FR3032560B1 - ELECTROLYTE FOR LITHIUM ION BATTERY COMPRISING A PARTICULAR IONIC LIQUID - Google Patents

Abstract

The subject of the invention is an electrolyte comprising: - lithium hexafluorophosphate, - a mixture of solvents comprising ethylene carbonate, methyl and ethyl carbonate and dimethyl carbonate, and - 1-butyl 1-methylpyrrolidinium bis (trifluoro-methanesulfonyl) imide.

Description

Electrolyte for lithium-ion battery comprising a particular ionic liquid

The invention relates to the general field of rechargeable lithium-ion (Li-ion) batteries.

The invention more specifically relates to electrolytes for Li-ion batteries comprising a positive electrode based on platy laminated oxide, a negative electrode based graphite and a separator.

The invention also relates to a method for preparing lithium-ion batteries.

Finally, the invention relates to a method for cycling lithium-ion batteries with moderate capacities for improving the life of a Li-ion battery cell and the stability of the average voltage of said cell.

Conventionally, the Li-ion batteries comprise one or more positive electrodes, one or more negative electrodes, an electrolyte and a separator composed of a porous polymer or any other suitable material in order to avoid any direct contact between the electrodes.

Li-ion batteries are increasingly being used as an autonomous power source, particularly in applications related to electric mobility. This trend can be explained in particular by densities of mass and volume energy which are much higher than those of conventional nickel cadmium (Ni-Cd) and nickel-metal hydride (Ni-MH) accumulators, a lack of memory effect, a low self-discharge compared to to other accumulators and also by a drop in costs per kilowatt-hour related to this technology.

The electrolytes generally used in Li-ion batteries include one or more lithium salt (s) and one or more solvent (s).

The most common lithium salt is an inorganic salt, namely lithium hexafluorophosphate (LiPF ₆ ). Other inorganic salts are suitable and may be chosen from L1CIO4, LiAsF _6, or Lil L1BF4. Organic salts are also suitable and may be selected from lithium bis [(trifluoromethyl) sulfonyl] imide (LiN (CF ₃ SO ₂ ) ₂ ), lithium trifluoromethanesulfonate (L1CF3SO3), lithium bis (oxalato) borate (LiBOB), lithium fluoro (oxolato) borate (LiFOB), lithium difluoro (oxolato) borate (LiDFOB), lithium bis (perfluoroethylsulfonyl) imide (LiN (CF ₃ CF ₂ SO ₂ ) ₂ ), L 1 CH ₃ SO ₃ , LiR _F SOSR _F , LiN (R _F SO ₂ ) ₂ , LiC (R _F SO ₂ ) 3, R _f being a group selected from a fluorine atom and a perfluoroalkyl group having between one and eight carbon atoms.

The lithium salt (s) are preferably dissolved in one or more solvents chosen from aprotic polar solvents, for example, ethylene carbonate (denoted "EC"), propylene carbonate (denoted "PC "), Dimethyl carbonate (denoted" DMC "), diethyl carbonate (denoted" DEC ") and methyl and ethyl carbonate (denoted" EMC ").

Among this list of solvents, ethylene carbonate is the essential solvent for the formation of a solid and stable layer called "Solid Electrolyte Interphase" (SEI) on the surface of the negative electrode. This SEI is an essential element for the proper functioning of the Li-ion battery, although it is responsible for the large irreversible capacity observed during the first cycle, because it not only conducts lithium ions very well but it also has the advantage of stop the catalytic decomposition of the solvent.

Moreover, the development of materials for positive electrode based on platy oxide overlithiated, generally represented by the formula xLiMO ₂ (lx) Li ₂ MnO 3, x being between 0 and 1, appeared interesting thanks to their specific discharge capacities high, typically up to 250 mAh / g, during a very large number of charging and discharging cycles at high voltages (greater than 4.2 V).

The so-called "activation" process, necessary for the proper functioning of a battery, of the overlited laminated oxide-based positive electrode material generally takes place during the initial cycles of a cycling process of a Li-ion battery. at voltages greater than 4.5 V. This "activation" is in particular accompanied by a release of oxygen in the structure of the materials for positive electrode and phase transformation of these materials.

Nevertheless, the application of this high voltage represents a major problem for Li-ion batteries.

In fact, the electrolytes generally used, namely LiPF ₆ dissolved in one or more carbonate solvents, as described above, are not thermodynamically stable at these voltages, which leads to a rapid degradation of the battery cell L1. ion.

Thus, an alternative electrolyte must be proposed to answer this problem.

In addition, problems related to the safety of Li-ion batteries have been raised due to the storage of an ever greater amount of energy in smaller volumes.

This problem is aggravated by the large scale implementation of Li-ion batteries. A short circuit can easily lead to spontaneous combustion and an explosion of the battery.

Moreover, the decomposition of the SEI at the negative electrode at an elevated temperature, of the order of 100 ° C., is followed by an exothermic interaction with electrolytes generally used.

Thus, it would be advantageous to provide a particular electrolyte for improving the thermal stability of the various components of a Li-ion battery. This thermal stability is indeed essential for the proper functioning of said batteries.

Recently, ionic liquids have been considered as an electrolyte component. Indeed, their good electrochemical properties or their low volatilities compared to carbonate solvents can make it an excellent candidate to improve thermal stability.

Thus, an electrolyte comprising N-butyl-N-methylpyrrolidinium bis (fluorosulfonyl) imide (Pyr14 FSI) as the solvent and lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) as the lithiated salt was developed for lithium batteries. as indicated in the publication entitled "Improved electrochemical performance of L1MO2 (M = Mn, Ni, Coj-IAMnCh cathode materials in ionic liquid-based electrolyte" in Journal of Power Sources, 239 (2013) 490-495.

Another electrolyte composition has been developed as the document "Lithium bis (fluorosulfonyl) imide-Pyr 14 TFSI ionic liquid compatible electrolyte with graphite" in Journal of Power Sources, 196 (2011) 7700-7706, describes it. It comprises 1-butyl-1-methylpyrrolidinium bis (trifluoromethanesulfonyl) imide (PyrI 4 TFSI) as a solvent and lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) as the lithiated salt.

The document "The effects of N-methyl-N-butylpyrrolidinium bis (trifluoromethylsulfonyl) imide-based electrolyte on the electrochemical performance of high capacity method material Li [Lio.2Mno.54NiO.i3Coo.i3] 02" in Electrochimica Acta, 59 ( 2012) 14-22, discloses an electrolyte comprising LiPF ₆ dissolved in a mixture of carbonate solvents comprising ethylene carbonate and dimethyl carbonate in proportions of 1/1 volume in the presence of N-methyl-N-butylpyrrolidinium. bis (trifluoromethylsulfonyl) imide. More specifically, this document describes an electrolyte comprising at least 40% by volume of N-methyl-N-butylpyrrolidinium bis (trifluoromethylsulfonyl) imide relative to the total volume of the electrolyte.

Application US 2014/0134501 describes an electrolyte comprising one or more ionic salts, at least one SEI-forming agent, at least one fluorinated compound and one or more non-aqueous solvent (s).

The application US 2010/0028785 relates to an electrolyte comprising an organic solvent, a lithiated salt, an ionic liquid and an additive.

The Applicant has surprisingly discovered that a Li-ion battery electrolyte comprising a platy-laminated oxide-based positive electrode, a graphite-based negative electrode and a separator, whose composition comprises hexafluorophosphate. of lithium (LiPF ₆ ) dissolved in a solvent mixture comprising ethylene carbonate (EC), methyl and ethyl carbonate (EMC) and dimethyl carbonate (DMC), and 1-butyl-1 Methylpyrrolidinium bis (trifluoromethanesulfonyl) imide (PyrI4 TFSI) allowed to improve the thermal stability of the battery. Obtaining good electrochemical performance is also observed thanks to this particular electrolyte.

The subject of the invention is therefore an electrolyte comprising lithium hexafluorophosphate, a mixture of solvents comprising ethylene carbonate, methyl and ethyl carbonate and dimethyl carbonate, and 1-butyl-1 methylpyrrolidinium bis (trifluoromethanesulfonyl) imide.

The invention also relates to a method for preparing the electrolyte according to the invention and the use of said electrolyte for a Li-ion battery.

Another object of the invention is a Li-ion battery comprising the electrolyte according to the invention.

The subject of the invention is also a process for preparing a Li-ion battery cell comprising the electrolyte according to the invention as well as a method for manufacturing a Li-ion battery.

Finally, the invention also relates to a particular cycling method for batteries comprising an electrolyte according to the invention.

Other advantages and features of the invention will appear more clearly on examining the detailed description and the attached drawings in which:

FIG. 1 is a graph comparing the specific discharge capacities of Li-ion battery cells comprising a platy-oxide surlithiated positive electrode, a graphite-based negative electrode and different electrolyte compositions, as a function of the number of FIG. 2 is a graph comparing the average discharge voltages of Li-ion battery cells comprising a platy-laminated oxide-based positive electrode, a graphite-based negative electrode, and various electrode compositions. electrolytes, as a function of the number of charging and discharging cycles, FIG. 3 is a graph comparing the different impedances of Li-ion battery cells comprising a positive electrode based on platy-laminated oxide, a negative electrode based on graphite. and different electrolyte compositions, measured at room temperature and at a voltage of 3.75V, the FIG. 4 is a graph comparing heat fluxes of laminate oxide-based positive electrode materials in a charged state of 4.7 V in the presence of different electrolyte compositions as a function of temperature.

In the description of the invention, the term "based on" is synonymous with "comprising predominantly".

It is furthermore specified that the expressions "between ... and ..." and "from ... to ..." used in the present description must be understood as including each of the mentioned terminals.

Li-ion batteries generally include a positive electrode, a negative electrode, a separator between the electrodes and an electrolyte.

The electrolyte according to the invention comprises:

- lithium hexafluorophosphate,

a solvent mixture comprising ethylene carbonate, methyl and ethyl carbonate and dimethyl carbonate, and

1-butyl-1-methylpyrrolidinium bis (trifluoromethanesulfonyl) imide.

In a preferred embodiment, the electrolyte according to the invention comprises from 0.1 to 50% by volume, preferably from 15 to 25% by volume, of 1-butyl-1-methylpyrrolidinium bis (trifluoromethanesulfonyl) imide relative to to the total volume of said electrolyte.

According to one particular embodiment of the invention, the solvent mixture comprises ethylene carbonate, methyl and ethyl carbonate and dimethyl carbonate in volume proportions of 1/1/1, that is, ie in an identical volume for each solvent.

Preferably, the electrolyte according to the invention further comprises lithium difluoro (oxolato) borate.

Advantageously, the weight percentage of lithium difluoro (oxolato) borate is between 0.005 and 10% relative to the total weight of the electrolyte.

The subject of the invention is also a process for preparing the electrolyte according to the invention for a Li-ion battery comprising a platy-oxide layer-based positive electrode material and a graphite-based negative electrode material, said hexafluorophosphate. of lithium and said 1-butyl-1-methylpyrrolidinium bis (trifluoromethanesulfonyl) imide being dissolved in a solvent mixture comprising ethylene carbonate, methyl and ethyl carbonate and dimethyl carbonate.

Another object of the invention is the use of the electrolyte according to the invention for a Li-ion battery.

The invention also relates to a Li-ion battery comprising the electrolyte according to the invention.

The Li-ion battery, comprising the electrolyte according to the invention, comprises a material for positive electrode based platy oxide overlithiated. Said laminate oxide surlithiated positive electrode material comprises an active material which is generally a lithiated metal oxide selected from nickel, cobalt and / or manganese and optionally another doping metal.

The ophthalmic platy-oxide-based positive electrode active material is of the formula Li 1 + _x (M _a Db) 1- _x O 2, wherein M represents a metal or several metals selected from nickel, manganese and cobalt, x is between 0.01 and 0.33. When b is between 0 and 0.05 and a + b = 1, then D is a member selected from

Na, Zn, Cd, Mg, Ti, Ca, Zr, Sr, Ba, Al or K or a mixture of these elements.

Advantageously, the active material for positive electrode based platinum oxide overlithiated is the Εΐι ^ Νΐο, ιΜηο, όΟι.

In addition to the active material, the platy-laminated oxide-based positive electrode material may also include carbon fibers. Preferably, these are vapor phase growth carbon fibers (VGCF for "Vapor Grown Carbon Fibers") marketed by the company Showa Denko. Other types of suitable carbon fibers may be carbon nanotubes, doped nanotubes (possibly with graphite), carbon nanofibers, doped nanofibers (possibly with graphite), single-walled carbon nanotubes or multi-walled carbon nanotubes . Synthetic methods for these materials may include arc discharge, laser ablation, plasma torch, and chemical vapor phase decomposition.

The platy-laminated oxide-based positive electrode material may further comprise one or more binders.

Preferably, the binder (s) may be chosen from polybutadiene-styrene latices and organic polymers, and preferably from polybutadiene-styrene latices, polyesters, polyethers, methylmethacrylate polymer derivatives, polymeric derivatives of acrylonitrile, carboxyl methyl cellulose and its derivatives, polyvinyl acetates or polyacrylate acetate, polyvinylidene fluoride, and mixtures thereof.

Preferably, the binder is polyvinylidene fluoride (PVdF).

The Li-ion battery, comprising the electrolyte according to the invention, comprises an active material for negative electrode based on graphite. The graphite carbon may be chosen from synthetic graphite carbons, and natural from natural precursors followed by purification and / or post-treatment.

Other active carbon-based materials can be used such as pyrolytic carbon, amorphous carbon, activated carbon, coke, coal tar pitch and graphene. Mixtures of graphite with one or more of these materials are possible. Materials having a core-shell structure may be used when the core comprises high capacity graphite and when the shell comprises a carbon-based material protecting the core from degradation related to the repeated phenomenon of intercalation / deintercalation of lithium ions .

Advantageously, the negative electrode active material is graphite supplied by the company Hitachi (SMGHE2).

The graphite-based negative electrode material may further comprise one or more binders as for the positive electrode.

The binders described above for the positive electrode can be used for the negative electrode.

Preferably, the binders used are carboxyl methyl cellulose (CMC) and latex Styrofan®, that is to say a carboxylated styrene-butadiene copolymer.

The Li-ion battery, comprising the electrolyte according to the invention, also comprises a separator located between the electrodes. It plays the role of electrical insulation.

Several materials can be used as separators. The separators are generally composed of porous polymers, preferably polyethylene and / or polypropylene.

Advantageously, the separator used is the Celgard® 2500 separator, that is to say a microporous single-layer membrane with a thickness of 25 μm composed of polypropylene.

The subject of the invention is also a method for preparing a Li-ion battery cell, comprising the electrolyte according to the invention, comprising the following steps:

- assembling a cell by stacking a positive-laminated oxide-based positive electrode material, a graphite-based negative electrode material and a separator located between the two electrodes,

impregnation of the separator with the electrolyte as previously described.

Preferably, the platy-laminated oxide-based positive electrode material comprises the active material of the formula Li 1 NioyMnO 2 -CU.

Preferably, the graphite-based negative electrode material comprises graphite supplied by Hitachi (SMGHE2) as an active material.

Preferably, the separator is the Celgard® 2500 separator, i.e. a 25 μm monolayer microporous membrane composed of polypropylene.

The invention also relates to a method for manufacturing a Li-ion battery, comprising the electrolyte according to the invention, by assembling one or more cells such as previously prepared.

The invention also relates to a particular cycling process of a Li-ion battery comprising the electrolyte according to the invention comprising the following steps:

the first two activation cycles between a higher voltage (T _sup ) strictly greater than 4.40 V, preferably between 4.40 V excluded and 4.70 V, and a lower voltage (Tinf) between 1 , 60 and 2.50 V, preferably equal to 2 V,

the following charging and discharging cycles at voltages lying between a voltage T _sup of between 4.30 and 4.45 V, preferably equal to 4.40 V, and a voltage Ti _n f of between 2 and 2, 50 V, preferably equal to 2.30 V;

the cycles being carried out at a cycling speed of between C / 20 and 3C, C designating the cycling regime of the Li-ion battery.

In a preferred embodiment, the first two activation cycles are carried out at a C / 10 cycling rate.

In another preferred embodiment, the following charge and discharge cycles are performed at a C / 2 cycling rate.

The present invention is illustrated in a nonlimiting manner by the following examples.

Examples

Preparation of the positive electrode

1 L of water are added to a reactor, and the water is heated to a temperature between 50 and 70 ° C. 12.5 L of a solution of nickel sulphate and manganese sulphate (at a molar ratio of 1/3), the concentration of which is 0.8 mol / L, is injected continuously into the stirred reactor constant.

The pH of the reactor is regulated between 7 and 8.5 by adding a solution of sodium carbonate and an ammonia solution.

The appearance of a precipitate is observed during the injection. At the end of the injection, the reactor is kept under the same constant stirring and at the same temperature for a period of between 4 and 10 hours.

The precipitate is then separated from the liquid phase by filtration, then washed extensively with water and dried on a drying filter.

The material thus formed is then intimately mixed with lithium carbonate. The resulting mixture is then calcined at a temperature between 850 and 900 ° C for 24 hours.

The active material obtained is an ophthalmic lamellar oxide of formula Εΐι ^ Νΐο, ιΜηο, όΟι being in the form of a powder.

The positive electrode is prepared by mixing 86% by weight of active material, 6% by weight of a Super P® carbon additive, and 8% by weight of polyvinylidene fluoride.

The electrode is made by depositing the mixture on an aluminum foil. The electrodes are dried and calendered at 80 ° C.

The density of material for positive electrode material is 6.1 mg / cm ² . The area of the positive electrode used in the cell is 10.24 cm ² .

Preparation of the negative electrode

Active graphite material is provided by Hitachi (SMGHE2). The electrode is made by mixing 96% by weight of graphite, 2% by weight of carboxyl methyl cellulose (CMC) and 2% by weight of Styrofan latex, that is to say a carboxylated styrene butadiene copolymer.

The resulting mixture is deposited on a copper foil and then dried and calendered at 80 ° C.

The density of negative electrode material is 4.4 mg / cm ² . The area of the negative electrode used in the cell is 12.25 cm ² .

Separator

The Celgard® 2500 separator is used to prevent short circuits between the positive electrode and the negative electrode during charging and discharging cycles. The Celgard® 2500 separator is a 25 μm thick monolayer microporous membrane made of polypropylene.

Electrolyte

The 1-butyl-1-methylpyrrolidinium bis (trifluoromethanesulfonyl) imide (PyrI 4 TFSI) used is supplied by Fluka (Pyrl4 TFSI,> 98.5%).

Four electrolytes are used to carry out the comparative tests, whose compositions are reported in Table 1:

Electrolyte AT (comparative) B (comparative) C (invention) D (invention) LiPF ₆ (mol / L) 1 1 1 1 EC / EMC / DMC (volume ratio) 1/1/1 1/1/1 1/1/1 1/1/1 Pyrl4 TFSI (% volumetric) - - 20 20 LiDFOB (% by mass) - 0.3 - 0.3

Table 1

Electrolyte A is composed of 1M LiPF ₆ lithium salt dissolved in a mixture of ethylene carbonate, methyl carbonate and ethyl and dimethyl carbonate (EC / EMC / DMC) in a ratio of 1 / 1/1 by volume.

Electrolyte B is composed of 1M LiPF ₆ lithium salt dissolved in a mixture of ethylene carbonate, methyl carbonate and ethyl and dimethyl carbonate (EC / EMC / DMC) in a ratio of 1 / 1/1 by volume and 0.3% by weight of lithium difluoro (oxolato) borate (LiDFOB).

Electrolyte C is composed of 1M LiPF ₆ lithium salt dissolved in a mixture of ethylene carbonate, methyl carbonate and ethyl and dimethyl carbonate (EC / EMC / DMC) in a ratio of 1 / 1/1 by volume and 20% by volume of 1-butyl-1-methylpyrrolidinium bis (trifluoromethanesulfonyl) imide (PyrI 4 TFSI).

The electrolyte D is composed of 1M LiPF ₆ lithium salt dissolved in a mixture of ethylene carbonate, methyl carbonate and ethyl and dimethyl carbonate (EC / EMC / DMC) in a ratio 1 / 1/1 by volume, 20% by volume of 1-butyl-1-methylpyrrolidinium bis (trifluoromethanesulfonyl) imide (PyrI 4 TFSI) and 0.3% by weight of lithium difluoro (oxolato) borate (LiDFOB).

Cell

The cells are finally assembled by stacking the positive electrode, the negative electrode as prepared above, and the Celgard® 2500 separator, located between the two electrodes. The separator is impregnated with the electrolyte as previously described.

Thus, four cells, which will be named cell A, B, C or D, respectively comprising the electrolyte A, B, C or D are prepared. Cell A contains the reference electrolyte A.

Electrochemical performance of Li-ion battery cells

Evaluation of the specific discharge capacities according to the number of cycles

FIG. 1 represents a graph comparing the specific discharge capacities of Li-ion battery cells, each comprising a platy-oxide surlithiated positive electrode, a graphite-based negative electrode and having different electrolyte compositions, as a function of number of charge and discharge cycles. Battery cells A, B, C and D respectively contain the electrolytes A, B, C and D.

Method

A cycling process was used. The first two cycles, or activation cycles, took place between 4.6 and 2 V at a C / 10 cycling rate. The following charging and discharging cycles were conducted at reduced voltages between 4.4 and 2.3 V at a C / 2 cycling rate.

Result

Thus, Figure 1 clearly shows that the battery cell A (curve A) has a very unstable electrochemical behavior. A drop in electrochemical performance is observed and a specific discharge capacity of about 110 mAh / g is measured after about 650 cycles.

Battery cell B (curve B) exhibits improved electrochemical behavior with the measurement of a specific discharge capacity of about 130 mAh / g after 638 cycles.

The battery cell C (curve C) has a very good electrochemical behavior with the measurement of a specific discharge capacity of about 140 mAh / g after about 650 cycles.

Finally, the battery cell D (curve D) also has a very good electrochemical behavior with the measurement of a specific discharge capacity of about 155 mAh / g after about 650 cycles.

It is further noted that the specific discharge capacity for the cells A and B, increases between the 50 ^th and the 150 ^th cycle. This phenomenon is due to the process of "activation" of the positive electrode material which persists.

On the contrary, this phenomenon is only observed for the first 50 cycles for cells C and D. In addition to the 50 ^th cycle, Figure 1 shows that the electrochemical behavior is stable for cells C and D.

This observation in the electrochemical behavior is very important for the practical application of a cell. Indeed, cells capable of rapidly delivering a stable performance, and therefore an activation process completed as soon as possible, are sought.

It is clearly established that the presence of LiDFOB in an electrolyte has a beneficial effect since the electrochemical performances of cell B are better than those of reference cell A but remain insufficient.

It should be noted that the replacement of LiDFOB by Pyrl4 TFSI is even more beneficial because cell C delivers a faster stable performance and a higher specific discharge capacity than that of cell B. These results are very satisfactory.

The additional presence of the LiDFOB in addition to the Pyrl4 TFSI does not improve the result concerning the delivery of a faster stable performance. Nevertheless, from the ^550th cycle, the specific discharge capacity of cell D is better than that of cell C.

Evaluation of the average discharge voltages according to the number of cycles

FIG. 2 represents a graph comparing the average discharge voltages of the four Li-ion battery cells A, B, C and D as a function of the number of charge and discharge cycles.

The same cycling method as that described for Figure 1 is used.

One of the major problems encountered in positive electrode materials based on lamellar oxides is the lowering of the average discharge voltage of the cell with the charging and discharging cycles.

Figure 2 shows a very large discharge voltage drop for cells A and B with the measurement of an average discharge voltage respectively of 3.23 and 3.18 V after about 650 cycles.

Cells C and D, meanwhile, have a significantly lower average discharge voltage drop with the measurement of an average discharge voltage of about 3.3 V after about 650 cycles for both cells.

The beneficial effect of the presence of Pyrl4 TFSI on the electrochemical behavior of a Li-ion battery cell is thus once again clearly established.

Evaluation of the impedance of the Li-ion battery cells

FIG. 3 is a Nyquist graph comparing the different impedances of the Li-ion battery cells A, B, C and D, measured at ambient temperature and at a voltage of 3.75V.

It should be noted that the diameter of the semicircles characterizing cells A and B is significantly larger than that of the half-circles characterizing cells C and D.

Thus, an SEI sufficiently strong and stable to withstand the exposure of a high voltage, seems to have formed with respect to cells C and D in contrast to cells A and B.

Evaluation of thermal properties of positive electrode materials based on lamellar oxides in the presence of electrolyte

FIG. 4 is a graph comparing heat fluxes of positive electrode materials based on overlapped layered oxides, in a charged state of 4.7 V, in the presence of electrolytes A, C and D, as a function of temperature.

The heat fluxes are measured by differential scanning calorimetry. The apparatus used is the DSC 404 Fl Pegasus® marketed by NETZSCH. The rise in temperature of the measured sample is carried out at 5K / min.

Migration of lithium ions from the positive electrode material during cell operation causes significant heat generation and is the primary source of thermal runaway of the cell.

Figure 4 shows that the heat released for the positive electrode material:

in the presence of the reference electrolyte A is 1043 J / g,

in the presence of the reference electrolyte C is 888 J / g,

in the presence of the reference electrolyte D is 747 J / g

Thus, the heat generation rate between 150 and 250 ° C is reduced by about 15% for the positive electrode material in the presence of the electrolyte C relative to the positive electrode material in the presence of the electrolyte A.

The heat generation rate between 150 and 250 ° C is reduced by about 28% for the positive electrode material in the presence of the electrolyte D relative to the positive electrode material in the presence of the electrolyte A.

The beneficial effect of the presence of Pyrl4 TFSI in an electrolyte on the thermal properties of a Li-ion battery cell is thus clearly established.

Claims (13)

1. Electrolyte comprising: - lithium hexafluorophosphate, a mixture of solvents comprising ethylene carbonate, methyl and ethyl carbonate and dimethyl carbonate, 1-butyl-1-methylpyrrolidinium bis (trifluoromethanesulfonyl) imide, and lithium difluoro (oxolato) borate . 2. Electrolyte according to claim 1, characterized in that said electrolyte comprises from 0.1 to 50% by volume, preferably from 15 to 25% by volume, of 1-butyl-1-methylpyrrolidinium ΒΪ8 (ΐπΑυο ^ πιόί1ΐηπ68η1Γοηγ1) ίιη1ά6 relative to the total volume of said electrolyte. 3. Electrolyte according to one of claims 1 or 2, characterized in that the solvent mixture comprises ethylene carbonate, methyl and ethyl carbonate and dimethyl carbonate in proportions by volume of 1/1 / 1. 4. Electrolyte according to any one of the preceding claims, characterized in that the weight percentage of lithium difluoro (oxolato) borate is between 0.005 and 10% relative to the total weight of the electrolyte. 5. A method for preparing an electrolyte as defined in any one of the preceding claims, for a Li-ion battery comprising a platy-oxide-surlithieed positive electrode material and a graphite-based negative electrode material, characterized in that said lithium hexafluorophosphate, 1-butyl-1-methylpyrrolidinium bis (trifluoromethanesulfonyl) imide and said lithium difluoro (oxolato) borate are dissolved in a solvent mixture comprising ethylene carbonate, methyl and ethyl carbonate and dimethyl carbonate. 6. Use of the electrolyte as defined in any one of claims 1 to 4 for a Li-ion battery. 7. A Li-ion battery comprising a platy-oxide-surlithiated oxide-based positive electrode material, a graphite-based negative electrode material, and a separator, characterized in that it comprises an electrolyte as defined in any one of: Claims 1 to 4, 8. Battery according to claim 7, characterized in that said laminated oxide-based surlithie positive electrode material comprises an active material of formula Li _{J + x} (M _a D _b ) ₁ . _x O ₂ , in which: M represents a metal or several metals chosen from nickel, manganese and cobalt, x is between 0.01 and 0.33, when b is between 0 and 0.05 and a + b = 1, then D is an element selected from Na, Zn, Cd, Mg, Ti, Ca, Zr, Sr, Ba, Al or K or a mixture of these elements. 9. A process for preparing a Li-ion battery cell, characterized in that said method comprises the following steps: - assembling a cell by stacking a positive-laminated oxide-based positive electrode material, a graphite-based negative electrode material and a separator located between the two electrodes, impregnation of the separator with the electrolyte as defined in any one of claims 1 to 4. 10. A method of manufacturing Li-ion battery, characterized in that said method comprises a step of assembling one or more cell (s) as defined in claim 9. 11. A method of cycling a Li-ion battery as defined in one of claims 7 or 8, characterized in that said method comprises the following steps: first two activation cycles between a higher voltage (T _sup ) strictly greater than 4.40 V, preferably between 4.40 V excluded terminal and 4.70 V, and a lower voltage (T _inf ) between 1, 60 and 2.50 V, preferably equal to 2 V, the following charging and discharging cycles at voltages between a voltage T _sup of between 4.30 and 4.45 V, preferably equal to 4.40 V , and a voltage Tj _n f of between 2 and 2.50 V, preferably equal to 2.30 V; the cycles being carried out at a cycling speed of between C / 20 and 3C, C designating the cycling regime of the Li-ion battery. 12. The method of claim 11, characterized in that said first two activation cycles are performed at a C / 10 cycling regime. 13. Method according to one of claims 11 or 12, characterized in that said subsequent charge and discharge cycles are performed at a C / 2 cycling regime.