Synthesis process for Prussian Blue analogues useful as cathode active material.

Abstract

The invention relates to a process for synthesizing particles of a Prussian Blue analog of formula AxM1yM2z(CN)6 comprising at least the steps of: Having, on the one hand, an aqueous solution A comprising at least one water-soluble salt of a transition metal M1 and at least one water-soluble salt of a transition metal M2, and on the other hand, an aqueous solution B containing at least potassium or sodium cyanide; Injecting said solutions A and B simultaneously and separately into a reactor containing at least one aqueous medium; and Maintaining the mixture of solutions A and B thus formed in said reactor, under stirring and under conditions conducive to the formation of the particles of said Prussian Blue analog by co-precipitation, said steps b and c being carried out under an inert atmosphere and at a controlled pH value, varying from 8 to 11. Figure for the abstract: None

Description

Title of the invention: Process for the synthesis of Prussian Blue analogues useful as cathodic active material. Technical field

[0001] The present invention relates to the field of electrochemical devices of the metal-ion accumulator or battery type, in particular sodium-ion or potassium-ion batteries.

[0002] It aims more specifically to propose a new process for synthesizing Prussian Blue analogue particles which are particularly useful as active cathode material in sodium-ion or potassium-ion batteries. Prior art

[0003] Electrochemical devices of the metal-ion battery type currently dominate the market for rechargeable electrochemical devices. They find multiple applications, in particular in the power supply of thin embedded systems, such as credit cards, smart labels, in the power supply of mobile phones, the storage of energy from photovoltaic cells or in the power supply of electric vehicles. Various electrochemical storage systems or electrochemical generators have therefore been developed, in particular sodium-ion, lithium-ion, potassium-ion, magnesium-ion batteries.

[0004] Currently, lithium-ion batteries are dominating the rechargeable battery market, particularly due to the exceptional electrochemical properties of lithium. However, this technology has drawbacks, particularly due to the relative scarcity of lithium resources, which is now considered a critical metal.

[0005] Sodium or potassium ion batteries represent interesting alternatives and are also viable means to support renewable energy sources for the purpose of leveling the charge and storing excess energy. The performance of these sodium or potassium ion batteries is highly dependent on the properties of the electrode active materials. However, Prussian Blue analogue cathode active materials (PBA) stand out precisely as promising materials for use in sodium or potassium ion batteries. Sodium and potassium Prussian Blue analogues are mostly synthesized by precipitation in water of a hexacyanometalate complex A4Mi(CN)6 with a salt of the other transition metal M2. Application JP2018106911 A or patents CN110002466B B and CN110002466B describe such a synthesis from hexacyanometalate and the salt of the other transition metal M2.However, the . The synthesis routes described in these documents do not allow effective control of particle morphology: micrometric aggregates of nanometric particles are obtained, of non-homogeneous sizes and shapes, leading to a material with a low volumetric energy density and low cyclability. The addition of a chelating agent (potassium citrate, EDTA, oxalate, etc.) to the solution of the transition metal salt M2 allows better control of particle morphology. The chelating agent forms a complex with the transition metal M2 of the transition metal salt, making it less available for the precipitation reaction and therefore slowing down the kinetics of the latter. Another synthesis process consists of first forming manganese hydroxide nanospheres using a polymer (polyacrylic acid) and then, in a second step, reacting these spheres with potassium hexacyanoferrate (K4Fe(CN)6).However, by analogy with the aforementioned methods, this method also systematically requires the use of the precursor of K4Fe(CN)6 which is complex to synthesize. In application JP2012046399A, particles of Ki 9Mni i[Mn(CN)6] and K2MnFe(CN)6 are synthesized by adding dropwise an aqueous solution containing a mixture of the two transition metal salts directly into a solution of potassium cyanide KCN. However, the particulate material thus obtained consists of aggregates of nanoparticles of various sizes and is therefore not entirely satisfactory as an electrode material. Statement of the invention

[0006] The present invention aims precisely to propose a new method for synthesizing particles of a Prussian Blue analogue, which is simple to implement, economical and reproducible.

[0007] In particular, the present invention aims to avoid the use of precursors such as K4Fe(CN)6 which are complex to synthesize.

[0008] The present invention also aims to propose a method making it possible to access PB A particles of controlled shape in size and homogeneity.

[0009] The present invention also aims to propose a method suitable for such control by controlling the kinetics of the reaction between the cyanide and the metal salts and in particular by regulating the germination and growth phases of the particles which result therefrom.

[0010] The inventors have now discovered that it is possible to meet these expectations provided that a particular synthesis technique is used. Summary of the invention

[0011] Thus, the present invention relates, according to its main aspect, to a process for synthesizing particles of a Prussian Blue analogue, also called PB A, of formula (I):

[0012] AxMlyM2z(CN)6(I)

[0013] in which: - A represents an atom of sodium, Na, or potassium, K, - Ml and M2, identical or different, are chosen from the transition metals Ti, Nb, V, Cr, Mn, Fe, Co, Ni, Cu, Zn and preferably from Fe and Mn, - x is non-zero and varies from 0 to 2.2, and is preferably close to 2

[0014] - y varies from 0 to 2 and is preferably non-zero,

[0015] - z varies from 0 to 2 and is preferably non-zero,

[0016] - y+z = 2

[0017] and its hydrates,

[0018] comprising at least the steps of: a. Having on the one hand, an aqueous solution A comprising at least one water-soluble salt of a transition metal M1 and at least one water-soluble salt of a transition metal M2 and on the other hand, an aqueous solution B containing at least potassium or sodium cyanide; b. Inject simultaneously, separately from each other and at a controlled flow rate, said solutions A and B into a so-called precipitation reactor, containing at least one aqueous medium and in particular water and c. Maintain in said reactor, the mixture of solutions A and B thus formed, with stirring, in particular under an inert atmosphere, and under conditions conducive to the formation of particles of said Prussian Blue analogue of formula (I) by co-precipitation,

[0019] said steps b and c being carried out under an inert atmosphere, at a controlled pH value varying from 8 to 11, preferably of the order of 9.9.

[0020] According to a particular embodiment, Ml and M2 in the general formula (I) are chosen from Fe and Mn, and x is close to or even equal to 2, y varies from 0 to 2 and preferably is 1, and z varies from 0 to 2 and preferably is 1 with y+z being equal to 2.

[0021] As is clear from the above, one of the characteristics of the process of the invention is the fact that the salts, respectively present in solutions A and B, are introduced in a dissociated manner into the reactor and this is advantageous in several ways.

[0022] The fact of injecting the KCN solution B in parallel with the metallic solution A, proves in fact to be particularly decisive for controlling the reaction kinetics between the metals of the two solutions A and B and the cyanide.

[0023] It thus proves possible to regulate the germination and growth phases of the particles by intervening on the one hand, on the injection flow rates of the two solutions A and B, and on the other hand, on the concentrations of the reactive solutions within the reactor.

[0024] In addition, a control of their respective concentrations in the reactor, in particular via their injection flow rate into this reactor, advantageously allows the expected germination and growth phases to be regulated. For example, in the case of a reactor with a volume varying from 500 mL to 2 L, it is advantageous for solutions A and B to be injected in parallel into said reactor with a flow rate varying from 1 mL / min to 5 mL / min and preferably from 2 mL / min to 4 mL / min.

[0025] Furthermore, the method according to the invention is compatible with the adjustment of an imposed and constant pH, either by adjusting the flow rate of the anion source, in this case CN, or by considering a controlled addition of a base. This results in control of the precipitation kinetics. It is important to note that such control cannot be achieved by dripping one of the solutions A or B into the other. In such an embodiment, the pH is necessarily "subjected".

[0026] Thus, according to an alternative embodiment, the pH is adjusted in the reactor by controlled injection of a KOH or NaOH solution.

[0027] According to another embodiment variant, step c) is carried out in the presence of at least one chelating agent.

[0028] The method according to the invention, which is therefore based on a co-precipitation operation, makes it possible to obtain particles of controlled and homogeneous size.

[0029] Advantageously, the synthesis method of the present invention makes it possible to directly obtain particles having a spherical shape.

[0030] In particular, the particles advantageously have a D50 of 0.5 pm to 25 pm, preferably of 0.5 pm to 3 pm. This size can in particular be characterized by laser granulometry.

[0031] These adjustments in morphology and size are precisely and advantageously controllable by the process according to the invention and its synthesis parameters such as the concentrations of reagents, the pH or even the concentration of chelating agent if present.

[0032] According to another of its aspects, the present invention also relates to particles of a Prussian Blue analogue of formula (I):

[0033] AxMlyM2z(CN)6(I)

[0034] in which: -A represents an atom of sodium, Na, or potassium, K, -Ml and M2, identical or different, are chosen from Fe and Mn, -x is non-zero and varies from 0 to 2.2, preferably is of the order of 2

[0035] -y varies from 0 to 2, and preferably is non-zero,

[0036] - z varies from 0 to 2 and preferably is non-zero and

[0037] - y+z = 2

[0038] said particles having a D50 of 0.5 pm to 25 pm, preferably of 0.5 pm to 3 pm, characterized by laser granulometry.

[0039] Another aspect of the invention relates to the particles of a Prussian Blue analogue of formula (I) in accordance with the invention directly obtained by a process according to the invention.

[0040] As is apparent from the examples below, these particles are particularly useful as active material for cathodes.

[0041] Thus, another aspect of the invention aims at the use of these particles in a sodium-ion or potassium-ion battery.

[0042] Other characteristics, variants and advantages of the composite materials according to the invention, their preparation and their implementation, will become more apparent on reading the description, examples and figures which follow, given for illustrative and non-limiting purposes of the invention.

[0043] In the rest of the text, the expressions “between ... and ...”, “ranging from ... to ...” and “varying from ... to ...” are equivalent and are intended to mean that the limits are included, unless otherwise stated. Brief description of the drawings

[0044] [Fig.l] shows the diffractogram, obtained by X-ray diffraction, of the microparticles of PB A type material obtained in example 1.

[0045] [Fig.2] shows the galvanostatic profile obtained in the first cycle of a button cell in a glove box, using as cathode the electrodes thus formed in example 2, and whose cycling regime is C / 20.

[0046] [Fig.3] shows the cycling curve (discharge capacity as a function of the number of cycles) of a button cell in a glove box, using as cathode the electrodes thus formed in example 2, and whose cycling regime is C / 20. Detailed description

[0047] As mentioned previously, the method of the invention aims to form an active material PB A of general formula (I)

[0048] AxMlyM2z(CN)6(I)

[0049] in which: - A represents an atom of sodium, Na, or potassium, K, - Ml and M2, identical or different, are chosen from the transition metals Ti, Nb, V, Cr, Mn, Fe, Co, Ni, Cu, Zn and preferably from Fe and Mn, - x is non-zero and varies from 0 to 2.2, preferably close to 2

[0050] - y varies from 0 to 2 and is preferably non-zero,

[0051] - z varies from 0 to 2 and preferably is non-zero and

[0052] - y+z = 2

[0053] For the purposes of the invention, the terms atom of Na, K, and the symbols M1 and M2 are intended to cover the charged forms of the elements considered. For example, Na covers Na+ and Ml covers Mn++.

[0054] In particular, Ml and M2 of general formula (I) are chosen from Fe and Mn, and x is close to 2 and preferably equal to 2; y varies from 0 to 2 and preferably is 1, and z varies from 0 to 2 and preferably is 1 and y+z=2.

[0055] According to a preferred embodiment variant, Ml and M2 are different and in particular Ml is Mn and M2 is Fe. In this variant the process is advantageous for forming K2Mn[Fe(CN)6] or Na2Mn[Fe(CN)6] and preferably K2Mn[Fe(CN)6].

[0056] As is evident from the examples below, the K2Mn[Fe(CN)6] formed according to the process of the invention is characterized by a monoclinic structure of space group P21 / n according to the Hermann-Mauguin notation (International Tables for Crystallography (2016). Volume A, Space-group symmetry)

[0057] In particular, the particles have a D50 of 0.5 pm to 25 pm, preferably of 0.5 pm to 3 pm. This size can in particular be characterized by laser granulometry.

[0058] According to the invention, the compounds of general formula (I) are formed in a reactor, called a precipitation reactor, into which are injected, simultaneously and separately, solution A containing at least one water-soluble salt of M1 and at least one water-soluble salt of M2 and solution B containing at least one cyanide salt chosen from potassium cyanide and sodium cyanide.

[0059] In other words, the water-soluble salts of M1 and M2 are not introduced into a reactor already containing at least one cyanide salt and are therefore not added to said cyanide salt. Solution A

[0060] In particular, solution A contains, as Ml, a manganese salt chosen from the salts Mn(NO3)2, Mn(SO4)2, MnCl2, Mn(CH3CO2)2 and their hydrates and preferably the salt MnSO4.H2O.

[0061] In particular, solution A contains, as M2, an iron salt chosen from the salts Fe(NO3)2, Fe(SO4)2, FeCl2 and their hydrates and preferably FeSO4.7H2O.

[0062] Solution A can advantageously have a molar concentration of salts M1 and M2 varying from 0.1 to 2.5 M. Of course, it is possible to consider higher concentrations provided that the reaction temperature is adjusted to a temperature conducive to the interaction of these salts with the anions of solution B.

[0063] Solution A is used in a deoxygenated form to avoid any oxidation of the salts it contains. Solution B

[0064] As regards solution B containing said cyanide salt, it is also used in a deoxygenated form.

[0065] Solution B may advantageously have a molar concentration of salts of cyanide varying from 0.1 to 7.5 M. However, as mentioned for solution A, a higher concentration is possible provided that the reaction temperature is adjusted to a temperature conducive to the interaction of these anions with the salts of solution A.

[0066] The cyanide salt of solution B and the transition metal salts M1 and M2 of solution A can be brought together in a molar ratio of salts M1+M2 / CN of K or Na varying from 2.5 to 3.5, preferably 3.

[0067] Solutions A and B can be introduced with a respective flow rate varying from 1 mL / min to 5 mL / min and preferably from 2 mL / min to 4 mL / min. For obvious reasons, these flow rates can also be adjusted outside the proposed range depending on the capacity of the reactor. These adjustments are within the competence of a person skilled in the art.

[0068] In particular, solutions A and B are introduced into the reactor at flow rates of equivalent values.

[0069] The reaction is carried out in a precipitation reactor, already containing an aqueous medium, preferably water, under an inert atmosphere, in particular under argon.

[0070] Once all of the solutions A and B have been introduced into the reactor, the mixture thus formed is kept stirring until all of the expected compound of general formula (I) has precipitated.

[0071] This mixture can in particular be kept stirring in step c) from 1 hour to 24 hours, preferably from 2 hours to 6 hours.

[0072] The temperature in the reactor during step c) can be maintained at 20°C to 70°C, preferably at 25°C to 35°C.

[0073] The particles of the Prussian Blue analogue of formula (I) obtained at the end of step c) are recovered, in particular by centrifugation and generally washed and dried. These operations clearly fall within the skills of a person skilled in the art and will therefore not be detailed in the present description.

[0074] As specified above, it is advantageous, in order to control the particle size of the Prussian Blue analogue of formula (I), for the synthesis process to be carried out at a controlled pH value, in particular varying from 8 to 11, preferably 9.9.

[0075] As specified above, this pH control of the reactor can in particular be adjusted directly by controlling the flow rate of solution B and / or by adding a third basic solution NaOH or KOH.

[0076] It may also be advantageous for the reaction, carried out within the reactor, to be carried out in the presence of at least one chelating agent.

[0077] This chelating agent can be chosen for example from potassium citrate, ethylenediaminetetraacetic acid, EDTA, and oxalates.

[0078] It can be introduced into the reactor independently of solutions A and B or not.

[0079] As specified above, the particles of a Prussian Blue analogue of formula (I) obtained according to the invention are particularly interesting as an active material for an electrode and in particular a cathode.

[0080] As illustrated in the examples which follow, a cathode comprising particles of a Prussian Blue analogue of formula (I) obtained according to the method of the invention, makes it possible to access an electrochemical system, such as an ion battery, having good electrochemical performances, in particular in terms of cycling stability and resistance to high charge / discharge rates.

[0081] The invention also relates to an electrochemical system comprising at least one electrode comprising, as active material, particles of a Prussian Blue analogue of formula (I) obtained according to the invention.

[0082] The electrochemical system in which the electrode according to the invention is implemented may in particular be a rechargeable electrochemical accumulator.

[0083] Advantageously, an electrode according to the invention can be implemented in a battery in cation-ion configuration, in particular a sodium-ion or potassium-ion battery.

[0084] Other characteristics, variants and advantages of the composite materials according to the invention, of their preparation and of their implementation, will emerge more clearly on reading the examples and figures which follow, given for illustrative and non-limiting purposes of the invention. Examples

[0085] Example 1: Synthesis of K2Mn[Fe(CN)6] microparticles

[0086] The following two solutions were prepared: - Solution A: 8.09 g of FeSO4.7H2O and 4.95 g of MnSO4.H2O in 200 mL of water - Solution B: 11.45 g of KCN in 200 mL of water

[0087] Solutions A and B were added simultaneously to a coprecipitation reactor containing 1 l of water, at a flow rate of 3.0 ml / min and at a pH of 9.9. The temperature in the reactor was maintained at 30 °C and its contents were stirred at 1000 rpm. Solutions A and B as well as the reactor were, beforehand, deoxygenated under argon. A flow of argon was maintained throughout the experiment to avoid oxidation of the products. Once the two solutions were introduced into the reactor, the contents of the reactor were left stirring for 4 hours under an inert atmosphere.

[0088] The microparticles of K2Mn[Fe(CN)6], corresponding to the precipitate resulting from the reaction, were recovered by centrifugation, washed with 400 ml of deoxygenated water and left to dry under vacuum at 100°C overnight.

[0089] They were characterized by X-ray diffraction. The diffractogram obtained, illustrated in [Fig.l], is characteristic of an analogous Prussian Blue material of the “Prussian White” type, with a monoclinic structure and space group P2i / n (Hermann-Mauguin notation).

[0090] The microparticles were also characterized by scanning electron microscopy in order to characterize their shape. They have a spherical appearance and a D50 of 0.7 micrometers.

[0091] Example 2: Use of the particles formed in Example 1 as active material of a cathode of a button cell

[0092] The electrochemical performances of the particles obtained in Example 1 as a component of a cathode of a button cell were evaluated.

[0093] The particles were mixed with a carbonaceous conductive additive (Carbone super P C65 ™) and polyvinylidene fluoride, PVDF 5130™, as a binder polymer, in N-Methyl-2-Pyrrolidone, NMP. The mass composition of the mixture was as follows: 70 / 20 / 10 (Active material / carbonaceous conductive additive / polyvinylidene fluoride). The mixture was coated on aluminum and then allowed to dry at 65°C in air overnight.

[0094] Electrodes of 14 mm diameter were cut, calendered at 10 tonnes and dried under vacuum at 80°C for 48 hours.

[0095] The button cells were manufactured in a glove box, using the electrodes thus formed as cathode, potassium metal as anode, Whatman GF / D as separator and an organic electrolyte (0.7 M KPF6 in a 1:1 mixture of ethylene carbonate, EC, and diethyl carbonate, DEC, + 2 m% fluoroethylene carbonate, FEC). The cycling regime is C / 20. The galvanostatic profile obtained in the first cycle is illustrated in [Fig.2]. We clearly find the two potential plateaus around 4 V vs. K+ / K corresponding to the oxidations / reductions of iron and manganese in the K2Mn[Fe(CN)6] material. We initially obtain 105 mAh.g 1 of reversible capacity.

[0096] The cycling curve (discharge capacity as a function of the number of cycles) is illustrated in [Fig.3]. After 50 cycles, the reversible capacity then reaches approximately 80 mAh.g-1.

Claims

Claims

1. A process for synthesizing particles of a Prussian Blue analogue of formula (I): AxMlyM2z(CN)6(I) in which: - A represents an atom of sodium, Na, or potassium, K, - Ml and M2, identical or different, are chosen from the transition metals Ti, Nb, V, Cr, Mn, Fe, Co, Ni, Cu, Zn and preferably from Fe and Mn, - x is non-zero and varies from 0 to 2.2, and is preferably close to 2 - y varies from 0 to 2 - z varies from 0 to 2 - y+z = 2 and its hydrates, comprising at least the steps of: a. Having on the one hand, an aqueous solution A comprising at least one water-soluble salt of a transition metal M1 and at least one water-soluble salt of a transition metal M2 and on the other hand, an aqueous solution B containing at least potassium or sodium cyanide; b. Inject simultaneously, separately from each other and at a controlled flow rate, said solutions A and B into a so-called precipitation reactor, containing at least one aqueous medium and in particular water and c. Maintain in said reactor, the mixture of solutions A and B thus formed, with stirring, in particular under an inert atmosphere, and under conditions conducive to the formation of particles of said Prussian Blue analogue of formula (I) by coprecipitation, said steps b and c being carried out under an inert atmosphere, at a controlled pH value varying from 8 to 11.

2. Method according to the preceding claim in which said particles of the Prussian Blue analogue of formula (I) are recovered at the end of step (c), in particular by centrifugation and, if necessary, washed and dried.

3. A method according to claim 1 or 2 wherein the pH within said reactor is adjusted directly by controlling the flow rate of solution B.

4. A method according to any preceding claim wherein the pH within said reactor is adjusted by adding a basic solution of NaOH or KOH.

5. Method according to any one of the preceding claims, characterized in that the precipitation reaction is carried out within said reactor in the presence of at least one chelating agent, in particular chosen from potassium citrate, ethylenediaminetetraacetic acid, EDTA, and oxalates.

6. Process according to the preceding claim in which the chelating agent is introduced into the reactor independently of solutions A and B or

7. no. Method according to any one of the preceding claims, characterized in that the mixture is kept stirring in step c) from 1 hour to 24 hours, preferably from 2 hours to 6 hours.

8. Process according to any one of the preceding claims in which the temperature in the reactor during step c) is maintained at 20°C to 70°C, preferably at 25°C to 35°C.

9. Method according to any one of the preceding claims in which said solution A contains, as salt of Ml, a manganese salt chosen from the salts Mn(NO3)2, Mn(SO4)2, MnCl2, Mn(CH3 CO2)2 and their hydrates and preferably the salt MnSO4.H2O and as salt of M2, an iron salt chosen from the salts Fe(NO3)2, Fe(SO4)2, FeCl2 and their hydrates and preferably FeSO4.7H2O.

10. Process according to any one of the preceding claims in which said cyanide salt of solution B and the transition metal salts M1 and M2 of solution A are brought together in a molar ratio of salts M1+M2 / CN of K or Na varying from 2.5 to 3.5, preferably 3.

11. A method according to any one of the preceding claims wherein the particles of said Prussian Blue analogue of formula (I) are particles of K2Mn[Fe(CN)6], Na2Mn[Fe(CN)6] and preferably K2Mn[Fe(CN)6].

12. Particles of a Prussian Blue analogue of formula (I): AxMlyM2z(CN)6(I) in which: - A represents a sodium atom, Na, or a potassium atom, K, - Ml and M2, identical or different, are chosen from Fe and Mn, - x is non-zero and varies from 0 to 2.2, and preferably is of the order of 2 - y varies from 0 to 2 - z varies from 0 to 2 and - y+z = 2 said particles having a D50 of 0.5 pm to 25 pm, characterized by laser granulometry.

13. Prussian Blue analogue particles of formula (I) as defined in claim 1 or 12 obtained according to any one of claims 1 to 11.

14. Use of the particles according to claim 12 or 13 as active material for cathode.

15. Use of the particles according to claim 12 or 13 in a sodium-ion or potassium-ion battery.

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