Liquid electrolyte composition comprising a salt, electrochemical cell comprising the electrolyte composition, salt, and use of the salt in the electrochemical cell

Abstract

The invention relates to a liquid electrolyte composition comprising a salt of formula (I) which has an anionic complex comprising three bidentate ligands. The complex comprises, as the central ion Z, tantalum or niobium ions. The invention also relates to an electrochemical cell comprising the liquid electrolyte composition, and to the salt. The subject matter of the invention is also the use of the salt in an electrochemical cell.

Description

[0001] 23-2034 Liquid electrolyte composition with a salt, electrochemical cell with the electrolyte composition, salt and use of the salt in the electrochemical cell The present invention relates to an electrolyte composition with a salt, an electrochemical cell with the electrolyte composition, a salt and a use of the salt in the electrochemical cell. Electrochemical cells are of great importance in many technical fields. In particular, electrochemical cells are often used for applications in which low voltages are required, such as for operating laptops or mobile phones. One advantage of electrochemical cells is that many individual cells can be connected to one another. For example, cells connected in series can deliver a high voltage, while connecting the cells in parallel results in a high nominal capacity.Such interconnections result in batteries with higher energy. Such battery systems are also suitable for high-voltage applications and can, for example, enable the electric drive of vehicles or be used for stationary energy storage. In the following, the term "electrochemical cell" is used synonymously for all terms commonly used in the art for rechargeable galvanic elements, such as cell, battery, battery cell, accumulator, battery accumulator and secondary battery. An electrochemical cell is capable of providing electrons to an external circuit during the discharge process. Conversely, an electrochemical cell can be charged during the charging process by supplying electrons via an external circuit. An electrochemical cell has at least two different electrodes: a positive electrode (cathode) and a negative electrode (anode).Both electrodes are in contact with an electrolyte composition. The most commonly used electrochemical cell is the lithium-ion cell, also called a lithium-ion battery. Lithium-ion cells known from the prior art have a composite anode, which very often comprises a carbon-based anode active material, typically graphitic carbon, deposited on a metallic copper carrier foil. The cathode typically comprises metallic aluminum coated with a cathode active material, such as a layered oxide. The layered oxide can be, for example, LiCoO2 or LiNi. 1 / 3 Mn 1 / 3 Co 1 / 3O2 is used, which is coated onto a rolled aluminum carrier foil. The electrolyte composition plays a key role in the safety and performance of an electrochemical cell. This ensures charge balance between the cathode and anode during charging and discharging. The necessary current flow is achieved through the ion transport of a conducting salt in the electrolyte composition. In lithium-ion cells, the conducting salt is a lithium conducting salt, and lithium ions serve as the ions that transport the current. Therefore, it is necessary to select a suitable conducting salt that can be dissolved in the electrolyte composition to a sufficient extent and has suitable ionic conductivity to maintain effective charge balance during operation. The most common conducting salt in lithium-ion cells is lithium hexafluorophosphate (LiPF6).In addition to the lithium conducting salt, electrolyte compositions contain a solvent that enables dissociation of the conducting salt and sufficient mobility of the lithium ions. Liquid organic solvents consisting of a selection of linear and cyclic dialkyl carbonates are known from the state of the art. Mixtures of ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), propylene carbonate (PC), and ethyl methyl carbonate (EMC) are typically used. It is important to note that each solvent has a specific stability range for the cell voltage, also known as a "voltage window." Within this voltage window, the electrochemical cell can operate stably. As the cell voltage approaches the upper voltage limit, electrochemical oxidation of the components of the electrolyte composition occurs. At the lower end of the voltage window, however, reductive processes occur.Both redox reactions are unwanted, reduce the performance and reliability of the cell, and in the worst case, lead to its failure. The processes in question are particularly relevant for the deep discharge and overcharging of a lithium-ion cell. Lithium-ion cells with state-of-the-art organic electrolyte compositions tend to gassing during charging and discharging processes. "Gassing" refers to the electrochemical decomposition of the electrolyte components into volatile and gaseous compounds due to the use of an excessively high cell voltage. Gassing reduces the electrolyte content and leads to the formation of undesirable decomposition products, resulting in a shorter service life and lower performance of the lithium-ion cell.To enable the cell to operate over the widest possible potential range, fluorinated solvents or additives are added to the electrolyte compositions in the prior art. Fluorinated solvents such as fluoroethylene carbonate (FEC) are chemically inert and electrochemically stable with respect to the operating voltages of lithium-ion cells. A widespread disadvantage of fluorinated electrolytes is that, in the event of a thermal failure in the cell, they can lead to increased heat release and the formation and emission of harmful gases such as hydrogen fluoride (HF). Due to these disadvantages, lithium-ion cells have a variety of regulation and control mechanisms to keep the cells in an optimal voltage range for the respective solvent during operation, thus stabilizing the electrolyte composition. Various approaches to stable electrolyte compositions are known in the prior art.EP 1 689 756 B1 describes a process for the preparation of weakly coordinating anions of the formula X(OR. F )m, wherein X is selected from the group consisting of B, Al, Ga, In, P, As and Sb, m is 3 or 5 and R Frepresents a straight-chain or branched-chain, partially or fully fluorinated alkyl or aryl radical. The weakly coordinating anions form salts with monovalent or divalent cations, preferably with alkali metal ions. Due to their chemical stability, particularly of the anion, the disclosed salts have been proposed, among other things, for use as inert lithium conducting salts in lithium-ion batteries. However, an electrolyte composition containing the weakly coordinating anions for use in lithium-ion batteries has not been demonstrated. Electrochemical properties of weakly coordinating anions, however, were investigated in the following article: "Fluorinated Materials for Energy Conversion" T. Nakajima and H. Groult (Editors), 195, 2005, Elsevier.It was found that the conductivity of lithium conducting salts with weakly coordinating anions in carbonate-containing solvents does not approach the conductivity of reference conducting salts such as lithium hexafluorophosphate. In addition to the selection of a chemically inert conducting salt, the stability of battery cells can also be increased by selecting a suitable solvent. Sulfur dioxide (SO2) is currently being discussed as an inorganic solvent in electrolyte compositions. Sulfur dioxide-based electrolyte compositions, in particular, exhibit increased ionic conductivity and thus enable the operation of battery cells at high discharge currents without negatively affecting cell stability. Furthermore, electrolyte compositions based on sulfur dioxide are characterized by high energy density, a low self-discharge rate, and limited overcharging and deep discharge.A disadvantage of sulfur dioxide is that it only insufficiently dissolves many lithium conducting salts that are readily soluble in organic solvents. Therefore, for example, the widely used lithium conducting salt lithium hexafluorophosphate cannot be used for electrolyte compositions containing sulfur dioxide. EP 1201004 B1 discloses a rechargeable electrochemical cell with a sulfur dioxide-based electrolyte. Sulfur dioxide is not added as an additive, but rather represents the main component of the electrolyte composition. Therefore, it is intended to at least partially ensure the mobility of the ions of the conducting salt, which effect charge transport between the electrodes.In the proposed cells, lithium tetrachloroaluminate is used as a lithium-containing conducting salt in combination with a cathode active material made of a metal oxide, in particular an intercalation compound such as lithium cobalt oxide (LiCoO2). By adding a salt additive, for example, an alkali halide such as lithium fluoride, sodium chloride, or lithium chloride, to the sulfur dioxide-containing electrolyte composition, functioning and rechargeable cells were obtained. EP 2534719 B1 shows a rechargeable lithium battery cell with a sulfur dioxide-based electrolyte in combination with lithium iron phosphate as the cathode active material. Lithium tetrachloroaluminate was used as the preferred conducting salt in the electrolyte composition. Tests with cells based on these components demonstrated high electrochemical stability of the cells.WO 2021 / 019042 A1 describes rechargeable battery cells with an active metal, a layered oxide as the cathode active material, and a sulfur dioxide-containing electrolyte. Due to the poor solubility of many common lithium conducting salts in sulfur dioxide, a conducting salt of the formula M was used in the cells. + [Z(OR)4]- is used, where M is a metal selected from the group consisting of alkali metal, alkaline earth metal, and a metal from group 12 of the Periodic Table, and R is a hydrocarbon radical. The alkoxy groups -OR are each monovalently bonded to the central atom Z, which can be aluminum or boron. In a preferred embodiment, the cells contain a perfluorinated conducting salt of the formula Li +[Al(OC(CF3)3)4] cells consisting of the described components demonstrate stable electrochemical performance in experimental studies. Furthermore, the conducting salts, especially the perfluorinated anion, exhibit surprising hydrolytic stability. Furthermore, the electrolytes are said to be oxidation-stable up to an upper potential of 5.0 V. It has also been shown that cells with the disclosed electrolytes can be discharged or charged at low temperatures, down to −41°C. However, no measurements of the electrochemical performance at high temperatures were performed. The thermal stability of perfluorinated lithium aluminates at high temperatures was investigated in one of the publications by Malinowski et al. (Dalton Trans., 2020, 49, 7766). In the study, the authors characterized various properties of [Al(OC(CF3)3)4] salts, including the thermal stability of the lithium derivative.Thermogravimetric studies showed that the compound Li[Al(OC(CF3)3)4] exhibits a mass loss already at 105°C, which is caused by the incipient decomposition of the fluorinated anion. To date, only conducting salts for sulfur dioxide-based electrolytes have been discussed, whose anions form a chemical complex. Mono-, bidentate, or polydentate ligands can be used to form these complexes. Bidentate or polydentate ligands are generally also known as chelate ligands, and the complexes composed of them are known as chelate complexes. For example, EP 4037056 A1 describes an SO2-based electrolyte for a rechargeable battery cell. The electrolyte contains at least one conducting salt, which can have at least one substituent configured as a chelate ligand. The chelate ligands coordinate to a central ion, which is either boron or aluminum.Further electrolyte salts with chelate ligands in electrolyte compositions for electrochemical cells are known from applications DE102021118811.3 and PCT / EP2022 / 069660. Chelate complexes are chemically more stable than their monovalent derivatives. The bonds between the chelate ligand and the central ion are difficult to break, which is why chelate complexes are chemically inert to external chemical and physical influences. Due to these properties, chelate complexes, especially the salts composed of them, are considered both temperature- and hydrolysis-resistant. Furthermore, electrolyte salts consisting of certain chelate complexes exhibit greater oxidation stability and can be safely operated at higher cell voltages.For the use of such electrolyte salts in commercially available batteries, especially in batteries used as power sources for electric vehicles, it is necessary that the electrolyte salts meet certain process-related and performance-related criteria in addition to the safety-related requirements discussed above. Firstly, electrolyte salts must consist of readily available and inexpensive ligands. Otherwise, the batteries produced from them are too expensive to manufacture and cannot be used economically. Secondly, electrolyte salts must exhibit good solubility in sulfur dioxide as a solvent, since salts with higher solubility are easier to process.Another criterion is the sufficient conductivity of the electrolyte salts in sulfur dioxide to ensure sufficient electrical efficiency in a battery made from them. Therefore, efforts are being made to develop new electrolyte salts that meet the above-mentioned requirements. The invention is therefore based on the object of providing an electrolyte composition for an electrochemical cell, and in particular rechargeable batteries, which meets the above-mentioned requirements and can be operated safely at various operating voltages. This object is achieved according to the invention by a liquid electrolyte composition for an electrochemical cell according to claim 1. Advantageous embodiments of the electrolyte composition according to the invention are specified in the subclaims, which can optionally be combined with one another.According to the invention, this object is achieved by a liquid electrolyte composition for an electrochemical cell. The electrolyte composition comprises the following components: (A) sulfur dioxide; (B) at least one salt, wherein the salt contains an anionic complex with three bidentate ligands and the salt has the following formula (I). Where M represents a metal cation selected from the group consisting of alkali metals, alkaline earth metals, and metals of group 12 of the periodic table. m represents an integer from 1 to 2. Z represents the element niobium (Nb) or tantalum (Ta) and represents the central ion of the anionic complex. L 1 , L 2 and L 3each independently represent a perfluorinated aliphatic or aromatic bridging radical. The bridging radical forms a five- to eight-membered ring with the central ion Z and two oxygen atoms bonded to the central ion Z and the bridging radical, and wherein the ring contains a sequence of 2 to 5 carbon atoms, optionally interrupted by an oxygen atom. The salts proposed according to the invention have an anion which contains three bidentate ligands. For the purposes of the invention, a bidentate ligand is understood to be a molecule which has at least two oxygen atoms and which binds to the central ion Z via the at least two oxygen atoms. Multidentate ligands which have a different dentateness, such as tridentate, tetradentate, pentadentate or hexadentate, are not within the meaning of the invention.Bidentate ligands are generally also known as chelate ligands, and the complexes composed thereof are known as chelate complexes. The anion of the salt of formula (I) is thus a chelate complex. Within the scope of this invention, chelate complexes and the salts formed therefrom have various advantages over complexes prepared from monovalent ligands and the salts formed therefrom. Chelate complexes are chemically more stable than their monovalent derivatives. The bonds between the chelate ligand and the central ion are difficult to break, which is why the chelate complexes according to the invention are chemically inert to external chemical and physical influences. According to the invention, a chelate complex represents the anion of the at least one salt of formula (I), wherein the salt serves as the conductive salt of the electrolyte composition. The electrolyte composition thus enables charge equalization between the two electrodes with which it is in contact.A further advantage is the high affinity of the chelate ligand to the central ion Nb or Ta. The chelate complexes used according to the invention are chemically and electrochemically stable compounds that, due to the strong coordinating properties of the ligand to the central ion, exhibit a low affinity for binding to positively charged ions. The chelate complexes themselves are therefore weakly coordinating anions. Therefore, the conducting salt in the electrolyte composition can dissociate almost completely without reverting to the original salt, forming ions with high mobility and correspondingly high ionic conductivity in solution. This, in turn, increases the electrochemical performance of the electrochemical cell. The chelate ligands to the central ion preferably have a binding energy of 700–800 kJ / mol per central ion-ligand bond, resulting in particularly stable anions.For example, a Nb-O bond (oxygen belonging to the ligand) can achieve a bond energy of 726 kJ / mol per central ion-ligand bond. A Ta-O bond can achieve a bond energy of 761 kJ / mol. As a result, chelate ligands form more stable compounds with the central ions Nb or Ta than with other central ions, which exhibit a comparatively weaker central ion-ligand bond. Bond energy values ​​can be found in the specialist literature. Due to these properties, the chelate complexes used according to the invention, in particular the salts composed thereof, are both temperature- and hydrolysis-resistant. According to the invention, the described salts dissolve sufficiently in liquid sulfur dioxide, which represents the inorganic solvent of the electrolyte composition.Within the scope of the invention, sulfur dioxide is not only included as an additive in low concentrations in the electrolyte composition, but is present in such a quantity that, as a solvent, it can ensure the mobility of the ions of the conducting salt. Sulfur dioxide is gaseous at room temperature under atmospheric pressure and forms stable liquid solvate complexes with lithium conducting salts, which have a noticeably lower vapor pressure than sulfur dioxide as a pure substance. The gaseous sulfur dioxide is thus bound in liquid form and can be handled safely and comparatively easily. A particular advantage is the non-flammability of sulfur dioxide itself and the solvate complexes, which increases the operational reliability of electrolyte compositions based on such solvate complexes and of the cells manufactured using the electrolyte composition.The described salt with the chelate complex of formula (I) is non-flammable. Therefore, the electrolyte compositions according to the invention are also non-flammable and enable safe operation of an electrochemical cell comprising the disclosed components of the electrolyte composition. Should sulfur dioxide escape from the cell due to mechanical damage, it cannot ignite outside the cell. Furthermore, the electrolyte composition according to the invention is also cost-effective compared to conventional organic electrolytes. The increased temperature stability and hydrolysis resistance enable direct and almost complete recycling of the electrolyte composition from used batteries without increased effort. Hydrothermal processes under high pressure and at high temperatures are usually used to recycle used batteries.Conventional electrolyte compositions are usually not hydrolysis-resistant and therefore require other processing methods. For this purpose, the electrolyte compositions are extracted from batteries in a complex process, for example, by flushing the cells with supercritical carbon dioxide. Newer electrolyte formulations based on aluminate, borate, or gallate salts, as described in the prior art, are generally not sufficiently temperature-stable. The electrolyte composition proposed here is temperature-stable and hydrolysis-resistant and can therefore be cost-effectively recycled directly from the electrochemical cells using water-based extraction methods. Due to the water solubility of the proposed components, the electrolyte composition proposed here has a high recycling potential with a high recycling rate.Recycling reduces both the consumption of primary raw materials and the energy required to produce a freshly prepared electrolyte composition, and thus also the carbon dioxide emissions caused during this manufacturing process. Thus, the manufacturing costs of the electrolyte composition according to the invention and of the electrochemical cell produced using the electrolyte composition can be kept low. According to the invention, the electrolyte composition comprises at least one salt of formula (I), wherein the salt contains an anionic complex with three bidentate ligands. In formula (I), the charge of the anion is stoichiometrically balanced by a positively charged metal cation M selected from the group consisting of alkali metals, alkaline earth metals, and metals of group 12 of the Periodic Table.Preferably, the metal cation is a lithium ion, and the salt is a lithium salt. Accordingly, m is an integer from 1 to 2, where m is stoichiometrically determined by the oxidation number of the metal cation used. In formula (I), the central ion Z is formed by a niobium or tantalum ion. The salts of formula (I) formed therefrom are accordingly niobates or tantalates and are singly negatively charged. Borates and aluminates, as well as other central ions besides niobium or tantalum, are not within the scope of the invention. This exclusion is based on the finding that electrolyte salts of boron or aluminum with commercially readily available bidentate ligands, for example, perfluoropinacol (PFP), exhibit only low conductivity in sulfur dioxide as a solvent. In other words, the conductivity does not correlate with the solubility of such electrolyte salts in sulfur dioxide.Without being bound to any scientific theory, it is assumed that due to the high surface charge of anionic chelate complexes with boron or aluminum as the central ion, sulfur dioxide is unable to dissociate such complexes into solvated ions. By using niobium or tantalum as the central ion, in contrast to aluminates or borates, the number of bidentate chelate ligands can be increased from two to three, thereby increasing the radius of the niobate or tantalate or reducing the surface charge. A reduced surface charge leads to a lower charge density at the anion, and the dissociation of the conductive salt in the sulfur dioxide solvent is increased compared to the prior art. The higher degree of dissociation corresponds to a higher electrical efficiency in a battery produced from the electrolyte composition according to the invention.By using niobium or tantalum as the central ion, the synthesis for the corresponding niobate or tantalate salt can be simplified and carried out more selectively. Furthermore, the yield in these two syntheses is significantly higher compared to salts of formula (I) with other central ions, thus eliminating the need for additional methods for purification and isolation of the resulting synthesis products. The bidentate chelate ligand has at least two oxygen atoms and a bridging group L. 1 , L 2 or L 3 which binds to both oxygen atoms. L 1 , L 2 and L 3each independently represent a perfluorinated aliphatic or aromatic bridging group. Accordingly, no hydrogen atoms are provided in the bridging groups. The complete fluorination of the bridging groups ensures that the ligands are overall stable towards electrolysis and higher cell voltages. The bridging group forms a five- to eight-membered ring with the central ion Z and two oxygen atoms bonded to the central ion Z and the bridging group. The ring contains a sequence of 2 to 5 carbon atoms, optionally interrupted by an oxygen atom. In other words, the ring can in particular have at least one ether group. By adding such an ether group, the fluorine content of the ring can advantageously be reduced. This also reduces the overall fluorine content of the ligand.Although fluorinated compounds exhibit good electrochemical stability, their synthesis is complex and costly. The inventors have recognized that the fluorine content in the ring, and thus also in the ligand, can be reduced without compromising the electrochemical stability of the ligand by incorporating heteroatoms into the ring. Ether groups, which are also stable to oxidative potentials, are particularly suitable for this purpose, so that the ligand remains electrochemically stable despite the reduced fluorine content. In a further development of the invention, the bridging groups L. 1 , L 2 and / or L 3 each have a linear, branched, or cyclic, saturated hydrocarbon skeleton. The term "hydrocarbon skeleton" is understood here and below as "perfluorinated hydrocarbon skeleton." The hydrocarbon skeleton of the bridging radicals L 1 , L 2and / or L 3 preferably has 3 to 16 carbon atoms, more preferably 6 to 9 carbon atoms. Hydrocarbon frameworks which have a number of hydrocarbon atoms in the stated range produce anions which form particularly stable salts of the formula (I). The bonding of the bridging radicals via the oxygen atoms to the central ion Z can be regarded as a coordinate bond within the meaning of the invention. The bonding of the ligand to the central ion Z forms a ring consisting of a bridging radical, the two oxygen atoms bonded to the bridging radical, and the central ion Z. According to one aspect of the invention, the ring has at least one continuous sequence of 2 to 5 carbon atoms, preferably 2, 3 or 5 carbon atoms. In this embodiment, no heteroatom is provided in the ring. Such rings form salts of the formula (II) where n = 0, 1, 2 or 3 and R represents a radical. M is a metal cation selected from the group consisting of alkali metals, alkaline earth metals and metals of group 12 of the Periodic Table. m is 1 or 2 and Z represents a central ion which is niobium or tantalum. The anion of the salt of the formula (II) has a total of three polycyclic rings according to the bonding situation according to formula (I). The radicals R can be identical or different and independently of one another are selected from the group consisting of C1-C10 perfluoroalkyl and fluorine. For the purposes of the invention, the term C1-C10 perfluoroalkyl encompasses linear, branched or cyclic saturated perfluorinated hydrocarbon radicals having 1 to 10 carbon atoms. Examples of suitable perfluoroalkyl radicals are trifluoromethyl, perfluoroethyl, perfluoropropyl, perfluoroisopropyl, perfluoro-n-butyl, perfluoro-sec-butyl, perfluoro-iso-butyl and perfluoro-tert-butyl.If n in formula (II) is 0, the ring formed by the central ion Z, the bridging group, and the two oxygen atoms bonded to the bridging group is pentacyclic and has a continuous sequence of 2 carbon atoms. If n in formula (II) is 1, the ring formed by the central ion Z, the bridging group, and the two oxygen atoms bonded to the bridging group is hexacyclic and has a continuous sequence of 3 carbon atoms. If n in formula (II) is 3, the ring formed by the central ion Z, the bridging group, and the two oxygen atoms bonded to the bridging group is eight-membered and has a continuous sequence of 5 carbon atoms. In a preferred embodiment, n in formula (II) is 0, and the R groups are identical and optionally correspond to fluorine-substituted methyl groups. Such chelating ligands are derived from pinacol, the simplest representative.In an advantageous embodiment of the invention, component (B) of the electrolyte composition comprises at least one lithium salt of formula (I). Lithium salts are particularly suitable for use as lithium conducting salts in lithium-ion batteries.

[0002] The lithium salt can preferably be selected from the group consisting of Z(O2C3(CF3)6)3of formula (III), and Z(O2C2(CF3)4)3of formula (V) where Z is Ta or Nb as the central ion, as well as combinations thereof. The proposed lithium salts dissolve well in liquid sulfur dioxide as a solvent. The electrolyte compositions prepared from them are non-flammable and possess extremely good ionic conductivity over a wide temperature range. The conductivity of the lithium salts can be determined using conductive measurement methods. For this purpose, various concentrations of the lithium salts (III) - (V) are prepared in sulfur dioxide. The conductivities of the solutions are then determined using a two-electrode sensor immersed in the solution at constant room temperature. For this purpose, the conductivity of the solution containing the lithium salts (III) - (V) is measured in a range of 0 - 100 mS / cm. Due to the high electrochemical stability of the lithium salts, they do not participate in cyclic and calendar aging processes in the battery cell.Furthermore, the proposed lithium salts exhibit increased thermal, chemical, and electrochemical stability, as well as particularly pronounced hydrolysis resistance. Thermal stability can be investigated, for example, using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The increased thermal, chemical, and electrochemical stability of the proposed conducting salts increases the service life of lithium-ion batteries. The electrolyte compositions produced from the lithium salts are therefore also more cost-effective to operate. Furthermore, the aforementioned properties of the lithium conducting salts enable the selection of a suitable recycling process. A recycling process based on water as a solvent is preferably used. The lithium conducting salts can thus be fully recovered from the used batteries.The improved recyclability of the electrolyte saves costs in the battery manufacturing process, which can be offset against the manufacturing costs of the electrolyte salts. In a further embodiment, the electrolyte composition contains component (B) in a concentration of 0.01 to 15 mol / L, preferably 0.1 to 10 mol / L, particularly preferably 0.2 to 1.5 mol / L, based on the total volume of the electrolyte composition. The electrolyte composition may further comprise at least one further additive in a proportion of 0-10 wt.%, preferably 0.1-2 wt.%, based on the total weight of the electrolyte composition.In one embodiment, the further additives comprise compounds selected from the group consisting of 2-vinylpyridine, 4-vinylpyridine, cyclic exomethylene carbonates, sulfones, cyclic and acyclic sulfonates, acyclic sulfites, cyclic and acyclic sulfinates, organic esters of inorganic acids, acyclic and cyclic alkanes, aromatic compounds, halogenated cyclic and acyclic sulfonylimides, halogenated cyclic and acyclic phosphate esters, halogenated cyclic and acyclic phosphines, halogenated cyclic and acyclic phosphites, halogenated cyclic and acyclic phosphazenes, halogenated cyclic and acyclic silylamines, halogenated cyclic and acyclic halogenated esters, halogenated cyclic and acyclic amides, halogenated cyclic and acyclic anhydrides and halogenated organic heterocycles.The additional additives can contribute to the stability of the electrolyte composition during operation in an electrochemical cell. The additional additives can also provide at least one additional lithium-containing conducting salt to the electrolyte composition. In one embodiment, the additional lithium-containing conducting salt can contribute to adapting the conductivity of the electrolyte composition to the requirements of the respective cell or increasing the corrosion resistance of the cathodic metal carrier foil.Preferred lithium-containing conducting salts include lithium tetrafluoroborate (LiBF4), lithium trifluoromethanesulfonate, lithium fluoride, lithium bromide, lithium sulfate, lithium oxalate, lithium (bisoxalato)borate, lithium difluoro(oxalato)borate, lithium tetrahalogenoaluminate, lithium hexafluorophosphate, lithium tris-(perfluoroethyl)trifluorophosphate, lithium tris-(perfluoropropyl)trifluorophosphate, lithium tris-(perfluorobutyl)trifluorophosphate, lithium tris-(perfluoropentyl)trifluorophosphate, lithium bis-(trifluoromethanesulfonyl)imide (LiTFSI), and lithium bis-(fluorosulfonyl)imide (LiFSI). Also included are possible isomers of the aforementioned compounds. The other additives may also include other solvents. Other solvents can contribute to adjusting the solubility of the electrolyte composition with respect to polar or nonpolar components.The further solvents preferably comprise vinylethylene carbonate (VEC), ethylmethyl carbonate (EMC), vinylene carbonate (VC) and 4-fluoro-1,3-dioxolan-2-one (FEC). In another embodiment, the further additives can also comprise at least one solid inorganic lithium ion conductor (solid electrolyte). Suitable examples of solid inorganic lithium ion conductors include perovskites, garnets, sulfides and amorphous compounds such as glasses, as well as combinations thereof. In a particularly preferred embodiment, the electrolyte composition comprises the following components: (A) sulfur dioxide; (B) at least one salt of the above formula (I) in a concentration of 0.01 - 15 mol / L, preferably 0.1 - 10 mol / L, based on the total volume of the electrolyte composition, wherein the salt is preferably a lithium salt, particularly preferably selected from the group consisting of the compounds of the formulas (III), (IV) and (V) and combinations thereof; (C) 0 - 10 wt.-%, preferably 0.1 - 2 wt.%, of at least one additive, wherein the additive is preferably selected from the group consisting of vinylene carbonate (VC), 4-fluoro-1,3-dioxolan-2-one (FEC), lithium hexafluorophosphate, cis-4,5-difluoro-1,3-dioxolan-2-one (cDFEC), 4-(trifluoromethyl)-1,3-dioxolan-2-one, lithium tris-(perfluoroethyl)trifluorophosphate, lithium tris-(perfluoropropyl)trifluorophosphate, lithium tris-(perfluorobutyl)trifluorophosphate, lithium tris-(perfluoropentyl)trifluorophosphate, bis-(trifluoromethanesulfonyl)imide (LiTFSI) and bis-(fluorosulfonyl)imide (LiFSI), including isomers and combinations thereof, based on the total weight of the electrolyte composition. Compared to an electrolyte composition with electrolyte salts of boron or aluminum and commercially readily available bidentate ligands, the electrolyte composition according to the invention has improved hydrolysis resistance during the recycling process and higher conductivity.The invention further relates to an electrochemical cell having a cathode, an anode, and the described electrolyte composition, which is in contact with the cathode and the anode. In an advantageous development of the invention, the electrochemical cell is a lithium-ion cell, wherein the electrolyte composition comprises the following components: (A) sulfur dioxide; (B) 0.5–2 mol / L of a salt of formula (I), preferably a salt of formula (III), (IV), and (V), and combinations thereof, based on the total volume of the electrolyte composition; (C) 0.1–2 wt.% lithium hexafluorophosphate and 0.1–2 wt.% 4-fluoro-1,3-dioxolan-2-one (FEC), each based on the total weight of the electrolyte composition. The proposed lithium-ion cells are cost-effective and can be operated safely at various operating voltages. The associated electrochemical properties can be determined by measurements on test cells.The cyclic aging resistance of the test cells can be determined by the number of cycles. The test cells are first charged with a constant charging current up to a maximum permissible cell voltage. The upper cutoff voltage is kept constant until the charging current drops to a specified value or the maximum charging time is reached. This is also known as I / U charging. The test cells are then discharged with a constant discharge current up to a specified cutoff voltage. The charging can be repeated depending on the desired number of cycles. The upper cutoff voltage and the lower cutoff voltage, as well as the specified charging or discharging currents, must be selected experimentally. This also applies to the value to which the charging current has dropped.The aging stability and the extent of self-discharge can be determined by storing a fully charged battery cell, particularly at elevated temperatures. For this purpose, the battery cell is charged up to the permissible upper voltage limit and maintained at this voltage until the charging current has dropped to a predetermined limit. The cell is then disconnected from the power supply and stored in a temperature chamber at elevated temperatures, for example, 45°C, for a specific period of time, for example, one month (variant 1). The cell is then removed from the temperature chamber, and the remaining capacity is determined under defined conditions. For this purpose, a discharge current is selected that, for example, numerically corresponds to one-third of the nominal capacity, thus discharging the cell down to the lower discharge limit.This process can be repeated as often as desired, for example until the detectable residual capacity has dropped to a previously defined value, such as 70% of the nominal capacity. In a second storage variant (variant 2), storage takes place in a temperature chamber with a power supply connected, whereby the voltage corresponds to the upper voltage limit and this voltage must be maintained. Tests are carried out according to both storage variants. From these tests, the actual calendar aging and self-discharge of the battery cell are then determined: Calendar aging corresponds to the capacity loss due to storage according to variant 2 and is calculated by subtracting the determined residual capacity 2 from the nominal capacity. The self-discharge rate is determined from the difference between the residual capacities 1 and 2 determined by storage according to variants 1 and 2 in relation to the nominal capacity of the battery cell.The cathode of the lithium-ion cell preferably comprises a cathode active material. Preferred cathode active materials for the electrochemical cell include lithium cobalt oxide (LCO), lithium nickel oxide (LNO), lithium nickel cobalt aluminum oxide (NCA), lithium nickel manganese cobalt oxide (NMC), lithium manganese oxide (LMO), lithium iron phosphate (LFP), lithium nickel manganese oxide (LMR), lithium nickel manganese oxide spinel (LNMO), and combinations thereof. Lithium nickel manganese cobalt compounds are also known by the abbreviation NMC, and occasionally alternatively by the technical abbreviation NCM. NMC-based cathode materials are used particularly in lithium-ion batteries for vehicles.NMC as a cathode material exhibits an advantageous combination of desirable properties, such as high specific capacity, a reduced cobalt content, high current capability, and high intrinsic safety, as demonstrated, for example, by sufficient stability during overcharging. NMC can be denoted by the general formula unit Li. α Ni x Mn y Co z O2 with x+y+z = 1, where α denotes the stoichiometric proportion of lithium and is usually between 0.95 and 1.05. Certain stoichiometries are given in the literature as triples, for example, NMC 811, NMC 622, NMC 532, and NMC 111. The triples indicate the relative nickel:manganese:cobalt content. In other words, NMC 811, for example, is a cathode material with the general formula unit LiNi. 0,8 Mn 0,1 Co 0,1O2, i.e. with α = 1. Furthermore, the so-called lithium- and manganese-rich NMCs with the general formula unit Li 1+ε (No x Mn y Co z ) 1-εO2 can be used, where ε is in particular between 0.1 and 0.6, preferably between 0.2 and 0.4. These lithium-rich layered oxides are also known as overlithitated (layered) oxides (OLO). In addition to the cathode active material, the cathode can comprise further components and additives, such as a foil carrier (rolled metal foil) or a metal-coated polymer film, an electrode binder, and / or an electrical conductivity enhancer, for example, conductive carbon black. All conventional compounds and materials known in the prior art can be used as further components and additives. The anode of the lithium-ion cell preferably comprises an anode active material.In particular, the anode active material can be selected from the group consisting of carbonaceous materials, soft carbon, hard carbon, natural graphite, synthetic graphite, silicon, silicon suboxide, silicon alloys, lithium, lithium alloys, aluminum alloys, indium, indium alloys, tin, tin alloys, cobalt alloys, niobium pentoxide, titanium dioxide, titanates, for example lithium titanates (Li4Ti5O12 or Li2Ti3O7), tin dioxide, and mixtures thereof. Preferably, the anode active material is selected from the group consisting of synthetic graphite, natural graphite, graphene, mesocarbon, doped carbon, hard carbon, soft carbon, fullerene, silicon-carbon composite, silicon, surface-coated silicon, silicon suboxide, silicon alloys, lithium, aluminum alloys, indium alloys, tin alloys, cobalt alloys, and mixtures thereof.In addition to the anode active material, the anode may comprise further components and additives, such as a foil carrier, an electrode binder, and / or an electrical conductivity enhancer, for example conductive carbon black, conductive graphite, so-called "carbon nanotubes" (CNTs), carbon fibers, and / or graphene. All conventional compounds and materials known in the art can be used as further components and additives. Furthermore, the invention relates to a salt with an anionic complex comprising three bidentate ligands, wherein the salt has the following formula (I). Where M represents a metal cation selected from the group consisting of alkali metals, alkaline earth metals, and metals of group 12 of the periodic table. m represents an integer from 1 to 2. Z represents a central ion, which is niobium or tantalum. L 1 , L 2 and L 3each independently represents a perfluorinated aliphatic or aromatic bridging radical. The bridging radical forms a five- to eight-membered ring with the central ion Z and two oxygen atoms bonded to the central ion Z and the bridging radical, and the ring contains a sequence of 2 to 5 carbon atoms, optionally interrupted by an oxygen atom. Regarding the properties and advantages of the salt, reference is made to the previous statements, which also apply analogously to the salt itself. The salt of formula (I) is preferably a lithium triperfluoropicanolatoniobate (LiNb(PFP)3) of the following formula (V): The salt of formula (V) is a lithium salt and is characterized by high conductivity in sulfur dioxide. Furthermore, the niobate according to formula (V) can be synthesized more easily than salts of formula (V) with phosphorus or antimony as the central ion. In particular, compared to the phosphorus- or antimony-based salts, time-consuming and costly purification of the resulting synthesis product can be dispensed with because the synthesis of niobium-containing salts according to formula (V) provides a significantly higher yield. This can simplify the synthesis overall. The invention further relates to the use of the above-mentioned salts of formula (I) in an electrochemical cell. The salt is preferably used as a lithium ion-conducting conducting salt in the electrochemical cell. The lithium salt of formula (V) is preferably used as a conducting salt in an electrochemical cell.The lithium salt of formula (V) is inexpensive, easy to produce, and exhibits higher conductivity than other conducting salts with anionic chelate complexes. In a particularly advantageous embodiment, the electrochemical cell is based on sulfur dioxide as a solvent. The invention further relates to the use of the above-mentioned salts of formula (I) as X-ray contrast agents in electrochemical cells. Particular preference is given to using a salt of formula (I) in which Z is tantalum. For example, a tantalate according to formula (I) can be used to monitor the cell-internal processes in a lithium-ion cell in combination with an imaging method. Computed tomography methods, in particular, can be used as imaging methods. Examples The invention is explained below using examples, which, however, are not to be interpreted in a limiting sense.Synthesis of lithium triperfluoropicanolatoniobate (LiNb(PFP)3): The synthesis of LiNb(PFP)3 can be carried out as described below. First, 1,1,1,3,3,3-hexafluoro-2-propanol is converted into the lithium salt. For this purpose, an excess of 1,1,1,3,3,3-hexafluoro-2-propanol (99%, dried for 4 hours over 3Å molecular sieves and then distilled) is placed in a flask equipped with a reflux condenser. Lithium metal is added in small portions. The lithium reacts with the evolution of hydrogen, and the solution begins to boil over time. After the reaction, the clear and colorless solution is evaporated, yielding lithium 1,1,1,3,3,3-hexafluoro-2-propanolate as a white powder. The powder is dried overnight in a vacuum at 60 °C. 2.70 g of NbCl5 are placed in 150 ml of 1,2-dichloroethane (99%, dried for 4 h over 3Å molecular sieves and then distilled), whereby the NbCl5 dissolves only partially.Subsequently, 10.02 g of lithium 1,1,1,3,3,3-hexafluoro-2-propanol are added portionwise while stirring. A whitish precipitate forms, and the yellow color of the NbCl5 disappears quite quickly. The mixture is then slowly heated to reflux. While still heating (approx. 1 h), a mixture of 10.01 g of anhydrous perfluoropinacol in 50 ml of 1,2-dichloroethane is added dropwise with continuous stirring. A white substance continually forms at the bottom of the flask. After 2-3 h, the resulting 1,1,1,3,3,3-hexafluoro-2-propanol is slowly distilled off through a 30 cm Vigreux column. After virtually all of the 1,1,1,3,3,3-hexafluoro-2-propanol has been distilled off, an additional 20 ml of 1,2-dichloroethane are distilled off. During the removal of 1,1,1,3,3,3-hexafluoro-2-propanol and some of the 1,2-dichloroethane, the amount of white precipitate increases. This is kept under gentle reflux overnight.The reaction mixture is then transferred to an extractor and extracted with the filtrate for 5 hours. The extract is then evaporated and dried (35 °C, < 10-5 mbar). The yield of white product is approximately 9.8 g (90%), leaving a residue of approximately 2.9 g in the extractor, which consists predominantly of LiCl. The yield is quantitative based on the lithium used. The purity of the resulting product was verified by HPLC and high-resolution MS. The data of the isotopic pattern found can be clearly assigned to the anion Nb(PFP)3- (measured masses and relative intensities): 1.088.8168 (100%); 1.089.8185 (19.6%); 1.090.8210 (1.8%); 1.091.8234 (0.4%). The purity found is 97.5%. Conductivity of LiNb(PFP)3: The conductivity of LiNb(PFP)3 can be determined using conductive measurement methods. For this purpose, a 0.5 M electrolyte solution of LiNb(PFP)3 in sulfur dioxide is prepared.The conductivity of this solution is then determined as a function of temperature using a two-electrode arrangement immersed in the solution. The conductivity of the solution is measured in a range of 0–100 mS / cm. The results shown in Table 1 below were obtained. Table 1: Conductivity of LiNb(PFP)3Temperature [°C] Conductivity [mS / cm] -10 16 +20 24 Test cells with LiNb(PFP)3: When used in full cells, several cycles could be performed without any loss of capacity. In particular, no lithium deposits were observed on the anode side.

Claims

Claims 1. A liquid electrolyte composition for an electrochemical cell, the electrolyte composition comprising the following components: (A) sulfur dioxide; (B) at least one salt, the salt containing an anionic complex with three bidentate ligands and the salt having the following formula (I) corresponds to; wherein − M is a metal cation selected from the group consisting of alkali metals, alkaline earth metals and metals of group 12 of the Periodic Table; − m is 1 or 2; − Z represents a central ion which is tantalum or niobium; and − L 1 , L 2 and L 3each independently represents a perfluorinated aliphatic or aromatic bridging radical, wherein the bridging radical forms a five- to eight-membered ring with the central ion Z and with two oxygen atoms bonded to the central ion Z and the bridging radical, and wherein the ring contains a sequence of 2 to 5 carbon atoms, optionally interrupted by an oxygen atom.

2. The electrolyte composition according to claim 1, characterized in that the metal cation M is lithium and component (B) is a lithium salt.

3. Electrolyte composition according to claim 1 or 2, characterized in that L 1 , L 2 and / or L 3independently of one another each comprise a linear, branched or cyclic, saturated hydrocarbon skeleton, wherein the hydrocarbon skeleton preferably has 3 to 16 carbon atoms, preferably 6 to 9 carbon atoms.

4. Electrolyte composition according to one of the preceding claims, characterized in that the ring contains a continuous sequence of 2 to 5 carbon atoms, preferably 2 to 3 carbon atoms.

5. Electrolyte composition according to one of the preceding claims, characterized in that component (B) of the electrolyte composition comprises at least one lithium salt of the formula (I), wherein the lithium salt is preferably selected from the group consisting of Z(O2C3(CF3)6)3 of the formula (III) , Z(O2C2(CF3)4CF2)3 of formula (IV) and Z(O2C2(CF3)4)3of formula (V) where Z is Ta or Nb as the central ion, and combinations thereof.

6. Electrolyte composition according to one of the preceding claims, characterized in that the electrolyte composition contains component (B) in a concentration of 0.01 to 15 mol / L, preferably 0.1 to 10 mol / L, particularly preferably 0.2 to 1.5 mol / L, based on the total volume of the electrolyte composition.

7. Electrolyte composition according to one of the preceding claims, characterized in that the electrolyte composition comprises at least one further additive in a proportion of 0 - 10 wt.%, preferably 0.1 - 2 wt.-%, based on the total weight of the electrolyte composition, wherein the further additive is preferably selected from the group consisting of vinylene carbonate (VC), 4-fluoro-1,3-dioxolan-2-one (FEC), lithium hexafluorophosphate, cis-4,5-difluoro-1,3-dioxolan-2-one (cDFEC), 4-(trifluoromethyl)-1,3-dioxolan-2-one, lithium tris-(perfluoroethyl)trifluorophosphate, lithium tris-(perfluoropropyl)trifluorophosphate, lithium tris-(perfluorobutyl)trifluorophosphate, lithium tris-(perfluoropentyl)trifluorophosphate, bis-(trifluoromethanesulfonyl)imide (LiTFSI) and bis-(fluorosulfonyl)imide (LiFSI), including isomers and combinations thereof.

8. Liquid electrolyte composition according to one of the preceding claims, characterized in that the electrolyte composition comprises the following components: (A) sulfur dioxide;. (B) at least one salt of formula (I) in a concentration of 0.01-15 mol / L, preferably 0.1-10 mol / L, based on the total volume of the electrolyte composition, wherein the salt is preferably a lithium salt, particularly preferably selected from the group consisting of the compounds of formula (III), (IV) and (V) and combinations thereof; (C) 0-10 wt.%, preferably 0.1-2 wt.-%, of at least one additive, wherein the additive is preferably selected from the group consisting of vinylene carbonate (VC), 4-fluoro-1,3-dioxolan-2-one (FEC), lithium hexafluorophosphate, cis-4,5-difluoro-1,3-dioxolan-2-one (cDFEC), 4-(trifluoromethyl)-1,3-dioxolan-2-one, lithium tris-(perfluoroethyl)trifluorophosphate, lithium tris-(perfluoropropyl)trifluorophosphate, lithium tris-(perfluorobutyl)trifluorophosphate, lithium tris-(perfluoropentyl)trifluorophosphate, bis-(trifluoromethanesulfonyl)imide (LiTFSI) and bis-(fluorosulfonyl)imide (LiFSI), including isomers and combinations thereof, based on the total weight of the electrolyte composition.

9. An electrochemical cell comprising a cathode, an anode, and an electrolyte composition according to any one of the preceding claims in contact with the cathode and the anode. 10.Electrochemical cell according to claim 9, characterized in that the electrochemical cell is a lithium-ion cell, and wherein the electrolyte composition comprises the following components: (A) sulfur dioxide; (B) 0.5-2 mol / L of a salt of formula (I), based on the total volume of the electrolyte composition; (C) 0.1-2 wt% lithium hexafluorophosphate and 0.1-2 wt% 4-fluoro-1,3-dioxolan-2-one (FEC), each based on the total weight of the electrolyte composition.

11. Salt, with an anionic complex comprising three bidentate ligands, wherein the salt has the following formula (I). corresponds to; wherein − M is a metal cation selected from the group consisting of alkali metals, alkaline earth metals and metals of group 12 of the Periodic Table; − m is 1 or 2; − Z represents a central ion which is tantalum or niobium; and − L 1 , L 2 and L 3each independently represents a perfluorinated aliphatic or aromatic bridging radical, wherein the bridging radical forms a five- to eight-membered ring with the central ion Z and with two oxygen atoms bonded to the central ion Z and the bridging radical, and wherein the ring contains a sequence of 2 to 5 carbon atoms, optionally interrupted by an oxygen atom.

12. Salt according to claim 11, characterized in that the salt is lithium triperfluoropicanolatoniobate (LiNb(PFP)3) of the following formula (V) 13. Use of the salt according to one of claims 11 or 12 in an electrochemical cell.

14. Use of the salt according to claim 13, characterized in that the salt is used as a lithium ion-conducting conducting salt in the electrochemical cell.

15. Use of the salt according to claim 13 or 14, characterized in that the electrochemical cell is based on sulfur dioxide as a solvent.

16. Use of the salt according to claim 11 as an X-ray contrast agent in an electrochemical cell.

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