A fluorinated polyacrylonitrile-based solid electrolyte membrane, and a preparation method and application thereof

Abstract

The application belongs to the technical field of solid electrolyte materials and solid-state batteries, and particularly relates to a fluorinated polyacrylonitrile-based solid electrolyte film and a preparation method and application thereof. The fluorinated polyacrylonitrile-based solid electrolyte film is obtained by fluorinating a polyacrylonitrile electrolyte film with fluorine gas. The introduction of fluorine elements significantly improves the dielectric constant of the matrix and reduces the crystallinity, thereby improving the ionic conductivity without sacrificing the mechanical strength, relieving the lithium dendrite growth problem, effectively improving the interface stability of the solid electrolyte and the lithium metal electrode, and improving the electrochemical performance of the solid-state lithium battery.

Description

Technical Field

[0001] This invention belongs to the field of solid electrolyte materials and solid battery technology, and particularly relates to a fluorinated polyacrylonitrile-based solid electrolyte membrane, its preparation method and application. Background Technology

[0002] With the increasing demand for high-energy-density and high-safety energy storage systems from portable electronic devices and electric vehicles, traditional liquid lithium-ion batteries pose a risk of thermal runaway due to the use of flammable organic electrolytes. Solid-state batteries, with their advantages of being non-flammable, leak-free, and capable of suppressing lithium dendrite formation, are considered the core of next-generation energy storage technology. However, while current mainstream oxide and sulfide inorganic solid electrolytes possess high room-temperature ionic conductivity, they are brittle, have high interfacial impedance, and are expensive to process. Polymer solid electrolytes such as polyethylene oxide (PEO), although flexible and easy to form films, have low room-temperature ionic conductivity (typically <10). - 5 Poor interfacial stability (S / cm) leads to dendrite growth, severely limiting cycle life and coulombic efficiency. Polyacrylonitrile (PAN), due to its polar cyano groups which can form strong coordination with lithium salts, theoretically possesses high lithium salt solubility and excellent mechanical strength, making it a potential matrix for polymer solid electrolytes. However, pure PAN has high crystallinity and restricted chain segment movement, resulting in a room temperature ionic conductivity of only 10. -6 The S / cm range is significant; however, the poor interfacial stability between PAN and lithium metal leads to high interfacial impedance, uneven lithium deposition, dendrite penetration, and rapid capacity decay. Although existing technologies attempt to modify PAN-based electrolytes using plasticizers, blends, or nanofillers, plasticizers are volatile, blends have poor compatibility, and nanofillers are difficult to disperse, making it difficult to simultaneously achieve high ionic conductivity, high mobility number, good interfacial compatibility, and long-term cycling stability. Therefore, developing a polyacrylonitrile-based solid-state electrolyte that combines high room-temperature ionic conductivity, excellent mechanical strength, and electrochemical performance remains a critical technological bottleneck in the field of solid-state batteries. Summary of the Invention

[0003] The purpose of this invention is to address the key technical bottlenecks of existing polyacrylonitrile-based solid electrolytes, such as low ionic conductivity at room temperature, poor interfacial stability with lithium metal anodes, and the resulting low initial coulombic efficiency and short cycle life of solid-state batteries. The invention proposes a high-performance solid electrolyte based on fluorinated polyacrylonitrile and its preparation method, and further provides a high-energy-density solid-state battery containing this solid electrolyte.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for preparing a fluorinated polyacrylonitrile-based solid electrolyte membrane includes the following steps: S1. Acrylonitrile monomer, free radical initiator and lithium salt are mixed in a polar solvent and subjected to free radical copolymerization under an inert atmosphere to obtain fluorinated polyacrylonitrile-lithium salt composite solution; S2. The composite solution obtained in step S1 is cast onto the surface of the substrate and dried to form a solid electrolyte membrane with a thickness of 10-100 μm; S3. Add the solid electrolyte membrane obtained in step S2 into the fluorination device, purge the air with nitrogen, and then purge with a mixture of nitrogen and fluorine gas to react and obtain a fluorinated polyacrylonitrile-based solid electrolyte membrane.

[0005] Furthermore, to improve ionic conductivity, an ion transport promoter accounting for 0.1-5% of its mass can be introduced into the composite solution. The ion transport promoter is carbon nanotube or graphene. The carbon nanotube or graphene is added to the fluorinated polyacrylonitrile-lithium salt composite solution prepared in step S1 and dispersed uniformly by ultrasonication. This additive can construct a continuous lithium ion rapid transport channel.

[0006] Furthermore, the polar solvent is one of N,N-dimethylformamide, N-methylpyrrolidone, acetonitrile, or acetone.

[0007] Furthermore, the lithium salt is at least one of LiTFSI, LiFSI, LiPF6 or LiClO4, and the molar ratio of the lithium salt to the acrylonitrile unit is 0.1 to 0.5.

[0008] Furthermore, the free radical initiator is azobisisobutyronitrile or benzoyl peroxide.

[0009] Furthermore, in step S1, the free radical copolymerization reaction temperature is 50-80℃ and the reaction time is 4-12 hours, which can suppress side reactions while ensuring the degree of polymerization.

[0010] Furthermore, in step S2, the drying temperature is 60-120°C under vacuum for 6-24 hours to completely remove residual solvent and improve the film density.

[0011] To achieve better fluorination results, in step S3, the nitrogen gas flow rate is 0.8–6 mL / min and the gas flow time is 1–3 h to remove air; the fluorine gas content in the nitrogen and fluorine gas mixture is 5 wt%–30 wt%; the mixed gas flow rate is 4–10 mL / min; the reaction temperature is -20–0 °C; and the reaction time is 72–192 h.

[0012] Another object of the present invention is to provide a fluorinated polyacrylonitrile-based solid electrolyte membrane prepared by the above method.

[0013] Another object of the present invention is to provide a solid-state battery comprising the aforementioned fluorinated polyacrylonitrile-based solid electrolyte membrane, a lithium metal anode, and a cathode. The cathode material may be selected from at least one of lithium iron phosphate, lithium cobalt oxide, lithium nickel cobalt manganese oxide, or sulfur-carbon composite materials, with lithium iron phosphate being particularly outstanding due to its high safety and long lifespan. The thickness of the solid electrolyte membrane is preferably 20-80 μm, achieving a balance between energy density and mechanical strength. To further improve interfacial stability, a 5-50 nm thick lithium fluoride protective layer may be pre-coated onto the surface of the lithium metal anode. This protective layer can be formed through in-situ reaction or physical vapor deposition, effectively suppressing lithium dendrite penetration and reducing interfacial resistance.

[0014] Mechanism: Fluorination with fluorine gas replaces the CH on the PAN chain with fluorine, introducing CF bonds on the PAN film surface, which significantly reduces the surface energy and promotes the formation of a stable SEI film rich in LiF when in contact with lithium metal, thereby improving the interfacial stability problem; Fluorinated PAN can reduce the crystallinity of PAN, which is conducive to chain segment movement, lithium ion migration, and improve ionic conductivity.

[0015] The advantages of this invention are: by introducing fluorine, the dielectric constant of the matrix is ​​significantly improved and the crystallinity is reduced, thereby increasing the ionic conductivity without sacrificing mechanical strength and alleviating the problem of lithium dendrite growth; the interfacial stability between the solid electrolyte and the lithium metal electrode is effectively improved, thus enhancing the electrochemical performance of solid-state lithium batteries; this invention achieves a synergistic improvement in room temperature performance, interfacial compatibility, and cycle life of polyacrylonitrile-based solid electrolytes, providing key technical support for the practical application of high-energy-density and high-safety solid-state batteries. Detailed Implementation

[0016] Example 1 1. Raw material preparation Acrylonitrile (AN) 10.0 g (0.189 mol) and LiTFSI 3.5 g (0.012 mol) were weighed according to a LiTFSI / AN molar ratio of 0.15. Azobisisobutyronitrile (AIBN) 0.12 g (0.0007 mol) was used as a free radical initiator. Anhydrous N,N-dimethylformamide (DMF) 40 mL was used as the solvent.

[0017] 2. Free radical copolymerization In an argon-filled glove box, AN, LiTFSI, and AIBN were added sequentially to a 100 mL container and magnetically stirred until completely dissolved. The system was then sealed and placed in a 70°C oil bath for 6 h under nitrogen protection. After the reaction was complete, a homogeneous, precipitate-free, pale yellow viscous solution (PAN-LiTFSI composite solution) was obtained.

[0018] 3. Film formation and drying The composite solution was injected into a polytetrafluoroethylene mold (effective area 50 mm × 50 mm) and dried in a vacuum oven at 80°C for 16 h. After the solvent was completely evaporated, a transparent flexible solid electrolyte membrane PAN-SSE-1 with a thickness of about 30 μm was obtained.

[0019] 4. Fluorine gas fluorination PAN-SSE-1 was added to the fluorination device, and nitrogen gas was introduced at a flow rate of 3 mL / min for 2 h to remove air. Then, a mixture of nitrogen and fluorine gas (fluorine content 10 wt%) was introduced at a flow rate of 6 mL / min. The reaction temperature was controlled at -20℃ and the reaction was carried out for 96 h to obtain a fluorinated polyacrylonitrile solid electrolyte membrane (hereinafter abbreviated as FPAN-SSE-1).

[0020] 5. Solid-state battery assembly In an argon-filled glove box, the FPAN-SSE-1 electrolyte membrane obtained in step 4 was punched into Φ16 mm discs. Using lithium metal foil (0.5 mm thick) as the negative electrode, and aluminum foil coated with lithium iron phosphate (LiFePO4, specific capacity 160 mAh / g) / Super P / polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1 as the positive electrode, the electrode compaction density was 2.3 g cm⁻¹. -3 A lithium fluoride protective layer of approximately 20 nm thickness is pre-deposited on the surface of the lithium anode by magnetron sputtering. The coin cell solid-state battery is then packaged in the following order: anode shell - spring sheet - gasket - lithium anode - electrolyte - cathode - cathode shell.

[0021] Example 2 Preparation of an electrolyte containing a carbon nanotube ion transport promoter (FPAN-SSE-2) 0.3 g of multi-walled carbon nanotubes (MWCNTs) (3% of the polymer matrix mass) were added to the composite solution obtained in step 2 of Example 1. The solution was dispersed in an ice-water bath using an ultrasonic disruptor for 30 min, and then cast and dried according to step 3 of Example 1 to obtain an electrolyte membrane FPAN-SSE-2 with a thickness of approximately 25 μm.

[0022] The solid-state battery assembly steps are the same as in Example 1.

[0023] Example 3 The PAN-SSE-1 membrane from Example 1 was placed in a fluorination device, and nitrogen gas was introduced at a flow rate of 4 mL / min for 1 hour to remove air. Then, a mixture of nitrogen and fluorine gas (fluorine content 20 wt%) was introduced at a flow rate of 8 mL / min. The reaction temperature was controlled at -10℃, and the reaction was carried out for 168 hours to obtain FPAN-SSE-1.

[0024] Example 4 The PAN-SSE-1 membrane from Example 1 was placed in a fluorination device, and nitrogen gas was introduced at a flow rate of 5 mL / min for 1 hour to remove air. Then, a mixture of nitrogen and fluorine gas (fluorine content 30 wt%) was introduced at a flow rate of 4 mL / min. The reaction temperature was controlled at -15°C, and the reaction was carried out for 192 hours to obtain FPAN-SSE-1.

[0025] Example 5 The PAN-SSE-1 membrane from Example 1 was placed in a fluorination device, and nitrogen gas was introduced at a flow rate of 4 mL / min for 1 hour to remove air. Then, a mixture of nitrogen and fluorine gas (fluorine content 20 wt%) was introduced at a flow rate of 4 mL / min. The reaction temperature was controlled at -10℃, and the reaction was carried out for 144 hours to obtain FPAN-SSE-1.

[0026] Comparative Example 1 Most of the steps in Comparative Example 1 and Example 1 are the same, except that no fluorination treatment is performed, and the solid electrolyte membrane PAN-SSE-1 is finally obtained.

[0027] Comparative Example 2 Comparative Example 2 and Example 2 followed most of the same steps, except that no fluorination treatment was performed, resulting in the final solid electrolyte membrane PAN-SSE-2.

[0028] The solid electrolyte membranes and solid batteries prepared in Examples 1-5 and Comparative Examples 1-2 were subjected to performance tests, and the test results are shown in Table 1.

[0029] 1. Tensile property test: Tensile tests were performed on the solid electrolyte membrane using a universal testing machine. A dumbbell-shaped specimen, 20 mm in length and 4 mm in width, was used according to ISO 527-2. The tensile speed during testing was 50 mm / min.

[0030] 2. Ionic conductivity: The test was conducted at 25℃ using a stainless steel blocking electrode to measure AC impedance (frequency range 1 MHz - 0.1 Hz, amplitude 10 mV). The ionic conductivity data were then calculated using the formula σ = L / (R × S), as shown in Table 1. Where σ is the ionic conductivity, L is the thickness of the electrolyte membrane, R is the resistance of the electrolyte membrane, and S is the contact area between the electrolyte membrane and the stainless steel electrode.

[0031] 3. Electrochemical performance testing: Constant current charge-discharge test at 25℃ and 0.5C, voltage range of 3.75-2.5V, 100 cycles, and record capacity retention rate; rate performance test is also performed at 5C high rate.

[0032] Table 1 Test Results The above embodiments fully demonstrate that by introducing fluorine-containing units into the polyacrylonitrile framework, and optionally introducing carbon nanotubes / graphene, the fluorinated polyacrylonitrile solid electrolyte provided by the present invention can achieve high ionic conductivity at room temperature. Solid-state batteries assembled from it possess high energy density, high rate performance, and long cycle life, thus comprehensively verifying that the solid electrolyte provided by the present invention has excellent electrochemical performance. Its application in batteries can significantly improve the room temperature ionic conductivity and enhance cycle and rate performance. In Example 2, the introduction of carbon nanotubes, utilizing lithium-ion transport, significantly improved ionic conductivity. In Example 4, the reaction conditions were harsh, resulting in a thicker fluorinated layer, which affected the mechanical properties of the membrane itself. The reaction process required controlling the fluorination depth by adjusting the fluorine concentration, reaction temperature, and time.

Claims

1. A method for preparing a fluorinated polyacrylonitrile-based solid electrolyte membrane, characterized in that, Includes the following steps: S1. Acrylonitrile monomer, free radical initiator and lithium salt are mixed in a polar solvent and subjected to free radical copolymerization under an inert atmosphere to obtain fluorinated polyacrylonitrile-lithium salt composite solution; S2. The composite solution obtained in step S1 is dried to form a solid electrolyte membrane with a thickness of 10-100 μm; S3. Add the solid electrolyte membrane obtained in step S2 into the fluorination device, purge the air with nitrogen, and then purge with a mixture of nitrogen and fluorine gas to react and obtain a fluorinated polyacrylonitrile-based solid electrolyte membrane.

2. The method as described in claim 1, characterized in that: It also includes an ion transport promoter, which is a carbon nanotube or graphene. The carbon nanotube or graphene is added to the fluorinated polyacrylonitrile-lithium salt composite solution prepared in step S1 and dispersed evenly by ultrasonication.

3. The method as described in claim 1, characterized in that: The polar solvent is one of N,N-dimethylformamide, N-methylpyrrolidone, acetonitrile, or acetone.

4. The method as described in claim 1, characterized in that: The lithium salt is at least one of LiTFSI, LiFSI, LiPF6 or LiClO4, and the molar ratio of lithium salt to acrylonitrile unit is 0.1 to 0.

5.

5. The method as described in claim 1, characterized in that: The free radical initiator is azobisisobutyronitrile or benzoyl peroxide.

6. The method as described in claim 1, characterized in that: In step S1, the free radical copolymerization reaction temperature is 50-80℃, and the reaction time is 4-12 hours.

7. The method as described in claim 1, characterized in that: In step S2, the drying temperature is 60-120℃ under vacuum for 6-24 hours.

8. The method as described in claim 1, characterized in that: In step S3, the nitrogen gas flow rate is 0.8–6 mL / min and the gas flow time is 1–3 h; the fluorine gas content in the nitrogen and fluorine gas mixture is 5 wt%–30 wt%; the mixed gas flow rate is 4–10 mL / min; the reaction temperature is -20–0 °C; and the reaction time is 72–192 h.

9. Fluorinated polyacrylonitrile-based solid electrolyte membrane prepared by the method according to any one of claims 1-8.

10. A solid-state battery, characterized in that: It includes the fluorinated polyacrylonitrile-based solid electrolyte membrane as described in claim 9.

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