Method of producing a sodium iron(II)-hexacyanoferrate(II) material

Abstract

The present invention relates to a method of producing a sodium iron(II)-hexacyanoferrate(II) (Na.sub.2-xFe[Fe(CN).sub.6].mH.sub.2O), where x is <0.4) material commonly referred to as Prussian White. The method comprises the steps of acid decomposition of Na.sub.4Fe(CN).sub.6.10H.sub.2O to a powder of Na.sub.2-xFe[Fe(CN).sub.6].mH.sub.2O, drying and enriching the sodium content in the Na.sub.2-xFe[Fe(CN).sub.6].mH.sub.2O powder by mixing the powder with a saturated or supersaturated solution of a reducing agent containing sodium in dry solvent under an inert gas. The steps of acid decomposition and enriching the sodium content are performed under non-hydrothermal conditions.

Claims

1. A method of producing a sodium iron(II)-hexacyanoferrate(II) material, Na.sub.2-xFe[Fe(CN).sub.6].mH.sub.2O, where x is <0.4, the method comprising the steps of: acid decomposition of Na.sub.4Fe(CN).sub.6.10H.sub.2O to a powder of Na.sub.2-xFe[Fe(CN).sub.6].mH.sub.2O, where x is <0.4 and m is between 0 and 10; filtering and drying the Na.sub.2-xFe[Fe(CN).sub.6].mH.sub.2O powder; and enriching the sodium content in the Na.sub.2-xFe[Fe(CN).sub.6].mH.sub.2O powder, resulting in a Na.sub.2-yFe[Fe(CN).sub.6].mH.sub.2O powder, where y<x; separating and drying the enriched Na.sub.2-yFe[Fe(CN).sub.6].mH.sub.2O powder resulting in the sodium iron(II)-hexacyanoferrate(II) material, wherein the steps of acid decomposition and enriching the sodium content are performed under non-hydrothermal conditions, and in that the step of enriching the sodium content comprises mixing the dried Na.sub.2-xFe[Fe(CN).sub.6].mH.sub.2O powder with a saturated or supersaturated solution of a reducing agent containing sodium in dry solvent under an inert gas.

2. The method according to claim 1, wherein the saturated or supersaturated solution comprises a sodium salt.

3. The method according to claim 1, wherein the dry solvent in the saturated or supersaturated solution is anhydrous organic solvent.

4. The method according to claim 1, wherein the enriching step comprises enriching the sodium content, 2-y, to above 1.8.

5. The method according to claim 1, wherein an electrode comprising the sodium iron(II)-hexacyanoferrate(II) material is formed, the method comprising the further steps of: mixing the enriched, separated and dried Na.sub.2-yFe[Fe(CN).sub.6].mH.sub.2O powder with solvent, conductive additive and binder by milling, forming a slurry; forming the slurry to a desired shape and removing the solvent by drying.

6. The method according to claim 2, wherein the sodium salt is sodium iodide or sodium bromide.

7. The method according to claim 3, wherein the anhydrous organic solvent is anhydrous acetone, tetrahydrofuran (THF), propylene carbonate, or acetonitrile.

8. The method according to claim 4, wherein the enriching step comprises enriching the sodium content, 2-y, to above 1.9.

9. The method according to claim 4, wherein the enriching step comprises enriching the sodium content, 2-y, to 1.92.

Description

DESCRIPTION OF DRAWINGS

(1) A more complete understanding of the above mentioned and other features and advantages of the present invention will be apparent from the following detailed description of preferred embodiments in conjunction with the appended drawings, wherein:

(2) FIG. 1 is a flowchart illustrating the method according to the invention for synthesising Na.sub.2-yFe[Fe(CN).sub.6].mH.sub.2O, wherein y is lower than x and preferably lower than 0.2;

(3) FIG. 2a-b are graphs representing the electrochemical behavior of Na.sub.2-xFe[Fe(CN).sub.6].mH.sub.2O synthesised via the modified acid decomposition synthesis procedure;

(4) FIG. 3a-b are X-ray diffraction patterns of: (a) the pure rhombohedral Prussian white phase and a comparison with Prussian blue (b).

(5) FIG. 4 is a SEM image of Prussian white obtained according to the present disclosure (EHT=5.00 kV; WD=8.7 mm; Signal A=InLens; Mag=43.60 K X; I Probe=30 pA).

DETAILED DESCRIPTION

(6) The method according to the present invention of producing a sodium iron(II)-hexacyanoferrate(II) material, Na.sub.2-yFe[Fe(CN).sub.6].mH.sub.2O, wherein y is below x and preferably below 0.2, comprises two stages: (A) acid decomposition of Na.sub.4Fe(CN).sub.6 and drying to a powder material and (B) enriching the sodium content of the powder material, Na.sub.2-xFe[Fe(CN).sub.6].mH.sub.2O where x is <0.4. The method according to the present invention of producing a positive electrode for a sodium battery comprises a further stage (C) of forming an electrode comprising the sodium-enriched powder material.

(7) The first stage (A) of the method according to the invention comprises acid decomposition of Na.sub.4Fe(CN).sub.6.10H.sub.2O, which is known in the art. However, a significant aspect is that the chemical reaction occurring during the method according to the invention is performed below 100 C. and at, or near, ambient pressure. The reaction begins with the acid decomposition, for example using HCl, of Na.sub.4Fe(CN).sub.6.10H.sub.2O in deoxygenated H.sub.2O at between 40-100 C. and in the presence of a saturated solution of sodium ions. As appreciated by the skilled person, other acids may be utilized. The reaction is kept under an inert gas, e.g. N.sub.2, and left for some time (generally 12-36 hrs). Inert gas should be interpreted as a gas, or gas mixture, that does not react with the used substances. The reaction mixture was then cooled to room temperature (RT) and filtered in air. The residue was rinsed with deionised water and ethanol. The resulting powder, Na.sub.2-xFe[Fe(CN).sub.6].mH.sub.2O, is then dried at 100-120 C. under vacuum overnight.

(8) The stage of increasing the sodium content (B) makes it possible to omit the hydrothermal synthesis utilized in prior methods. The dried sodium iron(II)-hexacyanoferrate(II) powder is mixed with a solution of a reducing agent containing sodium in dry solvent under an inert gas for several days. If complete sodiation is desired then a saturated solution of the reducing agent should be employed. A preferred sodiation agent is sodium iodide, NaI. Alternatively other sodium containing reducing agents are suitable, for example NaBr. A preferred dry solvent is anhydrous acetonitrile, however anhydrous methanol or anhydrous acetone could also be used. The resulting Prussian White powder was separated by centrifugation and decanting the solvent under inert atmosphere and washed with dry solvent (for example anhydrous acetonitrile) and can be readily used directly in the production of electrodes for sodium ion batteries.

(9) The third stage (C) comprises forming of an electrode comprising the Prussian white powder. Electrodes are be prepared by conventional slurry casting where the Prussian white material is mixed with conductive additive, binder and solvent in a ball mill. The slurry is then deposited onto a current collector; the film thickness is controlled by the doctor blade technique. One or more electrodes comprising Prussian white are arranged in a battery cell and will form high voltage and high capacity positive electrode(s).

(10) Devices similar to the above described electrode, for example fuel cell electrodes could advantageously comprise Prussian white produced by the method according to the invention.

(11) Prussian white powder could advantageously be utilized also in electrochromic devices and sensors.

(12) The method according to invention will be described in detail with references to the flow chart of FIG. 1. As realized by the skilled person, the processing times and temperatures in the individual steps should be seen as non-limiting guidance. The skilled person, given the information of the essential steps of the method, will be able to adapt the process, to the present conditions and requirements, for example reaction vessel sizes, heating capabilities etc. 1) A specific volume of water is deoxygenated by bubbling N.sub.2 gas through it for an hour. This solution was then saturated with NaCl. The entire reaction vessel is kept under flowing N.sub.2. 2) To the saturated solution a given quantity (dependent on the desired yield) of Na.sub.4Fe(CN).sub.6.10H.sub.2O was added and allowed to dissolve. HCl is then added to the solution to control the pH to be less than 6.5. The reaction vessel is heated to a temperature of 40-100 C. and allowed to react for a period of time between 12 and 24 hrs. 3) a. The resulting powder is separated and washed. Preferably the powder is separated by filtering in air and washed with deionised, deoxygenated water and then ethanol. Alternatively, the resulting powder can be separated by centrifugation, decanting the solution and then washing with water and ethanol followed by further centrifugation and decanting. Also other commonly used separation and washing methods can be utilized b. A concentrated solution of reducing NaX is produced in anhydrous acetonitrile under an inert atmosphere of Ar or N.sub.2. 4) The powder is added to the dry solvent and allowed to stir under a dry inert atmosphere of Ar or N.sub.2 until a white powder is obtained, Prussian white (Na.sub.2-yFe[Fe(CN).sub.6].mH.sub.2O) with a Na-content above 1.8 (i.e. y is below 0.2) and with a negligible water content. 5) The resulting Na.sub.2-yFe[Fe(CN).sub.6].mH.sub.2O powder is separated from the dry solvent and washed with additional dry solvent typically 3-4 times under an inert atmosphere. The resulting powder is dried again at a moderate temperature, for example 120 C. for 12 hrs. The so produced Prussian white material may be formed into an electrode by the additional steps of: 6) The dried Prussian white powder is mixed with solvent, conductive additive and binder by milling, for example ball milling, under inert atmosphere for about 1 hrs. 7) Forming the resulting slurry to the desired shape. For example by applying the resulting slurry is to a metal foil and evenly distributed by a doctor blade. The solvent is removed from the electrodes by drying at 120 C. for 12 hrs. Alternatively various casting or pressing procedures may be used.

EXAMPLES/RESULTS

(13) Using the above described synthesis method Prussian white can be synthesized via a method that uses similar reagents without the need for the expensive hydrothermal synthesis procedures. Evidence that Prussian white is synthesized is shown in both the X-ray diffraction pattern (FIG. 3a-b) and the characteristic voltage profiles (FIG. 2a-b), both of which are similar to the material produced via the hydrothermal synthesis method in the prior art. The voltage profiles were measured with the standard method galvanostatic cycling using a Digatron BTS from Digatron Power Electronics. The galvanostatic cycling of multiple cells was performed between 2 & 4.2 Volts at a current of 11.5 mA*g.sup.1. X-ray diffraction patterns were obtained by measurement of samples sealed in borosilicate glass capillaries, the instrument was a STOE-STADI P diffractometer with a Mythen Dectris 1K strip detector with a 0.15 angular resolution. Samples were measured between 10-60 two-theta and the X-ray source used was a single wavelength Cu K1.

(14) The X-ray diffraction pattern is distinctly Prussian White and not Prussian Blue because these two materials have different crystal structures. Specifically, Prussian Blue's crystal structure has cubic symmetry (space group Fm-3m) while Prussian White exhibits rhombohedral symmetry (space group R-3m). This symmetry difference produces a different characteristic diffraction pattern (FIG. 3b). Additionally, the two voltage plateaus have only ever been observed for Prussian White. As seen in FIG. 2a a stable capacity of 130-140 mAh g.sup.1 is achieved for the material according to the invention.