High-conductivity microcapsule, preparation method thereof and application of high-conductivity microcapsule in silicon negative electrode

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a high-conductivity microcapsule as well as a preparation method and application thereof in a silicon negative electrode. According to the invention, the conductive ionic liquid is encapsulated into microcapsules for the first time, and the microcapsules are added into the silicon negative electrode of the lithium ion battery as an additive, so that the silicon negative electrode is converted from a liquid state to a solid state, and the problem of poor dispersity of the ionic liquid is solved. Secondly, by utilizing the high-viscosity fluid-state conductive ionic liquid released after the microcapsules are broken when silicon is subjected to volume expansion, on one hand, stress caused by silicon particle expansion can be effectively reduced, an ionic conduction path can be reconstructed, and the electrochemical performance of the silicon negative electrode is remarkably improved; and on the other hand, the expanded silicon particles are not easy to fall off by utilizing the high viscosity characteristic of the ionic liquid, so that the attenuation of a conductive path is reduced. The invention provides a new path for the application of the silicon negative electrode of the lithium ion battery.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly to a highly conductive microcapsule, a preparation method thereof, and an application thereof in a silicon anode. Background Art

[0002] Lithium-ion batteries (LIBs), as an efficient energy storage technology, have been widely used in fields such as consumer electronics, electric vehicles, and energy storage systems due to their high energy density, long cycle life, and low self-discharge rate. With the increasing demand for higher energy density, traditional graphite anode materials (theoretical specific capacity of 372 mAh / g) have gradually become difficult to meet the actual application requirements. Therefore, finding anode materials with high specific capacity has become one of the key focuses in current lithium-ion battery research.

[0003] Silicon (Si), as a highly potential anode material, has attracted the attention of many researchers due to its extremely high theoretical specific capacity (4200 mAh / g, about more than 10 times that of graphite) and relatively low discharge plateau (~0.45 V vs. Li / Li+). However, silicon anodes face many challenges in practical applications. First, silicon undergoes significant volume changes during charge and discharge, with an expansion rate of about 300%; this huge volume change can lead to the destruction of the electrode structure, causing silicon particles to fall off from the electrode, thereby increasing the contact impedance between the electrode and the electrolyte and reducing ion conductivity. Second, silicon has poor conductivity and is prone to forming an unstable solid electrolyte interface (SEI) layer during charge and discharge, further exacerbating the capacity decay of the electrode. These factors work together to make the cycle stability and rate performance of silicon anodes difficult to meet the actual application requirements.

[0004] To solve the above problems, researchers have explored various strategies.

[0005] Among them, nanosizing is one of the earliest proposed methods. By preparing silicon into a nanostructure, the diffusion path of lithium ions can be effectively shortened, and the stress caused by volume expansion can be alleviated. In addition, the nanosizing process is complex and costly, which limits its large-scale application.

[0006] Alloying is another method to improve the performance of silicon anodes. By alloying silicon with other metals (such as tin, aluminum, germanium, etc.), the conductivity and structural stability of the electrode can be improved. However, the alloyed materials also undergo volume expansion during charge and discharge. Although the expansion degree is relatively lower than that of pure silicon, the problem of electrode structure stability still cannot be completely solved.

[0007] Silicon-carbon composite materials have also been a research hotspot in recent years. By combining silicon with carbon materials, the high electrical conductivity and good structural stability of carbon can be utilized to buffer the volume expansion of silicon. However, the addition of carbon materials will occupy a certain volume, thereby reducing the overall specific capacity of the electrode.

[0008] In addition to the above methods, researchers have also tried to improve the performance of silicon anodes by introducing conductive or functional additives. For example, adding materials such as conductive polymers, carbon nanotubes, and graphene can improve the electrical conductivity of the electrode and relieve the volume expansion of silicon particles. However, the addition of these additives often has limited improvement effects, and there are problems such as poor dispersibility of the additives, small stress buffering effect, and low adhesion, resulting in the cycling performance and rate performance of silicon anodes still being difficult to meet the actual application requirements. Summary of the Invention

[0009] In view of the above existing problems or deficiencies, in order to solve the problem that the cycling stability and rate performance of existing silicon anodes for lithium-ion batteries are difficult to meet the actual application requirements, the present invention provides a highly conductive microcapsule, its preparation method, and its application in silicon anodes. The present invention synthesizes a polystyrene shell microcapsule encapsulating an ionic liquid through the microcapsule technology and introduces it as an additive into the silicon anode, which can significantly improve the cycling stability and ionic conduction performance of the silicon anode, providing a new technical approach for the development of high-performance lithium-ion battery anode materials.

[0010] A highly conductive microcapsule and its preparation method, comprising the following steps:

[0011] Step 1, Preparation of materials:

[0012] Dissolve polystyrene and a conductive ionic liquid in a good solvent to form an oil-phase solution A.

[0013] Dissolve polyvinyl alcohol in deionized water to form an aqueous-phase solution B, where the mass fraction of polyvinyl alcohol is 1.5%-3%.

[0014] Step 2, Emulsify the solution A and solution B prepared in Step 1 completely at room temperature according to a volume ratio of 0.5-1.5:1-2.

[0015] Step 3, Centrifuge the product obtained in Step 2, then wash it with deionized water and ethanol and dry it. After it cools, highly conductive microcapsule powder is obtained.

[0016] Preferably, the conductive ionic liquid in Step 1 is at least one of [EMIM][FSI], [EMIM][TFSI], and [HMIM][BF4].

[0017] Preferably, the emulsification conditions in Step 2 are specifically 5500-6500 rpm for 1-1.5 h and standing for 12-15 h under atmospheric conditions.

[0018] Preferably, the drying temperature in step 3 is 50-100°C.

[0019] Preferably, a silicon anode for a lithium-ion battery includes the material components of a conventional silicon anode for a lithium-ion battery (silicon particles, conductive agent, and binder), and the highly conductive microcapsules provided by the present invention; wherein, relative to the mass of silicon, the addition amount of the highly conductive microcapsules is 2%-10%, and the highly conductive microcapsules are uniformly dispersed in the material components of the silicon anode for the lithium-ion battery.

[0020] Ionic liquids (ILs) have gradually become a research hotspot in the field of electrochemical energy storage due to their unique physical and chemical properties, such as high thermal stability, high ionic conductivity, and good electrochemical stability. As electrolytes or additives, ionic liquids have been proven to significantly improve the performance of lithium-ion batteries; however, the high viscosity of ionic liquids limits their uniform dispersion in electrodes, resulting in slow ion transport kinetics and restricting their application in lithium-ion batteries.

[0021] In summary, in the present invention, the conductive ionic liquid is encapsulated into microcapsules for the first time and then added as an additive to the silicon anode of a lithium-ion battery, converting it from a liquid state to a solid state, which solves the problem of poor dispersibility of ionic liquids. Secondly, taking advantage of the negative factor that the silicon in the silicon anode material will undergo volume expansion, when the silicon undergoes volume expansion, it will cause the microcapsules to rupture, and the highly viscous fluid-state conductive ionic liquid flowing out after the microcapsules rupture: on the one hand, it can effectively relieve the stress caused by the expansion of silicon particles and reconstruct the ion conduction path, significantly improving the electrochemical performance of the silicon anode; on the other hand, it also utilizes the high viscosity characteristic of ionic liquids to make the expanded silicon particles not easily fall off, reducing the attenuation of the conductive path. The present invention greatly improves the problem that the cycle stability and rate performance of the existing silicon anode for lithium-ion batteries are difficult to meet the actual application requirements, providing a new path for the application of silicon anodes for lithium-ion batteries. Description of the Drawings

[0022] Figure 1 SEM image of the highly conductive microcapsules prepared in Example 1.

[0023] Figure 2 DLS particle size analysis chart of the highly conductive microcapsules prepared in Example 1.

[0024] Figure 3 Infrared spectra of the highly conductive microcapsules, core materials, and shell materials prepared in Example 1.

[0025] Figure 4 Thermogravimetric analysis charts of the highly conductive microcapsules, core materials, and shell materials prepared in Example 1.

[0026] Figure 5Cycling test curve of the silicon anode battery prepared in Example 1.

[0027] Figure 6 Cycling test curve of the silicon anode battery prepared in Example 2.

[0028] Figure 7 Cycling test curve of the silicon anode battery prepared in Comparative Example 1.

[0029] Figure 8 Cycling test curve of the silicon anode battery prepared in Comparative Example 2.

[0030] Figure 9 Rate test curve of the silicon anode battery prepared in Example 1.

[0031] Figure 10 Rate test curve of the silicon anode battery prepared in Example 2.

[0032] Figure 11 Rate test curve of the silicon anode battery prepared in Comparative Example 1.

[0033] Figure 12 Rate test curve of the silicon anode battery prepared in Comparative Example 2. Detailed implementation mode

[0034] Next, the technical solution of the present invention will be described in detail through examples and drawings.

[0035] Example 1

[0036] A preparation method of a highly conductive microcapsule, characterized by comprising the following steps:

[0037] Step 1, prepare materials:

[0038] Dissolve 1.28 g of polystyrene and 3.87 g of ionic liquid [EMIM][FSI] in 70.54 g of dichloromethane to form an oil-phase solution A.

[0039] Dissolve 5.00 g of polyvinyl alcohol in 250 mL of deionized water to form an aqueous-phase solution B.

[0040] Step 2, add the solution A prepared in Step 1 to 76 mL of the solution B prepared in Step 2, and emulsify at a speed of 6000 rpm for 1 h at room temperature;

[0041] Then, add 120 mL of the solution B prepared in Step 2 to the emulsion for dilution, and then let it stand for 12 h under atmospheric conditions to fully emulsify it.

[0042] Step 3, centrifuge the sample obtained in Step 2, and then wash it 3 times with deionized water and ethanol respectively; and dry it completely at 55 °C, and wait for it to cool to obtain the highly conductive microcapsule powder.

[0043] Preparation of a silicon negative electrode containing 10% microcapsules:

[0044] Mix nano-silicon particles, conductive agent Super P, and binder sodium alginate in a mass ratio of 6:2:2, and ball mill them 3 times using a mechanical ball mill, with each ball milling for 20 minutes and an interval of 5 minutes each time to obtain a silicon negative electrode slurry. Add the above-prepared highly conductive microcapsules to the silicon negative electrode slurry, with a mass ratio of 10% of the nano-silicon mass; then stir the slurry using magnetic stirring at a speed of 300 rpm for 0.5 hours, and finally use an ultrasonic disperser to ultrasonically disperse for 0.5 hours to make the microcapsules uniformly dispersed in the slurry.

[0045] Uniformly coat the above silicon negative electrode slurry containing highly conductive microcapsules evenly on a copper foil, let it stand overnight at room temperature, and then let it stand in an 80°C drying oven for 8 hours to obtain a silicon negative electrode containing 10% microcapsules.

[0046] Example 2

[0047] The difference between Example 2 and Example 1 is only that: when preparing the silicon negative electrode, the addition amount of microcapsules is different, and the microcapsule content is 5%.

[0048] Comparative Example 1

[0049] The difference between Comparative Example 1 and Example 1 is only that: a silicon electrode without microcapsules is used as the negative electrode of a lithium-ion battery.

[0050] Comparative Example 2

[0051] The difference between Comparative Example 2 and Example 1 is only that: when preparing the silicon negative electrode, the addition amount of microcapsules is different, and the microcapsule content is 20%.

[0052] Perform SEM testing on the microcapsules prepared in Example 1, and the test results are as Figure 1 shown. It can be seen from Figure 1 that there is no obvious agglomeration between the microcapsules and the surface is smooth; the locally enlarged drawing in the upper right corner also shows the shape of the broken microcapsules, indicating that the microspheres have a typical core-shell hollow spherical structure, and the shell thickness is about 100 nm.

[0053] Perform DLS particle size analysis on the microcapsules prepared in Example 1, and the test results are as Figure 2 shown. It can be seen from Figure 2 that the monodisperse size of the microcapsules is about 1.46 μm, there is no obvious size difference, and the dispersion is good.

[0054] Perform infrared spectrum analysis on the microcapsules prepared in Example 1, and the test results are as Figure 3 shown. It can be seen from Figure 3It can be seen that in the spectrum of the polystyrene shell, the C-H stretching vibration peak of the aromatic group, the C-H bending of the alkyl group, and the C═C stretching peak of the benzene ring are shown, etc., which confirms its chemical structure. In the spectrum of the [EMIm][FSI] ionic liquid core material, the stretching vibration of the C-H bond, the vibration peak of the aliphatic group on the imidazole ring, the stretching vibration peak of the imidazole ring, the characteristic peak of the S-N-S bond, etc. are shown. The spectrum of the microcapsule simultaneously shows the characteristic peaks of the core material and the shell, which confirms that the [EMIm][FSI] ionic liquid is effectively encapsulated in the spherical shell.

[0055] Thermogravimetric analysis was carried out on the microcapsules prepared in Example 1, and the test results are as Figure 4 shown. It can be seen from Figure 4 that when the [EMIm][FSI] ionic liquid is encapsulated into the polystyrene shell, the initial weight loss temperature of the encapsulated [EMIm][FSI] is increased from 220 °C to 305 °C, indicating that the thermal stability at higher temperatures has been improved.

[0056] Electrochemical performance tests were carried out on Examples 1-2 and Comparative Examples 1-2: A coin-type half-cell was used to test the electrochemical performance of the electrode. The positive electrode was a lithium sheet, the negative electrode was the above-mentioned electrode, and the separator was a pp separator. The above negative electrode sheet was cut into a circular sheet with a diameter of 10 mm, weighed after drying; a coin-type half-cell was assembled with a silicon negative electrode containing 10% microcapsules, a separator, and a lithium sheet. Cyclic charge and discharge tests were carried out between 0.01 V and 1.2 V, activated for 3 cycles, and the activation current density was 0.1 C; cycled for 200 cycles, and the cycle current density was 0.2 C (1 C = 4200 mA / g), and the charge and discharge test results are as Figures 5-8 shown. The silicon negative electrodes containing 5% and 10% microcapsules showed discharge specific capacities of 1732.1 mAh / g and 1401.8 mAh / g after 200 cycles at 0.2 C, while the pure silicon electrode only had a discharge specific capacity of 1177 mAh / g. This is attributed to the fact that the rupture of the microcapsules can effectively relieve the stress caused by the swelling of the silicon particles and reconstruct the ion transport pathway in the electrode. When the content of the microcapsules in the silicon negative electrode of the silicon electrode is further increased to 20%, the cycle stability decreases because the presence of an excessive polystyrene shell leads to the formation of a discontinuous electron path in the electrode. These results indicate that the incorporation of an appropriate concentration of ionic liquid core microcapsules can improve the cycle stability of the silicon negative electrode.

[0057] Rate charge and discharge tests were carried out between 0.01 V and 1.2 V, activated for 3 cycles, and the activation current density was 0.1 C; the subsequent parameter settings were 0.2 C for 10 cycles, 0.5 C for 10 cycles, 1 C for 10 cycles, 0.5 C for 10 cycles, 0.2 C for 10 cycles, and the rate test results are as Figures 9-12As shown, the silicon anode containing 10% microcapsules exhibits superior rate capacity, i.e., the discharge capacities at 0.2, 0.5, 1, 0.5, and 0.2 C are 2243.5, 1745.1, 1240.9, 1881.0, and 2194.8 mAh / g, respectively. When the cycling current density increases from 0.2 C to 1 C and then decreases to 0.2 C, the specific capacities of the pure silicon anode are 2185.0, 1510.9, 924.8, 1123.2, and 1085.2 mAh / g, respectively. The superior rate performance of the silicon anode containing 10% microcapsules is related to the high ionic conductivity and structural integrity of the electrode at various current densities generated by the addition of microcapsules. Compared with the pure silicon anode, the silicon anode containing 20% microcapsules exhibits a lower discharge specific capacity, which is due to the generation of a large number of polystyrene shells caused by the rupture of microcapsules in the electrode, and the relatively low concentration of the released ionic liquid resulting in a relative decrease in the ionic conduction performance in the electrode. These results correspond to the battery cycling performance, and adding an appropriate concentration of ionic liquid core microcapsules can maintain the cycling performance of the silicon anode at high current densities.

[0058] In the present invention, for the first time, a conductive ionic liquid is encapsulated into microcapsules and then added as an additive to the silicon anode of a lithium-ion battery, converting it from a liquid state to a solid state, thus solving the problem of poor dispersibility of the ionic liquid. Secondly, taking advantage of the negative factor that the silicon in the silicon anode material will undergo volume expansion, when the silicon undergoes volume expansion, it will cause the microcapsules to rupture. The highly viscous fluid conductive ionic liquid flowing out after the rupture of the microcapsules: on the one hand, can effectively relieve the stress caused by the expansion of silicon particles and reconstruct the ionic conduction path, significantly improving the electrochemical performance of the silicon anode; on the other hand, it also utilizes the high viscosity characteristic of the ionic liquid to make the expanded silicon particles not easily fall off, reducing the attenuation of the conductive path. The present invention greatly improves the problem that the cycling stability and rate performance of the existing silicon anode of lithium-ion batteries are difficult to meet the actual application requirements, providing a new path for the application of the silicon anode of lithium-ion batteries.

Claims

1. A method for preparing highly conductive microcapsules, characterized in that: The following steps are involved: Step 1: Prepare the ingredients: Dissolving polystyrene and conductive ionic liquid in a good solvent to form an oil phase solution A; Dissolve polyvinyl alcohol in deionized water to form an aqueous solution B, wherein the mass fraction of polyvinyl alcohol is 1.5%-3%; Step 2, emulsifying the solution A and solution B prepared in step 1 at a volume ratio of 0.5-1.5:1-2 at room temperature; Step 3: Centrifuge the product obtained in step 2, wash it with deionized water and ethanol, and then dry it. After cooling, a highly conductive microcapsule powder is obtained.

2. The method for preparing highly conductive microcapsules according to claim 1, characterized in that: In step 1, the conductive ionic liquid is at least one of [EMIM][FSI], [EMIM][TFSI] and [HMIM][BF4].

3. The method for preparing highly conductive microcapsules according to claim 1, characterized in that: The specific emulsification conditions in step 2 are 5500-6500 rpm for 1-1.5 hours, and standing under atmospheric conditions for 12-15 hours.

4. The method for preparing highly conductive microcapsules according to claim 1, characterized in that: The drying temperature in step 3 is 50-100°C.

5. A highly conductive microcapsule, characterized in that: Prepared by any of the methods described in rights 1-4.

6. A highly conductive microcapsule, characterized in that: The silicon negative electrode used in lithium-ion batteries includes silicon particles, a conductive agent, a binder and microcapsules; the addition amount of the high-conductivity microcapsules is 2%-10% relative to the mass of silicon, and the high-conductivity microcapsules are uniformly dispersed in the silicon negative electrode material components of the lithium-ion battery.

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