Method for producing a sintered layer which can be used as a solid separator or part of an electrode in a battery cell, and battery cell having the sintered layer

Abstract

Proposed is a method for producing a sintered layer which can be used as a solid separator or part of an electrode in a battery cell, comprising: a) mixing (S1) particles of a solid electrolyte material and particles of a polymeric binder to form a pulverulent composition, wherein shear force is introduced into the pulverulent composition during or after the mixing, whereby the polymeric binder is obtained at least in part in fibrillary form in the pulverulent composition, wherein the pulverulent composition comprises ≤ 1 wt.% of the polymeric binder; b) forming (S2) a precursor layer from the composition obtained in a); and c) sintering (S3) the precursor layer to form the sintered layer (1).

Description

[0001] Description

[0002] Method for producing a sintered layer that can be used as a solid separator or part of an electrode in a battery cell, and battery cell comprising the sintered layer

[0003] The invention relates to a method for producing a sintered layer that can be used as a solid separator or part of an electrode in a battery cell, its use, and a battery cell comprising the sintered layer.

[0004] Due to the rapid market development of electric vehicles and grid-based energy storage, high-performance, cost-effective lithium-ion batteries are currently one of the most promising options for large-scale energy storage. Solid-state batteries offer many advantages for use in electric cars. Compared to conventional lithium-ion batteries, they offer a higher energy density, allowing for smaller batteries or, for the same size, more powerful designs. They also offer greater safety and faster charging. However, the production of solid-state separators for solid-state batteries is particularly time-consuming, energy-intensive, and expensive.

[0005] In the production of oxide-solid separators, solid electrolyte particles can be mixed with a binder in a solvent-based process, cast into foils, and then sintered. In the first heating step, the binder is burned off. This process requires very high temperatures and is therefore energy-intensive and expensive. Furthermore, the decomposing binder produces gas, which must escape slowly and thus disrupts the process.

[0006] DE 102022 114 925 A1 relates to a process for manufacturing a solid separator for a battery cell, which includes: feeding a substrate, applying a slurry of a ceramic solid separator to the substrate, drying the substrate coated with the slurry, whereby a dried coating is formed, and sintering the dried coating of the substrate, whereby a ceramic solid separator is formed.

[0007] WO 2020 / 127223 A2 concerns a process for manufacturing a solid electrolyte membrane or an anode unit for a solid-state battery, wherein the solid electrolyte membrane is a powder mixture of a solid electrolyte material and polytetrafluoroethylene, and the anode unit is a powder mixture of an electrode material and a

[0008] A solid electrolyte material, an electrically conductive conductivity additive and polytetrafluoroethylene is produced, by applying shear forces to the powder mixture at least partially fibrillated polytetrafluoroethylene is formed and the powder mixture is formed into a flexible composite layer.

[0009] The object of the invention is to simplify the manufacture of a solid-state separator for a battery cell and to provide a battery cell with a solid-state separator manufactured in this way.

[0010] The problem is solved by a method for producing a sintered layer, its use in a battery cell, and a battery cell according to the independent claims. Specific embodiments and further developments are described in the dependent claims and this description.

[0011] The invention relates to a method for producing a sintered layer that can be used as a solid separator or part of an electrode in a battery cell, comprising: a) mixing particles of a solid electrolyte material and particles of a polymeric binder to form a powdered composition, wherein shear force is introduced into the powdered composition during or after mixing, whereby the polymeric binder is obtained at least partially in fibrillar form in the powdered composition, wherein the powdered composition contains < 1 wt.% of the polymeric binder; b) forming a precursor layer (also referred to as a "precursor layer") from the composition obtained in a); c) sintering the precursor layer to form the sintered layer.

[0012] According to the invention, a drying process is described in which only a very small amount of binder is used. Firstly, this eliminates an energy- and time-intensive drying step. Due to the very small amount of binder, it has been shown that the binder can be removed, or largely removed, during sintering without the need for further, separate heating. Secondly, it has been shown that the separator function is little to not affected at all if binder or its decomposition products remain in the sintered layer. Sintering has the advantage of reducing the porosity of the precursor layer. Preferably, the sintered layer has a lower porosity than the precursor layer.

[0013] Furthermore, the invention offers the advantage that mechanical stability can be achieved through sintering. The sintered layer can be obtained as a free-standing layer (layer without a support layer) or on a support. The precursor layer can be formed on a support layer and then separated from the support layer and then sintered, or it can remain on the support layer during sintering. The support layer can be a metallic support layer. In another embodiment, the precursor layer can be formed on a support layer and sintered. In a further embodiment, the precursor layer can be formed without a support layer, preferably on another substrate, and the precursor layer can then be sintered.

[0014] A powdered composition is defined as a granular or lumpy mixture or bulk material. The powdered composition may be in dry form to simplify handling. It may also be non-free-flowing as defined by DIN EN ISO 6186. For the purposes of this document, "dry" means that the components of the powder mixture are solids, free from liquids or materials in a liquid state. The powder mixture may be solvent-free, i.e., composed without solvents. The powdered composition contains no liquid phase in which mineral particles and binders are dispersed or dissolved. Residual moisture in the form of water, e.g., from the ambient atmosphere, may be present in the composition. No liquid is used as a solvent or dispersant in the process.The process preferably does not include a drying step for removing a liquid.

[0015] When shear force is introduced into the composition in a), as mentioned, the polymeric binder is retained at least partially in fibrillar form within the composition. Fibrils are understood to be microscopically small fibers. In other words, fibers that are recognizable as fibers under microscopic magnification, preferably at least tenfold. The polymeric binder can be present at least partially as monoaxially and / or biaxially oriented fibrils. However, the polymeric binder can also be present as completely monoaxially or completely biaxially oriented fibrils.

[0016] The application of shear force causes particles of the binder to be at least partially transformed into a fibrous form. Mineral particles can be broken down by the application of shear force. Fibrils of the binder preferably adhere to mineral particles, and mineral particles are preferably bound together by fibrils of the binder. Preferably, a fibrillar network is formed in which mineral particles are embedded.

[0017] The mixing and application of shear force in a) whereby the polymeric binder is obtained at least partially, or alternatively completely, in fibrillar form in the powdered composition, can be carried out by friction milling, mixing in a screw conveyor or calender roller device, kneading device, mortar device, or a combination of the aforementioned methods to ensure efficient fibrillation. The formation of the at least partially fibrillated polymeric binder typically takes place at room temperature, preferably at elevated temperatures of 60°C to 100°C, and particularly preferably at 90°C to 100°C.

[0018] The formation of the precursor layer in b) can be achieved through forming. This forming can be done by rolling, pressing, or extrusion. A combination of these methods can also be used.

[0019] The thickness of the precursor layer is preferably in the range of 14 - 100 pm, preferably 20 - 50 pm.

[0020] The thickness of the sintered layer is preferably in the range of 10 - 50 pm, preferably 10 - 20 pm.

[0021] The particles of the solid electrolyte material preferably have a size in the range of up to 25 pm, more preferably up to 10 pm, more preferably up to 5 pm, and most preferably up to 1 pm. An exemplary lower limit is 0.1 or 0.2 pm. According to the invention, the composition obtained in a) comprises < 1 wt.% of the polymeric binder, wherein more than 0 wt.% of the polymeric binder is present. The remainder is preferably the solid electrolyte material.

[0022] In one embodiment of the process, the composition obtained in a) contains < 0.5 wt.% of the polymeric binder (where > 0 wt.% polymeric binder is present). The remainder is preferably the solid electrolyte material.

[0023] In one embodiment of the process, the composition obtained in a) contains < 0.1 wt.% of the polymeric binder (where > 0 wt.% polymeric binder is present). The remainder is preferably the solid electrolyte material.

[0024] In one embodiment of the process, the composition obtained in a) contains < 0.05 wt.% of the polymeric binder (where > 0 wt.% polymeric binder is present). The remainder is preferably the solid electrolyte material.

[0025] The advantages of particularly low proportions of polymeric binder are mentioned elsewhere in this description.

[0026] In one embodiment of the process, the polymeric binder is a fluorine-containing polymer with a carbon backbone, such as polytetrafluoroethylene, a copolymer of tetrafluoroethylene and a comonomer, preferably selected from chlorotrifluoroethylene, hexafluoropropylene, fluoroalkylethylene, perfluoroalkylethylene, and fluoroalkylfluorovinyl ether. Acrylic resins can also be used as polymeric binders.

[0027] In one embodiment of the method, the solid electrolyte material is an oxide, sulfide, or halide solid electrolyte material containing lithium and / or sodium and being conductive for lithium or sodium. An oxide solid electrolyte material is preferred. With an oxide solid electrolyte material, a sintered layer with particularly good separator properties was obtained.

[0028] The following are examples of solid electrolyte materials, which are to be understood as exemplary only and not as limiting.

[0029] In one example, the solid electrolyte material can be an oxide, ion-conducting material, such as an oxide, lithium-ion-conducting ceramic or glass ceramic, in particular lithium lanthanum zirconium oxide (LLZO) or doped LLZO, a lithium aluminum titanium phosphate, a lithium lanthanum titanate or a derivative.

[0030] The solid electrolyte material can have a NASICON structure. Suitable lithium-analogous structures of NASICON include, in particular, lithium phosphates of the formula LiM2(PO4)3, where M represents a base element selected from the group consisting of Ti, Ge, Zr, Hf, or Sn. To increase the ionic conductivity, the lithium phosphates can be doped, preferably using Al, Cr, Ga, Fe, Sc, In, Lu, Y, and La as dopants. Particularly preferred are LiZr2(PO4)3 (LZP) doped with La, Ti, or Al; LiTi2(PO4)3 (LTP); Lii + xAl x Ti2.x(PO4)3 with x = 0.3 - 0.5 (LATP), Lii +x AlxGe2-x(PO4)3 with x = 0.4 - 0.5 (LAGP) and Lii.4Alo.4Geo.2Tii.6(P04)3 LAGTP.

[0031] The solid electrolyte material can exhibit a LISICON structure. LISICON is an acronym for Lithium Super Ionic Conductor and originally referred to a family of minerals with the chemical formula Li 2+2xZni-xGeO4. Solid-state separators with a LISICON structure also enable additional uptake of lithium and can make this lithium available for use in the battery cell of a solid-state battery.

[0032] The solid electrolyte material can exhibit a garnet structure. Structurally, garnets are orthosilicates of the general composition X3Y2(SiO4)3, which crystallize in the cubic crystal system, where X and Y represent eight- and six-coordinate cation sites, respectively. The individual SiO4 tetrahedra are linked to each other by ionic bonds via the interstitial B cations. Garnet-like compounds with an excess of lithium are good lithium ion conductors. Particularly suitable examples of ion conductors with a garnet-like structure are lithium lanthanum zirconium oxide Li111La3Zr20112(LLZO) and lithium lanthanum zirconium titanate Li661La3Zr16Ta04O12.

[0033] The solid electrolyte material can exhibit an argyrodite structure. Argyrodite is a mineral with an orthorhombic crystal system and the chemical composition AgsGeSe.

[0034] The term is used here for lithium-ion conductors that have a comparable crystal system. Examples include Liy. x ZCh6-xX x with x = 0 to 1, Z = P or As, Ch = S or Se, and X = CI, Br, or I. The Li-argyrodites LiePSsX (X = CI, Br, and I), LiiPSe, LiyPSee, and Li6,6Po4Geo,6S5l are particularly preferred. Generally, in all compounds mentioned in this application, lithium can be exchanged for sodium.

[0035] In one embodiment of the method, the sintered layer has a porosity of < 5%, preferably < 3%, more preferably < 1%. The porosity is the percentage ratio of cavity volume to total volume and is preferably determined according to the following relationship:

[0036] Porosity (%) =

[0037] (1 - [Bulk density of the sintered layer] / [Pure density of the solid electrolyte material])x100

[0038] The density can be determined using pycnometers, by determining the density of solids or liquids by measuring the displaced liquid volumes.

[0039] These low porosities result in particularly good ion conductivity and stability against dendrite formation.

[0040] In one embodiment of the process, the polymeric binder is at least partially decomposed by the sintering step. In one variant of the process, decomposition products of the polymeric binder can remain in the sintered layer. The content of at least partially decomposed polymeric binder (undecomposed polymeric binder and decomposition products) in the sintered layer is > 0 to 1 wt.%, preferably > 0 to 0.5 wt.%, even more preferably > 0 to 0.1 wt.%, and most preferably > 0 to 0.05 wt.%.

[0041] In one embodiment of the process, the polymeric binder content is reduced during sintering. In this case, the polymeric binder and / or its decomposition products do not remain, at least partially, in the sintered layer. For example, gaseous decomposition products may partially or completely escape from the sintered layer. In this variant of the process, the polymeric binder content in the sintered layer is preferably 0 to 0.05 wt.%, more preferably 0 to 0.02 wt.%, even more preferably 0 to 0.01 wt.%, and most preferably 0 to 0.005 wt.%. Specifically, the combined content of the polymeric binder and its decomposition products in the sintered layer is preferably 0 to 0.05 wt.%, more preferably 0 to 0.02 wt.%, even more preferably 0 to 0.01 wt.%, and most preferably 0 to 0.005 wt.%. A content of 0 wt.% is not considered a significant difference.A reduction of -% can be achieved if no polymeric binder or its decomposition products remain in the sintered layer. The low polymeric binder content in this process has the advantage that decomposition products of the polymeric binder can remain in the sintered layer without impairing the separator's function. The entire original mass of the polymeric binder can remain in the sintered layer in the form of decomposition products and undecomposed binder.

[0042] In particular, with the low polymeric binder content according to the invention, little or no gas formation occurs during sintering, even with at least partial thermal decomposition of the polymeric binder. The process is advantageous because the low amount of polymeric binder used minimizes gas formation.

[0043] The application of temperature preferably leads to at least partial thermal decomposition of the polymeric binder, resulting in the formation of degradation products such as CFx and LiF (especially when using perfluorinated binders like PTFE). The formation of LiF actually has a negative impact on the separator material (in the sintered layer); therefore, it is advantageous according to the invention to use a small amount of polymeric binder to avoid these negative effects. Furthermore, this results in a faster and more efficient process.

[0044] In one embodiment of the invention, the sintering of the precursor layer to form the sintered layer takes place under an oxygen-free or oxygen-reduced atmosphere, specifically under a protective gas. This prevents gas formation through oxidation of the polymeric binder (e.g., CO2 formation in the presence of air / oxygen). An oxygen-reduced atmosphere is understood to be one containing less oxygen than the ambient air, preferably less than 20% by volume, more preferably less than 10% by volume, even more preferably less than 5% by volume, and most preferably less than 1% by volume.

[0045] The procedure may also exhibit:

[0046] Heating the precursor layer and / or the sintered layer to thermally decompose the polymeric binder contained therein. Heating the precursor layer or the sintered layer refers to heating independent of the sintering process, which may also involve heating. This additional heating is also known as "binder burnout." Such a binder burnout is optional and not mandatory. Due to the low proportion of polymeric binder, the binder burnout can occur quickly. The binder burnout can, for example, take place in an oven. In one embodiment of the process, no measures are taken between steps b) and c) to reduce the polymeric binder content, in particular, no measures are taken to reduce the polymeric binder content in the composition or the precursor layer. This results in a particularly efficient process.The previously mentioned binder burnout is omitted here. It has been shown that, with the low proportions of polymeric binder used, the energy input from sintering can be sufficient to effectively reduce the polymeric binder content and thus eliminate the need for a separate binder burnout, making the process fast and efficient. With the low proportions of polymeric binder used in the invention, degradation products of the decomposed polymeric binder do not have a detrimental effect, for example, through gas formation or remaining in the microstructure.

[0047] In the prior art process of binder burnout, which is necessary to reduce the binder content, a comparatively large amount of gas is produced as a degradation product. This gas must escape slowly through the porous structure. A high and open porosity is required to allow this gas to escape. In the prior art, this process must be slow because otherwise the precursor layer (in ceramics: green compact) foams or swells. The inventive method counteracts this, as the low binder content results in little or no gas formation, and a burnout step can be omitted. This also allows the use of very fast sintering processes, such as the laser sintering preferably employed in this invention.

[0048] Another advantage of the low binder content in the invention is a possible low porosity of the sintered layer (mentioned elsewhere), because the low binder content results in little to no gas formation through thermal decomposition of the binder, and therefore less gas needs to be transported away.

[0049] Sintering involves a densification of the microstructure below the melting temperature through diffusion at the grain boundaries and grain growth.

[0050] Sintering is preferably carried out by irradiating the precursor layer with high-energy radiation. The sintered layer is preferably heated, at least at the surface where the radiation strikes, and more preferably throughout, to a process temperature in the range of 700°C to 1400°C. Sintering is preferably carried out by laser sintering. In laser sintering, the precursor layer is irradiated with laser radiation, preferably over a surface and in sections. The high-energy radiation mentioned here is laser radiation. In one embodiment of the process, sintering is carried out by irradiating the precursor layer with radiation of a wavelength in the range of 100–490 nm, preferably 315–450 nm. This method is also known as black light sintering.

[0051] The aforementioned wavelengths are advantageous because the solid electrolyte material, particularly an oxide solid electrolyte material, absorbs very well at these wavelengths, thus enabling good energy coupling. This allows for a rapid sintering process, which is advantageous due to time savings and is possible because, due to the low binder content in the process according to the invention, little gas is generated by decomposing binder.

[0052] In a further aspect, the invention relates to the use of a sintered layer produced according to a method according to the invention or as previously described as a solid-state separator or part of an electrode in a battery cell, preferably a solid-state battery cell. An exemplary battery cell is an anode-free battery cell. An anode-free battery cell is a battery cell manufactured without an anode. An anodic current collector layer (also referred to as a current collector layer) is in direct contact with a separator layer when the cell is discharged. During charging, a metal anode is formed. In a zero-excess anode, the sintered layer is applied to a current collector layer, which is preferably made of copper, nickel, or stainless steel. During a charging process, a metal layer, preferably made of lithium or sodium, is formed between the current collector layer and the sintered layer, which forms the aforementioned anode.

[0053] In a further aspect, the invention relates to a battery cell, preferably a solid-state battery cell, comprising a sintered layer, obtainable or obtained by a method according to the invention, as a solid separator or part of an electrode. An exemplary battery cell is an anode-free battery cell mentioned above.

[0054] The invention is described below with reference to exemplary embodiments. The figures shown are:

[0055] Fig. 1 shows a process sequence according to the invention.

[0056] Fig. 2 shows a battery cell according to the invention. In step S1 of the process according to Fig. 1, particles of a solid electrolyte material, in this example LLZO, and polytetrafluoroethylene particles are mixed to form a powdered composition. 99.92 wt.% solid electrolyte material and 0.08 wt.% polytetrafluoroethylene are used. Mixing takes place under shear force in an extruder. The shear forces acting on the powdered composition cause fibrils to form from at least a portion of the polytetrafluoroethylene.

[0057] The composition obtained in S1 is then rolled out to a desired layer thickness of 50 pm to form (S2) a precursor layer using a calender.

[0058] The precursor layer thus obtained is irradiated section by section in S3 with a laser beam of wavelength 445 nm to produce a sintered layer. The sintered layer has a porosity of 1%, determined from the bulk density of the sintered layer and the known density of the solid electrolyte material using the formula mentioned above.

[0059] The sintered layer obtained from the process of Fig. 1 is incorporated into a battery cell as a separator layer. An exemplary battery cell 2 is shown in Fig. 2. Fig. 2 is not a scale drawing, but a schematic representation. The battery cell 2 is constructed as a solid-state battery cell, specifically as a solid-state accumulator, and initially has an anode-free structure. The sintered layer 1 produced according to the invention is a separator and is located on a stainless steel support layer 3 before the formation of the anode active layer 4. The battery cell 2 is shown with a cathode made of cathode active material NCM, conductive carbon black, and ion-conducting catholyte as the cathode active layer 6 on an aluminum support 5. The ion conduction occurs through the sintered layer 1, as an ion-conducting separator, and the catholyte.The anode active material (lithium) is formed as anode active layer 4 during charging between the stainless steel support layer 3 and the sintered layer 1, which acts as a separator. Reference numerals: sintered layer, battery cell, stainless steel support layer, anode active layer, aluminum support, cathode active layer.

Claims

Patent claims 1. A process for producing a sintered layer that can be used as a solid separator or part of an electrode in a battery cell, comprising: a) mixing (S1) particles of a solid electrolyte material and particles of a polymeric binder to form a powdered composition, wherein shear force is introduced into the powdered composition during or after mixing, whereby the polymeric binder is obtained at least partially in fibrillar form in the powdered composition, wherein the powdered composition contains < 1 wt.% of the polymeric binder; b) forming (S2) a precursor layer from the composition obtained in a); c) sintering (S3) the precursor layer to form the sintered layer (1).

2. The method according to claim 1, wherein the composition obtained in a) comprises < 0.5 wt.% of the polymeric binder.

3. Method according to claim 1 or 2, wherein the composition obtained in a) comprises < 0.1 wt.% of the polymeric binder.

4. Method according to any of the preceding claims, wherein the sintered layer (1) has a lower porosity than the precursor layer.

5. Method according to any of the preceding claims, wherein the sintered layer (1) has a porosity < 5%.

6. Method according to any of the preceding claims, wherein the sintered layer (1) has a porosity of < 1%.

7. Method according to one of the preceding claims, wherein the sintering (S3) of the precursor layer to form the sinter layer takes place under an oxygen-free or oxygen-reduced atmosphere.

8. Method according to one of the preceding claims, wherein the sintering process involves a reduction in the content of polymeric binder.

9. Method according to one of the preceding claims, wherein no measures to reduce the content of polymeric binder are taken between b) and c), in particular no measures to reduce the content of polymeric binder in the composition or the precursor layer are taken.

10. Method according to one of the preceding claims, wherein the sintering is carried out by irradiating the precursor layer with rays of a wavelength in the range of 100 - 490 nm.

11. Method according to any of the preceding claims, wherein the solid electrolyte material is an oxide solid electrolyte material.

12. Use of a sintered layer (1) produced according to a method of claims 1 to 11 as a solid separator or part of an electrode in a battery cell (2).

13. Battery cell (2) comprising a sintered layer (1) obtainable by a method according to any one of claims 1 to 11, as a solid separator or part of an electrode.

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