A chelate-modified polyolefin diaphragm and its preparation method and application

Abstract

The present invention discloses a chelate-modified polyolefin membrane and its preparation method and application. The preparation method comprises the following steps: (1) dissolving a pH buffer, a water-soluble metal salt, and a water-soluble oxidant in water, controlling the pH to be between 7.5 and 8.5, and adding dopamine hydrochloride to dissolve and obtain a modified precursor solution; (2) adding a polyolefin membrane to the modified precursor solution obtained in step (1), and standing to obtain the chelate-modified polyolefin membrane. The present invention converts the original weakly interacting π-π stacking structure of polydopamine into a strongly interacting coordination bonding structure through a method of chelating polydopamine with metal ions, thereby greatly improving the chemical stability of the modified membrane in the electrolyte. PDA chelate has high thermal stability and strength, can enhance the heat shrinkage resistance of the membrane, and improve the safety of battery use.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries, and in particular relates to a chelate-modified polyolefin diaphragm and a preparation method and application thereof. Background Art

[0002] Thanks to its advantages of high energy, high cost-effectiveness, long life and low pollution, lithium-ion batteries have become the backbone of the new energy battery industry and are widely used in new energy vehicles, energy storage power stations, consumer electronics and other fields.

[0003] Lithium-ion batteries primarily consist of a positive electrode, negative electrode, separator, electrolyte, current collector, and packaging material. As a crucial component of lithium-ion batteries, the separator is closely linked to battery safety, lifespan, and sustainable energy storage. High-performing separators require excellent insulation, outstanding electrochemical stability, high thermal stability, appropriate pore size and porosity, good mechanical strength, and electrolyte wettability.

[0004] At present, the production materials of lithium-ion battery separators for large-scale commercialization are mainly polyolefins, including polypropylene (PP), polyethylene (PE), and polypropylene (PP) and polyethylene (PE) composite materials. As a commonly used separator material in lithium-ion batteries, although polyolefin battery separators have many advantages, they also have some disadvantages:

[0005] (1) The thermal deformation temperature of polyolefin separators is relatively low. For example, the thermal deformation temperature of polypropylene is around 100°C, while that of polyethylene is less than 80°C. At relatively low temperatures, polyolefins will melt and flow, causing the pores of the microporous membrane to change and even rupture the separator, making it impossible to isolate the positive and negative electrodes, resulting in a short circuit inside the battery and leading to safety accidents.

[0006] (2) Polyolefin separators have poor wettability to polar electrolytes. Since polyolefin separators do not contain polar groups in their molecular chain structure, they have poor affinity with commonly used ester and ether electrolytes, resulting in low liquid absorption rate of micropores and affected ion transmission efficiency.

[0007] (3) The mechanical strength of polyolefin separator is poor. During the charge and discharge cycle of lithium battery, due to excessive charge rate, over discharge, temperature change, Li + Factors such as uneven current distribution can cause lithium dendrites to form at the battery's negative electrode. Polyolefins have poor puncture resistance and are easily pierced by lithium dendrites, causing the battery to short-circuit.

[0008] In order to improve the comprehensive performance of polyolefin separators, the industry has adopted a large-scale surface coating modification method of inorganic or organic materials such as boehmite, alumina, polyvinylidene fluoride, and aramid. However, these dense coatings formed on the separators can easily block the pores of the separators, hindering the effective migration of lithium ions on the composite separators, resulting in high polarization and low power output of lithium-ion batteries. In addition, these improved separators can only operate at limited current densities (<2mA / cm 2 ) to achieve stable lithium plating / stripping behavior. Moreover, under the existing coating process, the thickness of the coating layer on one side is basically above 1μm, which is not conducive to the high capacity and lightweight of the battery.

[0009] Therefore, there is an urgent need to study high-performance modification methods for diaphragms so that polyolefin diaphragms have both good thermal stability and sufficient ion conduction channels, which can not only ensure the rapid transmission of lithium ions but also improve the safety of the battery.

[0010] At present, there are existing technologies that improve the battery performance of some polyolefin membranes by in situ polymerization of dopamine on the surface of polyolefin membranes such as PE and PP. However, the self-assembled polymerized polydopamine has poor chemical stability in ester and ether electrolytes and is prone to depolymerization, which affects the long-term cycle performance of the battery. Summary of the Invention

[0011] In view of the above-mentioned deficiencies in the prior art, the present invention provides a chelate-modified polyolefin diaphragm and a preparation method and application thereof.

[0012] The present invention first provides a method for preparing a chelate-modified polyolefin membrane, wherein the chelate is a polydopamine chelate, and the preparation method comprises the following steps:

[0013] (1) dissolving a pH buffer, a water-soluble metal salt, and a water-soluble oxidant in water, controlling the pH to be between 7.5 and 8.5, and adding dopamine hydrochloride to dissolve the solution to obtain a modified precursor solution;

[0014] (2) Adding a polyolefin membrane to the modified precursor solution obtained in step (1), and allowing the solution to stand to obtain the chelate-modified polyolefin membrane.

[0015] Dopamine begins to polymerize upon addition to the solution, so it's best to fully dissolve the other reagents before adding dopamine. In weakly alkaline metal salt solutions, dopamine self-polymerizes and chelates with metal ions to form polydopamine chelates. Thanks to the strong adhesion of polydopamine chelates, they can grow in situ on the surface of polyolefin separators, resulting in chelate-modified polyolefin separators.

[0016] Preferably, the pH buffer is at least one of a phosphate buffer, a borax buffer, a sodium barbital buffer, a Tris buffer, or an aqueous ammonia buffer. The pH buffer maintains the solution pH within the range of 7.5 to 8.5. A pH of approximately 8.5 is preferred, as under these weakly alkaline conditions, the polymerization rate and yield of dopamine are both optimal.

[0017] The water-soluble metal salt is used to provide metal ions. Preferably, the water-soluble metal salt is at least one of a lithium salt, a copper salt, an aluminum salt, a zinc salt, a cobalt salt, an iron salt, a cerium salt, a zirconium salt, and a nickel salt. The anion in the water-soluble metal salt is selected from at least one of a sulfate, a hydrochloride, and a nitrate. More preferably, the sulfate metal salt is at least one of lithium sulfate, copper sulfate, aluminum sulfate, zinc sulfate, nickel sulfate, and their hydrates.

[0018] The water-soluble oxidant promotes the self-polymerization of dopamine. Preferably, the water-soluble oxidant is at least one of persulfate, potassium permanganate, potassium perchlorate, and potassium dichromate. The persulfate is at least one of ammonium persulfate, potassium persulfate, and sodium persulfate.

[0019] Preferably, the molar ratio of the metal ion to dopamine hydrochloride in the water-soluble metal salt is not less than 1:2. The chelation ratio of the metal ion to dopamine hydrochloride is 1:2, with two dopamine hydrochlorides typically chelating one metal ion. To ensure complete chelation, the water-soluble metal salt is preferably added in a slight excess.

[0020] Preferably, in step (2), the polyolefin membrane is one of a polypropylene membrane, a polyethylene membrane, and a polyethylene and polypropylene composite membrane.

[0021] Preferably, in step (2), the temperature during the standing period is 20-60°C, and the standing period is 2-48 hours. More preferably, the temperature during the standing period is 25°C, and the standing period is 24 hours. Generally speaking, the longer the reaction time and the higher the temperature, the faster the in-situ growth rate and the thicker the film produced. However, the thickness of the film has an upper limit and will not grow further after reaching a certain thickness.

[0022] Preferably, the chelate-modified polyolefin membrane obtained after standing in step (2) is further cleaned and dried.

[0023] The present invention further provides a chelate-modified polyolefin diaphragm prepared by the preparation method.

[0024] The present invention also provides application of the chelate-modified polyolefin separator in the preparation of lithium batteries.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) Improving the chemical stability of the modified membrane: The present invention transforms the original weakly interacting π-π stacking structure of polydopamine into a strongly interacting coordination bonding structure by chelating polydopamine with metal ions, thereby significantly improving the chemical stability of the modified membrane in the electrolyte;

[0027] (2) Improve the wettability of polyolefin diaphragm electrolyte: Polydopamine chelate contains rich catechol and amino functional groups and has good affinity for polar electrolytes such as esters and ethers, which can improve the electrolyte wettability and liquid retention rate of the diaphragm;

[0028] (3) Improve the thermal stability of the separator: PDA chelate has high thermal stability and strength, which can enhance the separator's resistance to thermal shrinkage and improve the safety of battery use;

[0029] (4) Reduce resource consumption and environmental pollution: Through the research and application transformation of room temperature aqueous synthesis of PDA chelates, it is expected to reduce resource consumption and environmental pollution, and promote the green production and sustainable development of modified diaphragms. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a scanning electron microscope image, where Figure 1 (a) is a commercial polypropylene diaphragm. Figure 1 (b) is the polydopamine modified membrane prepared in Example 3, Figure 1 (c) is the polydopamine nickel chelate modified membrane prepared in Example 2; Figure 1 (d) is the polydopamine copper chelate modified diaphragm prepared in Example 1.

[0031] Figure 2 Digital photo of the electrolyte wettability test.

[0032] Figure 3 Digital photo of the electrolyte resistance test of the separator modified layer.

[0033] Figure 4 Digital photos for heat shrink performance testing.

[0034] Figure 5 This is a graph of the cycle performance of button batteries. DETAILED DESCRIPTION

[0035] The present invention will be described in detail below with reference to the accompanying drawings and examples. However, the examples do not limit the present invention in any form. Unless otherwise specified, the reagents, methods and equipment involved in the present invention are conventional reagents, methods and equipment in the art.

[0036] Example 1

[0037] Preparation of polydopamine copper chelate modified membrane (PP@PDA(Cu)):

[0038] 5.5 g of tris(hydroxymethyl)aminomethane, 4.8 g of copper sulfate pentahydrate, and 3.6 g of ammonium persulfate were added to 900 ml of deionized water, and after stirring and dissolving, the pH was adjusted to 8.5 with dilute hydrochloric acid, and 1.8 g of dopamine hydrochloride was added, and the mixture was stirred and dissolved again to obtain a modified precursor solution;

[0039] Take 0.02m 2 A polypropylene (PP) membrane was added to the modified precursor solution and allowed to react at 25°C for 24 hours to obtain a polyolefin membrane in situ modified with polydopamine copper chelate.

[0040] The polydopamine copper chelate modified membrane was washed with deionized water four times, and then placed in a vacuum oven at 50° C. to dry for 24 h. The dried modified membrane was cut into discs with a diameter of 19 mm.

[0041] Application of polydopamine copper chelate modified diaphragm in lithium batteries:

[0042] LiFePO4 is used as the positive electrode, lithium sheet is used as the negative electrode, polydopamine copper chelate modified diaphragm is used as the battery diaphragm, electrolyte is added dropwise, and a button battery is assembled.

[0043] Example 2

[0044] Preparation of polydopamine nickel chelate modified membrane (PP@PDA(Ni)):

[0045] 5.5 g of tris(hydroxymethyl)aminomethane, 5.0 g of nickel sulfate hexahydrate, and 3.6 g of ammonium persulfate were added to 900 ml of deionized water, and after stirring and dissolving, the pH was adjusted to 7.5 with dilute hydrochloric acid, and 1.8 g of dopamine hydrochloride was added, and the mixture was stirred and dissolved again to obtain a modified precursor solution;

[0046] Take 0.02m 2 A polypropylene (PP) membrane was added to the modified precursor solution and allowed to react at 25°C for 24 hours to obtain a polyolefin membrane in situ modified with polydopamine nickel chelate.

[0047] The polydopamine nickel chelate modified membrane was washed with deionized water four times, and then placed in a vacuum oven at 50° C. to dry for 24 h. The dried modified membrane was cut into discs with a diameter of 19 mm.

[0048] Application of polydopamine nickel chelate modified diaphragm in lithium battery:

[0049] LiFePO4 is used as the positive electrode, lithium sheet is used as the negative electrode, polydopamine nickel chelate modified diaphragm is used as the battery diaphragm, electrolyte is added dropwise, and a button battery is assembled.

[0050] Example 3

[0051] Preparation of polydopamine modified membrane (PP@PDA):

[0052] 5.5 g of tris(hydroxymethyl)aminomethane) and 3.6 g of ammonium persulfate were added to 900 ml of deionized water, and after stirring and dissolving, the pH was adjusted to 8.5 with dilute hydrochloric acid, and 1.8 g of dopamine hydrochloride was added, and the mixture was stirred and dissolved again to obtain a modified precursor solution;

[0053] Take 0.02m 2 A polypropylene (PP) membrane was added to the modified precursor solution and allowed to react at 25°C for 24 hours to obtain a polydopamine-modified polyolefin membrane.

[0054] The polydopamine modified membrane was washed with deionized water four times, and then placed in a vacuum oven at 50° C. to dry for 24 h. The dried modified membrane was cut into discs with a diameter of 19 mm.

[0055] Application of polydopamine modified diaphragm in lithium battery:

[0056] LiFePO4 is used as the positive electrode, lithium sheet as the negative electrode, polydopamine modified diaphragm as the battery diaphragm, electrolyte is added dropwise, and a button battery is assembled.

[0057] Example 4

[0058] Preparation of polypropylene separator (PP) assembled battery:

[0059] Use LiFePO4 as the positive electrode, lithium sheet as the negative electrode, polypropylene separator as the battery separator, add electrolyte, and assemble into a button battery.

[0060] Test Example 1

[0061] 1. Experimental Materials

[0062] The polydopamine copper chelate modified membrane prepared under the conditions of Example 1 and the battery assembled with the polydopamine copper chelate modified membrane, the polydopamine nickel chelate modified membrane prepared under the conditions of Example 2 and the battery assembled with the polydopamine nickel chelate modified membrane, the polydopamine modified membrane prepared under the conditions of Example 3 and the battery assembled with the polydopamine modified membrane, and the polypropylene membrane of Example 4 and the battery assembled with the polypropylene membrane were used as experimental materials.

[0063] 2. Experimental methods

[0064] 1) Scanning electron microscopy:

[0065] The membranes of Examples 1-4 were attached to a sample stage of an electron microscope covered with conductive adhesive and placed in a gold spraying device for gold spraying. The morphology of the samples was then observed using a scanning electron microscope (Scios2 Hivac) at a voltage of 10 kV and a magnification of 20,000.

[0066] 2) Electrolyte wettability:

[0067] 5 μL of electrolyte was dropped onto a 19 mm diaphragm disc. After 5 minutes, the wettability of the electrolyte on the diaphragm was observed and photographed.

[0068] 3) Electrolyte resistance:

[0069] Different modified diaphragms were cut into discs with a diameter of 19 mm and immersed in 1 ml of electrolyte respectively. After immersion for 24 hours and 14 days, the color change of the electrolyte was observed.

[0070] 4) Heat shrinkage performance:

[0071] Place a stainless steel plate and two pieces of high-temperature filter paper in the middle of a constant-temperature oven. Control the temperature so that the stainless steel plate and the high-temperature filter paper reach 140±2°C or 160±2°C. After measuring the longitudinal and transverse lengths of the diaphragm, place it flat on one of the high-temperature filter papers on the stainless steel plate in the middle of the blast-type constant-temperature oven. Press it down with another piece of high-temperature filter paper, close the constant-temperature oven door, and start counting the time. Maintain the test temperature for 30min±2min. After heating, remove the diaphragm. After the diaphragm returns to room temperature, measure the longitudinal and transverse lengths again and calculate the thermal shrinkage. Repeat the test three times and take the average value.

[0072] 5) Cycle performance:

[0073] The button cells assembled with different separators were installed in the Xinwei battery testing system. In the rate mode, the battery was first run at 0.1C for 3 cycles to activate the battery, and then the battery was cycled at 0.5C.

[0074] 3. Experimental results

[0075] 1) Scanning electron microscope test: Figure 1 The figures are scanning electron microscope images, in which (a) is a commercial polypropylene membrane, (b) is a polydopamine-modified membrane prepared in Example 3, (c) is a polydopamine-nickel chelate-modified membrane prepared in Example 2; and (d) is a polydopamine-copper chelate-modified membrane prepared in Example 1.

[0076] Figure 1Scanning electron microscopy results show that the pore size of the membrane decreases slightly and becomes more uniform after modification due to the in situ growth of polydopamine and polydopamine chelate on the membrane surface. Some pores in the polydopamine-modified membrane are blocked by polydopamine, while the pores in the polydopamine chelate-modified membrane are not blocked.

[0077] 2) Electrolyte wettability test: Figure 2 The following are digital photos of commercial polypropylene diaphragms and modified diaphragms after 5 μL of electrolyte was added and allowed to stand for 5 minutes. The upper left corner is a commercial polypropylene diaphragm, the upper right corner is the polydopamine modified diaphragm prepared in Example 3, the lower left corner is the polydopamine nickel chelate modified diaphragm prepared in Example 2, and the lower right corner is the polydopamine copper chelate modified diaphragm prepared in Example 1. As can be seen from the figure, compared with the unmodified commercial diaphragm, the electrolyte diffusion area on the modified diaphragm is larger, among which the electrolyte diffusion area on the polydopamine chelate modified diaphragm is larger than that on the polydopamine modified diaphragm, indicating that the polydopamine chelate modification can significantly improve the electrolyte wettability of the diaphragm.

[0078] 3) Electrolyte resistance test: Figure 3 These are digital photos of polydopamine and polydopamine chelate modified diaphragms immediately after being immersed in electrolyte, after being immersed for 24 hours, and after being immersed for 14 days. As can be seen from the figure, after the polydopamine modified diaphragm was immersed in the electrolyte for 24 hours, the electrolyte changed from colorless and transparent to brown. This is because polydopamine is unstable in the electrolyte and will depolymerize into small molecular polymers, which will dissolve in the electrolyte and change the color of the electrolyte. However, after the polydopamine chelate modified diaphragm was immersed in the electrolyte for 14 days, the electrolyte in which it was located remained transparent and clear. This is due to the chelation effect of metal ions, which transforms the original weakly interacting π-π stacking structure of polydopamine into a strongly interacting coordination bonding structure, greatly improving the chemical stability of the modified diaphragm in the electrolyte.

[0079] 4) Thermal shrinkage performance test: Figure 4 This is a digital photo of the heat shrinkage performance test of commercial polypropylene diaphragm and chelate modified diaphragm, Figure 4 As shown in Table 1, the heat shrinkage resistance of the polydopamine chelate modified diaphragm is significantly improved, which is due to the thermal stability of the polydopamine chelate itself and the support layer structure formed on the surface of the diaphragm and inside the pores.

[0080] Table 1 Thermal shrinkage of commercial diaphragms and chelate-modified diaphragms

[0081]

[0082] 5) Electrochemical performance test: Figure 5The following graph shows the cycling performance of button cells assembled with commercial polypropylene separators, polydopamine-modified separators, polydopamine-nickel chelate-modified separators, and polydopamine-copper chelate-modified separators. As can be seen from the graph, the rate capability and cycling performance of button cells assembled with polydopamine-chelate-modified separators are superior to those assembled with commercial separators and polydopamine-modified separators. This is due to the excellent electrolyte wettability and chemical stability of the polydopamine-chelate-modified separator, as well as the improvement of the separator's pore structure.

[0083] In summary, for polyolefin separators, polydopamine chelate modification can improve the electrochemical performance and safety performance of the assembled batteries, and polydopamine chelate modified polyolefin separators have great commercial potential.

[0084] Finally, it should be noted that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art will be able to modify the technical solutions described in the aforementioned embodiments or substitute equivalent features for some of the technical features. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for preparing a chelate-modified polyolefin membrane, characterized in that: The chelate is a polydopamine chelate, and the preparation method comprises the following steps: (1) dissolving a pH buffer, a water-soluble metal salt, and a water-soluble oxidant in water, controlling the pH to be between 7.5 and 8.5, and adding dopamine hydrochloride to dissolve the solution to obtain a modified precursor solution; (2) Adding a polyolefin membrane to the modified precursor solution obtained in step (1), and allowing the solution to stand to obtain the chelate-modified polyolefin membrane.

2. The method for preparing a chelate-modified polyolefin membrane according to claim 1, wherein: The pH buffer is at least one of a phosphate buffer, a borax buffer, a sodium barbital buffer, a tris buffer, and an ammonia buffer.

3. The method for preparing the chelate-modified polyolefin membrane according to claim 1, wherein: The water-soluble metal salt is at least one of lithium salt, copper salt, aluminum salt, zinc salt, cobalt salt, iron salt, cerium salt, zirconium salt and nickel salt.

4. The method for preparing the chelate-modified polyolefin membrane according to claim 1, wherein: The water-soluble oxidant is at least one of persulfate, potassium permanganate, potassium perchlorate, and potassium dichromate; The persulfate is at least one of ammonium persulfate, potassium persulfate, and sodium persulfate.

5. The method for preparing the chelate-modified polyolefin membrane according to claim 1, wherein: The molar ratio of metal ions to dopamine hydrochloride in the water-soluble metal salt is not less than 1:

2.

6. The method for preparing the chelate-modified polyolefin membrane according to claim 1, wherein: In step (2), the polyolefin membrane is one of a polypropylene membrane, a polyethylene membrane, and a polyethylene and polypropylene composite membrane.

7. The method for preparing the chelate-modified polyolefin membrane according to claim 1, characterized in that: In step (2), the temperature during standing is 20 to 60° C., and the standing time is 2 to 48 hours.

8. The method for preparing the chelate-modified polyolefin membrane according to claim 1, wherein: The chelate-modified polyolefin membrane obtained in step (2) after standing is further cleaned and dried.

9. The chelate-modified polyolefin membrane prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the chelate-modified polyolefin separator according to claim 9 in the preparation of lithium batteries.

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