Polypropylene diaphragm modification method for energy storage of lithium ion battery

Abstract

The invention provides a modification method of a polypropylene diaphragm for energy storage of a lithium ion battery. The modification method comprises the following steps: step S01, selecting a polypropylene diaphragm sample; and S02, cleaning the polypropylene diaphragm sample, and drying the cleaned polypropylene diaphragm sample by using a vacuum drying oven. And S03, opening a gas conveying system, and adjusting the oxygen content and the gas flow rate. And step S04, placing the polypropylene diaphragm sample in the center of the DBD reactor, starting a nanosecond pulse power supply, and adjusting power supply parameters. And step S05, after the treatment is completed, taking out the polypropylene diaphragm sample, storing the polypropylene diaphragm sample in a dry dust-free sealing box, and standing to complete modification. The surface wettability, the roughness and the liquid absorption rate of the lithium ion battery diaphragm are improved, and the problems that an existing lithium ion battery diaphragm surface treatment technology is large in chemical reagent usage amount, not ideal in treatment effect and the like are solved. The method can comprehensively improve the performance and operation reliability of the lithium battery, and realizes effective combination of a plasma treatment process and other treatment processes.

Description

Technical Field

[0001] The present invention relates to a polypropylene diaphragm for lithium ion battery energy storage, and in particular to a method for modifying a polypropylene diaphragm for lithium ion battery energy storage. Background Art

[0002] The performance of lithium-ion batteries is largely influenced by the separator. The main problem with polypropylene (PP) separators is their non-polar surface, resulting in poor wettability with the electrolyte. This results in insufficient electrolyte absorption capacity, preventing the separator from rapidly absorbing the electrolyte, which in turn affects the battery's ionic conductivity, increases internal resistance, and reduces charge and discharge efficiency. Secondly, the low compatibility between the separator and the electrolyte increases interfacial contact resistance, leading to uneven local current density and, in turn, affecting the overall performance of the battery. Furthermore, due to insufficient wettability, the electrolyte may be unevenly distributed during cycling, which can lead to localized overcharge or over-discharge of the electrode, ultimately affecting the battery's cycle life.

[0003] To overcome these issues and improve the wettability of polypropylene separators, common modification methods include surface chemical treatment, graft polymerization, nanocoating, and plasma treatment. Plasma technology, due to its high efficiency, environmental friendliness, and minimal impact on the overall material performance, is an ideal choice for improving the wettability of polypropylene separators. Plasma technology effectively modifies the chemical composition, energy state, and physical structure of the material surface, thereby improving its wettability and liquid absorption. Plasma treatment of polypropylene separators can introduce polar functional groups (such as hydroxyl, carboxyl, and amide groups) onto the surface, increasing its affinity for electrolytes. In battery separator applications, dielectric barrier discharge (DBD) plasma is widely used for modifying polypropylene separators due to its simple equipment, convenient operation, and suitability for continuous production. Researchers have found that DBD plasma treatment can reduce the water contact angle of PP separators from an initial 110° to approximately 40°, significantly improving the material's wettability and electrolyte absorption rate. At the same time, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis showed that after plasma treatment, oxide groups (such as C=O, OC=O) were introduced on the surface of the PP membrane, thereby improving its hydrophilicity.

[0004] The modification of the plasma direct treatment technology only occurs in the surface layer and will not change the overall physical properties of the material. It only affects the surface range of a few nanometers to a few microns. At the same time, plasma technology meets the current needs of energy conservation and environmental protection. It has no special requirements for the materials being treated, has universal applicability, and has a short processing time of only a few seconds to a few minutes. Although plasma treatment is an efficient method, there are still certain challenges in the modification of battery separators. After plasma treatment, the surface energy of the material may gradually decrease over time, resulting in a decrease in hydrophilicity. The uniformity of the treatment is affected by the equipment and process parameters, which may lead to uneven modification effects in local areas. Summary of the Invention

[0005] 1. Technical problems to be solved: How to use plasma to treat the surface of lithium-ion battery separators to increase their roughness, wettability, and liquid absorption rate, thereby improving the safety and cycle performance of lithium batteries.

[0006] 2. Technical solution: In order to solve the above problems, the present invention provides a method for modifying a polypropylene diaphragm for lithium-ion battery energy storage, comprising the following steps: Step S01: Select a polypropylene diaphragm sample.

[0007] Step S02: Clean the polypropylene membrane sample to remove contaminants on the surface of the material, and dry it in a vacuum drying oven after cleaning.

[0008] Step S03: Turn on the gas delivery system and adjust the oxygen content and gas flow rate.

[0009] Step S04: Place the polypropylene membrane sample in the center of the DBD reactor, start the nanosecond pulse power supply and adjust the power supply parameters.

[0010] Step S05: After the treatment is completed, the polypropylene membrane sample is taken out and stored in a dry, dust-free sealed box for standing to complete the modification.

[0011] It also includes wettability analysis of the modified polypropylene membrane and evaluation of the modification effect through water contact angle measurement. The specific method is: using a multi-point detection method, with the center of the polypropylene membrane sample as the pole, taking a ray every 30°, and taking points on each ray at intervals of 5 mm, for a total of 61 points; using a 1.5 μL deionized water droplet, the CCD camera collects images, calculates the average contact angle, and draws a polar coordinate distribution diagram of the water contact angle.

[0012] In step S01 : a plurality of polypropylene separator samples having the same shape and thickness are prepared.

[0013] In step S02, an ultrasonic cleaning apparatus is used for cleaning, and deionized water is used to clean the material.

[0014] In step S03, the oxygen content is adjusted by mixing pure argon with an argon-oxygen mixed gas, wherein the argon-oxygen mixed gas contains 1% O2.

[0015] In step S04, cylindrical metals are placed at both ends of the DBD reactor as discharge electrodes, with the upper end being the positive electrode and the lower end being the negative electrode. The neutral wire passes through the current coil and is grounded. The reaction chamber is composed of a quartz glass plate with air inlet and outlet and a quartz glass dish. The reaction chamber is located between the positive and negative electrodes, and the polypropylene diaphragm sample is placed in the reaction chamber.

[0016] In step S04, the nanosecond pulse power supply has the following parameter specifications: output amplitude range is 0-15 kV, output waveform is high voltage square wave; output frequency range is 0-20 kHz; pulse width adjustment range is 0 ns-1 ms; rising edge and falling edge time adjustment range is 50 ns-250 ns.

[0017] In step S04, the processing time is 55-65 seconds.

[0018] In step S05, the polypropylene membrane sample is stored in a dry, dust-free sealed box and allowed to stand for 9-11 hours.

[0019] 3.Beneficial effects: The present invention proposes a method for modifying a polypropylene diaphragm for lithium-ion battery energy storage. The method utilizes plasma to achieve material surface treatment, physical etching, and chemical modification, thereby improving the surface wettability, roughness, and liquid absorption rate of the lithium-ion battery diaphragm. This method solves the problems of large amounts of chemical reagents used and unsatisfactory treatment effects in existing lithium-ion battery diaphragm surface treatment technologies.

[0020] The present invention provides a process flow and method suitable for the surface treatment of lithium battery separators. According to the different requirements of different plasma parameters for the treatment effect, the correlation between the external applied voltage, treatment time and the surface treatment effect, roughness, wettability and liquid absorption rate of the material is established, the treatment efficiency and effect are optimized, and the performance and operational reliability of lithium batteries can be comprehensively improved. The effective combination of plasma treatment process and other treatment processes is realized, and the key technologies in the industrialization process are solved. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a flow chart of the present invention.

[0022] Figure 2 It is a structural diagram of the DBD reactor.

[0023] Figure 3 This is the polar coordinate distribution diagram of water contact angle before polypropylene membrane treatment.

[0024] Figure 4 This is the polar coordinate distribution diagram of water contact angle after polypropylene membrane treatment.

[0025] Figure 5 It is the discharge specific capacity curve.

[0026] Explanation of reference numerals: 1. positive electrode; 2. negative electrode; 3. quartz glass plate; 4. quartz glass dish; 5. polypropylene diaphragm sample; 6. gas inlet. DETAILED DESCRIPTION

[0027] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0028] like Figure 1 As shown, a method for modifying a polypropylene diaphragm for lithium-ion battery energy storage comprises the following steps: Step S01: Select a polypropylene diaphragm sample 5.

[0029] In one embodiment, a plurality of polypropylene membrane samples 5 having the same shape and thickness are prepared, and each polypropylene membrane sample 5 is cut into a diameter of 70 mm and a thickness of 1 mm.

[0030] Multiple polypropylene diaphragm samples can be compared with the modified polypropylene diaphragm sample 5 and the unmodified polypropylene diaphragm sample to obtain the improved data after modification.

[0031] Step S02: Clean the polypropylene membrane sample 5 to remove contaminants on the surface of the material, and dry it in a vacuum drying oven after cleaning.

[0032] In one embodiment, an ultrasonic cleaner is used for cleaning; deionized water is used to clean the material for 10 minutes to remove contaminants on the surface of the material; and a vacuum drying oven is used to dry the material after cleaning to avoid interference from dust and oil.

[0033] Step S03: Turn on the gas delivery system and adjust the oxygen content and gas flow rate.

[0034] The gas flow meter can measure the flow rate of the gas flowing into the reactor to control the experimental variables and maintain the stability of the reaction. Preferably, argon and oxygen are used as working gases.

[0035] In one embodiment, when opening a gas cylinder, slowly introduce gas. Once the cylinder's ventilation stabilizes, adjust the oxygen content. Use pure argon (Ar) and an argon-oxygen mixture (1% O2) to prepare an argon-oxygen working gas with the desired oxygen content. To increase the oxygen content, introduce more argon-oxygen mixture. To reduce the oxygen content, introduce more pure argon. The argon-oxygen mixture contains 1% O2.

[0036] The oxygen content required for the experiment was 0.08%, and the argon gas flow rate of the mass flow monitor was fixed at 1 L / min.

[0037] Step S04: Place the polypropylene membrane sample in the center of the DBD reactor, start the nanosecond pulse power supply and adjust the power supply parameters to generate Ar / O2 plasma.

[0038] In one embodiment, the DBD reactor is provided with cylindrical metal discharge electrodes at both ends, with the upper end being the positive electrode 1 and the lower end being the negative electrode 2. A neutral line passes through a current coil and is then grounded. The reaction chamber in the reaction system consists of a quartz glass plate 3 with an air inlet and outlet 6 and a quartz glass dish 4. The reaction chamber is located between the two electrodes, with the lid and bottom of the dish serving as barrier media. The polypropylene membrane sample 5 is placed in the reaction chamber. The DBD structure has an upper plate thickness of 1 mm, a lower dish height of 4 mm, and a diameter of 80 mm. The electrode radius is 25 mm, with both positive and negative electrodes exposed, and a discharge air gap of 2 mm.

[0039] In one embodiment, a nanosecond pulse power supply is used as the power supply. The power supply has the following specifications: an output amplitude range of 0-15 kV, a high-voltage square wave waveform, preferably 3.8 kV; an output frequency range of 0-20 kHz, preferably 2.5 kHz; a pulse width adjustment range of 0 ns-1 ms, preferably 1500 ns; rising and falling edge time adjustment range of 50 ns-250 ns, preferably 50 ns; and a processing time of 60 s.

[0040] Step S05: After the treatment is completed, the polypropylene membrane sample is taken out and stored in a dry, dust-free sealed box for standing to complete the modification.

[0041] In one embodiment, the treatment time is 60 s. After the treatment is completed, the sample is taken out and stored in a dry, dust-free sealed box for 10 hours to reduce the influence of aging effects on the test results.

[0042] The wettability of the modified polypropylene diaphragm was analyzed, and the modification effect was evaluated by water contact angle (WCA) measurement. A multi-point detection method was used, with the center of the sample as the pole, a ray was taken every 30°, and points were taken on each ray at intervals of 5 mm, for a total of 61 points. A 1.5 μL deionized water droplet was used, and the image was captured by a CCD camera. The average contact angle was calculated, and the polar coordinate distribution diagram of the water contact angle was plotted.

[0043] Polar coordinate distribution of water contact angle before polypropylene membrane treatment Figure 3 As shown, the average water contact angle of the untreated polypropylene material surface is 90.15°; Figure 4As shown, the average water contact angle is 71.60°. Plasma treatment can effectively improve hydrophilicity. Therefore, plasma is an efficient and environmentally friendly method for polypropylene surface modification, suitable for applications in high-performance materials such as lithium battery separators.

[0044] Prepare four groups of polypropylene separators, cut them into shapes suitable for the size of lithium batteries, select one group as the control group, and use plasma treatment on the other three groups at voltages of 4000 V, 6000 V, and 8000 V.

[0045] The battery assembly process mainly involves electrolyte, lithium cathode material, button cell housing, stainless steel springs, and gaskets. The separator uses a polypropylene separator that has undergone DBD treatment. These materials are assembled into a complete lithium battery.

[0046] The assembled button batteries were subjected to cycle charge and discharge and rate performance test experiments. The charge and discharge performance test was carried out using the Wuhan Blue Electric Battery Test System with a constant current rate of 0.1 C and 50 cycles to evaluate the effect of the modified polypropylene separator on battery performance.

[0047] The discharge capacity curve is as follows Figure 5 As shown in the figure, with the increase of voltage, the discharge capacity of the battery shows a nonlinear change trend. All treated lithium-ion batteries show better performance in discharge capacity than the untreated group. When the voltage treatment condition is 6000 V, the curve is the most stable, indicating that under this treatment condition, the optimization effect of the diaphragm structure and electrolyte wettability is the best, and the battery does not show obvious performance degradation within 50 cycles. When the voltage treatment conditions are 4000V and 8000V, the discharge capacity of the lithium-ion battery is lower than that of 6000 V, but it is still significantly higher than the performance of the untreated group. The results show that plasma treatment improves the wettability of the diaphragm, thereby optimizing the charge and discharge characteristics of the battery.

[0048] This invention utilizes a purely physical modification technology that eliminates the use of any chemical solvents or hazardous chemicals during the entire modification process, thus achieving a green, environmentally friendly, and pollution-free production method. Furthermore, this technology features low energy consumption, meeting the requirements of green manufacturing and representing an environmentally friendly and innovative approach. The modification process of this invention only occurs on the surface layer of the material, with the impact range limited to a few nanometers to micrometers. Therefore, it does not affect the overall physical properties of the polypropylene material, and key characteristics such as mechanical strength and thermal stability are maintained. This modification method ensures the stability of the physical properties of the separator while not affecting its overall structure, thereby improving the surface performance of the separator while maintaining its original advantages. This invention uses plasma technology for surface modification. Due to its high energy density, it can achieve uniform modification of the membrane surface, ensuring consistent hydrophilicity across the entire membrane surface. This treatment method not only achieves high uniformity but is also suitable for roll-to-roll continuous production processes, requiring no additional vacuum equipment and can be directly integrated into existing production lines, making it ideal for large-scale industrial applications. The plasma surface modification technology used in this invention is highly controllable, allowing precise control of the modification effect by adjusting discharge parameters such as voltage, frequency, gas composition, and treatment time. This flexibility enables the technology to customize modification parameters according to different lithium battery application requirements, such as high-rate discharge, long cycle life, and high safety, thereby meeting diverse industrial production needs. The plasma treatment technology of the present invention has wide applicability and can be applied to a variety of lithium battery separator materials, including polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET), polycarbonate (PC), polymethyl methacrylate (PMMA) and other different polymer materials, thereby providing a universal solution for the modification of different types of lithium battery separators.

Claims

1. A method for modifying a polypropylene diaphragm for lithium-ion battery energy storage, comprising the following steps: Step S01: Select a polypropylene diaphragm sample (5); Step S02: cleaning the polypropylene diaphragm sample (5) to remove contaminants on the surface of the material, and drying it in a vacuum drying oven after cleaning; Step S03: Turn on the gas delivery system and adjust the oxygen content and gas flow rate; Step S04: placing the polypropylene membrane sample (5) in the center of the DBD reactor, starting the nanosecond pulse power supply and adjusting the power supply parameters; Step S05: After the treatment is completed, the polypropylene membrane sample (5) is taken out and stored in a dry, dust-free sealed box for standing to complete the modification.

2. The method for modifying a polypropylene diaphragm for lithium-ion battery energy storage according to claim 1, wherein: It also includes wettability analysis of the modified polypropylene membrane and evaluation of the modification effect by water contact angle measurement. The specific method is as follows: using a multi-point detection method, with the center of the polypropylene membrane sample (5) as the pole, taking a ray every 30°, and taking points on each ray at intervals of 5mm, for a total of 61 points; using a 1.5 μL deionized water droplet, the CCD camera collects images, calculates the average contact angle, and draws a polar coordinate distribution diagram of the water contact angle.

3. The method for modifying a polypropylene diaphragm for lithium-ion battery energy storage according to claim 1 or 2, wherein: In step S01: a plurality of polypropylene separator samples (5) of the same shape and thickness are prepared.

4. The method for modifying a polypropylene diaphragm for lithium-ion battery energy storage according to claim 1 or 2, wherein: In step S02, an ultrasonic cleaning apparatus is used for cleaning, and deionized water is used to clean the material.

5. The method for modifying a polypropylene diaphragm for lithium-ion battery energy storage according to claim 1 or 2, wherein: In step S03, the oxygen content is adjusted by mixing pure argon with an argon-oxygen mixed gas, wherein the argon-oxygen mixed gas contains 1% O2.

6. The method for modifying a polypropylene diaphragm for lithium-ion battery energy storage according to claim 1 or 2, wherein: In step S04, cylindrical metals are placed at both ends of the DBD reactor as discharge electrodes, the upper end is the positive electrode (1), the lower end is the negative electrode (2), the neutral line passes through the current coil and is grounded, the reaction chamber is composed of a quartz glass plate (3) with an air inlet and outlet (6) and a quartz glass dish (4), the reaction chamber is located between the positive electrode and the negative electrode, and the polypropylene diaphragm sample is placed in the reaction chamber.

7. The method for modifying a polypropylene diaphragm for lithium-ion battery energy storage according to claim 1 or 2, wherein: In step S04, the nanosecond pulse power supply has the following parameter specifications: output amplitude range is 0-15 kV, output waveform is high voltage square wave; output frequency range is 0-20 kHz; pulse width adjustment range is 0 ns-1 ms; rising edge and falling edge time adjustment range is 50 ns-250 ns.

8. The method for modifying a polypropylene diaphragm for lithium-ion battery energy storage according to claim 7, wherein: In step S04, the processing time is 55-65 seconds.

9. The method for modifying a polypropylene diaphragm for lithium-ion battery energy storage according to claim 1 or 2, wherein: In step S05, the polypropylene diaphragm sample (5) is stored in a dry, dust-free sealed box and allowed to stand for 9-11 hours.

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