A method for reducing the swell of a pvdf copolymer

Abstract

The present application relates to a method for reducing the swelling degree of PVDF copolymer, comprising the following steps: the PVDF copolymer is reacted under the condition of emulsion state, 70-95 DEG C and pH 10-12. In the method, the PVDF copolymer in emulsion state is reacted under the condition of specific temperature and pH, not only the molecular chain of the PVDF copolymer is dehydrofluorinated to generate double bond, but also the generated double bond is crosslinked, so that the crosslinking degree of the PVDF copolymer is improved, and the swelling degree of the PVDF copolymer is reduced.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and more specifically, to a method for reducing the swelling degree of PVDF copolymers. Background Technology

[0002] Wet-process separators have become the most widely used separator type in the lithium-ion battery market due to their excellent overall performance. To further improve separator performance, coating technology is widely used for separator surface modification. Currently, commonly used coating materials mainly include inorganic ceramic materials (such as boehmite and alumina) and organic polymer materials (such as PVDF, PMMA, and aramid). Among organic polymer materials, PVDF and its copolymers have become a research hotspot due to their good swelling properties, enabling them to adsorb more electrolyte and improve the ionic conductivity of the separator.

[0003] PVDF copolymers are typically copolymers of VDF and HFP, such as Arkema LBG, where the copolymerization ratio (molar percentage) of HFP is less than 2.5%. As the copolymerization ratio of HFP increases, although more electrolyte can be adsorbed, the swelling degree of PVDF copolymer in the electrolyte may increase significantly, and it may even dissolve, thereby damaging the structural integrity of the membrane and affecting its performance as a coating material.

[0004] Chinese patents on organic-inorganic composite PVDF resin emulsions, coating solutions and their preparation methods also focus on swelling performance. The specification states that when the proportion of HFP in the comonomer is high, the swelling is high, it is easy to dissolve, and it cannot maintain the core-shell structure, but it does not further solve this problem.

[0005] Therefore, there is an urgent need to develop a new technology that can reduce the swelling degree of PVDF copolymers. Summary of the Invention

[0006] The primary objective of this invention is to overcome the problem of excessive swelling in existing PVDF copolymers as the copolymerization ratio of HFP increases, and to provide a method for reducing the swelling degree of PVDF copolymers. In this method, under specific temperature and pH conditions, the PVDF copolymer in emulsion state not only undergoes HF removal to generate double bonds in its molecular chains, but the generated double bonds also undergo cross-linking, thereby increasing the degree of cross-linking of the PVDF copolymer and thus reducing its swelling degree.

[0007] The above-mentioned objective of the present invention is achieved through the following technical solution:

[0008] A method for reducing the swelling degree of PVDF copolymers,

[0009] Includes the following steps:

[0010] The PVDF copolymer was reacted in an emulsion state at 70–95°C and pH 10–12.

[0011] Through research, the inventors discovered that under specific temperature and pH conditions, the PVDF copolymer in emulsion state not only loses HF to generate double bonds, but the generated double bonds also undergo cross-linking, thereby increasing the degree of cross-linking of the PVDF copolymer and reducing the degree of swelling of the PVDF copolymer.

[0012] If the reaction temperature is too low (e.g., 60℃), the degree of crosslinking of the PVDF copolymer will be low, resulting in a negligible reduction in the swelling degree of the PVDF copolymer. If the reaction temperature is too high (e.g., 100℃), the PVDF copolymer in the emulsion state will demulsify and agglomerate, greatly reducing the uniformity of the reaction and essentially preventing crosslinking, thus failing to reduce the swelling degree of the PVDF copolymer.

[0013] Preferably, the molar percentage of HFP units in the PVDF copolymer is ≥4%.

[0014] When the molar percentage of HFP units in a PVDF copolymer is ≥4%, the PVDF copolymer typically has difficulty removing HF to form double bonds. However, the inventors of this invention have discovered that the method of this invention is applicable to PVDF copolymers with a molar percentage of HFP units ≥4%.

[0015] More preferably, the molar percentage of HFP units in the PVDF copolymer is 4-10%.

[0016] It should be understood that the molar percentage of HFP units in the PVDF copolymer of the present invention refers to the ratio of the amount of HFP units in the PVDF copolymer to the sum of the amounts of HFP units and VDF units.

[0017] The molar percentage of HFP units in the PVDF copolymer described in this invention was determined by nuclear magnetic resonance fluorine spectroscopy (F... 19 Measured by NMR.

[0018] Preferably, the pH is 11-12. When the pH is controlled within this range, the PVDF copolymer treated by the method of the present invention exhibits lower swelling.

[0019] Preferably, the PVDF copolymer is reacted at 85–95°C. By controlling the temperature within this range, the PVDF copolymer treated by the method of the present invention exhibits lower swelling degree.

[0020] Preferably, the reaction time is 1 to 2 hours.

[0021] Preferably, the pH is adjusted by adding an alkali.

[0022] More preferably, the alkali is at least one of NaOH, LiOH, KOH or NH4OH.

[0023] Preferably, the PVDF copolymer forms the emulsion state with water in the presence of an emulsifier.

[0024] More preferably, the emulsifier is at least one of sodium perfluoropolyether carboxylate, perfluorooctanoate, perfluoropolyether carboxylate, or hexafluoropropylene oxide polymer.

[0025] More preferably, the volume ratio of the water to the mass of the emulsifier is (7.5-10.5) L: 1 g.

[0026] Preferably, the PVDF copolymer is prepared by emulsion polymerization of VDF and HFP.

[0027] More preferably, the conditions for emulsion polymerization include maintaining the reaction system pressure at 2.5–4.5 MPa and the temperature at 70–90°C.

[0028] More preferably, the emulsion polymerization is carried out in an atmosphere with an oxygen content of less than 50 ppm.

[0029] More preferably, the emulsion polymerization is terminated by stopping the addition of the mixed monomers VDF and HFP, maintaining the temperature for 3 to 6 minutes, and then depressurizing to 0.09 to 0.11 MPa.

[0030] More preferably, the emulsifier used in the emulsion polymerization is at least one of sodium perfluoropolyether carboxylate, perfluorooctanoate, perfluoropolyether carboxylate, or hexafluoropropylene oxide polymer.

[0031] More preferably, the emulsifier accounts for 0.040 to 0.240% of the total mass of VDF and HFP.

[0032] More preferably, the mass ratio of VDF to HFP is (80-91):(9-20).

[0033] More preferably, the ratio of the volume of water to the total mass of VDF and HFP during the emulsion polymerization process is (3.0-4.2) L: 1 kg.

[0034] More preferably, the initiator used in the emulsion polymerization is at least one of ammonium persulfate, potassium persulfate, or sodium persulfate.

[0035] More preferably, the initiator accounts for 0.024 to 0.120% of the total mass of VDF and HFP.

[0036] More preferably, the chain transfer agent used in the emulsion polymerization is at least one of diethyl malonate, ethyl acetate, diethyl carbonate, or dimethyl carbonate.

[0037] More preferably, the total mass of the chain transfer agents VDF and HFP is 0.048 to 0.120%.

[0038] Preferably, the average particle size of the PVDF copolymer treated by the method is 2 to 10 μm.

[0039] Preferably, the reaction further includes at least one step selected from filtration, washing, drying, or pulverization.

[0040] Compared with the prior art, the beneficial effects of the present invention are:

[0041] In the method of the present invention, under specific temperature and pH conditions, the PVDF copolymer in emulsion state not only loses HF to generate double bonds, but the generated double bonds also undergo cross-linking, thereby increasing the degree of cross-linking of the PVDF copolymer and reducing the degree of swelling of the PVDF copolymer. Detailed Implementation

[0042] To more clearly and completely describe the technical solution of the present invention, the present invention will be further described in detail below through specific embodiments. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention. Various changes can be made within the scope of the claims of the present invention.

[0043] Example 1

[0044] This embodiment provides a method for reducing the swelling degree of PVDF copolymers, including the following steps:

[0045] NaOH solution was added to the PVDF copolymer 1# in emulsion state to adjust the pH of the reaction system to 11. The mixture was stirred at 85℃ for 1 hour, then filtered, washed, dried, and pulverized to reduce the swelling degree of the PVDF copolymer. The average particle size of the PVDF copolymer treated by this method was 9 μm.

[0046] The preparation method of PVDF copolymer 1# includes the following steps: In a 150L high-pressure reactor, add 105L of deionized water, then add 10g of sodium perfluoropolyether carboxylate. Vacuum the reactor until the oxygen content is below 50ppm. Start stirring at 45rpm. After the reactor temperature reaches 85℃, add VDF / HFP mixed monomers (VDF to HFP mass ratio of 91:9) to the reactor until the system pressure reaches 4.5MPa. Add 6g of ammonium persulfate initiator and 12g of diethyl malonate chain transfer agent to begin emulsion polymerization. Continue to add VDF / HFP mixed monomers to maintain the reaction pressure at 4.5MPa. When the VDF / HFP mixed monomer addition reached 2.5 kg, 7.5 kg, and 12.5 kg, 2 g, 1 g, and 1 g of initiator were added respectively; until the VDF / HFP mixed monomer addition reached 25 kg, the addition of VDF / HFP mixed monomer was stopped, and after holding at the temperature for 5 min, the system pressure was released to 0.10 MPa to obtain PVDF copolymer 1#, which was in an emulsion state. (The last sentence appears to be incomplete and possibly refers to a process involving F...) 19 NMR analysis showed that the molar percentage of HFP units in PVDF copolymer #1 was 4.32%.

[0047] Example 2

[0048] This embodiment provides a method for reducing the swelling degree of PVDF copolymers. The difference from Embodiment 1 is that PVDF copolymer 1# is replaced with PVDF copolymer 2#, and the molar percentage of HFP units in PVDF copolymer 2# is 6.72%.

[0049] The difference between the preparation method of PVDF copolymer 2# and PVDF copolymer 1# lies in the fact that the mass ratio of VDF to HFP in the added VDF / HFP mixed monomers is 85:15; through F 19 NMR analysis showed that the molar percentage of HFP units in PVDF copolymer #2 was 6.72%.

[0050] Example 3

[0051] This embodiment provides a method for reducing the swelling degree of PVDF copolymers. The difference from Embodiment 1 is that PVDF copolymer 1# is replaced with PVDF copolymer 3#, and the molar percentage of HFP units in PVDF copolymer 23# is 10%.

[0052] The difference between the preparation method of PVDF copolymer 3# and PVDF copolymer 1# lies in the fact that the mass ratio of VDF to HFP in the added VDF / HFP mixed monomers is 80:20; through F 19 NMR analysis showed that the molar percentage of HFP units in PVDF copolymer #3 was 10%.

[0053] Example 4

[0054] This embodiment provides a method for reducing the swelling degree of PVDF copolymers, which differs from Example 1 in that:

[0055] Add NaOH solution to the PVDF copolymer 1# in emulsion state to adjust the pH of the reaction system to 11, stir at 70℃ for 1 hour, and then filter, wash, dry and pulverize to complete the reduction of PVDF copolymer swelling degree.

[0056] Example 5

[0057] This embodiment provides a method for reducing the swelling degree of PVDF copolymers, which differs from Example 1 in that:

[0058] Add NaOH solution to the PVDF copolymer 1# in emulsion state to adjust the pH of the reaction system to 11, stir at 95℃ for 1 hour, and then filter, wash, dry and pulverize to complete the reduction of PVDF copolymer swelling degree.

[0059] Example 6

[0060] This embodiment provides a method for reducing the swelling degree of PVDF copolymers, which differs from Example 1 in that:

[0061] Add NaOH solution to the PVDF copolymer 1# in emulsion state to adjust the pH of the reaction system to 10, stir at 85℃ for 1 hour, and then filter, wash, dry and pulverize to complete the reduction of the swelling degree of PVDF copolymer.

[0062] Example 7

[0063] This embodiment provides a method for reducing the swelling degree of PVDF copolymers, which differs from Example 1 in that:

[0064] Add NaOH solution to the PVDF copolymer 1# in emulsion state to adjust the pH of the reaction system to 12, stir at 85℃ for 1 hour, and then filter, wash, dry and pulverize to complete the reduction of PVDF copolymer swelling degree.

[0065] Comparative Example 1

[0066] This comparative example provides a comparative PVDF copolymer, which is PVDF copolymer 1# in Example 1, i.e., without any treatment to reduce the swelling degree of PVDF copolymer 1#.

[0067] Comparative Example 2

[0068] This comparative example provides a comparative PVDF copolymer, which is PVDF copolymer 2# in Example 2, i.e., without any treatment to reduce the swelling degree of PVDF copolymer 2#.

[0069] Comparative Example 3

[0070] This comparative example provides a comparative PVDF copolymer, which is PVDF copolymer 3# in Example 3, i.e., without any treatment to reduce the swelling degree of PVDF copolymer 3#.

[0071] Comparative Example 4

[0072] This comparative example provides a method for reducing the swelling degree of PVDF copolymers, which differs from Example 1 in that:

[0073] Add NaOH solution to the PVDF copolymer 1# in emulsion state to adjust the pH of the reaction system to 9, stir at 85℃ for 1 hour, and then filter, wash, dry and pulverize.

[0074] Comparative Example 5

[0075] This comparative example provides a method for reducing the swelling degree of PVDF copolymers, which differs from Example 1 in that:

[0076] Add NaOH solution to the PVDF copolymer 1# in emulsion state to adjust the pH of the reaction system to 13, stir at 85℃ for 1 hour, and then filter, wash, dry and pulverize.

[0077] Comparative Example 6

[0078] This comparative example provides a method for reducing the swelling degree of PVDF copolymers, which differs from Example 1 in that:

[0079] Add NaOH solution to the PVDF copolymer 1# in emulsion state to adjust the pH of the reaction system to 11, stir at 60℃ for 1 hour, and then filter, wash, dry and pulverize.

[0080] Performance testing

[0081] Method for testing the degree of swelling: The PVDF copolymers prepared in each example or the comparative PVDF copolymers prepared in each comparative example were completely dissolved in N-methylpyrrolidone. After drying at 70°C for 4 days, PVDF films were obtained. The PVDF films were cut into 2g square films and weighed, which were recorded as the original film mass. The films were then immersed in an electrolyte (a solution formed by mixing ethylene carbonate, propylene carbonate, dimethyl carbonate, and methyl ethyl carbonate in a mass ratio of 35:5:5:55) at 70°C. After equilibration, the films were weighed, and the weight of the films after swelling equilibrium was recorded as the film mass after swelling equilibrium. The degree of swelling was calculated according to the formula (film mass after swelling equilibrium - original film mass) / original film mass. The results are shown in Table 1.

[0082] Table 1. Test results of swelling degree

[0083]

[0084]

[0085] As shown in Table 1, with the same molar percentage of HFP units in the PVDF copolymer, the swelling degree of the PVDF copolymers prepared in Examples 1, 4-7 was significantly reduced by 56%, 44%, 59%, 39%, and 46%, respectively, compared to Comparative Example 1. In Comparative Examples 2-3, the films prepared from the comparative PVDF copolymers dissolved or cracked when immersed in the electrolyte during swelling degree testing, making it impossible to test the swelling degree. The pH in Comparative Example 4 was too low, and the temperature in Comparative Example 6 was too low, both resulting in low crosslinking degree of the PVDF copolymer, thus making the reduction in swelling degree of the comparative PVDF copolymer insignificant. The pH in Comparative Example 5 was too high, causing the molecular chains of the PVDF copolymer to easily break, thus making the reduction in swelling degree of the comparative PVDF copolymer insignificant. The above demonstrates that the method of the present invention can significantly reduce the swelling degree of PVDF copolymers.

[0086] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for reducing the swelling degree of PVDF copolymers, characterized in that, Includes the following steps: The PVDF copolymer was reacted in an emulsion state at 70–95°C and pH 10–12.

2. The method as described in claim 1, characterized in that, The molar percentage of HFP units in the PVDF copolymer is ≥4%.

3. The method as described in claim 2, characterized in that, The molar percentage of HFP units in the PVDF copolymer is 4-10%.

4. The method as described in claim 1, characterized in that, The reaction time is 1 to 2 hours.

5. The method as described in claim 1, characterized in that, The pH is adjusted by adding an alkali.

6. The method as described in claim 5, characterized in that, The base is at least one of NaOH, LiOH, KOH or NH4OH.

7. The method as described in claim 1, characterized in that, In the presence of an emulsifier, the PVDF copolymer forms an emulsion with water.

8. The method as described in claim 7, characterized in that, The emulsifier is at least one of sodium perfluoropolyether carboxylate, perfluorooctanoate, perfluoropolyether carboxylate, or hexafluoropropylene oxide polymer.

9. The method as described in claim 1, characterized in that, The PVDF copolymer was prepared by emulsion polymerization of VDF and HFP.

10. The method as described in claim 1, characterized in that, The average particle size of the PVDF copolymer treated by the method is 2–10 μm.

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