PVDF binder, preparation method therefor and use thereof

Abstract

Disclosed in the present invention are a PVDF binder, a preparation method therefor and a use thereof. The PVDF binder of the present invention comprises a first repeating unit derived from a first monomer and a second repeating unit derived from a second monomer; the first monomer is a vinylidene fluoride monomer; the PVDF binder comprises from 0.05-10% by mole of repeating units derived from the second monomer, and is characterized in that the fraction of randomly distributed second monomer units is greater than 22% and less than 40%. The PVDF binder prepared in the present invention greatly enhances the adhesion of positive electrode sheets, and maintains the uniformity and stability of active materials during slurry preparation.

Description

A PVDF adhesive, its preparation method and application Technical Field

[0001] This invention relates to the field of new energy battery technology, and more specifically, to a PVDF binder, its preparation method, and its application. Background Technology

[0002] Binders are inactive materials in lithium-ion batteries. Although they do not contribute to battery capacity, they are a crucial component of the lithium-ion battery electrode and have a significant impact on battery performance. In the electrode, the main function of the binder is to bond and retain the active materials, providing the necessary electron conduction within the electrode. This plays a vital role in maintaining battery capacity, lifespan, and stability. Many types of binders are used in lithium batteries, with polyvinylidene fluoride (PVDF) being the most common. The bonding properties of PVDF have a significant impact on lithium-ion battery performance; increased viscosity increases cycle life, and the amount of high-viscosity binder used is less than in conventional batteries, resulting in batteries with high capacity and high charge density. Homopolymerized PVDF binders have relatively poor bonding strength. To improve the bonding strength of PVDF binders, polar comonomers, such as acrylic acid and maleic anhydride ester monomers, are usually introduced into PVDF.

[0003] For example, patent document CN101679563B discloses a vinylidene fluoride copolymer, which prepares polyvinylidene fluoride resin by copolymerizing hydrophilic (meth)acrylic acid monomers with vinylidene fluoride. The polymerization process involves the continuous addition of an aqueous solution of (meth)acrylic acid monomers to obtain a polyvinylidene fluoride resin with a random distribution unit (MA) fraction of at least 40%. However, the continuous addition of (meth)acrylic acid comonomers significantly reduces the reaction rate, thereby noticeably reducing the production efficiency of PVDF, and the adhesive properties of the polyvinylidene fluoride resin still need improvement.

[0004] Summary of the Invention

[0005] The primary objective of this invention is to overcome the shortcomings and deficiencies of the prior art and to provide a PVDF adhesive.

[0006] Another object of the present invention is to provide a method for preparing the above-mentioned PVDF adhesive.

[0007] Another object of the present invention is to provide an application of the above-mentioned PVDF binder in the preparation of battery cathode.

[0008] The above-mentioned objective of the present invention is achieved through the following technical solution:

[0009] A PVDF adhesive includes a first repeating unit derived from a first monomer and a second repeating unit derived from a second monomer; the first monomer is vinylidene fluoride monomer;

[0010] The general structural formula of the second monomer is as follows:

[0011] Wherein, X is an H atom or a C1-C5 hydrocarbon moiety including at least one hydroxyl group; R 1 R 2 R 3 Each can be independently a H atom, a halogen atom, or a C1-C5 hydrocarbon group;

[0012] The PVDF binder contains 0.05% to 10% of the second repeating unit on a molar basis, wherein the random distribution of the second monomer unit portion in the second repeating unit is greater than 22% and less than 40%.

[0013] The PVDF binder obtained by this invention reduces the inhibitory effect of the second monomer during the copolymerization reaction by controlling the addition method of the second monomer, thereby accelerating the reaction rate and improving production efficiency. Furthermore, the randomly distributed second monomer unit portion in the obtained PVDF binder is greater than 22% and less than 40%, which greatly improves the adhesion of the positive electrode sheet when applied to the positive electrode of a battery.

[0014] Preferably, the first monomer may also be a mixture of vinylidene fluoride monomer and monomers that can copolymerize with vinylidene fluoride.

[0015] Preferably, the monomer that can be copolymerized with vinylidene fluoride is selected from one or more of vinyl fluoride, trifluoroethylene, trifluorochloroethylene, tetrafluoroethylene, hexafluoropropylene, and fluoroalkyl vinyl ethers.

[0016] Specifically, the amount of the vinylidene fluoride monomer in the monomer mixture is at least 70 mol%, so as not to affect the excellent properties of the vinylidene fluoride resin, such as chemical resistance, weather resistance and heat resistance.

[0017] Preferably, the second monomer is selected from:

[0018] One or more of them.

[0019] Preferably, the PVDF adhesive contains at least 0.1% molar of repeating units derived from the second monomer.

[0020] Preferably, the PVDF adhesive contains up to 10% molar of repeating units derived from the second monomer.

[0021] The present invention also relates to a method for preparing the PVDF adhesive, the method comprising copolymerizing the first monomer and the second monomer in a reaction vessel in an aqueous medium in the presence of a free radical initiator;

[0022] The preparation method includes adding an aqueous solution of the second monomer in two separate steps during the reaction;

[0023] The preparation method includes maintaining the pressure in the reaction vessel greater than the critical pressure of the first monomer.

[0024] Preferably, the copolymerization reaction is emulsion polymerization or suspension polymerization; more preferably, the copolymerization reaction is suspension polymerization.

[0025] Preferably, the free radical initiator is an organic peroxide initiator; more preferably, the organic peroxide initiator is diisopropyl peroxide dicarbonate, di-n-propyl peroxide dicarbonate, diisobutyryl peroxide, tert-butyl peroxypentanoate, or tert-pentyl peroxypentanoate.

[0026] Preferably, the free radical initiator for the emulsion polymerization can also be a persulfate initiation system or a persulfate / sodium bisulfite initiation system; more preferably, the persulfate initiation system is an ammonium persulfate initiation system.

[0027] Specifically, a chain transfer agent may also be added to the polymerization system of the present invention to adjust the molecular weight of the polymer.

[0028] Preferably, the chain transfer agent is selected from one or more of ethyl acetate, diethyl malonate, diethyl carbonate, dimethyl carbonate, acetone, ethanol, and n-propanol.

[0029] Preferably, the chain transfer agent accounts for 0.01% to 1% of the polymer monomer by mass.

[0030] Preferably, the amount of the second monomer added for the first time is 10% to 90% of the total amount of the second monomer.

[0031] Preferably, the amount of the second monomer added is 10% to 90% of the total amount of the second monomer.

[0032] This invention also protects the application of the above-mentioned PVDF binder or the PVDF binder prepared by the above-mentioned preparation method in the preparation of battery cathode.

[0033] Compared with the prior art, the beneficial effects of the present invention are:

[0034] 1. This invention reduces the polymerization inhibition effect of the second monomer on the reaction system by adding it in two stages during the reaction, thereby increasing the reaction rate of the system and significantly improving the production efficiency of PVDF.

[0035] 2. In the PVDF binder prepared by the preparation method of the present invention, the randomly distributed second monomer unit portion is greater than 22% and less than 40%, and the prepared electrode sheet has higher bonding strength. Detailed Implementation

[0036] 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.

[0037] The randomly distributed second monomer unit portion represents the percentage between the mean (%) of the second monomer sequence (the sequence includes two repeating units derived from the first monomer) and the total mean (%) of the second monomer units:

[0038] When each second monomer repeating unit is separate, i.e. contained between two repeating units of the first monomer, the average number of the second monomer sequence is equal to the average total number of second monomer units. In this case, the randomly distributed second monomer unit portion is 100%.

[0039] The total average number of second monomer units in the polymer can be determined by acid-base titration of the carboxyl groups. 0.5 g of the polymer sample is weighed into acetone at 70–80 °C. 5 mL of water is added dropwise with thorough stirring to prevent polymer aggregation. The solution is then titrated with 0.1 N NaOH until complete neutralization, with a titration point of approximately -270 mV. Based on the measured acid equivalent, the content of the second monomer is determined, further confirming the total average number of second monomer units (i.e., the percentage molar number of second monomer units).

[0040] The mean (%) of the second monomer sequence can be obtained by... 19 F-NMR confirmed that the signal associated with the CF2 portion of the vinylidene fluoride unit of the adjacent separated second monomer unit (bold in the following chemical formula) corresponds to a peak at -94.8 ppm. This is based on -CH2CF2-[CH2CH(COOH)] n The intensity of the -CH2CF2-CH2- signal is proportional to the proportion of all peaks in the spectrum. The average number of second monomer sequences per 100 first monomer units (i.e., the % moles of the second monomer sequence) is determined, and from this, the proportion of randomly distributed second monomer units to the % moles of the second monomer sequence is finally determined.

[0041] Example 1

[0042] A method for preparing a PVDF adhesive specifically includes the following steps:

[0043] In a 20L vertical polymerization reactor, 10kg of deionized water and 4g of methylcellulose were added. The reactor was closed, evacuated, and purged several times with nitrogen until the oxygen content inside the reactor was less than 10ppm. 5g of diisopropyl peroxide and 4kg of vinylidene fluoride were added until the reactor pressure reached 6.0MPa. The reactor was heated to 45℃, and the stirrer was started at 800r / min. At the start of the reaction, 10g of acrylic monomer I-1 was dissolved in 500g of deionized water at 45℃ and added to the reactor via an additive pump, maintaining a pressure of 6.0MPa during the addition process. After 2 hours of reaction, 90g of acrylic monomer I-1 was added. After 4 hours, the reactor was depressurized, the suspension was collected, and repeatedly washed until the conductivity of the filtrate dropped below 0.1. Finally, the mixture was dried in a 95℃ oven for 24 hours to obtain the PVDF binder.

[0044] The prepared PVDF has a molecular weight of 1 million, an average total number of acrylic monomer units of 1.5%, and a random distribution of acrylic monomer units of 30%.

[0045] Example 2

[0046] A method for preparing a PVDF adhesive specifically includes the following steps:

[0047] In a 20L vertical polymerization reactor, 10kg of deionized water and 4g of methylcellulose were added. The reactor was closed, evacuated, and purged several times with nitrogen until the oxygen content inside the reactor was less than 10ppm. 5g of diisopropyl peroxide and 4kg of vinylidene fluoride were added until the reactor pressure reached 6.0MPa. The reactor was heated to 45℃, and the stirrer was started at 800r / min. At the start of the reaction, 50g of acrylic monomer I-1 was dissolved in 500g of deionized water at 45℃ and added to the reactor via an additive pump, maintaining a pressure of 6.0MPa during the addition process. After 2 hours of reaction, 50g of acrylic monomer I-1 was added again. After 4 hours, the reactor was depressurized, the suspension was collected, and repeatedly washed until the conductivity of the filtrate dropped below 0.1. Finally, the mixture was dried in a 95℃ oven for 24 hours to obtain the PVDF binder.

[0048] The prepared PVDF has a molecular weight of 1 million, an average total number of acrylic monomer units of 1.5%, and a random distribution of acrylic monomer units of 25%.

[0049] Example 3

[0050] A method for preparing a PVDF adhesive specifically includes the following steps:

[0051] In a 20L vertical polymerization reactor, 10kg of deionized water and 4g of methylcellulose were added. The reactor was closed, evacuated, and purged several times with nitrogen until the oxygen content inside the reactor was less than 10ppm. 5g of diisopropyl peroxide and 4kg of vinylidene fluoride were added until the reactor pressure reached 6.0MPa. The reactor was heated to 45℃, and the stirrer was started at 800r / min. At the start of the reaction, 30g of acrylic monomer I-1 was dissolved in 500g of deionized water at 45℃ and added to the reactor via an additive pump, maintaining a pressure of 6.0MPa during the addition process. After 2 hours of reaction, 30g of acrylic monomer I-1 was added again. After 4 hours, the reactor was depressurized, the suspension was collected, and repeatedly washed until the conductivity of the filtrate dropped below 0.1. Finally, the mixture was dried in a 95℃ oven for 24 hours to obtain the PVDF binder.

[0052] The prepared PVDF has a molecular weight of 1 million, an average total number of acrylic monomer units of 0.9%, and a random distribution of acrylic monomer units of 25%.

[0053] Example 4

[0054] A method for preparing a PVDF adhesive specifically includes the following steps:

[0055] In a 20L vertical polymerization reactor, 10kg of deionized water and 4g of methylcellulose were added. The reactor was closed, evacuated, and purged several times with nitrogen until the oxygen content inside the reactor was less than 10ppm. 5g of diisopropyl peroxide dicarbonate and 4kg of vinylidene fluoride / hexafluoropropylene monomer (molar ratio of vinylidene fluoride to hexafluoropropylene was 98:2) were added until the reactor pressure reached 6.0MPa. The reactor was heated to 45℃, and the stirrer was started at 800r / min. At the start of the reaction, 30g of acrylic monomer I-1 was dissolved in 500g of deionized water at 45℃ and added to the reactor via an additive pump, maintaining a pressure of 6.0MPa during the addition process. After 2 hours of reaction, 30g of acrylic monomer I-1 was added again. After 4 hours, the reactor was depressurized, the suspension was collected, and repeatedly washed until the conductivity of the filtrate dropped below 0.1. Finally, the mixture was dried in a 95℃ oven for 24 hours to obtain the PVDF binder.

[0056] The prepared PVDF has a molecular weight of 1 million, an average total number of acrylic monomer units of 0.9%, and a random distribution of acrylic monomer units of 25%.

[0057] Comparative Example 1

[0058] A method for preparing a PVDF adhesive specifically includes the following steps:

[0059] In a 20L vertical polymerization reactor, 10kg of deionized water and 4g of methylcellulose were added. The reactor was closed, evacuated, and purged several times with nitrogen until the oxygen content inside the reactor was less than 10ppm. 5g of diisopropyl peroxide dicarbonate and 4kg of vinylidene fluoride / hexafluoropropylene monomer (molar ratio of vinylidene fluoride to hexafluoropropylene was 98:2) were added until the reactor pressure reached 6.0MPa. The reactor was heated to 45℃, and the stirrer was started at 800r / min. 60g of acrylic acid monomer I-1 was dissolved in 500g of deionized water at 45℃ and continuously added to the reactor via an additive pump throughout the reaction, maintaining a pressure of 6.0MPa. After reacting for 10 hours, the reactor was depressurized, the suspension was collected, and repeatedly washed until the conductivity of the filtrate dropped below 0.1. Finally, the PVDF binder was dried in a 95℃ oven for 24 hours.

[0060] The prepared PVDF has a molecular weight of 1 million, an average total number of acrylic monomer units of 0.9%, and a random distribution of acrylic monomer units of 70%.

[0061] Comparative Example 2

[0062] A method for preparing a PVDF adhesive specifically includes the following steps:

[0063] In a 20L vertical polymerization reactor, 10kg of deionized water and 4g of methylcellulose were added. The reactor was closed, evacuated, and purged several times with nitrogen until the oxygen content inside the reactor was less than 10ppm. 5g of diisopropyl peroxide dicarbonate and 4kg of vinylidene fluoride / hexafluoropropylene monomer (molar ratio of vinylidene fluoride to hexafluoropropylene was 98:2) were added until the reactor pressure reached 6.0MPa. The reactor was heated to 45℃, and the stirrer was started at 800r / min. At the start of the reaction, 60g of acrylic monomer I-1 was dissolved in 500g of deionized water at 45℃ and added to the reactor using an additive pump, maintaining a pressure of 6.0MPa during the addition process. After 6 hours of reaction, the reactor was depressurized, the suspension was collected, and repeatedly washed until the conductivity of the filtrate dropped below 0.1. Finally, the PVDF binder was dried in a 95℃ oven for 24 hours.

[0064] The prepared PVDF has a molecular weight of 1 million, an average total number of acrylic monomer units of 0.9%, and a random distribution of acrylic monomer units of 22%.

[0065] Comparative Example 3

[0066] A method for preparing a PVDF adhesive specifically includes the following steps:

[0067] In a 20L vertical polymerization reactor, 10kg of deionized water and 4g of methylcellulose were added. The reactor was closed, evacuated, and purged several times with nitrogen until the oxygen content inside the reactor was less than 10ppm. 5g of diisopropyl peroxide and 4kg of vinylidene fluoride were added until the reactor pressure reached 6.0MPa. The reactor was heated to 45℃, and the stirrer was started at 800r / min. 100g of acrylic monomer I-1 was dissolved in 500g of deionized water at 45℃ and continuously added to the reactor via an additive pump throughout the reaction, maintaining a pressure of 6.0MPa. After reacting for 10 hours, the reactor was depressurized, the suspension was collected, and repeatedly washed until the conductivity of the filtrate dropped below 0.1. Finally, the PVDF binder was dried in a 95℃ oven for 24 hours.

[0068] The prepared PVDF has a molecular weight of 1 million, an average total number of acrylic monomer units of 1.5%, and a random distribution of acrylic monomer units of 70%.

[0069] Comparative Example 4

[0070] A method for preparing a PVDF adhesive specifically includes the following steps:

[0071] In a 20L vertical polymerization reactor, 10kg of deionized water and 4g of methylcellulose were added. The reactor was closed, evacuated, and purged several times with nitrogen until the oxygen content inside the reactor was less than 10ppm. 5g of diisopropyl peroxide and 4kg of vinylidene fluoride were added until the reactor pressure reached 6.0MPa. The reactor was heated to 45℃, and the stirrer was started at 800r / min. At the start of the reaction, 100g of acrylic monomer I-1 was dissolved in 500g of deionized water at 45℃ and added to the reactor via an additive pump, maintaining a pressure of 6.0MPa during the addition process. After 6 hours of reaction, the reactor was depressurized, the suspension was collected, and repeatedly washed until the conductivity of the filtrate dropped below 0.1. Finally, the PVDF binder was dried in a 95℃ oven for 24 hours.

[0072] The prepared PVDF has a molecular weight of 1 million, an average total number of acrylic monomer units of 1.5%, and a random distribution of acrylic monomer units of 22%.

[0073] Performance testing

[0074] The PVDF binders of the above embodiments and comparative examples were used to prepare electrode compositions, including electrode materials, conductive agents, and PVDF binders.

[0075] 1. Melting point test:

[0076] The melting peak value of the polymer was tested using DSC.

[0077] 2. Preparation process of cathode material (NCM8 series):

[0078] 2g of PVDF mixture was dissolved in 100g of NMP solution and stirred until fully dissolved. Then, 2.8g of conductive carbon black and 56.2g of NCM811 were added while stirring, and the mixture was ultrasonically stirred for 2 hours to obtain a uniform slurry. The slurry was coated onto a 12μm thick aluminum electrode foil using a coating machine, and the aluminum foil was then dried in a vacuum oven at 60℃ for 12 hours to obtain the positive electrode.

[0079] 3. Bond strength test:

[0080] The adhesive strength test was conducted according to the test method in the national standard GB / T2790-1995 "Test method for 180° peel strength of adhesives, flexible materials versus rigid materials".

[0081] Table 1 Comparison of test performance results between Examples 1-4 and Comparative Examples 1-4

[0082] As can be seen from the test results in Table 1 above, the PDVF binder of the present invention has excellent stability of the positive electrode slurry, does not exhibit gelation, has a low viscosity increase rate over 24 hours, and the slurry is uniform and stable. Furthermore, the PDVF binder of the present invention maintains a bonding strength of 20.1–22.5 N / m to the positive electrode sheet, demonstrating good bonding strength.

[0083] Compared to the embodiments of the present invention, the PVDF binders of Comparative Examples 1 to 4 have a high viscosity increase rate over 24 hours, exhibit gelation in the slurry, and have a low bonding strength of only 12.5 N / m to 13.5 N / m for the positive electrode sheet. Furthermore, the PVDF binders of Comparative Examples 1 and 3 require a longer preparation time and have lower production efficiency.

[0084] 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 PVDF adhesive, characterized in that, It includes a first repeating unit derived from a first monomer and a second repeating unit derived from a second monomer; the first monomer is vinylidene fluoride monomer; The general structural formula of the second monomer is as follows: Wherein, X is an H atom or a C1-C5 hydrocarbon moiety including at least one hydroxyl group; R 1 R 2 R 3 Each can be independently a H atom, a halogen atom, or a C1-C5 hydrocarbon group; The PVDF binder contains 0.05% to 10% of the second repeating unit on a molar basis, wherein the random distribution of the second monomer unit portion in the second repeating unit is greater than 22% and less than 40%.

2. The PVDF adhesive according to claim 1, characterized in that, The second monomer is selected from: One or more of them.

3. The PVDF adhesive according to claim 1 or 2, characterized in that, It contains at least 0.1% molar of a second repeating unit derived from the second monomer.

4. The PVDF adhesive according to claim 1 or 2, characterized in that, It contains up to 10% moles of a second repeating unit derived from the second monomer.

5. A method for preparing the PVDF adhesive according to claim 1 or 2, characterized in that, The preparation method includes copolymerizing the first monomer and the second monomer in a reaction vessel in an aqueous medium in the presence of a free radical initiator. The preparation method includes adding an aqueous solution of the second monomer in two separate steps during the reaction; The preparation method includes maintaining the pressure in the reaction vessel greater than the critical pressure of the first monomer.

6. The preparation method according to claim 5, characterized in that, The copolymerization reaction is emulsion polymerization or suspension polymerization.

7. The preparation method according to claim 5, characterized in that, The free radical initiator is an organic peroxide initiator.

8. The preparation method according to claim 5, characterized in that, The amount of the second monomer added for the first time is 10% to 90% of the total amount of the second monomer.

9. The preparation method according to claim 5, characterized in that, The amount of the second monomer added is 10% to 90% of the total amount of the second monomer.

10. The application of a PVDF binder according to any one of claims 1 to 4 or a PVDF binder prepared by any one of claims 5 to 9 in the preparation of a battery cathode.

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