Electrolyte composition comprising a phosphazene family additive suitable for use in supercapacitors

Abstract

The invention relates to a composition comprising at least one nitrile solvent, at least one salt, and further comprising at least one phosphazene additive containing at least one fluorine atom. This composition is used as an electrolyte for energy storage devices.

Description

Title of the invention: ELECTROLYTIC COMPOSITION COMPRISING AN ADDITIVE OF THE FAMILY PHOSPHAZENES USABLE FOR SUPERCAPACITORS Technical field

[0001] The present invention relates to new electrolytic compositions resulting from the original association between an additive from the phosphazene family and a specific type of organic solvent which can be used as electrolytes for energy storage devices, these compositions allowing an improvement in the capacity of such devices even after a very large number of charge-discharge cycles. State of the prior art

[0002] There are three main types of energy storage devices for reversibly storing electrical energy: conventional dielectric capacitors, secondary electrochemical accumulators or generators, and supercapacitors.

[0003] From an operational point of view, supercapacitors operate on the principle of the electrochemical double layer, hence the Anglo-Saxon name sometimes encountered of "Electrochemical double layer capacitor" (also known by the abbreviation ED LC), or in other words on the principle of energy storage by distribution of ions coming from an electrolyte in the vicinity of the surface of two porous electrodes impregnated with electrolyte, separated by an insulating and porous membrane ensuring ionic conduction.

[0004] Thus, a basic cell of a supercapacitor can be summarized as follows: -a positive electrode; -a positive electrode / electrolyte interface forming a double electrical layer; - an insulating and porous membrane impregnated with said electrolyte; -a negative electrode; and -a negative electrode / electrolyte interface forming a double electrical layer.

[0005] Due to the existence of these two interfaces, each forming a double electrochemical layer, a supercapacitor can be considered schematically as the series association of two capacitors, one at the positive electrode and the other at the negative electrode, these two capacitors being created by applying a current to the terminals of the supercapacitor, which creates a charge zone at the two interfaces. electrode-electrolyte, the energy being stored electrostatically and not electrochemically.

[0006] Knowing that the energy stored and the power delivered by a supercapacitor are a function of the square of the applicable nominal voltage, it is therefore understood that the performance of a supercapacitor can be greatly improved by increasing the nominal voltage applicable to the terminals of the supercapacitor with the constraint of finding an electrolyte adapted to the maximum potential difference existing at the terminals of the supercapacitor (in other words, the electrolyte must remain stable in the electrochemical window offered by the supercapacitor) while presenting the characteristics expected for an electrolyte, namely: -good ionic conductivity; -a high temperature range; and -a relatively low viscosity so as to allow good mobility of the ions.

[0007] Currently, two types of electrolytes are mainly used in supercapacitors: -aqueous electrolytes, consisting of one or more salts dissolved in water; and -organic electrolytes, consisting of one or more salts dissolved in an organic solvent.

[0008] Concerning aqueous electrolytes, whether acidic (for example, a sulfuric acid solution) or basic (for example, a potash solution), the applicable nominal voltage range, for reasons of decomposition of water, is limited to approximately 1 V, which requires, in order to reach conventional voltages (for example, 12 V), complex arrangements of several supercapacitor units. In addition, the accessible temperature range is limited due to the low solubility of certain salts in an aqueous medium, which does not allow these electrolytes to be used at temperatures below -20°C.

[0009] Concerning organic electrolytes, they have a larger electrochemical stability window than aqueous electrolytes and therefore prove to be immediately more interesting for use in supercapacitors.

[0010] Also, based on this observation, the inventors set themselves the objective of proposing new non-aqueous compositions which can be used as electrolytes for supercapacitors and which allow an improvement in the energy storage properties and, more particularly, an improvement in the capacity even after a large number of charge and discharge cycles. Statement of the invention

[0011] The inventors were able to achieve the above-mentioned objective by developing a new composition comprising the combination of a specific organic solvent and a specific organic additive.

[0012] Thus, the invention relates to a composition, preferably non-aqueous, comprising at least one nitrile solvent, at least one salt and further comprising at least one additive from the phosphazene family comprising at least one fluorine atom.

[0013] The nitrile solvent(s) are solvents comprising at least one nitrile function -CN and more specifically can be: - a dinitrile solvent (i.e. comprising two nitrile functions -CN), such as adiponitrile or glutaronitrile; - a mononitrile solvent (i.e. comprising a single nitrile function -CN), such as a solvent corresponding to the following formula (I): R*-CN (I) in which R1 represents an alkyl group comprising from 1 to 5 carbon atoms (which covers linear or branched alkyl groups) and one or more hydrogen atoms of which may be substituted by fluorine (in which case the alkyl groups concerned may be described as fluoroalkyl groups).

[0014] A nitrile solvent which is particularly suitable in the context of the invention is acetonitrile of formula CH3-CN (also known by the abbreviation ACN), this solvent being particularly advantageous because it is not very viscous, dissolves salts very well and is very dissociating.

[0015] In addition: -it is very stable, both in oxidizing and reducing conditions; -it has a dipole moment, which allows the solvation of ions; and -it has both a high donor number and a high acceptor number, which means that it can behave as both a Lewis acid and a Lewis base.

[0016] As for the salt, it may be a lithium salt, a sodium salt, a potassium salt or even a salt comprising a cation containing at least one nitrogen atom, the positive charge of which is carried by said nitrogen atom (this cation being able to be described as an ammonium cation).

[0017] As regards lithium salts, mention may be made, without being exhaustive, of LiClO4, LiBF4, LiPF6, lithium Z?A(trifluoromethanesulfonyl)imide (known by the abbreviation LiTFSI), lithium Z?A(fluorosulfonyl)imide (known by the abbreviation LiFSI), lithium Z?A(oxalato)borate (known by the abbreviation LiBOB), Li A1C14, LiSO3CF3 and mixtures thereof.

[0018] As regards sodium salts, there may be mentioned, without being exhaustive, NaC104, NaBF4, NaPF6, sodium (trifluoromethanesulfonyl)imide, sodium bis(fluorosulfonyl)imide, sodium (oxalato)borate, NaAlCl4, NaSO3 CF3, NaSCN and mixtures thereof.

[0019] As regards potassium salts, mention may be made, without being exhaustive, of KC1O4, KBF4, KPF6, potassium Z?A(trifluoromethanesulfonyl)imide, potassium bis(fluorosulfonyl)imide, potassium Z?zXoxalato)borate, KA1C14, KSO3CF3, KSCN and mixtures thereof.

[0020] Finally, concerning salts comprising a cation containing at least one nitrogen atom, this can correspond to the following general formula (II): [Chem 1] R3 I R2---N+---R4 R5 (II) in which R2, R3, R4 and R5, which may be identical or different, represent an alkyl group comprising from 1 to 12 carbon atoms.

[0021] As an example of ammonium cations, the tetraethylammonium cation may be cited (namely, a cation of formula (II) mentioned above with R2 to R5 representing an ethyl group).

[0022] The salt comprising the above-mentioned type of cation further comprises an anion so as to neutralize the cation, this anion possibly being a compound comprising a heteroatom carrying a negative charge, for example, chosen from a nitrogen atom or a boron atom.

[0023] For example, it may be a perfluorinated borate compound, such as a tetrafluoroborate compound of the following formula (III): [Chem 2] F I. F----B----F F (HQ

[0024] A particularly advantageous salt is a salt resulting from the association of a cation of formula (II) mentioned above and a perfluorinated borate anion, such as, for example, tetraethylammonium tetrafluoroborate (also known by the abbreviation TEABF4).

[0025]

[0026]

[0027]

[0028] The salt(s) included in the compositions of the invention may be present at a concentration ranging from 0.1 mol / L to 2 mol / L. Furthermore, the compositions of the invention comprise at least one additive from the phosphazene family comprising at least one fluorine atom. It is specified that, by phosphazenes, we mean compounds comprising at least one group comprising a pentavalent phosphorus atom and a nitrogen atom linked directly by a double bond (that is, in other words a -P=N- group), these compounds can also be qualified as iminophosphorane compounds or phosphine imide compounds. More specifically, the additive may be a phosphazene compound comprising a cycle incorporating at least one group comprising a pentavalent phosphorus atom and a nitrogen atom directly linked by a double bond and, more particularly incorporating three of these groups, such a compound being able to correspond to the following formula (IV): [Chem 3]

[0029] in which R6 to R11 represent, independently of one another, a halogen atom (preferably a fluorine atom), a hydrogen atom, an alkyl group comprising from 1 to 12 carbon atoms or an isocyanate group -NCO. A particularly advantageous additive in the context of this invention is a compound of formula (IV), in which R6 to R11 represent a fluorine atom, this compound thus corresponding to the following specific formula (V): [Chem 4]

[0030] (V) this compound also being called hexafluorocyclotriphosphazene. Said additive(s) may be present in the composition in an amount of 0.01 to 10%, preferably 0.1% to 5% by mass relative to the total mass of the composition.

[0031] According to a particular embodiment of the invention, the electrolytic compositions may consist exclusively of at least one nitrile solvent, at least one salt and at least one additive from the phosphazene family comprising at least one fluorine atom.

[0032] Even more specific compositions in accordance with the invention are the following compositions: -a composition comprising acetonitrile, a tetraethylammonium tetrafluoroborate salt (for example, at 1 mol / L) and a hexafluorocyclotriphosphazene additive at a content of 1% by mass relative to the total mass of the composition; -a composition comprising acetonitrile, a tetraethylammonium tetrafluoroborate salt (for example, at 1 mol / L) and a hexafluorocyclotriphosphazene additive at a content of 2% by mass relative to the total mass of the composition; -a composition comprising acetonitrile, a tetraethylammonium tetrafluoroborate salt (for example, at 1 mol / L) and a hexafluorocyclotriphosphazene additive at a content of 5% by mass relative to the total mass of the composition; - a composition comprising acetonitrile, a tetraethylammonium tetrafluoroborate salt (for example, at 1 mol / L) and a hexafluorocyclotripho sphazene additive at a content of 10% by mass relative to the total mass of the composition.

[0033] These specific compositions are advantageously composed exclusively of the above-mentioned ingredients.

[0034] Thanks to the combination of the constituent ingredients of the compositions of the invention, the inventors were able to demonstrate a significant improvement in the performance of energy storage devices, in which the compositions of the invention are introduced as electrolytes. More specifically, when the compositions are introduced as electrolytes in a device of the supercapacitor type, it is possible to obtain an improvement in the discharge capacities of the supercapacitor, in particular after 10,000 galvanostatic cycling test cycles and an improvement in the capacities, in particular over 30 cyclic voltammetry test cycles, while improving the safety of the device, in particular the non-flammable nature in contact with a flame. Also, the compositions of the invention advantageously constitute electrolytic compositions.

[0035] The invention also relates to an energy storage device comprising at least one cell comprising two electrodes of opposite polarity (respectively, a positive electrode and a negative electrode) between which is arranged an electrolytic composition as defined above, said energy storage device being, advantageously, a supercapacitor.

[0036] Both electrodes may be based on activated carbon and may be of identical composition.

[0037] In addition to the presence of activated carbon, the electrodes may comprise one or more organic binders, which will contribute to ensuring the mechanical cohesion of said electrode.

[0038] These organic binders may be, in particular, polymeric binders comprising one or more polymers chosen from: *fluoropolymers, such as polytetrafluoroethylene (known as PTFE), polyvinylidene fluoride (known as PVDF), poly(vinylidene fluoride-co-hexafluoropropene) copolymer (known as PVDF-HFP), fluorinated ethylene-propylene copolymer (known as FEP), a copolymer resulting from the copolymerization of tetrafluoroethylene and perfluoroalkoxyvinyl ether (known as PFA); *polyimides; *polyacrylonitriles; *cellulosic polymers, such as carboxymethylcellulose; and *mixtures of these.

[0039] The electrodes may also comprise an electrically conductive carbon additive other than activated carbon, selected from carbon blacks, acetylene blacks, graphite, carbon nanotubes, carbon fibers and mixtures thereof, for example, vapor-formed carbon fibers (known by the abbreviation VGCF).

[0040] The electrodes may each be associated with an electrically conductive current collector, this current collector being able to be in the form of a metal strip affixed to one of the faces of said electrodes, this metal strip being able to be an aluminum strip.

[0041] Finally, the electrolytic composition disposed between the two electrodes advantageously impregnates a separator, which serves as an impregnation matrix for the composition and allows it to be held between the two electrodes. This separator may be in the form of a single-layer or multi-layer porous polymer film coated or not with a compound capable of improving the wettability of the electrolyte or may be in the form of a porous film, for example, consisting of an interlacing of polyolefin fibers (for example, polyethylene fibers) or an interlacing of glass fibers.

[0042] The invention will now be described with reference to the examples provided below, given for illustrative and non-limiting purposes.

[0043] Detailed description of particular embodiments EXAMPLE 1

[0044] This example illustrates the preparation of different electrolytes: one electrolyte not in accordance with the invention (Paragraph a) below) and four electrolytes in accordance with the invention (Paragraphs b) to e) below).

[0045] a) Electrolyte not in accordance with the invention (called Electrolyte E o}

[0046] The electrolyte Eo is prepared in a glove box by dissolving 1 mol / L of tetraethylammonium tetrafluoroborate salt in an acetonitrile solvent (10 mL).

[0047] b) Electrolyte in accordance with the invention (called Electrolyte E À )

[0048] Electrolyte Ei is prepared from electrolyte Eo, to which hexafluorocyclotriphosphazene (0.1511 g) is added, whereby the content of hexafluorocyclotriphosphazene is 1% by mass relative to the total mass of the electrolyte.

[0049] c) Electrolyte in accordance with the invention (called Electrolyte E 21

[0050] Electrolyte E2 is prepared from electrolyte Eo, to which hexafluorocyclotriphosphazene (0.3053 g) is added, whereby the content of hexafluorocyclotriphosphazene is 2% by mass relative to the total mass of the electrolyte.

[0051] d) Electrolyte in accordance with the invention (called Electrolyte E 3 )

[0052] Electrolyte E3 is prepared from electrolyte Eo, to which hexafluorocyclotriphosphazene (0.7873 g) is added, whereby the content of hexafluorocyclotriphosphazene is 5% by mass relative to the total mass of the electrolyte.

[0053] e) Electrolyte in accordance with the invention (called Electrolyte E 4 )

[0054] Electrolyte E4 is prepared from electrolyte Eo, to which hexafluorocyclotriphosphazene (1.662 g) is added, whereby the content of hexafluorocyclotriphosphazene is 10% by mass relative to the total mass of the electrolyte. EXAMPLE 2

[0055] This example illustrates the preparation of a supercapacitor.

[0056] The assembly is carried out in a glove box by successively superimposing a lower stainless steel cover provided with a polypropylene seal, a stainless steel shim, a commercial JCK activated carbon electrode disc (the current collector, which is an aluminum sheet being in contact with the stainless steel shim and the face coated with activated carbon in the upper part), a commercial Freudenberg polyolefin fiber separator soaked in 150 μL of electrolyte (namely, the electrolyte Eo, Eb E2, E3 and E4), an activated carbon electrode disc (the face coated with activated carbon being in contact with the separator and the current collector, which is an aluminum sheet at the top), a stainless steel shim, a spring and a stainless steel top cover.

[0057] The assembly is then crimped in a glove box. EXAMPLE 3

[0058] In this example, cyclic voltammetry tests are carried out with supercapacitors prepared in accordance with Example 2 with respectively the electrolyte Eo (not in accordance with the invention) and the electrolyte Ei.

[0059] These tests consist of subjecting the supercapacitors to a voltage sweep at a speed of 100 mV / s between 0 V and 2.7 V, repeated thirty times at a temperature of 20°C. The measurement of the current during these sweeps makes it possible to calculate, at the end of the test, the specific capacity of an electrode in F / g.

[0060] The results of the specific capacities in F / g are reported in the table below. [Table 1] Electrolyte EiS Electrolyte Es ■Cycle 1 112.3 N Cycle 10 110.3 124.2 Cycle 20 110.4 124.3 Cycle 30 110.5 124.1

[0061] It clearly appears that the electrolyte Ei in accordance with the invention allows the supercapacitor, through this cyclic voltammetry test, to have an improved capacity of 12% (at the thirtieth cycle) compared to the results obtained with the electrolyte Eone containing no additive. EXAMPLE 4

[0062] In this example, galvanostatic cycling tests are carried out with supercapacitors prepared in accordance with Example 2 with respectively the electrolyte Eo (not in accordance with the invention) and the electrolyte Ep.

[0063] These tests consist of imposing a current density of 1 A / g on the supercapacitors, charging them alternately up to 2.7 V then discharging them down to 0 V (one charge followed by one discharge corresponding to one cycle), for a number of 10,000 cycles at a temperature of 20°C. Measuring the discharge time of each cycle makes it possible to calculate, at the end of the test, the capacity of the supercapacitor in F / g.

[0064] The results of the capacities in F / g are reported in the table below: [Table 2] Electrolyte Electrolyte Cycle 1 32.8 33.3 Cycle. 500 31.3 31.8 Cycle 1000 30.9 31.3 Cycle 2000 30.3 30.7 Cycle 3000 29.8 30.2 Cycle 4000 29.2 29.6 Cycle 5000 28.7 29.1 Cycle 6000 28 28.5 Cycle 7000 27r3 28.0 Cycle 8000 26.7 27.5 Cycle 9000 26.2 27.0 Cycle 10000 25.8 26.6

[0065] It clearly appears that the electrolyte Ei in accordance with the invention allows the supercapacitor, through this galvanostatic cycling test, to have an improved capacity of 3% (at the 10,000th cycle) compared to the results obtained with the electrolyte Eo. In addition, during cycling, the reduction in the capacities of the supercapacitor containing the electrolyte Ei is less high, thus predicting a better operating life. EXAMPLE 5

[0066] In this example, flammability tests are carried out on the electrolytes, the preparation of which is set out in Example 1.

[0067] To do this, 1 mL of electrolyte is placed in an aluminum dish. The electrolyte is spread by stirring so as to completely cover the bottom of the dish. Immediately after this operation, a flame is brought into contact with the electrolyte and a stopwatch is used to determine how long it takes for the electrolyte to self-extinguish, the start of the time measurement being set as soon as a flame is visible and the stopwatch is stopped as soon as the flame is extinguished.

[0068] The results of the self-extinction times (in seconds) of the electrolytes are reported in the table below. [Tables 3] Electrolyte Self-extinguishing time in seconds Eô 29 Ei 2. Ea Q e5 0 E* G

[0069] It is clear that the addition of the additive of the invention to an Eo type electrolyte has a safety effect. From the use of 1% of additive (Electrolyte EJ) the combustion time is reduced by 93% compared to an electrolyte containing no additive (Eo). In addition, as soon as a concentration greater than or equal to 2% of additive is formulated in an Eo type electrolyte, no ignition of the electrolyte is observed upon contact with a flame.

Claims

Claims

1. Supercapacitor comprising at least one cell comprising two electrodes of opposite polarity (respectively a positive electrode and a negative electrode) based on activated carbon between which is arranged an electrolytic composition comprising at least one nitrile solvent, at least one salt and further comprising at least one additive from the phosphazene family corresponding to the following formula (V): [Chem 4] ? F w ,p ^li C' NF 'F (V)

2. Supercapacitor according to claim 1, in which the at least one nitrile solvent corresponds to the following formula (I): R*-CN (I) in which R1 represents an alkyl group comprising from 1 to 5 carbon atoms and one or more hydrogen atoms of which may be substituted by fluorine.

3. A supercapacitor according to claim 1 or 2, wherein the nitrile solvent is acetonitrile.

4. A supercapacitor according to any preceding claim, wherein the salt is a lithium salt, a sodium salt, a potassium salt or a salt comprising a cation having at least one nitrogen atom, the positive charge of which is carried by said nitrogen atom.

5. A supercapacitor according to any preceding claim, wherein the salt is a salt comprising a cation having at least one nitrogen atom, the positive charge of which is carried by said nitrogen atom.

6. A supercapacitor according to claim 5, wherein the cation corresponds to the following formula (II): [Chem 1] R3 R2--N*---R4 (II) in which R2, R3, R4 and R5, identical or different, represent an alkyl group comprising from 1 to 12 carbon atoms.

7. A supercapacitor according to claim 5 or 6, wherein the cation is a tetraethylammonium cation.

8. A supercapacitor according to claim 5, wherein the salt comprises an anion, which is a compound comprising a heteroatom carrying a negative charge, which heteroatom is selected from a nitrogen atom or a boron atom.

9. The supercapacitor of claim 8, wherein the anion is a perfluorinated borate compound.

10. A supercapacitor according to claim 8 or 9, wherein the anion is the tetrafluoroborate compound of the following formula (III): [Chem 2] F F---B”---FF (III)

11. A supercapacitor according to any preceding claim, wherein the salt is tetraethylammonium tetrafluoroborate.

12. A supercapacitor according to any preceding claim, wherein the salt(s) are present at a concentration ranging from 0.1 mol / L to 2 mol / L.

13. A supercapacitor according to any preceding claim, wherein the additive(s) are present in an amount of 0.01 to 10% by mass relative to the total mass of the composition.

14. A supercapacitor according to any preceding claim, wherein the electrolytic composition is: -a composition comprising acetonitrile, a tetraethylammonium tetrafluoroborate salt and an additive hexafluorocyclotriphosphazene at a content of 1% in mass relative to the total mass of the composition; - a composition comprising acetonitrile, a tetraethylammonium tetrafluoroborate salt and a hexafluorocyclotriphosphazene additive at a content of 2% by mass relative to the total mass of the composition; - a composition comprising acetonitrile, a tetraethylammonium tetrafluoroborate salt and a hexafluorocyclotriphosphazene additive at a content of 5% by mass relative to the total mass of the composition; or - a composition comprising acetonitrile, a tetraethylammonium tetrafluoroborate salt and a hexafluorocyclotriphosphazene additive at a content of 10% by mass relative to the total mass of the composition.