WO2014177702A1

WO2014177702A1 - Fluorinated carbonates as solvent for lithium sulfonimide-based electrolytes

Info

Publication number: WO2014177702A1
Application number: PCT/EP2014/059005
Authority: WO
Inventors: Julian Kalhoff; Dominic BRESSER; Stefano Passerini; Marco Bolloli; Fannie Alloin; Jean-Yves Sanchez
Original assignee: Westfaelische Wilhelms Universitaet Muenster; Centre National de la Recherche Scientifique CNRS; Institut Polytechnique de Grenoble
Current assignee: Westfaelische Wilhelms Universitaet Muenster; Centre National de la Recherche Scientifique CNRS; Institut Polytechnique de Grenoble
Priority date: 2013-05-02
Filing date: 2014-05-02
Publication date: 2014-11-06
Anticipated expiration: 2015-11-02
Also published as: CA2911079A1; WO2014177704A1; EP2800197B1; EP2800197A1; JP2016532237A; US20160118690A1

Abstract

The present invention relates to an electrolyte solution comprising an electrolyte salt and an electrolyte solvent, wherein the electrolyte solvent comprises an n-fluoro diethyl carbonate according to formula (1) as follows: C₂H_5-xF_xCO₃C₂H_5-yF_y (1) wherein 1 ≤ x ≤ 5 and 0 ≤ y ≤ 5 in an amount in the range of ≥ 20 wt % to ≤ 100 wt%, referring to a total amount of the electrolyte solvent of 100 wt%, and the use of such n-fluoro diethyl carbonate for the prevention of aluminum current collector corrosion in an alkali or alkaline earth metal-based electrochemical energy storage device, particularly in a lithium-ion battery or lithium polymer battery containing an electrolyte solution comprising an alkali or alkaline earth metal sulfonimide or sulfonmethide salt.

Description

Fluorinated Carbonates as Solvent for Lithium Sulfonimide-based Electrolytes

The present invention relates to an electrolyte solution comprising an electrolyte salt, particularly a sulfonimide salt, and an electrolyte solvent for use in lithium and lithium-ion batteries.

Lithium-ion batteries are nowadays the leading battery technology, since they offer efficient and high energy storage as well as high power density, and thus, they dominate the market for batteries used in portable electronic devices. However, future large-scale applications like stationary energy storage and electric vehicles still require further improvement of the existing technology in terms of energy density, supplied power, and in particular in terms of safety. One of the major safety issues regarding the up-scaling of lithium-ion battery technology, particularly with respect to its application in electric vehicles, is related to the use of LiPF₆ as lithium salt in currently commercially available batteries. In fact, LiPF₆ does not have any single exceptional property, making it particularly attractive for application as lithium salt in commercial batteries. However, besides a well-balanced combination of chemical and electrochemical properties it offers one major advantage relatively to other lithium salts, the initial passivation of the aluminum surface of the cathodic current collector, caused by the partial decomposition of the lithium salt and oxidation of the metallic aluminum at high potentials, forming a dense film of A1F₃ on the top of the air-formed surface layer of AI₂O₃. Nevertheless, there arise some major issues by the use of LiPF₆, as it is thermally unstable, resulting in the formation of gaseous PF₅, a strong Lewis acid. Moreover, its hydrolysis results in the formation of HF, which is not only highly toxic, but furthermore has a deleterious effect on the electrolyte solvent as well as the electrodes active materials. Finally, a reaction of ethylene carbonate and LiPF₆ at high temperatures and in presence of transition metal based cathodes has been reported to result in the formation of highly toxic fluoroethanol derivatives.

Hence, research on electrolytes for lithium-based batteries has focused on the development and investigation of new lithium salts, replacing LiPF₆ and offering an enhanced thermal, chemical, and electrochemical stability. One of the most promising salts is certainly lithium

bis(trifluoromethanesulfonyl)imide or briefly LiTFSI as it provides a high ionic conductivity and a significantly improved thermal and electrochemical stability relatively to LiPF₆. Additionally, the immediate formation of HF by hydrolysis of the salt is prevented. However, LiTFSI severely suffers aluminum current collector corrosion (aluminum dissolution) at potentials higher than 3.5 V, which has so far inhibited its application in commercial lithium-based batteries. This oxidative

decomposition of the aluminum current collector causes an increase of the internal resistance of the cell, resulting in a continuous capacity fading and thus a decrease of the specific energy. Moreover, a continuous decomposition of the aluminum might eventually result in a loss of the mechanical integrity of the current collector to the outer circuit.

Several approaches have been pursued in order to overcome the issue of aluminum current collector corrosion for LiTFSI-based lithium battery electrolytes, for example the utilization of ether-based solvents, such as e.g. THF or DME, which allow for a reduction of the aluminum corrosion and a shift of oxidative current collector decomposition towards higher potentials but can not completely suppress the corrosion phenomenon, or the use of nitrile based electrolyte solvents, comprising at least one cyano group, which appeared to have a beneficial effect on the suppression of aluminum corrosion, resulting in an overpotential of around 0.4 V for the aluminum dissolution. However, aluminum corrosion still takes place at potentials higher than 4.1 V. Also, the use of ionic liquids as electrolyte solvent has shown a significant suppression of the aluminum current collector corrosion. However, the commercial use of ionic liquids as electrolyte solvents is still hampered by its high cost and low ionic conductivity at ambient temperature. Furthermore, a coating of the aluminum current collector has been reported to suppress the aluminum dissolution upon cycling of lithium(-ion) cells. However, such a coating of the current collector leads to the requirement of additional processing steps and thus increasing cost, particularly if rather expensive materials are used for the coating.

US 2005/0031963 Al addresses the problem of the safety of lithium battery and the use of flame retardant electrolytes. It is disclosed that an electrolyte solvent comprising 20 to 60% by volume of a cyclic fluoroethylene carbonate and a linear ester solvent, such as for instance dimethyl or diethyl carbonate, results in an enhanced safety of the battery comprising the same.

Thus, there still is need for the suppression and prevention of aluminum current collector corrosion resulting from the use of LiTFSl as lithium salt in liquid organic electrolytes in order to improve the safety of lithium(-ion) batteries, particularly with respect to large scale applications, as for instance electric vehicles or stationary energy storage.

Therefore, the object underlying the present invention was to provide a means usable for the suppression or prevention of aluminum current collector corrosion resulting from the use of LiTFSl as lithium salt in liquid organic electrolytes.

The problem is solved by an electrolyte solution comprising an electrolyte salt and an electrolyte solvent, wherein the electrolyte solvent comprises an n-fluoro diethyl carbonate according to formula (1) as follows: C2H5.xFxCO3C2H5._yF_y (1) wherein 1 < x < 5 and 0 < y < 5 in an amount in the range of > 20 wt % to < 100 wt%, referring to a total amount of the electrolyte solvent of 100 wt%.

Surprisingly, it was found that the prevention of aluminum current collector corrosion while using LiTFSI-based electrolytes could be achieved by utilizing linear fluorinated carbonates as electrolyte solvent. Surprisingly, the utilization of linear fluorinated carbonates as electrolyte solvent for LiTFSI- based electrolytes revealed a similar electrochemical behavior as observed for the utilization of LiPF₆ as lithium salt. Advantageously, no aluminum corrosion could be observed on the aluminum foil current collector subsequently, even after 100 potentiodynamic cyclic sweeps. Such prevention of aluminum corrosion in combination with the use of LiTFSl is surprising and has never been reported before. Further, electrochemical studies using common lithium-ion battery cathode materials confirmed that electrolyte compositions comprising > 20 wt % of linear fluorinated carbonates can be readily used for lithium-ion cells and are sufficiently stable towards oxidation, also in presence of transition metal oxides and phosphates, delivering a highly similar specific capacity, cycling stability, and

electrochemical performance as observed for commercial lithium-ion cells using 1M LiPF₆ in EC:DMC (1 : 1).

Particularly, the electrolyte solution comprising > 20 wt % of an n-fluoro diethyl carbonate according to formula (1) C2H5.xFxCO3C2H5._yF_y wherein 1 < x < 5 and 0 < y < 5 is usable for electrolyte solutions comprising a sulfonimide electrolyte salt and using an aluminum current collector.

In an embodiment, the electrolyte salt is an alkali or alkaline earth metal sulfonimide or sulfonmethide salt. The sulfonimide salt preferably is selected from lithium, sodium, potassium, magnesium, or calcium metal sulfonimides. Preferably, the sulfonimide salt is a lithium salt selected from the group comprising lithium bis(trifluoromethanesulfonyl)imide LiN(S0₂CF₃)₂ (LiTFSl), lithium

bis(fluorosulfonyl)imide (LiFSI), LiN(FS0₂)2, lithium trifluoromethanesulfonate Li(CF₃)S0₃ (LiTf), lithium (trifluoromethylsulfonyl)(nonafluorobutanesulfonyl)imide LiN(S0₂CF₃)(S0₂C₄F₉), lithium (fluorosulfonyl)(nonafluorobutanesulfonyl)imide LiN(S0₂F)(S0₂C₄F₉), lithium (nonafluoro butan-2- one sulfonyl)(trifluoromethylsulfonyl)imide LiN(S0₂C₂F₄OC₂F₅)(S0₂CF₃), and lithium (nonafluoro butan-2-one sulfonyl)(fluorosulfonyl)imide LiN(S0₂C₂F₄OC₂F₅)(S0₂F). Preferred lithium sulfonimide salts are bis(trifluoromethanesulfonyl)imide LiN(S0₂CF₃)₂ (LiTFSl), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium trifluoromethanesulfonate Li(CF₃)S0₃ (LiTf). Sulfonimide-based lithium salts provide a high ionic conductivity and show enhanced thermal and electrochemical stability. Further, the immediate formation of HF by hydrolysis as may occur with LiPF₆ is prevented. Particularly the anodic aluminum dissolution, which usually is referred to as "corrosion" and from which the utilization of sulfonimide-based lithium salts suffered, can be prevented by using a linear fluorinated carbonate according to the invention. Hence, the use of sulfonimide-based lithium salts such as LiTFSl will result in significantly safer lithium-ion batteries, as the severe anodic dissolution of aluminum current collectors particularly at potentials above 3.5 V is prevented using the linear fluorinated carbonates of the invention. Particularly LiTFSI, LiFSI (lithium bis(fluorosulfonyl)imide) and Li Triflate, which are known to suffer anodic aluminum dissolution, can advantageously be utilized as lithium salt for (fluorinated) organic carbonate-based electrolytes in lithium-ion batteries by adding a linear fluorinated carbonate according to the invention. Further, it could be shown that also for lithium (nonafluoro butan-2-one

sulfonyl)(trifluoromethylsulfonyl) imide LiN(S0₂C₂F₄OC₂F₅)(S0₂CF₃) in a solvent mixture of 4- fluoro-l,3-dioxolan-2-one (FiEC), ethyl (1-fluoroethyl) carbonate (FiDEC) and 1-fluoroethyl (2,2,2- trifluoroethyl) carbonate (F₄DEC) aluminum dissolution could be prevented.

The sulfonmethide salt preferably is LiC(CF₃S0₂)3. Also sulfonmethide salts such as LiC(CF₃S0₂)3 are electrolyte salts that likely will induce aluminum corrosion.

The n-fluoro diethyl carbonate C₂H₅._xF_xCO₃C₂H₅._yF_y (1) wherein 1 < x < 5 and 0 < y < 5 comprises at least one fluoro substituent. The fluorinated linear carbonate at each ethyl group may comprise one, two, three or more fluoro substituents and/or at least a 1-fluoroethyl group, a 2-fluoroethyl group, a 2,2,2-trifluoroethyl group or a pentafluoroethyl group. Hence, in formula (1) x may be an integer of 1, 2, 3, 4 or 5, while y may be an integer of 0, 1, 2, 3, 4 or 5. In an embodiment, the n-fluoro diethyl carbonate is selected from the group comprising ethyl (1-fluoroethyl) carbonate, 1-fluoroethyl (2,2,2- trifluoroethyl) carbonate, bis(2-fluoroethyl) carbonate, bis(2,2,2-trifluoroethyl) carbonate, bis(l,2,2- trifluoroethyl) carbonate and mixtures thereof. Preferably the n-fluoro diethyl carbonate is selected from the group comprising ethyl (1-fluoroethyl) carbonate, 1-fluoroethyl (2,2,2-trifluoroethyl) carbonate and mixtures thereof. The corresponding chemical formulas of ethyl (1-fluoroethyl) carbonate (FiDEC) and 1-fluoroethyl (2,2,2-trifluoroethyl) carbonate (F₄DEC), respectively, are given below:

uoroethyl) carbonate 1-fluoroethyl (2,2,2-trifluoroethyl) carbonate

'F.DEC' 'F₄DEC

(2) (3).

It could be shown that particularly the utilization of ethyl (1 -fluoroethyl) carbonate (FiDEC) or 1 - fluoroethyl (2,2,2-trifluoroethyl) carbonate (F₄DEC) as electrolyte solvent for LiTFSI-based electrolytes resulted in the prevention of anodic aluminum dissolution and no corrosion was observed for the aluminum foil used as current collector, even after 100 potentiodynamic cyclic sweeps. Hence, an electrolyte solution may comprise an unary solution of ethyl (1 -fluoroethyl) carbonate or 1 - fluoroethyl (2,2,2-trifluoroethyl) carbonate as the electrolyte solvent.

Advantageously, a study on the conductivity of these primary electrolyte solvents comprising LiTFSI as lithium salt revealed that the ionic conductivity can be further improved by utilizing secondary or ternary solvent mixtures of linear and cyclic fluorinated carbonates. So, in an embodiment, the electrolyte solvent further comprises at least one cyclic fluorinated carbonate selected from the group comprising 4-fluoro-l ,3-dioxolan-2-one, 4,5-difluoro-l ,3-dioxolan-2-one particularly d,y-4,5-difluoro- l ,3-dioxolan-2-one or ira« -4,5-difluoro-l ,3-dioxolan-2-one, and mixtures thereof. 4-Fluoro-l ,3- dioxolan-2-one (FiEC) and ira« -4,5-difluoro-l ,3-dioxolan-2-one (F₂EC) are represented by the following formulas (4) and (5), respectively:

In embodiments, the electrolyte solution comprises a binary solvent mixture of an n-fluoro diethyl carbonate according to formula C₂H₅._xF_xCO₃C₂H₅._yF_y (1) wherein 1 < x < 5 and 0 < y < 5 and a cyclic fluorinated carbonate selected from the group comprising 4-fluoro-l,3-dioxolan-2-one (FiEC), cis-4,5- difluoro-l,3-dioxolan-2-one, and ira« -4,5-difluoro-l,3-dioxolan-2-one. Preferably, the electrolyte solution comprises a binary solvent mixture of ethyl (1-fluoroethyl) carbonate or 1 -fluoroethyl (2,2,2- trifluoroethyl) carbonate and a cyclic fluorinated carbonate selected from the group comprising 4- fluoro-l,3-dioxolan-2-one, cis-4,5-difluoro-l,3-dioxolan-2-one, and ira« -4,5-difluoro-l,3-dioxolan-2- one. It could be shown that the conductivity of binary solvent mixtures was higher than that of the linear carbonates alone.

The binary solvent mixture may comprise a mixture of ethyl (1-fluoroethyl) carbonate or 1-fluoroethyl (2,2,2-trifluoroethyl) carbonate and 4,5-difluoro-l,3-dioxolan-2-one, particularly ira¾y-4,5-difluoro- l,3-dioxolan-2-one (F₂EC), in a ratio of 1 : 1. Advantageously, for a binary solvent mixture of 1M LiTFSI in F₁DEC/F₁EC (1 : 1) no pitting corrosion was observed by a subsequent SEM analysis of the electrochemically studied aluminum foil. In a preferred embodiment of a binary solvent mixture, the electrolyte solution comprises a mixture of ethyl (1-fluoroethyl) carbonate (FiDEC) or 1-fluoroethyl (2,2,2-trifluoroethyl) carbonate (F₄DEC) and 4-fluoro-l,3-dioxolan-2-one (FiEC), in a ratio of 1 : 1. It could be shown that the conductivity of binary mixtures of FiDEC or F₄DEC with FiEC were even higher than that of mixtures with F₂EC.

If not indicated otherwise, the ratios of electrolyte solvents or compounds as given refer to a respective weight ratio. Weight percent, abbreviated wt% or wt.-% are synonyms that refer to the concentration of a compound as the weight of the compound divided by the weight of the composition and multiplied by 100. The weight- % (wt.-% or wt%) of the components are calculated based on the total weight amount of the composition, if not otherwise stated. The total amount of all solvents of the solution does not exceed 100 wt.-%.

In further embodiments, the electrolyte solution comprises a ternary solvent mixture of at least one n- fluoro diethyl carbonate according to formula (1): C₂H₅._xF_xCO₃C₂H₅._yF_y wherein 1 < x < 5 and 0 < y < 5, and at least one cyclic fluorinated carbonate selected from the group comprising 4-fluoro-l,3- dioxolan-2-one, cis-4,5-difluoro-l,3-dioxolan-2-one, ira« -4,5-difluoro-l,3-dioxolan-2-one and mixtures thereof. Preferably the linear fluorinated carbonate is selected from the group comprising ethyl (1-fluoroethyl) carbonate, 1 -fluoroethyl (2,2,2-trifluoroethyl) carbonate and mixtures thereof. The ternary solvent mixture may either comprise an n-fluoro diethyl carbonate and two different cyclic fluorinated carbonates, or may comprise two linear fluorinated carbonates and one cyclic fluorinated carbonate. Particularly, a mixture of ethyl (1-fluoroethyl) carbonate, 4-fluoro-l,3-dioxolan-2-one, and ira« -4,5-difluoro-l,3-dioxolan-2-one proved useful in regard of conductivity, the resulting current and inhibition of corrosion. In preferred embodiments, the electrolyte solution comprises a ternary solvent mixture of ethyl (1-fluoroethyl) carbonate or 1-fluoroethyl (2,2,2-trifluoroethyl) carbonate, 4- fluoro-l,3-dioxolan-2-one, and ira« -4,5-difluoro-l,3-dioxolan-2-one in a ratio of 1 : 1 : 1. The F₂EC can have a beneficial effect on the resulting current of a ternary mixture. Also in such ternary mixture the aluminum foil did not show any indication of severe pitting corrosion.

Further, it could advantageously be shown that the linear fluorinated carbonates according to the invention also are usable for the inhibition of aluminum current collector corrosion in non- fluorinated organic carbonates. This is of particular benefit as non- fluorinated organic carbonates are used as standard solvents in commercial lithium-ion batteries. Hence, another preferred embodiment refers to an electrolyte solution further comprising a non- fluorinated organic carbonate selected from the group comprising ethylene carbonate, ethyl methyl carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and mixtures thereof. It could be shown that the linear fluorinated carbonates, particularly ethyl (1-fluoroethyl) carbonate (FiDEC) or 1-fluoroethyl (2,2,2-trifluoroethyl) carbonate (F₄DEC), in a standard electrolyte solvent mixture of ethylene carbonate and dimethyl carbonate in a 1 : 1 ratio efficiently could prevent an aluminum current collector from anodic aluminum dissolution. Preferably, the electrolyte solution comprises a solvent mixture of ethylene carbonate, dimethyl carbonate and ethyl (1-fluoroethyl) carbonate or 1-fluoroethyl (2,2,2-trifluoroethyl) carbonate in a ratio of 1 : 1 : 1.

It was investigate whether for the beneficial effect of the linear fluorinated carbonates already the addition of only little amounts to common organic electrolytes might be sufficient. However, it appeared that its use as simple additive, for example in amount of about 4 wt.%, referring to the total weight of the electrolyte solvent, either in cyclic fluorinated carbonates or in standard non-fluorinated carbonates was not sufficient in order to suppress aluminum current collector corrosion. Further, also an amount of about 11 wt.-% of ethyl (1-fluoroethyl) carbonate (FiDEC) in cyclic fluorinated or non- fluorinated carbonate-based electrolytes was not sufficient to protect the aluminum current collector from corrosion. Only an amount of > 20 wt.-% of ethyl (1-fluoroethyl) carbonate (FiDEC) either in cyclic fluorinated carbonates or in standard non-fluorinated carbonates showed a clear improvement in the passivation of an aluminum current collector.

In embodiments, the electrolyte solution comprises the n-fluoro diethyl carbonate in a range of > 25 wt% to < 100 wt%, preferably in a range of > 25 wt% to < 75 wt%, more preferably in a range of > 30 wt% to < 50 wt%, referring to a total amount of the electrolyte solvent of 100 wt%.

Studies using 1 M LiTFSI and cyclic fluorinated solvent mixtures with FiDEC, for example

FiEC/F₂EC (1 :1), showed that to properly protect the aluminum current collector from anodic aluminum dissolution the electrolyte composition should comprise at least about 20 wt.-%, while improved protection could be achieved using an electrolyte solution comprising about 25 wt.-% and even more preferably about 30 wt.-% of FiDEC in case fluorinated cyclic carbonates were used as electrolyte solvents. About 33 wt.-% of FiDEC appeared to be more than sufficient for a proper passivation of the aluminum current collector. Hence, for binary or ternary solvent mixtures with fluorinated cyclic carbonates the electrolyte solution may comprise the n-fluoro diethyl carbonate in a range of > 20 wt% to < 50 wt%, preferably range of > 25 wt% to < 50 wt%, more preferably in a range of > 25 wt% to < 33 wt%, referring to a total amount of the electrolyte solvent of 100 wt%. Using a low amount of linear fluorinated carbonates will reduce the total cost for the solvent. Referring to non-fluorinated carbonate solvents, aluminum passivation within the first cycles by using 25 wt.% of FiDEC could be achieved. However, a FiDEC content of around 33 wt.-% appeared preferable with respect to the occurring current within the initial cycles, indicating a more effective aluminum passivation. To properly protect the aluminum current collector from anodic aluminum dissolution the electrolyte composition should comprise at least about 20 wt.%, while improved protection could be achieved using an electrolyte solution comprising about 25 wt.% and even more preferably about 30 wt.% of FiDEC in case non-fluorinated organic carbonates are used as electrolyte solvents. Hence, for binary or ternary solvent mixtures with non-fluorinated carbonates the electrolyte solution may comprise the n-fluoro diethyl carbonate in a range of > 20 wt% to < 50 wt%, preferably in a range of > 25 wt% to < 50 wt%, referring to a total amount of the electrolyte solvent of 100 wt%. Particularly referring to mixtures of ethylene carbonate and dimethyl carbonate, in embodiments, the electrolyte solution comprises a mixture of ethylene carbonate and dimethyl carbonate in a ratio of 1 : 1 and the n-fluoro diethyl carbonate in a range of > 20 wt% to < 50 wt%, preferably in the range of > 25 wt% to < 50 wt%, referring to a total amount of the electrolyte solvent of 100 wt%. Another aspect of the invention refers to an alkali or alkaline earth metal-based electrochemical energy storage device, particularly a lithium battery, a lithium-ion battery, a lithium-ion accumulator, a lithium polymer battery or a lithium-ion capacitor, comprising an electrolyte solution according to the invention. Preferably, the electrolyte solution according to the invention is usable for a lithium or lithium-ion battery. A lithium-ion battery for example comprises a first electrode of a cathodic material, a second electrode of an anodic material and an electrolyte.

The term "energy storage device" comprises primary batteries and rechargeable batteries or accumulators. However, colloquially accumulators are also denoted with the term "battery" which usually is used as a generic term. Hence, for simplicity if not denoted otherwise, in the present invention the term "battery" is used synonymous to also designate "accumulators".

Surprisingly, it was found that the electrolyte compositions are not only usable in combination with common lithium-ion battery cathode materials such as LiFeP0₄ (LFP) or LiNii_/3Mni_/3Coi/302 (NMC) but also provide a promising electrolyte solvent for high voltage cathode materials, as for instance LiNio.4Mn1.6O4. Advantageously, all electrolyte compositions were stable at least up to 5 V. Further, it could be shown that cells comprising the electrolyte compositions exhibited a high efficiency and high capacity. Particularly, in NMC half cells 1 M LiTFSl in FiDEC showed slightly higher efficiency than for commercial LP30 cells. Also, the electrochemical performance of NMC in 1 M LiTFSl in binary mixtures of fluorinated carbonates such as FiDEC /FiEC (1 : 1) as well as in ternary mixtures of fluorinated carbonates such as F iDEC/F iEC/F₂EC (1 : 1 :1) showed a high efficiency comparable with LP30 cells. Moreover, binary mixtures of linear and cyclic fluorinated carbonates have been confirmed to be also suitable for graphite-based anodes.

Consequently, the electrolyte compositions can be readily used for lithium-ion cells and are sufficiently stable towards oxidation, also in presence of transition metal oxides and phosphates, delivering at least a highly similar specific capacity, cycling stability, and electrochemical performance as observed for commercial available cells.

Particularly, the electrolyte solution comprising an electrolyte salt and an electrolyte solvent, wherein the electrolyte solvent comprises an n-fluoro diethyl carbonate according to formula (1) as follows: C2H5.xFxCO3C2H5._yF_y (1) wherein 1 < x < 5 and 0 < y < 5 in an amount in the range of > 20 wt% to < 100 wt%, referring to a total amount of the electrolyte solvent of 100 wt%, is usable for alkali or alkaline earth metal-based batteries containing an electrolyte solution comprising a sulfonimide electrolyte salt and using an aluminum current collector.

The electrolyte salt can be an alkali or alkaline earth metal sulfonimide or sulfonmethide salt. The sulfonimide salt preferably is selected from lithium, sodium, potassium, magnesium, or calcium metal sulfonimide salts. Preferably the sulfonimide salt is a lithium salt selected from the group comprising lithium bis(trifluoromethanesulfonyl)imide LiN(S0₂CF₃)₂ (LiTFSl), lithium bis(fluorosulfonyl)imide (LiFSI), LiN(FS0₂)₂, lithium trifluoromethanesulfonate Li(CF₃)S0₃ (LiTf), lithium

(trifluoromethylsulfonyl)(nonafluorobutanesulfonyl)imide LiN(S0₂CF₃)(S0₂C₄F₉), lithium

(fluorosulfonyl)(nonafluorobutanesulfonyl)imide LiN(S0₂F)(S0₂C₄F₉), lithium (nonafluoro butan-2- one sulfonyl)(trifluoromethylsulfonyl)imide LiN(S0₂C₂F₄OC₂F₅)(S0₂CF₃), and lithium (nonafluoro butan-2-one sulfonyl)(fluorosulfonyl)imide LiN(S0₂C₂F₄OC₂F₅)(S0₂F). Preferred lithium sulfonimide salts are bis(trifluoromethanesulfonyl)imide LiN(S0₂CF₃)₂ (LiTFSl), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium trifluoromethanesulfonate Li(CF₃)S0₃ (LiTf). The sulfonmethide salt preferably is LiC(CF₃S0₂)₃.

The fluorinated linear carbonate at each ethyl group may comprise one, two, three or more fluoro substituents and/or at least a 1 -fluoroethyl group, a 2-fluoroethyl group, a 2,2,2-trifluoroethyl group or a pentafluoroethyl group. Hence, in formula (1) x may be an integer of 1, 2, 3, 4 or 5, while y may be an integer of 0, 1, 2, 3, 4 or 5. The n-fluoro diethyl carbonate can be selected from the group comprising ethyl (1-fluoroethyl) carbonate, 1 -fluoroethyl (2,2,2-trifluoroethyl) carbonate, bis(2- fluoroethyl) carbonate, bis(2,2,2-trifluoroethyl) carbonate, bis(l,2,2-trifluoroethyl) carbonate and mixtures thereof. Preferably, the n-fluoro diethyl carbonate is selected from the group comprising ethyl (1-fluoroethyl) carbonate, 1-fluoroethyl (2,2,2-trifluoroethyl) carbonate and mixtures thereof.

The electrolyte solvent further can comprises at least one cyclic fluorinated carbonate selected from the group comprising 4-fluoro-l,3-dioxolan-2-one, 4,5-difluoro-l,3-dioxolan-2-one particularly cis- 4,5-difluoro-l,3-dioxolan-2-one or ira« -4,5-difluoro-l ,3-dioxolan-2-one, and mixtures thereof.

The electrolyte solution can comprise a binary solvent mixture of an n-fluoro diethyl carbonate according to formula C2H5.xFxCO3C2H5._yF_y (1) wherein 1 < x < 5 and 0 < y < 5 and a cyclic fluorinated carbonate selected from the group comprising 4-fluoro-l,3-dioxolan-2-one (FiEC), d,y-4,5-difluoro- l,3-dioxolan-2-one, and ira« -4,5-difluoro-l,3-dioxolan-2-one. Preferably, the electrolyte solution may comprises a binary solvent mixture of ethyl (1-fluoroethyl) carbonate or 1-fluoroethyl (2,2,2- trifluoroethyl) carbonate and a cyclic fluorinated carbonate selected from the group comprising 4- fluoro-l,3-dioxolan-2-one, cis-4,5-difluoro-l,3-dioxolan-2-one, and ira« -4,5-difluoro-l,3-dioxolan-2- one. It could be shown that the conductivity of binary solvent mixtures was higher than that of the linear carbonates alone. The binary solvent mixture may comprise a mixture of ethyl (1-fluoroethyl) carbonate or 1-fluoroethyl (2,2,2-trifluoroethyl) carbonate and 4,5-difluoro-l,3-dioxolan-2-one, particularly ira« -4,5-difluoro-l,3-dioxolan-2-one (F₂EC), in a ratio of 1 :1. In a preferred embodiment of a binary solvent mixture, the electrolyte solution comprises a mixture of ethyl (1-fluoroethyl) carbonate (FiDEC) or 1-fluoroethyl (2,2,2-trifluoroethyl) carbonate (F₄DEC) and 4-fluoro-l,3- dioxolan-2-one (FiEC), in a ratio of 1 : 1.

Further, the electrolyte solution can comprises a ternary solvent mixture of at least one n-fluoro diethyl carbonate according to formula (1): C2H5.xFxCO3C2H5._yF_y wherein 1 < x < 5 and 0 < y < 5, and at least one cyclic fluorinated carbonate selected from the group comprising 4-fluoro-l,3-dioxolan-2-one, cis- 4,5-difluoro-l,3-dioxolan-2-one, ira¾y-4,5-difluoro-l,3-dioxolan-2-one and mixtures thereof. Preferably the linear fluorinated carbonate is selected from the group comprising ethyl (1-fluoroethyl) carbonate, 1 -fluoroethyl (2,2,2-trifluoroethyl) carbonate and mixtures thereof. A ternary solvent mixture may either comprise an n-fluoro diethyl carbonate and two different cyclic fluorinated carbonates, or may comprise two linear fluorinated carbonates and one cyclic fluorinated carbonate. Particularly, a mixture of ethyl (1-fluoroethyl) carbonate, 4-fluoro-l,3-dioxolan-2-one, and trans-4,5- difluoro-l,3-dioxolan-2-one proved useful in regard of conductivity, the resulting current and inhibition of corrosion. The electrolyte solution may comprise a ternary solvent mixture of ethyl (1- fluoroethyl) carbonate or 1-fluoroethyl (2,2,2-trifluoroethyl) carbonate, 4-fluoro-l,3-dioxolan-2-one, and ira« -4,5-difluoro-l,3-dioxolan-2-one in a ratio of 1 :1 : 1.

The electrolyte solution may comprise the n-fluoro diethyl carbonate in a range of > 25 wt % to < 100 wt%, preferably in a range of > 25 wt% to < 75 wt%, more preferably in a range of > 30 wt% to < 50 wt %, referring to a total amount of the electrolyte solvent of 100 wt %. For binary or ternary solvent mixtures with fluorinated cyclic carbonates the electrolyte solution may comprise the n-fluoro diethyl carbonate in a range of > 20 wt% to < 50 wt%, preferably range of > 25 wt% to < 50 wt%, more preferably in a range of > 25 wt% to < 33 wt%, referring to a total amount of the electrolyte solvent of 100 wt %. The electrolyte solution further may comprise a non- fluorinated organic carbonate selected from the group comprising ethylene carbonate, ethyl methyl carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and mixtures thereof. For binary or ternary solvent mixtures with non- fluorinated carbonates the electrolyte solution may comprise the n-fluoro diethyl carbonate in a range of > 20 wt% to < 50 wt%, preferably in a range of > 25 wt% to < 50 wt%, more preferably in a range of > 25 wt% to < 33 wt%, referring to a total amount of the electrolyte solvent of 100 wt%.

Particularly referring to mixtures of ethylene carbonate and dimethyl carbonate, in embodiments, the electrolyte solution comprises a mixture of ethylene carbonate and dimethyl carbonate in a ratio of 1 : 1 and the n-fluoro diethyl carbonate in a range of > 20 wt% to < 50 wt%, preferably in the range of > 25 wt% to < 50 wt%, referring to a total amount of the electrolyte solvent of 100 wt%. Another aspect of the invention refers to the use of an n-fluoro diethyl carbonate according to formula (1) as follows: C2H5.xFxCO3C2H5._yF_y (1) wherein 1 < x < 5 and 0 < y < 5, in an amount in the range of > 20 wt % to < 100 wt%, referring to a total amount of an electrolyte solvent of 100 wt%, for the prevention of aluminum current collector corrosion in an alkali or alkaline earth metal-based electrochemical energy storage device, particularly a lithium-ion battery or lithium polymer battery containing an electrolyte solution comprising an alkali or alkaline earth metal sulfonimide or sulfonmethide salt.

The alkali or alkaline earth metal sulfonimide salt in preferred embodiments is selected from lithium, sodium, potassium, magnesium, or calcium metal sulfonimide salts. Preferably the sulfonimide salt is a lithium salt selected from the group comprising lithium bis(trifluoromethanesulfonyl)imide

LiN(S0₂CF₃)₂ (LiTFSl), lithium bis(fluorosulfonyl)imide (LiFSI), LiN(FS0₂)₂, lithium

trifluoromethanesulfonate Li(CF₃)S0₃ (LiTf), lithium

The fluorinated linear carbonate at each ethyl group may comprise one, two, three or more fluoro substituents and/or at least a 1 -fluoroethyl group, a 2-fluoroethyl group, a 2,2,2-trifluoroethyl group or a pentafluoroethyl group. Hence, in formula (1) x may be an integer of 1, 2, 3, 4 or 5, while y may be an integer of 0, 1, 2, 3, 4 or 5. The n-fluoro diethyl carbonate can be selected from the group comprising ethyl (1 -fluoroethyl) carbonate, 1 -fluoroethyl (2,2,2-trifluoroethyl) carbonate, bis(2- fluoroethyl) carbonate, bis(2,2,2-trifluoroethyl) carbonate, bis(l,2,2-trifluoroethyl) carbonate and mixtures thereof. Preferably the n-fluoro diethyl carbonate is selected from the group comprising ethyl (1 -fluoroethyl) carbonate, 1 -fluoroethyl (2,2,2-trifluoroethyl) carbonate and mixtures thereof. In an embodiment of the use, the electrolyte solvent further comprises at least one cyclic fluorinated carbonate selected from the group comprising 4-fluoro-l,3-dioxolan-2-one, 4,5-difluoro-l,3-dioxolan- 2-one particularly d -4,5-difluoro-l,3-dioxolan-2-one or ira« -4,5-difluoro-l,3-dioxolan-2-one and mixtures thereof, and/or a non- fluorinated organic carbonate selected from the group comprising ethylene carbonate, ethyl methyl carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and mixtures thereof.

The electrolyte solvent further can comprises at least one cyclic fluorinated carbonate selected from the group comprising 4-fluoro-l,3-dioxolan-2-one, cis-4,5-difluoro-l,3-dioxolan-2-one, trans-4,5- difluoro-l,3-dioxolan-2-one, and mixtures thereof.

The electrolyte solution can comprise a binary solvent mixture of an n-fluoro diethyl carbonate according to formula C2H5.xFxCO3C2H5._yF_y (1) wherein 1 < x < 5 and 0 < y < 5 and a cyclic fluorinated carbonate selected from the group comprising 4-fluoro-l,3-dioxolan-2-one (FiEC), d,y-4,5-difluoro- l,3-dioxolan-2-one, and ira« -4,5-difluoro-l,3-dioxolan-2-one. Preferably, the electrolyte solution may comprises a binary solvent mixture of ethyl (1-fluoroethyl) carbonate or 1 -fluoroethyl (2,2,2- trifluoroethyl) carbonate and a cyclic fluorinated carbonate selected from the group comprising 4- fluoro-l,3-dioxolan-2-one, cis-4,5-difluoro-l,3-dioxolan-2-one, and ira« -4,5-difluoro-l,3-dioxolan-2- one. It could be shown that the conductivity of binary solvent mixtures was higher than that of the linear carbonates alone. The binary solvent mixture may comprise a mixture of ethyl (1-fluoroethyl) carbonate or 1-fluoroethyl (2,2,2-trifluoroethyl) carbonate and 4,5-difluoro-l,3-dioxolan-2-one, particularly ira« -4,5-difluoro-l,3-dioxolan-2-one (F₂EC), in a ratio of 1 :1. In a preferred embodiment of a binary solvent mixture, the electrolyte solution comprises a mixture of ethyl (1-fluoroethyl) carbonate (FiDEC) or 1-fluoroethyl (2,2,2-trifluoroethyl) carbonate (F₄DEC) and 4-fluoro-l,3- dioxolan-2-one (FiEC), in a ratio of 1 : 1.

Further, the electrolyte solution can comprises a ternary solvent mixture of at least one n-fluoro diethyl carbonate according to formula (1): C2H5.xFxCO3C2H5._yF_y wherein 1 < x < 5 and 0 < y < 5, and at least one cyclic fluorinated carbonate selected from the group comprising 4-fluoro-l,3-dioxolan-2-one, cis- 4,5-difluoro-l,3-dioxolan-2-one, ira¾y-4,5-difluoro-l,3-dioxolan-2-one and mixtures thereof. Preferably the linear fluorinated carbonate is selected from the group comprising ethyl (1-fluoroethyl) carbonate, 1 -fluoroethyl (2,2,2-trifluoroethyl) carbonate and mixtures thereof. An ternary solvent mixture may either comprise an n-fluoro diethyl carbonate and two different cyclic fluorinated carbonates, or may comprise two linear fluorinated carbonates and one cyclic fluorinated carbonate. Particularly, a mixture of ethyl (1-fluoroethyl) carbonate, 4-fluoro-l,3-dioxolan-2-one, and trans-4,5- difluoro-l,3-dioxolan-2-one proved useful in regard of conductivity, the resulting current and inhibition of corrosion. The electrolyte solution may comprise a ternary solvent mixture of ethyl (1- fluoroethyl) carbonate or 1-fluoroethyl (2,2,2-trifluoroethyl) carbonate, 4-fluoro-l,3-dioxolan-2-one, and ira« -4,5-difluoro-l,3-dioxolan-2-one in a ratio of 1 :1 : 1.

Particularly referring to mixtures of ethylene carbonate and dimethyl carbonate, in embodiments, the electrolyte solution comprises a mixture of ethylene carbonate and dimethyl carbonate in a ratio of 1 : 1 and the n-fluoro diethyl carbonate in a range of > 20 wt% to < 50 wt%, preferably in the range of > 25 wt% to < 50 wt%, referring to a total amount of the electrolyte solvent of 100 wt%. Unless otherwise defined, the technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

The examples which follow serve to illustrate the invention in more detail but do not constitute a limitation thereof.

In the figures show:

Figure 1 Cyclovoltammograms of fluorinated linear carbonates as only solvents for a 1 M solution of LiTFSI. Figure la) shows the cyclovoltammogram for FiDEC, and Figure lb) for F₄DEC for 100 cycles using aluminum as working electrode.

Figure 2 the cyclovoltammogram of 1 M LiTFSI in a binary mixture of FiDEC and FiEC in a ratio of 1 : 1 for 100 cycles using aluminum as working electrode.

Figure 3 the cyclovoltammogram of 1 M LiTFSI in a ternary mixture of FiDEC, FiEC and F₂EC in a ratio of 1 : 1 : 1 for 100 cycles using aluminum as working electrode.

Figure 4 Ionic conductivities of 1 M LiTFSI in linear fluorinated carbonates and binary solvent mixtures of linear and cyclic fluorinated carbonates.

Figure 5 the electrochemical performance of a carbon coated LiFeP0₄ (LFP) electrode in a solution of 1 M LiTFSI in FiDEC. Figure 5a) shows the galvanostatic cycling of the Li/LiFeP0₄ half cell with Li as reference electrode. Cut-off potentials were at 2.8-4.0 V. Plotted is the specific discharge capacity (left ordinate) against the efficiency (right ordinate) against the cycle number. Figure 5b) shows the potential profile comparison of Li/LiFeP0₄ half cell with Li as reference electrode with 1 M LiPF₆ in EC/DMC (1 : 1) (LP30) as electrolyte; Cut-offs: 2.8-4.0 V; 1st cycle (C/10). A C rate of 1C corresponds to an applied specific current of 170 mA g^"1, allowing a full charge or discharge of the electrode within one hour.

Figure 6 the electrochemical performance of a LiNii_/3Mni_/3Coi/302 (NMC) half cell in a solution of

1 M LiTFSI in FiDEC. Figure 6a) shows the galvanostatic cycling comparison of the Li/LiNii_/3Mni_/3Coi/302 half cell with Li as reference electrode and 1 M LiPF₆ in EC/DMC (1 : 1) (LP30) as electrolyte; Cut-offs: 3.0-4.3 V. Figure 6b) shows the potential profile comparison of Li/LiNii_/3Mni_/3Coi/302 half cell with Li as reference electrode with 1 M LiTFSI in FiDEC and 1 M LiPF₆ in EC/DMC (1 :1) (LP30) as electrolyte; Cut-offs: 2.8- 4.0 V; 1st cycle (C/10). A C rate of 1C corresponds to an applied specific current of 160 mA g^"1, allowing a full charge or discharge of the electrode within one hour. Figure 7 a galvanostatic cycling comparison of a Li/LiNii_/3Mni_/3Coi/302 (NMC) half cell with Li as reference electrode with 1 M LiTFSI in a binary mixture of FiEC/FiDEC (1 :1) and 1 M

LiPF₆ in EC/DMC (1 : 1) (LP30) as electrolyte; Cut-offs: 3.0-4.3 V.

Figure 8 a galvanostatic cycling comparison of a Li/LiNii_/3Mni_/3Coi/302 (NMC) half cell with Li as reference electrode with 1 M LiTFSI in a ternary mixture of F iEC/F₂EC/F iDEC (1 : 1 : 1) and 1 M LiPF₆ in EC/DMC (1 : 1) (LP30) as electrolyte; Cut-offs: 3.0-4.3 V. Figure 9 the electrochemical performance of a LiNio.₄Mn₁.₆O₄ (LNMO) half cell in a solution of 1

M LiTFSI in F₄DEC. Figure 9a) shows the galvanostatic cycling of Li/LiNi₀.₄Mni.₆O₄ half cell with Li as reference electrode; Cut-offs: 3.5-4.95 V. Figure 9b) shows selected potential profiles of the Li/LiNi₀.₄Mn_{1 6}O₄ half cell; Cut-offs: 3.5-4.95 V; 2nd (C/10) and 3rd (C/10) cycle. A C rate of 1C corresponds to an applied specific current of around 147 mA g^"1, allowing a full charge or discharge of the electrode within one hour.

Figure 10 the electrochemical performance of a graphite (SLP30) half cell in a binary mixture of 1

M LiTFSI in FiDEC and FiEC in a ratio of 1 : 1. Figure 10a) shows the galvanostatic cycling comparison of graphite (SLP30) half cell with Li as reference electrode with 1 M LiTFSI in F₁EC/F₁DEC (1 : 1) and of 1 M LiPF₆ in EC/DMC (1 : 1) (LP30) as electrolyte; Cut-offs: 0.02-1.5 V. Figure 10b) shows selected potential profiles of the graphite

(SLP30) half cell at different C rates (C/10, C/5, C/2) in comparison for the two electrolytes. A C rate of 1C corresponds to an applied specific current of 372 mA g^"1, allowing a full charge or discharge of the electrode within one hour.

Figure 11 Cyclovoltammograms for different concentrations of linear fluorinated carbonates with cyclic fluorinated carbonates as solvent for a 1 M solution of LiTFSI. Figure 11a) shows the cyclovoltammogram for a mixture of 20 wt% of 20 wt% of FiDEC in FiEC and F₂EC in a weight ratio of 1 : 1 , and Figure 1 lb) for 33 wt % of FiDEC in FiEC and F₂EC in a weight ratio of 1 : 1 , each for 100 cycles using aluminum as working electrode. Figure 12 Cyclovoltammograms for different concentrations of linear fluorinated carbonates with non- fluorinated carbonates as solvent for a 1 M solution of LiTFSI. Figure 12a) shows the cyclovoltammogram for a mixture of 25 wt% of FiDEC and Figure 12b) for 33 wt% of FiDEC in EC/DMC in a ratio of 1:1, each for 100 cycles using aluminum as working electrode.

Figure 13 the determination of the electrochemical stability window (ESW) of fluorinated and non- fluorinated organic carbonate solvents with 1 M LiTFSI as conductive salt; the working electrode was a Platinum- wire with lithium metal foils as counter and reference electrodes; oxidative current limit: 0.01 mA.

Figure 14 the cyclovoltammogram of 1 M lithium (nonafluoro butan-2-one sulfonyl) (trifluoro- methylsulfonyl)imide (salt A) in a ternary mixture of F₁EC/F₁DEC/F₃DEC in a mol ratio of 3:0.5:0.5 for 100 cycles using aluminum as working electrode; scan rate: 2 mV sec^"1.

Figure 15 XPS studies on potentiodynamically polarized aluminum foils subjected to five cyclic sweeps. Figures a) and c) show the Al 2p depth profile (10 x 1 min Ar⁺ etching) using EC/DEC (1:1) and FiDEC as solvent, respectively; figures b) and d) the corresponding evolution of the atomic concentration of F, O, C, and AL upon Ar⁺ etching. Figure 16 Figure 16 a) shows the electrochemical performance of an NMC/graphite lithium-ion pouch cell subjected to six galvanostatic formation cycles at C/10 and then cycled at C/5 applying cut-off voltages of 2.8 and 4.2 V. Figure 16 b) shows the corresponding voltage profiles. The specific capacity values refer to the active mass loading of NMC only. Electrode preparation

NMC-based electrodes were prepared using commercial NMC powder (Toda), which was mixed with PVdF binder (5130, Solvay) and LITX^™ 200 conductive carbon (Cabot Corporation) in a weight ratio of 94:3:3 using NMP as solvent. The obtained electrode paste was coated on battery grade aluminum foil using a laboratory doctor blade technique. Subsequently, electrodes were punched having a diameter of 12 mm. After drying at 120 °C under vacuum over night, such electrodes had an average mass loading of around 11 mg cm^"2.

LFP-based electrodes were prepared by mixing commercial LFP powder (Sudchemie) with PVdF 5130 and LITX^™ 200 conductive carbon in a weight ratio of 91 :6:4 using NMP as solvent. After drying accordingly to the NMC-based electrodes, such electrode disks (0 = 12 mm) had an average mass loading of around 9.5 mg cm^"2. LNMO-based electrodes were prepared according to the previous description, having an overall composition of 85:5: 10 (LNMO:PVdF:Super C65 conductive carbon, TIMCAL) and an average mass loading of around 10 mg cm^"2.

Graphite-based electrodes were prepared using commercial graphite powder (SLP30, TIMCAL), PVdF (Polyvinylidene fluoride) binder (9200, Solvay), and Super C65 conductive carbon, having an overall weight ratio of 91 :6:3. Copper foil (battery grade, EVONIK) served as current collector. Punched and dried electrodes had an average mass loading of 4-5 mg cm^"2.

Electrochemical characterization

Electrochemical studies were performed utilizing three-electrode, Swagelok™-type cells. Lithium metal foil (Rockwood Lithium, battery grade) served as reference and counter electrode, if not otherwise specified. The cells were assembled in an MBraun glove box with oxygen and water contents below 0.5 ppm. A sheet of Whatman GF/D glass fiber drenched with the respective electrolyte solutions served as separator. All electrochemical studies were performed at 20°C ± 2°C. Cyclic voltammetry experiments were performed by means of a VMP3 potentiostat (BioLogic).

Galvanostatic cycling of NMC, LFP, and Graphite-based electrodes was carried out using a Maccor Battery Tester 4300. Since lithium foil was used as counter and reference electrode, all the potentials as given refer to the Li⁺/Li reference. For the investigation of the anodic aluminum dissolution ("corrosion") in various electrolyte compositions blank aluminum foil (battery grade, 99.99% purity, EVONIK) were utilized as working electrode.

SEM analysis

Ex situ scanning electron microscope (SEM) analysis of aluminum current collectors (EVONIK, 20 μηι, purity > 99.9%) was carried out on a ZEISS Auriga^® microscope.

Solvents and lithium salts Ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC), all battery grade, were purchased at UBE Corporation and Ferro Corporation. Readily prepared EC:DMC (1 : 1) 1M LiPF₆ electrolyte (LP30) was purchased at Merck KGaA.

4-fluoro-l,3-dioxolan-2-one (FiEC), ira« -4,5-difluoro-l,3-dioxolan-2-one (F₂EC), ethyl (1- fluoroethyl) carbonate (FiDEC) and 1 -fluoroethyl (2,2,2-trifluoroethyl) carbonate (F₄DEC), all battery grade, were provided by Solvay GmbH.

LiPF₆, lithium bis(trifluoromethanesulfonyl)imide LiN(S0₂CF₃)₂ (LiTFSl), and lithium (nonafluoro butan-2-one sulfonyl)(trifluoromethylsulfonyl)imide, all battery grade, were purchased at 3M as well as Acros Organics and provided by Eras Labo, respectively. The latter provided by Eras Labo was further purified by recrystallization in distilled anisole inside a glove box, then dried at 140 °C under a 2 mm Hg pressure.

Preparation of electrolyte solutions

Electrolyte solutions were prepared by providing or mixing the solvents in their respective weight ratios and dissolving the lithium salt in an appropriate amount to yield a 1M solution.

Example 1

Determination of aluminium corrosion in ethyl (1 -fluoroethyl) carbonate (FiDEC) An electrolyte solution of 1M LiTFSl in FiDEC was prepared and cyclic voltammetry was performed using an aluminum foil as working electrode for 100 cyclic potentiodynamic sweeps in a potential rang ranging from 3.3 V (cathodic limit) to 5.1 V (anodic limit). A scan rate of 5 mV sec^"1 was applied. Figure la) shows the cyclovoltammogram for 1M LiTFSl in FiDEC. As can be taken from the figure la), after the first cycle the observed current density was significantly decreasing and almost no current could be observed subsequently upon the continuous potentiodynamic sweeps, indicating the initial formation of a protective surface film on the aluminum surface and a thus prevented anodic aluminum dissolution upon further polarization of the aluminum foil. The prevention of aluminum dissolution was further confirmed by a subsequent SEM analysis of the aluminum electrode, which did not show any indication of anodic aluminum dissolution.

Example 2

Determination of aluminium corrosion in 1 -fluoroethyl (2,2,2-trifluoroethyl) carbonate (F₄DEC)

An electrolyte solution of 1 M LiTFSl in F₄DEC was prepared and cyclic voltammetry was performed using aluminum as working electrode for 100 cyclic potentiodynamic sweeps in a potential rang ranging from 3.3 V to 5.1 V. A scan rate of 5 mV sec^"1 was applied.

Figure lb) shows the cyclovoltammogram for 1M LiTFSl in F₄DEC. As can be taken from the figure lb), also for F₄DEC as the solvent only for the initial anodic potentiodynamic sweep an evolving current could be observed and almost no current could be detected subsequently upon the continuous potentiodynamic sweeps, indicating the initial formation of a protective surface film on the aluminum surface and a thus prevented anodic aluminum dissolution upon further polarization of the aluminum foil. The prevention of aluminum dissolution further was confirmed by a subsequent SEM analysis of the aluminum electrode, which showed no aluminum dissolution.

This shows that the utilization of linear fluorinated carbonates as electrolyte solvent for LiTFSI-based electrolytes revealed a similar electrochemical behavior as for the utilization of LiPF₆ as lithium salt. In fact, no aluminum corrosion could be observed on the aluminum foil subsequently, even after 100 potentiodynamic cyclic sweeps.

Example 3

Determination of aluminium corrosion in a binary mixture of FiDEC and FiEC

An electrolyte solution of 1M LiTFSl in a binary mixture of 50 wt.-% FiDEC and 50 wt.-% FiEC was prepared and cyclic voltammetry was performed using aluminum as working electrode for 100 cycles in a potential rang ranging from 3.3 V to 5.1 V. A scan rate of 5 mV sec^"1 was applied. Figure 2 shows the cyclovoltammogram of 1 M LiTFSI in the binary mixture of FiDEC and FiEC (1 : 1). As can be taken from the figure 2, for the binary solvent mixture of FiDEC and the cyclic fluorinated FiEC, the observed current density increased starting from around 3.8 V. However, no evolving current could be detected for the subsequent potentiodynamic sweeps, indicating the presence of a protective layer on the aluminum surface formed upon the initial anodic sweep, preventing continuous anodic aluminum dissolution ("corrosion"). Accordingly, no pitting corrosion could be observed for a subsequent ex situ SEM analysis of the electrochemically studied aluminum foil. Apparently, 50 wt.% of the linear fluorinated carbonate are sufficient to prevent anodic aluminum dissolution by the formation of a protective surface film, enabling an increased flexibility of tailoring suitable electrolyte formulations, possessing enhanced electrochemical characteristics in terms of e.g. ionic conductivity.

Example 4

Determination of aluminium corrosion in a ternary mixture of FiDEC, FiEC and F₂EC

An electrolyte solution of 1 M LiTFSI in a ternary mixture of FiDEC, FiEC, and F₂EC in a weight ratio of 1 : 1 : 1 was prepared and cyclic voltammetry was performed using aluminum as working electrode for 100 cycles in a potential rang ranging from 3.3 V to 5.1 V. A scan rate of 5 mV sec^"1 was applied.

Figure 3 shows the cyclovoltammogram of 1 M LiTFSI in a ternary mixture of FiDEC, FiEC and F₂EC (1 : 1 : 1). For the ternary solvent mixture the characteristic shape of the first cyclo voltammogram could be observed as was for the pure linear fluorinated carbonates (examples 1 and 2) as well as for the binary solvent mixture (example 3), indicating the initial passivation of the aluminum surface, leading to a prevention of subsequent anodic aluminum dissolution. Nevertheless, the initially detected evolving current is significantly lower than for the former electrolyte solutions, indicating a beneficial effect of F₂EC. Moreover, subsequent SEM analysis confirmed that the aluminum foil did not show any indication of severe pitting corrosion. Example 5

Determination of ionic conductivity of LiTFSI in fluorinated carbonates and mixtures therof A study on the conductivity of primary and binary electrolyte solvents comprising LiTFSI as lithium salt was performed using electrolyte solutions of 1M LiTFSI in cyclic FiEC, F₂EC, and FiEC and F₂EC (1 : 1), linear FiDEC and F₄DEC, and binary mixtures of FiDEC and FiEC (1 : 1), F₄DEC and FiEC (1 : 1), FiDEC and F₂EC (1 : 1), F₄DEC and F₂EC (1 : 1), as well as for 1 M LiTFSI in DMC and EC and DMC (1 : 1) as references.

For this purpose, electrical impedance measurements were carried out on a Hewlett-Packard 4192A impedance analyzer, using a two platinum electrode configuration in Radiometer^® cells.

Measurements were conducted in a temperature range of -30 °C to +60 °C in a frequency range of 13 MHz to 5 Hz applying a 0.03 V alternated voltage with an equilibration time of 1.5 hours.

Measurements were averaged on nine points per decade. All electrolyte samples were prepared in and transferred to Jacomex^® glove boxes with a water content of less than 10 ppm.

Figure 4 illustrates the ionic conductivities of 1M LiTFSI in the different solutions of linear fluorinated carbonates and binary solvent mixtures of linear and cyclic fluorinated carbonates. As can be taken from the figure 4, the ionic conductivity was further improved by utilizing secondary solvent mixtures of linear and cyclic fluorinated carbonates, illustrating the suitability of such electrolyte compositions for practical applications.

Example 6

Determination of the electrochemical performance of a carbon coated LiFeP0₄ electrode in an electrolyte of 1M LiTFSI in FiDEC

An electrolyte solution of 1M LiTFSI in FiDEC was prepared and electrochemical characterization was performed as described above. Figure 5 illustrates the electrochemical performance of a carbon coated LiFeP0₄ (LFP) electrode in a solution of 1M LiTFSI in FiDEC. Figure 5a) shows the galvanostatic cycling of the Li/LiFeP0₄ half cell with Li as reference electrode. Cut-off potentials were at 2.8-4.0 V. As can be taken from figure 5a), the LFP electrode showed a high efficiency. Figure 5b) shows the potential profile comparison of Li/LiFeP0₄ half cell with Li as reference electrode with 1 M LiPF₆ in EC/DMC (1 : 1) (LP30) as electrolyte; Cut-offs: 2.8-4.0 V; 1st cycle (C/10). As can be taken from figure 5b), the LFP electrode showed a higher capacity in the FiDEC electrolyte compared to the standard electrolyte with 1 M LiPF₆ in EC/DMC (1 : 1) (LP30), while the characteristic shape of the LFP potential profile was well preserved.

Example 7

Determination of the electrochemical performance of a LiNii_/3Mni_/3Coi/302 (NMC) half cell

An electrolyte solution of 1M LiTFSI in FiDEC was prepared and electrochemical characterization was performed as described above. Figure 6 summarizes the electrochemical performance of a LiNi_1/3Mn_1/3Coi/302 (NMC) half cell in a solution of 1 M LiTFSI in FiDEC. Figure 6a) shows the galvanostatic cycling comparison of the Li/LiNii_/3Mni_/3Coi/302 half cell with Li as reference electrode and 1 M LiPF₆ in EC/DMC (1 : 1) (LP30) as electrolyte; Cut-offs: 3.0-4.3 V. It can be taken from figure 6a) that the efficiency for FiDEC was slightly higher than for the standard LP30 electrolyte. Figure 6b) shows the potential profile comparison of Li/LiNii_/3Mni_/3Coi/302 half cell with Li as reference electrode with 1 M LiTFSI in FiDEC and 1 M LiPF₆ in EC/DMC (1 : 1) (LP30) as electrolyte; Cut-offs: 2.8-4.0 V; 1st cycle (C/10). As can be taken from figure 6b), there was no difference of the capacity in the 1^st cycle detected between the NMC electrode in the FiDEC electrolyte compared to the NMC electrode in the standard electrolyte of 1 M LiPF₆ in EC/DMC (1 : 1) (LP30), confirming that such electrolyte compositions are suitable for commercially used lithium-ion cathode materials. Example 8

Determination of the electrochemical performance of LiNii_/3Mni_/3Coi/302 (NMC) in a binary electrolyte solution of 1 M LiTFSI in FiDEC/FiEC

An electrolyte solution of 1M LiTFSI in FiDEC/FiEC in a weight ratio of 1 : 1 was prepared and electrochemical characterization was performed as described above. Figure 7 shows the electrochemical performance of the LiNii_/3Mni_/3Coi/302 (NMC) half cell in a binary mixture of 1 M LiTFSI in FiDEC and FiEC in a ratio of 1 : 1. The figure shows the galvanostatic cycling comparison of Li/LiNii_/3Mni_/3Coi/302 half cell with Li as reference electrode with 1 M LiTFSI in FiEC/FiDEC (1 : 1) and 1 M LiPF₆ in EC/DMC (1 : 1) (LP30) as electrolyte with Cut-offs at 3.0-4.3 V.

Figure 7 illustrates that the specific capacity as well as the cycling stability and high rate performance of the NMC electrode in the binary mixture of linear and cyclic fluorinated carbonate FiDEC/FiEC electrolyte is comparable to the NMC electrode in the standard electrolyte of 1 M LiPF₆ in EC/DMC (1 : 1) (LP30). Moreover, a coulombic efficiency of almost 100% is obtained, highlighting once more the suitability of such electrolyte compositions for practical lithium-ion applications.

Example 9

Determination of the electrochemical performance of LiNii_/3Mni_/3Coi/302 (NMC) in a ternary electrolyte solution of 1 M LiTFSI in F iDEC/F iEC/F₂EC

An electrolyte solution of 1M LiTFSI in FiDEC/FiEC/F₂EC in a weight ratio of 1 :1 : 1 was prepared and electrochemical characterization was performed as described above. Figure 8 presents the electrochemical performance of a LiNii_/3Mni_/3Coi/302 (NMC) half cell in a ternary mixture of 1M LiTFSI in FiDEC, FiEC and F₂EC in a ratio of 1 : 1 : 1. Figure 8 shows the galvanostatic cycling comparison of Li/LiNii_/3Mni_/3Coi/302 half cell with Li as reference electrode with 1 M LiTFSI in

F iEC/F₂EC/F iDEC (1 : 1 : 1) and 1 M LiPF₆ in EC/DMC (1 : 1) (LP30) as electrolyte; Cut-offs: 3.0 to 4.3 V. Once again, figure 8 illustrates that the specific capacity as well as the efficiency of the NMC electrode in the ternary mixture of linear and cyclic fluorinated carbonates is highly comparable to the electrode performance in the standard electrolyte.

The examples 7 to 9 using common lithium-ion battery cathode materials (LiFeP0₄ (LFP) and LiNii_/3Mni_/3Coi/302 (NCM) confirm that the electrolyte compositions can be readily used for lithium- ion cells and are sufficiently stable towards oxidation, also in presence of transition metal oxides and phosphates, delivering a highly similar specific capacity, cycling stability, and electrochemical performance as observed for commercial LP30 (EC:DMC (1 : 1), 1M LiPF₆). Example 10

Determination of the electrochemical performance of a LiNio.₄Mn₁.₆O₄ half cell An electrolyte solution of 1M LiTFSI in F₄DEC was prepared and electrochemical characterization was performed as described above. Figure 9 summarizes the electrochemical performance of a LiNio.₄Mn₁.₆O₄ half cell in a solution of 1 M LiTFSI in F₄DEC. Figure 9a) shows the galvanostatic cycling of Li/LiNi₀.₄Mni.₆O₄ half cell with Li as reference electrode; Cut-offs: 3.5-4.95 V. Figure 9b) shows the corresponding potential profiles of the Li/LiNi₀.₄Mni.₆O₄ half cell; Cut-offs: 3.5-4.95 V; 2nd (C/l 0) and 3rd (C/l 0) cycle.

These data indicate that F₄DEC appears as a promising base-electrolyte solvent for high voltage cathode materials, as for instance LiNio.₄Mn₁.₆O₄ after a first activation cycle. By combining a second or third co-solvent as illustrated for FiDEC in examples 3 to 5, the ionic conductivity and the solubility of LiTFSI in F₄DEC at ambient temperature can be further optimized.

Example 11

Determination of the electrochemical performance of a graphite half cell An electrolyte solution of 1M LiTFSI in F₁DEC/F₁EC in a weight ratio of 1 :1 was prepared and electrochemical characterization was performed as described above. Figure 10 summarizes the electrochemical performance of the graphite (SLP30) half cell in a binary mixture of 1 M LiTFSI in F₁DEC/F₁EC. Figure 10a) shows the galvanostatic cycling comparison of graphite (SLP30) half cell with Li as reference electrode with 1M LiTFSI in F₁EC/F₁DEC (1 : 1) and of 1 M LiPF₆ in EC/DMC (1 : 1) (LP30) as electrolyte; Cut-offs: 0.02 and 1.5 V. Comparing the electrochemical performance of graphite-based electrodes in fluorinated carbonate-based electrolyte and LP30 as a standard electrolyte, it is obvious that the graphite electrode shows a superior cycling stability, specific capacity, and rate capability, while both show a high coulombic efficiency of up to 100%. Hence, it is shown that such fluorinated carbonate-based electrolytes are not only suitable for current state-of-the-art lithium-ion cathode materials, but moreover for graphite as state-of-the-art lithium-ion anode, confirming that such electrolyte compositions can be readily utilized in state-of-the-art lithium- ion cells and batteries.

Figure 10b) shows selected potential profiles for graphite (SLP30) half cell comprising 1M LiTFSl - FiEC/FiDEC (1 : 1) and 1M LiPF₆ - EC/DMC (1 : 1) as electrolyte. In fact, both cells show the characteristic potential profile, indicating the different stages of lithium ion (de-)intercalation. Comparative Example 12

Measurement of low amounts of FiDEC in cyclic fluorinated carbonates

To investigate whether for the beneficial effect of the linear fluorinated carbonates already the addition of only little amounts to common organic electrolytes might be sufficient, electrolyte compositions of 1M LiTFSl in FiEC/F₂EC (1 : 1) comprising 4 wt% or 11 wt% of FiDEC, were tested.

Electrolyte solutions of 1 M LiTFSl in FiEC/F₂EC (1 :1) containing either 4 wt% or 11 wt% of FiDEC were prepared and cyclic voltammetry was performed using an aluminum foil as working electrode for 100 cycles in a potential rang ranging from 3.3 V (cathodic limit) to 5.1 V (anodic limit). A scan rate of 5 mV sec^"1 was applied.

The cyclovoltammograms showed a continuously increasing evolving current density during the continuous potentiodynamic sweeps, indicating a continuous anodic aluminum dissolution, which illustrates that 4 wt.% or 11 wt.% of FiDEC in cyclic fluorinated carbonates do not appear to be sufficient to protect the aluminum current collector. Considerable marks of aluminum dissolution

("pitting corrosion") were further confirmed by subsequent SEM analysis of the aluminum electrodes.

This shows that a use as simple additive (weight content of the linear fluorinated carbonate < 11 wt.%) is not sufficient to initially form a protective passivation layer on the aluminum surface in order to suppress aluminum current collector corrosion in solvents of cyclic fluorinated carbonates. Example 13

Measurement of 20 wt% and 33 wt% of FiDEC in cyclic fluorinated carbonates To further investigate the linear fluorinated carbonates, electrolyte compositions of 1M LiTFSI in FiEC/F₂EC (1 :1) comprising 20 wt% or 33 wt% of FiDEC, were tested. Electrolyte solutions of 1 M LiTFSI in FiEC/F₂EC (1 : 1) containing either 20 wt% or 33 wt% of FiDEC were prepared and cyclic voltammetry was performed using an aluminum foil as working electrode for 100 cycles in a potential rang ranging from 3.3 V (cathodic limit) to 5.1 V (anodic limit). A scan rate of 5 mV sec^"1 was applied.

Figure 11a) shows the cyclovoltammogram for a 1M solution of LiTFSI in the mixture of 20 wt% of FiDEC in a solvent mixture of cyclic fluorinated carbonates FiEC and F₂EC in a weight ratio of 1 : 1 after for 100 cycles. The cyclovoltammogram illustrates a clear improvement relatively to the use of 11 wt.%. As can be taken from figure 11a) the detected evolving current density increases upon the first five cyclic potentiodynamic sweeps before it decreases subsequently rather rapidly, indicating the formation of a passivation layer within the first five cyclic sweeps. Subsequent SEM analysis of the aluminum electrodes did not show any severe marks of aluminum corrosion. This confirms that 20 wt.% of FiDEC provide a just sufficient passivation of the aluminum current collector.

Figure l ib) shows the cyclovoltammogram for a 1M solution of LiTFSI in the mixture of 33 wt% of FiDEC in FiEC and F₂EC (1 :1) after for 100 cycles. As can be taken from the figure 1 lb) no evolving current was detected for the second and subsequent potentiodynamic sweeps. The prevention of aluminum dissolution further was confirmed by a subsequent SEM analysis of the aluminum electrode, which showed no marks of aluminum dissolution.

This shows that the utilization of 33 wt.% of the linear fluorinated carbonate FiDEC as electrolyte solvent for LiTFSI appears to be more than sufficient for a proper passivation of the aluminum current collector. Comparative Example 14

Measurement of low amounts of FiDEC in non-fluorinated carbonates

In parallel to comparative example 12, also the effect of 11 wt% of the linear fluorinated carbonate FiDEC in non-fluorinated carbonates was investigated.

An electrolyte solution of 1 M LiTFSI in EC/DMC (1 :1) containing 11 wt% of FiDEC was prepared and cyclic voltammetry was performed using an aluminum foil as working electrode for 100 cycles in a potential rang ranging from 3.3 V (cathodic limit) to 5.1 V (anodic limit). A scan rate of 5 mV sec^"1 was applied.

The cyclovoltammograms showed a continuously increasing evolving current density during the continuous potentiodynamic sweeps, which illustrates that 11 wt.% of FiDEC in non-fluorinated carbonates is not sufficient to protect the aluminum current collector. Furthermore, obvious marks of aluminum dissolution were illustrated by subsequent SEM analysis of the aluminum electrode.

This shows that a use of 11 wt% of the linear fluorinated carbonate FiDEC in non-fluorinated carbonate-based electrolytes is obviously not sufficient to initially form a suitable passivation layer on the aluminum surface, thus protecting the aluminum current collector from corrosion.

Example 15

Measurement of 25 wt% and 33 wt% of FiDEC in non-fluorinated carbonates

Further, electrolyte compositions of 1M LiTFSI in non-fluorinated carbonates comprising 25 wt% or 33 wt% of FiDEC were tested. Electrolyte solutions of 1 M LiTFSI in EC/DMC (1 :1) containing either 25 wt% or 33 wt% of FiDEC were prepared and cyclic voltammetry was performed using an aluminum foil as working electrode for 100 cycles in a potential rang ranging from 3.3 V (cathodic limit) to 5.1 V (anodic limit). A scan rate of 5 mV sec^"1 was applied. Figure 12a) shows the cyclovoltammogram for a 1M solution of LiTFSl in a mixture of 25 wt% of FiDEC in a solvent mixture of non-fluorinated carbonates EC and DMC in a weight ratio of 1 : 1 during 100 cycles. The cyclovoltammogram illustrates an aluminum passivation within the first cycles by using 25 wt% of FiDEC. Figure 12b) shows the cyclovoltammogram for a 1M solution of LiTFSl in a mixture of 33 wt % of FiDEC in EC / DMC in a ratio of 1 : 1 for 100 cycles. As can be taken from the figure 12b), a FiDEC content of around 33 wt. % appears preferable with respect to the occurring current within the initial cycles, indicating a more effective aluminum passivation.

Example 16

Determination of the electrochemical stability window (ES W)

For the determination of the ESW of several electrolyte compositions a platinum wire was used as working electrode while lithium foils were used as counter and reference electrodes and the cell was polarized up to 6.0 V. An evolving current of 0.01 niA was defined as stability limit, indicating the anodic decomposition of the investigated electrolyte compositions.

Figure 13 illustrates the determination of the electrochemical stability window (ESW) of fluorinated and non-fluorinated organic carbonate solvents with 1M LiTFSl as conductive salt. As can be taken from figure 13, for an electrolyte solution of 1 M LiTFSl in EC/DMC (1:1) the oxidative ESW maximum vs. Li/Li⁺ was determined as 5.84 V, in FiDEC the oxidative ESW maximum was 5.56 V, in FiDEC/FiEC (1 :1) it was 5.52 V, in EC/DMC (1 :1) 5.49 V, and in FiEC it was 5.45. This confirms that the electrolyte compositions comprising linear fluorinated carbonates are stable at least up to 5 V, which is well above the solid state redox potentials of currently used and investigated lithium-ion cathode materials.

Example 17

Investigation of lithium (nonafluoro butan-2-one sulfonyl)(trifluoromethylsulfonyl)imide

An electrolyte solution of 1M lithium (nonafluoro butan-2-one sulfonyl) (trifluoromethylsulfonyl) imide (salt A) in a ternary mixture of F₁EC/F₁DEC/F₃DEC in a mol ratio of 3:0.5:0.5 was prepared and cyclic voltammetry was performed using an aluminum foil as working electrode for 100 cycles in a potential rang ranging from 3 V to 5 V. A scan rate of 2 mV sec^"1 was applied.

Figure 14 presents the cyclovoltammogram for 100 cycles, showing the characteristic behaviour (see e.g. Figure 1 a and b) of an initially evolving current, indicating the formation of a protective passivation layer on the aluminum surface, successfully preventing a continuous anodic aluminum dissolution upon the subsequent cyclic potentiodynamic sweeps, which is confirmed by the zero current. Also the optical microscope images of the utilized Al foil, obtained by means of a Leica DM2700 M, showed no marks of aluminum dissolution ("pitting corrosion"). This shows that also for other sulfonimide-based lithium salts aluminum dissolution can be prevented by using the linear fluorinated carbonates of the invention. Example 18

Investigation of the aluminum surface after anodic polarization in EC/DEC and in in ethyl (1- fluoroethyl) carbonate (FiDEC)

Initially, the aluminum foil was anodically polarized in 1M LiTFSI in EC/DEC (1 :1 by weight) or in 1M LiTFSI in FiDEC for five cyclic sweeps. Cyclic voltammetry and SEM measurements showed a massive aluminum dissolution for the aluminum foil in EC/DEC. Subsequently, the cycled Swagelok^® cells were disassembled in an MBraun dry box under argon. The polarized Al foils were rinsed with diethyl carbonate (DEC), and dried under vacuum. X-ray photoelectron spectroscopy (XPS) was carried out by means of an Axis Ultra HAS spectroscope (Kratos, GB) using a mono-chromatic Al Ka source (10 mA, 12 kV). The measurements were performed at a pass energy of 40 eV. A charge neutralizer was used to compensate the charging of the samples. Depth profiling by etching the studied samples with argon ions (Ar⁺) was used to investigate the composition of the film formed at the electrode/electrolyte interface and its variation along different layers of the exposed aluminum foil. The analysis area on the sample surface was about 100 μηι x 100 μηι. Ar etching was carried out at 2 kV and 8 mA to obtain depth profiles of the polarized aluminum electrodes. All spectra were calibrated to 71.6 eV, which refers to the Al 2p peak of metallic aluminum. Figure 15 shows the results of the XPS studies. The figures 15a) and 15b) show the Al 2p depth profile of 10 x 1 min Ar⁺ etching and the corresponding evolution of the atomic concentration of F, O, C, and AL upon Ar⁺ etching for EC/DEC (1 :1) as solvent, and the figures 15c) and 15d) for FiDEC as solvent, respectively.

As can be seen from the Al 2p spectrum of Figure 15a), a signal at 71.6 eV, referring to metallic aluminum, and a peak at 74.0 eV, corresponding to AI₂O₃, were detected, having an atomic ratio of 1 :5. After 10 minutes of Ar⁺-etching, a decrease of the aluminum oxide intensity was observed, whereby the atomic concentration of metallic aluminum increased, as shown in Figure 15b). The O ls depth profile confirmed the decrease of AI₂O₃, by following the signal at 531.2 eV. Within three minutes of sputtering, the AI₂O₃ atomic concentration decreased to less than 50% of the initial value. Additionally, the fluorine and carbon content along the depth profile were always close to zero with the exception of a high initial carbon content, as is shown in Figure 15b). The latter, however, vanished immediately with the sputtering supporting the assumption of a contaminated surface. These results confirm that the surface layer coating the current collector was mostly composed of AI₂O₃. The thickness of this layer was identified to be about 6.7 nm as calculated by XPS MultiQuant. The aluminum foil anodized in FiDEC showed a rather different chemistry of the surface layer. As shown in Figure 15c), A1F₃ coexisted with metallic aluminum and AI₂O₃. The coexistence of these two materials, which has been proven to be effective against aluminum anodic dissolution in LiPF₆-based electrolytes, explains the observed suppression of the current collector corrosion. The depth profiles in Figure 15d indicated a rather thin layer of about 1.3 nm of A1F₃ and AI₂O₃.

This shows that ethyl (1-fluoroethyl) carbonate (FiDEC) provides the formation of a thin and protective layer of A1F₃ preventing aluminum corrosion.

Example 19

Determination of a binary mixture of FiDEC and FiEC in a full-cell lithium-ion configuration A NMC cathode and graphite anode were tested in a full-cell lithium-ion configuration, using 1M LiTFSI in F₁EC/F₁DEC as the electrolyte. For the full-cell assembly, pouch cells were used. The NMC-based electrodes were prepared analogous to the electrodes characterized in half-cells, having a final composition of 88 wt% NMC (TODA), 7 wt% Super C65^® (TIMCAL), and 5 wt% PVdF

(Kynar® HSV 900, ARKEMA) with an average active material mass loading of around 6 mg cm^"2. Likewise, graphite-based electrodes were prepared according to the equivalent method of the half-cell electrodes, of 91 wt% graphite (SLP30, TIMCAL), 3 wt% Super C65^® (TIMCAL), and 6 wt% PVdF (Kynar^® HSV 900, ARKEMA). The pouch cells were composed of 4.4 x 4.4 cm² electrodes separated by a Whatman glass fiber filter. Based on the electrode mass loading indicated above, the cells were cathode limited with a capacity of about 16 mAh.

As electrolyte a 1M solution of LiTFSI in FiEC/FiDEC (1 :1 by weight) was utilized. The

NMC/graphite lithium-ion pouch cell was subjected to six galvanostatic formation cycles at C/10 and then cycled at C/5 within the cut-off voltages of 2.8 and 4.2 V. The results of the 16 mAh pouch cell and the corresponding voltage profiles are illustrated in Figure 16a) and 16b). As can be taken from figure 16, upon the initial cycles at C/10, the formation of the cell took place. In particular, the low coulombic efficiency detected in the first charge- discharge cycle results from a SEI formation on the graphite electrode. The coulombic efficiency was higher than 97% in the second cycle and 99% in the fifth cycle. Upon the following cycles at C/5 rate a very stable cycling performance was observed. The cell delivered a capacity of 130 mAh g^"1, with respect to the NMC active material, combined with an excellent coulombic efficiency (> 99.7%).

This shows that the fluorinated electrolyte mixture comprising LiTFSI is a very well performing electrolyte composition in combination with state-of-the-art high voltage cathode materials and low voltage anodes. Particularly, also the full-cell showed a very stable cycling performance.

The work leading to this invention has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 29100329 AMELIE.

Claims

1. An electrolyte solution comprising an electrolyte salt and an electrolyte solvent, wherein the electrolyte solvent comprises an n-fluoro diethyl carbonate according to formula (1) as follows: C2H5.xFxCO3C2H5._yF_y (1) wherein 1 < x < 5 and 0 < y < 5 in an amount in the range of > 20 wt% to < 100 wt%, referring to a total amount of the electrolyte solvent of 100 wt%.

2. The electrolyte solution according to claim 1, wherein the electrolyte salt is an alkali or alkaline earth metal sulfonimide or sulfonmethide salt, wherein the lithium sulfonimide salt preferably is selected from the group comprising lithium bis(trifluoromethanesulfonyl)imide LiN(S0₂CF₃)₂, lithium bis(fluorosulfonyl)imide, LiN(FS0₂)2, lithium trifluoromethanesulfonate Li(CF₃)S0₃, lithium (trifluoromethylsulfonyl)(nonafluorobutanesulfonyl)imide LiN(S0₂CF₃)(S0₂C₄F₉), lithium

(fluorosulfonyl)(nonafluorobutanesulfonyl)imide LiN(S0₂F)(S0₂C₄F₉), lithium (nonafluoro butan-2- one sulfonyl)(trifluoromethylsulfonyl)imide LiN(S0₂C₂F₄OC₂F₅)(S0₂CF₃), and lithium (nonafluoro butan-2-one sulfonyl)(fluorosulfonyl)imide LiN(S0₂C₂F₄OC₂F₅)(S0₂F).

3. The electrolyte solution according to claim 1 or 2, wherein the n-fluoro diethyl carbonate is selected from the group comprising ethyl (1-fluoroethyl) carbonate, 1-fluoroethyl (2,2,2-trifluoroethyl) carbonate, bis(2-fluoroethyl) carbonate, bis(2,2,2-trifluoroethyl) carbonate, bis(l,2,2-trifluoroethyl) carbonate and mixtures thereof, preferably selected from the group comprising ethyl (1-fluoroethyl) carbonate, 1 -fluoroethyl (2,2,2-trifluoroethyl) carbonate and mixtures thereof.

4. The electrolyte solution according to any of the preceding claims, wherein the electrolyte solvent further comprises at least one cyclic fluorinated carbonate selected from the group comprising 4-fluoro-l,3-dioxolan-2-one, cis-4,5-difluoro-l,3-dioxolan-2-one, ira¾y-4,5-difluoro-l,3-dioxolan-2- one and mixtures thereof.

5. The electrolyte solution according to any of the preceding claims, wherein the electrolyte solution comprises a binary solvent mixture of an n-fluoro diethyl carbonate according to formula (1) as follows: C2H5.xFxCO3C2H5._yF_y (1) wherein 1 < x < 5 and 0 < y < 5, preferably selected from ethyl (1-fluoroethyl) carbonate or 1 -fluoroethyl (2,2,2-trifluoroethyl) carbonate, and a cyclic fluorinated carbonate selected from the group comprising 4-fluoro-l,3-dioxolan-2-one, cis-4,5-difluoro-l,3- dioxolan-2-one, and ira« -4,5-difluoro-l,3-dioxolan-2-one.

6. The electrolyte solution according to any of the preceding claims, wherein the electrolyte solution comprises a mixture of ethyl (1 -fluoroethyl) carbonate or 1 -fluoroethyl (2,2,2-trifluoroethyl) carbonate and 4-fluoro-l,3-dioxolan-2-one in a ratio of 1 : 1.

7. The electrolyte solution according to any of the preceding claims, wherein the electrolyte solution comprises a ternary solvent mixture of at least one n-fluoro diethyl carbonate according to formula (1) as follows: C2H5.xFxCO3C2H5._yF_y (1) wherein 1 < x < 5 and 0 < y < 5, preferably selected from the group comprising ethyl (1 -fluoroethyl) carbonate, 1 -fluoroethyl (2,2,2-trifluoroethyl) carbonate and mixtures thereof, and at least one cyclic fluorinated carbonate selected from the group comprising 4-fluoro-l,3-dioxolan-2-one, cis-4,5-difluoro-l,3-dioxolan-2-one, ira¾y-4,5-difluoro-l,3- dioxolan-2-one and mixtures thereof.

8. The electrolyte solution according to any of the preceding claims, wherein the electrolyte solution comprises a ternary solvent mixture of ethyl (1 -fluoroethyl) carbonate or 1 -fluoroethyl (2,2,2- trifluoroethyl) carbonate, 4-fluoro-l,3-dioxolan-2-one, and ira« -4,5-difluoro-l,3-dioxolan-2-one in a ratio of 1 : 1 :1.

9. The electrolyte solution according to any of the preceding claims, wherein the electrolyte solution comprises a non- fluorinated organic carbonate selected from the group comprising ethylene carbonate, ethyl methyl carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and mixtures thereof.

10. The electrolyte solution according to any of the preceding claims, wherein the electrolyte solution comprises the n-fluoro diethyl carbonate in a range of > 25 wt % to < 100 wt%, preferably in a range of > 25 wt% to < 75 wt%, more preferably in a range of > 30 wt% to < 50 wt%, referring to a total amount of the electrolyte solvent of 100 wt%.

11. The electrolyte solution according to any of the preceding claims, wherein the electrolyte solution comprises a mixture of ethylene carbonate and dimethyl carbonate in a ratio of 1 : 1 and the n- fluoro diethyl carbonate in the range of > 20 wt% to < 50 wt%, preferably in the range of > 25 wt% to < 50 wt%, referring to a total amount of the electrolyte solvent of 100 wt%.

12. An alkali or alkaline earth metal-based electrochemical energy storage device, particularly a lithium battery, a lithium ion battery, a lithium-ion accumulator, a lithium polymer battery or a lithium-ion capacitor, comprising an electrolyte solution according to any of the preceding claims.

13. Use of an n-fluoro diethyl carbonate according to formula (1) as follows: C2H5.xFxCO3C2H5._yF_y (1) wherein 1 < x < 5 and 0 < y < 5, in an amount in the range of > 20 wt% to < 100 wt%, referring to a total amount of a electrolyte solvent of 100 wt%, for the prevention of aluminum current collector corrosion in an alkali or alkaline earth metal-based electrochemical energy storage device, particularly a lithium-ion battery or lithium polymer battery containing an electrolyte solution comprising an alkali or alkaline earth metal sulfonimide or sulfonmethide salt.

14. The use according to claim 13, wherein the alkali or alkaline earth metal sulfonimide salt is selected from the group comprising lithium bis(trifluoromethanesulfonyl)imide, lithium

bis(fluorosulfonyl)imide, LiN(FS0₂)2, lithium trifluoromethanesulfonate Li(CF₃)S0₃, lithium

(fluorosulfonyl)(nonafluorobutanesulfonyl)imide LiN(S0₂F)(S0₂C₄F₉), lithium (nonafluoro butan-2- one sulfonyl)(trifluoromethylsulfonyl)imide LiN(S0₂C₂F₄OC₂F₅)(S0₂CF₃), lithium (nonafluoro butan- 2-one sulfonyl)(fluorosulfonyl)imide LiN(S0₂C₂F₄OC₂F₅)(S0₂F).

15. The use according to claims 13 or 14, wherein the electrolyte solvent further comprises at least one cyclic fluorinated carbonate selected from the group comprising 4-fluoro-l,3-dioxolan-2-one, cis- 4,5-difluoro-l,3-dioxolan-2-one, ira« -4,5-difluoro-l,3-dioxolan-2-one and mixtures thereof, and/or a non- fluorinated organic carbonate selected from the group comprising ethylene carbonate, ethyl methyl carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and mixtures thereof.