Description

The present invention relates to lithium sulphur batteries comprising an electrolyte containing lithium alkoxyborates and lithium alkoxyaluminates and the use of lithium alkoxyborates and lithium alkoxyaluminates as conducting salts in lithium sulphur batteries. There is a high demand for long lasting rechargeable electric current producing cells having high energy density. Such electric current producing cells are used for portable devices as notebooks or digital cameras and will play a major role in the future for the storage of electric energy produced by renewable sources. At the time being, lithium ion rechargeable batteries are the most common batteries used. A further kind of rechargeable batteries with promising characteristics is the lithium sulphur battery (Li/S-battery). In Li/S-batteries, the anode is formed by Li-metal and the cathode is formed by sulphur. In the discharge modus Li° dissociates into an electron and a Li ⁺ -ion which is dissolved in the electrolyte. This process is called lithium stripping. At the cathode the sulphur is initially reduced to polysulfides like L12S8, L12S6, L12S4, and L12S3. These polysulfides are soluble in the electrolyte. Upon further reduction L12S2 and L12S are formed which precipitate.

In the charge modus of the Li/S-battery the Li ⁺ -ion is reduced to Li° at the anode. The Li ⁺ -ion is removed from the electrolyte and precipitated on the anode, thereby. This called lithium plating. L12S2 and L12S are oxidized to polysulfides (like L12S4, L12S6, and L12S8) and sulphur (Ss) at the cathode.

Li/S-batteries have a four times higher theoretical specific energy than Li-ion batteries, especially their gravimetric energy density (Wh/kg) is higher than that of Li-ion batteries. This is an important feature for their possible use as rechargeable energy source for automobiles. In addition, the sulphur used as main material in the cathode of the Li/S-batteries is much cheaper than the Li-ion intercalation compounds used in Li-ion batteries.

In electrochemical cells the ions participating in the electrochemical reaction taking place in the electrochemical cell have to be transferred. For this purpose conducting salts are present in the electrochemical cell. In lithium sulphur batteries the charge transfer is performed by lithium ions, i.e. lithium ion containing conducting salts are present.

Salts used as conducting salts in lithium sulphur batteries should meet several requirements like good solubility in the solvent used and electrochemical and thermal stability. The solvated ions should have high ion mobility and low toxicity and should be economic with regard to price. It is difficult to meet all requirements at the same time. For example, if the diameter of the anion of the salt is increased to decrease the association of the ions, the conductivity of an electrolyte composition containing said salt usually decreases due to the lower mobility of the enlarged anion. Furthermore, the solubility of such salts frequently decreases considerably. The conducting salt best suited has to be determined for every particular application. In regard to lithium ion batteries not all lithium salts are suited, in particular not for the application in high- capacity lithium ion batteries in automotive engineering. The most simple lithium salts are the halides like LiF and LiCI or the oxide L1O2 which are easily available at low costs. However, their solubility in non-aqueous solvents as used for lithium ion batteries is poor. The preparation of complex lithium salts is laborious and the complex lithium salts are expensive. Lithium salts of strong Lewis acids like aluminum halides (e.g. UAIF4, and LiAICU) are not suited, since these salts react with the non-aqueous solvents and with other battery components. Lithium salts derived from strong Bronstedt acids are e.g. lithium trilfluoromethane sulfonate (LiOTF) and lithium bis(trifluoromethyl sulfonyl) imide (LiTFSI). Both are thermally and electrochemically very stable, are non-toxic and insensitive against hydrolysis. LiTFSI is a commonly used conductive salt for Li/S batteries. Unfortunately such compounds are comparably expensive and conductive salts at lower price but with similar performance are desirable.

Lithium salts comprising complex anions with lower Lewis acidity have been developed like lithium perchlorate (LiCIC ), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluoroarsenate

(LiAsFe) and lithium hexafluorophosphate (LiPFe). These salts show very good solubility and electrochemical stability. But the perchlorate anion (CIO4 ^" ) is highly reactive; L1BF4 has a very low conductivity, and LiAsF ₆ cannot be used commercially due to the toxicity of the products of the reaction of As(V) to As(lll) and As(0).

Despite the fact that there has been long and intense research in the field of Li/S-batteries, there is still the need for further improvements of this kind of batteries to obtain Li/S-batteries which are capable of being charged/dis-charged a high number of cycles without losing too much of their capacity. This is a prerequisite for a widespread commercial use of Li/S-batteries. It was thus an object of the present invention to provide compounds suited as lithium conducting salts which have improved properties in respect to at least one of the following requirements: high solubility, high electrochemical and thermal stability, low toxicity, high ion mobility, high compatibility with the other components of the electrolyte, low reactivity, little or no corrosive effect on the current collector materials, little or no adverse interactions with electrode materials like binders, conductive carbon blacks and electrode active materials, e.g. transition metal oxides or graphite, good wetting properties and low price for non-aqueous electrolyte compositions with improved quality and life time.

This object is achieved by a lithium sulphur battery comprising

(A) an electrolyte composition,

(B) at least one cathode comprising a cathode active material containing sulphur, and (C) at least one anode comprising an anode active material selected from lithium and lithium alloys, wherein the electrolyte composition (A) contains at least one aprotic organic solvent,

at least one compound of formula I),

(I)

wherein

Z is Al or B;

R ¹ , R ² , R ³ , and R ⁴ are selected independently from each other from C(0)R ⁵ , C1-C10 alkyl, C2-C10 alkenyl, C2-C10 alkynyl, C3-C10 cycloalkyl, C6-C14 aryl and C5-C14 heteroaryl, wherein alkyl, alkenyl, alkynyl, cycloalkyl, aryl, and heteroaryl may be substituted by one or more F, and/or by one or more optionally fluorinated groups selected from C1-C4 alkyl, C2-C4 alkenyl, C2-C4 alkynyl, phenyl, and benzyl;

or R ¹ and R ² are selected independently from each other from C(O) and C1-C3 alkylene and form together with the joining group OZO a 5- to 7-membered hetero cycle, wherein alkylene may be substituted by one or more F, and/or by one or more optionally fluorinated groups selected from C1-C4 alkyl, phenyl, and benzyl, and R ³ and R ⁴ are selected as defined above;

R ⁵ is selected from C1-C10 alkyl, C2-C10 alkenyl, C2-C10 alkynyl, C3-C10 cycloalkyl, C6-C14 aryl and C5-C14 heteroaryl, wherein alkyl, alkenyl, alkynyl, cycloalkyl, aryl, and heteroaryl may be substituted by one or more F, and/or by one or more optionally fluorinated groups selected from C1-C4 alkyl, phenyl, and benzyl; and

wherein at least one of R ¹ , R ² , R ³ , and R ⁴ carries at least one F;

(iii) optionally at least one conducting salt different from the at least one compound of formula (I); and

(iv) optionally at least one further additive.

This object is also achieved by the use of the compounds of formula (I) as conducting salts in electrolytes for lithium sulphur batteries.

Lithium sulphur batteries comprising the inventive electrolyte composition containing a compound of formula (I) instead of the standard conducting salt lithium bis(trifluoromethyl sulfonyl) imide (LiTFSI) show improved capacity retention. the following the invention is described

The term "C1-C10 alkyl" as used herein means a straight or branched saturated hydrocarbon group with 1 to 10 carbon atoms having one free valence and includes, e.g., methyl, ethyl, n- propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, iso-pentyl, 2,2-dimethylpropyl, n-hexyl, iso-hexyl, 2-ethyl hexyl, n-heptyl, iso-heptyl, n-octyl, iso-octyl, n-nonyl, n-decyl and the like. Preferred are C1-C6 alkyl groups, more preferred C1-C4 alkyl groups and most preferred are 2-propyl, methyl and ethyl. The term "C3-C10 cycloalkyl" as used herein means a cyclic saturated hydrocarbon group with 3 to 10 carbon atoms having one free valence and includes, e.g. cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl. Preferred are C3-C6 cycloalkyl.

The term "C2-C10 alkenyl" as used herein refers to an unsaturated straight or branched hydrocarbon group with 2 to 10 carbon atoms having one free valence, wherein the

hydrocarbon group contains at least one C-C double bond. C2-C10 alkenyl includes for example ethenyl, 1 -propenyl, 2-propenyl, 1 -n-butenyl, 2-n-butenyl, iso-butenyl, 1 -pentenyl, 1 -hexenyl and the like. Preferred are C2-C6 alkenyl, more preferred are C2-C4 alkenyl, in particular ethenyl and propenyl (allyl).

The term "C2-C10 alkynyl" as used herein refers to an unsaturated straight or branched hydrocarbon group with 2 to 10 carbon atoms having one free valence, wherein the

hydrocarbon group contains at least one C-C triple bond. C2-C10 alkynyl includes for example ethynyl, 1 -propynyl, 2-propynyl, 1 -n-butinyl, 2-n-butynyl, iso-butinyl, 1 -pentynyl, 1 -hexynyl and the like. Preferred are C2-C6 alkynyl, more preferred are C2-C4 alkynyl.

The term "C5-C14 aryl" as used herein denotes an aromatic 5- to 14-membered hydrocarbon cycle having one free valence. Examples of C5-C14 aryl are phenyl, naphtyl and anthracyl, preferred is phenyl.

The term "C5-C14 hetero aryl" as used herein denotes an aromatic 5- to 14-membered hydrocarbon cycle having one free valence wherein at least one C-atom is replaced by N, O or S. Preferred are C5-C7 hetero aryl. Examples of C5-C14 hetero aryl are pyrrolyl, furanyl, thiophenyl, pyridinyl, pyranyl, thiopyranyl and the like.

The term "C1-C3 alkylene" as used herein denotes a saturated hydrocarbon group with one to three carbon atoms having two free valences. The alkylene groups are derived from the respective alkyl groups and are also named alkanediyl. Examples are methylene (CH2), 1 ,2- ethylene (CH2CH2), 1 ,3-propylene (CH ₂ CH ₂ CH ₂ ), and 1 ,2-propylene (CH2CHCH3).

The term "C(O)" as used herein denotes the carbonyl group C=0. In some embodiments R ¹ and R ² are selected independently from each other from C(O) and Ci- C3 alkylene and form together with the joining group OZO a 5- to 7-membered hetero cycle which may be substituted by one or more F, C1-C4 alkyi, and/or C1-C4 alkyi substituted by one or

more F. Examples for such compounds are wherein both C(O)

and form with the joining OBO-group a 5 membered heterocycle, and wherein R ¹ is methylene and R ² is ethylene.

The term "wherein at least one of R ¹ , R ² , R ³ , and R ⁴ carries at least one F" means that at least one of the substituents R ¹ , R ² , R ³ , and R ⁴ is substituted by at least one F. It is preferred if at least one of R ¹ , R ² , R ³ , and R ⁴ contains at least one CF3-group.

The term "benzyl" means the group Chb-phenyl.

Preferred compounds of formula (I) are compounds of formula (I) wherein

R ¹ , R ² , R ³ , and R ⁴ are selected independently from each other from C(0)R ⁵ , C1-C6 alkyi, C1-C6 alkyi, C2-C6 alkenyl, C2-C6 akynyl, C3-C6 cycloalkyl, C6-C14 aryl and C5-C14 heteroaryl, wherein alkyi, alkenyl, akynyl, cycloalkyl, aryl, and heteroaryl may be substituted by one or more F, and/or by one or more optionally fluorinated groups selected from C1-C4 alkyi, C2- C4 alkenyl, C2-C4 akynyl, phenyl, and benzyl;

or R ¹ and R ² are selected independently from each other from C(O) and C1-C3 alkylene and form together with the joining group OZO a 5- to 7-membered hetero cycle, wherein alkylene may be substituted by one or more F, and/or by one or more optionally fluorinated groups selected from C1-C4 alkyi, phenyl, and benzyl, and R ³ and R ⁴ are selected independently from each other from C(0)R ⁵ , C1-C6 alkyi, C1-C6 alkyi, C2-C6 alkenyl, C2-C6 akynyl, C3-C6 cycloalkyl, C6-C14 aryl and C5-C14 heteroaryl, wherein alkyi, alkenyl, akynyl, cycloalkyl, aryl, and heteroaryl may be substituted by one or more F, and/or by one or more optionally fluorinated groups selected from C1-C4 alkyi, C2-C4 alkenyl, C2-C4 akynyl, phenyl, and benzyl;

R ⁵ is selected from a chemical bonding, C1-C6 alkyi, C2-C6 alkenyl, C2-C6 akynyl, C3-C6

cycloalkyl, C6-C14 aryl and C5-C14 heteroaryl, wherein alkyi, alkenyl, akynyl, cycloalkyl, aryl, and heteroaryl may be substituted by one or more F, and/or by one or more optionally fluorinated groups selected from C1-C4 alkyi, phenyl, and benzyl;

wherein at least one of R ¹ , R ² , R ³ , and R ⁴ carries at least one F.

More preferred compounds of formula (I) are compounds of formula (I)

wherein R ¹ , R ² , R ³ , and R ⁴ are independently selected from C1-C6 alkyi and C(0)Ci-C6 alkyi, wherein at least one alkyi is substituted by one or more F; or wherein R ¹ and R ² are selected independently from each other from C(O) and C1-C3 alkylene and form together with the joining group OZO a 5- to 7-membered hetero cycle, wherein alkylene may be substituted by one or more F, C1-C4 alkyl, and/or C1-C4 alkyl substituted by one or more F, and R ³ and R ⁴ are selected from optionally fluorinated C1-C6 alkyl and C(0)Ci-C6 alkyl, and wherein at least one of R ¹ , R ² , R ³ , and R ⁴ carries at least one F.

Even more preferred are compounds of formula (I) wherein at least one of R ¹ , R ² , R ³ , and R ⁴ contains at least one CF3-group. In particular preferred R ¹ , R ² , R ³ , and R ⁴ are selected from CH2CF3, CH(CH ₃ )CF ₃ , CH(CF ₃ ) ₂ , C(CH ₃ )(CF ₃ ) ₂ , and C(CF ₃ ) ₃ .

The central atom Z of the compounds formula (I) may be Al or B.

According to one embodiment Z is Al. For compounds of formula (I) wherein the central atom Z is Al it is preferred if

R ¹ , R ² , R ³ , and R ⁴ are selected independently from each other from C(0)R ⁵ , C1-C10 alkyl, C2-C10 alkenyl, C2-C10 alkynyl, C3-C10 cycloalkyl, C6-C14 aryl and C5-C14 heteroaryl, wherein alkyl, alkenyl, alkynyl, cycloalkyl, aryl, and heteroaryl may be substituted by one or more F, and/or by one or more optionally fluorinated groups selected from C1-C4 alkyl, C2-C4 alkenyl, C2-C4 alkynyl, phenyl, and benzyl, preferably R ¹ , R ² , R ³ , and R ⁴ are selected independently from each other from optionally fluorinated C1-C6 alkyl;

R ⁵ is selected from C1-C10 alkyl, C2-C10 alkenyl, C2-C10 alkynyl, C3-C10 cycloalkyl, C6-C14 aryl and C5-C14 heteroaryl, wherein alkyl, alkenyl, alkynyl, cycloalkyl, aryl, and heteroaryl may be substituted by one or more F, and/or by one or more optionally fluorinated groups selected from C1-C4 alkyl, phenyl, and benzyl; and

wherein at least one of R ¹ , R ² , R ³ , and R ⁴ carries at least one F.

Preferred compounds of formula (I) wherein Z is Al are compounds of formula (I) wherein R ¹ , R ² , R ³ , and R ⁴ are selected independently from each other from C1-C6 alkyl wherein at least one alkyl is substituted by one or more F. Even more preferred R ¹ , R ² , R ³ , and R ⁴ are the same.

Within this embodiment it is further preferred if at least one of R ¹ , R ² , R ³ , and R ⁴ contains at least one CF3-group. Even more preferred R ¹ , R ² , R ³ , and R ⁴ are selected from CH2CF3, CH(CH ₃ )CF ₃ , CH(CF ₃ ) ₂ , C(CH ₃ )(CF ₃ ) ₂ , and C(CF ₃ ) ₃ .

According to another embodiment Z is B. For such compounds of formula (I) it is preferred if R ¹ and R ² are selected independently from each other from C(O) and C1-C3 alkylene and form together with the joining group OZO a 5- to 7-membered hetero cycle, wherein alkylene may be substituted by one or more F, and/or by one or more optionally fluorinated groups selected from C1-C4 alkyl, phenyl, and benzyl; and

R ³ and R ⁴ are selected independently from each other from C(0)R ⁵ , C1-C10 alkyl, C2-C10 alkenyl, C2-C10 alkynyl, C3-C10 cycloalkyl, C6-C14 aryl and C5-C14 heteroaryl, wherein alkyl, alkenyl, alkynyl, cycloalkyl, aryl, and heteroaryl may be substituted by one or more F, and/or by one or more optionally fluorinated groups selected from C1-C4 alkyl, C2-C4 alkenyl, C2-C4 alkynyl, phenyl, and benzyl, preferably R ³ and R ⁴ are selected independently from each other from optionally fluorinated C1-C6 alkyl;

R ⁵ is selected from C1-C10 alkyl, C2-C10 alkenyl, C2-C10 alkynyl, C3-C10 cycloalkyl, C6-C14 aryl and C5-C14 heteroaryl, wherein alkyl, alkenyl, alkynyl, cycloalkyl, aryl, and heteroaryl may be substituted by one or more F, and/or by one or more optionally fluorinated groups selected from C1-C4 alkyl, phenyl, and benzyl; and

wherein at least one of R ¹ , R ² , R ³ , and R ⁴ carries at least one F.

Preferred compounds of formula (I) wherein Z is B are compounds of formula (I) wherein

R ¹ and R ² are selected independently from each other from C(O) and C1-C3 alkylene and form together with the joining group OZO a 5- to 7-membered hetero cycle, wherein alkylene may be substituted by one or more F, and/or by one or more optionally fluorinated groups selected from C1-C4 alkyl, phenyl, and benzyl, and

R ³ and R ⁴ are selected independently from each other from optionally fluorinated C1-C6 alkyl, in particular preferred R ³ and R ⁴ are the same, and

wherein at least one of R ¹ , R ² , R ³ , and R ⁴ carries at least one F. Within this embodiment it is further preferred if at least one of R ¹ , R ² , R ³ , and R ⁴ contains at least one CF3-group. In particular preferred R ³ , and R ⁴ are selected from CH2CF3, CH(CH3)CF3, CH(CF ₃ ) ₂ , C(CH ₃ )(CF ₃ ) ₂ , and C(CF ₃ ) ₃ . Even more preferred R ¹ and R ² are C(0)C(0) forming a 5 membered cycle with OBO and R ³ and R ⁴ are selected from CH2CF3, CH(CH ₃ )CF ₃ , CH(CF ₃ ) ₂ , C(CH ₃ )(CF ₃ ) ₂ , and C(CF ₃ ) ₃ .

Electrolyte compositions (A) containing compounds of formula (I) wherein Z is B according to the above described embodiment of the invention are especially preferred according to the present invention since these compounds are especially well suited for use in Li/S batteries. Especially preferred salts of formula (I) are Li[B(OC(0)C(0)0)(OCH(CF ₃ ) ₂ ) ₂ ], Li[B(OCH(CF ₃ ) ₂ )4], Li[B(OCH ₂ CF ₃ ) ₄ ], Li[AI(OCH(CF ₃ ) ₂ )4], Li[AI(OC(CH ₃ )(CF ₃ ) ₂ )4], and Li[AI(OC(CF ₃ ) ₃ )4].

The preparation of compounds of formula (I) is known to the person skilled in the art. Salts of formula (I) with Z is Al may prepared by the reaction of LiAII-U or L1BH4 and the appropriate alcohols as described in I. Krossing, Chem. Eur. J. (2001 ) Vol. 7 p. 490 and S. M. Ivanova et al., Chem. Eur. J. (2001 ) Vol. 7, p. 503. These may be further reacted by ligand exchange as described for borates in I. Krossing, Dalton Trans. (201 1 ) Vol. 40 p. 81 14.

The concentration of the at least one compound of formula (I) (ii) in the electrolyte composition (A) is usually 0.01 to 40 wt.-%, preferred 0.1 to 30 wt.-%, more preferred 0.5 to 25 wt.-%, and in particular 1 to 20 wt.-%, based on the total weight of the electrolyte composition (A). The electrolyte composition (A) further contains at least one aprotic organic solvent (i). The at least compound of formula (I) present in the electrolyte is usually solvated in the aprotic organic solvent(s) (i). Preferably electrolyte composition (A) contains at least two aprotic organic solvents (i) and more preferred at least three aprotic organic solvents (i). According to one embodiment the electrolyte composition (A) may contain up to ten aprotic organic solvents (i).

The at least one aprotic organic solvent (i) is preferably selected from

(a) cyclic and noncyclic organic carbonates, which may be partly halogenated,

(b) di-Ci-Cio-alkylethers, which may be partly halogenated,

(c) di-Ci-C4-alkyl-C2-C6-alkylene ethers and polyethers, which may be partly halogenated,

(d) cyclic ethers, which may be partly halogenated,

(e) cyclic and acyclic acetals and ketals, which may be partly halogenated,

(f) orthocarboxylic acids esters, which may be partly halogenated,

(g) cyclic and noncyclic esters of carboxylic acids, which may be partly halogenated,

(h) Cyclic and noncyclic sulfones, which may be partly halogenated,

(i) Cyclic and noncyclic nitriles and dinitriles, which may be partly halogenated, and

(j) Ionic liquids, which may be partly halogenated.

The aprotic organic solvents (a) to (j) may be partly halogenated, e.g. they may be partly fluorinated, partly chlorinated or partly brominated, and preferably they may be partly fluorinated. "Partly halogenated" means, that one or more H of the respective molecule is substituted by a halogen atom, e.g. by F, CI or Br. Preference is given to the substitution by F. The at least one solvent (i) may be selected from partly halogenated and non-halogenated aprotic organic solvents (a) to (j), i.e. the electrolyte composition may contain a mixture of partly halogenated and non-halogenated aprotic organic solvents.

More preferred the at least one aprotic organic solvent (i) is selected from di-Ci-Cio-alkylethers (b), di-Ci-C4-alkyl-C2-C6-alkylene ethers and polyethers (c), cyclic ethers (d), and cyclic and acyclic acetals and ketals, most preferred electrolyte composition (A) contains at least two aprotic organic solvents (i) selected from di-Ci-Cio-alkylethers (b), di-Ci-C4-alkyl-C2-C6-alkylene ethers and polyethers (c), cyclic ethers (d), and cyclic and acyclic acetals and ketals (e), in particular electrolyte composition (A) contains at least one solvent selected from di-Ci-Cio- alkylethers (b) and di-Ci-C4-alkyl-C2-C6-alkylene ethers and polyethers (c), and at least one solvent selected from cyclic ethers (d), and cyclic and acyclic acetals and ketals (e), e.g. the electrolyte composition contains dimethoxyethane (DME) and 1 ,3-dioxolane (DOL).

Examples of suitable organic carbonates (a) are cyclic organic carbonates according to the general formula (a1 ), (a2) or (a3)

wherein

R ^a , R ^b und R ^c being different or equal and being independently from each other selected from hydrogen; Ci-C4-alkyl, preferably methyl; F; and Ci-C4-alkyl substituted by one or more F, e.g. CF ₃ .

"Ci-C4-alkyl" is intended to include methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, sec- butyl and tert. -butyl. Preferred cyclic organic carbonates (a) are of general formula (a1 ), (a2) or (a3) wherein R ^a , R ^b and R ^c are H. Examples are ethylene carbonate, vinylene carbonate, and propylene carbonate. A preferred cyclic organic carbonate (a) is ethylene carbonate. Further preferred cyclic organic carbonate (a) are difluoroethylene carbonate (a4) and monofluoroethylene carbonate (a5)

(a4) (a5) Examples of suitable non-cyclic organic carbonates (a) are dimethyl carbonate, diethyl carbonate, methylethyl carbonate and mixtures thereof.

In one embodiment of the invention the electrolyte composition (A) contains mixtures of at least one solvent selected from di-Ci-Cio-alkylethers (b) and di-Ci-C4-alkyl-C2-C6-alkylene ethers and polyethers (c) and at least one solvent selected from cyclic ethers (d) and cyclic and acyclic acetals and ketals (e) at a ratio by weight of from 1 :10 to 10:1 , preferred of from 3:1 to 1 :1 .

Examples of suitable non-cyclic di-Ci-Cio-alkylethers (b) are dimethylether, ethylmethylether, diethylether, diisopropylether, and di-n-butylether.

Examples of di-Ci-C4-alkyl-C2-C6-alkylene ethers (c) are 1 ,2-dimethoxyethane, 1 ,2- diethoxyethane, diglyme (diethylene glycol dimethyl ether), triglyme (triethylenglycol dimethyl ether), tetraglyme (tetraethylenglycol dimethyl ether), and diethylenglycoldiethylether. Examples of suitable polyethers (c) are polyalkylene glycols, preferably poly-Ci-C4-alkylene glycols and especially polyethylene glycols. Polyethylene glycols may comprise up to 20 mol% of one or more Ci-C4-alkylene glycols in copolymerized form. Polyalkylene glycols are preferably dimethyl- or diethyl-end capped polyalkylene glycols. The molecular weight M _w of suitable polyalkylene glycols and especially of suitable polyethylene glycols may be at least 400 g/mol. The molecular weight M _w of suitable polyalkylene glycols and especially of suitable polyethylene glycols may be up to 5 000 000 g/mol, preferably up to 2 000 000 g/mol.

Examples of suitable cyclic ethers (d) are tetrahydrofurane and 1 ,4-dioxane.

Examples of suitable non-cyclic acetals (e) are 1 ,1-dimethoxymethane and 1 ,1 - diethoxymethane. Examples for suitable cyclic acetals (e) are 1 ,3-dioxane and 1 ,3-dioxolane.

Examples of suitable orthocarboxylic acids esters (f) are tri-Ci-C4 alkoxy methane, in particular trimethoxymethane and triethoxymethane.

Examples of suitable noncyclic esters of carboxylic acids (g) are ethyl acetate, methyl butanoate, esters of dicarboxylic acids like 1 ,3-dimethyl propanedioate. An example of a suitable cyclic ester of carboxylic acids (lactones) is γ-butyrolactone.

Examples of suitable noncyclic sulfones (h) are ethyl methyl sulfone and dimethyl sulfone.

Examples of suitable cyclic and noncyclic nitriles and dinitriles (i) are adipodinitrile, acetonitrile, propionitrile, and butyronitrile.

The inventive electrolyte composition (A) may contain at least one conducting salt (iii) different from the at least one compound of formula (I). The conducting salt(s) (iii) optionally present in the electrolyte are usually solvated in the aprotic organic solvent(s) (i). Preferably the conducting salt (iii) is a lithium salt. The conducting salt (iii) is preferably selected from the group consisting of

• Li[F6-xP(C _y F2y+i)x], wherein x is an integer in the range from 0 to 6 and y is an integer in the range from 1 to 20;

Li[B( ') ₄ ], Li[B(R') ₂ (OR"0)] and Li[B(OR ^M 0) ₂ ] wherein each R' is independently from each other selected from F, CI, Br, I, C1-C4 alkyl, C2-C4 alkenyl, C2-C4 alkynyl, OC1-C4 alkyl, OC2-C4 alkenyl, and OC2-C4 alkynyl, wherein alkyl, alkenyl, and alkynyl may be substituted by one or more OR'", wherein R ^m is selected from C1-C6 alkyl, C2-C6 alkenyl, and C2-C6 alkynyl, and

(OR"0) is a bivalent group derived from a 1 ,2- or 1 ,3-diol, a 1 ,2- or 1 ,3-dicarboxlic acid or a 1 ,2- or 1 ,3-hydroxycarboxylic acid, wherein the bivalent group forms a 5- or 6- membered cycle via the both oxygen atoms with the central B-atom;

• salts of the general formula Li[X(C _n F2n+iS02)m], where m and n are defined as follows: m = 1 when X is selected from oxygen and sulfur, m = 2 when X is selected from nitrogen and phosphorus,

m = 3 when X is selected from carbon and silicon, and

n is an integer in the range from 1 to 20,

LiCI0 ₄ ; LiAsF ₆ ; LiCF ₃ S0 ₃ ; Li ₂ SiF ₆ ; LiSbF ₆ ; LiAICU, Li[N(S0 ₂ F) ₂ ], lithium tetrafluoro (oxalato) phosphate; and lithium oxalate.

Suited 1 ,2- and 1 ,3-diols from which the bivalent group (OR"0) is derived may be aliphatic or aromatic and may be selected, e.g., from 1 ,2-dihydroxybenzene propane-1 ,2-diol, butane-1 ,2- diol, propane-1 ,3-diol, butan-1 ,3-diol, cyclohexyl-trans-1 ,2-diol or naphthalene-2,3-diol which are optionally are substituted by one or more F and/or by at least one straight or branched non fluorinated, partly fluorinated or fully fluorinated Ci-C ₄ alkyl group. An example for such 1 ,2- or 1 ,3-diol is 1 ,1 ,2,2-tetra(trifluoromethyl)-1 ,2-ethane diol.

Suited 1 ,2- or 1 ,3-dicarboxlic acids from which the bivalent group (OR"0) is derived may be aliphatic or aromatic, for example oxalic acid, malonic acid (propane-1 ,3-dicarboxylic acid), phthalic acid or isophthalic acid, preferred is oxalic acid. The 1 ,2- or 1 ,3-dicarboxlic acids are optionally are substituted by one or more F and/or by at least one straight or branched non fluorinated, partly fluorinated or fully fluorinated Ci-C ₄ alkyl group. Suited 1 ,2- or 1 ,3-hydroxycarboxylic acids from which the bivalent group (OR"0) is derived may be aliphatic or aromatic, for example salicylic acid, tetrahydro salicylic acid, malic acid, 2- hydroxy acetic acid, which are optionally are substituted by one or more F and/or by at least one straight or branched non fluorinated, partly fluorinated or fully fluorinated Ci-C ₄ alkyl group. An example for such 1 ,2- or 1 ,3-hydroxycarboxylic acids is 2,2-bis(trifluoromethyl)-2-hydroxy-acetic acid.

Examples of Li[B(R') ₄ ], Li[B(R ^l )2(OR"0)] and Li[B(OR ^M 0) ₂ ] are LiBF ₄ , lithium difluoro oxalato borate and lithium dioxalato borate. Preferably the at least one conducting salt (iii) is selected from UCF3SO3, Li(CFsS02)2N, LiC ₄ FgS03 and Li^FsSC^N, more preferred the conducting salt (iii) is Li(CFsS02)2N.

According to one embodiment of the present invention the electrolyte composition contains at least one conducting salt (iii) different from the compound of formula I. In this case the at least one conducting salt (iii) is usually present at a minimum concentration of at least 0.01 wt.-%, preferably of at least 0.5 wt.-%, more preferred of at least 1 wt.-%, and most preferred of at least 5 wt.-%, based on the total weight of the electrolyte composition. Usually the upper concentration limit for the at least one conducting salt (iii) is 25 wt.-%, based on the total weight of the electrolyte composition Moreover, the inventive electrolyte composition (A) may contain at least one further additive (iv). The at least one further additive (iv) may be selected e.g. from lithium nitrate, guanidine nitrate, and pyridinium nitrate. According to one embodiment of the present invention the electrolyte composition contains at least one further additive (iv). If one or more further additives (iv) are present in the electrolyte composition (A), the total concentration of further additives (iv) is at least 0.001 wt.-%, preferred 0.005 to 10 wt.-% and most preferred 0.01 to 5 wt.-%, based on the total weight of the electrolyte composition (A).

The inventive electrolyte composition is preferably essentially water free, i.e. the water content of the inventive electrolyte composition is below 100 ppm, more preferred below 50 ppm, most preferred below 30 ppm. The term "ppm" denotes parts per million based on the weight of the total electrolyte composition. Various methods are known to the person skilled in the art to determine the amount of water present in the electrolyte composition. A method well suited is the titration according to Karl Fischer, e.g. described in detail in DIN 51777 or ISO760: 1978.

The electrolyte composition (A) of the inventive lithium ion battery is preferably liquid at working conditions; more preferred it is liquid at 1 bar and 25 °C, even more preferred the electrolyte composition is liquid at 1 bar and -15 °C, most preferred the electrolyte composition is liquid at 1 bar and -30 °C, and in particular preferred the electrolyte composition is liquid at 1 bar and -50 °C.

In a preferred embodiment of the present invention the electrolyte composition (A) contains at least two aprotic solvents (i), one solvent selected from di-Ci-Cio-alkylethers (b) and di-Ci-C ₄ - alkyl-C2-C6-alkylene ethers and polyethers (c) and the other solvent selected from cyclic ethers (d) and cyclic and acyclic acetals and ketals (e), at least one compound of formula (I), at least one conducting salt (ii) selected from LiCF ₃ S0 ₃ , Li(CF ₃ S0 ₂ ) ₂ N, LiC ₄ F ₉ S0 ₃ and Li(C ₂ F ₅ S02)2N and at maximum up to 100 ppm water.

Preference is further given to electrolyte composition (A), wherein the electrolyte composition contains

(i) from 60 to 99.99 wt.-% of the at least one aprotic organic solvent,

(ii) from 0.01 to 40 wt.-% of the at least one compound of formula (I),

(iii) from 0 to 25 wt.-% of the at least one conducting salt different the at least one compound of formula (I), and

(iv) from 0 to 10 wt.-% of the at least one further additive,

based on the total weight of the electrolyte composition.

Li/S-batteries comprising electrolyte composition (A) as described above show increased cycling stability. A further object of the present invention is the use of the compounds of formula (I) as described above in detail as component (ii) of the electrolyte composition (A) as conducting salt(s) in electrolytes for lithium sulphur batteries. The at least one compound of formula (I) is usually used by adding the compound(s) of formula (I) to the electrolyte. Usually the compound(s) of formula (I) are added in amounts yielding electrolyte compositions containing the above described concentrations of compound(s) of formula (I) in the electrolyte composition (A).

The lithium sulphur battery of the present invention comprises

(A) the electrolyte composition as described above in detail,

(B) at least one cathode comprising a cathode active material containing sulphur, and

(C) at least one anode comprising an anode active material containing lithium.

Cathode (B) comprises a cathode active material containing sulphur. In a preferred embodiment of the invention the cathode contains elemental sulphur. Usually elemental sulphur is crystalline with Se-molecules at room temperature. In this case the Ss-molecule is the theoretical final oxidation state of the sulphur in the Li/S current producing cell. The corresponding theoretical final reduction state of sulphur is L12S. Sulphur shows complicated redox reactions wherein polysulfides with various chain lengths and different oxidation states are generated. The sulphides L12S2 and L12S are normally non-soluble in the electrolyte; the polysulfides formed during the complicated redox reaction are soluble in the electrolyte. As a result, the solid composite for the cathode of a Li/S electric current producing cell may emit a part of the sulphur as polysulfides into the electrolyte during the discharge of the electric current producing cell. The sulphur used for the preparation of the cathode of the present invention may be milled before the preparation e.g. in a ball mill.

In a preferred embodiment the cathode contains a mixture containing sulphur and one or more conductive agents to allow good accessibility of the sulphur during discharge/charge of the Li/S- battery. Said one or more conductive agents are usually selected from the group consisting of expanded graphite, carbon black, graphite, carbon fibres, carbon nanotubes, activated carbon, carbon prepared by heat treating cork or pitch, a metal powder, metal flakes, a metal compound or a mixture thereof. The carbon black may include ketjen black, denka black, acetylene black, thermal black and channel black. The metal powder and the metal flakes may be selected from Sc, Ti, V, Cr, Mn, Sn, Ag, Fe, Co, Ni, Cu, Zn, Al, etc. Furthermore, the conductive agent may be selected from electrically conductive polymers and electrically conductive metal chalcogenides.

The cathode may further comprise one or more binder. The binder binds the sulphur and the optionally one or more conductive agents tightly as network to maintain a conductive network structure of the mixture containing sulphur and conductive agent and to bind the solid composite to the current collector/supply. The one or more binder are preferably selected from the group consisting of polyvinylacetate, polyvinylalcohol, polyethylene oxide, polyvinyl pyrrolidone, alkylated polyethylene oxide, cross linked polyethylene oxide, polyvinyl ether, poly(methyl methacrylate), polyvinylidene fluoride, a copolymer of polyhexafluoropropylene and polyvinylidene fluoride, poly(ethyl acrylate), polytetrafluoroethylene, polyvinyl chloride, polyacrylonitrile, polyvinylpyridine, polystyrene, polypyrrole, polythiophene, derivatives thereof, blends thereof, and copolymers thereof. Furthermore, the cathode may comprise a current collector which further acts as current supply in the charge modus of the electric current producing cell. The current collector/current supply may be a metal wire, a metal grid, a metal web, a metal sheet, a metal foil or a metal plate and may be prepared from conductive materials like stainless steel, aluminium, copper or titanium. According to one embodiment of the present invention the cathode has a thickness of from 25 to 200 μηη, preferably of from 30 to 100 μηη, based on the whole thickness of the cathode without the thickness of the current collector.

Furthermore, the lithium sulphur battery according to the present invention comprises at least one anode, wherein the anode comprises an anode active material selected from lithium and lithium alloys, i.e. lithium containing alloys like lithium-aluminium alloys, lithium-tin alloys, Li-Mg- alloys, lithium-silicon alloy, and Li-Ag-alloys. A suited anode may be a lithium foil.

The inventive lithium sulphur battery may contain further constituents customary per se, for example output conductors, separators, housings, cable connections etc. Output conductors may be configured in the form of a metal wire, metal grid, metal mesh, expanded, metal, metal sheet or metal foil. Suitable metal foils are especially aluminum foils. The separator is typically a porous non-conductive or isolative material which separates or insulates the anode and the cathode from each other and which permits the transport of ions through the separator between the anode and the cathode of the battery. The separator is usually selected from the group consisting of porous glass, porous plastic, porous ceramic and porous polymer separators. Suited separators are for example glass fiber separators and polymer-based separators like polyolefin separators. The housing may be of any shape, for example cuboidal or in the shape of a cylinder. In another embodiment, inventive electrochemical cells have the shape of a prism. In one variant, the housing used is a metal-plastic composite film processed as a pouch.

The present invention therefore also further provides for the use of inventive electrochemical cells in devices, especially in mobile devices. Examples of mobile devices are vehicles, for example automobiles, bicycles, aircraft, or water vehicles such as boats or ships. Other examples of mobile devices are those which are portable, for example computers, especially laptops, telephones or electrical power tools, for example from the construction sector, especially drills, battery-driven screwdrivers or battery-driven tackers. The inventive

electrochemical cells may also be used for stationary power storage. The invention is illustrated by the following examples, which do not, however, restrict the invention. Preparation of the compounds of formula (I)

The compounds of formula (I) prepared are summarized in Table 1 . (1.1 ) Preparation of lithium alkoxyaluminates

Compounds 1 to 3: Li[AI(OR ^F ) ₄ ] with R ^F = CH(CF ₃ ) ₂ , C(CF ₃ ) ₃ ) and C(CH ₃ )(CF ₃ ) ₂

Li[AI(OR ^F ) ₄ ] with R ^F =CH(CF ₃ ) ₂ , C(CF ₃ ) ₃ and C(CH ₃ )(CF ₃ ) ₂ were prepared by reacting purified LiAIH ₄ and the appropriate commercially available alcohols (I. Krossing, Chem. Eur. J. (2001 ) Vol. 7 p. 490 and S. M. Ivanova et al., Chem. Eur. J. (2001 ) Vol. 7, p. 503.). Due to the very low boiling points of the alcohols, especially the perfluorinated HOC(CF ₃ ) ₃ (b.p. = 45°C), a double reflux condenser connected to a cryostat and set to a temperature of -20°C has been used. Instead of toluene hexane and heptane were used as solvent.

(1.2) Preparation of lithium alkoxyborates Compound 4: Li[B(OCH(CF ₃ ) ₂ ) ₄ ] Li[B(OCH(CF ₃ ) ₂ ) ₄ ] was prepared from LiBH ₄ and HOCH(CF ₃ ) ₂ . Dry toluene was added to a certain amount of LiBH ₄ . The lithium salt was only partially dissolved. As the reaction has a strong exothermal effect, the mixture had to be cooled to -10 °C. Then, an excess of HOCH(CF ₃ ) ₂ (5.5 equivalents) was added by gradually dropping. The mixture was allowed to return slowly to room temperature and was heated to 1 10 °C until no evolution of H ₂ was observed any more. The proper use of a condenser at -20 °C was essential to avoid the [B(OCH(CF ₃ ) ₂ ) ₃ ] loss during the heating process. When the reaction mixture returned to room temperature, a stoichiometric amount of the O-donor additive was added and the mixture was heated further two hours at 1 10 °C, 95 °C or 80 °C depending on the O-donor additive in use (ethylene carbonate (EC), dimethyl carbonate (DMC) or ethyl acetate (EA) respectively. Finally, the solvent was removed by reduced pressure and the product was filtered in a high-integrity glove box. The reaction works very well with yield exceeding 95 %.

Compound 5: Li[B(OCH ₂ CF ₃ ) ₄ ] LiBH ₄ (184 mg; 8.40 mmol) was added to 80 ml of dimethoxyethane in a three necked 250 ml flask equipped with a condenser. Afterwards, dry 1 ,1 ,1 -trifluoroethanol (3.10 ml, 4.28 g, 428 mmol) was added during 30 minutes by a dropping funnel. After adding the alcohol the mixture was heated up to 90 °C for 4.5 h. After removal of the main part of the solvent by condensing technique the residual solvent was removed under reduced pressure (10-3 mbar). The product was obtained as a colorless solid in 71 % yield. Compound 6: Li[BOx(OCH(CF ₃ ) ₂ )2]

Dry acetonitrile (30 mL) was added to 32.1 g Li(EC) ₄ [B(OCH(CF ₃ ) ₂ )4] (30 mmol, 1 .0 eq) at room temperature. 2.8 g of oxalic acid (30 mmol, 1 eq) were then added and the reaction mixture was stirred overnight at room temperature. The next day, NMR measurements confirmed the completed absence of [B(OCH(CF ₃ ) ₂ ) ₄ r. The product was subjected under reduced pressure. A turbid liquid was obtained and was filtered under argon. Finally, a clear homogeneous liquid was obtained. Table 1

Ox: Oxalato

LiTFSI: lithium bis(trifluoromethylsulfonyl)imide

(II) Electrochemical cycling

Pouch bag cells were used as test system with a metallic lithium sheet of 50 μηη thickness as anode. The cathode is made of 60 wt.-% elemental sulphur as cathode active material, 35 wt.-% carbon black (Vulcan XC72) as conductive material and 5 wt.-% binder (polyvinylidene difluo- ride). The sulphur loading within the cathode amounts to ~2mg/cm ² . The usual standard electro- lyte system for Li/S-batteries containing 8 wt.-% LiTFSI in a 1 :1 mixture (by weight ) of 1 ,2- dimethoxyethane (DME) and 1 ,3-dioxolane (DOL) was used to prepare the comparative Li/S- battery (comparative 1 ). For the inventive Li/S-battery the LiTFSI was completely replaced by the compounds of formula (I). The cells were discharged in the first cycle with 0.02 C rate. Further cycling was performed with charging at 0.1 C and discharging with 0.15 C within the volt- age range of 1 .7 to 2.5 V at room temperature between 20 to 25 °C.

The results of cycling a comparative lithium sulphur battery containing an electrolyte composition containing a mixture of DME/DOL (1 : 1 by weight) and 8 wt.-% LiTFSI and an inventive lithium sulphur battery containing an electrolyte composition containing 8 wt.-% of the com- pound of example 5 (Li[B(OCH ₂ CF ₃ ) ₄ ]) instead of the LiTFSI like the comparative example 1 are shown in table 2. The capacity after 40 cycles is based on the capacity after 5 cycles. The capacity of the inventive cell after 40 cycles is higher than that of the comparative cell. Table 2

Electrolyte composition capacity after 5 cycles capacity after 40 cycles

8wt.-% LiTFSI in (DOL/DME) (1:1)

100% 70.00% (comparative 1)

8 wt.-% Li[B(OCH ₂ CF ₃ ) ₄ ] in

100% 80.00% (DOL/DME) (1:1) (compound 5)