Preparation method and application of self-healing modified polyacrylic acid binder applied to lithium-sulfur battery positive electrode material

Abstract

The invention discloses a self-healing modified polyacrylic acid binder applied to a positive electrode material of a lithium-sulfur battery, and aims to solve the problems of poor surface coating performance, low binding strength and poor electrochemical performance of the battery after long-term circulation of the positive electrode material prepared by the existing binder. In order to prepare a binder which can effectively relieve volume expansion caused by a shuttle effect in a lithium ion battery and is excellent in ionic conductivity and high in charge-discharge performance, when a soft monomer formula is designed, a large number of carboxyl polar groups are introduced; strong chemical adsorption and hydrogen bonds can be formed between the prepared 2-EHA-AA-MMA composite functional binder and an active substance and between the prepared 2-EHA-AA-MMA composite functional binder and a current collector, and more active sites are provided. The rigid-flexible multi-network structure and the special self-repairing capability formed in the electrode preparation process not only provide enough mechanical support, but also can buffer the strain caused by the volume change of the positive electrode material in the charge-discharge process. The material has the remarkable characteristic of repeatable self-repairing capability, not only can effectively maintain the integrity of an electrode structure, but also can remarkably improve the cycling stability of the lithium-sulfur battery, so that the service life of a whole electrode material system is prolonged.

Description

Technical Field

[0001] The invention relates to a preparation method and application of a self-healing modified polyacrylic acid binder for lithium-sulfur battery positive electrode materials. Background Art

[0002] During the charge and discharge cycles of lithium-sulfur (Li-S) batteries, the volume of the positive and negative active materials expands due to the repeated insertion and extraction of lithium ions. This can lead to the destruction of the conductive network of the electrode material and the dispersion of the active material particles, which in turn leads to rapid capacity decay and a reduction in the number of cycles. Binders, as one of the key auxiliary components of the positive electrode of lithium-sulfur batteries, are carriers that effectively connect the positive electrode active material, conductive agent, and current collector. Although their content is small, they are crucial to the stability and integrity of the positive electrode structure. The currently commercially available positive electrode binder PVDF has poor coating performance and low bonding strength on the positive electrode surface due to its inherent van der Waals adhesion and semi-crystalline structure. After long-term cycling, this leads to the collapse of the positive electrode structure and a decrease in the battery's electrochemical performance. Compared with PVDF binders, modified polyacrylic acid molecules contain a large number of -COOH groups, which form a tight and uniform protective layer on the surface of the positive electrode material, effectively stabilizing the lattice structure and suppressing the voltage and capacity decay of lithium-sulfur batteries.

[0003] Therefore, this study designed a self-healing functional flexible binder rich in polar groups through molecular design and process optimization. Due to the presence of a large number of carboxylic acid functional groups, the polyacrylic acid binder can form strong chemical adsorption and hydrogen bonds with both the active material and the current collector, providing more active sites. Its multiple network structure that combines rigidity and flexibility and the special self-healing ability formed during the electrode preparation process not only provide sufficient mechanical support, but also buffer the strain caused by the volume change of the positive electrode material during the charge and discharge process. After mechanical damage or chemical degradation, it can restore its performance by reorganizing its own molecular structure or responding to external stimuli. The remarkable feature of this material is its repeatable self-healing ability, which can not only effectively maintain the integrity of the electrode structure, but also significantly improve the battery cycle stability, thereby extending the service life of the entire electrode system. Summary of the Invention

[0004] The present invention aims to address the problem of existing lithium-sulfur battery binders used as cathode materials, which suffer from excessive volume expansion during charge and discharge, leading to cracking and pulverization, which can degrade electrochemical performance. A method for preparing a flexible, self-healing modified polyacrylic acid binder material with high conductivity and cycle stability is provided to prepare a novel lithium-sulfur battery cathode binder with high conductivity and cycle stability.

[0005] A method for preparing a self-healing modified polyacrylic acid binder for positive electrode materials is completed by the following steps: (1) Pre-emulsification: methyl methacrylate, hydroxyethyl methacrylate, acrylic acid, methacrylic acid, , butyl acrylate, isooctyl acrylate, acrylonitrile and other monomers, an emulsifier (a composite emulsifier composed of OP-10, sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, etc.), an initiator (an oxidation-reduction system of ammonium persulfate-sodium bicarbonate, potassium persulfate-sodium bisulfite) and deionized water were sequentially added into the reactor, nitrogen was introduced, and the mixture was stirred for 1-2 hours to obtain a mixed emulsion I; (2) A portion of the emulsion I was transferred to the reactor, and the initiator was added at the same time. The mixture was stirred using a CNC mechanical stirrer. When the reaction temperature rose to about 65-80 °C, the remaining mixed solution I was slowly added dropwise (maintaining a high speed of 360 r / rad and continuously introducing nitrogen during the entire process) to carry out the emulsion polymerization reaction; (3) After the addition is complete, keep the temperature at 70-80 °C for several hours, adjust the pH of the emulsion to about 7-9, and then cool it to obtain the target product.

[0006] (4) The active material (S-PAN), binder, and conductive additive (carbon black) are assembled into a positive electrode sheet in a ratio of 7:2:1 and applied to lithium-sulfur batteries.

[0007] (5) The prepared positive electrode sheets were assembled in an argon-filled glove box, and the assembled battery was subjected to constant current charge and discharge tests on a LAND system with a voltage of 1.7 to 2.8 V. The constant current charge and discharge tests were performed on an electrochemical workstation at a voltage of 0.1 mV s -1 Cyclic voltammetry (CV) was measured at a scan rate of 10.5 Hz to 0.01 Hz. Electrochemical impedance spectroscopy (EIS) was measured in the frequency range of 105 Hz to 0.01 Hz.

[0008] Compared with the prior art, the present invention has the following beneficial effects: 1. The dynamic covalent bonds (such as disulfide bonds) or supramolecular interactions (such as hydrogen bond networks) in the molecular chains of the self-healing modified polyacrylic acid binder prepared by the present invention can repair electrode cracks caused by volume changes during the charge and discharge process, maintain the integrity of the conductive network, improve sulfur utilization, and alleviate volume expansion.

[0009] The self-healing modified polyacrylic acid binder prepared by the present invention is rich in a large number of polar groups (such as -OH, -COOH, etc.), and the molecular chain is elongated, providing more active sites, which can effectively chemically adsorb polysulfides (Li2S) generated during the charge and discharge process of the battery. n ), reducing its dissolution and diffusion, thereby inhibiting the shuttle effect. By controlling the length of the polymer molecular chain, it is given good bonding strength and good flexibility, effectively alleviating the pulverization and cracking of the positive electrode active material caused by charging and discharging.

[0010] 3. The self-healing modified polyacrylic acid binder prepared by the present invention is applied to the positive electrode of lithium-sulfur batteries and has a high specific capacity of 956.8 mAh g -1 After 100 cycles, the capacitance retention rate is 71%; even at a high rate of 0.5 C, it can still maintain 898 mAh g -1 The high rate capacity and average coulombic efficiency remain at 98.8%, which can be widely used in lithium-sulfur batteries. 4. The method for preparing the self-healing modified polyacrylate adhesive of the present invention has the characteristics of simple process, easy operation and low equipment requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 Schematic diagram of the synthesis of the self-healing modified polyacrylic acid adhesive in an example of the present invention; Figure 2 FTIR infrared spectrum test graphs of five groups of adhesives with different contents of initiator and emulsifier in different proportions of soft monomer in the self-healing modified polyacrylic acid adhesive in the examples of the present invention; Figure 3 This is a particle size test analysis diagram of the self-healing modified polyacrylic acid binder in an example of the present invention; Figure 4 : TGA thermogravimetric curve of the self-healing modified polyacrylic acid binder and PVDF in the example of the present invention; Figure 5 This is a 100-cycle constant current charge-discharge curve in a lithium-sulfur battery in which the self-healing modified polyacrylic acid binder is assembled into the positive electrode as the S-PAN electrode in an example of the present invention; Figure 6 The UV-Vis ultraviolet test curves of the supernatant after the adsorption of lithium polysulfide by the self-healing modified polyacrylic acid binder and PVDF in the examples of the present invention; Figure 7 At 0.2 mV s -1 The cyclic voltammetry comparison curves of PVDF and the synthesized self-healing modified polyacrylic acid binder are shown below; Figure 8 This is a long cycle performance test of the self-healing modified polyacrylic acid binder and PVDF at 0.1°C in the examples of the present invention; Figure 9 These are the SEM images of the S-PAN electrode containing PVDF before and after 200 cycles, and the SEM electron microscope images of the S-PAN electrode containing the self-healing modified polyacrylic acid binder in the example of the present invention before and after 200 cycles. DETAILED DESCRIPTION

[0012] The following examples further illustrate the present invention, but should not be construed as limiting the present invention. Without departing from the essence of the present invention, modifications and substitutions made to the methods, steps or conditions of the present invention are within the scope of the present invention.

[0013] Specific embodiment 1: This embodiment is a method for preparing an acrylic acid / isooctyl acrylate / methyl methacrylate composite functional binder for lithium-ion batteries, which is completed by the following steps: 1. Self-healing modified acrylic adhesive (acrylic acid / isooctyl acrylate / methyl methacrylate) ① Dissolve 25-30 g of isooctyl acrylate, 15-18 g of methyl methacrylate, and 5-6 g of acrylic acid in 60 mL of deionized water in reactor I. Add 0.25-0.3 g of alkylphenol polyoxyethylene ether-10 emulsifier and 0.15-0.2 g of sodium lauryl sulfate to reactor I and stir at a high speed of 300 r / rad~360 r / rad for 40 min under a nitrogen atmosphere. ② Pour 40-60 ml of the pre-emulsified liquid from reactor I into reactor II and stir with a CNC mechanical stirrer. Adjust the temperature to 70-75°C and maintain a stirring speed of 360 r / rad-400 r / rad under a nitrogen atmosphere. Add 0.25-0.3 g of ammonium persulfate and 0.15-0.25 g of sodium bicarbonate to reactor II. ③ When the temperature of the thermometer in reactor II rises to 72℃~75℃, the liquid in reactor II turns milky white or light blue; slowly add the remaining pre-emulsified liquid in reactor I to reactor II dropwise, controlling the addition speed, and the addition is completed in about 2 hours; ④ After the addition is complete, stir at a constant temperature of 75℃~80℃ for 1-2 hours, and adjust the pH to about 7-8 with ammonia water to obtain reaction product II;

[0014] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0015] from Figure 1 It can be seen that Figure 1 This is a schematic diagram of the synthesis route of the self-healing modified acrylic binder. As can be seen from the figure, a polymer rich in polar functional groups (such as -OH, -COOH, etc.) and hydrogen bonds is synthesized. Through dynamic chemical bonds or physical entanglement interactions, it can autonomously repair cracks in the electrode active material caused by volume changes during charging and discharging, maintain the integrity of the conductive network, and effectively alleviate the volume expansion during battery charging and discharging. Figure 2 It can be seen that Figure 2 The infrared spectrum characterization test spectra of five groups of control experiments to control the percentage of initiator and emulsifier in soft monomer are shown in the figure. It can be seen from the figure that the characteristic peaks of the synthesized self-healing modified acrylic adhesive are located at 1730, 1450 and 1384 cm -1 , which correspond to -C=O stretching, -CH2 stretching, and -COH in-plane bonding, respectively, indicating the successful synthesis of the binder; Figure 3 It can be seen that the particle size of the synthesized self-healing modified acrylic adhesive was tested and analyzed, and the uniformity of the particle size was analyzed by observing the shape of the particle size distribution graph. The closer the particle size distribution graph is to the normal distribution, the better the consistency of the powder and the more uniform the particle size distribution; Figure 4 It can be seen that Figure 4 The TGA curves of the synthesized self-healing modified acrylic binder and PVDF are shown in the figure. As can be seen from the figure, the PVDF binder begins to thermally decompose at approximately 350°C, while the synthesized self-healing modified acrylic binder begins to thermally decompose at 500°C, which can well adapt to the volume reaction heat during the charge and discharge process. Figure 5 It can be seen that the first cycle charge and discharge capacity of the synthesized self-healing modified acrylic adhesive reached 850 mAh g -1 The specific capacity has good charge and discharge performance; Figure 6 It can be seen that the synthesized self-healing modified acrylic binder, PVDF and blank control group were subjected to lithium polysulfide adsorption test experiments, and the supernatant after absorbing lithium polysulfide was measured by UV-Vis. According to the peak intensity, it can be concluded that the 2-EHA-AA-MMA binder is expected to absorb more LiPSs during the charge and discharge process, while blocking LiPSs and reducing capacity loss; Figure 7 As shown in Figure 2, compared with PVDF-based batteries, 2-EHA-AA-MMA-based batteries have a higher −1 Even at high scan rates, it still shows obvious redox peaks, indicating that the 2-EHA-AA-MMA binder has excellent electrochemical stability. Figure 8 As shown in the figure, the lithium-sulfur batteries assembled with the two binders were subjected to a long cycle test at a high rate of 0.5 C. The capacitance retention rate of PVDF was only 45%, while the capacitance retention rate of the 2-EHA-AA-MMA-based battery was maintained at 68%, with excellent long cycle performance and no obvious side reactions. Figure 9 As shown in the figure, SEM electron microscopy (SEM) images of PVDF and 2-EHA-AA-MMA positive electrodes were taken before and after 200 cycles. As can be seen from the figure, the surface of the fresh PVDF electrode is noticeably loose and rough, but becomes denser after 200 cycles, with large-scale deposition of insoluble products. These reaction byproducts clog the pores in the electrode, hindering ion diffusion. By comparison, before cycling, the colloidal layered binder in the 2-EHA-AA-MMA electrode encapsulates other particles, contributing to the electrode's structural stability. After 200 cycles, only a small amount of insoluble deposits remain on the electrode, maintaining good porosity and preserving available paths for charge transfer.

[0016] In summary: The self-healing modified acrylic binder (2-EHA / AA / MMA) material prepared in Example 1 has high specific capacity and capacitance performance. Through dynamic chemical bonds or physical interactions, it can autonomously repair electrode cracks caused by volume changes during the charging and discharging process. It can better adapt to the volume changes of the sulfurized polyacrylonitrile positive electrode during the battery reaction, promote the transmission of lithium ions, and improve the comprehensive electrochemical performance of lithium-sulfur batteries.

Claims

1. A preparation method and application of a self-healing modified polyacrylic acid binder for lithium-sulfur battery cathode materials, characterized in that This is done by: (1) Preparation of self-healing modified polyacrylic acid binder for lithium-sulfur battery cathode materials: ① Pre-emulsification: methyl methacrylate, hydroxyethyl methacrylate, acrylic acid, methacrylic acid, , butyl acrylate, isooctyl acrylate, acrylonitrile and other monomers, an emulsifier (composite emulsifier, composed of OP-10, sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, etc.), an initiator (the initiator adopts an oxidation-reduction system, ammonium persulfate-sodium bicarbonate, potassium persulfate-sodium bisulfite) and deionized water are added to the reactor in sequence, nitrogen is introduced, and the mixture is stirred for 1-2 hours to obtain mixed emulsion I; ② Transfer a portion of emulsion I to the reactor, add the initiator, and stir using a CNC mechanical stirrer. When the reaction temperature rises to about 65-80°C, slowly add the remaining mixed solution I dropwise (maintaining a high speed of 360 r / rad and continuously introducing nitrogen throughout the process) to carry out the emulsion polymerization reaction; ③ After the addition is completed, keep the temperature at 70-80 ° C for several hours, adjust the pH of the emulsion to about 7-9, and cool it to obtain the target product; ④ Assemble the active material (S-PAN), binder, and conductive additive (carbon black) in a ratio of 7:2:1 to form a positive electrode sheet for use in lithium-sulfur batteries; ⑤ The prepared positive electrode sheets were assembled in a glove box filled with argon. The assembled batteries were subjected to constant current charge and discharge tests on a LAND system with a voltage of 1.7-2.8 V; the voltage of the battery was 0.1 mV s on an electrochemical workstation. -1 The cyclic voltammetry (CV) was measured at a scan rate of 105 Hz to 0.01 Hz; the electrochemical impedance spectroscopy (EIS) was measured in the frequency range of 105 Hz to 0.01 Hz.

2. The preparation method and application of a self-healing modified polyacrylic acid binder for lithium-sulfur battery positive electrode materials according to claim 1, characterized in that In step 1①, the pre-emulsified monomers are selected from isooctyl acrylate, hydroxyethyl methacrylate and acrylic acid, and butyl acrylate, methyl methacrylate and acrylonitrile, and the mass ratio of the three monomers is 5:3:1; in step 1①, the percentage of the emulsifier in the total content of the soft monomer is 1.2%, and the percentage of the initiator in the total content of the soft monomer is 0.8%; in step 1①, under a nitrogen atmosphere, high-speed stirring is maintained at 300 r / rad~360 r / rad for 40 minutes to obtain a mixed solution I; in step 1②, the temperature in the intelligent temperature-controlled magnetic stirrer is pre-adjusted to 60°C, and 0.25 g of ammonium persulfate and 0.15 g of ammonium persulfate are immediately added after transferring 40 ml of the mixed solution I. g sodium bicarbonate, the mass ratio of ammonium persulfate and sodium bicarbonate is 5:3; in step 1 ②, when the thermometer in reactor Ⅱ shows 72 ℃ ~ 75 ℃, transfer the remaining liquid in reactor Ⅰ to the dropping funnel in reactor Ⅱ and slowly add it dropwise at a rate of 5 seconds per drop; in step 1 ②, maintain a high speed of 360 r / rad throughout the process and continuously introduce nitrogen; in step 1 ③, after the dropping funnel in reactor Ⅱ is added dropwise, stir at a constant temperature of 75 ℃ ~ 78 ℃ for 1 ~ 3 h, adjust the pH to about 7 ~ 9 with ammonia water, and obtain the target product after cooling; in step 1 ④, the active material (S-PAN), binder, and conductive additive (carbon black) are weighed in a ratio of 7:2:1, and are pre-grinded in a ball mill at a high speed of 300 r / rad ~ 360 r / rad for 30 min, and 1 ~ 2 ml NMP (N-methylpyrrolidone) is added to adjust to a suitable viscosity, and evenly coated on a 7 cm × 8 cm carbon-coated aluminum foil, and dried at a constant temperature of 50℃~60℃ for 7h~8h to obtain the positive electrode sheet required for assembling the battery; in step 1⑤, the dried positive electrode sheet was cut into a disc with a diameter of 13 mm, the sheet weight was 4.2 mg, and the weight of the active material on the sheet was kept at about 1.5~3 mg; in step 1⑤, in the glove box, the battery shell, positive electrode sheet, 30~40 μL electrolyte, diaphragm, 30~40 μL electrolyte, metal lithium sheet, gasket, spring gasket, and battery shell were assembled in sequence, and a tablet press was used at 800~1000 kg / cm 2 The assembled battery is pressed with a pressure of 1000 nm and left to rest for 8 to 24 hours before subsequent electrochemical performance tests.

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