ion exchange using Formula 1 containing hydroxyl groups can obtain a better synergistic effect. In addition, it can also be seen from Examples 1 and 6 that the substrate and the T source are treated by Formula 1 simultaneously, which can further enhance the synergy of the process and obtain better high-pressure, high-rate and long-cycle performance.

[0131] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from these are still within the protection scope of the invention.

Claims

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

2. The double in-situ coated modified iron-rich Na 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, preferably at least one of Mn, Ti, Sn, and Zr; Preferably, the Me2 is at least one of Ni, Cu, Co, Cr, V, Bi, Al, Mg, Ru, Sb, and Bi; Preferably, a is 0.4 to 0.5, and b is 0.2 to 0.4; Preferably, T is at least one of Nb, Ti, Ta and Sb; Preferably, the double in-situ coated modified iron-rich Na x Fe a Me1 b Me2 c In the O2 material, the content of the middle layer is 1wt% to 5wt%, and the content of the outer layer is 1wt% to 10wt%.

3. A double in-situ coated modified iron-rich Na x Fe a Me1 b Me2 c The method for preparing O2 material is characterized in that: Will Na x Fe a Me1 b Me2 c O2 undergoes surface ion exchange transformation in an organic solution containing Formula 1, forming Fe a Me1 b Me2 c O2 intermediate layer, and then calcined with T source to form the outer layer in situ on the intermediate layer to obtain the double in-situ coated modified iron-rich Na x Fe a Me1 b Me2 c O2 materials; The R is a C1-C6 alkyl group, a C2-C6 olefin group, an aromatic group, a carboxyl group or a carboxyalkyl group.

4. The double in-situ coated modified iron-rich Na as claimed in claim 3 x Fe a Me1 b Me2 c The method for preparing O2 material is characterized in that: The mixed raw material containing Na source, Fe source, Me1 source and Me2 source is sintered to obtain the Na x Fe a Me1 b Me2 c O2; Preferably, the sintering process is carried out in an oxygen-containing atmosphere; Preferably, the sintering temperature is 900-1200° C., and the sintering time is 8 h-20 h.

5. The double in-situ coated modified iron-rich Na x Fe a Me1 b Me2 c The method for preparing O2 material is characterized in that: Will Na x Fe a Me1 b Me2 c O2 is preliminarily subjected to surface ion exchange transformation treatment in a solution containing Formula 1 to form the intermediate layer in situ, and then mixed with the T source and calcined to form the outer layer in situ; Alternatively, Na x Fe a Me1 b Me2 c O2 and T source are pre-mixed to obtain a mixture, and then the mixture is placed in a solution containing formula 1 for treatment, so that Na x Fe a Me1 b Me2 c O2 is used for surface ion exchange transformation treatment, and then calcination treatment is carried out together.

6. The double in-situ coated modified iron-rich Na as claimed in claim 3 x Fe a Me1 b Me2 c The method for preparing O2 material is characterized in that: The formula 1 may specifically be at least one of formula 1A, formula 1B, formula 1C, and formula 1D; The R1 and R2 are independently methyl, ethyl, propyl, butyl or phenyl; Preferably, the organic solvent in the organic solution containing Formula 1 comprises at least one of acetone, ethanol, cyclohexane, ethyl acetate, and tetrahydrofuran; Preferably, the concentration of Formula 1 in the solution containing Formula 1 is 0.001 mol / L to 0.005 mol / L; Preferably, the surface ion exchange transformation treatment time is 2 to 20 seconds.

7. The double in-situ coated modified iron-rich Na x Fe a Me1 b Me2 c The method for preparing O2 material is characterized in that: The T source is an oxide of the T element; 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; Preferably, the calcination atmosphere is an oxygen-containing atmosphere; Preferably, the calcination temperature is 600°C to 1000°C; Preferably, the calcination time is 8 to 15 hours; further, it can be 9 to 12 hours.

8. A positive electrode material for a sodium ion battery, characterized in that: Containing the double in-situ coated modified iron-rich Na according to any one of claims 1 to 2 x Fe a Me1 b Me2 c O2 material or double in-situ coated modified iron-rich Na prepared by the preparation method of any one of claims 3 to 7 x Fe a Me1 b Me2 c O2 material.

9. A positive electrode of a sodium ion battery, comprising a current collector and a positive electrode material composited on the surface thereof, characterized in that: The positive electrode material is the positive electrode material according to claim 8.

10. A sodium ion battery, characterized in that: The positive electrode according to claim 9 is included.

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