Metal Cyanometallate Synthesis Method

Abstract

A method is provided for synthesizing metal cyanometallate (MCM). The method provides a solution of AXM1Y(CN)Z; where “A” is selected from a first group of metals and M1 is selected from a second group of metals. The method adds a material including M2 to the solution to form a liquid phase material that may be either a suspension or a solution. M2 is selected from the second group of metals. The method adds acid to the liquid phase material. The addition of acid to the liquid phase material decomposes the M2 material into M2-ions. Simultaneous with the addition of the acid, a precipitate of ANM1PM2Q(CN)R. FH2O is formed, where N is in a range of 1 to 2. A variation of the above-described synthesis method is also provided.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to electrochemical cells and, more particularly, to a method for synthesizing metal cyanometallates (MCMs) for use in battery electrodes.

2. Description of the Related Art

Large-capacity, cost-effective energy storage is the transformational technology needed to enable the large scale integration of renewable energy (e.g. wind, solar). The rechargeable battery offers efficient electrical energy storage (EES). The lithium (Li)-ion battery has been the leading option based on its performance, but it is too expensive for large-scale EES. Therefore, it is necessary to develop superior low cost, high-performance electrode materials to reduce the cost of lithium-ion batteries or to develop a new system of rechargeable metal-ion batteries to replace lithium-ion batteries.

Sodium and potassium-ion batteries have attracted a great deal of attention recently because the reserves of sodium/potassium are more plentiful than lithium in the crust of earth. This abundance makes it possible to develop low cost batteries for EES. However, it is impractical to copy the structures of lithium-ion (Li⁺)-host compounds for use as sodium-ion (Na⁺) or potassium-ion (K⁺)-host compounds. Sodium and potassium ions are much larger than lithium ions and severely distort the structures of the Li⁺-host compounds. Thus, it is very important that new Na⁺/K⁺-host materials be developed, with a large interstitial space into which sodium/potassium-ions can easily and reversibly move. Na⁺/K⁺-ions have been inserted into metal cyanide compounds. Transition metal hexacyanoferrates (TMHCFs) with large interstitial space have been investigated as cathode materials for rechargeable lithium-ion batteries [1,2], sodium-ion batteries [3,4], and potassium-ion batteries [5]. With an aqueous electrolyte containing the proper alkali-ions or ammonium-ions, copper and nickel hexacyanoferrates [(Cu,Ni)-HCFs] demonstrated robust cycling life with 83% capacity retention after 40,000 cycles at a charge/discharge current of 17C (1C=150 milliamp hours per gram (mAh/g) [6-8]. In spite of this, the materials demonstrated low capacities and energy densities because: (1) only one sodium-ion can be inserted/extracted reversibly into/from Cu-HCF or Ni-HCF per formula unit, and (2) these TM-HCFs electrodes must be operated below 1.23 V due to the water electrochemical window. The electrochemical window of a substance is the voltage range between which the substance is neither oxidized nor reduced. This range is important for the efficiency of an electrode, and once out of this range, water becomes electrolyzed, spoiling the electrical energy intended for another electrochemical reaction.

In order to compensate for these shortcomings, manganese hexacyanoferrate (Mn-HCF) and iron hexacyanoferrate (Fe-HCF) were used as cathode materials in non-aqueous electrolyte [9, 10]. When assembled with a sodium-metal anode, Mn-HCF and Fe-HCF electrodes cycled between 2.0V and 4.2 V delivered capacities of ˜110 mAh/g. Very recently, FeFe(CN)₆.4H₂O and Na_0.61Fe[Fe(CN)₆]_0.94 were reported to exhibit high energy density and power density and good stability during cycling [11, 12].

FIG. 1 is a diagram representing a metal cyanometallate (MCM) open framework (prior art). All the compounds mentioned above can be categorized as MCMs with the general formula A_XM1_MM2_N(CN)_Z. The open framework structure of the transition metal MCM facilitates both rapid and reversible intercalation processes for alkali and alkaline ions (A_X). Some typical “A” metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), calcium (Ca), strontium (Sr), barium (Ba), silver (Ag), aluminum (Al), magnesium (Mg), etc. Typical M1, M2 materials include titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), niobium (Nb), ruthenium (Ru), tin (Sn), indium (In), cadmium (Cd), Ca, and Mg.

The MCM capacity is determined by the available A-sites in the compounds into which the alkali or alkaline ions can be inserted reversibly in the range of working voltages. From the electric neutrality point of view, the valences of M1 and M2 mainly contribute to the amount of the available A-sites. For example, 2 sodium-ions can be inserted/extracted into/from Na₂MnFe(CN)₆ between 2-4 V vs. Na° because the valences of Mn- and Fe-ions can change between +2 and +3, and its theoretical capacity is 171 mAh/g. Noteworthy is the fact that a greater number of metal-ions, “A”, result in MCMs able to deliver a higher capacity for use in a metal-ion battery with a non-metal counter electrode. In addition, in order to neutralize charges, the transition metals are kept at low valances. In typical MCM materials (A_XM1_NM2_M(CN)_Z.dH₂O) the value of X is less than or equal to 1.

It would be advantageous if a synthesis process existed to increase the number of “A” metal-ions in MCM materials.

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SUMMARY OF THE INVENTION

Disclosed herein are methods for synthesizing metal cyanometallates (MCMs) with the general formula A_NM1_PM2_Q(CN)_R such that the value of N is greater than that obtained using conventional processes, for use in metal-ion batteries. In some cases, water molecules can occupy the interstitial or lattice positions to form A_NM1_PM2_Q(CN)_R. FH₂O. Typically, “A” may be lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), calcium (Ca), strontium (Sr), barium (Ba), silver (Ag), aluminum (Al), magnesium (Mg), or combinations thereof. M1 and M2 may be titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), niobium (Nb), ruthenium (Ru), tin (Sn), indium (In), cadmium (Cd), Ca, Mg, strontium (Sr), or barium (Ba). The synthesis methods increase the available interstitial positions for “A”-ion intercalation into MCM compounds and improve MCM capacities for their use in metal-ion batteries.

In one aspect, the materials are synthesized with a solution containing M1_a2(CN)_z2^b--ions and materials containing M2. Generally, a high concentration of “A”-ions is included in the solution. An acid is added into the synthesis solution to decompose the M2-containing materials to form M2-ions that interact with M1_a2(CN)_z2^b--ions to form a framework. The synthesis may be carried out at a controlled temperature from 0 Centigrade (C) to 100° C. in an inert atmosphere such as nitrogen and/or argon. To neutralize charges, “A”-ions occupy the interstitial positions of the framework. A precipitation is formed that can be vacuum dried at a temperature in the range of 0° C. to 200° C.

Accordingly, a first method is provided for synthesizing metal cyanometallate, (MCM). The method provides a solution of A_XM1_Y(CN)_Z;

where “A” is selected from a first group of metals;

where M1 is selected from a second group of metals;

where X is in the range of 0 to 10;

where Y is in the range of 1 to 10; and,

where Z is in the range of 1 to 10.

The method adds a material including M2 to the solution to form a liquid phase material that may be either a suspension or a solution. M2 is selected from the second group of metals. The material may be pure M2 and it may be an oxide, sulfide, sulfite, carbonate, cyanide, or fluoride compound. The method adds acid to the liquid phase material. The addition of acid to the liquid phase material decomposes the M2 material into M2-ions. In one aspect, the method additionally adds a reducing agent to the liquid phase material. Simultaneous with the addition of the acid, a precipitate of A_NM1_PM2_Q(CN)_R.FH₂O is formed:

where N is in the range of 1 to 2;

where P is less than or equal to 2;

where F is in the range of 0 to 20;

where Q is less than or equal to 2; and,

where R is less than or equal to 6.

Additional details of the above-described first method, as well as a second method, are presented below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram representing a metal cyanometallate (MCM) open framework (prior art).

FIG. 2 is a diagram generally depicting a process for synthesizing MCM with a high content of “A”-ions.

FIG. 3 is the energy-dispersive X-ray spectroscopy (EDX) spectrum of a Na₂FeFe(CN)₆ sample prepared by the above-described second precipitation method.

FIGS. 4 and 5 are charge and discharge curves of Na₂FeFe(CN)₆ prepared by Method 1 and Method 2, respectively.

FIG. 6 is a flowchart illustrating a method for synthesizing metal cyanometallate

FIG. 7 is a flowchart illustrating another method for synthesizing MCM.

DETAILED DESCRIPTION

Disclosed herein are methods for the synthesis of metal cyanometallates (MCMs) or A_NM1_PM2_Q(CN)_R.FH₂O as the electrode in rechargeable metal-ion batteries. The metal-ions can be from metal lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), calcium (Ca), and/or magnesium (Mg). Some examples of transition metal M1 and M2 include titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn) iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), niobium (Nb), ruthenium (Ru), tin (Sn), indium (In), and cadmium (Cd). In general, the precipitation method is used to synthesize

MCM materials as electrodes. However, noteworthy is the fact that metal-ions with low valence are susceptible to being oxidized, resulting in low-“A” containing MCM. The “A” deficiency causes a low capacity when MCM is used as an electrode in a rechargeable “A”-ion battery with a counter electrode without “A”-ions. For example, Prussian white, Na₂FeFe(CN)₆ is expected to be synthesized with ferrous ions and ferrocyanide. But, the fact is that only Prussian blue, Na_XFeFe(CN)₆ (x<1), can conventionally be obtained because ferrous ions are easily oxidized.

FIG. 2 is a diagram generally depicting a process for synthesizing MCM with a high content of “A”-ions. To obtain a MCM electrode with a high capacity for use as a battery electrode, new approaches must be adopted to reduce the possibility of metal oxidation during synthesis. Disclosed herein is a “simultaneous” synthesis process where M2-ions, from the decomposition of stable M2 materials, immediately react with an M1-cyanide with a minimum possibility of oxidization. A reductive environment is used to protect the M1-ions from oxidizing.

To clarify the process, two examples are given here.

Method 1:

0.7 grams (g) iron (Fe) powder was mixed with 100 milliliters (mL) 0.02 moles (mol) L⁻¹ Na₄Fe(CN)₄-101120 solution to form a suspension. As used herein, a suspension is understood to be a heterogeneous mixture containing solid particles that are sufficiently large for sedimentation. The internal phase (solid) is dispersed throughout the external phase (fluid) through mechanical agitation, with the use of certain excipients or suspending agents. Generally, without agitation, the solid particles will eventually settle. A solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. The solvent does the dissolving. The solution more or less takes on the characteristics of the solvent including its phase, and the solvent is commonly the major fraction of the mixture. The concentration of a solute in a solution is a measure of how much of that solute is dissolved in the solvent.

Continuing the description of the method, 10 mL 0.1 mol L⁻¹ hydrogen chloride (HCl) solution was added into the suspension and stirred at room temperature. The Fe was decomposed by the to form Fe²⁺, which reacted with ferrocynide immediately to form Na₂FeFe(CN)₂. The obtained precipitate was collected by centrifuge or filtration, washed by water, and dried at 0-200° C. in a vacuum oven for 6-20 hours.

Method 2:

In a typical synthesis, 2 millimoles (mmol) Na₄Fe(CN)₄.10H₂O and 0.1 mL HCl (37%) was dissolved in 500 mL of distilled water to obtain a homogenous solution. The solution was maintained at 0400° C. for 0.5-60 hours being vigorous stirred to obtain Na₂FeFe(CN)₆. In the reaction, Fe²⁺-ions result from the decomposition of ferrocynide by HCl. The precipitate was collected by filtration in air, washed by water, and dried at 0-200° C. in a vacuum over for 0.5-60 hours.

FIG. 3 is the energy-dispersive X-ray spectroscopy (EDX) spectrum of a Na₂FeFe(CN)₆ sample prepared by the above-described second precipitation method. A high concentration of Na was observed in the as-prepared material. Silver (Ag) was coated on the material to increase conductivity for scanning electron microscope (SEM) characterization.

FIGS. 4 and 5 are charge and discharge curves of Na₂FeFe(CN)₆ prepared by Method 1 and Method 2, respectively. Two plateaus at 3.0 and 3.4 V during charge and discharge were observed with a very small polarization. Na₂FeFe(CN)₆ prepared by precipitation in aqueous solution delivers a high capacity of 120-145 milliamp hours per gram (mAh/g).

FIG. 6 is a flowchart illustrating a method for synthesizing metal cyanometallate (MCM). Although the method is depicted as a sequence of numbered steps for clarity, the numbering does not necessarily dictate the order of the steps. It should be understood that some of these steps may be skipped, performed in parallel, or performed without the requirement of maintaining a strict order of sequence. Generally however, the method follows the numeric order of the depicted steps. The method starts at Step 600.

Step 602 provides a solution of A_XM1_Y(CN)_Z;

where “A” is selected from a first group of metals;

where M1 is selected from a second group of metals;

where X is in the range of 0 to 10;

where Y is in the range of 1 to 10; and,

where Z is in the range of 1 to 10.

Some examples from the first group of metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), calcium (Ca), strontium (Sr), barium (Ba), silver (Ag), aluminum (Al), magnesium (Mg), and combinations thereof.

Step 604 adds a material including M2 to the solution to form a liquid phase material that is either a suspension or a solution. M2 is selected from the second group of metals. The M2 material may be pure M2 or a compound of oxides, sulfides, sulfites, carbonates, cyanides, fluorides, or a combination of these compounds. M1 and M2 are each independently selected from the second group of metals, some examples of which include titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), niobium (Nb), ruthenium (Ru), tin (Sn), indium (In), cadmium (Cd), Ca, Mg, strontium (Sr), and barium (Ba). Note: the materials listed above are examples used to illustrate the invention and do not necessarily represent an exhaustive list of all possible materials.

Step 606 adds an acid to the liquid phase material. Some examples of acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), sulfurous acid (H₂SO₃), acetic acid (CH₃COOH), formic acid (CHOOH), oxalic acid (C₂H₂O₄), and ascorbic acid. In one aspect, adding the acid to the liquid phase material in Step 606 includes the acid decomposing the M2 material into M2-ions.

Optionally, adding the acid to the liquid phase material in Step 606 may additionally include adding a reducing agent to the liquid phase material. Some examples of reducing agents include sodium borohydride, sodium hyposulfite, sodium sulfite, and polyvinylpyrrolidon. In one aspect, adding the acid to the liquid phase material includes adding the acid to the liquid phase material in an inert atmosphere. For example, the process may be performed in an atmosphere of argon or nitrogen.

Simultaneously with the addition of the acid in Step 606, Step 608 forms a precipitate of A_NM1_PM2_Q(CN)_R.FH₂O;

where N is in the range of 1 to 2;

where P is less than or equal to 2;

where F is in the range of 0 to 20;

where Q is less than or equal to 2; and,

where R is less than or equal to 6.

In one aspect, Step 610 dries the precipitate in a vacuum environment at a temperature in the range between 0 and 200 degrees C.

FIG. 7 is a flowchart illustrating another method for synthesizing MCM. The method begins at Step 700. Step 702 provides a solution of A_XM1_Y(CN)_Z;

where “A” is selected from a first group of metals;

where M1 is selected from a second group of metals;

where X is in the range of 0 to 10;

where Y is in the range of 1 to 10; and,

where Z is in the range of 1 to 10.

As mentioned above in the description of FIG. 6, examples from the first group of metals include Li, Na, K, Rb, Cs, Ca, Sr, Ba, Ag, Al, Mg, and combinations thereof. Examples from the second group of metals include Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Nb, Ru, Sn, In, Cd, Ca, Mg, Sr, and Ba.

Step 704 adds an acid to the solution. In one aspect, Step 704 includes the following substeps. In step 704a the acid decomposes the A_XM1_Y(CN)_Z. Step 704b releases M1-ions into the solution. Some examples of acids include HCl, H₂SO₄, H₂SO₃, CH₃COOH, CHOOH, C₂H₂O₄, and ascorbic acid. In one aspect, Step 704 optionally includes adding a reducing agent to the solution, in addition to the acid. Some examples of reducing agents include sodium borohydride, sodium hyposulfite, sodium sulfite, and polyvinylpyrrolidon. In another aspect, the acid is added to the solution in an inert atmosphere. Note: the materials listed above are examples used to illustrate the invention and do not necessarily represent an exhaustive list of all possible materials.

Simultaneously with the addition of the acid in Step 704, Step 706 forms a precipitate of A_NM1_D(CN)_E.GH₂O;

where N is in the range of 1 to 2;

where C is in the range of 0 to 20;

where D is less than or equal to 2; and,

where E is less than or equal to 6.

In one aspect, Step 708 dries the precipitate in a vacuum environment at a temperature in the range between 0 and 200 degrees C.

Processes for the synthesis of MCM have been provided. Examples of particular materials and process details have been presented to illustrate the invention. However, the invention is not limited to merely these examples. Other variations and embodiments of the invention will occur to those skilled in the art.

Claims

1. A method for synthesizing metal cyanometallate (MCM), the method comprising: providing a solution of AXM1Y(CN)Z; where “A” is selected from a first group of metals;where M1 is selected from a second group of metals;where X is in a range of 0 to 10;where Y is in a range of 1 to 10;where Z is in a range of 1 to 10;adding a material including M2 to the solution to form a liquid phase material selected from a group consisting of a suspension and a solution; where M2 is selected from the second group of metals;adding an acid to the liquid phase material;simultaneously with the addition of the acid, forming a precipitate of ANM1PM2Q(CN)R.FH2O; where N is in a range of 1 to 2;where P is less than or equal to 2;where F is in a range of 0 to 20;where Q is less than or equal to 2; and,where R is less than or equal to 6.

2. The method of claim 1 wherein adding the acid to the liquid phase material includes the acid decomposing the material including M2 into M2-ions.

3. The method of claim 1 wherein adding the acid to the liquid phase material includes adding an acid selected from a group consisting of hydrochloric acid (HCl), sulfuric acid (H2SO4), sulfurous acid (H2SO3), acetic acid (CH3COOH), formic acid (CHOOH), oxalic acid (C2H2O4), and ascorbic acid.

4. The method of claim 1 wherein adding the acid to the liquid phase material includes additionally adding a reducing agent to the liquid phase material.

5. The method of claim 4 wherein adding the reducing agent to the liquid phase material includes adding a reducing agent selected from a group consisting of sodium borohydride, sodium hyposulfite, sodium sulfite, and polyvinylpyrrolidon.

6. The method of claim 1 wherein adding the acid to the liquid phase material includes adding the acid to the liquid phase material in an inert atmosphere.

7. The method of claim 1 wherein the first group of metals is selected from a group consisting of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), calcium (Ca), strontium (Sr), barium (Ba), silver (Ag), aluminum (Al), magnesium (Mg), and combinations thereof.

8. The method of claim 1 wherein M1 and M2 are each independently selected from the second group of metals consisting of titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), niobium (Nb), ruthenium (Ru), tin (Sn), indium (In), cadmium (Cd), Ca, Mg, strontium (Sr), and barium (Ba).

9. The method of claim 1 further comprising: drying the precipitate in a vacuum environment at a temperature in a range between 0 and 200 degrees Centigrade (C).

10. The method of claim 1 wherein adding the material including M2 to the solution includes adding a material selected from a group consisting of pure M2 and compounds selected from a group consisting of oxides, sulfides, sulfites, carbonates, cyanides, and fluorides.

11. A method for synthesizing metal cyanometallate (MCM), the method comprising: providing a solution of AXM1Y(CM)Z; where “A” is selected from a first group of metals;where M1 is selected from a second group of metals;where X is in a range of 0 to 10;where Y is in a range of 1 to 10;where Z is in a range of 1 to 10;adding an acid to the solution;simultaneously with the addition of the acid, forming a precipitate of ANM1D(CN)E.GH2O; where N is in a range of 1 to 2;where G is in a range of 0 to 20;where D is less than or equal to 2; and,where E is less than or equal to 6.

12. The method of claim 11 wherein adding the acid to the solution includes: the acid decomposing the AXM1Y(CN)Z; and,releasing M1-ions into the solution.

13. The method of claim 11 wherein adding the acid to the solution includes adding an acid selected from a group consisting of hydrochloric acid (HCl), sulfuric acid (H2SO4), sulfurous acid (H2SO3), acetic acid (CH3COOH), formic acid (CHOOH), oxalic acid (C2H2O4), and ascorbic acid.

14. The method of claim 11 wherein adding the acid to the solution includes additionally adding a reducing agent to the solution.

15. The method of claim 14 wherein adding the reducing agent to the solution includes adding a reducing agent selected from a group consisting of sodium borohydride, sodium hyposulfite, sodium sulfite, and polyvinylpyrrolidon.

16. The method of claim 11 wherein adding the acid to the solution includes adding the acid to the solution in an inert atmosphere.

17. The method of claim 11 wherein the first group of metals is selected from a group consisting of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), calcium (Ca), strontium (Sr), barium (Ba), silver (Ag), aluminum (Al), magnesium (Mg), and combinations thereof.

18. The method of claim 11 wherein the second group of metals is selected from a group consisting of titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), niobium (Nb), ruthenium (Ru), tin (Sn), indium (In), cadmium (Cd), Ca, Mg, strontium (Sr), and barium (Ba).

19. The method of claim 11 further comprising: drying the precipitate in a vacuum environment at a temperature in a range between 0 and 200 degrees Centigrade (C).