Method for treating polypropylene diaphragm for lithium battery by plasma

Abstract

The invention provides a method for treating a polypropylene diaphragm for a lithium battery by using plasma, which comprises the following steps: (1) pretreating the polypropylene diaphragm by using an antioxidant and plasma, and (2) carrying out self-assembly crosslinking on the pretreated diaphragm and a natural polymer solution to generate a natural polymer layer, and drying to obtain the lithium battery diaphragm. According to the method, molecular chain damage is remarkably reduced on the premise that the surface activation effect is kept, meanwhile, the influence of mechanical strength caused after plasma treatment is improved, and the affinity of a treated membrane surface relative to a base membrane and an electrolyte is improved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically a method for plasma treatment of polypropylene separators used in lithium batteries. Background Technology

[0002] As a key component, the performance of the lithium-ion battery separator has a decisive impact on the battery's internal resistance, safety, and capacity. Currently, lithium-ion battery separators are mainly made of polyethylene (PE) and polypropylene (PP). PE separators are generally prepared using a wet process, resulting in a controllable pore structure and excellent permeability. Dry-process PP separators offer higher thermal stability and lower cost. While traditional polyolefin separators possess basic performance advantages, they have inherent defects such as insufficient electrolyte wettability and are prone to shrinkage and deformation under high temperatures, potentially leading to safety hazards such as short circuits and thermal runaway. With the development of separators, the industry has widely adopted surface coating processes to address these technical bottlenecks. By coating the separator surface with materials such as ceramics, PAA, PVDF, or aramid fibers, the performance of the separator has been significantly improved. However, due to the low surface energy of the separator surface, the peel strength between the separator and the coating layer is poor, resulting in a large amount of powder shedding.

[0003] Plasma surface treatment is a dry process with advantages such as energy saving and pollution-free operation, meeting environmental protection requirements. Furthermore, its depth of action is within the nanometer range, allowing for easy control over the area and extent of modification. However, for polypropylene membranes, the tertiary carbon atoms in the molecular chain readily generate free radicals when subjected to plasma treatment. These carbon atoms react with oxygen to form peroxide free radicals, ultimately forming peroxides. These unstable peroxides continue to attack the main chain, causing chain segment breakage. This also leads to a decrease in the mechanical strength of the membrane after plasma treatment, damage to the micropores, and affects the normal use of the membrane. Summary of the Invention

[0004] To address the shortcomings of the prior art, this invention provides a method for plasma-treated polypropylene separators for lithium batteries. Before plasma treatment, an antioxidant is attached to the surface of the separator. The antioxidant on the separator surface effectively prevents the generation of free radical carbon during plasma treatment of the polypropylene chains. Simultaneously, the plasma-treated membrane surface carries a charge, which allows natural polymers to be deposited on the membrane surface through charge action. After heating and cross-linking, a natural polymer layer is formed.

[0005] Traditional polypropylene membranes have a hydrophobic surface and low surface energy. Plasma-treated polypropylene membranes generate a large number of polar groups on their surface, increasing the membrane's free energy and effectively improving its electrolyte affinity. Antioxidants inhibit free radical degradation to some extent, while the cross-linked natural polymer layer acts as a framework, effectively improving the membrane's mechanical strength.

[0006] The technical solution provided by this invention: A method for plasma treatment of polypropylene separators for lithium batteries, comprising the following steps:

[0007] (1) Pretreatment of polypropylene membrane with antioxidants and plasma;

[0008] (2) The pretreated separator is self-assembled and cross-linked with a natural polymer solution to generate a natural polymer layer, and then dried to obtain a lithium battery separator.

[0009] Furthermore, in step (1), the polypropylene diaphragm is soaked in an antioxidant solution or the antioxidant solution is sprayed onto the surface of the diaphragm, and then subjected to plasma treatment after natural air drying or baking.

[0010] Furthermore, the antioxidant is one or a mixture of two of 1010 and 1076, the solvent is one of ethyl acetate, acetone and chloroform, and the mass concentration of the antioxidant in the antioxidant solution is 100-5000 ppm.

[0011] Furthermore, the conditions for plasma treatment in step (1) are as follows: the plasma is a mixture of argon and air, the volume ratio of argon to air is 0.5 to 1.5:10, the treatment time is 3 to 300 seconds, and the equipment power is 100 to 1000W.

[0012] Furthermore, the natural polymer solution is prepared from natural polymer, crosslinking agent and water, with the mass concentration of natural polymer being 1-5% and the mass ratio of crosslinking agent to natural polymer being 0.1-0.5:100.

[0013] Furthermore, the natural polymer is selected from one of carboxymethyl chitosan, sodium carboxymethyl starch, sodium carboxymethyl cellulose, and chitin.

[0014] Furthermore, the crosslinking agent is selected from sodium citrate, succinic acid, 2-hydroxysuccinic acid, and oxalic acid.

[0015] Furthermore, in step (2), the plasma-treated separator is immersed in a natural polymer solution for 2-6 hours. After immersion, it is cleaned and dried at 60-85℃ for 1-5 hours to obtain a lithium battery separator.

[0016] Furthermore, the polypropylene diaphragm is a single-layer or multi-layer PP diaphragm with a thickness of 6 to 25 μm.

[0017] A lithium battery separator is prepared by the above-described processing method.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0019] After being immersed in an antioxidant solution, the antioxidant enters the micropores of the base membrane along with the solution, forming a free radical scavenging network. This reduces the impact of free radicals on the base membrane and decreases the depth of chemical micro-etching. Simultaneously, the base membrane surface becomes charged after plasma treatment. This charge attracts natural polymers to the base membrane surface, which, after thermal cross-linking, can serve as a framework layer to improve the mechanical properties of the base membrane. This invention improves the electrolyte wetting effect of the base membrane and solves the problem of decreased mechanical strength of polypropylene separators after plasma treatment. Detailed Implementation

[0020] To enhance understanding of the present invention, the present invention will be further described in detail below with reference to embodiments. These embodiments are only used to explain the present invention and do not constitute a limitation on the scope of protection of the present invention.

[0021] Example 1

[0022] A 1010 solution with a mass concentration of 1000 ppm was prepared using ethyl acetate as the solvent. A 12 μm thick polypropylene membrane was immersed in the 1010 solution until it was completely submerged, and then removed and air-dried. The dried membrane was then placed under plasma discharge conditions, with the air-argon ratio adjusted to 10:1, the treatment time set to 120 s, and the power set to 100 W, to obtain the plasma-treated membrane.

[0023] Sodium carboxymethyl cellulose was dissolved in water, and after stirring, sodium citrate was added to prepare an aqueous solution of the natural polymer. The mass concentration of sodium carboxymethyl cellulose was 3%, and the mass ratio of sodium citrate to sodium carboxymethyl cellulose was 0.3:100. The plasma-treated base membrane was then immersed in the sodium carboxymethyl cellulose aqueous solution for 2 hours. After rinsing with pure water, it was dried at 75°C for 3 hours to obtain a multilayer polypropylene membrane.

[0024] Example 2

[0025] The only difference between this embodiment and Embodiment 1 is that in this embodiment, the antioxidant used is a mixed solution of antioxidants 1010 and 1076, with a mass ratio of 1:1 and a mass concentration of 2000 ppm.

[0026] Example 3

[0027] The only difference between this embodiment and Embodiment 1 is that the ratio of air to argon is adjusted to 10:1.5, the processing time is 150 seconds, and the power is 100W.

[0028] Example 4

[0029] The only difference between this embodiment and Embodiment 1 is that the natural polymer used in this embodiment is carboxymethyl chitosan with a mass concentration of 5%, and the mass ratio of sodium citrate to carboxymethyl chitosan is 0.5:100.

[0030] Example 5

[0031] The only difference between this embodiment and Embodiment 1 is that in this embodiment, the natural polymer used is sodium carboxymethyl starch with a mass concentration of 1%, and the crosslinking agent is succinic acid, with a mass ratio of succinic acid to sodium carboxymethyl starch of 0.3:100.

[0032] Comparative Example 1

[0033] A 12μm polypropylene diaphragm was placed under plasma discharge conditions using an air atmosphere for 120 seconds and 100W to obtain a plasma-treated base film.

[0034] Comparative Example 2

[0035] The performance was directly tested using a dry-process 12μm polypropylene base membrane.

[0036] The mechanical strength, contact angle of the uncoated surface, and peel force of the coating prepared in Examples 1-5 and Comparative Examples 1-2 were tested respectively, as shown in the table below.

[0037]

[0038] The mechanical strength of the membranes treated with different methods was tested. The results showed that the untreated membrane had the highest mechanical strength. The mechanical strength of the membrane decreased significantly after plasma treatment. Immersion in antioxidants and the addition of a natural polymer layer mitigated the impact of plasma treatment on mechanical strength to some extent and effectively improved the electrolyte affinity of the membrane.

[0039] The above description is merely a preferred embodiment of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the inventive concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A method for plasma treatment of a polypropylene separator for lithium batteries, characterized in that, Includes the following steps: (1) Pretreatment of polypropylene membrane with antioxidants and plasma; (2) The pretreated separator is self-assembled and cross-linked with a natural polymer solution to generate a natural polymer layer, and then dried to obtain a lithium battery separator.

2. The method for plasma treatment of a polypropylene separator for lithium batteries according to claim 1, characterized in that, In step (1), the polypropylene diaphragm is soaked in an antioxidant solution or the antioxidant solution is sprayed onto the surface of the diaphragm, and then it is naturally air-dried or oven-dried before being subjected to plasma treatment.

3. The method for plasma treatment of a polypropylene separator for lithium batteries according to claim 2, characterized in that, The antioxidant is one or a mixture of two of 1010 and 1076, the solvent is one of ethyl acetate, acetone and chloroform, and the mass concentration of the antioxidant in the antioxidant solution is 100-5000 ppm.

4. The method for plasma treatment of a polypropylene separator for lithium batteries according to claim 1, characterized in that, The conditions for plasma treatment in step (1) are as follows: the plasma is a mixture of argon and air, the volume ratio of argon to air is 0.5 to 1.5:10, the treatment time is 3 to 300 seconds, and the equipment power is 100 to 1000W.

5. The method for plasma treatment of a polypropylene separator for lithium batteries according to claim 1, characterized in that, The natural polymer solution is prepared from natural polymer, crosslinking agent and water, with the mass concentration of natural polymer being 1-5% and the mass ratio of crosslinking agent to natural polymer being 0.1-0.5:

100.

6. The method for plasma-processing a polypropylene separator for lithium batteries according to claim 5, characterized in that, The natural polymer is selected from one of carboxymethyl chitosan, sodium carboxymethyl starch, sodium carboxymethyl cellulose, and chitin.

7. The method for plasma treatment of a polypropylene separator for lithium batteries according to claim 5, characterized in that, The crosslinking agent is selected from one of sodium citrate, succinic acid, 2-hydroxysuccinic acid, and oxalic acid.

8. The method for plasma treatment of a polypropylene separator for lithium batteries according to claim 1, characterized in that, In step (2), the plasma-treated separator is immersed in a natural polymer solution for 2-6 hours. After immersion, it is cleaned and dried at 60-85℃ for 1-5 hours to obtain the lithium battery separator.

9. The method for plasma treatment of a polypropylene separator for lithium batteries according to claim 1, characterized in that, The polypropylene diaphragm is a single-layer or multi-layer PP diaphragm with a thickness of 6 to 25 μm.

10. A lithium battery separator, characterized in that, It is prepared by the processing method according to any one of claims 1-9.

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