Air stable alkali metal iron (II)-hexacyanoferrate (II) and method of preparation thereof

Abstract

The present disclosure provides an air stable alkali metal iron (II)-hexacyanoferrate (II) of Formula AxMy[Fe(CN)6].zH2O, wherein A is an alkali metal or combinations thereof, M is Fe, x is 1.0 to 2.0, y is 0.7 to 1.0 and z is less than 5.0, wherein the alkali metal iron (II)- hexacyanoferrate (II) is treated with an electrochemically active material such that at least part of surface of particles of alkali metal iron (II)-hexacyanoferrate (II) is covered with said electrochemically active material. Aspects of the present disclosure also provides a method of preparation, an electrode, and a cell comprising the air stable alkali metal iron (II)- hexacyanoferrate (II).

Description

AIR STABLE ALKALI METAL IRON (II)-HEXACYANOFERRATE (II) AND METHOD OF PREPARATION THEREOFFIELD OF THE INVENTION

[0001] The present disclosure pertains to the field of chemical engineering and materials science. More particularly, the present disclosure relates to an air-stable alkali metal iron (II)-hexacyanoferrate (II) and a method of preparation thereof. Aspects of the present disclosure also provides an electrode comprising the air stable alkali metal iron (Il)-hexacyanoferrate (II), and a cell having a cathode comprising the air stable alkali metal iron (Il)-hexacyanoferrate (II). Still further aspect relates to use of the air stable alkali metal iron (Il)-hexacyanoferrate (II) as an active cathode material.BACKGROUND

[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.

[0003] A battery is an electrochemical cell that stores electric energy in its chemical form and is again converted back to electric form when required. Battery technology requires Large-capacity, cost-effective energy storage for transformational technology needed for wind, solar etc. The rechargeable battery has an efficient electrical energy storage (EES) capacity. The lithium (Li)-ion battery is the leading option for energy storage, but it is expensive for large-scale. While Lithium-ion batteries predominate the market for secondary battery technologies, Lithium-ion batteries suffer from several drawbacks, including, lack of lithium sources, less abundance, high cost, high energy processing cost, and not being environmentally friendly. Further, Lithium ions are not useful in large-scale stationary energy storage applications such as grid-level energy storage.

[0004] Sodium-ion batteries are an alternative to Li-ion batteries. Sodium ions have more abundance, are less costly and are environment friendly. Due to the high abundance of sodium compounds, it is multiple times less costly than lithium-ion. However, there is a deficiency in energy density and cyclability in both anode (negative) and cathode (positive) electrode materials for sodium ion batteries, limiting their widespread usage.

[0005] Metal hexacyanoferrates (MHCF) have been investigated as cathode materials for rechargeable sodium-ion batteries, and other metal-ion batteries applications. For example, US9745202B2 discloses a method for synthesizing metal cyanometallates (MCMs) with the general formula ANM1PM2Q(CN)R such that the value of N is greater than that obtained using conventional processes, for use in metal-ion batteries; CN105555712A discloses a method for synthesizing sodium iron(II)-hexacyanoferrate(II), where a Fe(CN)e material is mixed with the first solution and is either an anti-oxidant or a reducing agent to form sodium iron(II)-hexacyanoferrate(II) (Nai+xFe[Fe(CN)e]z.mH2O is formed, where x is less than or equal to 1, and where m is in a range between 0 and 7; CN105555712A discloses a method for synthesizing a high-sodium iron-based Prussian blue electrode material; and US10899632B2 discloses a method of producing a sodium iron (Il)-hexacyanoferrate(II) (Na2-xFe[Fe(CN)e].mH2O), where x is < 0.4) material commonly referred to as Prussian White, contents of each of these documents are incorporated herein, in their entirety, by way of reference.

[0006] While Metal hexacyanoferrates (MHCF), such as Prussian blue analogues, and Prussian white analogues, holds potential for battery applications, they do suffer from some drawbacks that hinder their widespread use like: lower energy density; structural instability, as they undergo structural changes during charging and discharging cycles; limited electron transfer; water and air sensitivity, and the likes. As known in the state of art, Prussian blue analogue (PBAs) are not stable when come in contact with air and / or moisture. Similarly, Prussian white is highly unstable in normal air due to changes in the oxidation state of iron. Prussian white is of white color at the time of synthesis; however, during the isolation process like filtration, the color changes from white to light blue, and after storage for a few days, it completely changes to dark blue owing to oxidation.

[0007] Accordingly, there is a long felt need in the art of alkali metal iron (Il)-hexacyanoferrate (II) that is air-stable. Need is also felt of a method for production of air stable alkali metal iron (II) -hexacyanoferrate (II).OBJECTS OF THE INVENTION

[0008] An objective of the present disclosure is to provide an air-stable alkali metal iron (II)-hexacyanoferrate (II).

[0009] Another objective of the present disclosure is to provide an air-stable alkali metal iron (II) -hexacyanoferrate (II) that is economical and industrially applicable.

[0010] Another objective of the present disclosure is to provide an air-stable alkali metal iron (II) -hexacyanoferrate (II) having higher sodium ion contents, high energy density and higher charge-discharge cycles, easy to manufacture, store, transport and handle for utility in sodium-ion batteries.

[0011] Further objective of the present disclosure is to provide a method of preparation of an air-stable alkali metal iron (Il)-hexacyanoferrate (II).

[0012] Still another objective of the present disclosure is to provide an electrode that has a high initial charge capacity and is capable of being recharged multiple times without significant loss in charge capacity.SUMMARY OF THE INVENTION

[0013] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description section. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

[0014] The present disclosure pertains to the field of chemical engineering and materials science. More particularly, the present disclosure relates to an air-stable alkali metal iron (II)-hexacyanoferrate (II) and a method of preparation thereof.

[0015] An aspect of the present disclosure provides an air stable alkali metal iron (II)-hexacyanoferrate (II) of Formula AxMy[Fe(CN)e].zH2O, wherein A is an alkali metal or combinations thereof, M is Fe, x is 1.0 to 2.0, y is 0.7 to 1.0 and z is less than 5.0, wherein the alkali metal iron (Il)-hexacyanoferrate (II) is treated with an electrochemically active material such that at least part of surface of particles of alkali metal iron (Il)-hexacyanoferrate (II) is covered with said electrochemically active material.

[0016] In some embodiments, the electrochemically active material is selected from: (a) a Prussian white analogue of formula AxNy[Fe(CN)6].zH2O, wherein A is an alkali metal or combinations thereof, N is Cu, Mn, Ni, Co, V or combinations thereof, x is 1.0 to 2.0, y is 0.7 to 1.0 and z is less than 5.0; (b) a Prussian white analogue of formula AxMyNz[Fe(CN)e].111H2O. wherein A is an alkali metal or combinations thereof, M is Fe, N is Cu, Mn, Ni, Co, V or combinations thereof, x is 1.0 to 2.0, y is 0.1 to 0.3, z is 0.7 to 1.0 and m is less than 5.0; (c) a sodium layered oxide cathode material; (d) a sodium polyanion cathode material; (e) an alkali metal salt of vanadium phosphate; (f) an alkali metal salt of halide; (g) a NASICON type cathode material; and combinations thereof.

[0017] In some embodiments, the sodium-layered oxide cathode material, the sodium polyanion cathode material, the alkali metal salt of vanadium phosphate, and the NASICON type cathode material are selected from Na4Fe3(PO4)2P2O?, Na3V2(PC>4)3, sodium iron phosphate (NaFcPO ). sodium iron sulphate (Na2FeSC>4), sodium vanadium phosphate, sodium vanadium titanium phosphate (Na2TiV(PC>4)3), sodium chromium titanium phosphate (Na2CrTi(PO4)3) and combinations thereof.

[0018] In some embodiments, the air stable iron (Il)-hexacyanoferrate (II) comprises said electrochemically active material in an amount ranging from 2 to 20 % by wt. of the air stable iron (Il)-hexacyanoferrate (II).

[0019] Another aspect of the present disclosure relates to a method for preparation of air stable alkali metal iron (II) -hexacyanoferrate (II) of Formula AxMy[Fe(CN)e].zH2O, wherein A is an alkali metal or combinations thereof, M is Fe, x is 1.0 to 2.0, y is 0.7 to 1.0 and z is less than 5.0, said method comprising a step of treating an alkali metal iron (Il)-hexacyanoferrate (II) with an electrochemically active material such that at least part of surface of particles of alkali metal iron (Il)-hexacyanoferrate (II) is covered with said electrochemically active material.

[0020] In some embodiments, the step of treating comprises any of: (i) coating the alkali metal iron (Il)-hexacyanoferrate (II) with the electrochemically active material; (ii) blending the alkali metal iron (Il)-hexacyanoferrate (II) with the electrochemically active material; (iii) effecting in-situ generation of alkali metal iron (II) -hexacyanoferrate (II) with the electrochemically active material; and (iv) treating precipitates of alkali metal iron (II)-hexacyanoferrate (II) with a solution containing the electrochemically active material.

[0021] In some embodiments, the electrochemically active material is selected from: (a) a Prussian white analogue of formula AxNy[Fe(CN)6].zH2O, wherein A is an alkali metal or combinations thereof, N is Cu, Mn, Ni, Co, V or combinations thereof, x is 1.0 to 2.0, y is 0.7 to 1.0 and z is less than 5.0; (b) a Prussian white analogue of formula AxMyNz[Fe(CN)e] mFbO, wherein A is an alkali metal or combinations thereof, M is Fe, N is Cu, Mn, Ni, Co, V or combinations thereof, x is 1.0 to 2.0, y is 0.1 to 0.3, z is 0.7 to 1.0 and m is less than 5.0; (c) a sodium layered oxide cathode material; (d) a sodium polyanion cathode material; (e) an alkali metal salt of vanadium phosphate; (f) an alkali metal salt of halide; (g) a NASICON type cathode material; and combinations thereof.

[0022] In some embodiments, the step of coating comprises: adding the electrochemically active material to a solution containing the alkali metal iron (Il)-hexacyanoferrate (II) to obtain the air stable alkali metal iron (Il)-hexacyanoferrate (II).

[0023] In some embodiments, the step of blending comprises: blending the electrochemically active material with the alkali metal iron (Il)-hexacyanoferrate (II), optionally, in presence of a solvent, to obtain the air stable alkali metal iron (Il)-hexacyanoferrate (II).

[0024] In some embodiments, the step of effecting in-situ generation comprises: mixing a solution containing the alkali metal iron (II) -hexacyanoferrate (II) of Formula A, a solution containing alkali metal ferrocyanide and a reducing agent, and a solution containing a metal precursor to obtain the air stable alkali metal iron (Il)-hexacyanoferrate (II).

[0025] Further aspect of the present disclosure is directed towards use of the air stable alkali metal iron (II) -hexacyanoferrate (II), realized in accordance with embodiments of the present disclosure, as an active cathode material.

[0026] Still further aspect of the present disclosure provides an electrode comprising the air stable alkali metal iron (II) -hexacyanoferrate (II) realized in accordance with embodiments of the present disclosure.

[0027] Still further aspect of the present disclosure provides a cell comprising: (a) an anode; (b) a cathode, said cathode comprising the air stable alkali metal iron (Il)-hexacyanoferrate (II) realized in accordance with embodiments of the present disclosure; (c) a separator; and (d) an electrolyte.

[0028] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like features.DETAILED DESCRIPTION

[0029] The following is a detailed description of embodiments of the disclosure. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.

[0030] Unless the context requires otherwise, throughout the specification which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising”, are to be construed in an open, inclusive sense that is as “including, but not limited to.”

[0031] As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its sense including “and / or” unless the content clearly dictates otherwise.

[0032] In some embodiments, the numbers expressing quantities of ingredients, properties such as concentration, reaction conditions, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in the light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable.

[0033] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it is individually recited herein.

[0034] All processes described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.

[0035] The headings and abstract of the invention provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.

[0036] The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus, if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.

[0037] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

[0038] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and / or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description that follows, and the embodiments described herein, is provided by way of illustration of an example, or examples, of particular embodiments of the principles and aspects of the present disclosure. These examples are provided for the purposes of explanation, and not of limitation, of those principles and of the disclosure.

[0039] Unless the context requires otherwise, throughout the specification which follows, the word “comprise” and variations thereof, such as “comprises” and “comprising” are to be construed in an open, inclusive sense that is as “including, but not limited to.”

[0040] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.

[0041] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. Furthermore, the ranges defined throughout the specification include the end values as well, i.e., a range of 1 to 10 implies that both 1 and 10 are included in the range. For the avoidance of doubt, the applicant shall be entitled to any equivalents according to applicable law.

[0042] Various terms as used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.

[0043] The present disclosure pertains to the field of chemical engineering and materials science. More particularly, the present disclosure relates to an air-stable alkali metal iron (II)-hexacyanoferrate (II) and a method of preparation thereof. Aspects of the present disclosure also provides an electrode comprising the air stable alkali metal iron (Il)-hexacyanoferrate (II), and a cell having a cathode comprising the air stable alkali metal iron (Il)-hexacyanoferrate (II). Still further aspect relates to use of the air stable alkali metal iron (Il)-hexacyanoferrate (II) as an active cathode material.

[0044] The term “alkali metal iron (Il)-hexacyanoferrate (II)” as used herein, throughout the present disclosure, denotes an alkali metal iron (Il)-hexacyanoferrate (II) represented by formula AxMy[Fe(CN)e].zH2O, where A is an alkali metal or combinations thereof, M is Fe, x is 1.0 to 2.0, y is 0.7 to 1.0 and z is less than 5.0.

[0045] The term “air stable” as used herein, throughout the present disclosure, denotes the property of resistance of an alkali metal iron (Il)-hexacyanoferrate (II) to oxidation when exposed to air or oxygen. More particularly, the air stability refers to the resistance against air oxidation of iron (II) (i.e. Fe2+) present in the alkali metal iron (Il)-hexacyanoferrate (II) to iron (III) (i.e. Fe3+) on exposure to air or oxygen.

[0046] The term “electrochemically active material” as used herein, throughout the present disclosure, denotes a material or a combination thereof, which is / are capable of sodium ion intercalation and deintercalation through electrochemical reduction and oxidation, respectively. Preferably, the material has a redox potential ranging from 1.5 to 4.5 V vs Na / Na+.

[0047] The present disclosure is on the premise, at least in part, of surprising observation of inventors of the present disclosure that dramatic improvement in stability / resistance against air oxidation can be afforded to alkali metal iron (Il)-hexacyanoferrate (II) when alkali metal iron (Il)-hexacyanoferrate (II) is treated with an electrochemically active material such that at least part of surface of particles of alkali metal iron (Il)-hexacyanoferrate (II) is covered with the electrochemically active material. It could also be noted that employment of electrochemically active material as a protective material, not only provides improved air stability, but also affords high energy density and high charge-discharge rate owing to their being electrochemically active, in contrast to materials such as sodium ascorbate, sodium citrate, PVP, ZnO, carbon based materials and the likes, which are electrochemically inactive.

[0048] Accordingly, an aspect of the present disclosure provides an air stable alkali metal iron (II) -hexacyanoferrate (II) of Formula AxMy[Fe(CN)e].zH2O, where A is an alkali metal or combinations thereof, M is Fe, x is 1.0 to 2.0, y is 0.7 to 1.0 and z is less than 5.0, wherein the alkali metal iron (Il)-hexacyanoferrate (II) is treated with an electrochemically active material such that at least part of surface of particles of alkali metal iron (Il)-hexacyanoferrate (II) is covered with said electrochemically active material.

[0049] In some embodiments, the electrochemically active material is selected from: (a) a Prussian white analogue of formula AxNy[Fe(CN)6].zH2O, wherein A is an alkali metal or combinations thereof, N is Cu, Mn, Ni, Co, V or combinations thereof, x is 1.0 to 2.0, y is 0.7 to 1.0 and z is less than 5.0; (b) a Prussian white analogue of formula AxMyNz[Fe(CN)e] mFbO, wherein A is an alkali metal or combinations thereof, M is Fe, N is Cu, Mn, Ni, Co, V orcombinations thereof, x is 1.0 to 2.0, y is 0.1 to 0.3, z is 0.7 to 1.0 and m is less than 5.0; (c) a sodium layered oxide cathode material; (d) a sodium polyanion cathode material; (e) an alkali metal salt of vanadium phosphate; (f) an alkali metal salt of halide; (g) a NASICON type cathode material; and combinations thereof.

[0050] In some embodiments, the sodium-layered oxide cathode material, the sodium polyanion cathode material, the alkali metal salt of vanadium phosphate, and the NASICON type cathode material are selected from Na4Fe3(PO4)2P2O?, Na3V2(PO4)3, sodium iron phosphate (NaFePO4), sodium iron sulphate (Na2FeSO4), sodium vanadium phosphate, sodium vanadium titanium phosphate (Na2TiV(PO4)3), sodium chromium titanium phosphate (Na2CrTi(PO4)3) and combinations thereof.

[0051] In some embodiments, the air stable iron (Il)-hexacyanoferrate (II) comprises said electrochemically active material in an amount ranging from 2 to 20 % by wt. of the air stable iron (Il)-hexacyanoferrate (II).

[0052] In some embodiments, the air stable iron (Il)-hexacyanoferrate (II) has a particle size below 10 microns, preferably, in the range of 1-7 microns and more preferably, in the range of 2-7 microns.

[0053] In some embodiments, the air stable alkali metal iron (Il)-hexacyanoferrate (II) is of Formula AxMy[Fe(CN)e].zH2O, where z ranges from 0 to 4.5, preferably, ranges from 0 to 4, more preferably, ranges from 0 to 3 and most preferably, ranges from 0 to 0.5. In an embodiment, the air stable alkali metal iron (Il)-hexacyanoferrate (II) is of Formula AxMy[Fe(CN)e].zH2O, where z ranges from 0 to 0.5. In an embodiment, the air stable alkali metal iron (Il)-hexacyanoferrate (II) is of Formula AxMy[Fe(CN)e].zH2O, where z ranges from 0 to 0.1.

[0054] In some embodiments, the air stable alkali metal iron (Il)-hexacyanoferrate (II) is of Formula AxMy[Fe(CN)e].zH2O, where x ranges from 1.2 to 2.0, preferably, x ranges from 1.5 to 2.0, and most preferably, x ranges from 1.7 to 2.0.

[0055] Another aspect of the present disclosure relates to a method for preparation of air stable alkali metal iron (II) -hexacyanoferrate (II) of Formula AxMy[Fe(CN)e].zH2O, wherein A is an alkali metal or combinations thereof, M is Fe, x is 1.0 to 2.0, y is 0.7 to 1.0 and z is less than 5.0, said method comprising a step of treating an alkali metal iron (Il)-hexacyanoferrate (II) with an electrochemically active material such that at least part of surface of particles of alkali metal iron (Il)-hexacyanoferrate (II) is covered with said electrochemically active material.

[0056] In some embodiments, the step of treating comprises any of: (i) coating the alkali metal iron (Il)-hexacyanoferrate (II) with the electrochemically active material; (ii) blending thealkali metal iron (Il)-hexacyanoferrate (II) with the electrochemically active material; (iii) effecting in-situ generation of alkali metal iron (II) -hexacyanoferrate (II) with the electrochemically active material; and (iv) treating precipitates of alkali metal iron (II)-hexacyanoferrate (II) with a solution containing the electrochemically active material.

[0057] In some embodiments, the electrochemically active material is selected from: (a) a Prussian white analogue of formula AxNy[Fe(CN)6].zH2O, wherein A is an alkali metal or combinations thereof, N is Cu, Mn, Ni, Co, V or combinations thereof, x is 1.0 to 2.0, y is 0.7 to 1.0 and z is less than 5.0; (b) a Prussian white analogue of formula AxMyNz[Fe(CN)e] mFbO, wherein A is an alkali metal or combinations thereof, M is Fe, N is Cu, Mn, Ni, Co, V or combinations thereof, x is 1.0 to 2.0, y is 0.1 to 0.3, z is 0.7 to 1.0 and m is less than 5.0; (c) a sodium layered oxide cathode material; (d) a sodium polyanion cathode material; (e) an alkali metal salt of vanadium phosphate; (f) an alkali metal salt of halide; (g) a NASICON type cathode material; and combinations thereof.

[0058] In some embodiments, the step of coating comprises: adding the electrochemically active material to a solution containing the alkali metal iron (Il)-hexacyanoferrate (II) to obtain the air stable alkali metal iron (Il)-hexacyanoferrate (II). In some embodiments, the step of coating comprises: adding particulate (i.e. solid particles) electrochemically active material to a solution containing precipitates of the alkali metal iron (Il)-hexacyanoferrate (II) to obtain the air stable alkali metal iron (Il)-hexacyanoferrate (II).

[0059] In some embodiments, the step of blending comprises: blending the electrochemically active material with the alkali metal iron (Il)-hexacyanoferrate (II), optionally, in presence of a solvent, to obtain the air stable alkali metal iron (Il)-hexacyanoferrate (II). In some embodiments, the step of blending comprises dry blending (i.e. without usage of the solvent). In some embodiments, the step of blending comprises wet blending (i.e. blending in presence of a solvent). In some embodiments, the step of blending is effected in a ball mill. In some embodiments, the step of blending comprises: blending the electrochemically active material with the alkali metal iron (II) -hexacyanoferrate (II) in a weight ratio ranging from 1:3 to 1:25, preferably, at a weight ratio ranging from 1:5 to 1:25, more preferably, weight ratio ranging from 1:5 to 1:20 and most preferably, at a weight ratio ranging from l:7 to 1:15.

[0060] In some embodiments, the step of effecting in-situ generation comprises effecting coprecipitation of the alkali metal iron (Il)-hexacyanoferrate (II) of Formula A and the electrochemically active material. In some embodiments, the step of effecting in-situ generation comprises effecting co-crystallization of the alkali metal iron (Il)-hexacyanoferrate (II) of Formula A and the electrochemically active material.

[0061] In some embodiments, the step of effecting in-situ generation comprises: mixing a solution containing the alkali metal iron (II) -hexacyanoferrate (II) of Formula A, a solution containing alkali metal ferrocyanide and a reducing agent, and a solution containing a metal precursor to obtain the air stable alkali metal iron (Il)-hexacyanoferrate (II). The reducing agent can be any conventionally known reducing agent to serve its intended purpose. In an embodiment, the reducing agent is a salt of citrate or ascorbate, for example, sodium citrate or sodium ascorbate.

[0062] In some embodiment, the step of treating precipitates of alkali metal iron (II)-hexacyanoferrate (II) with a solution containing the electrochemically active material comprises treating a solution containing precipitates of alkali metal iron (II) -hexacyanoferrate (II) with a suspension of the electrochemically active material.

[0063] In some embodiment, the step of treating precipitates of alkali metal iron (II)-hexacyanoferrate (II) with a solution containing the electrochemically active material comprises treating a solution containing precipitates of alkali metal iron (II) -hexacyanoferrate (II) with a solution of an alkali halide.

[0064] The alkali metal iron (Il)-hexacyanoferrate (II) used in the present disclosure can be prepared by any method known to the skilled artisan. In accordance with an embodiment, the alkali metal iron (II) -hexacyanoferrate (II) is prepared by the process comprising: a) subjecting an alkali metal ferrocyanide to acid decomposition to obtain a Prussian blue product; b) filtering the Prussian blue product followed by washing with a solvent to remove chloride content and to obtain a washed Prussian blue product; and c) reducing the washed Prussian blue product to obtain the alkali metal iron (Il)-hexacyanoferrate (II).

[0065] In some embodiments, the alkali metal ferrocyanide is subjected to acid decomposition in presence of an acid selected from hydrochloric acid, sulfuric acid, acetic acid and combination thereof. The acid decomposition may be effected at a temperature in the range of 20 to 100 °C.

[0066] In some embodiments, the step of washing includes washing with a solvent selected from a group comprising: water, ethanol, methanol, isopropanol and combination thereof.

[0067] In some embodiments, the step of reducing comprises reacting the washed Prussian blue with a reducing agent selected from a group consisting of sodium borohydride, sodium hydrosulfite, sodium amalgam, sodium metal, sodium thiosulfate, sodium hydride, sodium cyanoborohydride, sodium polysulfide, sodium formate, sodium sulfite, sodium dithionite, sodium iodide, potassium borohydride, potassium hydride, potassium hydrosulfite, potassiumsulfite, potassium dithionite, potassium iodide and combination thereof. In some embodiments, the step of reducing is carried out at a temperature in the range of 10 to 60 °C.

[0068] Further aspect of the present disclosure is directed towards use of the air stable alkali metal iron (Il)-hexacyanoferrate (II), realized in accordance with embodiments of the present disclosure, as an active cathode material.

[0069] Still further aspect of the present disclosure provides an electrode comprising the air stable alkali metal iron (II) -hexacyanoferrate (II) realized in accordance with embodiments of the present disclosure.

[0070] Still further aspect of the present disclosure provides a cell comprising: (a) an anode; (b) a cathode, said cathode comprising the air stable alkali metal iron (Il)-hexacyanoferrate (II) realized in accordance with embodiments of the present disclosure; (c) a separator; and (d) an electrolyte. The anode can be a sodium metal based anode. Any conventionally known electrolyte and separator that may serve the desired purpose may be used.

[0071] While the foregoing describes various embodiments of the disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof. The invention is not limited to the described embodiments, versions, or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.EXAMPLES

[0072] The present invention is further explained in the form of the following examples. However, it is to be understood that the following examples are merely illustrative and are not to be taken as limitations upon the scope of the invention.

[0073] Example A: Preparation of Sodium Iron (II)-Hexacyanoferrate (II) -Na2Fe(Fe(CN)6

[0074] In a 5000 mL round-bottom flask, 46 g (1260.2 mmol) of concentrated hydrochloric acid was charged, which was mixed with 4688 mL of water and fitted with a reflux condenser. To this solution, 100 g (329.0 mmol) of sodium ferrocyanide and 50 g (283.8 mmol) of ascorbic acid were added at room temperature. The reaction mixture was then heated to 85°C and stirred for 6 hours. The reaction mass was cooled to about 25 °C. The resultant product was filtered and washed with 300 mL of water repeatedly. Finally, the solid was washed with 500 mL of ethanol twice. The wet weight of the blue colored precipitates (Prussian Blue) was 26.02 gm.

[0075] The obtained wet Prussian Blue was transferred into a 5000 mL round-bottom flask. To this, 2065 mL of ethanol and 2065 mL of water were added and the mixture was stirred at about 25°C for about 10 minutes. Subsequently, 16 g (328.83 mmol) of NaBH4was added in small portions while maintaining the temperature at about 25°C and the mixture was stirred for an additional 45 minutes. After completion of the reaction, the reaction mass was filtered and the solid product was collected. The wet weight of the product was recorded as 24.32 g, which was then dried in a vacuum oven at about 130°C for 22 hours. After drying, the weight of the product was 22.40 g. The dried material was stored under inert atmosphere. Yield = 33.93 %. Elemental analysis of the dried product revealed as having: Sodium: 14.5 %, Iron: 35.04%, Carbon: 22.35 % and Nitrogen 26.21%.

[0076] Example 1: Preparation of air stable Sodium Iron (II)-Hexacyanoferrate (II) treated with Manganese Prussian White Analogue (Na2Mn(Fe(CN)6) by in-situ generation

[0077] Sodium Iron (Il)-hexacyanoferrate (II) was prepared as per Example A till the step of filtration after completion of reaction with NaBH4. Rather than effecting the filtration of thereaction mass, the supernatant aqueous layer was removed from the reaction mass and 500 mlwater was added. The resultant solution contains about 22.4 gm of Sodium Iron (II)-hexacyanoferrate (II) (as this was the dry wt. of product obtained in Ex. A). A solution of 6.2gm (12.8 mmol) of sodium ferrocyanide and sodium citrate 35 gm (135.6 mmol) was preparedin 200 ml water. Separately, a solution of MnSO412 gm (70.9 mmol) was prepared in 200 ml water. Both the solutions were added separately dropwise into the reaction mass comprising sodium Iron (II) -hexacyanoferrate (II) at about 25°C for about 65 minutes. The reaction mass was stirred for about 5 hrs at about 25 °C. The product was filtered and washed with a 500 ml mixture of ethanol and water solution (1:1). Wet wt. = 26.4 gm. The product was dried at about 145 °C to obtain 24.1 gm of air stable sodium Iron (Il)-hexacyanoferrate (II) treated with Manganese Prussian White Analogue. Elemental analysis of the dried product revealed as having: Sodium: 14.35%, Iron: 33.04%, Manganese: 2.03% Carbon: 22.48 % and Nitrogen 26.65%.

[0078] Example 2: Preparation of air stable Sodium Iron (II)-Hexacyanoferrate (II) treated with alkali halide

[0079] Sodium iron (Il)-hexacyanoferrate (II) was prepared as per Example A till the step of filtration after completion of reaction with NaBH4. Rather than effecting filtration of the reaction mass, the supernatant aqueous layer was removed from the reaction mass, and 300 ml ethanol and 25 gm (166.7 mmol) Sodium iodide were added thereto. The reaction mass wasstirred for about 4 hours at a temperature of about 35°C. The product was fdtered. Wet wt. = 25.3 gm. The product was dried at about 170°C to obtain 23.6 gm of metal halide treated air stable sodium iron (Il)-hexacyanoferrate (II). Elemental analysis of the dried product revealed as having: Sodium: 14.75 %, Iron: 34.94%, Carbon: 22.15 % Nitrogen 26.05%, and iodide: 1.65%.

[0080] Example B: Preparation of Potassium Iron (II)-Hexacy anoferrate (II)

[0081] In a 5000 mL round-bottom flask, concentrated hydrochloric acid 34 g (932.5 mmol) was charged and mixed with 4500 mL of water and fitted with a reflux condenser. To this solution, 70 g (165.7 mmol) of Potassium ferrocyanide Trihydrate and 43 g (244.1 mmol) of ascorbic acid were added at room temperature. The reaction mixture was then heated to about 75°C and stirred for about 12 hours. The reaction mass was cooled to about 20°C. Product was filtered and washed with 500 mL of water. Finally, the solid was washed with 500 mL of ethanol twice. The wet weight = 37 gm.

[0082] The wet Prussian Blue obtained in the previous step was transferred into a 5000 mL round-bottom flask. To this, 1650 mL of ethanol and 1650 mL of water were added and the mixture was stirred at about 25°C for about 10 minutes. Subsequently, potassium borohydride 7.0 gm (129.77 mmol) was added in small portions while maintaining the temperature at about 25°C. The mixture was stirred for an additional 90 minutes. After completion, the reaction mass was filtered and the solid product was collected. The wet weight of the product was recorded as 36 gm, which was then dried in a vacuum oven at about 150°C for about 20 hours. After drying, the isolated product was 32.4 g. The dried material was stored under inert atmosphere. Yield = 33.93 %.

[0083] Example 3: Preparation of air stable Potassium Iron (II)-Hexacyanoferrate (II) treated with Manganese Prussian White Analogue - K2Mn(FeCN)e by in-situ generation

[0084] Potassium Iron (Il)-hexacyanoferrate (II) was prepared as per Example B till the step of filtration after completion of reaction with NaBH4. Rather than effecting the filtration of thereaction mass, the supernatant aqueous layer was removed from the reaction mass and 300 mlwater was added. A solution of Potassium Ferrocyanide Trihydrate 3.2 gm (7.5 mmol) andPotassium citrate tribasic monohydrate 15 gm (46.2 mmol) was prepared in 200 ml water.Separately, a solution of MnSO4·H2O 3.0 gm (17.7 mmol) was prepared in 100 ml water. Both the solutions were added separately dropwise into the reaction mass comprising potassium Iron (Il)-hexacyanoferrate (II) at about 25°C for about 65 minutes. The reaction mass was stirred for about 4 hrs. at about 30°C. The product was filtered and washed with a 500 ml mixture of ethanol and water solution (1:1). Wet wt. = 37.2 gm. The product was dried at about 140°C toobtain 34.50 gm of air stable potassium Iron (Il)-hexacyanoferrate (II) treated with Manganese Prussian White Analogue.

[0085] Example 4: Preparation of air stable Potassium Iron (II)-Hexacyanoferrate (II) treated with alkali halide

[0086] Potassium iron (Il)-hexacyanoferrate (II) was prepared as per Example B till the step of filtration after completion of reaction with NaBH4. Rather than effecting the filtration of the reaction mass, the supernatant aqueous layer was removed from the reaction mass, and 400 ml ethanol and 30 gm (180.7 mmol) Potassium iodide was added thereto at about 30°C. The reaction mass was heated to 40°C, and the reaction mass was stirred for about 4 hours at a temperature of about 40°C. The reaction mass was filtered to obtain the wet product. Wet wt. = 35.3 gm. The product was dried at about 170°C to obtain 32.9 gm of metal halide treated air stable potassium iron (Il)-hexacyanoferrate (II).

[0087] Example C: Preparation of Manganese Prussian White Analogue - Na2Mn(FeCN)e

[0088] In a 2000 ml RBF sodium ferrocyanide decahydrate 17 gm (35.11 mmol) was charged and sodium citrate 30 gm (116.25 mmol) in 1500 ml water was added thereto. A solution of Manganese (II) sulfate monohydrate 12 gm (70.99 mmol) and sodium chloride 56 gm (430.76 mmol) in 500 ml water was prepared. The solution was added to the RBF at about 35°C for about 60 min. The reaction mass was stirred for about 4 hrs. at about 35°C. The reaction mass was filtered and the product was washed with a mixture of ethanol and water solution (1: 1) 500 ml. The resultant product was dried at about 150°C to obtain 8.2 gm of Manganese Prussian White Analogue - Na2Mn(FeCN)e with a yield of 73.2%.

[0089] Example 5: Preparation of air stable Sodium Iron (II)-Hexacyanoferrate (II) treated with Manganese Prussian white Analogue by blending

[0090] 9.5 gm of sodium iron (Il)-hexacyanoferrate (II), prepared in Example A above, was blended with 0.5 gm of Manganese Prussian White Analogue, prepared in Example C above, in a planetary ball mill jar for a period of about 60 min to obtain air stable Sodium Iron (II)-Hexacyanoferrate (II) treated with Manganese Prussian white analogue.

[0091] Example D: Preparation of Nickel Prussian white Analogue - Na2Nio.3Feo.?(FeCN)6

[0092] In a 2000 ml RBF 23.65 gm (145.8 mmol) of FeCh and 5.34 (41.2 mmol) of NiCh in 500 ml of water was charged. A solution of 10 gm (20.65 mmol) sodium ferrocyanide decahydrate in 100 ml water was prepared, and the same was added dropwise in the solution prepared in RBF at temperature of about 30°C in about 90 min. The reaction mass was stirred for about 12 hours. The reaction mass was filtered to obtain the wet product. Wet Wt. = 4.8gm. The product was dried under vacuum at about 90°C for a period of about 24 hrs. Dry Wt. = 4.3 gm. Yield = 64.5 %.

[0093] Example E: Preparation of Copper Prussian white Analogue - Na2Cu(FeCNe)

[0094] In a 1000 ml RBF 15 gm (111.56 mmol) of CuCh was charged in 500 ml of water. A solution of 40 gm (82.63 mmol) of sodium ferrocyanide decahydrate and 20 gm sodium chloride in 200 ml water was prepared and the same was added dropwise in above solution (in RBF) at temperature about 30°C in about 90 min. The reaction mass was stirred for about 12 hours. The product was filtered. Wet Wt. = 26.6 gm. The product was dried under vacuum at about 120°C for about 24 hrs. Dry Wt. = 21.6 gm. Yield = 80.0 %.

[0095] Example F: Preparation of Cobalt Prussian white Analogue - Na2Co(FeCNe)

[0096] In a 1000 ml RBF 20 gm (154.03 mmol) of CoCh and 10 gm (38.75 mmol) oftri sodium citrate in 200 ml of water was charged. A solution of 20 gm (41.31 mmol) of sodium ferrocyanide decahydrate and 20 gm sodium chloride in 200 ml water was prepared and the same was added dropwise in above solution (in RBF) at temperature about 30°C in about 80 min. The reaction mass was stirred for about 12 hours. The reaction mass was filtered and the product was washed with mixture of 100 ml of ethanol and 100 ml of water. Wet Wt. = 13.4 gm. The product was dried under vacuum at about 120°C for about 32 hrs. Dry Wt. = 10.0 gm. Yield = 75.2 %.

[0097] Example G: Preparation of Nickel Prussian white Analogue - Na2Ni(FeCNe)

[0098] In a 1000 ml RBF 17 gm (108.43 mmol) of NiSO4·6H2O and 50 gm (193.75 mmol) of trisodium citrate dihydrate in 500 ml of water was charged. A solution of 25 gm (51.64 mmol) of sodium ferrocyanide decahydrate in 250 ml water was prepared and the same was added dropwise in above solution (in RBF) at temperature about 40°C in about 50 min. The reaction mass was stirred for about 5 hours at 60°C. The reaction mass was cooled to 30°C. The reaction mass was held for overnight. The product was fdtered and washed with a mixture of 100 ml ethanol and 100 ml water. Wet Wt. = 11.65 gm. The product was dried under vacuum at 120°C for about 32 hrs. Dry Wt. = 8.63 gm. Yield = 52 %.

[0099] Examples 6-9: Preparation of air stable Sodium Iron (II)-Hexacyanoferrate (II) treated with Prussian White Analogues by blending

[0100] 9.5 gm of sodium iron (Il)-hexacyanoferrate (II) (prepared in Example A) was mixed with 0.5 gm of Prussian White Analogues samples, prepared in Examples D-G, in a planetary ball mill jar for a time period of about 60 minutes.

[0101] Example 10: Preparation of air stable Sodium Iron (II)-Hexacyanoferrate (II) treated with Manganese Prussian White Analogue by coating

[0102] Sodium iron (Il)-hexacyanoferrate (II) was prepared as per Example A till the step of filtration after completion of reaction with NaBH4. Rather than effecting filtration of the reaction mass, the supernatant aqueous layer was removed from the reaction mass, and 500 mL water was added. 1.0 gm Manganese Prussian White Analogue (prepared in Example C above) was added in the reaction mass and stirred for about 3 hours. The resultant product was filtered and washed with 500 ml water and then washed with 500 ml ethanol, wet wt. = 22.4 gm. The product was dried at about 150°C to obtain 21.0 gm of air stable Sodium Iron (II)-hexacyanoferrate (II) treated with Manganese Prussian White Analogue.

[0103] Examples 11-14: Preparation of air stable Sodium Iron (II)-Hexacyanoferrate (II) treated with metal Prussian White Analogues by coating

[0104] Using the method as mentioned in Example 10 i.e. by treating precipitates of Sodium iron (II) -hexacyanoferrate (II) with samples of 1 gm of Prussian White Analogues prepared in Examples D-G, respectively, different samples of air stable Sodium Iron (II)-Hexacyanoferrate (II) were prepared and the same are labelled as Examples 11-14.

[0105] Example H: Preparation of Na-sFesCPO^zPzO?

[0106] In a 1000 mL round-botom flask, 33.72 g (223.60 mmol) of ferric orthophosphate, 20.0 g (11.80 mmol) of ferrous oxalate dihydrate, 47.61 g (335.40 mmol) of disodium hydrogen phosphate, and 28.80 g (145.34 mmol) of dextrose monohydrate were dissolved in 600 mL of double-distilled water. The mixture was stirred for about 60 minutes to form a uniform slurry. This slurry was then spray-dried using an inlet temperature of about 230°C and an outlet temperature of about 110°C to obtain a precursor powder. Later, the powder was sintered at about 350°C for about 3.5 hours, and then at about 550°C for about 16 hours under an inert atmosphere, with a heating rate of 2°C min−1. After sintering, 47 gm of Na4Fe3(PO4)2P2O7was obtained.

[0107] Example I: Preparation of NasVzCPO^

[0108] In a 500 mL round bottom flask, 10.0 g (20.62 mmol) of soluble starch, 14.0 g (116.68 mmol) of NaH2PO4, and 6.37 g (35.04 mmol) of V2O5were mixed by ball milling with 200 mL ethanol at 450 rpm for about 10 h and dried at about 80°C for 5 h. The dried mixture was heated at 350°C for about 3 h and calcinated at 800°C for about 10 h in an inert atmosphere to obtain about 18.0 g of the final product.

[0109] Example J: Preparation of Na2TiV(PC>4)3

[0110] In a 250 mL RBF, 11.7 gm (34.3 mmol) tetrabutyl titanate was slowly added to 200 mL of ethyl alcohol while stirring strongly for 10 minutes to form a clear, uniform solution. A solution of 10 gm (73.5 mmol) of sodium acetate trihydrate, 4.0 gm (34.2 mmol) of ammoniummetavanadate, 12 gm (104.3 mmol) of ammonium dihydrogen phosphate, and 12 gm (57.1 mmol) of citric acid monohydrate were added one by one to 250 mL of deionized water. This mixture was stirred until a yellow solution was formed. Afterwards, the solution was quickly poured into the reaction mass (solution of tetrabutyl titanate) and stirred for about 60 minutes. Further, the reaction mixture was heated at about 80°C to evaporate the water, forming dark green precursor powder. Later, the precursor was heated at about 350 °C for 5 hours and then annealed at about 800°C for 12 hours in an argon atmosphere (heating rate: 5°C per minute) to obtain the 14.5 gm of Na2TiV(PC>4)3 product.

[0111] Example K: Preparation of Na2CrTi(PO4)3

[0112] In a clean RBF, 50 gm (237.9 mmol) citric acid was added in 1000 ml water. The reaction mass was stirred for about 30 min. at about 30°C. 2.54 g (24.0 mmol) of sodium carbonate, 10 g (24.99 mmol) of chromium(III) nitrate nonahydrate, 6.82 g (24.0 mmol) of titanium isopropoxide, and 8.28 g (72.0 mmol) of ammonium dihydrogen phosphate were added in reaction mass at about 30°C. The reaction mixture was stirred at about 70°C until it formed a dry precursor. Next, this precursor was dried at about 120°C for about 12 hours to remove any remaining moisture. Finally, the material was heated first at about 350°C for about 5 hours, and then at about 800°C for about 12 hours under a continuous nitrogen atmosphere. After heating, the product was allowed to cool naturally to room temperature in nitrogen, to obtain the 8.5 gm of Na2CrTi(PO4)3 product.

[0113] Examples 15-18: Preparation of air stable Sodium Iron (II)-Hexacyanoferrate (II) treated by blending

[0114] 9.5 gm of sodium iron (Il)-hexacyanoferrate (II) (prepared in Example A) was mixed with 0.5 gm of electrochemically active materials prepared in Examples H-K in a planetary ball mill jar for a time period of about 60 minutes to obtain air stable.

[0115] Cell Fabrication and Testing Protocol

[0116] Following materials were used for fabrication of the cell:Active cathode material (Prepared in Examples 1-18 and A-G above): 7.5 g Super P conductive carbon: 2.0 gPoly vinylidene fluoride (PVDF): 0.5 gN-Methyl-2-pyrrolidone (NMP): 32 mLAluminum foil: 15 pm thicknessGlass fiber separator: 19 mm diameterSodium metal chips: 18 mm diameterElectrolyte: 100 pL of 1 M NaPFe in ethylene carbonate: propylene carbonate (EC: PC)CR2032 coin cell components (case, spacer, spring)

[0117] Electrode Preparation

[0118] Ball milling: 7.5 g active cathode material (prepared in Examples 1-18 and A-G above), 2.0 g Super P conductive carbon, and 0.5 g PVDF were charged into a ball mill jar with 100 g milling balls. The mixture was stirred at 100 RPM for 8 h, then unloaded. Slurry preparation: The ball-milled powder was transferred to a vacuum mixer jar, and 32 mL NMP was added. The slurry was mixed at 500 RPM for about 10 h under vacuum, then unloaded.Coating: The slurry was coated onto 15-pm-thick aluminum foil using a doctor blade (TOB model) to achieve a 370-pm wet thickness. The coated electrode was dried under vacuum at 100°C for about 10 hours. Calendaring: The dried electrode was calendered to 75-80 pm thickness. Punching: Electrode discs (16 mm diameter) were punched using a punching machine. Final drying: Punched electrodes were dried under vacuum at about 100°C.

[0119] Cell Fabrication: CR2032 coin cells were assembled in an argon-filled glovebox. The lower case was placed, followed by the cathode electrode (mass recorded), 19-mm glass fiber separator, 100 pL electrolyte (added via micropipette), 18-mm sodium metal anode, spacer, and spring. The upper case was added, and the cell was crimped using a crimping machine.

[0120] Cell Testing Protocol: Assembled cells were aged for 24 hours. Electrochemical testing was performed using a Neware CT8000 potentiostat from 4.2 to 2.2 V vs. Na / Na+. The protocol included 5 cycles each at C / 20, C / 10, C / 5, C / 2, 1C rates, followed by 5 cycles at C / 10.

[0121] The experimental data showing initial discharge capacity of the cells fabricated using materials prepared in Examples 1-5 and Examples A-G (Comparative Examples) above as active cathode material and stored at ambient conditions at a temperature of 25 °C ±3 °C at RH 65% for 0, 5, 10, 15 and 30 days are provided in Table 1 below.Table 1: Initial Discharge Capacity of the Cells at C / 10 C-rate Immediate Cell Cell Cell Cell Cell Fabriction Fabriction Fabriction Fabriction Examplefabrication after 5 after 10 after 15 after 30 (0 Days) Days Days Days Days Example119 mah / g 108 mah / g 96 mah / g 90 mah / g 84 mah / g AExample118 mah / g 122 mah / g 115 mah / g 112 mah / g 111 mah / g 1Example121 mah / g 117 mah / g 113 mah / g 112 mah / g 113 mah / g 2Example105 mah / g 97 mah / g 91 mah / g 83 mah / g 78 mah / gBExample107 mah / g 104 mah / g 105 mah / g 102 mah / g 99 mah / g 3Example103 mah / g 104 mah / g 102 mah / g 99 mah / g 97 mah / g 4Example94 mah / g 93 mah / g 94 mah / g 92 mah / g 91 mah / g CExample110 mah / g 108 mah / g 107 mah / g 105 mah / g 102 mah / g 5Example97 mah / g 98 mah / g 94 mah / g 94 mah / g 92 mah / g DExample90 mah / g 88 mah / g 89 mah / g 88 mah / g 86 mah / g EExample96 mah / g 95 mah / g 95 mah / g 92 mah / g 91 mah / g FExample90 mah / g 88 mah / g 89 mah / g 85 mah / g 86 mah / g GExample104 mah / g 103 mah / g 101 mah / g 101 mah / g 98 mah / g 11Example98 mah / g 96 mah / g 97 mah / g 93 mah / g 92 mah / g 12Example109 mah / g 110 mah / g 108 mah / g 108 mah / g 107 mah / g 13Example114 mah / g 112 mah / g 111 mah / g 112 mah / g 109 mah / g 14Example110 mah / g 108 mah / g 108 mah / g 106 mah / g 106 mah / g 15Example106 mah / g 104 mah / g 105 mah / g 102 mah / g 104 mah / g 16Example116 mah / g 118 mah / g 113 mah / g 112 mah / g 112 mah / g 17Example109 mah / g 108 mah / g 108 mah / g 107 mah / g 105 mah / g18

[0122] Based on the results, provided in Table 1, it is concluded that the alkali metal iron (II) -hexacyanoferrate (II) treated with electrochemically active material exhibit marked stability against air oxidation in comparison to the untreated alkali metal iron (II)-hexacyanoferrate (II) and Prussian White Analogues.

[0123] The foregoing examples are merely illustrative and are not to be taken as limitations upon the scope of the invention. Various changes and modifications to the disclosed embodiments will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the scope of the invention.ADVANTAGES OF THE PRESENT INVENTION

[0124] The present disclosure provides an air-stable alkali metal iron (II) -hexacyanoferrate (II).

[0125] The present disclosure provides an air-stable alkali metal iron (II) -hexacyanoferrate (II) that is economical and industrially applicable.

[0126] The present disclosure provides an air-stable alkali metal iron (II) -hexacyanoferrate (II) having higher sodium ion contents, high energy density and higher charge-discharge cycles, easy to manufacture, store, transport and handle for utility in sodium-ion batteries.

[0127] The present disclosure provides a method of preparation of an air-stable alkali metal iron (Il)-hexacyanoferrate (II) that is economical and industrially applicable.

[0128] The present disclosure provides an electrode that has a high initial charge capacity and is capable of being recharged multiple times without significant loss in charge capacity.

Claims

We Claim:

1. An air stable alkali metal iron (Il)-hexacyanoferrate (II) of Formula AxMy[Fe(CN)6].zH2Owherein A is an alkali metal or combinations thereof, M is Fe, x is 1.0 to 2.0, y is 0.7 to 1.0 and z is less than 5.0,wherein the alkali metal iron (Il)-hexacyanoferrate (II) is treated with an electrochemically active material such that at least part of surface of particles of alkali metal iron (II)- hexacyanoferrate (II) is covered with said electrochemically active material.

2. The air stable alkali metal iron (Il)-hexacyanoferrate (II) of claim 1, wherein the electrochemically active material is selected from:(a) a Prussian white analogue of formula AxNy[Fe(CN)6].zH2O, wherein A is an alkali metal or combinations thereof, N is Cu, Mn, Ni, Co, V or combinations thereof, x is 1.0 to 2.0, y is 0.7 to 1.0 and z is less than 5.0;(b) a Prussian white analogue of formula AxMyNz[Fe(CN)6].mH2O, wherein A is an alkali metal or combinations thereof, M is Fe, N is Cu, Mn, Ni, Co, V or combinations thereof, x is 1.0 to 2.0, y is 0.1 to 0.3, z is 0.7 to 1.0 and m is less than 5.0;(c) a sodium layered oxide cathode material;(d) a sodium polyanion cathode material;(e) an alkali metal salt of vanadium phosphate;(f) an alkali metal salt of halide;(g) a NASICON type cathode material; and combinations thereof.

3. The air stable alkali metal iron (Il)-hexacyanoferrate (II) of claim 1, wherein the sodium- layered oxide cathode material, the sodium polyanion cathode material, the alkali metal salt of vanadium phosphate, and the NASICON type cathode material are selected from Na4Fe3(PO4)2P2O7, Na3V2(PO4)3, sodium iron phosphate (NaFePO4), sodium iron sulphate (Na2FeSO4), sodium vanadium phosphate, sodium vanadium titanium phosphate (Na2TiV(PO4)3), sodium chromium titanium phosphate (Na2CrTi(PO4)3) and combinations thereof.

4. The air stable alkali metal iron (Il)-hexacyanoferrate (II) of claims 1-3, wherein the air stable iron (Il)-hexacyanoferrate (II) comprises said electrochemically active material in an amount ranging from 2 to 20 % by wt. of the air stable iron (Il)-hexacyanoferrate (II).

5. A method for preparation of air stable alkali metal iron (Il)-hexacyanoferrate (II) of FormulaAxMy[Fe(CN)6].zH2Owherein A is an alkali metal or combinations thereof, M is Fe, x is 1.0 to 2.0, y is 0.7 to 1.0 and z is less than 5.0,said method comprising a step of treating an alkali metal iron (Il)-hexacyanoferrate (II) with an electrochemically active material such that at least part of surface of particles of alkali metal iron (II) -hexacyanoferrate (II) is covered with said electrochemically active material.

6. The method as claimed in claim 5, wherein the step of treating comprises any of:(i) coating the alkali metal iron (Il)-hexacyanoferrate (II) with the electrochemically active material;(ii) blending the alkali metal iron (Il)-hexacyanoferrate (II) with the electrochemically active material;(iii) effecting in-situ generation of alkali metal iron (Il)-hexacyanoferrate (II) with the electrochemically active material; and(iv) treating precipitates of alkali metal iron (Il)-hexacyanoferrate (II) with a solution containing the electrochemically active material.

7. The method as claimed in claims 5-6, wherein the electrochemically active material is selected from: (a) a Prussian white analogue of formula AxNy[Fe(CN)6].zH2O, wherein A is an alkali metal or combinations thereof, N is Cu, Mn, Ni, Co, V or combinations thereof, x is 1.0 to 2.0, y is 0.7 to 1.0 and z is less than 5.0; (b) a Prussian white analogue of formula AxMyNz[Fe(CN)e] mFbO, wherein A is an alkali metal or combinations thereof, M is Fe, N is Cu, Mn, Ni, Co, V or combinations thereof, x is 1.0 to 2.0, y is 0.1 to 0.3, z is 0.7 to 1.0 and m is less than 5.0; (c) a sodium layered oxide cathode material; (d) a sodium polyanion cathode material; (e) an alkali metal salt of vanadium phosphate;(f) an alkali metal salt of halide; (g) a NASICON type cathode material; and combinations thereof.

8. The method as claimed in claims 6-7, wherein the step of coating comprises: adding the electrochemically active material to a solution containing the alkali metal iron (II)- hexacyanoferrate (II) to obtain the air stable alkali metal iron (Il)-hexacyanoferrate (II).

9. The method as claimed in claims 6-7, wherein the step of blending comprises: blending the electrochemically active material with the alkali metal iron (Il)-hexacyanoferrate (II), optionally, in presence of a solvent, to obtain the air stable alkali metal iron (II)- hexacyanoferrate (II).

10. The method as claimed in claims 6-7, wherein the step of effecting in-situ generation comprises: mixing a solution containing the alkali metal iron (Il)-hexacyanoferrate (II) of Formula A, a solution containing alkali metal ferrocyanide and a reducing agent, and a solution containing a metal precursor to obtain the air stable alkali metal iron (II)- hexacyanoferrate (II).

11. Use of the air stable alkali metal iron (Il)-hexacyanoferrate (II) of claim 1-4 as an active cathode material.

12. An electrode comprising the air stable alkali metal iron (Il)-hexacyanoferrate (II) of claim 1-4.

13. A cell comprising:an anode;a cathode, said cathode comprising the air stable alkali metal iron (II)- hexacyanoferrate (II) of claim 1-4;a separator; andan electrolyte.

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