Electrolyte film and method for making the same

Abstract

The present application provides an electrolyte film and a method for making the electrolyte film. The method includes mixing a first polymer, an ionic liquid, and an organic solvent, to obtain a first solution. A solid electrolyte is added into the first solution, stirring, to obtain a slurry body A. Many second polymers having different molecular weights are mixed in proportion, to obtain a polymer mixture. The polymer mixture, the ionic liquid, and the organic solvent are mixed, to obtain a colloidal fluid B. The slurry body A and the colloidal fluid B are mixed in proportion, to obtain an intermediate colloid C. The lithium salt, the ionic liquid, and the organic solvent are mixed, to obtain a composite solution. The intermediate colloid C and the composite solution are mixed and stirred, to obtain a composite. The composite is placed on a substrate, and the electrolyte film is formed after drying the composite.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to an electrolyte membrane and its preparation method. Prior Technology

[0002] With increasing market demand, lithium-ion batteries are facing increasingly stringent requirements in terms of energy density, rate performance, and cycle life. High-energy-density materials are often accompanied by high specific surface area or high activity, making them prone to reaction within the battery system. Therefore, there is a desire to develop materials that reduce electrolyte content, thereby introducing solid-state electrolyte materials to address battery safety issues.

[0003] However, current electrolyte membranes generally suffer from problems such as excessive thickness, brittleness, poor ionic conductivity, insufficient toughness to resist dendrite growth, and low yield. Therefore, improving ionic conductivity to enhance cell rate performance and cycle life, increasing the mechanical strength and dendrite growth suppression ability of solid electrolyte membranes, and taking into account the thickness of the electrolyte membrane are all directions for future research. Summary of the Invention

[0004] In view of this, it is necessary to provide a solid electrolyte membrane and its preparation method that can improve the poor ionic conductivity, insufficient toughness, and inability to resist crystal dendrites of existing solid electrolyte membranes.

[0005] This invention proposes a method for preparing an electrolyte membrane, which includes the following steps: The first polymer, ionic liquid, and organic solvent are mixed to obtain the first solution; A solid electrolyte was added to the first solution and stirred to obtain a slurry A. A polymer mixture is obtained by mixing multiple second polymers with different weight average molecular weights. The polymer mixture, the ionic liquid, and the organic solvent are then mixed to obtain a colloidal solution B. The slurry A is mixed with the colloidal solution B to obtain an intermediate colloid C. The slurry A is added to the colloidal solution B in multiple portions, with mixing for a period of time after each addition. During the process of adding the slurry A to the colloidal solution B, the pH value of the entire solution is controlled to be between 6.8 and 7.5. A composite solution is obtained by mixing a specific ratio of lithium salt and the ionic liquid with the organic solvent; The intermediate colloid C was mixed with the composite solution and stirred to obtain a composite material; The composite was placed on a substrate and then dried.

[0006] An electrolyte membrane, prepared by the method for preparing the electrolyte membrane.

[0007] Compared with existing technologies, the electrolyte membrane prepared by the method of the present invention has good ionic conductivity, good toughness, and stable mechanical properties, which are sufficient to resist the formation of dendrites. Furthermore, the preparation method of the electrolyte membrane provided by the present invention is simple and can be mass-produced. Simple Explanation of the Diagram

[0008] Figure 1 is a flowchart of the preparation method of the electrolyte membrane provided by the present invention.

[0009] Figure 2 shows the mixing process of slurry A provided by the present invention.

[0010] Figure 3 is a schematic diagram of the grinding machine provided by the present invention.

[0011] Figure 4 is a particle size analysis diagram of the slurry A provided by the present invention after being micronized by a grinding machine.

[0012] Figure 5 is a process flow diagram for preparing colloidal liquid B provided by the present invention.

[0013] Figure 6 is a flowchart of the slurry mixing process in the preparation of the electrolyte membrane provided by the present invention.

[0014] Figure 7 is a schematic diagram of the interaction between the ionic liquid and the solid ceramic powder in the composite provided by the present invention.

[0015] Figure 8 is a schematic diagram of the RTR (roll-to-roll) of the composite provided by the present invention coated on the substrate.

[0016] Figure 9 shows a schematic diagram of the PS test curves of the electrolyte membrane with added ionic liquid and the electrolyte membrane without added ionic liquid. Implementation

[0017] The electrolyte membrane and its preparation method provided by the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0018] Please refer to Figure 1. This invention provides a method for preparing an electrolyte membrane, which includes the following steps: S1, mix the first polymer, ionic liquid, and organic solvent to obtain the first solution; S2, add the solid electrolyte to the first solution and stir to obtain a slurry A; S3, mix multiple second polymers with different average molecular weights to obtain a polymer mixture, then mix the polymer mixture, the ionic liquid, and the organic solvent to obtain a colloidal solution B; S4, the slurry A is mixed with the colloidal solution B to obtain an intermediate colloid C, wherein the slurry A is added to the colloidal solution B in multiple portions, and the mixture is stirred for a period of time after each addition. During the process of adding the slurry A to the colloidal solution B, the pH value of the entire solution is controlled to be 6.8~7.5; S5, a specific ratio of lithium salt and the ionic liquid is mixed with the organic solvent to obtain a composite solution; S6, the intermediate colloid C is mixed with the composite solution and stirred to obtain a composite material; S7, The composite is placed on a substrate and dried.

[0019] In step S1, the first polymer is a high molecular weight polymer or a low molecular weight polymer with a molecular weight between 300,000 and 10,000. For example, the first polymer can be polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyvinyl alcohol (PVA), polylactic acid (PLA), polyethylene glycol (PEG), polymethyl methacrylate (PMMA), propylene glycol methyl ether acetate (PMA), polyacrylic acid (PAA), etc., or combinations thereof. The ionic liquid is an ionic compound in its liquid state, mainly composed of charged ions and transiently existing ion pairs, and is a liquid salt at room temperature. The organic solvent is a solvent capable of dissolving the main high molecular weight polymers, for example, the organic solvent is N,N-dimethylacetamide (DMAC), N-methylpyrrolidone (NMP), or N,N-dimethylformamide (DMF), etc., or combinations thereof. In one specific embodiment, the first polymer is a mixture of PVDF and PMMA, the ionic liquid is 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI), and the organic solvent is DMAC.

[0020] In step S2, the solid electrolyte is lithium lanthanum zirconium oxide (LLZO), tantalum-doped lithium lanthanum zirconium oxide (LLZTO), lithium titanium aluminum phosphate (LATP), or a combination thereof. In a specific embodiment, the solid electrolyte is LLZO. LLZO is added to the first solution, and the mixed liquid is injected into a mill containing zirconium beads. Grinding is performed at a speed of 2400 rpm, wherein the zirconium bead filling amount is 1.5 kg, the material temperature in the mill is controlled below 30°C, and nitrogen sealing protection is required throughout the process, for example, by injecting inert gas to eliminate oxygen or moisture present in the atmosphere.

[0021] In step S3, in one specific embodiment, the second polymer is a polymer with a molecular weight between 600,000 and 2,000,000, such as PVDF, polyacrylonitrile (PAN), and polyethylene oxide (PEO). The molecular weights of the four PVDFs with different weight average molecular weights are 2,000,000, 1,500,000, 1,300,000, and 1,000,000, respectively. The mass ratio of the four PVDFs with different weight average molecular weights is 1-5% : 5-10% : 60-90% : 10-20%. In one specific embodiment, the polymer mixture is mixed with DMAC containing 0.1% ionic liquid at a powder-to-solvent mass ratio of 0.35-0.2. That is, the ratio of the polymer mixture to DMAC containing 0.1% ionic liquid is 0.35-0.2. In one specific embodiment, a defoaming mixer is used to mix the polymer mixture, ionic liquid, and organic solvent until homogeneous, wherein the temperature is controlled at 60°C, the revolution:rotation ratio is 9:9, and the mixing time is 30 minutes.

[0022] In step S4, the slurry A and the colloidal liquid B are mixed at a mass ratio of 0.8 to 1.65. In one specific embodiment, the slurry A is added to the colloidal liquid B in three portions, and homogenization is performed using a degassing mixer with a revolution:rotation ratio of 9:9. Each addition and homogenization time is approximately 30 minutes, with the temperature controlled at approximately 60°C throughout the process, and the pH maintained at approximately 6.8-7.5 throughout.

[0023] In step S5, the lithium salt can be lithium fluoride (LiF), lithium difluorosulfonyl imide (LiO4NS2F2, abbreviated as LiFSI), lithium bis(trifluoromethanesulfonyl imide) (LiN(CF3SO2)2, abbreviated as LiTFSI), lithium bis(perfluoroethylsulfonyl imide) (Li(C2F5SO2)2N, abbreviated as LiBETI), lithium dioxaborate (LiB(C2O4)2, abbreviated as LiBOB), or combinations thereof. It is understood that conventional lithium salts used in lithium-ion batteries satisfy the requirements of this invention. A specific ratio of lithium salt and the ionic liquid is mixed with the organic solvent, i.e., the mass of the lithium salt is greater than the mass of the ionic liquid, and the total mass of the lithium salt and the ionic liquid is controlled to be less than 30% of the electrolyte membrane mass.

[0024] In step S6, in one specific embodiment, the intermediate colloid C is added to the composite solution and mixed using a degassing mixer.

[0025] In step S7, the material of the substrate is not limited and can be a polymer material, etc. The method for depositing the composite on the substrate is not limited, such as spin coating, spray coating, etc. In one specific embodiment, the composite is coated on the substrate using a blade coating method, with a blade gap of 200µm~500µm, a water droplet angle of 70°~120°, and a thickness of 50µm~150µm. The drying method is not limited. In one specific embodiment, the substrate coated with the composite is placed in an oven and baked at three temperature stages: Stage 1: Heating temperature 80℃~110℃, fan intake 30Hz, exhaust 25Hz; Stage 2: Initial solvent evaporation, heating temperature 130℃~150℃, fan intake 30Hz, exhaust 25Hz; Stage 3: Heating temperature 130℃~150℃, fan intake 43Hz, exhaust 25Hz. Higher fan frequency results in greater airflow, which can affect the solvent concentration and evaporation effect within the oven.

[0026] Furthermore, the method for preparing the electrolyte membrane may further include step S8 of depositing a release film on the surface of the electrolyte membrane away from the substrate, facilitating subsequent transportation and use of the electrolyte membrane. In a specific embodiment, step S8 involves attaching the release film to the surface of the electrolyte membrane away from the substrate. The release film has a droplet angle of 70° to 120° and a thickness of 10µm to 80µm. This results in an electrolyte membrane sheet with a thickness of 15µm to 20µm, an elongation of 30% to 40%, and a maximum tensile stress of 6 MPa to 12 MPa.

[0027] The following specific embodiment illustrates the preparation method of the electrolyte membrane, but it is not limited thereto.

[0028] The first step involves dissolving 1%–3% PVDF with a molecular weight of 1.3 million, 10%–50% PMMA relative to PVDF by mass, and 0.2% ionic liquid EMITFSI in DMAC to obtain the first solvent.

[0029] The second step involves mixing LLZO in powder form at a ratio of 1% to 20% into the first solution. Specifically, the mixture is pre-mixed uniformly for 15 minutes using an emulsifying homogenizer, then stirred using a planetary mixer. Finally, the mixed liquid is injected into a mill containing zirconium beads and ground at a speed of 2400 rpm. The zirconium bead loading is 1.5 kg, and the temperature of the grinding material is controlled below 30°C. Nitrogen sealing protection is applied throughout the process to obtain a homogenized slurry A with 50% of the particles having a diameter less than 100 nm (D50 < 100 nm) and 90% of the particles having a diameter less than 150 nm (D90 < 150 nm). Figure 2 shows the mixing process of slurry A. Figure 3 is a schematic diagram of the mill when the mixed liquid is added to the mill for nano-micronization treatment. The grinding mill contains 1.5 kg of zirconium beads. The process parameters are: nano-micronization at a speed of 2400 rpm; the material temperature in the grinding mill is controlled below 30℃ throughout the grinding process; the process time is 1 to 5 hours; and nitrogen gas is introduced throughout the process to avoid interference from water vapor. Figure 4 shows the particle size analysis of the slurry A after nano-micronization in the grinding mill. D50 < 100 nm and D90 < 150 nm indicate that the slurry A has become a homogenized slurry.

[0030] The third step involves mixing four types of PVDF with different average molecular weights (MWHMs) in a specific ratio. The MWHMs of these four PVDFs are 2 million, 1.5 million, 1.3 million, and 1 million, respectively, and the ratio of these four PVDFs is 1%–5% : 5%–10% : 60%–90% : 10%–20%, resulting in the polymer mixture. Then, the polymer mixture is mixed with DMAC containing 0.1% ionic liquid at a powder-to-solvent ratio of 0.35–0.2, and the mixture is stirred until homogeneous using a degassing mixer (revolution:rotation = 9:9, mixing time 30 min), yielding colloidal solution B. Figure 5 shows the process flow diagram for preparing colloidal solution B. Specifically, four types of PVDF with different weight-average molecular weights (2 million: 1.5 million: 1.3 million: 1 million) are mixed in a ratio of 1%~5%: 5%~10%: 60%~90%: 10%~20%, that is, PVDF with a molecular weight of 2 million: PVDF with a molecular weight of 1.5 million: PVDF with a molecular weight of 1.3 million: PVDF with a molecular weight of 1 million (mass ratio) = 1%~5%: 5%~10%: 60%~90%: 10%~20%, thereby obtaining the polymer mixture. The inclusion of 0.1% ionic liquid DMAC means that the mass ratio of the ionic liquid in the solvent formed by mixing the ionic liquid and DMAC is 0.1%. The polymer mixture is added to the solvent formed by mixing the ionic liquid and DMAC, and the polymer mixture:solvent formed by mixing the ionic liquid and DMAC (mass ratio) is 0.35~0.2.

[0031] Step 4: Mix the slurry A and the colloidal solution B at a ratio of 0.8 to 1.65. The slurry A is added to the colloidal solution B in three separate additions, each mixing for 30 minutes. During this process, the pH of the solution must be controlled at 6.8 to 7.5, and the temperature must be maintained at 60°C throughout, to obtain intermediate colloid C.

[0032] Step 5: Mix DMAC, ionic liquid, and LiTFSI to obtain the composite solution. The mass ratio of LiTFSI to ionic liquid is 3:4, and the total mass of LiTFSI and ionic liquid is controlled to be less than 30% of the electrolyte membrane mass.

[0033] Step 6: Add the composite solution to the intermediate colloid C and mix with a degassing mixer to obtain the composite.

[0034] Step 7: The composite is coated onto the substrate (with a water droplet angle of 70° to 120° and a thickness of 50µm to 150µm) using a blade coating method (blade gap of 200µm to 500µm). Then, it is dried using a three-stage baking temperature process to obtain the electrolyte membrane. The three-stage baking temperature process is as follows: Stage 1: Heating temperature of 80℃ to 110℃, air intake of 30, air outlet of 25; Stage 2: Initial solvent evaporation, heating temperature of 130℃ to 150℃, air intake of 30, air outlet of 25; Stage 3: Heating temperature of 150℃ to 180℃, air intake of 43, air outlet of 25. A release film is attached to the surface of the electrolyte membrane (the droplet angle of the release film is 70°~120° and the thickness is 10µm~80µm) to obtain a membrane with a thickness of 15µm~20µm (elongation is 30%~40% and maximum tensile stress is 6MPa~12MPa).

[0035] Figure 6 is a flow chart of the slurry mixing process during the preparation of the electrolyte membrane. That is, Figure 6 shows the process from steps four to six above. In Figure 6, slurry A represents slurry body A, slurry B represents colloidal liquid B, slurry C represents intermediate colloid C in step four, and slurry D represents the complex in step six. As shown in Figure 6, slurry body A is added to colloidal liquid B in three equal portions and mixed uniformly using a degassing mixer to form intermediate colloid C (revolution:rotation = 9:9, 30 minutes is the process time for one slurry addition, during which the pH value must be controlled at 6.8~7.5). Then, DMAC, ionic liquid, and LiTFSI are mixed and added to intermediate slurry body C, and mixed uniformly using a degassing mixer to form slurry D (i.e., the complex in step six).

[0036] Figure 7 is a schematic diagram of the interaction between the ionic liquid and the solid electrolyte LLZO powder in the slurry D (i.e., the composite in step 6). In Figure 7, "Ceramic" represents LLZO powder, and "Ionic liquid" represents ionic liquid. The LLZO powder carries a negative charge in the DMAC solvent, while the ionic liquid carries a positive charge. The positive and negative charges attract each other, causing the ionic liquid to coat the surface of the LLZO powder. This allows the LLZO powder surface to carry a positive charge, preventing secondary agglomeration of the LLZO powder. After the membrane is formed, the LLZO powder is uniformly distributed in the electrolyte membrane, improving the conductivity and mechanical stability of the electrolyte membrane.

[0037] Figure 8 is a schematic diagram of the RTR (roll-to-roll) of the composite coating on the substrate. As shown in Figure 8, the composite is coated onto the substrate using a blade coating method and dried using a three-stage baking temperature process. The first stage heating temperature is 80℃~110℃, the second stage heating temperature is 130℃~150℃, and the third stage heating temperature is 150℃~180℃, ultimately yielding an electrolyte film. This electrolyte film contains an ionic liquid. For comparison, a separate electrolyte film without added ionic liquid is also provided; no ionic liquid was added during any step of the preparation process.

[0038] Figure 9 shows the PS test curves of the electrolyte film with and without ionic liquid. The first part of the test curve was tested under the following conditions: charging for 1 hour at a current density of 0.5 mA / cm², stopping for 10 minutes, and then discharging for 1 hour, repeated for 3 cycles. The second part of the test curve was tested under the following conditions: charging for 1 hour at a current density of 1.25 mA / cm², stopping for 10 minutes, and then discharging for 1 hour, with repeated cyclic testing. Specific test results are shown in Figure 9. Figure 9 clearly shows that the electrolyte film with ionic liquid significantly reduces the voltage bias and increases the cycle life of the lithium metal PS test.

[0039] The present invention further provides an electrolyte membrane, which is prepared by the same method used for preparing the electrolyte membrane. The electrolyte membrane is a solid electrolyte membrane, suitable for lithium-ion batteries, etc.

[0040] The electrolyte membrane and its preparation method have the following advantages: First, the electrolyte membrane prepared by the method provided by this invention has good ionic conductivity, good toughness, and stable mechanical properties, which are sufficient to resist the formation of dendrites. The ionic conductivity of the solid electrolyte membrane without ionic liquid is about E-05, and the film elongation is unstable between 10% and 50%; the ionic conductivity of the solid electrolyte membrane with added ionic liquid can be increased to E-04, and the film elongation is stable between 30% and 40% because the solid electrolyte powder is uniformly distributed in the electrolyte membrane; Second, the method is simple and can be mass-produced.

[0041] In conclusion, this invention meets the requirements for an invention patent, and therefore a patent application is filed in accordance with the law. However, the above description is merely a preferred embodiment of this invention and should not be construed as limiting the scope of this patent application. All equivalent modifications or variations made by those skilled in the art in accordance with the spirit of this invention should be covered within the scope of the following patent application.

[0042] S1-S8: Steps

[0043] none

Claims

1. A method for preparing an electrolyte membrane, comprising the following steps: A first polymer, an ionic liquid, and an organic solvent are mixed to obtain a first solution. A solid electrolyte is added to the first solution and stirred to obtain a slurry A. The solid electrolyte is lithium lanthanum zirconium oxide, tantalum-doped lithium lanthanum zirconium oxide, lithium titanium aluminum phosphate, or a combination thereof. Multiple second polymers with different weight average molecular weights are mixed to obtain a polymer mixture. The polymer mixture, the ionic liquid, and the organic solvent are mixed to obtain a colloidal solution B. Slurry A is mixed with colloidal solution B to obtain an intermediate colloid C. Slurry A is added to colloidal solution B in multiple additions, with mixing for a period of time after each addition. During the addition of slurry A to colloidal solution B, the pH of the entire solution is controlled to be 6.8-7.

5. A specific ratio of lithium salt and the ionic liquid is mixed with the organic solvent to obtain a composite solution. The intermediate colloid C is mixed with the composite solution and stirred to obtain a complex. The complex is placed on a substrate and dried.

2. The method for preparing the electrolyte membrane as described in claim 1, wherein, The first polymer is polyvinylidene fluoride, (polyvinylidene fluoride-hexafluoropropylene) copolymer, polyvinyl alcohol, polylactic acid, polyethylene glycol, polymethyl methacrylate, propylene glycol methyl ether acetate, polyacrylic acid, or a combination thereof.

3. The method for preparing the electrolyte membrane as described in claim 2, wherein, The first polymer is a mixture of polyvinylidene fluoride and polymethyl methacrylate.

4. The method for preparing the electrolyte membrane as described in claim 1, wherein, The organic solvent is N,N-dimethylacetamide, N-methylpyrrolidone, N,N-dimethylformamide, or a combination thereof.

5. The method for preparing the electrolyte membrane as described in claim 1, wherein, The second polymer is polyvinylidene fluoride (PVDF). Four types of PVDF with different weight-average molecular weights are mixed. The molecular weights of the four types of PVDF with different weight-average molecular weights are 2 million, 1.5 million, 1.3 million, and 1 million, respectively. The mass ratio of PVDF with a molecular weight of 2 million: PVDF with a molecular weight of 1.5 million: PVDF with a molecular weight of 1.3 million: PVDF with a molecular weight of 1 million is 1%~5% : 5%~10% : 60%~90% : 10%~20%.

6. The method for preparing the electrolyte membrane as described in claim 1, wherein, A specific ratio of lithium salt and the ionic liquid is mixed with the organic solvent, wherein the mass of the lithium salt is greater than the mass of the ionic liquid, and the sum of the masses of the lithium salt and the ionic liquid is less than 30% of the mass of the electrolyte membrane.

7. The method for preparing the electrolyte membrane as described in claim 1, wherein, The composite was placed on the substrate and dried using a three-stage baking temperature method. The heating temperature in the first stage was 80℃~110℃, the heating temperature in the second stage was 130℃~150℃, and the heating temperature in the third stage was 150℃~180℃.

8. The method for preparing the electrolyte membrane as described in claim 1, wherein, The method further includes the step of providing a release film on the surface of the electrolyte membrane away from the substrate.

9. An electrolyte membrane, improved in that the electrolyte membrane is prepared by the method for preparing an electrolyte membrane as described in any one of claims 1 to 8.

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