Metal cyanometallates

Abstract

Methods are presented for synthesizing metal cyanometallate (MCM). A first method provides a first solution of A.sub.XM2.sub.Y(CN).sub.Z, to which a second solution including M1 is dropwise added. As a result, a precipitate is formed of A.sub.NM1.sub.PM2.sub.Q (CN).sub.R..sub.FH.sub.2O, where N is in the range of 1 to 4. A second method for synthesizing MCM provides a first solution of M2.sub.C(CN).sub.B, which is dropwise added to a second solution including M1. As a result, a precipitate is formed of M1[M2.sub.S(CN).sub.G].sub.1/T..sub.DH.sub.2O, where S/T is greater than or equal to 0.8. Low vacancy MCM materials are also presented.

Claims

1. A metal cyanometallate (MCM) material, prior to fabrication in a battery electrode, comprising:
A.sub.NM1.sub.PM2.sub.Q(CN).sub.R..sub.FH.sub.2O; where A is an alkaline earth metal; where M1 and M2 are metals selected from the group consisting of transition metals, tin (Sn), indium (In), calcium (Ca), magnesium (Mg), strontium (Sr), and barium (Ba); where N is in a range of greater than 0.5 to 1; 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 MCM material of claim 1 wherein the alkaline earth metal is selected from the group consisting of calcium (Ca), strontium (Sr), barium (Ba), magnesium (Mg), and combinations thereof.

3. The MCM material of claim 1 where M1 and M2 are metals selected from the group consisting of titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), niobium (Nb), and ruthenium (Ru).

4. The MCM material of claim 1 wherein M1 is Mn; and, wherein M2 is Fe.

5. A metal cyanometallate (MCM) powder, prior to exposure to an electrochemical stimulus, comprising:
A.sub.NM1.sub.PM2.sub.Q(CN).sub.R..sub.FH.sub.2O; where A is an alkaline earth metal; where M1 and M2 are metals selected from the group consisting of transition metals, tin (Sn), indium (In), calcium (Ca), magnesium (Mg), strontium (Sr), and barium (Ba); where N is in a range of greater than 0.5 to 1; 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.

6. The MCM powder of claim 5 wherein the alkaline earth metal is selected from the group consisting of calcium (Ca), strontium (Sr), barium (Ba), magnesium (Mg), and combinations thereof.

7. The MCM powder of claim 5 where M1 and M2 are metals selected from the group consisting of titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), niobium (Nb), and ruthenium (Ru).

8. The MCM powder of claim 5 wherein M1 is Mn; and, wherein M2 is Fe.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of the framework of ideal A.sub.xM.sub.1M.sub.2(CN).sub.z (prior art).

(2) FIG. 2 is a schematic diagram of a MCM with the formula of M1[M2.sub.S(CN).sub.G].sub.1/T..sub.DH.sub.2O.

(3) FIG. 3 is a graph comparing the capacities of the BG-1 and BG-2 samples.

(4) FIG. 4 is a graph depicting the cycling performance of the BG-2 Berlin Green electrode obtained from the synthesis method disclosed herein.

(5) FIG. 5 is a flowchart illustrating a method for synthesizing metal cyanometallate (MCM).

(6) FIG. 6 is a flowchart illustrating a second method for synthesizing MCM.

DETAILED DESCRIPTION

(7) Returning to FIG. 1, the schematic diagram can be used to represent a metal cyanometallate (MCM) with the formula of A.sub.NM1.sub.PM2.sub.Q(CN(.sub.R..sub.FH.sub.2O, with the exception that the interstitial spaces, shown as filled with A metal elements in the ideal case, are not necessarily completely filled. In the formula the variables are as follows:

(8) A is metal selected from a first group of metals including alkali and alkaline earth metals;

(9) M1 and M2 are independently selected (M1 and M2 may be the same or a different metal) from a second group of metals including transition metals;

(10) N is in the range of 1 to 4;

(11) P is less than or equal to 2;

(12) F is in the range of 0 to 20;

(13) Q is less than or equal to 2; and,

(14) R is less than or equal to 6.

(15) This MCM material is unique in that the value of N is larger than any other known, non-theoretical MCM material. Details concerning the synthesis of this MCM are presented in detail below.

(16) FIG. 2 is a schematic diagram of a MCM with the formula of M1[M2.sub.S(CN).sub.G].sub.1/T..sub.DH.sub.2O. This figure is similar to the ideal MCM representation of FIG. 1, except that that some or all of the elements enclosed by the dotted line may be vacant. The variables in the formula are as follows:

(17) M1 and M2 are independently selected from a group of metals including transition metals;

(18) S/T is greater than or equal to 0.8;

(19) D is in the range of 0 to 20; and,

(20) G is less than or equal to 6.

(21) This MCM material is unique in that the value of Q/P is larger than any other known, non-theoretical MCM material. Details concerning the synthesis of this MCM are presented in detail below.

(22) Regarding the notation: Prussian Blue is conventionally represented as Fe.sub.4[Fe(CN).sub.6].sub.3. However, this formula does not accurately reflect the crystal structure that is conventionally obtained. Conventional Prussian Blue is more accurately represented with the formula of Fe[Fe(CN).sub.6].sub.3/4 in which there are 0.25 vacancies of Fe(CN).sub.6. The MCM materials described herein have fewer vacancies than conventional materials. To that end, the formula M1[M2.sub.S(CN).sub.G].sub.1/T is used, where S/T is at least 0.8, meaning there are less than 0.2 M2 vacancies per M1[M2.sub.S(CN).sub.G].sub.1/T molecule.

(23) In general, a precipitation method is used to synthesize MCM materials as electrodes. Briefly, a solution including excess M1-ions reacts with the A.sub.XM2.sub.Y(CN).sub.Z solution to form A.sub.NM1.sub.PM2.sub.Q(CN).sub.R..sub.FH.sub.2O. However, under some circumstances, the sizes of metal-ions with high valences are very small, so it is most likely that the excess M1-ions occupy the interstitial space of M.sub.1M.sub.2(CN).sub.R to reduce its capability of containing alkaline or alkali-ions during charge/discharge in metal-ion batteries.

(24) The conventional precipitation method for synthesizing Berlin Green uses a solution including excess Fe.sup.3+ ions to react with a Fe(CN).sub.6.sup.3 solution to form FeFe(CN).sub.6. However, since the size of Fe.sup.3+-ions is even smaller than Li.sup.+ and Na.sup.+-ions, it is likely that the excess Fe.sup.3+-ions occupy the interstitial space of FeFe(CN).sub.6 to reduce its capability of containing sodium-ions during charge/discharge in sodium-ion batteries.

(25) As an improvement over the conventional synthesis method, one process described herein creates M1M2(CN).sub.R materials with a solution containing excess M2.sub.YCN).sub.Z-ions and a solution containing M1-ions. The solution containing the M1-ions is added dropwise into the solution containing the excess M2.sub.Y(CN).sub.Z-ions to form M1M2(CN).sub.R. As soon as the solution containing the M1-ions drops into the solution containing the M2.sub.Y(CN).sub.Z-ions, M1-ions are coordinated with M2.sub.Y(CN).sub.Z-ions so that just a very few M1-ions occupy the interstitial space of M1M2(CN).sub.R. The solution (Solution 1) of M2.sub.Y(CN).sub.Z-ions, with a concentration of 0.001 moles (M) to 2 M mixes with the solution (Solution 2) containing M1-ions having a concentration of 0.001 M-10 M. Typically, the ratio of M2.sub.Y(CN).sub.Z-ions to M1-ions in moles is larger than 1. The reaction temperature is usually between 20 C. and 100 C., and Solution 2 is dropped into Solution 1. The final product can be washed with water and acetone several times, and dried between 20 C. and 150 C. under air or vacuum conditions.

(26) The synthesis of Berlin Green (BG, FeFe(CN).sub.6) is given here as an example. Two BG samples were synthesized with the conventional process described above (designated BG-1) and the method disclosed herein (designated BG-2). In the conventional process [11], Solution 1 was a 0.1 M K.sub.3Fe(CN).sub.6 water solution. Solution 2 was a 0.1 M FeCl.sub.3 water solution. Solution 1 was dropped into Solution 2 slowly to form the precipitate BG-1. The volume of Solution 2 was as twice as that of Solution 1. In other words, Fe.sup.3+-ions were in excess in the reaction. In the method disclosed herein, the same molarities of Solutions 1 and 2 were used, but there were other differences. One difference was that the volume of Solution 1 was more than that of Solution 2. The other was that Solution 2 was added dropwise into Solution 1 to obtain BG-2. As used herein, dropwise added means that the solution was added in a controlled, step-by step manner. In other words, Solution 2 was not added to Solution 1 in a single step. After being separated, washed, and dried, BG-1 and BG-2 were used as electrodes in sodium-ion batteries with an electrolyte of saturated NaClO.sub.4 ethylene carbonate/diethylene carbonate (EC/DEC).

(27) FIG. 3 is a graph comparing the capacities of the BG-1 and BG-2 samples. With a current of 100 mA/g, the capacity at the first discharge of BG-2 was 30 mAh/g higher than that of BG-1. The method disclosed herein greatly improves the capacity of a Berlin Green electrode in a sodium-ion battery.

(28) FIG. 4 is a graph depicting the cycling performance of the BG-2 Berlin Green electrode obtained from the synthesis method disclosed herein. In the first nine cycles, the capacity of Berlin Green stabilized at 137.7 mAh/g with the charge/discharge current of 100 mAh/g. In the 10.sup.th cycle, the current was changed to 300 mA/g. After 520 cycles, greater than 80% of the capacity was still retained.

(29) FIG. 5 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, where noted, 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.

(30) Step 602 provides a first solution of A.sub.XM2.sub.Y(CN).sub.Z; where A is selected from a first group of metals including alkali and alkaline earth metals; where M2 is selected from a second group of metals including transition 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.

(31) More explicitly, the first group of metals includes 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. M1 and M2 are each independently selected from the second group of metals, which includes 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). As used herein, independently selected means that M1 and M2 may be the same or different types of metal.

(32) Step 604 dropwise adds a second solution including M1 to the first solution, where M1 is selected from the second group of metals. Step 606 forms a precipitate of A.sub.NM1.sub.PM2.sub.Q(CN).sub.R..sub.FH.sub.2O; where N is in the range of 1 to 4; 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.

(33) In one aspect, the molar ratio of A.sub.XM2.sub.Y(CN).sub.Z in the first solution is less than M1 in the second solution. Typically, Steps 602 through 606 are performed in a process environment having a temperature in a range of 20 to 100 degrees Centigrade (C). It is also typical that Step 604 is performed in an inert gas process environment. Step 608 dries the precipitate in a vacuum environment including an inert gas, at a temperature in the range between 0 and 200 degrees C.

(34) In one aspect, prior to Step 604, Step 603a adds an organic solvent to the first solution, the second solution, or both the first and second solutions. Some examples of organic solvents include alcohol (e.g. methanol, ethanol, isopropyl alcohol), acetone, acetonitrile, and ether.

(35) In another aspect, prior to the performance of Step 604, Step 603b adds a reducing agent to the first solution, second solution, or both the first and second solutions. Some examples of reducing agents include sodium borohydride, sodium hyposulfite, sodium sulfite, ascorbic acid, glucose, and polyvinylpyrrolidon. Note: this is not an exhaustive list of organic solvents or reducing agents that might be used. In some aspect, both Steps 603a and 603b are performed.

(36) FIG. 6 is a flowchart illustrating a second method for synthesizing MCM. The method begins at Step 700. Step 702 provides a first solution of M2.sub.C(CN).sub.B; where M2 is selected from a group of metals including transition metals; where C is in the range of 1 to 10; and, where B is in the range of 1 to 10.

(37) M1 and M2 are each independently selected from the group of metals that includes Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Nb, Ru, Sn, In, Cd, Ca, Mg, Sr, and Ba. Step 704 dropwise adds the first solution to a second solution including M1, where M1 is selected from the group of metals. Step 706 forms a precipitate of M1[M2.sub.S(CN).sub.G].sub.1/T..sub.DH.sub.2O; where S/T is greater than or equal to 0.8; where D is in the range of 0 to 20; and, where G is less than or equal to 6.

(38) Step 708 dries the precipitate in a vacuum environment including an inert gas, at a temperature in the range between 0 and 200 degrees C.

(39) In one aspect, the molar ratio of M2.sub.C(CN).sub.B in the first solution is greater than M1 in the second solution. Typically, Steps 702 through 706 are performed in a process environment having a temperature in a range of 20 to 100 degrees C. It is also typical that Step 704 is performed in an inert gas process environment.

(40) In one aspect, prior to Step 704, Step 703a adds an organic solvent to the first solution, the second solution, or both the first and second solutions. Some examples of organic solvents include alcohol (e.g. methanol, ethanol, isopropyl alcohol), acetone, acetonitrile, and ether.

(41) In another aspect, prior to the performance of Step 704, Step 703b adds a reducing agent to the first solution, second solution, or both the first and second solutions. Some examples of reducing agents include sodium borohydride, sodium hyposulfite, sodium sulfite, ascorbic acid, glucose, and polyvinylpyrrolidon. Note: this is not an exhaustive list of organic solvents or reducing agents that might be used.

(42) Processes for the synthesis of MCM have been provided.

(43) 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.