Solid electrolytes with improved dendrite stability

Abstract

The present invention relates to a solid electrolyte comprising a lithium ion conducting material, in particular a glass-ceramic, with improved dendrite stability (stability against dendrite formation), and to the use and method of manufacturing the same.

Description

[Technical Field]

[0001] The present invention relates to a solid electrolyte comprising a lithium ion conducting material, in particular a glass-ceramic, with improved dendrite stability (stability against dendrite formation), and to the use and method of manufacturing the same. [Background technology]

[0002] All-solid-state batteries (ASSBs) are considered the future of energy storage technology and electric vehicles due to their high energy density and safety. The core of this innovation is the replacement of liquid electrolytes with solid, ion-conducting separators, which allows the use of Li metal as the anode material, especially if the separator itself is lithium-stable (e.g., lithium lanthanum zirconate (LLZO)). However, during charging, Li deposited on the anode can penetrate the separator and grow onto the cathode side. This phenomenon is called dendrite formation. Such dendrite formation can cause short circuits. The higher the current density during charging, the more likely undesirable dendrites will form. Therefore, it is necessary to develop separator materials that suppress dendrite formation even at relatively high current densities.

[0003] The solid, ionically conductive separator comprises an ionically conductive solid, also referred to as a “solid electrolyte,” “solid-state electrolyte,” or “solid electrolyte.” In the case of solid-state batteries, lithium ion conductive materials are used.

[0004] In the field of solid electrolytes, a distinction is made between organic solid electrolytes (usually based on polymers) and inorganic solid electrolytes. There are also composites with organic and inorganic parts. In such composites, LLZO particles are often embedded in a lithium-ion conductive polymer matrix. Inorganic solid electrolytes are used as separators in solid-state batteries, usually in sintered form. The lithium-ion conductive material is crushed and subsequently pressed into a compact, which is sintered at a specific sintering temperature. Alternatively, the crushed lithium-ion conductive material can also be mixed with a binder and a solvent and processed in a "tape casting" process to form a green film. This green film is subsequently sintered, thus producing the separator. Sintering leads to densification. For separator applications, a relative density of at least 90% is targeted. This is due, among other things, to the fact that dendrite growth mainly occurs within the cavities at the grain boundaries of the sintered body.

[0005] Known inorganic solid electrolytes require relatively high sintering temperatures to achieve sufficient densification. However, these high sintering temperatures lead to excessive grain growth. However, large grain sizes are also associated with poor dendritic stability. Therefore, the very measures that are supposed to improve dendritic stability actually lead to poor dendritic stability. Summary of the Invention [Problem to be solved by the invention]

[0006] It is therefore an object of the present invention to overcome the drawbacks of the prior art and to provide a material with improved dendritic stability. This object is achieved by the subject matter of the claims. [Means for solving the problem]

[0007] Solid electrolytes with high relative density and small grain size can enhance dendrite stability. However, high density and small grain size in sintered solid electrolytes are contradictory in that they are determined by temperature and time but behave inversely. High temperature / long time leads to high density but also promotes grain growth, while low temperature / short time prevents grain growth but does not achieve sufficiently high density.

[0008] It would therefore be advantageous to have available a material that can be sintered to a high relative density at a relatively low temperature in a relatively short time. Furthermore, the sintered material should provide low interfacial resistance at the grain boundaries. The present invention provides such a material.

[0009] The lithium ion conducting material can be especially glass ceramic.Glass ceramic in the sense of the present invention is a material that is produced by starting from a homogeneous melt of components, cooling and spontaneous crystallization, or by cooling and subsequent controlled ceramicization process.Before, during or after cooling, a forming step or a crushing step can be carried out.

[0010] According to one aspect of the present disclosure, the present invention relates to a solid electrolyte comprising or consisting of a lithium ion conducting material, the lithium ion conducting material comprising a crystalline phase and an amorphous phase, the crystalline phase comprising a predominant crystalline phase, the predominant crystalline phase having a proportion of crystalline phase of at least 50% by mass, the solid electrolyte having a relative density of at least 90%, and the solid electrolyte having a microstructure such that less than 10% of the cross-sectional area of ​​the solid electrolyte is composed of grains having a diameter greater than 30 μm.

[0011] Such solid electrolytes can be produced from lithium-ion conducting materials in a simple and short sintering step at relatively low temperatures, and the amorphous phase has a softening point in the range of 850°C to 1100°C. The amorphous phase may also be referred to as a residual glassy phase.

[0012] The relative density of the solid electrolyte can be determined in particular as follows: The density of the powder is determined using the He pycnometer method. "Powder density" refers to the density of the material, not the bulk density. In the case of LLZO, the powder density is, for example, about 5 g / cm 3 After sintering, the density of the solid electrolyte is determined by gravitational and geometrical measurements (density = mass per volume). The relative density is the quotient of the density of the solid electrolyte and the density of the powder.

[0013] The diameter of a grain in a cross section of the solid electrolyte is determined as the maximum Ferret diameter (Ferret-Max), i.e., the maximum distance between two parallel tangents to the grain contour in the cross section of the solid electrolyte being examined.

[0014] The softening point of the amorphous phase associated with a lithium-ion conductive material is not the same as the softening point of the corresponding glass itself (without a crystalline phase). Therefore, the softening point of the amorphous phase associated with the lithium-ion conductive material must be determined. This is done, according to the present invention, using dynamic differential scanning calorimetry (DSC). The lithium-ion conductive material is subjected to the following DSC program: 20 to 100 mg of the lithium-ion conductive material is placed in a platinum DSC crucible. The measurement is carried out under argon at a heating rate of 10 K / min from room temperature to at least 1100°C. The softening point of the amorphous phase associated with the lithium-ion conductive material is also referred to as the "densification point" in this disclosure.

[0015] An endothermic DSC signal in the range of 850°C to 1100°C indicates softening of the amorphous phase of the lithium-ion conductive material, especially the residual glass phase in glass ceramics. If this occurs at a sufficiently low temperature (850°C to 1100°C), the material can be densified, and any voids that may exist can be filled with the residual glass phase. If the material does not exhibit such a DSC signal or only exhibits it at temperatures above 1100°C, a correspondingly high sintering temperature is required for densification, which also involves undesirable grain growth and thus reduced dendrite stability. The DSC signal can be determined, in particular, using heat flow DSC, for example, using a DSC 404 F1 Pegasus® measuring device from NETZSCH-Geraetebau GmbH. The signal is a positive (endothermic) or negative (exothermic) deviation from the baseline.

[0016] To ensure sufficient confidence that the amorphous phase has a densification point in the range of 850°C to 1100°C, the endothermic DSC signal in the range of 850°C to 1100°C should preferably have a value of at least 10 J / g. If the endothermic DSC signal in the range of 850°C to 1100°C has a magnitude of at least 10 J / g, the amorphous phase has a densification point in the range of 850°C to 1100°C.

[0017] Therefore, it is particularly important that the amorphous phase has a densification point in the range of 850°C to 1100°C. If the densification point is too low, the amorphous phase will become too liquid at the sintering temperature and will "flow" out of the compact before the sintering activity begins. In contrast, if the densification point is too high, too high a sintering temperature will be required for densification, which will result in undesirable grain growth and a corresponding decrease in dendritic stability. Preferably, the densification point of the amorphous phase is in the range of 900°C to 1050°C.

[0018] Furthermore, it is important that the proportion of amorphous phase in the lithium ion conductive material is sufficiently large to ensure sufficient densification during softening of the amorphous phase. Therefore, the proportion of amorphous phase in the lithium ion conductive material is preferably at least 0.1% by mass, more preferably at least 0.3% by mass. The proportion of amorphous phase is preferably at most 5.0% by mass. If the proportion of amorphous phase is too high, the Li-ion conductivity decreases. The proportion of amorphous phase in the lithium ion conductive material can be, for example, in the range of 0.1 to 5.0% by mass, or 0.3 to 5.0% by mass.

[0019] The amorphous phase preferably comprises LiO and at least one glass former selected from AlO, SiO, P, O, B, O, and combinations of two or more thereof. Preferably, the amorphous phase consists of LiO and at least one glass former selected from AlO, SiO, P, O, B, O, and combinations of two or more thereof. LiO is required for Li-ion conductivity. The glass former stabilizes the amorphous phase. BO is the least preferred of the glass formers. Therefore, B is preferably absent or present only in small proportions. SiO and P are preferred over B. The most preferred glass former is AlO. AlO can be the only glass former in the lithium-ion conductive material. However, one or more additional glass formers, particularly SiO and / or P, O, may be present.

[0020] Since the glass former is present in the amorphous phase but not in the crystalline phase, the glass former content of the amorphous phase (relative to the total mass of the lithium ion conductive material) corresponds to the glass former content of the lithium ion conductive material (relative to the total mass of the lithium ion conductive material). The exception here is that Al2O3 may also be present in the crystalline phase when the crystalline phase has a garnet structure, particularly lithium lanthanum zirconate (LLZO). However, the solubility of Al2O3 in the crystalline phase with a garnet structure is limited. This is due to the fact that, depending somewhat on the composition and manufacturing process, there is approximately 0.1 mole of Al2O3 per formula unit of LLZO, i.e., Li 6.4 Al 0.2 La3Zr 12 O 12 This is a doped version of this garnet, containing one or more divalent cations M II , one or more trivalent cations M III , one or more tetravalent cations M IV , and one or more pentavalent cations M V This also applies to those doped with AlO. Thus, when calculating the amorphous phase from the AlO composition, amounts up to 0.1 moles of AlO per formula unit of garnet are counted in the crystalline phase. Excess AlO, i.e., amounts greater than 0.1 moles of AlO per formula unit of garnet, are counted in the amorphous phase.

[0021] The amorphous phase can be produced by a melt-based manufacturing process, where, in addition to the crystalline phase (especially crystalline LLZO), an amorphous phase containing, especially consisting of, excess LiO and glass formers is also formed during solidification. Due to their small ionic radius, the glass formers do not "fit" into the crystalline structure, especially that of LLZO. Furthermore, the amorphous phase can be produced separately, for example, via a melting process, and then added to and mixed with the crystalline phase. This can be achieved, for example, by grinding the crystalline and amorphous phases. Other manufacturing and mixing processes are also possible.

[0022] Enhanced dendrite stability can be achieved by combining a lithium-ion conducting crystalline phase, such as Li-stable LLZO (especially Ta- or Al-doped), with an amorphous phase, especially at the grain boundaries of the sintered separator. The amorphous phase may contain, for example, LiO (for lithium-ion conductivity) and SiO (as a glass former), and optionally one or more additional glass formers, especially selected from the group consisting of AlO, PO, BO, and combinations thereof.

[0023] The proportion of crystalline and amorphous phases in a lithium-ion conducting material is determined, inter alia, based on the composition of the lithium-ion conducting material. For this purpose, the formula of the crystalline phase is taken into account and the composition is converted from mass % to atomic %. Then, the elements that form the crystalline phase according to the formula are substituted into this (the same applies to multiple crystalline phases). Excess Li, O, and glass formers are included in the amorphous phase.

[0024] The procedure for identifying the proportions of crystalline and amorphous phases is simplified by normalizing the composition in atomic percent to one of the stoichiometric coefficients from the formula of the crystalline phase. This is hereafter referred to as the formula of LLZO, Li 7-3x+y-z Al x M y II M 3-y III M 2-z IV M z V O 12 In the example shown below, the composition in atomic percent is first determined as M II +M III = 3 (or alternatively M IV +M V= 2) to obtain the composition in pfu (parts per formula unit). This composition is divided into crystal-forming components and components that are not incorporated into the stoichiometric crystal, i.e., excess Li and O as well as Si, P, B, and Al (Al in amounts up to 0.2 pfu is considered a crystal-forming component due to its limited solubility in the garnet structure. If more Al is contained, the difference from 0.2 pfu is counted as amorphous phase). Subsequently, the amorphous phase in pfu is again converted to mass % of the resulting oxide using the respective atomic masses. The mass fraction of the amorphous phase in a lithium ion conducting material is the sum of the mass fractions in mass % of the oxides in the amorphous phase (relative to the total mass of the lithium ion conducting material).

[0025] The composition of the amorphous phase can be selected to avoid undesirable interactions with crystals, particularly LLZO crystals (e.g., a transition from LLZO to the tetragonal modification, which has poor conductivity). For example, if the LiO content of the amorphous phase is too high, the cubic modification may transition to the tetragonal modification of LLZO, which has poor conductivity. The LiO content of the amorphous phase can be limited to, for example, at most 5.00 wt%, at most 4.50 wt%, at most 4.00 wt%, or at most 3.50 wt%, based on the total weight of the lithium ion conductive material. The LiO content of the amorphous phase can be, for example, at least 0.05 wt%, at least 0.20 wt%, at least 0.40 wt%, or at least 0.60 wt%, based on the total weight of the lithium ion conductive material. The Li2O content of the amorphous phase can be in the range of, for example, 0.05 to 5.00 mass%, 0.20 to 4.50 mass%, 0.40 to 4.00 mass%, or 0.60 to 3.50 mass%, relative to the total mass of the lithium ion conductive material.

[0026] The proportion of Li2O in the lithium ion conductive material of the present invention can be, for example, at least 10.0 mass%, at least 10.5 mass%, or at least 11.0 mass%. The proportion of Li2O in the lithium ion conductive material of the present invention can be, for example, up to 15.0 mass%, up to 14.5 mass%, or up to 14.0 mass%. The proportion of Li2O in the lithium ion conductive material of the present invention can be, for example, in the range of 10.0 to 15.0 mass%, 10.5 to 14.5 mass%, or 11.0 to 14.0 mass%.

[0027] The total proportion of ZrO2 and HfO2 in the lithium ion conductive material of the present invention can be, for example, at least 17 mass%, at least 18 mass%, or at least 19 mass%. The total proportion of ZrO2 and HfO2 in the lithium ion conductive material of the present invention can be, for example, up to 35 mass%, up to 33 mass%, or up to 31 mass%. The total proportion of ZrO2 and HfO2 in the lithium ion conductive material of the present invention can be, for example, in the range of 17 to 35 mass%, 18 to 33 mass%, or 19 to 31 mass%.

[0028] The total proportion of Ta2O5, Nb2O5, and Al2O3 in the lithium ion conductive material of the present invention can be, for example, at least 0.5 mass%, at least 0.75 mass%, or at least 1 mass%. The total proportion of Ta2O5, Nb2O5, and Al2O3 in the lithium ion conductive material of the present invention can be, for example, up to 15 mass%, up to 13.5 mass%, or up to 12 mass%. The total proportion of Ta2O5, Nb2O5, and Al2O3 in the lithium ion conductive material of the present invention can be, for example, in the range of 0.5 to 15 mass%, 0.75 to 13.5 mass%, or 1 to 12 mass%.

[0029] SiO2 has been found to be an advantageous component for obtaining the desired amorphous phase. In particular, the mass fraction of SiO2 in the lithium ion conductive material is greater than the mass fraction of B2O3 in the lithium ion conductive material. Preferably, the ratio of the mass fraction of B2O3 in the lithium ion conductive material to the mass fraction of SiO2 in the lithium ion conductive material is at most 0.9, more preferably at most 0.75, even more preferably at most 0.5, even more preferably at most 0.25, even more preferably at most 0.1, even more preferably at most 0.05, even more preferably at most 0.01.

[0030] The total mass proportion of SiO2 and B2O3 is preferably at least 0.1% by weight, more preferably at least 0.2% by weight, more preferably at least 0.3% by weight, more preferably at least 0.4% by weight, more preferably at least 0.5% by weight, more preferably at least 0.6% by weight, and even more preferably at least 0.7% by weight, based on the total mass of the lithium-ion conductive material. The total mass proportion of SiO2 and B2O3 is, for example, at most 5.0% by weight, in particular at most 4.5% by weight, at most 4.0% by weight, at most 3.5% by weight, at most 3.0% by weight, at most 2.5% by weight, or at most 2.0% by weight, based on the total mass of the lithium-ion conductive material. The total mass proportion of SiO2 and B2O3 is, for example, 0.1 to 5.0 mass%, in particular 0.2 to 4.5 mass%, 0.3 to 4.0 mass%, 0.4 to 3.5 mass%, 0.5 to 3.0 mass%, 0.6 to 2.5 mass%, or 0.7 to 2.0 mass%, based on the total mass of the lithium ion conductive material.

[0031] The mass proportion of SiO2 is preferably at least 0.1% by weight, more preferably at least 0.2% by weight, more preferably at least 0.3% by weight, more preferably at least 0.4% by weight, more preferably at least 0.5% by weight, more preferably at least 0.6% by weight, and even more preferably at least 0.7% by weight, based on the total mass of the lithium-ion conducting material. The mass proportion of SiO2 is, for example, at most 5.0% by weight, in particular at most 4.5% by weight, at most 4.0% by weight, at most 3.5% by weight, at most 3.0% by weight, at most 2.5% by weight, or at most 2.0% by weight, based on the total mass of the lithium-ion conducting material. The mass proportion of SiO2 is, for example, 0.1 to 5.0% by mass, in particular 0.2 to 4.5% by mass, 0.3 to 4.0% by mass, in particular 0.4 to 3.5% by mass, in particular 0.5 to 3.0% by mass, 0.6 to 2.5% by mass or in particular 0.7 to 2.0% by mass, based on the total mass of the lithium ion conductive material.

[0032] As mentioned above, SiO2 is considered to be a glass former of the amorphous phase. Thus, if a lithium ion conductive material contains, for example, 0.5% by weight of SiO2, this 0.5% by weight is considered to be an amorphous phase. Similarly, as mentioned above, excess Li2O is considered to be an amorphous phase. The proportion of the amorphous phase in a lithium ion conductive material corresponds in particular to the sum of the proportion of Li2O in the amorphous phase (relative to the total weight of the lithium ion conductive material) and the proportion of at least one glass former in the amorphous phase (relative to the total weight of the lithium ion conductive material).

[0033] The amorphous phase can be, for example, 2.0 wt% Li2O (based on the total mass of the lithium-ion-containing material) and 0.5 wt% SiO2 (based on the total mass of the lithium-ion-containing material). In this case, the proportion of the amorphous phase in the lithium-ion conductive material is 2.5 wt% (2.0 wt% Li2O + 0.5 wt% SiO2). The mass proportion of SiO2 relative to the total mass of the amorphous phase is 20 wt% (0.5 wt% SiO2 divided by 2.5 wt% of the total amorphous phase). Conversely, the mass proportion of Li2O relative to the total mass of the amorphous phase is 80 wt%.

[0034] Preferably, the mass proportion of SiO2 relative to the total mass of the amorphous phase is at least 1.0 mass%, more preferably at least 2.0 mass%, even more preferably at least 5.0 mass%, even more preferably at least 7.5 mass%, even more preferably at least 10.0 mass%, and even more preferably at least 12.5 mass%. SiO2 is advantageous for stabilizing the amorphous phase. However, particularly for high Li-ion conductivity, it is advantageous to select an SiO2 proportion that is not too high. Preferably, the mass proportion of SiO2 relative to the total mass of the amorphous phase is at most 60.0 mass%, more preferably at most 55.0 mass%, even more preferably at most 50.0 mass%, even more preferably at most 45.0 mass%, even more preferably at most 40.0 mass%, and even more preferably at most 35 mass%. Preferably, the mass proportion of SiO2 relative to the total mass of the amorphous phase is in the range of 1.0 to 60.0 mass%, more preferably 2.0 to 55.0 mass%, even more preferably 5.0 to 50.0 mass%, even more preferably 7.5 to 45.0 mass%, even more preferably 10.0 to 40.0 mass%, and even more preferably 12.5 to 35.0 mass%.

[0035] Preferably, the mass proportion of Li2O relative to the total mass of the amorphous phase is at least 40.0% by mass, more preferably at least 45.0% by mass, even more preferably at least 50.0% by mass, even more preferably at least 55.0% by mass, even more preferably at least 60.0% by mass, and even more preferably at least 65.0% by mass. Li2O is advantageous for lithium ion conductivity. However, for a particularly stable amorphous phase, it is advantageous to select a Li2O proportion that is not too high. Preferably, the mass proportion of Li2O relative to the total mass of the amorphous phase is at most 99.0% by mass, more preferably at most 98.0% by mass, even more preferably at most 95.0% by mass, even more preferably at most 92.5% by mass, even more preferably at most 90.0% by mass, and even more preferably at most 87.5% by mass. The mass proportion of Li2O relative to the total mass of the amorphous phase is preferably in the range of 40.0 to 99.0 mass%, more preferably 45.0 to 99.0 mass%, even more preferably 50.0 to 95.0 mass%, even more preferably 55.0 to 92.5 mass%, even more preferably 60.0 to 90.0 mass%, and even more preferably 65.0 to 87.5 mass%.

[0036] Preferably, the lithium ion conductive material does not contain B2O3 or contains only a very small amount. B2O3 has been found to lead to an increase in interfacial resistance. Therefore, the lithium ion conductive material preferably has a B2O3 content of less than 0.4% by weight, in particular at most 0.3% by weight, at most 0.2% by weight, or at most 0.1% by weight. Particularly preferably, the lithium ion conductive material of the present invention is free of B2O3.

[0037] In this disclosure, when a material is described as being free of or not containing a certain component, it means that this component may only be present as an impurity. This means that they are not added in an essential amount. An insubstantial amount is, according to the present invention, an amount of up to 0.05% by weight, or up to 0.04% by weight.

[0038] The lithium ion conductive material comprises a crystalline phase and an amorphous phase. The crystalline phase may comprise a predominant crystalline phase. The predominant crystalline phase is the crystalline phase that has the highest proportion by mass in the crystalline phase of the lithium ion conductive material. The predominant crystalline phase particularly has a proportion of at least 50% by mass, for example, more than 50% by mass, at least 60% by mass, at least 70% by mass, at least 80% by mass, at least 90% by mass, at least 95% by mass, or even 100% by mass in the crystalline phase of the lithium ion conductive material. Thus, the crystalline phase of the lithium ion conductive material may consist of the predominant crystalline phase.

[0039] The primary crystalline phase may have, in particular, a garnet structure. However, the primary crystalline phase may also have a rock salt structure, a perovskite structure, an antiperovskite structure, or a NASICON structure. The primary crystalline phase may, for example, be in the cubic system. The primary crystalline phase may, for example, comprise or consist of lithium lanthanum zirconate (LLZO).

[0040] The main crystalline phase of the lithium ion conductive material is particularly a crystalline phase of the composition formula Li 7-3x+y-z Al x M y II M 3-y III M 2-z IV M z V O 12±δ wherein M II is one or more divalent cations, M III is one or more trivalent cations, M IV is one or more tetravalent cations, and M V contains one or more pentavalent cations, and x+z>0, y<1, and δ<0.5. Particularly preferably, M III contains one or more lanthanides and / or yttrium. Particularly preferably, M IV contains zirconium or hafnium. Particularly preferably, M V contains niobium or tantalum. Particularly preferably, M III contains one or more lanthanides and / or yttrium, and M IVcontains zirconium or hafnium, and M V includes niobium or tantalum.

[0041] The lithium ion conducting material comprises a crystalline phase and an amorphous phase. The crystalline phase may be present in the lithium ion conducting material, for example, in the form of crystallites separated by grain boundaries. The amorphous phase may be present, in particular, at the grain boundaries. The amorphous phase may, for example, have a density of at least 1.5 g / cm. 3 The density may be

[0042] The present invention relates to a solid electrolyte comprising or consisting of a lithium ion conducting material. The solid electrolyte is in particular a sintered compact. Preferably, the solid electrolyte is an inorganic solid electrolyte.

[0043] The solid electrolyte has a relative density of at least 90%, more preferably at least 91%, more preferably at least 92%, more preferably at least 93%, more preferably at least 94%, and even more preferably at least 95%. The relative density of the solid electrolyte can be, for example, up to 100%, particularly up to 99.9%, up to 99.5%, up to 99.0%, up to 98.5%, or up to 98.0%. The relative density of the solid electrolyte is preferably in the range of 90% to 100%, for example, 91% to 99.9%, 92% to 99.5%, 93% to 99.0%, 94% to 98.5%, or 95% to 98.0%.

[0044] The solid electrolyte has a microstructure such that less than 10% of the cross-sectional area of ​​the solid electrolyte is made up of grains having a diameter greater than 30 μm. Thus, the solid electrolyte has a fine-grained microstructure.

[0045] Preferably, up to 9%, more preferably up to 8%, more preferably up to 7%, even more preferably up to 6%, and even more preferably up to 5% of the cross-sectional area of ​​the solid electrolyte is made up of grains having a diameter greater than 30 μm. In some embodiments, at least 0.01%, e.g., at least 0.02%, at least 0.05%, at least 0.1%, at least 0.2%, at least 0.5%, or at least 1% of the cross-sectional area of ​​the solid electrolyte is made up of grains having a diameter greater than 30 μm. In some embodiments, 0.01% to <10%, e.g., 0.02% to 9%, 0.05% to 8%, 0.1% to 7%, 0.2% to 6%, 0.5% to 5%, or 1% to 5% of the cross-sectional area of ​​the solid electrolyte is made up of grains having a diameter greater than 30 μm.

[0046] Preferably, less than 10% of the cross-sectional area of ​​the solid electrolyte is made up of grains having a diameter of at least 25 μm, more preferably at least 20 μm, more preferably at least 15 μm, more preferably at least 10 μm.

[0047] Preferably, at least 90% of the cross-sectional area of ​​the solid electrolyte is made up of grains having a diameter of at most 30 μm, more preferably at most 25 μm, even more preferably at most 20 μm, even more preferably at most 15 μm, even more preferably at most 10 μm. Preferably, at least 90% of the cross-sectional area of ​​the solid electrolyte is made up of grains having a diameter in the range of 0.1 to 30 μm, for example, 0.1 to 25 μm, 0.2 to 20 μm, 0.2 to 15 μm, or 0.5 to 10 μm.

[0048] Preferably, at least 91%, more preferably at least 92%, more preferably at least 93%, even more preferably at least 94%, and even more preferably at least 95% of the cross-sectional area of ​​the solid electrolyte is made up of grains having a diameter of up to 30 μm. In some embodiments, up to 99.99%, e.g., up to 99.98%, up to 99.95%, up to 99.9%, up to 99.8%, up to 99.5%, or up to 99% of the cross-sectional area of ​​the solid electrolyte is made up of grains having a diameter of up to 30 μm. In some embodiments, 90% to 99.99%, e.g., 91% to 99.98%, 92% to 99.95%, 93% to 99.9%, 94% to 99.8%, 95% to 99.5%, or 95% to 99% of the cross-sectional area of ​​the solid electrolyte is made up of grains having a diameter of up to 30 μm.

[0049] The microstructure can be examined as follows: A sample of the sintered solid electrolyte with a diameter of 8.5 mm and a height of 1 mm is cut using a diamond glass cutter. The resulting fractured edge is examined using a scanning electron microscope (SEM). The resulting SEM image is a top view of the fractured edge. The fractured edge is also used to examine the cross-sectional area of ​​the solid electrolyte.

[0050] For scanning electron microscopes, in particular a ZEISS LEO1550 equipped with a field emission source can be used to generate corresponding SEM images using a secondary electron detector at an acceleration voltage of 10-20 keV, with a magnification of preferably 500x or 1000x.

[0051] The resulting SEM images are used to identify the microstructure of the sintered solid electrolyte. 2 For each sample, two images are evaluated.

[0052] According to the present disclosure, a distinction is made between fine-grained and coarse-grained microstructures. If relatively large grains (>30 μm) occupy at least 10% of the area of ​​the fractured edge, the microstructure is coarse-grained. Conversely, if relatively large grains (>30 μm) occupy less than 10% of the area of ​​the fractured edge, the microstructure is fine-grained. Unless otherwise specified, the terms "particle" and "grain" are used synonymously in the present disclosure. The "size" of a grain refers to the diameter of the grain, and thus the above-mentioned maximum Ferret diameter (Ferret-Max), i.e., the maximum distance between two parallel tangents at the grain's contour in the cross section of the examined solid electrolyte.

[0053] Correspondingly, in the case of a fine-grained microstructure, at least 90% of the area of ​​the fractured edge of the solid electrolyte consists of grains having a size of up to 30 μm, especially in the size range of 0.1 μm to 30 μm. Relatively large grains (>30 μm) occupy less than 10% of the area of ​​the fractured edge. In the case of a coarse-grained microstructure, at least 90% of the area of ​​the fractured edge of the solid electrolyte consists of grains having a size of up to 30 μm, especially in the size range of 0.1 μm to 30 μm. Relatively large grains (>30 μm) occupy at least 10% of the area of ​​the fractured edge.

[0054] The investigation is carried out on SEM images of the respective fracture edges. If relatively large particles (>30 μm) occupy at least 10% of the area of ​​the investigated image, this corresponds correspondingly to the area of ​​the fracture edges and therefore to the cross-sectional area of ​​the solid electrolyte. The microstructure is coarse-grained. If relatively large particles (>30 μm) occupy less than 10% of the area of ​​the investigated image, this corresponds correspondingly to the area of ​​the fracture edges and therefore to the cross-sectional area of ​​the solid electrolyte. The microstructure is fine-grained.

[0055] The present invention also relates to a method for producing a solid electrolyte, in particular the solid electrolyte of the invention.

[0056] The method comprises in particular the following steps: melting the starting materials; cooling the melt; The resulting material is crushed to a particle size d in the range of 0.1 μm to 10 μm.50 pulverizing the mixture to a powder having the formula: producing an intermediate product comprising or consisting of said powder; and sintering the intermediate product, the sintering temperature being in particular up to 1100°C. may include:

[0057] The solid electrolyte is obtained by sintering the intermediate product.

[0058] The production of the intermediate product may be, for example, by pressing a powder into a molding, so that the intermediate product can be a molding.

[0059] The method comprises in particular the following steps: melting the starting materials; cooling the melt; The resulting material is crushed to a particle size d in the range of 0.1 μm to 10 μm. 50 pulverizing the mixture to a powder having the formula: pressing the powder into a compact; and sintering the shaped body, the sintering temperature being in particular up to 1100°C. may include:

[0060] The compact is sintered to obtain a solid electrolyte.

[0061] The method of the present invention may be a tape casting method, in which the intermediate product is not produced by pressing the powder into a compact. Instead, the intermediate product is produced by converting the powder into a tape (also called a "film" or "green film"). The conversion of the powder into a tape preferably comprises the following steps: mixing the powder with one or more solvents and one or more organic binders, as well as any plasticizers and / or further additives, to produce a slurry (also called a "slip"); optionally degassing the slurry; Pouring the slurry onto a substrate to form a film (also called a "tape"); removing the solvent from the tape by evaporation; peeling the tape from the support; cutting the tape into individual pieces according to the intended use; optionally hot pressing or laminating one or more layers of tape at a temperature between 40 and 200°C; a step of removing the organic binder and any plasticizers and further additives present, as well as any solvent residues that may be present, from the tape or laminate by temperature treatment, the temperature being between 250°C and 500°C, Includes.

[0062] The solid electrolyte is then obtained by sintering the tape. Thus, in the case of the tape casting method, the intermediate product is a tape.

[0063] The starting materials (also referred to as raw materials) can be melted, for example, in a skull crucible (especially open-topped). Preferably, the raw materials are mixed and the resulting mixture is preheated. For this purpose, burner heating can be used, in particular. Preheating can achieve minimal conductivity. After the so-called coupling temperature is reached, further heating and homogenization of the melt can be achieved, in particular by high-frequency coupling via an induction coil. To improve the homogenization of the melt, it can be stirred, in particular with a water-cooled stirrer. After complete homogenization, for example, a sample can be removed directly from the melt (rapid cooling), while the remaining melt can be slowly cooled by switching off the high-frequency.

[0064] The materials produced in this way can be converted into lithium-ion conducting glass-ceramic materials, especially those with a predominant garnet-type crystal phase, by direct solidification from the melt or by rapid cooling following a temperature treatment (ceramization). If the sample taken directly from the melt shows spontaneous crystallization independent of cooling, the subsequent ceramization treatment can be omitted.

[0065] Preferably, the sintering temperature is at most 1090°C, more preferably at most 1080°C, more preferably at most 1070°C, more preferably at most 1060°C, more preferably at most 1050°C, more preferably at most 1040°C, more preferably at most 1030°C, more preferably at most 1020°C, more preferably at most 1010°C, more preferably at most 1000°C. The sintering temperature can be, for example, at least 850°C, at least 875°C, at least 900°C, at least 925°C, at least 950°C, or at least 975°C. Preferably, the sintering temperature is in the range of 850°C to 1100°C, for example, 850°C to 1090°C, 850°C to 1080°C, 850°C to 1070°C, 850°C to 1060°C, 850°C to 1050°C, 875°C to 1040°C, 900°C to 1030°C, 925°C to 1020°C, 950°C to 1010°C, or 975°C to 1000°C.

[0066] The sintering time is preferably up to 4 hours, more preferably up to 3 hours, even more preferably up to 2 hours, even more preferably up to 1 hour, even more preferably up to 45 minutes, even more preferably up to 40 minutes. The sintering time is preferably at least 5 minutes, even more preferably at least 10 minutes, even more preferably at least 15 minutes, even more preferably at least 20 minutes, even more preferably at least 25 minutes, even more preferably at least 30 minutes. The sintering time is preferably in the range of 5 minutes to 4 hours, for example, 10 minutes to 3 hours, 15 minutes to 2 hours, 20 minutes to 1 hour, 25 minutes to 45 minutes, or 30 minutes to 40 minutes.

[0067] The present invention also relates to a tape casting method for producing a solid electrolyte, in particular a solid electrolyte according to the invention, said method in particular comprising the following steps: melting the starting materials; cooling the melt; The resulting material is crushed to a particle size d in the range of 0.1 μm to 10 μm. 50 pulverizing the mixture to a powder having the formula: mixing said powder with one or more solvents and one or more organic binders, as well as optional plasticizers and / or further additives (e.g., dispersing and / or rheological additives) to produce a slurry (also called a "slip"); optionally degassing the slurry; Pouring the slurry onto a substrate to form a film (also called a "tape"); removing the solvent from the tape by evaporation; peeling the tape from the support; cutting the tape into individual pieces according to the intended use; optionally hot pressing or laminating one or more layers of tape at a temperature between 40 and 200°C; a step of removing the organic binder, and optionally any plasticizers and further additives present, and any solvent residues present, from the tape or laminate by temperature treatment, the temperature being between 250°C and 500°C, Sintering the tape, wherein the sintering temperature is up to 1100°C. may include:

[0068] The present invention also relates to the use of the solid electrolyte of the present invention in a solid-state lithium-ion battery, in particular in or as a separator. The lithium-ion conducting material can also be used in the anode and / or cathode, in particular after co-sintering with an electrode material.

[0069] The solid electrolyte, especially when sintered alone or together with other battery materials, can be used in rechargeable lithium-ion batteries, especially for inorganic ceramic electrolytes in solid-state lithium-ion batteries (ASSB). On the one hand, it can be used as a separator, i.e., inserted between the electrodes, where it protects the electrodes from undesired short circuits and thereby ensures the functionality of the entire system. The separator according to the invention is particularly characterized by improved dendrite stability, which allows charging at high current densities without short circuits (fast charging). On the other hand, it can be co-sintered with the electrode material, where the solid electrolyte transports the associated charge carriers (lithium ions) to or from the electrode material and the conductive electrode, depending on whether the battery is being discharged or charged.

[0070] Unless otherwise indicated, the terms "electrolyte" and "solid electrolyte" are used interchangeably in this disclosure.

[0071] The present invention also relates to a separator comprising or consisting of the solid electrolyte of the present invention.

[0072] The present invention also relates to a solid-state lithium-ion battery containing the solid electrolyte of the present invention. [Brief explanation of the drawings]

[0073] [Figure 1] Figure 1 shows the dependence of the magnitude of the endothermic DSC signal on relative density in the range of 850°C to 1100°C when sintered at a sintering temperature of 1000°C for 30 minutes. The magnitude of the endothermic DSC signal is shown on the x-axis in J / g. A measurement value of 0 J / g indicates that no endothermic DSC signal was observed in the range of 850°C to 1100°C. The y-axis shows relative density in percent. [Example]

[0074] The examples described below relate to three LLZO glass ceramics according to the invention (Examples A, B and C) and to an LLZO glass ceramic not according to the invention (Comparative Example V1).

[0075] 1. Fabrication of LLZO glass-ceramics The raw materials were mixed according to composition and charged into an open-top skull crucible. The mixture first had to be preheated to achieve a certain minimum conductivity. For this purpose, burner heating was used. After reaching the coupling temperature, further heating and homogenization of the melt was achieved by high-frequency coupling via an induction coil. To improve homogenization of the melt, it was stirred with a water-cooled stirrer. After complete homogenization, samples were taken directly from the melt (rapid cooling), while the remaining melt was cooled slowly by shutting off the high-frequency.

[0076] Materials produced in this way can be converted into glass-ceramic materials with a predominant garnet-type crystalline phase by direct solidification from the melt or by rapid cooling following a temperature treatment (ceramization). Samples taken directly from the melt showed spontaneous crystallization independent of cooling, allowing the subsequent ceramization treatment to be omitted.

[0077] 2. Densification point of amorphous phase The samples of Examples A, B, and C, as well as Comparative Example V1, were each investigated using dynamic differential scanning calorimetry (DSC). The experiments were carried out as follows: Samples (20-100 mg) of the LLZO glass ceramic obtained in 1 above were placed in platinum DSC crucibles. DSC measurements were carried out under argon at a heating rate of 10 K / min from room temperature to at least 1100°C. The DSC signal was determined using heat flow DSC, using a measuring device DSC 404 F1 Pegasus® from NETZSCH-Geraetebau GmbH.

[0078] In the case of both Examples A, B and C according to the invention, an endothermic signal of more than 10 J / g was detected in the range of 850°C to 1100°C, respectively. In contrast, in the case of Comparative Example V1, no endothermic signal of a corresponding magnitude was observed. The results are summarized in the table below.

[0079] [Table 1]

[0080] Therefore, Examples A, B and C according to the present invention are clearly different from Comparative Example V1 in terms of the occurrence of endothermic signals in the range of 850°C to 1100°C. The amorphous phases of Examples A, B and C have densification points in the range of 850°C to 1100°C. In contrast, the amorphous phase of Comparative Example V1 does not have a densification point in the range of 850°C to 1100°C.

[0081] 3. Microstructure and relative density of sintered solid electrolytes The LLZO glass ceramic samples of Examples A, B and C and Comparative Example V1 were 50 The powder was crushed to a size of 1 μm and then pressed into a compact, which was sintered at a sintering temperature of 1000° C. for 30 minutes to form a solid electrolyte.

[0082] For comparison, the LLZO glass ceramic samples of Example A and Comparative Example V1 were 50 The powder was crushed to a size of 1 μm and subsequently pressed into a compact, which was then sintered at a sintering temperature of 1130° C. for 30 minutes to form a solid electrolyte.

[0083] For comparison, a sample of the LLZO glass ceramic of Example B was 50 The powder was crushed to a size of 1 μm and then pressed into a compact, which was sintered at a sintering temperature of 1070° C. for 30 minutes to form a solid electrolyte.

[0084] a) Microstructure For all four LLZO glass ceramics (A, B, C, V1), a fine-grained microstructure was obtained for the sintered solid electrolyte at a sintering temperature of 1000°C. In contrast, at a sintering temperature of 1130°C, the microstructure was not fine-grained in the case of Comparative Example V1 and Example A. Large domains appeared. For Example B, the microstructure was fine-grained at sintering temperatures of 1000°C and 1070°C.

[0085] The microstructure was investigated as follows: LLZO glass ceramic samples with a diameter of 8.5 mm and a height of 1 mm were cut using a diamond glass cutter. The resulting fractured edges were examined using a scanning electron microscope (SEM). The resulting SEM images are therefore top views of the fractured edges. The fractured edges were also used to examine the cross-sectional area of ​​the solid electrolyte.

[0086] For scanning electron microscopy, a ZEISS LEO1550 equipped with a field emission source was used to generate corresponding SEM images using a secondary electron detector at an accelerating voltage of 10–20 keV, with magnifications of 500x or 1000x.

[0087] The resulting SEM images were used to characterize the microstructure of the sintered solid electrolyte. 2 Images with an area of ​​0.01 mm were evaluated. For each sample, two images were evaluated.

[0088] Based on all four LLZO glass-ceramics (A, B, C, V1), here, a sintered solid electrolyte with a fine-grained microstructure was obtained at a sintering temperature of 1000 °C. Relatively large grains (>30 μm) occupied less than 10% of the area of ​​each examined image.

[0089] In contrast, at a sintering temperature of 1130° C., the microstructure was coarse-grained in both the case of Comparative Example V1 and Example A. Relatively large grains (>30 μm) occupied at least 10% of the area of ​​each examined image.

[0090] Thus, the desired fine grained microstructure was achieved at a sintering temperature of 1000°C but not at a sintering temperature of 1130°C for a sintering time of 30 minutes.

[0091] In the case of Example B, the desired fine grained microstructure was also achieved at sintering temperatures of 1000°C and 1070°C.

[0092] b) Relative density The sintered solid electrolyte was examined for its relative density, which was determined as follows: The density of the powder was determined using the He pycnometer method. After sintering, the density of the solid electrolyte was determined by weighing and geometric measurements (density = mass per volume). The relative density is the quotient of the density of the solid electrolyte and the density of the powder.

[0093] As expected, a high relative density of 96% (Example A) or 91% (Comparative Example V1) was found for the solid electrolytes according to Example A and Comparative Example V1 sintered at 1130° C. for 30 minutes. However, as mentioned above, the corresponding solid electrolytes did not have the desired fine-grained microstructure.

[0094] For the solid electrolytes sintered at 1000°C, significant differences in relative density were found depending on the appearance of the endothermic DSC signal in the range of 850°C to 1100°C.

[0095] The results are shown in Figure 1 and summarized in Table 2 below.

[0096] [Table 2]

[0097] It is shown that the relative density clearly depends on the presence of endothermic DSC. When such a signal is absent (Comparative Example V1), a relatively low relative density is obtained. In contrast, when the endothermic DSC signal appears and exceeds 10 J / g (Examples A, B, and C), a sintering temperature of only 1000°C and a sintering time of only 30 minutes can result in a sintered solid electrolyte with a high overall density of over 90%.

Claims

1. 1. A solid electrolyte comprising a lithium ion conducting material, the lithium ion conducting material comprising a crystalline phase and an amorphous phase, the crystalline phase comprising a predominant crystalline phase, the predominant crystalline phase having a proportion of crystalline phase of at least 50% by mass, the solid electrolyte having a relative density of at least 90%, and the solid electrolyte having a microstructure such that less than 10% of a cross-sectional area of ​​the solid electrolyte is composed of grains having a diameter greater than 30 μm.

2. 10. The solid electrolyte of claim 1, wherein the lithium ion conducting material is a glass ceramic.

3. 3. The solid electrolyte according to claim 1, wherein the main crystalline phase is in the cubic system and / or has a garnet structure.

4. 4. The solid electrolyte of claim 1, wherein the predominant crystalline phase comprises lithium lanthanum zirconate (LLZO).

5. The main crystalline phase has the composition formula Li 7-3x+y-z Al x M y II M 3-y III M 2-z IV M z V O 12±δ wherein M II is one or more divalent cations, M III is one or more trivalent cations, M IV is one or more tetravalent cations, and M V 5. The solid electrolyte of claim 1 , wherein x+z>0, y<1, and δ<0.

5.

6. The amorphous phase is Li 2 6. The solid electrolyte according to claim 1, wherein the lithium ion conductive material contains O and the proportion of the amorphous phase in the lithium ion conductive material is at least 0.1% by mass.

7. 7. The solid electrolyte of claim 1, wherein the amorphous phase has a densification point in the range of 850°C to 1100°C.

8. SiO relative to the total mass of the lithium ion conductive material 2 and B 2 O 3 The total mass ratio of SiO 2 The mass ratio of B 2 O 3 The solid electrolyte according to claim 1 , wherein the mass proportion of the cations ...

9. The lithium ion conductive material has a maximum proportion of 0.1% by weight of B 2 O 3 9. The solid electrolyte according to claim 1, wherein

10. The lithium ion conductive material has a proportion of at least 0.1% by weight of SiO 2 10. The solid electrolyte according to claim 1, wherein

11. SiO relative to the total mass of the amorphous phase 2 The solid electrolyte according to any one of claims 1 to 10, wherein the mass ratio of is in the range of 1.0 to 60.0 mass%.

12. 12. A method for producing a solid electrolyte according to any one of claims 1 to 11, comprising the steps of: - melting the starting materials; - cooling the melt; The resulting material is crushed to a particle size d in the range of 0.1 μm to 10 μm. 50 pulverizing the mixture to a powder having the formula: - producing an intermediate product comprising or consisting of said powder; and sintering the intermediate product, the sintering temperature being up to 1100°C; The method comprising:

13. 13. The method of claim 12, wherein the production of the intermediate product is by pressing a powder into a compact.

14. The intermediate product is produced by processing the powder into a tape, the processing of the powder into a tape comprising the steps of: mixing said powder with one or more solvents and one or more organic binders to produce a slurry; pouring the slurry onto a substrate to form a tape; removing the solvent from the tape by evaporation; peeling the tape from the support; removing the organic binder and any residues of the solvent from the tape by thermal treatment, the temperature being between 250°C and 500°C; 13. The method of claim 12, comprising:

15. 15. The method according to any one of claims 12 to 14, wherein the sintering time is up to 4 hours.

16. 12. Use of a solid electrolyte according to any one of claims 1 to 11 in a solid-state lithium-ion battery, in particular in or as a separator.

17. A separator comprising or consisting of the solid electrolyte according to any one of claims 1 to 11.

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