HK1237995B - Active electrode material for a li-s battery - Google Patents

Description

Active electrode materials for LI-S batteries

Technical Field

The present invention relates to the field of Li/S batteries. More particularly, it relates to an active material for producing an electrode comprising a carbon-based nanofiller uniformly (homogeneously) dispersed in a host of a sulfur-based material, the active material being obtainable according to a process via a melting route.

Background Art

Lithium/sulfur batteries (hereinafter referred to as Li/S batteries) consist of a positive electrode (cathode) of elemental sulfur or another electroactive sulfur-based material, a negative electrode (anode) formed of lithium metal or a lithium-based metal alloy, and an organic liquid electrolyte.

Typically, the positive electrode is prepared from an active material comprising elemental sulfur S ₈ (hereinafter referred to as natural sulfur) and optionally various additives, which is mixed with a solvent and a binder to form a paste that is applied to a current collector and then dried to remove the solvent. The resulting composite structure is optionally subjected to a compression stage and then cut to the desired dimensions of the positive electrode.

Li/S batteries are obtained by depositing a separator on the positive electrode and then depositing a lithium negative electrode on said separator. Subsequently, an electrolyte, which generally comprises at least one lithium salt dissolved in a solvent, is introduced into the battery.

Since 2000, Li/S batteries have been the subject of extensive research and are considered a promising alternative to conventional Li-ion batteries. The advantages of these batteries stem from the high volumetric storage capacity of the sulfur electrode, which enables energy densities ranging up to 500 Wh·kg ^-1 to be achieved. Furthermore, natural sulfur exhibits the non-negligible advantages of abundance, low cost, and non-toxicity, which makes the large-scale development of Li/S batteries conceivable.

The discharge and charge mechanism of Li/S batteries is based on the reduction/oxidation of sulfur at the positive electrode and the oxidation/reduction of lithium at the negative electrode.

During discharge, the sulfur molecules S ₈ are reduced and form lithium polysulfide chains of the general formula Li _{2 Sn} (n ≥ 2) dissolved in the organic electrolyte. The final stage of sulfur reduction consists of the formation of lithium sulfide Li ₂ S, which precipitates from the organic electrolyte and deposits on the negative electrode. The opposite electrochemical reaction occurs during charging.

In order for the electrochemical reactions to occur rapidly at the electrodes, the positive and negative electrodes must generally be good electron conductors. In fact, since sulfur is an electron insulator (σ= ^5.10-30 S.cm ^-1 at 25°C), the discharge rate is relatively slow.

Several improvement routes have been envisaged with the goal of overcoming this low electronic conductivity of the active material, in particular the addition of electronically conductive additives, such as carbon-based conductive materials. However, if the sulfur/additive mixture is not optimal or if the additive content is too low, the reaction kinetics at the positive electrode remain limited.

Among the conductive additives, carbon black, activated carbon, carbon fibers or carbon nanotubes are generally used. Carbon black is conventionally used.

The mixing of the active material and the conductive additive can be performed in a variety of ways.

For example, mixing can be performed directly during the preparation of the electrode. Thus, the sulfur is mixed with the conductive additive and the binder by mechanical stirring before the electrode is formed. Due to this homogenization stage, it is believed that the carbon-based additive is distributed around the sulfur particles, thereby creating a percolating network. A grinding stage can also be used and allows for a more intimate mixing of the materials. However, this additional stage can lead to a loss of porosity in the electrode.

Another method of mixing the active material with the carbon-based additive consists in grinding the sulfur and the carbon-based additive by a dry route, thereby coating the sulfur with carbon.

From the same point of view, carbon can be deposited around sulfur particles by deposition in the gas phase. Conversely, core-shell structures can also be prepared from carbon black on which a layer of sulfur is deposited, for example by precipitating sulfur on carbon black nanoparticles.

For example, document FR 2 948 233 describes a conductive composite material obtained by chemically treating sulfur and carbon, introduced into a sealed reactor without external pressure regulation at a temperature between 115° C. and 400° C. for a sufficient time to melt the sulfur and reach equilibrium. The material is in the form of sulfur particles covered with carbon, exhibiting a low specific surface area. However, the method for introducing carbon into sulfur described in this document is applicable only to carbon-based nanofillers without form factors or aggregation, and does not result in carbon-based nanofillers uniformly dispersed throughout the sulfur matrix.

US 2013/0161557 describes a method for preparing an electrode active material for rare earth lithium-sulfur batteries. This method produces a composite material comprising molten sulfur absorbed within carbon nanotubes under high temperature and vacuum. This composite material is then subjected to various treatments, including dissolution in alcohol, grinding, drying, and calcination, to form the electrode active material. The method described in this document is relatively complex to implement.

Unlike carbon black, additives of the carbon nanotube (CNT) type exhibit the advantage of having a beneficial adsorbent effect on the active material by limiting its dissolution in the electrolyte and thus promoting better cyclability.

For example, in the article Electrochimica Acta, 51 (2006), pp. 133-1335, Zheng W. et al. describe the preparation of a sulfur/carbon nanotube (CNT) composite material by melt blending at high temperatures with long residence times. However, the cycling tests carried out with this material were carried out for only 60 cycles, which did not allow it to be shown that the carbon nanotubes were indeed uniformly dispersed in the sulfur matrix to have an effect on the life of the electrode.

Introducing CNTs into electrode formulations also creates numerous problems. This is because CNTs have proven difficult to handle and disperse due to their small size, their powdered state, and, possibly, their entangled structure when obtained by chemical vapor deposition (CVD), which also creates strong van der Waals interactions between their molecules. Despite the addition of conductive materials, the low dispersion of CNTs limits the effectiveness (efficiency) of charge transfer between the positive electrode and the electrolyte and, therefore, the performance of Li/S batteries.

This is why it is advantageous for formulators to have available active materials comprising CNTs well dispersed in sulfur, and more generally sulfur-based materials, in the form of ready-to-use (RT-to-use) active materials that can be used directly in formulations for making electrodes for Li/S batteries to effectively increase electronic conductivity.

The Applicant Company has now discovered that an active material comprising carbon nanotubes homogeneously dispersed in a host of a sulphur-based material, such as sulphur, makes it possible to increase the conductive filler/sulphur interface and thus the charge and discharge capacity of the batteries incorporating this active material.

The Applicant Company has also discovered that such active materials can be obtained by contacting CNTs with a sulphur-based material by a melt route, for example in a compounding device, thereby forming an improved active material which can be used for the preparation of electrodes.

Furthermore, it also becomes clear that the invention is also applicable to carbon-based nanofillers other than CNTs, in particular carbon nanofibers and graphene, or mixtures thereof in all ratios.

Summary of the Invention

A subject of the present invention is an active material for producing an electrode, comprising:

- Sulfur-based materials;

- 1 to 25 wt% of carbon-based nanofiller uniformly dispersed in the bulk of the sulfur-based material.

According to one embodiment, the electrode active material includes 5 to 25 weight percent of a carbon-based nanofiller uniformly dispersed in a bulk of a sulfur-based material.

Another subject of the present invention is an electrode active material comprising:

- Sulfur-based materials;

- 1 to 25% by weight of carbon-based nanofillers uniformly dispersed in the body of the sulfur-based material,

It is characterized in that it exhibits a porosity of less than 40%.

Another subject of the present invention is an electrode active material comprising:

- Sulfur-based materials;

- 1 to 25% by weight of carbon-based nanofillers uniformly dispersed in the body of the sulfur-based material,

It is characterized in that it exhibits a density greater than 1.6 g/cm ³ .

According to one embodiment of the invention, the active material is obtained by a melting route, in particular with mechanical energy which may be between 0.05 kWh/kg and 1 kWh/kg of active material, preferably between 0.2 and 0.5 kWh/kg of active material.

"Carbon-based nanofiller" denotes a carbon-based filler, the smallest dimension of which is between 0.1 and 200 nm, preferably between 0.1 and 160 nm and more preferably between 0.1 and 50 nm, as measured by light scattering.

"Carbon-based nanofiller" may mean a filler comprising at least one component from the group consisting of carbon nanotubes, carbon nanofibers and graphene, or mixtures of these in all proportions. Preferably, the carbon-based nanofiller comprises at least carbon nanotubes.

"Sulfur-based materials" are understood to mean sulfur-donating (sulfur-donating) compounds selected from natural (or elemental) sulfur, sulfur-based organic compounds or polymers and sulfur-based inorganic compounds.

According to a preferred embodiment of the present invention, the sulfur-based material comprises at least natural sulfur, either alone or in a mixture with at least one other sulfur-based material.

The active material according to the present invention includes carbon-based nanofillers that are well-infiltrated into a molten sulfur-based matrix (matrix), and the carbon-based nanofillers are uniformly distributed throughout the bulk of the sulfur-based material, as can be visualized, for example, by electron microscopy. The sulfur-based material/nanofiller mixture has a morphology suitable for optimizing the function (operation, effect) of a Li/S battery electrode.

Thus, the active material according to the present invention can provide efficient transfer of electricity from the current collector of the electrode and provide an active interface for electrochemical reactions during operation of the battery.

Therefore, the present invention provides active materials that show a better combination of sulfur-donating materials and carbon-based nanofiller particles to promote sulfur entry into (conducting) electrochemical reactions. In addition, electrodes incorporating the active materials according to the present invention provide good maintenance of battery operation over time.

According to one embodiment of the present invention, the active material further comprises at least one additive selected from the group consisting of a rheology modifier, a binder, an ion conductor, a carbon-based electrical conductor, an electron-donating component, or a combination thereof. As with the carbon-based nanofillers, the additive is introduced into the material via a melt route.

Another aspect of the invention is the use of the active material described above in an electrode, in particular in a positive electrode of a Li/S battery.The active material according to the invention makes it possible to improve the electronic conductivity of the electrode formulation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 represents the particle size distribution of the powder obtained in Example 1 according to the present invention.

FIG. 2 illustrates the morphology of the electrode active material obtained in Example 1 according to the present invention by SEM.

DETAILED DESCRIPTION

The invention is now described in more detail and without limitation in the following description.

Carbon-based nanofillers

According to the invention, the carbon-based nanofiller can be carbon nanotubes, carbon nanofibers, graphene or mixtures of these in all proportions. The carbon-based nanofiller is preferably carbon nanotubes, alone or in a mixture with at least one other carbon-based nanofiller.

The carbon nanotubes (CNTs) that constitute the active material are single-walled, double-walled, or multi-walled, preferably multi-walled (MWNT).

The carbon nanotubes used according to the invention generally have an average diameter ranging from 0.1 to 200 nm, preferably from 0.1 to 100 nm, more preferably from 0.4 to 50 nm and even better still from 1 to 30 nm, in fact even from 10 to 15 nm, and advantageously have a length greater than 0.1 μm and advantageously from 0.1 to 20 μm, preferably from 0.1 to 10 μm, for example of about 6 μm. Their length/diameter ratio is advantageously greater than 10 and generally greater than 100. Their specific surface area is, for example, between 100 and 300 ^{m 2} /g, advantageously between 200 and 300 m ² /g, and their apparent density may in particular be between 0.01 and 0.5 g/cm ³ and more preferably between 0.07 and 0.2 g/cm ^3. The MWNTs may, for example, comprise 5 to 15 sheets and more preferably 7 to 10 sheets.

The carbon nanotubes are obtained in particular by chemical vapor deposition, for example according to the method described in document WO 06/082325. Preferably, they are obtainable from renewable raw materials, in particular of plant origin, as described in patent application EP 1980530.

These nanotubes may be treated or untreated.

Examples of crude (unprocessed) carbon nanotubes are in particular available under the trade name C100 from Arkema.

These nanotubes may be purified and/or treated (eg oxidized) and/or ground and/or functionalized.

The grinding of the nanotubes can be carried out in particular under cold or hot conditions and according to known techniques used in devices such as ball mills, hammer mills, wheel mills (milling wheel mills), choppers or gas jet mills or any other grinding system capable of reducing the size of the entangled network of nanotubes. Preferably, this grinding stage is carried out according to the gas jet milling technique and in particular in an air jet mill.

Crude or ground nanotubes can be purified by washing with a sulfuric acid solution to remove any residual inorganic and metallic impurities, such as iron, from their preparation process. The weight ratio of nanotubes to sulfuric acid can particularly be between 1:2 and 1:3. The purification operation can also be carried out at a temperature ranging from 90°C to 120°C, for a period of, for example, 5 to 10 hours. This operation can advantageously be followed by a stage in which the purified nanotubes are rinsed with water and dried. Alternatively, the nanotubes can be purified by a high-temperature heat treatment (typically at temperatures exceeding 1000°C).

The oxidation of the nanotubes is advantageously carried out by contacting them with a sodium hypochlorite solution comprising 0.5% to 15% by weight of NaOCl, and preferably 1% to 10% by weight of NaOCl, for example in a weight ratio of nanotubes to sodium hypochlorite ranging from 1:0.1 to 1:1. The oxidation is advantageously carried out at a temperature below 60° C. and preferably at room temperature for a period of time ranging from a few minutes to 24 hours. This oxidation operation can advantageously be followed by a stage in which the oxidized nanotubes are filtered and/or centrifuged, washed, and dried.

Functionalization of the nanotubes can be performed by grafting reactive units, such as vinyl monomers, onto the surface of the nanotubes.

In the present invention, preference is given to using crude carbon nanotubes, in other words nanotubes which have neither been oxidized nor purified nor functionalized and which have also not undergone any further chemical and/or thermal treatment, which are optionally ground.

Like carbon nanotubes, the carbon nanofibers that can be used as carbon-based nanofillers in the present invention are nanowires (filaments) produced by chemical vapor deposition (or CVD) starting from a carbon-based source that is decomposed in the presence of hydrogen (gas) at temperatures between 500° C. and 1200° C. over a catalyst comprising a transition metal (Fe, Ni, Co, Cu). However, the structures of these two carbon-based fillers are different, since carbon nanofibers consist of stacks of more or less organized graphitic regions (or turbostratic stacks), the planes of which are inclined at variable angles relative to the axis of the fiber. These stacks can take the form of plates (platelets), fishbones or disks, which are stacked to form structures with diameters typically in the range of 100 nm to 500 nm, and in practice even larger.

Examples of carbon nanofibers that can be used have in particular a diameter of 100 to 200 nm, for example about 150 nm, and advantageously a length of 100 to 200 μm. Nanofibers such as those available from Showa Denko can be used.

Graphene means a flat, isolated and separated (individual) graphene sheet, and by extension means a collection of between one and several dozen sheets and showing a flat or more or less wavy structure. Therefore, this definition includes FLG (few-layer graphene), NGP (nanographene plate (sheet)), CNS (carbon nanosheet) and GNR (graphene nanoribbon). On the other hand, it does not include carbon nanotubes and nanofibers, which are respectively composed of the coaxial winding of one or more graphene sheets and the turbostratic stacking of these sheets. In addition, preferably, the graphene used according to the present invention does not pass through the stage of additional chemical oxidation or functionalization.

The graphene used according to the invention is obtained by chemical vapor deposition, or CVD, preferably using a mixed oxide-based powdered catalyst. It is typically provided in the form of particles having a thickness of less than 50 nm, preferably less than 15 nm, more preferably less than 5 nm, and having lateral dimensions of less than one micrometer, preferably from 10 nm to less than 1000 nm, more preferably from 50 to 600 nm, and indeed even from 100 to 400 nm. Each of these particles typically comprises 1 to 50 flakes, preferably 1 to 20 flakes, more preferably 1 to 10 flakes, and indeed even from 1 to 5 flakes, which can be separated from one another in the form of individual flakes, for example during ultrasound treatment.

Sulfur-based materials

The sulfur-based material may be natural sulfur, a sulfur-based organic compound or polymer, a sulfur-based inorganic compound, or a mixture of these in all proportions.

Natural sulfur is commercially available from various sources. The particle size of the sulfur powder can vary within wide limits. The sulfur can be used as is, or it can be pre-purified according to various techniques such as refining, sublimation or precipitation. The sulfur, or more generally sulfur-based materials, can also undergo preliminary stages of grinding and/or sieving to reduce the particle size and narrow its distribution.

Sulfur-based inorganic compounds that can be used as the sulfur-based material are, for example, alkali metal anion polysulfides, preferably lithium polysulfides represented by the formula Li _{2 Sn} (wherein n≧1).

Sulfur-based organic compounds or polymers that can be used as sulfur-based materials can be selected from organic polysulfides, organic polythiolates including, for example, functional groups such as dithioacetals, dithioketals or trithioorthocarbonates, aromatic polysulfides, polyether-polysulfides, salts of polysulfide acids, thiosulfonates [—S(O) ₂ —S—], thiosulfinates [—S(O)—S—], thiocarboxylates [—C(O)—S—], dithiocarboxylates [—RC(S)—S—], thiophosphates, thiophosphonates, thiocarbonates, organometallic polysulfides or mixtures thereof.

Examples of such organosulfur-based compounds are described in particular in document WO 2013/155038.

According to a particular embodiment of the present invention, the sulfur-based material is an aromatic polysulfide.

Aromatic polysulfides correspond to the following general formula (I):

in:

_-R1 to _R9 , in the same or different manner, represent a hydrogen atom, an -OH or -O ^- M ⁺ group, a saturated or unsaturated carbon-based chain comprising 1 to 20 carbon atoms, or an _-OR10 group, wherein _R10 may be an alkyl group, an aralkyl group, an acyl group, a carboxyalkoxy group, an alkyl ether group, a silyl group, or an alkylsilyl group comprising 1 to 20 carbon atoms,

-M represents an alkali metal or an alkaline earth metal,

- n and n' are two identical or different integers, each greater than or equal to 1 and less than or equal to 8,

-p is an integer between 0 and 50,

- and A are a nitrogen atom, a single bond, or a saturated or unsaturated carbon-based chain of 1 to 20 carbon atoms.

Preferably, in formula (I):

-R ₁ , R ₄ and R ₇ are O ^- M ⁺ groups,

- R ₂ , R ₅ and R ₈ are hydrogen atoms,

- R ₃ , R ₆ and R ₉ are saturated or unsaturated carbon-based chains comprising 1 to 20 carbon atoms, preferably 3 to 5 carbon atoms,

- The average value of n and n' is about 2,

- the average value of p is between 1 and 10, preferably between 3 and 8. (These average values are calculated by a person skilled in the art from proton NMR data and by determining the weight of sulfur),

-A is a single bond connecting the sulfur atom to the aromatic ring.

Such poly(alkylphenol) polysulfides of the formula (I) are known and can be prepared, for example, in two stages:

1) Reaction of sulfur monochloride or sulfur dichloride with an alkylphenol at a temperature between 100 and 200°C according to the following reaction:

The compound of formula (II) is sold in particular by Arkema under the name HYDROGEN(R).

2) Reaction of compound (II) with a metal derivative containing a metal M (eg an oxide, hydroxide, alkoxide or dialkylamide of the metal) to obtain an O ⁻ M ⁺ group.

According to a more preferred alternative form, R is tert-butyl or tert-amyl.

According to another preferred alternative form of the invention, mixtures of compounds of formula (I) are used in which two of the R groups present on each aromatic unit are carbon-based chains comprising at least one tertiary carbon via which R is linked to the aromatic nucleus.

Active Materials

The amount of carbon-based nanofillers in the active material is 1 to 25 wt%, preferably 10 to 15 wt%, for example 12 to 14 wt%, relative to the total weight of the active material.

The active material according to the present invention is a solid final product (finished product) comprising an intimate mixture of particles, said carbon-based nanofillers being dispersed in a homogeneous manner in a bulk of the sulphur-based material.

The active material advantageously exhibits a density greater than 1.6 g/cm ³ , measured according to standard NF EN ISO 1183-1. The density is generally less than 2 g/cm ³ .

It also advantageously exhibits a porosity of less than 40%, preferably less than 20%.Porosity can be determined from the difference between theoretical density and measured density.

The electrode active material as defined according to the invention makes it possible to increase the specific capacity of the electrode, which is denser, and to increase the charge and discharge capacity of the electrode.

Subsequently, the homogeneous mixture of particles may be milled to obtain a powder exhibiting no particles having a size greater than 100 μm, preferably no particles having a size greater than 50 μm, to facilitate the electrode manufacturing process.

Carbon-based nanofillers, such as CNTs, are mixed with a sulfur-based material, in particular sulfur, preferably by melting. However, since the melting of the mixture is limited by the density difference between CNTs (0.1 g/cm ³ ) and sulfur (2 g/cm ³ ), it is generally necessary to add strong mechanical energy to carry out this mixing, which may be between 0.05 kWh/kg and 1 kWh/kg of active material, preferably between 0.2 and 0.5 kWh/kg of active material. Thus, the carbon-based nanofiller is uniformly dispersed throughout the bulk of the particle (throughout the bulk of the particle) and is not present only at the surface of the sulfur-based particle, as described in document FR 2 948 233.

To do this, preferably a compounding device is used, in other words an apparatus conventionally used in the plastics industry for melt blending of thermoplastic polymers and additives to produce composites.

Thus, the active material according to the invention can be prepared according to a process comprising the following stages:

(a) introducing at least one sulfur-based material and a carbon-based nanofiller into a compounding apparatus;

(b) melting sulfur-based materials;

(c) kneading of the melted sulfur-based material and the carbon-based nanofiller;

(d) recovery of the mixture obtained in the form of an agglomerated solid physical substance;

(e) Grinding of the mixture in powder form.

In a compounding apparatus, the sulfur-based material and the carbon-based nanofiller are mixed using a high shear device such as a co-rotating twin-screw extruder or a co-kneader. The molten material typically leaves the apparatus in an agglomerated solid physical form, such as pellets, or in the form of rods (which are cut into pellets after cooling).

Examples of usable co-kneaders are the MDK 46 co-kneader sold by Buss AG and the co-kneaders of the MKS or MX series, all of which consist of a screw shaft with threads (wings) located in a heated barrel, optionally consisting of several parts, the inner wall of which has kneading teeth adapted to interact with the threads to produce shearing of the kneaded material. The shaft is rotationally driven by a motor and is provided with an axial swing. These co-kneaders can be equipped with a system for producing pellets, for example connected to their outlet opening, which can consist of an extrusion screw or a pump.

Co-kneaders which can be used preferably have a screw ratio L/D of 7 to 22, for example 10 to 20, while co-rotating extruders advantageously have an L/D ratio of 15 to 56, for example 20 to 50.

The compounding stage is carried out at a temperature greater than the melting point of the sulfur-based material. In the case of sulfur, the compounding temperature can range from 120°C to 150°C. In the case of other types of sulfur-based materials, the compounding temperature depends on the specific material used, the melting point of which is usually quoted by the supplier of the material. The residence time will also be adjusted to the properties of the sulfur-based material.

This method allows for efficient and uniform dispersion of large amounts of carbon-based nanofillers in sulfur-based materials despite density differences between the components of the active material.

According to one embodiment of the present invention, the active material further comprises at least one additive selected from the group consisting of a rheology modifier, a binder, an ion conductor, a carbon-based electrical conductor, an electron-donating component, or a combination thereof. These additives are advantageously introduced during the compounding stage in order to obtain a homogeneous active material.

In this embodiment, the sulphur-based material and the carbon-based nanofillers then represent from 20% to 100% by weight, preferably from 20% to 80% by weight, relative to the total weight of the active material.

In particular, additives that modify the rheological properties of the molten sulfur can be added during mixing during the complexation phase in order to reduce the self-heating (autogenous heating) of the mixture in the compounding apparatus. Such additives having a fluidizing effect on liquid sulfur are described in application WO 2013/178930. By way of example, mention may be made of dimethyl sulfide, diethyl sulfide, dipropyl sulfide, dibutyl sulfide, dimethyl disulfide, diethyl disulfide, dipropyl disulfide, dibutyl disulfide, their trisulfide homologues, their tetrasulfide homologues, their pentasulfide homologues or their hexasulfide homologues, alone or as mixtures of two or more thereof in all proportions.

The amount of rheology-modifying additive is generally between 0.01% and 5% by weight, preferably 0.1% to 3% by weight, relative to the total weight of the active material.

The active material may comprise a binder, in particular a polymeric binder, chosen for example from halogenated polymers, preferably fluorinated polymers, functional polyolefins, polyacrylonitrile, polyurethanes, polyacrylic acid and its derivatives, polyvinyl alcohol and polyethers, or blends of these in all proportions.

As examples of fluorinated polymers, mention may be made of poly(vinylidene fluoride) (PVDF), preferably in the alpha form, poly(trifluoroethylene) (PVF3), polytetrafluoroethylene (PTFE), copolymers of vinylidene fluoride with hexafluoropropylene (HFP) or trifluoroethylene (VF3) or tetrafluoroethylene (TFE) or chlorotrifluoroethylene (CTFE), fluoroethylene/propylene (FEP) copolymers, copolymers of ethylene with fluoroethylene/propylene (FEP) or tetrafluoroethylene (TFE) or chlorotrifluoroethylene (CTFE), perfluoropropyl vinyl ether (PPVE), perfluoroethyl vinyl ether (PEVE), 2,3,3,3-tetrafluoropropylene, and copolymers of ethylene with perfluoromethyl vinyl ether (PMVE), or blends thereof.

As examples of polyethers, mention may be made of poly(alkylene oxides) such as poly(ethylene oxide) PEO, polyalkylene glycols such as polyethylene glycol PEG, polypropylene glycol PPG, polytetramethylene glycol (PTMG), polytetramethylene ether glycol (PTMEG), and the like.

Preferably, the binder is PVDF or PEO.

The active material may include an ion conductor that interacts favorably with the surface of the sulfur-based material to increase the ionic conductivity of the active material. Examples of ion conductors include, but are not limited to, lithium organic salts, such as lithium imidazole or lithium sulfite. Poly(alkylene oxide)s may also be mentioned, which, in addition to their role as binders, may contribute ionic conductivity properties to the active material.

The active material may comprise an electrical conductor, advantageously a carbon-based electrical conductor such as carbon black, graphite or graphene, typically in a proportion that may range from 1% to 10% by weight relative to the sulphur-based material. Preferably, carbon black is used as the electrical conductor.

The active material may include electron-donating components to improve electron exchange and regulate the length of polysulfides during charging, which optimizes the charge/discharge cycle of the battery.

As electron-donating components, it is advantageous to use components from groups IVa, Va and Via of the periodic table, preferably selected from Se, Te, Ge, Sn, Sb, Bi, Pb, Si or As, in powder form or in salt form.

The active material according to the invention is advantageously provided in the form of a powder comprising particles exhibiting a mean size of less than 150 μm, preferably less than 100 μm; a median (median) diameter d ₅₀ between 1 and 60 μm, preferably between 10 and 60 μm, more preferably between 20 and 50 μm; a median diameter d ₉₀ of less than 100 μm; and preferably a diameter d ₁₀₀ of less than 50 μm, these characteristics being determined by laser diffraction.

To obtain this powder morphology, use is generally made of equipment of the hammer mill, pin mill (pin mill) or bead mill type, air jet mills or other methods for the micronization of solid materials.

The active material according to the invention, preferably in the form of a powder as characterized above and advantageously showing a porosity of less than 20% and/or a density greater than 1.6 g/cm ³ , can be used to prepare Li/S battery electrodes; it generally represents approximately 20% to 95% by weight, preferably 35% to 80% by weight, relative to the total formulation of the electrode.

The invention will now be illustrated by the following examples, which are not intended to limit the scope of the invention as defined by the appended claims.

Experimental part

Example 1 : Preparation of S/CNT active material

CNTs (C100 from Arkema) and solid sulfur (50 to 800 μm) were introduced into the first hopper of an MDK 46 (L/D=11) co-kneader equipped with a take-up extrusion screw and a pelletizing device.

The set temperature values in the co-kneader were as follows: zone 1: 140°C; zone 2: 130°C; screw: 120°C.

At the exit of the die, the mixture consisting of 87.5% by weight of sulfur and 12.5% by weight of CNTs was in the form of pellets obtained by pelletization which were cooled by air.

The pellets were then ground in a hammer mill with cooling provided by nitrogen.

Scanning electron microscopy (SEM) observations showed that the CNTs were well dispersed in the sulfur.

The pellets were ground in a high-speed pin mill (12,000 to 14,000 rpm) and cooled with liquid nitrogen at -30°C introduced into the pellets through the mill feed screw. The powder was sieved using a cylindrical 80 μm sieve. The particle size distribution, determined by laser diffraction in a Malvern-type apparatus, is shown in FIG1 . The size of the largest particles was less than 100 μm, and the median diameter d ₅₀ was between 20 and 50 μm.

The powder was sieved in a second test using a cylindrical 50 μm sieve. The particle size distribution showed that the diameter d ₁₀₀ was less than 50 μm. The morphology of the electrode active material thus obtained is shown in FIG2 .

This powder, consisting of 87.5% by weight of sulfur and 12.5% by weight of CNTs, is the active material for producing electrodes for Li/S batteries.

Example 2 : Preparation of S/DMDS/CNT active material

CNTs (C100 from Arkema) and solid sulfur (50 to 800 μm) were introduced into the first hopper of an MDK 46 (L/D=11) co-kneader equipped with a take-up extrusion screw and a pelletizing device.

Liquid dimethyl disulfide (DMDS) was injected into the first zone of the co-kneader.

The set temperature values in the co-kneader were as follows: zone 1: 140°C; zone 2: 130°C; screw: 120°C.

At the outlet of the die, a masterbatch composed of 83% by weight of sulfur, 2% by weight of DMDS and 15% by weight of CNTs was in the form of pellets obtained by granulation cooled by water spraying.

The pellets obtained were dried down to a moisture content of <100 ppm.

The dried pellets were then ground in a hammer mill with nitrogen providing cooling.

Powders are obtained which exhibit a median diameter d ₅₀ between 30 and 60 μm and which can be used for the preparation of electrodes for Li/S batteries.

Example 3 : Preparation of S/poly(tert-butylphenol) disulfide/CNT active material

CNTs (C100 from Arkema) and solid sulfur (50 to 800 μm) were introduced into the first hopper of an MDK 46 (L/D=11) co-kneader equipped with a take-up extrusion screw and a pelletizing device.

Liquid dimethyl disulfide (DMDS) was injected into the first zone of the co-kneader.

Poly(tert-butylphenol) disulfide sold by Arkema under the name AUTOMATO® and a lithium salt sold by Arkema under the name LOA (lithium 4,5-dicyano-2-(trifluoromethyl)imidazolium) were premixed and then introduced into the first hopper using a third metering device.

The set temperature values in the co-kneader were as follows: zone 1: 140°C; zone 2: 130°C; screw: 120°C.

At the outlet of the die, the mixture is in the form of pellets obtained by granulation which are cooled by spraying water.

The pellets obtained were dried down to a moisture content of <100 ppm.

The dried pellets were then ground in a hammer mill with nitrogen providing cooling.

A powder consisting of 77 wt% of sulfur, 2 wt% of DMDS, 15 wt% of CNTs, 5% of Silane and 1% of LOA was obtained, which was used to prepare electrodes for Li/S batteries.

Example 4 : Preparation of S/POE/Li ₂ S/CNT active material

CNTs (C100 from Arkema) and solid sulfur (50 to 800 μm) were introduced into the first hopper of an MDK 46 (L/D=11) co-kneader equipped with a take-up extrusion screw and a pelletizing device.

Polyethylene oxide WSR N-60K (manufactured by Dow) was premixed with Li ₂ S (supplied by Sigma). This mixture was introduced into the first hopper through the third metering device.

The set temperature values in the co-kneader were as follows: zone 1: 140°C; zone 2: 130°C; screw: 120°C.

At the exit of the die, the mixture consisting of 70% by weight of sulfur, 15% by weight of CNTs, 10% by weight of WSR N-60K and 5% by weight of Li ₂ S is in the form of pellets obtained by crossing conveyor belts via an indexer of rods without contact with water.

The dried pellets were then ground in a hammer mill with nitrogen providing cooling.

A powder consisting of 70 wt% sulfur, 15 wt% CNTs, 10 wt% WSR N-60K and 5 wt% _Li2S was obtained, comprising particles showing an average size of less than 150 μm, median diameters _d50 and _d90 suitable for powders used as positive active material for Li/S batteries.

Example 5 : Evaluation of active materials

Active material evaluation tests were performed in a Li/S battery model, including:

1) Anode made of Li metal, 100 μm thick

2) Separator/membrane (20 μm)

3) Sulfolane-based electrolyte with 1M Li ⁺

4) Sulfur-based cathode supported by Al current collector

Two cathode formulations were tested:

- a reference formulation comprising: 70 wt% sulfur, 10 wt% carbon black and 20 wt% PEO (WSR N-60K), representing the prior art,

The formulation comprises: 80 wt% of the active material of Example 1, 5 wt% of carbon black and 15 wt% of PEO.

The positive electrode formulation is applied to the electrode via a paste in a solvent, followed by drying.

The capacity of the positive electrodes of the test cells was between 1.5 and 3 mAh/ ^cm2 .

The test cells were placed under charge/discharge conditions.

The performance of the positive electrode was evaluated after 150 cycles:

- Positive electrode prepared from reference formulation: 78%, relative to the initial capacity,

Positive electrode prepared from a formulation comprising an active material according to the invention: 88%, relative to the initial capacity.

These results demonstrate that the inclusion of carbon-based nanofillers in the active material according to the invention makes it possible to improve the lifetime and therefore the performance of Li/S batteries.