A method for manufacturing prussian white particles

Abstract

The present disclosure generally relates to a method for manufacturing Prussian White particles comprising: a) providing an aqueous solution of a precursor having the formula A4X1(CN)6, wherein A is Na or K, and wherein X1 is Fe or Mn; b) reacting said precursor with an acid under conditions that allow for an aqueous reaction product comprising Prussian White particles to be formed; and c) drying said aqueous reaction product, wherein step b) is performed at a temperature of from 75 to 95°C, preferably from 80 to 95°C, more preferably from 85 to 95°C and at a pH of from 1.8 to 4.5. The present disclosure also relates to a Prussian White particle obtainable by the method, to a cathode comprising a plurality of Prussian White particles, and to an electrochemical energy storage device, preferably a battery cell comprising the cathode. Furthermore, the present disclosure relates to a method for manufacturing a cathode comprising the Prussian White particles.

Description

[0001] A METHOD FOR MANUFACTURING PRUSSIAN WHITE PARTICLES

[0002] TECHNICAL FIELD

[0003] The present disclosure generally relates to a method for manufacturing Prussian White particles, and to Prussian White particles obtainable by the method. The present disclosure also relates to a cathode comprising a plurality of Prussian White particles, and to an electrochemical energy storage device, preferably a battery cell comprising the cathode. Furthermore, the present disclosure relates to a method for manufacturing a cathode comprising the Prussian White particles.

[0004] BACKGROUND

[0005] To date, lithium-ion based batteries (LIB) dominate the market for rechargeable batteries. However, the technology is associated with drawbacks, particularly since the availability of lithium resources is limited. Furthermore, the disposal of spent LIBs is challenging both from a recycling and waste management perspective. Specific precautions are typically required to handle spent LIBs in a safe manner, and improper disposal may have a negative environmental impact.

[0006] Sodium ion batteries (SIBs) have emerged as attractive alternatives to LIBs. SIBs offer environmental advantages due to the abundance of sodium resources and the reduced environmental impact associated with sodium extraction and processing.

[0007] The performance of a sodium ion battery is largely dependent on the properties of the cathode material.

[0008] Prussian White (PW) is an environmentally friendly cathode material, which has gained considerable attention due to its high theoretical capacity and rate performance. Prussian White has a crystal structure with an open three-dimensional framework and large interstitial voids, which makes it capable of storing sodium ions in an efficient manner.

[0009] Various methods to synthesize Prussian White are known. For example, Prussian White may be produced using a co-precipitation method.

[0010] Common to the known methods of synthesizing Prussian White is the challenge of providing a high yield of the Prussian White material, and in particular a high yield of a high-quality Prussian White material.

[0011] Furthermore, conventional Prussian White synthesis methods typically require lengthy drying and recovering steps, separate sodium enrichment steps, and the use of complex mechanochemical post-synthetic modifications, all of which may be regarded as inconvenient for large scale production.

[0012] Accordingly, there is a need to provide an improved method to synthesize Prussian White. Such a process should generate Prussian White in a high yield, be simple and inexpensive, and suitable for large scale production.

[0013] SUMMARY

[0014] In view of above-mentioned and other drawbacks of the prior art, it is an object of the present disclosure to provide improvements in relation to synthesizing a high yield of Prussian White material in a facilitated and inexpensive manner.

[0015] According to a first aspect of the present disclosure, there is provided a method for forming Prussian White particles comprising: a) providing an aqueous solution of a precursor having the formula A4X1(CN)6, wherein A is Na or K, and wherein X1is Fe or Mn; b) reacting the precursor with an acid under conditions that allow for an aqueous reaction product comprising Prussian White particles to be formed; c) drying the aqueous reaction product, wherein step b) is performed at a temperature of from 75 to 95°C, preferably from 80 to 95°C, more preferably from 85 to 95°C and at a pH of from 1.8 to 4.5.

[0016] The present disclosure is based on the realization that Prussian White can be obtained in a high yield and with a high sodium (or potassium) content by the method defined hereinabove.

[0017] The inventors have found that the specific reaction conditions, i.e. a temperature of from 75 to 95°C, and a pH of from 1.8 to 4.5 results in a significantly higher yield compared to a temperature below 75°C and / or a pH above 4.5.

[0018] Another advantage associated with the method of the present disclosure is that time-consuming drying steps and labor-intensive separation, washing, and recovering steps can be avoided. In the method of the present disclosure, only one drying step is required (step c).

[0019] Surprisingly, the inventors have also identified a new Prussian White crystal structure, obtainable at these reaction conditions. X-ray powder diffraction (XRD) characterization of the Prussian White material has revealed a material having distinct crystallographic features compared to known Prussian White materials, and which evidences the introduction of an enhanced amount of sodium and a decreased number of vacancies in the Prussian White structure.

[0020] The new Prussian White material, obtainable by the method described hereinbefore, is associated with a higher sodium content, an improved specific electrical charge and discharge capacity and thermal stability. Thus, the new Prussian White material (when used as a cathode active material) can be exposed to relatively high process and / or operating temperatures without breaking down and consequently has a decreased risk of contributing to escalating a thermal runaway in the case of failure of a battery cell.

[0021] Preferably, step b) is performed at a pH of from 2 to 3.5, more preferably from 2 to 3.

[0022] This pH range generates an improved yield of the Prussian White particulate material.

[0023] The Prussian White particles may be defined by AaX1[X2(CN)e]i-y-mH2O, wherein A is Na or K, 1.8 < a < 2, preferably 1.9 < a < 2, 0 < m < 3, 0 < y < 0.025, and wherein X1and X2are selected from Fe and Mn.

[0024] Each Prussian White particle comprises iron (Fe) or manganese (Mn) ions which are linked through cyanide (CN) bridges, forming a three-dimensional framework. The framework forms a cubic arrangement, in which each iron (or manganese) ion is surrounded by six cyanide groups. Sodium or potassium ions reside within the cavities or interstitial sites of the cubic arrangement formed by the [Fe(CN)e] or [Mn(CN)e] units.

[0025] At least one crystalline domain of each of the Prussian White particles may be defined by AaX1[X2(CN)6]i-y-m^O, wherein A is Na or K, 1.95 < a < 2, preferably 1.97 < a < 2, more preferably 1.99 < a < 2, 0 < m < 3, 0 < y < 0.013, and wherein X1and X2are Fe or Mn.

[0026] Accordingly, in such crystalline domain(s), the sodium or potassium content is significantly higher compared to crystalline domain(s) having, e.g., a cubic crystal structure; i.e. a higher amount of sodium or potassium has been introduced into the Prussian White structure. The enhanced capability in storing sodium (and potassium) ions allows for the specific capacity (Ah / kg) of a battery cell incorporating the material to be improved.

[0027] The method of the present disclosure, and the precise reaction conditions (pH and temperature range) increase the reaction rate of forming the Prussian White particles, the amount of sodium (or potassium) that is present in the structure and influences the crystal structure of the Prussian White particles. In exemplary embodiments, the method may further comprise a step b’) of coating the Prussian White particles prior to step c) of drying by adding a salt solution comprising a manganese, nickel, aluminum, silver, cobalt, and / or titanium salt to the aqueous reaction product.

[0028] The inventors have found that the provision of such a coating may significantly improve the moisture stability of the Prussian White particles. Furthermore, the coating process is associated with ease of manufacturing, reduced waste (by consuming unreacted precursor), and prevention of the formation of undesired by-products.

[0029] Many traditional coating methods (e.g. physical vapor deposition, chemical vapor deposition, sol-gel process) involve complex set-ups, high energy consumption, and expensive equipment. Furthermore, conventional coating methods typically involve applying the coating as a final, separate step after the Prussian White particles have been formed. An advantage of the method of the present disclosure is that the coating process can take place “in the same pot” (in situ)., i.e. directly after (or during the final step of) the formation of the Prussian White particles.

[0030] In exemplary embodiments, the salt solution in step b) may be added when at least 70%, preferably at least 85%, more preferably at least 95% of the precursor has reacted with the acid and formed the Prussian White particles.

[0031] Most preferably, the salt solution in step b) may be added when at least 99% of the precursor has reacted with the acid and formed the Prussian White particles.

[0032] If the salt is added too early; i.e. before a substantial proportion of the precursor has been consumed, this may lead to undesirably small Prussian White particles. Furthermore, if the salt is added when a too small amount of the precursor has been consumed, a substantial amount of the salt may react with the precursor (without the Prussian White being involved), and this may result in the formation of undesired by-products.

[0033] During the acid decomposition step (step b) hereinbefore), where the Prussian White particles are formed, the Prussian White particles typically continue to grow in size. On the outer surface of each particle, there are “dangling” bonds (CN-ligands) which have a tendency to react further, and thereby form larger particles. However, the cations of the salt solution (manganese, titanium, nickel, silver, cobalt, or aluminum) “attach” to the outer surface of the particles and prevent these from further growth. It is contemplated that the growth is prevented as the reaction rate of the coating formation is orders of magnitude faster than the acid decomposition step. In this regard, the acid decomposition is “quenched”. It is understood that a coated Prussian White particle is larger than the same Prussian White particle before coating.

[0034] According to another aspect, there is provided a Prussian White particle obtainable by the method described hereinbefore.

[0035] The Prussian White particle may comprise at least one crystalline domain defined by AaX1[X2(CN)6]i-y-mPhO, wherein A is Na or K, 1.95 < a < 2, preferably 1.97 < a < 2, more preferably 1.99 < a < 2 , 0< m < 3, 0 < y < 0.013 and wherein X1and X2are selected from Fe and Mn.

[0036] The at least one crystalline domain may have a monoclinic crystal structure.

[0037] As mentioned hereinbefore, the inventors have identified a new Prussian White crystal structure with distinct crystallographic features compared to known Prussian White materials. The new Prussian White material has shown a high thermal phase stability.

[0038] The inventors have found that the monoclinic crystal structure of the Prussian White particle of the present disclosure is stable at temperatures up to at least 100°C. In other words, the monoclinic crystal structure (indicative of a high sodium content) does not convert to the rhombohedral crystal structure upon heating at a temperature of up to, e.g., 100°C.

[0039] The distinct crystallographic features of the Prussian White particle of the present disclosure can be observed through XRD analysis even after e.g., drying, transport and shipping at elevated temperatures.

[0040] The Prussian White particle may comprise at least one crystalline domain having a monoclinic crystal structure and exhibiting a peak, Pl, at a diffraction angle of 34.1 plus or minus 0.2 degrees, and a peak, P2, at a diffraction angle of 34.4 plus or minus 0.2 degrees, when analyzed with X-ray Powder diffraction using Cu Ka radiation.

[0041] In some embodiments, the peak intensity ratio between the peak intensity of Pl and the peak intensity of P2 may be from 1.6 to less than 1.9.

[0042] The peak intensity of Pl and P2 may be abbreviated Ipi and Ip2, respectively.

[0043] The peak intensity ratio between Ipi and Ip2 may be expressed as IPI / IP2. The peak intensity ratio may also be understood as the relative intensity.

[0044] The monoclinic structure of the at least one crystalline domain is confirmed by the presence of specific peaks in X-ray powder diffraction (XRD), which indicates the precise orientation and arrangement of atoms within the crystal structure. The structure of the new Prussian White is associated with an enhanced incorporation of sodium (or potassium) ions and a low vacancy content in the Prussian White structure. When provided in an electrochemical cell, i.e., a battery cell, the high amount of (intercalatable / deintercalatable) sodium (potassium) ions advantageously contributes to a higher specific capacity.

[0045] The Prussian White particle may be defined by an inner core and an outer surface. The Prussian White particle may further comprise a coating arranged on at least a portion of the outer surface, wherein the coating comprises manganese, nickel, aluminum, silver, cobalt, and / or titanium.

[0046] Hence, the resulting Prussian White particles comprise a manganese, nickel, aluminum, silver, cobalt, or titanium coating on its outer surface.

[0047] The coating provided on at least a portion of the outer surface prevents the Prussian White structure from degrading and improves the shelf-life. The coating further slows down the degrading process of the particles when arranged in a slurry, such as an aqueous slurry. The inventors have found that the degradation over time is significantly reduced with a coating as defined hereinabove. Furthermore, the coating provided on the outer surface does not affect the overall Prussian White structure or impair the ionic conductivity of the material.

[0048] The coated Prussian White particles, as described hereinabove, are safe to use as a cathode active material in a battery cell. The coating does not introduce any flammable oxygen into the battery cell, which may e.g. be the case with coatings based on oxides.

[0049] The coating may be defined by Naa’X3[X2(CN)6]i-y’ -m’^O, wherein A is Na or K, 1.8 < a’ < 2, preferably 1.9 < a’ < 2, 0< m’ < 3, 0 < y’ <0.025 wherein X2is selected from Fe and Mn, and wherein X3is Mn, Ti, Ni, Ag, Co, and / or Al.

[0050] According to another aspect, there is provided a cathode comprising a plurality of Prussian white particles as defined hereinabove or manufactured by the method described hereinabove.

[0051] According to yet another aspect, there is provided a method for manufacturing a cathode comprising:

[0052] - providing a slurry comprising a solvent, a conductive additive, and a plurality of Prussian white particles as defined hereinabove or manufactured by the method described hereinabove; and

[0053] - applying the slurry onto a current collector.

[0054] According to another aspect, there is provided an electrochemical energy storage device, preferably a battery cell comprising the cathode defined hereinabove or manufactured as described hereinabove. According to yet another aspect, there is provided a Prussian White particle comprising at least one crystalline domain defined by AaX1[X2(CN)e] i-y-ml O, wherein A is sodium or potassium, 1.95 < a < 2, preferably 1.97 < a < 2, more preferably 1.99 < a < 2, more preferably 1.99 < a < 2 0 < m < 3, 0 < y < 0.013, and wherein X1and X2are selected from Fe and Mn; the at least one crystalline domain having a monoclinic crystal structure and exhibiting a peak, Pl, at a diffraction angle of 34.1 and a peak, P2, at a diffraction angle of 34.4 plus or minus 0.2 degrees, when analyzed by X-ray powder diffraction using Cu Ka radiation.

[0055] Further features of, and advantages with, the present disclosure will become apparent when studying the appended claims and the following description. The skilled addressee realizes that different features of the present disclosure may be combined to create embodiments other than those described in the following, without departing from the scope of the present disclosure.

[0056] BRIEF DESCRIPTION OF THE DRAWINGS

[0057] The various aspects of the present disclosure, including its particular features and advantages, will be readily understood from the following detailed description and the accompanying drawings, in which:

[0058] Figures la-f illustrate the XRD patterns observed for Examples 1-3 and 12 (figs, la-c and le), and for Comparative Example 3 and 4 (fig. Id and If) in Example 1 : Evaluation of different reaction conditions.

[0059] Figure 2 schematically illustrates a sodium or potassium ion battery cell according to an exemplary embodiment of the present disclosure.

[0060] Figures 3 a-f illustrate the SEM images of uncoated Prussian White particles (Reference Example 1, fig 3 a), and the coated Prussian White particles of Examples 4-8 (figs 3b-f) as synthesized; i.e. at week 0 (Example 2: Evaluation of coated Prussian White particles).

[0061] Figures 4a-f illustrate the SEM images of uncoated Prussian White particles (Reference Example 1, fig 4a), and the coated Prussian White particles of Examples 4-6 (figs 4b-e) at week 2 and Example 7 (fig 4f) at week 1 (Example 2: Evaluation of coated Prussian White particles).

[0062] Figure 5 illustrates a thermogravimetric analysis (TGA) plot of Example 3 and Comparative Example 3. DETAILED DESCRIPTION

[0063] The present disclosure will now be described more fully hereinafter with reference to the accompanying drawings. The present disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the present disclosure to the skilled person.

[0064] In a first aspect, the present disclosure relates to a method for manufacturing Prussian White particles comprising: a) providing an aqueous solution of a precursor defined by A4Xx(CN)6, wherein A is Na or K, and wherein X1is Fe or Mn; b) reacting the precursor with an acid under conditions that allow for an aqueous reaction product comprising Prussian White particles to be formed; and c) drying the aqueous reaction product, wherein step b) is performed at a temperature of from 75 to 95°C, preferably from 80 to 95°C, more preferably from 85 to 95°C and at a pH of from 1.8 to 4.5.

[0065] The term “Prussian White particle” means a particle defined by the formula AaX1[X2(CN)e] i-y-mH20. The Prussian White particle comprises iron (Fe) or manganese (Mn) ions which are linked through cyanide (CN) bridges, forming a three-dimensional framework. The framework forms a cubic-like arrangement, in which each iron (or manganese) ion is surrounded by six cyanide groups. Sodium or potassium ions reside within the cavities or interstitial sites of the cubic lattice formed by the Fe[Fe(CN)e] or Mn[Mn(CN)e] units. The Prussian White particle is a solid and may be understood as a discrete portion of solid matter.

[0066] In the formula AaX1[X2(CN)6]i-y-m^O, A is sodium (Na) or potassium (K). Preferably, A is sodium. Sodium has a smaller ionic radius than potassium and may diffuse faster through the Prussian White material structure.

[0067] Furthermore, in the formula AaX1[X2(CN)e] i-y-m^O, X1and X2are individually selected from Fe or Mn. Preferably, X1and X2are Fe.

[0068] For example, when X1and X2are Fe, A is Na, and a is 2, 0 < m < 3, and 0 <y < 0.025 the Prussian white particles may in be defined by the formula Na2Fe[Fe(CN)e]i-y•mH20. It is understood that the two Fe moieties of the formula may be provided from precursor molecules.

[0069] It is contemplated that in examples where the precursor is Na4Fe(CN)e and the acid is H2SO4 reacted under conditions that allow for an aqueous reaction product comprising Prussian White particles to be formed, the following reaction occurs:

[0070] 2 Na4Fe(CN)6+ 3 H2SO4Na2Fe[Fe(CN)6] + 3 Na2SO4+ 6 HCN.

[0071] In other words, one Fe moiety from one Na4Fe(CN)e takes the position of X1and another Fe moiety from another Na4Fe(CN)e takes the position of X2in the formed Prussian White particle.

[0072] It is understood that “an aqueous solution of a precursor defined by A4X1(CN)e” comprises one or more precursor moieties, preferably two or more precursor moieties. It is further understood that the aqueous solution of a precursor may be understood as an aqueous solution comprising at least partially and / or fully dissolved precursor.

[0073] The precursor in step a) may be a first precursor, wherein the aqueous solution comprises at least a second precursor.

[0074] The first precursor is defined by A4Xx(CN)6, and the second precursor may be defined by A4X2(CN)e, wherein A is Na or K, and wherein X1and X2is Fe or Mn.

[0075] X1and X2may be different or the same.

[0076] For example, X1may be Fe and X2may be Mn.

[0077] Accordingly, the first precursor may be Na4Fe(CN)e or K4Fe(CN)e, and the second precursor may be Na4Mn(CN)e or K4Mn(CN)e.

[0078] In such embodiments, the Prussian White particles may be defined by the formula Na2Mn[Fe(CN)e] or K2Mn[Fe(CN)e].

[0079] For example, step b) may comprise the reaction:

[0080] Na4Fe(CN)6+ Na4Mn(CN)6+ 3 H2SO4^Na2Mn[Fe(CN)6] + 6 HCN + 3 Na2SO4.

[0081] Herein the term “aqueous solution” refers to a solution in which water constitutes the predominant solvent by volume. The aqueous solution may comprise water, such as tap water, deionized water, or milli-q water.

[0082] The “aqueous reaction product” is typically an aqueous suspension comprising Prussian White particles. The aqueous reaction product may be a mixture in which solid particles are dispersed in water without being fully dissolved.

[0083] The H2O of the formula “AaX1[X2(CN)6]i-y-mH2O” (as well as “Aa’X3[X2(CN)6]i-y’-m’H2O” described hereinbelow) may herein refer to crystal water. The term “crystal water” refers to water molecules arranged in voids and / or interstitial spaces of the crystal structure of the Prussian White particles. A water molecule of the crystal water may be bonded through a hydrogen bond with the nitrogen of the (CN)e, a metal-ligand bond with one of the transition metal cations, or an ion-dipole bond with sodium. The Prussian White particles may have a Dv50 value of from 1 to 50pm, preferably from 10 to 30pm, more preferably in the range of from 12 to 25pm, as determined according to ISO 13320:2020.

[0084] Herein, the term “Dv50 value” refers to a particle diameter below which 50% of the sample volume exists. The Dv50 value may also be understood as the median particle size by volume or the volumetric particle size distribution. Dv50 as such is known to the person skilled in the art. A Dv50 value of a sample volume (a plurality of particles) may be measured using, e.g., a PSA 1090 L / D (Anton Paar) instrument.

[0085] The Prussian White particles of the present disclosure may form the active material of a cathode in a sodium or potassium ion battery cell. The Prussian White particles improve the performance of the cathode and facilitate the intercalation and deintercalation of sodium (or potassium) ions during the charging and discharging cycles. This process is crucial for the movement of ions that generates electrical current in a battery. The open Prussian White framework allows for easy movement of ions, thereby contributing to an improved cycling stability and capacity of the battery cell. In particular, the Prussian White has a very high sodium (or potassium) content which advantageously allows for improved specific capacity.

[0086] Herein, the terms “intercalation / deintercalation” and “insertion / deinsertion” may be used interchangeably and are known to the skilled artisan.

[0087] The method of the present disclosure is associated with ease of manufacturing, cost effectiveness, and environmental friendliness. Furthermore, the use of toxic and costly organic solvents can be avoided.

[0088] Furthermore, with the method of the present disclosure, no undesirable byproducts are formed, and the method is associated with an enhanced yield.

[0089] Another advantage associated with the method of the present disclosure is that time-consuming drying steps and labor-intensive separation and washing steps can be avoided. In the method of the present disclosure, only one drying step is required (step c).

[0090] An additional advantage of the method of the present disclosure is that the obtained Prussian White particles have a surprisingly high sodium (or potassium) content, and a surprisingly low vacancy content. Accordingly, labour-intensive separate sodium (or potassium)-enrichment steps and / or mechanochemical post-processing steps can be avoided.

[0091] Herein, the term “vacancy content” is to be understood as the amount [X2(CN)e] unit(s) absent from the AaX1[X2(CN)6]i.y-mH2O or Naa’X3[X2(CN)6]i-y’-m’H2O. A high vacancy content may comprise the structural integrity of the material making it more prone to structural degradation and / or collapsing during cycling in battery applications. Furthermore, with increasing vacancy content the material’s capacity for storing sodium (or potassium) is reduced as the charge balance must be maintained in the material.

[0092] The precursor may be selected from Na4Fe(CN)e, K4Fe(CN)e, Na4Mn(CN)e, and K4Mn(CN)6.

[0093] These precursors are suitable for providing Prussian white particles, i.e. sodium iron hexacyanoferrate, potassium iron hexacyanoferrate, sodium manganese hexacyanomanganate, or potassium manganese hexacyanomanganate particles, in a high yield.

[0094] Preferably, the precursor is Na4Fe(CN)e.

[0095] As mentioned hereinbefore, the precursor in step a) may be a first precursor, wherein the aqueous solution comprises at least a second precursor.

[0096] For example, the first precursor may be Na4Fe(CN)e or K4Fe(CN)e, and the second precursor may be Na4Mn(CN)e or K4Mn(CN)e. Alternatively, the first precursor may be Na4Mn(CN)e or K4Mn(CN)e and the second precursor may be Na4Fe(CN)e or K4Fe(CN)e.

[0097] The acid in step b) may be selected from the group consisting of hydrochloric acid, formic acid, acetic acid, citric acid, sulfuric acid, hydroiodic acid, hydrobromic acid, nitric acid, phosphoric acid, or combinations thereof.

[0098] Preferably, the acid in step b) is sulfuric acid or phosphoric acid, more preferably the acid in step b) is sulfuric acid.

[0099] The acid may be added continuously or sequentially during step b) to secure that the pH is kept in the range of from 1.8 to 4.5, preferably from 2 to 3.5, more preferably from above 2 to below 3. It is understood that the volumetric flow rate (measured in volume unit per time unit), of acid added continuously or sequentially may vary over a time period for which step b) is conducted.

[0100] Herein, the expression “added sequentially” is to be understood as added at at least two discrete occasions in time. In other words, the acid may be added in separate, discrete occasions.

[0101] Herein, the term “added continuously” is to be understood as added over a period of time without interruption. Expressed differently, “added continuously” may be understood as added at one single occasion over a period of time, without deliberate interruption. The addition may occur at a controlled rate.

[0102] Reacting the precursor with an acid “under conditions that allow for an aqueous reaction product comprising Prussian White particles to be formed” means performing step b) at a temperature of from 75 to 98°C, preferably from 80 to 95°C, preferably from 85 to 95°C and at a pH of from 1.8 to 4.5.

[0103] The inventors have found that a significantly higher yield may be obtained with these reaction conditions. The electrochemical performance of a battery cell incorporating the Prussian White material is also improved (see Example 1).

[0104] Typically, step b) is performed under inert conditions.

[0105] In some embodiments, step b) may be performed during 1 to 24 hours, such as 2 to 20 hours, preferably 3 to 12 hours, more preferably 4 to 8 hours.

[0106] Accordingly, the precursor reacts with the acid and / or salt solution in an inert atmosphere, which prevents undesired exposure to ambient air which could yield undesired side-reactions.

[0107] Preferably, steps a-c) of the method are performed under inert conditions.

[0108] Step a), b) and / or c) of the method may be performed in the presence of an inert gas, e.g. nitrogen and / or argon.

[0109] The method may comprise a separation step between step b) and c). In other words, the aqueous reaction product obtained in step b) may be separated prior to step c) of drying. The separation step may for example be performed using a filter press or a centrifuge.

[0110] The step c) of drying may be performed by means known to the skilled person.

[0111] The step c) of drying may for example be performed using a rotary dryer, a fluidized bed dryer, a spray dryer, a vacuum dryer, or a conveyor dryer.

[0112] Preferably, step b) is performed at a pH of from 2 to 3.5, more preferably from 2 to 3.

[0113] At least one crystalline domain of each of the Prussian White particles may be defined by AaX1[X2(CN)e] i-y-m^O, wherein A is Na or K, 1.95 < a < 2, preferably 1.97 < a < 2, more preferably 1.99 < a < 2, 0 < m < 3, 0 < y < 0.013 and wherein X1and X2are selected from Fe and Mn.

[0114] A “crystalline domain” is a distinct region within the Prussian White structure where the constituent atoms are arranged in a regular, repeating pattern, characteristic of a crystalline structure. In the Prussian White structure, each crystalline domain consists of iron (or manganese) ions and cyanide (CN) ligands arranged in a specific, repeating structure, with sodium (or potassium) ions interspersed within the structure to maintain charge balance. The Prussian White structure of the present disclosure is associated with an enhanced capability of storing sodium (and potassium) ions.

[0115] The at least one crystalline domain may have a monoclinic crystal structure. As mentioned hereinbefore, in the crystalline domain(s) as defined hereinbefore, the sodium or potassium content is significantly higher. Accordingly, the specific capacity of a battery cell comprising the Prussian White material may be improved.

[0116] The method may further comprise a step b’) of coating the Prussian White particles prior to step c) of drying by adding a salt solution comprising a manganese, nickel, aluminum, silver, cobalt, and / or titanium salt to the aqueous reaction product.

[0117] Accordingly, the Prussian White particles may be formed and coated in the same reaction vessel (or at least in the same chain of reactions), under substantially the same reaction conditions (temperature and pH). This is beneficial to avoid the addition of further chemicals for pH adjustment, as well as lengthy and complex temperature adjustment, drying, recovering, and coating steps and thus renders the method suitable for large-scale production of coated Prussian White particles.

[0118] In some embodiments, the step b’) is conducted at a temperature of 75 to 95°C, and a pH of from 1.8 to 4.5. In other words, step b’) may be carried out at substantially the same process conditions as step b). This advantageously allows for avoiding timeconsuming steps of changing the process conditions, e.g., pH and / temperature, to form the coating.

[0119] The salt solution may e.g. comprise manganese sulphate, manganese chloride, manganese acetate, manganese phosphate, titanium sulphate, titanium chloride, titanium acetate, titanium phosphate, aluminum sulphate, aluminum chloride, aluminum acetate, aluminum phosphate, silver sulphate, silver chloride, silver acetate, silver phosphate, cobalt sulphate, cobalt chloride, cobalt acetate, and / or cobalt phosphate.

[0120] Preferably the salt solution comprises a manganese salt. For example, the salt solution may comprise manganese sulphate.

[0121] The salt solution in step b’) may be added before the precursor has been “fully consumed”. Hence the “aqueous reaction product comprising Prussian White particles” may comprise a proportion of precursor and a proportion of formed Prussian white particles. For example, the molar ratio between Prussian white unit cells / moieties of the Prussian White particles and precursor moieties in the aqueous reaction product may be 35:30, e.g. 85:30, 95: 10, e.g. 99:2.

[0122] The aqueous reaction product is typically an aqueous suspension comprising Prussian White particles. For example, the salt solution of step b’) may be added when at least 70%, preferably at least 85%, more preferably at least 95%, most preferably at least 99%, of the precursor has reacted with the acid and formed the reaction product in step b).

[0123] This way, the salt primarily reacts with the Prussian White particles to form coated Prussian White particles of a suitable size, instead of reacting (almost) exclusively with the precursor (forming undesired by-products).

[0124] In some embodiments, it may be desirable to provide a coating having more than one layer, e.g. two or three layers. If more than one layer is desirable, the salt may be added to the aqueous precursor solution at an earlier stage, i.e. before a substantial proportion of the precursor has been consumed. For example, the salt may be added when 50 to 70% of the precursor has reacted with the acid and formed the Prussian White particles.

[0125] The ratio between the manganese, titanium, silver, cobalt, and / or aluminum salt and the precursor in the aqueous reaction product may be 1 :1 to 4: 1, preferably from 1 : 1 to 1.5: 1, more preferably about 1 : 1.

[0126] For complete consumption of the precursor, the molar ratio of salt in the salt solution to unreacted precursor is equal to or above 1 : 1. It is understood that in the cases of no precursor left in solution said molar ratio is unapplicable. A ratio of below 1 :1 leads to unreacted precursor which needs to be handled downstream the process during, e.g., a washing and / or filtration step. Introducing too high ratios of salt to precursor may lead to remaining salt or by-products thereof in the reaction vessel which must be handled downstream during, e.g., waste treatment.

[0127] For example, the salt solution in step b’) may be added when the precursor has reacted with the acid for at least 4 hours, such as from about 4 to 25 hours, such as from about 4 to 15 hours, such as from about 5 to 10 hours.

[0128] The reaction time may depend on the temperature, pH, and other reaction conditions.

[0129] In another aspect, there is provided a Prussian White particle obtainable by the method described hereinbefore.

[0130] The Prussian White particle may comprise at least one crystalline domain defined by AaX1[X2(CN)6]i-y-m^O, wherein A is Na or K, 1.95 < a < 2, preferably 1.97 < a < 2, more preferably 1.99 < a < 2, 0 < m < 3, 0 < y < 0.013 and wherein X1and X2are selected from Fe and Mn.

[0131] As mentioned hereinbefore, the inventors have identified a new Prussian White structure, obtainable by the method of the present disclosure. The new Prussian White material is associated with distinct crystallographic features compared to known Prussian White materials. The new Prussian White material is associated with a higher sodium content, an improved capacity and thermal phase stability (see Example 1).

[0132] Herein the term “thermal phase stability” refers to a measure of the ability of a material to maintain a specific crystal structure when subjected to different temperatures, such as elevated temperatures of 100°C.

[0133] The chemical composition of the Prussian White particle may be determined by methods known in the art. For example, an inductively coupled plasma (ICP) spectrometer (e.g., PerkinElmer Avio 200), CHNOS elemental analysis and / or Mdssbauer spectrometry (e.g., an MS 96 spectrometer) may be used to measure a content of element(s) in a sample of the Prussian White particle.

[0134] The Prussian White particle may comprise at least one crystalline domain, such as two or more crystalline domains. At least one crystalline domain has a monoclinic crystal structure.

[0135] At least one of the crystalline domains may exhibit a peak, Pl, at a diffraction angle of 34.1 plus or minus 0.2 degrees, and a peak, P2, at a diffraction angle of 34.4 plus or minus 0.2 degrees, when analyzed by X-ray powder diffraction using Cu Ka radiation (see figures la-c and le).

[0136] Exhibiting a peak when analyzed with X-ray powder diffraction Cu Ka radiation as referred to herein is to be understood that the peak is observable in the XRD pattern / diffractogram. Expressed differently, an X-ray diffraction pattern of the at least one crystalline domain includes peaks at 2 theta values of 34.1 and 34.4 plus or minus 0.2 degrees.

[0137] Pl represents a (400) lattice plane and P2 represents a (022) lattice plane. It is understood that the notation system of lattice planes (400) and (022) are Miller indices. Miller indices, presented as (hkl), as such are known in the art.

[0138] Hence, the diffraction peaks at a 2-theta value of 34.1 and 34.4 plus or minus 0.2 degrees may represent a (400) and (022) lattice plane, respectively.

[0139] In some embodiments, the peak intensity ratio between the peak intensity of Pl and the peak intensity of P2 is from 1.6 to less than 1.9.

[0140] Herein, the term “peak intensity” refers to the integrated area under a peak observed in a diffractogram. The peak intensity of Pl and P2 may be abbreviated Ipi and Ip2, respectively. The peak intensity ratio between IPI and IP2 may be expressed as IPI / IP2. The peak intensity ratio may also be understood as the relative intensity.

[0141] In some embodiments, the at least one of the crystalline domain may exhibit a peak, Pl, at a d-spacing (d) of 2.63 A plus or minus 0.02 A, and a peak, P2, at a d-spacing 2.60 A plus or minus 0.01 A, when analyzed by X-ray powder diffraction using Cu Ka radiation ( I = 1.540596 A, X2 = 1.544390 A).

[0142] It is understood that “A” refers to the unit Angstrom, i.e., 10'10m.

[0143] Herein, d-spacing (d) refers to the interplanar spacing calculated using Bragg’s law nX=2dsin0, where n is the diffraction order, X is the wavelength, and 0 as the glancing angle. The d-spacing represent the distance between adjacent, parallel planes of atoms in a crystal structure. The terms “d-spacing” and “d-value” may be used interchangeably herein. The use of Bragg’s Law and the concept of d-spacing are known in the art.

[0144] One suitable scanning step length when determining the d-spacing using X-ray powder diffraction using Cu Ka radiation (XI = 1.540596 A, X2 = 1.544390 A) is 0.021°.

[0145] In some embodiments, the Prussian white particle may exhibit one or more diffraction peaks selected from P3 at 26.80; P4 at 27.32; P5 at 27.74; P6 at 27.95; P7 at 28.29; P8 at 28.32; P9 at 28.85; P10 at 29.25; and / or Pl 1 at 29.29; plus or minus 0.2 degrees, when analyzed by X-ray powder diffraction using Cu Ka radiation.

[0146] The peak intensity ratio between the peak intensity of P3, P4, P5, P6, P7 and P8, and P9 to the combined peak intensity of P10 and Pl 1 may be 1.3-1.7, 1.4-1.7, 2.2-2.6, 2.5-3.1, 2.2-2.8, and 1.2-2.0, respectively.

[0147] In some embodiments, the Prussian White particle exhibits diffraction peaks P3, P4, P5, P6, P7, P8, P9, P10, and Pl 1 at d-spacing values of 3.324 A, 3.262 A, 3.213 A, 3.190 A, 3.152 A, 3.149 A, 3.092 A, 3.051 A, and 3.047 A; plus or minus 0.02 A, respectively, when analyzed by X-ray powder diffraction using Cu Ka radiation. It is understood that some Prussian White particles can have a (different) monoclinic crystal structure which is associated with relatively low sodium content. A Prussian White particle consisting of such a monoclinic structure with a low sodium content does not exhibit a peak, Pl, at a diffraction angle of 34.1 plus or minus 0.2 degrees, and a diffraction peak, P2, at a diffraction angle of 34.4 plus or minus 0.2 degrees, when analyzed with X-ray powder diffraction using Cu Ka radiation, and wherein the peak intensity ratio between the peak intensity of Pl and the peak intensity of P2 is from 1.5 to less than 1.95. The Prussian White particle may further comprise at least one second crystalline domain, wherein the second crystalline domain has a rhombohedral structure.

[0148] Figure Id illustrates a Prussian White material (particles) having a pure rhombohedral structure.

[0149] Rhombohedral crystalline domains in Prussian White are characterized by a rhombohedral crystal structure with an R3 space group symmetry.

[0150] The presence of both monoclinic and rhombohedral domains may improve the specific capacity and thermal stability of a battery cell incorporating the Prussian White (cathode) material.

[0151] In some embodiments, the at least one crystalline domain may be defined by NaaFe[Fe(CN)6]i-y-mPhO, wherein 1.99 < a < 2, 0 < m < 3, 0 < y < 0.013.

[0152] In some embodiments, the Prussian White particle comprises two or more crystalline domains defined by NaaFe[Fe(CN)6]i-y-mbhO, wherein 1.99 < a < 2, 0 < m < 3, 0 < y < 0.013; and has a monoclinic crystal structure and exhibits a peak, Pl, at a diffraction angle of 34.1 plus or minus 0.2 degrees, and a diffraction peak, P2, at a diffraction angle of 34.4 plus or minus 0.2 degrees, when analyzed with X-ray Powder diffraction using Cu Ka radiation.

[0153] Each of the two or more crystalline domains may be separated by grain boundaries. This may advantageously allow for a Prussian White particle having an improved ionic conductivity as the Na ions may migrate relatively easily via such grain boundaries.

[0154] The Prussian White particle may be defined by an inner core and an outer surface, wherein the Prussian White particle further comprises a coating arranged on at least a portion of the outer surface, wherein the coating comprises manganese, nickel, silver, cobalt, aluminum, and / or titanium.

[0155] The coating may be defined by Naa’X3[X2(CN)e] i-y-m^O, wherein 1.8 < a’ < 2, preferably 1.9 < a’ < 2, 0 < m < 3, 0 < y < 0.025 wherein X2is Fe or Mn, and wherein X3is Mn, Ti, Ni, Ag, Co, and / or Al.

[0156] Preferably, X2is Fe

[0157] Preferably, X3is Mn.

[0158] Preferably, X3is different from X1and / or X2.

[0159] Herein, the term “X1is different from X3” may be denoted as X X3, and understood as that X1and X3are not the same.

[0160] The coating secures that the structural integrity of the inner core is maintained. Accordingly, the Prussian White particle is prevented from degradation, which could otherwise lead to capacity loss in battery applications. Prussian White is generally prone to degradation, and the provision of a coating may suppress degradation mechanisms and secure that the electrochemical performance of the Prussian White particles is maintained during long-term storage. The coating primarily serves to protect the particles from degradation, but may also improve the electrochemical properties of the particles, e.g. by enhancing ion exchange rates, energy density, and contributing to the overall charge / discharge capacity in a battery cell.

[0161] Degradation may herein be understood as a decrease of sodium in the particles and / or the formation of unwanted compounds, such as potentially toxic cyanide-containing compounds. Without wishing to be bound by any particular theory, it is contemplated that through a series of reactions initiated by the Prussian White particle (in this one example a Na2Fe[Fe(CN)e] particle), coming into contact with oxygen and water (moisture), results in the formation of iron hydroxide and Na4Fe(CN)e. It is further contemplated that other reactions may occur during the degradation.

[0162] The coating may be a continuous coating or a discontinuous coating on the outer surface of the Prussian white particle.

[0163] In exemplary embodiments, the coating has a non-uniform thickness.

[0164] In other words, the outer surface of the Prussian White particle may comprise regions in which the coating is thicker, and regions in which the coating is thinner.

[0165] This may optimize the interface between the particle and the conductive additives and / or (polymeric) binders of a cathode and / or the electrolyte in a battery cell. Furthermore, the non-uniform thickness may be beneficial to improve the (sodium) ion transfer kinetics across the coating, thereby improving the ion diffusion in a battery cell.

[0166] The thickness of the coating may be from 0.001 nm to 500 nm.

[0167] At least a portion of the coating may have a thickness in the range of from 50 to 500 nm, e.g. from 80 to 300 nm. The portion(s) of the coating having a thickness in this range corresponds to the thickest parts of the coating.

[0168] At least a portion of the coating may have a thickness in the range of from 0.001 nm to 50 nm, e.g. from 1 nm to 40 nm. The portion(s) of the coating having a thickness in this range corresponds to the thinnest parts of the coating.

[0169] The concentration of manganese, titanium, nickel, silver, cobalt, and / or aluminum may vary across the thickness of the coating.

[0170] Accordingly, the charge and discharge capacity and cycling stability of a battery cell comprising the coated Prussian White particles may be improved. The coating may be arranged to cover at least 60%, preferably at least 70%, more preferably at least 80%, most preferably at least 90% of the outer surface of the Prussian White particle.

[0171] Accordingly, a substantial proportion of the particle’s outer surface is covered by the coating. Hence, the protection against degradation and the overall stability of the Prussian White particles may be improved. Furthermore, the coating may prevent agglomeration of the Prussian White particles when, e.g., arranged in a slurry.

[0172] The coating may be a monolayer coating.

[0173] The monolayer may herein be understood as a single layer with the thickness of one unit cell of Naa’X3[X2(CN)e], wherein X3is Mn, Ti, Ag, Co, and / or Al and wherein X2is Fe or Mn. In contrast, a coating thicker than a monolayer has the thickness of at least two unit cells of Naa’X3[X2(CN)e], wherein X3is Mn, Ti, Ag, Co, and / or Al, and wherein X2is Fe or Mn.

[0174] A monolayer may be beneficial to substantially maintain the particle’s size, morphology, and surface area, which may be important for the reactivity and interaction with electrolytes in battery applications. A monolayer coating advantageously allows for a coated Prussian White particle wherein the dominating intercalation / deintercalation properties stem from the Prussian White particle.

[0175] A monolayer coated particle is relatively cost-effective to manufacture as only a small amount of coating salt is required while still obtaining a particle having an improved moisture stability. The relatively low amounts of coating salt associated with a monolayer coating allows for a material which is relatively easy to recycle.

[0176] In another aspect, there is provided a cathode comprising a plurality of Prussian white particles as described hereinbefore or manufactured by the method described hereinbefore.

[0177] In yet another aspect, there is provided a method for manufacturing a cathode comprising:

[0178] - providing a slurry comprising a solvent, a conductive additive, and a plurality of Prussian white particles as described hereinbefore or manufactured by the method as described hereinbefore; and

[0179] - applying the slurry onto a current collector.

[0180] The Prussian White particles form the active material in the cathode.

[0181] The step of providing a slurry may comprise mixing the Prussian White particles with a conductive additive and a binder. The mixing may be performed by stirring and / or mixing for at least one hour. The mixing may be performed in an inert atmosphere to remove potential bubbles in the slurry prior to applying the slurry to a current collector.

[0182] The conductive additive may be any type of conductive additive known to the skilled person. For example, various types of carbon compounds, e.g. super P, C65, C45, carbon black, e.g. ketjen black may be utilized.

[0183] The binder is not limited to a particular binder. For example, alginate, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVdF) may be used.

[0184] The step of applying the slurry onto a current collector may be performed by any coating technique known to the skilled person. For example, slot-die coating may be utilized.

[0185] The coating weight (mass loading) may e.g. be from 5 mg / cm2to 30 mg / cm2, preferably of from 10 mg / cm2to 20 mg / cm2.

[0186] The current collector may e.g. be a metal foil or a metal sheet, such as an aluminum foil.

[0187] The thickness of the coating on the current collector may vary depending on the specific application and purpose. For example, the thickness of the coating may be in the range of from 50 pm to 500 pm, e.g. from 100 pm to 250 pm.

[0188] After the slurry has been applied onto the current collector, the solvent may be removed by drying.

[0189] The cathode may subsequently be assembled with an anode and a separator to form an electrode stack. The electrode stack may be arranged in a battery casing, such as a cylindrical cell or a pouch cell, and an electrolyte may be added. After electrolyte injection, the battery casing may be sealed.

[0190] According to another aspect, there is provided an electrochemical energy storage device, preferably a battery cell comprising the cathode defined hereinabove or manufactured as described hereinabove.

[0191] The electrochemical energy storage device may be selected from a battery cell, a pseudocapacitor, or a hybrid supercapacitor.

[0192] Preferably, the electrochemical energy storage device is a secondary battery cell. Battery cells, pseudocapacitors, and hybrid supercapacitors as such are known to the skilled artisan. A battery cell comprises at least a positive electrode; i.e. a cathode, a negative electrode; i.e. an anode, and a separator. In the battery cell, chemical energy is transformed into electricity by reduction and oxidation (redox) reactions at the electrodes.

[0193] Figure 2 illustrates the schematic principles of a sodium or potassium ion battery 300 utilizing sodium or potassium ions 201 as charge carriers. The battery stores energy in chemical bonds of the negative electrode, i.e. the anode 202. Charging the battery 200 forces Na+or K+ions 201 to de-intercalate from the positive electrode; i.e. the cathode 203 and migrate towards the anode 202. During discharge, the process reverses. Once a circuit is completed, electrons pass back from the anode 202 to the cathode 203 and the Na+or K+ions 201 travel back to the cathode 203. During discharge of the battery, as illustrated in figure 2, oxidation takes place at the anode 202, while reduction takes place at the cathode 203. The current flow is determined by the potential difference between the cathode 203 and the anode 202, the cell voltage.

[0194] The cathode 203 utilized in a sodium or potassium ion battery of the present disclosure comprises a current collector coated with the Prussian White particulate material as described hereinbefore.

[0195] The negative electrode material, i.e. the anode 202, is not particularly limited as long as it is a material capable of storing / releasing sodium or potassium ions. Examples include metal composite oxides, sodium metal, sodium alloys, silicon, silicon-based alloys, tin-based alloys, bismuth-based alloys, metal oxides, conductive polymers, Na-Co-Ni -based materials, hard carbon and the like. The anode may be a metal foil coated with an active anode material.

[0196] The battery cell may further comprise a separator 205 to prevent electrical short circuit between the negative and the positive electrodes, and to provide mechanical stability to the cell.

[0197] The separator material can be any material which is chemically stable and electrically insulating, e.g. polymer films commonly made from polypropylene, polyethylene or combinations of these. The separator is preferably heat-stable at temperatures ranging up to 250°C.

[0198] In figure 2, the electrolyte is denoted 204.

[0199] Preferably, the electrolyte is a non-aqueous electrolyte. A non-aqueous electrolyte prevents water from interfering with the Prussian White coated cathode. Furthermore, a non-aqueous electrolyte offers a larger voltage window compared to aqueous electrolytes. The present disclosure is not limited to the use of a specific non-aqueous electrolyte. The non-aqueous electrolyte solution typically comprises a solvent or blends of solvents and at least one dissolved salt. For example, the non-aqueous electrolyte may e.g. comprise ethylene carbonate, diethyl carbonate, dimethyl carbonate and / or any mixtures thereof.

[0200] The non-aqueous electrolyte may comprise a salt, such as sodium hexafluorophoshate (NaPFe) or sodium tetrafluoroborate (NABF4). The non-aqueous electrolyte may also comprise one or more additives.

[0201] According to yet another aspect, there is provided a Prussian White particle comprising at least one crystalline domain defined by AaX1[X2(CN)e] i-y-mPhO, wherein A is sodium or potassium, 1.95 < a < 2, preferably 1.97 < a < 2, more preferably 1.99 < a < 2, more preferably 1.99 < a < 2 0 < m < 3, 0 < y < 0.013, and wherein X1and X2are selected from Fe and Mn.

[0202] The Prussian White particle comprises at least one crystalline domain having a monoclinic crystal structure and exhibiting a peak, Pl, at a diffraction angle of 34.1 and a peak, P2, at a diffraction angle of 34.4 plus or minus 0.2 degrees, when analyzed by X-ray powder diffraction using Cu Ka radiation.

[0203] The peak intensity ratio between the peak intensity of Pl and the peak intensity of P2 may be from 1.6 to 1.9.

[0204] The at least one crystalline domain of the Prussian White particle exhibits a peak, Pl, at a d-spacing (d) of 2.63 A plus or minus 0.02 A, and a peak, P2, at a d-spacing 2.60 A plus or minus 0.01 A, when analyzed by X-ray powder diffraction using Cu Ka radiation ( I = 1.540596 A, X2 = 1.544390 A).

[0205] The at least one crystalline domain may have a P2i / n space group symmetry.

[0206] EXAMPLES

[0207] Example 1: Evaluation of different reaction conditions

[0208] Synthesis of Examples 1-3

[0209] An aqueous solution comprising a Na4Fe(CN)e was heated to a temperature of 90°C. Sulfuric acid was introduced to the aqueous solution to obtain a pH of 2.9. The precursor was then reacted with the sulfuric acid for a period of 6 hours to form an aqueous reaction product comprising Prussian White particles. Additional sulfuric acid was introduced during the period of 6 hours to maintain a pH of about 2.9. The temperature of 90°C was maintained through the period of six hours. The aqueous reaction product was filtered and dried to obtain a powder of Prussian White particles comprising Na2Fe[Fe(CN)e].

[0210] Three repetitions of the synthesis described above were conducted thereby obtaining three sets of Prussian White particles (Examples 1-3). The specific process conditions and yields for Examples 1-3 are presented in table 1 below.

[0211] Herein the term “yield”, refers to the obtained amount of Prussian White particles versus the theoretical amount of obtainable Prussian White particles based on the amounts of precursor and sulfuric acid provided.

[0212] Synthesis of Example 12

[0213] Prussian white particles of Example 12 were manufactured in a similar manner as Examples 1-3 with the difference being employing a pH of 3.5 and a temperature of 95°C.

[0214] Synthesis of Comparative examples 1-2 and 4

[0215] Comparative example 1 was prepared in a similar manner to Examples 1-3, the difference being utilizing a temperature of 70 °C instead of 90 °C. Comparative example 2 was prepared in a similar manner to Examples 1-3, the difference being utilizing a pH of about 5 instead of a pH of about 2.9. Comparative Example 4 was prepared in a similar manner to Examples 1-3, the difference being utilizing a temperature of 70°C and a pH of 5.0.

[0216] The process conditions and yields are presented in table 1 below.

[0217] Table 1 : Summary of reaction conditions and yield of examples 1-3 and 12 and comparative examples 1, 2 and 4. It can be concluded that the processes of Examples 1-3 and 12 provide a significantly higher yield than the processes of Comparative examples 1, 2 and 4.

[0218] Characterization

[0219] X-ray Powder Diffraction (XRD) was conducted to determine the crystal structure of the Prussian White particles obtained in Examples 1-3 and 12. The XRD was conducted using a Bruker D8 Advance diffractometer equipped with a Cu Ka (XI = 1.540596 A, X2 = 1.544390 A) source and a LYNXEYE XE-T detector over the range of 10-120 (20) with a scanning step of 0.021°.

[0220] Each of Examples 1-3 exhibited peaks at diffraction angles (23) at approximately 34.1 and 34.4, when analyzed with X-ray powder diffraction using Cu Ka radiation as described above. Through XRD and Rietveld analysis, it can be concluded that the Prussian White particles of each of Examples 1-3 and 12 comprise at least one crystalline domain having a monoclinic crystal structure with a P2i / n space group symmetry. It can further be concluded that some Prussian White particles comprise at least one second crystalline domain with a rhombohedral crystal structure with an R3 space group symmetry. The XRD patterns for Examples 1-3 and 12 are shown in figures la-c and le, respectively. As can be seen in the zoomed-in insets (top right comer) of the figures la-c and le, the diffractogram of the particles exhibit peaks at diffraction angles (23) at approximately 34.1° and 34.4°. The peak intensity ratio between the intensity of the peak at 34.1° to the intensity of the peak at 34.4° was calculated to be within the range of from 1.6 to less than 1.9.

[0221] Each of the particles of the Example 1-3 and 12 exhibited peaks at a d-spacing of 2.63 and 2.60 A.

[0222] Using the XRD process described above, a third and a fourth set of comparative Prussian White particles (Comparative example 3 and Comparative example 4) were evaluated. Through XRD and Rietveld analysis it was concluded that the particles of Comparative example 3 and Comparative example 4 did not comprise crystalline domains having the monoclinic crystal structure described above. The particles of Comparative example 3 and Comparative example 4 consisted substantially of crystalline domains having the rhombohedral crystal structure described above. The XRD patterns of Comparative example 3 and Comparative example 4 are shown in figure Id and If. As can be seen from the zoomed-in inset (top right corner) of figure Id and If, Comparative examples 3 and 4 do not exhibit peaks at diffraction angles (23) at approximately 34.1° and 34.4°. Neither of the particles of Comparative examples 3 and 4 exhibited peaks at d-spacings of 2.63 or 2.60 A.

[0223] It can be concluded that the processes of Examples 1-3 and 12 provide Prussian White particles having a higher sodium-content than the Prussian White particles of Comparative examples 3 and 4.

[0224] Electrochemical measurements

[0225] Twelve coin cells, three for each of Examples 1-3 and Comparative example 3, were assembled using the procedure described below.

[0226] An aqueous slurry comprising the Prussian White particles was prepared. The slurry was disposed on an aluminum foil to provide cathode coated foil. The cathode coated foil was dried at an elevated temperature to form a cathode.

[0227] A cathode, a separator, and a metallic sodium disc were stacked and arranged in a casing. After assembly a standard electrolyte was introduced into the casing and the cell was sealed, thereby obtaining a coin cell.

[0228] The twelve coin cells were subsequently electrochemically evaluated through cycling. Galvanostatic cycling using a Neware BTS4000 galvanostat was utilized to charge / discharge the half cells under controlled conditions. A constant current equal to C / 10 was used with voltage cut-off limits set to 2.0 and 3.8. The average specific charge capacity (SCC), average specific discharge capacity (SDC), and average efficiency for each set of three cells are presented in table 2.

[0229] Table 2: Summary of average SCC, average SDC, and average Coulombic efficiency of the coin cells comprising Prussian White particles of Examples 1-3, and comparative example 3 respectively. Coulombic efficiency (abbreviated Eff.) is calculated as SCC divided by SDC.

[0230] SCC and SDC are presented in mAh / g. Examples 1-3 exhibit higher SCC and SDC as compared to Comparative example 3. Thus, it can be concluded that Examples 1-3 when provided as cathode active material results in superior cells in terms of specific capacity.

[0231] Example 2: Evaluation of coated Prussian White particles

[0232] Synthesis of Prussian white particles

[0233] Prussian white particles were manufactured according to the following. An aqueous solution comprising H2SO4 (sulfuric acid) and Na4Fe(CN)e (precursor), having a pH of 2.9 was heated to a temperature of 90°C. The precursor was then reacted with the acid for a period of 5 hours to form a suspension of aqueous reaction product comprising Prussian white particles. Additional sulfuric acid was introduced during the period of five (5) hours to maintain a pH of about 2.9. The temperature of 90°C was maintained throughout the period of five hours. The resulting aqueous reaction product was filtered and dried thereby obtaining the Prussian white particles comprising Na2Fe[Fe(CN)e].

[0234] Synthesis of coated Prussian white particles of Examples 4-11

[0235] The synthesis of Prussian white particles described above was conducted up until and including obtaining the suspension. Subsequently, a predetermined amount of salt solution or salt solution and Na4Fe(CN)e (precursor) was added to the suspension to form a coated aqueous reaction product. The resulting coated aqueous reaction product was filtered and dried thereby obtaining the coated Prussian white particles.

[0236] By varying the salt solution or the amount of salt and precursor added, eight sets of coated Prussian White particles (examples 4-11) were manufactured. The salts used are presented in table 1. In Examples 4-7 and 10, the suspensions were prepared to comprise salt and precursor in a ratio of 1 : 1. In Examples 8, 9, and 11, the suspension was prepared to comprise salt and precursor in a ratio of about 4: 1.

[0237] Each precursor amount is presented in table 3 as a fraction (converted to percentage) of the starting precursor amount in the Prussian White particles synthesis. In other words, a precursor amount of 5% refers to 5% (in moles) of the total starting precursor amount in the synthesis of forming Prussian particles. In table 3, the coating amount is also presented as the corresponding fraction (converted to percentage) of precursor reacted to form the Prussian White particles.

[0238] Herein, “Na2Mn[Fe(CN)e]“ may also be referred to as “Mn-Fe PBA”. precursor reacted to form Prussian White particles, and formed coatings of Examples 4-11 and Reference example 1.

[0239] Coating degradation resistance assessment

[0240] The moisture resistance of Examples 4-8 and Reference example 1 was evaluated through a degradation / aging study. In the degradation study, ~2 g of sample was weighed and placed into a sealed container containing a saturated solution of NaCl, producing a 75% RH atmosphere together with oxygen. After storage in an environment of 75% RH for zero, one and / or two weeks respectively, sample was collected for Scanning Electron Microscopy (SEM) and X-ray Powder Diffraction (XRD) analysis to assess degradation by examining the occurrence of surface reactions and alterations of crystalline phases.

[0241] Scanning electron microscopy (SEM)

[0242] SEM images of Reference example 1 and the Example 4-8 at week 0 (as- synthesized) are shown in figure 3a-f. In figure 3 a, the uncoated particles of Reference example 1 can be observed. The uncoated particles observed are relatively sharp, cubic primary particles intergrown into larger secondary aggregates. In figures 3b-e, corresponding to Examples 4-7 respectively, a non-uniform coating comprising dispersed islands of Na2Mn[Fe(CN)e] formed on the Prussian White particles can be observed. Figure 3f shows the coated Prussian White particles of Example 8, where no dispersed islands of Na2Mn[Fe(CN)e] can be observed, indicating a relatively thin coating.

[0243] SEM images of Reference example 1 and the Examples 4-8 at week 2 are shown in figure 4a-f. The Prussian White particles shown in figure 4a show a severe degradation through occurrences of surface reactions, pitting and smearing of and edges and intersects. In comparison, the coated Prussian White particles of Examples 4-8 shown in figures 4b-f show significantly lower degree of degradation. In Examples 4-8, degradation can only be observed in a portion of the surface fractures, cracks and cube intersects of the particles. In contrast to Reference example 1, the flat surfaces of the cubes of Example 4-8 appear relatively unaffected. It can thus be concluded that Examples 4-8 have a superior moisture stability as compared to Reference example 1.

[0244] X-ray Powder Diffraction (XRD)

[0245] XRD and Rietveld refinements were conducted to determine the crystal structure of the coated respectively uncoated Prussian white particles obtained in Examples 4-8 and Reference example 1. The XRD was conducted using a Bruker D8 Advance diffractometer equipped with a Cu Ka (XI = 1.540596 A, X2 = 1.544390 A) source and a LYNXEYE XE-T detector over the range of 10-120 (20) with a scanning step of 0.021°.

[0246] Tables 2-5 show the weight fractions of Mn-Fe PBA, and hydrated rhombohedral (RH), monoclinic (MC), and cubic (CU) phases of the Na2Fe[Fe(CN)e] of Examples 4-8 and Reference example 1 as-synthesized (week 0) and after one or two weeks of aging respectively. Additionally, tables 4-7 show the relative phase changes of each material over time. Moisture stability can thus be evaluated based on the degree of change in crystal structure over time. The higher the change in crystal structure the higher the degree of degradation.

[0247] Out of the MC, RH, and CU phases, MC comprises the most sodium followed by RH and subsequently CU. A high amount of sodium is preferred when the Prussian White particles are to be used as cathode active material. Herein, the monoclinic phase is to be understood as a phase exhibiting peaks at diffraction angles (23) at approximately 34.1 and 34.4. The monoclinic phase shall exhibit a peak intensity ratio between the intensity of the peak at 34.1 to the intensity of the peak at 34.4 is from 1.6 to less than 1.9. It is understood that further peaks belonging to the monoclinic phase are present in the diffraction pattern. It is noted that no Mn-Fe PBA can be observed through XRD of Example 8. It is believed that this is due to the thin coating obtained and the amount of Mn-Fe PBA being below the detection limit of the instrument used. The degradation of Example 8 was only evaluated at week 0 and week 1.

[0248] 4-8 and Reference example 1 and exposure times (* No data exists)

[0249] Table 5: Monoclinic phase weight fractions obtained from Rietveld refinement for Examples 4-8 and Reference example 1 and exposure times

[0250] Examples 4-8 and Reference example 1 and exposure times

[0251] Table 7: Cubic phase weight fractions obtained from Rietveld refinement for Examples 4-8 and Reference example 1 and exposure times.

[0252] Reference example 1 exhibited a complete conversion of the high-sodium content monoclinic phase to rhombohedral and cubic already after one week. After two weeks exposure time to 75 % RH, only 9 wt. % of the sample exists in the form of the rhombohedral phase, while the rest is made up of cubic phase. No cubic phase could be observed for any of the Mn-containing Examples 4-8. Examples 4-8 degraded to some extent, which can be seen from a decrease in the monoclinic phase weight fraction in favour of an increase in the rhombohedral phase weight fraction. It can be concluded that moisture stability and thereby also the shelf-life is improved by coating the Prussian White particles.

[0253] Safety assessment

[0254] Unreacted Na4Fe(CN)e (precursor) in an acidic aqueous solution, such as the filtrate, may over time lead to the formation of undesirable cyanide-containing by-products. Therefore, the presence of unreacted precursor in, e.g., the filtrate after coating of the Prussian White particles is preferably kept to a minimum.

[0255] Staining tests were performed on the suspensions of Examples 6, 7 and 10 both prior to and during the addition of the salt solution, and after the formation of the coated Prussian White particles.

[0256] The staining tests were conducted by combining 1 ml of extracted suspension with FeSCU Prior to the addition of the salt solution, blue precipitates, indicating the presence of precursor, were observed in all three extracted suspensions when combined with FeSO4. During the subsequent addition of salt solution to the three suspensions, additional portions of the suspensions were extracted and subsequently combined with FeSCU The amounts of blue precipitates in the extracted suspensions decreased with the increasing amount of salt solution added to the suspension. Upon complete addition of the salt solution, the extracted suspensions were transparent in all cases, indicating the absence of precursor.

[0257] Thus, it can be concluded that the addition of a salt solution leads to the consumption of Na4Fe(CN)e and the formation of coated Prussian white particles. Furthermore, it can be concluded that forming the coating advantageously allows for a filtrate having a decreased risk of developing undesirable by-products and additionally to a reaction having an increased yield.

[0258] Particle size distribution

[0259] Particle size distribution (PSD) of Examples 4-8 and a reference example of uncoated Prussian white was measured using a PSA 1090 L / D (Anton Paar). By comparing the PSD of Examples 4-8 to Reference example 1 it was investigated whether coatings (nucleation and growth of, e.g., Na2Mn[Fe(CN)e] on the Prussian White particles) or byproducts (through, e.g., homogenous nucleation in solution) had formed. No drastic changes could be observed between the PSD of Examples 4-8 and Reference example. Consequently, it can be concluded that coating on the Prussian White particles has been formed. Thermogravimetric analysis

[0260] Thermogravimetric analysis (TGA) was carried out using a TA Instruments TGA 5500 on Example 3 and Comparative Example 3. Roughly 5 mg of sample was weighed into an aluminum pan and then heated from room temperature to 350°C at a constant heating rate of 5°C / min under N2 flow (25 mL / min).

[0261] As can be observed in figure 5, a major weight loss below 250°C (-10-11 weight %) can be observed in both Example 3 and Comparative Example 3. It is contemplated that said weight loss is mainly because the release of crystal water from the material. The additional weight loss above 250°C is due to decomposition of the respective materials (Prussian White particles), more specifically of the ferrocyanide network structure, resulting in release of CN-containing species. As seen from Figure 5, Example 3 exhibits a smaller weight loss overall at temperatures up to 350°C in comparison to Comparative Example 3, and specifically at temperatures above 250°C. It can be concluded that Example 3 has a superior thermal stability and thus a lesser formation of CN-containing species in the presented temperature range as compared to the Comparative Example 3.

[0262] Terms, definitions and embodiments of all aspects of the present disclosure apply mutatis mutandis to the other aspects of the present disclosure.

[0263] Even though the present disclosure has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art.

[0264] Variations to the disclosed embodiments can be understood and effected by the skilled addressee in practicing the present disclosure, from a study of the drawings, the disclosure, and the appended claims. Furthermore, in the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.

Claims

CLAIMS1. A method for manufacturing Prussian White particles comprising: a) providing an aqueous solution of a precursor defined by A4X1(CN)e, wherein A is Na or K, and wherein X1is Fe or Mn; b) reacting said precursor with an acid under conditions that allow for an aqueous reaction product comprising Prussian White particles to be formed; and c) drying said aqueous reaction product, characterized in that said step b) is performed at a temperature of from 75 to 95°C, preferably from 80 to 95°C, more preferably from 85 to 95°C and at a pH of from 1.8 to 4.5.

2. The method according to claim 1, wherein said step b) is performed at a pH of from 2 to 3.5, preferably from 2 to 3.

3. The method according to claim 1 or claim 2, wherein said Prussian White particles are defined by AaX1[X2(CN)6]i-y-m^O, wherein A is Na or K, 1.8 < a < 2, preferably 1.9 < a < 2, 0 < m < 3, 0 <y < 0.025, and wherein X1and X2are selected from Fe and Mn.

4. The method according to any one of claims 1-3, wherein at least one crystalline domain of each of said Prussian White particles is defined by AaX1[X2(CN)6]i-y-m^O, wherein A is Na or K, 1.95 < a < 2, preferably 1.97 < a < 2, more preferably 1.99 < a < 2, 0 < m < 3, 0 < y < 0.013 and wherein X1and X2are selected from Fe and Mn.

5. The method according to any one of claims 1-4, further comprising a step b’) of coating said Prussian White particles prior to said step c) of drying by adding a salt solution comprising a manganese, nickel, aluminum, silver, cobalt, and / or titanium salt to said aqueous reaction product.

6. The method according to claim 5, wherein said salt solution of said step b’) is added when at least 70%, preferably at least 85%, more preferably at least 95% of said precursor has reacted with said acid and formed said reaction product in said step b).

7. A Prussian White particle obtainable by the method according to any one of claims 1-6.

8. The Prussian White particle according to claim 7, wherein said Prussian White particle comprises at least one crystalline domain defined by AaX1[X2(CN)e] i-y-mPhO, wherein A is sodium or potassium, 1.95 < a < 2, preferably 1.97 < a < 2, more preferably 1.99 < a < 2, more preferably 1.99 < a < 2 0 < m < 3, 0 < y < 0.013, and wherein X1and X2are selected from Fe and Mn.

9. The Prussian White particle according to claim 7 or claim 8, wherein said Prussian White particle comprises at least one crystalline domain having a monoclinic crystal structure and exhibiting a peak, Pl, at a diffraction angle of 34.1 and a peak, P2, at a diffraction angle of 34.4 plus or minus 0.2 degrees, when analyzed by X-ray powder diffraction using Cu Ka radiation.

10. The Prussian White particle according to claim 9, wherein the peak intensity ratio between the peak intensity of Pl and the peak intensity of P2 is from 1.6 to 1.9.

11. The Prussian White particle according to any one of claims 8-10, wherein said at least one crystalline domain exhibits a peak, Pl, at a d-spacing (d) of 2.63 A plus or minus 0.02 A, and a peak, P2, at a d-spacing 2.60 A plus or minus 0.01 A, when analyzed by X-ray powder diffraction using Cu Ka radiation.

12. The Prussian White particle according to any one of claims 7-11, wherein said Prussian White particle is defined by an inner core and an outer surface, wherein said Prussian White particle further comprises a coating arranged on at least a portion of said outer surface, wherein said coating comprises manganese, nickel, aluminum, silver, cobalt, and / or titanium.

13. The Prussian White particle according to claim 12, wherein said coating is defined by Naa’X3[X2(CN)e] i-y’ -m’FhO, wherein A is Na or K, 1.8 < a’ < 2, preferably 1.9 < a’ < 2, 0 < m’ < 3, 0 < y’ < 0.025, wherein X2is selected from Fe and Mn, and wherein X3is Mn, Ti, Ni, Ag, Co, and / or Al.

14. A cathode comprising a plurality of Prussian White particles according to any one of claims 7-13, or manufactured by the method according to any one of claims 1-6,15. A method for manufacturing a cathode comprising:- providing a slurry comprising solvent, a conductive additive, and a plurality of Prussian White particles according to any one of claims 7-13, or manufactured by the method according to any one of claims 1-6,- applying said slurry onto a current collector.

16. An electrochemical energy storage device, preferably a battery cell, comprising the cathode according to claim 14 or manufactured by the method according to claim 15.

17. A Prussian White particle comprising at least one crystalline domain defined by AaX1[X2(CN)e] i-y-mFhO, wherein A is sodium or potassium, 1.95 < a < 2, preferably 1.97 < a < 2, more preferably 1.99 < a < 2, more preferably 1.99 < a < 2 0 < m < 3, 0 < y < 0.013, and wherein X1and X2are selected from Fe and Mn; said at least one crystalline domain having a monoclinic crystal structure and exhibiting a peak, Pl, at a diffraction angle of 34.1 and a peak, P2, at a diffraction angle of 34.4 plus or minus 0.2 degrees, when analyzed by X- ray powder diffraction using Cu Ka radiation.

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