WO2015110333A1

WO2015110333A1 - Electrochemical cells comprising alkali-ion conducting composite membranes

Info

Publication number: WO2015110333A1
Application number: PCT/EP2015/050595
Authority: WO
Inventors: Oliver Gronwald; Johan Ter Maat; Klaus Leitner; Klaus MÜHLBACH; Werner Goedel; Lutz Reinhardt; Peggy SCHUMANN
Original assignee: Basf Se
Priority date: 2014-01-23
Filing date: 2015-01-14
Publication date: 2015-07-30

Abstract

The present invention relates to a rechargeable electrochemical cell comprising (a) at least one cathode (a) comprising at least one electroactive sulfur-containing material, (b) at least one anode (b) comprising at least one alkali metal, (c) at least one electrolyte composition (c) comprising (c1) at least one solvent (c1), and (c2) at least one alkali metal salt (c2), and (d) at least one alkali-ion conducting separator assembly (d) comprising (A) a continuous matrix (A) of at least one polymer, and (B) particles (B) of an alkali-ion conducting material, which are embedded in the continuous matrix (A), wherein at least 50 % of the embedded particles (B) penetrate both sides of the continuous matrix (A) and are uncovered by matrix (A). The present invention further relates to a device comprising such a rechargeable electrochemical cell.

Description

Electrochemical cells comprising alkali-ion conducting composite membranes

Description The present invention relates to a rechargeable electrochemical cell comprising

(a) at least one cathode (a) comprising at least one electroactive sulfur-containing material,

(b) at least one anode (b) comprising at least one alkali metal,

(c) at least one electrolyte composition (c) comprising

(c1 ) at least one solvent (c1 ), and

(c2) at least one alkali metal salt (c2),

and

(d) at least one alkali-ion conducting separator assembly (d) comprising

(A) a continuous matrix (A) of at least one polymer, and

(B) particles (B) of an alkali-ion conducting material, which are embedded in the contin- uous matrix (A),

wherein at least 50 % of the embedded particles (B) penetrate both sides of the continuous matrix (A) and are uncovered by matrix (A).

The present invention further relates to a device comprising such a rechargeable electrochemi- cal cell.

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

However, the specific energy of conventional lithium ion accumulators which have a carbon anode and a cathode based on metal oxides is limited. New dimensions in respect of the specific energy have been opened up by lithium-sulfur cells. Ideally, in lithium-sulfur cells, sulfur (Ss) is reduced in the sulfur cathode via polysulfide ions to S^2_ (i.e. L12S), which on charging of the cell is reoxidized to form sulfur-sulfur bonds.

In electrochemical cells, the positively and negatively charged electrode compositions are me- chanically separated from one another by layers which are not electrically conductive, known as separators, to avoid internal discharge. Due to their microporous structure many commonly used separators like polymer membranes or nonwovens do not only allow the transport of positive ionic charges like lithium cations as basic prerequisite for continuing offtake of current during operation of the electrochemical cell but also open the unwanted migration of polysulfide ions from the cathode to the metal anode. In order to avoid these parasitic processes alternative separators have been discussed. Solid lithium electrolytes like solid Li-ion conductors have been proposed and investigated as separators in electrochemical cells. Basic requirements, which such separators have to meet, are chemical and electrochemical stability toward both the active electrode compositions, resistance to dendrite growth and high cell temperatures, imper- meability for mobile electrode components and liquid electrolyte components, and high ion conductivity even at room temperature and below.

US 8,323,817 describes a galvanic cell comprising a water-impermeable, alkali-ion-conductive ceramic membrane as separator.

US 2012/02701 12 describes a composite solid electrolyte that includes a monolithic solid electrolyte base component that is a continuous matrix of an inorganic active metal ion conductor and a filler component used to eliminate through porosity in the solid electrolyte.

DE102007049203A1 and J. Am. Chem. Soc, 2013, 135 (1 1 ), pp 4380^1388 describe membranes, which comprise particles embedded in a continuous matrix, wherein at least 50 percent of the embedded particles at both and opposite surfaces of the membrane, are uncovered by the matrix. The membranes can be used for the separation of a desired compound from a mix- ture comprising that desired compound.

US201 1027642A describes a microporous polyolefin composite film with a thermally stable porous layer at high temperature, which is used as a separator for a high-capacity/high-power lithium secondary battery.

J. Eur. Ceram. Soc. 24 (2004) 1385-1387 describes PEO-based solid polymer electrolytes comprising nanosized Zr02 particles for building rechargeable lithium metal batteries.

US 8,334,075 describes a composite solid electrolyte, which includes a monolithic solid electro- lyte base component that is a continuous matrix of an inorganic active metal ion conductor and a filler component used to eliminate through porosity in the solid electrolyte.

US 8,383,268 describes a lithium ion secondary battery, which includes a positive electrode, a negative electrode and a thin film solid electrolyte including lithium ion conductive inorganic substance. The thin film solid electrolyte has thickness of 20 μηη or below and is formed directly on an electrode material or materials for the positive electrode and/or the negative electrode.

Solid State Ionics 2004, 175, 243-245 describes the polysulfide shuttle in Li/S battery systems, which is responsible for reducing the lifetime and charge efficiency in these systems.

Separators known from the literature, which comprise alkali-ion conducting materials, still have deficiencies in respect of one or more of the properties desired for such separators, for example low thickness, low weight per unit area, good mechanical stability during processing or in operation of the battery in respect of metal dendrite growth, good heat resistance, good ion conductiv- ity and complete impermeability for organic solvents. Some of the deficiencies of the known separators are ultimately responsible for a reduced life or limited performance of the electrochemical cells comprising them. Furthermore, separators in principle have to be not only mechanically but also chemically stable toward the cathode materials, the anode materials and the electrolytes. In the field of lithium-sulfur cells, separators which also prevent early cell death of lithium-sulfur cells, which is brought about particularly by migration of polysulfide ions from the cathode to the anode and by decomposition of electrolyte by a metallic lithium anode, are desirable.

It was therefore an object of the invention to provide a sulfur-containing electrochemical cell comprising a separator, which has advantages in respect of one or more properties of a known separator, in particular a separator which displays sufficient ion conductivity, low thickness, high thermal stability and good mechanical properties, for example sufficient flexibility and sufficient stability with respect to growing metal dendrites, and which by efficiently separating anode and cathode chemistry prevents the migration of polysulfide ions from the cathode to the anode and prevents decomposition of electrolyte caused by parasitic reactions of electrolyte components with an anode, in particular a metallic lithium anode. This object is achieved by a rechargeable electrochemical cell comprising

(b) at least one anode (b) comprising at least one alkali metal, at least one electrolyte composition (c) comprising at least one solvent (c1 ), and

(c2) at least one alkali metal salt (c2), and at least one alkali-ion conducting separator assembly (d) comprising

(A) a continuous matrix (A) of at least one polymer, and

(B) particles (B) of an alkali-ion conducting material, which are embedded in the continuous matrix (A), wherein at least 50 % of the embedded particles (B) penetrate both sides of the continuous matrix (A) and are uncovered by matrix (A).

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 rechargeable electrochemical cell comprises at least one cathode (a) comprising at least one electroactive sulfur-containing material. In the context of the present invention, this cathode (a) comprising at least one electroactive sulfur-containing material is also called cathode (a) 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 ff 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 is elemental sulfur.

In one embodiment of the present invention, the inventive rechargeable electrochemical cell is characterized in that the electroactive sulfur-containing material of cathode (a) is elemental sulfur.

During the charging process of an inventive rechargeable electrochemical cell cathode (a) comprises usually a mixture of different electroactive sulfur-containing materials since more and more S-S-bonds are formed.

Cathode (a) may comprise one or further constituents. For example, cathode (a) may comprise carbon in a conductive polymorph, for example 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 rechargeable electrochemical cell is characterized in that cathode (a) contains a material based on electrically conductive carbon.

In addition, cathode (a) may comprise one or more binders, for example one or more organic polymers. Suitable binders are described in WO 2012/168851 page 6, line 40 to page 7, line 30.

Particularly suitable binders for the cathode (a) 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.

In one embodiment of the present invention, cathode (a) of the inventive cell comprises in the range from 10 to 80% by weight, preferably 50 to 70% by weight, of sulfur, determined by elemental analysis, based on the total mass of the sum of all electroactive sulfur-containing materials, all carbon in a conductive polymorph and all binders.

In one embodiment of the present invention, cathode (a) of the inventive 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 mass of the sum of all electroactive sulfur-containing materials, all carbon in a conductive polymorph and all binders. 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, cathode (a) of the inventive 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 10% by weight, based on the total mass of the sum of all electroactive sulfur-containing materials, all carbon in a conductive polymorph and all binders. In addition, cathode (a) 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, cathode (a) has a thickness in the range from 25 to 200 μηη, preferably from 30 to 100 μηη, based on the thickness without current collector.

The inventive rechargeable electrochemical cell further comprises, as well as cathode (a), at least one anode (b) comprising at least one alkali metal like lithium, sodium or potassium, also called anode (b) for short. Preferably anode (b) comprises lithium. The alkali metal of anode (b) 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. Anode (b) 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 anode (b) 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 rechargeable electrochemical cell is characterized in that the alkali metal of anode (b) is lithium.

Anode (b) can further comprise a current collector. Suitable current collectors are, e.g., metal wires, metal grids, metal gaze and preferably metal foils such as copper foils.

Anode (b) 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, eth- ylene-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 copolymers, 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, anode (b) 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 rechargeable electrochemical cell further comprises, as well as cathode (a) and anode (b), at least one electrolyte composition (c) comprising

(c1 ) at least one solvent (c1 ), and

(c2) at least one alkali metal salt (c2).

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.

Solvent (c1 ) can be chosen from a wide range of solvents, in particular from solvents which dissolve alkali metal salts (c2) easily. Solvents or solvent systems, which dissolve alkali metal salts (c2) 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. Example 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 acetonitril, lactams like NMP, lac- tones, linear or cyclic peralkylated urea derivatives like TMU or DMPU, fluorinated ether, fluori- nated carbamates, fluorinated carbonated or fluorinated esters.

Possible solvents (c1 ) may be liquid or solid at room temperature and are preferably liquid at room temperature.

In one embodiment of the present invention the inventive rechargeable electrochemical cell is characterized in that the solvent (c1 ) is a dipolar aprotic solvent.

Solvents (c1 ) are preferably selected from polymers, cyclic or noncyclic ethers, cyclic or noncy- die acetals, cyclic or noncyclic organic carbonates and ionic liquids.

In one embodiment of the present invention the inventive rechargeable electrochemical cell is characterized in that the solvent (c1 ) 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 (c2), which are used as conductive salts, have to be soluble in the solvent (c1 ). Preferred alkali metal salts (c2) are lithium salts or sodium salts, in particular lithium salts. In one embodiment of the present invention the inventive rechargeable electrochemical cell is characterized in that the alkali metal salt (c2) 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(CnF_2n+iS02)3, lithium imides such as

LiN(C_nF2n+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_nF2n+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.

The inventive rechargeable electrochemical cell further comprises, as well as cathode (a), anode (b) and electrolyte composition (c) at least one alkali-ion conducting separator assembly (d) comprising

(A) a continuous matrix (A) of at least one polymer, and

(B) particles (B) of an alkali-ion conducting material, which are embedded in the continuous matrix (A), wherein at least 50 % of the embedded particles (B) penetrate both sides of the continuous trix (A) and are uncovered by matrix (A). The alkali-ion conducting separator assembly (d) comprises a continuous matrix (A) of at least one polymer, also called matrix (A) for short, and particles (B) of an alkali-ion conducting material, also called particles (B) for short, which are embedded in the continuous matrix (A), where- in at least 50 %, preferably at least 80 %, more preferably at least 90 %, in particular at least 95% of the embedded particles (B) penetrate both sides of the continuous matrix (A) and are uncovered by matrix (A).

Preferably the particles, which penetrate both sides of the continuous matrix (A) and which are uncovered by matrix (A), expose a fraction of 10% to 40% of their total surface on each side of the continuous matrix (A), in particular these particles expose on each side of the continuous matrix (A) a similar fraction of their surface.

Matrix (A) together with embedded particles (B) forms a layer or membrane which is permeable for alkali-ions, in particular for lithium ions, and which is electrically insulating. While particles (B) are alkali-ion conducting the matrix (A) itself can in principle be either alkali-ion conducting or non-alkali-ion conducting, depending on the nature of the polymer or mixture of polymers forming matrix (A). Preferably matrix (A) is a non-ion conducting matrix, in particular a non-alkali-ion conducting matrix.

In another embodiment matrix (A) is blocking the transport of anions, in particular the transport of polysulfide and sulfide ions.

In the context of the present invention the expression "electrically insulating" means, that the electrical conductivity of the alkali-ion conducting separator assembly (d) is less than 10^-8 S/cm at 25 °C.

In one embodiment of the present invention the inventive rechargeable electrochemical cell is characterized in that the continuous matrix (A) of the alkali-ion conducting separator assembly (d) is a non-ion conducting matrix, in particular a non-alkali-ion conducting matrix.

The polymer or mixture of polymers forming matrix (A) can be chosen from a wide range of polymers, for example organic polymers or inorganic polymers like polyphosphazenes or poly(organo)siloxanes, providing that the chosen polymer or mixture of polymers is insoluble or non-swellable, in particular insoluble in such solvents to which the inventive alkali-ion conducting separator assembly (d) is exposed in its designated application, in particular in electrochemical cells. Preferably the polymer is insoluble in dipolar aprotic solvents, more preferably insoluble in ethers, carbonates, amides, sulfoxides, sulfones or mixtures thereof, in particular insoluble in ethers, carbonates or mixtures thereof.

In one embodiment of the present invention the alkali-ion conducting separator assembly is characterized in that the polymer of the continuous matrix (A) is a non-swellable polymer. Suitable polymers are preferably hydrophobic polymers, which are obtainable from appropriate monomers, which are in particular polymerizable by UV initiators. Preferred examples of such monomers are trimethylolpropane triacrylate (Laromer^®TMPTA), trimethylolpropane trimethacry- late (TMPTMA), mixture of 7,9,9 and 7,7,9-trimethyl-4,13-dioxo-3,14-dioxa-5,12- diazahexadecan-1 ,16-diol-dimethylacrylate (Plex 6661 -0^®, HEMATMDI), 1 ,3- butanedioldimethylacrylat (1 ,3-BDDMA), 1 ,4-butanedioldimethylacrylat (1 ,4-BDDMA), eth- yleneglycoldimethylacrylate (EGDMA), divinylbenzene or mixtures thereof.

In principle the polymer forming matrix (A) can be linear, branched, ladder-like or cross-linked. Preferably the polymer of the continuous matrix (A) is a cross-linked polymer, in particular a cross-linked polyacrylate or polymethacrylate.

In one embodiment of the present invention rechargeable electrochemical cell is characterized in that the polymer of the continuous matrix (A) of the alkali-ion conducting separator assembly (d) is a cross-linked polymer.

The shape of the alkali-ion conducting separator assembly (d) is preferably the shape of a sheet or flat body. In the context of the present invention, the expression "flat" means that the alkali- ion conducting separator assembly (d) described, which is a three-dimensional body, is smaller in one of its three spatial dimensions (extents), namely the thickness, with respect to the two other dimensions, the length and width. Typically, the thickness of the alkali-ion conducting separator assembly (d) is less than the second-greatest dimension at least by a factor of 5, preferably at least by a factor of 10, more preferably at least by a factor of 20. Since the alkali-ion conducting separator assembly (d) is flat, it can not only be incorporated as flat layer between cathode (a) and anode (b), but can also, as required, be rolled up, wound up or folded as desired.

The thickness of matrix (A) of the alkali-ion conducting separator assembly (d) can be varied in a wide range. In particular the thickness of matrix (A) depends on the average diameter of particles (B), since both sides of matrix (A) should be penetrated by particles (B). Preferably matrix (A) has an average thickness in the range from 0.01 to 100 μηη, preferably in the range from 0.1 to 10 μηη. In one embodiment of the present invention the inventive rechargeable electrochemical cell is characterized in that the continuous matrix (A) of the alkali-ion conducting separator assembly (d) has an average thickness in the range from 0.01 to 100 μηη.

Since at least 50 % of particles (B) of the alkali-ion conducting separator assembly (d) penetrate both sides of the continuous matrix (A) and are uncovered by matrix (A) preferably particles (B) form a monolayer in order to achieve that result. In one embodiment of the present invention the inventive rechargeable electrochemical cell is characterized in that the particles (B) of the alkali-ion conducting separator assembly (d) form a monolayer. Particles (B) consist of an alkali-ion conducting material. Alkali-ion conducting materials, in particular lithium ion conducting materials are known to the person skilled in the art. Non limiting examples of suitable alkali-ion conducting materials are described in US 8,383,268, col. 3, line 42 to col. 4, line 60. Preferably the alkali-ion conducting material is selected from the group consisting of ceramics, sintered ceramics, glass-ceramics and glasses, more preferably well- known Li ion conducting inorganic solid lithium ion conductors as described by P. Knauth in Solid State Ionics 180 (2009) 91 1 -916 or by A. Hayashi and M. Tatsumisago in Electronic Materials Letters 8 (2012) 199-207. In particular, ceramic materials with the perovskite, Nasicon, Thio- Lisicon or garnet crystal structure offer good conductivities, but also inorganic sulfide glasses in powder form are good candidates.

In one embodiment of the present invention the inventive rechargeable electrochemical cell is characterized in that the alkali-ion conducting material of the alkali-ion conducting separator assembly (d) is selected from the group consisting of ceramics, sintered ceramics, glass- ceramics and glasses.

The average diameter of particles (B) can be varied in a wide range. Preferably the average diameter of particles (B) is in the range from 0.1 to 10 μηη, more preferably in the range from 0.3 to 5 μηη, in particular in the range from 0.5 to 2 μηη. The particle size distribution was determined by means of laser diffraction technology in powder form to DIN ISO 13320-1 with a Mastersizer from Malvern Instruments GmbH, Herrenberg, Germany. The crucial value for the mean particle size is what is called the d90 value. The d90 value of the volume-weighted distribution is that particle size for which 90% of the particle volume of particles are smaller than or equal to the d90 value.

In one embodiment of the present invention the inventive rechargeable electrochemical cell is characterized in that the average diameter of particles (B) of the alkali-ion conducting separator assembly (d) is in the range from 0.1 to 10 μηη. In principle the shape of particles (B) can be freely chosen, but platelets and in particular cubes offer a better contact area for lithium ion transfer and can be arranged into a very high volume percentage of matrix (A).

In one embodiment of the present invention the rechargeable electrochemical cell is character- ized in that the shape of particles (B) is the shape of platelets or of cubes, in particular of cubes.

Especially if particles (B) are arranged in a monolayer it is extremely important to have a narrow particle size distribution, because particles which are significantly thinner or smaller than the average particles would not protrude on both sides of matrix (A). Particles (B) of an alkali-ion conducting material can be obtained from the corresponding material in macroscopic size by grinding processes resulting in a very wide particle size distribution. In such a case

over/undersize particles may be removed by suitable and well-established methods like filtra- tion, sieving and sifting. The ratio of d50 to d10 and also d90 to d50 should be below 3, preferably below 2.

In one embodiment of the present invention the rechargeable electrochemical cell is characterized in that the ratio of the d50 value to the d10 value and of the d90 value to the d50 value of the particle size distribution of particles (B) of the alkali-ion conducting separator assembly (d) is below 3, preferably below 2.

Particles in the shape of platelets or cubes are preferably obtained by using wet-chemical synthesis routes, which offer better control of the particle morphology than grinding methods.

The ratio of the average thickness of the continuous matrix (A) to the average diameter of particles (B) can be varied in a wide range. Preferably the ratio of the average thickness of the continuous matrix (A) to the average diameter of particles (B) is in the range from 0.1 to 2, more preferably in the range from 0.5 to 1.2, in particular in the range from 0.75 to 1.

In one embodiment of the present invention the inventive rechargeable electrochemical cell is characterized in that the ratio of the average thickness of the continuous matrix (A) to the average diameter of particles (B) is in the range from 0.1 to 2, preferably in the range from 0.5 to 1 .2, in particular in the range from 0.75 to 1 .

The ratio of the total volume of the particles (B) to the total volume of the continuous matrix (A) can be varied in a wide range depending on the alkali-ion conducting properties of these two components. Preferably the ratio of the total volume of the particles (B) to the total volume of the continuous matrix (A) is in the range from 95 / 5 to 20 / 80, more preferably in the range from 80 / 20 to 40 / 60. This means in other words that preferably the ratio of the volume fraction of the particles (B) to the volume fraction of the continuous matrix (A) is in the range from 19 to 0.25, more preferably in the range from 4 to 0.66

In one embodiment of the present invention the inventive rechargeable electrochemical cell is characterized in that the ratio of the total volume of the particles (B) to the total volume of the continuous matrix (A) is in the range from 95 / 5 to 20 / 80, preferably in the range from 80 / 20 to 40 / 60.

The total mass of all particles (B) in the alkali-ion conducting separator assembly (d) is prefera- bly least 20% by weight, more preferably at least 40% by weight, in particular in the range from 60% to 95% by weight based on the total weight of the alkali-ion conducting separator assembly (d). This means in other words that the mass fraction of all particles (B) in the inventive alkali-ion conducting separator assembly is preferably least 0.2, more preferably at least 0.4, in particular in the range from 0.60 to 0.95.

The sum of the total mass of matrix (A) and of the total mass of all particles (B) is preferably least 60% by weight, more preferably at least 80% by weight, in particular in the range from 90% up to 100% by weight based on the total weight of the alkali-ion conducting separator assembly (d). This means in other words that the sum of the mass fraction of matrix (A) and of the mass fraction of all particles (B) is preferably least 0.60, more preferably at least 0.80, in particular in the range from 0.90 up to 1.

The alkali-ion conducting separator assembly (d), which is an electrical insulator, shows preferably good wettability with respect to electrolytes, in particular to non-aqueous electrolytes, which are used in the inventive electrochemical cells. In addition the alkali-ion conducting separator assembly (d) is preferably chemically inert against the components of the electrodes, more preferably chemically inert against anode components, in particular chemically inert against lithium in form of lithium metal or an alloy of lithium. Particularly preferred the alkali-ion conducting separator assembly (d) is impermeable to organic solvents, ensuring that only naked lithium cations can cross the separator, in particular through particles (B). In one embodiment of the present invention the inventive rechargeable electrochemical cell is characterized in that the alkali-ion conducting separator assembly (d) is impermeable to organic solvents.

The alkali-ion conducting separator assembly (d) is preferably produced as a free-standing sep- arator, that is the separator assembly (d) is preferably produced independently of any electrode. The free-standing separator is combined with other parts of the inventive rechargeable electrochemical cell, like cathode (a) or anode (b), in a subsequent production step by the cell producer. In one embodiment of the present invention the rechargeable electrochemical cell is characterized in that the alkali-ion conducting separator assembly (d) is combined as a free-standing separator with other parts of the inventive rechargeable electrochemical cell.

In one embodiment of the present invention, separator assembly (d) is positioned between cathode (a) and anode (b) in a way that it is like a layer to either a major part of one surface of cathode (a) or anode (b).

In one embodiment of the present invention, separator assembly (d) is positioned between cathode (a) and anode (b) in a way that it is like a layer to both a major part of one surface of cathode (a) and anode (b). In a preferred embodiment of the present invention, separator assembly (d) is positioned between cathode (a) and anode (b) in a way that it is like a layer to one surface of cathode (a) or of anode (b).

In another preferred embodiment of the present invention, separator assembly (d) is positioned between anode (a) and cathode (b) in a way that it is like a layer to one surface of both cathode (a) and of anode (b).

The alkali-ion conducting separator assembly (d), which comprises as a first component (A) a continuous matrix (A) of at least one polymer, and as a second component (B) particles (B) of an alkali-ion conducting material, which are embedded in the continuous matrix (A), wherein at least 50 % of the embedded particles (B) penetrate both sides of the continuous matrix (A) and are uncovered by matrix (A), can be prepared in analogy to the processes described in

DE102007049203A1 . Preferably the process for producing an alkali-ion conducting separator assembly (d) comprises the process steps of

(a) depositing particles (B) of an alkali-ion conducting material and a liquid phase (A2) comprising at least one polymer or at least one polymerizable compound on a smooth surface of a solid or liquid phase (C), (b) solidifying liquid phase (A2) by evaporating volatile components, crystallization, vitrification or polymerization, and

(c) separating the continuous matrix (A) with the embedded particles (B) from the surface of phase (C).

The description and preferred embodiments of the alkali-ion conducting separator assembly (d) and its components, in particular the description of the continuous matrix (A) as a first component and of the particles (B) as a second component, in the process correspond to the above description of these components for the alkali-ion conducting separator assembly (d).

Different embodiments of the process comprising process steps a), b) and c) are presented in figures 1 to 4 of DE102007049203A1 and in J. Am. Chem. Soc, 2013, 135 (1 1 ), pp 4380-4388.

In process step (a) particles (B) of an alkali-ion conducting material and a liquid phase (A2) comprising at least one polymer or at least one polymerizable compound are deposited on a smooth surface of a solid or liquid phase (C).

The particles (B) of an alkali-ion conducting material, which are deposited on a smooth surface of a solid or liquid phase (C), have been described above. Depending on the nature of phase (C) and depending on the nature of liquid phase (A2) the particles (B) are preferably modified on their surface, in particular to adjust the hydrophilicity and hydrophobicity respectively, for example by coating the particles with a thin layer of a very hydrophobic material in order to minimize the contact of the particles with an aqueous phase (C) and to ensure a close contact to liquid phase (A2). Preferably Lisicon-type Ι_Η .3ΑΙΟ.3ΤΊΙ.₇(Ρ04)3 (LATP) is used as alkali-ion conducting material due to its non-sensitivity to ambient conditions (humidity, carbon dioxide). Preferably the surface of the alkali-ion conducting material is modified to increase its hydrophobicity. Surface modification is preferably carried out by treatment with silanes such as 1 H, 1 H, 2H, 2H- perflurooctyltriethoxysilane (PFOTES), but also halogenated silanes or phosphonic acids can be used for surface modification.

Liquid phase (A2) comprises a compound or a mixture of compounds, that can be solidified by evaporating volatile components like solvents, or by crystallization, vitrification or polymerization of an appropriate component. Preferably liquid phase (A2) comprises at least one polymer or at least one polymerizable compound, which is also called monomer, preferably an organic compound, which can be polymerized in a radical polymerization, preferably using thermal initiators in particular using photoinitiators. Particularly preferred suitable polymerizable compounds are for example acrylates or methacrylates. Examples for preferred polymerizable organic com- pounds are trimethylolpropane triacrylate (Laromer^®TMPTA), trimethylolpropane trimethacrylate (TMPTMA), mixture of 7,9,9 and 7,7,9-trimethyl-4,13-dioxo-3, 14-dioxa-5,12-diazahexadecan- 1 ,16-diol-dimethylacrylate (Plex 6661 -0^®, HEMATMDI), 1 ,3-butanedioldimethylacrylat (1 ,3- BDDMA), 1 ,4-butanedioldimethylacrylat (1 ,4-BDDMA), ethyleneglycoldimethylacrylate (EGDMA) and divinylbenzene. Preferred solvents in liquid phase (A2) are non-polar, aprotic compounds such as toluene, which can be used in particular in combination with a liquid phase (C) comprising water or consisting of water.

The solid or liquid phase (C) is usually a phase that is immiscible with liquid phase (A2) and that can be easily separated from solidified phase (A2). Examples of a suitable solid phase (C) are plates of salt, frozen liquid like ice, polyethylene, polypropylene or polytetrafluoroethylene or the electrode materials described in detail above. Examples of a liquid phase (C) are water, ionic liquids, salt melts, aqueous salt solutions or liquid metals like mercury. Preferably phase (C) neither reacts with particles (B) nor reacts with liquid phase (A2). Preferably water is used as liquid phase (C) due to its non-toxic properties and high surface tension. Depending on the den- sity of the alkali-ion conducting material, the density of liquid phase (C) can be adjusted by dissolution of salts, such as metal halogenides to ensure that at least 50 % of the embedded particles (B) penetrate both sides of the continuous matrix (A) and are uncovered by matrix (A). In one embodiment zinc bromide (ZnBr2) is added to water in order to increase density of liquid phase (C). As alternative, the average particle size can be adjusted to the surface tension of liquid phase (C) in order to obtain the alkali-ion conducting separators (d).

In process step (b) liquid phase (A2) is solidified by evaporating volatile components, crystallization, vitrification or polymerization in order to form matrix (A). Process step (b) can be also described as hardening or curing. The person skilled in the art is aware of different systems and methodologies the convert a liquid phase under controlled conditions into a solid phase. Depending on the physical properties of particles (B) or surface modified particles (B), in particular depending on their size in combination with their density and the hydrophilicity of their surface, the liquid phase (A2) and the solid or liquid phase (C) have to be chosen properly. In process step (c) the continuous matrix (A) with the embedded particles (B) is separated from the surface of phase (C). Depending on the physical nature of phase (C) process step (c) can be varied, for example the separation is simply done mechanically by taking off the alkali-ion conducting separator assembly (d) from either a solid or liquid phase (C) or after melting phase (C) or even after evaporating phase (C).

By the above-described process the separator assemblies (d) can be obtained in the form of continuous belts which are processed further by the battery manufacturer, especially assembling the separator assemblies (d) with appropriate flat cathodes and flat anodes in order to produce inventive rechargeable electrochemical cells.

In one embodiment of the present invention, inventive rechargeable 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 rechargeable electrochemical cells can have a disc-like shape. In another embodiment, inventive rechargeable electrochemical cells can have a prismatic shape.

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

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

In one embodiment of the present invention, inventive rechargeable electrochemical cells are selected from pouch cells. Inventive rechargeable electrochemical cells, in particular rechargeable lithium sulfur cells, comprising at least one alkali-ion conducting separator assembly (d) have overall advantageous properties. They have a long duration with very low loss of capacity, good cycling stability and a reduced tendency towards short circuits after longer operation and/or repeated cycling. A further aspect of the present invention refers to batteries, in particular to rechargeable lithium sulfur batteries, comprising at least one inventive rechargeable electrochemical cell, for example two or more. Inventive rechargeable 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 have a long duration with very low loss of capacity, good cycling stability, and high temperature stability. A further aspect of the present invention is the use of inventive rechargeable electrochemical cells or inventive batteries according 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, calculators, 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 rechargeable electrochemical cell.

The present invention further provides a device comprising at least one inventive rechargeable electrochemical cell as described above. Figures in percent are each based on % by weight, unless explicitly stated otherwise.

I. Preparation of hydrophobized Particles Lithium-Aluminium-Titanium-Phosphate (LATP) particles of average diameter of DV 10 0.7 μηη, DV 50 1 .9 μηη, DV 90 5.2 μηη (1 g) were dispersed in demineralized water (75 ml.) and stirred for 1 h. Subsequently, the dispersion was centrifugalized and the water was decanted. This procedure was repeated 3 times. Then the particles were dried at ambient temperature under ambient atmosphere, and pressure overnight and then at 120 °C for 5 h. They were added to a solution of 1 H,1 H,2H,2H-perfluorooctyltriethoxysilane (PFOTES) (0.27 g) in toluene (43.5 g) and stirred for 48 h. Subsequently, the particles were recovered by centrifugation and decanting the supernatant, washed with toluene, acetone and ethanol (each time followed by centrifugation and decanting the supernatant). Afterwards they were dried at ambient temperature under ambient atmosphere, and pressure at 130 °C for 5 h.

II. Preparation of composite membranes by float-casting without lateral compression on a Petri dish A glass Petri dish of 5 cm diameter and 1 .5 cm height was filled to half its height with demineralized water, placed on a dark surface and illuminated with a bright white light source from the top. Approximately 1 .5 g of a mixture comprising LATP particles prepared in Example I., monomer (Visiomer^®-HEMATMDI, Evonic industries), photo initiator system (Esacure^® ITX/Esacure^® A 198, Lamberti S.p.A.,) and ethyl acetate (mass ratios = 1 : 0.5 : 0.02 : 100) was applied to the water surface of the half-filled Petri dish using a syringe with a stainless steel needle, the needle tip touching the water surface. The exact amount to be spread was determined by visual inspection: Spreading initially gives rise to translucent patches on the water surface that differ in reflectivity from the original water surface and are visible if inspected at a shallow angle. Towards the end of the spreading these patches merge into a continuous layer. If the amount of solution applied exceeds the desired amount, white opaque schlieren appear on the water surface. If even more solution is applied, one observes in addition white opaque patches on the water surface. After spreading, the layer was exposed to air for 1 h (to evaporate the volatile com- pounds). Subsequently an arc discharge lamp with a primary emission wavelength at 395 nm (PC-2000/38003 of Dymax corp) was mounted 10 cm above the petri dish. The lamp was operated for 30 min, illuminating the layer with an intensity of 50 mW/cm². This illumination gave rise to a solidification of the layer. Subsequently the layer was lifted off the water surface using either continuous silicon or metal substrates, filter paper or metal grids.

III. Preparation of composite membranes by float-casting with lateral compression on a

Langmuir trough Preparation of the membrane was conducted similar to the way of preparation detailed in example II with the following deviations. A Langmuir trough (14.9 cm x 40.3 cm, KSV 3000) was filled with demineralized water and the movable barrier was positioned at the outmost position. Approximately 9 g of a mixture comprising LATP particles, monomer, photo initiator and ethyl acetate (mass ratios = 1 : 0.5 : 0.02 : 100) was applied to the water surface drop by drop using a syringe with a stainless steel needle within a period of approximately one minute. Immediately after application of this mixture the movable barrier was moved inwards at a speed of 0.2 to 0.5 m min-¹ until the area between the movable and the stationary barriers was approximately 14.9 cm x 8.5 cm. The endpoint of lateral compression was determined by visual inspection, using the same criteria as detailed above.

Illumination and transfer was conducted as detailed above.

Claims

A rechargeable electrochemical cell comprising

(b) at least one anode (b) comprising at least one alkali metal,

(c) at least one electrolyte composition (c) comprising (c1 ) at least one solvent (c1 ), and

(c2) at least one alkali metal salt (c2),

and

(d) at least one alkali-ion conducting separator assembly (d) comprising

(A) a continuous matrix (A) of at least one polymer, and

(B) particles (B) of an alkali-ion conducting material, which are embedded in the continuous matrix (A), wherein at least 50 % of the embedded particles (B) penetrate both sides of the con tinuous matrix (A) and are uncovered by matrix (A).

The rechargeable electrochemical cell according to claim 1 , wherein the electroactive sulfur-containing material of cathode (a) is elemental sulfur.

The rechargeable electrochemical cell according to claim 1 or 2, wherein the alkali metal of anode (b) is lithium.

The rechargeable electrochemical cell according to any of claims 1 to 3, wherein the solvent (c1 ) is selected from polymers, cyclic or noncyclic ethers, noncyclic or cyclic acetals and cyclic or noncyclic organic carbonates.

The rechargeable electrochemical cell according to any of claims 1 to 4, wherein the alkali metal salt (c2) is a lithium salt.

The rechargeable electrochemical cell according to any of claims 1 to 5, wherein the continuous matrix (A) of the alkali-ion conducting separator assembly (d) is a non-ion conducting matrix.

7. The rechargeable electrochemical cell according to any of claims 1 to 6, wherein the polymer of the continuous matrix (A) of the alkali-ion conducting separator assembly (d) is a cross-linked polymer. 8. The rechargeable electrochemical cell according to any of claims 1 to 7, wherein the continuous matrix (A) of the alkali-ion conducting separator assembly (d) has an average thickness in the range from 0.01 to 100 μηη.

9. The rechargeable electrochemical cell according to any of claims 1 to 8, wherein the parti- cles (B) of the alkali-ion conducting separator assembly (d) form a monolayer.

10. The rechargeable electrochemical cell according to any of claims 1 to 9, wherein the alkali-ion conducting material of the alkali-ion conducting separator assembly (d) is selected from the group consisting of ceramics, sintered ceramics, glass-ceramics and glasses.

1 1 . The rechargeable electrochemical cell according to any of claims 1 to 10, wherein the average diameter of particles (B) of the alkali-ion conducting separator assembly (d) is in the range from 0.1 to 10 μηη. 12. The rechargeable electrochemical cell according to any of claims 1 to 1 1 , wherein the ratio of the average thickness of the continuous matrix (A) to the average diameter of particles (B) is in the range from 0.1 to 2.

13. The rechargeable electrochemical cell according to any of claims 1 to 12, wherein the ratio of the total volume of the particles (B) to the total volume of the continuous matrix (A) is in the range from 95 / 5 to 20 / 80.

14. The rechargeable electrochemical cell according to any of claims 1 to 13, wherein the alkali-ion conducting separator assembly (d) is impermeable to organic solvents.

15. A device comprising at least one rechargeable electrochemical cell according to any of claims 1 to 14.