IT202000009835A1 - IONIC LIQUID AND ELECTROLYTIC COMPOSITION FOR ELECTROCHEMICAL DEVICES - Google Patents

Description

DESCRIPTION

Attached to a patent application for INDUSTRIAL INVENTION having the title

?Ionic liquid and electrolyte composition for electrochemical devices?

TECHNICAL FIELD

The present invention relates to an ionic liquid consisting of a tetra-alkyl-phosphonium cation and a perfluoro-alkyl-sulfonyl-imide anion, a process for obtaining it and a composition comprising it. The present invention also relates to the use of said ionic liquid or of said composition as an electrolyte in an electrochemical device.

STATE OF ART

In the last twenty years, the growing energy demand has pushed many countries, especially the most? industrialized, to concentrate more and more? efforts towards research and development of new energy sources and/or vectors. The exploitation of fossil fuels, from which most of the energy produced globally currently comes, in fact involves a considerable environmental impact due both to their non-renewable nature and to the pollution effects deriving from their use such as, for example, the production of high carbon dioxide emissions or water pollution. Another problem now of global importance linked to the use of fossil fuels concerns the automotive sector in which internal combustion engines are mainly used which cause considerable emissions of polluting substances with negative consequences for the environment. The community has therefore intensified its efforts towards the search for eco-compatible energy sources, which are renewable and which allow, in the near future, the transition towards new types of auto-traction with almost zero emissions, first of all electric traction. However, the development and diffusion of electric traction so? how, more in general, of renewable energy sources, are closely linked to the possibility? to develop energy storage systems that are efficient, low-cost and capable of guaranteeing performance, in terms of range and speed, comparable to that of internal combustion vehicles. Among the various possible accumulation systems currently studied and developed, a prominent place? occupied by rechargeable batteries (secondary) or accumulators, and more? especially from lithium batteries. The advantages of the latter compared to other types of accumulators mainly concern their performance in terms of energy (5 and 2 times higher than lead-acid and Ni-MH batteries, respectively) and number of life cycles (over double that of other electrochemical systems). In the current state of the art, among the lithium batteries, the lithium-ion batteries occupy a leading place among the electrochemical storage systems and are currently used in almost all? of electronic and portable devices. The biggest problem with lithium-ion batteries? For? constituted by their safety since? they use electrolytes based on organic solvents which are by their nature intrinsically flammable, volatile and, consequently, potentially harmful for the user. Furthermore, an inadequate or insufficient dissipation of the heat developed during battery operation (especially in the event of overcharging) can result in a rapid increase in the internal temperature of the device with a consequent increase in internal pressure due to the volatility? of the organic solvents present therein up to generating an uncontrolled process of (exothermic) reactions called ?thermal runaway? what can? degenerate into fire and explosion of the battery itself. From there? it logically follows that the safety level of the device decreases almost exponentially with the increase in the size of the lithium-ion battery, thus hindering its widespread use in many sectors and applications such as for example automotive and stationary accumulation systems. Therefore, the need remains in the sector to identify, develop and implement new electrolytic systems capable of combining high performance in terms of ionic conduction and stability? electrochemical and which possess at the same time a high intrinsic safety and a reduced environmental impact. A promising approach in this area? represented by the replacement of organic solvents, used in the electrolytes of commercial batteries, with the so-called ionic liquids. Ionic liquids are salts made up of ions, having organic or inorganic origin and which, thanks to the presence of asymmetry in the structure of the constituent ions, have a melting point lower than room temperature (in some cases down to temperatures below -60 ?C ).

At the current state of the art, ionic liquids are proposed as a possible alternative system to organic solvents, which are instead flammable and volatile, in laboratory and industrial scale processes in order to increase safety in plants and devices and minimize need? recovery of solvents that could be released into the external environment. As far as lithium batteries are concerned, ionic liquids have been proposed as alternative electrolyte components instead of the traditional organic solvents used in commercial lithium-ion batteries in order to create electrical storage devices with high safety and performance. However, the ionic liquids currently used in the sector have various disadvantages. Firstly, the traditional synthesis processes of ionic liquids, both on a laboratory and industrial scale, commonly employ organic solvents (such as dichloromethane, acetonitrile, acetone, ethyl acetate) with high environmental impact, toxicity, volatility? and flammability and therefore must be recovered and recycled resulting in an increase in the final cost of the ionic liquid. Furthermore, the currently known ionic liquids, such as for example those described in US2015/0166587 or in US20170077557, while showing good stability? ends and electrochemistry, they can not for? be used in combination with cathodes operating at high potentials (for example of? the order of 5V) since? are subject to oxidation phenomena even in the initial cycles of the battery.

The present invention therefore solves the problems of the prior art by making available an ionic liquid, consisting of a tetra-alkylphosphonium cation and an asymmetric perfluoro-alkyl-sulfonyl-imide anion, characterized by possessing a high stability in the properties of the prior art. thermal and electrochemical as well as? a? high conductivity? ionic and that can? be effectively used as an electrolyte inside an electrochemical device operating at high voltages and temperatures.

SUMMARY OF THE INVENTION

The present invention relates to an ionic liquid consisting of a tetra-alkyl-phosphonium cation and a perfluoro-alkyl-sulfonyl-imide anion and represented by the general formula (I):

wherein R1, R2, R3 and R4 are selected, independently of each other, from the group consisting of: an alkyl or cycloalkyl group, linear or branched, with a number of carbon atoms between 2 and 6, preferably between 2 and 5;

R5 ? a linear fluorinated alkyl group and R6 ? a linear or branched fluorinated alkyl group, respectively of formula CnF2n+1 and CmF2m+1, wherein n ? an integer chosen between 1 or 2, preferably 1, and m ? an integer between 3 and 5, preferably between 3 and 4.

The present invention also relates to an electrolyte composition comprising at least one ionic liquid of formula (I) as described above, optionally in combination with at least one additive, preferably present in an amount between 10 and 30% by weight, preferably between 12 and 26% by weight with respect to the total weight of the composition, said additive being selected from the group consisting of: a lithium salt, an organic solvent, a polymer, a further ionic liquid having different chemical formula than the at least one ionic liquid having formula (I); or a combination thereof.

A further object of the present invention is a process, in aqueous solvent, for obtaining said ionic liquid and a use of said ionic liquid or of said electrolytic composition as electrolyte in an electrochemical device, preferably operating up to voltages higher than 5V and temperatures higher than 200 ?C. Finally, the present invention also relates to an electrochemical storage or energy conversion device comprising said ionic liquid or said electrolytic composition.

DESCRIPTION OF THE FIGURES

Figure 1 depicts the thermogram obtained from the thermogravimetric analysis of the ionic liquid obtained as per Example 1 in which the value of the percentage variation of the sample weight (solid line) and the value of the applied temperature (dashed line), are reported as a function of the heating time, as described in Example 1.4.

Figure 2 shows a graph of the conductivity trend? ionic of the ionic liquid obtained as in Example 1 as a function of the temperature, as described in Example 1.5.

Figure 3 shows a graph of the voltammetric measurements performed as described in Example 1.6 on the ionic liquid obtained as per Example 1, in which the value of the density? current (flowing through the cell) ? reported as a function of the applied voltage.

Figure 4 shows the charge/discharge profiles of a lithium battery prototype operating up to voltage values equal to 5V which uses the ionic liquid of the invention (Example 1) as electrolyte, as described in Example 1.7.

Figure 5 shows a snapshot of the performance comparison following the flammability tests. and volatility? as described in Example 1.8, of an organic electrolyte commonly used in commercial batteries (left panel a)) compared to the ionic liquid of the present invention obtained as per Example 1 (right panel b)).

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of the present invention the terms ?electrolyte? and ?electrolytic system? are used as perfectly interchangeable synonyms. For the purposes of the present invention, the terms ?battery? or ?electrochemical storage device? are used as perfectly interchangeable synonyms.

The present invention relates to an ionic liquid consisting of a tetra-alkyl-phosphonium cation and a perfluoro-alkyl-sulfonyl-imide anion and represented by the general formula (I):

Formula (I)

wherein R1, R2, R3 and R4 are selected, independently of each other, from the group consisting of: an alkyl or cycloalkyl group, linear or branched, with a number of carbon atoms between 2 and 6, preferably between 2 and 5;

R5 ? a linear fluorinated alkyl group and R6 ? a linear or branched fluorinated alkyl group, respectively of formula CnF2n+1 and CmF2m+1, wherein n ? an integer chosen between 1 or 2, preferably 1, and m ? an integer between 3 and 5, preferably between 3 and 4.

According to the formula (I), the tetra-alkyl-phosphonium cation of the ionic liquid of the present invention, can therefore be chosen between a symmetrical cation, ie having R1 = R2 = R3 = R4, or asymmetrical, ie a cation in which at least one group among R1, R2, R3 and R4 ? different.

According to a preferred embodiment of the invention, the groups R1, R2, R3 and R4 are selected, independently of each other, from the group consisting of: n-propyl, 2-propyl, n-butyl, 2 -methylpropyl, 1,1-dimethylethyl, cyclobutyl.

According to a particularly preferred embodiment of the invention, the tetra-alkyl-phosphonium cation of the ionic liquid of the present invention, according to formula (I), is? selected from the group of cations represented by the following formulas (C-1 ? C-16):

According to one embodiment, the tetra-alkyl-phosphonium cation of the ionic liquid of the present invention, according to the formula (I), is a symmetrical cation characterized by having R1 = R2 = R3 = R4.

According to a particularly preferred embodiment, the tetra-alkyl-phosphonium cation of the ionic liquid of the present invention, according to formula (I), ? represented by the formula C-1, i.e. where R1 = R2 = R3 = R4 = n-butyl:

As regards the perfluoro-alkyl-sulfonyl-imide anion of the ionic liquid of the present invention, according to the formula (I), said anion ? necessarily an asymmetric anion, i.e. characterized by R5 ? R6. According to a preferred embodiment of the invention, the integers n and m of formula (I) are selected from any of the following combinations:

n = 1 and m = 3 (that is: R5 = CF3 and R6 = C3F7);

n = 2 and m = 3 (that is: R5 = C2F5 and R6 = C3F7);

n = 1 and m = 4 (that is: R5 = CF3 and R6 = C4F9);

n = 2 and m = 4 (that is: R5 = C2F5 and R6 = C4F9);

n = 1 and m = 5 (that is: R5 = CF3 and R6 = C5F11);

n = 2 and m = 5 (that is: R5 = C2F5 and R6 = C5F11).

R6 ? a linear or branched, preferably linear, fluorinated alkyl group CmF2m+1.

According to a particularly preferred embodiment of the invention the perfluoro-alkyl-sulfonyl-imide anion of the ionic liquid of the present invention, according to the formula (I), ? chosen from the group of anions represented by the following formulas (A-1 ? A-6):

According to a particularly preferred embodiment, the perfluoro-alkyl-sulfonyl-imide anion of the ionic liquid of the present invention, according to formula (I), is? represented by the formula (A-4), i.e. where n = 1 and m = 4:

Without wanting to be bound to a specific theory, the Applicant has found that the asymmetry of the anion inside the ionic liquid turns out to be particularly advantageous since, allowing to lower the melting point of the ionic liquid at temperatures well below the environment, preferably at temperatures below 0 ?C, pi? preferably below ? 10 ?C, even more? preferably between 0 and ? 30 ?C, gives the ionic liquid itself a? High stability? thermal (as well as absence of ignition and absence of evaporation) and makes it particularly favorable for applications inside electrochemical devices which operate in conditions of high temperatures and voltages. According to a preferred embodiment of the invention, the ionic liquid ? consisting of a tetra-alkyl-phosphonium cation of formula (C-1) and a perfluoro-alkyl-sulfonyl-imide anion of formula (A-4):

According to one embodiment, the ionic liquid of the present invention is characterized by a purity content greater than or equal to 99.9%, preferably greater than or equal to 99.95%. The ionic liquid of the present invention therefore preferably has a content of impurities, such as by-products and/or halides, lower than 0.1%, preferably lower than 10 ppm (0.00001%). Without wanting to be bound to a specific theory, the Applicant has nevertheless found that said purity content does s? that the ionic liquid of the invention is particularly advantageous for applications in electrochemical devices, such as for example electrochemical storage devices, preferably lithium-ion batteries or capacitors, or energy conversion devices, preferably fuel cells.

According to a particularly preferred embodiment, the ionic liquid of the present invention ? characterized by a halide content of less than 0.000002% (2 ppm). Said extremely reduced halide content inside the ionic liquid of the invention turns out to be particularly advantageous since the possible presence of a higher halide content would lead to a decrease in stability. electrochemistry of the ionic liquid.

According to one embodiment of the invention, the ionic liquid ? furthermore characterized by a humidity content, measured by coulometric titration, lower than 2 ppm (0.000002%). Said reduced humidity content? turns out to be particularly advantageous for applications of the ionic liquid of the invention in electrochemical devices (preferably lithium-ion batteries or capacitors, or energy conversion devices, preferably fuel cells), since the use of an ionic liquid with a humidity content ? higher than the aforementioned value would affect the performance and safety of the electrochemical device itself.

The ionic liquid according to the present invention ? preferably characterized by possessing a stability? thermal degradation, measured by thermo-gravimetric analysis, up to temperatures higher than 200°C, preferably up to temperatures higher than 250°C.

Furthermore, without wanting to be tied to a specific theory, it is however, it can be argued that, precisely thanks to the specific combination of the tetra-alkyl-phosphonium cation and the perfluoro-alkyl-sulfonyl-imide anion according to formula (I), in particular thanks to the specific asymmetry of the anion and, possibly, of the cation, the ionic liquid according to the present invention is preferably characterized by a conductivity? ionic between 10<-4 > and 10<-2 >S?cm<-1>, measured in a temperature range between 30 and 120 ?C. Furthermore, the ionic liquid according to the present invention is preferably characterized by a stability? electrochemical oxidation, measured by voltammetry, up to voltage values higher than 5V, preferably between 5 and 6V, and, preferably, for a number of charge/discharge cycles equal to or greater than 5, preferably equal to or greater than 10, more preferably equal to or greater than 100, even more? preferably equal to or greater than 1000, said charge/discharge cycles being furthermore preferably characterized by having a regular profile, an indication of the absence of oxidation processes and/or electrochemical degradation of the ionic liquid.

Furthermore, preferably, the ionic liquid according to the present invention is characterized by a density value of current (which passes through the electrochemical cell used for the voltammetric measurement), before the beginning of the electrochemical decomposition, extremely low or lower than 4 ?A?cm<-2>, preferably lower than 2 ?A?cm<-2> , confirming the remarkable and optimal stability? of the ionic liquid of the present invention.

In light of the above, the ionic liquid of the present invention is particularly advantageous for applications in electrochemical devices, such as for example electrochemical storage devices, preferably lithium-ion batteries, or energy conversion devices, since it is possible to use it as an electrolyte. even in conditions of high temperatures and voltages without it undergoing undesirable degradation phenomena, also resulting stable for a prolonged number of charge/discharge cycles.

A further object of the present invention relates to an electrolytic composition comprising at least one ionic liquid of formula (I) as previously described, optionally in combination with at least one additive selected from the group consisting of:

- a lithium salt, preferably selected from LiPF6, LiN(SO2F)2, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiOSO2CF3, LiB(C2O4)2 or a combination thereof;

- an organic solvent, preferably an alkyl carbonate selected from dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, fluoroethylene carbonate, vinylene carbonate; ?butyrolactone, dimethoxyethane, or a combination thereof;

- a polymer, preferably selected from polyethylene oxide, vinylidene polydifluoride, vinylidene hexafluoropropylene polydifluoride, polyethylene oxide, polypropylene oxide, polymethylmethacrylate, polyacrylonitrile or a combination thereof;

- at least one further ionic liquid having a different chemical formula than the at least one ionic liquid of formula (I), said at least one further ionic liquid being preferably selected from: methylpropylpyrrolidinium bis(fluorosulphonyl)imide, butylmethylpyrrolidinium bis(fluorosulphonyl)imide, methylpropylpyrrolidinium bis (trifluoromethylsulfonyl)imide, butylmethylpyrrolidinium bis(trifluoromethylsulfonyl)imide, ethylmethylimidazolium bis(fluorosulfonyl)imide, ethylmethylimidazolium bis(trifluoromethylsulfonyl)imide, methylpropylpyrrolidinium trifluoromethysulfonate, butylmethylpyrrolidinium trifluoromethysulfonate, ethylmethylimidazolium trifluoromethylsulfonate;

or a combination thereof.

According to a preferred embodiment, said at least one additive ? present in a quantity between 10 and 30% by weight, preferably between 12 and 26% by weight with respect to the total weight of the electrolyte composition.

The present invention also relates to a process for obtaining the ionic liquid of formula (I) in accordance with what has been previously described.

This process includes the phases of:

a) making available a tetraalkyl phosphonium salt represented by the general formula (II) (also called, for the purposes of the present invention, "precursor?"):

Formula (II)

wherein R1, R2, R3 and R4 are selected, independently of each other, from the group consisting of: an alkyl or cycloalkyl group, linear or branched, with a number of carbon atoms between 2 and 6, preferably between 2 and 5; And

X ? a halogen, preferably selected from the group consisting of: chlorine, bromine, iodine;

b) treating said salt with a mixture comprising activated carbon and alumina (Al2O3) in an aqueous solvent, preferably distilled water, preferably in a weight ratio salt:activated carbon between 1:0.5 and 1:0.8, salt :alumina between 1:0.8 and 1:1 and aqueous solvent:(carbon alumina) between 2.5:0.5 and 1.9:1, preferably equal to 2:1, until an aqueous solution is obtained of the purified salt of Formula (II), preferably characterized by a purity content ? 99.9%, preferably ? 99.95%;

c) adding a perfluoro-alkyl-sulfonyl-imide acid represented by the general formula (II I):

R5 ? a linear fluorinated alkyl group and R6 ? a linear or branched fluorinated alkyl group, respectively of formula CnF2n+1 and CmF2m+1, wherein n ? an integer chosen between 1 or 2, preferably 1, and m ? an integer between 3 and 5, preferably between 3 and 4;

said acid being added in a percentage excess of between 1.5 and 3.5% by weight, preferably between 2 and 3% by weight with respect to the 1:1 stoichiometric ratio necessary for the metathesis reaction in aqueous solution for the formation of the ionic liquid of formula (I) as represented in the following diagram (Scheme 1).

Scheme 1: Metathesis reaction to obtain the ionic liquid of formula (I) according to the present invention.

According to one embodiment of the invention, the tetraalkylphosphonium salt represented by the general formula (II) has an initial purity content of less than or equal to 98%.

Step b) of the process according to the present invention therefore allows to obtain the tetraalkyl phosphonium salt represented by the general formula (II) with a purity content greater than or equal to 99.9%, preferably greater than or equal to 99.95%. , functional for obtaining the ionic liquid of the invention preferably having a purity content and halide content as previously described.

According to a preferred embodiment, before being used in step b), the activated carbon is subjected to a prepurification, preferably carried out with distilled water both at room temperature and at boiling temperature and repeated at least 3 times, more than 3 times. preferably for at least 5 times, in order to eliminate any possible foreign substance initially adsorbed on the surface of the activated carbon which could possibly contaminate the tetraalkyl phosphonium salt in the subsequent step b). Preferably, said activated carbon ? used in granular format with dimensions of average diameter of the granules between 2 and 4 mm.

According to an embodiment of the invention, in step c), the salt:activated carbon weight ratio ? between 1:0.6 and 1:0.7, salt:alumina between 1:0.9 and 1:1, and aqueous solvent:(carbon alumina) between 2.5:0.8 and 1.9: 0.9, preferably equal to 2:1.

Advantageously, the Applicant has found that the use of an aqueous solvent:(alumina activated carbon) ratio between 2.5:0.5 and 1.9:1, preferably between 2.5:0.8 and 1.9 :0.9, even more? preferably equal to 2:1, at the end of purification step b), it allows not only an effective purification and separation of the tetra alkyl phosphonium salt of general formula (II) from the impurities initially contained therein but also an almost complete recovery, preferably equal to 100%, of the tetraalkylphosphonium salt from the mixture comprising activated carbon and alumina in aqueous solvent, thus increasing? the final yield of the reaction.

According to a preferred embodiment, the process according to the present invention comprises, after step c), a further step d) of purifying the ionic liquid obtained in step c) by means of an extraction and decantation process comprising at least two consecutive washings with water distilled. Preferably, said decantation ? carried out at a temperature between 3 and 10 ?C, preferably between 4 and 7 ?C. Preferably said extraction and decantation process ? repeated at least 3 times, preferably at least 5 times.

The present invention also relates to the use of the ionic liquid or electrolyte composition as described above as an electrolyte in an electrochemical device. Preferably, said electrochemical device ? an electrochemical storage device, preferably a lithium-ion battery or a capacitor, or an energy conversion device, preferably a fuel cell.

According to a preferred embodiment of the invention, said electrochemical device ? operating up to voltages higher than 5V, preferably between 5 and 6V. Preferably, said electrochemical device ? also operating at temperatures higher than 200 ?C, preferably higher than 250 ?C.

A further object of the present invention is an electrochemical storage or energy conversion device, comprising: - a positive electrode;

- a negative electrode;

- a separator element disposed between the positive electrode and the negative electrode and in contact with them;

- the ionic liquid or electrolyte composition as described above.

Preferably, said positive electrode ? selected from the group consisting of: LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiNixMnyCo1-x-yO2, Li1+xNiyMnzO2, V2O5, LiFePO4, LiFePO4, LiCoPO4, sulphur, coal.

Preferably, said negative electrode ? chosen from the group consisting of: metallic lithium, graphite, silicon, Li4Ti5O12, TiO2, Sn, ZnFe2O4, Li4.4Sn, carbon.

According to an embodiment of the present invention, the ionic liquid or electrolyte composition is in liquid form at room temperature, preferably in a temperature range of 10 to 40°C. In this embodiment, the separator element ? preferably soaked with said ionic liquid or with said electrolytic composition.

Preferably said separator element ? a porous separator element, preferably a glass fiber or a polyolefin (Celgard?). According to a preferred embodiment of the invention, said separator element has a thickness of less than 300 m, preferably less than or equal to 100 m, more preferably less than or equal to 30 µm.

According to a particularly preferred embodiment, the electrochemical storage device according to the present invention ? able to deliver capacity? greater than or equal to 80% of the theoretical value, measured by battery cycler.

Preferably, said electrochemical storage device ? a lithium-ion battery or a capacitor.

Preferably, said energy conversion device ? a fuel cell.

According to a preferred embodiment of the invention, said electrochemical storage device or said energy conversion device ? operating up to voltages higher than 5V, preferably between 5 and 6V. Preferably, said electrochemical storage device ? also operating at temperatures higher than 200 ?C, preferably higher than 250 ?C.

EXAMPLES

Example 1 ? Synthesis of an ionic liquid of formula (I)

A tetra-butyl-phosphonium nonafluorobutylsulfonyltrifluoromethylsulfonyl-imide ionic liquid, consisting of a tetra-alkyl-phosphonium cation of formula (C-1) and a perfluoro-alkyl-sulfonyl-imide anion of formula (A-4) according to the general formula (THE)

? been synthesized through four distinct steps according to the process of the present invention:

- purification of the precursor (steps a) and b) of the previously described process);

- anion exchange reaction (metathesis reaction ? step c) of the previously described process);

- washing of the ionic liquid obtained (step d)) of the previously described process).

The chemicals used in the ionic liquid synthesis process according to the present invention are listed in Table 1. Deionized water ? was used as the only process solvent.

Table 1

The precursor tetra-butyl-phosphonium bromide (according to Formula II, previously described in step a) of the process according to the present invention), commercially available with a purity content equal to 98%, ? been purified before use, by treatment with absorbent materials. For this purpose, activated carbon and alumina have been used, which (thanks to the high development and chemical nature of their surface area) exhibit a good capacity? (especially coal) to retain substances (liquid and gaseous). However, since Does the surface of the activated carbon generally contain foreign substances (adsorbed) which can contaminate the precursor, the carbon (before it is used) ? been purified using distilled water. The activated carbon (in granular format with dimensions equal to 2-3 mm) and distilled water (20 ml H2O for each gram of carbon) were introduced into a beaker (Pyrex glass) and placed under magnetic stirring for ten minutes. The aqueous phase? was then removed by vacuum filtration and, subsequently, a further aliquot of distilled water (in a similar ratio 20 ml: 1 g)? been added to the coal.

The process ? been repeated five times in order to completely remove the impurities adsorbed on the carbon surface (verified by UV-Vis spectrophotometric measurements). Finally, the coal ? was placed in water, kept boiling, and kept there for an hour (facilitating the removal of the air contained within the pores of the activated carbon) in order to activate the surface and make it immediately available for the purification of the precursor. The aqueous phase? was finally removed by vacuum filtration. What is activated carbon? purified and alumina (Al2O3) were added, inside a glass reactor, to the precursor tetra-butyl-phosphonium bromide (precursor:coal weight ratio = 1:0.6; precursor:Al2O3 weight ratio = 1:0.9 ) and a volume of distilled water approximately equal to the total volume of solid materials (absorbent precursor) ? been added. What is the mixture? obtained ? was placed under magnetic stirring at room temperature for three hours. The aqueous phase (containing the purified precursor) ? subsequently separated by vacuum filtration from the absorbent materials which, subsequently, are washed in-situ (in the filtering system) with distilled water in order to also recover the mechanically retained precursor fraction. The ratio by weight of water: (coal and alumina), adopted in the washing phase of the absorbent materials, is equal to 2:1. Without wanting to bind to a specific theory, ? it has been found that this ratio is preferred as it allows the complete recovery of the trapped precursor fraction (allowing a yield in the purification process of tetra-butyl-phosphonium bromide equal to 100%) avoiding, at the same time, the extraction of impurities (coming by the forerunner himself) previously withheld. The aqueous solution, coming from the washing phase of the absorbent materials, is was then added to that coming from the initial filtration.

The anion exchange (metathesis) provides for the reaction (represented in Scheme 1) of the precursor (in the aqueous phase after the purification process) with the nonafluorobutylsulphonyl-trifluoromethylsulphonimide acid to obtain the ionic liquid according to the present invention.

Scheme 1:

Nonafluorobutylsulfonyl-trifluoromethylsulfonyl-imide acid is been added to the aqueous solution of tetra-butyl-phosphonium bromide in slight excess (3% by weight) with respect to the quantity? stoichiometric reaction in order to maximize the yield of the metathesis reaction. Without intending to be bound by a specific theory, the Applicant has found that an excess greater than 3.5% (in particular greater than 10%) would involve both a decrease in the yield and a greater loss of the compound containing the anion of interest. The formation of the ionic liquid of the invention (consisting of a tetra-alkyl-phosphonium cation of formula (C-1) and a perfluoro-alkyl-sulfonyl-imide anion of formula (A-4):

insoluble in water, ? snapshot: however the solution ? was placed under vigorous magnetic stirring for one hour at room temperature in order to bring the metathesis reaction to completion. Next, the solution? was left to decant in a climatic chamber (about 5 ?C) for about 30 minutes. A clear phase separation is observed: the ionic liquid (denser than water) is stratified as the lower phase while the upper one is ? It consists of an aqueous solution containing the by-product HBr and the excess acid L? nonafluorobutylsulfonyl-trifluoromethylsulfonyl-imide, which are highly soluble in water.

What is the ionic liquid? obtained contains, however, quantities, albeit very modest, of HBr and of L?acid nonafluorobutylsulfonyltrifluoromethylsulfonyl-imide. In particular, the presence of halides ? unwanted because? would it lead to a decrease in stability? electrochemistry of the ionic liquid. The removal of the aforementioned substances? was carried out by means of an extraction process, carried out by means of consecutive washings with distilled water (ratio by volume of ionic liquid : water = 1:1 at each washing stage). The mixture, consisting of ionic liquid and H2O, is subjected to vigorous stirring (for 15 minutes) and then left to decant at a temperature of about 5? C for 30 minutes. A clear phase separation is obtained between the ionic liquid (the underlying liquid phase) and the aqueous solution (containing the substances extracted from the ionic liquid), which ? been removed by vacuum aspiration. This procedure ? performed five times consecutively. The absence of bromides in the aqueous phase? was quickly verified, after the last wash, using a 0.1 N aqueous solution of silver nitrate (AgNO3) which, in the event of the presence of Br- ions, would give rise to a very insoluble precipitate of AgBr or would make the solution opalescent .

The ionic liquid obtained after the washing phases? was finally filtered under vacuum (using a polyamide filter having a porosity of less than 0.2 µm) in order to remove any residual particulate. Subsequently, the ionic liquid ? dried under vacuum to eliminate excess water (residual humidity) according to the following protocol: 2 hours at room temperature; 2 hours at 50 ?C; 24 hours at 120?C. Finally, the ionic liquid ? been sealed in glass containers in controlled atmosphere environments and subsequently characterized.

Example 1.1 ? X-ray fluorescence

X-ray fluorescence? was used to determine the content of halides, in the specific case bromides, in the ionic liquid obtained as per Example 1. The spectroscopic measurements were performed (at room temperature) by means of an energy dispersion EDX-720 Schimadzu spectrometer. As a source of X-rays, ? A rhodium plate was used which operates with an energy level of 15 kV (for sodium to titanium elements) or 50 kV (titanium to uranium). The ionic liquid obtained as per Example 1 ? been housed in a cylindrical polypropylene cuvette closed at one end? by a circular window (10 mm in diameter and 6 ?m thick) in Mylar polyester, which (blocked by a suitable ring) ? transparent to radiation. Special care? been placed in the introduction of the sample cos? prepared in order to cover the window in a compact and uniform way. Before making measurements on the ionic liquid, ? A calibration curve of the instrument was created using aqueous solutions of potassium bromide KBr at known concentration. The bromide concentration (expressed in mMol<-1>) of the KBr solutions? been reported as a function of the number of counts recorded during the execution of the measures, from which ? was a linear dependence observed (regression coefficient equal to 0.999 and absolute error equal to 0.005) which allowed to determine the bromide content in the ionic liquid which ? found to be less than 2ppm.

Example 1.2 ? UV-Vis spectrophotometry

UV-Vis spectrophotometry ? was used in order to determine the purity content of the ionic liquid obtained as per Example 1.

For the analysis, said ionic liquid ? been diluted in absolute ethanol in an ionic liquid:ethanol volumetric ratio of 1:5 so that? the absorbance values did not exceed the full scale value of the instrument. The measurements were carried out at room temperature using a Schimadzu UV-1800 spectrophotometer in a wavelength range between 700 and 190 nm. The sample to be investigated (ionic liquid diluted in ethanol) ? been housed inside a quartz cuvette (having an optical path of 10 mm). The reference cuvette (identical to the one containing the sample under examination) ? filled with pure absolute ethanol. The spectrophotometer? able to subtract, from the alcoholic solution containing the ionic liquid, the absorbance due to ethanol and no interference on the absorbance measurements ? due to ethanol. The purity content measured for the sample ? was equal to 99.9%.

Example 1.3 ? Coulometric titration

The ionic liquid obtained as per Example 1 ? been subjected to coulometric titration in order to determine the moisture content?. A quantity? of ionic liquid (previously weighed using an analytical balance) equal to 0.5 g ? been introduced into the titrant solution. The humidity value? ? calculated by the instrument after about 20 seconds from the introduction of the ionic liquid under investigation. The titration, performed using a titrator having sensitivity? equal to 1 ?g of water, ? been carried out inside an environment with humidity? controlled, having a humidity content? lower than 10 ppm in order to avoid contamination (of humidity) from the outside. In order to correctly determine the moisture content? (and avoid underestimation), the ionic liquid under test must be soluble in the titrant solution (based on methanol) and the water (in it) must be released easily from the sample (absence of strong interactions between ionic liquid and water) at in order to be fully available for titration. The moisture content? measured for the sample ? been less than 2 ppm (0.000002 %).

Example 1.4 ? Thermo-gravimetric analysis

The stability? temperature of the ionic liquid obtained as per Example 1 ? was investigated by thermo-gravimetric analysis. The sample investigated (3-4 mg) ? been housed in an alumina crucible. Before performing each measurement, the instrument ? been tared with the weight of the empty crucible (subsequently filled with the sample which was weighed using an external scale).

The ionic liquid, introduced into a calorimeter at room temperature, is been subjected to three successive heating periods (each lasting 3 hours) according to the following protocol:

i) rapid increase of the temperature from the ambient value up to 250 ?C (< 5 minutes);

ii) isothermal heating at 250 ?C for 3 hours;

iii) rapid temperature increase up to 275 ?C;

iv) isothermal heating at 275 ?C for 3 hours;

v) rapid increase in temperature up to 300 ?C;

vi) isothermal heating at 300 ?C for 3 hours.

The value of the change in the mass of the sample ? been recorded as a function of the heating time and the results of the thermogravimetric analysis are shown in Figure 1.

Example 1.5 ? conductivity ionic

The conductivity? of the ionic liquid obtained as per Example 1 ? been determined by means of sealed glass cells, equipped with two porous platinum electrodes, inside which ? the sample of ionic liquid under examination was housed (handled inside a controlled humidity environment). The conductivity tests, shown in Figure 2, were performed in the temperature range 0-120 ?C using a climatic chamber. The cells were placed inside the climatic chamber and kept at a temperature of 0 ?C for 24 hours. The conductivity? of the ionic liquid ? was measured by performing a heating scan at 1.5 ?C/hour: conduction values were measured at temperature intervals of 5 ?C. The value of the constant (K) of the conductivity cells? been determined by solutions of potassium chloride (KCl) at concentrations and conductivity? specification (?SP) notes, in order to obtain a calibration line. The conductivity value? (?) of each KCl solution? was measured with a conductivity meter. The cell constant (K) ? been calculated using the equation:

?SP = ?R

or on the basis of the slope of the straight line ?SP vs. ?. K values close to 1.00 cm<-1> were obtained (error bar equal to ? 0.05 cm<-1>).

Does the ionic liquid obtained as in Example 1 have excellent conductivity values? ionic (between 10<-4 > and 10<-2 >S?cm<-1>) in the temperature range between 30 and 120 ?C

Example 1.6 ? Stability test electrochemistry

The stability? electrochemistry of the ionic liquid obtained as per Example 1 ? was evaluated by voltammetric measurements performed on cells consisting of a nickel working electrode (having an area equal to approximately 1x1 cm<2 > and a thickness equal to 100 ?m), a glass fiber separator (having an area equal to approximately 1, 5x1.5 cm<2 >and thickness equal to 100 ?m) soaked with the ionic liquid and a metallic lithium counter-electrode (having an area equal to approximately 1x1 cm<2 >and thickness equal to 50 ?m). The components of the cell (working electrode, separator, counter electrode) were superimposed in the sequence described. The current collectors of the cell consist of a copper network having a thickness of 30 ?m). The cell? been housed inside a container made of multilayer material (polymer/aluminum) and vacuum sealed (after evacuation for one hour). The assembly of the cell ? been performed in a humid environment? controlled.

The voltammetric measurements were performed by linearly increasing (with a scanning speed equal to 5 mVs<-1>) the cell voltage from the open circuit value (OCV) towards potentials lower than the voltage. positive (anodic limit). As shown in Figure 3, where the value of the density? current (flowing through the cell) ? reported as a function of the applied voltage, the ionic liquid has shown stability? to oxidation up to voltage values equal to 6V. ? it is also important to consider how the value of density? of current through the cell, before the beginning of the electrochemical decomposition (at voltage values higher than 6V), ? extremely reduced (less than 4 ?A?cm<-2>) confirming the remarkable and optimal stability? of the ionic liquid of the present invention. The tests were repeated more? times in order to confirm the results obtained. For each test, clean electrodes and fresh ionic liquid samples were used. Voltammetric analyzes were performed at 40 ?C at least twice for each fresh sample, using a potentiostat/galvanostat.

Example 1.7 ? Charge/discharge test

The performances of the ionic liquid obtained as per Example 1 with electrodes (cathodes) operating in proximity? of values of voltages of 5V, and the properties? of stability to the oxidation processes in these conditions, have been validated in electrochemical cells made by coupling a metallic lithium anode (having an area equal to about 1x1 cm<2 > and a thickness equal to 50 ?m), a glass fiber separator (having an equal to approximately 1.5x1.5 cm<2 > and thickness equal to 100 ?m) soaked in the ionic liquid and a cathode (having an area equal to approximately 1x1 cm<2 > and thickness equal to 50 ?m) based on oxide of manganese-nickel (85%) deposited on an aluminum substrate having a thickness of 30 ?m. The current collectors of the cell consist of networks having a thickness equal to 30 ?m in copper (anode) and aluminum (cathode), respectively. The cell? been housed inside a container made of multilayer material (polymer/aluminum) and vacuum sealed (after evacuation for one hour). The assembly of the cell ? been performed in a humid environment? controlled. The charge-discharge cycles were performed at 40 ?C according to the following protocol:

i) application of a current equal to 0.1 mA cm<-2 > from the open circuit cell voltage value (OCV, about 3V) up to 4.8 V (cell charge);

ii) application of a current equal to 0.1 mA cm<-2 > from the value of 4.8 V up to 2.5 V (cell discharge);

iii) consecutive repetition of the aforementioned charge (i)/discharge (ii) cycles.

As shown in Figure 4, the value of the cell voltage ? been reported according to the quantity? of electric charge (expressed in mA h g<-1>) accumulated in the charge cycle (or released in the discharge one) per gram of cathodic active material (manganese-nickel oxide). The tests were performed at 40 ?C at least twice (in order to verify the reproducibility of the results) on identical electrochemical cells (always using ionic liquid and fresh electrodes) using a multiple cycler for batteries.

The results of the aforementioned charge/discharge tests demonstrate that the batteries (i.e. electrochemical storage devices) operating with the ionic liquid of the invention are capable of delivering higher than 80% of the theoretical value. Also, how? possible to deduce from the regular charge-discharge profiles, no oxidation and/or electrochemical degradation process is evident. In fact, in the event that such oxidation and/or electrochemical degradation processes occur, the cell voltage would not reach values of 5V (as instead shown in Figure 4 in the case of the present invention) instead presenting a continuous fluctuation around clearly inferior.

Example 1.8 ? Flammability test and volatility? (comparison)

The performance of the ionic liquid obtained as per Example 1 as regards the flammability? and the volatility? were tested and compared with those of an organic electrolyte commonly used in commercial batteries (1M LiPF6 in an ethylene carbonate-dimethyl carbonate mixture). A blowtorch? approached to two open containers (at room temperature and pressure) containing, respectively, the ionic liquid of the present invention and the organic electrolyte. As ? possible to appreciate from Figure 5, the ionic liquid of the present invention does not undergo the ignition phenomenon (right side container in Figure 4, panel b)), on the contrary of the organic electrolyte which, following the approach of the oxyhydrogen flame ( depicted only in the right-hand box b) of Figure 4) at its surface, it develops an open flame (indicated by the arrow in Figure 4, left-hand side container, box a)).

Claims (16)

CLAIMS 1. An ionic liquid consisting of a tetra-alkyl-phosphonium cation and a perfluoro-alkyl-sulfonyl-imide anion and represented by the general formula (I): Formula (I) wherein R1, R2, R3 and R4 are selected, independently of each other, from the group consisting of: an alkyl or cycloalkyl group, linear or branched, with a number of carbon atoms between 2 and 6, preferably between 2 and 5; R5 ? a linear fluorinated alkyl group and R6 ? a linear or branched fluorinated alkyl group, respectively of formula CnF2n+1 and CmF2m+1, wherein n ? an integer chosen between 1 or 2, preferably 1, and m ? an integer between 3 and 5, preferably between 3 and 4. 2. Ionic liquid according to claim 1, wherein R1, R2, R3 and R4 are independently selected from the group consisting of: npropyl, 2-propyl, n-butyl, 2-methylpropyl, 1,1-dimethylethyl, cyclobutyl. 3. Ionic liquid according to claim 1 or 2 wherein the integers n and m are selected from any of the following combinations: n = 1 and m = 3; n = 2 and m = 3; n = 1 and m = 4; n = 2 and m = 4; n = 1 and m = 5; n = 2 and m = 5; and in which R6 ? preferably a linear fluorinated alkyl group. 4. Ionic liquid according to any of the preceding claims wherein n? = 1 and m = 4, and where R6 ? preferably a linear fluorinated alkyl group. 5. Ionic liquid according to any one of the preceding claims wherein R1 = R2 = R3 = R4. 6. Ionic liquid according to any one of the preceding claims wherein R1 = R2 = R3 = R4 = n-butyl. 7. Ionic liquid according to any one of the preceding claims characterized by a purity content greater than or equal to 99.9%, preferably greater than or equal to 99.95%. 8. Ionic liquid according to any one of the preceding claims characterized by a halide content lower than 2 ppm. 9. Ionic liquid according to any one of the preceding claims characterized by having a stability? thermal degradation, measured by thermo-gravimetric analysis, up to temperatures above 200 ?C, preferably 250 ?C. 10. Ionic liquid according to any one of the preceding claims characterized by a conductivity? ionic between 10<-4 >e 10<-2 >S?cm<-1 >measured in a temperature range between 30 and 120 ?C and by a stability? electrochemical oxidation, measured by voltammetry, up to voltage values higher than 5V, preferably between 5 and 6V, and for a number of charge/discharge cycles equal to or greater than 5, preferably equal to or greater than 10, plus? preferably equal to or greater than 100, even more? preferably equal to or greater than 1000. 11. Electrolytic composition comprising at least one ionic liquid according to any of the preceding claims optionally in combination with at least one additive selected from the group consisting of: a lithium salt, preferably selected from LiPF6, LiN(SO2F)2, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiOSO2CF3, LiB(C2O4)2 or a combination thereof; an organic solvent, preferably an alkyl carbonate selected from dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, ethylene carbonate, fluoroethylene carbonate, vinylene carbonate; ?-butyrolactone, dimethoxyethane, or a combination thereof; a polymer, preferably selected from polyethylene oxide, vinylidene polydifluoride, vinylidene-hexafluoropropylene polydifluoride, polyethylene oxide, polypropylene oxide, polymethyl methacrylate, polyacrylonitrile or a combination thereof; at least one further ionic liquid having a different chemical formula than the at least one ionic liquid according to any one of the preceding claims, said at least one further ionic liquid being preferably selected from: methylpropylpyrrolidinium bis(fluorosulphonyl)imide, butylmethylpyrrolidinium bis(fluorosulphonyl)imide, methylpropylpyrrolidinium bis (trifluoromethylsulfonyl)imide, butylmethylpyrrolidinium bis(trifluoromethylsulfonyl)imide, ethylmethylimidazolium bis(fluorosulfonyl)imide, ethylmethylimidazolium bis(trifluoromethylsulfonyl)imide, methylpropylpyrrolidinium trifluoromethysulfonate, butylmethylpyrrolidinium trifluoromethysulfonate, ethylmethylimidazolium trifluoromethylsulfonate; or a combination thereof. 12. The electrolyte composition according to claim 11 wherein said at least one additive is present in a quantity between 10 and 30% by weight, preferably between 12 and 26% by weight with respect to the total weight of the composition. 13. Process for obtaining the ionic liquid according to any one of claims 1 to 10 comprising the steps of: a) making available a tetra-alkyl-phosphonium salt represented by the general formula (II): Formula (II) wherein R1, R2, R3 and R4 are selected, independently of each other, from the group consisting of: an alkyl or cycloalkyl group, linear or branched, with a number of carbon atoms between 2 and 6, preferably between 2 and 5; And X ? a halogen, preferably selected from the group consisting of: chlorine, bromine, iodine; b) treating said salt with a mixture comprising activated carbon and alumina (Al2O3) in an aqueous solvent, preferably distilled water, preferably in a weight ratio salt:activated carbon between 1:0.5 and 1:0.8, salt :alumina between 1:0.8 and 1:1 and aqueous solvent:(carbon alumina) between 2.5:0.5 and 1.9:1, preferably 2:1, until an aqueous solution of the salt is obtained purified, preferably characterized by a purity content greater than or equal to 99.9%, preferably greater than or equal to 99.95%; c) adding a perfluoro-alkyl-sulfonyl-imide acid represented by the general formula (III): Formula (III) R5 ? a linear fluorinated alkyl group and R6 ? a linear or branched fluorinated alkyl group, respectively of formula CnF2n+1 and CmF2m+1, wherein n ? an integer chosen between 1 or 2, preferably 1, and m ? an integer between 3 and 5, preferably between 3 and 4; said acid being added in a percentage excess of between 1.5 and 3.5% by weight, preferably between 2 and 3% by weight with respect to the 1:1 stoichiometric ratio necessary for the metathesis reaction in aqueous solution for the formation of the ionic liquid of formula (I). The use of the ionic liquid according to any of claims 1 to 10 or the electrolyte composition according to any of claims 11 to 12, as an electrolyte in an electrochemical device, preferably said electrochemical device being an electrochemical storage device, preferably a battery lithium-ion or a capacitor, or an energy conversion device, preferably a fuel cell. 15. The use according to claim 14 wherein said electrochemical device is operating up to voltages higher than 5V, preferably between 5 and 6V, and temperatures higher than 200°C, preferably higher than 250°C; . 16. Electrochemical storage or energy conversion device including: - a positive electrode, preferably selected from the group consisting of: LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiNixMnyCo1-x-yO2, Li1+xNiyMnzO2, V2O5, LiFePO4, LiFePO4, LiCoPO4, sulphur, carbon; - a negative electrode, preferably selected from the group consisting of: metallic lithium, graphite, silicon, Li4Ti5O12, TiO2, Sn, ZnFe2O4, Li4.4Sn, carbon; - a separator element disposed between the positive electrode and the negative electrode and in contact with them; - the ionic liquid according to any one of claims 1 to 10 or the electrolyte composition according to any one of claims 11 to 12; said separator element being preferably soaked with the ionic liquid or with the electrolytic composition; said separator element preferably being glass fiber or a polyolefin.