Solid electrolyte with improved dendritic stability

Abstract

The invention relates to a solid electrolyte having improved dendritic stability (stability against dendritic formation), comprising a lithium ion conductive material, in particular a glass ceramic, and to the use and to a method for the production thereof.

Description

Technical Field

[0001] The present invention relates to a solid electrolyte having improved dendrite stability (stability against dendrite formation, gegenüber der Bildung von Dendriten), its use and preparation method, and the solid electrolyte comprises a lithium ion conductive material, in particular a glass ceramic. Background Art

[0002] All-Solid-State Batteries (ASSBs) are considered the future development direction in the field of energy storage technology and electric mobility due to their high energy density and safety. The core of this innovation is to replace the liquid electrolyte with a solid ion conductive separator. If the separator itself is stable to lithium (for example, when using lithium lanthanum zirconium oxide (LLZO)), it also allows lithium metal to be used as the anode material. However, during charging, the lithium deposited on the anode may grow into the interior of the separator and even penetrate to the cathode side. This is the so-called dendrite formation. When such dendrites form, a short circuit occurs. The higher the current density during charging, the easier it is to form unnecessary dendrites. Therefore, it is necessary to develop a separator material that can suppress dendrite formation even at a relatively high current density.

[0003] The solid ion conductive separator contains an ion-conductive solid substance, also known as a "solid electrolyte", "solid-state electrolyte" or "solid-substance electrolyte". Regarding solid-state batteries, a lithium ion conductive material is used.

[0004] In the field of solid electrolytes, there is a distinction between organic solid electrolytes (usually based on polymers) and inorganic solid electrolytes. In addition, there are composite materials containing organic and inorganic components. In such composite materials, LLZO particles are usually embedded in a lithium ion conductive polymer matrix. Inorganic solid electrolytes are usually used as separators in solid-state batteries in a sintered form. The lithium ion conductive material is ground and then pressed into a compact, and sintered at a certain sintering temperature. Alternatively, the ground lithium ion conductive material can also be mixed with a binder and a solvent, and then processed into a green film by the "tap-casting process". Then, the green film is sintered to produce the separator in this way. Densification is achieved through sintering. To be used as a separator, a relative density of at least 90% is required. One of the reasons is that dendrite growth mainly occurs in the cavities of the sintered body grain boundaries.

[0005] ​Regarding existing inorganic solid electrolytes, relatively high sintering temperatures are required to achieve sufficient densification. However, these high sintering temperatures lead to excessive grain growth. Large grain sizes are also associated with poor dendrite stability. Measures that should be taken to improve dendrite stability are instead associated with a decrease in dendrite stability. Summary of the Invention

[0006] Accordingly, an object of the present invention is to overcome the disadvantages in the prior art and provide a material with improved dendrite stability. The above object is achieved by the subject matter of the claims.

[0007] Solid electrolytes with high relative density and low grain size can improve dendrite stability. However, there is a contradictory relationship between high density and low grain size in sintered solid electrolytes. Therefore, although both are determined by temperature and time, their trends are opposite: high temperature / long time leads to high density but also promotes grain growth; low temperature / short time inhibits grain growth but cannot achieve a sufficiently high density.

[0008] Therefore, it is advantageous to provide a material that can be sintered to a relatively high relative density even at relatively low temperatures and relatively short times. In addition, the sintered material should have low interfacial resistance at grain boundaries. The present invention provides such a material.

[0009] In particular, the lithium-ion conductive material can be a glass-ceramic. A glass-ceramic in the sense of the present invention refers to a material made from a homogeneous melt of starting components through cooling and spontaneous crystallization or through cooling and subsequent controllable ceramization processes. Forming steps or comminution operations can be carried out before, during, or after cooling.

[0010] According to one aspect of the present disclosure, the present invention relates to a solid electrolyte comprising or consisting of a lithium-ion conductive material, wherein the lithium-ion conductive material comprises a crystalline phase and an amorphous phase, wherein the crystalline phase comprises a main crystalline phase, wherein the content of the main crystalline phase in the crystalline phase is at least 50 wt.%, wherein the relative density of the solid electrolyte is at least 90%, and wherein the solid electrolyte has a microstructure such that less than 10% of the cross-sectional area of the solid electrolyte is occupied by grains with a diameter greater than 30 μm.

[0011] Such a solid electrolyte can be made from a lithium-ion conductive material with an amorphous phase softening point in the range of 850 °C to 1100 °C through a simple, short sintering step at a relatively low temperature. The amorphous phase can also be referred to as the residual glass phase.

[0012] Specifically, the relative density of the solid electrolyte can be determined as follows: The powder density is determined using a helium pycnometer. "Powder density" refers to the material density, not the bulk density. Taking LLZO as an example, its powder density is about 5 g / cm3 After sintering, the density of the solid electrolyte is determined by weighing and geometric measurement (density = mass / volume). The relative density is the quotient of the solid electrolyte density and the powder density.

[0013] The grain diameter in the cross-section of the solid electrolyte is determined as the maximum Feret diameter (Feret-Max), that is, the maximum distance between two parallel tangents of the grain contour in the measured cross-section of the solid electrolyte.

[0014] In the context of lithium-ion conductive materials, the softening point of the amorphous phase is not the same as that of the corresponding glass (without crystalline phase). Therefore, in the context of lithium-ion conductive materials, the softening point of the amorphous phase must be determined. According to the present invention, this process can be completed by differential scanning calorimetry (DSC). In this case, the lithium-ion conductive material undergoes the following DSC procedure: 20 mg to 100 mg of the lithium-ion conductive material is placed in a platinum DSC crucible; in an argon atmosphere, measurements are carried out at a heating rate of 10 K / min from room temperature to a temperature of at least 1100 °C. In the context of lithium-ion conductive materials, the softening point of the amorphous phase in the present disclosure is also referred to as the "densification point (Verdichtungspunkt)".

[0015] The DSC endothermic peak signal that appears in the range of 850 °C to 1100 °C indicates that the amorphous phase of the lithium-ion conductive material, especially the residual glass phase in the glass-ceramic, softens. If this occurs at a sufficiently low temperature (850 - 1100 °C), the material will densify, and any cavities will be filled with the residual glass phase. If such a DSC signal does not appear in the material or only appears when the temperature exceeds 1100 °C, the sintering temperature needs to be increased accordingly to achieve densification, but this will also lead to undesired grain growth, thereby reducing the stability of the dendrites. Specifically, the DSC signal can be determined using heat flow-DSC, for example, using DSC 404 F1 from GmbH measuring instrument. This signal is a positive deviation (endothermic) or negative deviation (exothermic) of the phase from the baseline.

[0016] In order to reliably confirm that the densification point of the amorphous phase is in the range of 850 °C to 1100 °C, preferably, the value of the DSC signal that is endothermic in the range of 850 °C to 1100 °C is at least 10 J / g. If the value of the DSC signal that is endothermic in the range of 850 °C to 1100 °C is at least 10 J / g, then the densification point of the amorphous phase is in the range of 850 °C to 1100 °C.

[0017] Therefore, it is particularly important that the densification point of the amorphous phase must be in the range of 850 °C to 1100 °C. If the densification point is too low, the amorphous phase has too high fluidity at the sintering temperature, so it "flows out" from the pressling before sintering starts. On the other hand, if the densification point is too high, the sintering temperature needs to be increased for densification, which is accompanied by undesired grain growth and correspondingly reduces the stability of the dendrites. Preferably, the densification point of the amorphous phase is in the range of 900 °C to 1050 °C.

[0018] Equally important is that the content of the amorphous phase in the lithium-ion conductive material is high enough to ensure sufficient densification when the amorphous phase softens. Therefore, the content of the amorphous phase in the lithium-ion conductive material is preferably at least 0.1 wt.%, more preferably at least 0.3 wt.%. The content of the amorphous phase is preferably at most 5.0 wt.%. If the content of the amorphous phase is too high, the lithium-ion conductivity will decrease. The content of the amorphous phase in the lithium-ion conductive material can be, for example, from 0.1 wt.% to 5.0 wt.%, or from 0.3 wt.% to 5.0 wt.%.

[0019] The amorphous phase preferably contains Li2O and at least one glass former selected from the group consisting of Al2O3, SiO2, P2O5, B2O3, and combinations of two or more thereof. Preferably, the amorphous phase consists of Li2O and at least one glass former selected from the group consisting of Al2O3, SiO2, P2O5, B2O3, and combinations of two or more thereof. Li2O is necessary for lithium-ion conduction. The glass former stabilizes the amorphous phase. As a glass former, B2O3 is the most unfavorable one. Therefore, preferably, B2O3 is absent or present only in a small amount. Compared with B2O3, SiO2 and P2O5 are more preferred. The most preferred glass former is Al2O3. Al2O3 can be the only glass former in the lithium-ion conductive material. However, one or more additional glass formers can also be provided, especially SiO2 and / or P2O5.

[0020] The glass former is present in the amorphous phase and not in the crystalline phase. Therefore, the content of the glass former in the amorphous phase (based on the total mass of the lithium-ion conductive material) corresponds to the content of the glass former in the lithium-ion conductive material (based on the total mass of the lithium-ion conductive material). An exception is Al2O3. If the crystalline phase has a garnet structure, especially lithium lanthanum zirconium oxide (LLZO), then Al2O3 may also be present in the crystalline phase. However, the solubility of Al2O3 in the crystalline phase with a garnet structure is limited, which slightly depends on the specific composition and preparation process. Per formula unit of LLZO (i.e., Li 6.4 Al 0.2 La3Zr 12 O 12)It can dissolve about 0.1 mol of Al2O3. This also applies to the doped variants of this garnet, i.e., doped with 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 . Thus, when calculating the amorphous phase based on the composition of Al2O3, at most 0.1 mol of Al2O3 per formula unit of garnet is attributed to the crystalline phase. The excess Al2O3, i.e., the part exceeding 0.1 mol of Al2O3 per formula unit of garnet, is attributed to the amorphous phase.

[0021] The amorphous phase can be generated during the preparation process by the melt, so that in addition to the crystalline phase, especially crystalline LLZO, an amorphous phase will be formed during the solidification process, especially an amorphous phase composed of excess Li2O and glass formers. The above-mentioned glass formers cannot "fit into" the crystal structure, especially the LLZO crystal structure, due to their small ionic radii. In addition, the amorphous phase can also be prepared separately, for example, by a melting process, and then added to and mixed with the crystalline phase. For example, this process can be achieved by grinding the crystalline phase and the amorphous phase. In addition, other preparation processes and mixing processes can also be considered.

[0022] By combining a lithium-ion conducting crystalline phase (such as lithium-stabilized LLZO, especially LLZO doped with Ta or Al) with an amorphous phase (especially in the grain boundaries of the sintered separator), the stability of dendrites can be improved. For example, the amorphous phase contains: Li2O (for lithium-ion conduction) and SiO2 (as a glass former); and optionally one or more additional glass formers, especially selected from the group consisting of: Al2O3, P2O5, B2O3 and combinations thereof.

[0023] Determine the content of the crystalline phase and the amorphous phase in the lithium-ion conducting material, especially based on the composition of the lithium-ion conducting material. For this purpose, use the molecular formula of the crystalline phase and convert the composition from wt.% to at.%. Then, according to the molecular formula, attribute the elements forming the crystalline phase to the crystalline phase (if there are multiple crystalline phases, handle them in the same way). The excess Li2O and glass formers are attributed to the amorphous phase.

[0024] If the composition of unit at.% is normalized to a certain stoichiometric factor in the molecular formula of the crystalline phase, the process of determining the content of the crystalline phase and the amorphous phase can be simplified. The following takes the LLZO molecular formula Li 7-3x+y-z- Al x My II M 3-y III M 2-z IV M zV O 12 is taken as an example for illustration: First, the component in at.% is normalized to M II +M III = 3 (or M IV +M V = 2), so as to obtain a component with the unit of pfu (Parts per Formula Unit). This component can be divided into the components forming the crystal and the components not incorporated into the stoichiometric crystal: excess Li2O and Si, P, B, Al (the Al content is at most 0.2 pfu, and because of its limited solubility in the garnet structure, it is classified as a crystal-forming component. If the Al content is more, the difference from 0.2 pfu will be attributed to the amorphous phase). Then, through their respective atomic masses, the component in pfu of the amorphous phase is converted back to the weight percentage (wt.%) of the contained oxides. The weight percentage of the amorphous phase in the lithium-ion conductive material is the sum of the weight percentages of the oxides in the amorphous phase (based on the total mass of the lithium-ion conductive material), with the unit of wt.%.

[0025] The composition of the amorphous phase can be selected such that it does not have an undesired interaction with the crystal (especially the LLZO crystal) (for example, transforming into a less conductive tetragonal modification of LLZO). For example, if the Li2O content in the amorphous phase is too high, the cubic modification of LLZO will transform into a less conductive tetragonal modification of LLZO. Based on the total mass of the lithium-ion conductive material, the Li2O content in 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 mass of the lithium-ion conductive material, the Li2O content in 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 mass of the lithium-ion conductive material, the Li2O content in the amorphous phase can be, for example, 0.05 wt.% to 5.00 wt.%, 0.20 wt.% to 4.50 wt.%, 0.40 wt.% to 4.00 wt.%, or 0.60 wt.% to 3.50 wt.%.

[0026] The content of Li2O in the lithium-ion conductive material of the present invention can be, for example, at least 10.0 wt.%, at least 10.5 wt.%, or at least 11.0 wt.%. The content of Li2O in the lithium-ion conductive material of the present invention can be, for example, at most 15.0 wt.%, at most 14.5 wt.%, or at most 14.0 wt.%. The content of Li2O in the lithium-ion conductive material of the present invention can be, for example, from 10.0 wt.% to 15.0 wt.%, from 10.5 wt.% to 14.5 wt.%, or from 11.0 wt.% to 14.0 wt.%.

[0027] The content of the sum of ZrO2 and HfO2 in the lithium-ion conductive material of the present invention can be, for example, at least 17 wt.%, at least 18 wt.%, or at least 19 wt.%. The content of the sum of ZrO2 and HfO2 in the lithium-ion conductive material of the present invention can be, for example, at most 35 wt.%, at most 33 wt.%, or at most 31 wt.%. The content of the sum of ZrO2 and HfO2 in the lithium-ion conductive material of the present invention can be, for example, from 17 wt.% to 35 wt.%, from 18 wt.% to 33 wt.%, or from 19 wt.% to 31 wt.%.

[0028] The content of the sum of Ta2O5, Nb2O5 and Al2O3 in the lithium-ion conductive material of the present invention can be, for example, at least 0.5 wt.%, at least 0.75 wt.%, or at least 1 wt.%. The content of the sum of Ta2O5, Nb2O5 and Al2O3 in the lithium-ion conductive material of the present invention can be, for example, at most 15 wt.%, at most 13.5 wt.%, or at most 12 wt.%. The content of the sum of Ta2O5, Nb2O5 and Al2O3 in the lithium-ion conductive material of the present invention can be, for example, from 0.5 wt.% to 15 wt.%, from 0.75 wt.% to 13.5 wt.%, or from 1 wt.% to 12 wt.%.

[0029] It has been confirmed that SiO2 is a favorable component for obtaining the desired amorphous phase. In particular, the weight percentage content of SiO2 in the lithium-ion conductive material is greater than the weight percentage content of B2O3 in the lithium-ion conductive material. Preferably, the ratio of the weight percentage content of B2O3 in the lithium-ion conductive material to the weight percentage content of SiO2 in the lithium-ion conductive material is at most 0.9, more preferably at most 0.75, more preferably at most 0.5, more preferably at most 0.25, more preferably at most 0.1, more preferably at most 0.05, more preferably at most 0.01.

[0030] Based on the total mass of the lithium ion conductive material, the sum of the weight percentages of SiO2 and B2O3 is preferably at least 0.1 wt.%, more preferably at least 0.2 wt.%, more preferably at least 0.3 wt.%, more preferably at least 0.4 wt.%, more preferably at least 0.5 wt.%, more preferably at least 0.6 wt.%, more preferably at least 0.7 wt.%. Based on the total mass of the lithium ion conductive material, the sum of the weight percentages of SiO2 and B2O3 is, for example, at most 5.0 wt.%, particularly at most 4.5 wt.%, at most 4.0 wt.%, at most 3.5 wt.%, at most 3.0 wt.%, at most 2.5 wt.%, or at most 2.0 wt.%. Based on the total mass of the lithium ion conductive material, the sum of the weight percentages of SiO2 and B2O3 is, for example, from 0.1 wt.% to 5.0 wt.%, particularly from 0.2 wt.% to 4.5 wt.%, from 0.3 wt.% to 4.0 wt.%, from 0.4 wt.% to 3.5 wt.%, from 0.5 wt.% to 3.0 wt.%, from 0.6 wt.% to 2.5 wt.%, or from 0.7 wt.% to 2.0 wt.%.

[0031] Based on the total mass of the lithium ion conductive material, the weight percentage of SiO2 is preferably at least 0.1 wt.%, more preferably at least 0.2 wt.%, more preferably at least 0.3 wt.%, more preferably at least 0.4 wt.%, more preferably at least 0.5 wt.%, more preferably at least 0.6 wt.%, more preferably at least 0.7 wt.%. Based on the total mass of the lithium ion conductive material, the weight percentage of SiO2 is, for example, at most 5.0 wt.%, particularly at most 4.5 wt.%, at most 4.0 wt.%, at most 3.5 wt.%, at most 3.0 wt.%, at most 2.5 wt.%, or at most 2.0 wt.%. Based on the total mass of the lithium ion conductive material, the weight percentage of SiO2 is, for example, from 0.1 wt.% to 5.0 wt.%, particularly from 0.2 wt.% to 4.5 wt.%, from 0.3 wt.% to 4.0 wt.%, from 0.4 wt.% to 3.5 wt.%, from 0.5 wt.% to 3.0 wt.%, from 0.6 wt.% to 2.5 wt.%, or from 0.7 wt.% to 2.0 wt.%.

[0032] As described above, SiO2 is classified as a glass former in the amorphous phase. For example, if the lithium ion conductive material contains 0.5 wt.% of SiO2, then this 0.5 wt.% is classified as the amorphous phase. As described above, the excess Li2O is classified as the amorphous phase. In particular, the content of the amorphous phase in the lithium ion conductive material corresponds to the sum of the content of Li2O in the amorphous phase (based on the total mass of the lithium ion conductive material) and the content of at least one glass former in the amorphous phase (based on the total mass of the lithium ion conductive material).

[0033] The amorphous phase may consist of, for example, 2.0 wt.% Li2O (based on the total mass of the lithium ion conductive material) and 0.5 wt.% SiO2 (based on the total mass of the lithium ion conductive material). In this case, the content of the amorphous phase in the lithium ion conductive material is 2.5 wt.% (2.0 wt.% Li2O + 0.5 wt.% SiO2). The weight percentage of SiO2 in the total weight of the amorphous phase is 20 wt.% (0.5 wt.% SiO2 ÷ 2.5 wt.% total amorphous phase). Conversely, the weight percentage of Li2O in the total weight of the amorphous phase is 80 wt.%.

[0034] Preferably, the weight percentage of SiO2 in the total weight of the amorphous phase is at least 1.0 wt.%, more preferably at least 2.0 wt.%, more preferably at least 5.0 wt.%, more preferably at least 7.5 wt.%, more preferably at least 10.0 wt.%, more preferably at least 12.5 wt.%. SiO2 is beneficial for stabilizing the amorphous phase. However, in order to obtain a particularly high lithium ion conductivity, it is not advisable to select too high a content of SiO2. Preferably, the weight percentage of SiO2 in the total weight of the amorphous phase is at most 60.0 wt.%, more preferably at most 55.0 wt.%, more preferably at most 50.0 wt.%, more preferably at most 45.0 wt.%, more preferably at most 40.0 wt.%, more preferably at most 35 wt.%. Preferably, the weight percentage of SiO2 in the total weight of the amorphous phase is from 1.0 to 60.0 wt.%, more preferably from 2.0 to 55.0 wt.%, more preferably from 5.0 to 50.0 wt.%, more preferably from 7.5 to 45.0 wt.%, more preferably from 10.0 to 40.0 wt.%, more preferably from 12.5 to 35.0 wt.%.

[0035] Preferably, the weight percentage of Li2O in the total weight of the amorphous phase is at least 40.0 wt.%, more preferably at least 45.0 wt.%, more preferably at least 50.0 wt.%, more preferably at least 55.0 wt.%, more preferably at least 60.0 wt.%, more preferably at least 65.0 wt.%. Li2O is beneficial for lithium ion conduction. However, in order to obtain a particularly stable amorphous phase, it is not advisable to select too high a content of Li2O. Preferably, the weight percentage of Li2O in the total weight of the amorphous phase is at most 99.0 wt.%, more preferably at most 98.0 wt.%, more preferably at most 95.0 wt.%, more preferably at most 92.5 wt.%, more preferably at most 90.0 wt.%, more preferably at most 87.5 wt.%. Preferably, the weight percentage of Li2O in the total weight of the amorphous phase is from 40.0 wt.% to 99.0 wt.%, more preferably from 45.0 wt.% to 99.0 wt.%, more preferably from 50.0 wt.% to 95.0 wt.%, more preferably from 55.0 wt.% to 92.5 wt.%, more preferably from 60.0 wt.% to 90.0 wt.%, more preferably from 65.0 wt.% to 87.5 wt.%.

[0036] Preferably, the lithium ion conductive material does not contain B2O3 or contains only a very small amount of B2O3. It has been confirmed that B2O3 will cause an increase in the interfacial resistance. Therefore, the content of B2O3 in the lithium ion conductive material is preferably less than 0.4 wt.%, especially at most 0.3 wt.%, at most 0.2 wt.% or at most 0.1 wt.%. Particularly preferably, the lithium ion conductive material of the present invention does not contain B2O3.

[0037] When the present disclosure states that the material has no certain component or does not contain a certain component, it means that the component may exist only as an impurity. This means that the component is not added in a necessary amount. According to the present invention, the non-necessary amount is at most 0.05 wt.% or at most 0.04 wt.%.

[0038] The lithium ion conductive material contains a crystalline phase and an amorphous phase. The crystalline phase may contain a main crystalline phase. The main crystalline phase refers to the crystalline phase with the highest weight percentage in the crystalline phase of the lithium ion conductive material. In particular, the content of the main crystalline phase in the crystalline phase of the lithium ion conductive material is at least 50 wt.% (for example, greater than 50 wt.%), at least 60 wt.%, at least 70 wt.%, at least 80 wt.%, at least 90 wt.%, at least 95 wt.%, or even 100%. The crystalline phase of the lithium ion conductive material can thus consist of the main crystalline phase.

[0039] In particular, the main crystal phase may have a garnet structure. However, the main crystal phase may also have a rock salt structure, a perovskite structure, an anti-perovskite structure, or a NASICON structure. The main crystal phase may be, for example, cubic. The main crystal phase may, for example, include lithium lanthanum zirconium oxide (LLZO) or consist of lithium lanthanum zirconium oxide (LLZO).

[0040] In particular, the main crystal phase in the crystalline phase of the lithium ion conductive material may have the molecular 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 , where M II contains one or more divalent cations, M III contains one or more trivalent cations, M IV contains one or more tetravalent cations, M V contains one or more pentavalent cations, where x + z > 0, y < 1, and δ < 0.5. Particularly preferably, M III contains one or more lanthanide elements and / or yttrium. Particularly preferably, M IV contains zirconium or hafnium. Particularly preferably, M V contains niobium or tantalum. Particularly preferably, M III includes one or more lanthanide elements and / or yttrium, M IV contains zirconium or hafnium, and M V contains niobium or tantalum.

[0041] The lithium ion conductive material contains a crystalline phase and an amorphous phase. The crystalline phase may exist, for example, in the form of microcrystals separated by grain boundaries in the lithium ion conductive material. In particular, the amorphous phase may exist on the grain boundaries. The density of the amorphous phase may be, for example, at least 1.5 g / cm 3 .

[0042] The present invention relates to a solid electrolyte containing or consisting of a lithium ion conductive material. In particular, the solid electrolyte is a sintered molded body. Preferably, the solid electrolyte is an inorganic solid electrolyte.

[0043] The relative density of the solid electrolyte is at least 90%, more preferably at least 91%, more preferably at least 92%, more preferably at least 93%, more preferably at least 94%, more preferably at least 95%. The relative density of the solid electrolyte can be, for example, at most 100%, in particular at most 99.9%, at most 99.5%, at most 99.0%, at most 98.5% or at most 98.0%. The relative density of the solid electrolyte is preferably from 90% to 100%, for example from 91% to 99.9%, from 92% to 99.5%, from 93% to 99.0%, from 94% to 98.5%, or from 95% to 98.0%.

[0044] The solid electrolyte has a microstructure in which less than 10% of the cross-sectional area of the solid electrolyte is occupied by grains having a diameter greater than 30 μm. The solid electrolyte has a fine-grained microstructure.

[0045] Preferably, at most 9%, more preferably at most 8%, more preferably at most 7%, more preferably at most 6%, more preferably at most 5% of the cross-sectional area of the solid electrolyte is occupied by grains having a diameter greater than 30 μm. In certain embodiments, at least 0.01%, for example 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 occupied by grains having a diameter greater than 30 μm. In certain embodiments, from 0.01% to less than 10%, for example from 0.02% to 9%, from 0.05% to 8%, from 0.1% to 7%, from 0.2% to 6%, from 0.5% to 5%, or from 1% to 5% of the cross-sectional area of the solid electrolyte is occupied by grains having a diameter greater than 30 μm.

[0046] Preferably, less than 10% of the cross-sectional area of the solid electrolyte is occupied by 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 occupied by grains having a diameter of at most 30 μm, more preferably at most 25 μm, more preferably at most 20 μm, more preferably at most 15 μm, more preferably at most 10 μm. Preferably, at least 90% of the cross-sectional area of the solid electrolyte is occupied by grains having a diameter from 0.1 μm to 30 μm, for example from 0.1 μm to 25 μm, from 0.2 μm to 20 μm, from 0.2 μm to 15 μm, or from 0.5 μm to 10 μm.

[0048] Preferably, at least 91%, more preferably at least 92%, more preferably at least 93%, more preferably at least 94%, more preferably at least 95% of the cross-sectional area of the solid electrolyte is occupied by grains having a diameter of at most 30 μm. In some embodiments, up to 99.99%, such as 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 occupied by grains having a diameter of at most 30 μm. In some embodiments, 90% to 99.99%, such as 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 occupied by grains having a diameter of at most 30 μm.

[0049] The microstructure analysis is as follows:

[0050] A sintered solid electrolyte sample with a diameter of 8.5 mm and a height of 1 mm was fractured using a diamond glass cutter. The resulting fracture edge was analyzed using a Scanning Electron Microscopy (SEM). The obtained SEM images represent a top view of the fracture edge. The fracture edge was then used to analyze the cross-section of the solid electrolyte.

[0051] Regarding the scanning electron microscope, in particular, a ZEISS LEO 1550 with a field emission source can be used to generate the corresponding SEM images with a secondary electron detector at an acceleration voltage of 10 - 20 keV, and the magnification is preferably 500 times or 1000 times.

[0052] The obtained SEM images can be used to determine the microstructure of the sintered solid electrolyte. For this purpose, images with an area of at least 0.04 mm 2 are evaluated. For each sample, 2 images are evaluated.

[0053] According to the present disclosure, the microstructure can be classified into a fine-grained structure and a coarse-grained structure. If the larger particles (>30 μm) account for at least 10% of the fracture edge area, the microstructure is a coarse-grained structure. If the larger particles (>30 μm) account for less than 10% of the fracture edge surface area, the microstructure is a fine-grained structure. In the present disclosure, the terms "particle" and "grain" are synonyms unless otherwise stated. The "size" of a particle refers to the grain diameter, that is, the above-mentioned maximum Feret diameter (Feret-Max), which is the maximum distance between two parallel tangents of the grain contour in the measured cross-sectional area of the solid electrolyte.

[0054] Regarding the fine-grained microstructure, at least 90% of the fracture edge area of the solid electrolyte is occupied by particles with a particle size of at most 30 μm, in particular with a particle size of 0.1 μm to 30 μm. Less than 10% of the fracture edge area is occupied by larger particles (>30 μm). Regarding the coarse-grained microstructure, less than 90% of the fracture edge area of the solid electrolyte is occupied by particles with a particle size of at most 30 μm, in particular with a particle size of 0.1 μm to 30 μm. At least 10% of the fracture edge area is occupied by larger particles (>30 μm).

[0055] The SEM images of the fracture edges are analyzed. If larger particles (>30 μm) account for at least 10% of the area of the analyzed image, this thus applies to both the fracture edge area and the cross-sectional area of the solid electrolyte. The microstructure is a coarse-grained structure. If larger particles (>30 μm) account for less than 10% of the area of the analyzed image, this thus applies to both the fracture edge area and the cross-sectional area of the solid electrolyte. The microstructure is a fine-grained structure.

[0056] The invention also relates to a method for preparing a solid electrolyte, in particular the solid electrolyte according to the invention.

[0057] In particular, the method may include the following steps: · Melting the starting materials; · Cooling the melt; · Grinding the obtained material into a powder with a particle size d 50 ranging from 0.1 μm to 10 μm; · Preparing an intermediate product comprising or consisting of the powder; and · Sintering the intermediate product, in particular at a sintering temperature of at most 1100 °C.

[0058] The solid electrolyte is obtained by sintering the intermediate product.

[0059] Preparing the intermediate product may be pressing the powder into a compact. The intermediate product can be the compact.

[0060] In particular, the method may include the following steps: · Melting the starting materials; · Cooling the melt; · Grinding the obtained material into a powder with a particle size d 50 ranging from 0.1 μm to 10 μm; · Pressing the powder into a compact; and · Sintering the compact, in particular at a sintering temperature of at most 1100 °C.

[0061] The solid electrolyte is obtained by sintering the compact.

[0062] The method of the present invention can also be the tape casting method. In this case, the intermediate product is not prepared by pressing the powder into a compact. Instead, the intermediate product is prepared by processing the powder into a tape (also known as "foil" or "green film"). Processing the powder into a tape preferably includes the following steps:

[0063] Mix the powder with one or more solvents, one or more organic binders, and optionally a plasticizer and / or other additives to form a slurry; · Optionally, degas the slurry; · Cast the slurry onto a carrier substrate to form a foil (also known as "tape"); · Remove the solvent from the tape by evaporation; · Peel the tape from the carrier substrate; · Optionally, cut the tape individually into a form suitable for the application; · Optionally, hot press or laminate one or more layers of tape at a temperature of 40°C to 200°C; · Remove the organic binder, optionally the plasticizer and other additives, and any remaining solvent from the tape or laminated tape by heat treatment, where the temperature is 250°C to 500°C.

[0064] Then, a solid electrolyte is obtained by sintering the tape. In the tape casting method, the intermediate product is the tape.

[0065] For example, the starting materials (also known as raw materials) can be melted in a cold crucible, especially an open-top crucible. Preferably, the raw materials are mixed, and the obtained mixture is preheated. For this purpose, heating can be carried out especially using a burner. The lowest conductivity can be achieved through preheating. After reaching the so-called coupling temperature, further heating and homogenization of the melt can be achieved by high-frequency coupling, especially through an induction coil. To improve the melt homogenization, stirring can be carried out, especially using a water-cooled stirrer. For example, after complete homogenization, a sample can be taken directly from the melt (rapid cooling), while the remaining melt can be slowly cooled by turning off the high frequency.

[0066] The material prepared in this way can be converted into a lithium-ion conductive material, especially a glass-ceramic material with a garnet-based main crystal phase, by direct solidification of the melt or by heat treatment (ceramicization) after quenching. If the sample directly extracted from the melt shows spontaneous crystallization regardless of cooling, the subsequent ceramicization treatment can be omitted.

[0067] The sintering temperature is preferably 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. The sintering temperature is preferably from 850 °C to 1100 °C, for example from 850 °C to 1090 °C, from 850 °C to 1080 °C, from 850 °C to 1070 °C, from 850 °C to 1060 °C, from 850 °C to 1050 °C, from 875 °C to 1040 °C, from 900 °C to 1030 °C, from 925 °C to 1020 °C, from 950 °C to 1010 °C, or from 975 °C to 1000 °C.

[0068] The sintering time is preferably at most 4 hours, more preferably at most 3 hours, more preferably at most 2 hours, more preferably at most 1 hour, more preferably at most 45 minutes, more preferably at most 40 minutes. The sintering time is preferably at least 5 minutes, more preferably at least 10 minutes, more preferably at least 15 minutes, more preferably at least 20 minutes, more preferably at least 25 minutes, more preferably at least 30 minutes. The sintering time is preferably from 5 minutes to 4 hours, for example from 10 minutes to 3 hours, from 15 minutes to 2 hours, from 20 minutes to 1 hour, from 25 minutes to 45 minutes, or from 30 minutes to 40 minutes.

[0069] The present invention also relates to a doctor blade forming method for preparing a solid electrolyte, in particular the solid electrolyte of the present invention. In particular, the method may include the following steps: · Melting the initial raw materials; · Cooling the melt; · Grinding the obtained material into a powder with a particle size d 50 of 0.1 μm to 10 μm; · Mixing the powder with one or more solvents, one or more organic binders and optionally a plasticizer and / or other additives (such as a dispersion additive and / or a rheological additive) to form a slurry; · Optionally, degassing the slurry; · Casting the slurry on a carrier substrate into a foil (also called a "green tape"); · Removing the solvent from the green tape by evaporation; · Peeling the green tape from the carrier substrate; · Optionally, cutting the green tape individually into a form suitable for application; · Optionally, hot pressing or laminating one or more layers of green tape at a temperature of 40 °C to 200 °C; · By heat treatment, remove the organic binder, optional plasticizer and other additives, as well as any remaining solvents from the tape or laminate, at a temperature of 250 °C to 500 °C; · Sinter the tape, where the sintering temperature is at most 1100 °C.

[0070] The present invention also relates to the use of the solid electrolyte described above in a solid-state lithium-ion battery, particularly for use as a separator or as part of a separator. The lithium-ion conductive material can also be used in the anode and / or cathode, particularly after co-sintering with the electrode material.

[0071] In particular, the solid electrolyte can be used alone or co-sintered with other battery materials to form an inorganic ceramic electrolyte for a rechargeable lithium-ion battery, particularly an all-solid-state battery (ASSB). On the one hand, the solid electrolyte can be used as a separator: placed between the electrodes, it can protect the electrodes from accidental short circuits, thus ensuring the functionality of the entire system. The characteristics of the separator of the present invention, particularly the stability of dendrites, are improved, so that charging can be carried out at a higher current density without short circuits (fast charging). On the other hand, co-sintering with the electrode material can be envisaged: in this case, the solid electrolyte manages the reciprocating transport of relevant charge carriers (lithium ions) between the electrode material and the conductive electrode according to the discharge or charge state of the battery.

[0072] In the present disclosure, the terms "solid electrolyte" and "solid-state electrolyte" are synonyms unless otherwise stated.

[0073] The present invention also relates to a separator comprising or consisting of the solid electrolyte of the present invention.

[0074] The present invention also relates to a solid-state lithium-ion battery containing the solid electrolyte of the present invention. Description of the Drawings

[0075] Figure 1 Shows the relationship between the relative density after sintering for 30 minutes at a sintering temperature of 1000 °C and the magnitude of the DSC endothermic signal in the range of 850 °C to 1100 °C. The x-axis represents the magnitude of the DSC endothermic signal in J / g. A measured value of 0 J / g indicates that no DSC endothermic signal appears in the range of 850 °C to 1100 °C. The y-axis represents the relative density percentage. Detailed Description

[0076] The following examples relate to three LLZO glass ceramics according to the present invention (Example A, Example B, Example C) and an LLZO glass ceramic not according to the present invention (Control Example V1).

[0077] 1. Preparation of LLZO Glass Ceramic

[0078] Mix the raw materials according to the composition and load them into an open-top cold crucible. First, the mixture must be preheated to reach a certain minimum conductivity. For this purpose, use a burner for heating. After reaching the coupling temperature, further heating and homogenization of the melt are achieved through high-frequency coupling by an induction coil. To improve the melt homogenization, a water-cooled stirrer is used for stirring. After complete homogenization, samples are directly taken from the melt (rapid cooling), while the remaining melt is slowly cooled by turning off the high frequency.

[0079] The material prepared in this way can be transformed into a glass-ceramic material with a garnet-type main crystal phase by direct solidification of the melt or by heat treatment (ceramization) after quenching. Samples directly extracted from the melt will spontaneously crystallize regardless of whether they are cooled or not, so subsequent ceramization treatment is not required.

[0080] 2. Densification points of the amorphous phase

[0081] Analyze the samples of Example A, Example B, Example C and Comparative Example V1 by Differential Scanning Calorimetry (DSC). The experimental procedure is as follows: Place the LLZO glass-ceramic samples (20 - 100 mg) obtained in item 1 in a platinum DSC crucible. Under an argon atmosphere, perform DSC measurements from room temperature to at least 1100 °C at a heating rate of 10 K / min. The DSC signal is determined using a thermal flow-DSC DSC 404 F1 from GmbH.

[0082] In Examples A, B, and C according to the present invention, endothermic signals greater than 10 J / g were detected in the range of 850 °C to 1100 °C. In contrast, no endothermic signals of the same magnitude appeared in Comparative Example V1. The results are shown in the following table.

[0083] Table 1

[0084] It can be seen that there are significant differences between Examples A, B, and C according to the present invention and Comparative Example V1 in terms of the appearance of endothermic signals in the range of 850 °C to 1100 °C. The densification points of the amorphous phase in Examples A, B, and C are in the range of 850 °C to 1100 °C. In contrast, the amorphous phase in Comparative Example V1 does not have a densification point in the range of 850 °C to 1100 °C.

[0085] 3. Microstructure and relative density of the sintered solid electrolyte

[0086] The LLZO glass-ceramic samples of Example A, Example B, Example C, and Comparative Example V1 were ground to a particle size d 50 = 1 μm, and then pressed into compacts and sintered at a sintering temperature of 1000 °C for 30 minutes to form a solid electrolyte.

[0087] For comparison, the LLZO glass-ceramic samples of Example A and Comparative Example V1 were ground to a particle size d 50 = 1 μm, then pressed into compacts and sintered at a sintering temperature of 1130 °C for 30 minutes to form a solid electrolyte.

[0088] For comparison, the LLZO glass-ceramic sample of Example B was ground to a particle size d 50 = 1 μm, then pressed into compacts and sintered at a sintering temperature of 1070 °C for 30 minutes to form a solid electrolyte.

[0089] (a) Microstructure

[0090] Based on all four LLZO glass-ceramics (A, B, C, V1), sintered solid electrolytes with a fine-grained microstructure were obtained at a sintering temperature of 1000 °C. In contrast, at a sintering temperature of 1130 °C, in Comparative Example V1 and even in Example A, the microstructure was not a fine-grained structure, and large crystal domains appeared. In Example B, at sintering temperatures of 1000 °C and 1070 °C, the microstructure was a fine-grained structure.

[0091] The microstructure analysis was as follows:

[0092] LLZO glass-ceramic samples with a diameter of 8.5 mm and a height of 1 mm were fractured using a diamond glass cutter. The resulting fracture edges were analyzed using a Scanning Electron Microscopy (SEM). The obtained SEM images represent top views of the fracture edges. The fracture edges were then used to analyze the cross-section of the solid electrolyte.

[0093] Regarding the scanning electron microscope, a ZEISS LEO 1550 with a field emission source was used to generate the corresponding SEM images with a secondary electron detector at an acceleration voltage of 10 - 20 keV and a magnification of 500 times or 1000 times.

[0094] The obtained SEM images can be used to determine the microstructure of the sintered solid electrolyte. For this purpose, images with an area of at least 0.04 mm 2 were evaluated. Two images were evaluated for each sample.

[0095] At this time, based on all four kinds of LLZO glass ceramics (A, B, C, V1), a sintered solid electrolyte with a fine-grained microstructure was obtained at a sintering temperature of 1000 °C. Larger particles (>30 μm) accounted for less than 10% of the fracture edge area.

[0096] In contrast, at a sintering temperature of 1130 °C, in Comparative Example V1 and even in Example A, the microstructure was a coarse-grained structure, and larger particles (>30 μm) accounted for at least 10% of the analyzed image area.

[0097] The desired fine-grained microstructure can be achieved when the sintering time is 30 minutes and the sintering temperature is 1000 °C, but it cannot be achieved when the sintering temperature is 1130 °C.

[0098] In Example B, the desired fine-grained microstructure was obtained at both sintering temperatures of 1000 °C and 1070 °C.

[0099] (b) Relative density

[0100] Analyze the relative density of the sintered solid electrolyte. The relative density is determined as follows:

[0101] The powder density was determined by the helium pycnometer method. After sintering, the density of the solid electrolyte was determined by weighing and geometric measurement (density = mass / volume). The relative density is the quotient of the solid electrolyte density and the powder density.

[0102] As expected, according to Example A and Comparative Example V1, after sintering at 1130 °C for 30 minutes, the relative densities of the solid electrolytes were as high as 96% (Example A) and 91% (Comparative Example V1), respectively. However, the corresponding solid electrolytes did not exhibit the above-mentioned desired fine-grained microstructure.

[0103] Among the solid electrolytes sintered at 1000 °C, the relative density showed significant differences, which were related to the intensity of the DSC endothermic signal in the range of 850 °C to 1100 °C.

[0104] The results are as Figure 1 shown and summarized in Table 2 below.

[0105] Table 2

[0106] The relative density clearly depends on the presence or absence of the DSC endothermic signal. If such a signal does not exist (Comparative Example V1), the obtained relative density is relatively low. In contrast, when the DSC endothermic signal is significantly greater than 10 J / g (Example A, Example B, Example C), a sintered solid electrolyte with a relative density greater than 90% can be obtained even when the sintering temperature is only 1000 °C and the sintering time is only 30 minutes.

Claims

1. A solid electrolyte comprising a lithium ion conductive material, wherein, The lithium-ion conductive material comprises a crystalline phase and an amorphous phase. Among them, the crystalline phase comprises a main crystalline phase, and the content of the main crystalline phase in the crystalline phase is at least 50 wt.%. The relative density of the solid electrolyte is at least 90%, and the solid electrolyte has a microstructure such that less than 10% of the cross-sectional area of the solid electrolyte is occupied by grains with a diameter greater than 30 μm.

2. The solid electrolyte according to claim 1, wherein, The lithium-ion conductive material is a glass-ceramic.

3. The solid electrolyte according to any one of the preceding claims, wherein, The main crystalline phase has a cubic crystal system and / or has a garnet structure.

4. The solid electrolyte according to any one of the preceding claims, wherein, The main crystalline phase comprises lithium lanthanum zirconium oxide LLZO.

5. The solid electrolyte according to any one of the preceding claims, wherein, The molecular formula of the main crystal phase is Li 7-3x+y-z- Al x M y II M 3-y III M 2-z IV M z V O 12±δ , where M II contains one or more divalent cations, M III contains one or more trivalent cations, M IV contains one or more tetravalent cations, M V contains one or more pentavalent cations, and where x + z > 0, y < 1, δ < 0.

5.

6. The solid electrolyte according to any one of the preceding claims, wherein, The amorphous phase comprises Li2O, and the content of the amorphous phase in the lithium-ion conductive material is at least 0.1 wt.%.

7. The solid electrolyte according to any one of the preceding claims, wherein, The densification point of the amorphous phase is in the range of 850 °C to 1100 °C.

8. The solid electrolyte according to any one of the preceding claims, wherein, Based on the total mass of the lithium-ion conductive material, the sum of the weight percentages of SiO2 and B2O3 is at least 0.1 wt.%, and the weight percentage of SiO2 is greater than the weight percentage of B2O3.

9. The solid electrolyte according to any one of the preceding claims, wherein, The content of B2O3 in the lithium-ion conductive material is at most 0.1 wt.%.

10. The solid electrolyte according to any one of the preceding claims, wherein, The content of SiO2 in the lithium-ion conductive material is at least 0.1 wt.%.

11. The solid electrolyte according to any one of the preceding claims, wherein, Based on the total weight of the amorphous phase, the weight percentage of SiO2 is 1.0 wt.% to 60.0 wt.%.

12. A method for preparing a solid electrolyte according to any one of the preceding claims, comprising the following steps: · Melting the starting materials; · Cooling the melt; · Grind the obtained material into a powder with a particle size d 50 ranging from 0.1 μm to 10 μm; · Preparing an intermediate product comprising or consisting of the powder; and · Sintering the intermediate product, wherein the sintering temperature is at most 1100 °C.

13. The method according to claim 12, wherein, The preparation of the intermediate product is to press the powder into a compact.

14. The method according to claim 12, wherein The preparation of the intermediate product is by processing the powder into a tape. Processing the powder into a tape comprises the following steps: · Mixing the powder with one or more solvents and one or more organic binders to form a slurry; · Casting the slurry onto a carrier substrate to form a tape; · Removing the solvent from the tape by evaporation; · Peeling the tape from the carrier substrate; · Removing the organic binder and any remaining solvent from the tape by heat treatment, wherein the temperature is 250 °C to 500 °C.

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

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