Highly crystalline sodium-rich rhombohedral structure prussian blue material and method of making

Abstract

The application discloses a high-crystallinity sodium-rich rhombohedral structure Prussian blue material and a preparation method thereof, and comprises the following steps: S1, preparation of a first precursor solution: uniformly dissolving and mixing transition metal salt, doped active metal salt and a chelating agent to obtain the first precursor solution; S2, preparation of a second precursor solution: dissolving ferrocyanide salt in water to obtain the second precursor solution; S3, preparation of a precursor mixture: injecting the first precursor solution and the second precursor solution into a buffer solution containing inorganic sodium salt and a surfactant, and continuously stirring to obtain the precursor mixture; S4, separation of the Prussian blue material, and the Prussian blue material is obtained; wherein the stoichiometric ratio of the transition metal salt, the doped active metal salt, the ferrocyanide salt and the chelating agent is (5-x):x:5:y, y is greater than or equal to 3, and the value range of x is 0.1-5. The application has the characteristics of high crystallinity, excellent electrochemical performance and controllable process.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of sodium ion batteries, in particular to a high-crystallinity sodium-rich rhombohedral structure Prussian blue material and a preparation method thereof. BACKGROUND

[0002] Lithium ion batteries stand out among numerous secondary battery systems due to their high voltage (3.7 V), high energy density (120-160 Wh kg -1 ), high power density (150-300 W kg -1 ), long cycle life (1000-2000 times), no memory effect, low self-discharge rate and other advantages. However, the prospect of large-scale application of lithium ion batteries is still widely questioned. The first problem is the lithium resource problem. The content of lithium element in the earth's crust is only 0.0065%, and it is estimated that the total amount of lithium resources that can be proved to be mined is about 15 million tons (converted into metallic lithium). The second problem is the cost of lithium resources.

[0003] Accordingly, sodium ion batteries (SIBs) have become the focus of attention due to their abundance of sodium resources and their wide distribution around the world. People have been trying to explore various positive electrode materials for SIBs, including Prussian blue analogues (PBAs), polyanion compounds, transition metal oxides and organic materials.

[0004] Among them, PBAs are the most commonly used positive electrode materials for SIBs due to their high theoretical capacity, excellent performance, simple synthesis, high theoretical capacity, good performance, simple synthesis and other advantages. Generally, PBAs can be divided into single electron (SE-PBAs) and double electron (DE-PBAs) transfer types, and the theoretical specific capacity is 85 and 170 mAh g -1 , respectively. For SE-PBAs, M can be Zn, Ni, and M' can be Fe, Co, Mn, while DE-PBAs are characterized by M and M' being Mn, Fe, Co. The large channels and three-dimensional rigid framework of PBAs can realize high Na + deintercalation.

[0005] Na2M[Fe(CN)6] is generally prepared by a rapid precipitation reaction of M 2+ and Na4Fe(CN)6 in an aqueous solution. During the rapid crystallization process, a large number of lattice water molecules exist in the Prussian blue lattice. The existence of water molecules will cause serious side reactions during the charging and discharging process of the battery, resulting in poor cycle performance of the material. At the same time, the transition metal ions will be dissolved and undergo oxidation and reduction during the charging and discharging process, causing the Jahn-Teller effect, lattice distortion and even structure collapse. SUMMARY

[0006] The application aims to provide a preparation method of a high-crystallinity sodium-rich rhombohedral structure Prussian blue material, which has the characteristics of high crystallinity, excellent electrochemical performance and controllable process.

[0007] The application can be implemented by the following technical solutions.

[0008] The application discloses a preparation method of a high-crystallinity rhombohedral structure Prussian blue material, which comprises the following steps.

[0009] S1, preparation of a first precursor solution: transition metal salt, doped active metal salt and chelating agent are dissolved and uniformly mixed to obtain the first precursor solution;

[0010] S2, preparation of a second precursor solution: ferrocyanide salt is dissolved in water to obtain the second precursor solution;

[0011] S3, preparation of a precursor mixture: the first precursor solution and the second precursor solution are injected into a buffer solution containing inorganic sodium salt and surfactant, and continuous stirring is performed to obtain the precursor mixture;

[0012] S4, separation of the Prussian blue material: after the precursor mixture is placed in an ice water bath and settled, the precipitate is obtained through solid-liquid separation, and the Prussian blue material can be obtained through washing and drying;

[0013] The stoichiometric ratio of the transition metal salt, the doped active metal salt, the ferrocyanide salt and the chelating agent is (5-x):x:5:y, y is greater than or equal to 3, and the value range of x is 0.1-5.

[0014] Further, in step S3, the inorganic sodium salt is one or two or more of Na2SO4, NaCl, NaNO3 and Na2CO3, and the concentration of the inorganic sodium salt is 0.2-6 mol / L; a small amount of inorganic sodium salt will result in a small amount of sodium salt in the synthesis environment, and a sodium-poor Prussian blue is synthesized. A large amount of inorganic salt will cause the surfactant to precipitate from the solution, and the material is prone to agglomeration; the surfactant is one or two or more of P123, CTAB and PEG, and the mass of the added surfactant is 20-80% of the mass of Na4Fe(CN)6; the surfactant molecules can divide the solid particle aggregates into fine particles and make them disperse and suspend in the solution, thereby playing a role in promoting uniform dispersion of the solid particles. If the amount of the surfactant is small, the surfactant cannot prevent agglomeration; if the amount of the surfactant is too large, part of the surfactant molecules cannot contact water, and therefore the part of the surfactant does not play a role. The buffer solution is a citric acid / sodium citrate buffer solution.

[0015] Further, in step S3, the injection rate is 1-50 mL / h, and a faster dropping speed will cause the precipitation to be too fast, and the crystallization water of the material will increase; if the dropping speed is too slow, the doped active metal ions are easy to be oxidized. The stirring time is 1-20 h, and a shorter stirring time will cause the reaction to be incomplete, and a longer stirring time will increase the amount of by-products.

[0016] Further, in step S4, the standing and settling time is 10-24 h, and a shorter settling time will cause the crystal growth to be incomplete; a longer settling time will increase the amount of by-products with time; and the drying condition is: drying at 80-150 ℃ for 2-24 h. A lower or shorter time will cause the adsorbed water in the material to be unable to be completely removed; and a higher or longer time will cause the structure of the material to collapse and lose the electrochemical activity.

[0017] Further, in step S1, the chelating agent is one or two or more of ethylenediaminetetraacetic acid (EDTA), ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), ethylenediaminetetraacetic acid tetrasodium salt (EDTA-4Na), nitrilotriacetic acid disodium salt (NTA), or sodium citrate; the transition metal salt is a sulfate salt and / or a chloride salt of a transition metal ion Mn 2+ , Ni 2+ , Co 2+ , or Fe 2+ ; and the doped active metal salt is a sulfate salt or a chloride salt of Co 2+ or Fe 2+ .

[0018] Further, in step S2, the ferrocyanide salt is sodium ferrocyanide Na4Fe(CN)6 and / or potassium ferrocyanide K4Fe(CN)6.

[0019] Further, in step S4, the solvent for washing the precipitate is deionized water and ethanol.

[0020] Further, in step S4, the solid-liquid separation method is suction filtration or centrifugation.

[0021] Further, the crystal point group of the rhombohedral structure Prussian blue material is Fm-3m, and the unit cell parameters are a=10.2971 Å, b=7.3989 Å, and c=7.1651 Å.

[0022] Another aspect of the present application is to protect a Prussian blue material prepared by the above preparation method.

[0023] The rhombohedral structure Prussian blue material with high crystallinity and the preparation method thereof have the following beneficial effects:

[0024] The optimization is carried out in the process of the precipitation generation and the crystal growth, and the Prussian blue material with perfect morphology, high crystallinity and low water content is formed. The Prussian blue precipitate is stable under neutral conditions, and the cubic morphology is damaged by acid / alkali. Therefore, the agglomeration phenomenon is inevitable in the process of adding citric acid / sodium citrate buffer and chelating agent to generate the Prussian blue precipitate with perfect cubic morphology. The surfactant is added to promote the uniform dispersion of the Prussian blue particles, and the exposed reaction sites are more in the charging and discharging process, so that the charging and discharging capacity is higher. After the precipitation is generated, the crystal needs to grow for a long time, and the slowly grown crystal has the characteristics of low defects and high crystallinity. Therefore, after the reaction is completed, it is placed in a low temperature condition to stand, so that the crystal grows slowly, and the process controllability is high.

[0025] To solve the problem of structural instability caused by the oxidation and reduction of transition metals in the charging and discharging process, the second transition metal (T, T=Fe, Co) is introduced in the patent to selectively remove a part of the transition metal ions. The active oxidation and reduction site of T can endow the skeleton with superior performance and can also play a role in stabilizing the material skeleton. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 The SEM graph of Example 1;

[0027] Figure 2 The XRD graph of Example 1;

[0028] Figure 3 The first charge-discharge curve of Example 1;

[0029] Figure 4 The cycle curve of Example;

[0030] Figure 5 The XRD graph of Example 2;

[0031] Figure 6 The first charge-discharge curve of Example 2;

[0032] Figure 7 The cycle curve of Example 2;

[0033] Figure 8 The first charge-discharge curve of Example 3;

[0034] Figure 9 The cycle curve of Example 4. DETAILED DESCRIPTION

[0035] In order to make the person skilled in the art better understand the technical scheme of the present application, the product of the present application will be further described in detail below in combination with examples.

[0036] The application discloses a preparation method of a high-crystallinity pruss blue material with a rhombohedron structure.

[0037] S1, preparation of a first precursor solution: transition metal salt, doped active metal salt and chelating agent are dissolved and uniformly mixed to obtain the first precursor solution;

[0038] S2, preparation of a second precursor solution: ferrocyanide salt is dissolved in water to obtain the second precursor solution;

[0039] S3, preparation of a precursor mixture: the first precursor solution and the second precursor solution are injected into a buffer solution containing inorganic sodium salt and surfactant, and continuous stirring is carried out to obtain the precursor mixture;

[0040] S4, separation of the pruss blue material: after the precursor mixture is placed in an ice water bath and settled, the precipitate is obtained through solid-liquid separation, and the pruss blue material can be obtained through washing and drying;

[0041] The stoichiometric ratio of the transition metal salt, the doped active metal salt, the ferrocyanide salt and the chelating agent is (5-x):x:5:y, y is greater than or equal to 3, and the value range of x is 0.1-5.

[0042] Further, in step S3, the inorganic sodium salt is one or more than two of Na2SO4, NaCl, NaNO3 and Na2CO3, and the concentration of the inorganic sodium salt is 0.2-6 mol / L; too little inorganic sodium salt will result in too little sodium salt in the synthesis environment, and a sodium-poor pruss blue is synthesized. Too much inorganic salt will cause the surfactant to precipitate from the solution, and the material is prone to agglomeration; the surfactant is one or more than two of P123, CTAB and PEG, and the mass of the added surfactant is 20-80% of the mass of Na4Fe(CN)6; the surfactant is prone to settlement in water, the molecules of the surfactant can divide the solid particle aggregates into fine particles, and the surfactant can make the fine particles disperse and suspend in the solution, thereby promoting the uniform dispersion of the solid particles. Too little surfactant cannot prevent agglomeration; too much surfactant cannot play a role because part of the surfactant molecules cannot contact water. The buffer solution is a citric acid / sodium citrate buffer solution.

[0043] Further, in step S3, the injection rate is 1-50 mL / h; too fast dropping speed will cause too fast precipitation, and the crystal water of the material will increase; too slow dropping speed will cause the doped active metal ions to be easily oxidized; the continuous stirring time is 1-20 h; too short stirring time will cause incomplete reaction, and too long stirring time will increase the amount of by-products.

[0044] Further, in step S4, the standing and settling time is 10-24 h, a shorter settling time can result in incomplete crystal growth, and a longer settling time can result in more by-products over time; the drying condition is at 80-150℃ for 2-24 h, a lower or shorter time can result in incomplete removal of adsorbed water in the material, and a higher or longer time can result in collapse of the structure of the material and loss of electrochemical activity.

[0045] Further, in step S1, the chelating agent is one or two or more of ethylenediaminetetraacetic acid (EDTA), ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), ethylenediaminetetraacetic acid tetrasodium salt (EDTA-4Na), nitrilotriacetic acid disodium salt (NTA), or sodium citrate; the transition metal salt is a sulfate and / or chloride salt of a transition metal ion Mn 2+ , Ni 2+ , Co 2+ , or Fe 2+ ; and the doped active metal salt is a sulfate or chloride salt of Co 2+ or Fe 2+ .

[0046] Further, in step S2, the ferrocyanide salt is sodium ferrocyanide Na4Fe(CN)6 and / or potassium ferrocyanide K4Fe(CN)6.

[0047] Further, in step S4, the solvent for washing the precipitate is deionized water and ethanol.

[0048] Further, in step S4, the solid-liquid separation method is suction filtration or centrifugation.

[0049] Further, the rhombohedral Prussian blue material has a crystal point group of Fm-3m, and a unit cell parameter of a = 10.2971 Å, b = 7.3989 Å, and c = 7.1651 Å.

[0050] Another aspect of the present application is to protect a Prussian blue material prepared by the above method.

[0051] In the present application, the doped active metal ion in the doped active metal salt has the following effects:

[0052] ②The metal ion can participate in electrochemical redox reactions and provide capacity, thereby increasing the specific capacity.

[0053] ②The metal ion competes with the original metal ion during synthesis, making the generation reaction of the target product slower, reducing the generation of Fe(CN)6 vacancies and lattice water molecules during synthesis, and improving the cycle stability.

[0054] ③ The metal ions occupy the same sites as the metal ions in the original Prussian blue material, and can replace some unstable ions, thus significantly improving its cycle stability.

[0055] Example 1

[0056] This embodiment relates to a highly crystalline, sodium-rich, rhombohedral Prussian blue material, the preparation method of which includes the following steps:

[0057] (1) Under continuous stirring, MnSO4, FeSO4, and sodium citrate (C6H5Na3O7·6H2O) were dissolved in water to obtain solution A;

[0058] (2) Dissolve Na4Fe(CN)6·10H2O in water to obtain solution B;

[0059] (3) Mix solution A and solution B at a rate of 10 mL / h -1 The solution was injected at a rate that allowed it to dissolve in 1 mol / L Na₂SO₄ and 2 g of P123 surfactant. During the reaction, sodium citrate / citric acid buffer solution was added. The mixture was stirred continuously for 5 hours to obtain mixture C.

[0060] (4) Place the resulting mixture in an ice-water bath and let it stand for 12 h;

[0061] (5) The mixture C was separated into solid and liquid by centrifugation to obtain a precipitate, which was then repeatedly washed with deionized water and ethanol and dried at 85°C for 24 h to obtain the Prussian blue target product.

[0062] In step (3) above, the stoichiometric ratio of reactants MnSO4, FeSO4, Na4Fe(CN)6·10H2O, and sodium citrate (C6H5Na3O7·6H2O) is maintained at (5-x):x:5:5. Specific amounts added are detailed in Table 1. Among them, Fe... 2+ Compared to Mn 2+ The doping ratio at each site is converted to a percentage using x / 5.

[0063] Table 1. Ingredient list of the 6 samples in Example 1

[0064]

[0065] The samples from Example 1 were subjected to SEM testing, XRD testing, initial charge-discharge testing, and cycle performance testing. The results are as follows: Figures 1-4 As shown:

[0066] Figure 1 Fe in Example 1 2+ NaMn with doping ratios of 0%, 5%, 10%, 15%, 20%, and 25% (5-x) / 5 Fex / 5 SEM data of HCF material, which always keeps cubic morphology.

[0067] Figure 2 Fe 2+ NaMn (5-x) / 5 Fe x / 5 XRD pattern of HCF material, which always keeps sodium-rich Prussian blue structure, the diffraction peaks of (220), (420), (440) and (620) are all split into two peaks with similar intensity, according to the relevant knowledge of crystallography, this structure can be attributed to rhombohedral structure, which is derived from face-centered cubic structure along <111> crystal plane to store more Na + .

[0068] Figure 3 NaMn (5-x) / 5 Fe x / 5 First cycle charge-discharge data of HCF. Fe 2+ NaMn (5-x) / 5 Fe x / 5 HCF, the first cycle discharge specific capacity is 133, 134, 135, 139, 148, 139.5 mAh g -1 ; wherein, Fe 2+ NaMn (5-x) / 5 Fe x / 5 HCF, the discharge specific capacity is improved the most.

[0069] Figure 4 NaMn (5-x) / 5 Fe x / 5 Electrochemical cycle data of HCF. The material of this example can achieve good long cycle performance. Fe 2+ NaMn (5-x) / 5 Fe x / 5 HCF, the cycle 300 cycle capacity retention rate is 50%, 56%, 54%, 66%, 79%, 64%, wherein Fe 2+ NaMn (5-x) / 5 Fe x / 5 HCF, the cycle stability is more excellent than other doping ratios;

[0070] Example 2

[0071] This example relates to a high crystallinity sodium-rich rhombohedral structure Prussian blue material, the preparation method comprises the following steps:

[0072] (1) Under continuous stirring, NiCl2, CoCl2 and ethylenediaminetetraacetic acid (EDTA) are dissolved in water to obtain solution A;

[0073] (2) Dissolve K4Fe(CN)6·10H2O in water to obtain solution B;

[0074] (3) Mix solution A and solution B at a rate of 25 mL / h -1 The solution was added at an injection rate of 5 mol / L NaCl and 1 g of P123 surfactant. During the reaction, sodium citrate / citric acid buffer solution was added. The mixture was stirred continuously for 20 h to obtain mixture C.

[0075] (4) Place the resulting mixture in an ice-water bath and let it stand for 16 h;

[0076] (5) The mixture C was separated into solid and liquid by vacuum filtration to obtain a precipitate, which was repeatedly washed with deionized water and ethanol and dried at 140°C for 3 h to obtain the Prussian blue target product.

[0077] In step (3) above, the stoichiometric ratio of the reactants NiSO4, CoCl2, K4Fe(CN)6·10H2O, and ethylenediaminetetraacetic acid (EDTA) is maintained at (5-x):x:5:3. The specific amounts added are detailed in Table 2; among which Co... 2+ Compared to Ni 2+ The doping ratio at each site is converted to a percentage using x / 5.

[0078] Table 2. Ingredient list of the three samples in Example 2

[0079]

[0080] The samples from Example 2 were subjected to XRD tests, initial charge-discharge tests, and cycle performance tests. The results are as follows: Figures 5-7 As shown:

[0081] Figure 5 The XRD patterns of NaNiFeHCF synthesized with 0%, 10% and 20% FeSO4 in Example 2 are shown. The diffraction peaks (220), (420), (440) and (620) are split into two peaks with similar intensities, and the surface material maintains the sodium-rich Prussian blue structure.

[0082] Figure 6 The first-cycle charge-discharge data for NaNiFeHCF in Example 2 are shown. The first-cycle discharge specific capacities of NaNiFeHCF synthesized with doping ratios of 0%, 10%, and 20% FeSO4 are 76.8, 79.8, and 83.5 mAh g, respectively. -1Among them, NaNiFeHCF synthesized with 20% FeSO4 doping ratio has the highest discharge specific capacity.

[0083] Figure 7 The cycling data for NaNiFeHCF in Example 2 are shown. NaNiFeHCF synthesized with doping ratios of 0%, 10%, and 20% FeSO4 showed capacity retention rates of 89%, 92%, and 94% after 1000 cycles, respectively, demonstrating a significant improvement in cycling performance.

[0084] Example 3

[0085] This embodiment relates to a highly crystalline, sodium-rich, rhombohedral Prussian blue material, the preparation method of which includes the following steps:

[0086] (1) Under continuous stirring, CoSO4, FeSO4, and disodium aminotriacetate (NTA) were dissolved in water to obtain solution A;

[0087] (2) Dissolve Na4Fe(CN)6·10H2O in water to obtain solution B;

[0088] (3) Mix solution A and solution B at a rate of 45 mL / h -1 The solution was added at the injection rate of 1.5 mol / L Na2SO4 and 1.5 g CTAB surfactant, and sodium citrate / citric acid buffer solution was added during the reaction, and the mixture was stirred for 10 h to obtain mixture C.

[0089] (4) Place the obtained mixture in an ice water bath and let it stand for 20 hours;

[0090] (5) The mixture C was separated into solid and liquid by centrifugation to obtain a precipitate, which was then repeatedly washed with deionized water and ethanol and dried at 110°C for 20 h to obtain the Prussian blue target product.

[0091] In step (3) above, the stoichiometric ratio of the reactants CoSO4, FeSO4, Na4Fe(CN)6·10H2O, and disodium aminotriacetate (NTA) is maintained at (5-x):x:5:5. Specific amounts added are detailed in Table 3; among which Fe... 2+ Compared to Co 2+ The doping ratio at each site is converted to a percentage using x / 5.

[0092] Table 3. Ingredient list of the three samples in Example 3

[0093]

[0094] The samples from Example 3 were subjected to initial charge-discharge tests and cycle performance tests, and the results are as follows: Figures 8-9 As shown:

[0095] Figure 8 The first week charge-discharge data of NaCoFeHCF in Example 3. The NaCoFeHCF synthesized by doping 0%, 10% and 20% FeSO4, the first week discharge specific capacity is: 138.1, 129.5, 120.2 mAh g -1 , the NaCoFeHCF synthesized by doping 20% FeSO4 has the highest capacity.

[0096] Figure 9 The cycle data of NaCoFeHCF in Example 3. The cycle performance of NaCoFeHCF synthesized by doping 0%, 10% and 20% FeSO4 is obviously improved, and the capacity retention rate after 1000 cycles is: 80%, 85% and 89% respectively.

[0097] The above examples are only specific embodiments of the present application, which are described in detail and in detail, but cannot be understood as a limitation on the scope of the patent of the present application. It should be noted that for those skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, and these obvious alternative forms all belong to the protection scope of the present application.

Claims

1. A method for preparing a high-crystallinity rhombohedral Prussian blue material, characterized by The method comprises the following steps: S1, preparation of a first precursor solution: uniformly dissolving and mixing a transition metal salt, a doped active metal salt and a chelating agent to obtain the first precursor solution; S2, preparation of a second precursor solution: dissolving a ferrocyanide salt in water to obtain the second precursor solution; S3, preparation of a precursor mixture: injecting the first precursor solution and the second precursor solution into a buffer solution containing an inorganic sodium salt and a surfactant, and continuously stirring to obtain the precursor mixture, the concentration of the inorganic sodium salt being 0.2-6 mol / L, the injection rate being 1-50 mL / h, and the continuous stirring time being 1-20 h; S4, separation of the Prussian blue material: after the precursor mixture is placed in an ice water bath and allowed to settle, the precipitate is obtained through solid-liquid separation, and the Prussian blue material is obtained after washing and drying; the settling time is 10-24 h, and the drying condition is: drying at 80-150℃ for 2-24 h; The stoichiometric ratio of the transition metal salt, the doped active metal salt, the ferrocyanide salt and the chelating agent is (5-x):x:5:y, y≥3, and the value range of x is 0-5.

2. The method of claim 1, wherein the method is characterized by: In step S3, the inorganic sodium salt is one or two or more of Na2SO4, NaCl, NaNO3 and Na2CO3; and the buffer solution is a citric acid / sodium citrate buffer solution.

3. The method of claim 1, wherein the method is characterized by: In step S1, the chelating agent is one or two or more of ethylenediaminetetraacetic acid, ethylenediaminetetraacetic acid disodium salt, ethylenediaminetetraacetic acid tetrasodium salt, nitrilotriacetic acid disodium salt, or sodium citrate; the transition metal salt is a sulfate and / or a chloride salt of a transition metal ion Mn 2+ , Ni 2+ , Co 2 + , or Fe 2+ ; and the doped active metal salt is a sulfate or a chloride salt of Co 2+ or Fe 2+ .

4. The method of claim 1, wherein the method is characterized by: In step S2, the ferrocyanide salt is sodium ferrocyanide Na4Fe(CN)6 and / or potassium ferrocyanide K4Fe(CN)6.

5. The method of claim 1, wherein the method is characterized by: In step S4, the solvent for washing the precipitate is deionized water and ethanol.

6. The method of claim 1, wherein the method is characterized by: In step S4, the solid-liquid separation method is suction filtration or centrifugation.

7. The method of claim 1, wherein the method is characterized by: The rhombohedral structure Prussian blue material has a crystal point group of Fm-3m and a unit cell parameter of a=10.2971 Å, b=7.3989 Å and c=7.1651 Å.

8. A Prussian blue material characterized in that: The Prussian blue material is prepared by the method in any one of claims 1-7.

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