Ultra-high speed, high temperature sintering method

Abstract

The present invention relates to a method for producing a sintered inorganic substrate, the method comprising: providing an inorganic substrate between first and second thermally conductive substrates comprising carbon; providing the first and second thermally conductive substrates and the inorganic substrate between third and fourth thermally conductive substrates; heating the third and / or fourth thermally conductive substrates to a temperature of 750°C to 1400°C at a heating rate of at least 50°C / s, thereby heating the first and / or second thermally conductive substrates, respectively; and sintering the inorganic substrates by heating the inorganic substrate with the heated first and / or second thermally conductive substrates at a temperature of 750°C to 1400°C, wherein the third and fourth thermally conductive substrates independently comprise one or more of a single crystal metal nitride and / or a single crystal metal oxide.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a sintered inorganic substrate, and further to an apparatus for sintering an inorganic substrate. [Background technology]

[0002] Inorganic materials, such as ceramics, are widely used in electronics, energy storage, and extreme environments due to their high thermal, mechanical, and chemical stability. Conventional synthesis of such inorganic materials, especially ceramics, often involves solid-state reactions to form inorganic components from precursors, and sintering the inorganic components to obtain solid (inorganic) components. Each step typically requires high temperatures and long processing times. This can lead to undesirable non-uniform grain growth, which can be an obstacle to high-throughput (mass production).

[0003] Long processing times are also a problem with conventional methods for the production of inorganic-based (e.g., ceramic-based) solid electrolytes (SSEs), which present, for example, safety issues and are promising alternatives to leaky Li-ion batteries.

[0004] Traditional sintering methods are typically carried out in so-called bulk furnaces, where the inorganic material composition to be sintered is heated to the required sintering temperature. Disadvantages of such bulk furnaces include long heating and cooling times (i.e., slow heating and cooling rates) and difficulty in controlling the temperature and heat distribution (i.e., uniform temperature).

[0005] More recently, novel sintering methods have been developed, including microwave-assisted sintering, spark plasma sintering, and flash sintering. However, microwave-assisted sintering primarily depends on the microwave absorption characteristics of the material to be sintered, which limits its applicability. Spark plasma sintering requires dies to compress the inorganic material during sintering, thus limiting the shape and scalability of the component to be sintered. Furthermore, the application of pressure makes it unsuitable for sintering complex three-dimensional structures. Flash sintering can achieve heating rates of up to 10,000 °C / min, but requires expensive platinum electrodes. Flash sintering is also difficult to apply to specimens with complex shapes, such as three-dimensional structures. In particular, the specific flash sintering conditions strongly depend on the electrical properties of the inorganic material, which limits the sintering of components with unknown compositions.

[0006] Another recently developed sintering method is ultra-rapid high temperature sintering.

[0007] “A general method to synthesize and sinter bulk ceramics in seconds”, C. Wang, W. Ping, et al., Science 2020, 368 (6490), pp. 521-526, discloses the sintering of ceramic materials between two Joule-heated carbon felts, where the heating is radiative heating and the sintering is carried out under an inert atmosphere.

[0008] WO2020 / 236767 discloses a high-temperature sintering system and method. The substrate to be sintered is placed between two conductive carbon elements, with a distance of 0-10 mm between each conductive carbon element and the substrate. The conductive carbon elements are heated to a temperature of 500-3000°C by an electric current, and the heated conductive carbon elements heat the substrate, resulting in sintering for 1 second to 1 hour.

[0009] "High-temperature ultrafast sintering: exploiting a new kinetic region to fabricate porous solid-state electrolyte scaffolds," R. Wang, Q. Dong, et al., Advances materials 2021, 33 (34), 2100726, discloses a method for sintering 3D porous scaffolds with a range of ceramic solid electrolytes on various substrates at high temperatures for a few seconds. For example, co-sintering with an alumina (Al2O3) substrate is performed. However, (co-)sintering on various substrates limits its applicability in all-solid-state battery cells.

[0010] "Ultrafast high-temperature sintering (UHS) of fine-grained α-Al2O3," M. Kermani, J. Dong, et al., Journal of the European Ceramic Society 2021, 41, 6626-6633, discloses an ultrafast high-temperature sintering method and apparatus for sintering α-Al2O3. The α-Al2O3 substrate is placed between carbon felt, which is placed between Al2O3 fiberboards, ensuring excellent thermal contact between the carbon felt and the α-Al2O3. An electric current is applied to the carbon felt to heat the α-Al2O3 to be sintered.

[0011] "Upscaling Ultrafast High-Temperature Sintering (UHS) to Consolidate Large-Sized and Complex-Shaped Ceramics," F. Zuo, Q. Wang, et al., Scripta Materialia 2022, 221, 114973, discloses an ultrafast, high-temperature sintering apparatus for complex-shaped Al2O3 samples. The Al2O3 samples are embedded in a graphite powder bed, which is heated at a heating rate of up to 770 °C / min. To reduce heat loss to the outside, the powder bed is placed inside an Al2O3 thermal insulator.

[0012] A disadvantage of the aforementioned methods and apparatus is that it is difficult to sinter free-standing substrates, i.e., substrates without the presence of a support or carrier. In particular, in the case of flat free-standing substrates, it is difficult for the aforementioned methods and apparatus to maintain the flatness of the substrate during sintering. In other words, the sintered substrate obtained by the aforementioned methods and / or apparatus, when sintered without the presence of a carrier or support, tends to bend, exhibit curves, and may even exhibit cracks or may begin to crack when attempting to flatten the sintered substrate after sintering. Summary of the Invention

[0013] The present invention aims to overcome one or more of the above-mentioned drawbacks. It is an object of the present invention to provide a method for sintering inorganic substrates, said method requiring a reduced sintering time. It is a further object to provide a sintering method that maintains the microstructure and / or stoichiometry of the inorganic substrate and / or that results in uniform sintering. It is a further object to provide a sintering method that allows for the sintering of thin, i.e., having a thickness of less than 100 μm, and / or flat, inorganic substrates without damaging or deforming the sintered inorganic substrate, thereby maintaining flatness. It is a further object of the present invention to provide an apparatus capable of sintering inorganic substrates in a uniform manner.

[0014] The term "substrate" as part of the sintering process, eg, "thermally conductive substrate," is used in this disclosure with respect to a component or layer, particularly a layer having a sheet-like shape.

[0015] According to a first aspect of the present disclosure, there is disclosed a method for producing a sintered inorganic substrate according to the accompanying claims.

[0016] Preferably, the sintering method of the present disclosure is an ultra-rapid, high-temperature sintering method. The term "ultra-rapid sintering" is used in the present disclosure to refer to a sintering method that includes heating at a heating rate of at least 50°C / s and, optionally, cooling at a cooling rate of at least 50°C / s. The term "high-temperature sintering" is used in the present disclosure to refer to a sintering method that includes heating to a temperature of at least 750°C, preferably at least 900°C, for example, from 750°C to 1400°C, more preferably from 900°C to 1250°C.

[0017] The inorganic substrate, i.e., the article or object, may comprise or consist essentially of any known inorganic material. Preferably, the inorganic substrate comprises or consists essentially of a ceramic material. Preferably, the ceramic material may comprise one or more alkali metals and / or alkaline earth metals. In other words, the inorganic substrate preferably comprises or consists of a ceramic material containing one or more alkali metals or alkaline earth metals.

[0018] Preferably, the alkali metal includes one or more of lithium, sodium, or potassium. Preferably, the alkaline earth metal includes one or more of magnesium or calcium. Preferably, when the ceramic material including one or more alkali metals and / or alkaline earth metals includes lithium, the lithium exists as a lithium garnet structure. In other words, the ceramic material including one or more alkali metals and / or alkaline earth metals preferably includes a lithium garnet structure. Preferably, the lithium garnet structure is lithium lanthanum zirconium oxide (LLZO).

[0019] According to the method of the present disclosure, an inorganic substrate to be sintered is provided. The inorganic substrate is provided between a first thermally conductive substrate and a second thermally conductive substrate. Suitably, at least one, and preferably both, of the first and second thermally conductive substrates comprises carbon.

[0020] Preferably, the distance between the inorganic substrate and the first and second thermally conductive substrates is 0 mm (i.e., in contact) to 20 mm, preferably 0 mm to 10 mm. Preferably, the first and / or second thermally conductive substrates are at least partially, for example, and preferably completely (i.e., over the entire surface area) in contact with the respective surfaces of the inorganic substrate.

[0021] The method further includes providing the first and second thermally conductive substrates and the inorganic substrate (to be sintered) between a third thermally conductive substrate and a fourth thermally conductive substrate.

[0022] The third thermally conductive substrate and the fourth thermally conductive substrate each comprise or consist essentially of one or more metal nitrides and / or metal oxides. Preferably, the third and fourth thermally conductive substrates each comprise, independently of one another, one or more single-crystal metal nitrides and / or single-crystal metal oxides. "Individually comprise or consist essentially of" in this disclosure means that the two substrates may be the same, i.e., have the same composition, or may be different, i.e., have different compositions.

[0023] Preferably, the metal nitride comprises boron nitride and / or aluminium nitride. Preferably, the metal oxide comprises alumina and / or sapphire.

[0024] The method further comprises heating the third and / or fourth thermally conductive substrate to a temperature of 500°C to 2000°C, for example 600°C to 1500°C, preferably 750°C to 1400°C, more preferably 900°C to 1250°C. Suitably, the third and fourth thermally conductive substrates may be heated independently of each other. Suitably, one or both of the third and fourth thermally conductive substrates may be heated. Suitably, when the third and fourth thermally conductive substrates are heated, they may be heated to the same or different temperatures.

[0025] Suitably, the third and / or fourth thermally conductive substrate is heated at a heating rate of at least 40°C / s, preferably at least 50°C / s, for example at least 55°C / s, at least 60°C / s, or at least 65°C / s. Such heating rates are considered in the art to be ultrafast heating rates, i.e., the heating is ultrafast. When the third and / or fourth thermally conductive substrate is heated, the first and / or second thermally conductive substrate is also heated.

[0026] The method further comprises sintering the inorganic substrate by heating the inorganic substrate with the heated first and / or second thermally conductive substrates. Suitably, the inorganic substrate is heated at a temperature of 500°C to 2000°C, for example, 600°C to 1500°C, preferably 750°C to 1400°C, more preferably 900°C to 1250°C.

[0027] Preferably, the heating is carried out in the presence of an inert gas. Preferably, the sintering is carried out in the presence of an inert gas. Preferably, the inert gas comprises or consists essentially of one or more of argon, helium, and nitrogen, or a combination of two or more thereof.

[0028] Preferably, the third and fourth thermally conductive substrates, the first and second thermally conductive substrates, and the inorganic substrate are provided between the first conductor and the second conductor, and preferably at least one, and preferably both, of the first and second conductors comprise carbon.

[0029] According to the first embodiment, heating the third and / or fourth thermally conductive substrates preferably comprises inducing an electric current in the first and / or second conductors. Preferably, upon inducing an electric current in the first and / or second conductors, the third and / or fourth thermally conductive substrates, respectively, are heated, preferably by Joule heating. Joule heating is also known as resistive heating or ohmic heating. Preferably, upon inducing an electric current in the first and / or second conductors (using a device), any ohmic or resistive losses in the first and / or second conductors are dissipated in the form of heat, which heats the third and / or fourth thermally conductive substrates.

[0030] Preferably, current is induced in the first and / or second conductors by providing a third conductor at the proximal end of the first conductor and / or at the proximal end of the second conductor. Additionally, a fourth conductor is preferably provided at the distal end of the first conductor and / or at the distal end of the second conductor. Preferably, current is induced in the third and fourth conductors, thereby inducing current in the first and / or second conductors.

[0031] Preferably, the third and fourth conductors each, ie, independently of each other, comprise copper, tungsten, or a combination thereof.

[0032] According to a second embodiment, the third and / or fourth thermally conductive substrate is suitably heated by radiation, preferably infrared (IR) radiation.

[0033] Preferably, when the third and fourth thermally conductive substrates, and the first and second thermally conductive substrates, and the inorganic substrate are provided between the first conductor and the second conductor, the third and / or fourth thermally conductive substrates can be heated by IR radiation by heating the first and / or second conductors, respectively, by IR radiation.

[0034] Preferably, the step of providing an inorganic substrate to be sintered comprises the steps of producing a green structure and debinding the green structure.

[0035] Preferably, the step of manufacturing, i.e. preparing, producing or obtaining, the green structure comprises preparing a mixture by adding a compound containing one or more alkali metals and / or alkaline earth metals and a binder to a solvent, wherein the alkali metals and alkaline earth metals are preferably as described herein above.

[0036] The mixture is then film-cast, thereby obtaining a green structure. Film-casting in the present disclosure includes casting methods known in the art, such as tape-casting, screen-printing, and spray-printing. Preferably, film-casting the mixture includes tape-casting the mixture, respectively. Preferably, tape-casting is performed by a method known in the art.

[0037] Debinding the green structure involves at least partially removing one or more, preferably all, of the binder and solvent. Suitably, at least 50%, preferably at least 75%, more preferably at least 80%, such as at least 85%, at least 90%, at least 95%, at least 98%, or at least 99% of the binder and solvent is removed.

[0038] Debinding is suitably carried out or performed in an atmosphere containing at least 20% by volume of oxygen, such as air or substantially pure oxygen (e.g. industrial grade oxygen). Debinding is suitably carried out at a temperature between 250°C and 800°C, preferably between 400°C and 700°C.

[0039] Preferably, sintering of the inorganic substrate obtained as described herein above by preparing a mixture, film-casting the mixture, and debinding the resulting green structure allows for porous (sintered) inorganic substrates, dense (sintered) inorganic substrates, and substrates with porosity that varies throughout the thickness of the inorganic substrate, i.e., (sintered) inorganic substrates with a porosity gradient.

[0040] As used herein, a "porous substrate" refers to a substrate having a porosity of at least 40%, where the porosity is measured by X-ray computed tomography. A "dense substrate" refers to a substrate having a porosity of 40% or less as measured by X-ray computed tomography. A "substrate with a porosity gradient" refers to a substrate having a first region with a first porosity and a second region with a second porosity. The first and second regions may be, for example, first and second surfaces, for example, two opposite surfaces.

[0041] Preferably, a sintered inorganic substrate having a porosity of at least 40% as measured by X-ray computed tomography is obtained by preparing a mixture, film-casting the mixture, and debinding the resulting green structure as described herein above to provide an inorganic substrate, and heating the inorganic substrate at a temperature between 750°C and 1400°C for a period of time between 1 second and 200 seconds.

[0042] Preferably, when the temperature is in the lower half of the temperature range, the duration is in the upper half of the duration range. Preferably, when the temperature is in the upper half of the temperature range, the duration is in the lower half of the duration range. However, as will be appreciated, the temperature and duration will depend on several factors, including but not limited to the composition and structure of the green structure being debound and the predetermined porosity to be obtained after sintering.

[0043] Preferably, the period is 60 seconds to 200 seconds when the temperature is 750° C. to 1050° C. Preferably, the period is 1 second to 60 seconds when the temperature is 1050° C. to 1400° C.

[0044] Alternatively, and preferably, a sintered inorganic substrate having a first porosity at a first surface and a second porosity different from the first porosity at a second surface opposite the first surface can be obtained by preparing a mixture as described hereinabove, film-casting the mixture, and debinding the resulting green structure to provide an inorganic substrate, and heating the inorganic substrate by heating third and fourth thermally conductive substrates to different temperatures and / or for different durations. As a result, the first surface and the second surface opposite the first surface are heated to different temperatures and / or for different durations. Preferably, each temperature is between 750°C and 1400°C, and each duration is between 1 s and 200 s.

[0045] The present invention further discloses the use of the sintering method of the present disclosure to produce an inorganic solid electrolyte.

[0046] Advantages of the method of the present invention include, but are not limited to: - Significantly reducing sintering times and thereby reducing energy consumption when compared to existing sintering methods; - maintaining the microstructure and / or stoichiometry of the inorganic substrate; -Uniform and homogeneous sintering - be able to sinter thin, i.e., less than 100 μm thick, and / or flat inorganic substrates without damaging or deforming them and by maintaining their flatness; Versatility, in particular the ability to obtain porous or dense (sintered) inorganic substrates, or (sintered) inorganic substrates with a porosity gradient or a dense-porous bilayer structure, from the same inorganic substrate to be sintered, in particular thereby eliminating the need for the use of pore-forming components or agents. Includes:

[0047] The advantage of the disclosed method of being able to sinter thin and / or flat substrates without damage or deformation, and in particular while maintaining flatness, is realized by the combination of third and fourth thermally conductive substrates comprising one or more metal nitrides and / or metal oxides and which are heated, and first and second thermally conductive substrates comprising carbon, which are provided between the substrate (i.e., article or object) to be sintered and the third and fourth thermally conductive substrates and which transfer heat from the third and fourth thermally conductive substrates to the substrate to be sintered. [Brief explanation of the drawings]

[0048] Aspects of the present invention will now be described in more detail with reference to the accompanying drawings, in which like reference numbers illustrate like features. [Figure 1] 1 to 6 show schematic diagrams of various embodiments of the sintering method of the present invention. [Figure 2] 1 to 6 show schematic diagrams of various embodiments of the sintering method of the present invention. [Figure 3] 1 to 6 show schematic diagrams of various embodiments of the sintering method of the present invention. [Figure 4] 1 to 6 show schematic diagrams of various embodiments of the sintering method of the present invention. [Figure 5] 1 to 6 show schematic diagrams of various embodiments of the sintering method of the present invention. [Figure 6] 1 to 6 show schematic diagrams of various embodiments of the sintering method of the present invention. [Figure 7] 7A and 7B show SEM images at different magnifications of a cross section of a substrate that was sintered according to methods known in the art. [Figure 8] 8A and 8B show SEM images at different magnifications of a cross section of a substrate sintered according to the method of the present invention. [Figure 9] 9A and 9B show SEM images of the cross section of the porous structure before and after sintering, respectively. [Figure 10]10A and 10B show SEM images of the cross section of the bilayer structure before and after sintering, respectively. [Figure 11A] 11A-11E show SEM images of cross sections of sintered structures showing varying porosity. [Figure 11B] 11A-11E show SEM images of cross sections of sintered structures showing varying porosity. [Figure 11C] 11A-11E show SEM images of cross sections of sintered structures showing varying porosity. [Figure 11D] 11A-11E show SEM images of cross sections of sintered structures showing varying porosity. [Figure 11E] 11A-11E show SEM images of cross sections of sintered structures showing varying porosity. [Figure 12] FIG. 12A shows the heating and sintering temperature profile, and FIG. 12B shows an SEM image of a cross section of the sintered structure obtained following the temperature profile of FIG. 12A. [Figure 13] FIG. 13A shows the heating and sintering temperature profile, and FIG. 13B shows an SEM image of a cross section of the sintered structure obtained following the temperature profile of FIG. 13A. [Figure 14] FIG. 14 shows the limiting current density measurements for the symmetric cell. [Figure 15] FIG. 15 shows the cycling stability measurements of a solid-state battery containing a solid electrolyte obtained according to the method of the present invention as a function of the number of cycles. [Figure 16] FIG. 16 shows an SEM image of a cross section of a further sintered structure obtained by the method of the present invention. [Figure 17] FIG. 17 shows cycling stability measurements of further solid-state batteries containing a solid electrolyte obtained according to the method of the present invention as a function of the number of cycles. DETAILED DESCRIPTION OF THE INVENTION

[0049] 1 shows a schematic setup 100 suitable for carrying out the method of the present invention. An inorganic substrate 1 to be sintered is provided in a space 101 between a first thermally conductive substrate 2 and a second thermally conductive substrate 3. A third thermally conductive substrate 4 is provided adjacent to the first thermally conductive substrate 2. A first conductor 6 is provided adjacent to the third thermally conductive substrate 4. A fourth thermally conductive substrate 5 is provided adjacent to the second thermally conductive substrate 3. A second conductor 7 is provided adjacent to the fourth thermally conductive substrate 5.

[0050] The inorganic substrate 1 is preferably as described above. The inorganic substrate 1 may be a substrate that forms a solid electrolyte upon sintering.

[0051] The inorganic substrate may have any possible shape. In particular, and preferably, the inorganic substrate has a planar shape, such as a plate or a layer. Preferably, the inorganic substrate is substantially flat. "Substantially flat" in the present disclosure means that the thickness of the substrate at each position is 80% to 120% of the average thickness of the substrate.

[0052] Preferably, at least the space 101 is filled with an inert gas. The inert gas comprises or consists essentially of argon, helium, nitrogen, or a combination of two or more thereof. Preferably, the inert gas comprises or consists essentially of argon. In other words, the sintering is preferably carried out in an inert atmosphere.

[0053] Preferably, the first 2 and second 3 thermally conductive substrates, independently of one another, comprise or consist essentially of carbon, e.g., they may comprise or consist essentially of graphite, carbon fiber, or carbon nanotubes.

[0054] The first 2 and second 3 thermally conductive substrates preferably have a planar shape, such as, but not limited to, a film, foil, sheet, or foil. Preferably, the first 2 and second 3 thermally conductive substrates are substantially flat. Examples of thermally conductive substrates that include carbon include, but are not limited to, graphite, carbon fiber, carbon nanotubes, or a combination of two or more thereof.

[0055] As shown in Figure 1, the dimensions of space 101 are preferably such that the surface of inorganic substrate 1 does not come into contact with the first 2 and second 3 thermally conductive substrates during heating and sintering. Alternatively, and also preferably, one or both of first 2 and second 3 thermally conductive substrates at least partially contact inorganic substrate 1 during heating and sintering, as shown in Figure 2. Preferably, the distance between inorganic substrate 1 and each one of first 2 and second 3 thermally conductive substrates is, independently of one another, 0 mm to 25 mm, preferably 0 mm to 20 mm, more preferably 0 mm to 15 mm, or 0 mm to 10 mm.

[0056] The third and fourth thermally conductive substrates may have the same or different compositions. Preferably, the third and fourth thermally conductive substrates, independently of one another, have a thermal conductivity of at least 25 W / (m * K), e.g., at least 35 W / (m * K), at least 50W / (m * K), e.g., at least 100 W / (m * K), at least 150W / (m * K), preferably at least 200 W / (m * K), e.g., at least 250 W / (m * Preferably, the third and fourth thermally conductive substrates comprise or consist essentially of a metal nitride or a metal oxide. Non-limiting examples of metal nitrides include boron nitride (751 W / (m * K) or aluminum nitride (321 W / (m * Non-limiting examples of metal oxides include aluminum oxide (thermal conductivity of 26 W / (m *K) or sapphire (35W / (m * K), such as single crystal sapphire.

[0057] Preferably, the metal nitride comprises or consists essentially of a single crystal metal nitride. Preferably, the metal oxide comprises or consists essentially of a single crystal metal oxide. The inventors have discovered that single crystal metal nitrides and single crystal metal oxides are better able to withstand the high heating rates (i.e., at least 50°C / s) of the methods of the present invention, i.e., without significant damage or degradation, when compared to non-single crystal metal nitrides and metal oxides. They also appear to better withstand the cooling rates of the present invention.

[0058] The first 2 and third 4 thermally conductive substrates may be in at least partial or complete contact with one another (as shown in FIG. 2). Similarly, the second 3 and fourth 5 thermally conductive substrates may be in at least partial or complete contact with one another (as shown in FIG. 2).

[0059] The inventors have discovered that by using the third and fourth conductive substrates as described above, and particularly when comprising boron nitride, it is possible to process the inorganic substrate 1 without placing it on a carrier or support. In other words, it is possible to calcinate so-called free-standing inorganic substrates, i.e., substrates that do not need to be on a carrier or support structure and are readily available.

[0060] Furthermore, the inventors have surprisingly discovered that the conductive substrates of the third and fourth aspects of the present invention allow thin inorganic substrates to be heated and sintered in a uniform manner without any structural damage.

[0061] The inventors have also surprisingly discovered that the conductive substrates of the third and fourth aspects of the present invention make it possible to sinter a substantially flat inorganic substrate, thereby obtaining a substantially flat sintered inorganic substrate. In other words, the sintering method of the present invention is capable of maintaining the flatness of the sintered substrate.

[0062] It has further been found that by providing the thermally conductive substrates 2, 3 comprising carbon between the third 4 and fourth 5 thermally conductive substrates, it is possible to prevent any solid-state reaction between the third 4 and fourth 5 thermally conductive substrates and the inorganic substrate 1 during heating and sintering.

[0063] Preferably, at least one of the first 6 and second 7 conductors, and preferably both, comprises or consists essentially of carbon. For example, the first 6 and second 7 conductors may, independently of one another, comprise or consist essentially of graphite, carbon fiber, or carbon nanotubes. The first 6 and second 7 conductors may be of the same composition or different compositions.

[0064] The first 6 and second 7 conductors preferably have a planar shape, such as, but not limited to, a film, foil, sheet, or foil. Preferably, the first 6 and second 7 conductors are fibrous carbon materials. For example, but not limited to, these carbon materials may be sponge-like carbon, also known in the art as carbon felt. Furthermore, the fibrous carbon material may include an optional conductive layer capable of dissipating heat to any surrounding object, such as an adjacent substrate or layer, through resistive losses, also known as ohmic losses.

[0065] 1 and 2, the third 4 and fourth 5 thermally conductive substrates are heated, for example, by inducing (e.g., applying) an electric current 102 applied to the first 6 and second 7 conductors, such that the electric current 102 flows through the first 6 and second 7 conductors.

[0066] The current 102 may be continuous or pulsed. The current may be direct current (DC) or alternating current (AC). A power source is preferably provided to supply and direct the current. Preferably, the current 102 is provided by a power source 103.

[0067] Preferably, when an electric current is passed through the first 6 and second 7 conductors, the third 4 and fourth 5 thermally conductive substrates are heated by Joule heating, as described above. Preferably, the third 4 and fourth 5 thermally conductive substrates are uniformly heated. Because they are thermally conductive, heat is transferred to the first 2 and second 3 thermally conductive substrates, respectively. Because the first 2 and second 3 thermally conductive substrates are also thermally conductive, they preferably transfer heat to the inorganic substrates that are heated and sintered.

[0068] Suitably, the heating is carried out at a heating rate of 50°C / s to 120°C / s, for example 55°C / s to 110°C / s, preferably 60°C / s to 100°C / s, for example 60°C / s to 90°C / s, or 60°C / s to 80°C / s.

[0069] Suitably, at least one of the heating and sintering, preferably both, is carried out in an inert atmosphere, i.e. in the presence of one or more inert gases, such as argon, helium or nitrogen, which reduces the risk of any undesired side effects, such as oxidation or calcination reactions, taking place.

[0070] Preferably, the current returns to 0 A once sintering is complete, i.e., once the predetermined sintering period has been reached. Preferably, the absence of current causes no more resistive losses and therefore no heat to be generated, so the temperatures of the third 4 and fourth 5 thermally conductive substrates decrease, i.e., the third 4 and fourth 5 thermally conductive substrates cool down. During the cooling down of the third 4 and fourth 5 thermally conductive substrates, the first 2 and second 3 thermally conductive substrates and the sintered inorganic substrate also cool down.

[0071] Preferably, the method includes the step of actively cooling the thermally conductive substrate and the sintered inorganic substrate. The active cooling may be performed by methods known in the art. Preferably, the active cooling includes passing a flow of a low-temperature inert gas over one or more of the thermally conductive substrates and / or over the sintered inorganic substrate. The inert gas may be as described herein above.

[0072] Figure 3 shows a further embodiment 120 of the method of the present invention. Preferably, a first 2, a second 3, a third 4 and a fourth 5 thermally conductive substrate and a first 6 and a second 7 conductor are provided as described in the method shown in Figure 2. Furthermore, a space 101 between the first 2 and second 3 thermally conductive substrates is provided so that the inorganic substrate 1 to be sintered is in contact with the first 2 and second 3 thermally conductive substrates.

[0073] Preferably, a first current 103a is induced in the second conductor 7. Preferably, the first current 103a is generated by a first power supply 103a. The first power supply 103a is preferably as described above. When the first current 102a is induced in the second conductor 7, the current 102a passes through the second conductor 7, thereby generating ohmic losses. Preferably, the ohmic losses heat the fourth thermally conductive substrate 5.

[0074] Preferably, a second current 103b is induced in the first conductor 6. Preferably, the second current 103b is generated by a second power supply 103b. The second power supply 103b is preferably as described above. When the second current 102b is induced in the first conductor 6, the current 102b passes through the first conductor 6, thereby generating ohmic losses. Preferably, the ohmic losses heat the third thermally conductive substrate 4.

[0075] Preferably, the embodiment of the method of the present invention as shown in Figure 3 allows the third 4 and fourth 5 thermally conductive substrates to be heated independently of each other. As a result, and preferably, the first 2 and second 3 thermally conductive substrates can be heated independently of each other. Preferably, this embodiment of the method according to the present invention allows the first surface of the inorganic substrate 1 and the second surface of the inorganic substrate 1 opposite the first surface to be heated at different heating rates and / or to different temperatures. Preferably, this embodiment of the method allows the first surface of the inorganic substrate 1 and the second surface of the inorganic substrate 1 opposite the first surface to be sintered at different temperatures and / or for different periods of time.

[0076] Figure 4 further illustrates a further embodiment 130 of the method of the present invention. Preferably, a first 2, a second 3, a third 4 and a fourth 5 thermally conductive substrate and a first 6 and a second 7 conductor are provided as described in the method as shown in Figure 2. Furthermore, a space 101 between the first 2 and second 3 thermally conductive substrates is provided such that the inorganic substrate 1 to be sintered is in contact with the first 2 and second 3 thermally conductive substrates.

[0077] Preferably, a current 102 is induced in the first conductor 6 and the second conductor 7 through the third conductor 8 and the fourth conductor 9. The current 102 is preferably provided by a power source 103 as described herein above.

[0078] Preferably, the third 8 and fourth 9 conductors are 10 -3 S / cm~75 * 10 4 Preferably, the third 8 and / or fourth 9 conductors, independently of one another, comprise or consist essentially of copper, a copper alloy, silver, a silver alloy, tungsten, a tungsten alloy, iron, an iron alloy, or a combination of two or more thereof.

[0079] Preferably, the third conductor 8 is provided at the proximal end 60 of the first conductor 6 and at the proximal end 70 of the second conductor. Preferably, the third conductor 8 is in at least partial, preferably complete contact with the first conductor 6 at its proximal end 60 and / or in at least partial, preferably complete contact with the second conductor 7 at its proximal end 70.

[0080] Preferably, the fourth conductor 9 is provided at the distal end 61 of the first conductor 6 and at the distal end 71 of the second conductor. Preferably, the fourth conductor 9 is in at least partial, preferably complete contact with the first conductor 6 at its distal end 61 and / or in at least partial, preferably complete contact with the second conductor 7 at its distal end 71.

[0081] Preferably, resistive losses generated when current 102 is induced or applied to first 6 and second 7 conductors through third 8 and fourth 9 conductors uniformly heat third 4 and fourth 5 thermally conductive substrates. The heated third 4 and fourth 5 thermally conductive substrates preferably heat first 2 and second 3 thermally conductive substrates. Preferably, inorganic substrate 1 to be sintered is heated and sintered by heated first 2 and heated second 3 thermally conductive substrates.

[0082] Figure 5 shows a further embodiment 140 of the method of the present invention. Preferably, the first 2, second 3, third 4 and fourth 5 thermally conductive substrates and the first 6 and second 7 conductors are provided as described in the method shown in Figure 2. Furthermore, a space 101 between the first 2 and second 3 thermally conductive substrates is provided so that the inorganic substrate 1 to be sintered is in contact with the first 2 and second 3 thermally conductive substrates.

[0083] Preferably, a first current 102a is induced in the second conductor 7 through the second portion 82 of the third conductor and the second portion 92 of the fourth conductor. Preferably, the current 102a is provided by a first power source 103a. The power source 103a is preferably as described above. Preferably, the second portion 82 of the third conductor is provided at the proximal end 70 of the second conductor 7, and in particular, at least partially in contact with this proximal end. Preferably, the second portion 92 of the fourth conductor is provided at the distal end 71 of the second conductor 7, and in particular, at least partially in contact with this distal end.

[0084] Preferably, second current 102b is induced in first conductor 6 through third conductor first portion 81 and fourth conductor first portion 91. Preferably, current 102b is provided by second power source 103b. Power source 103b is preferably as described above. Preferably, third conductor first portion 81 is provided at proximal end 60 of first conductor 6, and in particular at least partially in contact with this proximal end. Preferably, fourth conductor first portion 91 is provided at distal end 61 of first conductor 6, and in particular at least partially in contact with this distal end.

[0085] Preferably, the third and fourth conductors are 10 -3 S / cm~75 * 10 4 Preferably, the third and fourth conductors independently comprise copper, a copper alloy, silver, a silver alloy, tungsten, a tungsten alloy, iron, an iron alloy, or a combination of two or more thereof.

[0086] Preferably, and as explained above, providing the first 102a and second 102b electric currents allows the first surface of the inorganic substrate 1 and the second surface of the inorganic substrate 1 opposite the first surface to be heated and sintered independently of each other.

[0087] Figure 6 shows yet another embodiment of the method of the present invention. Preferably, a first 2, a second 3, a third 4 and a fourth 5 thermally conductive substrate are provided as shown in the method of Figure 1. The inorganic substrate to be sintered is provided in the space 101 between the first 2 and second 3 thermally conductive substrates.

[0088] Preferably, at least a first 2 and a second 3 thermally conductive substrate are heated by radiation 10. Alternatively, or in addition, at least a third 4 and a fourth 5 thermally conductive substrate are heated by radiation 10. Preferably, the radiation is infrared radiation. The radiation may be provided by one or more radiation sources, in particular light sources, for example IR light sources.

[0089] Preferably, the inorganic substrate to be sintered is provided by providing a mixture, film-casting the mixture, and preferably debinding the film-cast mixture.

[0090] Preferably, the mixture is obtained by adding a compound containing one or more alkali metals and / or alkaline earth metals and a binder to a solvent.

[0091] Preferably, the alkali metal comprises or consists essentially of lithium or sodium. Preferably, the alkaline earth metal comprises or consists essentially of magnesium.

[0092] Preferably, the solvent comprises a polar solvent. Non-limiting examples of suitable polar solvents include isopropanol, 1-propanol, 2-propanol, butanol, ethanol, methanol, acetone, xylene, methyl ethyl ketone, toluene, 1,1,1-trichloroethane, chlorohexane, cyclohexane, and water.

[0093] Non-limiting examples of binders include polyvinyl alcohol, polyvinyl butyral, polyvinyl chloride, polyurethane, cellulose acetate-butyrate, polyacrylate esters, polytetrafluoroethylene, polypropylene carbonate, vinyl chloride acetate, methyl cellulose, and ethyl cellulose.

[0094] Optionally, the mixture may include one or more additional compounds, such as, but not limited to, a plasticizer, a dispersant (ie, a surfactant).

[0095] Non-limiting examples of plasticizers include (poly)propylene glycol, (poly)ethylene glycol, butyl benzyl phthalate, butyl stearate, a mixture of phthalates, polypropylene carbonate, tricresyl phosphate, and triethylene glycol.

[0096] Non-limiting examples of dispersants include polyvinyl butyral, phosphate esters, ethoylate, aliphatic hydrocarbons, linoleic acid, polyisobutylene, polyethylene glycol, sodium sulfosuccinate, and 2-amino-2-methyl-1-propanol.

[0097] Optionally and preferably, the mixture may further comprise one or more 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. Specific examples of such compounds are alkali metal carbonates or alkaline earth metal carbonates. For example, if the mixture comprises lithium, the mixture may comprise lithium carbonate (Li2CO3).

[0098] Suitably, when the mixture comprises one or more of these compounds, the compounds are present in an amount of from 0.1% to 10% by weight, preferably from 0.5% to 9% by weight, more preferably from 1% to 8% by weight, such as from 2% to 7% by weight, for example from 2.5% to 5% by weight, based on the total weight of the mixture.

[0099] Suitably, the mixture is obtained by mixing the ingredients, for example by ball milling or other techniques known in the art.

[0100] The mixture may be in the form of a slurry, suspension (ie, suspension solution), solution, or dispersion.

[0101] Preferably, the mixture is film-cast as described herein above, thereby obtaining a green structure. Preferably, the green structure is debound as described herein above.

[0102] Advantageously, the method of the present invention makes it possible to produce (sintered) inorganic substrates with a porosity that can vary depending on the application or use of the (sintered) inorganic substrate. In particular, such (sintered) inorganic substrates can be obtained by providing the inorganic substrate to be sintered by mixing, film-casting and suitable debinding as explained above.

[0103] The inventors have surprisingly discovered that, when sintering an inorganic substrate (to be sintered) provided by mixing, film-casting and suitable debinding as described above according to the method of the present invention, it is possible to obtain sintered inorganic substrates that are dense (i.e. have a porosity of 40% or less), porous (i.e. have a porosity of at least 40% as measured by X-ray tomography) or have a porosity gradient (i.e. a porosity that varies throughout the thickness of the (sintered) inorganic substrate) from a single inorganic substrate (i.e. have the same composition) to be sintered.

[0104] In other words, the inventors have surprisingly discovered that it is not necessary to add a pore-forming compound to the mixture to enable the production or manufacture of a porous sintered inorganic substrate, and therefore, and preferably, the mixture does not comprise a pore-forming compound.

[0105] Preferably, the porous (sintered) inorganic substrate can be obtained by heating the inorganic substrate at a temperature between 750°C and 1400°C for a period between 1 s and 200 s.

[0106] Preferably, a sintered inorganic substrate with a porosity gradient can be obtained by heating the inorganic substrate, i.e., by inducing a first current 102a and a second current 102b independently of each other, for example according to the method shown in Figures 3 and 5.

[0107] A preferred example of a sintered inorganic substrate with a porosity gradient is a dense-porous bilayer inorganic substrate, i.e., an inorganic substrate having at least 40% porosity (i.e., porous) on one surface or side and up to 40% porosity (i.e., dense) on the opposite surface or side. Suitably, the thickness of the dense layer and the thickness of the porous layer can be varied by varying the heating and sintering conditions on both sides, in particular the heating rate, temperature, and / or sintering duration.

[0108] Optionally, when a highly porous inorganic substrate is desired, a pore-forming compound may be added to the mixture. When a pore-forming compound is used, the mixture suitably comprises 20% to 90% by volume, preferably 30% to 80% by volume, more preferably 50% to 75% by volume of the pore-forming compound.

[0109] Non-limiting examples of pore-forming compounds include synthetic organic materials such as polymethyl methacrylate (PMMA), polyvinyl chloride, polystyrene, polyethylene oxide, polyvinyl butyral, and PMMA-polyethylene glycol. Preferably, these 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 materials such as sucrose, dextrin, starch, water, and emulsion oils, salts (e.g., NaCl, BaSO4, SrSO4, K2SO4), and metal ceramics (e.g., ZnO, SiO2). A preferred example of a pore-forming compound is PMMA.

[0110] The sintering method of the present invention may be used to produce a solid electrolyte containing one or more alkali metals and / or alkaline earth metals. Preferably, the SSE contains an alkali metal. Preferably, the alkali metal contains one or more of lithium, sodium, or potassium. Preferably, when the alkali metal contains lithium, the lithium exists as a lithium garnet-type structure. Preferably, the lithium garnet-type structure is lithium lanthanum zirconium oxide (LLZO). In other words, the SSE obtained by the method of the present invention preferably contains LLZO.

[0111] The SSE may be a single layer SSE, where the single layer is preferably a dense layer, i.e., a layer having a porosity of up to 40%, or a porous layer, i.e., a layer having a porosity of 40% or more as measured by X-ray computed tomography.

[0112] Alternatively, the SSE may be a multilayer solid electrolyte. In the context of the present invention, a "multilayer SSE" includes any SSE having at least two layers, e.g., three, four, five, or more layers. Preferably, the multilayer solid electrolyte includes alternating dense and porous layers. For example, if the number of layers is two, the multilayer SSE includes a dense layer adjacent to a porous layer. For example, if the number of layers is three, the multilayer SSE includes 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.

[0113] The advantages of such multilayer solid electrolytes are that they are non-flammable, in addition to high ionic conductivity and high voltage stability. Therefore, a safe SSE is thus provided. Such multilayer SSEs can also reduce, inhibit, or even prevent dendrite formation of alkali metals or alkaline earth metals, especially lithium. This significantly reduces the risk of short circuits in solid-state batteries (SSBs) containing such SSEs, thus improving the safety of the SSBs. [Example]

[0114] Example 1 The reference porous substrate and the porous substrate of the present invention were fabricated, and the difference between them was the sintering method. First, 3 g of Li 6.25 Al 0.25 La3Zr2O 12 A mixture was prepared by mixing aluminum-doped LLZO (or Al-LLZO), 0.075 g of Li2CO3 (2.5 wt%), 0.56 mL of plasticizer, 0.59 g of surfactant, and 2.07 g of poly(methyl methacrylate) (PMMA) as a pore-forming compound with 5.9 mL of a solvent containing 5% by volume isopropanol, 87% by volume ethanol, and 8% by volume 1-propanol with a spatula and then 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.

[0115] The mixture was film-cast by tape casting onto a glass substrate. This was done twice to obtain two green structures (i.e., one for each sintering method). The resulting green structures were left at ambient conditions for 1 hour to allow the solvent to evaporate, and then removed from the glass substrate.

[0116] The green structure was then placed between two alumina plates. Debinding of the green structure was carried out at 600 °C in air to completely remove the solvent (evaporation temperature up to 150 °C), PMMA (approximately 350 °C), and residual organic compounds such as binders and plasticizers (approximately 600 °C).

[0117] A reference (sintered) LLZO substrate was fabricated by placing the first green structure between two graphite foils sandwiched between two carbon plates. Sintering was carried out at 1250 °C in a nitrogen atmosphere for 30 seconds.

[0118] SEM images of the cross section of the resulting reference sintered LLZO substrate (Figures 7A and 7B at different magnifications) clearly show that the resulting reference sintered LLZO substrate is not flat but highly curved. Several cracks were also observed. SEM images were recorded using a Hitachi TM3030Plus tabletop microscope at an accelerating voltage of 10 kV.

[0119] Similar results were also obtained with the same setup by sintering in nitrogen atmosphere at temperatures between 1000°C and 1250°C for durations between 30 and 120 seconds.

[0120] The (sintered) LLZO substrate of the present invention was obtained by placing the second green structure between two carbon foils, which were then sandwiched between two boron nitride plates and then inserted between two carbon felts. The proximal and distal ends of both carbon felts were then clamped between two copper conductors. An AC / DC power supply (Aim-TTi CPX400DP dual 420-watt PowerFlex DC power supply) was used to induce current through the copper electrodes and thus through the carbon felts. Sintering was carried out in an argon-filled glove box. The sintering temperature was monitored by an IR camera (MAURER Pyrometer KTRD 4085-1). Current was applied to the carbon felt through the copper electrodes. Sintering was carried out at 1200°C for 90 seconds, with a heating rate of approximately 70°C / s.

[0121] The sintered LLZO substrate was then heat-treated in air at 600 °C for 30 min to remove graphite residues from the LLZO surface, followed by heat-treatment at 900 °C for 10 min under argon atmosphere to remove any contaminants resulting from the presence of Li2CO3 or LiOH on the LLZO surface.

[0122] SEM images (Figures 8A and 8B at different magnifications) of the cross section of the obtained sintered LLZO substrate of the present invention clearly show that the sintered LLZO substrate of the present invention is substantially flat. Similar results were also obtained by sintering in the same apparatus at temperatures between 1000°C and 1250°C for periods between 30 and 120 seconds.

[0123] Figure 9A shows an SEM image of a cross section of a debound LLZO substrate before sintering, containing obvious pores, and Figure 9B shows an SEM image of a cross section of a debound LLZO substrate after sintering, with pores clearly visible.

[0124] Example 2 A two-layer dense-porous inorganic substrate was prepared. 3 g of Li 6.25 Al 0.25 La3Zr2O 12 A first mixture was prepared by mixing aluminum-doped LLZO (or Al-LLZO), 0.15 g of Li2CO3 (2.5 wt%), 0.56 mL of plasticizer, 0.59 g of surfactant, and 5.9 mL of a solvent containing 5% by volume isopropanol, 87% by volume ethanol, and 8% by volume 1-propanol with a spatula and then 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 binder solution was added to the mixture (suspension), followed by an additional ball milling at 200 rpm for 2 hours.

[0125] 3g of Li 6.25 Al 0.25 La3Zr2O 12 A second mixture was prepared by mixing aluminum-doped LLZO (or Al-LLZO), 0.15 g of Li2CO3 (2.5 wt%), 0.56 mL of plasticizer, 0.59 g of surfactant, and 2.07 g of poly(methyl methacrylate) (PMMA) as a pore-forming compound with a spatula and then 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.

[0126] The first mixture was film-cast onto a glass substrate by tape casting. After 60 seconds, the second mixture was tape-cast onto the first mixture (i.e., continuous tape casting). The resulting green structure was left at ambient conditions for 1 hour to allow the solvent to evaporate, and then removed from the glass substrate.

[0127] The green structure was then placed between two alumina plates. Debinding of the green structure was carried out at 600 °C in air to completely remove the solvent (evaporation temperature up to 150 °C), PMMA (approximately 350 °C), and residual organic compounds such as binders and plasticizers (approximately 600 °C).

[0128] The debound green structure was then placed between two carbon foils, which were placed between two boron nitride plates, and then inserted between two carbon felts. The carbon felts were then clamped between two copper electrodes. Sintering was carried out in an argon-filled glove box using a DC power supply. Electric current was applied to the carbon felt through the copper electrodes, which resulted in ultra-rapid heating of the carbon felt. Sintering was carried out at 1200°C for approximately 90 seconds, with a heating rate of approximately 60°C / s.

[0129] The sintered LLZO substrate was then heat-treated in air at 600 °C for 30 min to remove graphite residues from the LLZO surface, followed by heat-treatment at 900 °C for 10 min under argon atmosphere to remove any contaminants resulting from the presence of Li2CO3 or LiOH on the LLZO surface.

[0130] Figure 10A shows an SEM image of a debound two-layer LLZO substrate before sintering, including a dense layer 200 and a porous layer 201. Figure 10B shows an SEM image of the debound LLZO substrate after sintering. The sintered dense layer 202 and sintered porous layer 203 are clearly visible.

[0131] The porous layer was analyzed for its degree of porosity and pore size by X-ray computed tomography. The porosity varied between 40% and 55% within the porous layer, and the pores ranged in size from 1 μm to 9 μm. X-ray computed tomography measurements were performed on an EasyTom XL Ultra 230-160 micro / nano CT scanner (RX Solutions, Chavanod, France). The scanner was operated at 90 kV and a current of 160 μA. The sample was scanned over a full 360° with a rotation step of 0.2° and a frame average of 10. The nominal resolution was set to an 850 nm voxel size. Image reconstruction was performed using X-Act computed tomography software (RX Solutions, Chavanod, France).

[0132] Example 3 To demonstrate the ability of the disclosed sintering method to produce dense and porous inorganic structures from the same starting material and without the need for the addition of pore-forming compounds, 3 g of Li 6.25 Al 0.25 La3Zr2O 12 A mixture was prepared by mixing aluminum-doped LLZO (or Al-LLZO), 0.075 g of Li2CO3 (2.5 wt%), 0.408 mL of plasticizer, 0.43 g of surfactant, and 4.3 mL of a solvent containing 5% by volume isopropanol, 87% by volume ethanol, and 8% by volume 1-propanol with a spatula and then 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. 1.83 g of the binder solution was added to the mixture (suspension), followed by an additional ball milling at 200 rpm for 2 hours.

[0133] The mixture was then tape-cast onto a glass substrate using a doctor blade with a 300 μm opening. The green substrate was dried in ambient air for 30 minutes and removed from the glass substrate.

[0134] The green structure was then placed between two alumina plates and heated to 200° C. in air for 2 hours to remove the solvent, followed by debinding at 600° C. in air for 2 hours.

[0135] The debound green structure was then placed between two carbon foils, which were placed between two boron nitride plates, and then inserted between two carbon felts. The carbon felt was then clamped between two copper electrodes at its proximal and distal ends. Sintering was carried out in an argon-filled glove box using a DC power supply. Electric current was applied to the carbon felt through the copper electrodes, which resulted in ultra-rapid heating of the carbon felt. Sintering was carried out at 1200°C for various durations, from 10 to 90 seconds.

[0136] The sintered LLZO substrate was then heat-treated in air at 600 °C for 30 min to remove graphite residues from the LLZO surface, followed by heat-treatment at 900 °C for 10 min under argon atmosphere to remove any contaminants resulting from the presence of Li2CO3 or LiOH on the LLZO surface.

[0137] FIG. 11A shows an SEM image of a cross section of the LLZO film obtained after sintering at 1200°C for 10 seconds. The porosity was 44% as determined by X-ray computed tomography. In other words, a porous LLZO film was obtained. FIGS. 11B, 11C, 11D, and 11E show SEM images of the cross sections of the obtained LLZO films with porosities of 34%, 21%, 16%, and 8%, respectively, after sintering at 1200°C for 40 seconds, 50 seconds, 70 seconds, and 90 seconds, respectively. In other words, dense LLZO films were obtained at these sintering times.

[0138] The same debound green LLZO structure was sintered according to the profile shown in Figure 12A, which indicates a sintering temperature of 1225°C and a sintering period of 27 seconds. Figure 12B shows an SEM image of the cross section of the sintered LLZO film, which has a porosity of 27% and is therefore a dense LLZO film.

[0139] The same debound green LLZO structure was also sintered according to the profile shown in Figure 13A, which indicates a sintering temperature of 1225°C and a sintering period of 120 seconds. Figure 13B shows an SEM image of the cross section of the sintered LLZO film, which has a porosity of 2%, i.e., a fully dense LLZO film.

[0140] Example 4 To determine the lithium ion (Li ion) conductivity of the LLZO film obtained by the sintering method of the present invention, a fully dense LLZO film was fabricated according to Example 3. The LLZO substrate had a density of 5.1 g / cm 3 The substrate had a density that was 95% to 98% of the theoretical density of 100 nm. The substrate was then coated with gold (Au) electrodes (each electrode 50 nm thick) by thermal evaporation to a total thickness of 100 nm.

[0141] Electrochemical impedance spectroscopy (EIS) measurements were carried out on the dense LLZO substrate using a frequency range of 1 MHz to 0.1 Hz with a sinus amplitude of 10 mV. 2 The ionic conductivity was calculated from the Re(Z) value (bulk and grain boundary resistivity), the thickness of the LLZO film (38 μm), and the diameter of the symmetric thermally evaporated 500 nm Au electrodes (5 mm). The resulting ionic conductivity was 1.9 x 10 -4 S / cm.

[0142] Example 5 To evaluate the electrochemical performance of the porous layer obtained according to the present invention in terms of lithium plating / stripping, a symmetric battery cell was fabricated containing a lithium metal anode and a lithium metal cathode and a single porous layer. The porous layer was as obtained in Example 1. The symmetric battery cell was fabricated by thermal evaporation of 200 nm of metallic lithium (using a Covap thermal evaporation device), after which lithium foil was cold isostatically pressed onto the porous layer on both sides of the porous LLZO layer at approximately 71 MPa for 5 minutes (using a PW 100EH cold isostatic press). This allowed for the impregnation of up to approximately 15 μm of lithium into the porous layer. This resulted in a capacity of approximately 1.5 mAh / cm. 2 It corresponds to the areal capacity of

[0143] The limiting current density (CCD) of a symmetric cell, i.e., the current density at which Li dendrite / filament propagation begins, is set to 0.1 mA / cm 2 ~8mA / cm 2 The current density was determined by galvanostatic cycling experiments at different current densities of 0.1 mA / cm. 2 in steps of 0.1 to 1.5 mA / cm 2 , 0.5mA / cm 2 in steps of 1.5 to 3 mA / cm 2 , and 1 mA / cm 2 in steps of 3 to 10 mA / cm 2 The same amount of Li was transferred for each half cycle (0.1 mAh / cm 2 , i.e., the areal capacity limit applied during the test). Tests were performed at room temperature without any stack pressure applied. Figure 14 shows that the symmetric cell with a single porous layer can deliver up to 1.7 mA / cm 2 The results show that the high limiting current density was observed.

[0144] 0.1mAh / cm per half cycle 2 with an areal capacitance limit of 0.1 mA / cm 2Galvanostatic cycling experiments were performed at a constant current density of 0.05 V. The results are shown in Figure 15. High cycling stability of the cell for approximately 100 h and a relatively low voltage polarization of 30-45 mV during cycling were observed.

[0145] Example 6 To demonstrate the ability of the disclosed sintering method in producing porous LLZO substrates with low pore size and high porosity, 3 g of Li 6.25 Al 0.25 La3Zr2O 12 A mixture was prepared by mixing 0.69 g to 2.07 g of aluminum-doped LLZO (or Al-LLZO), 0.408 mL of plasticizer, 0.43 mL of surfactant, and 4.3 mL of a solvent containing 5% by volume isopropanol, 87% by volume ethanol, and 8% by volume 1-propanol in a ball mill jar with a spatula. The mixture was then further ball milled at 165 rpm for 18 hours. The Al-LLZO was a powder with an average particle size of 500 nm (i.e., nanopowder). The pore former was a mixture of narrow-disperse acrylic particles (NDAP) of different diameters (1.5 μm, 5 μm, and 15 μm).

[0146] Then, 2 mL of a binder solution of polyvinyl butyral in isopropanol (3 g of polyvinyl butyral per 8.9 mL of isopropanol) was added, and the resulting suspension was ball milled at 200 rpm for 2 hours.

[0147] The suspension was then tape-cast onto a glass substrate. The resulting green substrate was dried under ambient air for 1 hour to remove the solvent, and then removed from the glass substrate.

[0148] The green structure was then placed between two alumina plates and heated to 150°C in air to remove the solvent, followed by debinding at 600°C in air for 2 hours, resulting in decomposition of the pore-forming agent and removal of residual organic components, such as binders and plasticizers.

[0149] The debound green structure was then placed between two graphite foils, which were placed between two boron nitride plates, and then inserted between two carbon felts. The carbon felt was then clamped between two copper electrodes at its proximal and distal ends. Sintering was carried out in an argon-filled glove box using an AC / DC power supply. Sintering was carried out at 1150 °C for 20 seconds. The sintering temperature was monitored by an IR camera.

[0150] The sintered LLZO substrate was then heat-treated in air at 600 °C for 30 min to remove graphite residues from the LLZO surface, followed by heat-treatment at 900 °C for 10 min under argon atmosphere to remove any contaminants resulting from the presence of Li2CO3 or LiOH on the LLZO surface.

[0151] Figure 16 shows SEM images of the cross section of the sintered LLZO substrate, where the SEM images were recorded using a Hitachi TM3030Plus tabletop microscope at an accelerating voltage of 10 kV. From Figure 16A, it is clear that the sintered LLZO substrate exhibits excellent flatness.

[0152] The porosity was 50% by volume within the entire thickness of the sintered LLZO substrate, and the average pore size was approximately 2 μm as determined by X-ray computed tomography (RX Solutions Easy Tom XL system using a Deben CT5000-RT stage). In other words, a porous LLZO film with small pores was obtained.

[0153] Mechanical stability was tested by a three-point bending test on a Tinius Olsen 1ST electromechanical testing machine. The crosshead speed and strain rate applied to the samples were 1 μm s -1 and 1.56x10-3s -1 A high breaking load of 146 mN was measured and excellent mechanical stability was observed, i.e., the sintered substrate was shown to be self-supporting.

[0154] To evaluate the electrochemical performance of the porous LLZO substrate with respect to lithium plating / stripping, symmetric battery cells were fabricated containing a lithium metal anode and a lithium metal cathode and a single porous layer of LLZO substrate. The symmetric battery cells were fabricated by thermal evaporation of 200 nm of metallic lithium (using a Covap thermal evaporation system), followed by cold isostatic pressing of lithium foil onto the porous layer on both sides of the porous LLZO layer at approximately 71 MPa for 5 minutes (using a PW 100 EH cold isostatic press). This allowed for the impregnation of up to approximately 5 μm of lithium into the porous layer.

[0155] 0.1mAh / cm per half cycle 2 0.1 mA / cm at room temperature and without external pressure, with an areal capacitance limit of 2 Galvanostatic cycling experiments were performed at a constant current density of 0.05 V. The results are shown in Figure 17. High cycling stability of the cells was observed for over 625 h. Furthermore, the symmetric cell exhibited a stable overpotential of ≈10 mV throughout the cycling measurements.

[0156] naming 1.Inorganic base material 2. First thermally conductive substrate 3. Second thermally conductive substrate 4. Third thermally conductive substrate 5. Fourth thermally conductive substrate 6. First Conductor 7. Second Conductor 8.Third Conductor 9. Fourth Conductor 10. Infrared (IR) light source 60. Proximal end of first conductor 61. Distal end of first conductor 70. Proximal end of second conductor 71. Distal end of second conductor 81. Third Conductor (First Part) 82. Third Conductor (Second Part) 91. Fourth Conductor (First Part) 92. The second part of the fourth conductor 100.Schematic sintering arrangement 101. A space between the first and second thermally conductive substrates 102. Means for inducing electric current 102a. Means for inducing electric current 102b. Means for inducing electric current 110.Schematic sintering arrangement 120.Schematic sintering arrangement 130.Schematic sintering arrangement 140.Schematic sintering arrangement 150.Schematic sintering arrangement 200.Dense layer before sintering 201.Porous layer before sintering 202. Sintered dense layer 203. Sintered Porous Layer

Claims

1. 1. A method for producing a sintered inorganic substrate, comprising: - providing an inorganic substrate (1) to be sintered; - providing said inorganic substrate (1) between a first thermally conductive substrate (2) and a second thermally conductive substrate (3), said first (2) and second (3) thermally conductive substrates comprising carbon; - providing said first (2) and second (3) thermally conductive substrates and said inorganic substrate (1) between a third thermally conductive substrate (4) and a fourth thermally conductive substrate (5); - heating said third (4) and / or said fourth (5) thermally conductive substrate at a heating rate of at least 50°C / s to a temperature of between 750°C and 1400°C, preferably between 900°C and 1250°C, thereby heating said first (2) and / or said second (3) thermally conductive substrate, respectively; - sintering said inorganic substrate (1) by heating it with said heated first (2) and / or second (3) thermally conductive substrate at a temperature between 750°C and 1400°C, preferably between 900°C and 1250°C; Including, The method, wherein the third (4) and fourth (5) thermally conductive substrates independently comprise one or more metal nitrides and / or metal oxides.

2. 10. The method for producing a sintered inorganic substrate according to claim 1, wherein the third (4) and fourth (5) thermally conductive substrates independently comprise one or more single crystal metal nitrides and / or single crystal metal oxides.

3. 10. A method for producing a sintered inorganic substrate according to any one of the preceding claims, wherein the metal nitride comprises boron nitride and / or aluminium nitride.

4. 10. A method for producing a sintered inorganic substrate according to any one of the preceding claims, wherein the metal oxide comprises alumina and / or sapphire.

5. 10. The method for producing a sintered inorganic substrate according to any one of the preceding claims, wherein the third (4) and fourth (5) thermally conductive substrates are heated, wherein the third (4) and fourth (5) thermally conductive substrates are heated independently of each other.

6. 10. The method for manufacturing a sintered inorganic substrate according to any one of the preceding claims, further comprising providing the third (4) and fourth (5) thermally conductive substrates, the first (2) and second (3) thermally conductive substrates, and the inorganic substrate (1) between a first conductor (6) and / or a second conductor (7), preferably wherein the first (6) and second (7) conductors comprise carbon.

7. 7. The method for producing a sintered inorganic substrate according to claim 6, wherein heating the third (4) and / or fourth (5) thermally conductive substrate comprises inducing an electric current (102, 102a, 102b) in the first (6) and / or second conductor (7), thereby heating the third (4) and / or fourth (5) thermally conductive substrate.

8. 8. A method for producing a sintered inorganic substrate according to claim 7, comprising: - providing a third conductor (8, 81, 82) at the proximal end (60) of said first conductor (6) and / or at the proximal end (70) of said second conductor (7), preferably said third conductor (8, 81, 82) comprising one or more of copper and tungsten; - providing a fourth conductor (9, 91, 92) at the distal end (61) of said first conductor (6) and / or at the distal end (71) of said second conductor (7), preferably said fourth conductor (9, 91, 92) comprising one or more of copper and tungsten; - inducing a current (102, 102a, 102b) in the third (8, 81, 82) and fourth (9, 91, 92) conductors, thereby inducing the current (102, 102a, 102b) in the first (6) and / or second (7) conductors; wherein the current (102, 102a, 102b) is induced in the first (6) and / or second (7) conductors by

9. 7. The method for producing a sintered inorganic substrate according to any one of claims 1 to 6, wherein the third (4) and / or the fourth (5) thermally conductive substrate is heated by infrared (IR) radiation.

10. 10. The method for producing a sintered inorganic substrate according to claim 9, taking into consideration claim 6, wherein heating the third (4) and / or fourth (5) thermally conductive substrate by IR radiation comprises heating the first (6) and / or second (7) conductor by IR radiation, thereby heating the third (4) and / or fourth (5) thermally conductive substrate, respectively.

11. 10. A method for producing a sintered inorganic substrate according to any one of the preceding claims, wherein said heating and said sintering are carried out in the presence of an inert gas.

12. 10. A method for producing a sintered inorganic substrate according to any one of the preceding claims, comprising providing the inorganic substrate (1) to be sintered, - adding a compound containing one or more alkali metals and / or alkaline earth metals and a binder to a solvent, thereby obtaining a mixture; - film-casting said mixture, thereby obtaining a green structure; - debinding said green structure, thereby at least partially removing said binder and said solvent, thereby obtaining said inorganic substrate (1); A method comprising:

13. 13. A method for obtaining a sintered inorganic substrate having a porosity of at least 40% as measured by X-ray computed tomography, the method comprising providing the inorganic substrate (1) according to claim 12, and further comprising heating the inorganic substrate (1) at a temperature between 750°C and 1400°C for a period of time between 1 s and 200 s.

14. 13. A method for obtaining a sintered inorganic substrate having a first porosity at a first surface and a second porosity at a second surface opposite the first surface, the method comprising providing the inorganic substrate (1) of claim 12, and further comprising heating the third (4) and fourth (5) thermally conductive substrates to different temperatures and / or for different durations, whereby the first surface and the second surface opposite the first surface are heated to different temperatures and / or for different durations, the temperature being between 750°C and 1400°C, and the duration being between 1 s and 200 s.

15. Use of the method according to any one of claims 1 to 14 for producing an inorganic solid electrolyte.

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