Method for producing multilayer solid electrolyte and multilayer solid electrolyte

Abstract

Disclosed is a method for producing a multilayer solid electrolyte (SSE) comprising alternating dense and porous layers wherein the number of layers is at least 2, the method comprising: adding a first compound comprising one or more alkali and / or alkaline earth metals and a first binder to a first solvent, thereby obtaining a first mixture; adding a second compound comprising one or more alkali metals and / or alkaline earth metals, a second binder and a pore forming compound to a second solvent, thereby obtaining a second mixture; film casting the first mixture and the second mixture on a substrate until the number of layers is obtained, thereby obtaining a green (multilayer) structure; degreasing the green (multilayer) structure in an atmosphere comprising at least 20 vol% oxygen at a temperature between 250 DEG C and 800 DEG C, and sintering the green (multilayer) structure, thereby obtaining the multilayer SSE. The invention further discloses a multilayer SSE obtained by the method of the invention and a solid state battery comprising such SSE.

Description

[0001] Technical field of the invention

[0002] The present invention relates to a method for producing a multilayer solid electrolyte comprising alternating dense layers and porous layers, to a multilayer solid electrolyte comprising alternating dense layers and porous layers, and to a solid-state battery comprising such a multilayer solid electrolyte.

[0003] background

[0004] Solid-state batteries, especially lithium metal solid-state batteries (SSBs), are now considered as promising, non-toxic and safer alternatives to lithium-ion batteries containing liquid electrolytes. However, the performance of solid-state electrolytes (SSEs) used in such SSBs today does not seem to be sufficient to meet commercial requirements.

[0005] Based on lithium garnet, especially Li7La3Zr2O 12 SSEs of lithium lanthanum zirconium oxide (LLZO) have attracted interest as an attractive approach to improve one or more of the energy density, cycling stability, and safety of lithium-ion batteries. However, even these SSEs have not met commercial requirements. Lithium metal anodes combined with dense LLZO ceramics are characterized by high cycling stability at >0.5 mA / cm 2 Current density and >0.5mAh / cm 2 Under these electrochemical conditions, the electrodeposited lithium quickly penetrates into the LLZO solid electrolyte, forming lithium dendrites that eventually short-circuit the battery.

[0006] To reduce the formation of lithium dendrites, double-layer dense / porous LLZO structures and three-layer porous / dense / porous LLZO structures have been recently developed. These structures can mitigate the dynamic volume changes of the lithium anode and the formation of voids, both of which are considered to be the main causes of lithium dendrite formation. In addition, lithium can be stored in the pores of the LLZO scaffold during lithium deposition, thereby avoiding dynamic changes in the battery volume. In addition, the void formation during stripping can be mitigated by the larger surface area of ​​the LLZO / lithium interface in the scaffold compared to dense LLZO ceramics. It is also speculated that the dense layer can act as an additional protective layer, which reduces the risk of potential short circuits during battery charging.

[0007] "High-rate lithium cycling in a scalable trilayer Li-garnet-electrolyte architecture", GTHitz, DWMcOwen et al., Materials Today, Vol. 22 (2019), pp. 50-57 discloses a three-layer porous / dense / porous solid electrolyte comprising a doped LLZO ceramic lithium conductor. The SSE is obtained by tape-casting a dense layer from a first slurry and a porous layer from a second slurry and a third slurry, respectively, thereby obtaining three separate layers. The layers are then laminated to each other by hot pressing at 1.17 MPa and 60°C for 15 minutes, followed by degreasing and sintering.

[0008] US2022 / 0093904 discloses a method for producing a double-layer dense / porous SSE, which includes the steps of preparing a first slurry comprising LLZO and a binder in a solvent and a second slurry comprising LLZO, a binder and poly(methyl methacrylate) (PMMA) in a solvent. The first slurry is tape-cast to obtain a dense layer. The second slurry is then tape-cast on the dense layer to obtain a porous layer. Next, the entire green structure is immersed in lithium-saturated water to induce a phase inversion to remove the solvent. The resulting structure is then dried and sintered to remove PMMA and any binder. Alternatively, the two layers can be tape-cast separately, then laminated together, and then sintered.

[0009] US2022 / 0093910 discloses a multilayer solid electrolyte comprising two or more alternating dense-porous layers. Both layers may contain Li 0.35 La 0.55 TiO3. The density of the dense layer is at least 95% of the theoretical density of the dense layer. The porosity of the porous layer is between 20% and 80% by volume. Two or more alternating dense layers are formed as pellets using only powdered material.

[0010] CN104916869 discloses a multilayer solid electrolyte comprising alternating dense / porous layers and at least two layers that may contain LLZO. The density of the dense layer is at least 90% of the theoretical density of the dense layer. The porosity of the porous layer is at least 35%. The multilayer solid electrolyte is prepared by using solid particle forming technology, and other methods are mentioned, such as casting a mixture of powder in a solvent, then casting a mixture of powder and a pore-forming agent in a solvent, and then sintering.

[0011] One of the drawbacks of the aforementioned methods is that the resulting double-layer or triple-layer SSEs exhibit significant delamination between the layers. Another drawback is that the resulting SSEs are difficult to keep flat and crack-free, which is a prerequisite for obtaining large-area solid-state batteries (SSBs) for high-energy batteries. In addition, the resulting SSEs in some cases show depletion of metals in LLZO, such as alkali metals (such as lithium) or alkaline earth metals (such as magnesium). SUMMARY OF THE INVENTION

[0013] The present invention aims to overcome one or more of the above disadvantages. One object of the present invention is to provide a method for producing a multilayer solid electrolyte (SSE) with substantially reduced delamination of the layers, i.e. providing improved interfaces between the layers. Another object is to provide a method for producing a SSE with improved mechanical properties. Another object is to provide a method for producing a SSE as a membrane, in particular a free-standing membrane, i.e. a membrane that does not require a support or carrier.

[0014] The term "free-standing" is used interchangeably with "self-supporting" in this disclosure and is used to refer to a film that has sufficient mechanical strength to be handled (eg, cut or shaped or processed), i.e., does not require a carrier or support to place the film on to avoid damage to the film.

[0015] According to a first aspect of the present invention, a method for producing a multilayer solid state electrolyte (SSE) according to the accompanying claims is disclosed.

[0016] A "multilayer SSE" according to the present invention comprises any SSE having at least two layers, such as three, four, five or more layers. The multilayer solid electrolyte according to the present disclosure (i.e., obtained by the method of the present disclosure) comprises alternating dense layers and porous layers. For example, when the number of layers is 2, the multilayer SSE comprises a dense layer adjacent to a porous layer. For example, when the number of layers is 3, the multilayer SSE may comprise a sequence of a dense layer, a porous layer and a dense layer, or a sequence of a porous layer, a dense layer and a porous layer.

[0017] The method comprises the preparation of a first mixture and a second mixture. The first mixture is obtained by adding a first compound comprising one or more alkali metals and / or alkaline earth metals and a first binder to a first solvent. Advantageously, the first mixture is used to obtain (one or more) dense layers of a multilayer SSE.

[0018] The second mixture is obtained by adding a second compound comprising one or more alkali metals and / or alkaline earth metals, a second binder and a pore-forming compound in a second solvent. Advantageously, the second mixture is used to obtain (one or more) porous layers of a multilayer SSE. The pore-forming agent enables to provide a layer of the SSE with pores, i.e. a porous layer. Advantageously, the composition and amount of the pore-forming agent enables to control the degree of porosity and / or the pore size of the resulting layer.

[0019] Advantageously, the second mixture comprises between 20 and 90% by volume of pore-forming compound, preferably between 30 and 80% by volume, more preferably between 50 and 75% by volume.

[0020] The first compound and the second compound comprising one or more alkali metals and / or alkaline earth metals may be different or the same. Advantageously, the first compound and / or the second compound comprising one or more alkali metals and / or alkaline earth metals comprise an alkali metal. Advantageously, the alkali metal comprises one or more of lithium, sodium or potassium. Advantageously, when the first compound and / or the second compound comprising one or more alkali metals and / or alkaline earth metals comprise lithium, the lithium exists as a lithium garnet type structure. In other words, the first compound and / or the second compound comprising one or more alkali metals and / or alkaline earth metals advantageously comprise a lithium garnet type structure. Advantageously, the lithium garnet type structure is lithium lanthanum zirconium oxide (LLZO).

[0021] The first and second solvents may be identical to or different from each other. Advantageously, the first and / or second solvents comprise polar solvents. Non-limiting examples of suitable polar solvents include isopropanol, 1-propanol, 2-propanol, butanol, ethanol, methanol, acetone, dimethylbenzene, methyl ethyl ketone, toluene, 1,1,1-trichloroethane, hexyl chloride, cyclohexane and water.

[0022] Advantageously, the first and / or second mixture further comprises one or more of the carbonates, oxides, tungsten oxides or zirconium oxides of alkali metals and / or alkaline earth metals contained in the first and / or second mixture, respectively. Advantageously, when the first and / or second mixture comprises one or more such compounds, the mixture comprises between 0.1 wt % and 10 wt % of the compound, preferably between 0.5 wt % and 9 wt %, more preferably between 1 wt % and 8 wt %, for example between 2 wt % and 7 wt %, or between 2.5 wt % and 5 wt %, based on the total weight of the first and / or second mixture, respectively.

[0023] According to the method of the present invention, the first mixture is subjected to film casting, thereby obtaining a green layer. Then, the second mixture is subjected to film casting on the green layer (i.e., on the film-cast first mixture), thereby obtaining a green structure.

[0024] Film casting in the present disclosure includes casting methods known in the art, such as casting, screen printing and spray printing. Advantageously, the film casting of the first mixture and / or the second mixture includes casting of the first mixture and / or the second mixture, respectively. Advantageously, casting is performed by methods known in the art.

[0025] Optionally, and when the number of layers in the multilayer solid electrolyte to be obtained is 3 or more, the film casting of the first mixture and the second mixture is repeated until the number of layers is obtained. For example, when the number of layers is 4, it is advantageous to film cast the first mixture twice on the green structure, and then film cast the second mixture twice, thereby obtaining a four-layer (i.e., multi-layer) green structure.

[0026] The green structure, or when the number of layers is 3 or more, the green multilayer structure is degreased, thereby obtaining a degreased multilayer structure comprising alternating dense layers and porous layers. During the degreasing of the green (multilayer) structure, one or more of the first and second binders, the first and second solvents and the pore-forming compound are at least partially removed, and preferably all. Advantageously, at least 50%, preferably at least 75%, more preferably at least 80%, for example at least 85%, at least 90%, at least 95%, at least 98% or at least 99% of one or more of the first and second binders, the first and second solvents and the pore-forming compound are removed, and preferably all.

[0027] Advantageously, debinding is carried out or occurs in an atmosphere comprising at least 20% by volume of oxygen, for example in air or substantially pure oxygen (for example technical grade oxygen). Advantageously, debinding is carried out at a temperature between 250°C and 800°C, preferably between 400°C and 700°C.

[0028] The degreased multilayer structure comprising alternating dense and porous layers is sintered in the next step. A multilayer solid electrolyte comprising alternating dense and porous layers is obtained.

[0029] Advantageously, sintering is performed or occurs in an inert atmosphere, for example in an atmosphere comprising an inert gas such as argon, helium or nitrogen. Advantageously, sintering is performed in an atmosphere consisting essentially of argon. Advantageously, sintering is performed at a temperature between 500° C. and 2000° C., such as between 600° C. and 1500° C., preferably between 750° C. and 1400° C., more preferably between 900° C. and 1250° C.

[0030] According to a second aspect of the present invention, a multilayer solid electrolyte according to the appended claims is disclosed. Advantageously, the multilayer solid electrolyte is obtained by the method according to the first aspect of the present disclosure.

[0031] The multilayer solid electrolyte comprises at least two layers, namely a dense layer and a porous layer. The multilayer solid electrolyte may comprise three, four, five or more layers. As explained above, these layers are alternating dense layers and porous layers. Advantageously, when the number of layers is 3, the multilayer SSE has a dense-porous-dense configuration, i.e. a porous layer is provided between two dense layers.

[0032] The dense layer of the multilayer SSE or each dense layer when the number of dense layers in the multilayer SSE is two or more, comprises a first compound, wherein the first compound comprises one or more alkali metals and / or alkaline earth metals. This compound may be the same or different for each dense layer.

[0033] Advantageously, the dense layer(s) have a porosity of 40% or less, preferably 35% or less, such as 30% or less, as measured by X-ray computed tomography.

[0034] The porous layer of the multilayer SSE, or each porous layer when the number of porous layers in the multilayer SSE is two or more, comprises a second compound containing one or more alkali metals and / or alkaline earth metals. The compromise compound may be the same or may be different for each porous layer.

[0035] Advantageously, the porosity of the (one or more) porous layers is at least 40%, preferably at least 45%, for example at least 50%, at least 55%, more preferably at least 60%, for example at least 65%, at least 70%, at least 75% or at least 80%, as measured by X-ray computed tomography.

[0036] The first and second compounds comprising one or more alkali metals and / or alkaline earth metals are advantageously as described above. Advantageously, one or both of the first and second compounds comprising one or more alkali metals and / or alkaline earth metals comprise one or more of lithium, sodium or potassium. Advantageously, one or both of the first and second compounds comprising one or more alkali metals and / or alkaline earth metals comprise a lithium garnet type structure, preferably LLZO.

[0037] According to another aspect of the present invention, a solid state battery (SSB) according to the appended claims is disclosed. The SSB comprises a multi-layer SSE according to the present disclosure.

[0038] An advantage of the method according to the invention is that it is possible to control the stoichiometry of the alkali metal and / or alkaline earth metal in the resulting solid electrolyte, as well as the microstructure and thickness of the layers (particularly the porous layers) of the SSE. In other words, it is possible to control the degree and thickness of the porosity of both (one or more) dense layers and the porous layers. The method enables solid electrolytes with high ionic conductivity and high voltage stability to be obtained. They also enable the production of solid-state batteries with improved energy density.

[0039] In addition to high ionic conductivity and high voltage stability, an advantage of the solid electrolytes of the present invention is that they are non-toxic and non-flammable, thereby providing safe SSEs. They are further able to reduce, even inhibit or avoid the formation of dendrites of alkali metals or alkaline earth metals (especially lithium), thereby reducing the risk of short circuits in batteries containing such SSEs and thereby improving their safety.

[0040] Description of the drawings

[0041] Aspects of the present invention will now be described in more detail with reference to the drawings, wherein like reference numerals represent like features.

[0042] - Figure 1 The process steps of the method disclosed in the present invention are schematically disclosed;

[0043] - Figure 2 schematically shows a solid-state battery cell comprising a double-layer solid-state electrolyte according to the present invention;

[0044] - Figure 3 and Figure 4 SEM image showing a cross section of a double-layer dense-porous solid electrolyte obtained by a method known in the art;

[0045] - Figure 5 SEM image showing a cross section of a double-layer dense-porous solid electrolyte obtained by the method of the present disclosure;

[0046] - Figure 6 shows the variation of the porosity of a double-layer solid electrolyte with its thickness, as measured by means of X-ray computed tomography;

[0047] - Fig. 7A -C shows SEM images of cross sections of porous layers obtained from mixtures containing different amounts of Li2CO3;

[0048] - Figure 8 Critical current density measurements showing symmetric cells;

[0049] - Fig. 9 shows the voltage distribution of the solid-state battery cell according to the present invention;

[0050] - Fig.10 shows the cycling stability measurements of solid-state batteries according to the present invention as a function of cycle number;

[0051] - Fig.11 Critical current density measurements of Li / LLZO / Li symmetric cells are shown, comparing cells heat treated at different temperatures. DETAILED DESCRIPTION OF THE INVENTION

[0053] Figure 1 The process steps of the method according to the invention for producing a multilayer solid-state electrolyte (SSE) are schematically represented.

[0054] The method comprises a step 10 of preparing a first mixture. To this end, at least a first compound comprising one or more alkali metals and / or alkaline earth metals and a first binder are added to a first solvent. Advantageously, the first mixture is substantially free of any compound capable of forming pores. Advantageously, the first mixture can be used or is suitable for obtaining (one or more) dense layers of a multilayer solid electrolyte.

[0055] Advantageously, the first mixture is obtained by mixing the components, for example by ball milling or other techniques known in the art.

[0056] The first mixture may be in the form of a slurry, a suspension (ie, a suspending solution), a solution, or a dispersion.

[0057] Advantageously, the first compound comprising one or more alkali metals and / or alkaline earth metals conducts ions of the alkali metals and / or alkaline earth metals, respectively.

[0058] Advantageously, when the first compound comprising one or more alkali metals and / or alkaline earth metals comprises an alkali metal, the alkali metal comprises one or more of lithium, sodium or potassium. Advantageously, when the first compound comprising one or more alkali metals and / or alkaline earth metals comprises an alkaline earth metal, the alkaline earth metal advantageously comprises one or more of magnesium or calcium.

[0059] Advantageously, when the first compound comprises an alkali metal containing lithium (i.e., the first compound comprises lithium), at least a portion of the lithium is present as a lithium garnet-type structure. A specific example of a lithium garnet-type structure is LLZO. Optionally, LLZO may be doped, as is known in the art. The dopant may comprise one or more of aluminum (Al), gallium (Ga), neodymium (Nd), strontium (Sr), chromium (Cr), titanium (Ti), tantalum (Ta), tellurium (Te), niobium (Nb), or tungsten (W). Advantageously, when LLZO is doped, it may be according to formula (I):

[0060] Li 7-x α x La 3-y β y Zr 2-z δ z O 12 (I)

[0061] in

[0062] α is Al or Ga, β is Nd or Sr, δ is Cr, Ti, Ta, Nb, W or Te,

[0063] x is between 0 and 6, y is between 0 and 2, z is between 0 and 1, and

[0064] At least one of x, y, and z is different from zero.

[0065] Alternatively or additionally, at least a portion of the lithium may be present as Li 1.3 Al 0.3 Ti 1.7 (PO4)3 (lithium aluminum titanium phosphate, abbreviated as LATP, CAS No. 120479-61-0) exists.

[0066] Advantageously, when the first compound comprises an alkali metal containing sodium (i.e. the first compound comprises sodium), at least a portion of the sodium is present as sodium-β-alumina. Sodium-β-alumina is known as a general term used to describe compositions ranging from Na2O·11Al2O3 (β-alumina) to Na2O·5Al2O3 (β"-alumina).

[0067] Advantageously, when the first compound comprises an alkali metal containing sodium (i.e., the first compound comprises sodium), at least a portion of the sodium is present as potassium-β-alumina. Potassium-β-alumina is a general term used to describe compositions known as K-β-Al2O3 and K-β"-Al2O3.

[0068] Non-limiting examples of binders include polyvinyl alcohol, polyvinyl butyral, polyvinyl chloride, polyurethane, cellulose acetate-butyrate, polyacrylates, polytetrafluoroethylene, polypropylene carbonates, vinylchloride acetate, methyl cellulose, and ethyl cellulose.

[0069] The method further comprises a step 11 of preparing a second mixture. To this end, at least a second compound comprising one or more alkali metals and / or alkaline earth metals, a second binder and a pore-forming compound are added to a second solvent. Advantageously, the second mixture can be used or is suitable for obtaining (one or more) porous layers of a multilayer solid electrolyte.

[0070] The second mixture can be obtained by the same means as the first mixture, for example by means of ball milling.

[0071] The second mixture may have the same or a different form than the first mixture and is advantageously in the form of a slurry, a suspension (ie a suspended solution), a solution or a dispersion.

[0072] The second compound comprising one or more alkali metals and / or alkaline earth metals is advantageously as described above with respect to the first compound comprising one or more alkali metals and / or alkaline earth metals. The first and second compounds comprising one or more alkali metals and / or alkaline earth metals may be identical, for example they may both comprise LLZO or consist essentially of LLZO, or may be different. In particular, the first and second compounds each comprise LLZO or consist essentially of LLZO, which may be doped as described above.

[0073] The second adhesive may be according to the first adhesive as described above.The first and second adhesives may be the same or different.

[0074] The pore former makes it possible to provide a layer of SSE with pores, i.e. a porous layer. Advantageously, the composition and amount of the pore former makes it possible to control the degree of porosity and / or the pore size of the resulting layer. Advantageously, the second mixture comprises between 20% and 90% by volume of pore-forming compound, preferably between 30% and 80% by volume, more preferably between 50% and 75% by volume.

[0075] Non-limiting examples of pore-forming compounds include synthetic organic materials such as polymethyl methacrylate (PMMA), polyvinyl chloride, polystyrene, polyethylene oxide, polyvinyl butyral, PMMA-polyethylene glycol. Advantageously, such synthetic organic materials are in the shape of beads or spheres. Other non-limiting examples of pore-forming compounds include phenolic resins, polymer gels, cellulose acetate, natural organic substances such as sucrose, dextrin, starch, water, and emulsion oil. A preferred example of a pore-forming compound is PMMA.

[0076] Optionally, the first and / or second mixture may contain one or more additional compounds, such as, but not limited to, plasticizers, dispersants (ie, surfactants).

[0077] Non-limiting examples of plasticizers include (poly)propylene glycol, (poly)ethylene glycol, butyl benzyl phthalate, butyl stearate, mixtures of phthalates, polypropylene carbonates, tricresyl phosphate, and triethylene glycol.

[0078] Non-limiting examples of dispersants include polyvinyl butyral, phosphate esters, ethoxylates, aliphatic hydrocarbons, linoleic acid, polyisobutylene, polyethylene glycol, sodium sulfosuccinate, and 2-amino-2-methyl-1-propanol.

[0079] Optionally and advantageously, the first and / or second mixture may further comprise one or more of the carbonates, oxides, tungsten oxides or zirconium oxides of the alkali metals and / or alkaline earth metals contained in the first and / or second mixtures, respectively. A specific example of such a compound is an alkali metal carbonate or an alkaline earth metal carbonate. For example, when the mixture comprises lithium, the mixture may comprise lithium carbonate (Li2CO3).

[0080] Advantageously, when the first and / or second mixture contains one or more such compounds, the compound is present in an amount between 0.1 wt.-% and 10 wt.-%, preferably between 0.5 wt.-% and 9 wt.-%, more preferably between 1 wt.-% and 8 wt.-%, for example between 2 wt.-% and 7 wt.-%, such as between 2.5 wt.-% and 5 wt.-%, based on the total weight of the first and / or second mixture, respectively.

[0081] The inventors have surprisingly found that by adding such a compound to one or both of the first and second mixtures, the compound acts as an additional source of alkali metal or alkaline earth metal. In particular, the addition of such a compound enables a solid electrolyte having a favorable microstructure to be obtained.

[0082] In particular when the alkali metal in the first and second mixtures comprises LLZO, the inventors have surprisingly found that the addition of such a compound, in particular Li2CO3, makes it possible to obtain, after sintering, a multilayer SSE of LLZO containing substantially no impurities in its cubic phase.

[0083] It is believed that lithium loss occurs during the debinding and sintering process, resulting in a portion of the LLZO being converted to La2Zr2O7 (LZO). Therefore, the addition of such a compound as described above in the first and / or second mixture enables the LZO formed during the debinding process to be converted back to LLZO during the sintering process. The addition of such a compound further provides a cubic phase LLZO present in the SSE after sintering, which is the preferred phase of LLZO for use as an electrolyte, as opposed to tetragonal phase LLZO, which has a higher ionic conductivity as a cubic phase. It is known that cubic phase LLZO generally has a relative low conductivity of about 10 -4 S / cm to 10 -3 S / cm at room temperature, while tetragonal LLZO typically has an ionic conductivity of about 10 -6 S / cm ionic conductivity at room temperature.

[0084] The first mixture and the second mixture are film cast in a sequential manner, i.e. by means of sequential film casting. Sequential film casting comprises film casting the first mixture 12 and then film casting the second mixture 13 on the film cast first mixture. After sequential film casting, a (multi-layer) green structure is obtained.

[0085] Advantageously, the first tape-casting step, i.e. the first (if the desired number of layers is greater than two) tape-casting step of the first mixture, is carried out on a substrate or support material. The substrate may be any substrate known in the art of film casting, including but not limited to glass, a substrate consisting of polytetrafluoroethylene (PTFE) or comprising a PTFE layer on the surface, or an alumina substrate. Advantageously, the substrate is inert to the first mixture, i.e. the first mixture does not bind or react with the substrate.

[0086] Advantageously, film casting comprises or consists essentially of tape casting. Advantageously, at least one and preferably all of the film casting steps (one or more) of the first mixture and the film casting steps (one or more) of the second mixture are performed by means of tape casting. Advantageously, tape casting is performed using methods and devices known in the art. Advantageously, the first tape casting step, i.e. the (first if the desired number of layers is greater than two) tape casting step of the first mixture, is performed on a substrate or carrier material as described above.

[0087] When the required or desired number of layers (i.e., the preset number of layers) is greater than two, the steps of tape casting the first mixture 12 and tape casting the second mixture 13 are advantageously repeated until the preset number of layers is reached. For example, when the number of layers is 3, the second step of tape casting the first mixture 12 is advantageously performed after tape casting 13 of the second mixture.

[0088] Optionally and when the (multi-layer) green structure film is cast onto a substrate, the (multi-layer) green structure is dried 17 before removing 14 the (optionally dried) (multi-layer) green structure from the substrate. The drying may be performed at atmospheric pressure or under reduced pressure. The drying may be performed at a temperature between room temperature and 100° C. The drying may be performed in an open space. After drying, the first solvent and the first portion of the second solvent may be removed.

[0089] When film casting of the first mixture is performed on a substrate, the green body is removed 14 from the substrate after film casting 12, 13 or after optional drying 17 of the (multilayer) green body. The green body may be removed from the substrate by methods known in the art, for example by peeling.

[0090] The (multi-layer) green body is then debound 15 or calcined. Debinding 15 or calcining removes at least partially one or more organic components contained in the green body. In particular, during debinding 15, the first and second solvents, the first and second binders and the pore-forming compound are at least partially, preferably completely, removed. After removal of the pore-forming compound, pores are obtained in the layers of the green body achieved by film casting the second mixture. Thus, debinding 15 of the green body results in a debound multi-layer structure comprising alternating dense layers and porous layers.

[0091] Advantageously, after at least partial removal of the first and second binders and the first and second solvents during the debinding process, better accessibility to the first and second compounds comprising one or more alkali metals and / or alkaline earth metals is obtained in the multilayer solid electrolyte, thereby improving the functionality of the multilayer SSE.

[0092] Advantageously, the debinding is carried out or occurs in an atmosphere comprising at least 20% by volume of oxygen, for example in air or substantially pure oxygen (for example technical grade oxygen).

[0093] Advantageously, debinding is performed at a temperature between 250°C and 800°C, preferably between 300°C and 750°C, more preferably between 400°C and 700°C, for example between 500°C and 650°C, such as about 600°C.

[0094] Advantageously, degreasing does not comprise or is substantially not comprised of a phase inversion process. In fact, the presence of the pore-forming compound in the green body structure enables the formation of pores in the porous layer, thereby eliminating the need for phase inversion. It is generally known that phase inversion is a technique for inducing pores in a compound by removing a solvent without removing any binder material. For example, phase inversion may comprise exposing the green body to a non-solvent for a binder material, which results in demixing of the binder material and its solidification (and therefore not removing it), as well as the removal of the solvent, thereby inducing pores in the solidified structure.

[0095] Advantageously, the presence of the pore-forming compound in the green structure enables a one-step debinding (i.e. a debinding requiring a single processing step) and in particular comprises at least partial removal of the first and second solvents, (and) the first and second binders (and) the pore-forming compound in one step, thereby making the process less complex and shorter.

[0096] Next, the degreased multilayer structure is subjected to sintering 16. During sintering 16, the multilayer structure is densified by fusion of particles, and a multilayer solid electrolyte comprising alternating dense layers and porous layers is obtained. Sintering can be performed by placing the degreased multilayer structure on a support and placing the support in a sintering device or sintering environment. The support is made of or realized by a material or combination of materials that can withstand the temperature at which sintering is performed. For example, the support may contain or consist essentially of a carbonaceous component.

[0097] Advantageously, sintering is performed or occurs in an inert atmosphere, for example in an atmosphere comprising or consisting essentially of an inert gas such as argon, helium or nitrogen or a combination of any two or more thereof. Advantageously, sintering is performed in an atmosphere consisting essentially of argon.

[0098] “Essentially consisting of an inert gas” means that according to the present invention the atmosphere comprises at least 98% by volume of an inert gas, preferably at least 99% by volume, more preferably at least 99.5% by volume.

[0099] Advantageously, sintering is performed at a temperature between 500°C and 2000°C, such as between 600°C and 1500°C, preferably between 750°C and 1400°C, more preferably between 900°C and 1250°C, such as about 1100°C.

[0100] Advantageously, the multilayer SSE after sintering has sufficient mechanical strength, which enables the multilayer SSE to be handled without placing it on a carrier or support to avoid damage (such as cracks). The handling of the multilayer SSE includes transporting the multilayer SSE, storing the multilayer SSE and manufacturing solid-state battery cells containing the multilayer SSE. Advantageously, the multilayer solid electrolyte has the shape of a membrane, in particular a so-called self-supporting or free-standing membrane, i.e. having sufficient mechanical strength so that no carrier or support is needed to place the membrane on it to avoid damage to the membrane.

[0101] Optionally, the multilayer SSE can be further exposed 18 to an atmosphere comprising at least 20% by volume oxygen, such as air or substantially pure oxygen (e.g., technical grade oxygen). Similar to debinding 15, this further step 18 enables the removal of any carbonaceous residues present in and / or on the multilayer SSE. For example, during sintering on a carbonaceous support, some residues from the support may be deposited on or in the SSE.

[0102] Advantageously, the exposure step 18 is performed at a temperature between 250°C and 800°C, preferably between 300°C and 750°C, more preferably between 400°C and 700°C, for example between 450°C and 650°C, such as about 600°C.

[0103] Advantageously, the exposure step 18 is carried out for a period of between 1 minute and 1 hour, for example between 2 minutes and 45 minutes, preferably between 5 minutes and 30 minutes, more preferably between 10 minutes and 20 minutes, for example about 15 minutes.

[0104] Optionally, the multilayer SSE may be subjected to an annealing 19 after sintering 16 or after the optional exposure 18 (if performed). The annealing step 19 is particularly advantageously performed when the multilayer SSE is subjected to the exposure step 18, since carbonates and / or hydroxides of alkali metals or alkaline earth metals present in the multilayer SSE may be formed during the exposure step 18. Annealing enables any carbonaceous residues, hydroxides and carbonates present in and / or on the multilayer SSE to be removed. In other words, the annealing 19 may be considered as a cleaning step of the SSE, for example before its assembly into a battery cell.

[0105] Advantageously, annealing 19 is performed in an inert atmosphere, for example in an atmosphere comprising or consisting essentially of an inert gas such as argon, helium or nitrogen or a combination of any two or more thereof. Advantageously, sintering is performed in an atmosphere consisting essentially of argon.

[0106] Advantageously, annealing 19 is performed at a temperature between 400°C and 1250°C, preferably between 500°C and 1200°C, more preferably between 600°C and 1100°C, for example between 700°C and 1000°C, or between 800°C and 900°C, for example about 900°C.

[0107] Advantageously, the annealing step 19 is performed for a period of between 1 minute and 1 hour, for example between 2 minutes and 45 minutes, preferably between 5 minutes and 30 minutes, more preferably between 10 minutes and 20 minutes, for example about 15 minutes.

[0108] The annealing is carried out in particular shortly before the multilayer solid electrolyte is brought into contact with the cathode and / or anode, in particular the anode. By doing so, a low interface resistance between the multilayer SSE and the electrode can be achieved. A low interface resistance enables a battery cell to be obtained which exhibits or has a lower polarization, which is advantageous for the lifetime of the battery cell.

[0109] The inventors have noticed that by using the method of the present invention, the resulting dense layer(s) are substantially free of pinholes. "Substantially free of pinholes" means that the surface area of ​​the dense layer contains less than 1% of pinholes. In other words, when the surface area is 100 μm 2 When the 2 Made up of pinholes.

[0110] Advantageously, the thickness of the dense layer(s) in the multilayer SSE of the present disclosure is at least 5 μm, such as at least 8 μm, such as about 10 μm, wherein the thickness of the dense layer is calculated from a SEM image of a cross section of the multilayer SSE. Advantageously, the thickness of the dense layer is at most 17 μm, preferably at most 15 μm. When the multilayer SSE comprises two or more dense layers, they may have the same or different thicknesses.

[0111] It is known that the dense layer in multilayer SSEs prevents dendrites formed in the electrolyte during use of the battery cell from reaching the cathode, thereby improving the lifetime of the battery comprising the multilayer solid electrolyte of the present invention. Contrary to what is generally believed, the inventors have surprisingly found that dendrites are prevented from reaching the cathode even for a dense layer having a thickness equal to or below 17 μm, such as 10 μm to 15 μm. This allows the thickness of the dense layer to be reduced to values ​​lower than those of the dense layers in the multilayer solid electrolytes of the prior art.

[0112] Advantageously, the dense layer(s) have as low a thickness as possible while being substantially pinhole-free. The inventors further found that a minimum thickness of 5 μm is required to prevent dendrites from reaching the cathode. In addition, it was found that a minimum thickness of the dense layer of 5 μm provides sufficient mechanical stability for the multilayer SSE. It will be understood in the art that a lower thickness means that a smaller amount of the first mixture can be used, resulting in reduced costs. In addition, a lower thickness enables battery cells comprising the SSEs of the present disclosure to be obtained, wherein the battery cells have a lower thickness and a lower weight.

[0113] Advantageously, the porosity of the dense layer(s) in the multilayer solid electrolyte of the present disclosure is 40% or less, preferably 35% or less, such as 30% or less, more preferably 25% or less, most preferably 20% or less, such as 15%, 10% or 5%, as measured by X-ray computed tomography. When the multilayer SSE comprises two or more dense layers, they may have the same or different (degrees of) porosity.

[0114] Advantageously, the thickness of the (one or more) porous layers in the multilayer solid electrolyte of the present disclosure is between 30 μm and 75 μm, for example between 32 μm and 70 μm, preferably between 35 μm and 65 μm, more preferably between 35 μm and 50 μm, most preferably between 35 μm and 45 μm, for example between 37 μm and 38 μm, wherein the thickness of the dense layer is calculated from the SEM image of the cross section of the multilayer SSE. When the multilayer SSE comprises two or more porous layers, they may have the same or different thicknesses. As is known in the art, the thickness of the porous layer depends inter alia on the degree of porosity and the capacity per cathode surface area of ​​the cathode.

[0115] The inventors have surprisingly found that even for porous layer thicknesses equal to or below 75 μm, and in particular for thicknesses between 35 μm and 40 μm, the ionic conductivity provided by the porous layer is very good when compared to prior art multilayer SSEs which typically have thicker porous layers. Without wishing to be bound by any theory, the inventors believe that the high ionic conductivity is obtained because the porous layer of the present invention is less prone to dendrite formation.

[0116] It will be understood in the art that lower thickness means that a smaller amount of the second mixture can be used, resulting in reduced cost. In addition, lower thickness enables battery cells containing the SSEs of the present disclosure to be obtained, wherein the battery cells have lower thickness and lower weight.

[0117] The inventors have further found that at porous layer thicknesses below 30 μm, the ionic conductivity becomes too low for the multilayer SSE to be useful in battery cells. A minimum thickness of 30 μm also provides sufficient mechanical stability of the porous layer.

[0118] Advantageously, the total thickness of the multilayer solid electrolyte is at most 80 μm, preferably at most 75 μm, more preferably at most 60 μm, for example between 45 μm and 55 μm, or 50 μm. Advantageously, the total thickness of the multilayer SSE is at least 40 μm, for example at least 42 μm. The term "total thickness of the multilayer solid electrolyte" is used in the present disclosure for the sum of the thicknesses of the layers (porous layer and dense layer) of the multilayer solid electrolyte. It will be understood that the thickness of each layer is selected to correspond to the total thickness of the multilayer solid electrolyte. The inventors have found that for multilayer SSEs with a total thickness exceeding 80 μm, the ionic conductivity decreases significantly, making the multilayer SSE unsuitable for use in battery cells. The inventors further noted that when the total thickness is greater than 80 μm, the multilayer SSE becomes very susceptible to dendrite formation.

[0119] Advantageously, the thickness of the dense layer is at most 75%, preferably at most 70%, such as at most 65%, more preferably at most 60%, at most 55%, most preferably at most 50% of the thickness of the porous layer. When the multilayer solid electrolyte comprises two or more dense layers and / or two or more porous layers, the thickness of each dense layer is advantageously at most 75%, such as at most 65%, more preferably at most 60%, at most 55%, most preferably at most 50% of the minimum thickness of the porous layer. The inventors have found that for dense layers having a thickness greater than 75% of the thickness of the porous layer (or the minimum thickness of the porous layer), the weight and volume energy densities increase to values ​​that make the multilayer SSE unsuitable for use in battery cells.

[0120] Advantageously, the porosity of the (one or more) porous layers in the multilayer solid electrolyte of the present disclosure is at least 40%, preferably at least 45%, for example at least 50%, at least 55%, more preferably at least 60%, for example at least 65%, at least 70%, at least 75% or at least 80%, as measured by X-ray computed tomography.

[0121] Advantageously, the porosity of the porous layer(s) is substantially constant throughout the thickness of the porous layer(s). The term "substantially constant" is used in the present disclosure to mean that the porosity is contained within ±5% of the average porosity throughout the thickness of the layer. For example, when the average porosity is 80%, a substantially constant porosity means that at every point in the layer the porosity is contained between 75% and 85%.

[0122] It is known that multi-layer SSEs having substantially constant porosity of the porous layer(s) are less likely to form dendrites, thereby improving resistance to high current densities of a battery cell and the life of the battery cell when compared to other SSEs.

[0123] Advantageously, the pores contained in the (one or more) porous layers have a pore size between 1 μm and 20 μm, for example between 2 μm and 10 μm, as measured by X-ray computed tomography. Advantageously, at least a portion of the pores contained in the (one or more) porous layers, and preferably at least 50%, for example at least 75%, more preferably at least 90% or at least 95%, are open-cell pores. Open-cell pores are pores that are open to at least one surface of the layer and are therefore accessible from outside the layer.

[0124] Figure 2 A solid-state battery cell 20 according to the invention is schematically shown. The solid-state battery cell 20 comprises a cathode 21, an anode 24 and a double-layer solid electrolyte 27. Advantageously, the battery cell is a secondary battery cell.

[0125] According to the present invention, the terms "battery cell" and "battery" are used interchangeably and thus have the same meaning.

[0126] Advantageously, cathode 21 comprises cathode current collector 22 (e.g., aluminum foil) and cathode layer 23. Cathode layer 23 advantageously comprises cathode active material, electronically conductive compound (e.g., carbon black) and binder. The cathode active material is advantageously ionically conductive. The cathode active material may be any cathode active material known in the art. Examples of cathode active materials for lithium-based battery cells include, but are not limited to, lithium nickel cobalt manganese oxide (LiNiCoMnO2, abbreviated as NMC), lithium iron phosphate (LiFePO4, abbreviated as LFP), lithium nickel manganese spinel (LiNi 0.5 Mn 1.5 O4, abbreviated as LNMO), lithium nickel cobalt aluminum oxide (LiNiCoAlO2, abbreviated as NCA), lithium manganese oxide (LiMn2O4, abbreviated as LMO) and lithium cobalt oxide (LiCoO2, abbreviated as LCO). The binder can be any binder known in the art, such as polyvinylidene fluoride (PVDF).

[0127] Advantageously, anode 24 comprises an anode current collector 25 (e.g., copper foil) and a metal layer 26 comprising an alkali metal and / or an alkaline earth metal. Examples of alkali metals include lithium, sodium, and potassium. Examples of alkaline earth metals include magnesium, beryllium, and calcium. A preferred example of metal layer 26 comprises lithium or consists essentially of lithium.

[0128] The double-layer solid electrolyte 27 comprises one dense layer 28 and one porous layer 29. It will be understood that the double-layer SSE 27 may also be any multi-layer SSE according to the present invention, comprising three or more layers.

[0129] Alternatively, the solid-state battery may include a stack of multiple anodes, cathodes, and multilayer solid electrolytes. For example, the SSB may include an electronic insulating layer, which may be any electronic insulating layer known in the art, such as a polyethylene film or a polypropylene foil. Applied to the electronic insulating layer is an anode 24, which includes a first anode current collector 25 and a first lithium metal layer 26, a first multilayer SSE 27, a first cathode layer 23, a first cathode current collector 22, a second cathode layer 23, a second multilayer SSE 27, a second lithium metal layer 26, a second anode current collector 25, a third lithium metal layer 26, a third multilayer SSE 27, a third cathode layer 23, a second cathode current collector 22, a fourth cathode layer 23, a third multilayer SSE 27, a fourth lithium metal layer 26, and a third anode current collector 25. The solid-state battery may also include two or more of the aforementioned stacks, wherein the stacks are electronically insulated from each other by an insulating layer. Example

[0130] Example 1

[0131] A reference double-layer solid electrolyte comprising one dense layer and one porous layer was produced by tape-casting the dense layer and the porous layer separately. The mixture for the dense layer comprises LLZO. The mixture for the porous layer also comprises LLZO and further comprises PMMA as a pore former. Both layers were removed from their respective supports and degreased separately at 650°C for 2 hours and then sintered together by contacting them (so-called co-sintering). Sintering was performed at a temperature between 1120°C and 1150°C for 5 hours.

[0132] Figure 3 and 4 SEM images of the cross section of the obtained reference double-layer solid electrolyte at different magnifications are shown (SEM equipment: Hitachi TM3030 Plus Tabletop microscope, with an accelerating voltage of 10 kV). Obvious delamination in the form of gaps 3 can be seen between the dense layer 1 and the porous layer 2.

[0133] A double-layer solid electrolyte comprising a dense layer and a porous layer is also manufactured according to the method of the present invention. A first mixture is prepared by mixing 3 grams of aluminum-doped LLZO, 0.15 grams of Li2CO3, 0.43 grams of a surfactant, 0.408 milliliters of a plasticizer, and 4.3 milliliters of a solvent comprising 5 volume % of isopropanol, 87 volume % of ethanol, and 8 volume % of 1-propanol with a spatula, and then ball milling for 18 hours at 165 rpm. A binder solution is prepared by adding 3 grams of polyvinyl butyral to 8.89 milliliters of isopropanol. 1.83 grams of the binder solution is added to the mixture (suspension), and then further ball milled for 2 hours at 200 rpm.

[0134] A second mixture was prepared by mixing 3 g of aluminum-doped LLZO, 2.07 g of PMMA as a pore-forming compound, 0.15 g of Li2CO3, 0.59 g of a surfactant, 0.56 ml of a plasticizer, and 5.9 ml of a solvent comprising 5 vol% isopropanol, 87 vol% ethanol, and 8 vol% 1-propanol with a spatula, followed by ball milling at 165 rpm for 18 hours. A binder solution was prepared by adding 3 g of polyvinyl butyral to 8.89 ml of isopropanol. 2.51 g of the binder solution was added to the mixture (suspension), followed by further ball milling at 200 rpm for 2 hours.

[0135] The first mixture was film cast by tape casting on a glass substrate. After 60 seconds, the second mixture was tape cast onto the first mixture (i.e., sequential tape casting). The resulting green structure was kept at ambient conditions for 1 hour to allow the solvent to evaporate and then removed from the glass substrate.

[0136] The green structure was then placed between two alumina plates. Debinding of the green structure was performed at 600°C in air to completely remove the solvent (evaporation temperature up to 150°C), PMMA (at about 350°C) and residual organic compounds, such as binders and plasticizers (at about 600°C). The debinded structure was then placed between two graphite foils, sandwiched between two carbon plates and sintered at 1100°C for 15 minutes in a nitrogen atmosphere.

[0137] Thereafter, the sintered multilayer LLZO SSE was heated at 600 °C in air to remove any carbon residues from the LLZO surface and then annealed in an argon-filled glove box for 1 h to remove any contamination on the LLZO surface originating from the presence of Li2CO3 or LiOH. The annealing was performed at three different temperatures varying between 600 °C and 900 °C.

[0138] Figure 5 A SEM image of a cross section of a double layer SSE obtained after annealing is shown. The boundary between the dense layer 30 and the porous layer 31 is sharp and there is no visible delamination of the layers 30, 31. The total thickness of the double layer SSE is about 70 μm.

[0139] The extent of porosity was also measured on the entire double-layer solid electrolyte sintered and annealed by means of X-ray computed tomography. An EasyTom XL Ultra 230-160 micro / nanoCT scanner was used, operating at 90 kV and 160 μA. The sample was scanned at full 360° with a rotation step of 0.2° and a frame average of 10. The nominal resolution was set to 850 nm voxel size. Figure 6The porosity is shown as a function of its thickness, starting with a dense layer of thickness 0. A clear difference in porosity between the dense and porous layers is visible.

[0140] Example 2

[0141] In order to study the effect of adding an alkali or alkaline earth metal carbonate to the first and / or second mixture, three mixtures for the porous layer were prepared with different amounts of Li2CO3. Mixture 1 contained no Li2CO3, mixture 2 contained 5 wt% Li2CO3, and mixture 3 contained 10 wt% Li2CO3, wherein the wt% is based on the total weight of the mixture.

[0142] Each mixture further comprises aluminum-doped LLZO, PMMA as a pore-forming compound, a surfactant, a plasticizer in the amounts described above. The composition and amount of the solvent and binder solution are also described above. The mixtures are prepared as described above. Each mixture is then tape-casted on a glass support, dried, degreased, and sintered as described in Example 1.

[0143] Fig. 7A , 7B 7C and 7C show SEM images of cross sections of three porous layers obtained from mixtures without any Li2CO3, with 5 wt% Li2CO3, and with 10 wt% Li2CO3, respectively. As determined by X-ray diffraction (XRD) measurements, the porous layer without any Li2CO3 ( Fig. 7A ) shows the presence of LZO instead of LLZO. This indicates a significant loss of lithium during the degreasing process. As determined by XRD, Figure 7B The porous layer showed good porosity and the presence of cubic LLZO, while as determined by XRD, Figure 7C The layers show low porosity and the presence of tetragonal LLZO. It is speculated that the decrease in porosity with increasing amount of Li2CO3 is related to the melting of Li2CO3, which enables the sintering process to be initiated at a lower temperature, resulting in a denser layer. It is therefore clear that the addition of Li2CO3 to the reaction mixture for the porous layer can modify the degree of porosity of the resulting porous layer and the phase of LLZO.

[0144] Example 3

[0145] In order to evaluate the electrochemical performance of the porous layer in terms of lithium plating / stripping, a symmetric battery cell with a lithium metal anode and a lithium metal cathode and a single porous layer was prepared. The porous layer was manufactured according to Example 1 by tape casting the second mixture of Example 1 onto a glass support, followed by drying, stripping, degreasing, sintering and annealing. The symmetric battery cell was manufactured by thermally evaporating 200nm of metallic lithium (using a Covap thermal evaporator) and then cold isostatically pressing lithium foil at about 71MPa (using a PW 100EH cold isostatic press) onto the porous layer for 5 minutes on both sides of the porous LLZO layer. This enables lithium impregnation into the porous layer up to about 10μm. This corresponds to about 1mAh / cm 2 Surface capacity.

[0146] The critical current density (CCD) achievable by the symmetric battery cell obtained by annealing LLZO was determined by constant current cycling experiments at different current densities, i.e., the current density at which Li dendrites / filaments begin to grow. The current density is 0.1 mA / cm 2 The step size is from 0.1 to 1.5 mA / cm 2 , at 0.5mA / cm 2 The step size is from 1.5 to 3 mA / cm 2 And at 1mA / cm 2 The step size is from 3 to 10 mA / cm 2 Increase, for each half cycle the same amount of Li (0.1 mAh / cm 2 , i.e. the areal capacity limit imposed during the test). The test was conducted at 60°C without any stacking pressure applied. Figure 8 Results are presented, which show that a symmetric cell with a single porous layer exhibits high current density up to 6 mA / cm 2 high critical current density.

[0147] The impact of the annealing step was also tested by measuring the critical current density of symmetric battery cells containing LLZO SSEs annealed at different temperatures. Galvanostatic cycling experiments were performed at different current densities. 0.1 mAh / cm was applied for each half cycle. 2 The capacity limit is 0.04–0.4 mA / cm 2 The corresponding current density was varied within a range of . The tests were performed at room temperature without applying any stacking pressure.

[0148] Fig.11Results for symmetric battery cells containing unannealed LLZO (a) and LLZO annealed for 1 h at 600 °C (b), 750 °C (c), and 900 °C (d) are shown from top (a) to bottom (d). The critical current density increases from 0.04 mA / cm 2 (without annealing) increased to 0.40mA / cm 2 (for annealing at 900°C), it is clearly shown that the critical current density increases with increasing annealing temperature.

[0149] Example 4

[0150] A battery cell was prepared using a double-layer SSE according to Example 1 made according to the present invention. To apply an anode on the porous side of the SSE, 200 nm of metallic lithium was evaporated on the porous side of the SSE, and then the lithium foil was cold isostatically pressed at about 71 MPa for 5 minutes. 2 mg of LiFePO4 (LFP), 2 mg of carbon black and 0.3 M LiTFSI were mixed in a mortar and pestle in 119 μl PY 14 The active material loading in the obtained paste-type LFP cathode is about 3 mg / cm 2 A 20 μm thick cellulose separator was placed on the dense side of the double-layer SSE and then covered with a paste-type LFP cathode and aluminum foil as a cathode current collector, thereby obtaining a Li / double-layer LLZO SSE / LFP battery cell.

[0151] The battery cells were tested at room temperature and without applied external pressure. The applied current was normalized to the surface area of ​​the Li anode. + All electrochemical measurements were performed by galvanostatic cycling in the voltage range of 2.5V / Li. Fig. 9 Shown is the voltage profile of a battery cell measured at 0.1C rate as a function of the number of test cycles. Fig.10 Cycling stability measurements of the battery cell measured at 0.1C rate are shown as a function of the number of test cycles. The battery cell achieves an LFP capacity of approximately 100-150 mAh / g, which corresponds to approximately 0.3-0.45 mAh / cm 2 Surface capacity.

[0152] Reference numerals

[0153] 1. Dense layer

[0154] 2. Porous layer

[0155] 3. Clearance

[0156] 10. Preparation Step – First Mixture

[0157] 11. Preparation Step – Second Mixture

[0158] 12. Film casting of the first mixture

[0159] 13. Film casting of the second mixture

[0160] 14. Remove from the base

[0161] 15. Skim

[0162] 16. Sintering

[0163] 17. Drying

[0164] 18. Exposure to air

[0165] 19. Annealing

[0166] 20. Solid-state battery pack

[0167] 21. Cathode

[0168] 22. Cathode current collector

[0169] 23. Cathode layer

[0170] 24. Anode

[0171] 25. Anode current collector

[0172] 26.Metal layer

[0173] 27. Multilayer solid electrolyte

[0174] 28.Dense layer

[0175] 29.Porous layer

[0176] 30.Dense layer

[0177] 31.Porous layer

Claims

1. A method for producing a multilayer solid electrolyte (26) comprising alternating dense layers and porous layers, wherein the number of layers is at least two, the method comprising: - adding (10) a first compound comprising one or more alkali metals and / or alkaline earth metals and a first binder to a first solvent, thereby obtaining a first mixture, - adding (11) a second compound comprising one or more alkali metals and / or alkaline earth metals, a second binder and a pore-forming compound to a second solvent, thereby obtaining a second mixture, - film casting (12) said first mixture, thereby obtaining a green layer, - film casting (13) the second mixture on the green layer, thereby obtaining a green structure, - optionally, when the number of layers is three or more, repeating the step of film casting (12) the first mixture on the green structure and the step of film casting (13) the second mixture on the green structure until the number of layers is obtained, thereby obtaining a green multilayer structure, - debinding (15) the green structure or, when the number of layers is three or more, the green multilayer structure in an atmosphere comprising at least 20% by volume of oxygen at a temperature between 250° C. and 800° C., preferably between 400° C. and 700° C., thereby at least partially removing the first and second binders, the first and second solvents and the pore-forming compound, thereby obtaining a debinded multilayer structure comprising alternating dense and porous layers, - sintering (16) the degreased multilayer structure comprising alternating dense and porous layers, thereby obtaining the multilayer solid electrolyte (26) comprising alternating dense and porous layers.

2. The method of claim 1, wherein the first compound and / or the second compound comprising one or more alkali metals and / or alkaline earth metals comprises one or more of lithium, sodium or potassium.

3. The method of claim 2, wherein the first compound and / or the second compound comprising one or more alkali metals and / or alkaline earth metals comprises a lithium garnet-type structure.

4. The method of claim 3, wherein the lithium garnet-type structure is lithium lanthanum zirconium oxide (LLZO).

5. The method according to any one of the preceding claims, wherein the first solvent and / or the second solvent comprises one or more of isopropanol, ethanol or 1-propanol.

6. A method according to any preceding claim, wherein the second mixture comprises between 50% and 75% by volume of the pore forming compound.

7. The method according to any one of the preceding claims, wherein film casting of the first mixture and / or the second mixture comprises tape casting of the first mixture and / or the second mixture, respectively.

8. The method according to any one of the preceding claims, wherein the first and / or the second mixture further comprises one or more of alkali metal or alkaline earth metal carbonates, oxides, tungsten oxide or zirconium oxide between 0.1 wt % and 10 wt % based on the total weight of the first and / or the second mixture, respectively.

9. Method according to any of the preceding claims, wherein sintering (16) of the debinded multilayer structure comprising alternating dense and porous layers occurs in an inert atmosphere at a temperature between 750°C and 1400°C, preferably between 900°C and 1250°C.

10. A multilayer self-supporting solid electrolyte (26) obtained by a method according to any one of the preceding claims, comprising alternating dense layers and porous layers, wherein the number of layers is at least two, wherein each dense layer (27) comprises a first compound containing one or more alkali metals and / or alkaline earth metals and has a porosity of 40% or less, and wherein each porous layer (28) comprises a second compound containing one or more alkali metals and / or alkaline earth metals and has a porosity of at least 40%, wherein the porosity is measured by X-ray computed tomography.

11. The multilayer self-supporting solid electrolyte (26) according to claim 10, wherein the number of layers is three, and wherein the multilayer solid electrolyte (26) has a dense layer-porous layer-dense layer structure.

12. A multilayer, self-supporting solid electrolyte (26) according to any one of claims 10 to 11, wherein one or both of the first and second compounds comprising one or more alkali metals and / or alkaline earth metals comprises one or more of lithium, sodium or potassium.

13. The multilayer free-standing solid electrolyte (26) according to claim 12, wherein one or both of the first and second compounds comprising one or more alkali metals and / or alkaline earth metals comprises a lithium garnet type structure, preferably LLZO.

14. A multilayer free-standing solid electrolyte (26) according to any one of claims 10 to 13, wherein each porous layer (28) has a thickness between 30 μm and 75 μm, wherein the thickness of the porous layer (28) is calculated from a SEM image of a cross-section of the multilayer SSE.

15. A solid-state battery (20) comprising the multilayer solid-state electrolyte (26) according to any one of claims 10 to 14 or a multilayer solid-state electrolyte (26) obtained by the method according to any one of claims 1 to 9.

16. The solid-state battery (20) of claim 15, wherein the solid-state battery is a secondary battery.

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