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 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 synthesizing Prussian Blue analogues useful as cathodic active material. technical field

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

[0002] It aims more specifically to propose a new process for the synthesis of Prussian Blue analogue particles which are particularly useful as an active material for cathodes in sodium-ion or potassium-ion batteries. Previous technique

[0003] Metal-ion battery-type electrochemical devices currently dominate the market for rechargeable electrochemical devices. They have numerous applications, including powering thin embedded systems such as credit cards and smart tags, powering mobile phones, storing energy in photovoltaic cells, and powering electric vehicles. Various electrochemical storage systems and electrochemical generators have therefore been developed, including sodium-ion, lithium-ion, potassium-ion, and 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, notably due to the relative scarcity of lithium resources, which are now considered a critical metal.

[0005] Sodium- or potassium-ion batteries represent attractive alternatives and are also viable means of supporting renewable energy sources for load balancing and excess energy storage. The performance of these sodium- or potassium-ion batteries depends heavily on the properties of the active electrode materials. Cathode active materials analogous to Prussian Blue (PBA) stand out precisely as promising materials for use in sodium- or potassium-ion batteries. Sodium- and potassium-based Prussian Blue analogs are mainly synthesized by precipitation in water of a hexacyanometallate complex A4Mi(CN)6 with a salt of the other transition metal M2. Application JP2018106911 A, as well as patents CN110002466B B and CN110002466B, describe such a synthesis starting from hexacyanometalate and the salt of the other transition metal M2. However,The synthesis methods described in these documents do not allow for effective control of particle morphology: micrometric aggregates of nanometric particles are obtained, with inhomogeneous sizes and shapes, leading to a material with low volumetric energy density and low cyclability. The addition of a chelating agent (potassium citrate, EDTA, oxalate, etc.) to the M2 transition metal salt solution allows for better control of particle morphology. The chelating agent forms a complex with the M2 transition metal in the transition metal salt, making it less available for the precipitation reaction and thus slowing down its kinetics. Another synthesis method involves first forming manganese hydroxide nanospheres using a polymer (polyacrylic acid) and then reacting these spheres with potassium hexacyanoferrate (K4Fe(CN)6).However, by analogy with the aforementioned processes, this process also systematically requires the use of the K4Fc(CN)6 precursor, 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 potassium cyanide (KCN) solution. However, the resulting particulate material consists of aggregates of nanoparticles of various sizes and is therefore not entirely satisfactory as an electrode material. Description 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 eliminate the need for the implementation of precursors such as K4pe(CN)6, which are complex to synthesize.

[0008] The present invention also aims to provide a method for obtaining PB A particles of controlled shape in size and homogeneity.

[0009] The present invention also aims to provide a method suitable for such control through control of the kinetics of the reaction between cyanide and metallic salts and in particular a regulation of the germination and growth phases of the resulting particles.

[0010] The inventors have now discovered that it is possible to meet these expectations provided that a particular synthesis technique is chosen. 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 a sodium atom, Na, or a potassium atom, K, - M1 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. Have 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 reactor, called a 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, under agitation, 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 in the order of 9.9.

[0020] According to a particular embodiment, M1 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 can be seen from the above, one of the characteristics of the process of the invention is 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 respects.

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

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

[0024] Furthermore, controlling their respective concentrations in the reactor, particularly via their injection rate, advantageously allows for regulating the germination and growth phases of the expected particles. For example, assuming a reactor with a volume ranging from 500 mL to 2 L, it is advantageous for solutions A and B to be injected in parallel into said reactor at a flow rate ranging from 1 mL / min to 5 mL / min, and preferably from 2 mL / min to 4 mL / min.

[0025] Furthermore, the process according to the invention is compatible with adjusting a fixed 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 "subject to" input.

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

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

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

[0029] Advantageously, the synthesis process 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 from 0.5 pm to 3 pm. This size can notably be characterized by laser particle size analysis.

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

[0032] According to another aspect of it, 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 a sodium atom, Na, or a potassium atom, K, -M1 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) according to the invention directly obtained by a process according to the invention.

[0040] As can be seen from the examples below, these particles are particularly useful as an active material for a cathode.

[0041] Thus, another aspect of the invention relates to 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 implementation, will become clearer from the description, examples and figures which follow, given by way of illustration and not limitation of the invention.

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

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

[0045] [Fig.2] presents 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 a cathode the electrodes thus formed in example 2, and whose cycling regime is C / 20. Detailed description

[0047] As mentioned previously, the process 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 a sodium atom, Na, or a potassium atom, K, - M1 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 preferably is non-zero,

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

[0052] - y+z = 2

[0053] For the purposes of this invention, the terms Na atom, K atom, and the symbols Ml 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, M1 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, M1 and M2 are different and in particular M1 is Mn and M2 is Fe. In this embodiment the process is advantageous for forming K2Mn[Fe(CN)6] or Na2Mn[Fe(CN)6] and preferably K2Mn[Fe(CN)6].

[0056] As can be seen 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 from 0.5 pm to 3 pm. This size can notably be characterized by laser particle size analysis.

[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 selected 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 among the salts Fe (NO3)2, Fe(SO4)2, FeCl2 and their hydrates and preferably FeSO4.7H2O.

[0062] Solution A may 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 favorable to the interaction of these salts with the anions of solution B.

[0063] Solution A is implemented 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 implemented in a deoxygenated form.

[0065] Solution B may advantageously have a molar concentration of cyanide salts ranging from 0.1 to 7.5 M. However, as mentioned for solution A, a higher concentration is conceivable provided that the reaction temperature is adjusted to a temperature favorable 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 at respective flow rates ranging 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 reactor capacity. These adjustments are within the expertise 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 under agitation until all of the expected compound of general formula (I) has precipitated.

[0071] This mixture can in particular be kept under agitation in step c) from 1h to 24h, preferably from 2h to 6h.

[0072] The temperature in the reactor during step c) can be maintained from 20°C to 70°C, preferably from 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 competence of a person skilled in the art and will therefore not be detailed in this description.

[0074] As specified above, it is advantageous, in order to control the particle size of the Prussian Blue analogue of formula (I), that the synthesis process be carried out at a controlled pH value and 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 take place 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 stated 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 following examples, a cathode comprising particles of a Prussian Blue analogue of formula (I) obtained according to the process of the invention, makes it possible to access an electrochemical system, such as an ion battery, exhibiting good electrochemical performance, 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 can 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, their preparation and implementation, will become clearer from reading the examples and figures that follow, given by way of illustration and not limitation 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 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 previously deoxygenated under argon. An argon flow was maintained throughout the experiment to prevent oxidation of the products. Once the two solutions were introduced into the reactor, the reactor contents were left under stirring for 4 hours in an inert atmosphere.

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

[0089] They were characterized by X-ray diffraction. The diffractogram obtained, illustrated in [Fig.1], is characteristic of an analogous material Prussian Blue type "Prussian White", of monoclinic structure and of 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 the active material of a button cell cathode

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

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

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

[0095] The button cells were manufactured in a glove box, using the electrodes thus formed as the cathode, potassium metal as the anode, Whatman GF / D as the 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]. The two potential plateaus around 4 V vs. K7K corresponding to the oxidation / reduction of iron and manganese in the K2Mn[Fe(CN)6] material are clearly visible. An initial reversible capacity of 105 mAh.g is obtained.

[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

Demands

1. Process for synthesizing 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, - M1 and M2, identical or different, are chosen from among 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 preferably is close to 2 - y varies from 0 to 2 - z varies from 0 to 2 - y+z = 2 and its hydrates, including at least the steps of: a. Have 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. Injecting simultaneously, separately from each other and at a controlled flow rate, said solutions A and B into a reactor, called a precipitation reactor, so as to regulate the germination and growth phases of the particles, said 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, under agitation and under conditions conducive to the formation of particles of said Prussian Blue analogue of formula (I) by co-precipitation, the conducive conditions being achieved by an inert atmosphere, and a controlled pH value varying from 8 to 11, step b is also carried out under an inert atmosphere, at a controlled pH value varying from 8 to 11.

2. A method according to the preceding claim wherein said particles of the Prussian Blue analogue of formula (I) are recovered at the end of step (c), in particular by centrifugation and where appropriate 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 one of the preceding claims wherein the pH within said reactor is adjusted by adding a basic solution of NaOH or KOH.

5. A process 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 selected from potassium citrate, ethylenediaminetetraacetic acid, EDTA, and oxalates.

6. Method according to the preceding claim in which the chelating agent is introduced into the reactor independently of solutions A and B or not.

7. A method according to any one of the preceding claims characterized in that the mixture is kept under agitation in step c) from 1h to 24h, preferably from 2h to 6h.

8. A method according to any one of the preceding claims wherein the temperature in the reactor during step c) is maintained from 20°C to 70°C, preferably from 25°C to 35°C.

9. A process according to any one of the preceding claims wherein said solution A contains, as salt of M1, a manganese salt selected from the salts Mn(NO3)2, Mn(SO4)2, MnCl2, Mn(CH3CO2)2 and their hydrates and preferably the salt MnSO4.H2O and as salt of M2, an iron salt selected from the salts Fe(NO3)2, Fe(SO4)2, FeCl2 and their hydrates and preferably FeSO4.7H2O.

10. A process according to any one of the preceding claims wherein said cyanide salt from solution B and the transition metal salts M1 and M2 from solution A are brought together in a molar ratio of salts M1+M2 / CN of K or Na ranging 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

12.

13.

14. (I) are particles of K2Mn[Fe(CN)6], Na2Mn[Fe(CN)6] and preferably of K2Mn[Fe(CN)6]. K2Mn[Fe(CN)6] particles obtained by the process according to claims 1 to 11, said particles having a monoclinic structure of space group P21 / n according to the Hermann-Mauguin notation and having a D50 of 0.5 pm to 25 pm, characterized by laser granulometry. Use of the particles according to claim 12 as an active material for cathode. Use of the particles according to claim 12 in a sodium-ion or potassium-ion battery.

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