WO2016012275A1

WO2016012275A1 - Composites comprising mxenes for cathodes of lithium sulfur cells

Info

Publication number: WO2016012275A1
Application number: PCT/EP2015/065833
Authority: WO
Inventors: Arnd Garsuch; Linda Nazar; Xiao Liang
Original assignee: Basf Se; University Of Waterloo The Guelph-Waterloo Centre
Priority date: 2014-07-22
Filing date: 2015-07-10
Publication date: 2016-01-28

Abstract

The present invention relates to an electroactive composite for an electrochemical cell comprising (A1) an electroactive sulfur-containing material, and (A2) nanosheets of an early transition metal compound comprising an array of crystal cells, wherein each crystal cell has an empirical formula (I) of M_n+1X_n, such that each X is positioned within an octahedral array of M, wherein M is at least one early transition metal selected from the group consisting of Sc, Y, La, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, X is C and/or N, and n is 1, 2 or 3. The present invention further relates to a cathode material for an electrochemical cell comprising said electroactive composite, to a cathode and an electrochemical cell comprising said cathode material and to a process for preparing said electroactive composite.

Description

Composites comprising MXenes for cathodes of lithium sulfur cells Description The present invention relates to an electroactive composite for an electrochemical cell comprising

(A1 ) an electroactive sulfur-containing material, and (A2) nanosheets of an early transition metal compound comprising an array of crystal cells, wherein each crystal cell has an empirical formula (I) of M_n+iX_n, such that each X is positioned within an octahedral array of M, wherein

M is at least one early transition metal selected from the group consisting of Sc, Y, La, Ti, Zr,

Hf, V, Nb, Ta, Cr, Mo and W,

X is C and / or N, and

n is 1 , 2 or 3.

The present invention further relates to a cathode material for an electrochemical cell comprising said electroactive composite, to a cathode and an electrochemical cell comprising said cathode material and to a process for preparing said electroactive composite.

Secondary batteries, accumulators or rechargeable batteries are just some embodiments by which electrical energy can be stored after generation and used when required. Due to the significantly better power density, there has been a move in recent times away from the water- based secondary batteries to development of batteries in which the charge transport in the elec- trical cell is accomplished by lithium ions.

However, the energy density of conventional lithium ion batteries which have a carbon anode and a cathode based on metal oxides is limited. New horizons with regard to energy density have been opened up by lithium-sulfur cells. In lithium-sulfur cells, sulfur in the sulfur cathode is reduced via polysulfide ions to S^2_, which is reoxidized when the cell is charged to form sulfur- sulfur bonds.

A problem, however, is the solubility of the polysulfides, for example L12S4 and L12S6, which are generally soluble in the solvent and can migrate to the anode. The consequences may include: loss of capacitance and deposition of electrically insulating material on the sulfur particles of the electrode. The migration of the polysulfide ions from the cathode to anode can ultimately lead to discharge of the affected cell and to cell death in the battery. This unwanted migration of polysulfide ions is also referred to as "shuttling", a term which is also used in the context of the present invention. Carbon sulfur composites are important components of the cathodes of lithium sulfur cells contributing significantly to the overall performance of lithium sulfur cells in particular with respect to lowering the internal impedance by providing a conductive element. Depending on the porous carbon architecture and its surface modification, the carbon framework can increase the cou- lombic efficiency, lower the degree of capacity fading, and help limit self-discharge by physically trapping polysulfide ions within the cathode although these effects are usually limited to short- term cycling,

US 6,210,831 describes solid composite cathodes which comprise (a) sulfur-containing cathode material which, in its oxidized state, comprises a polysulfide moiety of the formula, -S_m-, wherein m is an integer from 3 to 10; and (b) a non-electroactive particulate material having a strong adsorption of soluble polysulfides.

US 8,173,302 describes an electrode material having carbon and sulfur, wherein the carbon is in the form of a porous matrix having nanoporosity and the sulfur is sorbed into the nanoporosity of the carbon matrix.

US 2013/0065127 describes sulfur cathodes for use in an electric current producing cells or rechargeable batteries. The sulfur cathode comprises an electroactive sulfur containing materi- al, an electrically conductive filler and a non-electroactive component.

WO 2012/177712 describes compositions comprising free standing and stacked assemblies of two dimensional crystalline solids, and methods of making the same. The sulfur-containing electroactive composites or cathode materials described in the literature still have shortcomings with regard to one or more of the properties desired for such materials and the electrochemical cells produced therefrom. Desirable properties are, for example, high electrical conductivity of the cathode materials, maintenance of cathode capacity during lifetime, reduced self-discharge of the electrochemical cells during storage, an increase in the lifetime of the electrochemical cell, an improvement in the mechanical stability of the cathode or a reduced change in volume of the cathodes during a charge-discharge cycle. In general, the desired properties mentioned also make a crucial contribution to improving the economic viability of the electrochemical cell, which, as well as the aspect of the desired technical performance profile of an electrochemical cell, is of crucial significance to the user.

It was thus an object of the present invention to provide a beneficial sulfur-containing electroactive composite or the corresponding cathode material for a lithium-sulfur cell, which have advantages over one or more properties of a known materials, more particularly a sulfur- containing electroactive composite and accordingly the cathode material which enable the con- struction of cathodes with an improved electrical conductivity, combined with high cathode capacity, high mechanical stability and long lifetime. This object is achieved by an electroactive composite for an electrochemical cell comprising

M is at least one early transition metal selected from the group consisting of Sc, Y, La, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, preferably consisting of Ti, V, Cr and Ta, more preferably consisting of Ti and V, and in particular consisting of Ti,

X is C and / or N, and

n is 1 , 2 or 3, preferably 2 or 3, more preferably 2 in case of a transition metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, in particular Ti, or 3 in case of a transition metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, in particular Ta.

In the context with the present invention, the electrode where during discharging a net negative charge occurs is called the anode and the electrode where during discharging a net positive charge occurs is called the cathode. An inventive electroactive composite for an electrochemical cell, also referred to hereinafter as electroactive composite (A), for short, comprises, as component (A1 ), an electroactive sulfur- containing material, also referred to hereinafter as electroactive material (A1 ) for short, and, as component (A2), nanosheets of an early transition metal compound comprising an array of crystal cells, wherein each crystal cell has an empirical formula (I) of M_n+iX_n, as described above, also referred to hereinafter as MXene-nanosheets (A2) for short.

Electroactive sulfur-containing materials are either covalent compounds like elemental sulfur, composites produced from elemental sulfur and at least one polymer or polymers comprising polysulfide bridges or ionic compounds like salts of sulfides or polysulfides.

Elemental sulfur is known as such.

Composites produced from elemental sulfur and at least one polymer, which find use as a constituent of electrode materials, are likewise known to those skilled in the art. Adv. Funct. Mater. 2003, 13, 487 - 492 describes, for example, a reaction product of sulfur and

polyacrylonitrile, which results from elimination of hydrogen from polyacrylonitrile with simultaneous formation of hydrogen sulfide. Polymers comprising divalent di- or polysulfide bridges, for example polyethylene tetrasulfide, are likewise known in principle to those skilled in the art. J. Electrochem. Soc, 1991 , 138, 1896 - 1901 and US 5,162,175 describe the replacement of pure sulfur with polymers comprising disulfide bridges. Polyorganodisulfides are used therein as materials for solid redox

polymerization electrodes in rechargeable cells, together with polymeric electrolytes.

Salts of sulfides or polysulfides are examples of ionic compounds comprising at least one Li-S- group like L12S, lithium polysulfides (Li2S2 to 8) or lithiated thioles (lithium thiolates). A preferred electroactive sulfur-containing material (A1 ) is elemental sulfur. Particularly preferred is elemental sulfur in the form of colloidal sulfur, wherein the size of the sulfur particles is in the nano-range.

In one embodiment of the present invention, the inventive electroactive composite (A) is charac- terized in that the electroactive sulfur-containing material (A1 ) is elemental sulfur.

Nanosheets of an early transition metal compound comprising an array of crystal cells, wherein each crystal cell has an empirical formula (I) of M_n+iX_n, as described above, and processes to produce such MXene-nanosheets (A2) are known to the person skilled in the art. As mentioned in the introduction WO 2012/177712 discloses the synthesis of MXene-nanosheets (A2) by removing layers of main group element A from a MAX-phase composition having formula (II)

wherein M, X and n are defined as described above and A is a main group element selected from the group of elements consisting of Al, Si, P, S, Ga, Ge, As, Cd, In, Sn, Tl and Pb, preferably consisting of Al, Ga, In, Tl, Si, Ge, Sn and Pb, more preferably consisting of Al and Si, in particular Al, followed by delamination of stacked nanosheets of the early transition metal compound.

The crystal cells of empirical formula (I) of MXene-nanosheets (A2) are usually arranged to each other in the same way as in MAX-phase compositions of formula (II). Preferably the crystal cells of empirical formula (I) form a substantially two-dimensional array. A single MXene- nanosheet (A2) can be arranged in different ways like a sheet of paper, which can be arranged in different ways. Preferably a single MXene-nanosheet (A2) is in the form of a plane, a scroll, or a tube. Furthermore the single MXene-nanosheets (A2), which have two major surfaces can be coated with a coating comprising alkoxide, carboxylate, halide, hydroxide, hydride, oxide, suboxide, nitride, sub-nitride, sulfide, thiol, or a combination thereof, depending on the chemical treatment during or after the removal of layers of main group element A from a MAX-phase composition having formula (II) as described above. In one embodiment of the present invention, the inventive electroactive composite (A) is characterized in that the crystal cells of empirical formula (I) form a substantially two- dimensional array. In another embodiment of the present invention, the inventive electroactive composite (A) is characterized in that a single nanosheet of the early transition metal compound is in the form of a plane, a scroll, or a tube.

In a further embodiment of the present invention, the inventive electroactive composite (A) is characterized in that at least one surface of the nanosheets is coated with a coating comprising alkoxide, carboxylate, halide, hydroxide, hydride, oxide, suboxide, nitride, sub-nitride, sulfide, thiol, or a combination thereof.

Preferred examples the empirical formula (I) of M_n+iX_n are T13C2, T13CN , T12C, Ta₄C3, or (Vi 2Cri 2)3C2, more preferably T12C, T13C2, T13CN or (Vi 2Cri 2)3C2, in particular T12C.

In one embodiment of the present invention, the inventive electroactive composite (A) is characterized in that the empirical formula (I) of M_n+iX_n is T13C2, T13CN , T12C, Ta₄C3, or

(Vi 2Cri 2)3C2, more preferably T12C, T13C2, T13CN or (Vi 2Cri 2)3C2, in particular T12C.

The thickness of the MXene-nanosheets (A2) can be varied in a wide range depending on the applied process of production, in particular depending on the reaction conditions of the removal of layers of main group element A from a MAX-phase composition having formula (II) as described above and on the degree of delamination of the resulting stacked MXene-nanosheets (A2). The thickness of a single layer MXene-nanosheet depends usually on the dimensions of the crystal cells forming said single layer, and is preferably in the range from 0.2 to 0.7 nm. Preferably the MXene-nanosheets (A2), which comprise at least of a single layer MXene- nanosheet, have an average thickness in the range from 0.2 nm to 50 nm, more preferably a thickness in the range from 1 to 20 nm, in particular a thickness in the range from 1 to 10 nm.

In one embodiment of the present invention, the inventive electroactive composite (A) is characterized in that the nanosheets of an early transition metal compound (A2) have an average thickness in the range from 0.2 nm to 50 nm, more preferably a thickness in the range from 1 to 20 nm, in particular a thickness in the range from 1 to 10 nm.

Accompanied by the decrease of the thickness of the MXene-nanosheets (A2) the BET surface area of the MXene-nanosheets (A2) is increased. The BET surface area of the MXene- nanosheets (A2) can be varied in a wide range by the methods described above. Preferably the MXene-nanosheets (A2), which comprise at least of a single layer MXene-nanosheet, have a BET surface area in the range from 10 m²/g to 500 m²/g, more preferably a BET surface area in the range from 40 m²/g to 300 m²/g, in particular 50 m²/g to 100 m²/g. The ratio of the mass fraction of all electroactive sulfur-containing material (A1 ) to the mass fraction of the MXene-nanosheets (A2) can be varied in a wide range. Preferably the ratio of the mass fraction of all electroactive sulfur-containing material (A1 ) to the mass fraction of the MXene-nanosheets (A2) is in the range from 0.2 to 0.95, preferably in the range from 0.35 to 0.9, in particular in the range from 0.5 to 0.85.

In one embodiment of the present invention, the inventive electroactive composite (A) is characterized in that the ratio of the mass fraction of all electroactive sulfur-containing materials (A1 ) to the mass fraction of the nanosheets of an early transition metal compound (A2) is in the range from 0.2 to 0.95, preferably in the range from 0.35 to 0.9, in particular in the range from 0.5 to 0.85.

The sum of the mass fraction of all electroactive sulfur-containing material (A1 ) and of the mass fraction of the MXene-nanosheets (A2) based on the total mass of the electroactive composite (A) can be varied in a wide range depending on the amount of components in addition to components (A1 ) and (A2). Preferably the sum of the mass fraction of all electroactive sulfur- containing material (A1 ) and of the mass fraction of the MXene-nanosheets (A2) based on the mass of the electroactive composite (A) is in the range from 0.5 to 1 , preferably in the range from 0.8 to 1 , in particular in the range from 0.9 to 1 .

In one embodiment of the present invention, the inventive electroactive composite for an electrochemical cell is characterized in that the electroactive sulfur-containing material (A1 ) is elemental sulfur, preferably elemental sulfur in the form of colloidal sulfur, and the empirical formula (I) of M_n+iXn is Ti₃C₂, Ti₃CN, Ti₂C, Ta₄C₃, or (Vi ₂Cri 2)3C₂, more preferably Ti₂C, Ti₃C₂, Ti₃CN or (Vi ₂Cri ₂)3C₂, in particular Ti₂C, wherein the ratio of the mass fraction of elemental sulfur to the mass fraction of MXene-nanosheets (A2) is in the range from 0.5 to 0.85 and the sum of the mass fraction of elemental sulfur and of the mass fraction of MXene-nanosheets (A2) is in the range from 0.90 to 1. The inventive electroactive composite for an electrochemical cell can be further improved with respect to higher conductivity, higher sulfur content due to a higher surface area of the early transition metal compound or reduced capacity fade rates of the corresponding electrochemical cells comprising said inventive electroactive composite by mixing the nanosheets of the early transition metal compound (A2) with an electrically conductive additive as component (A3) be- fore contacting the resulting mixture with the electroactive sulfur-containing material (A1 ).

In one embodiment of the present invention, the inventive electroactive composite (A) additionally comprises as a further component (A3) an electrically conductive additive, preferably an electrically conductive, carbonaceous material, more preferably selected from the group consist- ing of graphite, activated carbon, carbon black, carbon nanotubes, graphene and mixtures of at least two of the aforementioned substances, in particular carbon nanotubes. Electrically conductive, carbonaceous materials, in particular carbon nanotubes which improve the electrical conductivity of the inventive electroactive composite are known as such and are described in WO 2012/168851 page 4, line 30 to page 6, line 22. The amount of the electrically conductive additive (A3) in the inventive electroactive composite can be varied in a wide range. Preferably the electrically conductive additive (A3) is used in such amounts that the mass fraction of all electrically conductive additive (A3) to the sum of the mass fraction of MXene-nanosheets (A2) and of the mass fraction of all electrically conductive additives (A3) is in the range from 0.05 to 0.3, preferably from 0.08 to 0.25.

In one embodiment of the present invention, the inventive electroactive composite for an electrochemical cell is characterized in that the electroactive sulfur-containing material (A1 ) is elemental sulfur, preferably elemental sulfur in the form of colloidal sulfur, the empirical formula (I) of Mn+iXn is T13C2, T13CN , T12C, Ta₄C3, or (Vi 2Cn 2)3C2, more preferably T12C, T13C2, T13CN or (Vi 2Cn 2)3C2, in particular T12C, and the electrically conductive additive (A3) are carbon nanotubes, wherein the ratio of the mass fraction of elemental sulfur to the mass fraction of MXene- nanosheets (A2) is in the range from 0.5 to 0.85 and the mass fraction of the carbon nanotubes to the sum of the mass fraction of MXene-nanosheets (A2) and of the mass fraction of the carbon nanotubes is in the range from 0.08 to 0.25.

The invention further provides a process for preparing an electroactive composite for an electrochemical cell comprising

(A1 ) an electroactive sulfur-containing material, and

(A2) nanosheets of an early transition metal compound comprising an array of crystal cells, wherein each crystal cell has an empirical formula (I) of M_n+iX_n, such that each X is positioned within an octahedral array of M, wherein

X is C and / or N, and

n is 1 , 2 or 3, preferably 2 or 3, more preferably 2 in case of a transition metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, in particular Ti, or 3 in case of a transition metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, in particular Ta. comprising the process step of (a) loading the electroactive sulfur-containing material (A1 ) onto the nanosheets of the early transition metal compound (A2).

The description and preferred embodiments of the electroactive composite for an electrochemi- cal cell (electroactive composite (A)) and its components, in particular the description of the electroactive sulfur-containing material (A1 ) as a first component and of the nanosheets of an early transition metal compound (A2) as a second component, in the inventive process correspond to the above description of these components for the electroactive composite (A) of the present invention.

In one embodiment of the present invention, the inventive process for preparing an electroactive composite (A) is characterized in that in process step (a) the electroactive sulfur-containing material (A1 ) is elemental sulfur, particularly colloidal sulfur. In one embodiment of the present invention, the inventive process for preparing an electroactive composite (A) is characterized in that the nanosheets of the early transition metal compound (A2) in the electroactive composite are MXene nanosheets, which are obtained by removing layers of main group element A from a MAX-phase composition having formula (II)

wherein

M is at least one early transition metal selected from the group consisting of Sc, Y, La, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, preferably Ti, V, Cr and Ta, more preferably Ti and V, and in particular Ti,

A is a main group element selected from the group of elements consisting of Al, Si, P, S, Ga, Ge, As, Cd, In, Sn, TI and Pb, preferably Al, Ga, In, TI, Si, Ge, Sn and Pb, more preferably Al and Si, in particular Al,

X is C and / or N, and

n is 1 , 2 or 3, preferably 2 or 3, more preferably 2 in case of the transition metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, in particular Ti, or 3 in case of the transition metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, in particular Ta, followed by delamination of stacked nanosheets of the early transition metal compound.

In one embodiment of the present invention, the inventive process for preparing an electroactive composite (A) is characterized in that the nanosheets of the early transition metal compound (A2) in the electroactive composite are MXene nanosheets, which are obtained by removing layers of main group element A from a MAX-phase composition having formula (II)

wherein M is at least one early transition metal selected from the group consisting of Sc, Y, La, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, preferably Ti, V, Cr and Ta, more preferably Ti and V, and in particular Ti,

X is C and / or N, and

n is 1 , 2 or 3, preferably 2 or 3, more preferably 2 in case of the transition metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, in particular Ti, or 3 in case of the transition metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, in particular Ta, followed by delamination of stacked nanosheets of the early transition metal compound, and wherein in process step (a) the electroactive sulfur-containing material (A1 ) is elemental sulfur in form of colloidal sulfur.

The removal of layers of main group element A from a MAX-phase composition is known to the person skilled in the art. Usually the removal is performed with the aid of a strong aqueous acid, preferably a strong mineral acid, preferably a strong non-oxidizing mineral acid. After removing main group element A from the treated MAX-phase a MX-material or exfoliated MX material is obtained that consists essentially of stacked nanosheets or so-called multilayers, having a thickness of in the range of 10 nm and more.

Methods for further delaminating the stacked nanosheets, which have been obtained by remov- al of layers of main group element A from a MAX-phase composition, in order to obtain MXene- nanosheets having a thickness in the range from 0.2 to 10 nm, preferably 1 to 8 nm are known to the person skilled in the art. Particles of stacked MXene-nanosheet can be suspended in an organic solvent, preferably a polar organic solvent, in particular an aprotic dipolar solvent, like DMSO.

Process step (a) can be executed in a wide temperature range. Preferably process step (a) comprises a heat treatment at a temperature in the range from 100 to 200°C, preferably in the range from 120 to 180°C, in particular in the range from 150 to 160°C. In one embodiment of the present invention, the inventive process for preparing an electroactive composite (A) is characterized in that process step (a) comprises a heat treatment at a tempera- ture in the range from 100 to 200°C, preferably in the range from 120 to 180°C, in particular in the range from 150 to 160°C.

In one embodiment the inventive process for preparing an electroactive composite (A) is char- acterized in that in process step (a) as component (A1 ) elemental sulfur in form of colloidal sulfur is loaded on MXene-nanosheets (A2) comprising an array of crystal cells, wherein each crystal cell has an empirical formula (I) of M_n+iX_n, wherein the empirical formula (I) of M_n+iX_n is T13C2, Ti₃CN, Ti₂C, Ta₄C3, or (Vi ₂Cri ₂)3C2, more preferably Ti₂C, T13C2, Ti₃CN or (Vi ₂Cri ₂)3C2, in particular T12C, as component (A2) comprising a heat treatment at a temperature in the range from 100 to 200°C, preferably in the range from 120 to 180°C, in particular in the range from 150 to 160°C.

In order to improve the inventive electroactive composite for an electrochemical cell the inventive process is complemented by mixing the nanosheets of the early transition metal com- pound (A2) with an electrically conductive additive (A3) before performing process step (a).

In one embodiment of the present invention, the inventive process for preparing an electroactive composite (A) is characterized in that the nanosheets of the early transition metal compound (A2) are mixed with an electrically conductive additive (A3) before performing process step (a).

The description and preferred embodiments of the electrically conductive additive (A3) in the inventive process correspond to the above description of this component.

The preparation of a mixture of the nanosheets of the early transition metal compound (A2), in particular MXene-nanosheets (delaminated MXene), with an electrically conductive additive (A3), in particular carbon nanotubes, can be done in different ways known to the person skilled in the art, preferably by adding component (A3) to a suspension of component (A2) in a super acid solution such as chlorosulfonic acid. As described above the MXene-nanosheets (A2) can be used as a component for the preparation of an electroactive composite for an electrochemical cell comprising an electroactive sulfur- containing material.

The invention further provides the use of nanosheets of an early transition metal compound (A2) comprising an array of crystal cells, wherein each crystal cell has an empirical formula (I) of such that each X is positioned within an octahedral array of M, wherein M is at least one early transition metal selected from the group consisting of Sc, Y, La, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, preferably consisting of Ti, V, Cr and Ta, more preferably consisting of Ti and V, and in particular consisting of Ti,

X is C and / or N, and

n is 1 , 2 or 3, preferably 2 or 3, more preferably 2 in case of a transition metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, in particular Ti, or 3 in case of a transition metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, in particular Ta as a component for the preparation of an electroactive composite for an electrochemical cell comprising as a further component an electroactive sulfur-containing material.

The description and preferred embodiments of the nanosheets of an early transition metal compound (A2) correspond to the above description of this component for the electroactive compo- site (A) of the present invention. Particularly preferred is the use of MXene-nanosheets (A2), which comprise an array of crystal cells, wherein each crystal cell has an empirical formula (I) of Mn+iXn, which is T13C2, T13CN , T12C, Ta₄C3, or (Vi 2Cri 2)sC2, more preferably T12C, T13C2, T13CN or (Vi 2Cri 2)3C2, in particular T12C, as a component for the preparation of an electroactive composite for an electrochemical cell comprising as a further component elemental sulfur.

The inventive electroactive composite (A) can ultimately be used as an essential constituent of cathode materials for electrochemical cells, especially lithium-sulfur cells. Preferably the inventive electroactive composite (A) is combined with a carbon, which improves the electrical conductivity of the cathode material, and optionally at least one binder, which is typically an or- ganic polymer. The binder serves principally for mechanical stabilization of the components of the electrode, by virtue of electroactive composite (A) particles and carbon particles being bonded to one another by the binder, and also has the effect that the cathode material has sufficient adhesion to an output conductor. The binder is preferably chemically inert toward the chemicals with which it comes into contact in an electrochemical cell.

The present invention further also provides a cathode material for an electrochemical cell comprising

(A) an electroactive composite as described above,

(B) carbon in a polymorph comprising at least 60% sp²-hybridized carbon atoms, and

(C) optionally at least one polymer as a binder.

The inventive cathode material for an electrochemical cell comprises in addition to the inventive electroactive composite (A), which has been described above, as a second component, carbon in a polymorph comprising at least 60% sp²-hybridized carbon atoms, also referred to hereinaf- ter as carbon (B) for short, and optionally as component (C) a polymer as a binder, also referred to hereinafter as binder (C) for short.

The description and preferred embodiments of the electroactive composite (A) and its compo- nents correspond to the above description.

Carbon (B), which improves the electrical conductivity of the inventive cathode material, can be selected from graphite, carbon black, carbon nanotubes, graphene or mixtures of at least two of the aforementioned substances. Suitable carbons in a conductive polymorph are described in WO 2012/168851 page 4, line 30 to page 6, line 22.

In one embodiment of the present invention, the inventive cathode material for an electrochemical cell is characterized in that carbon (B) is selected from graphite, graphene, activated carbon and especially carbon black.

In one embodiment of the present invention, the inventive cathode material for an

electrochemical cell comprises at least one polymer as a binder.

Binder (C) can be selected from a wide range of organic polymers. Suitable binders are described in WO 2012/168851 page 6, line 40 to page 7, line 30.

Particularly suitable binders for the inventive cathode material for an electrochemical cell are especially polyvinyl alcohol, poly(ethylene oxide), carboxymethyl cellulose (CMC) and halogenated (co)polymers, for example polyvinyl chloride or polyvinylidene chloride, especially fluorinated (co)polymers such as polyvinyl fluoride and especially polyvinylidene fluoride, lithiated Nafion and polytetrafluoroethylene.

electrochemical cell comprises in the range from 10 to 80% by weight, preferably 30 to 75% by weight, of sulfur, determined by elemental analysis, based on the total weight of the cathode material for an electrochemical cell.

electrochemical cell comprises in the range from 0.1 to 60% by weight of carbon in a conductive polymorph, preferably 1 to 30% by weight based on the total weight of the cathode material for an electrochemical cell. This carbon can likewise be determined by elemental analysis, for example, in which case the evaluation of the elemental analysis has to take into account the fact that carbon also arrives in organic polymers representing binders, and possibly further sources.

In one embodiment of the present invention, the inventive cathode material for an electrochemical cell comprises in the range from 0.1 to 20% by weight of binder, preferably 1 to 15% by weight and more preferably 3 to 12% by weight, based on the total weight of the cathode material for an electrochemical cell.

Inventive electroactive composite (A) and inventive cathode material are particularly suitable as or for production of cathodes, especially for production of cathodes of lithium-containing batteries. The present invention provides for the use of inventive electroactive composites (A) or inventive cathode materials as or for production of cathodes for electrochemical cells.

The present invention further also provides a cathode which has been produced from or using a cathode material as described above.

The inventive cathode may have further constituents customary per se, for example an current collector, which may be configured in the form of a metal wire, metal grid, metal mesh, expanded metal, metal sheet, metal foil or carbon paper/cloth. Suitable metal foils are especially aluminum foils.

In one embodiment of the present invention, the inventive cathode has a thickness in the range from 25 to 200 μηη, preferably from 30 to 100 μηη, based on the thickness without current collector.

A further feature of inventive electroactive composites (A) or inventive cathode materials is that it is possible in accordance with the invention to produce battery cells which are preferably stable over at least 30 cycles, more preferably over at least 50 cycles, even more preferably over at least 100 cycles, especially over at least 200 cycles or over at least 500 cycles.

In one embodiment of the present invention, electroactive composite (A) or inventive cathode material is processed to cathodes, for example in the form of continuous belts which are processed by the battery manufacturer. Inventive cathodes produced from electroactive composite (A) or inventive cathode material may have, for example, thicknesses in the range from 20 to 500 μηη, preferably 40 to 200 μηη. They may, for example, be in the form of rods, in the form of round, elliptical or square columns or in cuboidal form, or in the form of flat cathodes. The present invention further provides electrochemical cells comprising at least one inventive cathode as described above, which has been produced from or using at least one inventive electroactive composite (A) or at least one inventive cathode material as described above.

During the charging process of an inventive electrochemical cell the inventive cathode comprises usually a mixture of different electroactive sulfur-containing materials since more and more S-S-bonds are formed. In one embodiment of the present invention, inventive electrochemical cells comprise, as well as inventive cathode, which comprises inventive electroactive composite (A) respectively inventive cathode material, at least one anode comprising at least one alkali metal like lithium, sodium or potassium. Preferably the anode of the inventive electrochemical cell comprises lithi- um.

The alkali metal of anode of the inventive electrochemical cell can be present in the form of a pure alkali metal phase, in form of an alloy together with other metals or metalloids, in form of an intercalation compound or in form of an ionic compound comprising at least one alkali metal and at least one transition metal.

The anode of the inventive electrochemical cell can be selected from anodes being based on various active materials. Suitable active materials are metallic lithium, carbon-containing materials such as graphite, graphene, charcoal, expanded graphite, in particular graphite, furthermore lithium titanate (Li4Ti₅0i2), anodes comprising In, Tl, Sb, Sn or Si, in particular Sn or Si, for example tin oxide (Sn02) or nanocrystalline silicon, and anodes comprising metallic lithium.

In one embodiment of the present invention the electrochemical cell is characterized in that the anode of the inventive electrochemical cell is selected from graphite anodes, lithium titanate anodes, anodes comprising In, Tl, Sb, Sn or Si, and anodes comprising metallic lithium.

In one embodiment of the present invention, the inventive electrochemical cell is characterized in that the alkali metal of the anode is lithium. The anode of the inventive electrochemical cell can further comprise a current collector. Suitable current collectors are, e.g., metal wires, metal grids, metal gauze and preferably metal foils such as copper foils.

The anode of the inventive electrochemical cell can further comprise a binder. Suitable binders can be selected from organic (co)polymers. Suitable organic (co)polymers may be halogenated or halogen-free. Examples are polyethylene oxide (PEO), cellulose, carboxymethyl cellulose, polyvinyl alcohol, polyethylene, polypropylene, polytetrafluoroethylene, polyacrylonitrile-methyl methacrylate, styrene-butadiene copolymers, tetrafluoroethylene-hexafluoropropylene copolymers, vinylidene fluoride-hexafluoropropylene copolymers (PVdF-HFP), vinylidene fluoride- tetrafluoroethylene copolymers, perfluoroalkyl vinyl ether copolymers, ethylene- tetrafluoroethylene copolymers, vinylidene fluoride-chlorotrifluoroethylene copolymers, ethylene- chlorofluoroethylene copolymers, ethylene-acrylic acid copolymers, optionally at least partially neutralized with alkali metal salt or ammonia, ethylene-methacrylic acid copolymers, optionally at least partially neutralized with alkali metal salt or ammonia, ethylene-(meth)acrylic ester co- polymers, polysulfones, polyimides and polyisobutene. Suitable binders are especially polyvinyl alcohol and halogenated (co)polymers, for example polyvinyl chloride or polyvinylidene chloride, especially fluorinated (co)polymers such as polyvinyl fluoride and especially polyvinylidene fluoride and polytetrafluoroethylene. The average molecular weight M_w of binder may be selected within wide limits, suitable examples being 20,000 g/mol to 1 ,000,000 g/mol.

In one embodiment of the present invention, the anode of the inventive electrochemical cell can have a thickness in the range of from 15 to 200 μηη, preferably from 30 to 100 μηη, determined without the current collector.

The inventive electrochemical cell further comprises, as well as the inventive cathode and an anode, at least one electrolyte composition comprising at least one solvent and at least one alkali metal salt.

As regards suitable solvents and further additives for nonaqueous liquid electrolytes for lithium- based rechargeable batteries reference is made to the relevant prior art, e.g. Chem Rev. 2004, 104, 4303-4417, in particular table 1 on page 4307, table 2 on page 4308 and table 12 on page 4379.

The solvents of the electrolyte composition can be chosen from a wide range of solvents, in particular from solvents which dissolve alkali metal salts easily. Solvents or solvent systems, which dissolve alkali metal salts, are for example ionic liquids, polar solvents or combinations of apolar solvents combined with polar additives like crown ethers, like 18-crown-6, or cryptands. Examples of polar solvents are polar protic solvents or dipolar aprotic solvents.

Examples of polar protic solvents are water, alcohols like methanol, ethanol or iso-propanol, carbonic acids like acetic acid, ammonia, primary amines or secondary amines. Polar protic solvents can only be used in electrochemical cell comprising an anode, which comprises an alkali metal, if any contact between that anode and the polar protic solvent is strictly precluded by an appropriate separator.

Examples of dipolar aprotic solvents are organic carbonates, esters, ethers, sulfones like DMSO, sulfamides, amides like DMF or DMAc, nitriles like acetonitrile, lactams like NMP, lac- tones, linear or cyclic peralkylated urea derivatives like TMU or DMPU, fluorinated ether, fluorinated carbamates, fluorinated carbonated or fluorinated esters.

Suitable solvents of the electrolyte composition may be liquid or solid at room temperature and are preferably liquid at room temperature.

In one embodiment of the present invention the inventive electrochemical cell is characterized in that the solvent is a dipolar aprotic solvent. A suitable solvent is preferably selected from polymers, cyclic or noncyclic ethers, cyclic or noncyclic acetals, cyclic or noncyclic organic carbonates and ionic liquids. In one embodiment of the present invention the inventive electrochemical cell is characterized in that the solvent is selected from polymers, cyclic or noncyclic ethers, noncyclic or cyclic acetals and cyclic or noncyclic organic carbonates.

Examples of suitable polymers are especially polyalkylene glycols, preferably poly-Ci-C4- alkylene glycols and especially polyethylene glycols. Polyethylene glycols may comprise up to 20 mol% of one or more Ci-C4-alkylene glycols in copolymerized form. Polyalkylene glycols are preferably doubly methyl- or ethyl-capped polyalkylene glycols.

The molecular weight M_w of suitable polyalkylene glycols and especially of suitable polyethylene glycols may be at least 400 g/mol.

The molecular weight M_w of suitable polyalkylene glycols and especially of suitable polyethylene glycols may be up to 5 000 000 g/mol, preferably up to 2 000 000 g/mol. Examples of suitable noncyclic ethers are, for example, diisopropyl ether, di-n-butyl ether, 1 ,2- dimethoxyethane, 1 ,2-diethoxyethane, preference being given to 1 ,2-dimethoxyethane.

Examples of suitable cyclic ethers are tetrahydrofuran and 1 ,4-dioxane. Examples of suitable noncyclic acetals are, for example, dimethoxymethane, diethoxymethane, 1 ,1 -dimethoxyethane and 1 ,1 -diethoxyethane.

Examples of suitable cyclic acetals are 1 ,3-dioxane and especially 1 ,3-dioxolane. Examples of suitable noncyclic organic carbonates are dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate. Examples of suitable cyclic organic carbonates are compounds of the general formulae (X) and (XI)

in which R¹, R² and R³ may be the same or different and are each selected from hydrogen and Ci-C4-alkyl, for example methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl and tert- butyl, where R² and R³ are preferably not both tert-butyl.

In particularly preferred embodiments, R¹ is methyl and R² and R³ are each hydrogen, or R¹, R² and R³ are each hydrogen.

Another preferred cyclic organic carbonate is vinylene carbonate, formula (XII).

Preference is given to using the solvent(s) in what is called the anhydrous state, i.e. with a water content in the range from 1 ppm to 0.1 % by weight, determinable, for example, by Karl Fischer titration.

Possible alkali metal salts, which are used as conductive salts, have to be soluble in the solvent. Preferred alkali metal salts are lithium salts or sodium salts, in particular lithium salts.

In one embodiment of the present invention the inventive electrochemical cell is characterized that the alkali metal salt is a lithium salt or sodium salt, preferably a lithium salt.

Suitable alkali metal salts are especially lithium salts. Examples of suitable lithium salts are LiPF₆, LiBF₄, UCIO4, LiAsFe, UCF3SO3, LiC(CnF₂n₊iS0₂)₃, lithium imides such as

LiN(CnF_2n+iS0₂)₂, where n is an integer in the range from 1 to 20, LiN(S02F)₂, Li₂SiF₆, LiSbF₆, LiAICU, and salts of the general formula (C_nF_2n+iS0₂)mXLi, where m is defined as follows:

m = 1 when X is selected from oxygen and sulfur, m = 2 when X is selected from nitrogen and phosphorus, and

m = 3 when X is selected from carbon and silicon.

Preferred alkali metal salts are selected from LiC(CF₃S0₂)₃, LiN(CF₃S0₂)₂, LiPF₆, LiBF₄, LiCI0₄, and particular preference is given to LiPF6 and LiN(CFsS02)2.

In one embodiment of the present invention, the concentration of conductive salt in electrolyte is in the range of from 0.01 M to 5 M, preferably 0.5 M to 1 .5 M. In one embodiment of the present invention, inventive electrochemical cells comprise one or more separators by which the electrodes are mechanically separated from one another. Suitable separators are polymer films, especially porous polymer films, which are unreactive toward metallic lithium and toward lithium sulfides and lithium polysulfides. Particularly suitable materials for separators are polyolefins, especially porous polyethylene films and porous polypropyl- ene films.

Polyolefin separators, especially of polyethylene or polypropylene, may have a porosity in the range from 35 to 45%. Suitable pore diameters are, for example, in the range from 30 to 500 nm.

In another embodiment of the present invention, the separators selected may be separators composed of PET nonwovens filled with inorganic particles. Such separators may have a porosity in the range from 40 to 55%. Suitable pore diameters are, for example, in the range from 80 to 750 nm.

In one embodiment of the present invention, inventive electrochemical cells can contain additives such as wetting agents, corrosion inhibitors, or protective agents such as agents to protect any of the electrodes or agents to protect the salt(s). In one embodiment of the present invention, inventive electrochemical cells can have a disc-like shape. In another embodiment, inventive electrochemical cells can have a prismatic shape.

In one embodiment of the present invention, inventive electrochemical cells can include a housing that can be from steel or aluminium.

In one embodiment of the present invention, inventive electrochemical cells are combined to stacks including electrodes that are laminated.

In one embodiment of the present invention, inventive electrochemical cells are selected from pouch cells. Inventive electrochemical cells, in particular rechargeable lithium sulfur cells, comprising the inventive electroactive composite (A) have overall advantageous properties. They exhibit good capacity, a low capacity fade rate per cycle, and good cycling stability on extended cycling. A further aspect of the present invention refers to batteries, in particular to rechargeable lithium sulfur batteries, comprising at least one inventive electrochemical cell, for example two or more. Inventive electrochemical cells can be combined with one another in inventive batteries, for example in series connection or in parallel connection. Series connection is preferred. Inventive batteries, in particular rechargeable lithium sulfur batteries, have advantageous properties. They exhibit good capacity, a low capacity fade rate per cycle, and good cycling stability on extended cycling.

The inventive electrochemical cells or inventive batteries can be used for making or operating cars, computers, personal digital assistants, mobile telephones, watches, camcorders, digital cameras, thermometers, calculators, laptop BIOS, communication equipment or remote car locks, and stationary applications such as energy storage devices for power plants. A further aspect of the present invention is a method of making or operating cars, computers, personal digital assistants, mobile telephones, watches, camcorders, digital cameras, thermometers, cal- culators, laptop BIOS, communication equipment, remote car locks, and stationary applications such as energy storage devices for power plants by employing at least one inventive battery or at least one inventive electrochemical cell.

A further aspect of the present invention is the use of the electrochemical cell as described above in motor vehicles, bicycles operated by electric motor, aircraft, ships or stationary energy stores.

The use of inventive electrochemical cells in devices gives the advantage of prolonged run time before recharging and a smaller loss of capacity in the course of prolonged run time. If the in- tention were to achieve an equal run time with electrochemical cells with lower energy density, a higher weight for electrochemical cells would have to be accepted.

The present invention further provides a device comprising at least one inventive electrochemical cell as described above. Preferred are mobile devices such as are vehicles, for example automobiles, bicycles, aircraft, or water vehicles such as boats or ships. Other examples of mobile devices are those which are portable, for example computers, especially laptops, telephones or electrical power tools, for example from the construction sector, especially drills, battery-driven screwdrivers or battery-driven tackers. The invention is illustrated by the examples which follow, but these do not restrict the invention.

Figures in percent are each based on % by weight, unless explicitly stated otherwise. Characterization: The SEM studies were carried out on a Zeiss Ultra field emission SEM instrument. TEM was carried out on a Jeol 201 OF TEM/STEM operating at 200KeV, thermogravimet- ric analysis to determine the sulfur content of the material was performed on a TA Instruments SDT Q600 at a heating rate of 10 °C/min from room temperature to 500 °C in air. FTIR analysis was performed on a Bruker Tensor 37 spectrometer. XPS analysis was performed by using a multi-technique ultra-high vacuum Imaging XPS Microprobe system (Thermo VG Scientific ES- CALab 250), where the samples were sealed in a vial before being transferred to the chamber. The surface area was determined from nitrogen adsorption and desorption isotherms performed on a Quantachrome Autosorb-1 instrument. Before measurement the samples were degassed at 90 °C on a vacuum line. Surface areas were calculated using Brunauer-Emmett-Teller (BET) method.

I. Synthesis of electroactive composites 1.1 Synthesis of MXene-nanosheets

1.1.a Synthesis of Ti2C-MXene-nanosheets A1 -1

To exfoliate T12AIC, commercial T12AIC (3-ONE-2) was sieved (325 mesh) prior use. 10 g of T12AIC were immersed in 370 mL of 10 wt% hydrofluoric acid in an ice bath for 10 hours. The black powder (termed e-Ti2C or A1 -2) was centrifuged and washed three times by deionized (Dl) water. The BET surface area of A1 -2 was 20.2 m²/g.

To fully delaminate the T12C, 300 mg of e-Ti2C were stirred in 5 mL dimethyl sulfoxide (DMSO) at room temperature for 18 hours. The mixture was then centrifuged (10000 rpm, 10 minutes) and the DMSO supernatant was decanted to leave a residue, to which 100 mL Dl water was added. After 4 hours of sonication, centrifugation was carried out again (2000 rpm, 6 minutes) to discard the less dispersed sheets. The supernatant was filtered and "d-Ti2C" (A1 -1 ) was obtained by drying at 60 °C overnight. The BET surface area of A1 -1 was 67.9 m²/g. 1 .1.b Synthesis of Ti₃C2-MXene-nanosheets A2-1

T13AIC2 was prepared by heating a mixture of T12AIC (3-ONE-2), and TiC (Sigma-Aldrich), with a mole ratio of 1 : 1 , at 1350 °C for 2 hours under Ar protection. To exfoliate T13AIC2, as-prepared T13AIC2 was sieved (325 mesh) prior to use. 10 g of T13AIC2 were immersed in 100 mL of 48 wt% hydrofluoric acid for 2 hours. The black powder (termed e-Ti3C2 or A2-2) was centrifuged and washed three times by deionized (Dl) water.

To fully delaminate the T13C2, 300 mg of e-Ti3C2 were stirred in 5 mL dimethyl sulfoxide (DMSO) at room temperature for 18 hours. The mixture was then centrifuged (10000 rpm, 10 minutes) and the DMSO supernatant was decanted to leave a residue, to which 40 mL Dl water was added. After 4 hours of sonication, centrifugation was carried out again (2000 rpm, 6 minutes) to discard the less dispersed sheets. The supernatant was filtered and "d-Ti3C2" (A2-1 ) was obtained by drying at 60 °C overnight. 1 .1 .c Synthesis of TisCN-MXene-nanosheets A3-1

T13CN was prepared by heating a mixture of Ti powder (Alfa Aesar) , AIN (Sigma-Aldrich), and graphite (Alfa Aesar) with a mole ratio of 3 : 1 : 1 , at 1500 °C for 2 hours under Ar protection. To exfoliate T13AICN , as-prepared T13AICN was sieved (325 mesh) prior to use. 10 g of T13AICN were immersed in 65 mL of 30 wt% hydrofluoric acid for 18 hours. The black powder (termed e- T13CN or A3-2) was centrifuged and washed three times by deionized (Dl) water.

To fully delaminate the T13CN , 300 mg of e-Ti3CN were stirred in 5 mL dimethyl sulfoxide (DMSO) at room temperature for 18 hours. The mixture was then centrifuged (10000 rpm, 10 minutes) and the DMSO supernatant was decanted to leave a residue, to which 40 mL Dl water was added. After 4 hours of sonication, centrifugation was carried out again (2000 rpm, 6 minutes) to discard the less dispersed sheets. The supernatant was filtered and "d-Ti3CN" (A3- 1 ) was obtained by drying at 60 °C overnight. 1 .1 .d Synthesis of (Vi/2Cri/2)sC2-MXene-nanosheets A4-1

(Vi 2Cn 2)3AIC2 was prepared by heating a mixture of Ti powder (Alfa Aesar) , Cr powder (Alfa Aesar), Al powder (Alfa Aesar) and graphite (Alfa Aesar) with a mole ratio of 1.5 : 1 .5: 1.2: 1 , at 1550 °C for 2 hours under Ar protection. To exfoliate (Vi 2Cri 2)sAIC2, as-prepared T13AICN was sieved (325 mesh) prior to use. 10 g of Vi.₅Cri.₅AIC2 were immersed in 100 mL of 48 wt% hydrofluoric acid for 65 hours. The black powder (termed e-(Vi 2Cri 2)3C2 or A4-2) was centrifuged and washed three times by deionized (Dl) water.

To fully delaminate the (Vi/2Cr-i/2)3C2, 300 mg of e-(Vi 2Cri 2)3C2 were stirred in 5 mL dimethyl sulfoxide (DMSO) at room temperature for 18 hours. The mixture was then centrifuged (10000 rpm, 10 minutes) and the DMSO supernatant was decanted to leave a residue, to which 40 mL Dl water was added. After 4 hours of sonication, centrifugation was carried out again (2000 rpm, 6 minutes) to discard the less dispersed sheets. The supernatant was filtered and "d- (Vi ₂Cri 2)3C₂" (A4-1 ) was obtained by drying at 60 °C overnight. Table A shows the surface area of the exfoliated and delaminated MXene sheets (m²/g)

1.2 Synthesis of electroactive composites comprising an electroactive sulfur-containing material and materials comprising MXene-nanosheets 1.2. a Synthesis of a sulfur-Ti2C-MXene-nanosheet composite A-1

For sulfur loading onto the Ti2C-MXene-nanosheets A1 -1 , nano-sized sulfur was first synthesized by reacting of 255 mg Na2S203 with 278 μΙ_ concentrated hydrochloric acid (37 wt%) and 17 mg poly (vinylpyrrolidinone) (PVP) in 85 ml. Dl water. Ti2C-MXene-nanosheets A1 -1 and nano-sized sulfur were dispersed separately in 15 mL Dl water by sonication before being mixed to obtain a homogenous suspension. The weight ratio of T12C to sulfur was set to (3:7). The suspension was filtered and then dried at 60 °C. The S/T12C composite A-1 was obtained by heating the mixtures at 155 °C overnight. l.2.b Synthesis of a sulfur-e-Ti2C-composite A-2

For sulfur loading onto the e-Ti2C (A1 -2), nano-sized sulfur was first synthesized by reacting of 255 mg Na2S203 with 278 μΙ_ concentrated hydrochloric acid (37 wt%) and 17 mg poly (vinylpyr- rolidinone) (PVP) in 85 mL Dl water. The e-Ti2C A1 -2 and nano-sized sulfur were dispersed separately in 15 mL Dl water by sonication before being mixed to obtain a homogenous suspension. The weight ratio of T12C to sulfur was set to (3:7). The suspension was filtered and then dried at 60 °C. The S/T12C composite A-2 was obtained by heating the mixtures at 155 °C overnight.

I.2.C Synthesis of a sulfur-Ti3C2-MXene-nanosheet composite A-3

For sulfur loading onto the Ti3C2-MXene-nanosheets A2-1 , nano-sized sulfur was first synthesized by reacting of 255 mg Na2S203 with 278 μί concentrated hydrochloric acid (37 wt%) and 17 mg poly (vinylpyrrolidinone) (PVP) in 85 mL Dl water. Ti3C2-MXene-nanosheets A2-1 and nano-sized sulfur were dispersed separately in 15 mL Dl water by sonication before being mixed to obtain a homogenous suspension. The weight ratio of T13C2 to sulfur was set to (2.5:7.5). The suspension was filtered and then dried at 60 °C. The S/T13C2 composite A-3 was obtained by heating the mixture at 155 °C overnight. l.2.d Synthesis of a sulfur-TisCN-MXene-nanosheet composite A-4

For sulfur loading onto the TisCN-MXene-nanosheets A3-1 , nano-sized sulfur was first synthesized by reacting of 255 mg Na2S203 with 278 μί concentrated hydrochloric acid (37 wt%) and 17 mg poly (vinylpyrrolidinone) (PVP) in 85 mL Dl water. TisCN-MXene-nanosheets A3-1 and nano-sized sulfur were dispersed separately in 15 mL Dl water by sonication before being mixed to obtain a homogenous suspension. The weight ratio of T13CN to sulfur was set to (3.7:6.3). The suspension was filtered and then dried at 60 °C. The S/T13CN composite A-4 was obtained by heating the mixture at 155 °C overnight. 1.2. e Synthesis of a sulfur-(Vi/2Cri/2)3C2-MXene-nanosheet composite A-5

For sulfur loading onto the (Vi/2Cri/2)3C2-MXene-nanosheets A4-1 , nano-sized sulfur was first synthesized by reacting of 255 mg Na2S203 with 278 μΙ_ concentrated hydrochloric acid (37 wt%) and 17 mg poly (vinylpyrrolidinone) (PVP) in 85 mL Dl water. (Vi/2Cr-i/2)3C2-MXene- nanosheets A4-1 and nano-sized sulfur were dispersed separately in 15 mL Dl water by sonication before being mixed to obtain a homogenous suspension. The weight ratio of (Vi 2Cri 2)3C2 to sulfur was set to (5.9:4.1 ). The suspension was filtered and then dried at 60 °C. The

SA/i.5Cri.₅C2 composite A-5 was obtained by heating the mixture at 155 °C overnight.

1.3 Synthesis of electroactive composites comprising an electroactive sulfur-containing material, materials comprising MXene-nanosheets and carbon nanotubes 1.3. a Synthesis of a sulfur-Ti2C-carbon nanotubes (CNT) composite A-6

Preparation of CNT-Ti₂C-MXene (A1 -3): CNT (10 wt%; 50 nm CNT) were added to Ti₂C- MXene-nanosheets A1 -1 (delaminated MXenes sheets) by stirring in a super acid solution (chlorosulfonic acid). The MXene nanosheets are proved to have no chemical reaction with the super acid. In details, MXene and CNTs were stirred in 10 mL super acid overnight. The suspension was filtered on an AAO membrane and dried at 60 °C overnight.

Preparation of sulfur-Ti2C-carbon nanotubes (CNT) composite A-6:

Sulfur-Ti2C-CNT were prepared by melt diffusion: nano-sized sulfur was first synthesized by reacting of 255 mg Na2S2C"3 with 278 μί concentrated hydrochloric acid (37 wt%) and 17 mg poly (vinylpyrrolidinone) (PVP) in 85 mL Dl water. The CNT-MXene (A1 -3) and nano-sized sulphur were dispersed separately in 15 mL Dl water by sonication before being mixed to obtain a homogenous suspension. The suspension was filtered and then dried at 60 °C. The S/ CNT- MXene composite A-6 was obtained by heating the mixtures at 155 °C overnight in a pellet die.

1.3. b Synthesis of a sulfur-Ti3C2-carbon nanotubes composite A-7

Preparation of CNT-Ti₃C₂-MXene (A2-3): CNT (10 wt%; 8 nm CNT) were added to Ti₃C₂- MXene-nanosheets A2-1 (delaminated MXenes sheets) by stirring in a super acid solution (chlorosulfonic acid). The MXene nanosheets are proved to have no chemical reaction with the super acid. In details, MXene and CNTs were stirred in 10 mL super acid overnight. The suspension was filtered on an AAO membrane and dried at 60 °C overnight. Preparation of sulfur-Ti3C2-carbon nanotubes (CNT) composite A-7:

Sulfur- T13C2-CNT were prepared by melt diffusion: nano-sized sulfur was first synthesized by reacting of 255 mg Na2S203 with 278 μΙ_ concentrated hydrochloric acid (37 wt%) and 17 mg poly (vinylpyrrolidinone) (PVP) in 85 mL Dl water. The CNT-MXene (A2-3) and nano-sized sul- phur were dispersed separately in 15 mL Dl water by sonication before being mixed to obtain a homogenous suspension. The suspension was filtered and then dried at 60 °C. The S/ CNT- MXene composite A-7 was obtained by heating the mixtures at 155 °C overnight in a pellet die.

I.3.C Synthesis of a sulfur-TisCN-carbon nanotubes composite A-8

Preparation of CNT- Ti₃CN-MXene (A3-3): CNT (20 wt%; 8 nm CNT) were added to T13CN- MXene-nanosheets A3-1 (delaminated MXenes sheets) by stirring in a super acid solution (chlorosulfonic acid). The MXene nanosheets are proved to have no chemical reaction with the super acid. In details, MXene and CNTs were stirred in 10 mL super acid overnight. The sus- pension was filtered on an AAO membrane and dried at 60 °C overnight.

Preparation of sulfur- TisCN-carbon nanotubes (CNT) composite A-8:

Sulfur-TisCN -CNT were prepared by melt diffusion: nano-sized sulfur was first synthesized by reacting of 255 mg Na2S2C"3 with 278 μί concentrated hydrochloric acid (37 wt%) and 17 mg poly (vinylpyrrolidinone) (PVP) in 85 mL Dl water. The CNT-MXene (A3-3) and nano-sized sulphur were dispersed separately in 15 mL Dl water by sonication before being mixed to obtain a homogenous suspension. The suspension was filtered and then dried at 60 °C. The S/ CNT- MXene composite A-8 was obtained by heating the mixtures at 155 °C overnight in a pellet die. Table B shows the surface area of the MXene and CNT-MXene composites (m²/g)

Table C shows the sulfur content on the MXene and CNT-MXene composites (wt%)

Ti₂C T13C2 TisCN

MXene 70 (A-1 ) 64 (A-3) 75 (A-4)

CNT-MXene 83 (A-6) 79 (A-7) 83 (A-8) 11. Production of cathodes

11.1 Production of cathodes K.1 from A-1 The cathodes were prepared by casting a dimethylformamide slurry containing sulfur-Ti2C-

MXene-nanosheet composite A-1 with carbon black (Super P, commercially available from Tim- cal AG, 6743 Bodio, Switzerland) and PVDF binder in an 8:1 :1 respective weight ratio onto carbon paper (AVCarb P50) as current collector. The average sulfur loading on the electrode K.1 is about 0.7 - 1 .0 mg cnr².

11.2 Production of cathodes K.2 from A-2

The cathodes were prepared by casting a dimethylformamide slurry containing sulfur-e-Ti2C- composite A-2 with carbon black (Super P, commercially available from Timcal AG, 6743 Bodio, Switzerland) and PVDF binder in an 8:1 :1 respective weight ratio onto carbon paper (AVCarb P50) as current collector. The average sulfur loading on the electrode K.2 is about 0.7 - 1.0 mg cm^"2.

11.3 Production of cathodes K.3 from A-3

The cathodes were prepared by casting a dimethylformamide slurry containing sulfur-Ti3C2- MXene-nanosheet composite A-3 with carbon black (Super P, commercially available from Timcal AG, 6743 Bodio, Switzerland) and PVDF binder in an 8:1 :1 respective weight ratio onto carbon paper (AVCarb P50) as current collector. The average sulfur loading on the electrode K.3 is about 0.7 - 1 .0 mg cm-².

11.4 Production of cathodes K.4 from A-4

The cathodes were prepared by casting a dimethylformamide slurry containing sulfur-TisCN- MXene-nanosheet composite A-4 with carbon black (Super P, commercially available from Timcal AG, 6743 Bodio, Switzerland) and PVDF binder in an 8:1 :1 respective weight ratio onto carbon paper (AVCarb P50) as current collector. The average sulfur loading on the electrode K.4 is about 0.7 - 1 .0 mg cnr². 11.5 Production of cathodes K.5 from A-5

The cathodes were prepared by casting a dimethylformamide slurry containing sulfur- (Vi 2Cri 2)3C2-MXene-nanosheet composite A-5 with carbon black (Super P, commercially available from Timcal AG, 6743 Bodio, Switzerland) and PVDF binder in an 8:1 :1 respective weight ratio onto carbon paper (AVCarb P50) as current collector. The average sulfur loading on the electrode K.5 is about 0.7 - 1 .0 mg cm^"2. 11.6 Production of cathodes K.6 from A-6

The cathodes were prepared by casting a dimethylformamide slurry containing sulfur-Ti2C MXene-nanosheet-CNT composite A-6 with carbon black (Super P, commercially available from Timcal AG, 6743 Bodio, Switzerland) and PVDF binder in an 8:1 :1 respective weight ratio onto carbon paper (AVCarb P50) as current collector. The average sulfur loading on the electrode K.6 is about 1.5 mg cm^"2.

11.7 Production of cathodes K.7 from A-7

The cathodes were prepared by casting a dimethylformamide slurry containing sulfur-Ti3C2 MXene-nanosheet-CNT composite A-7 with carbon black (Super P, commercially available from Timcal AG, 6743 Bodio, Switzerland) and PVDF binder in an 8:1 :1 respective weight ratio onto carbon paper (AVCarb P50) as current collector. The average sulfur loading on the electrode K.7 is about 1.5 mg cm^"2.

11.8 Production of cathodes K.8 from A-8

The cathodes were prepared by casting a dimethylformamide slurry containing sulfur-TisCN MXene-nanosheet-CNT composite A-8 with carbon black (Super P, commercially available from Timcal AG, 6743 Bodio, Switzerland) and PVDF binder in an 8:1 :1 respective weight ratio onto carbon paper (AVCarb P50) as current collector. The average sulfur loading on the electrode K.8 is about 1.5 mg cnr².

III. Testing of the cathodes in electrochemical cells

The electrochemical tests were carried on 2325 coin cells with lithium foil as the anode and Celgard^® 3501 separator sheets (25 μηη, microporous monolayer PP membrane). The electro- lyte used was 1 M LiTFSI in 1 : 1 volume of DME: DOL, with 2 wt% LiNOs. The electrochemical properties were tested on an Arbin Battery Cycler at room temperate between 1.8- 3.0 V except for extremely high rate 2C, 3C (1.7- 3.0 V) and 4C (1.6 - 3.0 V).

The resulting test data of the electrochemical cells are summarized in Table 1 and Table 2. Table 1 : Test results of an inventive electrochemical cell at C/2

Table 2: Test results of an inventive electrochemical cells at C/2

Discharge Discharge Discharge Discharge Discharge capacity capacity capacity capacity capacity

Example 5th cycle 100th cy200th cy400th cy800th cy¬

[mA-h/g S] cle cle cle cle

[mA-h/g S] [mA-h/g S] [mA-h/g S] [mA-h/g S]

Cathode

K.6 based 934 930 875 718 - on A-6

Cathode

K.7 based 964 871 830 - - on A-7

Cathode

K.8 based 946 882 810 662 542 on A-8

Claims

An electroactive composite for an electrochemical cell comprising (A1 ) an electroactive sulfur-containing material, and

M is at least one early transition metal selected from the group consisting of Sc, Y, La,

Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W,

X is C and / or N, and

n is 1 , 2 or 3

The electroactive composite according to claim 1 , wherein the electroactive sulfur- containing material (A1 ) is elemental sulfur.

The electroactive composite according to claim 1 or 2, wherein the empirical formula (I) of M_n+iXn is Ti₃C₂, T13CN, Ti₂C, Ta₄C₃, or (Vi/2Cri/2)₃C2.

The electroactive composite according to any of claims 1 to 3, wherein the nanosheets of an early transition metal compound (A2) have an average thickness in the range from 0.2 nm to 50 nm.

The electroactive composite according to any of claims 1 to 4, wherein the ratio of the mass fraction of all electroactive sulfur-containing materials (A1 ) to the mass fraction of the nanosheets of an early transition metal compound (A2) is in the range from 0.2 to 0.95.

The electroactive composite according to any of claims 1 to 5, wherein the electroactive composite additionally comprises

(A3) an electrically conductive additive.

A cathode material for an electrochemical cell comprising

(A) an electroactive composite according to any of claims 1 to 6, carbon in a polymorph comprising at least 60% sp²-hybridized carbon atoms, and (C) optionally at least one polymer as a binder.

8. A cathode which has been produced from or using a cathode material according to claim 7.

9. An electrochemical cell comprising at least one cathode according to claim 8.

10. A battery comprising at least one electrochemical cell according to claim 9.

1 1 . The use of the electrochemical cell according to claim 9 in motor vehicles, bicycles operated by electric motor, aircraft, ships or stationary energy stores.

12. A device comprising at least one electrochemical cell according to claim 9.

13. A process for preparing an electroactive composite for an electrochemical cell comprising

M is at least one transition metal selected from the group consisting of Sc, Ti, Zr, Hf, V,

Nb, Ta, Cr, Mo and W,

X is C and / or N, and

n is 1 , 2 or 3, according to any of claims 1 to 6, comprising the process step of

(a) loading the electroactive sulfur-containing material (A1 ) onto the nanosheets of the early transition metal compound (A2).

14. The process according to claim 13, wherein the nanosheets of the early transition metal compound (A2) in the electroactive composite are MXene nanosheets, which are obtained by removing layers of main group element A from a MAX-phase composition having for- mula (II)

M_n+iAXn (II), wherein

Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W,

A is a main group element selected from the group of elements consisting of Al, Si, P,

S, Ga, Ge, As, Cd, In, Sn, TI and Pb,

X is C and / or N, and

n is 1 , 2 or 3, followed by delamination of stacked nanosheets of the early transition metal compound, and wherein in process step (a) the electroactive sulfur-containing material (A1 ) is elemental sulfur in form of colloidal sulfur.

The process according to 13 or 14, wherein process step (a) comprises a heat treatment at a temperature in the range from 120 to 180°C.

The process according to any of claims 13 to 15, wherein the nanosheets of the early transition metal compound (A2) are mixed with an electrically conductive additive (A3) before performing process step (a).

The use of nanosheets of an early transition metal compound (A2) comprising an array of crystal cells, wherein each crystal cell has an empirical formula (I) of M_n+iX_n, such that each X is positioned within an octahedral array of M, wherein

Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W,

X is C and / or N, and

n is 1 , 2 or 3, as a component for the preparation of an electroactive composite for an electrochemical cell comprising an electroactive sulfur-containing material.