Method for producing a composite polymer electrolyte membrane and a composite polymer electrolyte membrane obtainable by the method

Abstract

The present invention relates to a process for producing a composite polymer electrolyte membrane, wherein the process comprises reacting nucleophilic groups of an α,ω-terminated polymer with a silyl isocyanate, reacting the resulting silane-terminated polymer with -OH groups of oxide nanoparticles using a conducting salt, thereby obtaining a paste from which a composite polymer electrolyte membrane is obtained by a rolling process and subsequent drying. The present invention further relates to a composite polymer electrolyte membrane obtainable by the process, a battery comprising the composite polymer electrolyte membrane, and the use of the composite polymer electrolyte membrane in a battery.

Description

[0001] The present invention relates to a method for producing a composite polymer electrolyte membrane, a composite polymer electrolyte membrane obtainable by the method, a battery comprising the composite polymer electrolyte membrane, and a use of the composite polymer electrolyte membrane in a battery.

[0002] Composite polymer electrolyte membranes (CPEs) play a crucial role in energy storage and conversion, such as in fuel cells and batteries. CPEs are ion-conducting membranes composed of a polymer, a conducting salt, and a filler (e.g., (in)active nanoparticles), often offering improved performance compared to conventional polymer membranes. They can be characterized by increased ion permeability, mechanical stability, and chemical resistance. These properties enable efficient ion conduction and contribute to the reliability and performance of fuel cells and batteries. CPEs offer promising opportunities for a wide range of applications where an efficient electrolyte membrane is required.

[0003] Solid-state batteries, in particular, are a promising technology for energy storage, as they potentially offer higher energy density, improved safety, and longer lifespans compared to conventional lithium-ion batteries. The thickness of the chemical energy equivalent (CPE), which can be used as the electrolyte layer, plays a crucial role in the performance and efficiency of these batteries. In solid-state batteries, ions (typically lithium ions) must migrate through the electrolyte to store and release energy during charging and discharging. A thinner electrolyte layer allows for faster diffusion of ions between the electrodes, resulting in improved conductivity. This enables solid-state batteries to achieve higher power density and faster charging times. The internal resistance of the entire cell is strongly dependent on the ionic conductivity and thus the thickness of the electrolyte layer.Therefore, thinner electrolyte membranes are advantageous in the production of high-performance solid-state batteries (X. Lu, Y. Wang, X. Xu, B. Yan, T. Wu, L. Lu, Polymer-Based Solid-State Electrolytes for High-Energy-Density Lithium-Ion Batteries - Review. Adv. Energy Mater. 2023, 13, 2301746.).

[0004] Conventional methods for producing CPEs typically use volatile organic solvents, raising environmental and safety concerns. CPEs are usually produced by dissolving silanes in solution and subjecting them to self-condensation with the addition of acids or bases. A common method is solvent casting, where the polymers and conducting salts are dissolved in a solvent and the nanoparticles are dispersed. The solvent is then slowly evaporated. This process is only feasible on a laboratory scale for many solvents, as both the amount of solvent required and the evaporation time are unsuitable for industrial production.

[0005] Furthermore, the poor miscibility of oxide nanoparticles with polymers is a problem in CPEs production processes. The nanoparticles form agglomerates, leading to inhomogeneous membranes. This poor miscibility is based on the high chemical potential difference at the interface (R. Fang, Y. Li, N. Wu, B. Xu, Y. Liu, A. Manthiram, JB Goodenough, Ultra-Thin Single-Particle-Layer Sodium Beta-Alumina-Based Composite Polymer Electrolyte Membrane for Sodium-Metal Batteries. Adv. Funct. Mater. 2023, 33, 2211229).

[0006] Furthermore, KR 10 2013 0 134 630 A describes an organic-inorganic multilayer microporous film comprising a porous polyolefin-based substrate and a coating layer on at least one surface of the substrate, wherein the coating layer comprises inorganic particles, an organometallic coupling agent chemically bonded to the inorganic particles, and an organic linker chemically bonded to the organometallic coupling agent. US 10 414 890 B2 further describes a hybrid organic-inorganic fluoropolymer composite, a process for producing the hybrid organic-inorganic fluoropolymer composite and films and membranes made from it, as well as the use of the hybrid organic-inorganic fluoropolymer composite and the films and membranes made from it in various applications.

[0007] The present invention is therefore based on the objective of providing a method for the production of composite polymer electrolyte membranes (CPEs) with a thickness ≤ 40 µm, preferably with a thickness ≤ 30 µm, which overcomes the aforementioned disadvantages of the known methods and also allows for scalability.

[0008] The aforementioned problems are solved by the embodiments of the present invention characterized in the claims.

[0009] In particular, according to the invention a method for producing a composite polymer electrolyte membrane with a thickness ≤ 40 µm, preferably with a thickness ≤ 30 µm, is provided, wherein the method comprises the steps: (a) Reacting an α,ω-terminated polymer with -OH, -SH or -NH2, preferably an α,ω-OH-terminated polymer, with a silyl isocyanate, thereby obtaining a modified silane-terminated polymer, (b) Reacting the polymer obtained in step (a) with oxide nanoparticles having -OH groups on their surface, using a conducting salt selected from alkali metal and alkaline earth metal salts, but without the use of a solvent, to obtain a paste, (c) Subjecting the paste obtained in step (b) to a rolling process using a rolling press with an adjustable gap, and (d) Drying the paste after rolling in step (c), obtaining a composite polymer electrolyte membrane.

[0010] The inventive process, in which oxide nanoparticles are introduced into a polymer matrix using silane-terminated polymers, allows for the improvement of the ionic conductivity and mechanical properties of the composite polymer electrolyte membrane. The absence of solvents in step (b) of the process reduces the need for solvent evaporation steps, thereby minimizing environmental impact, increasing process safety, and accelerating the production of the CPE. Conductivity measurements show that composite polymer electrolyte membranes produced according to the process exhibit competitive ionic conductivity compared to conventionally produced CPEs. On the other hand, the production of thin CPEs using known methods such as solvent casting or electrospinning is extremely complex and poorly scalable.Furthermore, the mechanical stability of the membranes is often very low when the particle size is on the order of the layer thickness. Reducing the CPE layer thickness by rolling typically results in film destruction upon compression.

[0011] The inventive process for producing a composite polymer electrolyte membrane with a thickness ≤ 40 µm, preferably with a thickness ≤ 30 µm, is explained in more detail below.

[0012] According to the present invention, the process as such is not subject to any particular limitations. With regard to step (a) of the process, the process is also not further limited. First, the silyl isocyanate is added to the polymer α,ω-terminated with -OH, -SH or -NH2 ( Fig. 1).

[0013] Step (a) can be carried out using solvents or solvent-free, with solvent-free execution being preferred. Solvents that can be used in step (a) preferably have high solubility for the silyl isocyanate and for the -OH, -SH, or -NH₂ α,ω-terminated polymer. These solvents can be organic or inorganic, preferably organic, and are used to generate a homogeneous solution or suspension of said polymers and the silyl isocyanate. Typically, such solvents have sufficient polarity to dissolve the polymers substituted with polar -OH, -SH, or -NH₂ groups, while simultaneously exhibiting low viscosity and evaporation rates to facilitate easy handling and processing.Examples of solvents that exhibit these properties include, but are not limited to, dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), dimethylformamide (DMF), acetonitrile, acetone, ethyl acetate, dichloromethane, and chloroform.

[0014] For a solvent-free reaction, the polymers preferably have a melting point below 150 °C, more preferably below 100 °C, even more preferably below 85 °C, and most preferably below 75 °C. When a polymer is melted, the polymer chains are thoroughly mixed due to their increased mobility in the melt, resulting in a homogeneous mixture with the silyl isocyanate. Reacting with a molten polymer does not require the use of solvents, thus reducing environmental impact from solvent evaporation or disposal. This contributes to the promotion of more environmentally friendly production processes. Since no solvents are needed, using a reaction in the molten state can be more resource-efficient compared to a solvent-based reaction.Processing molten polymers is safer than using solvents because it reduces the risk of chemical exposure, especially with toxic or volatile solvents. Reacting with a molten polymer can lead to products of higher purity due to the reduced risk of solvent contamination. Furthermore, processing molten polymers allows for more precise process control, as reaction temperature and process conditions can be more easily set and monitored. This can result in improved reproducibility of results and higher product quality. Processing molten polymers can also often be faster than the corresponding solvent reaction, as no additional time is required for solvent separation and drying. This enables faster production speeds and higher throughput rates.

[0015] In general, α,ω-terminated polymers feature -OH, -SH, or -NH₂ groups, preferably -OH groups, as terminal groups. -OH groups react with isocyanate groups in high product yields. This addition reaction leads to the formation of urethane bonds. The reaction of alcohol groups with isocyanate groups typically proceeds rapidly and efficiently under mild reaction conditions, resulting in short reaction times and ease of handling. Furthermore, alcohol groups are generally chemically stable and do not readily react with other functional groups besides the aforementioned isocyanate groups, leading to improved long-term stability of the polymers.

[0016] Silyl isocyanate is a compound comprising at least one silyl functionality (SiOR3) and at least one isocyanate functionality (NCO), preferably one functionality each, wherein the three substituents R of the silyl functionality are each independently selected from the group consisting of alkyl, aryl, alkenyl and alkynyl groups, each of which may have further substituents.

[0017] Preferably, the substituent R is an alkyl group, wherein the number of carbon atoms of the alkyl group is preferably in the range of 1 to 30, more preferably in the range of 1 to 10, even more preferably in the range of 1 to 3, and most preferably 2. Large and long-chain substituents R on the silyl group lead to steric hindrance and impair the accessibility of the silyl residue to the -OH group in a further reaction with the oxide nanoparticles. This can reduce the reactivity in step (b) because the interaction between the silyl and the -OH group is hindered. The at least one silyl functionality is linked to the at least one isocyanate functionality via a structural element comprising an alkyl and / or aryl group.Preferably, the structural element is an alkyl group, wherein the number of carbon atoms is in the range of 1 to 30, more preferably in the range of 1 to 15, even more preferably in the range of 1 to 6, and most preferably 3. High molecular weight silyl isocyanates are poorly soluble in solvents or, in the case of a solvent-free reaction, in the molten polymer, which leads to an uneven reaction and hinders step (a). The silyl isocyanate 3-(triethoxysilyl)propyl isocyanate is particularly preferred. By using 3-(triethoxysilyl)propyl isocyanate, an addition reaction with 99% conversion can be carried out, and silyl groups can be efficiently introduced into the α- and ω-positions of the polymers.With regard to the reaction conditions such as the reaction temperature, the reaction time and the mixing procedure of the reactants of step (a) of the process according to the invention, conditions can be chosen that correspond to the conditions of step (b) of the process according to the invention.

[0018] With regard to the next step (b), the process is also not further restricted. In this step, the silane-terminated polymer from step (a), with the structural features defined above, is mechanically stirred with oxide nanoparticles, a conducting salt, and optionally with additives. On a laboratory scale, a mortar is suitable for stirring. For industrial processes, stirring can be carried out using industrial mixing methods, preferably in a screw extruder. The conducting salt dissolves in the polymer during stirring, making it flowable. This allows a viscous dispersion, referred to as a paste, to be formed. An amount of up to 50 wt.% conducting salt is preferred, an amount of up to 40 wt.% conducting salt is more preferred, and an amount of up to 30 wt.% conducting salt is even more preferred with respect to the total weight of the paste in step (b). Furthermore, there is no lower limit to the amount of conducting salt.Values ​​for the lower limit of the amount of conducting salt are, in particular, 5 wt.%, 10 wt.%, 15 wt.%, 20 wt.% and 25 wt.%. A sufficient amount of conducting salt leads to advantageous properties of the CPE in a battery, such as increased ionic conductivity.

[0019] The silane-terminated polymer obtained from step (a), with the structural features defined above (i.e., with the SiOR3 groups in the α,ω position defined above), exhibits the property of reacting excellently with -OH groups on surfaces, thus generating a covalent bond on the surface. In the dry state, these silane-terminated polymers are stable and unreactive. When the silane-terminated polymers are mixed with a conducting salt (e.g., lithium bis(trifluoromethylsulfonyl)amide (LiTFSI)) and oxide nanoparticles, a sticky paste is formed that can be extruded or otherwise deformed and processed. Small amounts of additives can be added, if necessary, to increase fluidity. Furthermore, the addition of additives allows the finished composite polymer electrolyte membrane from step (d) to be improved with regard to its processing and electrochemical performance. A maximum amount of 40 wt.A maximum of 35 wt.% additives is preferred, a maximum of 35 wt.% additives is more preferred, a maximum of 25 wt.% additives is even more preferred, and a maximum of 3 wt.% additives based on the total weight of the paste in step (b) is most preferred. Possible additives include, for example, plasticizers, stabilizers, flame retardants, inorganic salts, liquid electrolytes, dyes, and pigments. In particular, the additives include inorganic salts, preferably LiNO3 and / or SbF2, which improve and stabilize the interface between the electrolyte and the Li metal on the anode side. Furthermore, possible additives include in particular plasticizers such as short-chain polymers, preferably polyethylene glycol dimethyl ether 500 (PEGDME500, CAS 24991-55-7), succinonitrile, adiponitiril or known liquid electrolytes, preferably at least one selected from the group consisting of ethylene carbonate, propylene carbonate and dimethyl carbonate.The addition of additives improves processability and increases ion mobility, leading to increased ionic conductivity.

[0020] According to the present invention, step (b) takes place without the addition of solvents. "Without the addition of solvents" or "solvent-free" in relation to the process according to the invention means that no additional solvent is added. However, the presence of traces of solvents, in particular alcohols produced by the condensation reaction, cannot be ruled out. The addition of solvents leads to a decrease in viscosity, which results in undesirable agglomeration and segregation of the oxide nanoparticles. Even with coatings for improved compatibility, the oxide nanoparticles, due to their high density, tend to accumulate in the lower region of the membrane when solvents are used. Furthermore, in a solvent-free step (b), all starting materials are solids, which simplifies storage, handling, transport, dosing, and processing.

[0021] It is also preferred to carry out step (b) of the process without adding any additional acids or bases. By selectively using nanoparticles with pH values ​​higher and lower than 7, the cross-linking of the silyl groups with the surface of the nanoparticles can be accelerated without the addition of further acids and bases. Usually, this requires first preparing an acidic or basic solution and activating the silyl ethers to silanols, a step that can be omitted in this case. For example, lithium lanthanum zirconium oxide (LLZO) is basic due to the formation of small amounts of Li₂CO₃ on its surface, which is why the silyl groups react directly with the oxide nanoparticles without the addition of further acids or bases. This effect can be observed with many oxide nanoparticles, but its rate of reaction varies depending on the oxide used.

[0022] With regard to step (c), the process is also not further restricted. Very thin CPEs can be produced reproducibly by processing the paste obtained from step (b) with a roller press. For the production of ultra-thin CPEs, it is essential to use a roller press with an adjustable gap, for example, a roller press from Xiamen Tmax Battery Equipments Limited. By subjecting the paste obtained in step (b) to a rolling process using a roller press with an adjustable gap, it is possible to obtain mechanically durable CPEs at layer thicknesses ≤ 40 µm, preferably at layer thicknesses ≤ 30 µm. The processing after mixing the paste can be carried out in a single step. For example, the paste can be applied between non-stick films, such as Mylar® film, of known thickness.The gap width of the rolling press can then be adjusted to the desired thickness plus twice the film thickness, and the paste can be pressed through the rollers to achieve the desired thickness. For example, the Mylar® film can have a thickness of 100 µm. To achieve a thickness of 30 µm, a gap width of 230 µm is set. A schematic representation of the rolling process is shown in [reference missing]. Fig. 3 shown.

[0023] With regard to step (d), the process is also not further restricted. After rolling, the paste is dried. During this process, the silyl groups of the silane-terminated polymers react with -OH groups on the surface of the oxide nanoparticles ( Fig.2) Drying preferably takes place in ambient air. Furthermore, a reaction temperature of ≤ 150 °C is preferred for drying, more preferred is a reaction temperature of ≤ 100 °C, even more preferred is a reaction temperature of ≤ 60 °C, and most preferred is a reaction temperature of ≤ 40 °C. Excessively high temperatures can lead to undesirable side reactions or impair the stability of the nanoparticles. Additionally, a reaction time of ≤ 20 h is preferred, a reaction time of ≤ 8 h is more preferred, a reaction time of ≤ 3 h is even more preferred, and a reaction time of ≤ 45 min is most preferred. Excessively long reaction times can promote undesirable side reactions.

[0024] After drying, the CPE can be removed from the non-stick film without tearing. The CPE layer thickness after drying is ≤ 40 µm, preferably ≤ 30 µm. There is no further lower limit to the layer thickness. Typical values ​​for the lower limit of the layer thickness are 5 µm, 10 µm, 15 µm, 20 µm, and 25 µm. The thickness of the resulting CPE can be determined using any suitable technique. While alternative techniques are conceivable, the thickness is particularly measured using measuring instruments such as a dial gauge or a thickness gauge blade, which are applied directly to the membrane. Multiple measurements at several points on the CPE and determination of the average value are preferred. Such a measurement procedure and its adaptation to different CPEs and thicknesses are known to those skilled in the art.The thickness of the resulting CPE can deviate from the set gap size, which is related to the concentration of the oxide nanoparticles. The higher the proportion of oxide nanoparticles, the greater the deviation of the resulting CPE thickness from the set gap size. This effect becomes more pronounced at a mass ratio of silane-terinated polymer to oxide nanoparticles of 1:3 or higher.

[0025] Step (b) can further include applying the paste to a cathode. If the paste is applied directly to a cathode, the rolling process in step (c) can improve the contact between the CPE and the cathode after drying. The paste can infiltrate the cathode, thereby improving the stability, contact, and electrochemical performance of the CPE after drying.

[0026] For example, to produce a cathode, a slurry of active material (e.g., lithium iron phosphate (LFP), lithium nickel manganese cobalt oxide (NMC), lithium manganese nickel oxide (LMNO), etc.) can be mixed with a binder (e.g., polyvinylidene fluoride (PVDF), (PVDF-HFP), cellulose, etc.) and electrically conductive graphite ("carbon black") in solvents such as N-methyl-2-pyrrolidone (NMP), applied to an aluminum foil, and dried. The porous cathode material can then be compacted with a roller press, the paste applied to the cathode, and subsequently rolled again to press it deep into the cathode. Preferably, the paste can be applied directly to the uncompacted cathode and then subjected to roller pressing, thus eliminating the step of compacting the cathode.This step allows for a denser compaction of the electrolyte and cathode, resulting in good contact and thus improved performance of a battery comprising a CPE according to the invention. Furthermore, it allows for the creation of a gradient from cathode material on the aluminum foil to the electrolyte membrane at the surface. This leads to better compatibility between the two components.

[0027] The polymer reacted in step (a) can also comprise at least one selected from the group consisting of polyethylene glycol (PEO), polycaprolactone, and polytetrahydrofuran (PTHF). Polyethylene glycol and polytetrahydrofuran generally already possess -OH groups, which are required for the reaction in step (a), while polycaprolactone diol (PCL) can be used in particular with respect to polycaprolactone. The aforementioned polymers and their silane-terminated reaction products have a low melting point, making them particularly suitable for a solvent-free reaction in steps (a) and (b) of the process according to the invention. Furthermore, the polarity of these polymers results in good miscibility with the oxide nanoparticles.

[0028] Potentially, all active and inactive oxide nanoparticles with -OH groups on their surface are suitable as oxide nanoparticles for conversion in step (b). Active oxide nanoparticles are capable of conducting ions, while inactive oxide nanoparticles are not. However, the conductivity at the interface of the inactive oxide nanoparticles is increased compared to the pure polymer.

[0029] The oxide nanoparticles used in step (b) can, in particular, comprise at least one selected from the group consisting of lithium lanthanum zirconium oxide (LLZO), molybdenum(VI) oxide (MoO3), silicon dioxide, aluminum oxide, magnesium oxide, and lithium aluminum titanium phosphate (LATP), preferably LLZO. However, other active nanoparticles of the NASICON (sodium (Na) super ionic conductor) or LISICON (lithium (Li) super ionic conductor) class can also be used, such as LiZr2(PO4)3, LiTi2(PO4)3, and LiGe2(PO4)3. Equivalent materials for other battery classes (e.g., Na, K, Mg) with high ionic conductivities and the corresponding cation can also be used.

[0030] The oxide nanoparticles can have a particle size ≤ 20 µm, with a particle size ≤ the desired layer thickness being preferred. There is no lower limit for the particle size used, although a particle size ≥ 50 nm is preferred. By choosing a sufficiently small particle size, achieving the desired layer thickness can be ensured. For example, for a CPE layer thickness of approximately 16 µm, particles with a size distribution from 100 nm to approximately 20 µm (average: 5 µm) can be used. This means that the thickness of the CPE is only slightly more than the diameter of the largest particles, without compromising the mechanical stability of the CPE. The size of the nanoparticles thus influences the mechanical stability of the membrane.Larger nanoparticles can contribute to improved mechanical stability by acting as reinforcing materials and supporting the structure of the CPE, even for nanoparticles whose diameter is approximately equal to the thickness of the CPE. Conversely, small nanoparticles have a larger specific surface area compared to larger nanoparticles. This allows them to disperse more effectively within the polymer matrix, resulting in a more homogeneous dispersion. This is important for ensuring uniform conductivity and structure of the CPE. The size of the oxide nanoparticles can be determined using any suitable technique, such as sieve analysis or laser diffraction. While alternative techniques are conceivable, the size of the oxide nanoparticles has been determined primarily using sieve analysis.For sieve analysis, a suitable sieve set with different mesh sizes covering the expected size range of the nanoparticles was selected. A defined quantity of the oxide nanoparticles was placed on the top sieve of the sieve set. The sieve set was placed in a sieving machine and shaken for a defined time and speed to sift the oxide nanoparticles through the sieves. After sieving, the oxide nanoparticles remaining on each sieve were collected and weighed. The particle size distribution and average value were calculated based on the weight of the oxide nanoparticles on each sieve and the known mesh size of the sieve. Corresponding parameters for determining and adapting the measurement method to different oxide nanoparticles are known to those skilled in the art.

[0031] In particular, as mentioned above, the cross-linking of silyl groups with the surface of oxide nanoparticles can be promoted without the addition of additional acids and bases by the targeted use of oxide nanoparticles with pH values ​​higher and lower than 7. Furthermore, the nanoparticles can be treated with concentrated hydrochloric acid or 30% hydrogen peroxide solution to increase the number of active -OH groups.

[0032] The conducting salt used in step (b) is a salt consisting of cation(s) and anion(s) that dissociates in a polymer or solvent and contributes to the ionic conductivity of the CPE. Any known conducting salts selected from alkali metal and alkaline earth metal salts that dissociate in polymers can be used. Examples of lithium conducting salts include: LiPF6, LiClO4, LiB[C2O4]2, LiBF4, LiAsF6, and lithium trifluoromethanesulfonate. Alternatively, any other type of cation selected from alkali metal and alkaline earth metal salts used in battery technology (e.g., Mg, K, Na) can be used in combination with dissociating anions. Alkali metal and alkaline earth metal ions exhibit high ionic mobility within the polymer, resulting in efficient ionic conductivity. This enables rapid and effective transport of charge carriers between the electrodes during charging and discharging in a battery.Lithium and magnesium conducting salts are preferred, with lithium conducting salts being even more so. Lithium and magnesium have a high energy density, meaning they can store a large amount of energy per unit mass. This is crucial for developing efficient battery systems. Lithium has the lowest electrochemical potential of all metals, meaning it has the highest electrochemical voltage. Furthermore, lithium is the lightest metal and has a low density, which contributes to making lithium-ion batteries lighter and more compact. This is particularly important for applications where weight is a factor, such as in portable electronics and electric vehicles.In this context, the conducting salt can, in particular, comprise at least one selected from the group consisting of lithium bis(trifluoromethylsulfonyl)amide (LiTFSI), magnesium bis(trifluoromethylsulfonyl)amide (Mg(TFSI)₂), magnesium trifluoromethanesulfonate (Mg(OTf)₂), and magnesium chloride (MgCl₂). LiTFSI, Mg(TFSI)₂, Mg(OTf)₂, and MgCl₂ are chemically stable and do not tend to decompose easily or undergo undesirable side reactions. Furthermore, these salts exhibit high ionic conductivity, meaning they efficiently facilitate the transport of lithium or magnesium ions between the electrodes of a battery. Good ionic conductivity is crucial for the performance and charge / discharge cycles of a battery.

[0033] The polymer used in step (a) has a number-average molecular weight of 2000 g / mol or more and 50000 g / mol or less, preferably a number-average molecular weight of 2900 g / mol or more and 20000 g / mol or less. Polymers with a number-average molecular weight ≥ 2000 g / mol lead to stable CPEs at layer thicknesses ≤ 40 µm, and polymers with a number-average molecular weight ≥ 2900 g / mol lead to even more stable CPEs at layer thicknesses ≤ 40 µm. Polymers with a number-average molecular weight < 2000 g / mol are too small, resulting in poor crosslinking because a polymer chain statistically often reacts with only one oxide nanoparticle instead of several. Therefore, in this case, the CPE becomes brittle at layer thicknesses ≤ 40 µm. At a number-mean molecular weight ≥ 20000 g / mol, the mechanical stability of the CPE is also impaired.Furthermore, if the number-average molecular weight of the polymer is too high, the high viscosity makes mixing into a paste more difficult. These effects are even more pronounced with a number-average molecular weight ≥ 50,000 g / mol. However, this also depends on the polymer class and can be adjusted, if necessary, by adding plasticizers or additives. For this purpose, the additives defined above can be used, for example. The following polymers, for instance, can also be used in step (a) of the process: polyethylene glycol CAS: 25322-68-3 (Mn = 6,000 g / mol and 10,000 g / mol), polycaprolactone diol CAS: 36890-68-3 (Mn = 2,000 g / mol), polytetrahydrofuran CAS: 25190-06-1 (Mn = 2,900 g / mol). The number-average molecular weight of the polymers used in step (a) can be determined using any suitable technique.While alternative techniques are conceivable, the number-mean molecular weight was determined specifically using size exclusion chromatography (SEC) with THF as the eluent at 25 °C. SEC was performed using a Tosoh EcoSEC (Tokyo, Japan) SEC system equipped with an SDV 5 µm particle size column (50 × 8 mm) followed by three SDV 5 µm columns (300 × 7.5 mm, successively with pore sizes of 100, 1000, and 105 Å) and a refractive index detector. Furthermore, the number-mean molecular weight was determined by calibration with, for example, a polyethylene oxide standard in the range of 1000 to 875,000 g / mol. Adaptations of the measurement method to different polymer types are known to those skilled in the art.

[0034] The mass ratio of the oxide nanoparticles and polymers defined above can be freely chosen, and various polymers and oxide nanoparticles can be used. In particular, mass ratios of silane-terminated polymer to oxide nanoparticles in the range of 1:0.1 to 1:3, preferably in the range of 1:0.1 to 1:2, can be used. If the nanoparticle fraction is too high, flexible CPE cannot usually be obtained, and if the nanoparticle fraction is too low, the CPE generally does not exhibit sufficient cross-linking and thus low mechanical stability.

[0035] According to the present invention, the composite polymer electrolyte membrane obtainable by the process has a thickness ≤ 40 µm, preferably a thickness ≤ 30 µm. A CPE with a thickness ≤ 40 µm, which can be used in a battery according to the invention, particularly as an electrolyte layer, enables better utilization of the available volume within the battery, resulting in a higher volumetric energy density. This allows solid-state batteries to store a greater amount of energy in a smaller installation space, which is particularly important for space-constrained applications such as portable electronic devices or electric vehicles. The ionic conductivities and ion transfer numbers of the composite polymer electrolyte membranes are higher than those of the pure polymers. The mechanical stability depends on the particle size and the polymer used. Smaller particle sizes result in CPES with a higher shear modulus.CPEs using polyethylene glycol generally exhibit a higher shear modulus than CPEs using polycaprolactone. The thermal properties of CPEs are comparable to those of the pure polymers.

[0036] Here, "batteries" refers to rechargeable accumulators. For simplicity, the term "batteries" is always understood to mean rechargeable. A battery comprising the CPE according to the invention, as well as the use of a CPE according to the invention, offers extensive application possibilities in the field of battery technology. One main application area is lithium-metal accumulators for electric vehicles or other applications requiring compact accumulators. Further potential applications include all currently known applications for lithium-ion batteries with liquid electrolytes. In particular, the CPEs according to the invention can be used as a hybrid solid polymer electrolyte for solid-state batteries. For the use of CPEs according to the invention in solid-state batteries, total cell resistances of less than 30 Ω are preferred.To achieve this goal, a particularly thin electrolyte membrane is essential. Furthermore, CPEs with layer thicknesses < 60 µm are often very difficult to handle for use in batteries, as even very small forces can destroy the CPE, which is not the case with the CPEs according to the invention, even at a thickness of ≤ 30 µm.

[0037] The process according to the invention is not limited to the preceding steps, but may include further process steps. In this context, the process according to the invention may in particular include additional steps of transferring, purifying and drying reaction products or the paste defined above.

[0038] The figures show: Fig. Figure 1 shows the addition of 3-(triethoxysilyl)propyl isocyanate to α,ω-OH-terminated polymers of step (a), in particular showing Fig.1 a) a general reaction scheme, where R is an undefined polymer suitable as a polymer electrolyte and Fig. Figure 1 b) shows an example reaction with polyethylene glycol. Fig. Figure 2 shows a condensation reaction of a silane-terminated polymer obtained from step (a) with -OH groups of oxide nanoparticles with elimination of ethanol. Fig. Figure 3 shows a schematic drawing of a roller press; the rollers run in opposite directions at the same speed. Fig. Figure 4 shows the thermogravimetric analysis of examples 1 to 3, performed with the TA Instruments TGA5500; Platinum HT crucible from 30 - 800 °C at a heating rate of 10 K / min. Fig.Figure 5 shows the dynamic differential calorimetry of examples 1 to 3, performed with Netzsch 214 Polyma DSC with a heating or cooling rate of 10 K / min in the temperature range of -150 - 160 °C; the second heating run is shown to equalize the previous temperature history. Examples

[0039] The following general method and examples serve to further explain the present invention, without, however, being limited thereto. General procedure

[0040] Reaction of α,ω-OH-terminated polymers with a silyl isocyanate according to step (a): Polyethylene glycol (PEO, CAS: 25322-68-3, Mn = 10000 g / mol), polycaprolactone diol (PCL, CAS: 36890-68-3, Mn = 2000 g / mol), or polytetrahydrofuran (PTHF, CAS: 25190-06-1, Mn = 2900 g / mol) were each used as α,ω-OH-terminated polymers and heated to 75 °C with 2 equivalents of 3-(triethoxysilyl)propyl isocyanate while stirring. After the polymer melted, a homogeneous reaction mixture was formed. Stirring for 24 h at 75 °C, after cooling to room temperature, yielded the silane-modified polymers.

[0041] Converting the silane-terminated polymers obtained in step (a) with oxide nanoparticles according to step (b): The silane-terminated polymer obtained from step (a) was mixed with LLZO with an average particle size of 5 µm to achieve a defined ratio (silane-terminated polymer : LLZO). A defined amount of LiTFSI (wt.% of the conducting salt based on the total weight of the paste) was also added. Optionally, additives (wt.% based on the mass of the silane-terminated polymer) can be added. The mixture was ground to a paste at room temperature in a mortar using a pestle. After stirring for 2 to 4 minutes, a homogeneous paste was formed.

[0042] Subjecting the paste obtained in step (b) to a rolling process using a rolling press with adjustable gap size according to step (c): A roller press from Xiamen Tmax Battery Equipments Limited was used to subject the paste obtained in step (b) to a rolling process. After mixing, the paste was placed between sheets of 100 µm thick Mylar® film. The roller press gap was then set to the desired thickness plus twice the film thickness, and the paste was pressed through the rollers to achieve the desired thickness. The Mylar® film was 100 µm thick. To achieve a thickness of approximately 30 µm, a gap of 230 µm was set.

[0043] Drying the paste after rolling in step (c), obtaining a composite polymer electrolyte membrane according to step (d): Drying took place at a temperature of 40 °C for 45 minutes in ambient air. After drying, the resulting CPE was removed from the Mylar® film and subjected to thermal and electrochemical characterization. Characterization of the CPEs obtained by the general procedure

[0044] Table 1 shows the results of a thermogravimetric analysis ( Fig. 4) and a dynamic differential calorimetry ( Fig. 5) of CPEs obtained by the above general procedure. Thermogravimetric analysis was performed using a TA Instruments TGA5500 instrument; Platinum HT crucibles from 30 to 800 °C at a heating rate of 10 K / min, and differential scanning calorimetry was performed using a Netzsch 214 Polyma DSC instrument with a heating / cooling rate of 10 K / min in the temperature range of -150 to 160 °C (values ​​from the second heating run to equilibrate previous temperature history). The measured values ​​of the thermal decomposition temperature (T d ) and the glass transition temperature (T g ) are shown below. Table 1 Schirtt (a) Step (b) Achieved thickness of the CPE (µm) T d (°C) T g (°C) polymer Mn (g / mol) LiTFSI (wt.%) Mass ratio polymer : LLZO Additives, wt.% Example 1 PEO 10000 17 1: 1 - 35 µm 360 -34 Example 2 PCL 2000 17 1: 1 - 30 µm 203 -25 Example 3 PTHF 2900 25 1: 1 - 25 µm 247 -51

[0045] Examples 2 and 3, where PTHF and PCL were used as polymers, respectively, showed no melting point in DSC. For Example 1, which used PEO, a melting point of 47 °C was determined ( Fig. 5) The ionic conductivity depends on the volume fraction that is amorphous. The less crystalline the material, the more amorphous phase and thus the higher the conductivity of the CPE. The crystallization of the polymers is partially suppressed by the oxide nanoparticles.

[0046] Table 2 shows the results of the ionic conductivity at 60 °C and Li + Transfer number at 60 °C of CPEs obtained by the general procedure. Table 2 Schirtt (a) Step (b) Achieved thickness of the CPE (µm). ionic conductivity σ (S / cm) Li + Transfer figure polymer Mn (g / mol) LiTFSI (wt.%) Mass ratio polymer : LLZO Additives, wt.% Example 4 PEO 10000 22 1 :0,5 - 37 1,45E-04 0,16 Example 5 PEO 10000 17 1:1 - 40 1,16E-04 0,12 Example 6 PEO 10000 17 1:1 SBF3, 2.12 39 6,80E-05 0,1 Example 7 PCL 2000 21 1: 0,5 - 30 8,53E-06 0,45 Example 8 PTHF 2900 31 1: 0,5 - 25 3,29E-06 0,08 Example 9 PTHF 2900 25 1:1 - 28 2,28E-06 0,62 Example 10 PTHF 2900 18 1:2 - 34 1,01E-06 0,65 Comparative example 1 PEO 10000 8 1:4 - 52 7,05E-05 0,09 Comparative example 2 PEO 10000 9 1:3 PEGDME500, 25 53 5,12E-05 0,11 Comparative example 3 PEO 10000 8 1:4 PEGDME500, 25 48 1,03E-04 0,13

[0047] The thickness variation of the CPEs, despite a uniform gap size of 30 µm, is related to the different LLZO concentrations. The higher the proportion of LLZO, the greater the thickness of the resulting CPE. This undesirable effect becomes more pronounced at a mass ratio of silane-terminated polymer to LLZO of 1:3.

[0048] Electrochemical impedance spectroscopy was used to determine the ionic conductivity. CR2032 button cells in a symmetrical arrangement with stainless steel discs as electrodes, with the CPEs listed in Table 2 (prepared according to the general procedure) sandwiched between them, were measured on a Multi Autolab M204 with an FRA32M module (Deutsche Metrohm). The data were then acquired using the NOVA 2.1.4 software (Metrohm Autolab BV). Logarithmic frequency scans with 50 steps from 1 to 106 Hz and an AC voltage of 40 mV were performed at all temperature settings (20 to 80 °C), controlled via a climate chamber (MK 053, Binder GmbH, Germany), with temperature fluctuations not exceeding ±0.02 K. The ionic conductivity was calculated using the following formula: σ=1R∗dπ∗r2 where σ is the ionic conductivity, R is the ohmic resistance, d is the sample thickness and r is the radius of the sample.

[0049] Examples using PEO as the polymer (Examples 4, 5, 6, and the comparison examples) generally exhibit higher ionic conductivities than examples using PCL or PTHF. Examples with higher weight percentages of LiTFSI also tend to have higher ionic conductivities. In the context of batteries, ionic conductivity is particularly important because it affects an electrolyte's ability to transport ions between the electrodes. Higher ionic conductivity results in more efficient ion diffusion and allows for faster battery charge and discharge rates, which in turn improves performance and energy density.

[0050] The Li +Transfer numbers were measured using the Bruce-Vincent method. For this purpose, symmetrical CR2032 button cells with lithium electrodes were constructed, between which CPEs fabricated according to the general procedure, with the parameters listed in Table 2, were enclosed. The measurements were performed at 60 °C in a climate chamber (MK 053, Binder GmbH, Germany), with temperature fluctuations not exceeding ±0.02 K. Complete transfer experiments, including polarization and inter-impedance measurements, were performed on a Multi Autolab M204 with an FRA32M module (Deutsche Metrohm), and the data were acquired using NOVA software 2.1.4 (Metrohm Autolab BV). The cells were equilibrated for 24 hours, during which the polymer / lithium interface impedance was monitored to ensure the formation of a stable SEI. To determine the Li +For transfer numbers, a constant DC voltage of 10 mV (ΔV) was applied to the cell and the time-dependent current was monitored, with the initial current being denoted as I0 and the final steady state current (ss) being denoted as I. ss The cell impedance was analyzed before and after each individual DC polarization step by AC impedance measurements. For this purpose, logarithmic frequency sampling was performed in 50 steps from 1 to 106 Hz with an alternating voltage of 10 mV. The ohmic component of the electrolyte impedance and the impedance of the lithium electrolyte interface are denoted as R. el,0 and R Li,ss for the sample before or after polarization in the steady state. The calculation of Li + Transfer figures were calculated according to the following formula: tLi+=Rel,0∗IssΔV−RLi,ss∗Iss

[0051] The values ​​of the Li +Transfer numbers for PEO are somewhat higher than the values ​​reported in the literature for pure PEO, which is attributed to the use of oxide nanoparticles (LLZO). In particular, examples 9 and 10 (PTHF) showed very high Li + -transfer numbers, which is due to the advantageous combination of the polymer, the conducting salt, the amount used, the oxide nanoparticle, and its ratio to the polymer. Example 7 (PCL) also showed a high Li + -transfer number, which is also attributable to the optimized combination of the polymer used and process parameters. In contrast, comparative examples with a greater CPE thickness and a higher ratio show comparatively lower Li + -Transfer figures. A high Li + The transfer number is particularly advantageous for deposition processes of Li-metal on the anode and for suppressing side reactions in batteries. Reference symbol list 1 non-stick film 2 Paste 3. Direction of movement 4 Adjustable gap width 5 Direction of rotation 6 Identical Roles 7 frame

Claims

[1] A method for producing a composite polymer electrolyte membrane with a thickness ≤ 40 µm, preferably with a thickness ≤ 30 µm, the method comprising the steps of: (a) reacting a polymer terminated with -OH, -SH or -NH2 α,ω, preferably an α,ω-OH-terminated polymer, with a silyl isocyanate to obtain a modified silane-terminated polymer, (b) reacting the polymer obtained in step (a) with oxidic nanoparticles, the particles having -OH groups on their surface, using a conductive salt selected from alkali metal and alkaline earth metal salts, but without using a solvent, thereby obtaining a paste (2), (c) subjecting the paste (2) obtained in step (b) to a rolling process by means of a rolling press with an adjustable gap, and (d) drying the paste (2) after rolling in step (c), to obtain a composite polymer electrolyte membrane. [2] A method for producing a composite polymer electrolyte membrane according to claim 1, wherein step (b) further comprises applying the paste (2) to a cathode. [3] A process for producing a composite polymer electrolyte membrane according to claim 1 or 2, wherein the polymer reacted in step (a) comprises at least one selected from the group consisting of polyethylene glycol, polycaprolactone and polytetrahydrofuran. [4] A method for producing a composite polymer electrolyte membrane according to any one of claims 1 to 3, wherein the oxide nanoparticles comprise at least one selected from the group consisting of lithium lanthanum zirconium oxide, molybdenum(VI) oxide, silicon dioxide, aluminum oxide, magnesium oxide and lithium aluminum titanium phosphate. [5] A method for producing a composite polymer electrolyte membrane according to any one of claims 1 to 4, wherein the conductive salt comprises at least one selected from the group consisting of lithium bis(trifluoromethylsulfonyl)amide, magnesium bis(trifluoromethylsulfonyl)amide, magnesium trifluoromethanesulfonate and magnesium chloride. [6] A process for producing a composite polymer electrolyte membrane according to any one of claims 1 to 5, wherein the polymer used in step (a) has a number average molecular weight of 2000 g / mol or more and 50000 g / mol or less. [7] A method for producing a composite polymer electrolyte membrane according to any one of claims 1 to 6, wherein a mass ratio of the polymer obtained from step (a) to the oxide nanoparticles has a range of 1:0.1 to 1:

4. [8] Composite polymer electrolyte membrane with a thickness ≤ 40 µm, preferably with a thickness ≤ 30 µm, obtainable by the process according to any one of claims 1 to 7. [9] A battery comprising the composite polymer electrolyte membrane according to claim 8. [10] Use of the composite polymer electrolyte membrane according to claim 8 in a battery.

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