WO2017180059A1

WO2017180059A1 - Monoclinic sodium iron hexacyanoferrate

Info

Publication number: WO2017180059A1
Application number: PCT/SG2017/050203
Authority: WO
Inventors: Balaya PALANI; Ashish RUDOLA
Original assignee: National University Of Singapore
Priority date: 2016-04-12
Filing date: 2017-04-11
Publication date: 2017-10-19

Abstract

There is disclosed is a compound of formula (I): Na_2-xFe_2-w(CN)_6-y.(Vacancy)_v.zH₂O, which compound is provided in the form of a monoclinic lattice system, and where v, w, x, y and z are as defined herein. Methods of making this material and its use to form electrodes for sodium ion batteries are also disclosed. In addition, the compound of formula (I) can be converted into a compound of formula (II) in a rhombohedral lattice system: Na_2-x'Fe_2-w'(CN)_6-y'.(Vacancy)_v'.z'H₂O by a further method disclosed herein, where v', w', x', y' and z' for the compounds of formula (II) are as defined herein. The use of compounds of formula (I) in the formation of electrodes for sodium ion batteries and electrodes comprising compounds of formula (II) prepared from converting electrodes comprising compounds of formula (I) are also disclosed.

Description

MONOCLINIC SODIUM IRON HEXACYANOFERRATE

Field of Invention

This invention concerns the discovery of a new phase in the Prussian Blue Analogue (PBA) family with the formula Na₂-xFe₂(CN)₆.2H₂0 (with x « 0) belonging to the monoclinic lattice system with the space group P2₁/n. A synthesis method is described which yields pure phase M-Na₂Fe₂(CN)₆.2H₂0 (M is short for monoclinic). This material may be used as a cathode for sodium-ion batteries (NIBs) or it may be converted into the known high capacity rhombohedral Na₂Fe₂(CN)₆ phase of the space group R3 and its subsequent application as an electrode (e.g. a cathode) in NIBs. This conversion may be conducted on the M-phase material when it has already been applied to an electrode, allowing one to convert an M- phase electrode to an electrode containing rhombohedral Na₂Fe₂(CN)₆.

Background

One of the compelling requirements for mitigating C0₂ emission is to generate our primary electricity from clean and renewable energy sources such as solar or wind. This action will have a greater impact in curbing global warming than widespread use of electric vehicles (EVs): if EVs are charged from electricity originating from power plants relying on fossil fuels, then the cumulative C0₂ emission (upon considering emissions from the dirty power plants) will be higher than those from using traditional gasoline-powered vehicles. The urgent need of the hour, hence, is the mass adoption of renewable power plants. However, the intermittent energy generation from such renewable sources needs to be addressed if they are to ever become commercially viable. Large-scale electrochemical energy storage (EES) devices, or grid-storage batteries, are the most convenient and practically relevant means for handling this intermittent energy generation from such power plants. Combined costs of the power plant and storage systems would be the deciding factor for market penetration of renewable energy sources into our electricity grid. Apart from low cost, the other most desirable performance metrics for such EES batteries are long cycle life (a few thousand cycles), high efficiency and a high degree of safety. Gravimetric and volumetric energy densities of such batteries are only a secondary factor, as the footprint and weight are not major concerns. Sodium-ion batteries (NIBs) are very attractive for such large-scale EES applications owing to the expected cost reductions arising from the globally abundant Na resources. However, to truly reduce costs further, it is imperative that cathodes and anodes in NIBs targeted for grid-storage batteries also use other earth-abundant elemental resources, such as Fe, Mn and Ti. Costs can further be decreased if the electrode materials display air and water stability, scalable syntheses are employed preferably in water medium without the need for additional calcinations/firing steps at high temperatures, environmentally safe, non-toxic and inexpensive chemicals are used throughout the production process and sodium storage occurs in the battery with good thermal and chemical stability at different charged/discharged states at room temperature. These factors would ultimately reduce the costs associated with battery manufacturing, production, maintenance and management processes which can significantly affect battery costs over its lifetime. However, these requirements are quite stringent. Despite such harsh demands, there have been a few promising NIB electrode materials reported which meet most of the above requirements for grid-storage batteries. Among them, there is a class of cathodes belonging to the Prussian Blue Analogue (PBA) family, which is very appealing due to its reliance on Fe and/or Mn as the redox active centers and possession of high sodium storage capacities (theoretical capacity limit as high as 170.8 mAh g^"1 assuming two mole sodium storage per mole of material) at relatively high voltages.

The general formula for PBAs relevant for NIBs is

with 0 < x < 2 and 0 < y < 1. Here, Mi and M₂ are transition-metal ions strongly covalently bridged by cyano (C≡N) ligands with octahedrally coordinated to N, M₂ octahedrally coordinated to C and

□ refers to vacancies which may arise during the synthesis. The crystal structure of these compounds consists generally of either perfect or slightly distorted cubes with Mi and M₂ situated at the corners, bridged by the cyano ligands. This arrangement leaves eight sub- cube units within each unit cell where alkali ions such as Na⁺ and/or interstitial water molecules may reside. Most of the PBAs which have been reported to store sodium have less Na content in the as- synthesized compounds (0 < x < 1 ) demonstrating a cubic structure with a space group Fm- 3m. This may become a disadvantage in a full cell formulation against a suitable anode as a full cell relies on the cathode supplying Na during the first charging (sodium extraction from cathode and insertion into the anode). An x = 1 value would mean that the practically achieved capacity of such a cathode would be half of its theoretical capacity (corresponding to the capacity for an analogous cathode with x = 2) in a practical full cell assuming no loss of Na due to surface passivation and 100% coulombic efficiency for both cathode and anode. Recently, some papers have reported successful synthesis of Na and Fe containing PBAs with slightly richer Na content (1.5 < x < 1.6). Interestingly, for these PBAs, the cubic structure is still maintained (W.-J. Li, et a/., Chem. Mater., 27 (6), 1997-2003 (2015); Y. Liu, et at, Nano Energy, 12 386-393 (2015); M. J. Piernas-Munoz, er el., J. Power Sources, 324 766-773 (2016); and Y. Tang.ei a/., J. Mater. Chem. A, 4 (16), 6036-6041 (2016)).

To date, there have been only two reports which have displayed a value of 1.6 < x < 2 in their general formula NaxI ^M^CN^.n^O, particularly with both M† and M₂ as Fe. You er al. (Nano Res., 8 (1 ), 1 17-128 (2015)) synthesized Na_{1 63}Fei.89(CN)₆ with a rhombohedral crystal structure capable of delivering 153 mAh g^"1 during its first charge (Na extraction process). However, the authors did not provide details about the space group or the water content in the structure of

It has been recently reported three variations of the following Na rich PBAs- monoclinic M-Na₂-₅Mn[Fe(CN)₆].1.87H₂0, rhombohedral R-Na₂. eMn[Fe(CN)₆] and rhombohedral R-Na₁._g2Fe[Fe(CN)₆]- demonstrating high capacity of 150 - 160 mAh g^"1 at an attractive voltage of 3 - 3.6 V vs Na/Na⁺ (see L. Wang et al., Angew. Chem. Int. Ed., 52 (7), 1964-1967 (2013); J. Song er a/., J. Am. Chem. Soc, 137 (7), 2658- 2664 (2015); L. Wang er al., J. Am. Chem. Soc, 137 (7), 2548-2554 (2015)). Song er al. demonstrated the critical effect of interstitial water in influencing the structure of the Mn-Fe PBA; water expulsion was shown to raise the symmetry from monoclinic Ρ2_ΛΙη space group to rhombohedral R3~along with a flattening of the voltage profiles. Curiously, the monoclinic equivalent of an all Fe based PBA is not currently known. The sodium rich compound "Na₂-xFe₂(CN)₆ with x = 0" is known to exist only in the rhombohedral R3 phase (see L. Wang et al., J. Am. Chem. Soc, 137 (7), 2548-2554 (2015)). R3 Na₂Fe₂(CN)₆ is known as a very promising NIB cathode demonstrating energy density values comparable to existing lithium-ion battery (LIB) cathodes such as LiFeP0₄. However, R3 Na₂Fe₂(CN)₆ degrades quickly if it is exposed to ambient air.

US patent application No. 2015/0357630 discloses a cathode meant for application in NIBs. The active material in this cathode is described by the general formula Na_xFe Fe_N(CN)_z.d[H₂0]_ZEo e[H₂0]_BND, where 0.5 < x < 2, 0.5 < M < 1.5, 0.5 < N < 1.5, 5 < Z < 6, 0 < d < 14 and e > 0. In this formula "ZEO" refers to Zeolitic water and "BND" refers to bound water (lattice water which forms part of the crystal structure). Dehydration to get rid of the zeolitic water is effected by heating the composition at a temperature greater than 120 °C and less than 200 °C. As disclosed in this application, this removed the zeolite water to provide a formula where d = 0, but which retains at least some bound water. While the resulting stoichiometry is similar to that disclosed in the current application, as discussed in more detail hereinbelow, the electrochemical signatures provided are very different to those obtained in the current application for the monoclinic lattice material disclosed herein, but are consistent with the rhombohedral R3 phase materials (without any bound water) disclosed herein.

Thus, there remains a need for materials that may have advantages in terms of air stability and ease of synthesis, as well as ways of manufacturing existing materials in more convenient ways so as to significantly reduce the costs associated with production and manufacturing.

Summary of Invention

Aspects and embodiments of the invention are provided by the numbered clauses below.

1. A compound of formula (I): Na_2-xFe_2-w(CN)_6-y.(Vacancy)_v.zH₂0 (I)

wherein:

0 < x < 1 , 0 < w < 0.2, 0 < y < 0.2, 0 < v < 0.2 and 0 < z < 4; and

the compound of formula (I) is provided in the form of a monoclinic lattice system. 2. The compound of Clause 1 , wherein 0 < x < 0.5, 0 < w < 0.1 , 0 < y < 0.1 , 0 < v < 0.1 and 0 < z < 2.5.

3. The compound of Clause 2, wherein 0 < x < 0.2, w and y are 0, 0 < v < 0.1 and 0 < z < 2.2.

4. The compound of Clause 3, wherein the compound of formula (I) has the formula (la):

M-Na_2-xFe₂(CN)₆.2H₂0 (la),

where M is an indicator of the monoclinic lattice system and x is from 0 to 0.2.

5. The compound of Clause 4, wherein x is 0. 6. The compound of any one of the preceding clauses, wherein the monoclinic lattice system has the space group P2†/n.

7. The compound of Clause 6, wherein the space group is P2₁/n and the lattice parameters are a =10.45983(56) A, b = 7.51295(42) A, c = 7.27153(48) A and β =

92.7379(33)°.

8. The compound of any one of the preceding clauses, wherein the compound shows principal (most intense) peaks at angles of diffraction 2Θ of 17.0, 23.7, 24.5 and 34.3 in an X- ray powder diffraction pattern obtained by irradiation with copper Ka radiation (wavelength λ = 1.54 angstroms).

9. An electrode comprising the compound of formula (I) as described in any one of Clauses 1 to 8, optionally wherein the electrode is a cathode.

10. A method of preparing a compound of formula (I), as described in any one of Clauses 1 to 18, which method comprises the steps of:

(a) reacting a solution comprising Na₄Fe(CN)₆, or hydrates thereof, a reducing agent and water for a period of time to provide a precipitate;

(b) separating the precipitate from the solution; and

(c) drying the precipitate to provide a compound of formula (I).

1 1. The method of Clause 10, wherein the Na₄Fe(CN)₆ is provided as the hydrate Na₄Fe(CN)₆.10H₂O.

12. The method of Clause 10 or Clause 1 1 , wherein the reducing agent is provided in molar excess relative to the Na₄Fe(CN)₆ or hydrate thereof.

13. The method of any one of Clauses 10 to 12, wherein the reducing agent is selected from one of more of the group consisting of ascorbic acid, oxalic acid, glucose, starch, gluconic acid lactone, and formic acid, optionally wherein the reducing agent is ascorbic acid and/or formic acid.

14. The method of any one of Clauses 10 to 12, wherein the temperature of reaction step (a) is from 0 °C to 200 °C, such as from 80 °C to 170 °C, from 90 °C to 160 °C, from 100 °C to 150 °C (e.g. 140 °C). 15. A method of converting a compound of formula (I), according to any one of Clauses 1 to 8, into a compound of formula (II),

Na₂._x Fe₂._w.(CN)_6-y..(Vacancy)_v..z'H₂0 (II)

wherein:

0 < x' < 1 , 0 < w' < 0.2, 0 < y' < 0.2, 0 < v' < 0.2 and 0 < z' < 0.5; and

the compound of formula (II) is provided in the form of a rhombohedral R lattice system having the space group R3, which process comprises the steps of:

(a) providing a compound of formula (I); and

(b) heating the compound of formula (I) to a temperature from 50 °C to 500 °C for a period of time under vacuum, dry air or an inert atmosphere to provide the compound of formula (II).

16. The method of Clause 15, wherein the conversion process is conducted on an electrode comprising a compound of formula (I) to provide an electrode comprising the compound of formula (II), optionally wherein the electrode is a cathode.

17. A method of converting an electrode comprising a compound of formula (I), according to any one of Clauses 1 to 8, into an electrode comprising a compound of formula (II),

Na₂-x'Fe_2-w'(CN)_6-y..(Vacancy)_v..z'H₂0 (II)

wherein:

0 < x' < 1 , 0 < w' < 0.2, 0 < y' < 0.2, 0 < v' < 0.2 and 0 < z' < 0.5; and

(a) providing a compound of formula (I); and

18. The method of any one of Clauses 15 to 17, wherein 0 < x' < 0.5, 0 < w' < 0.1 , 0 < y' < 0.1 , 0 < v' < 0.1 , and 0 < z' < 0.2 (e.g. 0 < x' < 0.5, 0 < w' < 0.1 , 0 < y' < 0.1 , 0 < v' < 0.1 , and z' is 0). 19. The method of Clause 18, wherein the compound of formula (II) has the formula (Ha):

R-Na_2-x Fe₂(CN)₆ (lla), where R is an indicator of the rhombohedral lattice system having the space group R3 and x' is from 0 to 0.2, optionally wherein x' is 0.

20. A compound of formula (II),

Na₂._xFe_2-w<CN)_6-y.(Vacancy)_v..z'H₂0 (II)

wherein:

0≤S x'≤S 1 , 0 ¾Ξ w'≤ 0.2, 0≤ y' ¾S 0.2, 0≤ v'≤ 0.2 and 0≤S z' ¾S 0.5; and the compound of formula (II) is provided in the form of a rhombohedral R lattice system having the space group R3 and provided that the crystal lattice contains no zeolitic water.

21. The compound of Clause 20, wherein 0≤S x'≤ 0.5, 0 =¾ w'≤S 0.1 , 0 y'≤ 0.1, 0 5S v"≤Ξ 0.1 and 0≤ z'≤ 0.2.

22. The compound of Clause 21 , wherein 0≤£ x'≤S 0.5, 0≤ w' =¾ 0.1 , 0 y'≤ 0.1 , 0 ¾S v' 0.1 and z' is 0.

23. The compound of Clause 21 or Clause 22, wherein the compound of formula (II) has the formula (I la):

R-Na₂._x Fe₂(CN)₆ (Ma),

where R is an indicator of the rhombohedral lattice system having the space group R3, and x' is from 0 to 0.2, optionally wherein x' is 0.

24. A sodium ion battery comprising an electrode comprising a compound of formula (I) according to any one of Clauses 1 to 8 and/or a compound of formula (II) according to any one of Clauses 15 to 23.

In summary, this patent covers the following numbered features, which are expanded on further below.

1. This invention is concerned with the discovery of a new PBA phase which can be represented as Na2-xFe2(CN)₆.2H₂0 with x = 0. We have proved that it is a distinct phase, belonging to the monoclinic crystal family and hence, referred to as M- Na₂Fe₂(CN)₆.2H₂0. Its application to NIBs (both aqueous and non-aqueous) is to be protected. 2. The synthesis method described herein in order to synthesize M-Na₂Fe2(CN)6.2H₂0 is to be protected. Please do keep in mind that some tolerance of x, w, y, z and v (for vacancies) in the general formula of M-Na2_-xFe2-w(CN)_6-y.(\ acancy)_v.zH₂0 will be covered as well. As such, the following compound's synthesis method, as detailed in the "Results and Analysis" section, and subsequent application to NIBs is protected: M- Na₂-xFe2-w(CN)_6-y.(Vacancy)_v.zH₂0 with 0 < x < 1 , 0 < w < 0.2, 0 < y < 0.2, 0 < v < 0.2 and 0 < z < 4. It should be stated that though we did not observe any significant vacancies in this material based on the electrochemical curves and elemental analyses, a small amount of vacancies cannot be ruled out as they can be present in PBAs.

3. While claim 1 protects the application of M-Na2Fe₂(CN)₆.2H₂0 in NIBs, claim 2 protects the particular synthesis approach described herein with application not just limited to batteries; any application is protected if the synthesis method used to synthesize M-Na2Fe2(CN)₆.2H₂0 is the same as that described in this patent.

4. This patent protects the idea of converting an electrode fabricated with the active material being M-Na₂Fe₂(CN)₆.2H₂0 into an electrode with the high capacity active material phase R3 Na₂Fe₂(CN)₆ by a simple heating process at temperatures >50 °C and < 500 °C in either vacuum, air or inert atmosphere or in a dry room. This is due to the discovery revealed herein that the M-Na₂Fe2(CN)₆.2H₂0 phase transforms to the R3 Na₂Fe₂(CN)₆ phase upon heating. Furthermore, this patent also protects the method of obtaining R3 Na₂Fe₂(CN)₆ phase by heating the M-Na₂Fe2(CN)₆.2H₂0 phase (first synthesized by the method disclosed in this patent) when the M- Na₂Fe2(CN)₆.2H₂0 phase is in any form (not just limited to being coated on an electrode), including the powder form.

5. It should be mentioned that the electrode fabricated with M-Na₂Fe₂(CN)₆.2H₂0 to be used in NIBs could be on any conductive substrate with any conductive additive (if any at all) and binder material (both water soluble and otherwise) generally used in the NIB field. Furthermore, the final NIB with these two phases as cathodes can be coupled with any NIB anode in any electrolyte (both aqueous and non-aqueous) and in any cell configuration.

Drawings

Figure 1. Characterization of the as-synthesized powder, a) FESEM image of the as- synthesized M-Na2Fe₂(CN)₆.2H₂0 powder; b) TGA curve of the as-synthesized M-Na₂-xFe2(CN)₆.2H₂0 conducted in an inter N₂ atmosphere; c) TGA with Mass Spectroscopy (MS) of the as-synthesized M-Na₂-_xFe2(CN)₆.2H20 also conducted in N₂ atmosphere showing signals from water, HCN and C0₂. Please note that there is some delay in temperature recordings during the TGA-MS measurements, which is why, the temperatures are slightly higher with respect to that of the TGA experiment, this may also be due to use of a different instrument for TGA-MS; d) FTIR in transmission mode and e) the fitted XPS curve zoomed in to the Fe 2p₃₂ edge, f) XRD plot with Rietveld refinement using a monoclinic structural model (space group P2₁/n). The inset depicts the higher angles of 70 - 140 °2Θ for clarity. The vertical green ticks indicate the expected positions of the Bragg reflections. A reasonable fitting was obtained with reliable refinement factors (R_wp = 7.42 %, Bragg = 4.621 %, χ² = 3.39 %, and Re_Xp = 2.19 %) validating the monoclinic structural model.

Figure 2. XRD patterns of the as-synthesized M-Na₂Fe2(CN)₆.2H₂0 powder after being stirred in excess water for 17 h to show that the M-Na₂Fe₂(CN)₆.2H₂0 material is water- insoluble. No change in the XRD pattern was observed after immersing it in water with respect to that of the pristine material which was not stirred in water.

Figure 3. Galvanostatic cycling of M-Na₂Fe₂(CN)₆.2H20 vs Na metal in a half cell, a) C/1.8 cycling within 4.3 - 2.0 V window (charged first). The effect of discharging first is shown as well by the red curve - a negligible capacity was obtained reflecting the Na rich nature of the material. Electrolyte used was 1 M NaCI0₄ in EC:PC (1 :1 v/v). b) Cycling within the 3.9 - 2.0

V window corresponding to slightly in excess of one mole Na storage per mole of material. The discharge profiles at various rates are shown, with the charging cycle being conducted at C/4.5 rate, c) The corresponding discharge capacity values vs cycle number at various rates, d) Long term cycling of M-Na₂Fe₂(CN)6.2H₂0 within the 3.9 - 2.0 V window at 2.2C rate over 3,000 cycles. For the 3.9 - 2.0 V window, the electrolyte was 0.6 M NaPF₆ in EC: PC based.

Figure 4. Structural transformations occurring in M-Na₂Fe2(CN)6.2H₂0 during sodium storage, a) Ex-situ XRD plots at various points as indicated during charging and discharging within the 3.9 - 2.0 V window illustrating the mixture of two-phase and solid-solution reaction mechanisms, b) Ex-situ FTIR at selected states of charge and discharge in the high wavenumber region, highlighting that the structural water was preserved during the 3.9 - 2.0

V cycling but released if M-Na2Fe₂(CN)₆.2H₂0 was charged to 4.3 V. c) DSC curves on charged M-Na₂Fe2(CN)₆.2H₂0 to 3.9 V and 4.3 V illustrating the presence of an endothermic peak for the 3.9 V sample which was due to water loss from the structure triggered by heating during the DSC experiment. For the 4.3 V sample, no such endothermic reaction was observed indicating that water loss had already occurred during galvanostatic charging to 4.3 V. The DSC measurements were conducted in Ar atmosphere without any air exposure to the samples, d) Ex-situ FTIR plots zoomed to the cyanide stretching frequencies at corresponding points to that shown in panel b). The FTIR spectra clarify the involvement of HS-Fe²⁺ and LS-Fe²⁺ in the galvanostatic lower and upper charge/discharge voltage plateaus of M-Na₂Fe2(CN)₆.2H₂0, respectively.

Figure 5. Zoomed-in first galvanostatic cycle of M-Na₂Fe2(CN)6.2H₂0 at C/4.5 rate showing the flat voltage profile in the initial stage of charge or later stage of discharge and the sloping voltage profile thereafter in the charging cycle or during the initial-middle parts of the discharging cycle corresponding to two-phase and solid-solution reaction mechanisms, respectively. The electrolyte used was 0.6 M NaPF₆ in EC:PC (1 :1 volume ratio) with 5 volume % FEC added.

Figure 6. Ex-situ XRD characterization of the two moles sodium storage per mole of M- Na₂Fe₂(CN)₆.2H₂0 within the voltage window 4.3 - 2.0 V. a) Ex-situ XRD patterns at selected states of charge and discharge revealing an intensification of the XRD peaks for the 4.3 V charged sample with respect to that of 3.9 V sample, without any observed peak shifts, b) Ex-situ XRD patterns of M-Na₂Fe₂(CN)₆.2H₂0 electrodes discharged to 2.0 V (in the 4.3 - 2.0 V window) after the 1^st and 500^th cycles. The greatly reduced XRD peaks after 500 cycles indicate significant structural collapse, while the peak shifts indicate structural distortion.

Figure. 7 XRD pattern on electrodes with the two different phases of Na_2-xFe₂(CN)₆ (x « 0): the M-Na_2-xFe₂(CN)₆.2H₂0 and the R3 Na₂Fe₂(CN)₆ phase. The latter was obtained by heating the former above 200 °C in Ar atmosphere.

Figure 8. Thermal dehydration of the M phase in ambient air and the ambient air stabilities of the M and R phases, a) Variable temperature XRD patterns of M-Na₂Fe₂(CN)₆.2H₂0 at various temperatures in ambient air atmosphere, showing the conversion to the R phase upon the loss of structural water caused by heating, b) Air stability of M-Na₂Fe₂(CN)₆.2H₂0 powder in ambient air at various periods as indicated, c) Air stability of R-Na₂Fe₂(CN)₆ electrode (formed by heating M electrode above 240 °C in inert Ar atmosphere) when exposed to ambient air. Figure 9. Effect of thermal dehydration of M-Na₂Fe₂(CN)₆.2H₂0. a) Variable temperature XRD patterns obtained in high vacuum of 10^"2 mbar showing the complete conversion to the R-Na₂Fe₂(CN)₆ phase at 100 °C. b) XRD patterns of -Na₂Fe₂(CN)₆.2H₂0 powder which was dried in air at 70 °C and another XRD pattern of M-Na₂Fe2(CN)₆.2H₂0 powder dried in a milder vacuum of 1 mbar at 120 °C. These results indicate the importance of the degree of vacuum to facilitate the thermal dehydration of M-Na₂Fe₂(CN)₆.2H₂0: mild vacuum conditions would be ineffective for conversion of M to R phase even till 120 °C. c) The first two gaivanostatic cycles of R-Na₂Fe₂(CN)₆ shown at C/2 rate using 0.6 M NaPF₆ in EC:PC (1 :1 volume ratio) with 5 volume % VC added. The electrochemical signature and the capacity value close to 164 mAh g^"1 indicate the successful conversion of the M phase to the R phase. An M-Na₂Fe2(CN)6.2H₂0 electrode was heated above 240 °C in inert Ar atmosphere to convert it into the R phase and then cycled in a Na battery.

Figure 10. XRD patterns of M-Na₂Fe2(CN)₆.2H₂0 after storage in inert Ar atmosphere for 5 months indicating its stability in such atmospheres for several months. Hence, prolonged storage of this material in inert atmosphere should not be a concern. Figure. 11 First gaivanostatic cycle of an (a) M-Na₂Fe2(CN)₆.2H₂0 and (b) R3 Na₂Fe₂(CN)₆ electrode in a non-aqueous NIB in a half cell (Na metal was the reference and counter electrode). The sodium rich nature of the compounds is evident as the first process was charging (sodium extraction from the M or R3 Na₂Fe₂(CN)₆). Although the cycling rate for the M-Na₂Fe₂(CN)₆.2H₂0 phase is shown as C/5, it was actually C/4.5.

Figure. 12 Rate Performance of an M-Na₂Fe2(CN)₆.2H₂0 electrode at various rates from C/5 to 10 C with (a) the discharge capacity at different rates vs cycle number and (b) the corresponding cycling curves with a sample C/5 charging curve. The cells were cycled in a non-aqueous Na half cell with the charging cycle at C/5. Please note that the C rates mentioned in the figure are slightly lower than the actual values. The C rates of C/5, C/2, 1 , 2C, 3C, 5C and 10C shown in this figure were actually C/4.5, C/1.8, 1.1C, 2.2C, 3.3C, 5.6C and 11 .1 C, respectively.

Figure. 13 Rate Performance of an R3 Na₂Fe₂(CN)₆ electrode at various rates from C/5 to 5 C with (a) the discharge capacity at different rates vs cycle number and (b) the corresponding cycling curves with a sample C/5 charging curve. The cells were cycled in a non-aqueous Na half cell with the charging cycle at C/5.

Description

Features of the current invention and their potential advantages are discussed below. There is disclosed a compound of formula (I):

Na₂-xFe_2-w(CN)₆__y.(Vacancy)_v.zH₂0 (I)

wherein:

0 < x < 1 , 0 < w < 0.2, 0 < y < 0.2, 0 < v < 0.2 and 0 < z < 4; and

the compound of formula (I) is provided in the form of a monoclinic lattice system.

These compounds, having a monoclinic lattice system may be air stable, water insoluble and possess impressive sodium storage characteristics when used as an NIB cathode. Most importantly, these compounds are sodium rich, which means that it can be used as a NIB cathode straightaway.

While no significant amount of vacancies were observed in the compounds described in the examples section below based based on the electrochemical curves and elemental analyses, a small amount of vacancies cannot be ruled out as they can be present in Prussian Blue Analogues. When used herein "vacancy" is used to refer to a type of point defect in the crystal lattice of the compounds described herein caused when an atom is missing from one of the lattice sites of the crystal.

Compounds of formula (I) that may be mentioned herein may have 0 < x < 0.5, 0 < w < 0.1 , 0 < y < 0.1 , 0 < v < 0.1 and 0 < z < 2.5. More particular compounds of formula (I) that may be mentioned herein may have 0 < x < 0.2, w and y are 0, 0 < v < 0.1 and 0 < z < 2.2.

Specific compounds of formula (I) that may be mentioned herein may have the formula (la):

M-Na_2-xFe₂(CN)_6.2H₂0 (la),

where M is an indicator of the monoclinic lattice system and x is from 0 to 0.2 (e.g. x = 0).

M-Na_2-xFe₂(CN)₆.2H₂0 (x « 0 or is 0) as described herein is air stable, water insoluble and possesses impressive sodium storage characteristics when used as an NIB cathode. Most importantly, this material is sodium rich (x ^¾ 0, or is 0), which means that it can be used as a NIB cathode straightaway.

As noted above, the compound of formula (I) and formula (la) have a monoclinic lattice system. This monoclinic lattice system may have the space group P2₁/n. In particular examples of the compounds of formula (I) and formula (la), when the space group is P2 n and the lattice parameters may be a =10.45983(56) A, b = 7.51295(42) A, c = 7.27153(48) A and β = 92.7379(33)°. Particular compounds of the compound of formula (I) and formula (la) that may be mentioned herein may show principal (most intense) peaks at angles of diffraction 2Θ of 17.0, 23.7, 24.5 and 34.3 in an X-ray powder diffraction pattern obtained by irradiation with copper Ka radiation (wavelength λ = 1.54 angstroms). For example, these peaks may particularly correlate to a compound of formula (la) where x is 0.

As noted hereinbefore, the compound of formula (I) and formula (la) may be particularly suitable for use in the manufacture of an electrode. As such, there is also disclosed an electrode comprising the compound of formula (I) or formula (la). Said electrode may be a cathode. Details of how to manufacture such electrodes (e.g. cathodes) are provided in more detail hereinbelow.

Without wishing to be bound by theory, it is believed that the method of manufacture of the compound of formula (I) (and la) is crucial to provide the newly discovered monoclinic lattice system form disclosed herein. To that end, the process requires the steps of:

(b) separating the precipitate from the solution; and

(c) drying the precipitate to provide a compound of formula (I).

When used herein "or hydrates thereof is used to refer to hydrates of Na₄Fe(CN)₆. For example, the hydrate may be any hydrated form of Na₄Fe(CN)₆ according to the formula (X):

Na₄Fe(CN)₆.nH₂0 (X)

where 0 < n < 10. For example, the hydrated form may be the decahydrate (Na₄Fe(CN)₆.10H₂O).

Certain aspects of this synthesis which can be varied or must be kept the same are discussed below.

1. The relative amounts of the precursor elements added do not seem to matter much, though a greater quantity (i.e. an excess) of reducing agent (e.g. ascorbic acid) added with respect to Na₄Fe(CN)₆.10H₂O results in a greater yield of the final M- Na₂Fe₂(CN)₆.2H₂0 product. Thus, it may be advantageous to provide the reducing agent in a molar excess.

2. Without wishing to be bound by theory, the function of ascorbic acid in the reaction is to serve as a reducing agent that prevents oxidation of the Fe²⁺ ions in the solution to Fe³⁺ ions. This is quite critical in this synthesis. However, ascorbic acid is just one example of many reducing agents that could be used instead in this synthesis. Other examples of reducing agents that may be used include, but are not limited to oxalic acid, glucose, starch, formic acid, gluconic acid lactone or any organic or inorganic compound which may possess these reducing properties. Particular reducing agents that may be mentioned herein are ascorbic and fomic acids.

3. The reaction temperature can vary between 0 - 200°C. In practice, it was observed that the phase purity did not change if the reaction was carried out at high temperature (such as 180 °C) vs that carried out at lower temperatures (such as 140 °C). Suitable temperature ranges for the reaction may be from 80 °C to 70 °C, from 90 °C to 160 °C, from 100 °C to 150 °C (e.g. 140 °C). In particular embodiments, it may be useful to use reflux conditions to conduct the reaction and, as the solvent used is water, this would appear to require a temperature of at least 100 °C. However, as the reflux temperature for water is influenced by the type of salts/compounds dissolved in it, it is possible that the reflux temperature may in some instances be below 100 °C, such as to 80 °C or 90 °C, depending on the type of reducing agent used.

4. Any means of heating the reaction can be used, such as an oil bath, heating mantles, hot plates, heating coils, temperature chambers/ovens and the like.

5. In the example below, while stirring the reaction in the oil bath (for heating), a precipitate of the product seemed to form quite readily (within 5-15 min). Hence, the stirring time (i.e. the time under heating) can be varied between 5 min - 100 h and it is expected that stirring time would not influence phase purity but may just affect the yield.

6. The method of obtaining the precipitate from the solution was not found to influence phase purity. As such, any of the following methods can be used for this purpose: centrifugation, filtration, spray drying, roto-evapoartion, physically removing the supernatant by any means such as by a dropper or through a tube sucking the supernatant out operated by a pump or through any other means.

7. Drying of the as-obtained precipitate can be conducted at any temperature between room temperature to below 200 °C in either vacuum or air or in any inert gas. The end product is the desired compound of formula (I). In the examples described below, the desired product is M-Na₂Fe2(CN)6.2H₂0.

As the process to make the compounds of formula (I) (e.g. M-Na₂Fe2(CN)6.2H₂0) is an easy to scale up solution-based synthesis conducted in a water medium at atmospheric pressure, it is expected that the process will be beneficial because it obviates the need for high- pressure reactions, thus helping to reduce costs. In addition, the process provides essentially pure phase of the compounds of formula (I) (e.g. pure phase M-Na₂Fe₂(CN)₆.2H₂0), which can be used as-is or can be converted into a compound of formula (II) (e.g. R3 Na₂Fe₂(CN)₆) as discussed below.

While the synthesis method described herein may at first appear similar to what has been previously reported, the changes made to the method have a major impact in formation of the new phase described herein (e.g. M-Na₂Fe₂(CN)₆.2H₂0). The detailed differentiation of this synthesis from that existing in prior art is as follows:

1. Wang et al. Journal of the American Chemical Society, 2015, 137, 2548-2554 have described a pressure based reaction using similar precursor compounds of this patent (Na₄Fe(CN)₆, ascorbic acid and water). However, the outcome of that synthesis was that the R3 Na₂Fe₂(CN)₆ phase was obtained directly. The synthesis described herein differs from the Wang synthesis because it was conducted at atmospheric pressures using a refluxing approach. Even though the currently disclosed synthesis and Wang's synthesis were both conducted at 140 °C, it appears that the effect of pressure has an important role to play in the final product. Not only is the currently disclosed synthesis atmospheric pressure based (hence, it is safer and expected to be less expensive), but it also results in the discovery of the new M- Na₂Fe₂(CN)₆.2H₂0 phase. The M-Na₂Fe₂(CN)₆.2H₂0 phase is air stable and water insoluble in contrast to the air instability and extreme moisture sensitivity of the R3 phase. This is practically important as it ensures easy handling and also easy electrode fabrication processing for this compound.

2. US patent application publication No. 20150266745 describes a synthesis using Na₄Fe(CN)₆.10H₂O (the same compound used in this patent) and HQ in water with the mixture maintained at 0 - 100 °C to yield a compound (Na₂FeFe(CN)₆) which has the same electrochemical signature as R3 Na₂Fe₂(CN)₆, although they did not provide any XRD, TGA or space group information (it is inferred that it is R3 Na₂Fe₂(CN)<5 based on the electrochemical signature). The key difference between their synthesis and ours is the reaction temperature and the usage of HCL which result in the formation of the M-Na₂Fe₂(CN)₆.2H₂0 phase described here.

3. US patent application publication No. 20140050982 discloses a synthesis method for synthesizing Na _+xFe[Fe(CN)₆]_z (where x and z are each < 1 ) using a sodium salt, a separate ferrocyanide salt and a reducing agent (including ascorbic acid) in water medium heated anywhere between 20 - 1000 °C. Obviously, this synthesis method differs from that of the current invention because a single sodium and iron source (Na₄Fe(CN)₆) is used in the process disclosed herein and the resulting reaction product is also different (M-Na₂Fe₂(CN)6.2H₂0 phase obtained herein while Lu ef al. obtained a rhombohedral phase directly). It should be kept in mind that the electrochemical signature of their final compound, shown in Fig. 9, is identical to that of the R3 Na₂Fe₂(CN)₆ phase.

4. You ef al. Nano Research, 2014, 8, 117-128 disclose a compound with the stoichiometry Na .₆₃Fe .₈9(CN)₆ using water as solvent, ascorbic acid and Na₄Fe(CN)₆.10H₂O and HCI. Furthermore, the synthesis was conducted under a N₂ atmosphere and the solution was stirred at 80 °C for 4 h. The synthesis disclosed herein is not only simpler (no N₂ atmosphere required and less reagents), but it also yields a more sodium rich PBA, which has practical advantages as discussed herein. In addition, the crystal structure of the compound obtained by the process of You ef al. was rhombohedral (no space group was suggested), which is different from that disclosed herein and no evidence of lattice water in the crystal structure was shown either.

A further advantage associated with the active material of the compound of formula (I) is that it is possible to convert the compound of formula (I) as described hereinbefore, into a compound of formula (II),

Na_2-x Fe_2-w<CN)₆__y..(Vacancy)_v..z'H₂0 (II)

wherein:

0 < x' < 1 , 0 < w' < 0.2, 0 < y' < 0.2, 0 < v' < 0.2 and 0 < z' < 0.5; and

(a) providing a compound of formula (I); and

It will be appreciated that the actual temperature needed will depend on the conditions that the process is conducted under. For example, with a close to full vacuum, the temperature necessary to conduct the transformation from the compound of formula (I) to the compound of formula (II) may be close to 50 °C (e.g. from 50 °C to 150 °C) and the temperature will vary with the applied vacuum. If dry air or an inert gas is used under standard pressure, the temperature used to effect the transformation may be from, for example 200 °C to 500 °C. The process described above may result in a significant reduction in bound water within the resultant compound of formula (II). As such, the compound of formula (II) may fall within the following values 0 < x' < 0.5, 0 < w' < 0.1 , 0 < y' < 0.1 , 0 < v' < 0.1 , and 0 < z' < 0.2. In fact, the process described herein may result in the complete removal of bound water from the compound of formula (II) and as such, the values may also be 0 < x' < 0.5, 0 < w' < 0.1 , 0 < y' < 0.1 , 0 < v' < 0.1 , and z' is 0. For example, the compound of formula (II) may have the formula (I Is):

R-Na_2-x Fe₂(CN)₆ (lla),

where R is an indicator of the rhombohedral lattice system and x' is from 0 to 0.2 (i.e. x' is 0).

The above described synthesis method for the compounds of formula (I) (e.g. M- Na₂Fe₂(CN)₆.2H₂0) provides a water insoluble material that can be used to fabricate an electrode on any conductive substrate using any type of binder material (both water soluble and/or water insoluble) that is generally used in battery electrode fabrication in the field of NIBs (both aqueous and non-aqueous). Furthermore, the electrode may contain conductive additives or it may not, which is discussed in more detail below. Once such an electrode is prepared, it can be converted into the high capacity electrode comprising a compound of formula (II) (e.g. R3 Na₂Fe₂(CN)₆) by simply heating the compound of formula (I) (e.g. M- Na₂Fe₂(CN)₆.2H₂0) electrode in either vacuum, air or any inert atmosphere at any temperature between 50 to 500 °C (e.g. from 140 to 500 °C). Once the R3 phase of the compound of formula (II) is formed, the electrode ceases to be air stable. Hence, post- heating handling would need to be conducted either in inert atmosphere or in a dry room; if left to ambient air, the R3 phase would be lost within 15 - 45 min. As a non-limiting example, converting an electrode fabricated with the active material M- Na₂Fe₂(CN)₆.2H₂0 into an electrode with the active material R3 Na₂Fe₂(CN)₆ (a higher capacity active material) can be achieved by a method that involves simply heating the previously formed M-Na₂Fe₂(CN)₆.2H₂0 electrode. R3 Na₂Fe₂(CN)₆ is known as a very promising NIB cathode demonstrating energy density values comparable to existing lithium- ion battery (LIB) cathodes such as LiFeP0₄. However, this R3 Na₂Fe₂(CN)₆ is very air sensitive if exposed to ambient air. Fortunately, the M-Na₂Fe₂(CN)₆.2H₂0 phase is quite air stable. Hence, an electrode fabricated with the M-Na₂Fe₂(CN)₆.2H₂0 phase as the active material can be conveniently handled in air throughout the electrode fabrication process, thus significantly reducing manufacturing costs. If it is desired to fabricate an NIB with the active material being R3 Na₂Fe₂(CN)₆, then, all usual electrode fabrication steps (from synthesis of active material till the point just before cell assembly) could be conducted in ambient air with the active material being the air stable M-Na₂Fe2(CN)₆.2H₂0. The fabricated electrode could then be heated in an inert atmosphere, in vacuum or in a dry room which triggers a phase transformation from M-Na2Fe₂(CN)₆.2H₂0 to the R3 Na₂Fe₂(CN)₆ . The coated electrodes would now have R3 Na₂Fe₂(CN)₆ as the active material and it could be now assembled in a cell (in inert atmosphere/dry room). Hence, with this method, the need for inert atmosphere/dry room has been reduced only to the last step which is a huge practical advantage. Furthermore, as will be discussed in detail in the following sections, water-based binders cannot be used if R3 Na₂Fe₂(CN)₆ is used as the active material straight-away. This is because the R3 Na₂Fe₂(CN)₆ phase is extremely moisture sensitive. Hence, with the method disclosed herein, an electrode can be first fabricated with the M- Na₂Fe₂(CN)₆.2H₂0 phase as the active material using a water-based binder and then the electrode can simply be heated to convert the active material into the R3 Na₂Fe₂(CN)₆ phase. By adopting this methodology, a water-based binder can be used with the R3 Na₂Fe₂(CN)₆ phase too. This can be a huge advantage as using water-based binders (as opposed to binders needing more costly organic solvents) can potentially reduce electrode processing costs by an order of magnitude (D. L. Wood III et al., J. Power Sources, 275, 234-242 (2015)).

Thus, the synthesis methods outlined herein are capable of providing a choice between two different cathodes which could be used in very different battery applications. Due to the freedom of using the lower capacity, yet extremely stable and responsive compounds of formula (I) (e.g. M-Na₂Fe2(CN)6.2H₂0) as the active material or using the very high energy density compounds of formula II (e.g. R 3 Na₂Fe₂(CN)₆) as the active material, a researcher/product engineer can choose which phase would suit a particular application by deciding whether to transform the compounds of formula (I) (e.g. M-Na₂Fe2(CN)₆.2H₂0) into the R3 phase of the compounds of formula (II) or not, by utilizing the above method. Particularly, in those applications where energy density is not very important (but perhaps the cost is), the lower energy density of the compounds of formula (I) (e.g. M- Na₂Fe2(CN)6.2H₂0) may be a good choice since the costs associated with its precursor materials, the inexpensive and large-scale synthesis, electrode fabrication and cell assembly are expected to be much less.

The compounds of formula (I) (e.g. M-Na₂Fe₂(CN)₆.2H₂0) have excellent sodium storage characteristics in terms of cyclability, response and cost, however, such compounds as exemplified by M-Na₂Fe₂(CN)₆.2H₂0 have moderate capacity (about 85 mAh/g) at an average voltage of 3 V vs Na/Na⁺. Hence, the energy density of the compounds of formula (I) (e.g. M-Na₂Fe₂(CN)₆.2H₂0) is not very high. However, it could be sufficient for certain applications, such as for frequency regulation in grid storage or for mitigating the minute-by- minute power fluctuations from renewable power plants running on solar, wind etc. Furthermore, the lower energy density of the compounds of formula (I) (e.g. M- Na₂Fe₂(CN)₆.2H₂0) can always be converted into the high energy density R3 phase of the compounds of formula (II) as described herein, which could be used in far more applications than the compounds of formula (I) (e.g. M-Na₂Fe₂(CN)₆.2H₂0) as an NIB cathode.

It will be appreciated that the conversion process provides compounds of formula (II) (and lla) as defined hereinbefore. An important aspect of these compounds is that they are either entirely free of water (both zeolite water and bound water), that is z' = 0 or only contain a minimal amount of bound (but not zeolite) water, such that 0 < z' < 0.5 (e.g. 0 < z' < 0.2, such as 0 < z' < 0.002). In preferred examples of the compounds of formula (II) that may be disclosed herein, the compounds of formula (II) (or lla) are substantially free of water (e.g. 0 < z' < 0.2, such as 0 < z' < 0.002, that is z' may be 0).

Positive electrodes of the current invention may comprise a current collector with a layer of the active material thereon, which layer also comprises at least one of a binder and a conductive material in addition to the active material.

The current collector may be any suitable conductor for a positive electrode, for example, aluminium (Al), stainless steel, nickel-plated steel, and/or the like.

The binder improves binding properties of the positive active material particles (e.g. M- Na₂Fe₂(CN)6.2H₂0 or R-Na₂Fe₂(CN)₆) with one another and the current collector. The binder may be a non-aqueous binder, an aqueous binder, or a combination thereof. The binder is not particularly limited as long as it binds the positive active material and the conductive material on a current collector, and simultaneously (or concurrently) has oxidation resistance for high potential of a positive electrode and electrolyte stability.

Non-aqueous binders that may be mentioned herein include, but are not limited to, polyvinylchloride, carboxylated polyvinylchloride, polyvinylfluoride, an ethylene oxide- containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, or a combination thereof. Aqueous binders that may be mentioned herein include, but are not limited to, a rubber- based binder or a polymer resin binder. Rubber-based binders may be selected from a styrene-butadiene rubber, an acrylated styrene-butadiene rubber (SBR), an acrylonitrile- butadiene rubber, an acrylic rubber, a butyl rubber, a fluorine rubber, and a combination thereof. Polymer resin binders may be selected from ethylenepropylene copolymer, epichlorohydrin, polyphosphazene, polyacrylonitrile, polystyrene, ethylenepropylenediene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, a polyester resin, an acrylic resin, a phenolic resin, an epoxy resin, polyvinyl alcohol and a combination thereof. A cellulose-based compound may be used as the binder (or in combination with other materials). Examples of suitable cellulose-based materials includes, but is not limited to, one or more of carboxylmethyl cellulose, hydroxypropylmethyl cellulose, methyl cellulose, or alkali metal salts thereof. The alkali metal may be Na, K, or Li. Such a cellulose-based compound may be included in an amount of about 0.1 parts by weight to about 10 parts by weight based on 100 parts by weight of the active material. A particular cellulose-based binder that may be mentioned herein is the sodium salt of carboxylmethyl cellulose.

The conductive material improves conductivity of an electrode. Any electrically conductive material may be used as a conductive material, unless it causes a chemical change, and examples thereof may be natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber and/or like carbon-based material; copper, nickel, aluminum, silver, and/or like metal powder or metal fiber and/or like metal-based material; polyphenylene derivative and/or like conductive polymer; and/or a mixture thereof. Positive electrodes of the current invention may be manufactured using the following method. First, the active material, the conductive material, and the binder are mixed in a desirable ratio (e.g. and active:additive:binder ratio of from 80:10:10 to 96:2:2, specific ratios that may be mentioned include, but are not limited to 85:10:5 and 90:5:5) and dispersed in an aqueous solution and/or an organic solvent (such as N-methyl-2-pyrrolidone) to form a slurry. Additionally or alternatively, the amount of active substance in the positive electrode may be from 80 to 96 wt%, the amount of additive (e.g. conductive carbon) may be from 2 to 10 wt% and the amount of binder may also be from 2 to 10 wt%. Subsequently, the slurry is coated on a current collector and then dried to form an active material layer. Herein, the coating method is not particularly limited, and may be, for example, a knife coating method (e.g. Doctor knife coating), a gravure coating method, and/or the like. Then, the active material layer is compressed utilizing a compressor to a desirable thickness to manufacture an electrode. A thickness of the active material layer is not particularly limited, and may be any suitable thickness that is applicable to a positive active material layer of a rechargeable lithium or sodium battery. In embodiments using M-Na2Fe₂(CN)6.2H₂0 or R-Na₂Fe₂(CN)₆, the active material loading may be from 1 to 40 mg cm^"2, for example the active material loading may be from 20 to 40 mg cm^'2, such as from 25 to 30 mg cm^"2.

A full cell battery will comprise a positive electrode, a negative electrode, a separator and an electrolyte solution. The positive electrode may be formed from an electrode comprising M- Na₂Fe₂(CN)₆.2H₂0 or R-Na₂Fe₂(CN)₆ as the active material. The negative electrode may be formed in similar manner to that described herein before. That is the negative electrode may include a negative active material, and may further include a binder and a conductive additive.

The negative active material layer may be any suitable negative active material layer for a full cell battery (e.g. a NIB). For example, the negative active material may include a carbon- based material, a silicon-based material, a tin-based material, an antimony-based material, a lead-based material, a metal oxide (e.g. a lithium or sodium metal oxide), a sodium metal, and/or the like, which may be utilized singularly or as a mixture of two or more. The carbon- based material may be, for example, soft carbon or hard carbon or a graphite-based material such as artificial graphite, natural graphite, a mixture of artificial graphite and natural graphite, natural graphite coated with artificial graphite, and/or the like. The silicon-based material may be, for example, silicon, a silicon oxide, a silicon-containing alloy, a mixture of the graphite-based material with the foregoing materials, and/or the like. The silicon oxide may be represented by SiO_x (0<x<2). The silicon-containing alloy may be an alloy including silicon in the largest amount of the total metal elements (e.g., silicon being the metal element that is present in the largest amount of all the metal elements) based on the total amount of the alloy, for example, a Si-AI-Fe alloy. The tin-based material may be, for example, tin, a tin oxide, a tin-containing alloy, a mixture of the graphite-based material with the foregoing materials, and/or the like. Likewise for antimony and lead-based materials. The lithium metal oxide may be, for example, a titanium oxide compound such as Li₄Ti₅0₁₂, Li₂Ti₆0 ₃ or Li₂Ti₃0₇. The sodium metal oxide may be, for example, a titanium oxide compound such as Na₂Ti₃0₇ or Na₂Ti₆0i₃. Other metal oxides that may be mentioned herein as suitable include, but are not limited to, Ti0₂, Fe₂0₃, Mo0₃. According to one embodiment, among them, graphite may further improve cycle-life characteristics of a NIB. The binder and conductive additive (if any) are not particularly limited, and may be the same binder and conductive additive as that of the positive electrode. A weight ratio of the negative active material and the binder is not particularly limited, and may be a weight ratio of a related art NIB. The negative electrode may be manufactured as follows. The negative active material, conductive additive and the binder are mixed in a desired ratio and the mixture is dispersed in an appropriate solvent (such as water and/or the like) to prepare a slurry. Then, the slurry is applied on a current collector and dried to form a negative active material layer. Then, the negative active material layer is compressed to have a desired thickness by utilizing a compressor, thereby manufacturing the negative electrode. Herein, the negative active material layer has no particularly limited thickness, but may have any suitable thickness that a negative active material layer for a rechargeable lithium (or sodium) ion battery may have. In addition, when metal sodium is utilized as the negative active material layer, the metal sodium may be overlapped with (e.g., laminated or coated on) the current collector.

The separator is not particularly limited, and may be any suitable separator utilized for a NIB. For example, a porous layer or a nonwoven fabric showing excellent high rate discharge performance and/or the like may be utilized alone or as a mixture (e.g., in a laminated structure).

A substrate of the separator may include, for example, a polyolefin-based resin, a polyester- based resin, polyvinylidene difluoride (PVDF), a vinylidene difluoride-hexafluoropropylene copolymer, a vinylidene difluoride-perfluorovinylether copolymer, a vinylidene difluoride- tetrafluoroethylene copolymer, a vinylidene difluoride-trifluoroethylene copolymer, a vinylidene difluoride-fluoroethylene copolymer, a vinylidene difluoride-hexafluoroacetone copolymer, a vinylidene difluoride-ethylene copolymer, a vinylidene difluoride-propylene copolymer, a vinylidene difluoride-trifluoropropylene copolymer, a vinylidene difluoride- tetrafluoroethylene-hexafluoropropylene copolymer, a vinylidene difluoride-ethylene- tetrafluoroethylene copolymer, and/or the like. The polyolefin-based resin may be polyethylene, polypropylene, and/or the like; and the polyester-based resin may be polyethylene terephthalate, polybutylene terephthalate, and/or the like.

A porosity of the separator is not particularly limited, and may be any suitable porosity that a separator of a NIB may have.

The separator may include a coating layer including an inorganic filler may be formed on at least one side of the substrate. The inorganic filler may include Al₂0₃, Mg(OH)₂, Si0₂, and/or the like. The coating layer including the inorganic filler may inhibit direct contact between the positive electrode and the separator, inhibit oxidation and decomposition of an electrolyte on the surface of the positive electrode during storage at a high temperature, and suppress the generation of gas which is a decomposed product of the electrolyte. A suitable separator that may be mentioned herein is a glass fibre separator.

The electrolyte may include an electrolyte salt in a non-aqueous solvent.

The non-aqueous solvent may be, for example, cyclic carbonates (such as propylene carbonate, ethylene carbonate, butylene carbonate, fluoroethylene carbonate, chloroethylene carbonate, vinylene carbonate, and/or the like); linear carbonates (such as dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, and/or the like); cyclic esters (such as γ-butyrolactone, γ-valerolactone, and/or the like); linear esters (such as methyl formate, methyl acetate, methyl butyrate, and/or the like); tetrahydrofuran or a derivative thereof; ethers (such as 1 ,3-dioxane, 1 ,4-dioxane, 1 ,2-dimethoxy ethane, 1 ,4-dibutoxyethane, diglyme, tetraglyme, methyl diglyme, and/or the like); nitriles (such as acetonitrile, benzonitrile, and/or the like); dioxolane or a derivative thereof; ethylene sulfide; sulfolane; and/or sultone or a derivative thereof, which may be utilized singularly or as a mixture of two or more, without being limited thereto.

The electrolytic salt may be, for example, an inorganic ion salt including sodium (Na), such as NaCI0₄, NaAsF₆, NaPF₆, NaPF₆._x(C_nF_2n+1)_x (1 <x<6, n=1 or 2), NaSCN, NaBr, Nal, Na₂S0₄, Na₂B₁₀Clio, NaCI0₄, Nal, NaSCN, NaBr, NaPF₄, NaBF₄, NaPF₆, and/or the like; and/or an organic ion salt such as NaCF₃S0₃, NaN(CF₃S0₂)2, NaN(C₂F₅S02)2_> NaN(CF₃S0₂)(C₄F₉S0₂), NaC(CF₃S0₂)₃, NaC(C₂F₅S0₂)_3> (CH₃)₄NBF₄, (CH₃)₄NBr, (C₂H₅)₄NCI0₄, (C₂H₅)₄NI, (C₃H₇)₄NBr, (n-C₄H₉)₄NCI0_4l (n-C₄H₉)₄NI, (C₂H₅)₄N-maleate, (C₂H₅)₄N-benzoate, (C₂H₅) N-phtalate, sodium stearyl sulfonate, sodium octyl sulfonate, sodium dodecylbenzene sulfonate, and/or the like. The ionic compounds may be utilized singularly or in a mixture of two or more. Particular electrolyte salts that may be mentioned herein are NaCI0₄, NaPF₄, NaBF , and NaPF₆ which may be utilized singularly or in a mixture of two or more.

A concentration of the electrolyte salt is not particularly limited, and may be, for example, about 0.1 mol/L to about 10.0 mol/L.

The electrolyte may further include various suitable additives such as a negative electrode SEI (Solid Electrolyte Interface) forming agent, a surfactant, and/or the like. Such additives may be, for example, succinic anhydride, lithium bis(oxalato)borate, sodium bis(oxalato)borate, lithium tetrafluoroborate, sodium tetrafluoroborate a dinitrile compound, propane sultone, butane sultone, propene sultone, 3-sulfolene, a fluorinated allylether, a fiuorinated acrylate, carbonates such as vinylene carbonate, vinyl ethylene carbonate and fluoroethylene carbonate and/or the like. The concentration of the additives may be any suitable one that is utilized in a general NIB.

In a NIB, the separator is disposed between the positive electrode and the negative electrode to manufacture an electrode structure, and the electrode structure is processed to have a desired shape, for example, a cylinder, a prism, a laminate shape, a button shape, and/or the like, and inserted into a container having the same shape. Then, the non-aqueous electrolyte is injected into the container, and the electrolyte is impregnated in the pores in the separator, thereby manufacturing a rechargeable sodium or sodium-ion battery. Hereinafter, embodiments of the invention are illustrated in more detail with reference to the following examples. However, the present disclosure is not limited thereto. Furthermore, what is not described in this disclosure may be sufficiently understood by those who have knowledge in this field and will not be illustrated herein. Examples

Methodology

Synthesis Description: In a typical synthesis, Na₄Fe(CN)₆.10H₂0 (5 mmol) was first dissolved in 100 ml_ of Milli-Q water in a glass vessel. Then, ascorbic acid (22.5 mmol) was added to the solution and the entire vessel was placed in a silicone oil bath maintained at 140 °C such that the solution was refluxed (the reflux temperature of the solution, measured by a thermometer dipped into the solution, was around 107 °C and the solution displayed vigorous bubbling throughout). The solution was then refluxed for 4 h under constant stirring. Thereafter, the vessel was taken out of the oil bath and allowed to cool to room temperature. At this stage, the clear white precipitate formed during reflux settled below a pale yellow-coloured transparent solution. This precipitate is the desired M-Na₂Fe₂(CN)₆.2H₂0. The method for retrieval of this precipitate does not matter; it can be obtained via filtration, centrifugation or the supernatant can simply be removed by a dropper or other means or through any other precipitate retrieval technique (the precipitate retrieval method did not alter the phase purity). The obtained precipitate can then be dried in either vacuum or air or under inert atmosphere at any temperature ranging from room temperature to below 200°C. In a typical procedure, the precipitate was dried at 70°C in air for 3 h, resulting in the final as-synthesized compound.

The obtained dried precipitate is white in colour and can have a somewhat-strong tinge of cyan or light cream-yellow to it (depending on the chosen precipitate retrieval method, but this does not affect the phase purity).

As-synthesized Material Characterization: For Na:Fe molar ratio determination by inductively coupled plasma optical emission spectroscopy (ICP-OES), a Perkin Elmer Optima 5300 DV instrument was used while for C and N measurements by CHN elemental analysis, an Elementar vario MICRO Cube elemental analyzer was used. For both ICP and CHN experiments, measurements were repeated and yielded consistent results. Water content was measured by thermogravimetric analysis, TGA (TA instrument; model 2960), where the measurements were obtained till 450 °C in N₂ atmosphere at 10 °C/min ramp rate. TGA-MS (mass spectrometry) measurements were taken on a TGA-MSMettler-Toledo TGA/DSC coupled with Pifzer mass spectrometry instrument in N₂ atmosphere till 400 °C at 10 °C/min ramp rate. For field emission scanning electron microscopy (FESEM) measurements, a JEOL JSM-7000F model was used and operated at 15 kV and 20 mA while the energy-dispersive X-ray spectroscopy (EDX) were obtained on a JEOL JED-2300F Energy Dispersive Spectrometer. Fourier transform infrared spectroscopy (FTIR) was conducted on a Variant 3100 (Excalibur Series) instrument in transmission mode. X-ray photoelectron spectroscopy (XPS) was measured on the as-synthesized powder on a Kratos Analytical Axis Ultra DVD using monochromated Al Ka (1486.7 eV). The binding energy of C1 s was taken as 284.8 eV for calibration purposes. X-ray diffraction (XRD) patterns were acquired with a Bruker AXS D8 ADVANCE powder diffractometer using Cu Ka radiation source in the 2Θ range of 10-140 ° and operated at 25 mA and 40 kV. Rietveld refinement was conducted using the TOPAS academic version 4.2 software. For Rietveld refinement, the structural model of M-Na_2-5Mn[Fe(CN)₆].1.87H₂0 was used (see J. Song, et al., J. Am. Chem. Soc, 137 (7), 2658-2664 (2015)) with Fe atom in place of Mn atom and with the occupancy of Fe-C and Na freely refined. Variable temperature XRD patterns were obtained on a Bruker D8 Advance powder X-ray diffractometer equipped with an Anton Paar HTK1200 High-Temperature Oven-Chamber. The measurements were conducted in either atmospheric air or high vacuum (10^"2 mbar) as indicated. Electrode Preparation, Cell Assembly and Electrochemical Evaluation: Composite electrodes were made with the as-synthesized material as the active material, Ketjen Black (KB) (Lion Corporation) as the conductive additive and sodium salt of carboxymethyl cellulose, CMC (Alfa Aesar), as the binder in the weight ratio 85:10:5. In order to make the slurry, CMC was first dissolved in Milli-Q water to which a hand ground mixture of M- Na₂Fe₂(CN)₆.2H₂0 and KB were added. After stirring at 1200 rpm for 2 h, the slurry was coated on Al foil with the doctor blade technique and then dried overnight at 120 °C under 1 mbar vacuum. Upon drying, the coated electrode was pressed by a twin roller at a pressure of 37 psi. Electrodes were hence punched with an active material loading between 3 - 4 mg cm^"2. Coin cells of 2016 type (MTI Corporation) were fabricated with such electrodes as the working electrode and Na metal (Merck) as the counter and reference electrodes with a glass fiber (Whatman, grade GF/A) as a separator layer. Prior to cell assembly, the electrodes were dried at 120 °C in 1 mbar vacuum and brought inside an Ar filled glove box (MBraun, Germany) with H₂0 and 0₂ < 5 ppm. For the 4.3 - 2.0 V cycling with carbonate based electrolytes, 1 M NaCI0₄ (Alfa Aesar, 98+%, anhydrous) in ethylene carbonate, EC (Alfa Aesar): propylene carbonate, PC (Sigma Aldrich) in a 1 :1 volume ratio prepared in house with no further purification, was used. For the 3.9 - 2.0 V cycling with carbonate based electrolytes, the electrolyte used was 0.6 M NaPF₆ (Alfa Aesar, purity 99+%) in EC:PC in a 1 :1 volume ratio also prepared in house with no further purification. For rate performance studies, 5 volume % of fluoroethylene carbonate, FEC (Sigma Aldrich, 99%) was added while for long term cycling studies, 2.5 volume % of FEC and 2.5 volume % vinylene carbonate, VC (Alfa Aesar, 97%) were added. The coin cells were cycled in a computer controlled Arbin battery tester (model BT2000, USA) at room temperature.

Ex-situ XRD, FTIR and DSC Measurements: For ex-situ XRD measurements at various states of charge and discharge, the electrodes were cycled at C/9 to the appropriate charge/discharge state. The cells were then opened in the glove box, the electrodes retrieved and all XRD patterns reported were obtained within 30 s - 5 min air exposure. For the ex-situ FTIR and differential scanning calorimetry (DSC) measurements of charged/discharged M-Na₂Fe₂(CN)₆.2H₂0, special electrodes were made with just M- Na₂Fe₂(CN)₆.2H₂0 and KB in the weight ratio 90:10. No binder was used so as to eliminate its contribution to the FTIR/DSC spectra. After hand-grinding, the homogenously mixed powders were stirred in Milli-Q water and then hand coated on Al foils. After cycling to the required state of charge/discharge, the cells were opened in the glove box and the electrodes were washed 20 times with anhydrous PC to remove any electrolyte salt. The washed electrodes were then dried in 1 mbar vacuum for 16 h and were hence scratched. For FTIR measurement, the scratched powders were packed into Ar filled vials which were opened just prior to FTIR measurements. The air exposure time for FTIR measurements was about 5 min. However, air exposure was not a concern for the charged samples as they were found to be air-stable. For DSC measurements, the scratched powders were sealed in aluminum capsules inside the Ar-filled glove box itself. The DSC measurements were hence performed on a TA Instrument 2920 at 10 °C/min ramp rate. Hence, no air exposure occurred during these DSC measurements.

Results and Analysis

The as-synthesized material displayed homogenous particle size of cubic shape with dimensions below 3 pm (refer to Figure 1a), the particles generally being from less than or equal to 1 Mm to 2 pm in size. ICP-OES and EDX revealed a Na:Fe molar ratio of 1 :1 confirming the sodium rich nature of the as-synthesized material. Measured amounts of C and N were almost identical to expected quantities corresponding to the (CN)₆ backbone. These Elemental Analysis results for the as-synthesized M-Na₂Fe₂(CN)₆.2H₂0 powder are provided below in Table 1. For the Na:Fe molar ratio, two separate methods were used: ICP-OES and FESEM-EDX. For determining the C and N quantities, CHN analysis was conducted. For all the measurements, the percentage error from the theoretical value is also provided in paranthesis. Data for two separate samples have been provided. As can be seen, multiple measurements on different samples yielded consistent results.

Na:Fe Molar Ratio Elemental Analysis Results

Sample

ICP-OES (error %) EDX (error %) C (error %) N (error %)

#1 1.04:1 (4) 1.002:1 (0.2) 20.67 wt % (0.34) 23.84 wt % (0.79)

#2 1.06:1 (6) 1 .03:1 (3) 20.63 wt % (0.15) 23.73 wt % (1.26)

Table 1

Thermogravimmetric analysis (TGA) was conducted for the as-synthesized M-Na2-xFe2(CN)₆.2H₂0 to gauge how much water was present in the as-synthesized compound. The TGA curve is presented in Fig. 1b. As can be seen, the material was able to retain almost 99.3 % of its initial weight till 180 °C indicating that there was essentially no zeolitic water in the as-synthesized compound. A weight loss of about 10 % was observed between 180 - 250 °C. To ascertain the cause of this weight loss, Mass Spectroscopy (MS) was conducted along with TGA and the obtained results have been plotted in Fig. 1c. For MS analysis, the chosen compounds tracked as a function of temperature were water (H₂0), HCN gas and C0₂ gas, as the observed weight loss of 10 % could be caused either due to water removal from the PBA or could be due to the decomposition of the structure (e.g. HCN and C0₂ could be released). From Fig. 1c, it is clear that the 10 % weight loss between 180 - 250 °C arises due to water removal and not due to HCN or C0₂ release; the latter two showed negligible signal changes. This indicates that the type of water present in the as- synthesized compound is the so-called "bound water" which is basically lattice water that is a part of the crystal structure of the compound. The amount of water released (= 10 % by weight) indicates that the as-synthesized material bears about 2 moles of water per mole of the material. Hence, the formula for the as-synthesized compound is M-Na2_-xFe2(CN)₆.2H₂0 (with x ~ 0).

The presence of water was also confirmed by FTIR. The FTIR spectrum, displayed in Figure 1d, showed two sharp peaks at 1619 and 2071 cm^"1 and two broad peaks around 3445 and 3611 cm^"1. The peak at 2071 cm^"1 corresponds to the cyanide stretching vibration band coordinated to Fe²⁺ in such PBAs (e.g. see M. J. Piernas-Munoz, et al., J. Power Sources, 324 766-773 (2016)). The other peaks at 1619, 3445 and 361 1 cm^'1 are attributable to structural water present, with the 1619 cm^"1 peak corresponding to the O-H bending band and the broader peaks at high wavenumbers to the O-H stretching bands (e.g. see X. Sun et al., Adv. Sci., 3 1600044 (2016)..

As noted above, the weight loss of about 10 wt% indicates that about 2 moles of water were present per mole of the material. Hence, based on ICP-OES, EDX, CHN and TGA analyses, the stoichiometry of this material can be stated as Na₂Fe2(CN)₆.2H₂0 highlighting the Na- rich nature of the material and lack of vacancies resulting from the synthesis protocol disclosed hereinbefore. This stoichiometry implies that both Fe atoms should exist as Fe²⁺. For a second confirmation of this fact (apart from FTIR), XPS analysis was carried out to track the position of the Fe 2p_3/2 edge. The as-synthesized material displayed predominantly a single peak at 708.6 eV which is consistent with the Fe²⁺ oxidation state (e.g. see M. Datta and A. Datta, J. Phys. Chem., 94 (21 ), 8203-8207 (1990)), as depicted in Figure 1e. During the curve fitting, a minor Fe³⁺ peak was revealed at 710.3 eV. Since XPS is a surface analysis technique limited to the first 10 nm, the presence of the Fe³⁺ peak indicates a very slight Na loss from the surface occurring mainly during the precipitate retrieval step (either filtration or centrifugation) during the synthesis, while the bulk of the material remained in the Fe²⁺ state. This also explains why no Fe³⁺ peaks were observed in the FTIR spectrum as FTIR is a bulk characterization technique. In fact, the color change of the as-synthesized material mirrored this hypothesis: the precipitate while settled in the solution was white in color, however, after filtration/centrifugation, it acquired a faint cyan tinge, similar to the observation reported for the rhombohedral Prussian white Na₁.9₂Fe2(CN)₆ (e.g. see L. Wang, er a/., J. Am. Chem. Soc, 137 (7), 2548-2554 (2015)). The XRD pattern of the as-synthesized powder along with the calculated pattern as a result of Rietveld refinement is shown in Fig. 1f. In the Rietveld analysis, the structural model proposed for Na_2-5Mn[Fe(CN)6].zH₂0 (δ ~ 0 and z = 1.87, see J. Song, et al. Journal of the American Chemical Society, 2015, 137, 2658-2664) was used with the Mn sites replaced with Fe sites and the Na sites and Na and Fe occupancies along with the lattice parameters allowed to be refined with other parameters held constant. The initially-obtained refinement R values (R_exp = 1.58 %, R_p = 3.25 %, R_wp = 4.98 %, R_Bragg = 2.963 % and the Goodness of Fit = 3.15) indicated that the structural model proposed adequately describes the crystal structure of the M-Na_2-xFe₂(CN)6.2H₂0 phase. A more detailed analysis conducted with higher-quality XRD data provided the obtained refinement R values (R_wp = 7.42 %, R_Bra_gg = 4.621 %, x² = 3.39 %, and R_exp = 2.19 %), which validates the initial conclusion. The calculated lattice parameters for the M-Na₂-xFe₂(CN)₆.2H₂0 phase are shown in Table 2 and, as can be seen in Fig. 1f, these calculated values generally agree with the experimental result (see Table 3 for the atomic co-ordinates of the compound). Further, the XRD pattern of the as-synthesized compound is very different from that of the R3 phase reported by Wang ef al Journal of the American Chemical Society, 2015, 137, 2548-2554 (also shown in Figs. 7, Fig. 8a, Fig. 8c and Fig. 9a).

Table 2 Lattice parameters of M-Na₂-_xFe₂(CN)₆.2H₂0 as calculated. Please note that α =γ = 90°.

S.G. P2₁/n; a =10.45983(56) A, b = 7.51295(42) A, c = 7.27153(48) A and β = 92.7379(33) ⁰

Atom Wyck. X y z Occ. B,so (A^z)

Fe1 (Fe-N) 2a 0.50000 0.50000 0.50000 1 0.846

Fe2 (Fe-C) 2d 0.50000 0.0000 0.00000 0.9937 (60) 0.988

N1 4e 0.49939 0.31393 0.74298 1 0.391

N2 4e 0.29412 0.50590 0.50582 1 0.391

N3 4e 0.50173 0.26558 0.31835 1 0.391

C1 4e 0.49796 0.19145 0.83494 1 0.646

C2 4e 0.18931 0.49940 0.51831 1 0.646

C3 4e 0.50246 0.16216 0.20308 1 0.646

O 4e 0.25400 0.21607 0.28021 1 0.564

Na 4e 0.25775 0.44553 0.02722 0.8378 (45) 4

Table 3. Atomic coordinates for the as-synthesized M-Na₂Fe₂(CN)₆.2H₂0 powder from Rietveld refinement (R_wp = 7.42 %, R_Bragg = 4.621 %, χ² = 3.39 %, R_exp = 2.19 %). Sodium Storage Performance

To study the sodium storage performance of M-Na₂Fe2(CN)6.2H₂0, composite electrodes were fabricated with KB as the conductive additive and CMC as binder. As M- Na₂Fe2(CN)₆.2H₂0 was found to be water-insoluble (refer to Fig. 2), water was used as the slurry preparation medium, thus eliminating the need for toxic and costly n-methylpyrrolidone (NMP) as the binder solvent, which is the traditional solvent used for the most widely known polyvinylidene fluoride (PVDF) binder employed for lithium-ion battery (LIB) cathodes/anodes. The use of environmentally safe and non-toxic CMC, along with water as the medium, is expected to reduce the electrode processing cost for this material. In fact, a recent study on LIBs has suggested that switching to water-based binders could potentially reduce electrode processing costs by an order of magnitude (D. L. Wood III ei a/., J. Power Sources, 275 234-242 (2015)). The first charge-discharge galvanostatic cycle of the M-Na2Fe2(CN)₆.2H₂0 cathode against Na metal as the counter and reference electrodes (half-cell configuration) in a coin cell between 4.3 - 2.0 V vs Na/Na⁺ is depicted in Figure 3a. With a standard NIB electrolyte (1 M NaCI0₄ in EC:PC) which displays an electrochemical stability window till at least 4.4-4.5 V vs Na/Na⁺ (see: A. Ponrouch, et a/., Energy Environ. Sc/^'., 5 (9), 8572-8583 (2012)), the M- Na₂Fe2(CN)6.2H₂0 cathode could deliver a capacity of 170.9 mAh g^"1 during the first charge, in excess of its theoretical capacity of 153.2 mAh g^"1 (corresponding to two mole sodium storage per mole of material), indicating that apparently more than two moles of sodium were extracted per mole of cathode. In the first discharge (Na insertion into the cathode), about 150 mAh g^"1 was obtained, confirming that two moles of sodium were inserted back into the cathode, per mole of material. The cause of the excess charge capacity will be discussed in the following section. The Na rich nature of M-Na₂Fe₂(CN)₆.2H₂0 was further evidenced when the cathode was discharged first rather than charged: discharging first resulted in negligible capacity, as indicated in Figure 3a. The cyclability of M- Na₂Fe₂(CN)₆.2H₂0 in the voltage window 4.3 - 2.0 V was found to be quite poor, with capacity retention of just 77% of its initial discharge capacity in 10 cycles and 44% in 500 cycles at C/1.8 rate. The voltage profiles also changed significantly during cycling, with almost a complete loss of the charge-discharge plateau above 4.0 V within a few cycles. These facts indicate that the structure of M-Na₂Fe₂(CN)₆.2H₂0 changes when it is forced to store two moles of sodium per mole of the material (details discussed in the following section). In stark contrast, if the cycling of M-Na₂Fe₂(CN)₆.2H₂0 upon charging is stopped just before the start of the 4.0 V plateau (if cycled until 3.9 V vs Na/Na⁺), the galvanostatic profile of this material looked very different, as presented in Figure 3b. Within this voltage window (3.9 - 2.0 V vs Na/Na⁺), the M-Na₂Fe2(CN)₆.2H₂0 cathode could deliver 84 mAh g^"1 at a C/4.5 rate which indicates that 1.1 mole of sodium was stored per mole of material (76.6 mAh g^'1 corresponds to the theoretical capacity of one mole sodium storage) at an acceptable average discharge voltage of 3.03 V. As shown in Figure 3b and Figure 3c, almost 100 % capacity retention was observed at a slow C/4.5 (4.5 h) or fast 5.6 C (10.7 min) discharge with very stable capacities for all rates. For even faster response times such as 11 .1 C rate (5.4 min discharge), the as-synthesized material could still deliver 74 mAh g^"1, which was 88% of the delivered capacity at slower rates. It should be mentioned that polarization became significant only at 1 1.1 C as shown in Figure 3b, which indicates potential possibility for high power applications if paired with an equally responsive anode. More importantly, the cycle life of M-Na₂Fe2(CN)₆.2H₂0 within the 3.9 - 2.0 V voltage window was found to be dramatically improved with capacity retention of 82% and 67% observed after 2,000 and 3,000 cycles, respectively (see Figure 3d), with a highly stable coulombic efficiency above 99% throughout cycling. These results are indeed quite appealing for grid-storage applications.

Sodium Storage Mechanism

Structural Changes during Cycling

To understand the cause of the excellent sodium storage performance for M- Na₂Fe₂(CN)₆.2H₂0 within the 3.9 - 2.0 V voltage window and not within 4.3 - 2.0 V, insights were obtained by ex- situ XRD, DSC and FTIR at different states of charge/discharge. Figure 4a presents the ex-situ XRD plots for M-Na₂Fe₂(CN)₆.2H₂0 within the 3.9 - 2.0 V window. As the charging cycle proceeded, sodium extraction led to a merger of the peaks at 23.7 and 24.5° 2Θ into a single peak, signalling the symmetry transition from monoclinic to cubic; the cubic phase of PBAs is widely reported for Na poorer versions of NaxFe₂(CN)₆ in the literature (0 < x < 1.6), for example see: Y. You, et al., Energy Environ. Sci., 7 (5), 1643-1647 (2014) and S. J. R. Prabakar et al., RSC Adv., 5 (47), 37545-37552 (2015). During this transition, the voltage profile was quite flat (see Fig. 5), in accordance with the co-existence of these two phases dictated by the Gibbs Phase Rule. On further charging, sodium extraction caused the cubic peaks to shift to higher 2Θ values, indicating the expected volume contraction owing to the removal of sodium from the structure. This continuous shift, indicating a solid-solution reaction mechanism, is the reason for the sloping voltage profiles as shown in Fig. 5. During the discharge process, Na insertion caused the structure to first expand in volume while still maintaining the cubic symmetry before lowering the symmetry back to monoclinic at the later stages of discharge, confirming a smooth structural transition in the course of cycling. When cycled between 4.3 - 2.0 V such that both moles of sodium were extracted per mole of M-Na₂Fe₂(CN)₆.2H₂0 during charging, the ex-situ XRD plot for the fully charged cathode at 4.3 V displayed greatly intensified peaks (see the peaks at 17.4 and 35.1 ° 2Θ in Fig. 6a) with respect to that charged to 3.9 V, without any peak shift. Upon discharge down to 2.0 V, the XRD pattern was similar to that of M-Na₂Fe₂(CN)₆.2H₂0 but with slight differences in intensity particularly to the peaks at 23.7 and 24.5° 2Θ, indicating probably slight structural distortions in the course of cycling between 4.3 - 2.0 V. These observations are different from those reported for M-Na_2-6Mn[Fe(CN)₆].1.87H₂0 where its two mole sodium cycling resulted in a mixture of monoclinic and rhombohedral phases at the end of the first discharge as a consequence of the structural water loss during its cycling (see J. Song, et al., J. Am. Chem. Soc, 137 (7), 2658-2664 (2015)).

In order to gain a deeper understanding of the galvanostatic cycling effects on the structural water of M-Na₂Fe₂(CN)₆.2H₂0, the cause of the poor cycling stability within the 4.3 - 2.0 V window and the origin of the 4.0 V charge plateau, an ex-situ FTIR experiment was conducted at selected states of charge/discharge. Firstly, the obtained FTIR plots within the high wavenumber region are shown in Figure 4b. The O-H stretching bands at 3445 cm^"1 were preserved within the 3.9 - 2.0 V window, indicating that the structural water present in M-Na₂Fe₂(CN)₆.2H₂0 remained within the crystal structure during cycling in this restricted voltage window. On the other hand, when charged to 4.3 V, the O-H band significantly reduced, indicating release of most of its structural water during sodium extraction after the first charge itself, consistent with observations made for M-Na_2-5Mn[Fe(CN)₆].1.87H₂0 (see Song et al. ibid). This release of structural water was also reflected in the DSC curves of M-Na₂Fe₂(CN)₆.2H₂0 when charged to 3.9 and 4.3 V (refer to Figure 4c). While both the 3.9 and 4.3 V charged samples exhibited broad exothermic reactions around 198 °C, the DSC curve of the 3.9 V sample displayed an endothermic peak at 233 °C concomitant with the loss of its structural water caused by heating during the DSC experiment. Such an endothermic reaction was not observed for the 4.3 V charged sample further confirming that M-Na₂Fe₂(CN)₆.2H₂0 lost its structural water during the 4.0 V charge plateau upon cycling. This loss of structural water during charging to 4.3 V must contribute to the excess first charge capacity observed in Figure 3a due to the likely side reaction of released water molecules with the electrolyte at that high potential range (4.0 - 4.3 V vs Na/Na+). Moreover, this structural water loss probably distorted the structure after the first charge itself, explaining the slight differences in the intensities of the peaks at 23.7 and 24.5° 2Θ for M-Na₂Fe₂(CN)₆.2H₂0 discharged to 2.0 V after being charged to 4.3 V (shown in Figure 6a). Furthermore, this combined action of two mole sodium storage per mole of M-Na₂Fe₂(CN)₆.2H₂0 during cycling between 4.3 - 2.0 V and water loss brought about significant structural collapse and distortion after repeated cycling, as indicated by the greatly suppressed peaks along with slight peak shifts of the ex-situ XRD pattern of M-Na₂Fe₂(CN)₆.2H₂0 after 500 cycles between 4.3 - 2.0 V (shown in Figure 6b). Such structural collapse and distortion explains the poor cycling stability within this 4.3 - 2.0 V voltage window and the changing voltage profiles, respectively (shown in Figure 3a). Alternatively, the poor cycling stability and changing voltage profiles could also occur due to adverse changes to the surface of the M-Na₂Fe₂(CN)₆.2H₂0 particles. Additional high resolution transmission electron microscopy and FESEM studies on cycled electrodes would help in explaining such observations further. Conversely, retention of the structural water for the 3.9 - 2.0 V cycled M-Na₂Fe₂(CN)₆.2H₂0 not only keeps the structure intact as evidenced by its excellent cycle life, but also seems to be beneficial from a safety point of view as the endothermic reaction at 233 °C for the charged sample to 3.9 V may arrest any thermal runaway type scenarios. This inherent in-built "thermal safety-fuse" mechanism is quite appealing considering its ultimate application in large-scale grid-storage batteries. Redox Mechanism

The crystal structure of M-Na₂Fe₂(CN)₆.2H₂0 renders the Fe²⁺ coordinated to N (Fe2⁺-NC) in the high spin configuration (HS-Fe²⁺) and the Fe²⁺ coordinated to C (Fe²⁺-CN) in the low spin configuration (LS-Fe ⁺), see L. Wang, et al., J. Am. Chem. Soc, 137 (7), 2548-2554 (2015). To ascertain which Fe partakes in the low (3.0 - 3.3 V) and high (4.0 V) voltage plateaus witnessed during galvanostatic cycling, ex-situ FTIR measurements were conducted at relevant states of charge and discharge to follow the cyanide stretching vibration band as it is a sensitive indicator of the Fe oxidation state in such PBAs. Within the 3.9 - 2.0 V window, the cyanide stretching bands shift from 2072 cm^"1 for the pristine state to 2078 cm^"1 for the cathode charged to 3.9 V and then it shifts back to 2072 cm^"1 when discharged to 2.0 V (see Figure 4d). The stretching band at 2078 cm^"1 agrees well with the band at 2076 cm^"1 reported for Na₀.75Fe₂.08(CN)₆, indicating that one of the Fe²⁺ has been oxidized to Fe³⁺ (see M. J. Piernas-Munoz et al., Electrochim. Acta, 200 123-130 (2016)). On further charge to 4.3 V, the cyanide band shifts to a higher wavenumber of 2086 cm^"1 in good agreement with the band at 2090 cm^"1 observed for Fe³⁺Fe³⁺(CN)₆, indicating that at 4.3 V, both Fe²⁺ have been oxidized to Fe³⁺ (see X. Wu, et al., J. Mater. Chem. A, 1 (35), 10130-10134 (2013)). Furthermore, another minor peak at 2171 cm^"1 was detected for the cathode charged to 4.3 V. In previous reports of K_xFe₂(CN)₆ and Na_xFe₂(CN)₆, such a relatively weak peak at 2171 cm^"1 vs the 2085 cm^"1 peak was attributed to a splitting of the CN stretching bands and assigned to Fe³⁺ bonded to the C of C≡N^" anion. Rather expectediy, it appears that the high voltage plateau above 4.0 V vs Na/Na⁺ is caused due to the oxidation/reduction of LS-Fe²⁺ to LS-Fe³⁺ while the lower voltage plateau (3.0 - 3.3 V) is due to the corresponding processes between HS-Fe²⁺ and HS-Fe³⁺ for M-Na2Fe2(CN)₆.2H₂0. These results are consistent with theoretical calculations (Y. Jiang, et al., Adv. Fund. Mater., 26 5315 (2016)) and with other PBAs reported in NIBs where the HS-Fe²⁺ is active in the lower voltage regions while the LS- Fe²⁺ is responsible for the higher voltage plateaus (see Wang et al., ibid).

Thermal and Air Stability

TGA-MS data for M-Na₂Fe2(CN)₆.2H₂0 (refer to Figure 1 c) indicated that the structural water in this compound can be removed by heating this material above 200 °C. In order to investigate the change in the M-Na₂-xFe₂(CN)₆.2H₂0 compound due to water removal between 180 - 250 °C, XRD patterns were obtained on an electrode that was fabricated with M-Na2-xFe2(C )₆.2H₂0 and which was subsequently heated > 200 °C in Argon atmosphere. The corresponding XRD plots are shown in Fig. 7. A more comprehensive view of the changes are presented in Figure 8a, which provides XRD plots at temperatures between ambient temperature (i.e. 25 °C) and 250 °C.

Upon heating, the M-Na₂-xFe2(CN)₆.2H₂0 transforms to R3 Na₂Fe₂(CN)₆ above 200 °C. Hence, removal of water molecules from the crystal structure triggers a phase transition which changes the as-synthesized compound M-Na2_-xFe2(CN)₆.2H₂0 into the R 3 Na₂Fe₂(CN)₆ phase. As shown by Figure 8a, the change was more nuanced as, upon heating in ambient air, the XRD peaks of M-Na2Fe₂(CN)₆.2H₂0 began to decrease with a new set of XRD peaks appearing as the temperature reached 200 °C. Further heating caused these new peaks to grow at the expense of those of the pristine phase and by 250 °C, only the peaks of the new phase existed, signalling a thermally induced phase transformation above 200 °C. In fact, the peaks of this new phase correspond to the rhombohedral phase of Na₂Fe₂(CN)₆ with the space group R3^~ recently reported by Wang et al., in J. Am. Chem. Soc, 137 (7), 2548-2554 (2015) (abbreviated as R-Na₂Fe₂(CN)₆ henceforth). It is interesting to note that while thermal dehydration changed the structure from monoclinic to rhombohedral, electrochemical dehydration on the other hand (caused by galvanostatic charging to 4.3 V which releases the structural water), resulted in structure change from monoclinic to cubic (refer to Figure 6a) most probably due to the concomitant Na loss that also occurred during charging (Na loss did not occur during thermal dehydration). In a high vacuum environment of 10^~2 mbar, on the other hand, M-Na₂Fe2(CN)₆.2H₂0 lost its structural water completely at 100 °C to form once again R-Na₂Fe₂(CN)₆ (see Figure 9a). But, when heated in a milder 1 mbar vacuum, this conversion from M to R phase did not occur even till 120 °C, as shown in Figure 9b. These results indicate the importance of the atmosphere when Na rich PBAs are handled. From a practical consideration, all electrodes are typically heated in vacuum prior to fabrication. It is imperative that the degree of vacuum must be taken into account when Na rich PBAs in general, and the M-Na₂Fe₂(CN)₆.2H₂0 in particular, are heated. Hence, if a -Na₂Fe₂(CN)6.2H₂0 electrode is purposefully heated above 100 - 200 °C in vacuum/air/inert atmosphere, it can be made to easily transform to R-Na₂Fe₂(CN)₆. The advantage of the R phase is that it can deliver higher capacity (theoretical capacity of 170.8 mAh g^"1) within a narrow voltage window of 3.0 and 3.4 V: it displays two very flat charge-discharge plateaus centered at 3.1 and 3.3 V in accordance with the HS-Fe²⁺ and the LS-Fe²\ respectively. Indeed, when a R-Na₂Fe₂(CN)6 electrode was prepared by simply heating an already fabricated M-Na₂Fe₂(CN)₆.2H₂0 electrode above 240 °C in inert Ar atmosphere and then cycled in a sodium battery, a high capacity of 164 mAh g^"1 close to its theoretical value was obtained with the charge/discharge plateaus consistent with the previous report of Wang et al (refer to Fig. 9c). Note that the shifting minor voltage step observed during discharge (as indicated by the arrows in Figure 9c) is due to the voltage step phenomenon caused by an increased polarization of the sodium counter electrode in EC: PC based solutions during the discharge cycle in a half cell (see A. Rudola, et al., Electrochem. Commun., 46 56-59 (2014)). An advantage of the method disclosed herein to obtain the high capacity R phase by heating already fabricated M-Na₂Fe₂(CN)₆.2H₂0 electrodes lies in the ambient air stability of these two phases. To test the air stability of the as-prepared M-Na₂-xFe₂(CN)6.2H₂0, XRD patterns of this powder were collected at various intervals whilst it being fully exposed to actual ambient air (not in a dry room). The corresponding XRD plots are shown in Fig. 8b. Until 5 days, the XRD pattern of the material remained essentially the same. Some changes were observed around 2 weeks (as indicated by the appearance of a new XRD peak at the expense of the 23.7 and 24.5° 2Θ peaks and a corresponding quite significant peak shift of the 34.3° 2Θ peak) and by 5 weeks of air exposure, the M-Na₂-xFe₂(CN)₆.2H₂0 had completely transformed. The XRD pattern of this new phase is very similar to that reported for the cubic phase of Na_xFe₂(CN)₆.nH₂0 with the space group Fm-Zm, though with a greatly suppressed intensity of the 38.8° 2Θ peak compared to some of the previous reports (e.g. see Y. You er al., Energy Environ. Sci.,7 (5), 1643-1647 (2014)). Hence, it can be seen that M-Na_2-xFe₂(CN)₆.2H₂0 appears to be metastable in ambient air, though this phase was very stable for months in an inert atmosphere (see Fig. 10). However, it is air stable up till 5 days of air exposure which would give plenty of time for electrode fabrication in air thus aiding handling of this material. In contrast, an M-Na₂-xFe₂(CN)₆.2H₂0 electrode heated above 200 °C in Ar, so as to transform it to R3 Na₂Fe₂(CN)₆ phase, is very unstable when exposed to ambient air as seen from Fig. 8c. The R3 Na₂Fe₂(CN)₆ phase is essentially lost completely within 25 min of air exposure. It should be stated that in a dry room atmosphere, the R phase was reported to be stable for at least 20 h (see Wang et al ibid). Hence, it can be concluded that the R phase is actually moisture sensitive rather than air sensitive. It follows that water-based binders cannot obviously be used with the R phase if electrodes are fabricated directly with it. The above highlights the practical advantage of the method proposed in this patent to use the R3 Na₂Fe₂(CN)₆ phase in a battery. By dealing with the M-Na_2-xFe₂(CN)₆.2H₂0 phase first, all conventional electrode fabrication steps can be conducted in ambient air. When the moment comes to actually assemble a cell, the M-Na_2-xFe₂(CN)₆.2H₂0 electrode can be heated in either vacuum or air or inert atmosphere and henceforth, assembled in inert atmosphere. In doing this, the need for the inert atmosphere gets limited to just the last step which should ensure significant cost savings besides ensuring much easier handling. In addition, it is noted that the use of M-Na_2-xFe₂(CN)₆.2H₂0 to form an electrode also enables one to make use of water-based binders when seeking to ultimately construct an R-phase electrode. This may also help to significantly reduce the processing costs and environmental impact associated with the R phase material. Initial Testing of Electrodes

The below represents the initial tests that were conducted on these active materials and as such, they represent un-optimized results. They are included herein to provide context to the improvements made following optimization of the electrodes and the conditions of operation demonstrated by the rest of the disclosure. Nevertheless, the conclusions presented herein are consistent with the results described above.

The first cycle of an M-Na_2-xFe₂(CN)₆.2H₂0 electrode in a non-aqueous NIB is presented in Fig. 1 1 a. A first charge and discharge capacity of 81 and 82 mAh/g respectively were obtained, confirming the sodium rich nature of the M-Na_2-xFe₂(CN)₆.2H₂0 phase. The unique electrochemical signature of M-Na_2-xFe₂(CN)₆.2H₂0 is evident, with a single charge and discharge plateau at an average voltage of 3.0 V vs Na/Na⁺. However, Fig. 11a only reveals that the as-obtained material definitely does contain at least 1 mole of Na per mole of material. To confirm its Na rich nature, the first cycle of an R3 Na₂Fe₂(CN)₆ electrode (obtained by heating the M-Na_2-xFe₂(CN)₆.2H₂0 electrode > 200 °C in Argon) will be more relevant. Indeed, a first charge capacity of 167 mAh/g was obtained as shown in Fig. 11 b with the first discharge capacity being 166 mAh/g. The high first charge capacity of 167 mAh/g indicates that the 'x' in M-Na₂-xFe₂(CN)₆.2H₂0 is essentially 0. This is because the theoretical capacity of Na₂Fe₂(CN)₆ assuming complete removal of 2 moles of Na per mole of Na₂Fe₂(CN)₆ is 170.8 mAh/g. Hence, based on the electrochemical cycling curves and the TGA data, it can be unambiguously concluded that the formula of the as-synthesized compound is Na₂._xFe₂(CN)₆.2H₂0 with x « 0. Henceforth, this material will be referred to as "M-Na₂Fe₂(CN)₆.2H₂0".PIease do keep in mind that both electrodes were first charged (sodium was extracted from both types of Na₂Fe₂(CN)₆). The expected two charge and discharge plateaus centered at 3.3 and 3.0 V vs Na/Na⁺ can be seen in Fig. 11 b for the R3 Na₂Fe₂(CN)₆ electrode, in line with that reported by Wang et a/ (ibid).

The rate performance of an M-Na₂Fe₂(CN)₆.2H₂0 electrode is depicted in Fig. 12a with the charging performed at C/4.5 rate corresponding to 1 mole sodium storage per mole of Na₂Fe₂(CN)₆ resulting in a capacity of 76.6 mAh/g. An outstanding response is obtained even at high rates and stable discharge capacities of 80, 78, 77.3, 75.7, 73, 68 and 58 mAh/g could be obtained at 1/4.5, 1/1.8, 1.1 , 2.2, 3.3, 5.6 and 11.1 C rates respectively. Please note that the actual C rate values have been refined to more accurate values since the time the original patent application claiming priority was filed (Fig. 12 still depicts the original C rate values, but the updated C rate values have been mentioned in the figure caption). As can be seen, the adjusted and more accurate values are actually higher, which is even more favourable for battery application. Furthermore, as seen from Fig. 12b, the polarization of an M-Na₂Fe₂(CN)₆.2H₂0 electrode did not significantly increase at any rate < 5.6 C, pointing to a robust structure and opportunity for high power densities. In contrast to the M-Na₂Fe₂(CN)₆.2H₂0 phase, the R3 Na₂Fe₂(CN)₆ phase has double the capacity due to the involvement of two moles of sodium storage per mole of Na₂Fe₂(CN)₆. Similar to the excellent rate performance exhibited by M-Na₂Fe₂(CN)₆.2H₂0, the sodium storage performance of an R3 Na₂Fe₂(CN)₆ electrode was found to be extremely good at low rates < 2 C being above 130 mAh/g as presented in Fig. 13a (for this phase, the current was calculated based on two mole Na storage per mole of Na₂Fe₂(CN)₆ resulting in theoretical capacity of 170.8 mAh/g). The polarization of the R 3 Na₂Fe₂(CN)₆ electrode became significant only above 2 C, ensuring a sufficiently fast response (refer to Fig. 13b). These results indicate that the sodium storage performance of an R3 Na₂Fe₂(CN)₆ electrode prepared from the method described in this patent is very impressive and also ensures easy handling. Without wishing to be bound by theory, the impressive sodium storage performance of the two phases appears to arise due to the favourable morphology which is an outcome of this synthesis method. As seen from Fig. 1a, the field emission scanning electron microscopy (FESEM) image reveals relatively small particle size < 1-2 μηη in size.

Claims

1. A compound of formula (I):

Na₂-_xFe₂-w(CN)_6-y.(Vacancy)_v.zH₂0 (I)

wherein:

0 < x < 1 , 0 < w < 0.2, 0 < y < 0.2, 0 < v < 0.2 and 0 < z < 4; and

2. The compound of Claim 1 , wherein 0 < x < 0.5, 0 < w < 0.1 , 0 < y < 0.1 , 0 < v < 0.1 and 0 < z < 2.5.

3. The compound of Claim 2, wherein 0 < x < 0.2, w and y are 0, 0 < v < 0.1 and 0 < z < 2.2.

4. The compound of Claim 3, wherein the compound of formula (I) has the formula (la):

M-Na_2-xFe₂(CN)₆.2H₂0 (la),

5. The compound of Claim 4, wherein x is 0.

6. The compound of any one of the preceding claims, wherein the monoclinic lattice system has the space group P2 /n, optionally wherein the space group is P2†/n and the lattice parameters are a =10.45983(56) A, b = 7.51295(42) A, c = 7.27153(48) A and β = 92.7379(33)°.

7. The compound of Claim 1 , wherein the compound shows principal (most intense) peaks at angles of diffraction 2Θ of 17.0, 23.7, 24.5 and 34.3 in an X-ray powder diffraction pattern obtained by irradiation with copper Ka radiation (wavelength λ = 1.54 angstroms).

8. An electrode comprising the compound of formula (I) as claimed in Claim 1 , optionally wherein the electrode is a cathode.

9. A method of preparing a compound of formula (I), as claimed in Claim 1 , which method comprises the steps of:

(a) reacting a solution comprising Na₄Fe(CN)₆, or hydrates thereof, a reducing agent and water for a period of time to provide a precipitate; (b) separating the precipitate from the solution; and

(c) drying the precipitate to provide a compound of formula (I).

10. The method of Claim 9, wherein the Na₄Fe(CN)₆ is provided as the hydrate Na₄Fe(CN)₆.10H₂O.

11. The method of Claim 9, wherein the reducing agent is provided in molar excess relative to the Na₄Fe(CN)₆ or hydrate thereof.

12. The method of Claim 9, wherein the reducing agent is selected from one of more of the group consisting of ascorbic acid, oxalic acid, glucose, starch, gluconic acid lactone, and formic acid.

13. The method of Claim 12, wherein the reducing agent is ascorbic acid and/or formic acid.

14. The method of Claim 9, wherein the temperature of reaction step (a) is from 0 °C to 200 °C, such as from 80 °C to 170 °C, from 90 °C to 160 °C, from 100 °C to 150 °C (e.g. 140 °C).

15. A method of converting a compound of formula (I), according to Claim 1 , into a compound of formula (II),

Na2-x'Fe_2-w'(CN)_6-y..(Vacancy)v.z'H₂0 (II)

wherein:

0 < x' < 1, 0 < w" < 0.2, 0 < y' < 0.2, 0 < v' < 0.2 and 0 < i≤ 0.5; and

(a) providing a compound of formula (I); and

(b) heating the compound of formula (I) to a temperature from 50 °C to 500 °C for a period of time under vacuum, dry air or an inert atmosphere to provide the compound of formula (II), optionally wherein 0 < x' < 0.5, 0 < w' < 0.1 , 0 < y' < 0.1 , 0 < v' < 0.1 , and 0 < z' < 0.2 (e.g. 0 < x' < 0.5, 0 < w^* < 0.1 , 0 < y' < 0.1 , 0 < v' < 0.1 , and z' is 0).

16. The method of Claim 15, wherein the compound of formula (II) has the formula (I la):

R-Na_2-x.Fe₂(CN)₆ (Ma), where R is an indicator of the rhombohedral lattice system having the space group R3 and x' is from 0 to 0.2,

17. The method of Claim 15, wherein the compound of formula (II) has the formula (I Is):

R-Na_2-x Fe₂(CN)₆ (lla),

where R is an indicator of the rhombohedral lattice system having the space group R3 and x' is 0.

18. A method of converting an electrode comprising a compound of formula (I), according to Claim 1 , into an electrode comprising a compound of formula (II),

Na_2-x'Fe_2-w(CN)₆._y..(Vacancy)_v..zH₂0 (II)

wherein:

0 < x' < 1 , 0 < w' < 0.2, 0 < y' < 0.2, 0 < v' < 0.2 and 0 < z' < 0.5; and

(a) providing a compound of formula (I); and

(b) heating the compound of formula (I) to a temperature from 50 °C to 500 °C for a period of time under vacuum, dry air or an inert atmosphere to provide the compound of formula (II), optionally wherein 0 < x' < 0.5, 0 < w' < 0.1 , 0 < y' < 0.1 , 0 < ' < 0.1 , and 0 < z' < 0.2 (e.g. 0 < x' < 0.5, 0 < w' < 0.1 , 0 < y' < 0.1 , 0 < ' < 0.1 , and z' is 0).

19. The method of Claim 18, wherein the compound of formula (II) has the formula (I Is):

R-Na₂._xFe₂(CN)₆ (I la),

where R is an indicator of the rhombohedral lattice system having the space group R3 and x' is from 0 to 0.2, optionally wherein x' is 0.

20. A sodium ion battery comprising an electrode comprising a compound of formula (I) according to Claim 1.