CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to European application No.

17189133.6 filed on September 1 , 2017, the whole content of this application being incorporated herein by reference. Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

TECHNICAL FIELD

The present invention relates to electrochemical cells comprising lithium metal as an anode electrochemically active material.

TECHNICAL BACKGROUND

Rechargeable lithium-based electrochemical cells have been world-widely investigated as light-weight and high-energy-density energy storage devices. Those employing lithium metal as the anode electrochemically active material are known since the 1970s. Indeed, lithium metal has a high specific capacity of 3861 mAh g ^"1 . However, these cells have not achieved commercial success due to the following two main drawbacks :

First, lithium metal dendrites may form during the operation of the cell.

They tend to accumulate in the cell, puncture the separator and cause an internal short-circuit, leading to heat and possibly fire or explosion.

Second, lithium metal anodes continuously form a passivation layer (also called solid electrolyte interface SEI layer) on their surface during cycling, causing significant coulombic efficiency losses, consumption of lithium metal and increase of the cell resistance.

Thus, much research effort has been engaged with a view of reducing or suppressing the lithium dendrite formation and improving the cycling ability of the cell.

The use of a solid electrolyte polymer has been considered in place of a liquid electrolyte. For example, S. Liu et al. in Journal of Power Sources, 195, 6847 (2010) describe a lithium electrochemical cell comprising a lithium ion conducting polymer electrolyte of polyethylene oxide PEOis with lithium trifluoromethane sulfonimide LiN(CF ₃ S02)2 (LiTFSI). However, short-circuit has been observed even with such a solid polymer, although to a lesser extent than with a liquid electrolyte. Besides, no polymer electrolyte with high conductivity at room temperature has been reported yet.

Hydro-Quebec and 3M have recently developed lithium electrochemical cells comprising a polymer electrolyte, an anode made of a thin lithium foil and a cathode containing vanadium oxide as the active material. However, accidents have been reported on these cells, which were probably caused by the formation of dendrites during the charging process.

The use of a solid electrolyte has also been considered in place of a liquid electrolyte. For example R. Sudo et al. describe in Solid State Ionics, 262, 151 (2014) the use of Al-doped Li ₇ La ₃ Zr ₂ 0i2 as a solid electrolyte in a

electrochemical cell comprising a lithium anode. However, lithium dendrites were again observed.

Many approaches to the prevention of lithium dendrites formation have focused on improving the stability and uniformity of the passivation layer on the anode. For examples, D. Aurbach et al. in Solid State Ionics, 148, 405 (2002) and H. Ota et al. in Electrochimica Acta, 49, 565 (2004) report that additives such as C0 ₂ , S0 ₂ and vinylene carbonate help in improving the stability of the passivation layer. However, these additives are consumed during the operation of the cell. Thus, they do not offer a long-term solution to the problem of dendrites formation.

Some approaches consist in modifying the composition of the liquid electrolyte.

For example, the use of a liquid electrolyte with a high lithium salt concentration of (LiTFSI) in dimethoxyethane (DME)-l,3dioxolane (DOL) (1 : 1 v:v) for suppressing lithium dendrite formation has been described by L.Suo et al. in Nature Communications, DOI: 10.1038/ncomms2513 (2013).

The use of a liquid electrolyte with a high lithium salt concentration of lithium bis(fluorosulfonyl)imide LiN(FS0 ₂ ) ₂ (LiFSI) in dimethoxyethane (DME)-l,3dioxolane (DOL) (1 : 1 v:v) for enabling a high-rate cycling of a lithium metal electrode without dendrite growth has been described by J. Qian et al. in Nature Communications, DOI: 10.1038/ncomms7362 (2015).

H. Wang et al. report in ChemElectroChem, 2, 1144 (2015) that a cell containing lithium metal as the anode and a solvated ionic liquid of tetraglyme (G4) and LiFSI as the electrolyte exhibits excellent cycling performance. There is still a need to find a remedy to the growth of lithium dendrites at the surface of the anode, which eventually increases its impedance while reducing the volume of active chemicals within the cell. A lithium

electrochemical cell having lithium metal as an anode active material and a liquid electrolyte is therefore sought which allows reducing or even suppressing the growth of dendrites on the anode surface so that the battery cycle life, which is defined as the number of complete charge, that is, discharge cycles a battery can perform before its nominal capacity falls below 80% of its initial rated capacity, becomes longer.

SUMMARY OF THE INVENTION

A first object of the present invention is an electrochemical cell comprising:

a) at least one anode containing lithium metal as an electrochemically active material, and

b) an electrolyte composition comprising :

i) at least 50% by weight (wt%) of a solvent mixture based on the total weight of the electrolyte composition containing

- from 5 to 95 wt% of at least one fluorinated acyclic compound; and

- from 95 to 5 wt% of at least one fluorinated cyclic compound based on the total weight of the solvent mixture; and

ii) at least one lithium salt.

According to one embodiment, the electrolyte composition comprises at least 60 wt% of the solvent mixture, preferably at least 70 wt% of the solvent mixture, and more preferably at least 80 wt% of the solvent mixture.

According to one embodiment,

- the at least one fluorinated acyclic compound accounts for from 35 to 90 wt% based on the total weight of the solvent mixture; and

- the at least one fluorinated cyclic compound accounts for from 65 to 10 wt% based on the total weight of the solvent mixture.

According to one embodiment,

- the at least one fluorinated acyclic compound accounts for from 50 to 85 wt% based on the total weight of the solvent mixture; and

- the at least one fluorinated cyclic compound accounts for from 50 to 15 wt% based on the total weight of the solvent mixture.

According to one embodiment, - the at least one fluorinated acyclic compound accounts for from 60 to 80 wt% based on the total weight of the solvent mixture; and

- the at least one fluorinated cyclic compound accounts for from 40 to 20 wt% based on the total weight of the solvent mixture.

According to one embodiment, the fluorinated cyclic compound is a partially or fully fluorinated cyclic carbonate.

According to one embodiment, the fluorinated cyclic carbonate is selected from the group consisting of mono- or dif uoroethylene carbonate, mono- or dif uoropropylene carbonate, mono- or difluorobutylene carbonate, 3,3,3- trifluoropropylene carbonate and mixtures thereof, preferably mono- or difluoroethylene carbonate.

According to one embodiment, the fluorinated acyclic compound is a carboxylic acid ester represented by the formula

R'-COO-R ² , where R ¹ and R ² independently represent a linear or branched alkyl or an alkyl ether group, the sum of carbon atoms in R ¹ and R ² being from 2 to 7, at least one hydrogen in R ¹ and/or R ² being replaced by fluorine.

According to one embodiment, R ¹ is C¾- and R ² is CHF ₂ CH ₂ -.

According to one embodiment, the electrochemical cell further comprises at least one film-forming additive in an amount accounting for from 0.05 to 30 wt% of the electrolyte composition, preferably from 0.05 to 20 wt% of the electrolyte composition, more preferably from 2 to 15 wt% of the electrolyte composition, and even more preferably from 2 to 5 wt% of the electrolyte composition.

According to one embodiment, the film-forming additive is selected from the group consisting of salts based on tetrahedral boron compounds comprising lithium(bisoxalatoborate) (LiBOB) and lithium difluorooxalato borate

(LiDFOB); cyclic sulphites and sulfate compounds comprising 1,3- propanesultone (PS), ethylene sulphite (ES) and prop-l-ene-l,3-sultone (PES), sulfone derivatives comprising dimethyl sulfone, tetrametylene sulfone (also known as sulfolane), ethyl methyl sulfone and isopropyl methyl sulfone; nitrile derivatives comprising succinonitrile, adiponitrile, glutaronitirle and 4,4,4- trifluoronitrile; and vinyl acetate (VA), biphenyl benzene, isopropyl benzene, hexafluorobenzene, lithium nitrate (L1NO3), tris(trimethylsilyl)phosphate, triphenyl phosphine, ethyl diphenylphosphinite, triethyl phosphite, vinylene carbonate (VC), vinyl ethylene carbonate (VEC), ethyl propyl vinylene carbonate, dimethylvinylene carbonate, maleic anhydride (MA), and mixtures thereof.

In one embodiment, the electrolyte composition comprises :

- from 0.5 to 5 wt% of at least one of lithium(bisoxalatoborate) (LiBOB), lithium difluorooxalato borate (LiDFOB) and vinylene carbonate (VC); and/or

- from 1.5 to 10.0 wt% of at least one of 1,3-propanesultone (PS), prop-1- ene-l,3-sultone (PES) and maleic anhydride (MA), with respect to the total weight of the electrolyte composition.

According to one embodiment, the non-fluorinated solvent(s) if any, present in the electrolyte composition accounts for at most 20 wt%, preferably at most 10 wt%, even more preferably at most 5 wt% of the solvent mixture.

According to one embodiment, the electrochemical cell further comprises at least one cathode the electrochemically active material of which is selected from the group consisting of :

- LiaNi _x Mn _y Co _z 0 ₂ , where x+y+z=l and 0.5<a<l .3;

- LiaCo0 ₂ , where 0.5<a<1.3; and

- LiaMn ₂ _ _x Ni _x 0 ₄ , where 0<x<0,5 and 0.5<a<1.3.

According to one embodiment, the lithium salt is selected from the group consisting of lithium trifluoromethane sulfonate (L1CF ₃ SO ₃ ), lithium

hexafluorophosphate (LiPF ₆ ), lithium bis(fluorosulfonyl)imide Li(FS0 ₂ ) ₂ N (LiFSI), LiN(S0 ₂ C mF ₂ m ₊ i)(S0 ₂ C _n F _2n+ i) and

LiC(S0 ₂ C _k F _2k+1 )(S0 ₂ C _m F _2m+1 )(S0 ₂ C _n F _2n+1 ), wherein k=l- 10, m=l-10 and n=l- 10, LiN(S0 ₂ C _p F _2p S0 ₂ ) and LiC(S0 ₂ C _p F _2p S0 ₂ )(S0 ₂ C _q F _2q+1 ), wherein p=l-10 and q=l-10, and mixtures thereof.

A second object of the present invention is a lithium metal battery comprising at least one electrochemical cell as described above.

A third object of the present invention is the use of the electrolyte composition as described above, in an electrochemical cell comprising at least one anode containing lithium metal, for reducing or suppressing the growth of lithium dendrites on the anode surface and/or for stabilizing the solid electrolyte interface layer on the anode surface.

Battery cycle life is defined as the number of complete charge, that is, the number of discharge cycles a battery can perform before its nominal capacity falls below 80% of its initial rated capacity. The most significant cause of calendar life loss is the build-up of a passication layer of unwanted chemicals on the surface of the anode, which eventually increases its impedance while reducing the volume of active chemicals within the cell.

Indeed, it was surprisingly found by the inventors that the use of a solvent mixture containing at least one fluorinated acyclic compound and at least one fluorinated cyclic compound, and optionally non- fluorinated solvent(s) in the electrolyte of an electrochemical cell comprising lithium metal as an anode electrochemically active material results in reduction or even elimination of the formation of dendrites so that the risk of an internal short-circuit becomes dramatically reduced and the battery cycle life becomes longer. It also allows stabilizing the solid electrolyte interface SEI layer on the anode surface. As a result, the cycling ability of the cell is improved, which was clearly demonstrated in terms of number of cycles at 80% of capacity retention. It is believed that a decomposition of the fluorinated compounds present in the electrolyte creates a fluoride-rich solid electrolyte interface SEI at the surface of the anode and said fluoride-rich solid electrolyte interface stabilizes the lithium metal.

BRIEF DESCRIPTION OF THE FIGURE

Figure 1 shows on the left ordinate axis the variation of the capacity retention as a function of the cycle number of the electrochecmial cells for the Inventive Examples of E1-E6 with Li metal foil in 20 μιη as anode and 0.5 C of C rate. It shows on the right ordinate axis the variation of the coulombic efficiency as a function of the cycle number.

Figure 2 shows the same for Inventive Examples of El, E9 and E10 in comparison with the Comparative Example of CE2.

Figure 3 shows the same for Inventive Examples of El and E5 in comparison with the Comparative Examples of CE3, CE4, CE6 and CE7.

Figure 4 shows the same for Inventive Examples of El and E6 in comparison with the Comparative Example of CE5.

Figure 5 shows the same for Inventive Examples of E7 and E8 in comparison with the Comparative Example of CE1, except that Li metal foil in 300 μιη is used as anode and C rate is 1.0 C.

DETAILED DESCRIPTION OF THE INVENTION

The following constituents of the electrochemical cell according to the invention are described hereafter in details. It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention claimed. Accordingly, various changes, modifications, described herein will be apparent to those of ordinary skill in the art. Moreover, descriptions of well- known functions and constructions may be omitted for the sake of clarity and conciseness.

Electrolyte :

The electrolyte composition according to the invention typically comprises:

- a mixture of non-aqueous solvents ;

- at least one lithium salt ; and

- optionally at least one film- forming additive.

The mixture of non-aqueous solvents comprises at least one fluorinated acyclic compound and at least one fluorinated cyclic compound. In the present invention, the term "fluorinated compound", is intended to denote, in particular, a compound, wherein at least one hydrogen atom is replaced by fluorine. One, two, three or a higher number of hydrogen atoms may be replaced by fluorine.

This solvent mixture accounts for at least 50 wt% of the electrolyte composition.

In one embodiment, the solvent mixture accounts for at least 60 wt% of the electrolyte composition.

In one embodiment, the solvent mixture accounts for at least 70 wt% of the electrolyte composition.

In one embodiment, the solvent mixture accounts for at least 80 wt% of the electrolyte composition.

The fluorinated acyclic compound may be selected from the group consisting of fluorinated acyclic esters, fluorinated acyclic ethers, fluorinated acyclic carbonates and fluorinated alkanes.

In a first embodiment, the fluorinated acyclic compound is a fluorinated acyclic ester. Examples of fluorinated acyclic esters are esters of formula R ¹ - COO-R ² , where R ¹ and R ² independently represent a linear or branched alkyl group or an alkyl ether group, the sum of carbon atoms in R ¹ and R ² being from 2 to 7, at least one hydrogen in R ¹ and/or R ² being replaced by fluorine.

R ¹ may be selected from the group consisting of CH ₃ -, CH ₃ CH ₂ -,

CH ₃ CH ₂ CH ₂ -, CH(CH ₃ ) ₂ -, CFH ₂ -, CF ₂ H-, CF ₃ -, CHF ₂ CH ₂ -, CHF ₂ CH ₂ CH ₂ -.

R ² may be selected from the group consisting of CH ₃ -, CH ₃ CH ₂ -,

CH ₃ CH ₂ CH ₂ -, CH(CH ₃ ) ₂ -, CFH ₂ -, CF ₂ H-, CF ₃ -, CHF ₂ CH ₂ -, CHF ₂ CH ₂ CH ₂ -.

Preferably, the fluorinated ester is 2,2-difluoroethyl acetate, namely R ¹ is

CH ₃ - and R ² is CHF ₂ CH ₂ -. In one embodiment, the fluorinated acyclic ester is isopropyl-2,2,2- trifluoroacetate, namely R ¹ is CF ₃ - and R ² is CH(CH ₃ ) ₂ -.

In a second embodiment, the fluorinated acyclic compound is a fluorinated acyclic ether, which may be selected from :

3-(l , 1 ,2,2-tetrafluoroethoxy)-(l , 1 ,2,2-tetrafluoro)-propane,

pentafluoropropyl methyl ether,

pentafluoropropyl fluoromethyl ether,

pentafluoropropyl trifluoromethyl ether,

4,4,4,3,3,2,2-heptafluorobutyl difluoromethyl ether,

4,4,3 ,2,2-pentafluorobutyl 2,2,2-trifluoroethyl ether,

methyl nonafluorobutyl ether,

ethyl nonafluorobutyl ether,

2-difluoromethoxy-l, 1, 1-trifluoroethane,

2-difluoromethoxy-l, 1 , 1 ,2-tetrafluoroethane, and

mixtures thereof.

In a third embodiment, the fluorinated acyclic compound is a fluorinated acyclic carbonate. Suitable examples include fluorinated dimethyl carbonate, fluorinated diethyl carbonate, fluorinated ethyl methyl carbonate, fluorinated dipropyl carbonate, fluorinated dibutyl carbonate, fluorinated methyl propyl carbonate, fluorinated ethyl propyl carbonate. The fluorinated acyclic carbonate may be mono fluorinated or polyfluorinated.

In one embodiment, the fluorinated acyclic compound accounts for from 5 to 95 wt% of the total weight of the solvent mixture. In other embodiment, the fluorinated acyclic compounds accounts for from 35 to 90 wt% of the total weight of the solvent mixture, or from 40 to 85 wt%, or from 50 to 85 wt%, or from 60 to 80 wt% of the total weight of the solvent mixture.

The fluorinated cyclic compound may be selected from the group consisting of fluorinated cyclic saturated carbonates, fluorinated cyclic unsaturated carbonates, fluorinated cyclic esters, fluorinated cyclic ethers and fluorinated cyclic alkanes.

In one preferred embodiment, the fluorinated cyclic compound is a fluorinated cyclic carbonate. Suitable examples include mono- and difluorinated ethylene carbonate, mono- and difluorinated propylene carbonate, mono- and difluorinated butylene carbonate, 3,3,3-trifluoropropylene carbonate, and mixtures thereof. In one most preferred embodiment, the fluorinated cyclic compound is monofluorinated ethylene carbonate (4-fluoro-l,3-dioxolan-2-one) (FEC).

According to the invention, the fluorinated cyclic compound accounts for from 95 to 5 wt% of the total weight of the solvent mixture. In other

embodiment, the fluorinated cyclic compounds accounts for from 65 to 10 wt% of the total weight of the solvent mixture, or from 60 to 15 wt%, or from 50 to 15 wt%, or from 40 to 20 wt% of the solvent mixture.

The solvent mixture according to the present invention optionally comprises at least one non-fluorinated organic solvent(s). Such non- fluorinated organic solvents include non-fluorinated organic cyclic carbonates and non- fluorinated organic linear carbonates. Examples of non-fluorinated organic carbonates comprise, but not limited to, ethylene carbonate, also known as 1,3- dioxalan-2-one, 4-methylene-l,3-dioxolan-2-one and 4,5-dimethylene-l,3- dioxolan-2-one; ethyl methyl carbonate; dimethyl carbonate; diethyl carbonate; propylene carbonate; di-tert-butyl carbonate; dipropyl carbonate; methyl propyl carbonate; methyl butyl carbonate; ethyl butyl carbonate; propyl butyl carbonate; dibutyl carbonate; or mixtures thereof. In one embodiment, non-fluorinated organic solvent is dimethoxy ethane (DME). In another embodiment, non- fluorinated organic solvent is 1,3-dioxolane (DOL).

It has been observed that the presence of non-fluorinated solvents in the electrolyte composition may have a detrimental effect on the cycling ability of the lithium electrochemical cell. Accordingly, the wt% of non-fluorinated solvents is preferably at most 20 wt%, more preferably at most 10 wt% of the solvent mixture. In a particular embodiment, the solvent mixture contains at most 5 wt% of any of the following compounds : non-fluorinated carbonates, non-fluorinated esters, non-fluorinated ethers and non-fluorinated alkanes.

Typically, the solvent mixture contains a majority of fluorinated acyclic compounds and a minority of fluorinated cyclic compounds. The term "majority" refers to a weight percentage of fluorinated acyclic compound generally ranging from 50 to 85 wt% based on the weight of the solvent mixture. The term

"minority" refers to a weight percentage of fluorinated cyclic compound generally ranging from 50 to 15 wt% based on the weight of the solvent mixture. Preferably, the at least one fluorinated acyclic compound accounts for from 60 to 80 wt% based on the weight of the solvent mixture and the at least one fluorinated cyclic compound accounts for from 40 to 20 wt% based on the weight of the solvent mixture. The solvent mixture in the electrolyte composition according to the present invention is capable of dissolving a lithium salt, such as LiPF ₆ at ambient temperature and at atmospheric pressure, e.g., 23 °C and 1 bar, in an amount of at least 0.5 mol.L ¹ , preferably at least 0.8 mol.L ¹ and more preferably at least l .O moLL ^"1 .

According to one embodiment, the solvent mixture comprises

- from 5 to 95 wt% of at least one fluorinated acyclic compound;

- from 95 to 5 wt% of at least one fluorinated cyclic compound, and

- from 0 to 20 wt% of non- fluorinated solvent(s), with respect to the total weight of the solvent mixture.

According to one preferred embodiment, the solvent mixture comprises

- from 35 to 90 wt% of at least one fluorinated acyclic compound;

- from 65 to 10 wt% of at least one fluorinated cyclic compound, and

- from 0 to 10 wt% of non- fluorinated solvent(s), with respect to the total weight of the solvent mixture.

According to another preferred embodiment, the solvent mixture comprises

- from 60 to 80 wt% of at least one fluorinated acyclic compound;

- from 40 to 20 wt% of at least one fluorinated cyclic compound, and

- from 0 to 5 wt% of non- fluorinated solvent(s), with respect to the total weight of the solvent mixture.

The electrolyte composition may also comprise at least one film-forming additive which promotes the formation of the solid electrolyte interface SEI layer at the anode surface and/or cathode surface by reacting in advance of the solvents on the electrode surfaces. Main components of SEI hence comprise the decomposed products of electrolyte solvents and salts, which include L1 ₂ CO3, lithium alkyl carbonate, lithiu alkyl oxide and other salt moieties such as LiF for LiPF ₆ -based electrolytes. Usually, the reduction potential of the film-forming additive is higher than that of solvent when reactions occurs at the anode surface, and the oxidation potential of the film- forming additive is lower than that of solvent when reaction occurs at the cathode side.

In the present invention, the film- forming additive is not typically a fluorinated compound. For the sake of clarity, the film-forming additives of the present invention differ from the fluorinated cyclic compounds or fluorinated acyclic compounds and non- fluorinated solvents. Examples of film- forming additives include, but not limited to, salts based on tetrahedral boron compounds comprising lithium(bisoxalatoborate) (LiBOB) and lithium difluorooxalato borate (LiDFOB); cyclic sulphites and sulfate compounds comprising 1 ,3- propanesultone (PS), ethylene sulphite (ES) and prop-l-ene-l ,3-sultone (PES); sulfone derivatives comprising dimethyl sulfone, tetrametylene sulfone (also known as sulfolane), ethyl methyl sulfone and isopropyl methyl sulfone; nitrile derivatives comprising succinonitrile, adiponitrile glutaronitirle and 4,4,4- trifluoronitrile; and vinyl acetate (VA), biphenyl benzene, isopropyl benzene, hexafluorobenzene, lithium nitrate (L1NO3), tris(trimethylsilyl)phosphate, triphenyl phosphine, ethyl diphenylphosphinite, triethyl phosphite, vinylene carbonate (VC), vinyl ethylene carbonate (VEC), ethyl propyl vinylene carbonate, dimethyl vinylene carbonate, maleic anhydride (MA), and mixtures thereof.

The total amount of all the film- forming additive(s) generally accounts for from 0.05 to 30 wt% of the electrolyte composition, preferably from 0.05 to 20 wt% of the electrolyte composition, more preferably from 2 to 15 wt% of the electrolyte composition, and even more preferably from 2 to 5 wt% of the electrolyte composition.

In one embodiment, the total amount of film- forming additive(s) accounts for at least 2 wt% of the electrolyte composition.

In a certain embodiment, the electrolyte composition comprises :

- from 0.5 to 5 wt% of at least one of lithium(bisoxalatoborate) (LiBOB), lithium difluorooxalato borate (LiDFOB) and vinylene carbonate (VC); and/or - from 1.5 to 10.0 wt% of at least one of 1 ,3-propanesultone (PS), prop-1- ene-l ,3-sultone (PES) and maleic anhydride (MA), with respect to the total weight of the electrolyte composition.

Additives other than those favouring the formation of the film may be additionally present in the electrolyte composition. Examples of other additives include, but not limited to, cathode protection agents, LiPF ₆ salt stabilizer, safety protection agent, Li deposition improver, ionic salvation enhancer, Al corrosion inhivitor, wetting agent, viscosity diluter, anti-swelling agents, low temperature or high temperature performance enhancers.

In the present invention, the lithium salt is intended to denote, in particular, a lithium ion complex comprising, but not limited to, lithium trifluoromethane sulfonate (L1CF ₃ SO ₃ ), lithium hexafluorophosphate (LiPF ₆ ), lithium

bis(fluorosulfonyl)imide Li(FS0 ₂ ) ₂ N (LiFSI), LiN(S0 ₂ C _m F _2m+ i)(S0 ₂ C _n F _2n+ i) and LiC(S0 ₂ C _k F _2k+ i)(S0 ₂ C _m F _2m+ i)(S0 ₂ C _n F _2n+ i) wherein k=l-10, m=l-10 and n=l-10, LiN(S0 ₂ C _p F _2p S0 ₂ ) and LiC(S0 ₂ C _p F _2p S0 ₂ )(S0 ₂ C _q F _2q+ i) wherein p=l-10 and q=l-10, lithium perchlorate (LiC10 ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), lithium hexafluoroantimonate (LiSbF ₆ ), lithium hexafluorotantalate (LiTaF ₆ ), lithium tetrachloroaluminate (L1AICI ₄ ), lithium tetrafluoroborate (L1BF ₄ ), lithium chloroborate (Li ₂ B ₁₀ Cl ₁₀ ), lithium fluoroborate (Li ₂ B ₁₀ F ₁₀ ), Li ₂ Bi ₂ F _x Hi ₂ - _x wherein x=0-12 ; LiPF _x (R _F )6- _x and LiBF _y (R _F )4- _y wherein R _F represents perfluorinated Ci-C ₂ o alkyl groups or perfluorinated aromatic groups, x=0-5 and y=0-3, LiBF ₂ [0 ₂ C(CX ₂ ) _n C0 ₂ ], LiPF ₂ [0 ₂ C(CX ₂ ) _n C0 ₂ ] ₂ , LiPF ₄ [0 ₂ C(CX ₂ ) _n C0 ₂ ] wherein X is selected from the group consisting of H, F, CI, C ₁ -C ₄ alkyl groups and fluorinated alkyl groups, and n=0-4, lithium salts of chelated orthoborates and chelated orthophosphates such as lithium bis(oxalato)borate [LiB(C ₂ 0 ₄ ) ₂ ], lithium bis(malonato)borate [LiB(0 ₂ CCH ₂ C0 ₂ ) ₂ ], lithium bis(difluoromalonato) borate [LiB(0 ₂ CCF ₂ C0 ₂ ) ₂ ], lithium (malonatooxalato) borate

[LiB(C ₂ 0 ₄ )(0 ₂ CCH ₂ C0 ₂ )], lithium (difluoromalonatooxalato) borate

[LiB(C ₂ 0 ₄ )(0 ₂ CCF ₂ C0 ₂ )], lithium tris(oxalato) phosphate [LiP(C ₂ 0 ₄ ) ₃ ], lithium tris(difluoromalonato) phosphate [LiP(0 ₂ CCF ₂ C0 ₂ ) ₃ ], lithium difluorophosphate (LiP0 ₂ F ₂ ), and mixtures thereof.

The preferred lithium salts are lithium trifluoromethane sulfonate

(LiCF ₃ S0 ₃ ), lithium hexafluorophosphate (LiPF ₆ ), lithium

bis(fluorosulfonyl)imide Li(FS0 ₂ ) ₂ N (LiFSI), LiN(S0 ₂ C _m F _2m+ i)(S0 ₂ C _n F _2n+ i) and LiC(S0 ₂ C _k F _2k+ i)(S0 ₂ C _m F _2m+ i)(S0 ₂ C _n F _2n+ i) wherein k=l-10, m=l-10 and n=l-10, and LiN(S0 ₂ C _p F _2p S0 ₂ ) and LiC(S0 ₂ C _p F _2p S0 ₂ )(S0 ₂ C _q F _2q+ i) wherein p=l-10 and q=l-10, which may be used alone or in combination.

The concentration of the lithium salt(s) generally ranges from 0.1 to 3 mol per liter of the electrolyte composition and is typically about 1 mol per liter of the electrolyte composition.

It has been found surprisingly that the use of an electrolyte composition, as described above, in an electrochemical cell having an anode comprising lithium metal allows reducing or even eliminating the growth of dendrites at the surface of the anode. It also helps in stabilizing the solid electrolyte interface SEI layer on the anode surface As a result, the cycling ability of the electrochemical cell is enhanced.

Anode (negative electrode):

In the present invention, the term "anode" is intended to denote, in particular, the electrode of an electrochemical cell, where oxidation occurs during discharging. An anode comprises an anode active maerial which is capable of storing and releasing lithium ions. Examples of suitable anode electrochemically active materials comprise, but not limited to, lithium metal and lithium alloys. Lithium alloys include lithium-aluminum alloys, lithium-lead alloys, lithium-silicon alloys, lithium-tin alloys, LiZn, Li ₃ Bi, Li ₃ Cd, Li ₃ Sb and combinaisons thereof.

The lithium metal and the lithium alloy may be in the form of a foil, a rod or a mesh. The foil and the mesh may have a thickness typically ranging from 10 μιη to 500 μιη, preferably from 10 μιη to 400 μιη, and more preferably from 20 μιη to 300 μιη. In one embodiment, the thickness is about 20 μιη. In another embodiment, the thickness is about 300 μιη.

Cathode (positive electrode):

In the present invention, the term "cathode" is intended to denote, in particular, the electrode of an electrochemical cell, where reduction occurs during discharging. The cathodic active material is not particularly limited. It can be any cathodic active material known in the art of lithium electrochemical cells. It can be a lithium transition metal oxide (LiM0 ₂ , where M is at least one transition metal), a lithium transition metal phosphate (LiMP0 ₄ , where M is at least one transition metal) or a lithium transition metal fluorosilicate (LiM-SiO- F _y , where M is at least one transition metal).

Lithium transition metal oxides contain at least one metal selected from the group consisting of Mn, Co, Cr, Fe, Ni, V, and combinations thereof. For example, the following lithium transition metal oxides may be used in the cathode: LiaCo0 ₂ (0.5<a<1.3), LiaMn0 ₂ (0.5<a<1.3), LiMn ₂ 0 ₄ (0.5<a<1.3), Li ₂ Cr ₂ 0 ₇ , Li ₂ Cr0 ₄ , LiaNi0 ₂ (0.5<a<1.3), LiFe0 ₂ , LiaNii_ _x Coi_ _x 0 ₂ where

0.5<a<1.3, 0<x<l, LiaCoi_ _x Mn _x 0 ₂ , where 0.5<a<1.3, 0<x<l, LiaNii_ _x Mn _x 0 ₂ where 0.5<a<1.3, 0<x<l, which includes LiMno.5Nio.5Ch, LiMco.5Mn1.5O4, wherein Mc is a divalent metal, and LiNi _x Co _y Me _z 0 ₂ wherein Me may be one or more of Al, Mg, Ti, B, Ga, and Si and 0<x,y,z<l .

In one embodiment, the cathodic electrochemically active material is a compound having the formula Li _a (Ni _x Mn _y Co _z )0 ₄ , where 0.5<a<1.3 ; 0<x<2 ; 0<y<2 ; 0<z<2 and x+y+z=2.

A first preferred cathodic electrochemically active material is a compound having the formula: Li _a M0 ₂ , where M refers to Ni _x Mn _y Co _z M' _t where 0.5<a<1.3 ; x>0 ; y>0 ; z>0 ; t>0 and x+y+z+t=l; M' being selected from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Fe, Cu, Zn, Y, Zr, Nb, Mo or a mixture thereof.

In one embodiment, a=l, t=0 and x=l/3, y=l/3 and z=l/3. In one embodiment, a=l, t=0, x=0.8, y=0.1 and z=0.1.

In one embodiment, a=l, t=0, x=0.6, y=0.2 and z=0.2.

A second preferred cathodic electrochemically active material is a spinel type compound having formula Li _a Mn ₂ - _x M _x 0 ₄ where M is selected from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo; and 0.5 < a < 1.3, 0 < x < 2. In one embodiment, M is Ni, a=l , 0 < x < 0.7, preferably 0 < x < 0.5.

According to one embodiment, the electrochemical cell further comprises at least one cathode the electrochemically active material of which is selected from the group consisting of :

- LiaNi _x Mn _y Co _z 0 ₂ , where x+y+z=l and 0.5<a<1.3;

- LiaCo0 ₂ , where 0.5<a<1.3; and

- LiaMn ₂ _ _x Ni _x 0 ₄ , where 0<x<0,5 and 0.5<a<1.3.

Lithium transition metal phosphate encompasses compounds of formula LiaMP0 ₄ where 0.5<a<l .3 and M is selected from the group consisting of Fe, Mn, Co, Ni, Cu, Zn, Mg, Cr, V, Mo, Ti, Al, Nb and Ga. One example is LiMn _x Mc _y P0 ₄ , where Mc may be one metal selected from Fe, V, Ni, Co, Al, Mg, Ti, B, Ga, or Si and 0<x,y<l .

A possible cathodic active material is a compound having the formula xLiM0 ₂ . ₍ i- _x) Li ₂ M ₃ , where 0<x<l, M includes at least one metal element having an average oxidation number of +3 and includes at least one Ni element, and M' includes at least one metal element having an average oxidation number of +4.

Furthermore, transition metal oxides such as Mn0 ₂ and V ₂ 0 ₅ , transition metal sulfides such as FeS ₂ , MoS ₂ , and TiS ₂ , and conducting polymers such as polyaniline and polypyrrole may be used.

The structure of the cathode described herein is not particularly limited. The cathode is typically obtained by disposing the cathode electrode material on a current collector. To improve the adhesion of the particles of active material therebetween and the adhesion of these particles to the current collector, the cathode electrode material is generally mixed with a binder. Further, a conductive carbon is generally added in order to improve the electrode electrical conductivity. A cathode paste is thereby obtained.

The binder and the conductive carbon are known in the art. Suitable binders include, polyvinylidene fluoride (PVDF), styrene-butadiene rubber

(SBR), cellulose, polyamide, melamine resin or a mixture thereof. Binders made of polyvinylidene fluoride are preferred. A commercially available PVDF binder is Solef®5130. Depending on the characteristics of the binder, the binder is preferably present an amount of 1 to 9 wt% based on the total weight of the cathode paste. The binder is preferably present in the cathode paste in an average amount of 5 wt% or less based on the total weight of the cathode paste.

The conductive carbon is not particularly limited. Suitable conductive carbons include acetylene black. A commercially available carbon black is Super P® available from Alfa Aesar. Depending on the characteristics of the conductive carbon, the conductive carbon is preferably present in an amount of 1 to 10 wt% based on the total weight of the cathode paste. The conductive carbon is preferably present in an average amount of 5 wt% or less based on the total weight of the cathode paste.

The cathode current collector is a metallic foil, preferably made of aluminum or of an aluminum alloy.

Separator

It is well known that lithium electrochemical cells generally contain a separator between the anode and the cathode. A typical separator is a porous film made of polyethylene (PE), polypropylene (PP), or a composite film made of PE and PP layers, or cellulose fibers. The separator can also be made of a polyester, for example polyethylene terephthalate (PET). The porous separator functions to prevent short-circuit between the anode and the cathode.

The general structure and the methods of making a cell having lithium as an anode material are well known. The cell may be prismatic, cylindrical and coin-type. Other formats are also possible. The electrochemical cell according to the invention may belong to one of the following types of cells : lithium metal, lithium- sulfur and lithium-air cells.

One or more electrochemical cells according to the invention may be fitted with devices, for example a case, terminals, marking, bus bars and protective devices. The assembly formed by the cell(s) and the devices is a battery.

The electrochemical cell and the battery according to the invention exhibit a long life when used in cycling conditions. They are thus well suited as a source of electric energy in an electric vehicle.

EXAMPLES

The coin electrochemical cells of the 2032-type were prepared for the Inventive Examples of E1-E10 and Comparative Examples of CE1-CE8. Their constituents are summarized in the Tables 1 and 2 below : [Table 1]

Inventive

Solvent mixture Additive(s) Anode C rate Examples

FEC/DFEA

El (25/75)* - (22.16/66.49)**

FEC/DFEA

LiBOB/PS

E2 (25/75)*

(0.83/1.94)**

(21.55/64.65)**

FEC/DFEA

LiDFOB

E3 (25/75)*

(0.84)**

(21.98/65.93)** Li metal foil

0.5 C

FEC/DFEA in 20 μτη

LiDFOB/PS

E4 (25/75)*

(0.83/1.94)**

(21.55/64.65)**

FEC/DFEA

VC

E5 (25/75)*

(1.96)**

(21.73/65.18)**

FEC/DFEA

VC/PS

E6 (25/75)*

(1.95/0.49)**

(21.62/64.87)**

FEC/DFEA/PC

E7 (15/75/10)* - (13.27/66.35/8.85)** Li metal foil

1.0 C

FEC/DFEA/PC in 300 μτη

LiBOB/MA/PES

E8 (15/75/10)*

(0.83/0.49/1.46)**

(12.90/64.51/8.60)**

FEC/DFEA/DME

E9 (24.45/73.35/2.21)* - (21.73/65.18/1.96)** Li metal foil

0.5 C

FEC/DFEA/DME in 20 μτη

E10 (23.67/71/5.34)* - (21.11/63.32/4.76)** [Table 2]

* : wt% with respect to the total weight of the solvent mixture

** : wt% with respect to the total weight of the electrolyte composition

*** : parts in volume

FEC : monofluoroethylene carbonate

DFEA : 2,2-difluoroethyl acetate

PC : propylene carbonate DME : dimethoxy ethane

EC : ethylene carbonate

EMC : ethyl methyl carbonate

DMC : dimethyl carbonate

LiBOB : lithium (bisoxalatoborate)

LiDFOB : lithium difluorooxalato borate

VC : vinylene carbonate

PS : 1,3-propanesultone

PES : prop-l-ene-l,3-sultone (1 ,3,2-dioxathiane 2,2-dioxide)

MA : maleic anhydride

Li salt : LiPF ₆ (lithium hexafluorophosphate) in 1 mol.L ^"1

Anode : Li metal foil having a thickness of 20 μιη or 300 μιη

Cathode : LiNii/3Mni/ ₃ Coi/3C>2

A/ Formulation of the electrolyte compositions :

The electrolyte composition was prepared by mixing the different compounds using a magnetic stirrer.

LiPF ₆ was added to the solvent mixture of FEC and DFEA to prepare the electrolyte composition of El .

When preparing the electrolyte composition of E2, LiBOB was first dissolved in a solvent mixture of FEC and DFEA. After LiBOB was completely dissolved, LiPF ₆ and PS was added to the LiBOB solution.

When preparing the electrolyte composition of E3 and E4, LiDFOB was first dissolved in a solvent mixture of FEC and DFEA. After LiDFOB was completely dissolved, LiPF ₆ was added to the LiDFOB solution without (E3) or with PS (E4).

When preparing the electrolyte composition of E8 or CE1 , PES was first dissolved in a portion of DFEA (Solution A). Subsequently, LiBOB was then dissolved in a solvent mixture of FEC/DFEA/PC (Solution B). After LiBOB was dissolved completely, Li salt and then MA were added to the Solution B. Finally, Solution A and Solution B were mixed. When preparing the electrolyte composition of CE4, all the required compounds were added to one bottle and they were mixed until a transparent solution was obtained.

B/ Assembly of the coin cells :

1- Preparation of the electrodes:

1.1 Cathode The cathodic active material was a lithium nickel cobalt manganese oxide of formula LiNii _/3 Mni _/3 Coi _/3 0 ₂ . This cathodic active material was mixed with a conductive carbon and a binder to form a positive paste. The conductive carbon was carbon black (Super-P®). The binder was made of polyvinylidene fluoride (Solef®5130). The cathodic active material, the conductive carbon and the binder accounted respectively for 95 wt%, 3 wt% and 2 wt% of the total weight of the cathode paste. The cathode paste was deposited on an aluminum current collector at a loading level of 3.0 ± 0.1 mAh/cm ² to form a cathode. The cathode was vacuum dried at 100 °C overnight.

1.2 Anode

The anode was made of a lithium metal foil having a thickness of either 20 μιη or 300 μιη.

2- Assembly of the coin cells:

A separator made of a polyethylene sheet having a 20 μιη thickness was placed between the cathode and the anode. This assembly was cut in the dimensions corresponding to a 2032-type coin cell. The amount of electrolyte composition injected in each cell was 200 μί.

C / Electrical tests - Capacity retention:

The cycling ability of each cell was evaluated. Each cell was first subjected to an electrical test comprising a series of about 15 charge-discharge cycles carried out at different currents for the purpose of measuring the rated capacity C of the cell. Then, each cell was subjected to a repetition of cycles of charge and discharge. One cycle consisted in a charging phase at a charging current of C followed by a discharge phase at a discharge current of C. The following results were obtained as shown in the Tables 3 and 4 below :

[Table 3]

Figures 1-5 show the variation of the capacity retention and the Coulombic efficiency of E1-E10 and CE1-CE8 as a function of the cycle number.

Notably, it was observed that the discharged capacity of E1-E6, all according to the present invention, decreaseed slowly as the number of cycles increased (Figure 1). In particular, Figure 3 clearly shows that the the number of cycles at 80% of capacity retention for Inventive Examples, i.e., El and E5, each comprising the electrolyte composition according to the invention with or without VC, were much higher than those for Comparative Examples, i.e., CE3, CE4, CE6 and CE7, all with VC as a film- forming additive. Figure 4 also shows similar results, with both VC and PS as film- forming additives, notably between E6 and CE5.

Among Inventive Examples of E1-E6, E9 and E10, the number of cycles at 80% of capacity retention of El was the lowest, i.e., 57. However, such lowest number of 57 was much higher than the number of cycles of Comparative Examples of CE2-CE8. The highest number of cycles at 80% of capacity retention among CE2-CE8 was 38 from CE3. It was hence clearly demonstrated that the cycling ability was improved according to the present invention.

Similarly, the number of cycles at 80% of capacity retention showed even bigger difference between the Inventive Examples of E7 and E8 and the Comparative Examples CE1 and CE2, with Li metal foil having 300 μιη of thickness and 1.0 C of C rate.

Further, one can note that the coulombic efficiency of Inventive Examples shown on the right ordinates of Figures 1-5 remained essentially constant up to at least 40 cycles and decreased as the cycles increased, but very slowy, whereas the coulombic efficiency of Comparative Examples decreased rapidly around 40 cycles.