Patent ID: 12224400

DESCRIPTION OF EMBODIMENTS

The various constituents of an electrochemical cell according to the invention will be described below.

Cathode:

The cathode is a composite prepared from electrochemically-active solid elemental sulphur and non-electrochemically active compounds.

Solid elemental sulphur exists in different molecular forms. The preferred form is alpha sulphur Sα, of formula S8corresponding to cyclooctasulphur, which is the most thermodynamically-stable form.

Non-electrochemically active compounds comprise:carbon having a porous structure comprising pores with a mean diameter less than or equal to 6 nm andgenerally at least one electrical conductor compound and at least one binder.

The mean pore diameter may be less than or equal to 3 nm, or even less than or equal to 2 nm.

The carbon having a porous structure plays the role of electrical percolator. The pores of the porous structure house the elemental sulphur particles.

One possible method for the incorporation of the elemental sulphur particles into the pores of the porous structure of the carbon is as follows. The porous carbon is mixed with the solid elemental sulphur. Typically, the mass of the solid elemental sulphur represents from 30 to 80% or 55 to 65% of the sum of the masses of the solid elemental sulphur and carbon; The mass of the carbon typically represents from 70 to 20% or 45 to 35% of the sum of the masses of solid elemental sulphur and carbon.

The mixture is heated to a temperature close to 155° C. for approximately 5 hours under vacuum, in order to permit the sulphur molecules to penetrate into the open pores of the carbon. At around 155° C., the sulphur in the liquid state has its lowest viscosity.

The mixture is then heated under inert gas at a temperature of approximately 300° C. for approximately 30 minutes, which sublimates the sulphur and eliminates the excess. The product obtained is then generally mixed with at least one binder and at least one compound that is a good electrical conductor.

The binder may be chosen from carboxymethylcellulose (CMC), a butadiene-styrene copolymer (SBR), polytetrafluoroethylene (PTFE), polyamideimide (PAI), polyimide (PI), styrene-butadiene rubber (SBR), poly(acrylic acid) (PAA), polyvinyl alcohol, polyvinylidene fluoride (PVDF) and a mixture thereof.

The electrical conductor compound is generally carbon black.

An ink is obtained that is deposited on one or two faces of a current collector that can be an aluminum strip. The ink-coated current collector is laminated to adjust its thickness. A cathode is thus obtained.

A typical composition of the ink deposited on the current collector can be the following:from 30 to 80% by mass, preferably 50 to 55% by mass of solid elemental sulphur relative to the sum of the masses of the solid elemental sulphur, the carbon, the binder(s) and the electrical conductor compound(s);from 10 to 60% by mass, preferably 30 to 35% by mass of solid porous carbon relative to the sum of the masses of the solid elemental sulphur, the carbon, the binder(s) and the electrical conductor compound(s);from 3 to 8% by mass, preferably 5 to 7% by mass of binder relative to the sum of the masses of the solid elemental sulphur, the carbon, the binder(s) and the electrical conductor compound(s);from 2 to 7% by mass, preferably 3 to 5% by mass of electrical conductor compound relative to the sum of the masses of the solid elemental sulphur, the carbon, the binder(s) and the electrical conductor compound(s).

The quantity of elemental sulphur is measured in the composite by thermogravimetric analysis after treatment at 300° C.

Anode:

The active material of the anode is a strip made up of lithium metal or a metallic alloy of lithium.

Electrolyte:

The electrolyte is liquid and comprises one or more organic solvents, at least one of these organic solvents being a carbonate, in which one or more salts are dissolved, at least one of these salts being lithium bis(fluorosulfonyl)imide LiFSI of the formula:

The carbonate can be a cyclic or linear carbonate. The cyclic carbonate can be mono- or multi-substituted by one or more halogen atoms, such as fluorine.

Examples of cyclic carbonates are ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate (BC). Ethylene carbonate (EC) and propylene carbonate (PC) are preferred. The ethylene carbonate may be monosubstituted by fluorine (FEC). The electrolyte may also contain vinylene carbonate.

Examples of linear carbonates are dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and propyl methyl carbonate (PMC). Dimethyl carbonate (DMC) is preferred.

The percentage of carbonate(s) by volume can be greater than or equal to 50%, greater than or equal to 60%, greater than or equal to 70%, greater than or equal to 80% or greater than or equal to 90% of the sum of the organic solvent volumes. Preferably, the electrolyte does not contain any ester and/or does not contain any ether. More preferably still, the electrolyte does not contain any organic solvent other than one or more carbonates.

The electrolyte may comprise a first organic solvent that is a cyclic carbonate and a second organic solvent that is a linear carbonate. Preferably, the electrolyte does not contain any organic solvent other than one or more cyclic carbonate(s) and other than one or more linear carbonate(s).

The volume of said cyclic carbonate(s) can represent from 10 to 50% or 20 to 40% or 15 to 25% of the total volume of organic solvents; The volume of said at least one linear carbonate can represent from 90 to 50% or 80 to 60% or 85 to 75% of the total volume of organic solvents.

The cyclic carbonate may be ethylene carbonate, optionally monosubstituted by fluorine, and the linear carbonate may be dimethyl carbonate.

The electrolyte necessarily contains lithium bis(fluorosulfonyl)imide LiN(SO2F)2(LiFSI) but may also contain one or more other salts, such as lithium perchlorate LiClO4, lithium hexafluoroantimonate LiSbF6, hexafluorophosphate LiPF6, lithium tetrafluoroborate LiBF4, lithium hexafluoroarsenate LiAsF6, lithium trifluoromethanesulfonate LiCF3SO3, lithium bis(trifluoromethanesufonyl)imide LiN(CF3SO2)2(LiTFSI), lithium trifluoromethanesulfonemethide LiC(CF3SO2)3(LiTFSM), bisperfluoroethylsulfonimide LiN(C2F5SO2)2(LiBETI), lithium 4,5-dicyano-2-(trifluoromethyl) imidazolide (LiTDI), lithium bis(oxalatoborate) (LiBOB), lithium tris(pentafluoroethyl)trifluorophosphate LiPF3(CF2CF3)3(LiFAP) and mixtures thereof.

The percentage by mass of lithium bis(fluorosulfonyl)imide LiFSI may be greater than or equal to 50%, greater than or equal to 60%, greater than or equal to 70%, greater than or equal to 80%, greater than or equal to 90%, preferably equal to 100% of the sum of the masses of the salts dissolved in the organic solvent(s).

The lithium bis(fluorosulfonyl)imide LiFSI concentration in the electrolyte may be greater than or equal to 1 mol/L or less than or equal to 1 mol/L. It may also comprise between 0.1 and 3 mol/L, or between 0.5 and 1.5 mol/L or even be approximately 1 mol/L.

In order to prepare the electrolyte, the lithium bis(fluorosulfonyl)imide LiFSI salt is dissolved with, optionally, the other lithium salts in said at least one carbonate and, optionally, other organic solvents.

Separator:

A separator prevents electrical contact between an anode and a cathode but nevertheless allows ion transport between the electrodes. The separator material may be chosen from the following materials: a polyolefin, for example polypropylene and polyethylene, a polyester, glass fibers bound together by a polymer, polyimide, polyamide, polyaramid, polyamideimide and cellulose. The polyester may be chosen from polyethylene terephthalate (PET) and polybutylene terephthalate (PBT). Advantageously, the polyester or polypropylene or polyethylene contains or is coated with a material chosen in the group consisting of a metal oxide, a carbide, a nitride, a boride, a silicide and a sulfide. This material may be SiO2or Al2O3.

FIG.1is an exploded schematic sectional view of a stack of a cathode (2), a separator (3) and an anode (4) in an electrochemical cell (1) according to the invention. The cathode has a porous structure (5). The cathode pores house sulphur. A stable passivation layer (SEI for “solid electrolyte interface”) (6) is formed on the surface of cathode (2). It results from the reduction of the cathode chemical species contained in the electrolyte during discharge of the cell.

Preparation of the Electrochemical Bundle:

The electrochemical bundle is formed by interspersing a separator between at least one cathode and at least one anode. The electrochemical bundle is inserted in the cell container. The cell container may be in the parallelepiped or cylindrical format. In this latter case, the electrochemical bundle is coiled to form a cylindrical assembly of electrodes.

Filling the Container:

The container provided with the electrochemical bundle is filled with the electrolyte as described above.

Surprisingly, the Applicant observed that the association of a carbon cathode with a porous structure comprising pores with a mean diameter less than or equal to 6 nm in combination with an electrolyte comprising at least one carbonate and at least one lithium bis(fluorosulfonyl)imide (LiFSI) salt allowed significantly reducing the quantity of hydrogen sulfide that could be emitted in the event of accidental exposure of the cell.

Without wanting to be bound by the theory, the Applicant is of the opinion that during operation of the cell, long-chain lithium polysulfides, such as Li2S5or Li2S6remain confined in the pores of the carbon and are not present in the electrolyte. Furthermore, the Applicant is of the opinion that the chemical species located in the cathode passivation layer (SEI) in the cell according to the invention have a lower reactivity to water and that this lower reactivity is responsible for the lower emission of hydrogen sulfide.

In addition to the reduction of the quantity of hydrogen sulfide released, the invention allows reducing the quantity of electrolyte contained in the cell, indeed, it is customary when designing a lithium-sulphur cell to provide an excess of electrolyte. This excess serves to compensate for the increase in electrolyte viscosity induced by the dissolution of lithium polysulfides when the cell discharges. Remember that the discharge of a lithium/sulphur cell leads to the formation of lithium polysulfides that dissolve in the electrolyte. This dissolution leads to a reduction of the quantity of active cathode material and an increase of the electrolyte viscosity. The increase of electrolyte viscosity leads, in turn, to a reduction of ionic mobility, detrimental to the smooth functioning of the cell. In order to compensate for the increased viscosity of the electrolyte, it is customary when designing the cell to provide an excess of electrolyte. For example, the ratio between the electrolyte volume and the quantity of sulphur in a conventional lithium-sulphur cell can be at least 5 μL of electrolyte per milligram of sulphur. However, an increase in the quantity of electrolyte penalizes the specific energy and energy density of the electrochemical cell. The invention allows reducing the ratio between the electrolyte volume and the quantity of sulphur, for example by maintaining this ratio at a value less than or equal to 4 μL of electrolyte per milligram of sulphur, and consequently increasing the specific energy and energy density of the cell.

EXAMPLES

Six lithium/sulphur electrochemical cells A-F of 5 mAh capacity were created. The different cathode and electrolyte compositions used to create cells A to F are indicated in Table 1 below.

The carbon used in Examples A and C is available from Akzo Nobel under the tradename Ketjen Black EC 600J.

To characterize the porous volume and the pore size of the carbons, the technique of nitrogen adsorption at 77 K was used. The adsorption and desorption isotherms were measured with a Belsorp Mini II device. The carbon used in Examples A and C has a porous structure comprising pores with a mean diameter of 6.5 nm. The specific surface area developed of this carbon, measured by the Brunauer-Emmet-Teller (BET) method, is 1439 m2/g.

The carbon used in Examples B, D, E, F and G has the following pore distribution:mesoporous volume (denoted Vmesoporouscorresponding to the specific volume of the pores with a mean diameter greater than 2 nm and less than 50 nm) equal to 0.1 cm3/g measured according to the “Barrett-Joyner-Halenda” (BJH) method from nitrogen adsorption and desorption isotherms;microporous volume (denoted Vmicroporouscorresponding to the specific volume of the pores with a mean diameter less than or equal to 2 nm) equal to 0.9 cm3/g measured according to the “t-plot” from nitrogen adsorption and desorption isotherms;

The BJH method also allows estimating the mean diameter of the pores associated with the mesoporous volume, which will be denoted dmesoporous, from nitrogen adsorption and desorption isotherms.

The Horvath-Kawazoe method also allows estimating the mean diameter of the pores associated with the microporous volume, which will be denoted dmicroporous, from nitrogen adsorption and desorption isotherms.

When the carbon has both a mesoporous and microporous volume, the evaluation of the mean pore diameter of the carbon is calculated by the mean of the microporous and mesoporous diameters weighted by the associated pore volumes according to the following formula:
dporous mean=((Vmesoporous*dmesoporous)+(Vmicroporous*dmicroporous))/(Vmesoporous+Vmicroporous)

The mean pore diameter was evaluated at 0.88 nm for the carbon used in Examples B, D, E, F and G. The specific surface area of this carbon, measured by nitrogen adsorption and calculated by the Brunauer-Emmet-Teller (BET) method, is 2163 m2/g.

The carbon of Examples A and C does not have microporous volume. Its mean pore diameter therefore corresponds to dmesoporousmeasured by the BJH method, which is 6.5 nm.

The solid elemental sulphur used is available from the Sigma-Aldrich company, under reference 215198.

The sulphur and carbon composite was prepared by mixing the carbon with the solid elemental sulphur. The mixture was heated to a temperature of 155° C. for approximately 5 hours under vacuum, in order to permit the sulphur molecules to penetrate into the carbon pores. Heating of the mixture is continued at a temperature of 300° C. for 30 minutes under an inert gas flow in order to eliminate the excess sulphur. The sulphur content of the prepared carbon-sulphur composite is measured by thermogravimetric analysis (TGA) using a TA Instrument Q500 device.

In all the cells, the anode is lithium metal and the separator is a microporous polyolefin membrane sold by the Celgard company under the tradename Celgard® 2500.

TABLE 1Cathodemean carbonpercentageelectricalporeof porousconductorsulphurdiametercarboncompoundbinderElectrolyteCell(% m**)(nm)(% m**)(% m**)(% m**)(% vol***)A*536.53287LiPF61 mol/L inEC/DMC (20/80)B*520.883657LiPF61 mol/L inEC/DMC (20/80)C*536.53287LiTFSI1 mol/L indimethoxyethane(DME)/dioxolaneD*590.882957LiPF61 mol/L in FEC/DMC(20/80)E590.882957LiFSI1 mol/L in FEC/DMC(20/80)F*530.883557LiFSI1 mol/L in(DME)/dioxolaneG530.883557LiFSI1 mol/L inFEC/DMC (20/80)*example that is not part of the invention**the percentages are mass percentages expressed relative to the sum of the masses of the cathode constituents except for the current collector.***the percentages are volume percentages expressed relative to the total volume of organic solvents.

A) A first test was conducted on cells A and B for purposes of evaluating the effect of the mean diameter of the carbon pores of the cathode on the discharge performance of the cells. Cells A and B contain the same anode, the same separator and the same electrolyte. The carbon of the cell A cathode has a porous structure comprising pores with a mean diameter of 6.5 nm. The carbon of the cell B cathode has a porous structure comprising pores with a mean diameter of 0.88 nm. The discharge curves of cells A and B were plotted and the initial discharge mass capacities of these cells were measured. The discharge curves are shown inFIG.2. Cell A has a mass capacity of approximately 150 mAh per gram of sulphur while cell B has a mass capacity of approximately 1645 mAh per gram of sulphur. These results show that a cell comprising a cathode comprising carbon comprising pores with a mean diameter of 6.5 nm and a carbonate-based solvent does not have a high mass capacity.

B) A second test was performed in order to compare the quantities of hydrogen sulfide (H2S) emitted by cells C, D and E. For this purpose, cells C, D and E were discharged until a voltage of 1 V was attained. The low voltage of 1 V allowed maximizing the quantity of sulphur reduced to Li2S at the cathode. The electrodes were then removed from the cells and placed in a closed beaker containing a source of moisture in order to measure the quantity of hydrogen sulfide emitted by the reaction between lithium sulfide Li2S and moisture. The source of moisture consisted of water and acid poured into a beaker. Care was taken not to wash the surface of the electrodes in order not to eliminate any deposit of products that would be present at the electrode surface. The electrodes were dried before exposure to the source of moisture.

The cumulative quantities of hydrogen sulfide were recorded over time and are shown inFIG.3. It is observed that the electrode of cell E according to the invention is the one that emits the lowest quantity of hydrogen sulfide. It appears that the compounds resulting from the reduction of sulphur during discharge remain in the cathode pores.

Cells D and E differ only by the nature of the electrolyte salt, which is LiPF6for cell D and LiFSI for cell E. Comparison between the results obtained with cell D and those obtained with cell E show that the replacement of LiPF6with LiFSI permitted reducing the quantity of hydrogen sulfide emitted. The lowest quantity of hydrogen sulfide emitted by cell E shows that the invention permits reducing the toxicity risk for a user.

C) A third test was conducted on cells F and G. Cells F and G differ by the nature of the organic solvents of the electrolyte. The organic solvents of cell G are ethylene monofluorocarbonate and dimethyl carbonate, which are part of the carbonate family. The organic solvents of cell F are 1,2-dimethoxyethane (DME) and dioxolane, which are part of the ether family. The discharge curves of cells F and G were plotted and the initial mass discharge capacities of these cells were measured. The discharge curves are shown inFIG.4. Cell G has a mass capacity of approximately 1750 mAh per gram of sulphur while cell F has a mass capacity of approximately 330 mAh per gram of sulphur, much less than that of cell F. These results show that the use of an ether-based organic solvent does not allow obtaining a sufficient mass capacity.