Glassy-state binder as well as preparation method and application thereof

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a glassy binder as well as a preparation method and application thereof. According to the preparation method, the ionic liquid and the polymer interact to form glassy state gel for the first time, and the intermolecular interaction between a polymer gel network and a solvent is adjusted through strategies such as coordination of an ionic liquid solvent, in-situ phase separation or the synergistic effect of the ionic liquid solvent and stress induced crystallization; an ionic polymer gel network with high toughness and high adhesion strength is constructed; and a continuous high-conductivity ion transmission path is constructed through a high-adhesion high-strength network formed by the ionic polymer gel and flowing and pore filling of the free ionic liquid in the thick electrode, so that the conductivity and the structural stability of the thick electrode are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly to a vitreous binder, a preparation method thereof, and an application thereof. Background Art

[0002] Among various energy storage technologies, lithium-ion batteries, with their advantages such as high energy density, high power density, long cycle life, and no memory effect, lead and restrict the development of fields such as new energy vehicles and low-altitude economy. Among them, high-energy-density lithium-ion batteries lead the development of energy storage in China and play an important role in promoting the transformation of clean energy, improving transportation methods, supporting mobile electronic devices, and maintaining national security.

[0003] In the construction of high-energy-density lithium-ion batteries, microscopic structures such as electrode areal capacity, thickness, and porosity play a key role in the energy density and power density of lithium-ion batteries. In particular, reducing the electrode porosity, increasing the tap density, and increasing the thickness contribute to increasing the content of active materials in the electrode, thereby increasing the battery capacity. However, with the increase in electrode thickness and compaction, the ion diffusion path in the electrode becomes narrower, the ion diffusion resistance increases, and the internal electron transport of the electrode is also hindered, resulting in deteriorated rate performance of the electrode, low capacity utilization rate, and slow increase in energy density. In addition, thick electrodes are prone to fracture and delamination during the drying process, resulting in the failure of their structures. Therefore, how to improve the electron / ion transport rate and structural stability inside thick electrodes is the key to improving the energy density of lithium-ion batteries and one of the greatest scientific and engineering challenges for achieving long cycles of high-energy-density batteries.

[0004] Currently, the research on constructing thick electrodes with high structural stability and conductivity mainly focuses on tortuous structure design and patterned electrode construction.

[0005] In terms of tortuous structure design, by adjusting the content of conductive additives, introducing 1D or 2D materials (such as CNT and graphene) with a lower electron percolation threshold and better conductivity, etc., the percolation ability of the electrolyte and the migration rate of ions are improved, a thick electrode with a low tortuosity pore structure is constructed, and its electrochemical performance is improved. However, the method of introducing a low tortuosity pore structure usually inevitably increases the electrode porosity, thereby reducing the active material loading and areal capacity; in addition, the construction of the percolation network usually involves delicate modification designs (in-situ growth of 2D materials, self-assembly decoration, etc.) or complicated synthesis methods (sacrificial phase screening and optimization, pore structure regulation, etc.), and the scalability of the preparation process is limited. Therefore, there is still a lack of a thick electrode solution with a highly continuous ion / electron transport network, temperature-structurable, and scalable production.

[0006] In addition, through layer-by-layer preparation processes such as extrusion-based 3D printing or direct ink writing, the fabrication of patterned thick electrodes with both precise structures and adjustable compositions can be achieved. However, the structural stability of the patterned thick electrodes fabricated by this method is limited by the characteristics of the electrode slurry (such as high viscosity, low shear force, self-stabilization, and self-adhesion), and customized development is required for different electrodes.

[0007] Therefore, how to construct a binder that combines high toughness, electrochemical stability, and high ionic conductivity has great practical application prospects for the development of high-energy-density lithium-ion batteries, but also poses huge challenges. Summary of the Invention

[0008] Aiming at the above problems or deficiencies, to solve the problems of poor structural stability and limited ion conduction in silicon-based thick electrodes, the present invention provides a method for preparing a glassy binder and its application in silicon anodes. By optimizing the intermolecular interactions and swelling effects between polymer chains by ionic liquids, the present invention develops a binder technology with high deformation rate, high adhesiveness, and high ionic conductivity to ensure efficient ion transport and high-elastic stability of the structure in silicon-based thick electrodes.

[0009] A method for preparing a glassy binder, comprising the following steps:

[0010] Step 1, preparing materials:

[0011] Dissolve a polymer monomer, a crosslinking agent MBAA, and a photoinitiator in an ionic liquid at a mass ratio of 4500 - 5500:1:15 - 25 to form a mixed solution A.

[0012] Dissolve a high-molecular material rich in carboxyl functional groups in deionized water to form a solution B, where the mass fraction of the high-molecular material is 1.5% - 3%.

[0013] Step 2, mix the solution A and the solution B prepared in Step 1 at room temperature at a mass ratio of the polymer monomer to the high-molecular material rich in carboxyl functional groups of 6.5 - 7.5:2.5 - 3.5 to obtain a mixed solution C.

[0014] Step 3, cure the mixed solution C obtained in Step 2 by ultraviolet light to finally obtain a glassy binder.

[0015] Preferably, the polymer monomer in Step 1 is an acrylic acid substance, including at least one of acrylic acid AA, methyl acrylate MA, ethyl acrylate EA, butyl acrylate BA, acrylamide AAm, 2-hydroxyethyl acrylate HEA, and 2-hydroxyethyl methacrylate HEMA.

[0016] Preferably, the photoinitiator in step 1 is at least one of I2959, 1-hydroxycyclohexyl phenyl ketone, and diphenyl-(2,4,6-trimethylbenzoyl) phosphine oxide.

[0017] Preferably, the ionic liquid in step 1 is at least one of [EMIM][ESO4], [EMIM][DEP], [EMIM][TFSI], and [EMIM][PF6], or is compounded according to a mass ratio of 1:1.

[0018] Preferably, the polymer material rich in carboxyl functional groups in step 1 is at least one of sodium alginate Alg, sodium carboxymethyl cellulose CMC, polyacrylic acid PAA, or Alg-Li, CMC-Li, PAA-Li after lithiation dialysis treatment.

[0019] Preferably, the stirring and mixing conditions in step 2 are specifically stirring at 500-1000 rpm for 0.5-1.5 h.

[0020] Preferably, the parameters of ultraviolet curing in step 3 are a power of 40%-60% and a time of 10-60 s.

[0021] Preferably, a vitreous binder is prepared by the above method.

[0022] Preferably, for a silicon negative electrode of a lithium-ion battery, the above vitreous binder is used as the binder in the silicon negative electrode of the lithium-ion battery; the silicon negative electrode of the lithium-ion battery includes silicon particles, a conductive agent, and the vitreous binder provided by the present invention; wherein, ultraviolet curing needs to be carried out again after the electrode coating is completed.

[0023] The micro-nano structured polymer glass gel is composed of physically or chemically cross-linked polymer chains. By swelling with a solvent, the free volume and interaction between polymer chains are changed, and it has the soft and hard characteristics of glass and gel, and is expected to improve the mechanical structure and electrochemical performance stability of thick electrodes. Ionic liquids are room-temperature organic molten salts composed of organic cations and organic or inorganic anions, and have characteristics such as low volatility, non-flammability, good thermal and chemical stability, high ionic conductivity (1-10 mS / cm), and a wide electrochemical window (3-5 V). The present invention adjusts the intermolecular interaction between the polymer gel network and the solvent through strategies such as coordinating ionic liquid solvents, in-situ phase separation, or the synergistic effect of ionic liquid solvents and stress-induced crystallization, to construct an ionic polymer gel network with both high toughness and high adhesion strength.

[0024] In summary, the present invention is the first to use the interaction between ionic liquid and polymer to form a glassy gel, which is then used as a binder for the silicon anode of a lithium-ion battery. Through the high-adhesion and high-strength network formed by the ionic polymer gel and the flow and pore filling of free ionic liquid inside the thick electrode, a continuous and highly conductive ionic transport pathway is constructed, thereby improving the conductivity and structural stability of the thick electrode; this provides new ideas and solutions for the development of high-performance lithium-ion battery anode materials. Description of the Drawings

[0025] Figure 1 SEM image of the glassy binder PAA-Alg silicon anode prepared in Example 1.

[0026] Figure 2 SEM image of the glassy binder PAA-CMC silicon anode prepared in Example 2.

[0027] Figure 3 Electrochemical impedance spectra of the silicon anode batteries prepared in Example 1 and Example 2.

[0028] Figure 4 Electrochemical impedance spectra of the silicon anode batteries prepared in Example 3 and Example 4.

[0029] Figure 5 0.1C cycling test curves of the silicon anode batteries prepared in Example 3 and Example 4.

[0030] Figure 6 0.2C cycling test curves of the silicon anode batteries prepared in Example 3, Example 4 and the comparative example. Detailed Description of the Invention

[0031] Next, the technical solutions of the present invention will be described in detail through examples and drawings.

[0032] Example 1

[0033] A preparation method of a glassy binder includes the following steps:

[0034] Step 1. Prepare materials:

[0035] Dissolve 5 g of polymer monomer AA, 0.001 g of cross-linking agent MBAA, and 0.02 g of initiator (0.01 g of 1-hydroxycyclohexyl phenyl ketone and 0.01 g of diphenyl-(2,4,6-trimethylbenzoyl) phosphine oxide) in 4.34 mL of [EMIM][DEP] ionic liquid to form a mixed solution A.

[0036] Dissolve 2.14 g of sodium alginate in 107 mL of deionized water to form solution B.

[0037] Step 2: Mix solution A and solution B prepared in Step 1 at a mass ratio of AA to sodium alginate of 7:3 at room temperature with a stirring speed of 600 rpm for 0.5 h to obtain a mixed solution C.

[0038] Step 3: Carry out ultraviolet curing on the mixed solution C obtained in Step 2 under the parameters of a power of 50% and a time of 50 s to finally obtain a vitreous binder, denoted as PAA-Alg.

[0039] Preparation of silicon anode:

[0040] Mix nano-silicon particles, conductive agent Super P and vitreous binder PAA-Alg at a mass ratio of 6:2:2, and ball mill with a mechanical ball mill 3 times, with each ball milling for 15 minutes and an interval of 5 minutes each time to obtain a silicon anode slurry; uniformly coat the above slurry on a copper foil, and carry out ultraviolet curing again for 30 s with a power of 50% to obtain a silicon anode.

[0041] Example 2

[0042] The difference between Example 2 and Example 1 is only that: when preparing the vitreous binder, the polymer rich in carboxyl functional groups in Step 1 is changed to CMC, and the product is denoted as PAA-CMC; when preparing the silicon anode, the vitreous binder used is PAA-CMC.

[0043] Example 3

[0044] The difference between Example 3 and Example 1 is only that: when preparing the vitreous binder, sodium alginate in Step 1 is changed to Alg-Li after lithiation dialysis treatment, and the prepared vitreous binder is denoted as PAA-Alg-Li, specifically as follows:

[0045] Dissolve 3 g of sodium alginate in an ethanol solution (the volume ratio of ethanol to water is 95:5), and at the same time prepare a 20 wt% hydrochloric acid solution. Take out the corresponding hydrochloric acid according to the molar ratio of HCl:Na = 1:1 and add it to the ethanol solution of sodium alginate, and stir magnetically at 35 °C for 24 h. The obtained mixed solution is centrifugally washed with absolute ethanol and then placed in an oven at 60 °C for drying for 24 h. The dried powder is configured into a 2 wt% aqueous solution, and lithium hydroxide is taken according to the molar ratio of LiOH:-COOH = 1:1 and added thereto, and lithiation is carried out at 25 °C for 24 h. Finally, the required Alg-Li is obtained through dialysis treatment.

[0046] Example 4

[0047] The difference between Example 4 and Example 1 is only that: when preparing the vitreous binder, sodium alginate in Step 1 is changed to CMC-Li after lithiation dialysis treatment, and the prepared vitreous binder is denoted as PAA-CMC-Li, specifically as follows:

[0048] Dissolve 3 g of CMC in an ethanol solution (volume ratio of ethanol to water is 95:5). Meanwhile, prepare a 20 wt% hydrochloric acid solution. Take the corresponding hydrochloric acid according to the molar ratio of HCl:Na = 1:1 and add it to the ethanol solution of CMC. Stir magnetically at 35 °C for 24 h. The obtained mixed solution is centrifugally washed with anhydrous ethanol and then dried in an oven at 60 °C for 24 h. The dried powder is made into a 2 wt% aqueous solution, and lithium hydroxide is added according to the molar ratio of LiOH:-COOH = 1:1. Lithiation is carried out at 25 °C for 24 h. Finally, the required CMC-Li is obtained through dialysis treatment.

[0049] Comparative example

[0050] The difference between the comparative example and Example 1 is only that: instead of using a vitreous binder, sodium alginate is used as the binder when preparing the silicon negative electrode.

[0051] Scanning electron microscopy (SEM) tests were carried out on the silicon negative electrodes prepared with the vitreous binder in Example 1 and Example 2. The obtained images are respectively as Figure 1 and Figure 2 shown. It can be seen from the figures that the silicon particles are relatively uniformly distributed under the action of the two binders, the particle sizes are relatively consistent and the morphologies are regular, mostly spherical. This indicates that the two binders can connect the silicon particles to each other, forming a relatively tight network structure. This network structure not only enhances the mechanical strength of the electrode but also significantly improves its structural stability. During the charge and discharge process, the tightly connected silicon particles can effectively reduce the shedding phenomenon and avoid electrode pulverization, thereby prolonging the service life of the electrode. In addition, it can also be observed that conductive agent particles are uniformly distributed around the silicon particles. This tight connection helps to reduce the shedding of silicon particles and electrode pulverization during the charge and discharge process.

[0052] Electrochemical performance tests were carried out on Examples 1-4 and the comparative example: 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-mentioned negative electrode sheet was cut into a round sheet with a diameter of 10 mm and weighed after drying; the silicon negative electrodes using the binders in Examples 1-4 and the comparative example, the separator, and the lithium sheet were assembled into a coin-type half-cell. The set parameters of the electrochemical impedance were: the frequency range was 0.01 - 100000 Hz, and the amplitude of the AC voltage signal was 5 mV. The set parameters of the electrochemical cycle test were: 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; the cyclic current density was 0.1 C for 120 cycles, and the cyclic current density was 0.2 C for 60 cycles (1 C = 4200 mA / g).

[0053] Analysis was carried out on the electrochemical impedance spectra of the silicon negative electrode batteries prepared in Examples 1-4. The results are as Figure 3 and Figure 4As shown. It can be seen from the figure that the impedance spectrum curves of the silicon anodes prepared with the four binders show similar shapes, indicating that the several silicon anodes have similar electrochemical behaviors. The diameter of the semicircle in the high-frequency region of the impedance spectrum curve represents the charge transfer resistance (R ct ), and the oblique line in the low-frequency region represents the ion diffusion impedance. It can be seen from the figure that the R ct value of the PAA-Alg silicon anode is about 43 Ω, the R ct value of the PAA-CMC silicon anode is about 80 Ω, the R ct value of the PAA-Alg-Li silicon anode is about 42 Ω, the R ct value of the PAA-CMC-Li silicon anode is about 38 Ω. The R ct values of the four silicon anodes are relatively low, indicating that the charge transfer process is relatively easy to carry out and the electrode has good electrochemical activity. At the same time, the oblique line in the low-frequency region is relatively steep, indicating that the ion diffusion impedance is small and the diffusion rate of lithium ions in the electrode is fast.

[0054] Analyze the electrochemical cycling curves of the silicon anode batteries prepared in Examples 3 and 4 and the comparative example. The results are as Figure 5 and Figure 6 shown. Figure 5 is the electrochemical cycling test curve of the silicon anode batteries prepared in Example 3 and Example 4, with a current density of 0.1C; Figure 6 is the electrochemical cycling test curve of the silicon anode batteries prepared in Examples 3, 4 and the comparative example, with a current density of 0.2C. It can be seen from the two figures that the trends of the cycling curves of several groups of batteries are similar. After activation, they all experience capacity decay. The capacity decay is faster in the first 20 cycles, and the subsequent cycles tend to be stable. Specifically, for the silicon anodes prepared in Example 3 and Example 4, after 120 cycles at a current density of 0.1C, the discharge specific capacities of 1459.3 mAh / g and 1425.1 mAh / g are shown, and the capacity retention rate remains at about 37%. Thanks to the glassy binder effectively alleviating the volume expansion problem of silicon during charge and discharge, it promotes the rapid transmission of lithium ions in the electrode. When the current density increases to 0.2C, after 60 cycles of the silicon anode prepared in Example 3, the discharge specific capacity of 1488.1 mAh / g is shown, and after 60 cycles of the silicon anode prepared in Example 4, the discharge specific capacity of 2164.7 mAh / g is shown, which is still relatively high compared to the comparative example, proving that the glassy binder has a significant effect on improving the cycling stability and capacity retention rate of the silicon anode battery, especially under higher current density conditions, its advantages are more obvious.

[0055] The present invention pioneered the development of a glassy binder with high deformation rate, high adhesiveness and high ionic conductivity by optimizing the interaction between ionic liquid and polymer, and successfully applied it to silicon anodes. Experimental results show that the silicon anodes using this binder are significantly superior to traditional binders in terms of cycle stability and rate performance. Especially at high current densities, its capacity retention and cycle stability are both excellent, demonstrating the effectiveness and superiority of the present invention in improving the performance of silicon-based thick electrodes. This innovative technology provides new ideas and solutions for the development of high-performance anode materials for lithium-ion batteries.

Claims

1. A method for preparing a vitreous binder, characterized in that: The following steps are involved: Step 1: Prepare the ingredients: Dissolving the polymer monomer, the crosslinking agent MBAA and the photoinitiator in the ionic liquid at a mass ratio of 4500-5500:1:15-25 to form a mixed solution A; Dissolving a polymer material rich in carboxyl functional groups in deionized water to form a solution B, wherein the mass fraction of the polymer material is 1.5%-3%; Step 2, mixing the solution A and solution B prepared in step 1 at room temperature in a ratio of 6.5-7.5:2.5-3.5 by mass of the polymer monomer and the polymer material rich in carboxyl functional groups to obtain a mixed solution C; Step 3: Curing the mixed solution C obtained in step 2 by ultraviolet light to obtain a glassy adhesive.

2. The method for preparing a vitreous adhesive according to claim 1, characterized in that: The polymer monomer in step 1 is an acrylic substance, including at least one of acrylic acid AA, methyl acrylate MA, ethyl acrylate EA, butyl acrylate BA, acrylamide AAm, 2-hydroxyethyl acrylate HEA and hydroxyethyl methacrylate HEMA.

3. The method for preparing the vitreous binder according to claim 1, characterized in that: In the step 1, the photoinitiator is at least one of I2959, 1-hydroxycyclohexyl phenyl ketone, and diphenyl-(2,4,6-trimethylbenzoyl)phosphine oxide.

4. The method for preparing a vitreous binder according to claim 1, characterized in that: In the step 1, the ionic liquid is at least one of [EMIM][ESO4], [EMIM][DEP], [EMIM][TFSI] and [EMIM][PF6] or is compounded in a mass ratio of 1:

1.

5. The method for preparing the vitreous binder according to claim 1, characterized in that: The polymer material rich in carboxyl functional groups in step 1 is at least one of sodium alginate Alg, sodium carboxymethyl cellulose CMC, polyacrylic acid PAA, or Alg-Li, CMC-Li, and PAA-Li after lithiation dialysis treatment.

6. The method for preparing a vitreous adhesive according to claim 1, characterized in that: The mixing method in step 2 is specifically stirring at 500-1000 rpm for 0.5-1.5 h.

7. The method for preparing a vitreous adhesive according to claim 1, characterized in that: The parameters of the ultraviolet curing in step 3 are power 40%-60% and time 10-60s.

8. A vitreous adhesive, characterized in that: Prepared by any of the methods described in claims 1-7.

9. A lithium-ion battery silicon negative electrode, characterized in that: The glassy binder as described in claim 8 is used as the binder in the silicon negative electrode of the lithium-ion battery; wherein, after the electrode is coated, it needs to be cured by ultraviolet light again.

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