DE102019101343A1 - Electrolyte additive for lithium-ion batteries - Google Patents

Abstract

The invention relates to an electrolyte for an energy store comprising an electrolyte salt and a solvent, the electrolyte comprising at least one compound according to the general formula (1) as indicated below: wherein R, R, R, R are the same or independently selected from the group comprising hydrogen , Fluorine, linear or branched CC-alkyl, CC-alkenyl, CC-alkynyl, CC-cycloalkyl and / or phenyl, where the alkyl, alkenyl, alkynyl, cycloalkyl and phenyl groups are each unsubstituted or mono- or polysubstituted by fluorine are. The invention further relates to the use of a compound of the general formula (1) as an additive in an electrolyte to form a solid-electrolyte phase interface.

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. The energy requirements of the devices have increased rapidly, so that lithium-ion batteries with a higher energy density are required. Conventional lithium-ion batteries usually use a graphite anode and a liquid electrolyte, which contains a lithium salt dissolved in an organic carbonate solvent, mostly lithium hexafluorophosphate (LiPF6).

The operation of graphite anodes leads to reductive decomposition of the electrolyte. The reaction products can form an adherent and electronically insulating, but lithium ion conductive film on the electrode. Suitable electrolytes are characterized in that the formation of such a solid-state electrolyte phase interface, the so-called solid electrolyte interphase (SEI), is induced on the electrode. In the following, the SEI prevents the graphite from reacting further with the electrolyte and thereby protects the electrolyte from further reductive decomposition and the anode from destruction by co-intercalation of the solvent. In particular when using graphite anodes, the formation of such a film is necessary for the safe operation of the lithium-ion battery.

However, the reductive decomposition of the solvent propylene carbonate (IUPAC name 4-methyl-1,3-dioxolan-2-one) does not lead to the formation of an effective solid electrolyte interphase. Instead, a reductively induced gas development within the graphite layers, which is 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 compared to ethylene carbonate (IUPAC 1,3-dioxolan-2-one), for lithium-ion technology. Without the formation of a solid electrolyte interphase, a graphite anode is destroyed when propylene carbonate is used. Propylene carbonate can serve as a model system for other high-voltage electrolytes, which also show reductive decomposition without SEI formation and exfoliation of graphite.

It has already been proposed to prevent the exfoliation of the graphite and the reductive decomposition of the solvent by using 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 amount of conductive salt normally required. In addition, the concentration (usually> 3 mol 1 ^-1 ) greatly increases the viscosity of the electrolyte, which leads to a noticeable drop in the conductivity and performance of the battery. Furthermore, it is to be expected that if the operating temperature is lowered, the solubility product of the conductive salt in the electrolyte solution will be undershot, which will cause the salt in the interior of the batteries to fail. In addition, with a constant volume and increasing addition of conductive salt, the density and therefore the total mass of the electrolyte increases. This also leads to a decrease in the specific energy density (Ah kg ^-1 ) of the battery as a whole system.

Furthermore, the use of adequate performance additives such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC) or adequate co-solvents such as diglyme has been proposed. Reveal further J. Alvarado et al., Materials Today, Volume 21, Issue 4, May 2018, pages 341-353 , the use of a carbonate-free, sulfone-based electrolyte for high-voltage lithium-ion batteries. Here, tetramethylene sulfone or sulfanolane is used as a solvent for lithium bis (fluorosulfonyl) imide (LiFSI). Vinylene carbonate (VC) is the most used SEI additive in commercial batteries. However, it is known that vinylene carbonate can only be used in a lithium-ion battery up to a final voltage of max. 4.5 V, since it is at voltages above 4.5 V leads to an oxidative decomposition of this standard additive. There is therefore a need for further compounds which can prevent graphite exfoliation.

The object of the present invention was to provide an electrolyte which overcomes at least one of the aforementioned disadvantages of the prior art. In particular, the present invention was based on the object of providing a connection which supports the formation of a solid electrolyte interphase on graphite and thus enables reversible cyclization of electrolytes containing propylene carbonate.

This object is achieved by an electrolyte according to claim 1, an energy store according to claim 8, a method for forming a solid-electrolyte phase interface according to claim 9, and a use according to claim 10. Further advantageous embodiments of the invention result from the subclaims.

worin: R¹, R², R³, R⁴ gleich oder unabhängig voneinander ausgewählt sind aus der Gruppe umfassend Wasserstoff, Fluor, lineares oder verzweigtes C₁-C₆-Alkyl, C₂-C₆-Alkenyl, C₂-C₆-Alkinyl, C₃-C₆-Cycloalkyl und/oder Phenyl, wobei die Alkyl-, Alkenyl-, Alkinyl-, Cycloalkyl- und Phenylgruppen jeweils unsubstituiert oder einfach oder mehrfach mit Fluor substituiert sind.Here, an electrolyte for an energy store comprises a conductive salt and a solvent, the Solvent comprises at least one compound according to the general formula (1) as indicated below:

wherein: R ¹ , R ² , R ³ , R ⁴ are selected identically or independently of one another from the group comprising hydrogen, fluorine, linear or branched C ₁ -C ₆ alkyl, C ₂ -C ₆ alkenyl, C ₂ -C ₆ -alkynyl, C ₃ -C ₆ -cycloalkyl and / or phenyl, the alkyl, alkenyl, alkynyl, cycloalkyl and phenyl groups in each case being unsubstituted or mono- or polysubstituted by fluorine.

Surprisingly, it was found that 3-oxetanone and its derivatives according to the general formula (1) can cause an effective passivation of a graphite electrode even in small amounts by weight in carbonate-based electrolytes. Without being bound by any particular theory, it is believed that 3-oxetanone forms a solid electrolyte interphase (SEI) on graphite. This allows the use of 3-oxetanone as an additive for propylene carbonate-based electrolytes. In particular, it was shown that 3-oxetanone up to 5 V vs. Li / Li ^{+ was} stable. The oxidative stability can thus be increased compared to electrolytes containing vinylene carbonate. Furthermore, 2% by weight of 3-oxetanone in propylene carbonate already allows stable operation of a full cell for more than 700 cycles. Overall, 3-oxetanone and its derivatives provide a high-voltage stable electrolyte additive for lithium-ion battery technology.

The term “C ₁ -C ₆ alkyl” includes, unless stated otherwise, straight-chain or branched alkyl groups having 1 to 6 carbon atoms. C ₁ -C ₅ -alkyl groups are preferably selected from the group comprising methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, isopentyl and / or neopentyl. C ₁ -C ₃ alkyl substituents are particularly preferred. The term “C ₃ -C ₆ cycloalkyl” is to be understood as meaning cyclic alkyl groups with 3 to 6 carbon atoms. Unless otherwise stated, the terms “C ₂ -C ₆ -alkenyl” and “C ₂ -C ₆ -alkynyl” encompass straight-chain or branched alkenyl or alkynyl groups with 2 to 6 carbon atoms and in each case at least one double or triple bond .

The radicals R ¹ , R ² , R ³ and R ⁴ can be the same or different. R ¹ , R ² , R ³ and R ⁴ are preferably selected identically or independently of one another from the group comprising hydrogen, fluorine, linear or branched C ₁ -C ₆ alkyl, and / or phenyl. The C ₁ -C ₆ alkyl or phenyl groups can be unsubstituted or substituted one or more times with fluorine, preferably perfluorinated. The radicals R ¹ , R ² , R ³ and R ⁴ , preferably C ₁ -C ₆ -alkyl or phenyl groups, on different, preferably on the same, carbon atoms can be single or multiple or perfluorinated. In preferred embodiments, R ¹ , R ² , R ³ , R ⁴ are selected identically or independently of one another from the group comprising hydrogen, fluorine and / or in each case unsubstituted or mono- or polysubstituted by fluorine, C ₁ -C ₄ -alkyl or phenyl. Non-fluorinated compounds are usually cheaper and therefore more economical as an additive in lithium-ion batteries. In preferred embodiments, R ¹ , R ² , R ³ , R ⁴ are selected identically or independently of one another from the group comprising hydrogen, fluorine, CF ₃ , C ₂ F ₅ , methyl and / or ethyl. R ¹ , R ² , R ³ , R ⁴ are preferably selected identically or independently of one another from hydrogen, fluorine and / or CF ₃ . By replacing hydrogen with fluorine, the stability of the electrolyte additive can be specifically adapted to the requirements of a battery. In particular, one or more of the radicals R ¹ , R ² , R ³ and R ^{4 can be} fluorine, while the other radicals R ¹ , R ² , R ³ and R ^{4 are} hydrogen.

In preferred embodiments, the compound according to the general formula (1) is 3-oxetanone, or according to IUPAC oxetan-3-one. 3-Oxetanone can provide high electrochemical activity and reactivity and can provide an effective solid electrolyte interphase on graphite even in small amounts of, for example, 1 or 2% by weight in electrolytes. In particular, 3-oxetanone is up to 5 V vs. Li / Li ⁺ stable.

The compound according to the general formula (1) can be present in the electrolyte in a wide range from 0,0 0.01% by weight to ≤ 10% by weight, based on the total weight of the electrolyte. It is preferred that 3-oxetanone and its derivatives of the formula (1) are used in a proportion ≤ 10% by weight, based on the total weight of the electrolyte. Additions in amounts of or less than 10% by weight are referred to as an electrolyte additive or mostly as an additive. In embodiments, the electrolyte can the compound according to the general formula (1) in the range of ≥ 0.05 wt .-% to ≤ 10 wt .-%, preferably in the range of ≥ 0.5 wt .-% to ≤ 5 wt. -%, preferably in the range of ≥ 1 wt .-% to ≤ 2 wt .-%, based on the total weight of the electrolyte. It could be shown in an advantageous manner that fractions of 1% by weight of 3-oxetanone were able to form an effective SEI on the negative graphite electrode. This allows economical use.

The electrolyte has at least one conductive salt, preferably a lithium salt, a solvent and at least one compound according to the general formula (1). The solvent serves as a solvent for the electrolyte or lithium salt. The terms solvent and solvent are used synonymously here. The electrolyte can contain a solvent selected from non-fluorinated, fluorinated or partially fluorinated organic solvents, ionic liquids, a polymer matrix and / or mixtures thereof. The electrolyte preferably has an organic solvent, in particular a cyclic or linear carbonate. In preferred embodiments, the organic solvent is selected from the group comprising 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, propyl acetate, butyl acetate, ethyl methanesulfonate, dimethyl methylphosphonate, linear or cyclic sulfone, symmetrical or non-symmetrical alkyl phosphates, symmetrical or non-symmetrical organosilanes and siloxanes, organic esters, fluorinated cyclic and linear / linear carbonates, ethers, ethers, and / or linear carbonates, ethers or their mixtures. In preferred embodiments, the solvent is selected from the group comprising propylene carbonate, ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate and / or mixtures thereof. The solvent is further preferably selected from the group comprising fluorinated ethylene carbonate and propylene carbonate, their mixtures and / or mixtures with propylene carbonate, ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate and diethyl carbonate.

The electrolyte can in particular comprise solvents such as propylene carbonate, which do not lead to the formation of an effective solid electrolyte interphase. Solvents such as propylene carbonate and mixtures of propylene carbonate with ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate and / or diethyl carbonate are therefore preferred, in particular mixtures of propylene carbonate with dimethyl carbonate. Addition of 3-oxetanone to form an effective solid electrolyte interphase is particularly advantageous for use with these solvents.

The electrolyte can also be a polymer electrolyte, for example selected from the group comprising polyethylene oxide, polyacrylonitrile, polyvinyl chloride, polyvinylidene fluoride, poly (vinylidene fluoride-co-hexafluoropropylene) and / or polymethyl methacrylate with the addition of a conductive salt, or a gel-polymer electrolyte comprising a polymer the aforementioned organic solvent and / or an ionic liquid and a conductive salt. The electrolyte can also comprise an ionic liquid.

In addition to a solvent and at least one compound according to the general formula (1), the electrolyte has at least one electrolyte salt which is usually referred to as a conductive salt, in particular a lithium salt. The electrolyte salt is preferably 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 , preferably LiPF ₆ . The lithium salt is preferably selected from LiN (SO ₂ CF ₃ ) ₂ (LiTFSI, lithium bis (trifluoromethanesulfonyl) imide, LiN (SO ₂ F) ₂ (LiFSI) and LiPF _6. The concentration of the lithium salt in the electrolyte can be in customary ranges, for example in the range of ≥ 0.5 M to ≤ 1.5 M, mostly at 1 M.

In embodiments, the electrolyte can comprise a compound according to the general formula (1), 3-oxetanone, at least one lithium salt, and propylene carbonate or a mixture of organic solvents such as ethylene carbonate and dimethyl carbonate. The electrolyte can be produced, for example, by mixing the compound of the general formula (1) with the solvent and introducing the lithium salt into the solvent.

The electrolyte can contain further additives, for example selected from SEI formers, flame retardants and / or overcharge additives. For example, the electrolyte can contain a compound according to the general formula (1) and a further SEI former, for example selected from the group comprising fluoroethylene carbonate, chloroethylene carbonate, vinylene carbonate, vinyl ethylene carbonate, ethylene sulfite, propane sultone, propene sultone, sulfites, preferably dimethyl sulfite and propylene sulfite, ethylene sulfate, Propylene sulfate, methylene methane disulfonate, trimethylene sulfate, butyrolactones optionally substituted with F, Cl or Br, phenylethylene carbonate, vinyl acetate and / or trifluoropropylene carbonate. These connections can further improve battery performance, such as capacity, long-term stability or cycle life.

Compounds of the general formula (1), in particular 3-oxetanone, are commercially available or can be prepared by processes familiar to the person skilled in the art.

The present invention further relates to an energy store, in particular electrochemical energy store, selected from the group comprising lithium battery, lithium-ion battery, lithium-ion accumulator, lithium-polymer battery, lithium-ion capacitor, supercapacitor, dual -Ion battery or dual Graphite battery, comprising an electrolyte according to the invention. The electrolyte is particularly suitable for a secondary battery, in particular as an electrolyte for a lithium-ion battery.

For the description of the electrolyte, reference is made to the above description. In the context of the present invention, the term “energy store” encompasses primary and secondary energy storage devices, that is to say batteries (primary store) and accumulators (secondary store). In general parlance, accumulators are often referred to with the term "battery", which is often used as a generic term. Thus, the term lithium-ion battery is used synonymously with lithium-ion accumulator in the present case, unless stated otherwise. In the context of the present invention, the term “electrochemical energy store” in particular also includes electrochemical capacitors (English: electrochemical capacitors) such as supercapacitors (English: supercapacitors). Electrochemical capacitors, also referred to in the literature as supercapacitors, are electrochemical energy stores which are distinguished from batteries by a higher power density and from conventional capacitors by a higher energy density. Secondary electrochemical energy stores are preferred. The energy store is in particular a lithium-ion battery.

In particular, the energy store can comprise a compound according to the general formula (1), a carbon electrode, in particular a graphite electrode, and an electrolyte based on an organic aprotic carbonate solvent. Preferred is, for example, a lithium-ion battery which comprises a cathode, a graphite anode, a separator and an electrolyte comprising 3-oxetanone or its derivatives according to the general formula (1) in amounts of 1 to 2% by weight in propylene carbonate or mixtures of ethylene carbonate and dimethyl carbonate and preferably 1 M LiPF ₆ contains. It could be shown that a solid electrolyte phase interface formed on a graphite anode was stable for at least 700 cycles. This enables secondary batteries to be operated economically.

In principle, all electrolytes, solvents, conductive salts and electrodes known to the person skilled in the art, which are usually used in energy stores such as lithium-ion batteries, can be used. It could be shown that electrolytes containing 3-oxetanone have a wide oxidative stability window and, due to this high stability, can be used for applications with high-voltage electrode materials. It could be shown that 3-oxetanone as an additive in particular has a higher oxidative stability than vinylene carbonate and can be used in high-voltage electrolytes up to 5 V. This provides a great advantage, since vinylene carbonate and other conventional SEI formers do not allow graphite anodes with high-voltage cathode materials in lithium-ion batteries with switch-off potentials higher than 4.5 V vs. Li / Li ⁺ to operate. The compounds according to the invention are an additive in energy stores with common electrode materials such as LiCoO ₂ (LCO), LiMPO ₄ (M = Fe, Mn, Ni, Co), LiMn ₂ O _4th (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) can be used.

The invention further relates to a method for forming a solid-state electrolyte phase interface on an electrode of an electrochemical cell comprising an anode, a cathode and an electrolyte, the cell being operated using the electrolyte according to the invention. For the description of the electrolyte and the compound of formula (1), reference is made to the above description.

worin: R¹, R², R³, R⁴ gleich oder unabhängig voneinander ausgewählt sind aus der Gruppe umfassend Wasserstoff, Fluor, lineares oder verzweigtes C₁-C₆-Alkyl, C₂-C₆-Alkenyl, C₂-C₆-Alkinyl, C₃-C₆-Cycloalkyl und/oder Phenyl, wobei die Alkyl-, Alkenyl-, Alkinyl-, Cycloalkyl- und Phenylgruppen jeweils unsubstituiert oder einfach oder mehrfach mit Fluor substituiert sind, in einem Energiespeicher, insbesondere einem elektrochemischen EnergiespeicherThe invention further relates to a use of the compound of the general formula (1) as indicated below:

wherein: R ¹ , R ² , R ³ , R ⁴ are selected identically or independently of one another from the group comprising hydrogen, fluorine, linear or branched C ₁ -C ₆ alkyl, C ₂ -C ₆ alkenyl, C ₂ -C ₆ -alkynyl, C ₃ -C ₆ -cycloalkyl and / or phenyl, the alkyl, alkenyl, alkynyl, cycloalkyl and phenyl groups in each case being unsubstituted or mono- or polysubstituted by fluorine, in an energy store, in particular an electrochemical energy store

The energy store is preferably selected from the group comprising lithium battery, lithium-ion battery, lithium-ion battery, lithium-polymer battery, lithium-ion capacitor, supercapacitor, dual-ion battery and dual-graphite battery .

The compound of the general formula (1) can be used advantageously as an electrolyte additive, in particular in electrolytes which do not form an effective SEI without the addition of an additive. In particular, the compound of the general formula (1) can advantageously be used in an energy store, which carbon in particular Includes graphite as the electrode material. For the description of the compound according to the general formula (1), the energy store and electrolytes, reference is made to the above description. R ¹ , R ² , R ³ , R ⁴ are preferably selected identically or independently of one another from the group comprising hydrogen, fluorine and / or in each case unsubstituted or mono- or polysubstituted by fluorine, C ₁ -C ₄ -alkyl or phenyl, in particular from the Group comprising hydrogen, fluorine, CF ₃ , C ₂ F ₅ , methyl and / or ethyl. R ¹ , R ² , R ³ , R ⁴ are preferably selected identically or independently of one another from hydrogen, fluorine and / or CF ₃ .

In particular, one or more of the radicals R ¹ , R ² , R ³ and R ^{4 can be} fluorine, while the other radicals R ¹ , R ² , R ³ and R ^{4 are} hydrogen. 3-Oxetanone is preferred.

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

• 1 das oxidative Stabilitätsfenster von Elektrolyten umfassend 1M LiPF₆ in einem Gemisch von Ethylencarbonat und Dimethylcarbonat (EC:DMC, 1:1, Gew.-%) ohne Additiv (Linie) oder mit Zugabe von jeweils 2 Gew.-% 3-Oxetanon (Striche) oder Vinylencarbonat (Punkte) im Potentialbereich von 3,5 V bis 6,0 V vs. Li/Li⁺. Aufgetragen ist die Stromstärke gegen das Potential.
• 2 Zyklovoltammogramme von Halbzellen mit LNMO-Arbeitselektrode in einem Potentialbereich von 3 V bis 4,95 V vs. Li/Li⁺ unter Verwendung von Elektrolyten enthaltend 1M LiPF₆ in Gemischen von Ethylencarbonat und Dimethylcarbonat (EC:DMC, 1:1) in 2A, sowie enthaltend 1 Gew.-% 3-Oxetanon (Ox-one) in 2B oder 2 Gew.-% Vinylencarbonat (VC) in 2C.
• 3 in 3A das reduktive Stabilitätsfenster eines Elektrolyten enthaltend 1M LiPF₆ in einem Gemisch von Ethylencarbonat und Dimethylcarbonat (EC:DMC, 1:1) in einer Halbzellen mit Graphit-Arbeitselektrode im Potentialbereich von 3 V bis 0,025 V vs. Li/Li⁺, sowie des Elektrolyten zusätzlich enthaltend 1 Gew.-% 3-Oxetanon in 3B.
• 4 in 4A das reduktive Stabilitätsfenster eines Elektrolyten enthaltend 1M LiPF₆ in einem Gemisch von Ethylencarbonat und Dimethylcarbonat (EC:DMC, 1:1) in einer Halbzellen mit LTO-Arbeitselektrode im Potentialbereich von 3 V bis 1,0 V vs. Li/Li⁺, sowie des Elektrolyten zusätzlich enthaltend 1 Gew.-% 3-Oxetanon in 4B.
• 5 die Spannungskurven über die Zeit während des ersten Lade- und Entladezyklus einer Vollzelle unter Verwendung eines Vergleichselektrolyten enthaltend 1M LiPF6 in Propylencarbonat (PC) ohne Additiv (Linie), eines Vergleichselektrolyten mit 2 Gew.-% Vinylencarbonat (VC, Striche), eines Vergleichselektrolyten mit 2 Gew.-% Fluorethylencarbonat (FEC, Punkte), und eines Elektrolyten mit 2 Gew.-% 3-Oxetanon (Ox-one, Strich-Punkt-Punkt).
• 6 die Entladekapazität gegen die Anzahl der Lade-/Entladezyklen für eine Vollzelle unter Verwendung eines Vergleichselektrolyten enthaltend 1M LiPF₆ in Propylencarbonat mit 2 Gew.-% Vinylencarbonat (VC, Viereck), eines Vergleichselektrolyten mit 2 Gew.-% Fluorethylencarbonat (FEC, Kreis), und eines Elektrolyten mit 2 Gew.-% 3-Oxetanon (Oxetanone, Raute).

The figures show:

• 1 the oxidative stability window of electrolytes comprising 1M LiPF ₆ in a mixture of ethylene carbonate and dimethyl carbonate (EC: DMC, 1: 1,% by weight) without additive (line) or with the addition of 2% by weight of 3-oxetanone (lines ) or vinylene carbonate (points) in the potential range from 3.5 V to 6.0 V vs. Li / Li ⁺ . The current strength is plotted against the potential.
• 2nd Cyclic voltammograms of half cells with LNMO working electrodes in a potential range from 3 V to 4.95 V vs. Li / Li ⁺ using electrolytes containing 1M LiPF ₆ in mixtures of ethylene carbonate and dimethyl carbonate (EC: DMC, 1: 1) in 2A , as well as containing 1 wt .-% 3-oxetanone (Ox-one) in 2 B or 2nd % By weight vinylene carbonate (VC) in 2C .
• 3rd in 3A the reductive stability window of an electrolyte containing 1M LiPF ₆ in a mixture of ethylene carbonate and dimethyl carbonate (EC: DMC, 1: 1) in a half cell with graphite working electrode in the potential range from 3 V to 0.025 V vs. Li / Li ⁺ , and the electrolyte additionally containing 1% by weight of 3-oxetanone in 3B .
• 4th in 4A the reductive stability window of an electrolyte containing 1M LiPF ₆ in a mixture of ethylene carbonate and dimethyl carbonate (EC: DMC, 1: 1) in a half cell with LTO working electrode in the potential range from 3 V to 1.0 V vs. Li / Li ⁺ , and the electrolyte additionally containing 1% by weight of 3-oxetanone in 4B .
• 5 the voltage curves over time during the first charge and discharge cycle of a full cell using a reference electrolyte containing 1M LiPF6 in propylene carbonate (PC) without additive (line), a reference electrolyte with 2% by weight vinylene carbonate (VC, dashes), a reference electrolyte with 2% by weight of fluoroethylene carbonate (FEC, points), and an electrolyte with 2% by weight of 3-oxetanone (Ox-one, dash-dot point).
• 6 the discharge capacity versus the number of charge / discharge cycles for a full cell using a reference electrolyte containing 1M LiPF ₆ in propylene carbonate with 2% by weight vinylene carbonate (VC, square), a reference electrolyte with 2% by weight fluoroethylene carbonate (FEC, circle) , and an electrolyte with 2 wt .-% 3-oxetanone (oxetanone, rhombus).

example 1

Determination of the oxidative electrochemical stability of an electrolyte containing 3-oxetanone

The oxidative stability of the electrolytes was determined using linear sweep voltammetry. With this method, the electrode potential is changed continuously (linear sweep). For this, a three-electrode cell (Swagelok® type) using lithium manganese oxide (LMO, Customcells) was used as the working electrode. Lithium foil (Rockwood Lithium) served as a reference and counter electrode. A glass fiber fleece (Whatman, GFD) was used as the separator. To determine the oxidative stability, the potential between the working and reference electrodes was changed from the no-load potential to 6.0 V vs. Li / Li ⁺ increased. The feed rate of the potential was 0.025 mV s ^-1 . The potentiodynamic measurements were carried out on a VSP (biology) potentiostat at 25 ° C ± 0.1 ° C.

Electrolytes containing 1M LiPF ₆ (BASF, battery purity) in a mixture of ethylene carbonate (EC) (BASF) and dimethyl carbonate (DMC) (BASF) in a weight ratio of 1: 1 containing 2% by weight of 3-oxetanone (ABCR, 97 %). The electrolyte without additive and with the addition of 2% by weight of vinylene carbonate (UBE Industries) was tested as a reference electrolyte. The electrolytes were produced by mixing ethylene carbonate and dimethyl carbonate in a weight ratio of 1: 1 and dissolving the required amount of LiPF ₆ therein. The additives were then added in the various batches. 210 ul electrolyte were used in each case.

The 1 shows the oxidative stability window of the electrolytes containing 1M LiPF6 in the mixture EC: DMC in a weight ratio of 1: 1 without Additive, and with the addition of 2% by weight of 3-oxetanone or 2% by weight of vinylene carbonate in a potential range from 3.5 V to 6.0 V vs. Li / Li ⁺ . How to 1 removed, the electrolyte containing 2 wt .-% vinylene carbonate was up to about 4.75 V vs. Li / Li ⁺ stable, while the electrolyte with 2 wt .-% 3-oxetanone to 5 V vs. Li / Li ^{+ was} stable. This shows that 3-oxetanone has a higher oxidative stability than vinylene carbonate and is therefore suitable for use with high-voltage cathode material.

Example 2

Determination of the oxidative electrochemical stability of an electrolyte containing 3-oxetanone in a mixture of ethylene carbonate and dimethyl carbonate

Cyclic voltammetry was used to determine the oxidative stability of the electrolytes in half cells. For this, a three-electrode cell (Swagelok® type) using lithium-nickel-manganese oxide (LNMO, Customcells Itzehoe) was used as the working electrode. Lithium foil (Rockwood Lithium) served as a reference and counter electrode. A glass fiber fleece (Whatman, GFD) was used as the separator. The potential was reduced between the working and reference electrodes from the no-load potential to 4.95 V vs. Li / Li ⁺ increased and then to 3.0 V vs. Li / Li ⁺ decreased. The feed rate of the potential was 0.025 mV s ^-1 . The potentiodynamic measurements were carried out on a VSP (biology) potentiostat at 25 ° C ± 0.1 ° C.

An electrolyte made of 1M LiPF _{6 was} examined in a mixture of ethylene carbonate and dimethyl carbonate in a weight ratio of 1: 1 containing 1% by weight of 3-oxetanone. The electrolyte without additive and with the addition of 2% by weight of vinylene carbonate (VC) was tested as a reference electrolyte. 210 ul electrolyte were used in each case.

The 2nd shows the cyclic voltammograms of the half cells with LNMO working electrode using the electrolytes in a potential range from 3.0 V to 4.95 V vs. Li / Li ⁺ . Here shows the 2A the electrolyte containing 1M LiPF ₆ in ethylene carbonate and dimethyl carbonate (EC: DMC, 1: 1) without additive ("/"), the 2 B the electrolyte with the addition of 1% by weight of 3-oxetanone (Ox-one), and the 2C the electrolyte with the addition of 2 wt .-% vinylene carbonate (VC).

The 2A shows the oxidation of the electrode material, the signal at approx. 4 V indicating that manganese is oxidized and the signals at approx. 4.7 V the oxidation of nickel. How to 2 B removed, the electrolyte containing 3-oxetanone showed a current / voltage characteristic whose signals are similar to those of the electrolyte without an additive. This shows that 3-oxetanone is oxidatively stable on the active material used and does not form a harmful layer on the high-voltage spinel, or does not cause any increased electrochemical-parasitic reactions compared to the electrolyte without an additive. The 2C shows a sharp increase at 4.75 V. This signal is assigned to the decay of vinylene carbonate induced by oxidation. The 2nd shows overall that 3-oxetanone is compatible with high voltage cathode materials such as LNMO.

Example 3

Determination of the reductive electrochemical stability of an electrolyte containing 3-oxetanone in a mixture of ethylene carbonate / dimethyl carbonate in a graphite half cell

The reductive stability of the electrolyte in half cells was also determined using cyclic voltammetry. For this, a three electrode cell (Swagelok® type) using a graphite composite electrode (Customcells Itzehoe) was used as the working electrode. Lithium foil (Rockwood Lithium) served as a reference and counter electrode. A glass fiber fleece (Whatman, GFD) was used as the separator. To determine the reductive stability, the potential in the first cycle between the working and reference electrodes was changed from the no-load potential to 0.02 V vs. Li / Li ⁺ decreased and then to 1.5 V vs. Li / Li ⁺ increased. The potential range of the second cycle was 1.5 V - 0.02 V vs. Li / Li ⁺ . The feed rate of the potential was 0.025 mV s ^-1 . The potentiodynamic measurements were carried out on a VSP (biology) potentiostat at 25 ° C ± 0.1 ° C.

An electrolyte containing 1M LiPF _{6 was} examined in a mixture of ethylene carbonate and dimethyl carbonate in a weight ratio of 1: 1 containing 1% by weight of 3-oxetanone. The electrolyte without additive was tested as a reference electrolyte. 210 ul electrolyte were used in each case.

The 3A shows the reductive stability window of the control electrolyte containing 1M LiPF6 in a mixture of ethylene carbonate and dimethyl carbonate. The signal of the decomposition of the electrolyte can be seen at 0.8 to 0.4 V vs. Li / Li ⁺ . The 3B shows the reductive stability window of the electrolyte containing 1 wt .-% 3-oxetanone. How to 3B removed, an onset of reductive decomposition of 3-oxetanone was already visible at 1.2 V. This corresponds to a decomposition before a reduction of ethylene carbonate or propylene carbonate. This shows that 3-oxetanone forms a passivating protective layer, a so-called solid electrolyte interphase (SEI), on the graphite electrode. This result shows that 3-oxetanone is suitable as an additive.

Example 4

Determination of the reductive electrochemical stability of an electrolyte containing 3-oxetanone in a mixture of ethylene carbonate / dimethyl carbonate in an LTO half cell

The determination of the reductive stability of the electrolyte in half cells by means of cyclic voltammetry was repeated in an LTO half cell. For this purpose, a three-electrode cell (Swagelok® type) using a lithium titanate spinel (LTO) electrode (in-house production MEET battery research center) was used as the working electrode. Lithium foil (Rockwood Lithium) served as a reference and counter electrode. A glass fiber fleece (Whatman, GFD) was used as the separator. To determine the reductive stability, the potential in the first cycle between the working and reference electrodes was changed from the no-load potential to 1.0 V vs. Li / Li ⁺ decreased and then to 2.0 V vs. Li / Li ⁺ increased. The potential range of the second cycle was 2.0 V to 1.0 V vs. Li / Li ⁺ . The feed rate of the potential was 0.025 mV s ^-1 . The potentiodynamic measurements were carried out on a VSP (biology) potentiostat at 25 ° C ± 0.1 ° C.

An electrolyte made of 1M LiPF6 was examined in a mixture of ethylene carbonate and dimethyl carbonate in a weight ratio of 1: 1 containing 1% by weight of 3-oxetanone. The electrolyte without additive was tested as a reference electrolyte. 210 ul electrolyte were used in each case.

The 4A shows the reductive stability window of the control electrolyte containing 1M LiPF ₆ in a mixture of ethylene carbonate and dimethyl carbonate. Only the characteristic current / voltage characteristic of the LTO active material can be seen. The 4B shows the reductive stability window of the electrolyte containing 1 wt .-% 3-oxetanone. How to 4B after the signal of the active material there was another signal from approx. 1.3 V to approx. 1.1 V vs. Li / Li ⁺ to recognize which is assigned to the decomposition of the 3-oxetanone. This result shows that 3-oxetanone is also active on LTO.

Example 5

Investigation of the Coulomb efficiency in a full cell in propylene carbonate

The investigation of the Coulomb efficiency in the first cycle was carried out in full cells of a button cell structure (Hohsen, CR2032; 1.25 mm spring, 1 × 0.5 mm + 1 × 1 mm spacer) using LiNi _0.5 Mn _0.3 Co _{0 , 2} O ₂ (NMC532, Customcells Itzehoe) and graphite electrodes (Customcells Itzehoe). A polymer fleece (Freudenberg, FS2226) was used as the separator. The cycles were carried out in a voltage window of 4.2 V to 2.8 V. The measurements at constant current were carried out on a battery tester Series 4000 (Maccor) at 25 ° C ± 0.1 ° C.

In each case 90 μl of a solution of 1M LiPF ₆ in propylene carbonate (PC) containing 2% by weight of 3-oxetanone (Ox-one) and of comparative electrolytes containing 1M LiPF ₆ in propylene carbonate (PC) without additive, with 2 % By weight vinylene carbonate (VC), or with 2% by weight fluoroethylene carbonate (FEC). The electrolytes were produced by dissolving the required amount of LiPF ₆ in propylene carbonate and adding the corresponding amount of 3-oxetanone, vinylene carbonate or fluoroethylene carbonate.

The 5 shows the voltage curves of the investigated electrolytes over time during the first charge and discharge cycle at a C rate of 0.1C. Using the electrolyte with 2% by weight of 3-oxetanone, a Coulomb efficiency of 72.2 ± 0.1% was achieved in the first cycle. This was significantly higher than the achieved values of 26.1 ± 2.3% for the reference electrolyte with 2% by weight vinylene carbonate and 39.9 ± 0.2% for the reference electrolyte with 2% by weight fluoroethylene carbonate (FEC). The electrolyte without additive could not be cycled in propylene carbonate (PC) due to the exfoliation of the graphite electrode, so that no Coulomb efficiency was measured. The significantly higher Coulomb efficiency of 3-oxetanone shows that a more effective passivation layer was formed compared to the standard additives vinylene carbonate and fluoroethylene carbonate, thus suppressing the adverse co-intercalation of the solvent and the subsequent exfoliation.

Example 6

Investigation of the long-term cycle stability of a full cell using a graphite electrode in propylene carbonate

The long-term cycle stability was investigated in full cells of a button cell structure (Hohsen, CR2032; 1.25 mm spring, 1 × 0.5 mm + 1 × 1 mm spacer) using LiNi _0.5 Mn _0.3 Co _0.2 O ₂ (NMC532, Customcells Itzehoe) and graphite electrodes (Customcells Itzehoe). A polymer fleece (Freudenberg, FS2226) was used as the separator. The cycles were carried out in a voltage window of 4.2 V to 2.8 V. Two formation cycles were run at 0.1C, including maintaining the voltage at 4.2V until the current was below 0.0IC, followed by three conditioning cycles at 0.33C, including maintaining the voltage at 4.2V until the current was below 0. 05C was followed by several charge / discharge cycles at 1.0C. The measurements at constant current were on a Series 4000 battery tester (Maccor) at 25 ° C ± 0.1 ° C.

90 μl of a solution of 1M LiPF6 in propylene carbonate (PC) containing 2% by weight of 3-oxetanone (Ox-one) and of comparative electrolytes containing 2% by weight of vinylene carbonate (VC) or with 2% by weight were used as the electrolyte. -% fluoroethylene carbonate (FEC). The electrolytes were produced by dissolving the required amount of LiPF6 in propylene carbonate and adding the corresponding amount of 3-oxetanone, vinylene carbonate or fluoroethylene carbonate.

In 6 the respective discharge capacity for the tested electrolytes is shown against the number of charge / discharge cycles carried out. Experiments were carried out in two cells for the electrolyte with 2% by weight of 3-oxetanone. How to 6 removed, these each went through more than 500 or more than 700 charge / discharge cycles before the discharge capacity of the cell decreased to 80% compared to the sixth cycle. In contrast, the comparative electrolytes with 2% by weight of fluoroethylene carbonate or 2% by weight of vinylene carbonate showed a significantly lower discharge capacity. The electrolyte with vinylene carbonate showed a rapid collapse and a short cell life of less than 50 cycles. The electrolyte with fluoroethylene carbonate showed a lifespan of just over 100 cycles.

Overall, the electrolyte containing 2% by weight of 3-oxetanone in propylene carbonate showed a significantly higher discharge capacity and service life than the standard additives vinylene carbonate and fluoroethylene carbonate. It was also shown that there is good compatibility with NMC cathode material.

Overall, the results show that 3-oxetanone can form a passivating, lithium ion conductive protective layer on the surface of graphite. In addition, 3-oxetanone shows good oxidative stability. Furthermore, lithium-ion batteries could be operated stably using 3-oxetanone with good cycle stability for more than 700 cycles. In particular, it was found that 3-oxetanone has high oxidative stability up to 5 V vs. Li / Li ⁺ shows and is therefore suitable for use with high-voltage cathode material.

The invention on which this patent application is based was developed in a project which was funded by the BMBF under grant number 3120040800.

QUOTES INCLUDE IN THE DESCRIPTION

This list of documents listed by the applicant has been generated automatically and is only included for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Non-patent literature cited

• J. Alvarado et al., Materials Today, Volume 21, Issue 4, May 2018, pages 341-353 [0006]

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: R ¹ , R ² , R ³ , R ⁴ are selected identically or independently of one another from the group comprising hydrogen, fluorine, linear or branched C ₁ -C ₆ alkyl, C ₂ -C ₆ alkenyl, C ₂ -C ₆ -alkynyl, C ₃ -C ₆ -cycloalkyl and / or phenyl, the alkyl, alkenyl, alkynyl, cycloalkyl and phenyl groups in each case being unsubstituted or mono- or polysubstituted by fluorine. Electrolyte after Claim 1 , characterized in that R ¹ , R ² , R ³ , R ⁴ are selected identically or independently of one another from the group comprising hydrogen, fluorine and / or in each case unsubstituted or mono- or polysubstituted by fluorine, C ₁ -C ₄ -alkyl or phenyl . Electrolyte after Claim 1 or 2nd , characterized in that R ¹ , R ² , R ³ , R ⁴ are selected identically or independently of one another from the group comprising hydrogen, fluorine, CF ₃ , C ₂ F ₅ , methyl and / or ethyl. Electrolyte according to one of the preceding claims, characterized in that the compound according to the general formula (1) is 3-oxetanone. 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 ≥ 0.05 wt .-% to ≤ 10 wt .-%, preferably in the range of ≥ 0.5 wt .-% to ≤ 5 wt .-%, preferably in the range of ≥ 1 wt .-% to ≤ 2 wt .-%, based on the total weight of the electrolyte. Electrolyte according to one of the preceding claims, characterized in that the electrolyte comprises an organic solvent, preferably selected from the group comprising 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, propyl acetate, butyl acetate, ethyl methanesulfonate, dimethyl methyl phosphonate, linear or cyclic sulfone, symmetrical or non-symmetrical alkyl phosphates, symmetrical or non-symmetric organosilanes, organic and non-symmetric organosilanes cyclic or linear carbonates, ethers, lactones and esters and / or mixtures thereof. Electrolyte after Claim 6 , characterized in that the solvent is selected from the group comprising propylene carbonate, ethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate and / or mixtures thereof. Energy storage, in particular electrochemical energy storage selected from the group comprising lithium battery, lithium-ion battery, lithium-ion battery, lithium-polymer battery, lithium-ion capacitor, supercapacitor, dual-ion battery or dual-graphite A battery comprising an electrolyte according to one of the Claims 1 to 7 . A method of forming a solid-state electrolyte phase interface on an electrode of an electrochemical cell comprising an anode, a cathode and an electrolyte, wherein the cell is used using an electrolyte according to one of the Claims 1 to 7 operates. Use of a compound according to general formula (1) as indicated below:

wherein: R ¹ , R ² , R ³ , R ⁴ are selected identically or independently of one another from the group comprising hydrogen, fluorine, linear or branched C ₁ -C ₆ alkyl, C ₂ -C ₆ alkenyl, C ₂ -C ₆ -alkynyl, C ₃ -C ₆ -cycloalkyl and / or phenyl, the alkyl, alkenyl, alkynyl, cycloalkyl and phenyl groups in each case being unsubstituted or mono- or polysubstituted by fluorine, in an energy store, in particular an electrochemical energy store .

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