Ultrafast high-temperature sintering method

Abstract

The present invention relates to a method for producing a sintered inorganic substrate comprising disposing an inorganic substrate between first and second carbon-containing thermally conductive substrates, disposing 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 substrate to a temperature between 750 DEG C and 1400 DEG C at a heating rate of at least 50 DEG C / s, thereby heating the first and / or second thermally conductive substrate, respectively, and sintering the inorganic substrate by heating the inorganic substrate with the heated first and / or second thermally conductive substrate at a temperature between 750 DEG C and 1400 DEG C, wherein the third and fourth thermally conductive substrates independently of each other comprise one or more single crystal metal oxides and / or single crystal metal nitrides.

Description

[0001] Technical field of the invention

[0002] The invention relates to a method for producing a sintered inorganic substrate. The invention further relates to an apparatus for sintering an inorganic substrate.

[0003] background

[0004] 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) generally involves solid-state reactions to form inorganic components from precursors, and sintering the inorganic components to obtain solid (inorganic) components. Each step generally requires high temperatures and long processing times. This can lead to undesirable, non-uniform grain growth and can be an obstacle to high throughput (mass production).

[0005] Long processing times are also one of the problems of conventional methods in the production of inorganic (e.g. ceramic-based) solid-state electrolytes (SSEs). Such SSEs are a promising alternative to lithium-ion batteries, which have shown safety issues and possible leakage.

[0006] Conventional sintering methods are usually carried out in so-called bulk furnaces, where they are heated to the desired sintering temperature, which depends on the composition of the inorganic material to be sintered. Disadvantages of such bulk furnaces include long heating and cooling times (i.e., low heating rates and low cooling rates), and difficult to control temperature and heat distribution (i.e., temperature uniformity).

[0007] More recently, new sintering methods have been developed, including microwave assisted sintering, spark plasma sintering and flash sintering. However, microwave assisted sintering depends largely on the microwave absorption properties of the material to be sintered, which limits its applicability. Spark plasma sintering requires a mold to compress the inorganic material during the sintering process, and therefore limits the geometry and scalability of the components to be sintered. In addition, due to the applied pressure, it is not suitable 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 geometries, such as three-dimensional structures. In particular, the specific flash sintering conditions depend greatly on the electrical properties of the inorganic material, which limits the sintering of components with unknown composition.

[0008] Another recently developed sintering method is ultrafast high temperature sintering.

[0009] “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, wherein the heating is radiant heating and the sintering is carried out under an inert atmosphere.

[0010] WO2020 / 236767 discloses a rapid high temperature sintering system and method. The substrate to be sintered is placed between two conductive carbon elements, and the distance between each conductive carbon element and the substrate is 0 to 10 mm. The conductive carbon element is heated to a temperature between 500°C and 3000°C by an electric current, and sintering is performed within 1 second to 1 hour by heating the substrate with the heated conductive carbon element.

[0011] “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 within seconds at high temperatures. For example, co-sintering is performed with an alumina (Al2O3) substrate. However, (co)sintering on various substrates limits its applicability in all-solid-state battery cells.

[0012] "Ultrafast high-temperature sintering (UHS) offine 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 felts, which are placed between Al2O3 fiber sheets to ensure good thermal contact between the carbon felts and the α-Al2O3. An electric current is applied to the carbon felt to heat the α-Al2O3 to be sintered.

[0013] "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 device for complex-shaped Al2O3 samples. The Al2O3 sample is embedded in a graphite powder bed heated at a heating rate of up to 770°C / min. The powder bed is placed in an Al2O3 insulating material to reduce heat loss to the outside.

[0014] One disadvantage of the aforementioned method and apparatus is that self-supporting substrates (i.e. in the absence of a support or carrier) are difficult to sinter. In particular, in the case of flat, self-supporting substrates, it is difficult to maintain the flatness of the substrate during sintering using the aforementioned method and apparatus. In other words, the sintered substrate obtained by means of the aforementioned method and / or apparatus tends to bend when sintered in the absence of a carrier or support, exhibits warping, may even exhibit cracks, or may begin to crack when attempting to flatten the sintered substrate after sintering. SUMMARY OF THE INVENTION

[0016] The present invention aims to overcome one or more of the above-mentioned disadvantages. One object of the present invention is to provide a method for sintering an inorganic substrate, wherein the method requires a reduced sintering time. Another object is to provide a method for sintering, which maintains the microstructure and / or stoichiometry of the inorganic substrate, and / or wherein the sintering is carried out uniformly. Another object is to provide a sintering method that allows sintering of thin (i.e. having a thickness of less than 100 μm) and / or flat inorganic substrates without causing damage or deformation to the sintered inorganic substrate, i.e. thereby maintaining flatness. Another object of the present invention is to provide a device capable of sintering an inorganic substrate in a uniform manner.

[0017] The term "substrate" as part of a sintering process (eg "thermally conductive substrate"), is used in the present disclosure for components or layers, in particular layers having a sheet-like shape.

[0018] According to a first aspect of the present invention, a method for producing a sintered inorganic substrate according to the accompanying claims is disclosed.

[0019] Advantageously, the sintering method of the present disclosure is an ultrafast high temperature sintering method. The term "ultrafast sintering" is used in the present disclosure to include sintering methods that are heated at a heating rate of at least 50°C / s and optionally cooled at a cooling rate of at least 50°C / s. The term "high temperature sintering method" is used in the present disclosure to include sintering methods that are heated to a temperature of at least 750°C, preferably at least 900°C, such as between 750°C and 1400°C, more preferably between 900°C and 1250°C.

[0020] The inorganic substrate, i.e. the article or object, may comprise or consist essentially of any known inorganic material. Advantageously, the inorganic substrate comprises or consists essentially of a ceramic material. Advantageously, the ceramic material may comprise one or more alkali metals and / or alkaline earth metals. In other words, the inorganic substrate advantageously comprises or consists essentially of a ceramic material, the ceramic material comprising one or more alkali metals or alkaline earth metals.

[0021] Advantageously, the alkali metal comprises one or more of lithium, sodium or potassium. Advantageously, the alkaline earth metal comprises one or more of magnesium or calcium. Advantageously, when the ceramic material comprising one or more alkali metals and / or alkaline earth metals comprises lithium, the lithium exists as a lithium garnet type structure. In other words, the ceramic material comprising one or more alkali metals and / or alkaline earth metals advantageously comprises a lithium garnet type structure. Advantageously, the lithium garnet type structure is lithium lanthanum zirconium oxide (LLZO).

[0022] According to the method of the present disclosure, an inorganic substrate to be sintered is provided. The inorganic substrate is arranged between a first thermally conductive substrate and a second thermally conductive substrate. Advantageously, at least one and preferably both of the first and second thermally conductive substrates contain carbon.

[0023] Advantageously, the distance between the inorganic substrate and the first and second thermally conductive substrates is between 0 mm (i.e. in contact) and 20 mm, preferably between 0 mm and 10 mm. Advantageously, the first and / or second thermally conductive substrates at least partially, for example and preferably completely (i.e. over the entire surface area) contact the respective surface of the inorganic substrate.

[0024] The method further comprises disposing the first and second thermally conductive substrates and the inorganic substrate (to be sintered) between the third thermally conductive substrate and the fourth thermally conductive substrate.

[0025] The third thermally conductive substrate and the fourth thermally conductive substrate respectively contain one or more metal nitrides and / or metal oxides or are substantially composed of one or more metal nitrides and / or metal oxides. Advantageously, the third and fourth thermally conductive substrates independently of each other contain one or more single-crystalline metal nitrides and / or single-crystalline metal oxides. "Respectively contain ... or are substantially composed of ..." in the present disclosure means that the two substrates can be the same, i.e. have the same composition, or can be different, i.e. have different compositions.

[0026] Advantageously, the metal nitride comprises boron nitride and / or aluminum nitride. Advantageously, the metal oxide comprises aluminum oxide and / or sapphire.

[0027] The method further comprises heating the third and / or fourth thermally conductive substrate to a temperature between 500°C and 2000°C, for example between 600°C and 1500°C, preferably between 750°C and 1400°C, more preferably between 900°C and 1250°C. Advantageously, the third and fourth thermally conductive substrates may be heated independently of each other. Advantageously, one or both of the third and fourth thermally conductive substrates may be heated. Advantageously, when the third and fourth thermally conductive substrates are heated, they may be heated to the same or different temperatures.

[0028] Advantageously, 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 a heating rate is considered to be an ultrafast heating rate in the art, i.e. the heating is ultrafast heating. When the third and / or fourth thermally conductive substrate is heated, the first and / or second thermally conductive substrate is also heated.

[0029] The method further comprises sintering the inorganic substrate. The sintering is performed by heating the inorganic substrate with the heated first and / or second thermally conductive substrate. Advantageously, the inorganic substrate is heated at a temperature between 500°C and 2000°C, for example between 600°C and 1500°C, preferably between 750°C and 1400°C, more preferably between 900°C and 1250°C.

[0030] Advantageously, the heating is carried out in the presence of an inert gas. Advantageously, the sintering is carried out in the presence of an inert gas. Advantageously, the inert gas comprises or consists essentially of one or more of argon, helium and nitrogen or a combination of two or more thereof.

[0031] Advantageously, the third and fourth thermally conductive substrates, the first and second thermally conductive substrates and the inorganic substrate are arranged between the first conductor and the second conductor. Advantageously, at least one, and preferably both, of the first and second conductors comprise carbon.

[0032] According to a first embodiment, heating the third and / or fourth thermally conductive substrate advantageously comprises inducing an electric current into the first and / or second conductor. Advantageously, when inducing an electric current into the first and / or second conductor, the third and / or fourth thermally conductive substrate is heated, respectively, advantageously by means of Joule heating. Joule heating is also known as resistive heating or ohmic heating. Advantageously, when inducing an electric current into the first and / or second conductor (in use of the device), any ohmic or resistive losses in the first and / or second conductor are dissipated in the form of heat energy, which heats the third and / or fourth thermally conductive substrate.

[0033] Advantageously, the current is induced to the first and / or second conductor by providing a third conductor at the proximal end of the first conductor and / or at the proximal end of the second conductor. Furthermore, advantageously a fourth conductor is provided at the distal end of the first conductor and / or at the distal end of the second conductor. Advantageously, the current is induced to the third and fourth conductors, thereby inducing the current to the first and / or second conductor.

[0034] Advantageously, the third and fourth conductors respectively (ie independently of each other) comprise copper, tungsten or a combination thereof.

[0035] According to a second embodiment, the third and / or fourth heat conducting substrate is advantageously heated by means of radiation, preferably infrared (IR) radiation.

[0036] Advantageously, when the third and fourth thermally conductive substrates, the first and second thermally conductive substrates and the inorganic substrate are arranged between the first conductor and the second conductor, the first and / or second conductors can be heated by IR radiation respectively.

[0037] Advantageously, the step of providing an inorganic substrate to be sintered comprises producing a green structure and debinding the green structure.

[0038] Advantageously, manufacturing (ie preparing, producing or obtaining) the green structure comprises preparing a mixture by adding a compound comprising one or more alkali metals and / or alkaline earth metals and a binder to a solvent. Advantageously, the alkali metals and alkaline earth metals are as described above.

[0039] The mixture is then subjected to film casting, thereby obtaining a green body structure. Film casting in the present disclosure includes casting methods known in the art, such as tape casting, screen printing and spray printing. Advantageously, the film casting of the mixture includes tape casting of the mixture, respectively. Advantageously, tape casting is performed by methods known in the art.

[0040] Debinding of the green structure comprises at least partially removing one or more of the binder and the solvent, preferably all. Advantageously, removing one or more of the binder and the solvent, preferably at least 50% of all, 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%.

[0041] 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.

[0042] Advantageously, sintering the inorganic substrate obtained by preparing a mixture, casting a thin film of the mixture and debinding the resulting green structure as described above makes it possible to obtain a porous (sintered) inorganic substrate, a dense (sintered) inorganic substrate and a substrate having a porosity that varies throughout the thickness of the inorganic substrate, i.e. a (sintered) inorganic substrate having a porosity gradient.

[0043] As used herein, a "porous substrate" is a substrate having a porosity of at least 40%, wherein the porosity is measured by X-ray computed tomography. As used herein, a "dense substrate" is a substrate having a porosity of 40% or less, as measured by X-ray computed tomography. As used herein, a "substrate having a porosity gradient" is a substrate having a first region having a first porosity and a second region having a second porosity. The first and second regions can be, for example, a first and a second surface, for example, two opposing surfaces.

[0044] Advantageously, a sintered inorganic substrate having a porosity of at least 40% as measured by X-ray computed tomography is obtained by providing an inorganic substrate as described above by preparing a mixture, thin film casting the mixture and debinding the resulting green structure, and then heating the inorganic substrate at a temperature between 750°C and 1400°C for a duration between 1 second and 200 seconds.

[0045] Advantageously, when the temperature is in the lower half of the temperature range, the duration is in the upper half of the duration range. Advantageously, 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 depend on several aspects, including but not limited to the composition and structure of the debound green structure, and the predetermined porosity to be obtained after sintering.

[0046] Advantageously, when the temperature is between 750° C. and 1050° C., the duration is between 60 seconds and 200 seconds. Advantageously, when the temperature is between 1050° C. and 1400° C., the duration is between 1 second and 60 seconds.

[0047] Alternatively and advantageously, as explained above, by providing an inorganic substrate obtained by preparing a mixture, film casting the mixture and debinding the resulting green structure and heating the inorganic substrate by heating the third and fourth thermally conductive substrates to different temperatures and / or heating for different durations, 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 to the first surface can be obtained. Thus, the first surface and the second surface opposite to the first surface are heated to different temperatures and / or heated for different durations. Advantageously, each temperature is between 750°C and 1400°C and each duration is between 1 second and 200 seconds.

[0048] The present invention further discloses the use of the sintering method disclosed in the present invention for producing an inorganic solid electrolyte.

[0049] Advantages of the method of the present invention include but are not limited to:

[0050] - Significantly reduced sintering time compared to existing sintering methods, thereby reducing energy consumption;

[0051] - Maintaining the microstructure and / or stoichiometry of the inorganic substrate;

[0052] -Consistent, uniform sintering;

[0053] - Thin (i.e. thickness below 100 μm) and / or flat inorganic substrates can be sintered without damaging or deforming them and maintaining flatness;

[0054] - Multifunctionality, in particular the possibility of obtaining porous or dense (sintered) inorganic substrates, or (sintered) inorganic substrates with porosity gradients or dense-porous bilayer structures, from the same inorganic substrate to be sintered, in particular thereby eliminating the need for the use of pore-forming components or pore-forming agents.

[0055] An advantage of the method of the present disclosure is that thin and / or flat substrates can be sintered without damage or deformation, in particular, maintaining flatness, which is achieved by providing a third and fourth thermally conductive substrate and a combination of a first and a second thermally conductive substrate, wherein the third and fourth thermally conductive substrates contain one or more metal nitrides and / or metal oxides and the third and fourth thermally conductive substrates are heated, and the first and second thermally conductive substrates contain carbon, the first and second thermally conductive substrates are arranged between the substrate to be sintered (i.e., the article or object) and the third and fourth thermally conductive substrates, and transfer heat from the third and fourth thermally conductive substrates to the substrate to be sintered.

[0056] Description of the drawings

[0057] Aspects of the present invention will now be described in more detail with reference to the accompanying drawings, wherein like reference numerals represent like features, and wherein:

[0058] - Figures 1 to 6 Schematically showing various embodiments of the sintering method of the present invention;

[0059] - Fig. 7A and 7B SEM images at different magnifications showing a cross section of a substrate sintered according to methods known in the art;

[0060] - Fig. 8A and 8B SEM images showing a cross section of a substrate sintered according to the method of the present invention at different magnifications;

[0061] - Fig.9A and 9B SEM images showing the cross-section of the porous structure before and after sintering, respectively;

[0062] - Fig. 10A and 10B SEM images showing the cross-section of the bilayer structure before and after sintering, respectively;

[0063] - FIG. 11A to FIG. 11E SEM images showing cross-sections of sintered structures, showing varying porosity;

[0064] - Fig. 12A Displays heating and sintering temperature profiles, and Fig. 12B Display according to Fig. 12A SEM image of a cross section of the sintered structure obtained with a temperature profile of

[0065] - Fig.13A Displays heating and sintering temperature curves, and Fig. 13B Display according to Fig.13A SEM image of a cross section of the sintered structure obtained with a temperature profile of

[0066] - Fig.14 Critical current density measurements showing symmetric cells;

[0067] - Fig.15 shows the variation of cycling stability measurements of solid-state batteries comprising a solid-state electrolyte obtained according to the method of the present invention as a function of the number of cycles;

[0068] - Fig.16a SEM image showing a cross section of another sintered structure obtained by the method of the present invention; and

[0069] - Fig.17 The cycling stability measurements as a function of the number of cycles are shown for another solid-state battery comprising a solid-state electrolyte obtained according to the method of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0071] Figure 1 A schematic apparatus 100 suitable for carrying out the method of the present invention is shown. An inorganic substrate 1 to be sintered is arranged in a space 101 between a first heat-conducting substrate 2 and a second heat-conducting substrate 3. A third heat-conducting substrate 4 is arranged adjacent to the first heat-conducting substrate 2. A first conductor 6 is arranged adjacent to the third heat-conducting substrate 4. A fourth heat-conducting substrate 5 is arranged adjacent to the second heat-conducting substrate 3. A second conductor 7 is arranged adjacent to the fourth heat-conducting substrate 5.

[0072] The inorganic substrate 1 is advantageously as described above. The inorganic substrate 1 may be a substrate which forms a solid electrolyte after sintering.

[0073] The inorganic substrate may have any possible geometric shape. In particular and advantageously, the inorganic substrate has a planar shape, such as a plate or a layer. Advantageously, the inorganic substrate is substantially flat. "Substantially flat" in the present disclosure means that the thickness of the substrate at each position thereof is between 80% and 120% of the average thickness of the substrate.

[0074] Advantageously, at least space 101 is filled with an inert gas. The inert gas comprises argon, helium, nitrogen, or a combination of two or more thereof, or consists essentially of argon, helium, nitrogen, or a combination of two or more thereof. Advantageously, the inert gas comprises argon or consists essentially of argon. In other words, sintering is advantageously performed in an inert atmosphere.

[0075] Advantageously, the first thermally conductive substrate 2 and the second thermally conductive substrate 3 independently of one another comprise carbon or consist essentially of carbon. For example, they may comprise graphite, carbon fibers or carbon nanotubes or consist essentially of graphite, carbon fibers or carbon nanotubes.

[0076] The first thermally conductive substrate 2 and the second thermally conductive substrate 3 advantageously have a planar shape, such as a film, foil, sheet or foil, but not limited thereto. Advantageously, the first thermally conductive substrate 2 and the second thermally conductive substrate 3 are substantially flat. Examples of carbonaceous thermally conductive substrates include, but are not limited to, graphite, carbon fiber, carbon nanotubes, or a combination of two or more thereof.

[0077] like Figure 1 As shown in , the size of the space 101 is advantageously such that the surface of the inorganic substrate 1 does not contact the first thermally conductive substrate 2 and the second thermally conductive substrate 3 during the heating and sintering process. Alternatively and also advantageously, as Figure 2As shown in , one or both of the first thermally conductive substrate 2 and the second thermally conductive substrate 3 at least partially contacts the inorganic substrate 1 during the heating and sintering process. Advantageously, the distance between the inorganic substrate 1 and each of the first thermally conductive substrate 2 and the second thermally conductive substrate 3 is independently between 0 mm and 25 mm, preferably between 0 mm and 20 mm, more preferably between 0 mm and 15 mm, or between 0 mm and 10 mm.

[0078] The third thermally conductive substrate 4 and the fourth thermally conductive substrate 5 may have the same or different compositions. Advantageously, the third thermally conductive substrate 4 and the fourth thermally conductive substrate 5 independently have a thermal conductivity of at least 25 W / (m*K), such as at least 35 W / (m*K), at least 50 W / (m*K), such as at least 100 W / (m*K), at least 150 W / (m*K), preferably at least 200 W / (m*K), such as at least 250 W / (m*K). Advantageously, the third thermally conductive substrate 4 and the fourth thermally conductive substrate 5 contain metal nitrides or metal oxides or are substantially composed of metal nitrides or metal oxides. Non-limiting examples of metal nitrides include boron nitride (thermal conductivity 751 W / (m*K)) or aluminum nitride (thermal conductivity 321 W / (m*K)). Non-limiting examples of metal oxides include aluminum oxide (thermal conductivity 26 W / (m*K)) or sapphire (thermal conductivity 35 W / (m*K)), such as a sapphire single crystal.

[0079] Advantageously, the metal nitride comprises or consists essentially of a single crystalline metal nitride. Advantageously, the metal oxide comprises or consists essentially of a single crystalline metal oxide. The inventors have found that single crystalline metal nitrides and single crystalline metal oxides are better able to tolerate (i.e., do not show significant damage or degradation) the high heating rates (i.e., at least 50°C / s) of the method of the present invention when compared to non-single crystalline metal nitrides and metal oxides. They also appear to better tolerate the cooling rates of the method of the present invention.

[0080] The first heat-conducting substrate 2 and the third heat-conducting substrate 4 may be at least partially or completely (eg Figure 2 Similarly, the second thermally conductive substrate 3 and the fourth thermally conductive substrate 5 may be at least partially or completely (as shown in Figure 2 ) are in contact with each other.

[0081] The inventors have found that by using the third thermally conductive substrate 4 and the fourth thermally conductive substrate 5 as described above, in particular when comprising boron nitride, it is possible to process the inorganic substrate 1 without the need for a carrier or support to place the inorganic substrate 1 thereon. In other words, it is possible to sinter so-called free-standing inorganic substrates, i.e. readily available substrates that are not on a carrier or support structure.

[0082] Furthermore, the inventors have surprisingly found that the third and fourth thermally conductive substrates 4, 5 according to the invention enable heating and sintering of thin inorganic substrates in a uniform manner without causing any structural damage thereto.

[0083] The inventors have also surprisingly found that the third and fourth thermally conductive substrates 4 and 5 according to the present invention enable sintering of substantially flat inorganic substrates, thereby obtaining substantially flat sintered inorganic substrates. In other words, the sintering method of the present invention enables the flatness of the sintered substrates to be maintained.

[0084] Furthermore, it is noted that providing the carbonaceous thermally conductive substrates 2, 3 between the third thermally conductive substrate 4 and the fourth thermally conductive substrate 5 can prevent any solid-state reaction between the third thermally conductive substrate 4 and the fourth thermally conductive substrate 5 and the inorganic substrate 1 during heating and sintering.

[0085] Advantageously, at least one of the first conductor 6 and preferably both of the second conductor 7 comprise carbon or consist essentially of carbon. For example, the first conductor 6 and the second conductor 7 may independently of one another comprise graphite, carbon fibers or carbon nanotubes or consist essentially of graphite, carbon fibers or carbon nanotubes. The first conductor 6 and the second conductor 7 may have the same composition or different compositions.

[0086] The first conductor 6 and the second conductor 7 advantageously have a planar shape, such as a film, foil, sheet or foil, but not limited thereto. Advantageously, the first conductor 6 and the second conductor 7 are filamentous carbon materials. For example, but not limited thereto, these carbon materials may be sponge carbon, also known in the art as carbon felt. Furthermore, the filamentous carbon material may comprise any conductive layer that is capable of dissipating heat to any object around it (e.g., an adjacent substrate or layer) through resistive loss (also known as ohmic loss).

[0087] Advantageously, if Figure 1 and 2 As shown in FIG. 1 , the third thermally conductive substrate 4 and the fourth thermally conductive substrate 5 are heated by inducing (eg, applying) a current 102 applied to the first conductor 6 and the second conductor 7. Thus, the current 102 is caused to flow through the first conductor 6 and the second conductor 7.

[0088] The current 102 may be a continuous current or a pulsed current. The current may be a direct current (DC) or an alternating current (AC). Advantageously, a power source is provided to supply and sense the current. Advantageously, the current 102 is provided by means of a power source 103.

[0089] Advantageously, as explained above, the third thermally conductive substrate 4 and the fourth thermally conductive substrate 5 are heated by Joule heating when current passes through the first conductor 6 and the second conductor 7. Advantageously, the third thermally conductive substrate 4 and the fourth thermally conductive substrate 5 are uniformly heated. Since they are thermally conductive, heat is transferred to the first thermally conductive substrate 2 and the second thermally conductive substrate 3, respectively. Since the first thermally conductive substrate 2 and the second thermally conductive substrate 3 are also thermally conductive, they advantageously transfer heat to the inorganic substrate, heating and sintering it.

[0090] Advantageously, the heating is performed at a heating rate between 50°C / s and 120°C / s, for example between 55°C / s and 110°C / s, preferably between 60°C / s and 100°C / s, for example between 60°C / s and 90°C / s, or between 60°C / s and 80°C / s.

[0091] Advantageously, at least one of the heating and sintering, and 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. This reduces the risk of any unwanted side effects, such as oxidation or calcination reactions.

[0092] Advantageously, once sintering is complete, i.e. once a predetermined sintering duration has been reached, the current is returned to 0 A. Advantageously, the absence of current causes no more resistive losses and therefore no more heat to be generated, so that the temperature of the third and fourth heat-conducting substrates 4 and 5 is reduced, i.e. the third and fourth heat-conducting substrates 4 and 5 are cooled. When the third and fourth heat-conducting substrates 4 and 5 are cooled, the first and second heat-conducting substrates 2 and 3 and the sintered inorganic substrates are also cooled.

[0093] Advantageously, the method comprises the step of actively cooling the thermally conductive substrate and the sintered inorganic substrate. Active cooling can be performed by methods known in the art. Advantageously, active cooling comprises passing a cold inert gas stream over one or more thermally conductive substrates and / or over the sintered inorganic substrate. The inert gas may be as described above.

[0094] Figure 3 Represents another embodiment 120 of the method of the present invention. Advantageously, if Figure 2 In the method shown, a first heat conductive substrate 2, a second heat conductive substrate 3, a third heat conductive substrate 4 and a fourth heat conductive substrate 5 as well as a first conductor 6 and a second conductor 7 are provided. In addition, a space 101 between the first heat conductive substrate 2 and the second heat conductive substrate 3 is provided so that the inorganic substrate 1 to be sintered contacts the first heat conductive substrate 2 and the second heat conductive substrate 3.

[0095] Advantageously, the first current 103a is induced to the second conductor 7. Advantageously, the first current 103a is generated by a first power source 103a. The first power source 103a is advantageously as described above. When the first current 102a is induced to the second conductor 7, the current 102a passes through the second conductor 7, thereby generating resistive losses. Advantageously, the resistive losses heat the fourth thermally conductive substrate 5.

[0096] Advantageously, the second current 103b is induced to the first conductor 6. Advantageously, the second current 103b is generated by a second power source 103b. The second power source 103b is advantageously as described above. When the second current 102b is induced to the first conductor 6, the current 102b passes through the first conductor 6, thereby generating resistive losses. Advantageously, the resistive losses heat the third thermally conductive substrate 4.

[0097] Advantageously, if Figure 3 The embodiment of the method of the invention shown enables heating of the third heat-conducting substrate 4 and the fourth heat-conducting substrate 5 independently of one another. Thus and advantageously, the first heat-conducting substrate 2 and the second heat-conducting substrate 3 can be heated independently of one another. Advantageously, with this embodiment of the method according to the invention, it is possible to heat the first surface of the inorganic substrate 1 and the second surface of the inorganic substrate 1 opposite the first surface at different heating rates and / or to different temperatures. Advantageously, with this embodiment of the method, it is possible to sinter the first surface of the inorganic substrate 1 and the second surface of the inorganic substrate 1 opposite the first surface at different temperatures and / or for different durations.

[0098] Figure 4 Represents yet another embodiment 130 of the method of the present invention. Advantageously, if Figure 2 In the method shown, a first heat conductive substrate 2, a second heat conductive substrate 3, a third heat conductive substrate 4 and a fourth heat conductive substrate 5 as well as a first conductor 6 and a second conductor 7 are provided. In addition, a space 101 between the first heat conductive substrate 2 and the second heat conductive substrate 3 is provided so that the inorganic substrate 1 to be sintered contacts the first heat conductive substrate 2 and the second heat conductive substrate 3.

[0099] Advantageously, the current 102 is induced to the first conductor 6 and the second conductor 7 via the third conductor 8 and the fourth conductor 9. The current 102 is advantageously provided by means of a power supply 103, as described above.

[0100] Advantageously, the third conductor 8 and the fourth conductor 9 have a -3 S / cm to 75*10 4The electrical conductivity is between 1000 and 1000 s / cm. Advantageously, the third conductor 8 and / or the fourth conductor 9, independently of each other, comprise copper, copper alloy, silver, silver alloy, tungsten, tungsten alloy, iron, iron alloy or a combination of two or more thereof or consist essentially of copper, copper alloy, silver, silver alloy, tungsten, tungsten alloy, iron, iron alloy or a combination of two or more thereof.

[0101] Advantageously, a 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. Advantageously, the third conductor 8 at least partially, preferably completely contacts the first conductor 6 at the proximal end 60 of the first conductor 6 and / or at least partially, preferably completely contacts the second conductor 7 at the proximal end 70 of the second conductor 7.

[0102] Advantageously, a 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. Advantageously, the fourth conductor 9 at least partially, preferably completely contacts the first conductor 6 at the distal end 61 of the first conductor 6 and / or at least partially, preferably completely contacts the second conductor 7 at the distal end 71 of the second conductor 7.

[0103] Advantageously, the resistance loss generated when the current 102 is induced or applied to the first conductor 6 and the second conductor 7 via the third conductor 8 and the fourth conductor 9 uniformly heats the third heat-conducting substrate 4 and the fourth heat-conducting substrate 5. The heated third heat-conducting substrate 4 and the fourth heat-conducting substrate 5 advantageously heat the first heat-conducting substrate 2 and the second heat-conducting substrate 3. Advantageously, the inorganic substrate 1 to be sintered is heated and sintered by means of the heated first heat-conducting substrate 2 and the heated second heat-conducting substrate 3.

[0104] Figure 5 Represents yet another embodiment 140 of the method of the present invention. Advantageously, if Figure 2 In the method shown, a first heat conductive substrate 2, a second heat conductive substrate 3, a third heat conductive substrate 4 and a fourth heat conductive substrate 5 as well as a first conductor 6 and a second conductor 7 are provided. In addition, a space 101 between the first heat conductive substrate 2 and the second heat conductive substrate 3 is provided so that the inorganic substrate 1 to be sintered contacts the first heat conductive substrate 2 and the second heat conductive substrate 3.

[0105] Advantageously, the first current 102a is induced to the second conductor 7 via the second portion 82 of the third conductor and the second portion 92 of the fourth conductor. Advantageously, the current 102a is provided by means of a first power supply 103a. The power supply 103a is advantageously as described above. Advantageously, the second portion 82 of the third conductor is arranged at the proximal end 70 of the second conductor 7, and in particular, at least partially contacts the proximal end 70 of the second conductor 7. Advantageously, the second portion 92 of the fourth conductor is arranged at the distal end 71 of the second conductor 7, and in particular, at least partially contacts the distal end 71 of the second conductor 7.

[0106] Advantageously, the second current 102b is induced to the first conductor 6 via the first portion 81 of the third conductor and the first portion 91 of the fourth conductor. Advantageously, the current 102b is provided by means of a second power supply 103b. The power supply 103b is advantageously as described above. Advantageously, the first portion 81 of the third conductor is arranged at the proximal end 60 of the first conductor 6, and in particular, at least partially contacts the proximal end 60 of the first conductor 6. Advantageously, the first portion 91 of the fourth conductor is arranged at the distal end 61 of the first conductor 6, and in particular, at least partially contacts the distal end 61 of the first conductor 6.

[0107] Advantageously, the third and fourth conductors have -3 S / cm to 75*10 4 The electrical conductivity is between 200 Å and 100 Å S / cm. Advantageously, the third and fourth conductors independently of one another 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.

[0108] Advantageously and as explained above, the provision of the first current 102a and the second current 102b enables heating and sintering of the first surface of the inorganic substrate 1 and the second surface of the inorganic substrate 1 opposite to the first surface independently of each other.

[0109] Figure 6 Yet another embodiment of the method of the present invention is shown. Advantageously, if Figure 1 In the method, a first heat conductive substrate 2, a second heat conductive substrate 3, a third heat conductive substrate 4 and a fourth heat conductive substrate 5 are provided. An inorganic substrate to be sintered is arranged in a space 101 between the first heat conductive substrate 2 and the second heat conductive substrate 3.

[0110] Advantageously, at least the first thermally conductive substrate 2 and the second thermally conductive substrate 3 are heated by means of radiation 10. Alternatively or additionally, at least the third thermally conductive substrate 4 and the fourth thermally conductive substrate 5 are heated by means of radiation 10. Advantageously, the radiation is infrared radiation. The radiation may be provided by means of one or more radiation sources, in particular light sources such as IR light sources.

[0111] Advantageously, the inorganic substrate to be sintered is provided by providing a mixture, film casting the mixture and advantageously debinding the film cast mixture.

[0112] Advantageously, the mixture is obtained by adding a compound comprising one or more alkali metals and / or alkaline earth metals and a binder to a solvent.

[0113] Advantageously, the alkali metal comprises or consists essentially of lithium or sodium. Advantageously, the alkaline earth metal comprises or consists essentially of magnesium.

[0114] Advantageously, 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, hexyl chloride, cyclohexane and water.

[0115] Non-limiting examples of binders include polyvinyl alcohol, polyvinyl butyral, polyvinyl chloride, polyurethane, cellulose acetate-butyrate, polyacrylate, polytetrafluoroethylene, polypropylene carbonate, vinyl chloride acetate, methyl cellulose, and ethyl cellulose.

[0116] Optionally, the mixture may contain one or more additional compounds such as, but not limited to, plasticizers, dispersants (ie, surfactants).

[0117] Non-limiting examples of plasticizers include (poly)propylene glycol, (poly)ethylene glycol, butyl benzyl phthalate, butyl stearate, mixtures of phthalates, polypropylene carbonate, tricresyl phosphate, and triethylene glycol.

[0118] 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.

[0119] Optionally and advantageously, the mixture may further comprise one or more of the carbonate, oxide, tungsten oxide or zirconium oxide of the alkali metal and / or alkaline earth metal contained in the first and / or second mixture, 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).

[0120] Advantageously, when the mixture comprises one or more such compounds, the compound is present in an amount of between 0.1% and 10% by weight, preferably between 0.5% and 9% by weight, more preferably between 1% and 8% by weight, for example between 2% and 7% by weight, for example between 2.5% and 5% by weight, based on the total weight of the mixture.

[0121] Advantageously, the mixture is obtained by mixing the components, for example by ball milling or other techniques known in the art.

[0122] The mixture may be in the form of a slurry, a suspension (ie a suspended solution), a solution or a dispersion.

[0123] Advantageously, the mixture is film cast as described above, thereby obtaining a green structure. Advantageously, the green structure is degreased as described above.

[0124] Advantageously, the method of the present invention enables the production of a (sintered) inorganic substrate having a porosity that can vary based on the application or use of the (sintered) inorganic substrate. In particular, such a (sintered) inorganic substrate can be obtained by mixing, film casting and advantageously debinding to provide an inorganic substrate to be sintered as described above.

[0125] The inventors have unexpectedly found that sintering the (to be sintered) inorganic substrate provided by mixing, film casting and advantageously debinding as described above according to the method of the present invention makes it possible to obtain a dense (i.e. having a porosity of 40% or less), porous (i.e. having a porosity of at least 40%, as measured by X-ray tomography) or sintered inorganic substrate with a porosity gradient (i.e. a porosity that varies throughout the thickness of the (sintered) inorganic substrate) from a single (i.e. having the same composition) inorganic substrate to be sintered.

[0126] In other words, the inventors have surprisingly found that it is not necessary to add a pore-forming compound to the mixture in order to be able to produce or manufacture a porous sintered inorganic substrate.Therefore and advantageously, the mixture does not comprise a pore-forming compound.

[0127] Advantageously, the porous (sintered) inorganic substrate may be obtained by heating the inorganic substrate at a temperature between 750° C. and 1400° C. for a duration between 1 second and 200 seconds.

[0128] Advantageously, it is possible to obtain the following information by, for example, Figure 3 and Figure 5 A sintered inorganic substrate having a porosity gradient is obtained by heating the inorganic substrate using the method shown in , ie, by inducing the first current 102a and the second current 102b independently of each other.

[0129] 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 a porosity of at least 40% (i.e., porous) on one surface or side and a porosity of at most 40% (i.e., dense) on the opposite surface or side. Advantageously, the thickness of the dense layer and the thickness of the porous layer can be varied by changing the heating and sintering conditions (particularly the heating rate, temperature, and / or sintering duration) on both sides.

[0130] Optionally, when an inorganic substrate with high porosity is to be obtained, a pore-forming compound can be added to the mixture. When a pore-forming compound is used, the mixture advantageously contains between 20% and 90% by volume of the pore-forming compound, preferably between 30% and 80% by volume, more preferably between 50% and 75% by volume.

[0131] 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 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.

[0132] The sintering method of the present invention can be used to produce a solid electrolyte comprising one or more alkali metals and / or alkaline earth metals. Advantageously, the SSE comprises an alkali metal. Advantageously, the alkali metal comprises one or more of lithium, sodium or potassium. Advantageously, when the alkali metal comprises lithium, the lithium exists as a lithium garnet-type structure. Advantageously, the lithium garnet-type structure is lithium lanthanum zirconium oxide (LLZO). In other words, the SSE obtained by the method of the present invention advantageously comprises LLZO.

[0133] The SSE may be a single layer SSE, wherein the single layer is advantageously a dense layer (ie a layer having a porosity of at most 40%) or a porous layer (ie a layer having a porosity of 40% or more) as measured by X-ray computed tomography.

[0134] Alternatively, the SSE may be a multilayer solid electrolyte. A "multilayer SSE" according to the present invention comprises any SSE having at least two layers, such as three, four, five or more layers. Advantageously, the multilayer solid electrolyte 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.

[0135] In addition to high ionic conductivity and high voltage stability, one advantage of such a multilayer solid electrolyte is that it is non-flammable. Therefore, a safe SSE is provided. Such a multilayer SSE is further able to reduce, even inhibit or avoid the formation of dendrites of alkali metals or alkaline earth metals (especially lithium). This greatly reduces the risk of short circuits in solid-state batteries (SSBs) containing such SSEs and thereby improves the safety of SSBs. Example

[0136] Example 1

[0137] A reference porous substrate and a porous substrate according to the invention were prepared, the difference between the two being the sintering method. First, 3 g of Li 6.25 Al 0.25 La3Zr2O 12 (aluminum-doped LLZO or Al-LLZO), 0.075 g of Li2CO3 (2.5 wt%), 0.56 ml of a plasticizer, 0.59 g of a surfactant, and 2.07 g of poly(methyl methacrylate) (PMMA) as a pore-forming compound and 5.9 ml of a solvent comprising 5 vol% isopropanol, 87 vol% ethanol and 8 vol% 1-propanol, and then ball milled at 165 rpm for 18 hours to prepare a mixture. 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), and then further ball milled at 200 rpm for 2 hours.

[0138] The mixture was subjected to film casting by tape casting on a glass substrate. This was done twice to obtain two green structures (i.e., one green structure for each sintering method). 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.

[0139] The green structure is then placed between two alumina plates. Debinding of the green structure is 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.).

[0140] A reference (sintered) LLZO substrate was produced by placing the first green structure between two graphite foils sandwiched between two carbon plates. Sintering was performed at 1250° C. for 30 seconds in a nitrogen atmosphere.

[0141] The SEM image of the cross section of the obtained reference sintered LLZO substrate ( Fig. 7A and 7B , at different magnifications) clearly show that the reference sintered LLZO substrate thus obtained is not flat, but highly curved. Some cracks are also noted. The SEM images were recorded using a Hitachi TM3030Plus Tabletop microscope with an accelerating voltage of 10 kV.

[0142] Similar results were also obtained by sintering in the same setup in a nitrogen atmosphere at a temperature between 1000° C. and 1250° C. for a duration between 30 seconds and 120 seconds.

[0143] The (sintered) LLZO substrate of the present invention is obtained by placing the second green body structure between two carbon foils, which are then sandwiched between two boron nitride plates, and then inserting them between two carbon felts. The proximal and distal ends of the two carbon felts as conductors are then sandwiched between two copper conductors. An AC / DC power supply (Aim-TTi CPX400DP Dual 420watt PowerFlex DC power supply) is used to induce current to the copper electrodes and thus to the carbon felt. Sintering is carried out in an argon-filled glove box. The sintering temperature is monitored by an IR camera (MAURER Pyrometer KTRD 4085-1). Current is applied to the carbon felt via the copper electrodes. Sintering lasts for 90 seconds at 1200°C and the heating rate is about 70°C / s.

[0144] Thereafter, the sintered LLZO substrate was heat treated at 600°C for 30 minutes in air to remove graphite residues from the LLZO surface, and then heat treated at 900°C for 10 minutes under an argon atmosphere to remove any contamination on the LLZO surface originating from the presence of Li2CO3 or LiOH.

[0145] The SEM image of the cross section of the sintered LLZO substrate obtained in the present invention ( Fig. 8A and 8B , at different magnifications) clearly show that the sintered LLZO substrate of the present invention is substantially flat. Similar results were also obtained by sintering at a temperature between 1000° C. and 1250° C. for a duration between 30 seconds and 120 seconds in the same apparatus and settings.

[0146] Fig.9A A SEM image showing a cross section of a debound LLZO substrate before sintering, which contains clear pores. Fig. 9B SEM image showing a cross section of a debound LLZO substrate after sintering. The pores are clearly visible.

[0147] Example 2

[0148] The double-layer dense-porous inorganic substrate was prepared by mixing 3 g Li 6.25 Al 0.25 La3Zr2O 12(aluminum-doped LLZO or Al-LLZO), 0.15 g of Li2CO3 (2.5 wt%), 0.56 ml of a plasticizer, 0.59 g of a surfactant, and 5.9 ml of a solvent comprising 5 vol% isopropanol, 87 vol% ethanol, and 8 vol% 1-propanol, and then ball milled at 165 rpm for 18 hours to prepare a first mixture. 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), and then further ball milled at 200 rpm for 2 hours.

[0149] By mixing 3 g of Li 6.25 Al 0.25 La3Zr2O 12 A second mixture was prepared by adding 0.15 g of Li2CO3 (2.5 wt%), 0.56 ml of a plasticizer, 0.59 g of a surfactant, and 2.07 g of poly(methyl methacrylate) (PMMA) as a pore-forming compound and 5.9 ml of a solvent comprising 5 vol% isopropanol, 87 vol% ethanol, and 8 vol% 1-propanol, 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), and then further ball milled at 200 rpm for 2 hours.

[0150] 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.

[0151] The green structure is then placed between two alumina plates. Debinding of the green structure is 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.).

[0152] The degreased green structure was then placed between two carbon foils, which were placed between two boron nitride plates, which were then inserted between two carbon felts. The carbon felt was then sandwiched between two copper electrodes. A DC power supply was used, and sintering was performed in an argon-filled glove box. An electric current was applied to the carbon felt via the copper electrodes, which resulted in ultrafast heating of the carbon felt. Sintering lasted for approximately 90 seconds at 1200°C, and the heating rate was approximately 60°C / s.

[0153] Thereafter, the sintered LLZO substrate was heat treated at 600°C for 30 minutes in air to remove graphite residues from the LLZO surface, and then heat treated at 900°C for 10 minutes under an argon atmosphere to remove any contamination on the LLZO surface originating from the presence of Li2CO3 or LiOH.

[0154] Fig. 10A A SEM image of a debound double-layer LLZO substrate before sintering is shown, which includes a dense layer 200 and a porous layer 201 . Fig. 10B The SEM image of the debindered LLZO substrate after sintering is shown. The sintered dense layer 202 and the sintered porous layer 203 are clearly visible.

[0155] The porosity degree and pore size of the porous layer were analyzed by X-ray computed tomography. The porosity in the porous layer varied between 40% and 55%, and the pore size was between 1 μm and 9 μm. X-ray computed tomography measurements were performed on an EasyTom XL Ultra 230-160micro / nano-CT scanner (RX Solutions, Chavanod France). The scanner was operated at 90 kV and a current of 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. Image reconstruction was performed using X-Act computed tomography software (RX Solutions, Chavanod, France).

[0156] Example 3

[0157] To illustrate the ability of the disclosed sintering method to produce dense and porous inorganic structures from the same starting materials without the need for the addition of pore-forming compounds, 3 g of Li 6.25 Al 0.25 La3Zr2O 12 (aluminum-doped LLZO or Al-LLZO), 0.075 g of Li2CO3 (2.5 wt%), 0.408 ml of a plasticizer, 0.43 g of a surfactant, and 4.3 ml of a solvent comprising 5 vol% isopropanol, 87 vol% ethanol, and 8 vol% 1-propanol, and then ball milled at 165 rpm for 18 hours to prepare a mixture. 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), and then further ball milled at 200 rpm for 2 hours.

[0158] 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 then removed from the glass substrate.

[0159] 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.

[0160] The degreased green structure is then placed between two graphite foils, which are placed between two boron nitride plates, which are then inserted between two carbon felts. The carbon felt is then clamped between two copper electrodes at its proximal end and at its distal end. A DC power supply is used, and sintering is performed in an argon-filled glove box. An electric current is applied to the carbon felt via the copper electrodes, which results in ultrafast heating of the carbon felt. Sintering is performed at 1200°C for durations varying between 10 seconds and 90 seconds.

[0161] Thereafter, the sintered LLZO substrate was heat treated at 600°C for 30 minutes in air to remove graphite residues from the LLZO surface, and then heat treated at 900°C for 10 minutes under an argon atmosphere to remove any contamination on the LLZO surface originating from the presence of Li2CO3 or LiOH.

[0162] Fig.11A An SEM image of a cross section of a LLZO membrane obtained after sintering at 1200° C. for 10 seconds is shown. The porosity, as determined by means of X-ray computed tomography, is 44%. In other words, a porous LLZO membrane is obtained. Fig. 11B , 11C , 11D and 11E show SEM images of cross-sections of LLZO films obtained after sintering at 1200° C. for 40 seconds, 50 seconds, 70 seconds and 90 seconds, respectively, having porosities of 34%, 21%, 16% or 8%, respectively. In other words, dense LLZO films were obtained at these sintering durations.

[0163] The same degreased green LLZO structure is based on Fig. 12A The curve shown in sintering indicates a sintering temperature of 1225° C. and a sintering duration of 27 seconds. Fig. 12B The SEM image of the cross section of the sintered LLZO film is shown and has a porosity of 27%, ie, a dense LLZO film.

[0164] The same degreased green LLZO structure is also based on Fig.13A The curve shown in sintering indicates a sintering temperature of 1225° C. and a sintering duration of 120 seconds. Fig. 13B The SEM image of the cross section of the LLZO film after sintering is shown and has a porosity of 2%, i.e., a fully dense LLZO film.

[0165] Example 4

[0166] In order 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 manufactured according to Example 3. The density of the LLZO substrate was 5.1 g / cm 3 The substrate was then coated with gold (Au) electrodes (each electrode was 50 nm thick) with a total thickness of 100 nm by thermal evaporation.

[0167] Electrochemical impedance spectroscopy (EIS) measurements were performed on the dense LLZO substrate using a frequency range of 1 MHz to 0.1 Hz at a sinusoidal amplitude of 10 mV. 2 The ionic conductivity was calculated based on the Re(Z) value (bulk resistance and grain boundary resistance), the thickness of the LLZO film (38 μm), and the diameter of the symmetrically thermally evaporated 50 nm Au electrode (5 mm). The ionic conductivity thus obtained is 1.9×10 –4 S / cm.

[0168] Example 5

[0169] In order to evaluate the electrochemical performance of the porous layer obtained according to the present invention in terms of lithium plating / stripping, a symmetrical battery cell with a lithium metal anode and a lithium metal cathode and a single porous layer was manufactured. The porous layer obtained as in Example 1 was used. The symmetrical battery cell was manufactured by thermal evaporation of 200nm of metallic lithium (using a Covap thermal evaporator) and then cold isostatic pressing (using a PW 100EH cold isostatic press) of lithium foil onto the porous layer at about 71MPa for 5 minutes on both sides of the porous LLZO layer. This enables the lithium to be impregnated into the porous layer up to about 15μm. This corresponds to about 1.5mAh / cm 2 Surface capacity.

[0170] By 0.1mA / cm 2 Up to 8mA / cm 2 The critical current density (CCD) achievable by the symmetric battery pack is determined by constant current cycling experiments at different current densities between 0.1 mA / cm 2 The step size is from 0.1mA / cm 2 Increase to 1.5mA / cm 2 , at 0.5 mA / cm 2 The step size is from 1.5mA / cm 2 Increase to 3mA / cm 2 , and at 1mA / cm 2 The step size is from 3mA / cm 2 Increase to 10mA / cm2 , 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 tests were carried out at room temperature without any stacking pressure applied. Fig.14 Results are presented showing that a symmetric cell with a single porous layer exhibits up to 1.7 mA / cm 2 high critical current density.

[0171] At 0.1 mAh / cm per half cycle 2 The surface capacity is limited to 0.1 mA / cm 2 The constant current cycling experiment was carried out at a constant current density of . The results show Fig.15 A high cycling stability of the cells of approximately 100 h and a relatively low voltage polarization of 30–45 mV during cycling were observed.

[0172] Example 6

[0173] To demonstrate the ability of the disclosed sintering method to produce 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 an aluminum-doped LLZO or Al-LLZO, a pore former between 0.408 ml of a plasticizer, 0.43 ml of a surfactant, and 4.3 ml of a solvent comprising 5 vol% isopropanol, 87 vol% ethanol, and 8 vol% 1-propanol. The mixture was then ball milled at 165 rpm for 18 hours. The Al-LLZO was a powder (i.e., a nanopowder) having an average particle size of 500 nm. The pore former was a mixture of narrowly dispersed acrylic particles (NDAP) of different diameters (1.5 μm, 5 μm, and 15 μm).

[0174] Then 2 ml of a binder solution comprising polyvinyl butyral in isopropanol (3 g polyvinyl butyral / 8.9 ml isopropanol) was added. The resulting suspension was ball milled at 200 rpm for 2 hours.

[0175] 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.

[0176] 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 h, which resulted in decomposition of the pore formers and removal of residual organic components, such as binders and plasticizers.

[0177] The degreased green structure was then placed between two graphite foils, which were placed between two boron nitride plates, which were subsequently inserted between two carbon felts. The carbon felt was then clamped between two copper electrodes at its proximal end and at its distal end. An alternating current / direct current (AC / DC) power supply was used, and sintering was performed in an argon-filled glove box. Sintering was performed at 1150° C. for 20 seconds. The sintering temperature was monitored by an IR camera.

[0178] Thereafter, the sintered LLZO substrate was heat treated at 600°C for 30 minutes in air to remove graphite residues from the LLZO surface, and then heat treated at 900°C for 10 minutes under an argon atmosphere to remove any contamination on the LLZO surface originating from the presence of Li2CO3 or LiOH.

[0179] Fig.16 SEM images of cross sections of sintered LLZO substrates are shown, where the SEM images were recorded using a Hitachi TM3030 Plus Tabletop microscope at an accelerating voltage of 10 kV. Fig.16 It is clearly seen in A that the sintered LLZO substrate exhibits excellent flatness.

[0180] As determined by means of X-ray computed tomography (RX Solutions EasyTomXL system using Deben CT5000-RT stage), the porosity was 50% by volume throughout the thickness of the sintered LLZO substrate, and the average pore size was about 2 μm. In other words, a porous LLZO membrane with small pores was obtained.

[0181] The mechanical stability was tested by means of a three-point bending test on a Tinius Olsen 1ST electromechanical testing machine. The crosshead speed and strain rate applied to the sample were 1 μm s -1 and 1.56x10-3s -1 A high breaking force of 146 mN was measured, indicating that excellent mechanical stability was observed, ie the sintered substrate was free-standing.

[0182] To evaluate the electrochemical performance of the porous LLZO substrate in terms of lithium coating / stripping, a symmetric battery cell with a lithium metal anode and a lithium metal cathode and a single porous layer of the LLZO substrate was fabricated. The symmetric battery cell was fabricated by thermally evaporating 200 nm of metallic lithium (using a Covap thermal evaporator) and then cold isostatically pressing (using a PW100EH cold isostatic press) lithium foil at about 71 MPa on both sides of the porous LLZO layer for 5 minutes onto the porous layer. This enables lithium infiltration into the porous layer up to about 5 μm.

[0183] 0.1 mAh / cm per half cycle at room temperature without external pressure 2 The surface capacity is limited to 0.1 mA / cm 2 The constant current cycling experiment was carried out at a constant current density of . The results show Fig.17 A high cycling stability of the cell was observed for more than 625 h. In addition, the symmetric cell exhibited a stable overvoltage of ≈10 mV throughout the cycling measurement.

[0184] Reference numerals

[0185] 1. Inorganic substrate

[0186] 2. The first thermally conductive substrate

[0187] 3. Second thermally conductive substrate

[0188] 4. The third thermal conductive substrate

[0189] 5. The fourth thermal conductive substrate

[0190] 6. First conductor

[0191] 7. Second conductor

[0192] 8. Third conductor

[0193] 9. The fourth conductor

[0194] 10. Infrared (IR) light source

[0195] 60. Near end of first conductor

[0196] 61. Distal end of first conductor

[0197] 70. Proximal end of second conductor

[0198] 71. The far end of the second conductor

[0199] 81. Third conductor (first part)

[0200] 82. Third conductor (part 2)

[0201] 91. Fourth conductor (first part)

[0202] 92. The fourth conductor (part 2)

[0203] 100. Schematic sintering device

[0204] 101. Space between the first and second heat conductive substrates

[0205] 102. Device for inducing electric current

[0206] 102a. Device for inducing current

[0207] 102b. Device for inducing current

[0208] 110. Schematic sintering device

[0209] 120.Schematic sintering device

[0210] 130.Schematic sintering device

[0211] 140.Schematic sintering device

[0212] 150.Schematic sintering device

[0213] 200.Dense layer before sintering

[0214] 201.Porous layer before sintering

[0215] 202. Sintered dense layer

[0216] 203. Sintered porous layer

Claims

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

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

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

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

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

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

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

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

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

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

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

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

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

14. A method for obtaining a sintered inorganic substrate having a first porosity on a first surface and a second porosity different from the first porosity on a second surface opposite to the first surface, comprising providing an inorganic substrate (1) according to claim 12, and further wherein the third thermally conductive substrate (4) and the fourth thermally conductive substrate (5) are heated to different temperatures and / or for different durations, thereby heating the first surface and the second surface opposite to the first surface to different temperatures and / or for different durations, wherein the temperature is between 750°C and 1400°C, and the duration is between 1 second and 200 seconds.

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

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