Liquid electrolyte composition containing salt, electrochemical cell having electrolyte composition, salt and use of salt in electrochemical cell

Abstract

The present invention relates to a liquid electrolyte composition having a salt of formula (I), the salt having an anionic complex comprising three bidentate ligands. The complex comprises phosphorus as a central ion. The invention also relates to an electrochemical cell having the liquid electrolyte composition and to the salt. The invention further relates to the use of said salts in electrochemical cells. Formula (I) imgabs0 #

Description

Technical Field

[0001] The invention relates to a salt-containing electrolyte composition, an electrochemical cell having the electrolyte composition, a salt and use of the salt in an electrochemical cell. Background Art

[0002] Electrochemical cells are of great importance in many areas of technology. In particular, electrochemical cells are often used in applications that require low voltages, for example for driving laptops or mobile phones. The advantage of electrochemical cells is that many individual cells can be connected to one another. Cells can, for example, provide high voltages by connecting them in series, while parallel connection of cells results in a higher nominal capacity. Such a connection produces a battery with greater energy. Such battery systems are also suitable for high-voltage applications and can, for example, enable electric drives of vehicles or be used for stationary energy storage.

[0003] In the following, the term “electrochemical cell” is used as a synonym for the designations of all common rechargeable galvanic elements from the prior art, such as cells, batteries, cells, accumulators, battery accumulators and secondary batteries.

[0004] The electrochemical cell is able to provide electrons to an external circuit during the discharge process. Conversely, during the charging process, the electrochemical cell is charged by inputting electrons with the aid of the external circuit.

[0005] An electrochemical cell has at least two different electrodes, a positive electrode (cathode) and a negative electrode (anode). The two electrodes are in contact with an electrolyte composition.

[0006] The most commonly used electrochemical cell is the lithium-ion cell, also known as a lithium-ion battery.

[0007] Lithium-ion cells known from the prior art have an assembled anode which very often comprises a carbon-based anode active material (typically graphite-type carbon) deposited on a metallic copper support foil. The cathode generally comprises metallic aluminum coated with a cathode active material (e.g. a layered oxide). For example, LiCoO2 or LiNi coated onto a rolled aluminum support foil can be used. 1 / 3 Mn 1 / 3 Co 1 / 3 O2 as layered oxide.

[0008] The electrolyte composition plays an important role in the safety and performance of electrochemical cells. This ensures the charge balance between cathode and anode during the charge-discharge process. The current required for this is achieved by ion transport of the conductive salt in the electrolyte composition. In lithium-ion cells, the conductive salt is a lithium conductive salt and lithium ions are used as ions for the current transport.

[0009] Therefore, there is a need to select a suitable conducting salt that is not only sufficiently soluble in the electrolyte composition but also has suitable ionic conductivity to maintain effective charge balance during operation. The most common conducting salt in lithium-ion cells is lithium hexafluorophosphate (LiPF6).

[0010] In addition to the lithium conducting salt, the electrolyte composition also contains a solvent, which allows the dissociation of the conducting salt and sufficient mobility of the lithium ions. Liquid organic solvents consisting of a series of linear and cyclic dialkyl carbonates are known from the prior art. Generally, a mixture of the following substances is used: ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), propylene carbonate (PC) and ethyl methyl carbonate (EMC).

[0011] It should be noted here that each solvent has a specific cell voltage stability range, also referred to as a "voltage window". In this voltage window, the electrochemical cell can operate stably during operation. If the cell voltage approaches the upper voltage limit, electrochemical oxidation of the components of the electrolyte composition occurs. In contrast, at the lower end of the voltage window, reduction processes take place. Both redox reactions are undesirable, reduce the performance and reliability of the cell and lead to its failure in the worst case.

[0012] The processes mentioned here are particularly relevant for deep discharge and overcharging of lithium-ion cells.

[0013] During the charge-discharge process, prior art lithium-ion cells with organic electrolyte compositions tend to generate gas. "Gas generation" is understood to mean that the components of the electrolyte are electrochemically decomposed into volatile gaseous compounds due to the use of excessively high cell voltages. The generation of gas reduces the proportion of the electrolyte and leads to the generation of undesirable decomposition products, which results in a lower service life and lower performance of the lithium-ion cell.

[0014] In order to allow the battery to operate in a potential range as wide as possible, fluorinated solvents or additives are added to the electrolyte composition in the prior art. Fluorinated solvents such as fluoroethylene carbonate (FEC) are chemically inert and electrochemically stable relative to the operating voltage of lithium-ion batteries.

[0015] A well-known disadvantage of fluorinated electrolytes is that, in the event of thermal defects in the cell, they can lead to an increased exotherm and to the formation and emission of gases that are harmful to health, such as hydrogen fluoride (HF).

[0016] Due to this disadvantage, lithium-ion cells have a number of control mechanisms in order to keep the cell during operation in the voltage range that is optimal for the respective solvent and thereby stabilize the electrolyte composition.

[0017] In the prior art, various approaches for stable electrolyte compositions are known.

[0018] EP 1 689 756 B1 describes a method for preparing compounds of the formula X(OR F ) m A method for weakly coordinating anions, wherein X is selected from the group consisting of B, Al, Ga, In, P, As and Sb, m is 3 or 5, and R F is a linear or branched, partially or fully fluorinated alkyl or aryl group. The weakly coordinating anion forms a salt with a monovalent or divalent cation, preferably with an alkali metal ion. Due to the chemical stability, especially the chemical stability of the anion, the disclosed salt is particularly recommended for use as an inert lithium-conducting salt in lithium-ion batteries. However, no electrolyte composition with weakly coordinating anions for lithium-ion batteries is shown.

[0019] In addition to selecting a chemically inert conductive salt, the stability of the battery cell can also be improved by selecting a suitable solvent. Currently, sulfur dioxide (SO2) is being discussed as an inorganic solvent in an electrolyte composition. Sulfur dioxide-based electrolyte compositions have, in particular, improved ionic conductivity and can thereby enable operation of the battery cell at high discharge currents without negatively affecting the stability of the cell. In addition, sulfur dioxide-based electrolyte compositions are unique in that they have high energy density, low self-discharge rate, and limited overcharge and deep discharge.

[0020] A disadvantage of sulfur dioxide is that it can only dissolve many lithium-conducting salts poorly, whereas these can be dissolved well in organic solvents. Thus, for example, the widely used lithium-conducting salt lithium hexafluorophosphate cannot be used in electrolyte compositions containing sulfur dioxide.

[0021] EP 1 201 004 B1 discloses a rechargeable electrochemical cell with an electrolyte based on sulfur dioxide. Sulfur dioxide is not added as an additive here, but is the main component of the electrolyte composition. Therefore, it should at least partially ensure the mobility of the ions of the conductive salt, which leads to the charge transport between the electrodes. In the proposed cell, lithium tetrachloroaluminate is used as a lithium-containing conductive salt in combination with a cathode active material formed by a metal oxide, in particular an intercalation compound such as lithium cobalt oxide (LiCoO2). By adding a salt additive, such as an alkali metal halide such as lithium fluoride, sodium chloride or lithium chloride, to the electrolyte composition containing sulfur dioxide, a functional and rechargeable cell is obtained.

[0022] EP 2534719 B1 shows a rechargeable lithium battery cell having a sulfur dioxide-based electrolyte in combination with lithium iron phosphate as cathode active material. Lithium tetrachloroaluminate is used as the preferred conducting salt in the electrolyte composition. In studies on cells based on these components, it was possible to demonstrate a high electrochemical resistance of the cells.

[0023] WO 2021 / 019042 A1 describes rechargeable battery cells with active metals, layered oxides as cathode active materials and an electrolyte containing sulfur dioxide. Due to the poor solubility of many common lithium-conducting salts in sulfur dioxide, lithium-conducting salts of the formula M are used in these cells. + [Z(OR)4] - A conductive salt wherein M is a metal selected from the group consisting of alkali metals, alkaline earth metals and metals of Group 12 of the periodic table, and R is a hydrocarbon group. The alkoxy groups -OR are each single-bonded to a central atom Z, which may be aluminum or boron. In a preferred embodiment, the battery cell comprises a Li + [Al(OC(CF3)3)4] - Perfluorinated conductive salts. Cells consisting of the described components show stable electrochemical properties in experimental studies. In addition, the conductive salts, especially perfluorinated anions, have unexpected hydrolysis stability. In addition, the electrolyte should be stable to oxidation up to an upper potential of 5.0 V. It has further been shown that cells with the disclosed electrolyte can be discharged or charged at low temperatures up to -41°C. However, no electrochemical performance measurements were performed at high temperatures.

[0024] The thermal stability of lithium perfluoroaluminate at high temperatures was studied in a professional publication by Malinowski et al. (Dalton Trans., 2020, 49, 7766). In the study, the authors characterized various properties of [Al(OC(CF3)3)4] salts, especially the temperature stability of lithium derivatives. Thermogravimetric studies showed that the compound Li[Al(OC(CF3)3)4] already had a mass loss at 105°C, which was caused by the beginning of decomposition of fluoride anions.

[0025] So far, only conducting salts for sulfur dioxide-based electrolytes have been discussed, the anions of which form chemical complexes. These complexes can be formed using monodentate, bidentate or polydentate ligands. Bidentate or polydentate ligands are also generally known as chelate ligands and the complexes formed by their combination are called chelate complexes.

[0026] For example, EP 4 037 056 A1 describes an electrolyte based on SO2 for rechargeable battery cells. The electrolyte contains at least one conductive salt, which may have at least one substituent formed as a chelating ligand. The chelating ligand is coordinated to a central ion, which is boron or aluminum.

[0027] Other electrolyte salts with chelating ligands in electrolyte compositions for electrochemical cells are known from applications DE 10 2021 118 811.3 and PCT / EP2022 / 069660.

[0028] Chelate complexes are chemically more stable than their single-bond derivatives. The bond between the chelate ligand and the central ion is difficult to disengage, so the chelate complex is chemically inert to external chemical and physical influences. Due to these characteristics, chelate complexes, especially salts composed of them, are considered to have temperature tolerance and hydrolysis tolerance. Therefore, electrolyte salts composed of specific chelate complexes have higher oxidation stability and can therefore operate safely at higher cell voltages.

[0029] In order to use such electrolyte salts for commercially available batteries, especially for batteries as driving sources for electric vehicles, it is necessary that the electrolyte salts satisfy specific process and performance-related indicators in addition to the safety-related requirements discussed above.

[0030] On the one hand, a prerequisite is that the electrolyte salt consists of ligands that are easily available and inexpensive. Otherwise, the battery produced therefrom is too expensive to produce and cannot be used economically. On the other hand, the electrolyte salt must have good solubility in sulfur dioxide as a solvent, because salts with higher solubility can be processed more easily. Another indicator is sufficient conductivity of the electrolyte salt in sulfur dioxide, which can also ensure sufficient electrical efficiency in the battery produced therefrom.

[0031] Therefore, efforts have been made to develop new electrolyte salts that meet the above requirements. Summary of the invention

[0032] In this context, the present invention is based on the object of providing an electrolyte composition for electrochemical cells and in particular for rechargeable batteries which meets the above-mentioned requirements and can be operated safely at various operating voltages.

[0033] According to the invention, this object is achieved by a liquid electrolyte composition for an electrochemical cell according to claim 1 .

[0034] Advantageous embodiments of the electrolyte composition according to the invention are given in the dependent claims, which can optionally be combined with one another.

[0035] According to the present invention, the object is achieved by a liquid electrolyte composition for an electrochemical cell. The electrolyte composition comprises the following components:

[0036] (A) Sulfur dioxide;

[0037] (B) at least one salt, wherein the salt comprises an anionic complex having three bidentate ligands and the salt corresponds to the following formula (I):

[0038]

[0039] wherein 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 is an integer from 1 to 2. P represents elemental phosphorus and is the central ion of the anionic complex. 1 , L 2 and L 3 Each independently represents a perfluorinated aliphatic or aromatic bridge group. The bridge group forms a five-membered to eight-membered ring with the central ion P and with two oxygen atoms bonded to P and the bridge group, and wherein the ring comprises a sequence of 2 to 5 carbon atoms optionally interrupted by oxygen atoms.

[0040] The salt proposed according to the invention has an anion containing three bidentate ligands. In the sense of the present invention, bidentate ligands are understood to be molecules having at least two oxygen atoms and being connected to the central ion P via the at least two oxygen atoms. Multidentate ligands with different numbers of teeth (e.g. tridentate, tetradentate, pentadentate or hexadentate) are not within the scope of the present invention.

[0041] Bidentate ligands are also generally referred to as chelate ligands and the complexes formed by their combination are referred to as chelate complexes. The anions of the salts of formula (I) are therefore chelate complexes. Within the scope of the present invention, chelate complexes and salts formed therefrom have various advantages over complexes prepared from single-bond ligands and salts formed therefrom.

[0042] The chelate complex is chemically more stable than its single bond derivative. The bond between the chelate ligand and the central ion is difficult to release, so the chelate complex according to the present invention is chemically inert with respect to external chemical and physical influences.

[0043] According to the invention, the chelate complex is the anion of at least one salt of formula (I), wherein the salt serves as a conducting salt of an electrolyte composition. The electrolyte composition thus enables charge balancing between the two electrodes with which it is in contact.

[0044] Another advantage is the high affinity of the chelating ligand to the central ion P. The chelating complex used according to the invention is a chemically and electrochemically stable compound which has a lower affinity for bonds with positively charged ions due to the strong coordination properties of the ligand to the central ion. Therefore, the chelating complex itself is a weakly coordinated anion. Therefore, the conducting salt can be almost completely dissociated into the initial salt in the electrolyte composition without recovery, and ions with high mobility and correspondingly high ionic conductivity are formed in the solution. This in turn improves the electrochemical performance of the electrochemical cell.

[0045] Due to these properties, the chelate complexes used according to the invention, in particular the salts composed of the chelate complexes, are both temperature-resistant and hydrolysis-resistant.

[0046] According to the invention, the salts described are fully dissolved in liquid sulfur dioxide, which is an inorganic solvent for the electrolyte composition. Within the scope of the invention, sulfur dioxide is not only contained in the electrolyte composition as an additive in a small concentration, but is also present to such an extent that it can act as a solvent to ensure the mobility of the ions of the conductive salt.

[0047] Sulfur dioxide is gaseous at room temperature and atmospheric pressure and forms a stable, liquid solvate complex with a lithium conducting salt, which has a significantly reduced vapor pressure relative to sulfur dioxide as a pure substance. The gaseous sulfur dioxide is thus bound in liquid form and can be handled safely and relatively simply. The non-flammability of sulfur dioxide itself and of the solvate complex is a particular advantage, which increases the operating safety of electrolyte compositions based on such solvate complexes and of cells produced using such electrolyte compositions.

[0048] The described salts of the chelate complexes of formula (I) are non-flammable. Therefore, the electrolyte composition of the invention is also non-flammable and allows safe operation of electrochemical cells containing components of the disclosed electrolyte composition. If sulfur dioxide escapes from the cell in the event of mechanical damage, the sulfur dioxide cannot be ignited outside the cell either.

[0049] In addition, the electrolyte composition of the present invention is still inexpensive relative to conventional organic electrolytes. The improved temperature stability and hydrolysis resistance make it possible to directly and almost completely recycle the electrolyte composition from old batteries without increasing consumption. In order to recycle old batteries, hydrothermal methods at high pressure and high temperature are used in most cases. Conventional electrolyte compositions are mostly not resistant to hydrolysis and therefore must be post-processed in other ways. For this reason, the electrolyte composition is extracted from the battery in a highly costly manner, for example by flushing the battery cell with supercritical carbon dioxide. In contrast, as described in the prior art, newer electrolyte formulations based on aluminates, borates or gallates mostly do not have sufficient temperature stability.

[0050] The electrolyte composition proposed here is temperature-stable and hydrolysis-resistant and can therefore be recycled cost-effectively directly from 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 and a high recycling rate.

[0051] Recycling not only reduces the consumption of primary raw materials of the electrolyte composition and the energy requirements that must be spent to produce a new electrolyte composition, but also reduces the carbon dioxide emissions caused during the preparation process. As a result, the preparation costs of the electrolyte composition of the present invention and the preparation costs of the electrochemical cells prepared using the electrolyte composition are kept low.

[0052] According to the invention, the electrolyte composition comprises at least one salt of formula (I), wherein the salt comprises an anionic complex having three bidentate ligands.

[0053] In formula (I), the charge of the anion is balanced by a positively charged metal cation M in a stoichiometric ratio, the metal cation being selected from the group consisting of alkali metals, alkaline earth metals and metals of Group 12 of the periodic table. The metal cation is preferably a lithium ion, and the salt is preferably a lithium salt. Correspondingly, m is an integer from 1 to 2, wherein m is determined in a stoichiometric ratio by the oxidation number of the metal cation used.

[0054] In formula (I), the central ion is formed by phosphorus. Correspondingly, the salt of formula (I) thus formed is a phosphate and carries a single negative charge. Borates and aluminates and other central ions besides phosphorus are not within the scope of the present invention.

[0055] The main advantage when selecting phosphorus as the central ion of anion is that phosphorus is widely present in the earth's crust as an element. Therefore, phosphorus-containing initial materials are easily available for the synthesis of the proposed chelate complex. Correspondingly, they are also low in cost. In addition, the proposed compound according to formula (I) has a low molecular weight due to phosphorus as the central ion, which advantageously affects the energy density of the battery core thus prepared by the weight method. This is especially true when compared with the central ions such as Ga, In, As, Sb, Nb, Ta, V and La that form a stable oxidation valence of 3 or 5.

[0056] Compared with the above central ions, the compound of formula (I) can also be prepared at a low cost, thereby also reducing the cost of batteries prepared therefrom.

[0057] Boron or aluminum electrolyte salts with readily commercially available bidentate ligands, such as perfluoropinacol (PFP), have only low to moderate conductivity in sulfur dioxide as a solvent. In other words, the conductivity is not directly related to the solubility of such electrolyte salts in sulfur dioxide.

[0058] Without being bound by scientific theory, it is assumed that, due to the high unit area charge of anionic chelate complexes containing boron or aluminum as central ions, sulfur dioxide is not sufficiently capable of dissociating such complexes into solvated ions. Unlike aluminates or borates, the number of bidentate chelate ligands can be increased from two to three by using phosphorus as central ion, thereby increasing the radius of the phosphate or reducing the unit area charge. The reduced unit area charge leads to a lower charge density on the anion, and the dissociation of the conductive salt in the solvent sulfur dioxide is increased compared to the prior art. In a battery prepared from an electrolyte composition according to the invention, a higher degree of dissociation corresponds to a higher electrical efficiency.

[0059] The bidentate chelate ligand has at least two oxygen atoms and a bridging group L bonded to the two oxygen atoms. 1 , L 2 or L 3 .

[0060] L 1 , L 2 and L 3 Each independently of one another is a fully fluorinated aliphatic or aromatic bridge group. Correspondingly, there are no hydrogen atoms in the bridge group. The complete fluorination of the bridge group can ensure that the ligand is generally stable to electrolysis and higher cell voltages.

[0061] The bridge group forms a five- to eight-membered ring with the central ion P and with two oxygen atoms bonded to the central ion P and the bridge group.

[0062] The ring comprises a sequence of 2 to 5 carbon atoms, which may be interrupted by oxygen atoms. In other words, the ring may in particular have at least one ether group.

[0063] By adding such ether groups, the fluorine content of the ring can be advantageously reduced. This also reduces the fluorine content of the ligand as a whole. Although fluorinated compounds have good electrochemical tolerance, the synthesis of such compounds is highly laborious and cost-intensive. The inventors have recognized that by making the ring contain heteroatoms, the fluorine content in the ring and thus also the fluorine content of the ligand can be reduced without adversely affecting the electrochemical stability of the ligand. Suitable for this are ether groups that are also stable to the oxidation potential, so that despite the reduced fluorine content, the ligand is still electrochemically stable.

[0064] In an improved embodiment of the present invention, the bridge group L 1 , L 2 and / or L 3 Each has a linear, branched or cyclic saturated hydrocarbon backbone. Here and hereinafter, the term "hydrocarbon backbone" is to be understood as a "perfluorinated hydrocarbon backbone".

[0065] The bridging group L 1 , L 2 and / or L 3 The hydrocarbon backbone of preferably has 3 to 16 carbon atoms, preferably 6 to 9 carbon atoms. Hydrocarbon backbones having a number of carbon atoms in the ranges mentioned give rise to anions which form particularly stable salts of the formula (I).

[0066] In the sense of the present invention, the bonding of the bridge group to the central ion P via an oxygen atom is understood as coordinate bonding. The bonding of the ligand to the central ion P forms a ring consisting of the bridge group, the two oxygen atoms bonded to the bridge group and the central ion P.

[0067] 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 4 carbon atoms. In this embodiment, there are no heteroatoms in the ring.

[0068] Such rings form salts of formula (II)

[0069]

[0070] Wherein n=0,1,2 or 3 and R is a radical. M is a metal cation selected from the group consisting of alkali metals, alkaline earth metals and metals of the 12th group of the periodic table. m is 1 or 2 and P represents a central ion, which is phosphorus. The anion of the salt of formula (II) has a total of three polycyclic rings in the bonding state according to formula (I). The radicals R may be identical or different and independently selected from C1-C 10A group consisting of a perfluoroalkyl group and fluorine.

[0071] In the sense of the present invention, the term C1-C 10 The perfluoroalkyl group includes a straight chain, branched or branched saturated perfluorinated hydrocarbon group having 1 to 10 carbon atoms.

[0072] Examples of suitable perfluoroalkyl groups are trifluoromethyl, perfluoroethyl, perfluoropropyl, perfluoroisopropyl, perfluoro-n-butyl, perfluoro-sec-butyl, perfluoroisobutyl and perfluoro-tert-butyl.

[0073] If n in formula (II) is equal to 0, the ring formed by the central ion P, the bridge group and the two oxygen atoms bonded to the bridge group is pentacyclic and has a continuous sequence of 2 carbon atoms.

[0074] If n in formula (II) is equal to 1, the ring formed by the central ion P, the bridge group and the two oxygen atoms bonded to the bridge group is hexacyclic and has a continuous sequence of 3 carbon atoms.

[0075] If n in formula (II) is equal to 3, the ring formed by the central ion P, the bridge group and the two oxygen atoms bonded to the bridge group is eight-membered and has a continuous sequence of 5 carbon atoms.

[0076] In a preferred embodiment, n in formula (II) corresponds to 0 and the radicals R are identical and correspond to methyl radicals optionally substituted by fluorine. This type of chelating ligand is derived from pinacol as the simplest representative.

[0077] In an advantageous development of the invention, component (B) of the electrolyte composition comprises at least one lithium salt of the formula (I). The lithium salt is particularly suitable for use as a lithium-conducting salt in lithium-ion batteries.

[0078] The lithium salt may preferably be selected from the group consisting of:

[0079] P(O2C3(CF3)6)3 of formula (III)

[0080]

[0081] P(O2C2(CF3)4CF2)3 of formula (IV)

[0082]

[0083] And P(O2C2(CF3)4)3 of formula (V)

[0084]

[0085] and combinations thereof.

[0086] The proposed lithium salt dissolves well in liquid sulfur dioxide as a solvent. The electrolyte composition prepared therefrom is non-flammable and has excellent ionic conductivity over a wide temperature range.

[0087] The conductivity of lithium salts can be determined by conductivity measurement. For this purpose, lithium salts (III)-(V) are prepared in different concentrations in sulfur dioxide. The conductivity of the solution is then determined with the aid of a two-electrode sensor immersed in the solution at constant room temperature. For this purpose, the conductivity of solutions with lithium salts (III)-(V) is measured in the range of 0-100 mS / cm.

[0088] Due to the high electrochemical tolerance of lithium salts, lithium salts do not participate in the cyclic and daily aging processes in battery cells.

[0089] In addition, the proposed lithium salts have increased thermal, chemical and electrochemical resistance and particularly good hydrolysis resistance. The thermal resistance can be investigated, for example, by thermogravimetric analysis (TGA) and dynamic differential scanning calorimetry (DSC).

[0090] Due to the increased thermal, chemical and electrochemical stability of the proposed conductive salts, the service life of lithium-ion batteries is increased. Thus, electrolyte compositions prepared from lithium salts are also cost-effective in operation.

[0091] In addition, the properties of the lithium-conducting salts mentioned enable a range of suitable recycling processes. Preferably, recycling processes based on water as a solvent can be used. As a result, the lithium-conducting salts can be completely recovered again from the used batteries.

[0092] Due to the better recyclability of the electrolyte, costs are saved in the battery manufacturing process, and the production costs of the electrolyte salt can be included in these costs.

[0093] In another embodiment, the electrolyte composition comprises the component (B) at 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, relative to the total volume of the electrolyte composition.

[0094] The electrolyte composition may further comprise at least one additional additive in a proportion of 0-10 wt %, preferably 0.1-2 wt %, relative to the total weight of the electrolyte composition.

[0095] In one embodiment, the additional additive comprises a compound 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 sulfonyl imides, halogenated cyclic and acyclic phosphates, halogenated cyclic and acyclic phosphines, halogenated cyclic and acyclic phosphites, halogenated cyclic and acyclic phosphazenes, halogenated cyclic and acyclic silyl amines, halogenated cyclic and acyclic halogenated esters, halogenated cyclic and acyclic amides, halogenated cyclic and acyclic anhydrides and halogenated organic heterocycles.

[0096] The additional additives may contribute to the stability of the electrolyte composition during operation of the electrochemical cell.

[0097] The additional additive may also provide at least one additional lithium-containing conductive salt to the electrolyte composition. In one embodiment, the additional lithium-containing conductive salt may help to adapt the conductivity of the electrolyte composition to the requirements of the corresponding battery cell or to improve the corrosion resistance of the metal support foil of the cathode.

[0098] Preferred lithium-containing conducting salts include lithium tetrafluoroborate (LiBF4), lithium trifluoromethanesulfonate, lithium fluoride, lithium bromide, lithium sulfate, lithium oxalate, lithium (bisoxalate) borate, lithium difluoro (oxalate) borate, lithium tetrahaloaluminate, lithium hexafluorophosphate, tris-(perfluoroethyl) lithium trifluorophosphate, tris-(perfluoropropyl) lithium trifluorophosphate, tris-(perfluorobutyl) lithium trifluorophosphate, tris-(perfluoropentyl) lithium trifluorophosphate, bis-(trifluoromethanesulfonyl) imide lithium (LiTFSI) and bis-(fluorosulfonyl) imide lithium (LiFSI). Possible isomers of the compounds are also included.

[0099] The additional additives may also include additional solvents. The additional solvents may help to adjust the solubility of the electrolyte composition with respect to the polar or non-polar components therein.

[0100] The additional solvent preferably includes vinyl ethylene carbonate (VEC), ethyl methyl carbonate (EMC), vinylene carbonate (VC) and 4-fluoro-1,3-dioxolane-2-one (FEC).

[0101] In another embodiment, the additional additives may also include 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 and combinations thereof.

[0102] In a particularly preferred embodiment, the electrolyte composition comprises the following components:

[0103] (A) Sulfur dioxide;

[0104] (B) at least one salt of the above formula (I) at 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 compounds of formula (III), (IV) and (V) and combinations thereof;

[0105] (C) 0 to 10 wt. %, preferably 0.1 to 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-dioxolane-2-one (FEC), lithium fluoride, lithium hexafluorophosphate, cis-4,5-difluoro-1,3-dioxolane-2-one (cDFEC), 4-(trifluoromethyl)-1,3-dioxolane-2-one, tris-(perfluoroethyl) lithium trifluorophosphate, tris-(perfluoropropyl) lithium trifluorophosphate, tris-(perfluorobutyl) lithium trifluorophosphate, tris-(perfluoropentyl) lithium trifluorophosphate, bis-(trifluoromethanesulfonyl)imide (LiTFSI) and bis-(fluorosulfonyl)imide (LiFSI), including isomers thereof, and combinations thereof, relative to the total weight of the electrolyte composition.

[0106] The electrolyte composition according to the present invention has improved hydrolysis resistance during recycling and has higher conductivity compared to an electrolyte composition comprising an electrolyte salt formed of boron or aluminum and a bidentate ligand that is readily commercially available.

[0107] The invention also 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.

[0108] In an advantageous improvement of the present invention, the electrochemical cell is a lithium ion cell, wherein the electrolyte composition comprises the following components:

[0109] (A) Sulfur dioxide;

[0110] (B) 0.5-2 mol / L of a salt of formula (I) relative to the total volume of the electrolyte composition;

[0111] (C) 0.1-2 wt % of lithium hexafluorophosphate and 0.1-2 wt % of 4-fluoro-1,3-dioxolane-2-one (FEC), each relative to the total weight of the electrolyte composition.

[0112] The proposed lithium-ion battery cell is cost-effective and can be safely operated at various operating voltages. The relevant electrochemical properties can be determined by measuring the test cells.

[0113] The cyclic aging resistance of the test cell can be determined via the number of cycles. The test cell is first charged to the maximum permissible cell voltage with a constant charging current intensity. The upper disconnection voltage limit is kept constant until the charging current has dropped to the input value or the maximum charging time has been reached. This is also referred to as I / U charging. The test cell is then discharged with a constant discharge current intensity to a given disconnection voltage. The charging is repeated depending on the number of cycles to be achieved. Here, the upper and lower disconnection voltage limits as well as the given charging or discharging current intensity must be selected experimentally. This also applies to the value to which the charging current is reduced.

[0114] The resistance to day-to-day aging and the degree of self-discharge can be determined by storing fully charged battery cells, in particular at elevated temperatures.

[0115] For this purpose, the battery cell is charged to an upper permissible voltage limit and is held at this voltage until the charging current has dropped to a previously determined limit value.

[0116] The cell is then disconnected from the power source and stored in a temperature chamber at elevated temperature, for example 45° C., for a certain period of time, for example one month (variant 1).

[0117] The cell is then removed from the temperature chamber again and the residual capacity present is determined under defined conditions. For this purpose, a discharge current is selected, which corresponds to, for example, one third of the nominal capacity, and the cell is discharged as a result to the lower discharge limit. This process can be repeated as many times as desired, for example until the detectable residual capacity has fallen to a previously determined value, for example 70% of the nominal capacity, in order to determine the extent of the capacity loss caused by daily use or the self-discharge associated with the service life of the cell.

[0118] In a second storage variant (variant 2), storage takes place in a temperature chamber with the power supply connected, the voltage corresponding to the upper voltage limit being maintained.

[0119] Experiments were performed according to these two storage variation schemes.

[0120] The actual daily aging and self-discharge of the battery cells were then determined from these experiments: Daily aging corresponds to the capacity loss resulting from storage according to variant 2 and is calculated by subtracting the measured 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 relative to the nominal capacity of the battery cell.

[0121] The cathode of the lithium-ion cell preferably comprises a cathode active material.

[0122] Preferred cathode active materials for electrochemical cells 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.

[0123] Lithium nickel manganese cobalt compounds are also known by the abbreviation NMC, sometimes also using the technical abbreviation NCM instead. Cathode materials based on NMC are used in particular in lithium-ion batteries for vehicles. NMC as a cathode material has an advantageous combination of desirable properties, such as high specific capacity, reduced cobalt content, high maximum current capacity and high intrinsic safety, which is manifested, for example, in sufficiently high stability in the event of overcharging.

[0124] NMC can be used with the general unit Li α Ni x Mn y Co z O2, where x+y+z=1, where α represents the value of the stoichiometric ratio of lithium and is usually between 0.95 and 1.05. The literature gives a specific stoichiometric ratio as a three-digit number, such as NMC 811, NMC 622, NMC 532 and NMC 111. These three digits give the relative content of nickel: manganese: cobalt, respectively. In other words, for example, NMC 811 is a material with the general formula unit LiNi 0.8 Mn 0.1 Co 0.1 O2 cathode material, that is, where α = 1. In addition, it is also possible to use a cathode material having a general unit Li 1+ε (Ni x Mn y Co z ) 1-ε So-called lithium-rich and manganese-rich NMC of O2, wherein ε is in particular between 0.1 and 0.6, preferably between 0.2 and 0.4. Such lithium-rich layered oxides are also referred to as overlithiated (layered) oxides (OLO).

[0125] In addition to the cathode active material, the cathode may also have further components and additives, such as a foil support (rolled metal foil) or a polymer film coated with a metal, an electrode binder and / or a conductivity improver (e.g. conductive carbon black). All common compounds and materials known in the prior art can be used as further components and additives.

[0126] The anode of the lithium-ion cell preferably comprises an anode active material.

[0127] The anode active material can be selected from the group consisting of carbonaceous materials, soft carbon, hard carbon, natural graphite, synthetic graphite, silicon, silicon suboxides, silicon alloys, lithium, lithium alloys, aluminum alloys, indium, indium alloys, tin, tin alloys, cobalt alloys, niobium pentoxide, titanium dioxide, titanates such as lithium titanate (Li4Ti5O 12 or Li2Ti3O7), tin dioxide and mixtures thereof.

[0128] The anode active material is preferably selected from the group consisting of synthetic graphite, natural graphite, graphene, mesophase carbon, doped carbon, hard carbon, soft carbon, fullerene, silicon-carbon composites, silicon, surface-coated silicon, silicon suboxides, silicon alloys, lithium, aluminum alloys, indium alloys, tin alloys, cobalt alloys, and mixtures thereof.

[0129] In addition to the anode active material, the anode may also have further components and additives, such as a foil support, an electrode binder and / or a conductivity improver (e.g. conductive carbon black, conductive graphite, so-called "carbon nanotubes" (CNTs), carbon fibers and / or graphene). All common compounds and materials known in the prior art can be used as further components and additives.

[0130] The present invention also relates to a salt having an anionic complex comprising three bidentate ligands, wherein the salt corresponds to the following formula (I):

[0131]

[0132] wherein 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 is an integer from 1 to 2. P represents elemental phosphorus and is the central ion of the anionic complex. 1 , L 2 and L 3 Each independently represents a perfluorinated aliphatic or aromatic bridge group. The bridge group forms a five-membered to eight-membered ring with the central ion P and with two oxygen atoms bonded to P and the bridge group, and wherein the ring comprises a sequence of 2 to 5 carbon atoms optionally interrupted by oxygen atoms.

[0133] With regard to the properties and advantages of the salts, reference is made to the previous embodiments, which also apply in an analogous manner to the salts themselves.

[0134] The salt of formula (I) is preferably tris-perfluoro-pikanolato-lithium phosphate (LiP(PFP)3) of the following formula (V):

[0135]

[0136] The salt of formula (V) is a lithium salt and is unique in its high conductivity in sulfur dioxide.

[0137] The present invention also provides for the use of the abovementioned salts of the formula (I) in electrochemical cells.

[0138] The salt is preferably used as a conducting salt for conducting lithium ions in the electrochemical cell.

[0139] The lithium salts of the formula (V) are preferably used as conductive salts in electrochemical cells. The lithium salts of the formula (V) are inexpensive, simple to produce and have a higher conductivity than other conductive salts having anion chelate complexes.

[0140] In a particularly advantageous embodiment, it is provided that the electrochemical cell is based on sulfur dioxide as electrolyte. DETAILED DESCRIPTION

[0141] Example

[0142] The invention is explained below with the aid of examples, which are however not to be understood in a limiting sense.

[0143] Synthesis of Lithium Triperfluoropinacolato Phosphate (LiP(PFP)3)

[0144] LiP(PFP)3 can be obtained in a two-step synthesis, where P(PFP)2OH is first synthesized as an intermediate in the first step and converted into LiP(PFP)3 in the second step.

[0145] In order to more simply synthesize tetraalkoxy (hydroxy) phosphane P(PFP)2OH, the The synthetic route proposed by W. Storzer (Angew. Chem. 94 (1982) p. 212) was modified. All the steps described below were carried out under a protective atmosphere.

[0146] Perfluoropinacol (H2PFP) was placed in a flask and titrated with a stoichiometric solution of methyllithium in ether until exactly one proton of the perfluoropinacol was exchanged by a lithium ion, which can also be discerned from the slowing down of methane formation. The white LiHPFP thus formed was concentrated and the ether residue was removed in vacuo at 60° C. until constant weight.

[0147] 9.8 g of LiHPFP dissolved in 40 ml of acetonitrile were placed in advance. 3.0 g of PCl5 dissolved in 40 ml of acetonitrile were slowly added dropwise to the LiHPFP solution. A pure white, coarse precipitate quickly formed. The reaction mixture was allowed to continue boiling overnight under reflux. The precipitate was then allowed to settle and filtered out. The almost colorless filtrate was carefully evaporated under vacuum, leaving a light yellow, almost white material.

[0148] This material is dissolved in 30ml ether. Then, in the ethereal solution, a solution of 0.35g LiOH in 10ml water is slowly added dropwise under stirring. Then continue stirring for three hours, and then extract the aqueous phase with 10ml diethyl ether for many times. Through NaSO4 dry the ether extracts merged, then recrystallize P(PFP)2OH from ether. Can realize the improved purification of P(PFP)2OH by sublimation by means of a sublimation apparatus.

[0149] The P(PFP)2OH thus obtained can be converted to LiP(PFP)3 according to the following synthesis.

[0150] 10g P(PFP)2OH and 5.25g LiHPFP were dissolved in 40ml acetonitrile in a 100ml laboratory autoclave. 0.08g lithium tert-butoxide was added as a catalyst. The autoclave was then pressure-tightly closed, a stirring motor was turned on, and the contents were slowly heated to 120°C. After a reaction duration of 24 hours, the reaction was cooled to room temperature again and the solvent was subsequently evaporated. The reaction water still attached was removed by azeotropic distillation with hexane as an entrainer, and unconverted P(PFP)2OH was reacted with the slightly excessively added LiHPFP to form LiP(PFP)3 by dehydration.

[0151] Finally, the obtained LiP(PFP)3 is subjected to perfluorohexane extraction to remove insoluble components. LiP(PFP)3 is soluble in hot perfluorohexane and almost completely crystallizes out of it when cooled to -20°C. The remaining solvent can be removed in vacuo after filtration. 12.5 g of white fine powdered LiP(PFP)3 salt is obtained.

Claims

1. A liquid electrolyte composition for an electrochemical cell, wherein the electrolyte composition comprises the following components: (A) Sulfur dioxide; (B) at least one salt, wherein the salt comprises an anionic complex having three bidentate ligands and the salt corresponds to the following formula (I): in - 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; -P represents a central ion which is phosphorus; and -L 1 , L 2 and L 3 Each independently of one another is a perfluorinated aliphatic or aromatic bridging group, wherein the bridging group forms a five- to eight-membered ring with the central ion P and with two oxygen atoms bonded to P and the bridging group, and wherein the ring comprises a sequence of 2 to 5 carbon atoms optionally interrupted by oxygen atoms.

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

3. The electrolyte composition according to claim 1 or 2, characterized in that L 1 , L 2 and / or L 3 Each independently contains a linear, branched or cyclic saturated hydrocarbon backbone, wherein the hydrocarbon backbone preferably has 3 to 16 carbon atoms, more preferably 6 to 9 carbon atoms.

4. An electrolyte composition according to any one of the preceding claims, characterized in that The ring comprises a consecutive sequence of 2 to 5 carbon atoms, preferably 2 to 3 carbon atoms.

5. An electrolyte composition according to any one of the preceding claims, characterized in that Component (B) of the electrolyte composition comprises at least one lithium salt of formula (I), wherein the lithium salt is preferably selected from the group consisting of: P(O2C3(CF3)6)3 of formula (III) P(O2C2(CF3)4CF2)3 of formula (IV) And P(O2C2(CF3)4)3 of formula (V) and combinations thereof.

6. An electrolyte composition according to any one of the preceding claims, characterized in that The electrolyte composition comprises the component (B) at 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, relative to the total volume of the electrolyte composition.

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

8. A liquid electrolyte composition according to any 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 compounds of formula (III), (IV) and (V) and combinations thereof; (C) 0 to 10 wt %, preferably 0.1 to 2 wt %, of at least one additive, relative to the total weight of the electrolyte composition, wherein the additive is preferably selected from the group consisting of vinylene carbonate (VC), 4-fluoro-1,3-dioxolane-2-one (FEC), lithium fluoride, lithium hexafluorophosphate, cis-4,5-difluoro-1,3-dioxolane-2-one (cDFEC), 4-(trifluoromethyl)-1,3-dioxolane-2-one, tris-(perfluoroethyl) lithium trifluorophosphate, tris-(perfluoropropyl) lithium trifluorophosphate, tris-(perfluorobutyl) lithium trifluorophosphate, tris-(perfluoropentyl) lithium trifluorophosphate, bis-(trifluoromethanesulfonyl)imide (LiTFSI) and bis-(fluorosulfonyl)imide (LiFSI), including isomers thereof, and combinations thereof.

9. An electrochemical cell having a cathode, an anode and an electrolyte composition according to any one of the preceding claims, the electrolyte composition being in contact with the cathode and the anode.

10. The 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) relative to the total volume of the electrolyte composition; (C) 0.1-2 wt % of lithium hexafluorophosphate and 0.1-2 wt % of 4-fluoro-1,3-dioxolane-2-one (FEC), each relative to the total weight of the electrolyte composition.

11. A salt having an anionic complex comprising three bidentate ligands, wherein the salt corresponds to the following formula (I): in - 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; -P represents a central ion which is phosphorus; and -L 1 , L 2 and L 3 Each independently of one another is a perfluorinated aliphatic or aromatic bridging group, wherein the bridging group has a central ion P and forms a five-membered to eight-membered ring with two oxygen atoms bonded to the central ion P and the bridging group, and wherein the ring comprises a sequence of 2 to 5 carbon atoms optionally interrupted by oxygen atoms.

12. The salt according to claim 11, characterized in that The salt is a tri-perfluoropinacolato lithium phosphate (LiP(PFP)3) of the following formula (V):

13. Use of the salt according to any one of claims 11 or 12 in an electrochemical cell.

14. The use of the salt according to claim 13, characterized in that The salt is used as a conducting salt for conducting lithium ions 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 electrolyte.

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