A method for preparing a lithium battery grade polyvinylidene fluoride binder

Abstract

The present application relates to a kind of preparation methods of lithium battery grade polyvinylidene fluoride binder, belong to lithium battery adhesive field.The preparation method includes the following steps: the emulsion polymerization reaction of polymeric monomer containing vinylidene fluoride under composite emulsifier;The composite emulsifier is composed of perfluoropolyether carboxylate and non-fluorine non-ionic amine polyoxyethylene ether.It is surface active agent that perfluoropolyether carboxylate and non-fluorine non-ionic amine polyoxyethylene ether are compounded as composite emulsifier, so that the emulsion polymerization reaction of polymeric monomer containing vinylidene fluoride is carried out.The emulsion polymerization reaction does not use PFOA surface active agent and anti-sticking kettle agent, kettle wall gel is still less, simple operation, easy to separate.

Description

Technical Field

[0001] This invention relates to a method for preparing lithium-ion battery-grade polyvinylidene fluoride adhesive, belonging to the field of lithium battery adhesives. Background Technology

[0002] Polyvinylidene fluoride (PVDF) is a highly non-reactive thermoplastic fluoropolymer. PVDF exhibits excellent viscoelasticity and is used in lithium-ion battery binders. It has good adhesion in organic solvents, enabling the bonding of positive electrode materials and conductive agents to the current collector. The thickness of the positive and negative electrode sheets expands and contracts during battery charging and discharging, and the PVDF binder, after drying, retains good elasticity, providing a buffer against this expansion and contraction.

[0003] For lithium-ion grade PVDF, adhesion is the most important indicator. According to the Markhouwink equation for polymer adhesion, the viscosity of a polymer is directly related to its molecular weight. Therefore, achieving a high molecular weight in PVDF is crucial for the preparation of PVDF binders. The commonly used preparation method in current technology is the emulsion method. However, the molecular weight of vinylidene fluoride polymers produced by the emulsion method is affected by various factors, such as emulsifiers, chain transfer agents, reaction temperature, and pressure. Because vinylidene fluoride monomers are extremely hydrophobic, polymerization takes place in water, resulting in high mass transfer resistance. Industrially, this polymerization requires high pressure, special reactor types, and agitator designs.

[0004] The polymerization of fluorinated monomers generally uses perfluorooctanoate (PFOA) as an emulsifier. However, as is well known, PFOA has been banned in countries and regions such as Europe, the United States, Japan, and China due to environmental issues. Therefore, finding alternative products is very important.

[0005] Chinese invention patent CN101745338B discloses a method for preparing a fluoropolymer without perfluorooctanoic acid (PFOA). In this method, a surfactant made of hexafluoropropylene oxide polymer and its derivatives is used instead of PFOA. However, compared with the amount of PFOA, the amount of emulsifier made of hexafluoropropylene oxide polymer and its derivatives is relatively large. Moreover, the surfactant made of hexafluoropropylene oxide polymer and its derivatives needs to be prepared into a microemulsion before use, which makes it inconvenient to use and the post-processing of the polymerized product is relatively complicated. This makes it difficult to meet the requirements of lithium battery binders with strict impurity requirements.

[0006] Chinese invention patent CN103739757B discloses a method for preparing polyvinylidene fluoride resin without PFOA. This method uses anionic surfactants combined with nonionic surfactants to replace PFOA. However, the amount of the combined surfactants used in this method is small and the emulsifying ability is weak, which is insufficient to wet the hydrophobic polyvinylidene fluoride monomers to form micelles. Therefore, this method cannot prepare polyvinylidene fluoride with large molecular weight.

[0007] Furthermore, during the polymerization of polyvinylidene fluoride (PVDF), the mutual adhesion of PVDF can easily lead to the formation of gels, plasticizers, and sticking to the reactor. Generally, solid paraffin or silicone oil needs to be added as an antifouling agent. Paraffin is liquid during the polymerization reaction and turns back into a solid when the reaction ends and the temperature is lowered. This makes the post-processing of PVDF emulsion method more complicated, which is unacceptable when using PVDF binders for lithium batteries. Summary of the Invention

[0008] The purpose of this invention is to provide a method for preparing lithium-ion grade polyvinylidene fluoride (PVDF) adhesive, which solves the problem that the post-processing in the prior art is complicated and the obtained PVDF adhesive is difficult to meet the requirements of lithium-ion battery adhesive.

[0009] To achieve the above objectives, the technical solution of the present invention is as follows:

[0010] A method for preparing a lithium-ion battery-grade polyvinylidene fluoride (PVDF) adhesive includes the following steps: PVDF-containing monomers undergo emulsion polymerization under a composite emulsifier; the composite emulsifier is composed of perfluoropolyether carboxylates and nonfluoro nonionic amine polyoxyethylene ethers.

[0011] This invention uses a surfactant blend of perfluoropolyether carboxylates and nonfluorinated nonionic amine polyoxyethylene ethers as a composite emulsifier to induce emulsion polymerization of vinylidene fluoride-containing monomers. The nonfluorinated nonionic amine polyoxyethylene ethers play a role in chain transfer during polymerization, significantly controlling molecular weight and molecular weight distribution, reducing gelation and plasticizing, and ensuring the NMP viscosity of PVDF is at a reasonable value. This avoids the need for troublesome post-processing anti-sticking agents (anti-fouling agents) such as solid paraffin or silicone oil. This emulsion polymerization reaction does not use PFOA surfactants, results in less gel on the reactor walls, and is simple to operate and easy to separate.

[0012] The preparation method of the present invention improves the branching, molecular weight and viscosity of polyvinylidene fluoride, which meets the requirements of lithium electrode PVDF.

[0013] Preferably, the amount of the perfluoropolyether carboxylate added is 0.1% to 2% of the mass of the polymerizing monomer. More preferably, it is 0.15% to 0.5%. The perfluoropolyether carboxylate can prevent or reduce atomic migration between the growing polymer chains and the surfactant during the polymerization reaction. Atom migration can lead to a decrease in the molecular weight of the product and may inhibit the polymerization reaction. The molecular formula of the perfluoropolyether carboxylate is CF3O (C3F6O). m (CF2O) nCOOX; where 1≤m<6, 1≤n<6, and X represents alkali metal ions or ammonium ions; it belongs to the Y-type PFPE type anionic surfactant. Y-type perfluoropolyether refers to perfluoropolyether obtained by reacting hexafluoropropylene with oxygen under photoinitiation conditions, which is different from perfluoropolyether obtained by anionic ring-opening polymerization of hexafluoropropylene oxide.

[0014] Further preferred are one or more of Chenguang Boda FSA8000, FSA6000, and FSA4000.

[0015] Preferably, the non-fluorinated nonionic amine polyoxyethylene ether is one or more of oleylamine polyoxyethylene ether, octadecylamine polyoxyethylene ether, and ethylenediamine polyoxyethylene polyoxypropylene ether; the amount of the non-fluorinated nonionic amine polyoxyethylene ether added is 0.1% to 2% of the mass of the polymeric monomer. More preferably, it is 0.15% to 0.5%. The non-fluorinated nonionic amine polyoxyethylene ether is a nonionic surfactant with active hydrogen, capable of transfer between the polymer chain and free radicals. It can control the growth of the molecular chain backbone, improve molecular weight and its distribution, and can also combine with fluorinated surfactants to stabilize the emulsion and prevent sedimentation.

[0016] Preferably, the emulsion polymerization reaction is carried out under high pressure conditions of not less than 6 MPa for the monomers; the high pressure is 6–12 MPa. More preferably, it is 6–10 MPa. Because of the compatibilizing effect of the emulsifier in the emulsion polymerization reaction, emulsion polymerization is generally carried out at low pressure (compared to suspension polymerization) of 2–4.5 MPa. However, this invention has found that carrying out the emulsion polymerization reaction under high pressure conditions helps to increase the branching of the polymer. The branched PVDF will become entangled after dissolving in NMP, thereby increasing the viscosity of PVDF and improving the performance of lithium-ion battery-grade PVDF.

[0017] To promote the polymerization reaction of the monomers, the emulsion polymerization reaction is preferably carried out at a low temperature not exceeding 50°C; the low temperature is 10–50°C, preferably 40–50°C.

[0018] In order to provide suitable polymerization reaction conditions for the monomers and improve the polymerization rate, preferably, the emulsion polymerization reaction uses a pH buffer; the pH buffer is added to make the pH value 7-8.

[0019] More preferably, the pH buffer is one or more of sodium acetate, potassium acetate, or dipotassium hydrogen phosphate.

[0020] To initiate the polymerization of the monomers, the emulsion polymerization reaction preferably uses an initiator or an initiator and a reducing agent. The initiator is one or more selected from diisopropyl peroxide, di-n-propyl peroxide, di-tert-butyl peroxide, n-butyl peroxy(2-ethylhexanoate)valerate, tert-butyl peroxybenzoate, potassium persulfate, sodium persulfate, or ammonium persulfate. The amount of the initiator added is 0.001–1% of the monomer mass; preferably 0.03–0.4%. The reducing agent is one or more selected from sulfites, hypochlorites, ferrous salts, and hydrogen peroxide. The amount of the reducing agent added is 0.001–1% of the monomer mass; preferably 0.03–0.4%.

[0021] Preferably, the polymerizing monomer is vinylidene fluoride or a composite monomer composed of vinylidene fluoride and a comonomer; the comonomer is one or more selected from tetrafluoroethylene, vinyl fluoride, hexafluoropropylene, trifluorochloroethylene, perfluoropropyl vinyl ether, and perfluoromethyl vinyl ether; the comonomer accounts for 0.1% to 20% of the total mass of the polymerizing monomer, preferably 2% to 6%.

[0022] More preferably, the amount of the polymeric monomer added is 20% to 40% of the mass of water. More preferably, it is 25% to 40%.

[0023] Preferably, the non-fluorinated nonionic amine polyoxyethylene ether is added before the polymerization reaction, or added separately before and during the polymerization reaction; the amount of non-fluorinated nonionic amine polyoxyethylene ether added during the polymerization reaction accounts for 20-30% of the total amount added. During the polymerization reaction, the non-fluorinated nonionic amine polyoxyethylene ether added at different times has different effects on the polymerization reaction. In particular, adding non-fluorinated nonionic amine polyoxyethylene ether in batches can further improve the performance of PVDF.

[0024] Preferably, the polyvinylidene fluoride adhesive has a weight-average molecular weight of 600,000 or higher, and a molecular weight distribution P D Below 2.5. The weight-average molecular weight and molecular weight distribution P D The viscosity of PVDF in solution is relatively high.

[0025] More preferably, the polyvinylidene fluoride adhesive has a viscosity of 1030 mPa·s or higher in 6% NMP and a viscosity of 3345 mPa·s or higher in 8% NMP. Attached Figure Description

[0026] Figure 1 The 400MHz NMR F19 spectra of the lithium-ion grade polyvinylidene fluoride adhesives prepared in Examples 1 and 2 of this invention are shown. Detailed Implementation

[0027] The preparation method of the lithium-ion battery-grade polyvinylidene fluoride (PVDF) binder of the present invention includes an emulsion polymerization reaction of a PVDF-containing polymeric monomer under a composite emulsifier. The preparation method of the present invention does not use PFOA surfactants, nor does it use paraffin or silicone oil as antifouling agents, resulting in less residue sticking.

[0028] Furthermore, the mass ratio of perfluoropolyether carboxylate and nonfluorononionic amine polyoxyethylene ether in the composite emulsifier is (0.9–1.5):1.

[0029] During emulsion polymerization, a portion of the monomer is first introduced to bring the pressure to a preset level. After the pressure of the polymerization system decreases, the monomer is continued to be introduced to maintain the high pressure condition.

[0030] Furthermore, some of the polymerizing monomers account for 20% to 50% of the total amount of polymerizing monomers introduced.

[0031] Furthermore, if the introduced polymerizing monomer contains a comonomer, preferably some of the polymerizing monomer is pure vinylidene fluoride, and the continued introduced polymerizing monomer is a composite monomer composed of vinylidene fluoride and a comonomer.

[0032] Furthermore, the comonomer accounts for 4% to 6% of the total amount of polymeric monomers introduced.

[0033] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0034] I. Specific embodiments of the preparation method of the lithium-ion battery-grade polyvinylidene fluoride adhesive of the present invention are as follows:

[0035] Example 1

[0036] The preparation method of the lithium-ion battery-grade polyvinylidene fluoride adhesive in this embodiment adopts the following steps:

[0037] (1) Dissolve 6.8g of FSA8000 Y-type PFPE anionic surfactant, 5.6g of ethylenediamine polyoxyethylene polyoxypropylene ether (EDEP), and 5.0g of sodium acetate in a beaker containing 7.0kg of deionized water, with a pH value between 7.0 and 8.0, to obtain a mixed solution;

[0038] (2) Vacuum the 10L horizontal reactor equipped with a ribbon agitator, then draw in the mixed solution, then fill it with 0.3MPa nitrogen for replacement and vacuuming, repeat 3 times, and wait until the oxygen content measured by the oxygen content meter reaches 20ppm to be qualified. After the oxygen content is qualified, continue to vacuum.

[0039] (3) After heating the reactor to 40°C, introduce 0.58 kg of VDF monomer, pressurize to 6.5 MPa, and rotate the reactor's spiral agitator at 45 rpm;

[0040] (4) Take 1.0g of initiator ammonium persulfate and 1.0g of reducing agent Na2SO3 to prepare an aqueous solution, and then use a constant flow pump to pump it into the reactor at a speed of 3mL / min. After 20min, after observing that the pressure is decreasing, reduce the pumping speed of the initiator to 2mL / min. Then, continue to introduce VDF monomer, and adjust the valve to control the pressure to maintain at this pressure (6.5MPa). React for 4.5 hours, and a total of 2.23kg of VDF monomer is introduced. Stop the monomer feeding and stop the pumping of initiator. Continue to react for 1 hour to reduce the pressure of VDF monomer in the reactor. Discharge the unreacted VDF monomer to atmospheric pressure and cool to room temperature. After nitrogen replacement, discharge the emulsion with a solid content of 16.7% and 1.5g of gel on the reactor wall. After high-speed stirring to break the emulsion, wash with high-purity water until the conductivity of the washing water is qualified. Then dry in a vacuum oven at 100℃ overnight to obtain powdered lithium battery grade PVDF.

[0041] The lithium-ion grade polyvinylidene fluoride binder in this embodiment is the lithium-ion grade PVDF prepared in this embodiment. The 400MHz NMR F19 spectrum of this product was measured, and the results are as follows: Figure 1 As shown.

[0042] Example 2

[0043] The preparation method of the lithium-ion battery-grade polyvinylidene fluoride adhesive in this embodiment adopts the following steps:

[0044] (1) Dissolve 6.5g of FSA6000 Y-type PFPE-type anionic surfactant, 6.8g of octadecylamine polyoxyethylene ether AC-1812 and 4.0g of dipotassium hydrogen phosphate in a beaker containing 7.0kg of deionized water, with a pH value between 7.0 and 8.0, to obtain a mixed solution;

[0045] (2) Vacuum the 10L horizontal reactor equipped with a ribbon agitator, then draw in the mixed solution, then fill with 0.3MPa nitrogen for replacement and vacuuming, repeat 3 times, measure the oxygen content with an oxygen meter until it reaches 20ppm to be qualified, and continue vacuuming after the oxygen content is qualified.

[0046] (3) Heat the reactor to 45°C, introduce 0.58 kg of VDF monomer, pressurize to 6.5 MPa, and rotate the reactor's spiral agitator at 45 rpm;

[0047] (4) Dissolve 6.9g of 50% diisopropyl peroxide (IPP) in an aqueous solution containing 0.3g of FSA6000Y type PFPE type anionic emulsifier, and then pump it into the reactor at a constant flow pump at a speed of 3mL / min. After 20min, when the pressure is observed to be decreasing, reduce the pumping speed of the initiator to 2mL / min. Then, continue to introduce VDF monomer, and control the pressure by adjusting the valve to maintain the pressure. React for 4.5 hours, and a total of 2.57kg of VDF monomer is introduced. Stop feeding the monomer and stop pumping the initiator. Continue to react for 1 hour to reduce the pressure of VDF monomer in the reactor. Discharge the unreacted VDF monomer to atmospheric pressure and cool it. After nitrogen replacement, discharge the emulsion with a solid content of 18.3% and 2.1g of gel on the reactor wall. After the coagulated material is broken by high-speed stirring, wash it with high-purity water until the conductivity of the washing water is qualified. Then dry it overnight in a vacuum oven at 100℃ to obtain powdered lithium battery grade PVDF.

[0048] The lithium-ion grade polyvinylidene fluoride binder in this embodiment is the lithium-ion grade PVDF prepared in this embodiment. The 400MHz NMR F19 spectrum of this product was measured, and the results are as follows: Figure 1 As shown.

[0049] Example 3

[0050] The preparation method of the lithium-ion battery-grade polyvinylidene fluoride adhesive in this embodiment adopts the following steps:

[0051] (1) Dissolve 9.7g of FSA8000 Y-type PFPE anionic surfactant, 8.9g of ethylenediamine polyoxyethylene polyoxypropylene ether (EDEP), and 5.0g of sodium acetate in a beaker containing 7.0kg of deionized water, with a pH value between 7.0 and 8.0, to obtain a mixed solution;

[0052] (2) Vacuum the 10L horizontal reactor equipped with a ribbon agitator, then draw in the mixed solution, then fill with 0.3MPa nitrogen for replacement and vacuuming, repeat 3 times, measure the oxygen content with an oxygen meter until it reaches 20ppm to be qualified, and continue vacuuming after the oxygen content is qualified.

[0053] (3) Heat the reactor to 40°C, introduce 0.95 kg of VDF monomer, pressurize to 10 MPa, and rotate the reactor's spiral agitator at 45 rpm.

[0054] (4) Take 8.0g of initiator ammonium persulfate and 8.0g of reducing agent Na2SO3 to prepare an aqueous solution. Use a constant flow pump to pump it into the reactor at a speed of 3mL / min. After 20min, observe that the pressure is decreasing. Then reduce the pumping speed of the initiator to 2mL / min. Then continue to introduce VDF monomer. Adjust the valve to control the pressure and maintain it at this pressure. React for 5 hours. A total of 2.42kg of VDF monomer is introduced. Stop the monomer feeding and stop the pumping of initiator. Continue to react for 1 hour to reduce the pressure of VDF monomer in the reactor. Discharge the unreacted VDF monomer to atmospheric pressure and cool it. After nitrogen replacement, discharge the emulsion. The solid content is 16.9%. There is 0.9g of gel on the reactor wall. After high-speed stirring to break the emulsion, wash with high-purity water until the conductivity of the washing water is qualified. Then dry in a vacuum oven at 100℃ overnight to obtain powdered lithium battery grade PVDF.

[0055] Example 4

[0056] The preparation method of the lithium-ion battery-grade polyvinylidene fluoride adhesive in this embodiment adopts the following steps:

[0057] (1) Dissolve 6.8g of FSA8000 Y-type PFPE anionic surfactant, 4.2g of ethylenediamine polyoxyethylene polyoxypropylene ether (EDEP), and 5.0g of sodium acetate in a beaker containing 7.0kg of deionized water, with a pH value between 7.0 and 8.0, to obtain a mixed solution;

[0058] (2) Vacuum the 10L horizontal reactor equipped with a ribbon agitator, then draw in the mixed solution, then fill it with 0.3MPa nitrogen for replacement and vacuuming, repeat 3 times, measure the oxygen content with an oxygen meter until it reaches 20ppm to be qualified, and after the oxygen content is qualified, vacuum it.

[0059] (3) Heat the reactor to 40°C, introduce 0.53 kg of VDF monomer, pressurize to 6.5 MPa, and rotate the reactor's spiral agitator at 45 rpm;

[0060] (4) Take 1.0g of initiator ammonium persulfate and 1.0g of reducing agent Na2SO3 to prepare an aqueous solution. Use a constant flow pump to pump it into the reactor at a speed of 3mL / min. After 20min, observe that the pressure is decreasing. Then reduce the pumping speed of the initiator to 2mL / min. Then continue to introduce VDF monomer. Adjust the valve to control the pressure and maintain it at this pressure. After reacting for 2h, quickly pump in an aqueous solution containing 1.4g of ethylenediamine polyoxyethylene polyoxypropylene ether (EDEP). The reaction lasts for a total of 4.5h. A total of 2.25kg of VDF monomer is introduced. Stop the monomer feeding and stop the initiator pumping. Continue to react for 1 hour to reduce the pressure of VDF monomer in the reactor. Discharge the unreacted VDF monomer to atmospheric pressure and cool it. After nitrogen replacement, discharge the emulsion. The solid content is 16.3%. There is 0.9g of gel on the reactor wall. After high-speed stirring to break the emulsion, wash with high-purity water until the conductivity of the washing water is qualified. Then dry in a vacuum oven at 100℃ overnight to obtain powdered lithium battery grade PVDF.

[0061] Example 5

[0062] The preparation method of the lithium-ion battery-grade polyvinylidene fluoride adhesive in this embodiment adopts the following steps:

[0063] (1) Dissolve 6.8g of FSA8000 Y-type PFPE anionic surfactant, 5.6g of ethylenediamine polyoxyethylene polyoxypropylene ether (EDEP), and 5.0g of sodium acetate in a beaker containing 7.0kg of deionized water, with a pH value between 7.0 and 8.0, to obtain a mixed solution;

[0064] (2) Vacuum the 10L horizontal reactor equipped with a ribbon agitator, then draw in the mixed solution, then fill it with 0.3MPa nitrogen for replacement and vacuuming, repeat 3 times, measure the oxygen content with an oxygen meter until it reaches 20ppm to be qualified, and after the oxygen content is qualified, vacuum it.

[0065] (3) Heat the reactor to 45°C, introduce 1.03 kg of VDF monomer, pressurize to 6.5 MPa, and rotate the reactor's spiral agitator at 45 rpm;

[0066] (4) Dissolve 2.3g of 50% diisopropyl peroxide (IPP) in a solution containing 0.3g of FSA8000. In an aqueous solution of Y-type PFPE anionic emulsifier, the initiator was pumped into the reactor at a constant flow rate of 3 mL / min. After 20 minutes, when the pressure was observed to be decreasing, the pumping rate of the initiator was reduced to 2 mL / min. Then, a mixture of VDF and HFP (hexafluoropropylene) monomers was continuously introduced, and the pressure was maintained at this level by adjusting the valve. The reaction was carried out for 4.5 hours, during which a total of 2.07 kg of VDF monomer and 157 g of HFP monomer were introduced. The monomer feeding and initiator injection were stopped, and the reaction was continued for 1 hour to reduce the pressure of VDF monomer in the reactor. The unreacted VDF monomer was discharged to atmospheric pressure and cooled. After nitrogen purging, the emulsion was discharged with a solid content of 16.7% and 3.2 g of gel on the reactor wall. The HFP content in the copolymer was between 7% and 9%. After demulsification by high-speed stirring, the emulsion was washed with high-purity water until the conductivity of the washing water was qualified. Then, it was dried overnight in a vacuum oven at 100°C to obtain powdered lithium-ion grade PVDF / HFP copolymer.

[0067] Example 6

[0068] The preparation method of the lithium-ion battery-grade polyvinylidene fluoride adhesive in this embodiment adopts the following steps:

[0069] (1) Dissolve 6.8g of FSA8000 Y-type PFPE anionic surfactant, 4.2g of ethylenediamine polyoxyethylene polyoxypropylene ether (EDEP), and 5.0g of sodium acetate in a beaker containing 7.0kg of deionized water, with a pH value between 7.0 and 8.0, to obtain a mixed solution;

[0070] (2) Vacuum the 10L horizontal reactor equipped with a ribbon agitator, then draw in the mixed solution, then fill it with 0.3MPa nitrogen for replacement and vacuuming, repeat 3 times, measure the oxygen content with an oxygen meter until it reaches 20ppm to be qualified, and after the oxygen content is qualified, vacuum it.

[0071] (3) Heat the reactor to 45°C, introduce 0.95 kg of VDF monomer, pressurize to 10.0 MPa, and rotate the reactor's spiral agitator at 45 rpm;

[0072] (4) Dissolve 2.3g of 50% diisopropyl peroxide (IPP) in a solution containing 0.3g of... An aqueous solution of FSA6000Y type PFPE type anionic surfactant was pumped into the reactor at a constant flow rate of 3 mL / min. After 30 min, when the pressure was observed to be decreasing, the pumping rate of the initiator was reduced to 2 mL / min. Then, VDF monomer was continued to be introduced, and the pressure was maintained at this pressure by adjusting the valve. After 2 h of reaction, an aqueous solution containing 1.4 g of ethylenediamine polyoxyethylene polyoxypropylene ether (EDEP) was rapidly pumped in. The reaction lasted for a total of 4 h, and a total of 2.30 kg of VDF monomer was introduced. The monomer feed and initiator injection were stopped, and the reaction was continued for 1 hour to reduce the VDF monomer pressure in the reactor. The unreacted VDF monomer was discharged to atmospheric pressure and cooled. After nitrogen purging, the emulsion was discharged with a solid content of 17.1% and 1.6 g of gel on the reactor wall. After high-speed stirring to break the emulsion, it was washed with high-purity water until the conductivity of the washing water was qualified. Then, it was dried overnight in a vacuum oven at 100 °C to obtain powdered lithium battery grade PVDF.

[0073] II. Comparative Example

[0074] Comparative Example 1

[0075] The preparation method of the polyvinylidene fluoride adhesive in this comparative example is basically the same as that in Example 1, except that:

[0076] The mixed solution in step (1) does not contain ethylenediamine polyoxyethylene polyoxypropylene ether (EDEP), and the mass of sodium acetate is 4.0 g;

[0077] In step (3), the mass of the VDF monomer is 0.89 kg;

[0078] (4) Take 1.0g of initiator ammonium persulfate and 1.0g of reducing agent Na2SO3 to prepare an aqueous solution. Use a constant flow pump to pump it into the reactor at a speed of 3mL / min. After 30min, observe that the pressure is decreasing. Then reduce the pumping speed of the initiator to 2mL / min. Then continue to introduce VDF monomer. Adjust the valve to control the pressure to maintain at 6.5MPa. React for 5 hours. A total of 2.57kg of VDF monomer is introduced. Stop the monomer feeding and stop the pumping of initiator. Continue to react for 1 hour to reduce the pressure of VDF monomer in the reactor. Discharge the unreacted VDF monomer to atmospheric pressure and cool it. After nitrogen replacement, discharge the emulsion. The solid content is 15.3% and the reactor wall gel is 10.7g. After high-speed stirring to break the emulsion, wash with high-purity water until the conductivity of the washing water is qualified. Then dry in a vacuum oven at 100℃ overnight to obtain powdered PVDF.

[0079] In this comparative example, only Y-type PFPE anionic surfactants were used, resulting in a higher viscosity of the generated PVDF and a higher viscosity of the gel on the reactor wall compared to Example 1.

[0080] Comparative Example 2

[0081] The preparation method of the polyvinylidene fluoride adhesive in this comparative example is basically the same as that in Example 1, except that:

[0082] The mass of sodium acetate in the mixed solution of step (1) is 4.0 g;

[0083] In step (3), the reactor is heated to 83°C, and the mass of the VDF monomer is 0.53 kg.

[0084] (4) Take 1.0g of initiator ammonium persulfate to prepare an aqueous solution, and pump it into the reactor at a constant flow rate of 3mL / min. After 30min, observe that the pressure is decreasing, reduce the pumping rate of the initiator to 2mL / min. Then, continue to introduce VDF monomer, and adjust the valve to maintain the pressure at 6.5MPa. React for 5 hours, and a total of 2.23kg of VDF monomer is introduced. Stop feeding the monomer and stop pumping the initiator. Continue to react for 1 hour to reduce the pressure of VDF monomer in the reactor. Discharge the unreacted VDF monomer to atmospheric pressure and cool it. After nitrogen replacement, discharge the emulsion with a solid content of 16.7% and 8.9g of gel on the reactor wall. After high-speed stirring to break the emulsion, wash with high-purity water until the conductivity of the washing water is qualified. Then dry in a vacuum oven at 100℃ overnight to obtain powdered PVDF.

[0085] Comparative Example 3

[0086] The preparation method of the polyvinylidene fluoride adhesive in this comparative example is basically the same as that in Example 1, except that:

[0087] The mass of sodium acetate in the mixed solution of step (1) is 4.0 g;

[0088] In step (3), the reactor is heated to 83°C, the mass of VDF monomer is 0.23 kg, and the reaction pressure is 2.8 MPa.

[0089] (4) Take 1.0g of initiator ammonium persulfate to prepare an aqueous solution, and pump it into the reactor at a constant flow rate of 3mL / min. After 30min, observe that the pressure is decreasing, reduce the pumping rate of the initiator to 2mL / min. Then, continue to introduce VDF monomer, and adjust the valve to maintain the pressure at 2.8MPa. React for 6 hours, and a total of 2.01kg of VDF monomer is introduced. Stop feeding the monomer and stop pumping the initiator. Continue to react for 1 hour to reduce the pressure of VDF monomer in the reactor. Discharge the unreacted VDF monomer to atmospheric pressure and cool it. After nitrogen replacement, discharge the emulsion with a solid content of 13.1% and 6.4g of gel on the reactor wall. After high-speed stirring to break the emulsion, wash with high-purity water until the conductivity of the washing water is qualified. Then dry in a vacuum oven at 100℃ overnight to obtain powdered PVDF.

[0090] Comparative Example 4

[0091] The preparation method of the polyvinylidene fluoride adhesive in this comparative example is basically the same as that in Example 1, except that:

[0092] In step (1), the FSA8000 Y-type PFPE-type anionic surfactant in the mixed solution is replaced with a fluorinated surfactant, namely ammonium perfluorooctanoate, and the amount of perfluorooctanoate added is 5.0g.

[0093] In step (3), the mass of the VDF monomer is 0.57 kg;

[0094] (4) Take 1.0g of initiator ammonium persulfate and 1.0g of reducing agent Na2SO3 to prepare an aqueous solution. Use a constant flow pump to pump it into the reactor at a speed of 3mL / min. After 30min, observe that the pressure is decreasing. Then reduce the pumping speed of the initiator to 2mL / min. Then continue to introduce VDF monomer. Adjust the valve to control the pressure to maintain at 6.5MPa. React for 5 hours. A total of 2.32kg of VDF monomer is introduced. Stop the monomer feeding and stop the pumping of initiator. Continue to react for 1 hour to reduce the pressure of VDF monomer in the reactor. Discharge the unreacted VDF monomer to atmospheric pressure and cool it. After nitrogen replacement, discharge the emulsion. The solid content is 15.6%. There is 7.7g of gel on the reactor wall. After high-speed stirring to break the emulsion, wash with high-purity water until the conductivity of the washing water is qualified. Then dry in a vacuum oven at 100℃ overnight to obtain powdered PVDF.

[0095] III. Experimental Examples

[0096] In this experiment, the molecular weight of the polyvinylidene fluoride adhesives prepared in Examples 1-6 and Comparative Examples 1-4 was determined by gel permeation chromatography (GPC), and the results are shown in Table 1.

[0097] In this experiment, Brook 400MHz NMR F19 spectra of the polyvinylidene fluoride adhesives prepared in Examples 1-6 and Comparative Examples 1-4 were measured. The results are as follows: Figure 1 As shown.

[0098] This experiment determined the segment regularity, head-to-tail, head-to-head, and tail-to-tail content of the polyvinylidene fluoride adhesives prepared in Examples 1-6 and Comparative Examples 1-4. The results are shown in Table 1. Here, head-to-tail, head-to-head, and tail-to-tail structures refer to a molecular polymerization structure. The head-to-tail structure (-CF2-CH2-CF2-CH2-) has regular segments, while the head-to-head structure (-CF2-CH2-CH2-CF2-) and tail-to-tail structure (-CF2-CH2-CH2-CF2-) are irregular portions of the molecular chain. A higher proportion of head-to-tail structures indicates more regular segments and better stability.

[0099] In this experimental example, the viscosity of PVDF solutions was measured by dissolving PVDF samples in N,N-dimethylpyrrolidone to form N,N-dimethylpyrrolidone solutions with PVDF mass fractions of 6% and 8%, respectively. The viscometer used was a Brookfield DV2T, and the results are shown in Table 1.

[0100] This experiment also measured the peel strength of the electrodes obtained by the polyvinylidene fluoride adhesives prepared in Examples 1-6 and Comparative Examples 1-4, and the results are shown in Table 1.

[0101] Specific method: Take the electrode sheets before and after rolling and perform a 180° peel test. The peel force and speed are 100±10mm / min and the width is 20mm. LP5K Plus tensile tester. Reference standards: GB / T2790-1995 and JIS K6854-2:1999.

[0102] Table 1 Relevant performance of PVDF

[0103]

[0104] Compared to Comparative Example 1, Examples 1-6 used a blend of perfluoropolyether carboxylates and nonfluorinated nonionic amine polyoxyethylene ether surfactants to prepare PVDF. This significantly increased the molecular weight of PVDF while maintaining a narrow molecular weight distribution. Furthermore, it exhibited high viscosity and improved electrode peeling force in NMP, resulting in a higher proportion of "head-tail" structures and better stability within the chain segments. However, since Example 5 involved a copolymerization of vinylidene fluoride and hexafluoropropylene, rather than a homopolymerization, it was impossible to statistically analyze the head-tail, head-head, and tail-tail structures.

[0105] Compared with Examples 1-6, the polymerization temperature of Comparative Example 2 was set to 83°C, which is higher. The resulting PVDF showed precipitation upon standing, had a lower molecular weight and a wider molecular weight distribution, lower viscosity in NMP, and lower electrode peeling force. Correspondingly, the proportion of "head and tail" structures in the chain segments was also reduced, and the stability was poor, which could not meet the requirements of lithium electrode PVDF.

[0106] Compared with Example 1, the reaction pressure of Comparative Example 3 was lower, at 2.8 MPa. The obtained PVDF showed a small amount of precipitation after standing. Its molecular weight distribution was wider and the molecular weight was lower. The viscosity of the NMP dispersion was extremely low. Correspondingly, the proportion of "head and tail" structures in the chain segments was lower than that in Example 1, which means that the stability was relatively poor.

[0107] Compared with Example 1, Comparative Example 4 uses a compound of ammonium perfluorooctanoate and non-fluorinated nonionic amine polyoxyethylene ether surfactants to prepare PVDF. The obtained PVDF has a small amount of precipitation after standing. Its molecular weight and molecular weight distribution are similar to those of Example 1, but its viscosity dispersed in NMP is lower and the proportion of "head and tail" structures in the chain segments is lower than that of Example 1, that is, its stability is relatively poor.

Claims

1. A method for producing a lithium battery grade polyvinylidene fluoride binder, characterized by, The method comprises the following steps: vinylidene fluoride-containing polymerization monomers are subjected to emulsion polymerization under a composite emulsifier; the composite emulsifier is composed of a perfluoropolyether carboxylate and a non-fluorine non-ionic amine polyoxyethylene ether; the perfluoropolyether carboxylate is added in an amount of 0.1-2% of the mass of the polymerization monomers; the non-fluorine non-ionic amine polyoxyethylene ether is added in an amount of 0.1-2% of the mass of the polymerization monomers; the emulsion polymerization is carried out under high pressure of not less than 6 MPa; and the emulsion polymerization is carried out under low temperature of not higher than 50 DEG C.

2. The method for preparing lithium-ion grade polyvinylidene fluoride adhesive according to claim 1, characterized in that, The non-fluorine non-ionic amine polyoxyethylene ether is one or more of an oleyl amine polyoxyethylene ether, a stearyl amine polyoxyethylene ether and an ethylenediamine polyoxyethylene polyoxypropylene ether.

3. The method of making lithium battery grade polyvinylidene fluoride binder of claim 1, wherein, The high pressure is 6-12 MPa.

4. The method of producing a lithium battery grade polyvinylidene fluoride binder according to claim 1 or 3, characterized by, The low temperature is 10-50 DEG C.

5. The method of producing a lithium battery grade polyvinylidene fluoride binder according to claim 1 or 2, characterized by, The emulsion polymerization uses a pH buffer; the pH buffer is added so that the pH value is 7-8.

6. The method of producing a lithium battery grade polyvinylidene fluoride binder according to claim 1 or 2, characterized by, The emulsion polymerization uses a first initiator, which is diisopropyl peroxydicarbonate.

7. The method of producing a lithium battery grade polyvinylidene fluoride binder according to claim 1 or 2, characterized by, The emulsion polymerization uses a second initiator and a reducing agent; the second initiator is one or more of potassium persulfate, sodium persulfate and ammonium persulfate; and the reducing agent is one or both of a sulfite and a ferrous salt.

8. The method of producing a lithium battery grade polyvinylidene fluoride binder according to claim 1 or 2, characterized by, The polymerization monomers are vinylidene fluoride or a composite monomer composed of vinylidene fluoride and a comonomer; the comonomer is one or more of tetrafluoroethylene, fluoroethylene, hexafluoropropylene, trifluorochloroethylene, perfluoro-n-propyl vinyl ether and perfluoro-methyl vinyl ether; and the comonomer accounts for 0.1-20% of the total mass of the polymerization monomers.

9. The method of producing a lithium battery grade polyvinylidene fluoride binder according to claim 1 or 2, characterized by, The non-fluorine non-ionic amine polyoxyethylene ether is added before the polymerization or is added before and during the polymerization respectively; and the non-fluorine non-ionic amine polyoxyethylene ether is added in an amount of 20-30% of the total amount during the polymerization.

10. The method for preparing the lithium-ion grade polyvinylidene fluoride adhesive according to claim 1, characterized in that, The polyvinylidene fluoride binder has a weight average molecular weight of 600,000 or more, a molecular weight distribution P D is 2.5 or less.

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