Preparation method of silver@polypyrrole modified current collector, battery positive electrode and battery

Abstract

The present invention discloses a preparation method of a silver@polypyrrole modified current collector, a battery positive electrode and a battery. The silver@polypyrrole powder synthesized by a low-temperature redox method is mixed with a binder to form a current collector modification slurry; the current collector modification slurry is brush-coated on the surface of the battery current collector to obtain a silver@polypyrrole modified current collector. This current collector can achieve the functions of adsorbing polysulfides to inhibit the shuttle effect and buffering and protecting the structure of the current collector, and can also ensure efficient electron transfer from the external circuit; when applied to a lithium-sulfur battery, the current collector with a polypyrrole@silver modified coating can effectively improve the cycle stability of the battery and exhibit good battery life. In addition, the present invention utilizes the sulfur fixation effect brought by the large specific surface area and porous characteristics of the polypyrrole@silver functional layer to realize normal charge and discharge of the battery under a high sulfur loading and exhibit quite good charge and discharge capacities, and has great commercial application potential.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and relates to a preparation method of silver@polypyrrole composite material, a battery positive electrode and a battery. Background Art

[0002] Electric vehicles have entered a rapid development era, which brings new opportunities and challenges to the energy storage battery industry. Lithium-sulfur batteries have been widely studied and applied due to their extremely high theoretical capacity (1675 mAh g -1 ), natural abundance of sulfur, low toxicity and low cost, which have attracted strong interest. At present, lithium-sulfur batteries are still in the early stage of laboratory research and development for technical breakthroughs, and their large-scale practical applications still face huge challenges. The safety, cycle performance, stability, charge retention ability, etc. of the batteries need to be further improved. However, the commercialization process of lithium-sulfur batteries still faces many problems: the dissolution of active substances, the volume change of sulfur during charge and discharge, the shuttle effect of polysulfides (LiPSs), the unstable SEI layer, the growth of lithium dendrites on the negative electrode, etc. seriously affect the performance of lithium-sulfur batteries. In addition, due to the low sulfur loading and high electrolyte / sulfur (E / S) ratio of the lithium-sulfur batteries used in current research, it is difficult to achieve an energy density and cost that can compete with the most advanced lithium-ion batteries, which further limits the practical application of Li-S. In order to construct an ideal lithium-sulfur battery system with truly high energy density (close to the theoretical value), high stability (long charge and discharge cycles) and no safety hazards, there are still many key technical problems to be solved.

[0003] As an organic polymer material, conductive polymers have inherent properties, such as porous structure, doping and dedoping properties, etc., which make them have great potential in the application as lithium-sulfur battery materials. Although gratifying progress has been made, several factors in the modification by conductive polymers are still not satisfactory for practical applications. Due to the weak binding energy between non-polar carbon (from conductive polymers) and polar Li n S (0 < n ≤ 2), ordinary conductive polymers can only slow down the loss of polysulfides in the short term and maintain their internal chelation and adsorption, but cannot inhibit the "shuttle effect" of polysulfides in the long term. In addition, conductive polymers have a stable π-π conjugate structure, which makes the internal arrangement of their molecules, reducing the conductivity of conductive polymers, so that the conductive performance of pure conductive polymers cannot meet the requirements of lithium-sulfur battery positive electrode materials. Summary of the Invention

[0004] To overcome the problems in the prior art, the purpose of the present invention is to provide a silver@polypyrrole modified current collector, a battery positive electrode and a battery, and optimize the positive electrode reaction of the lithium-sulfur battery through a simple commercial electrode modification method, so as to achieve the effect of improving the overall performance of the lithium-sulfur battery.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A preparation method of a silver@polypyrrole modified current collector, comprising the following steps:

[0007] Mix the silver@polypyrrole powder synthesized by the low-temperature redox method with a binder to form a current collector modification slurry;

[0008] Brush the current collector modification slurry onto the surface of the battery current collector to obtain a silver@polypyrrole modified current collector.

[0009] Further, the binder is a PVDF-HFP or PVDF solution.

[0010] Further, the mass concentration of the binder solution is 5% - 15%; the mass ratio of the silver@polypyrrole powder to the binder solution is 1:4 - 1:15.

[0011] Further, the current collector is carbon fiber, aluminum foil or copper foil.

[0012] Further, the silver@polypyrrole powder is prepared through the following process:

[0013] Dissolve AgNO3 in ultrapure water, then add Py and an acid, and stir evenly to obtain a solution to be reacted;

[0014] Let the solution to be reacted stand in the dark at -20 to 10 °C and then transfer it to stand at 15 to 40 °C to precipitate a solid, and then purify it to obtain the silver@polypyrrole powder.

[0015] Further, the standing time at -20 to 10 °C is 4 - 64 h, and the standing time at 15 to 40 °C is 24 - 148 h.

[0016] Further, the purification is filtration, extraction or Soxhlet extraction.

[0017] Further, the molar ratio of AgNO3 to Py is 1:0.5 - 1:2; the acid is nitric acid, citric acid or ascorbic acid; the molar ratio of the amount of the acid used to AgNO3 is 1:0.5 - 1:4.

[0018] A lithium-sulfur battery cathode is prepared by using the silver@polypyrrole modified current collector prepared by the method as described above.

[0019] A lithium-sulfur battery includes the lithium-sulfur battery cathode as described above.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The present invention utilizes a silver@polypyrrole (Ag@PPy) material prepared by a low-temperature redox method, mixes it with a binder, and loads it on a battery current collector to form an Ag@PPy modified functional layer, thereby obtaining an Ag@PPy modified current collector. This functional layer can not only capture polysulfides but also act as a buffer layer to protect the current collector from adapting to the volume change of active sulfur during charge and discharge processes. It can also ensure efficient electron migration, accelerate the catalytic conversion of sulfur on the positive electrode side, and realize a high-sulfur-loaded lithium-sulfur battery. The mechanism of action of the Ag@PPy modified functional layer is as follows: During discharge, the lithium metal negative electrode is oxidized to form lithium ions and electrons, which reach the sulfur positive electrode through the electrolyte and the external circuit, respectively. Lithium ions will continuously combine with the active sulfur molecules in the positive electrode to form long-chain lithium polysulfides (Li2S x , where 4 ≤ x ≤ 8). Due to the nitrogen-rich characteristics and abundant adsorption sites of Ag@PPy, the long-chain lithium polysulfides will be anchored and adsorbed on the Ag@PPy modified functional layer. Subsequently, the long-chain lithium polysulfides are rapidly reduced to short-chain polysulfides (Li2S2 or Li2S) on the surface of the functional layer. During the conversion process, Ag@PPy actually acts as a reaction bed for lithium polysulfides, and the rapid electron transfer will catalyze the conversion of long-chain lithium polysulfides. Finally, most of the final product (Li2S) is reversely adsorbed on the Ag@PPy layer. Compared with ordinary current collectors, the Ag@PPy modified functional layer can effectively inhibit the shuttle effect of polysulfides. Due to the presence of the binder, this Ag@PPy modified functional layer can improve the mechanical strength of the current collector, making it more in line with the requirements of the current collector (Current collector, CC) for lithium-sulfur batteries used in flexible electronic devices. Therefore, when applied to lithium-sulfur batteries, the current collector with an Ag@PPy modified functional layer can effectively improve the charge and discharge capacity of the battery and achieve good cycle stability.

[0022] The Ag@PPy modified current collector exhibits stable charge and discharge capabilities and high areal capacity in high-sulfur-loaded lithium-sulfur batteries, showing great potential for commercial applications. The optimization effects of the Ag@PPy modified current collector on the positive electrode of lithium-sulfur batteries include, but are not limited to: restricting polysulfides on the positive electrode side through conductive polymer adsorption, leaving sufficient buffer space for volume expansion during discharge / charge processes, controlling the sulfur / polysulfide conversion reaction, improving electron transport efficiency by incorporating nano silver, increasing the mechanical strength of the current collector, etc. This method has the advantages of easy operation and low cost.

[0023] The optimization effects of Ag@PPy powder on the positive electrode of lithium-sulfur batteries include, but are not limited to: restricting polysulfides on the positive electrode side through conductive polymer adsorption, leaving sufficient buffer space for volume expansion during discharge / charge processes, controlling the sulfur / polysulfide conversion reaction, improving electron transport efficiency, and having good commercial advantages such as easy operation and low cost.

[0024] Furthermore, the present invention combines Ag nanoparticles and PPY (polypyrrole) to synthesize an Ag@PPy composite material, which is used as a coating layer to modify the battery current collector. The Ag@PPy composite material achieves comprehensive performance: (1) It has a good microscopic pore structure and excellent specific surface area characteristics, which can hinder the shuttle effect; (2) The nitrogen-rich property can effectively inhibit the shuttle effect by improving the chemical adsorption ability of polysulfide lithium; (3) A more stable material structure can adapt to the volume expansion of sulfur during the cycling process and protect the electrode structure; (4) On the one hand, Ag improves the conductivity of the conductive polymer, and on the other hand, it can achieve the purpose of improving the sulfur redox kinetics by reducing the reaction energy barrier. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The schematic diagrams in the specification forming a part of this application are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0026] Figure 1 is a schematic diagram of the preparation process of polypyrrole@silver modified current collector (CC / Ag@PPy / S);

[0027] Figure 2 is a schematic diagram of the structure of a lithium-sulfur battery current collector with a polypyrrole@silver functional layer (CC / Ag@PPy / S);

[0028] Figure 3 is a schematic diagram of the working principle of silver@polypyrrole modified current collector in a lithium-sulfur battery;

[0029] Figure 4 is the cyclic stability test of the CNF / Ag@PPy / S electrode in Example 1 of the present invention;

[0030] Figure 5 is the rate performance test of the CNF / Ag@PPy / S electrode in Example 1 of the present invention;

[0031] Figure 6 is the cycle life test of the CNF / Ag@PPy / S electrode in Example 1 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032] The present invention will be described in detail below with reference to the drawings and in combination with embodiments. It should be noted that, without conflict, the embodiments in this application and the features in the embodiments can be combined with each other.

[0033] The following detailed descriptions are all exemplary descriptions, aiming to provide further details of the present invention. Unless otherwise specified, all technical terms used in the present invention have the same meaning as commonly understood by those of ordinary skill in the art to which this application belongs. The terms used in the present invention are only for describing specific embodiments, and are not intended to limit the exemplary embodiments according to the present invention.

[0034] See Figure 1 , a preparation method of an Ag@PPy modified current collector, comprising the following steps:

[0035] Step 1: Completely dissolve AgNO3 in ultrapure water, use a pipette to transfer 0.1 - 100 mL of Py (pyrrole), add it to the reaction system at one time, and add a certain amount of acid to ensure an acidic reaction environment. Subsequently, under ice bath conditions, stir rapidly at 200 - 1000 rpm to prepare a solution to be reacted; wherein, the molar ratio of the AgNO3 solution to Py is 1:0.5 to 1:2;

[0036] The acid used can be, but is not limited to, nitric acid, citric acid, ascorbic acid, etc., and the molar ratio of the amount of acid used to AgNO3 ranges from 1:0.5 to 1:4.

[0037] Step 2: Carry out light-shielding treatment on the solution to be reacted obtained in Step 1, let it stand at low temperature for a period of time, and then transfer it to room temperature and let it stand again;

[0038] Among them, the temperature range for standing at low temperature is -20 to 10 °C, and the standing time is 4 to 64 h.

[0039] The temperature range for standing at room temperature is 15 to 40 °C, and the standing time is 24 to 148 h.

[0040] Step 3: After the standing in Step 2 is completed, a layer of solid attached to the inner wall of the beaker is the obtained product. The product is purified, washed, dried, ground and other processes to obtain Ag@PPy powder;

[0041] Among them, the purification method can be, but is not limited to, suction filtration, extraction and Soxhlet extraction, etc.;

[0042] Step 4: Mix the Ag@PPy powder obtained in Step 3 with a binder in a certain proportion and then ball-mill to form a current collector modification slurry;

[0043] Among them, the binder can be, but is not limited to, PVDF-HFP, PVDF solution. In the preferred embodiment of the present invention, the mass concentration of the binder solution is 5% - 15%, preferably 7% - 12%.

[0044] The mass ratio of the Ag@PPy powder to the binder solution is 1:15 to 1:4, preferably 3:16 or 1:10, 3:25.

[0045] Step 5: Dilute the current collector modified slurry obtained in Step 4 to the required concentration, brush it onto the surface of the battery current collector, and then dry it to form an Ag@PPy modified functional layer, obtaining an Ag@PPy modified current collector, i.e., CC / Ag@PPy;

[0046] The obtained Ag@PPy modified current collector is used to load active sulfur to prepare a battery positive electrode, i.e., CC / Ag@PPy / S. The battery assembled using it includes a button cell and a soft-pack battery. This battery can light a bulb, and the internal structure of the battery is as Figure 2 shown.

[0047] The Ag@PPy modified functional layer is applicable to several current collectors used in current lithium-sulfur battery research, including but not limited to carbon fiber, aluminum foil, copper foil, etc.

[0048] It should be noted that the performance of the prepared modified current collector is evaluated by the stability, cycle life, rate performance, and sulfur loading of the lithium-sulfur battery. The specific test methods are as follows:

[0049] 1. Evaluation method for the cycle stability of the sulfur positive electrode: By conducting the cycle stability test of the sulfur positive electrode on a LAND tester, in this invention, the charge and discharge are carried out at a charge-discharge rate of 0.2 C, and the capacity decay and retention after 200 cycles are measured. The average decay rate of the capacity is used to measure the stability of the sulfur positive electrode.

[0050] 2. Test method for the cycle life of the sulfur positive electrode: By conducting the cycle life test of the sulfur positive electrode on a LAND tester, in this invention, the charge and discharge are carried out at a high rate (0.5 C or 1 C) charge-discharge rate, and the capacity retention and Coulomb efficiency after 500 cycles are measured. Based on this, the life of the battery is evaluated.

[0051] 3. Test method for rate performance: By conducting a series of constant current charge-discharge cycles at different current densities on a LAND tester, the current density gradually increases from low to high and then returns to the low current density, and the capacity performance and capacity recovery ability of the battery at different current densities are observed.

[0052] 4. Evaluation method for the sulfur loading of the sulfur positive electrode: In the examples of this invention, the active sulfur slurry used contains about 70% sulfur. The weight of sulfur on the current collector is obtained by the gravimetric method and converted into the areal density (M = m s / S 集流体 , where M is the areal density, m s is the sulfur mass, and S 集流体 is the area of the current collector).

[0053] Example 1

[0054] Step 1: Dissolve 1.69 g of silver nitrate in 100 mL of ultrapure water. Pipette 6.5 mL of Py (pyrrole) and add it to the reaction system all at once. Then add 1.7 g of citric acid to ensure an acidic reaction environment. Subsequently, stir at 200 rpm for 1.5 h at 5 °C to prepare the solution to be reacted.

[0055] Step 2: Protect the solution to be reacted obtained in Step 1 from light. Let it stand at -2 °C for 24 h and then transfer it to room temperature (25 °C) and let it stand for another 48 h. The polymerization reaction will proceed rapidly at room temperature.

[0056] Step 3: After the polymerization reaction in Step 2 is completed, purify the product with 200 mL of ethanol and ultrapure water to remove impurities and oligomers. Then perform Soxhlet extraction with methanol for 24 h, THF for 24 h, and chloroform for 24 h for 72 h in total. Then dry it in vacuo at 60 °C for 48 h and grind it to obtain Ag@PPy powder.

[0057] Step 4: Mix the Ag@PPy powder obtained in Step 3 with a 10% (mass concentration) PVDF-HFP solution at a mass ratio of 3:16 and then ball mill for 6 h to obtain the modified current collector slurry.

[0058] Step 5: Brush the modified current collector slurry obtained in Step 4 onto the surface of the carbon fiber current collector. Then dry it at 60 °C for 10 h to form an Ag@PPy modified functional layer on the surface of the carbon fiber current collector, and obtain the Ag@PPy modified carbon fiber current collector.

[0059] After loading sulfur onto the Ag@PPy modified carbon fiber current collector by the coating method (sulfur loading is about 2 mg / cm 2 ), assemble it into a coin-type lithium-sulfur battery for testing. The test results are as Figure 4 、 Figure 5 and Figure 6 shown. It can be seen that:

[0060] 1. The initial discharge specific capacity of the CNF / Ag@PPy / S composite electrode at 0.2C is 1123.0 mAh / g. After 200 cycles, it remains at 831.0 mAh / g, and the capacity loss per cycle is 0.13%.

[0061] 2. The CNF / Ag@PPy / S composite electrode was tested for battery life at 1 C. After 500 charge-discharge cycles, the battery still maintained a discharge specific capacity of 680.4 mAh / g. The average capacity loss per cycle was as low as 0.048%, and the Coulombic efficiency remained at about 90% after 500 cycles.

[0062] 3. The CNF / Ag@PPy / S composite electrode was tested for the battery rate at different current densities (0.1, 0.2, 0.5, 1, 2 C). The reversible capacity of the battery gradually decreased from 1228.5 mAh / g to 1009.2, 901.1, 806.1 and 686.9 mAh / g. When the current density returned to 0.1 C, the battery still had a reversible capacity of 1006.3 mAh / g;

[0063] 4. The sulfur loading of the high-loading CNF / Ag@PPy / S composite electrode is about 6.5 - 7.2 mg / cm 2 .

[0064] Through the above steps, a modified carbon fiber current collector with Ag@PPy loaded on the surface can be obtained. Applying the modified current collector to a lithium-sulfur battery and comparing it with ordinary carbon fiber, it can be found that ordinary carbon fiber is difficult to inhibit the polysulfide shuttle and adapt to the change in sulfur volume. When the Ag@PPy functional layer is introduced onto the current collector, the porous property of PPy greatly increases the specific surface area of its carbon fiber surface. It can not only reversely adsorb lithium polysulfide on the layer, but also adapt to the change in sulfur volume and protect the structure of carbon fiber from being damaged. On the other hand, due to the introduction of Ag, the conductivity of the current collector is improved, which can accelerate the transmission of electrons in the external circuit and catalyze the conversion of active sulfur on the current collector. Compared with the lithium-sulfur battery using an ordinary carbon fiber current collector, the discharge specific capacity of the modified lithium-sulfur battery is increased by 15% - 25%, the number of cycling times is 2 - 2.5 times that of the ordinary lithium-sulfur battery, and it has a more stable operating voltage.

[0065] Example 2

[0066] Step 1: Dissolve 3.38 g of silver nitrate in 100 mL of ultrapure water. Use a pipette to transfer 6.5 mL of Py (pyrrole) and add it to the reaction system at one time, and then add 6.5 mL of concentrated nitric acid to ensure an acidic reaction environment. Subsequently, stir at 7°C and 1000 rpm for 0.5 h to prepare the solution to be reacted;

[0067] Step 2: Treat the solution to be reacted obtained in Step 1 in the dark. Let it stand at -2°C for 24 h and then transfer it to room temperature (25°C) and let it stand for another 64 h. The polymerization reaction will proceed rapidly at room temperature;

[0068] Step 3: After the polymerization reaction in Step 2 is completed, purify the product with 400 mL of ethanol and pure water to remove impurities and oligomers, and then dry it in vacuo at 60°C for 48 h and grind it to obtain Ag@PPy powder;

[0069] Step 4: Mix the Ag@PPy powder obtained in Step 3 with a 7% mass concentration PVDF-HFP solution at a mass ratio of 1:10, and then ball mill for 6 h to obtain a modified current collector slurry.

[0070] Step 5: Brush the modified current collector slurry obtained in Step 4 onto the surface of a copper foil current collector, and then dry at 60 °C for 10 h to obtain an Ag@PPy modified copper foil current collector.

[0071] After loading sulfur onto the Ag@PPy modified copper foil current collector by the coating method (sulfur loading is about 2 mg / cm 2 ), assemble it into a coin-type lithium-sulfur battery for testing. The test results show that:

[0072] 1. The initial discharge capacity of the Cu / Ag@PPy / S composite electrode at 0.2C is 1106.2 mAh / g, and it maintains 725 mAh / g after 200 cycles. The capacity loss per cycle is 0.17%.

[0073] 2. The Cu / Ag@PPy / S composite electrode is tested for battery life at 0.5 C. After 500 charge-discharge cycles, the battery still maintains 579.4 mAh / g. The average capacity loss per cycle is as low as 0.08%, and the Coulomb efficiency remains at about 90% after 500 cycles.

[0074] 3. The sulfur loading of the high-loading Cu / Ag@PPy / S composite electrode is about 4.5 - 6 mg / cm 2 .

[0075] Through the above steps, a modified copper foil current collector with Ag@PPy loaded on its surface can be obtained. When the modified current collector is applied to a lithium-sulfur battery and compared with a common copper foil, it can be found that although the common copper foil has high mechanical strength, its sulfur fixation property is poor, and it is difficult to inhibit the polysulfide shuttle and adapt to the change in sulfur volume. When the Ag@PPy functional layer is introduced onto the current collector, multiple characteristics greatly increase the specific surface area of its copper foil surface. It can not only reversely adsorb lithium polysulfide on the layer, but also adapt to the change in sulfur volume and protect the electrode structure from being damaged. On the other hand, the inherent conductive property of Ag@PPy can accelerate the transmission of electrons in the external circuit and catalyze the conversion of active sulfur on the current collector. In addition, Ag@PPy improves the ability of the copper foil to load active sulfur. Compared with a lithium-sulfur battery using a common copper foil current collector, the discharge specific capacity of the modified lithium-sulfur battery is increased by 15% - 25%, and it has a more stable operating voltage.

[0076] Example 3

[0077] Step 1: Dissolve 0.85 g of silver nitrate in 100 mL of ultrapure water. Pipette 6.5 mL of Py (pyrrole) and add it to the reaction system all at once. Then add 0.88 g of ascorbic acid to ensure an acidic reaction environment. Subsequently, stir at 500 rpm for 0.5 h at 10 °C to prepare the solution to be reacted.

[0078] Step 2: Protect the solution to be reacted obtained in Step 1 from light. Let it stand at -5 °C for 24 h and then transfer it to room temperature (25 °C) and let it stand for another 24 h. The polymerization reaction will proceed rapidly at room temperature.

[0079] Step 3: After the polymerization reaction in Step 2 is completed, purify the product with 400 mL of ethanol and pure water to remove impurities and oligomers. Then dry it under vacuum at 60 °C for 48 h and grind it to obtain Ag@PPy powder.

[0080] Step 4: Mix the Ag@PPy powder obtained in Step 3 with a 10% (by mass) PVDF solution (binder) at a mass ratio of 1:10 and then ball-mill for 6 h to obtain the modified slurry for the current collector.

[0081] Step 5: Brush the modified slurry for the current collector obtained in Step 4 onto the surface of the aluminum foil current collector. Then dry it at 60 °C for 10 h to obtain the modified aluminum foil current collector (Al / Ag@PPy).

[0082] After loading sulfur (sulfur loading is about 2 mg / cm 2 ) onto the modified aluminum foil current collector Al / Ag@PPy by the coating method, assemble it into a coin-type lithium-sulfur battery for testing. The test results show that:

[0083] 1. The initial discharge capacity of the Al / Ag@PPy / S composite electrode at 0.2C is 1100.1 mAh / g. After 200 cycles, it maintains 735 mAh / g, and the capacity loss per cycle is 0.16%.

[0084] 2. The Al / Ag@PPy / S composite electrode is tested for battery life at 1C. After 500 charge-discharge cycles, the battery still maintains 600.1 mAh / g. The average capacity loss per cycle is as low as 0.08%, and the Coulombic efficiency remains at about 93% after 500 cycles.

[0085] 3. The sulfur loading of the high-loading Al / Ag@PPy / S composite electrode is about 5.5 - 6.2 mg / cm 2 .

[0086] Through the above steps, a modified aluminum foil current collector with Ag@PPy loaded on its surface can be obtained. When the modified current collector is applied to a lithium-sulfur battery and compared with ordinary aluminum foil, it can be found that although ordinary aluminum foil has high mechanical strength, its sulfur fixation property is poor, and it is difficult to inhibit polysulfide shuttling and adapt to the change in sulfur volume. When the Ag@PPy functional layer is introduced onto the current collector, its porous property greatly increases the specific surface area of the aluminum foil surface. It can not only reversely adsorb lithium polysulfide onto the layer, but also adapt to the change in sulfur volume and protect the electrode structure from being damaged. On the other hand, the inherent conductive property of Ag@PPy can accelerate the transmission of electrons in the external circuit and catalyze the conversion of active sulfur on the current collector. In addition, Ag@PPy improves the ability of copper foil to load active sulfur. Compared with a lithium-sulfur battery using an ordinary aluminum foil current collector, the discharge specific capacity of the modified lithium-sulfur battery is increased by 5%-20%, the cycle life is 2-2.5 times that of an ordinary lithium-sulfur battery, and it has a more stable operating voltage.

[0087] Example 4 Performance Test of Current Collector under High Silver Loading

[0088] Step 1: Dissolve 1.69 g of silver nitrate in 100 mL of ultrapure water. Use a pipette to transfer 6.5 mL of Py (pyrrole) and add it to the reaction system at one time. Then add 6.5 mL of concentrated nitric acid (the mass concentration of concentrated nitric acid is 16 mol / L) to ensure an acidic reaction environment. Subsequently, stir at 700 rpm at 5 °C for 1.5 h to prepare the solution to be reacted.

[0089] Step 2: Treat the solution to be reacted obtained in Step 1 in the dark. Let it stand at -5 °C for 24 h and then transfer it to room temperature (25 °C) and let it stand for another 24 h. The polymerization reaction will proceed rapidly at room temperature.

[0090] Step 3: After the polymerization reaction in Step 2 is completed, purify the product with 400 mL of ethanol and ultrapure water to remove impurities and oligomers. Then dry it in vacuum at 60 °C for 48 h and grind it to obtain Ag@PPy powder.

[0091] Step 4: Mix the Ag@PPy powder obtained in Step 3 with a 10% mass concentration PVDF-HFP solution at a mass ratio of 3:25 and ball mill for 6 h to obtain the current collector modification slurry.

[0092] Step 5: Brush the current collector modification slurry obtained in Step 4 onto the surface of the carbon fiber current collector, and then dry it at 60 °C for 10 h to obtain the Ag@PPy modified carbon fiber current collector (the Ag@PPy loading accounts for 50% of the mass of the current collector).

[0093] After the Ag@PPy modified carbon fiber current collector is loaded with sulfur by the coating method (the sulfur loading is about 2 mg / cm2 ), assembled into a button lithium-sulfur battery for testing, and the test results are as follows:

[0094] 1. The initial discharge specific capacity of the CNF / Ag@PPy / S composite electrode at 0.1C is 1223.0 mAh / g, and it maintains 879.0 mAh / g after 200 cycles, with the capacity loss per cycle being 0.14%;

[0095] 2. The CNF / Ag@PPy / S composite electrode was tested for battery life at 1C. After 500 charge-discharge cycles, the battery still maintained a discharge specific capacity of 700.2 mAh / g, with the average capacity loss per cycle as low as 0.044%, and the Coulombic efficiency remained at about 91% after 500 cycles;

[0096] 3. The CNF / Ag@PPy / S composite electrode was tested for battery rate performance at different current densities (0.1, 0.2, 0.5, 1, 2C). The reversible capacity of the battery gradually decreased from 1152.0 mAh / g to 1095.2, 983.1, 879.1, and 657.9 mAh / g. When the current density returned to 0.1C, the battery still had a reversible capacity of 1081.3 mAh / g;

[0097] 4. The sulfur loading of the high-loading CNF / Ag@PPy / S composite electrode is about 7.0 - 7.7 mg / cm 2 .

[0098] Example 5

[0099] Step 1: Dissolve 1.69 g of silver nitrate in 100 mL of ultrapure water, then add Py (pyrrole), and add ascorbic acid to ensure an acidic reaction environment. Subsequently, stir at 200 rpm for 1.5 h at 5°C to prepare a solution to be reacted; among them, the molar ratio of silver nitrate to Py is 1:0.5, and the dosage of ascorbic acid is in a molar ratio of 1:2 to AgNO3.

[0100] Step 2: Protect the solution to be reacted obtained in Step 1 from light, let it stand at -2°C for 64 h, and then transfer it to stand at 40°C for 24 h for polymerization reaction;

[0101] Step 3: After the polymerization reaction in Step 2 is completed, purify the product with 200 mL of ethanol and ultrapure water to remove impurities and oligomers, then perform suction filtration, and vacuum dry at 60°C for 48 hours, and grind to obtain Ag@PPy powder;

[0102] Step 4: Mix the Ag@PPy powder obtained in Step 3 with a PVDF-HFP solution with a mass concentration of 15% at a mass ratio of 1:4, and then ball-mill for 6 h to obtain a modified current collector slurry.

[0103] Step 5: Brush the modified current collector slurry obtained in Step 4 onto the surface of the carbon fiber current collector, and then dry it at 60 °C for 10 h to form an Ag@PPy modified functional layer on the surface of the carbon fiber current collector, thereby obtaining an Ag@PPy modified carbon fiber current collector.

[0104] Example 6

[0105] Step 1: Dissolve 1.69 g of silver nitrate in 100 mL of ultrapure water, then add Py (pyrrole), and add citric acid to ensure an acidic reaction environment. Subsequently, stir at 200 rpm at 5 °C for 1.5 h to prepare a solution to be reacted; among them, the molar ratio of silver nitrate to Py is 1:1, and the dosage of citric acid is in a molar ratio of 1:0.5 to AgNO3.

[0106] Step 2: Treat the solution to be reacted obtained in Step 1 in the dark, let it stand at -20 °C for 4 h, and then transfer it to stand at 15 °C for 148 h for polymerization reaction.

[0107] Step 3: After the polymerization reaction in Step 2 is completed, purify the product with 200 mL of ethanol and ultrapure water to remove impurities and oligomers, then extract, and vacuum dry at 60 °C for 48 hours, and grind to obtain Ag@PPy powder.

[0108] Step 4: Mix the Ag@PPy powder obtained in Step 3 with a PVDF solution with a mass concentration of 5% at a mass ratio of 3:25, and then ball-mill for 6 h to obtain a modified current collector slurry.

[0109] Step 5: Brush the modified current collector slurry obtained in Step 4 onto the surface of the carbon fiber current collector, and then dry it at 60 °C for 10 h to form an Ag@PPy modified functional layer on the surface of the carbon fiber current collector, thereby obtaining an Ag@PPy modified carbon fiber current collector.

[0110] Example 7

[0111] Step 1: Dissolve 1.69 g of silver nitrate in 100 mL of ultrapure water, then add Py (pyrrole), and add nitric acid to ensure an acidic reaction environment. Subsequently, stir at 200 rpm at 5 °C for 1.5 h to prepare a solution to be reacted; among them, the molar ratio of silver nitrate to Py is 1:2, and the dosage of nitric acid is in a molar ratio of 1:4 to AgNO3.

[0112] Step 2: The reaction solution obtained in Step 1 was treated to avoid light, allowed to stand at 10 °C for 30 h, and then transferred to stand at 25 °C for 50 h for polymerization reaction;

[0113] Step 3: After the polymerization reaction in Step 2 was completed, the product was purified with 200 mL of ethanol and ultrapure water to remove impurities and oligomers, and then Soxhlet extracted with methanol for 24 h, THF for 24 h, and chloroform for 24 h for 72 h, and then vacuum dried at 60 °C for 48 h and ground to obtain Ag@PPy powder;

[0114] Step 4: The Ag@PPy powder obtained in Step 3 was mixed with a PVDF-HFP solution with a mass concentration of 7% at a mass ratio of 1:15 and then ball milled for 6 h to obtain a current collector modified slurry;

[0115] Step 5: The current collector modified slurry obtained in Step 4 was brushed onto the surface of the carbon fiber current collector, and then dried at 60 °C for 10 h to form an Ag@PPy modified functional layer on the surface of the carbon fiber current collector, obtaining an Ag@PPy modified carbon fiber current collector.

[0116] Example 8

[0117] Step 1: 1.69 g of silver nitrate was dissolved in 100 mL of ultrapure water, then Py (pyrrole) was added, and nitric acid was added to ensure an acidic reaction environment. Subsequently, it was stirred at 200 rpm at 5 °C for 1.5 h to prepare a reaction solution to be reacted; among them, the molar ratio of silver nitrate to Py was 1:1.2, and the dosage of nitric acid was in a molar ratio of 1:3 to AgNO3.

[0118] Step 2: The reaction solution obtained in Step 1 was treated to avoid light, allowed to stand at -10 °C for 45 h, and then transferred to stand at 35 °C for 92 h for polymerization reaction;

[0119] Step 3: After the polymerization reaction in Step 2 was completed, the product was purified with 200 mL of ethanol and ultrapure water to remove impurities and oligomers, and then Soxhlet extracted with methanol for 24 h, THF for 24 h, and chloroform for 24 h for 72 h, and then vacuum dried at 60 °C for 48 h and ground to obtain Ag@PPy powder;

[0120] Step 4: The Ag@PPy powder obtained in Step 3 was mixed with a PVDF-HFP solution with a mass concentration of 12% at a mass ratio of 3:16 and then ball milled for 6 h to obtain a current collector modified slurry;

[0121] Step 5: Brush the current collector modified slurry obtained in Step 4 onto the surface of the carbon fiber current collector, and then dry it at 60 °C for 10 h to form an Ag@PPy modified functional layer on the surface of the carbon fiber current collector, thereby obtaining an Ag@PPy modified carbon fiber current collector.

[0122] Through the above steps, a modified carbon fiber current collector with Ag@PPy loaded on its surface can be obtained. When the modified current collector is applied to a lithium-sulfur battery and compared with ordinary carbon fibers, it can be found that ordinary carbon fibers are difficult to inhibit the polysulfide shuttle and adapt to the change in sulfur volume. When the Ag@PPy functional layer is introduced onto the current collector, the porous property of PPy greatly increases the specific surface area on the surface of the carbon fiber. It can not only reversely adsorb lithium polysulfide on the layer, but also adapt to the change in sulfur volume and protect the structure of the carbon fiber from being damaged. On the other hand, due to the introduction of Ag, the conductive property of the current collector is improved, which can accelerate the transmission of electrons in the external circuit and catalyze the conversion of active sulfur on the current collector. Compared with a lithium-sulfur battery using an ordinary carbon fiber current collector, the discharge specific capacity of the modified lithium-sulfur battery is increased by 20% - 23%, and the cycle stability is greatly improved.

[0123] The action mechanism of the Ag@PPy functional layer is as Figure 2 and Figure 3 shown: During the discharge process, the lithium metal anode is oxidized to form lithium ions and electrons, which reach the sulfur cathode through the electrolyte and the external circuit respectively. Lithium ions will continuously combine with the active sulfur molecules in the cathode to form long-chain lithium polysulfide (Li2S x , 4 ≤ x ≤ 8). Due to the nitrogen-rich property and abundant adsorption sites of Ag@PPy, the long-chain lithium polysulfide will be anchored and adsorbed on the Ag@PPy functional layer. Subsequently, the long-chain polysulfide is rapidly reduced to short-chain polysulfide (Li2S2 or Li2S) on the surface of the functional layer. During the conversion process, Ag@PPy actually serves as a reaction bed for lithium polysulfide, and the rapid electron transfer will catalyze the conversion of long-chain lithium polysulfide. Finally, most of the final product (Li2S) is reversely adsorbed on the Ag@PPy layer. Compared with ordinary current collectors, the Ag@PPy functional layer can effectively inhibit the polysulfide shuttle effect. In addition, due to the presence of the binder, this functional layer can improve the mechanical strength of the current collector, making it more in line with the requirements of the current collector for the lithium-sulfur battery used in flexible electronic devices.

[0124] The present invention designs a new material based on conductive polymers. On the one hand, it has the original physical and chemical properties of conductive polymers, as well as strong physical / chemical adsorption ability and porous structure. On the other hand, by doping highly conductive electrocatalysts, the electron arrangement inside the polymer is changed, enabling the conductive polymer to have higher conductivity and electrocatalytic activity to accelerate sulfur conversion.

[0125] As is known by common technical knowledge, the present invention can be implemented by other embodiments that do not depart from its spiritual essence or essential features. Therefore, the above-disclosed embodiments are illustrative in all aspects and not exclusive. All changes within the scope of the present invention or within the scope equivalent to the present invention are encompassed by the present invention.

Claims

1. A preparation method of a silver@polypyrrole modified lithium-sulfur battery positive current collector, characterized in that It includes the following steps: Mix the silver@polypyrrole powder synthesized by the low-temperature redox method with the binder solution to form a current collector modification slurry; the silver@polypyrrole powder is prepared through the following process: Dissolve AgNO3 in ultrapure water, then add Py and an acid, and stir evenly to obtain a solution to be reacted; Let the solution to be reacted stand in the dark at -20~10°C and then transfer it to stand at 15~40°C to precipitate solids, and then purify to obtain the silver@polypyrrole powder; Brush the current collector modification slurry onto the surface of the battery current collector to obtain a silver@polypyrrole modified lithium-sulfur battery positive current collector.

2. The preparation method of a silver@polypyrrole modified lithium-sulfur battery positive current collector according to claim 1, characterized in that, The binder solution is a PVDF-HFP solution or a PVDF solution.

3. The preparation method of a silver@polypyrrole modified lithium-sulfur battery positive current collector according to claim 1, characterized in that, The mass concentration of the binder solution is 5%~15%; the mass ratio of the silver@polypyrrole powder to the binder solution is 1:4~1:

15.

4. The preparation method of a silver@polypyrrole modified lithium-sulfur battery positive current collector according to claim 1, characterized in that, The current collector is carbon fiber, aluminum foil or copper foil.

5. The preparation method of a silver@polypyrrole modified lithium-sulfur battery positive current collector according to claim 1, characterized in that, The standing time at -20~10°C is 4~64h, and the standing time at 15~40°C is 24~148h.

6. The preparation method of a silver@polypyrrole modified lithium-sulfur battery positive current collector according to claim 1, characterized in that, Purification is by filtration or extraction.

7. The preparation method of a silver@polypyrrole modified lithium-sulfur battery positive current collector according to claim 1, characterized in that The molar ratio of AgNO3 to Py is 1:0.5~1:2; the acid is nitric acid, citric acid or ascorbic acid; the molar ratio of the amount of the acid used to AgNO3 is 1:0.5~1:

4.

8. A lithium-sulfur battery cathode, characterized in that, It is obtained by using the silver@polypyrrole modified lithium-sulfur battery positive current collector prepared by the method described in any one of claims 1-7.

9. A lithium-sulfur battery, characterized in that, It includes the lithium-sulfur battery positive electrode described in claim 8.

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