DE102017107257A1 - Electrolyte additive for lithium-ion batteries - Google Patents

Abstract

The invention relates to the use of compounds according to the general formula (1), in particular 1,4,2-dioxoazol-5-one derivatives, as an additive in electrolytes for electrochemical energy storage such as lithium-ion batteries, and electrolytes containing compounds according to the general formula (1), especially 1,4,2-dioxoazol-5-one derivatives.

Description

The invention relates to the field of lithium-ion batteries.

Lithium-ion batteries (secondary batteries) are currently the leading technology in the field of rechargeable batteries, especially in the field of portable electronics. Conventional lithium-ion batteries use an anode of carbon, usually graphite. Charge transport occurs via an electrolyte comprising a lithium salt dissolved in a solvent. Various electrolytes and conductive salts are known in the prior art. Conventional lithium-ion batteries currently use mostly lithium hexafluorophosphate (LiPF ₆ ).

Suitable electrolytes are characterized in that the formation of a solid-electrolyte phase interface, the so-called solid electrolyte interphase (SEI) is induced on an electrode. In the case of graphite anodes, this leads to a reductive decomposition of the electrolyte and the reaction products can form an adhering and electronically insulating, but lithium ion-conducting film on the anode. The Solid Electrolyte Interphase subsequently prevents the electrode material from reacting further with the electrolyte and thereby protects the electrolyte from further reductive decomposition and the anode from destruction by the solvent. Especially when using graphite anodes, the formation of such a film is necessary for safe operation of the lithium-ion battery.

However, the reductive decomposition of the solvent propylene carbonate (IUPAC designation 4-methyl-1,3-dioxolan-2-one) does not lead to the formation of a solid electrolyte interphase. Thus, the reductively induced gas evolution within the graphite layers, induced by co-intercalation of propylene carbonate, causes the "exfoliation" and the irreversible destruction of the active material. This limits the use of propylene carbonate, despite its better thermal and physicochemical properties over ethylene carbonate, for lithium-ion technology. Furthermore, propylene carbonate can serve as a model system for electrolytes, which also show a reductive decomposition without SEI formation and exfoliation of graphite.

In the prior art, other electrolytes containing dioxolanes are known as organic solvents. The font DE 10 2010 020 992 A1 for example, discloses solvents comprising ethylene carbonate, propylene carbonate, butylene carbonate and others such as 3-methyl-1,3-oxazolidin-2-one, as well as mixtures of two or more of these solvents.

It has already been proposed to prevent the exfoliation of the graphite and the reductive decomposition of the solvent by the use of highly concentrated electrolytes. However, the use of highly concentrated electrolytes, also known as "solvent-in-salt" electrolytes, is not economical, since this approach requires a multiple of the normally required amount of electrolyte salt. Furthermore, the concentration of (usually> 3 mol 1 ^-1 ) increases the viscosity of the electrolyte greatly, which leads to a marked decrease in the conductivity and the performance of the battery. Furthermore, it is to be expected that when the operating temperature falls, the solubility product of the electrolyte salt in the electrolyte solution falls below the limit, which leads to precipitation of the salt inside the batteries. In addition, increases the volume and increasing the addition of conductive salt, the density and therefore the total mass of the electrolyte at a constant volume. This also means that the specific energy density (Ah kg ^-1 ) of the battery as a whole system decreases.

Further, the use of adequate performance additives has been suggested. Particularly important in commercial batteries are vinylene carbonate (VC) and fluoroethylene carbonate (FEC). However, the conventionally used additive vinylene carbonate can only be used up to a final voltage of 4.7 V maximum in lithium-ion batteries, otherwise there is an oxidative decomposition. Especially for the development of high-voltage electrolytes there is a need for SEI-forming electrolyte additives with increased oxidative stability.

The present invention was therefore based on the object to provide an electrolyte which overcomes at least one of the aforementioned disadvantages of the prior art. In particular, the object of the present invention was to provide a compound which supports the formation of a solid electrolyte interphase.

worin:

X

ist C, S oder S=O;

R¹

ist ausgewählt aus der Gruppe umfassend CN, C₁-C₁₀-Alkyl, C₁-C₁₀-Alkoxy, C₃-C₇-Cycloalkyl, C₆-C₁₀-Aryl und/oder -CO-O-R², wobei die Alkyl-, Alkoxy-, Cycloalkyl- und Arylgruppen jeweils unsubstituiert oder einfach oder mehrfach mit wenigstens einem Substituenten ausgewählt aus der Gruppe umfassend F, C_1-4-Alkyl und/oder CN substituiert sind; und

R²

ist ausgewählt aus der Gruppe umfassend C₁-C₁₀-Alkyl, C₃-C₇-Cycloalkyl, und/oder C₆-C₁₀-Aryl.

This object is achieved by an electrolyte for an electrochemical energy store comprising an electrolyte salt and a solvent, wherein the electrolyte comprises at least one compound according to the general formula (1) as indicated below:

wherein:

X

is C, S or S = O;

R ¹

is selected from the group consisting of CN, C ₁ -C ₁₀ -alkyl, C ₁ -C ₁₀ -alkoxy, C ₃ -C ₇ -cycloalkyl, C ₆ -C ₁₀ -aryl and / or -CO-OR ² , wherein the Alkyl, alkoxy, cycloalkyl and aryl groups are each unsubstituted or monosubstituted or polysubstituted by at least one substituent selected from the group consisting of F, C _1-4 -alkyl and / or CN; and

R ²

is selected from the group comprising C ₁ -C ₁₀ -alkyl, C ₃ -C ₇ -cycloalkyl, and / or C ₆ -C ₁₀ -aryl.

Further advantageous embodiments of the invention will become apparent from the dependent claims and the independent claims.

Surprisingly, it has been found that compounds according to the general formula (1) form a solid electrolyte interphase (SEI) on a graphite electrode. By using a compound according to the general formula (1) in electrolytes, the use of solvents such as propylene carbonate, which alone do not form solid-electrolyte interphase, is thus made possible in electrochemical energy stores such as lithium-ion batteries. It has been found that the compounds of general formula (1) can help form a stable solid electrolyte interphase that can protect graphite anodes from exfoliation and bulk electrolytes from continuous reductive decomposition over at least 50 charge and discharge cycles , In particular, it is advantageous that the decomposition of the compounds according to the general formula (1), such as 3-methyl-1,4,2-dioxoazol-5-one, occurs at higher potentials than that of vinylene carbonate. The higher oxidative stability of the SEI-forming compounds of general formula (1) thereby enables use in high-voltage batteries.

Advantageously, an inventive electrolyte can thus be used as a high-voltage electrolyte. Without being bound by any particular theory, it is believed that a polymerization reaction involving the double bond of the 1,4,2-dioxoazol-5-one derivatives according to the general formula (1) is involved in the presumed reaction mechanism of formation of the SEI , and thus is essential for the advantageous properties of the compounds.

The term "C ₁ -C ₁₀ -alkyl", unless stated otherwise, includes straight-chain or branched alkyl groups having 1 to 10 carbon atoms. The term "C ₆ -C ₁₀ -aryl" is to be understood as meaning aromatic radicals having 6 to 10 carbon atoms. The term "aryl" preferably includes carbocycles. C ₆ -C ₁₀ -aryl groups are preferably selected from the group comprising phenyl and / or naphthyl, preferably phenyl. C ₃ -C ₇ cycloalkyl groups are preferably selected from the group comprising cyclopentyl and / or cyclohexyl. C ₁ -C ₅ -alkoxy groups are preferably selected from the group comprising methoxy, ethoxy, linear or branched propoxy, butoxy and / or pentoxy.

The substituent X may be carbon or sulfur or a group S = O. Preferably, X is carbon.

The substituent R ¹ is preferably selected from the group comprising CN, C ₁ -C ₅ -alkyl, C ₁ -C ₅ -alkoxy, C ₅ -C ₆ -cycloalkyl and / or phenyl, where the alkyl, alkoxy, cycloalkyl - And phenyl groups are each unsubstituted or mono- or poly-substituted by fluorine. Fluorinated substituents R ¹ can support the formation of Solid Electrolyte Interphase. In particular, by fluorination, the stability of the electrolyte additive can be tailored to the requirements of a lithium-ion battery. In preferred embodiments, R ^{1 is} selected from the group consisting of CN and / or C ₁ -C ₅ alkyl or phenyl which is unsubstituted or mono- or poly-fluoro-substituted. Such groups R ¹ can advantageously assist the reactions of the double bond which lead to the formation of a solid electrolyte interphase (SEI) on an electrode. C ₁ -C ₅ -alkyl groups are in particular selected from the group comprising methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, isopentyl, tert-pentyl and / or neopentyl. In further preferred embodiments, R ^{1 is} selected from the group comprising CN, tert-butyl, phenyl and / or unsubstituted or mono- or poly-fluorine-substituted C ₁ -C ₂ -alkyl. In particularly preferred embodiments, R ^{1 is} selected from the group comprising CH ₃ , CF ₃ , CN, tert-butyl and / or phenyl.

When R ^{1 is} a group -CO-OR ² , R ^{2 is} preferably selected from the group comprising C ₁ -C ₅ -alkyl, C ₅ -C ₆ -cycloalkyl and / or phenyl.

X

ist C, S oder S=O; und/oder

R¹

ist ausgewählt aus der Gruppe umfassend CN, C₁-C₅-Alkyl, C₁-C₅-Alkoxy, C₅-C₆-Cycloalkyl, Phenyl und/oder -CO-O-R², wobei die Alkyl-, Alkoxy-, Cycloalkyl- und Phenylgruppen jeweils unsubstituiert oder einfach oder mehrfach mit Fluor substituiert sind, und/oder

R²

ist ausgewählt aus der Gruppe umfassend C₁-C₅-Alkyl, C₅-C₆-Cycloalkyl, und/oder Phenyl.

Preferably, the electrolyte comprises at least one compound according to the general formula (1) in which:

X

is C, S or S = O; and or

R ¹

is selected from the group comprising CN, C ₁ -C ₅ -alkyl, C ₁ -C ₅ -alkoxy, C ₅ -C ₆ -cycloalkyl, phenyl and / or -CO-OR ² , where the alkyl, alkoxy, Cycloalkyl and phenyl groups each unsubstituted or simple or substituted several times with fluorine, and / or

R ²

is selected from the group comprising C ₁ -C ₅ -alkyl, C ₅ -C ₆ -cycloalkyl, and / or phenyl.

Particularly preferably, the electrolyte comprises at least one compound according to the general formula (1) in which X is carbon and / or R ^{1 is} selected from the group comprising CN, phenyl and / or unsubstituted or mono- or polysubstituted with F-substituted C ₁ -C ₅ - Alkyl, in particular C ₁ -C ₂ -alkyl. Advantageously, small alkyl groups lead to compounds that can contribute to the efficient formation of a solid electrolyte interphase. In preferred embodiments, X is carbon and R ¹ is selected from the group comprising CH ₃ , CF ₃ , CN, tert-butyl and / or phenyl.

worin:

R¹

ist ausgewählt aus der Gruppe umfassend CN, C₁-C₅-Alkyl, C₁-C₅-Alkoxy, C₅-C₆-Cycloalkyl, Phenyl und/oder -CO-O-R², wobei die Alkyl-, Alkoxy-, Cycloalkyl- und Phenylgruppen jeweils unsubstituiert oder einfach oder mehrfach mit Fluor substituiert sind, und/oder

R²

ist ausgewählt aus der Gruppe umfassend C₁-C₅-Alkyl, C₅-C₆-Cycloalkyl, und/oder Phenyl.

X is particularly preferably carbon and the electrolyte comprises at least one 1,4,2-dioxoazol-5-one derivative according to the general formula (2)

wherein:

R ¹

is selected from the group comprising CN, C ₁ -C ₅ -alkyl, C ₁ -C ₅ -alkoxy, C ₅ -C ₆ -cycloalkyl, phenyl and / or -CO-OR ² , where the alkyl, alkoxy, Cycloalkyl and phenyl groups are each unsubstituted or mono- or poly-substituted by fluorine, and / or

R ²

is selected from the group comprising C ₁ -C ₅ -alkyl, C ₅ -C ₆ -cycloalkyl, and / or phenyl.

Preference is given to 1,4,2-dioxoazol-5-one derivatives in which R ^{1 is} selected from the group consisting of CN and / or unsubstituted or mono- or poly-fluorine-substituted C ₁ -C ₅ -alkyl or phenyl, in particular selected from Group comprising CN, tert-butyl, phenyl and / or unsubstituted or mono- or poly-fluorine-substituted C ₁ -C ₂ -alkyl. In particular, 1,4,2-dioxoazol-5-one derivatives in electrolytes can advantageously form a stable solid electrolyte interphase which, over a period of cyclization of at least 50 charging and discharging cycles, has graphite anodes before exfoliation and the bulk electrolyte before the continuous one Can protect reduction. A particularly preferred 1,4,2-dioxoazol-5-one derivative is 3-methyl-1,4,2-dioxoazol-5-one.

It has been shown that an electrolyte containing 3-methyl-1,4,2-dioxoazol-5-one has a wide oxidative stability window, and because of this high stability, it can also be used for applications with high-voltage electrode materials. This illustrates a particular advantage, since conventional SEI-formers do not allow graphite anodes with high-voltage cathode materials in lithium-ion batteries with turn-off potentials higher than 4.5V. Li / Li ⁺ to operate. The compounds of the invention are used as an additive in electrolytes with common electrode materials such as Li (Ni _1/3 Mn _1/3 Co _1/3 ) O ₂ (NMC (111)) cathodes, LiFePO ₄ (LFP), LiMPO ₄ (M = Mn , Ni, Co), LiMn ₂ O ₄ (LMO), LiNi _x Mn _y O ₄ (LNMO), Li (Ni _x Co _y Mn _z ) O ₂ (x + y + z = 1) (NMC (XYZ)) , (Li ₂ MnO ₃ ) _x (LiMO ₂ ) _1-x and Li (Ni _x Co _y Al _z ) O ₂ (x + y + z = 1) (NCA).

The compounds according to the general formula (1) are commercially available or can be prepared by methods familiar to the person skilled in the art.

In preferred embodiments, the electrolyte contains the compound of the general formula (1) in a range of ≥ 0.1 wt% to ≤ 10 wt%, preferably in the range of ≥ 0.5 wt% to ≤ 7 Wt .-%, preferably in the range of ≥ 3 wt .-% to ≤ 5 wt .-%, based on the total weight of the electrolyte. Data in wt .-% are each based on a total weight of the electrolyte of 100 wt .-%. Figures in percent correspond, unless otherwise indicated, in% by weight.

Advantageously, an electrolyte comprising such proportions of a compound of the general formula (1), in particular 1,4,2-dioxoazol-5-one derivatives such as 3-methyl-1,4,2-dioxoazol-5-one, show a very good formation of a Solid Electrolyte interphase. Advantageously, proportions in the range of ≥ 3 wt .-% to ≤ 5 wt .-%, based on the total weight of the electrolyte, already sufficient for an effective passivation of graphite. Such small amounts by weight of an additive allow for economical commercial application.

The electrolyte has, in addition to at least one electrolyte salt, preferably a lithium salt, and at least one compound according to the general formula (1) a solvent. The solvent preferably serves as a solvent for the electrolyte or lithium salt.

In preferred embodiments, the electrolyte comprises a solvent selected from the group comprising a non-fluorinated or partially fluorinated organic solvent, an ionic liquid, a polymer matrix and / or mixtures thereof.

Preferably, the electrolyte comprises an organic solvent, in particular a cyclic or linear carbonate. In preferred embodiments, the organic solvent is selected from the group consisting of ethylene carbonate, ethyl methyl carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, acetonitrile, propionitrile, 3-methoxypropionitrile, glutaronitrile, adiponitrile, pimelonitrile, gamma-butyrolactone, gamma-valerolactone, dimethoxyethane, 1,3- Dioxolane, methyl acetate, ethyl acetate, ethyl methanesulfonate, dimethyl methyl phosphonate, linear or cyclic sulfone such as ethyl methyl sulfone or sulfolane, symmetrical or non-symmetrical alkyl phosphates and / or mixtures thereof.

The electrolyte may in particular comprise solvents, such as propylene carbonate, which do not lead to the formation of a solid electrolyte interphase without an additive. For use with these solvents, it is particularly advantageous to add the compounds according to the invention to form an effective solid electrolyte interphase. Advantageously, using a compound of the general formula (1), an electrolyte comprising propylene carbonate as the solvent can have comparable oxidative stability and equally efficient SEI formation as known electrolytes which can form a solid electrolyte interphase without admixing other compounds. The electrolyte preferably comprises propylene carbonate, or a mixture comprising propylene carbonate and at least one further organic solvent selected from the group comprising ethylene carbonate, ethylmethyl carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, gamma-valerolactone, dimethoxyethane, 1,3-dioxolane, methyl acetate, ethyl acetate, Ethyl methanesulfonate, dimethyl methyl phosphonate, linear or cyclic sulfone such as ethyl methyl sulfone or sulfolane and / or symmetrical or non-symmetrical alkyl phosphates.

The electrolyte may also be a polymer electrolyte, for example, selected from the group consisting of polyethylene oxide, polyacrylonitrile, polyvinyl chloride, polyvinylidene fluoride, poly (vinylidene fluoride-co-hexafluoropropylene) and / or polymethylmethacrylate, or a gel-polymer electrolyte comprising a polymer and an aforementioned organic solvent , Also, the electrolyte may be an ionic liquid.

In addition to a solvent and at least one compound according to the general formula (1), the electrolyte according to the invention has at least one electrolyte salt, in particular a lithium salt.

In preferred embodiments, the electrolyte salt is selected from the group consisting of LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ , LiClO ₄ , LiPtCl ₆ , LiN (SO ₂ F) ₂ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , LiB (C ₂ O ₄ ) ₂ , LiBF ₂ (C ₂ O ₄ ) and / or LiSO ₃ CF ₃ . The lithium salt LiPF ₆ is preferred. The concentration of the lithium salt in the electrolyte may be in conventional ranges, for example in the range of ≥ 1.0 M to ≤ 1.2 M.

In a preferred embodiment, the electrolyte comprises a compound according to the general formula (1), in particular a 1,4,2-dioxoazol-5-one derivative such as 3-methyl-1,4,2-dioxoazol-5-one, at least a lithium salt, preferably LiPF ₆ , and propylene carbonate or a mixture of organic solvents comprising propylene carbonate as a solvent. The electrolyte can be prepared, for example, by introducing the lithium salt into the solvent and adding the at least one compound according to the general formula (1). Alternatively, the compound of the general formula (1) may be first mixed with the solvent, and then the lithium salt may be added.

The electrolyte may further contain at least one additive, in particular selected from the group comprising SEI-formers and / or flame retardants. For example, the electrolyte may contain compound of the general formula (1) and another SEI-formers. Suitable additives are, for example, selected from the group consisting of fluoroethylene carbonate, chloroethylene carbonate, vinylene carbonate, vinyl ethylene carbonate, ethylene sulfide, propanesultone, propensultone, sulfites, preferably dimethyl sulfite and propylene sulfite, ethylene sulfate, propylene sulfate, methylene methane disulfonate, trimethylene sulfate, optionally F, Cl or Br substituted butyrolactones, phenylethylene carbonate, Vinyl acetate and / or trifluoropropylene carbonate. For example, the electrolyte may contain compound according to the general formula (1) and another SEI-formner selected from the group comprising vinyl carbonate, fluoroethylene carbonate and / or ethylene sulfate. These connections can improve battery performance, such as capacity, long-term stability, or cycle life.

The electrolyte is particularly suitable for a battery or a rechargeable battery, in particular as an electrolyte for a lithium-ion battery or a lithium-ion rechargeable battery.

A further subject matter of the present invention relates to an electrochemical energy store, in particular a supercapacitor or lithium-ion-based electrochemical energy store, comprising an electrolyte according to the invention described above. The term "energy storage" in the context of the present invention comprises primary and secondary electrochemical energy storage devices, ie batteries (Primary storage) and accumulators (secondary storage). In common usage, accumulators are often referred to with the term "battery", often used as a generic term. Thus, the term lithium-ion battery is used synonymously with lithium-ion accumulator, unless otherwise specified. The term "electrochemical energy storage" in the context of the present invention also includes electrochemical capacitors (English: electrochemical capacitors) or supercapacitors (English: supercapacitors, short supercaps). Electrochemical capacitors, also referred to in the literature as double-layer capacitors or supercapacitors, are electrochemical energy stores, which are characterized by batteries by a significantly higher power density, compared to conventional capacitors by orders of magnitude higher energy density. Lithium-ion-based electrochemical energy stores are preferably selected from the group comprising lithium-ion batteries, lithium-ion accumulators and polymer batteries. The energy store is preferably a lithium-ion battery or a lithium-ion battery. More preferably, the electrochemical energy storage is an electrochemical capacitor, in particular supercapacitor. It could be shown that the formed solid-electrolyte phase interface was stable on a graphite anode for at least 50 cycles. This allows economical operation of rechargeable batteries.

Another object of the invention relates to a method for forming a solid-electrolyte phase interface on an electrode of an electrochemical cell comprising an anode, a cathode and an electrolyte, wherein the cell is operated using the inventive electrolyte described above. The solid electrolyte phase interface is preferably formed on the anode, in particular of a graphite anode, the negative electrode. It has become common in the consideration of batteries to use the discharge process as a definition of the terms anode and cathode.

worin:

X

ist C, S oder S=O;

R¹

ist ausgewählt aus der Gruppe umfassend CN, C₁-C₁₀-Alkyl, C₁-C₁₀-Alkoxy, C₃-C₇-Cycloalkyl, C₆-C₁₀-Aryl und/oder -CO-O-R², wobei die Alkyl-, Alkoxy-, Cycloalkyl- und Arylgruppen jeweils unsubstituiert oder einfach oder mehrfach mit wenigstens einem Substituenten ausgewählt aus der Gruppe umfassend F, C_1-4-Alkyl und/oder CN substituiert sind, und

R²

ist ausgewählt aus der Gruppe umfassend C₁-C₁₀-Alkyl, C₃-C₇-Cycloalkyl, und/oder C₆-C₁₀-Aryl,

in einem elektrochemischen Energiespeicher, insbesondere einem Superkondensator oder Lithium-Ionen-basierten elektrochemischen Energiespeicher.Another object of the invention relates to the use of a compound according to the general formula (1) as indicated below:

wherein:

X

is C, S or S = O;

R ¹

is selected from the group consisting of CN, C ₁ -C ₁₀ alkyl, C ₁ -C ₁₀ alkoxy, C ₃ -C ₇ cycloalkyl, C ₆ -C ₁₀ aryl and / or -CO-OR ² , wherein the Alkyl, alkoxy, cycloalkyl and aryl groups are each unsubstituted or monosubstituted or polysubstituted by at least one substituent selected from the group consisting of F, C _1-4 alkyl and / or CN, and

R ²

is selected from the group comprising C ₁ -C ₁₀ -alkyl, C ₃ -C ₇ -cycloalkyl, and / or C ₆ -C ₁₀ -aryl,

in an electrochemical energy store, in particular a supercapacitor or lithium-ion-based electrochemical energy store.

The compound according to the general formula (1) can be used advantageously as an electrolyte additive, in particular as a SEI-formers, in particular in electrolytes which form no SEI without addition of additive.

For the description of the compound according to the general formula (1), reference is made to the above description. The substituent X is preferably carbon. R ¹ is preferably selected from the group consisting of CN, C ₁ -C ₅ alkyl, C ₁ -C ₅ alkoxy, C ₅ -C ₆ cycloalkyl and / or phenyl, wherein the alkyl, alkoxy, cycloalkyl and Phenyl groups are each unsubstituted or mono- or poly-substituted by fluorine. In preferred embodiments, R ^{1 is} selected from the group consisting of CN and / or C ₁ -C ₅ alkyl or phenyl which is unsubstituted or mono- or poly-fluoro-substituted. In further preferred embodiments, R ^{1 is} selected from the group comprising CN, tert-butyl, phenyl and / or unsubstituted or mono- or poly-fluorine-substituted C ₁ -C ₂ -alkyl. In particularly preferred embodiments, R ^{1 is} selected from the group comprising CH ₃ , CF ₃ , CN, tert-butyl and / or phenyl. X is particularly preferably carbon and / or R ¹ is selected from the group consisting of CN, phenyl and / or unsubstituted or mono- or poly-F-substituted C ₁ -C ₅ -alkyl, in particular C ₁ -C ₂ -alkyl. In preferred embodiments, X is carbon and R ¹ is selected from the group comprising CH ₃ , CF ₃ , CN, tert-butyl and / or phenyl.

More preferably, the electrolyte comprises at least one 1,4,2-Dioxoazol-5-one derivative represented by the general formula (2) wherein R ¹ is selected from the group comprising CN, C ₁ -C ₅ -alkyl, C ₁ -C ₅ -alkoxy, C ₅ -C ₆ -cycloalkyl, phenyl and / or -CO-OR ² , where the alkyl, alkoxy, Cycloalkyl and phenyl groups are each unsubstituted or monosubstituted or polysubstituted by fluorine, and / or R ^{2 is} selected from the group comprising C ₁ -C ₅ -alkyl, C ₅ -C ₆ -cycloalkyl, and / or phenyl. Preference is given to 1,4,2-dioxoazol-5-one derivatives in which R ^{1 is} selected from the group consisting of CN and / or unsubstituted or mono- or polysubstituted fluorine-substituted C ₁ -C ₅ -alkyl or phenyl, in particular selected from the group comprising CN, tert-butyl, phenyl and / or unsubstituted or mono- or polysubstituted with fluorine-substituted C ₁ -C ₂ alkyl. In particular, 1,4,2-dioxoazol-5-one derivatives in electrolytes can advantageously form a stable solid electrolyte interphase which, over a period of cyclization of at least 50 charge and discharge cycles, has graphite anodes before exfoliation and the bulk electrolyte before continuous can protect reductive decomposition. A particularly preferred 1,4,2-dioxoazol-5-one derivative is 3-methyl-1,4,2-dioxoazol-5-one.

Examples and figures which serve to illustrate the present invention are given below.

• 1 zeigt den ersten Zyklus einer Graphit-Anode in einer Graphit/Lithium-Halbzelle unter Verwendung einer Elektrolytlösung gemäß einer Ausführungsform der Erfindung enthaltend 1 M LiPF₆ und 5 Gew.-% 3-Methyl-1,4,2-dioxoazol-5-on (Additiv) in Propylencarbonat (PC), sowie von Vergleichselektrolyten enthaltend 1 M LiPF₆ in Propylencarbonat oder einer 1:1-Mischung von Ethylencarbonat und Dimethylcarbonat (EC:DMC). Aufgetragen ist das Potential gegen die spezifische Kapazität.
• 2 zeigt das oxidative Stabilitätsfenster in LiMn₂O₄/Li Halbzellen des Elektrolyten enthaltend 1 M LiPF₆ und 5 Gew.-% 3-Methyl-1,4,2-dioxoazol-5-on in Propylencarbonat (PC), sowie von Vergleichselektrolyten enthaltend 1 M LiPF₆ in einer 1:1-Mischung von Ethylencarbonat und Dimethylcarbonat (EC:DMC) mit oder ohne 5 Gew.-% VC.
• 3 zeigt die Konstantstromzyklisierung einer NMC/Graphit-Vollzelle unter Verwendung der Elektrolytlösung enthaltend 1 M LiPF₆ und 5 Gew.-% 3-Methyl-1,4,2-dioxoazol-5-on (Additiv) in Propylencarbonat (PC), sowie eines Vergleichselektrolyten enthaltend 1 M LiPF₆ in einer 1: 1-Mischung von Ethylencarbonat und Dimethylcarbonat (EC:DMC) über 50 Zyklen. Aufgetragen ist die spezifische Entladekapazität in mAh g^-1 gegen die Anzahl der Zyklen. Nur jeder dritte Zyklus ist gezeigt.
• 4 zeigt jeweils die Coulombsche Effizienz aufgetragen gegen die Zyklenzahl für die NMC/Graphit-Vollzelle unter Verwendung der Elektrolytlösung enthaltend 1 M LiPF₆ und 5 Gew.-% 3-Methyl-1,4,2-dioxoazol-5-on (Additiv) in Propylencarbonat (PC), sowie eines Vergleichselektrolyten enthaltend 1 M LiPF₆ in einer 1: 1-Mischung von Ethylencarbonat und Dimethylcarbonat (EC:DMC) über 50 Zyklen. Nur jeder dritte Zyklus ist gezeigt.

Here are the figures:

• 1 Figure 3 shows the first cycle of a graphite anode in a graphite / lithium half-cell using an electrolyte solution according to one embodiment of the invention containing 1 M LiPF ₆ and 5% by weight of 3-methyl-1,4,2-dioxoazol-5-one (Additive) in propylene carbonate (PC), as well as comparative electrolyte containing 1 M LiPF ₆ in propylene carbonate or a 1: 1 mixture of ethylene carbonate and dimethyl carbonate (EC: DMC). The potential is plotted against the specific capacity.
• 2 shows the oxidative stability window in LiMn ₂ O ₄ / Li half-cells of the electrolyte containing 1 M LiPF ₆ and 5 wt .-% 3-methyl-1,4,2-dioxoazol-5-one in propylene carbonate (PC), as well as by comparative electrolyte containing 1 M LiPF ₆ in a 1: 1 mixture of ethylene carbonate and dimethyl carbonate (EC: DMC) with or without 5 wt% VC.
• 3 Figure 3 shows the constant current cycling of a full NMC / graphite cell using the electrolytic solution containing 1 M LiPF ₆ and 5% by weight of 3-methyl-1,4,2-dioxoazol-5-one (additive) in propylene carbonate (PC), as well as a Comparative electrolyte containing 1 M LiPF ₆ in a 1: 1 mixture of ethylene carbonate and dimethyl carbonate (EC: DMC) over 50 cycles. Plotted is the specific discharge capacity in mAh g ^-1 against the number of cycles. Only every third cycle is shown.
• 4 shows, respectively, the Coulomb efficiency versus number of cycles for the NMC / graphite full cell using the electrolytic solution containing 1 M LiPF ₆ and 5 wt% 3-methyl-1,4,2-dioxoazol-5-one (additive) in FIG Propylene carbonate (PC), and a comparative electrolyte containing 1 M LiPF ₆ in a 1: 1 mixture of ethylene carbonate and dimethyl carbonate (EC: DMC) over 50 cycles. Only every third cycle is shown.

example 1

Preparation of 3-methyl-1,4,2-dioxoazol-5-one

The synthesis was carried out as from S. Chang et al. in J. Am. Chem. Soc. 2015, 137, pages 4534-4542 described. To this was dissolved 50 mmol acetohydroxamic acid (Sigma-Aldrich) in 500 mL dichloromethane. To this was added 50 mmol of 1,1'-carbonyldiimidazole (Combi blocks) in one portion at room temperature (20 ° C ± 2 ° C). After stirring for 16 hours, the reaction mixture was quenched with 300 mL of 1 M HCl, extracted three times with 150 mL each of dichloromethane, and dried over magnesium sulfate. The solvent was removed under reduced pressure and 3-methyl-1,4,2-dioxoazol-5-one was obtained as a colorless, slightly yellowish oil.

The resulting reaction product was analyzed by ¹ H and ¹³ C NMR, with the ¹ H and ¹³ C signals corresponding to the expected values for 3-methyl-1,4,2-dioxoazol-5-one.

Example 2

Determination of specific capacity in a graphite / Li half-cell

For the determination of the specific capacity half-cell in a three electrode cell (Swagelok ^® type) was used. The electrodes used were graphite electrodes (SFG6L, Imerys SA). The separator used was a polymer flow (Freudenberg SE, FS2226). Lithium foil (Rockwood lithium, battery unit) served as reference and counter electrode.

An electrolyte containing 1 M LiPF ₆ (BASF, battery purity) and 5 wt .-% 3-methyl-1,4,2-dioxoazol-5-one obtained from Example 1 was prepared by adding the required amount of LiPF ₆ in propylene carbonate (PC, BASF, battery purity) and the corresponding amount of 3-methyl-1,4,2-dioxoazol-5-one was added. In the same way, comparative electrolytes containing 1 M LiPF ₆ in propylene carbonate and a mixture of ethylene carbonate (BASF, battery purity) and dimethyl carbonate (BASF, battery purity) (EC: DMC) were prepared in a weight ratio of 1: 1.

The first charging process and the first discharging process between 0.025 V and 1.5 V were investigated vs. Li / Li ⁺ at a C rate of 0.2C. In addition, the potential for one hour at 0.025 V vs. Li / Li ⁺ kept constant. The 1 shows the first cycle of the graphite / lithium half cell for the investigated electrolytes. How to get the 1 takes the comparison electrolyte showed 1 M LiPF ₆ in propylene carbonate no reversible capacity. Addition of 5% by weight of 3-methyl-1,4,2-dioxoazol-5-one to this propylene carbonate-based electrolyte allowed reversible cyclization, corresponding to the positive control of the comparative electrolyte containing 1 M LiPF ₆ in a 1: 1 ratio. Mixture of ethylene carbonate and dimethyl carbonate.

Example 3

Determination of oxidative electrochemical stability in a LMO / Li half-cell

The determination of the oxidative stability of the electrolytes in half cells was carried out by means of linear sweep voltammetry. In this method, there is a continuous change in the electrode voltage (linear sweep). For this purpose, a three-electrode cell (Swagelok ^® type) using lithium manganese oxide (LMO, Custom Cells GmbH) was used as working electrode. Lithium foil (Rockwood lithium, battery unit) served as reference and counter electrode. The separator used was a polymer flow (Freudenberg SE, FS2226). To determine the oxidative stability, the potential between the working and reference electrodes was increased from the no-load voltage to 6 V. The feed rate of the potential was 0.05 mV s ^-1 .

The electrolyte containing 1 M LiPF ₆ and 5 wt .-% of 3-methyl-1,4,2-dioxoazol-5-one obtained from Example 1 was prepared by dissolving the required amount of LiPF ₆ in propylene carbonate and the corresponding amount 3-methyl-1,4,2-dioxoazol-5-one was added as described in Example 2. Similarly, comparative electrolytes were prepared using a comparative electrolyte 1 M LiPF ₆ in a mixture of ethylene carbonate and dimethyl carbonate (EC: DMC) in a weight ratio of 1: 1, and a second comparative electrolyte additionally 5 wt .-% of the additive vinylene carbonate (VC, UBE Industries).

The 2 shows the oxidative stability window of the electrolytes for a potential vs. Li / Li ⁺ in the range of 3 V to 6 V. How to get the 2 The electrolyte containing 5% by weight of 3-methyl-1,4,2-dioxoazol-5-one in propylene carbonate was up to 5.6 V vs. Li / Li ⁺ stable, the electrolytes in ethylene carbonate / dimethyl carbonate with or without the addition of 5 wt .-% vinylene carbonate in each case to 4.7 V and 5.3 V vs.. Li / Li ⁺ .

The electrolyte according to the invention containing 5 wt .-% of 3-methyl-1,4,2-dioxoazol-5-one thus shows a significantly higher oxidative stability limit than the vinylene carbonate-based electrolyte. This indicates that 3-methyl-1,4,2-dioxoazol-5-one is later oxidized as vinylene carbonate, and thus has better high-voltage stability. The electrolyte according to the invention containing 5 wt .-% of 3-methyl-1,4,2-dioxoazol-5-one in propylene carbonate also shows a significantly higher oxidative stability than the reference electrolyte 1 M LiPF ₆ in a mixture of ethylene carbonate and dimethyl carbonate (EC: DMC). Accordingly, due to its high oxidative stability, 3-methyl-1,4,2-dioxoazol-5-one is particularly useful for high voltage electrode material applications.

Thus, an effective passivation of graphite can be achieved by the use of the dioxazolone derivatives according to the invention in small proportions by weight in the electrolyte. The fact that already 5 wt .-% of 3-methyl-1,4,2-dioxoazol-5-one in propylene carbonate sufficient for the formation of a solid electrolyte interphase (SEI) on graphite electrodes, the use of dioxazolones as an additive for Propylene carbonate-based electrolytes also economical.

Example 4

Investigation of long-term cycle stability under application of a constant current

Long-term cycle stability was assessed in whole cells of a button cell assembly (Hohsen Corp., CR2032) using lithium _1/3 manganese _1/3 cobalt _1/3 oxide (NMC (111), Customcells GmbH) and graphite electrodes. Customcells GmbH). The separator used was a polymer flow (Freudenberg SE, FS2226). The cyclizations were performed in a voltage window of 4.2V to 2.8V. Two cycles of formation were run at 0.1C (including holding the voltage at 4.2V until the current is <0.05C) followed by 3 conditioning cycles at 0.33C (including holding the voltage at 4.2V until the current <0 , 05C and one hour wait), followed by 95 1.0C charge / discharge cycles. The constant current measurements were made on a Battery Tester Series 4000 (Maccor) at 20 ° C ± 0.1 ° C.

The electrolyte containing 1 M LiPF ₆ and 5 wt .-% of 3-methyl-1,4,2-dioxoazol-5-one obtained from Example 1 was prepared by dissolving the required amount of LiPF ₆ in propylene carbonate and the corresponding amount 3-methyl-1,4,2-dioxoazol-5-one was added as described in Example 2. As a comparative electrolyte, a solution of 1 M LiPF ₆ in a mixture of ethylene carbonate and dimethyl carbonate (EC: DMC) was prepared in a weight ratio of 1: 1.

In 3 is the specific discharge capacity of the NMC / graphite full cell against the number of cycles and in 4 Coulombic efficiency vs. number of cycles using the respective electrolytes. As the 3 shows the electrolyte containing 5 wt .-% of 3-methyl-1,4,2-dioxoazol-5-one only a small capacity loss over 50 cycles. The capacity maintenance in the cycle 50 based on the first cycle at 1.0C (cycle 6 ) was 98.7%. The comparative electrolyte 1 M LiPF ₆ in a mixture of ethylene carbonate and dimethyl carbonate (EC: DMC) showed a capacity retention of 98.1%.

Out 4 One can further deduce that the Coulomb efficiency in the first cycle 85 3% for the electrolyte containing 5% by weight of 3-methyl-1,4,2-dioxoazol-5-one. The comparative electrolyte 1 M LiPF ₆ in a mixture of ethylene carbonate and dimethyl carbonate (EC: DMC) showed a Coulomb efficiency of 86.3% in the first cycle. The full cell with the electrolyte containing 5 wt .-% 3-methyl-1,4,2-dioxoazol-5-one thus showed good cycle stability, corresponding to the comparative electrolyte 1 M LiPF ₆ in a mixture of ethylene carbonate and dimethyl carbonate (EC: DMC).

Overall, the results show that 3-methyl-1,4,2-dioxoazol-5-one forms a passivating lithium ion conductive protective layer on the surface of graphite, is suitable for high voltage applications, and lithium ion batteries with good cycle stability can be operated.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• DE 102010020992 A1 [0005]

Cited non-patent literature

• S. Chang et al. in J. Am. Chem. Soc. 2015, 137, pages 4534-4542 [0043]

Claims (10)

Electrolyte for an electrochemical energy store comprising an electrolyte salt and a solvent, characterized in that the electrolyte comprises at least one compound according to the general formula (1) as indicated below:

wherein: X is C, S or S = O; R ¹ is selected from the group comprising CN, C ₁ -C ₁₀ -alkyl, C ₁ -C ₁₀ -alkoxy, C ₃ -C ₇ -cycloalkyl, C ₆ -C ₁₀ -aryl and / or -CO-OR ² , wherein the alkyl, alkoxy, cycloalkyl and aryl groups are each unsubstituted or monosubstituted or polysubstituted by at least one substituent selected from the group consisting of F, C _1-4 alkyl and / or CN, and R ² is selected from the group comprising C ₁ -C ₁₀ -alkyl, C ₃ -C ₇ -cycloalkyl, and / or C ₆ -C ₁₀ -aryl. Electrolyte after Claim 1 , characterized in that R ^{1 is} selected from the group comprising CN and / or unsubstituted or mono- or polysubstituted with fluorine-substituted C ₁ -C ₅ alkyl or phenyl. Electrolyte after Claim 1 or 2 , characterized in that X is carbon and R ^{1 is} selected from the group comprising CH ₃ , CF ₃ , CN, tert-butyl and / or phenyl. Electrolyte according to one of the preceding claims, characterized in that the electrolyte, the compound according to the general formula (1) in the range of ≥ 0.1 wt .-% to ≤ 10 wt .-%, preferably in the range of ≥ 0.5 wt .-% to ≤ 7 wt .-%, preferably in the range of ≥ 3 wt .-% to ≤ 5 wt .-%, based on the total weight of the electrolyte containing. Electrolyte according to one of the preceding claims, characterized in that the solvent is selected from the group comprising a non-fluorinated or partially fluorinated organic solvent, an ionic liquid, a polymer matrix and / or mixtures thereof. Electrolyte according to one of the preceding claims, characterized in that the solvent is an organic solvent selected from the group comprising ethylene carbonate, ethylmethyl carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, acetonitrile, propionitrile, 3-methoxypropionitrile, glutaronitrile, adiponitrile, pimelonitrile, gamma-butyrolactone, gamma-valerolactone, dimethoxyethane, 1,3-dioxolane, methyl acetate, ethyl acetate, ethyl methanesulfonate, dimethyl methyl phosphonate, linear or cyclic sulfone, symmetrical or non-symmetrical alkyl phosphates and / or mixtures thereof. Electrolyte according to one of the preceding claims, characterized in that the electrolyte salt is selected from the group comprising LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ , LiClO ₄ , LiPtCl ₆ , LiN (SO ₂ F) ₂ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , LiB (C ₂ O ₄ ) ₂ , LiBF ₂ (C ₂ O ₄ ) and / or LiSO ₃ CF ₃ LiPF ₆ . Electrochemical energy store, in particular supercapacitor or lithium-based electrochemical energy store, comprising an electrolyte according to one of Claims 1 to 7 , A method of forming a solid electrolyte phase interface on an electrode of an electrochemical cell comprising an anode, a cathode and an electrolyte, which method comprises using an electrolyte according to any one of Claims 1 to 7 operates. Use of a compound according to the general formula (1) as indicated below:

wherein: X is C, S or S = O; R ¹ is selected from the group comprising CN, C ₁ -C ₁₀ -alkyl, C ₁ -C ₁₀ -alkoxy, C ₃ -C ₇ -cycloalkyl, C ₆ -C ₁₀ -aryl and / or -CO-OR ² , wherein the alkyl, alkoxy, cycloalkyl and aryl groups are each unsubstituted or monosubstituted or polysubstituted by at least one substituent selected from the group consisting of F, C _1-4 alkyl and / or CN, and R ² is selected from the group comprising C ₁ -C ₁₀ -alkyl, C ₃ -C ₇ -cycloalkyl, and / or C ₆ -C ₁₀ -aryl, in an electrochemical energy store, in particular a supercapacitor or lithium-ion-based electrochemical energy store.

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