Flame-retardant polymer electrolyte membrane of solid-state lithium battery and preparation method of flame-retardant polymer electrolyte membrane

Abstract

The invention relates to the field of all-solid-state electrolyte, in particular to a flame-retardant polymer electrolyte membrane of a solid-state lithium battery and a preparation method of the flame-retardant polymer electrolyte membrane. The polymer electrolyte membrane mainly comprises the following components: polyethylene oxide (PEO), lithium salt, a flame retardant, a plasticizer, a photoinitiator and the like. Through a solvent-free hot-pressing ultraviolet curing method, PEO is subjected to a cross-linking reaction, the crystallinity of the PEO is greatly reduced at room temperature, the mechanical performance of the polymer electrolyte is improved, the prepared polymer electrolyte membrane is non-combustible, the flame retardant can generate a more stable SEI layer in the battery circulation process, formation of lithium dendrites is inhibited, the electrochemical window of the polymer electrolyte membrane is widened, and the service life of the polymer electrolyte membrane is prolonged. And the lithium metal battery can be stably circulated for more than 200 circles at the multiplying power of 0.5 C, the capacity retention ratio reaches 95.9%, and the cycle performance and the safety of the lithium metal battery are remarkably improved. The preparation method is simple in process flow, wide in application range and low in cost, and has a great application prospect.

Description

Technical Field

[0001] The present invention relates to the technical field of solid electrolytes, and in particular to a flame-retardant polymer electrolyte membrane of a solid lithium battery and a preparation method thereof. Background Art

[0002] With the booming development of new energy industries such as electric vehicles, higher requirements are being placed on the currently widely used liquid lithium-ion batteries, especially in terms of energy density and safety. Since all-solid-state batteries do not require liquid electrolytes, they can achieve higher energy density and will not have leakage problems like liquid batteries. They can greatly improve the safety of lithium batteries, so all-solid-state batteries have broad development prospects.

[0003] Polymer solid electrolytes mainly include inorganic electrolytes and polymer electrolytes. Compared with inorganic electrolytes, polymer electrolytes have better chemical stability and processability, so they have received widespread attention. Among polymer electrolytes, PEO-based polymer electrolytes are one of the most popular polymer electrolytes because of their good conductivity and the unique characteristics of polymer materials such as light weight, good elasticity, and easy film formation. However, the practical application of PEO-based polymer electrolytes in lithium metal batteries still faces two challenges: (1) The operating temperature range of all-solid-state batteries assembled from unmodified PEO-based electrolytes is narrow, and the electrochemical performance is poor at room temperature; (2) PEO has poor thermal stability, is flammable, and is prone to deformation at high temperatures. The above challenges lead to poor cycling stability and low safety of lithium batteries, which seriously hinder the commercialization of PEO-based polymer electrolytes. Therefore, it is of great significance to achieve uniform lithium deposition and excellent thermal stability in lithium batteries by optimizing the composition of polymer electrolytes. Summary of the invention

[0004] In view of the above-mentioned deficiencies in the prior art, an object of the present invention is to provide a flame retardant polymer electrolyte membrane for a solid-state lithium battery, aiming to solve the problems of poor chemical properties and poor thermal stability of PEO-based electrolyte solid-state batteries in the prior art at room temperature.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions: In a first aspect, a flame retardant polymer electrolyte membrane of a solid-state lithium battery is provided, wherein the flame retardant polymer electrolyte membrane of the solid-state lithium battery is mixed with polyethylene oxide, lithium salt, flame retardant, plasticizer and photoinitiator.

[0006] Furthermore, the mass fraction of the polyethylene oxide is 10%-50%, the mass fraction of the lithium salt is 5%-20%, the mass fraction of the flame retardant is 10%-50%, the mass fraction of the plasticizer is 10%-40%, and the mass fraction of the photoinitiator is 5%-20%.

[0007] Furthermore, the polyethylene oxide PEO has a molecular weight of one or more of 100,000, 300,000, 600,000, 1 million and 5 million.

[0008] Furthermore, the lithium salt is lithium bis(trifluoromethanesulfonyl)imide LiTFSI, lithium bis(fluorosulfonyl)imide LiFSI, lithium perchlorate LiClO 4 、Lithium hexafluorophosphate LiPF 6 and lithium tetrafluoroborate LiBF 4 One or more of .

[0009] Furthermore, the flame retardant is one or more of trimethyl phosphate TMP, triethyl phosphate TEP, triphenyl phosphate TPP, tricresyl phosphate TCP and tolyl diphenyl phosphate CDP.

[0010] Furthermore, the plasticizer is one or more of fluoroethylene carbonate FEC, polyethylene glycol PEG, dioctyl adipate DOA, triethylene glycol dimethyl ether TEGDMA and tetraethylene glycol dimethyl ether TEGDME.

[0011] In a second aspect, a method for preparing a flame-retardant polymer electrolyte membrane of a solid-state lithium battery as described above comprises: S1. Add lithium salt, flame retardant, plasticizer and photoinitiator into a brown bottle in the order of flame retardant, lithium salt, plasticizer and photoinitiator to prevent lithium salt and photoinitiator from sticking to the bottom. After all are added, stir at room temperature to form a uniform solution. The brown bottle can reduce the negative impact of premature cross-linking caused by visible light during the stirring process. Stir and mix evenly to obtain a first mixed solution. S2, adding polyethylene oxide to the first mixed solution and stirring to mix evenly (premature addition may cause agglomeration of polyethylene oxide with solids such as lithium salt and photoinitiator, resulting in uneven subsequent stirring), to obtain a second mixed solution; S3. Pour the second mixed liquid onto a polytetrafluoroethylene mold, cover it with a PET sheet, and heat press it into a translucent film with a thickness of 100-300 μm at a temperature of 40-100° C. under a pressure of 0.2-10 MPa by a hot pressing method, and then cure it with ultraviolet light for 2-20 minutes to generate a flame-retardant polymer electrolyte membrane.

[0012] Furthermore, the temperature of the hot pressing method is 40-100° C. and the pressure is 0.2-10 MPa.

[0013] Furthermore, the stirring in step S1 and step S2 is performed by mixing with a magnetic stirrer, and the stirring speed of the magnetic stirring is 100-200 rpm / min.

[0014] In a third aspect, a PEO-based flame-retardant polymer electrolyte prepared by the method described above is applied to a lithium metal battery at room temperature.

[0015] The technical solution adopted by the present invention has the following beneficial effects: 1. The present invention provides a polymer electrolyte membrane, which significantly reduces the crystallinity of a PEO-based polymer at room temperature by introducing a plasticizer, thereby having a more efficient lithium deposition / stripping process.

[0016] 2. The present invention provides a polymer electrolyte membrane, which introduces a flame retardant to make the SEI membrane more stable, less prone to deformation at high temperatures, and has excellent flame retardant properties.

[0017] 3. The LFP||Li battery assembled with the polymer electrolyte membrane prepared by the method proposed in the present invention has excellent electrochemical performance, and the capacity at room temperature and 0.1C rate reaches 160 mAh g -1 Above, the capacity reaches 140mAh g at 0.5C rate -1 As above, after 200 stable cycles, the capacity retention rate reaches 95.9%.

[0018] 4. The method for preparing the polymer electrolyte membrane proposed in the present invention is low-cost, highly safe, compatible with existing battery processes, and easy to mass-produce. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 The process flow chart of the method for preparing the flame-retardant polymer electrolyte membrane of the solid-state lithium battery of the present invention; Figure 2 This is a flame retardant test diagram of the polymer electrolyte in the experimental example of the present invention; Figure 3 is an X-ray diffraction test diagram of the polymer electrolyte in the experimental example of the present invention; Figure 4 The impedance diagram of the battery assembled with the polymer electrolyte in the experimental example of the present invention; Figure 5 It is a linear scan LSV diagram of a battery assembled with a polymer electrolyte in an experimental example of the present invention; Figure 6 This is a low-rate cycle test diagram of a battery assembled with a polymer electrolyte in an experimental example of the present invention; Figure 7 This is a high-rate cycle test diagram of a battery assembled with a polymer electrolyte in an experimental example of the present invention. DETAILED DESCRIPTION

[0020] In order to make the purpose, technical solution and effect of the present invention clearer and more specific, the present invention is further described in detail with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0021] The present invention provides a flame retardant polymer electrolyte membrane of a solid-state lithium battery. The polymer electrolyte membrane comprises the following components by mass fraction: 10%-50% polyethylene oxide, 5%-20% lithium salt, 10%-50% flame retardant, 10%-40% plasticizer and 5%-20% photoinitiator.

[0022] In the present invention, by controlling the contents of the respective components of the polymer matrix, lithium salt, flame retardant, plasticizer and photoinitiator, the prepared solid electrolyte membrane has a high room temperature ionic conductivity and excellent flame retardancy. If the content of the flame retardant and plasticizer is too high, it will make it difficult for the solid electrolyte to form a film, and if the content is too low, it will lead to too low room temperature ionic conductivity and poor flame retardancy.

[0023] Specifically, in an embodiment of the present invention, the polymer electrolyte membrane includes the following components in mass fraction: polyethylene oxide 20%-40% (for example, 20%, 24%, 27%, 30%, 34%, 37%, 40%), lithium salt 6%-10% (for example, 6%, 7%, 8%, 9%, 10%), flame retardant 20%-40% (for example, 20%, 24%, 27%, 30%, 34%, 37%, 40%), plasticizer 20%-40% (for example, 20%, 24%, 27%, 30%, 34%, 37%, 40%) and photoinitiator 6%-10% (for example, 6%, 7%, 8%, 9%, 10%).

[0024] Further, in an embodiment of the present invention, the polymer electrolyte membrane may be implemented according to a mass ratio of polyethylene oxide, lithium salt, flame retardant, plasticizer and photoinitiator of 4:1:4:3:1.

[0025] In the present invention, by further optimizing the contents of the polymer matrix, lithium salt, flame retardant and plasticizer, the prepared solid electrolyte membrane has higher room temperature ionic conductivity and excellent flame retardancy.

[0026] The polyethylene oxide PEO is at least one of the polyethylene oxide PEO with a molecular weight of 100,000, 300,000, 600,000, 1 million and 5 million, and types not listed in the polyethylene oxide range are also applicable.

[0027] The lithium salts include lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium perchlorate (LiClO 4), lithium hexafluorophosphate (LiPF 6 ) and lithium tetrafluoroborate (LiBF 4 ), and types not listed in the scope of lithium salts are also applicable.

[0028] The flame retardant includes at least one of trimethyl phosphate (TMP), triethyl phosphate (TEP), triphenyl phosphate (TPP), tricresyl phosphate (TCP) and cresyl diphenyl phosphate (CDP), and types not listed in the flame retardant range are also applicable.

[0029] The plasticizer includes at least one of fluoroethylene carbonate (FEC), polyethylene glycol (PEG), dioctyl adipate (DOA), triethylene glycol dimethyl ether (TEGDMA) and tetraethylene glycol dimethyl ether (TEGDME), and types not listed in the scope of plasticizers are also applicable.

[0030] A method for preparing a flame-retardant polymer electrolyte membrane of a solid-state lithium battery as described above comprises: S1. Add lithium salt, flame retardant, plasticizer and photoinitiator into a brown bottle in the order of flame retardant, lithium salt, plasticizer and photoinitiator to prevent lithium salt and photoinitiator from sticking to the bottom. After all are added, stir at room temperature to form a uniform solution. The brown bottle can reduce the negative impact of premature crosslinking caused by visible light during the stirring process. Stir and mix evenly to obtain a first mixed solution. S2, adding polyethylene oxide to the first mixed solution and stirring to mix evenly (premature addition may cause agglomeration of polyethylene oxide with solids such as lithium salt and photoinitiator, resulting in uneven subsequent stirring), to obtain a second mixed solution; S3. Pour the second mixed liquid onto a polytetrafluoroethylene mold, cover it with a PET sheet, and heat press it into a translucent film with a thickness of 100-300 μm at a temperature of 40-100° C. under a pressure of 0.2-10 MPa by a hot pressing method, and then cure it with ultraviolet light for 2-20 minutes to generate a flame-retardant polymer electrolyte membrane.

[0031] Among them, the above operation steps are all required to be carried out in a glove box, and the temperature of the hot pressing method is 40-100°C and the pressure is 0.2-10MPa; the stirring in step S1 and step S2 is mixed by a magnetic stirrer, and the stirring speed of the magnetic stirring is 100-200rpm / min.

[0032] If the hot pressing temperature is too low, the polymer electrolyte membrane may not reach a molten state, causing the polymer electrolyte membrane to contain unmelted polyethylene oxide particles. If the temperature is too high, the additive may volatilize. Therefore, when hot pressing is performed, the hot pressing temperature is selected to be 40-100°C, for example, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, or 100°C.

[0033] If the hot pressing pressure is too high, the polymer electrolyte membrane may be too thin to form a membrane, and if the hot pressing pressure is too low, the polymer electrolyte membrane may be too thick. + The transmission is more difficult; therefore, during the hot pressing process, the hot pressing pressure is determined to be 0.2-10MPa. For example, the pressure values ​​can be selected as 0.2MPa, 2MPa, 4MPa, 6MPa, 8MPa, and 10MPa according to the hot pressing state.

[0034] If the UV curing time is too short, the cross-linking may not be complete. If the UV curing time is too long, the membrane may be damaged or the resistance may increase. The UV curing time is 2-20 min. During the curing process, the curing time can be selected according to the state of the electrolyte membrane, for example, 2 min, 4 min, 6 min, 8 min, 10 min, 12 min, 14 min, 16 min, 18 min, 20 min.

[0035] The present invention utilizes the cross-linking reaction between the plasticizer in the polymer electrolyte membrane and the polymer matrix to improve the mechanical properties of the polymer and reduce the crystallinity of the polymer, so that the polymer electrolyte membrane can have more Li at room temperature. + By introducing flame retardants, on the one hand, the polymer electrolyte membrane can have excellent flame retardancy, and on the other hand, the polymer electrolyte membrane can generate byproducts on the negative electrode side that help stabilize the SEI film, inhibit the growth of lithium dendrites, and broaden its electrochemical window. Therefore, the polymer electrolyte membrane ensures an efficient and stable lithium deposition / stripping process, so that the lithium metal battery using the polymer electrolyte membrane can have high ionic conductivity, stable long cycle and excellent flame retardancy at room temperature.

[0036] Example 1 This embodiment provides a method for preparing a PEO-based flame-retardant polymer electrolyte membrane suitable for room temperature all-solid-state lithium batteries. The process flow chart of preparing a flame-retardant polymer electrolyte membrane for solid-state lithium batteries is as follows: Figure 1 As shown, the flame retardant polymer electrolyte membrane comprises the following components according to mass fraction: PEO (M W =100,000 g mol -1 ), LiFSI, TMP, TEGDMA and MBP. Among them, the mass fraction of PEO in the polymer electrolyte mixed solution is 30.2%, the weight of LiFSI accounts for 25% of the total mass of LiFSI and PEO, the weight of TMP accounts for 75% of the total mass of LiTFSI and PEO, the weight of TEGDMA accounts for 75% of the total mass of LiFSI and PEO, and the weight of MBP accounts for 15% of the total mass of LiFSI and PEO.

[0037] The preparation method of the flame retardant polymer electrolyte membrane is as follows: The polymer electrolyte mixed solution is coated on a polytetrafluoroethylene plate mold, heated at 80°C for 1 hour, and then pressed (1MPa) to obtain an initial film with a thickness of 100-300㎛. The film is irradiated with ultraviolet light for 4 minutes to obtain a flame-retardant polymer electrolyte membrane.

[0038] Example 2 This embodiment provides a flame retardant polymer electrolyte membrane for a solid-state lithium battery. The flame retardant polymer electrolyte membrane comprises the following components by mass fraction: PEO (M W =600,000 g mol -1 ), LiTFSI, TEP, PEG, FEC and MBP. Among them, the mass fraction of PEO in the polymer electrolyte mixed solution is 30.2%, the weight of LiTFSI accounts for 25% of the total mass of LiTFSI and PEO, the weight of TEP accounts for 50% of the total mass of LiTFSI and PEO, the weight of PEG accounts for 40% of the total mass of LiTFSI and PEO, the weight of FEC accounts for 40% of the total mass of LiTFSI and PEO, and the weight of MBP accounts for 15% of the total mass of LiTFSI and PEO.

[0039] The preparation method of the flame retardant polymer electrolyte membrane is as follows: The polymer electrolyte mixed solution is coated on a polytetrafluoroethylene plate mold, heated at 60°C for 1 hour, and then pressed (3MPa) to obtain an initial film with a thickness of 100-300㎛. The film is irradiated with ultraviolet light for 10 minutes to obtain a flame-retardant polymer electrolyte membrane.

[0040] The following is the performance test of the samples prepared in Example 2: 1. Flame retardant test The polymer electrolyte membrane prepared in Example 2 was subjected to a combustion test. A flame was brought into contact with the polymer electrolyte membrane and the flame was removed after 2 seconds. The changes of the polymer electrolyte membrane before and after the combustion test were as follows: Figure 2 As shown, the polymer electrolyte membrane is self-extinguishing.

[0041] 2. X-ray diffraction test The polymer electrolyte membrane prepared in Example 2 was subjected to an X-ray diffraction test. The angle was 5-90°, the scanning speed was 5° / min, and the corresponding X-ray diffraction test pattern was as follows: Figure 3 As shown, the crystallinity of the polymer electrolyte membrane is low.

[0042] 3. Impedance test The polymer electrolyte membrane prepared in Example 2 was subjected to impedance testing. Stainless steel sheets were added to both ends of the polymer electrolyte membrane to form a battery test. The diameter of the polymer electrolyte membrane was 16 mm, the diameter of the stainless steel sheet was 16 mm, and the test frequency range was 0.1 Hz-1000000 Hz (electrochemical workstation). The corresponding impedance diagram at room temperature is as follows: Figure 4 As shown, the polymer electrolyte membrane has a small impedance.

[0043] 4. Linear scan LSV test The polymer electrolyte membrane prepared in Example 2 was subjected to a linear scanning LSV test. A lithium sheet and a stainless steel sheet were added to both ends of the polymer electrolyte membrane to form a battery test. The diameter of the lithium sheet was 12 mm, the diameter of the polymer electrolyte membrane was 16 mm, and the diameter of the stainless steel sheet was 16 mm. The scanning rate was 0.01 V s -1 (Electrochemical workstation), the corresponding linear scan LSV test diagram, such as Figure 5 As shown, the electrochemical window of the polymer electrolyte membrane is 4.3 V.

[0044] Example 3 This example is to prepare an all-solid-state lithium metal button battery, and the process is as follows: Lithium iron phosphate, acetylene black and polyvinylidene fluoride were mixed uniformly in N-methylpyrrolidone at a mass ratio of 7:2:1 to obtain a positive electrode slurry, which was coated on one side of an aluminum foil, dried in a vacuum at 120° C., and the N-methylpyrrolidone was removed to obtain a positive electrode sheet.

[0045] The obtained positive electrode sheet was cut into a sheet with a diameter of 12 mm, and the active material was about 2.03 mg cm -2 The negative electrode uses a metal lithium sheet. The flame-retardant polymer electrolyte membrane in Example 1 or 2 is cut into a disc with a diameter of 16 mm, sandwiched between the positive electrode sheet and the negative electrode sheet, and loaded into a 2032 battery shell to assemble into a button battery for testing.

[0046] The following is the performance test of the samples prepared in Example 3: 1. Cycle test The lithium metal battery of Example 3 was tested at room temperature. The charge cut-off voltage was 3.8 V, and the discharge cut-off voltage was 2.5 V. The charge and discharge current was set to 0.1 C. Figure 6 As shown in Figure 2, the all-solid-state battery prepared according to Example 3 has a discharge capacity of 160 mAh g at room temperature. -1 , the coulombic efficiency reaches 99%; the charge and discharge current is set to 0.5C, such as Figure 7 As shown, the all-solid-state battery prepared according to Example 3 was charged and discharged 200 times at room temperature, and the capacity retention rate reached 95.9%.

[0047] The implementation results show that the polymer electrolyte membrane provided by the present invention, by introducing a plasticizer, undergoes a cross-linking reaction, not only increases the mechanical properties of the polymer electrolyte membrane, but also has high ionic conductivity at room temperature; by introducing a flame retardant, not only does the polymer electrolyte membrane have excellent flame retardancy, but also generates a more stable SEI membrane, inhibits the formation of lithium dendrites, and improves the electrochemical window of the PEO-based polymer electrolyte membrane, so that the lithium metal battery assembled with the flame retardant polymer electrolyte membrane has ultra-high cycle stability and coulomb efficiency, significantly improving the cycle performance and safety performance of the lithium metal battery. The preparation process of the present invention is simple and low-cost, compatible with existing processes, and has great application prospects.

[0048] Those skilled in the art will readily appreciate other embodiments of the present invention after considering the specification and practicing the schemes disclosed herein. The present invention is intended to cover any variations, uses or adaptations of the present invention that follow the general principles of the present invention and include common knowledge or customary techniques in the art that are not disclosed in this disclosure. The description and examples are to be considered exemplary only, and the true scope and spirit of the present invention are indicated by the claims.

Claims

1. A flame retardant polymer electrolyte membrane for a solid-state lithium battery, characterized in that: The flame-retardant polymer electrolyte membrane of the solid-state lithium battery is mixed with polyethylene oxide, lithium salt, flame retardant, plasticizer and photoinitiator; the mass fraction of polyethylene oxide is 10%-50%, the mass fraction of lithium salt is 5%-20%, the mass fraction of flame retardant is 10%-50%, the mass fraction of plasticizer is 10%-40%, and the mass fraction of photoinitiator is 5%-20%.

2. The flame-retardant polymer electrolyte membrane of a solid-state lithium battery according to claim 1, characterized in that: The polyethylene oxide PEO has a molecular weight of one or more of 100,000, 300,000, 600,000, 1 million and 5 million.

3. The flame-retardant polymer electrolyte membrane of a solid-state lithium battery according to claim 1, characterized in that: The lithium salt is one or more of lithium bis(trifluoromethanesulfonyl)imide LiTFSI, lithium bis(fluorosulfonyl)imide LiFSI, lithium perchlorate LiClO4, lithium hexafluorophosphate LiPF6 and lithium tetrafluoroborate LiBF4.

4. The flame-retardant polymer electrolyte membrane of a solid-state lithium battery according to claim 1, characterized in that: The flame retardant is one or more of trimethyl phosphate TMP, triethyl phosphate TEP, triphenyl phosphate TPP, tricresyl phosphate TCP and tolylene diphenyl phosphate CDP.

5. The flame-retardant polymer electrolyte membrane of a solid-state lithium battery according to claim 1, characterized in that: The plasticizer is one or more of fluoroethylene carbonate FEC, polyethylene glycol PEG, dioctyl adipate DOA, triethylene glycol dimethyl ether TEGDMA and tetraethylene glycol dimethyl ether TEGDME.

6. A method for preparing a flame-retardant polymer electrolyte membrane for a solid-state lithium battery as claimed in claim 1, characterized in that: include: S1. Add lithium salt, flame retardant, plasticizer and photoinitiator into a brown bottle in the order of flame retardant, lithium salt, plasticizer and photoinitiator to prevent lithium salt and photoinitiator from sticking to the bottom. After all are added, stir at room temperature to form a uniform solution. The brown bottle can reduce the negative impact of premature cross-linking caused by visible light during the stirring process. Stir and mix evenly to obtain a first mixed solution. S2, adding polyethylene oxide to the first mixed solution and mixing them evenly by stirring to obtain a second mixed solution; S3. Pour the second mixed liquid onto a polytetrafluoroethylene mold, cover it with a PET sheet, and heat press it into a translucent film with a thickness of 100-300 μm at a temperature of 40-100° C. under a pressure of 0.2-10 MPa by a hot pressing method, and then cure it with ultraviolet light for 2-20 minutes to generate a flame-retardant polymer electrolyte membrane.

7. The flame-retardant polymer electrolyte membrane of a solid-state lithium battery according to claim 6, characterized in that: The temperature of the hot pressing method is 40-100° C. and the pressure is 0.2-10 MPa.

8. The flame-retardant polymer electrolyte membrane of a solid-state lithium battery according to claim 6, characterized in that: The stirring in step S1 and step S2 is performed by mixing with a magnetic stirrer, and the stirring speed of the magnetic stirring is 100-200 rpm / min.

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