A process for preparing a material similar to Prussian Blue and a cathode ink containing it

Abstract

The present invention relates to a process for synthesizing an active material of the Prussian Blue type analogous to the general formula (I) AwM1xM2y(CN)6(I), comprising at least the step of bringing together, in an aqueous medium, under stirring and in an inert atmosphere, at least one cyanide ion source selected from compounds of formula (II) ACN or (III) AaM1(CN)b with an effective amount of at least one salt of formula (IV) M2v(X)k. It further relates to an electrode ink obtained directly in the reaction medium of this process. Figure: none

Description

Title of the invention: Process for preparing a Prussian Blue-like material and a cathode ink containing it Technical field

[0001] The present invention relates to the field of electrochemical devices of the metal-ion battery type. More specifically, it aims to propose a method for synthesizing an active material of the Prussian Blue analog type, useful as an ink in electrochemical systems, in particular in sodium-ion or potassium-ion batteries. Prior art

[0002] Electrochemical devices of the metal-ion battery type currently dominate the market for rechargeable electrochemical devices. They find multiple applications, notably 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.

[0003] Various electrochemical storage systems or electrochemical generators have been developed, including sodium-ion, lithium-ion, potassium-ion, magnesium-ion batteries. In particular, the active materials, including the active cathode materials, of such electrochemical devices are the subject of constant research, particularly to improve the electrochemical performance of such devices but also to perfect their production processes.

[0004] Among the promising materials for cathode synthesis in metal-ion batteries, Prussian Blue (PBL) analogues, in particular Prussian White materials, make it possible to obtain cathode materials whose electrochemical performances are particularly interesting. This is a category of materials with the general formula AxMi[M2(CN)6] with A being an alkali, Ml and M2 transition metals and x varying from 0 to 2. When x is close to 2, we speak of Prussian White materials.

[0005] For sodium and potassium-based BPAs, the materials are mainly synthesized by precipitation in water of a hexacyanometalate complex A4Mi(CN)6 with a salt of the other transition metal M2.

[0006] Thus, for an application in potassium-ion accumulator, one of the most promising materials is Prussian White K2Mn[Fe(CN)6], having a theoretical capacity of 155 mAh.g-1 at a potential close to 4 V vs. K+ / K. Its synthesis is generally carried out by precipitation in water of the hexacyanoferrate complex K4 Fe(CN)6 with a manganese salt such as for example Mn(NO3)2, Mn(SO4)2, MnCl2 or Mn(CH3CO2)2. A chelating agent is sometimes added to the reaction mixture to control the morphology of the resulting K2Mn[Fe(CN)6] particles. After precipitation of the K2Mn[Fe(CN)6] material, the reaction mixture is washed with water to remove impurities such as potassium, the manganese salt counterion, and any chelating agent, and the powder is then dried to remove residual water. Once washed and dried, the K2Mn[Fe(CN)6] powder is then mixed with a conductive additive and a binder to form an ink suitable for coating a current collector and thus forming a cathode in an electrochemical storage system or an electrochemical generator.

[0007] Thus, the publication by Hosaka et al. ChemSusChem, 2021, 14, 1166-1175 [1] describes an example of the synthesis of the material K2Mn[Fe(CN)6] from K4Fe(CN)6 and MnCl2. Similarly, the publication by Wu et al. Advanced Materials, 2022, 34, 2106876 [2] describes a process comprising the synthesis of a Prussian White material K2Mn[Fe(CN)6], from K4Fe(CN)6 and MnSO4 and several steps of washing the material thus obtained to remove impurities and then drying under vacuum at 60°C overnight. The preparation of the ink constitutes a second step in the electrode production process, quite distinct from this synthesis step. Applications WO201856890 A1 and EP4170741 A1 describe other examples of synthesis of the Prussian White material, Na2Fe[Fe(CN)6], and then the preparation of a cathode from this material still requiring a two-step process separated by several washing and drying steps.

[0008] As is clear from the above, these methods of synthesis require, firstly, producing the active material analogous to BPA, isolating it from its reaction medium to rid it of contaminating secondary products and only secondly, considering it, in a purified form, for the preparation of an electrode ink.

[0009] The present invention aims precisely to propose a method making it possible to carry out, in a single step, the synthesis of the active material and the preparation of the ink for cathode materials of the Prussian Blue (BPA) type, in particular for Na-ion and K-ion batteries.

[0010] More specifically, the object of the present invention is to propose a process which is simple to implement, inexpensive (minimization of steps and syntheses of complex precursors) and environmentally friendly (synthesis in water, at a temperature close to ambient).

[0011] It also aims to propose a process for synthesizing a cathode material of the Prussian Blue analogue (BPA) type, making it possible to avoid the washing and drying steps commonly required to eliminate the synthesis by-product or even the unreacted manganese or iron salt.

[0012] It further aims to propose a synthesis process making it possible to obtain an ink with improved electrochemical properties. Summary of the invention

[0013] Thus, according to a first of its aspects, the present invention provides a method for synthesizing an active material of the Prussian Blue analog type of general formula (I): AwMlxM2y(CN)6(I)

[0014] 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, - w varies from 0.5 to 2.2 and is preferably of the order of 2,

[0015] - x varies from 0 to 2

[0016] - y varies from 0 to 2 and

[0017] - x+y = 2

[0018] and its hydrates, said process comprising at least the step of bringing together, in an aqueous medium, with stirring and under an inert atmosphere, at least one source of cyanide ion chosen from the compounds of formula (II) and the compounds of formula (III):

[0019] (II) ACN

[0020] (III) AaMl(CN) and its hydrates

[0021] in which

[0022] A and Ml are as defined in formula (I) a varies from 2 to 4 and b varies from 4 to 6 in formula (III) with an effective amount of at least one salt of formula (IV) (IV) M2v(X)k in which - M2 is as defined in general formula (I) - X is chosen from the TFSI, FSI, PF6, C1O4 and BF4 units - v varies from 1 to 2 and - k varies from 1 to 4 said compound of formula (I) being formed in the precipitate state.

[0023] In an alternative embodiment, the source of cyanide ion is chosen from the compounds of formula (II) or the compounds of formula (III).

[0024] In the embodiment variant, where M1 and M2 are identical in formula (I), said process brings into contact at least one compound of formula (II) and one compound of formula (IV) or at least one compound of formula (III) and one compound of formula (IV) whose metal salts, M1 and M2, are of the same chemical nature.

[0025] In the embodiment variant, where M1 and M2 are different in formula (I), said process brings together a compound of formula (III) and a compound of formula (IV) whose metal salts, M1 and M2, are of different chemical natures.

[0026] In particular, said compound of general formula (I) is obtained as a mixture with at least one compound of formula (V) (V) A(X) in which - A represents a Na or K atom and - X is chosen from the TFSI, FSI, PF6, C1O4 and BF4 units

[0027] As is apparent from the examples below, the inventors have, against all expectations, found that the use of a compound of general formula (IV) as a transition metal salt precursor makes it possible to satisfy the objectives specified above.

[0028] The counterion of the transition metal salt M2, considered in formula (IV), forms an impurity of formula (V) which does not prove useful to remove by washing. A mixture of the compounds of general formula (I) and (V) can therefore be used as such to form an electrode ink.

[0029] Present in the ink, the compound of formula (V) will remain on the cathode, coated with the ink, and advantageously participate in the proper functioning of the accumulator during cycling. These compounds of formula (V), which are in the solid state by their chemical nature, can thus participate in the formation of the passive layer which develops between the negative electrode and the electrolyte during charging at low potential values, called solid electrolyte interphase (SEI). The formation of such a passive layer which sometimes also develops between the positive electrode and the electrolyte, called in English "Cathode Electrolyte Interphase" (CEI). In addition, these compounds of formula (V) can also act as an electrolyte salt.

[0030] According to another of its aspects, the present invention relates to an electrode ink, containing, in an aqueous or organic medium, and preferably aqueous, at least one active material of the Prussian Blue analog type of general formula (I): AwMlxM2y(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, - w varies from 0.5 to 2.2 and is preferably of the order of 2 - x varies from 0 to 2 - y varies from 0 to 2 and - x+y = 2 and its hydrates, in mixture with at least one compound of general formula (V) (V) A(X) in which - A represents an atom of Na or K, and - X is chosen from TFSI, FSI, PF6, C1O4 and BF4 and

[0031] at least one conductive additive and, where appropriate, at least one binding agent.

[0032] In particular, the ink according to the invention is directly formed in the synthesis medium of an active material of the Prussian Blue analog type of general formula (I) prepared according to a process in accordance with the invention.

[0033] According to another of its aspects, the present invention aims at the use of an ink as defined above to prepare a cathode.

[0034] The present invention further relates to a cathode characterized in that it comprises a current collector on which an ink in accordance with the invention is applied.

[0035] In particular, it relates to a cathode comprising a current collector on which is applied an active material of the Prussian Blue analog type of general formula (I) in a mixture with at least one compound of general formula (V), said mixture having been obtained according to the process of the invention.

[0036] According to another of its aspects, the present invention relates to a method for preparing an electrode, in particular a cathode, comprising at least the steps of: a)- Have an ink according to the invention b)- Applying said ink to the surface of a current collector, for example, of the aluminum foil type; and c)- Dry said collector coated with said ink.

[0037] More particularly, an electrode, in particular a cathode, can be obtained by a method comprising at least the steps of: a') Dispose of the synthesis medium in which a compound of the analogous type Prussian Blue of general formula (I) has been prepared according to the process of the invention and containing said compound of general formula (I) thus formed;

[0038] b') Applying at least in part said synthesis medium containing said analogue Prussian blue of general formula (I) formed, and further containing at least one conductive additive and where appropriate a binding agent, on the surface of a collector and;

[0039] c') Dry the collector coated with said synthesis medium.

[0040] The invention also relates, according to another of its aspects, to an electrochemical system comprising at least one cathode according to the invention.

[0041] Other characteristics, variants and advantages of the composite materials according to the invention, their preparation and their implementation, will emerge better from reading the description, examples and figures which follow.

[0042] 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

[0043] [Fig. 1] shows the diffractogram obtained by X-ray diffraction of the electrode developed in comparative example 1.

[0044] [Fig.2] shows the galvanostatic curves of the first five cycles obtained at C / 10 of the button cell developed in comparative example 1.

[0045] [Fig.3] shows the diffractogram obtained by X-ray diffraction of the electrode produced in example 2.

[0046] [Fig.4] shows the galvanostatic curves of the first five cycles obtained at C / 10 of the button cell developed in example 2.

[0047] [Fig.5] shows the diffractogram obtained by X-ray diffraction of the electrode produced in example 3.

[0048] [Fig.6] shows the galvanostatic curves of the first five cycles obtained at C / 10 of the button cell developed in example 3. Detailed description

[0049] Process for the synthesis of an active material of the Prussian Blue analogue type, known as BPA, according to the invention

[0050] As mentioned above, the method of the invention aims to form an active BPA material of general formula (I)

[0051] AwMlxM2y(CN)6(I)

[0052] 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, - w varies from 0.5 to 2.2 and is preferably of the order of 2 - x varies from 0 to 2 and preferably is different from 0 - y varies from 0 to 2 and preferably is different from 0 - x+y = 2

[0053] and its hydrates.

[0054] For the purposes of the invention, the expressions “active material of Prussian Blue analogue type of general formula (I)”, “active material of BPA type of general formula (I)”, “Prussian Blue analogues of formula (I)” and “compound of general formula (I)” are used interchangeably.

[0055] For the purposes of the invention, the terms “Na atom”, “K atom”, and the symbols Ml and M2 are intended to cover the charged or uncharged forms of the elements considered. For example, Na covers Na+ and Ml covers Mn++.

[0056] For the purposes of the invention, the term of the order of 2 means that the index considered, such as for example w, can be equal to 2 but also to decimal values ​​close to 2.

[0057] Advantageously, M1 and M2 are different.

[0058] According to a particular embodiment Ml is Mn and M2 is Fe.

[0059] According to another particular embodiment M1 and M2 are Fe.

[0060] As representatives of the compounds of general formula (I) the following compounds may in particular be cited: K2Mn[Fe(CN6)], K2Fe[Fe(CN6)], Na2Mn[Fe(CN6)] and Na2 Fe[Fe(CN6)].

[0061] According to the invention, the compounds of general formula (I) are formed by bringing together, in an aqueous medium, with stirring and under an inert atmosphere, at least one hexacyanoferrate complex chosen from the compounds of formula (II) and the compounds of formula (III)

[0062] (II) ACN (III) AaMl(CN)b and its hydrates In which - A and Ml are as defined in general formula (I) - a varies from 2 to 4 and b varies from 4 to 6 in formula (III) with an effective amount of at least one salt of formula (IV) (IV) M2v(X)k in which - M2 is as defined in general formula (I) - X is chosen from the TFSI, FSI, PF6, C1O4 and BF4 units _v varies from 1 to 2 and - k varies from 1 to 4. Representative compounds of general formula (II) may include KCN and NaCN.

[0063] As representatives of the compounds of general formula (III) which may in particular be cited are K4Fe(CN)6, K4Mn(CN)6, Na4Fe(CN)6, Na4Mn(CN)6, K2Ni(CN)4 and their hydrates.

[0064] As specified above, depending on whether M1 and M2 are or are not identical in general formula (I), the choice of the compound of general formula (II) or (III) is adjusted.

[0065] It is also possible to consider the joint implementation of at least one compound of general formula (II) and at least one compound of general formula (III).

[0066] In particular, when M1 and M2 are identical in formula (I), the process according to the invention can bring together in a first variant a compound of formula (II) and a compound of formula (IV) or in a second variant, a compound of formula (III) and a compound of formula (IV) whose metal salts, M1 and M2, are of the same chemical nature.

[0067] If Ml and M2 are different in formula (I), the process according to the invention brings together a compound of formula (III) and a compound of formula (IV) whose metal salts, Ml and M2, are of different chemical natures.

[0068] In an alternative embodiment, the process according to the invention brings together a compound of formula (III) and in particular K4Fe(CN)6.3H2O to form the compound of formula (I) which is K2Mn[Fe(CN)6].

[0069] As regards the compounds of general formula (III), they are not water-soluble.

[0070] According to a particular embodiment, X is TFSI, FSI or PF6.

[0071] According to a preferred embodiment of the invention, the molar ratio between the hexacya- complex noferrate of formula (II) or (III) and said salt of formula (IV) is such that the reaction between these two compounds is complete.

[0072] According to an advantageous embodiment, a compound of general formula (III) and a compound of general formula (IV) are brought together in a stoichiometric ratio of salt of formula (IV) / compound of formula (III) adjusted to condition the desired M1 / M2 ratio in formula (I).

[0073] Generally speaking, bringing at least one compound of formula (II) or formula (III) into contact with at least one salt of formula (IV) is carried out in an aqueous medium, with stirring and under an inert atmosphere.

[0074] In particular, the bringing together of the compound of formula (II) or (III) with a compound of formula (IV) is carried out in a reactor containing water, preferably deionized water.

[0075] Such a reaction in an aqueous medium aims to form an active BPA material of general formula (I) according to an environmentally friendly process.

[0076] The bringing together of all the reagents is also carried out under an inert atmosphere to avoid any oxidation of the salts it contains.

[0077] “Inert atmosphere” means an atmosphere free of reactive gases, composed of non-reactive (inert) gases such as nitrogen, carbon dioxide, argon or helium.

[0078] Generally, the bringing together of the two compounds of formulae (II) and (III) is carried out at a temperature close to room temperature. Nevertheless, higher temperatures may be considered provided that they are not detrimental to the expected reaction and to the stability of the reactants brought together as well as that of the expected compounds. As illustrated in the examples below, this bringing together may be carried out between 15°C and 35°C, preferably between 19°C. and 25°C.

[0079] The bringing together of the two compounds of formulas (II) and (III) is also carried out with stirring.

[0080] Advantageously, stirring is maintained until complete precipitation of the compound of general formula (I) thus formed.

[0081] At the end of the reaction, a precipitate is obtained comprising said compound of general formula (I) and at least one compound of general formula (V).

[0082] As previously specified, together with the compound of general formula (I) is in fact formed at least one compound of general formula (V) (V) A(X)

[0083] in which

[0084] -A represents an Na or K atom and

[0085] -X is chosen from the TFSI, FSI, PF6, C1O4 and BF4 units

[0086] According to an alternative embodiment, the process leads to the formation of the compound K2 Mn[Fe(CN)6] obtained in a mixture with at least one compound of general formula (V) chosen from KTFSI, KFSI, KPF6, KCLO4 and KBF4 and preferably KTFSI.

[0087] As illustrated in the examples below, the method according to the invention is particularly advantageous for forming an ink for an electrode, in particular a cathode.

[0088] This ink is advantageously directly formed, according to the process of the invention, in the synthesis medium of an active material of the Prussian Blue analog type of general formula (I).

[0089] More precisely, it then derives from the implementation, in said aqueous medium, of at least one conductive additive and at least one binding agent.

[0090] Thus, the method according to the invention may comprise the addition to its aqueous reaction medium of at least one conductive additive and, optionally, of at least one binding agent to form an electrode ink.

[0091] According to this embodiment, the conductive additive can be added to said aqueous medium before or after the addition of at least one compound of formula (IV) and at least one compound chosen from the compounds of general formula (II) and the compounds of general formula (III).

[0092] According to one embodiment, the binding agent is added to said aqueous medium before or after the addition of said compounds of general formula (II) or (III) and (IV).

[0093] According to another embodiment, the binding agent and the additive are added after the formation of the expected compound of formula (I). Conductive additive

[0094] The term “conductive additive” means an electronic conductive additive.

[0095] Thus, said conductive additive(s) may be chosen from fibers of carbon, carbon black, carbon nanotubes, graphene, graphite, and metal particles, such as metal nanowires or nanoparticles.

[0096] The metal particles may be, for example, aluminum, copper, nickel, silver, gold, platinum, titanium, palladium, zinc or alloys thereof.

[0097] Preferably, the electronically conductive additives are chosen from carbon additives, such as carbon nanofibers and carbon black. Binder

[0098] The binder is generally of a polymeric nature.

[0099] Such binders may be chosen from fluorinated binders, in particular from polytetrafluoroethylene, polyvinylidene fluoride, carboxymethylcellulose and its derivatives, polysaccharides, polyacrylates, latexes in particular of the styrene-butadiene type. Ink

[0100] Thus, the invention further relates to an ink for an electrode, in particular a cathode, containing, in an aqueous or organic medium, at least one active material of the Prussian Blue analog type of general formula (I) in a mixture with at least one compound of general formula (V) as defined above, at least one conductive additive and, where appropriate, at least one binding agent.

[0101] According to a particular variant, such an ink comprises, as active material of the Prussian Blue analog type, at least K2Mn[Fe(CN)6] and comprises at least one compound of general formula V chosen from KTFSI, KPF6, KFSI, KCLO4 and KBF4.

[0102] In particular, an ink in accordance with the invention may comprise, as a percentage by weight of dry matter:

[0103] a. 40 to 60% of at least one active material of Prussian Blue analog type of general formula I and preferably at least K2MnFe(CN)6,

[0104] b. 25 to 35% of at least one compound of general formula V preferably chosen from KTFSI and KFSI,

[0105] c. 10 to 20% of at least one conductive additive; and

[0106] d. 7 to 10% of at least one binding agent.

[0107] The Prussian Blue analogues of formula (I) and the ink obtained according to the invention are particularly interesting as active material for an electrode and in particular a positive electrode or cathode. Electrode

[0108] Conventionally, the electrode is in contact with a current collector. This collector may in particular be chosen from copper, aluminum, nickel, carbon felt or stainless steel for a positive electrode.

[0109] This electrode can in particular be obtained by a process comprising at least the steps consisting of: a) Have an ink according to the invention b) Applying said ink to the surface of a collector and c) Drying said collector coated with said ink, in particular at a temperature between 50°C and 100°C, preferably 60°C and 90°C, and in particular for 8 to 60 hours, preferably for 10 to 50 hours.

[0110] More particularly, this electrode can be obtained by a method comprising at least the steps consisting of: a') Arranging the synthesis medium for a compound of analogous type Prussian Blue of general formula (I) prepared according to the process of the invention with said medium containing said compound of general formula (I) thus formed;

[0111] b') Applying at least in part said synthesis medium containing said analogue Prussian blue of general formula (I) formed, and further containing at least one conductive additive, in particular as defined above, and preferably a binding agent, in particular as defined above, on the surface of a collector and;

[0112] c') Drying said collector coated with said synthesis medium, in particular at a temperature temperature between 50°C and 100°C, preferably 60°C and 90°C, in particular for 8 to 60 hours, preferably for 10 to 50 hours.

[0113] Step (b) or (b') of the preceding methods can be carried out by coating, by a printing technique, by extrusion or by co-lamination. A person skilled in the art is able to adjust the conditions for implementing these different techniques.

[0114] As illustrated in the examples which follow, a cathode comprising an ink 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. Electrochemical system

[0115] The invention therefore also relates to an electrochemical system comprising at least one cathode (I) in accordance with the invention.

[0116] The electrochemical system in which the cathode according to the invention is implemented may in particular be a rechargeable electrochemical accumulator.

[0117] Advantageously, such an electrode can be implemented in a battery in cation-ion configuration, in particular a sodium-ion or potassium-ion battery.

[0118] 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

[0119] In the following examples, the following products are used: - carbon black C65 (Super P), as a carbon conductive additive, marketed by MTI Corporation, - carboxymethylcellulose (CMC), as a binder, marketed by Ashland (ref 7HXF), and used in the form of CMC 7HXF at 2 m% in water.

[0120] Comparative Example 1: Synthesis of a cathode whose ink is formed from Mn(CH3COO)2

[0121] The various constituents below were added in their order of citation to a 25 ml beaker containing 7 ml of degassed water under argon and with magnetic stirring: - 285 mg of Carbon super P C65; - 1.24 g of K4Fe(CN)6.3H2O; - 0.71 g of Mn(CH3COO)2.

[0122] A white precipitate was obtained corresponding to a Prussian White K2 Mn[Fe(CN)6]-Carbon super P C65 composite. 143 mg of carboxymethyl cellulose (CMC) were then added to the mixture as well as 3 ml of water to obtain a satisfactory viscosity for the ink. The dispersion was carried out in Dispermat® for 15 minutes at 2000 rpm.

[0123] The ink thus formed was then coated onto an aluminum current collector. After drying in air at 70°C overnight, electrodes with a diameter of 14 mm were pelletized. Since the adhesion of the ink to the aluminum was quite weak, the electrodes were not calendered.

[0124] The total mass of dry matter is 1.997 g. The mass proportion of Prussian White in the electrode is therefore 1 / 1.997 or 50.1 m%.

[0125] The electrodes obtained were characterized by X-ray diffraction (XRD). The diffractogram obtained, illustrated in [Fig.l], is characteristic of a “Prussian White” type material, with a monoclinic structure and space group P2i / n. The two most intense peaks at 65° and 79° correspond to the aluminum of the current collector. No peak attributable to potassium acetate is observed, indicating that the compound is in amorphous form in the electrode.

[0126] The electrodes were then tested in button cells against potassium metal. The electrodes were dried under vacuum at 80 °C for 48 hours. The button cells were manufactured in a glove box, using as cathode the electrode obtained according to the invention, as anode potassium metal, as separator glass fiber (Whatman GF / D) and 100 μL of an organic electrolyte (0.7 M KPF6 in EC:DEC (1:1 vol) + 2 m% FEC). The galvanostatic curves of the first five cycles obtained at C / 10 are shown in [Fig.2].

[0127] The irreversible capacity during the first charge is significant. Obviously, the potassium acetate formed as a secondary product during the synthesis of K2Mn[Fe(CN)6] affects the performance of the system. From the 2nd cycle, the galvanostatic curve is more classic: the two plateaus relating to the oxidation (charge) or reduction (discharge) of iron and manganese are clearly visible. The initial reversible capacity is approximately 70 mAh.g, which is much lower than that obtained according to the invention (see examples 2 and 3).

[0128] Example 2: Synthesis of a cathode whose ink is formed according to the invention from the precursor Mn(TFSI)2,

[0129] In a 25 ml beaker containing 10 ml of degassed water under argon and with magnetic stirring are added 1.24 g of K4Fe(CN)6.3H2O and 1.79 g of Mn(TFSI)2>. A white precipitate is obtained and the solution thickened significantly corresponding to the obtaining of a Prussian White material K2Mn[Fe(CN)6] and a KTFSI impurity. 285 mg of super P carbon C65 and 143 mg of CMC were then added to the mixture as well as 4 mL of water to obtain a satisfactory viscosity for the ink. The dispersion was carried out in Dispermat® for 15 minutes at 2000 rpm.

[0130] The ink was then coated onto an aluminum current collector with a 150 μm thick doctor blade. After drying in air at 70°C overnight, electrodes with a diameter of 14 mm were pelletized. The electrodes were calendered at 10 tons.

[0131] The total mass of dry matter on the coated strip is 3.278 g. The mass proportion of Prussian White in the electrode is therefore 1 / 3.278 or 30.5 m%. Furthermore, the proportion of KTFSI in the electrode is 1.850 / 3.278 or 56.4 m%.

[0132] The mass of dry matter on the electrodes obtained after cutting is approximately 3.40 mg. We therefore have 1.04 mg of Prussian White and 1.92 mg of KTFSI.

[0133] The electrodes obtained were first characterized by X-ray diffraction (XRD). The diffractogram obtained is illustrated in [Fig.3]. The two most intense peaks at 65° and 79° correspond to the aluminum of the current collector. The other peaks are attributable either to Prussian White or to the KTFSI salt.

[0134] The electrodes were then tested in button cells against potassium metal. The electrodes were dried under vacuum at 80 °C for 48 hours. The button cells were manufactured in a glove box, using the previously obtained electrode as cathode, potassium metal as anode, glass fiber (Whatman GF / D) as separator and 100 pL of an organic electrolyte (0.7 M KPF6 in EC:DEC (1:1 vol) + 2 m% FEC). The galvanostatic curves of the first five cycles obtained at C / 10 are shown in [Fig.4]. The significant irreversible capacity observed in example 1 is not observed in the present case: the KTFSI impurity is not degraded during the first charge (unlike potassium acetate). The two Plateaus relating to the oxidation (charge) or reduction (discharge) of iron and manganese are clearly visible. Considering that Prussian White represents 30.5 m% of the total dry matter, an initial reversible capacity of approximately 125 mAh.g 1 was obtained, which is much higher than that obtained in example 1.

[0135] Example 3: Synthesis of a cathode whose ink is formed according to the invention from the precursor Mn(TFSI)2

[0136] In a 25 ml beaker containing 4 ml of degassed water under argon are added 143 mg of CMC (i.e. 7.145 g of CMC 7HXF at 2 m% in water) and 285 mg of super P carbon C65. The constituents were incorporated with a spatula and then dispersed using a Dispermat® at 2000 rpm for 5 minutes. The base of the ink was thus formed.

[0137] A 5 ml aqueous solution degassed under argon and containing 1.24 g of K4Fe(CN)6 .3H2O was then added dropwise to the ink base continuously dispersed in Dispermat® still at 2000 rpm for 5 minutes. Finally, a 5 ml aqueous solution degassed under argon and containing 1.79 g of Mn(TFSI)2 was added dropwise and then dispersed in Dispermat® still at 2000 rpm for 5 minutes. A fairly thick and heterogeneous ink was then obtained. The ink was once again dispersed in Dispermat®, this time at 5000 rpm for 5 minutes to obtain a homogeneous ink.

[0138] The ink was then coated onto an aluminum current collector with a 150 μm thick doctor blade. After drying in air at 70 °C overnight, electrodes with a diameter of 14 mm were pelletized. The electrodes were calendered at 10 tons.

[0139] The electrodes obtained were characterized by X-ray Diffraction (XRD). The diffractogram obtained is shown in [Fig.5]. The two most intense peaks at 65° and 79° correspond to the aluminum of the current collector. The other peaks are attributable either to Prussian White or to KTFSI.

[0140] The electrodes were then tested in button cells against potassium metal. The electrodes were dried under vacuum at 80 °C for 48 hours. The button cells were manufactured in a glove box, using the electrode obtained above as cathode, potassium metal as anode, glass fiber (Whatman GF / D) as separator and 100 pL of an organic electrolyte (0.7 M KPF6 in EC:DEC (1:1 vol) + 2 m% FEC). The galvanostatic curves of the first four cycles obtained at C / 10 are shown in [Fig.6]. Considering that Prussian White represents 30.5 m% of the total dry matter, an initial reversible capacity of approximately 120 mAh.g 1 was obtained, equivalent to that obtained in example 2.

Claims

Claims

1. Process for the synthesis of an active material of the Prussian Blue analogue type of general formula (I): (I) AwMlxM2y(CN)6 in which: - A represents a sodium atom, Na, or a potassium atom, 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, - w varies from 0.5 to 2.2 and preferably is of the order of 2 - x varies from 0 to 2 - y varies from 0 to 2 and - x+y = 2 and its hydrates, said process comprising at least the step of bringing together, in an aqueous medium, with stirring and under an inert atmosphere,at least one source of cyanide ion chosen from the compounds of formula (II) and the compounds of formula (III): (II) ACN (III) AaMl(CN)b and its hydrates in which A and Ml are as defined in formula (I) a varies from 2 to 4 and b varies from 4 to 6 in formula (III) with an effective amount of at least one salt of formula (IV) (IV) M2v(X)k in which - M2 is as defined in general formula (I) - X is chosen from the TFSI, FSI, PF6, C1O4 and BF4 units - v varies from 1 to 2 and - k varies from 1 and 4, said compound of formula (I) being formed in the precipitate state.,

2. Process according to the preceding claim in which said compound of general formula (I) is obtained in a mixture with at least one compound of formula (V) (V) A(X) in which -A represents an Na or K atom and -X is selected from the TFSI, FSI, PF6, C1O4 and BF4 units

3. A process according to any preceding claim wherein said compound of formula (III) is K4Fe(CN)6.3H2O and the compound of formula (I) obtained is K2Mn[Fe(CN)6].

4. Process according to the preceding claim in which said compound K2 Mn[Fe(CN)6] is obtained in a mixture with at least one compound of general formula (V) chosen from KTFSI, KFSI, KPF6, KCLO4 and KBF4 and preferably KTFSI.

5. A method according to any preceding claim further comprising adding to said aqueous medium at least one conductive additive and at least one binding agent to form an electrode ink.

6. Electrode ink, containing, in an aqueous or organic medium, at least one active material of the Prussian Blue analog type of general formula (I): (I) AwMlxM2y(CN)6 in which: A represents a sodium, Na, or potassium atom, 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, w varies from 0.5 to 2.2 and preferably is of the order of 2 x varies from 0 to 2 y varies from 0 to 2 and x+y = 2 and its hydrates, in a mixture with at least one compound of general formula (V) (V) A(X) in which -A represents a Na or K atom and - X is chosen from the TFSI, FSI, PF6, C1O4 and BF4 units and at least one conductive additive and where appropriate at least one binding agent.

7. Ink according to claim 6, the active material of the Prussian Blue analog type of which is at least K2Mn[Fe(CN)6] and comprising at least one compound of general formula V chosen from KTFSI, KPF6, KFSI, KCLO4 and KBF4.

8. Ink according to claim 6 or 7 formed directly in the synthesis medium of an active material of the Prussian Blue analog type of general formula (I) prepared according to the process as defined in claims 1 to 5.

9. Use of an ink according to any one of claims 6 to 8 for preparing an electrode, in particular a cathode.

10. A method of preparing an electrode, in particular a cathode, comprising at least the steps of: a) providing an ink according to any one of claims 6 to 8, b) applying said ink to the surface of a current collector and c) drying said collector coated with said ink.

11. A method for preparing an electrode, in particular a cathode, comprising at least the steps of: a') Arranging the synthesis medium in which has been prepared, according to the method according to one of claims 1 to 5, a compound of Prussian Blue analogue type of general formula (I) as defined in claim 1 with said medium containing said compound of general formula (I) thus formed, b') Applying at least in part said synthesis medium containing said Prussian Blue analogue of general formula (I) formed, and further containing at least one conductive additive and where appropriate a binding agent, to the surface of a collector and; c') Drying the collector coated with said synthesis medium.

12. Cathode characterized in that it comprises a current collector on which is applied an active material of the Prussian Blue analog type of general formula (I) as defined in claim 1 in a mixture with at least one compound of general formula (V) as defined in claim 2, said mixture having been obtained according to the process defined in claims 1 to 5.

13. Electrochemical system comprising at least one cathode according to the preceding claim.

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