WO2024110074A1

WO2024110074A1 - Electrochemical cell comprising a sulfur cathode with carbonaceous materials and sparingly solvating electrolytes, method of preparation and uses thereof

Info

Publication number: WO2024110074A1
Application number: PCT/EP2023/068117
Authority: WO
Inventors: Julen CASTILLO RUIZ DE AZÚA; Daniel CARRIAZO MARTÍN; María Ángeles MORENO FERNÁNDEZ; Xabier JUDEZ LÓPEZ; Juan Luis GÓMEZ URBANO; Alexander SANTIAGO SÁNCHEZ; Aitor VILLAVERDE OREJÓN; Chunmei Li; José Antonio González Marcos
Original assignee: Fundación Centro De Investigación Cooperativa De Energías Alternativas Cic Energigune Fundazioa; Universidad Del País Vasco/Euskal Herriko Unibertsitatea
Priority date: 2022-11-25
Filing date: 2023-06-30
Publication date: 2024-05-30

Abstract

The present invention relates to the development of an electrochemical cell comprising a sulfur cathode comprising a combination of at least a carbonaceous material and a graphene-type material, said cathode being synergistically combined with Sparingly Solvating Electrolytes (SSEs). Additionally, the invention relates to a method for preparing said electrochemical cell and its use in Li-S secondary batteries.

Description

ELECTROCHEMICAL CELL COMPRISING A SULFUR CATHODE WITH CARBONACEOUS MATERIALS AND SPARINGLY SOLVATING ELECTROLYTES, METHOD OF PREPARATION AND USES THEREOF.

FIELD OF THE INVENTION

The present invention belongs to the technical field of Lithium- Sulfur batteries (LSBs). In particular, the present invention relates to the development of an electrochemical cell comprising a sulfur cathode with a combination of carbonaceous materials, said cathode being synergistically combined with Sparingly Solvating Electrolytes (SSEs). Additionally, the invention relates to a method for preparing said electrochemical cell and its uses.

BACKGROUND

LSBs hold the potential to be the next generation of power cells to be used in electric cars, mobile phones or any application where weight is the critical factor (airplanes, high altitude long endurance unmanned aerial vehicles, high altitude pseudo-satellite, trucks) as they are lighter and cheaper than many of currently used batteries.

Despite its advantages, the Li-S technology has inherent issues due to sulfur undergoing a series of compositional and structural changes during cycling, which involve soluble polysulfides and insoluble sulfides (so-called “polysulfide shuttling”). As a result, researchers have struggled with the maintenance of a stable electrode structure, full utilization of the active material, and sufficient cycle life. Although significant progress has been made, the cycle life and efficiency problems hinder the use of Li-S technology in commercial cells.

Several patent documents have been published aiming to solve the above issues of LSBs. As an example, in W02017218150A1, a core-shell cathode for LSBs is disclosed, the cathode includes an electrically conductive, porous shell and a sulfur-based core enclosed within the shell. The electrically conductive, porous shell encloses the sulfur-based core and blocks the passage of polysulfides from the cathode.

With the aim to increase the percentage of sulfur in the cathode to have large energy density, W02017120391A1 reports a device, comprising: an anode that includes a lithiated silicon-based or lithiated carbon-based material or pure lithium metal or metal oxides and a sandwich-type sulfur-based cathode, wherein the anode and the cathode have porous structures. The sandwich-type Li-S batteries involve the use of exfoliated carbon nanotube sponges.

US2019027793A1 discloses positive electrodes for lithium batteries, particularly lithium sulfur batteries, and the manufacture thereof. Particularly, such electrodes have good performance characteristics, such as capacity retention, even at very high loading of sulfur (e.g. >5 mg/cm²), as well as flexibility. Exemplary manufacturing techniques include the electrospraying of sulfur (e.g., electrode active sulfur compounds), and an optional nanostructured conductive additive onto a porous, conductive substrate (e.g., a porous carbon substrate comprising multiple layers and/or domains).

CN107611395B discloses a positive electrode material for a lithium sulfur battery with good cycle performance, high safety, and high conductivity. The invention produces small-sized graphene by electrolyzing microcrystalline graphite powder; then small-size graphene or surface-modified small-size graphene, such as small-size graphene modified with sodium carboxymethylcellulose, sulfate, silicate, metal ion, metal oxide, non- metallic element or polymer material, is combined with sulfur to prepare a lithium-sulfur battery cathode material. A “graphene lithium sulfur battery” is claimed to be obtained as graphene is generally used as an additive in lithium sulfur batteries and state-of-art electrolytes are reported.

Recently, G. Jimenez-Martin et al. (Batteries & Supercaps 2022, 5, e202200167) reported on the impact of two graphene-based nanomaterials in combination with a DME/DOL standard electrolyte on the final performance of LSBs. The article showed that long-term cyclability was the major drawback of this battery configuration.

The above-mentioned documents focus on classical SoA electrolytes, or do not even mention specific electrolytes. Long-lived, efficient operation under low electrolyte to sulfur (E/S) ratios is a critical challenge for Li-S chemistry. The challenge of operating at low E/S ratios is proposed to be related to maximum solubility of polysulfide species ~3.9 mL/g (i.e. 8 mol S/L) in conventional electrolytes. In the state-of-the-art, and particularly in 1 M Li-bis(trifhioromethane)sulfonamide (LiTFSI) in l,2-dimethoxyethane/l,3- dioxolane (DME/DOL) electrolyte operated at moderate and high E/S ratios (i.e. >4 mL/g), the polysulfides are solution species undergoing electrochemistry and chemical equilibria in the catholyte regime. In this catholyte regime, only the primary reactant, S, and product, IJ2S, are believed to be precipitated.

One of the first approaches to overcome the short life of Li-S batteries with SoA electrolytes was reported as “solvent in salt” concept, which exploited high conductive salt concentrations, and, therefore, led to the dissolution-based mechanism being converted into a quasi-solid-state conversion. However, a limitation of this concept is the high viscosity of the electrolytes provoked by the high salt concentration. In this regard, new electrolyte alternatives need to be developed. Among them, SSEs have been recently studied in Lithium Metal Batteries (LMBs) and, specifically, in LSBs. Non-solvating “diluents” have been introduced in the “solvent in salt” concept to create sparingly solvating electrolytes. One example of such electrolyte system is the combination of hydrofluoroethers with sulfones.

By developing new electrolyte formulations with reduced LPS solubility, often a standard sulfur cathode containing commercial carbon black or self-developed carbon material is used. In a study on the impact of carbon porosity in LSB cathodes comprising Sparingly Solvating Electrolyte (C. Kensy et al. Batteries & Supercaps 2021, 4, 823-833), cathodes consisting of a hierarchical porous carbon and a SSE showed worse cycling stability and lower discharge capacity (<800 mA h/g_suifur after 50 cycles) compared to electrolytes comprising the standard DME/DOL system (Strubel et al., Carbon 107 (2016) 705-710). So far, little is known about the impact of the carbon materials on the electrochemical performance and active material utilization when the redox reaction is changed into a quasi- solid- state mechanism with SSEs. For example, the utility of graphene-based activated carbons has been demonstrated, until now, only in combination with SoA electrolytes, ultimately delivering decent discharge capacities but a poor cycling life for a real application.

Another requirement for a real-world application of sulfur cathodes in LSBs is the high sulfur loading. Watanabe’s research group (Batteries & Supercaps, 2022, vol. 5 (5), e202100409) reported the use of a cathode additive, so-called titanium black (mixture of TiCh and TiN) which allows the development of crack-free high loading sulfur cathodes. They combined a sulfur cathode with a sparingly solvated electrolyte. In this configuration, the titanium black plays a role in maintaining the original porosity of the electrode avoiding pore clogging, which facilitates the discharge reactions and enhances the LSB performance. However, the use of expensive transition-metals in the cathode increases the potential cost of the battery.

Overall, the main drawbacks associated to sulfur cathodes in LSBs, namely maintaining a high sulfur loading while ensuring high discharge capacity and cycling stability over time, still remain.

BRIEF DESCRIPTION OF THE INVENTION

The project leading to this application has received funding from the European Union’s Horizon2020 research and innovation programme under grant agreement No 881603.

The present invention is based on the synergistic combination of a particular type of electrolyte, known as Sparingly Solvating Electrolytes (SSEs), and carbonaceous materials, comprising a graphene oxide material, optionally combined with an additional carbonaceous material, for application in LSBs. The synergistic combination represents a solution to the inherent issues of Li-S technology and delivers high energy density and stable electrochemical cells.

Figure 1 shows different cell performances with different cathode and electrolyte configurations. The modification of the cathode formulation with a graphene oxide material possessing high specific surface area resulted in an improvement in sulfur utilization, which translates into high specific capacity of the cells. In particular, by using a graphene-oxide derived carbonaceous additive, the discharge capacity is exceptionally high. However, the standard state-of-art (SoA) electrolyte, which commonly refers to a lithium salt dissolved in a DME/DOL solvent mixture, does not provide the cathode with sufficient stability to achieve long life (more than 50 cycles) at such high current densities. The substitution of the SoA electrolyte by an SSE generates a stabilization of the system in terms of cyclability, contrary to what is observed in the prior art (see C. Kensy et al. vs Strubel et al.). Surprisingly, the use of a graphene oxide material as an additive, improves the wettability of the system, thus solving the viscosity problem. In addition, using other types of graphene-type materials (such as graphene nanoplatelets) an outstanding cycling stability is observed however at the cost of discharge capacity. As a result, a graphene oxide material, optionally in combination with additional carbonaceous materials, possessing high specific surface area together with SSEs produces cells with a high and sustained discharge capacity, outperforming all other combinations. The proposed invention combines sparingly solvated electrolytes with a graphene oxide material, optionally in combination with activated carbonaceous materials.

Thus, a first aspect of the invention relates to an electrochemical cell comprising a negative electrode, an electrolyte and a positive electrode, wherein the electrolyte comprises:

- at least one lithium salt, and

- a matrix, characterized in that the solubility of the sulfur species generated during the cycling of the electrochemical cell in said matrix is <5 wt.%; and wherein the positive electrode comprises:

- at least i) a graphene oxide material, wherein the specific surface area of the graphene oxide material is between 500 and 2500 m²/g; or ii) a mixture of a graphene oxide material and a carbonaceous material, wherein the specific surface area of said mixture is between 500 and 2500 m²/g, provided that the specific surface area of the graphene oxide material in said mixture is not higher than 2500 m²/g;

- sulfur, an organosulfur compound or a combination thereof; and

- at least one binder.

Another aspect of the invention relates to a method for preparing the electrochemical cell of the present invention, said method comprising the steps of: a) infiltrating sulfur, an organosulfur compound or a combination thereof into at least i) a graphene oxide material, or ii) a mixture of a graphene oxide material and a carbonaceous material, to give an infiltrated material; b) combining the infiltrated material from step a) with at least one binder to obtain a first mixture; c) optionally, combining an additional binder with the first mixture of step b) to obtain a second mixture; d) coating the mixture of step b) or c) on a current collector to obtain a positive electrode; e) assembling the positive electrode obtained in step d), an electrolyte as defined above and an anode to obtain the electrochemical cell of the present invention. Yet another aspect of the invention refers to the use of the electrochemical cell according to the present invention in Li-S batteries.

DESCRIPTION OF THE FIGURES

Figure 1 shows the gravimetric capacity (top) and the coulombic efficiency (bottom) of coin cells comprising different cathodes including those of the present invention (circle • and square ■ curves) and those representing comparative results (triangles, A and ▼, and rhombus, 0, curves). The first 5 cycles were performed at C/20 rate while the remaining cycles were performed at C/10 rate.

Figure 2 shows the gravimetric capacity (top) and the coulombic efficiency (bottom) of the 20 cm² pouch (prototype) cells comprising different cathodes including those of the present invention (circle • and square ■ curves) and those representing comparative results (triangles curves, A and ▼). The first 5 cycles were performed at C/20 rate while the remaining cycles were performed at C/10 rate.

DETAILED DESCRIPTION OF THE INVENTION

The inventors have surprisingly found the optimal combination of cathode and electrolyte components to increase the discharge capacity and cycling lifespan of the corresponding electrochemical cell. A stable and long cycling lifespan is a prerequisite for practical LSBs, yet it is normally restricted by side reactions between soluble polysulfides and the lithium-metal anode. The use of specific electrolytes encapsulating polysulfides in their solvation structure is a promising solution to suppress the parasitic reactions and allow to achieve long battery life. On the other hand, it is important to achieve a sufficiently high discharge capacity in combination with a long-life stability for practical applications.

The inventors have solved the problem of short-lived, sulfur-containing cathodes with low discharge capacity for LSBs with the present invention which relates to: an electrochemical cell comprising a negative electrode, an electrolyte and a positive electrode, wherein the electrolyte comprises:

- at least one lithium salt, and - a matrix, characterized in that the solubility of the sulfur species generated during the cycling of the electrochemical cell in said matrix is <5 wt.%; and wherein the positive electrode comprises:

- sulfur, an organosulfur compound or a combination thereof; and

- at least one binder.

In an embodiment of the present invention, the verb “comprise” throughout the description has to be interpreted with the meaning of “consist of’.

Electrolyte

The electrolyte of the electrochemical cell of the present invention comprises at least one lithium salt and a matrix characterized in that the solubility of the sulfur species generated during the cycling of the electrochemical cell in said matrix therein is <5 wt.%. The matrix can be in a gel form or a liquid form, preferably is a liquid matrix. It serves to dissolve and/or dilute to the desired concentration the at least one lithium salt. Preferred concentration ranges of said at least one lithium salt are detailed below.

The lithium salt may be an organic lithium salt, an inorganic lithium salt, or a combination thereof. Examples of organic lithium salts include, but are not limited to, LiN(SO2CF3)2, LiN(SO₂F)₂, LiN(SO₂CF3)(SO₂F), LiN(C2F₅SO2)(SO₂F) LiB(C₂O₄)2, LiBF2(C₂O₄), LiC(SO2CF3)3, LiPF3(C2Fs)3, and LiCEjSCL). Examples of inorganic lithium salts include, but are not limited to, LiClO₄, LiNCL, LiBF₄, LiAsFe, LiPFe, LiBFsCl, and LiF. In an embodiment, the at least one lithium salt is selected from LiClO₄, LiNCL, LiBF₄, LiAsF₆, LiPFe, LiBF₃Cl, LiF, LiN(SO₂CF₃)2 (LiTFSI), LiN(SO₂F)₂ (LiFSI), LiN(SO₂CF₃)(SO₂F), LiN(C₂F₅SO2)(SO₂F) LiB(C₂O₄)₂, LiBF₂(C₂O₄), LiC(SO₂CF₃)3, LiPF3(C2Fs)3, LiCFsSCL and a combination thereof. In another embodiment, the at least one lithium salt is selected from LiN(SO2CF3)2, LiN(SO₂F)₂, LiN(SO₂CF₃)(SO₂F), LiB(C₂O₄)2, LiBF₂(C₂O4), LiC(SO₂CF₃)3, LiPF₃(C2Fs)₃, LiCF₃SO₃, and LiNO₃ or a combination thereof.

In yet another embodiment, the at least one lithium salt is selected LiB(C2O4)2, LiN(SO₂F)₂, LiN(SO₂CF₃)₂, LiN(SO₂CF₃)(SO₂F), LiBF2(C₂O₄) (LiDFOB), LiNO₃ or a combination thereof.

In a preferred embodiment, the at least one lithium salt is a mixture of two lithium salts. The two lithium salts can be mixed in different proportions, preferably ranging from 50/50 to 99/1, from 60/40 to 95/5, from 70/30 to 90/10. Preferably, the two lithium salts are organic lithium salts of formula LifRpSChNSChRF], wherein RF is F or a partially fluorinated or perfluorinated alkyl group, meaning that the H atoms of an alkyl group (branched or linear) are partially or completely replaced by F atoms, respectively. Preferably, RF is F or a perfluorinated alkyl group. In a preferred embodiment, RF is F or a branched or linear Ci-8 perfluorinated alkyl group. In a more preferred embodiment, RF is F or a branched or linear Ci-₄ perfluorinated alkyl group. In a most preferred embodiment, RF is F or CF₃, corresponding to compounds LiFSI and LiTFSI, respectively. Most preferably, the two lithium salts are LiFSI and LiTFSI in a molar ratio of 80/20, respectively.

In a particular embodiment, the at least one lithium salt comprised in the electrolyte is combined with a matrix, preferably liquid matrix, and its concentration may vary within a broad molarity range. The molarity can be expressed in mol/L (M), wherein “mol” refers to the number of moles of the at least one lithium salt and L refers to the volume expressed in liters of the matrix comprised in the electrolyte.

In an embodiment, the concentration of the at least one lithium salt ranges from 0.1 M to 5 M; preferably, from 0.3 M to 4 M; more preferably, from 0.5 M to 3 M. In a preferred embodiment, the concentration of the at least one lithium salt is about 1 M.

As used herein, the term “about” or “approximately” means within 10%, preferably within 5%, of a given value or range.

The matrix comprised in the electrolyte is characterized in that the solubility of the sulfur species generated during the cycling of the electrochemical cell in said matrix is <5 wt.%. This means that <5 wt.% of the total sulfur species generated during the cycling of the electrochemical cell is soluble in the matrix comprised in the electrochemical cell. In other words, the matrix does not solubilize more than 5 wt.% of the total sulfur species generated during the cycling of the electrochemical cell. In other embodiments, the solubility of the sulfur species generated during the cycling of the electrochemical cell in said matrix is <4 wt.%, <3 wt.%, <2 wt.%, or <1 wt.%.

Alternatively, the solubility of the sulfur species generated during the cycling of the electrochemical cell in said matrix, preferably liquid matrix, is <100mM, <75 mM, <50mM, <25mM, wherein mM refers to mmol of total amount of sulfur species per liter of matrix. The concentration of the sulfur species may be determined by UV/vis spectroscopy by using a calibration curve. Electrolytes with such a matrix are known as Sparingly Solvating Electrolytes (or SSEs) and have been reviewed in literature (Lei Cheng et al., ACS Energy Lett. 2016, 1, 3, 503-509).

In this sense, another role of the matrix comprised in the electrolyte is ion conduction, for example Li⁺ conduction. The matrix only sparingly dissolves the intermediate or product sulfur species generated during the cycling of the electrochemical cell. Thus, a successful secondary battery electrode is achieved based on a quasi-solid reaction mechanism rather than on common precipitation-dissolution mechanism. The sulfur species that are generated during the cycling of the electrochemical cell include S and Li2S, which are poor electronic and ionic conductors, and polysulfides of formula Li2S_x, wherein x is comprised between 2 and 8. Examples of polysulfide are Li2Ss, Li2Se, Li2S4, and Li2Ss. In an embodiment, the matrix is characterized in that the solubility of S and Li2S_x (wherein l<x<8) therein is < 5wt.% with respect to the total weight of S and Li2S_x generated during the cycling of the electrochemical cell; alternatively, the solubility of S and Li2S_x (wherein l<x<8) generated during the cycling of the electrochemical cell in the matrix is <100 mM. In an embodiment, the sulfur species generated during the cycling of the electrochemical cell and the above stated solubility values refer solely to the Li2S_x species (wherein l<x<8). The dissolution of polysulfides, besides creating voids in the cathode, increases the viscosity of the electrolyte making it less conductive. Additionally, upon dissolution, polysulfides tend to shuttle back and forth to the anode side and precipitate in the form of insoluble and insulating low order Li2S_x. These species contribute to self-discharge and an overall poor cycle life with low specific capacity.

Thus, the matrix comprised in the electrolyte reduces the polysulfide solubility and prevents the increase of viscosity derived from the presence of the polysulfides in the electrolyte, and most importantly, possesses the ability to oppose undesired polysulfide shuttling.

In an embodiment, the matrix is characterized in that the solubility of S and Li2S_x species (wherein l<x<8) in said matrix therein is <100 mM. The solubility refers to the total sulfur species as defined above regardless of the electrochemical cell of the invention being operative or not. In one embodiment, the solubility refers to the sulfur species as defined above when the cell is operative. In another embodiment, the solubility refers to the sulfur species as defined above when the cell is not operative.

In an embodiment, the matrix is a liquid matrix comprising organic solvents and/or ionic liquids. In an embodiment, the liquid matrix comprises at least one organic solvent. In another embodiment, the liquid matrix comprises at least two organic solvents, more preferably the liquid matrix comprises two organic solvents. In yet another embodiment, the liquid matrix consists of two organic solvents.

When the liquid matrix comprises at least two organic solvents, at least a first organic solvent can dissolve the at least one lithium salt, while at least a second organic diluent (or co-solvent) only poorly solubilizes the at least one lithium salt and dilutes the at least one lithium salt dissolved in the at least a first organic solvent. Any organic solvent that could respectively dissolve or only poorly dissolve the at least one lithium salt would be suitable and a skilled person would know which one to select based on common knowledge. For example, the former can be any solvent that can dissolve the at least one lithium salt up to a 5M, up to a 4M, up to a 3M, up to a 2 M, and, preferably up to a IM concentration, whereas the latter can be any solvent that cannot do so, such as any solvent that can dissolve the at least one lithium salt at most up to a less than IM, up to a 0.75M, up to a 0.50M, or up to a 0.25M, concentration. Organic solvents that are suitable to dissolve the at least one lithium salt comprised in the electrolyte include ether-, sulfone-, carbonated-based solvents or a combination thereof. Organic diluents (or co-solvents) that display limited solubility for the at least one lithium salt comprised in the electrolyte include halogenated solvents and aromatic hydrocarbon solvents (both halogenated and non-halogenated variants).

In one embodiment, the liquid matrix does not comprise a non-fluorinated glyme, preferably a non-halogenated glyme, preferably it does not comprise a glyme. The term “glyme” refers to a glycol ether, more specifically to an alkyl ether of glycol, such as an alkyl ether of ethylene glycol or propylene glycol, which may or may not be halogenated. The term alkyl refers to a linear or branched hydrocarbon chain consisting of carbon and hydrogen atoms, containing no unsaturation, having from 1 to 6 carbon atoms (Ci-Ce alkyl), preferably from 1 to 3 carbon atoms (C1-C3 alkyl), and being attached to the rest of the molecule through a single bond. Non-limiting examples of alkyl are methyl, ethyl, n-propyl, i-propyl, cyclopropyl, n-butyl, t-butyl, n-pentyl or cyclohexyl. Non-limiting examples of glymes include dimethoxyethane, diethoxyethane, methoxyethoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methylethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl ethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methyl ethyl ether, polyethylene glycol dimethyl ether, polyethylene glycol diethyl ether and polyethylene glycol methyl ethyl ether.

More particularly, the liquid matrix does not comprise a non-fluorinated linear ether, preferably a non-halogenated linear ether, preferably it does not comprise a linear ether. Even more particularly, the liquid matrix does not comprise a non-fluorinated ether, preferably a non-halogenated ether, preferably it does not comprise an ether.

In one embodiment, the liquid matrix comprises: i) at least one organic solvent which does not comprise a non-fluorinated glyme, preferably a non-halogenated glyme, preferably a glyme; particularly does not comprise a non-fluorinated linear ether, preferably a non-halogenated linear ether, preferably a linear ether; and more particularly does not comprise an non- fluorinated ether, preferably a non-halogenated ether, preferably a ether; and ii) at least one organic diluent selected from halogenated solvents, aromatic solvents or a combination thereof.

In one embodiment, the liquid matrix comprises: i) at least one organic solvent selected from a linear or cyclic ether, a sulfone, a carbonate or a combination thereof; particularly from a cyclic ether, a sulfone, a carbonate or a combination thereof; more particularly from a sulfone, a carbonate or a combination thereof; and ii) at least one organic diluent selected from halogenated solvents, aromatic solvents or a combination thereof. Linear and cyclic ethers as the at least one organic solvent comprised in the liquid matrix include but are not limited to diethylether, methylbutylether, dimethoxy ethane (DME), 1,2-di ethoxy ethane (DEE), 1,3 -dioxolane (DOL), bis(2-methoxy ethyl)ether (DEGDME), triethylene glycol dimethyl ether (G3), tetraethylene glycol dimethyl ether (TEGDME), poly(ethylene glycol) dimethyl ether (PEGDME), tetrahydropyran (TEIP), tetrahydrofuran (THF) and mixtures thereof.

In one embodiment, the liquid matrix does not comprise linear and cyclic ethers as the at least one organic solvent, preferably non-halogenated, such as non-fluorinated, linear and cyclic ethers.

Sulfones as the at least a first organic solvent comprised in the liquid matrix include but are not limited to tetramethylene sulfone (TMS), dimethyl sulfone, allyl methyl sulfone, butadiene sulfone, dibutyl sulfone, dipropyl sulfone (DPS), ethyl methyl sulfone (EMS), methyl isopropyl sulfone (MiPS), ethyl isopropyl sulfone (EiPS), 3,3,3- trifluoropropylmethyl sulfone (FPMS), ethyl- sec-butyl sulfone (EsBS), isopropyl methyl sulfone (IPMS) and mixtures thereof.

Carbonates as the at least a first organic solvent comprised in the liquid matrix include but are not limited to dimethyl carbonate (DMC), propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), and mixtures thereof.

In a preferred embodiment, the at least a first organic solvent comprised in the liquid matrix is a cyclic or linear ether, a sulfone or a mixture thereof. In such embodiment, the at least one organic solvent comprised in the liquid matrix is selected from dimethoxy ethane (DME), 1,2-di ethoxy ethane (DEE), 1,3-dioxolane (DOL), bis(2-methoxy ethyl)ether (DEGDME), tetraethylene glycol dimethyl ether (TEGDME), or polyethylene glycol) dimethyl ether (PEGDME), sulfolane (SL), dimethyl sulfone and any mixture thereof.

In the most preferred embodiment, the at least a first organic solvent comprised in the liquid matrix is a sulfone according to the embodiments above, even more preferably sulfolane.

The at least a second organic diluent comprised in the liquid matrix is selected from bis(2,2,2-trifluoroethyl) ether (BTFE), l,l,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), tris(2,2,2-trifluoroethyl)orthoformate (TFEO), fluorobenzene (FB), 1,2 difluorobenzene (DFB), bis (2,2- difluoroethyl) ether (BDE), trifluorotoluene (TFT),

1.1.2.2-Tetrachloroethane, 1,2 dichloroethane, chlorobenzene, dichlorobenzene, trichlorobenzene, chlorotoluenes, a, a, a tri chlorotoluene, bromobenzene, dibromoethane, 1,2 dibromobenzene, iodobenzene, 1,2 diodobenzene, 1,3 iodobenzene, anisole, furan, toluene, ethoxybenzene, 1,2-dimethoxybenzene, and mixtures thereof.

In a preferred embodiment, the at least a second organic diluent comprised in the liquid matrix is selected from halogenated ethers and halogenated aromatic hydrocarbon solvents. In a more preferred embodiment, organic diluents comprised in the liquid matrix are partially or completely fluorinated. Preferably, the at least a second organic diluent comprised in the liquid matrix is selected from bis(2,2,2-trifluoroethyl) ether (BTFE),

1.1.2.2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), tris(2,2,2- trifluoroethyl)orthoformate (TFEO), fluorobenzene (FB), 1,2 difluorobenzene (DFB), bis (2,2- difluoroethyl) ether (BDE), trifluorotoluene (TFT). Most preferably, the organic diluent is l,l,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE).

In particular, the liquid matrix comprises at least one organic solvent selected from dimethoxy ethane (DME), 1,2-di ethoxy ethane (DEE), 1,3-dioxolane (DOL), bis(2- methoxy ethyl)ether (DEGDME), tetraethylene glycol dimethyl ether (TEGDME), polyethylene glycol) dimethyl ether (PEGDME), tetramethylene sulfone (TMS) (also called sulfolane), and dimethyl sulfone, preferably from tetramethylene sulfone (TMS) and dimethyl sulfone, and at least one organic diluent selected from bis(2,2,2- trifluoroethyl) ether (BTFE), l,l,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), tris(2,2,2-trifluoroethyl)orthoformate (TFEO), fluorobenzene (FB), 1,2 difluorobenzene (DFB), bis (2,2- difluoroethyl) ether (BDE), and trifluorotoluene (TFT). Most preferably, the liquid matrix comprises tetramethylene sulfone (TMS) and 1, 1,2,2- tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE).

In yet another embodiment, the liquid matrix consists of one organic solvent and one organic diluent according to the embodiments above. Most preferably, the liquid matrix consists of tetramethylene sulfone (TMS) (also called sulfolane) and 1, 1,2,2- tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE).

The volumetric ratio between the at least a first organic solvent and the at least a second organic diluent (or co-solvent) comprised in the liquid matrix may be adjusted in order to tune the solubility of the at least one lithium salt. In one embodiment, said volumetric ratio (organic solvent : organic diluent) ranges from 5:95 to about 99.5:0.5, preferably from 20:80 to 85: 15, more preferably from 30:70 to 70:30. Even more preferably, said volumetric ratio is about 40:60.

Positive electrode

As said above, the positive electrode comprised in the electrochemical cell of the invention comprises:

- sulfur, an organosulfur compound or a combination thereof; and

- at least one binder.

The at least a graphene oxide material, or mixture of a graphene oxide material and a carbonaceous material, is present altogether in at least 5 wt.% compared to the total weight of the positive electrode. In a preferred embodiment, the at least a graphene oxide material, or mixture of a graphene oxide material and a carbonaceous material, is present in the range 5-40 wt.% compared to the total weight of the positive electrode. In a more preferred embodiment, the at least graphene oxide material, or mixture of a graphene oxide material and a carbonaceous material, is present in the range 10-30% wt.% compared to the total weight of the positive electrode. In a most preferred embodiment, the at least a graphene oxide material, or mixture of a graphene oxide material and a carbonaceous material, is present in about 25 wt.% compared to the total weight of the positive electrode.

When a mixture of a graphene oxide material and a carbonaceous material is used in the cathode, the weight ratio graphene oxide material : carbonaceous material is between 5:95 and 95:5, preferably between 10:90 and 90: 10, more preferably between 25:75 and 75:25, respectively. Even more preferably, the weight ratio between the graphene oxide material and the carbonaceous material is between 30:70 and 50:50; most preferably the weight ratio is about 40:60. In one embodiment, the at least a carbonaceous material is selected from activated carbon, carbon black, and carbon nanofibers, nanoparticles, nanotubes, nanoflakes, and a combination thereof. Preferably, carbon black is employed. Carbonaceous materials are very versatile materials and are available in large surface area and hierarchical porosity. Their porosity can be attributed to “intrinsic porosity”, that is the presence of pores within the carbonaceous framework. The porosity can be also due to the presence of pores corresponding to inter-particle voids between packed particles of carbonaceous material (“extrinsic porosity”). The pore size can be categorized into three classes such as macropores, mesopores, and micropores. According to IUPAC, macroporous materials have pore diameter larger than 50 nm, mesoporous materials have pore diameter equal to or smaller than 50 nm and higher than or equal to 2 nm, and microporous materials have pore diameter lower than 2 nm. In one embodiment, the at least a carbonaceous material is a porous material which comprises pores equal to or smaller than 50 nm. In another embodiment, the at least a carbonaceous material is a mesoporous carbonaceous material. In an embodiment, the pores of the mesoporous carbonaceous material have a pore size distribution comprised between 5 and 40 nm, preferably between 15 and 30 nm, more preferably the average pore size is about 25 nm as determined by nitrogen adsorptiondesorption isotherms registered at -196 °C (according to ISO 9277:2010) using non-local density functional theory (NLDFT) calculations.

The use of high surface area graphene oxide material, optionally in combination with additional carbonaceous materials, facilitates producing high porosity cathodes. In an embodiment, the porosity of the positive electrode is equal to or greater than 40%, such as equal to or greater than 45%, more particularly equal to or greater than 50%; more particularly, the porosity of the positive electrode of the cell of the invention is comprised between 40 and 70%, such as between 45 and 65%, in particular 50 and 60%. Such embodiments apply to all other embodiments of the first aspect of the invention. Porosity can be determined according to ISO 9277:2010, more particularly by nitrogen adsorptiondesorption isotherms registered at -196 °C (according to ISO 9277:2010), in particular using non-local density functional theory (NLDFT) calculations.

In an embodiment, in any of the herein described embodiments, the positive electrode is not subjected to pressure, particularly to pressure by application of force such as mechanical force. In an embodiment, in any of the herein described embodiments, the positive electrode has not been subjected to pressing or to calendering, preferably to calendering.

The mixture of the graphene oxide material and carbonaceous material presents a specific surface area of said mixture between 500 and 2500 m²/g, provided that the specific surface area of the graphene oxide material in said mixture is not higher than 2500 m²/g.

The specific area of said mixture is preferably in the range 750 - 2250 m²/g, more preferably 1000 - 2000 m²/g, even more preferably 1250 - 1750 m²/g.

Any of these surface areas of the mixture is given provided that the specific surface area of the graphene oxide material in said mixture is not higher than 2500 m²/g, in particular is between 300 - 2500 m²/g, preferably 500 - 2500 m²/g, more preferably 750 - 2250 m²/g, even more preferably 1000 - 2000 m²/g, yet more preferably 1250 - 1750 m²/g.

Due to the porous structure (intrinsic and/or extrinsic), the at least a carbonaceous material, presents a high specific surface area. In an embodiment, the specific surface area is at least 300, at least 400, at least 500, at least 600, at least 700, at least 800, at least 900, at least 1000, at least 1100, at least 1200, at least 1300, at least 1400 m²/g. In another embodiment, the specific surface area is in the range 300 - 2500 m²/g, 500 - 2500 m²/g, preferably 750 - 2250 m²/g, more preferably 1000 - 2000 m²/g, even more preferably 1250 - 1750 m²/g. In a specific embodiment, the surface area of the at least a carbonaceous material is about 1400 m²/g. The specific surface area can be determined by techniques known to a person with ordinary skills in the art. In a specific embodiment, the specific surface area is determined by the BET method through the nitrogen adsorption/desorption isotherms recorded at -196 °C (according to ISO 9277:2010).

The at least a graphene oxide material is a material with bidimensional particle morphology. Two-dimensional (2D) materials are crystalline materials consisting of single- or few-layer atoms (such as 2-, 3-, 4-, 5-atom layers), in which the in-plane interatomic interactions are much stronger than those along the stacking direction. The particle morphology can be easily determined by electron microscopy techniques, in particular by SEM. These materials can be found under a variety of nanostructures such as nanoplatelets, nanoribbons, nanomeshes, nanospheres, nanoonions and nanoflakes. The at least a graphene oxide material is present in the positive electrode between 2.5 and 10 wt% with respect to the total weight of the positive electrode. The positive electrode comprises at least a graphene oxide material, or a mixture of a graphene oxide material and a carbonaceous material. In an embodiment, the at least a graphene oxide material is one, two or three, preferably one or two graphene oxide materials as described throughout the text. In a preferred embodiment, the at least a graphene oxide material is one graphene oxide material as described throughout the text. The graphene oxide material may be obtained by oxidation of graphite and subsequent exfoliation of the oxidized graphite.

Purified natural graphite powder (e.g., natural graphite powder of ultrahigh purity) may be used to for oxidised graphite.

Graphite may be oxidised such as by oxidation by treatment with an oxidizing agent such as potassium permanganate. Methods such as the Hummers method (Journal of the American Chemical Society, 1958, 80(6), 1339) or the modified Hummers method (Marcano et al., ACS nano, 2010, 4(8), 4806) may be employed.

The graphite oxide is exfoliated to produce sheets of graphene oxide. The exfoliation of the graphite oxide may be performed using exfoliation techniques and conditions known in the art, such as described in Casallas-Caicedo et al. 2019 J. Phys.: Conf. Ser. 1386 012016. In some embodiments, the graphite oxide is exfoliated by chemical exfoliation, i.e. by insertion of chemical species between the layers of the oxidized graphite; for instance, the graphite oxide can be suspended in a solvent and optionally sonicated or subjected to mechanical forces. In an embodiment, the graphite oxide is suspended in an organic or an aqueous solvent, preferably in an aqueous solvent, more preferably in water. The resulting graphene oxide solution comprises separated sheets of graphene oxide suspended in the solvent, which can be retrieved therefrom. The separated graphene oxide sheets may be in monolayer or few-layer form. Few-layer form may comprise from 2 to 10 sheets.

The graphene oxide material may also be obtained by chemical modifications of graphene oxide or graphene oxide precursors, for example by reduction or by polymerization of appropriate monomers (such as olefins or phenolic monomers) on the graphene oxide surface. In an embodiment, the graphene oxide material is a graphene oxide-polymer composite, wherein the polymer is grown on the graphene oxide surface. In a preferred embodiment, the graphene oxide material is graphene oxide or reduced graphene oxide, more preferably it is reduced graphene oxide. Graphene oxide materials, such as graphene oxide, may have an oxygen content between 5 and 30%, preferably between 10 and 30% (as determined by X-ray photoelectron spectroscopy (XPS), ISO 16129:2018). Preferably, the reduced graphene oxide has an oxygen content between 5 and 18%. Furthermore, the graphene oxide material, preferably graphene oxide or reduced graphene oxide, more preferably reduced graphene oxide, may also present high specific surface area and act as an active carbon, namely it is activated to feature micropores as defined above that increase the surface area available for adsorption or chemical reactions. In an embodiment, the specific surface area of the graphene oxide material, preferably graphene oxide or reduced graphene oxide, more preferably reduced graphene oxide, is at least 300, at least 400, at least 500, at least 600, at least 700, at least 800, at least 900, at least 1000, at least 1100, at least 1200, at least 1300, at least 1400 m²/g. In another embodiment, the specific surface area is from any of these values up to 2500, 2250, 2000 or 1750 m²/g, such as 300 - 2500 m²/g, 500 - 2500 m²/g, preferably 750 - 2250 m²/g, more preferably 1000 - 2000 m²/g, even more preferably 1250 - 1750 m²/g, as determined by the BET method through the nitrogen adsorption/desorption isotherms recorded at -196 °C (ISO 9277:2010). For the sake of clarity, a graphene oxide material as that used in Example 1 below is named as ResFarGO throughout the description.

In a preferred embodiment, the positive electrode comprises a mixture of a graphene oxide material, preferably graphene oxide or reduced graphene oxide, more preferably reduced graphene oxide, and a porous carbon black material, wherein the specific surface area of said mixture is between 500 and 2500 m²/g, provided that the specific surface area of the graphene oxide material in said mixture is not higher than 2500 m²/g. More particular and preferred specific surface areas are as described above for the mixture of the graphene oxide material and the carbonaceous material.

In an alternative embodiment, the positive electrode comprises a graphene oxide material, preferably graphene oxide or reduced graphene oxide, more preferably reduced graphene oxide, displaying a specific surface area of between 500 - 2500 m²/g, while the electrolyte comprises a matrix that does not comprise a non-fluorinated glyme, preferably a nonhalogenated glyme, preferably it does not comprise a glyme; particularly does not comprise a non-fluorinated linear ether, preferably a non-halogenated linear ether, preferably it does not comprise a linear ether; and more particularly does not comprise a non-fluorinated ether, preferably a non-halogenated ether preferably it does not comprise an ether; and even more particularly the electrolyte comprises a liquid matrix comprising: i) at least one organic solvent which does not comprise a non-fluorinated glyme, preferably it does not comprise a glyme; particularly does not comprise a non-fluorinated glyme, preferably a non-halogenated glyme, preferably it does not comprise a glyme; particularly does not comprise a non-fluorinated linear ether, preferably a non- halogenated linear ether, preferably it does not comprise a linear ether; and more particularly does not comprise a non-fluorinated ether, preferably a non-halogenated ether preferably it does not comprise an ether; and ii) at least one organic diluent selected from halogenated solvents, aromatic solvents or a combination thereof. Particular solvents and diluents were described hereinabove.

In a preferred embodiment, the positive electrode comprises the graphene oxide material and the carbonaceous material, each presenting a specific surface area of between 500 - 2500 m²/g, preferably 750 - 2250 m²/g, more preferably 1000 - 2000 m²/g, even more preferably 1250 - 1750 m²/g.

In the Examples, the present disclosure refers to a graphene-type material in the form of graphene nanoplatelets (GNPs). Graphene nanoplatelets are a two-dimensional allotropic modification of carbon, formed by a layer of carbon atoms one atom thick, in a state of sp² hybridization and connected via c and TI bonds to a hexagonal two-dimensional crystal lattice, usually with several (5-10) layers. GNPs may be obtained by mechanical exfoliation of graphite, thus their oxygen content is lower compared to graphene oxide materials. Preferably, the oxygen content in GNPs is less than 5%, less than 4%, less than 3%, more preferably less than 1% (as determined by X-ray photoelectron spectroscopy (XPS), ISO 16129:2018). The platelet-shaped graphene sheets are identical to those found in the walls of carbon nanotubes, but in a planar form. The edges of the platelets can be suitably functionalized with covalently bonded atoms or groups of atoms. Graphene nanoplatelets are usually 1-10 nm thick with a bulk density of 0.1 to 0.5 g/cc, an oxygen content of <1% and a carbon content of >99 wt% and a residual acid content of <0.5 wt%. Several methods of preparing graphene nanoplatelets are known in the art (e.g. Dong Wook Chang and Jong-Beom Baek, J. Mater. Chem. A, 2016,4, 15281-15293, and references cited therein), for example the reduction of graphene oxide and the direct exfoliation of graphene from pristine graphite under mild conditions. The positive electrode comprises sulfur, an organosulfur compound or a combination thereof, preferably it comprises sulfur or an organosulfur compound, even more preferably it comprises sulfur.

The sulfur, organosulfur compound or combination thereof is present in the positive electrode in at least 50 wt.% with respect to the total weight of the positive electrode. In a preferred embodiment, the sulfur, organosulfur compound or combination thereof are present in the range 50-85 wt.% with respect to the total weight of the positive electrode. In a more preferred embodiment, the sulfur, organosulfur compound or combination thereof are present in the range 50-75 wt.% with respect to the total weight of the positive electrode. In a most preferred embodiment, the sulfur, organosulfur compound or combination thereof are present in about 64 wt.% with respect to the total weight of the positive electrode.

The sulfur that may be incorporated in the positive electrode corresponds to the stable allotropic form S8; in particular, crystalline a-S8 (also called cyclooctasulfur) is the most thermodynamically stable sulfur allotrope.

The organosulfur compound that may be incorporated in the positive electrode refers to the following three main classes: organodisulfides, organosulfide polymers and thioethers. In a particular embodiment, the organosulfur compound is selected from sulfurized polyacrylonitrile (SPAN), sulfurized divinylbenzene (p(DVB)), and phenyl disulfide (DPDS). More preferably, the organosulfur compound is selected from sulfurized polyacrylonitrile (SPAN) and sulfurized divinylbenzene (p(DVB)).

In an embodiment, the sulfur loading comprised in the positive electrode is at least 1, at least 2 or at least 3 mg/cm². More preferably the sulfur loading comprised in the positive electrode is between 3 and 8 mg/cm², wherein the lower and upper value of the range are included.

The positive electrode further comprises at least one binder. Said at least one binder is present in at least 5 wt.% with respect to the total weight of the positive electrode. In a preferred embodiment, the at least one binder is present between 5-25 wt.% with respect to the total weight of the positive electrode. In a more preferred embodiment, the at least one binder is present between 7.5-15 wt.% with respect to the total weight of the positive electrode. In a most preferred embodiment, the at least one binder is present in about 10 wt.% with respect to the total weight of the positive electrode. In a particular embodiment, the at least one binder may comprise aqueous and nonaqueous based polymeric binder. In a preferred embodiment, the at least one binder is selected from carboxymethyl cellulose (CMC), polyacrylonitrile (PAN), polyacrylic latex (LAI 32), styrene-butadiene rubber (SBR), poly(vinylidenedifluoride) (PVDF), polyurethane (PU), polytetrafluoroethylene (PTFE), poly(vinylidene fluoride- hexafluoropropylene (PVDF-HFP), polyethylene oxide (PEO) and poly(acrylic acid) (PAA). More preferably, the at least one binder is a combination of two binders. In a most preferred embodiment, the two binders are carboxymethyl cellulose (CMC) and styrenebutadiene rubber (SBR).

Negative electrode

In an embodiment, the negative electrode (or anode) is selected from a metal anode, preferably an alkali metal anode, and more preferably a lithium metal anode.

In another embodiment, the anode comprises a metal or metalloid suitable for reversibly forming an alloy with metal cations, preferably alkali metal cations, and more preferably lithium cations. The skilled person knows how to select appropriate metals or metalloids suitable for forming the alloy. For instance, examples of metals or metalloids suitable for reversibly forming an alloy with lithium cations are Mg, Al, Zn, Bi, Cd, Sb, Ag, Si, Pb, Sn, or In which in particular can form alloys such as LiMg, LiAl, LiZn, LisBi, LisCd, LisSb, Li4Ag, Li^Si, Li^Pb or Li^Sn. In an embodiment, the anode comprises such alloys.

In a particular embodiment, the electrochemical cell of the present invention further comprises a separator.

The separator is a medium between the two electrodes of an electrochemical cell that has to fulfill at least two functions. One function is to store and accommodate the electrolyte and, simultaneously, to assure ionic conductivity within the electrodes and between the anode and cathode. The further function of the separator is to electrically insulate the two electrodes from one another, in order to avoid short circuits. The separator is a separating means, usually a plate, positioned between the anode and cathode in the electrochemical cell to avoid the electrical contact between them. Moreover, the separator is in contact with the electrolyte, particularly it is partially or completely contacted with the electrolyte, which favors the flow of ions from one electrode to the other one. In an embodiment, said separator is a polymeric membrane mainly based on a polyolefin, preferably polypropylene, (PP) polyethylene (PE), or any combination thereof (PP-PE). The polypropylene (or polyethylene or PP-PE) membrane may comprise one or more layer of polypropylene (or polyethylene or PP-PE), preferably it comprises one layer of polypropylene (or polyethylene or PP-PE). In a preferred embodiment, the separator of the electrochemical cell consists of a one-layer polypropylene (or polyethylene or PP-PE) membrane.

Nevertheless, the embodiments above may be combined to afford the electrochemical cell of the invention.

In one embodiment, the electrochemical cell of the invention comprises: a) a negative electrode comprising lithium metal; and b) an electrolyte comprising:

- a LiTFSI-LiFSI mixture, and

- a TTE-sulfolane mixture; and c) a positive electrode comprising:

- a mixture of a graphene oxide material and a porous carbon black material, preferably wherein the specific surface area of said mixture is between 500 - 2500 m²/g, provided that the specific surface area of the graphene oxide material in said mixture is not higher than 2500 m²/g, whereby more particular and preferred specific surface areas are as described above for the mixture of the graphene oxide material and the carbonaceous material;

- sulfur; and

- optionally a CMC-SBR mixture as binder.

In another embodiment, the electrochemical cell of the invention consists of: a) a negative electrode comprising lithium metal; and b) an electrolyte comprising:

- a LiTFSI-LiFSI mixture, and

- a TTE-sulfolane mixture; and c) a positive electrode comprising:

- sulfur; and

- optionally a CMC-SBR mixture as binder.

- a LiTFSI-LiFSI mixture, and

- a TTE-sulfolane mixture; c) a positive electrode comprising:

- a graphene oxide material displaying a specific surface area of between 500 - 2500 m²/g, whereby more particular and preferred specific surface areas are as described above for the graphene oxide material ;

- sulfur; and

- optionally a CMC-SBR mixture as binder.

Method for preparing the electrochemical cell

The method for preparing the electrochemical cell of the invention comprises the steps of: a) infiltrating sulfur, an organosulfur compound or a combination thereof into at least i) a graphene oxide material, or ii) a mixture of a graphene oxide material and a carbonaceous material, to give an infiltrated material; b) combining the infiltrated material from step a) with at least one binder to obtain a first mixture; c) optionally, combining an additional binder with the first mixture in step b) to obtain a second mixture; d) coating the mixture of step b) or c) on a current collector to obtain a positive electrode e) assembling the positive electrode with an electrolyte as defined throughout the present description and an anode to obtain the electrochemical cell of the present invention. In an embodiment, specific surface areas that were described in the first aspect of the invention for the graphene oxide material, or for its mixture with the carbonaceous material, apply to step a) described above.

The sulfur, organosulfur compound, and the at least one carbonaceous material and the graphene oxide material of step a) are defined as above.

In an embodiment, the sulfur, organosulfur compound or combination thereof are infiltrated into a pre-formed mixture of at least one carbonaceous material and a graphene oxide material via physical mixing or melt diffusion.

When sulfur is used, melt diffusion is the preferred technique for infiltration. In this technique the sulfur is mixed with the at least one carbonaceous material and a graphene oxide material, for example in a mortar, and subsequently the resulting mixture is heat- treated. Preferably, the temperature for melt diffusion is at least 50 °C, preferably is at least 100 °C. More preferably, the temperature for melt diffusion is from about 100° C to about 170° C. The time for melt diffusion is at least 10 minutes, at least 1 hour, at least 2 hours, at least 3 hours, at least 4 hours, at least 5 hours, at least 6 hours, at least 7 hours, at least 8 hours, at least 9 hours, at least 10 hours, at least 11 hours, at least 12 hours. Preferably the time for melt diffusion is at least 12 hours.

When an organosulfur compound or a combination of sulfur with an organosulfur compound is used in step a), infiltration may be carried out by physical mixing with or deposition/adsorption on the at least one carbonaceous material.

In another embodiment, the sulfur, organosulfur compound or combination thereof are infiltrated into at least one carbonaceous material, according to the techniques explained above, and subsequently at least a graphene oxide material, optionally in combination with additional carbonaceous materials, is added. In an embodiment, the at least a graphene oxide material is added via dry mixing, namely by mixing the at least one carbonaceous material infiltrated with the sulfur, organosulfur compound or combination thereof with the graphene oxide material (optionally, in mixture with additional carbonaceous materials) in a mortar in a solvent-free manner until all the mixture is homogenous. Additional carbonaceous materials include activated carbon, carbon black, and carbon nanofibers/nanoparticles/nanotubes/nanoflakes, preferably carbon black and carbon nanotubes. After step a) is concluded, an infiltrated material is obtained, wherein the S atoms are infiltrated within the pores of the at least one carbonaceous material and a graphene oxide material.

Once the infiltrated material is formed, step b) is carried out by combining the infiltrated material with an amount of at least one binder. The at least one binder may be first dissolved in an appropriate solvent. The solvent can be an organic solvent or water. In a preferred embodiment, the solvent is water, however any solvent suitable to dissolve the at least one binder could be used. The concentration of the at least one binder solution may vary within a broad range and will depend on the desired final wt.% of at least one binder compared to the total mass of electrode. For example, if a 10wt.% of at least one binder is targeted in a 100g electrode, then 10g of at least one binder are dissolved in a certain amount of solvent so to obtain a solution with a known concentration. Preferably, such concentration is in the range 0.01-1.0 M. The solution is further stirred until complete dissolution of the at least one binder.

Alternatively, the infiltrated material may be combined with the at least one binder while the latter is in a dry form, for example by physical mixing.

Further, the so-obtained first mixture may be further stirred.

An optional step c) involves the combination of an additional binder with the first mixture according to the procedure followed in step b). In a preferred embodiment, the additional binder is defined as above but is different from the at least one binder of step b). The additional binder is combined with the mixture from step b) in a dry form or dissolved in an organic solvent or in water, preferably in water (as described for the at least one binder above). In an embodiment, step c) is performed mandatorily and a second mixture is obtained.

In step d), the mixture from step b) or c) is coated on a current collector to obtain a positive electrode. In an embodiment, the current collector is a carbon-coated aluminium foil. An appropriate selection of the amount of first or second mixture will determine the final amount of sulfur in the electrode. This amount is at least 1, at least 2 or at least 3 mg/cm². More preferably the sulfur loading comprised in the positive electrode is between 3 and 8 mg/cm², wherein the lower and upper value of the range are included

Next, if solvents were used in step b) (and in the optional step after step b), the positive electrode is next dried to remove the solvent(s) and finally assembled with an electrolyte and an anode to obtain the electrochemical cell of the invention. In a particular embodiment, when a separator is used, this is also assembled along with the positive electrode, the electrolyte and the anode to provide an electrochemical cell.

Uses

Electrochemical cells of the invention can be assembled together to prepare a battery according to the present invention. The cells may be assembled in parallel or in series, or both. In an embodiment, any, several or each of the cells is connected to a module for monitoring cell performance, e.g. for monitoring cell temperature, voltage, charge status or current. Methods of battery assembly are well-known in the art and are reviewed for instance in Maiser, Review on Electrochemical Storage Materials and Technology, AIP Conf. Proc. 1597, 204-218 (2014).

In particular, the electrochemical cell of the invention is a secondary electrochemical cell that can be used to prepare a secondary battery, which refers to a battery in which charging and discharging operations are reversible. The charging and the discharging of the electrochemical cell and battery is accomplished by the reversible incorporation of metal cations at the negative electrode (anode) and positive electrode (cathode). During discharge, electrons are liberated at the anode by an oxidation process, resulting in an electron current, usually via an external load, to the cathode where the electrons are taken up by a reduction process. At the same time, charge carriers are released from the anode in the form of metal cations, which can migrate to the cathode where they are incorporated. Charge carrier migration is ensured by the conductive electrolyte. Conversely, during charge, the opposite reaction takes place, whereby the electrons and metal cations are released from the cathode and are incorporated at the anode.

In a particular embodiment, the electrochemical cell or battery is a Li-S electrochemical cell or battery. In a particular embodiment, the secondary electrochemical cell or secondary battery is a secondary Li-S electrochemical cell or battery.

The present invention is also directed to a vehicle, an electronic device or an electrical grid comprising a secondary electrochemical cell or battery as defined above.

Similarly, the invention is directed to the use of a secondary electrochemical cell or battery as defined above, for storing energy, and more particularly for storing energy in a vehicle, an electronic device or an electrical grid. T1

The vehicle can be an automobile, in particular a heavy automobile such as buses or trucks, a rail vehicle, a marine vehicle, an aircraft or a spacecraft.

Preferably, the electronic device is a portable electronic device, such as a laptop, a tablet, a cellular phone, a smart phone or a smart watch. Preferably, the electrical grid is associated to a solar panel or a wind turbine.

Other embodiments

1.- An embodiment refers to an electrochemical cell comprising a negative electrode, an electrolyte, and a positive electrode, wherein the electrolyte comprises:

- at least one lithium salt, and

- a matrix characterized in that the solubility therein of the sulfur species generated during the cycling of the electrochemical cell is <5 wt.%; and wherein the positive electrode comprises:

- at least a carbonaceous material and a graphene-type material;

- sulfur, an organosulfur compound or a combination thereof; and

- at least one binder.

2.- Another embodiments refers to the electrochemical cell according to embodiment 1, wherein the at least one lithium salt is selected from LiC10₄, LiNCh, LiBF4, LiAsFe, LiPF₆, LiBF₃Cl, LiF, LiN(SO₂CF₃)2, LiN(SO₂F)₂, LiN(SO₂CF₃)(SO₂F), LiN(C₂F₅SO₂)(SO₂F), LiB(C₂O₄)₂, LiBF₂(C₂O₄), LiC(SO₂CF₃)₃, LiPF₃(C₂F₅)₃, LiCF₃SO₃ and a combination thereof.

3.- Another embodiments refers to the electrochemical cell according to embodiment 1 or 2, wherein the matrix is a liquid matrix which comprises: i) at least one organic solvent selected from a linear or cyclic ether, a sulfone, a carbonate or a combination thereof; and ii) at least one organic diluent selected from halogenated solvents, aromatic solvents or a combination thereof. 4.- Another embodiments refers to the electrochemical cell according to embodiment 3, wherein the at least one organic solvent comprised in the liquid matrix is selected from diethylether, methylbutyl ether, dimethoxy ethane (DME), 1,2-di ethoxy ethane (DEE), 1,3-dioxolane (DOL), bis(2-methoxy ethyl)ether (DEGDME), triethylene glycol dimethyl ether (G3), tetraethylene glycol dimethyl ether (TEGDME), poly(ethylene glycol) dimethyl ether (PEGDME), tetrahydropyran (TEIP), tetrahydrofuran (THF), tetramethylene sulfone (TMS), dimethyl sulfone, allyl methyl sulfone, butadiene sulfone, dibutyl sulfone, dipropyl sulfone (DPS), ethyl methyl sulfone (EMS), methyl isopropyl sulfone (MiPS), ethyl isopropyl sulfone (EiPS), 3,3,3-trifluoropropylmethyl sulfone (FPMS), ethyl-sec-butyl sulfone (EsBS), isopropyl methyl sulfone (IPMS), dimethyl carbonate (DMC), propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), and mixtures thereof.

5.- Another embodiments refers to the electrochemical cell according to embodiment 3, wherein the at least one organic diluent comprised in the liquid matrix is selected from bis(2,2,2-trifluoroethyl) ether (BTFE), l,l,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), tris(2,2,2-trifluoroethyl)orthoformate (TFEO), fluorobenzene (FB), 1,2 difluorobenzene (DFB), bis (2,2- difluoroethyl) ether (BDE), trifluorotoluene (TFT), 1,1,2,2-Tetrachloroethane, 1,2 di chloroethane, chlorobenzene, dichlorobenzene, trichlorobenzene, chlorotoluenes, a, a, a trichlorotoluene, bromobenzene, dibromoethane, 1,2 dibromobenzene, iodobenzene, 1,2 diodobenzene, 1,3 iodobenzene, anisole, furan, ethoxybenzene, 1,2-dimethoxybenzene, and mixtures thereof.

6.- Another embodiments refers to the electrochemical cell according to embodiments 1 to 5, wherein the at least a carbonaceous material comprised in the positive electrode is selected from activated carbons, black carbons, carbon nanotubes, carbon nanofibers, carbon nanoflakes and combinations thereof, and wherein the graphene-type material is selected from graphene nanoplatelets, graphene oxide, graphene-oxide-polymer composites and combinations thereof. 7.- Another embodiments refers to the electrochemical cell according to embodiments 1 to 6, wherein the organosulfur compound is selected from sulfurized polyacrylonitrile (SPAN), sulfurized divinylbenzene (p(DVB)), and phenyl disulfide (DPDS).

8.- Another embodiments refers to the electrochemical cell according to embodiments 1 to 7, wherein the at least one binder is selected from carboxymethyl cellulose (CMC), polyacrylonitrile (PAN), polyacrylic latex (LAI 32), styrene-butadiene rubber (SBR), poly(vinylidenedifluoride) (PVDF), poly(acrylic acid) (PAA), polyurethane (PU), polytetrafluoroethylene (PTFE), poly(vinylidene fluoride-hexafluoropropylene (PVDF- HFP), polyethylene oxide (PEO) and combinations thereof.

9.- Another embodiments refers to the electrochemical cell according to embodiments 1 to 8, wherein the negative electrode comprises a lithium metal or a lithium metal alloy.

10.- Another embodiments refers to the electrochemical cell according to embodiments 1 to 9, wherein the sulfur loading comprised in the positive electrode is at least 1 mg/cm², preferably between 3 and 8 mg/cm².

11.- Another embodiments refers to the electrochemical cell according to embodiments 1 to 10, wherein the amount of a sulfur, organosulfur compound or a combination thereof is at least 50 wt%, preferably 50-85 wt%, more preferably 50-75 wt%, even more preferably about 64 wt%, with respect to the total weight of the positive electrode; wherein the amount of the at least one carbonaceous material and a graphene-type material is at least 5 wt%, preferably 10-40 wt%, more preferably 10-30 wt%, even more preferably about 26 wt%, with respect to the total weight of the positive electrode; and wherein the amount of at least one binder is at least 5 wt%, preferably 5-25 wt%, more preferably 5-15 wt%, even more preferably about 10 wt%, with respect to the total weight of the positive electrode.

12.- Another embodiments refers to the electrochemical cell according to embodiments 1 to 11, wherein the concentration of at least one lithium salt is comprised between 0.1 and 5.0 M, preferably between 0.3 and 4.0 M, more preferably between 0.5 and 2.5 M, even more preferably is about 1.0 M.

13.- Another embodiments refers to the electrochemical cell according to embodiments 1 to 12, wherein the negative electrode comprises lithium metal; and wherein the electrolyte comprises:

- a LiTFSI-LiFSI mixture, and

- a TTE-sulfolane mixture; and wherein the positive electrode comprises:

- a mixture of a graphene oxide material and a porous carbon black material;

- sulfur; and

- optionally a CMC-SBR mixture as binder.

14.- Another embodiments refers to a method for preparing the electrochemical cell of embodiments 1 to 13 comprising the steps of: a) infiltrating sulfur, an organosulfur compound or a combination thereof into at least one carbonaceous material and a graphene-type material; or infiltrating sulfur, an organosulfur compound or a combination thereof into at least one carbonaceous material and then adding at least a graphene-type material, optionally in combination with additional carbonaceous materials, to give an infiltrated material; b) combining the infiltrated material from step a) with at least one binder to obtain a first mixture; c) optionally, combining an additional binder with the first mixture in step b) to obtain a second mixture; d) coating the mixture of step b) or c) on a current collector to obtain a positive electrode; and e) assembling the positive electrode obtained in step d) with an electrolyte as defined in any of claims 1 to 5 and 12 to 13, and an anode to obtain the electrochemical cell. 15.- Another embodiments refers to the use of the electrochemical cell according to embodiments 1 to 13 in Li-S secondary batteries.

EXAMPLES

The following examples are intended to illustrate but not limit the disclosed embodiments.

Reagents and starting materials

The following chemicals were used as received without any pre-treatment:

• For S cathodes: Sulfur (99 wt.%, Sigma Aldrich), KetjenBlack® (EC-600JD,

Akzo Nobel, also known with the acronym KJ600), carbon black (super C-65, Imerys), graphene nanoplatelets (GNP, Sigma Aldrich), multi-walled carbon nanotubes (CNTs, Sigma Aldrich) sodium carboxymethyl cellulose (CMC, Sigma Aldrich), styrene butadiene rubber (SBR, 40 wt.°/o in water, Jingrui), and carbon- coated aluminum current collector (ARMOR). The synthetic route to graphene- oxide based activated porous carbon (ResFarGO) is detailed in G. Jimenez- Martin et al., Batteries & Supercaps 2022,5, e20220016

(doi.org/10.1002/batt.202200167). The specific surface area was determined by the BET method through the nitrogen adsorption/desorption isotherm measured at -196°C in a Micromeritics ASAP2020.

• For the SSE: Sulfolane (99%, Sigma Aldrich), l,l,2,2-tetrafhioroethyl-2,2,3,3- tetrafluoropropyl ether (TTE, 95%, Fluorochem), lithium bis- (trifluoromethanesulfonyl) imide (LiTFSI, 99.9 wt.%, Solvionic), lithium bis(fhiorosulfonyl)imide (LiFSI, 99.9 M7.%, Nippon Subokai).

• Lithium metal anode (China Energy Lithium).

LIST OF EXAMPLES

Example 1 refers to the Li-S battery with a cathode comprising a ResFarGO additive combined with the Sparingly Solvated electrolyte.

Example 2 (comparative) refers to the Li-S battery with a cathode comprising graphenebased material (GNPs) combined with the Sparingly Solvated electrolyte. Comparative Example 1 refers to the Li-S battery with a cathode comprising ResFarGO additive combined with the state of art electrolyte (DME/DOL).

Comparative Example 2 refers to the Li-S battery with a cathode comprising carbonaceous additives (KJ600 and GNPs) combined with the state of art electrolyte (DME/DOL).

Comparative Example 3 refers to the Li-S battery with a reference cathode (only KJ600 present) combined with the state of art electrolyte (DME/DOL).

CATHODE PREPARATION

Cathode of the Example 1 and Comparative Example 1 : General preparation of cathode with a graphene oxide-based activated porous carbon (ResFarGO)

Positive electrodes were prepared according to the procedure below and the final composition consisted of 64 wt.% sulfur, 26 wt.% carbon, and 10 wt.% CMC/SBR (carboxymethyl cellulose/ styrene-butadiene rubber). Sulfur (640 mg) was initially infiltrated into a mixture of porous carbon consisting of 160 mg KJ600 and 100 mg ResFarGO by melt diffusion at 155°C for 12 h. In parallel, CMC (50 mg) was magnetically stirred in water (2 mL) until it was totally dissolved. Afterward, the infiltrated material was added and vigorously stirred for 4 h and 4 ml of water were further added. Finally, 120 mg of SBR solution (corresponding to 50 mg SBR) was added and magnetically stirred for 24 h. The resulting slurries were used to coat carbon-coated aluminum foil and the sulfur loading was fixed at the desired amount. Finally, the electrodes were gently dried, initially at room temperature to avoid cracking and later at 50°C under vacuum overnight.

Cathode of the Example 2 and Comparative Example 2: General preparation of cathode with graphene nanoplatelets (GNPs).

The positive electrode was prepared following the same procedure as in Example 1, but infiltrating first sulfur into KJ600 (by melt diffusion of the S in the porous carbon structure of KJ600 to form an infiltrated material) and subsequently introducing 10 wt.% conductive carbon additives including graphene-containing materials (5 wt. % C-65 + 2.5 wt. % carbon nanotubes + 2.5 wt. % graphene nanoplatelets) into the composite by dry mixing in a mortar until all the mixture is homogenous.

Cathode of the Comparative Example 3: General preparation of cathode with KJ600 only (or Reference cathode).

In this cathode, only KJ600 is used as carbonaceous material. The positive electrode was prepared following the same procedure as in Example 1 and 2, but infiltrating the sulfur only into KJ600, by melt diffusion of the S in the porous carbon structure of KJ600 to form a sulfur-carbon composite material.

Example 3 : Electrolyte preparation

In the case of the state of art electrolyte, 0.5 M lithium bis- (trifluoromethanesulfonyl)imide (LiTFSI, 99.9 wt.%, Solvionic) and 0.5 M lithium nitrate (LiNCL, 99.99 wt.%, Sigma- Aldrich) solution in a 1 : 1 (v./v.) mixture of DME and dimethyl 1,3-dioxolane (DOL) was employed. For the preparation of this electrolyte, the desired amounts of both salts were dissolved in DME/DOL and left stirring for 24 h to ensure that all the lithium salt was fully dissolved.

In the case of the Sparingly Solvated Electrolyte, 0.8 M of lithium bis- (trifluoromethanesulfonyl)imide (LiTFSI, 99.9 wt.%, Solvionic) and 0.2 M of lithium bis(fluorosulfonyl)imide solution in a 1 : 1.5 (v./v.) mixture of sulfolane and TTE was employed. Regarding the electrolyte preparation, firstly, the desired amounts of LiTFSI and LiFSI were mixed in sulfolane and magnetically stirred for 24 h to ensure that all the Li salts were fully dissolved (thus preparing the highly concentrated electrolyte). Once the Li salts were dissolved, TTE was added in the desired proportion to the prepared the SSE. Finally, the mixture was stirred for another 24 h to ensure a homogeneous solution.

Example 4: Coin cell and pouch cell assembly

The coin cells were assembled in an argon-filled glove box using the above-outlined cathodes from Examples 1, 2, and the reference cathode, a 16 pm Celgard® 2500 separator, and a Li° disk (China Energy Lithium, 500 pm) anode. For the pouch cell tests, Celgard® 2500 and 39^55 mm Li° anode were used. In both coin and pouch cells the corresponding electrolyte (state-of-art electrolyte of Sparingly Solvating Electrolyte) is added to the top of the cathode to activate it before the cell closure and were cycled keeping an electrolyte-to-sulfur ratio (E/S) of 7 pl mg'¹.

Example 5: Electrochemical measurements All the battery cycling tests were conducted in a Maccor Battery Tester. The coin cells (Figure 1) and pouch cells (Figure 2) were cycled with 4 mgs cm'² electrode loading under a long-cycling test at C/10 with 5 cycles at C/20 of preconditioning. In the case of the cells employing the state-of-the-art electrolyte based on DME/DOL (comparative results 1, 2 and, 3), they were cycled using a cut-off voltage range between 2.6 and 1.7 V. On the other hand, in the case of cells that employ a SSE (Example 1 and 2), they were cycled using a cut-off voltage range between 2.7 and 1.4 V.

Contrary to what is observed for carbonaceous cathodes in the prior art (C. Kensy et al. vs Strubel et al.), the replacement of the DME/DOL electrolyte with the SSE rescues and greatly improves cyclability when the cathode comprises a carbonaceous material and a graphene-type material (Examples 1 and 2). Further unexpectedly, when the graphenetype material is a graphene oxide, exceptional discharge capacity is additionally observed (Example 1).

Claims

1.- An electrochemical cell comprising a negative electrode, an electrolyte, and a positive electrode, wherein the electrolyte comprises:

- at least one lithium salt, and

- sulfur, an organosulfur compound or a combination thereof; and

- at least one binder.

2. The electrochemical cell according to claim 1, wherein the positive electrode comprises a mixture of the graphene oxide material and the carbonaceous material, each presenting a specific surface area of between 500 - 2500 m²/g.

3.- The electrochemical cell according to claim 1 or 2, wherein the at least one lithium salt is selected from LiClO₄, LiNO₃, LiBF4, LiAsFe, LiPFe, LiBF₃Cl, LiF, LiN(SO₂CF₃)₂, LiN(SO₂F)₂, LiN(SO₂CF₃)(SO₂F), LiN(C₂F₅SO₂)(SO₂F), LiB(C₂O₄)₂, LiBF₂(C₂O₄), LiC(SO₂CF₃)₃, LiPF₃(C₂F₅)₃, LiCF₃SO₃ and a combination thereof.

4.- The electrochemical cell according to any one of claims 1 to 3, wherein the matrix does not comprise a non-fluorinated glyme, preferably it does not comprise a non-halogenated glyme, preferably it does not comprise a glyme.

5.- The electrochemical cell according to any one of claims 1 to 4, wherein the matrix is a liquid matrix which comprises: i) at least one organic solvent selected from a linear or cyclic ether, a sulfone, a carbonate or a combination thereof; and ii) at least one organic diluent selected from halogenated solvents, aromatic solvents or a combination thereof.

6.- The electrochemical cell according to claim 5, wherein the at least one organic solvent comprised in the liquid matrix is selected from tetramethylene sulfone (TMS), dimethyl sulfone, allyl methyl sulfone, butadiene sulfone, dibutyl sulfone, dipropyl sulfone (DPS), ethyl methyl sulfone (EMS), methyl isopropyl sulfone (MiPS), ethyl isopropyl sulfone (EiPS), 3,3,3-trifluoropropylmethyl sulfone (FPMS), ethyl-sec-butyl sulfone (EsBS), isopropyl methyl sulfone (IPMS), dimethyl carbonate (DMC), propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), and mixtures thereof.

7.- The electrochemical cell according to claim 5 or 6, wherein the at least one organic diluent comprised in the liquid matrix is selected from bis(2,2,2- trifluoroethyl) ether (BTFE), l,l,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), tris(2,2,2-trifluoroethyl)orthoformate (TFEO), fluorobenzene (FB), 1,2 difluorobenzene (DFB), bis (2,2- difluoroethyl) ether (BDE), trifluorotoluene (TFT), 1,1,2,2-Tetrachloroethane, 1,2 di chloroethane, chlorobenzene, dichlorobenzene, trichlorobenzene, chlorotoluenes, a, a, a trichlorotoluene, bromobenzene, dibromoethane, 1,2 dibromobenzene, iodobenzene, 1,2 diodobenzene, 1,3 iodobenzene, anisole, furan, ethoxybenzene, 1,2- dimethoxybenzene, and mixtures thereof.

8.- The electrochemical cell according to any one of claims 1 to 7, wherein the positive electrode comprises a mixture of a carbonaceous material selected from activated carbons, black carbons, carbon nanotubes, carbon nanofibers, carbon nanoflakes and combinations thereof; and the graphene oxide material.

9. The electrochemical cell according to any one of claims 1 to 8, wherein the graphene oxide material has an oxygen content between 5 and 30%.

10.- The electrochemical cell according to any one of claims 1 to 9, wherein the organosulfur compound is selected from sulfurized polyacrylonitrile (SPAN), sulfurized divinylbenzene (p(DVB)), and phenyl disulfide (DPDS).

11.- The electrochemical cell according to any one of claims 1 to 10, wherein the at least one binder is selected from carboxymethyl cellulose (CMC), polyacrylonitrile (PAN), polyacrylic latex (LAI 32), styrene-butadiene rubber (SBR), poly(vinylidenedifluoride) (PVDF), poly(acrylic acid) (PAA), polyurethane (PU), polytetrafluoroethylene (PTFE), poly(vinylidene fluoridehexafluor opropylene (PVDF-HFP), polyethylene oxide (PEO) and combinations thereof.

12.- The electrochemical cell according to any one of claims 1 to 11, wherein the negative electrode comprises lithium metal or a lithium metal alloy.

13.- The electrochemical cell according to claims 1 to 12, wherein the amount of a sulfur, organosulfur compound or a combination thereof is at least 50 wt%, preferably 50-85 wt%, more preferably 50-75 wt%, even more preferably about 64 wt%, with respect to the total weight of the positive electrode; wherein the amount of the graphene oxide material, or of the mixture of the graphene oxide material and the carbonaceous material, is at least 5 wt%, preferably 10-40 wt%, more preferably 10-30 wt%, even more preferably about 26 wt%, with respect to the total weight of the positive electrode; and wherein the amount of at least one binder is at least 5 wt%, preferably 5-25 wt%, more preferably 5-15 wt%, even more preferably about 10 wt%, with respect to the total weight of the positive electrode.

14.- The electrochemical cell according to any one of claims 1 to 13, wherein the concentration of at least one lithium salt is comprised between 0.1 and 5.0 M, preferably between 0.3 and 4.0 M, more preferably between 0.5 and 2.5 M, even more preferably is about 1.0 M.

15.- The electrochemical cell according to claims 1 to 14, wherein the negative electrode comprises lithium metal; and wherein the electrolyte comprises:

- a LiTFSI-LiFSI mixture, and

- a TTE-sulfolane mixture; and wherein the positive electrode comprises:

- a mixture of a graphene oxide material and a porous carbon black material, wherein the specific surface area of said mixture is between 500 and 2500 m²/g, provided that the specific surface area of the graphene oxide material in said mixture is not higher than 2500 m²/g;

- sulfur; and

- optionally a CMC-SBR mixture as binder.

16.- A method for preparing the electrochemical cell of any one of claims 1 to 15 comprising the steps of: a) infiltrating sulfur, an organosulfur compound or a combination thereof into at least i) a graphene oxide material, or ii) a mixture of a graphene oxide material and a carbonaceous material, to give an infiltrated material; b) combining the infiltrated material from step a) with at least one binder to obtain a first mixture; c) optionally, combining an additional binder with the first mixture in step b) to obtain a second mixture; d) coating the mixture of step b) or c) on a current collector to obtain a positive electrode; and e) assembling the positive electrode obtained in step d) with an electrolyte as defined in any one of claims 1, 3 to 7 and 13 to 15, and an anode to obtain the electrochemical cell.

17.- Use of the electrochemical cell according to claims 1 to 15 in Li-S secondary batteries.