A polyacrylic acid-based adhesive with self-healing function and preparation method thereof

Abstract

The present invention relates to a polyacrylic acid-based binder with self-healing function and a preparation method thereof, belonging to the technical field of lithium-ion batteries. The raw materials of the binder are polyacrylic acid, sodium alginate, and sodium borate. The carboxyl groups of the polyacrylic acid in the binder combine with the hydroxyl groups of the sodium alginate to form hydrogen bonds, and the sodium borate is bonded to the polyacrylic acid and the sodium alginate respectively through boron ester bonds, thereby forming a three-dimensional cross-linked network structure. Through the cross-linking reaction, reversible dynamic multiple hydrogen bonds and boron ester bonds are formed by chemical cross-linking, and a three-dimensional spatial skeleton is formed by the extension of the molecular chain. The self-healing function of the hydrogen bonds and boron ester bonds simultaneously improves the structural stability and electrochemical performance of the micron silicon negative electrode.

Description

Technical Field

[0001] The invention relates to a polyacrylic acid-based adhesive with a self-healing function and a preparation method thereof, and belongs to the technical field of lithium ion batteries. Background Art

[0002] At present, renewable energy technologies have been developed as supplementary or alternative technologies to the fossil fuels commonly used in modern society. However, renewable energy is generated intermittently, which makes it impossible for the power grid to supply electricity on a large scale, for a long time, and periodically. Therefore, the research and development of large-scale energy storage systems is particularly important for the storage and conversion of renewable energy. Among the many energy storage systems, lithium-ion batteries, which have the advantages of long cycle life, high power density and the fastest charge and discharge rate, have been widely used in various electronic devices such as computers, mobile phones, power tools, industrial equipment, energy-saving hybrid ships, airplanes, drones, plug-in hybrid and pure electric vehicles in the field of large-scale energy storage and conversion. However, traditional graphite anodes are difficult to meet the growing demand for high-energy batteries. Micronized silicon (Si) has high theoretical capacity and low cost (4200mAh g -1 ) has attracted great interest from researchers as a promising anode material for next-generation lithium-ion batteries. Unfortunately, the large volume expansion (~300%) leads to a series of problems, including electrode pulverization, active material shedding, and the formation of a continuous solid-electrolyte interface (SEI). As a result, micronized Si anodes exhibit rapid capacity loss, low Coulombic efficiency, and poor cycling stability. An effective approach to address this problem is to employ polymer binders with higher elasticity. Various polymer binders have been reported as alternatives to traditional polyvinylidene fluoride (PVDF). Polymer binders such as polyacrylic acid (PAA), carboxymethyl cellulose (CMC), and conductive polymers significantly enhance the capacity and cycling stability of Si anodes due to their inherent elastic properties. In addition, the affinity of the polymer functional groups with the native oxides on the Si particle surface further improves the cycling stability of the electrode. However, due to the repeated extreme volume expansion and contraction suffered by the micronized Si anode and polymer binders, these polymer binders eventually undergo irreversible mechanical degradation and cracking. Summary of the Invention

[0003] In view of this, the present invention aims to provide a polyacrylic acid-based binder with self-healing properties and its preparation method. Through a cross-linking reaction, reversible dynamic multiple hydrogen bonds and boron ester bonds are formed through chemical cross-linking. The molecular chain is extended to form a three-dimensional spatial skeleton. The self-healing properties of hydrogen bonds and boron ester bonds simultaneously improve the structural stability and electrochemical performance of the micron silicon anode.

[0004] To achieve the above object, the technical solution of the present invention is as follows:

[0005] A polyacrylic acid-based adhesive with self-healing function. The raw materials of the adhesive are polyacrylic acid (PAA), sodium alginate (SA) and sodium borate. The carboxyl groups of the polyacrylic acid in the adhesive combine with the hydroxyl groups of the sodium alginate to form hydrogen bonds, and the sodium borate is bonded to the polyacrylic acid and sodium alginate respectively through boron ester bonds, thereby forming a three-dimensional cross-linked network structure.

[0006] Preferably, the mass ratio of polyacrylic acid, sodium alginate and sodium borate is 16:1-4:1-4.

[0007] Preferably, the molecular weight of the polyacrylic acid is 240,000 to 1,000,000.

[0008] A method for preparing a polyacrylic acid-based adhesive having a self-healing function according to the present invention comprises the following steps:

[0009] The polyacrylic acid aqueous solution and the sodium alginate aqueous solution are mixed and stirred at a stirring rate of 100 to 300 rpm at 40 to 60° C. for 8 to 20 hours, and then the sodium borate aqueous solution is added. The above conditions are maintained and stirring is continued for 8 to 20 hours. After the stirring is completed, a polyacrylic acid-based adhesive with self-healing function is obtained.

[0010] Preferably, the mass fraction of the polyacrylic acid aqueous solution is 8-10%, the mass fraction of the sodium alginate aqueous solution is 1-3%, and the mass fraction of the sodium borate aqueous solution is 1-3%.

[0011] Preferably, polyacrylic acid is added to deionized water, heated to 70-90° C. and stirred for 1-3 hours at a stirring rate of 100-300 rpm until completely dissolved to obtain a polyacrylic acid aqueous solution.

[0012] Preferably, sodium alginate is added to deionized water, heated to 40-60° C. and stirred for 1-3 hours at a stirring rate of 100-300 rpm until completely dissolved to obtain a sodium alginate aqueous solution.

[0013] A lithium ion battery, wherein the negative electrode material of the battery is micron silicon, and the negative electrode binder is the polyacrylic acid-based binder with self-healing function described in the present invention.

[0014] Beneficial effects

[0015] The present invention provides a polyacrylic acid-based binder with self-healing function. The binder has a large number of hydrogen bonds inside, and sodium borate is bonded to polyacrylic acid and sodium alginate through boron ester bonds, thereby forming a three-dimensional cross-linked network structure. The binder has a large number of dynamic hydrogen bonds and thus has the ability to self-repair after fracture; it can repair cracks generated in silicon electrodes during the charge and discharge process, so that the electrodes have excellent cycle stability.

[0016] The present invention prepares a polyacrylic acid / sodium alginate (PAA / SA / PB) binder containing a large number of hydrogen bonds through chemical crosslinking. The carboxyl groups on PAA and the hydroxyl groups on SA first form hydrogen bonds, and then sodium borate is added. The sodium borate chelates with the alginate and PAA to form a three-dimensional crosslinked network structure. The method has a simple operation process and low cost.

[0017] The present invention provides a lithium-ion battery, in which the negative electrode binder adopts PAA / SA / PB binder to enhance the cycle stability of Si negative electrode. During the lithiation process, SA molecules are combined with PAA through hydrogen bonds, and sodium borate is cross-linked with SA and PAA skeletons, thereby avoiding the separation between the active material and the binder due to strong stress release. During the delithiation process, the volume of the active material is reduced. The hydrogen bonds between SA and PAA ensure that the Si particles move only within a very small range, thereby improving the long cycle life of the micron Si negative electrode. The strong self-healing ability of PAA / SA / PB can be applied to micron Si electrode materials with high loading (~1.2 mg -1 cm -2 ), improving the commercialization capabilities of micron Si. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is the SEM image of PAA / SA / PB prepared in Example 1.

[0019] Figure 2 This is the SEM image of PAA-Li prepared in Comparative Example 1.

[0020] Figure 3 This is the SEM image of PAA prepared in Comparative Example 2.

[0021] Figure 4 This is the SEM image of CMC+SBR prepared in Comparative Example 3.

[0022] Figure 5 This is the SEM image of PVDF prepared in Comparative Example 4.

[0023] Figure 6 This is the charge-discharge specific capacity diagram of PAA / SA / PB in Example 1 in the voltage range of 0.01-1.5V and the first three cycles of 0.3C cycling.

[0024] Figure 7 This is the charge-discharge specific capacity diagram of PAA-Li in Comparative Example 1 in the voltage range of 0.01-1.5V and the first three cycles of 0.3C cycle.

[0025] Figure 8 This is the charge-discharge specific capacity diagram of PAA in Comparative Example 2 in the voltage range of 0.01-1.5V and the first three cycles of 0.3C cycle.

[0026] Figure 9 This is the charge-discharge specific capacity diagram of CMC+SBR in Comparative Example 3 in the voltage range of 0.01-1.5V and the first three cycles of 0.3C cycle.

[0027] Figure 10 This is the charge-discharge capacity diagram of comparative example 4, PVDF in the voltage range of 0.01-1.5V and the first three cycles of 0.3C cycle.

[0028] Figure 11 Graph showing the discharge capacity of PAA / SA / PB, PAA-Li, PAA, CMC+SBR, and PVDF in Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4 within a voltage range of 0.01-1.5 V and 0.3C cycled for 100 cycles.

[0029] Figure 12 Graph showing the discharge capacity of PAA / SA / PB, PAA-Li, PAA, CMC+SBR, and PVDF in Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4 within a voltage range of 0.01-1.5 V and 300 cycles at 0.5C.

[0030] Figure 13 This is the SEM top view of PAA / SA / PB in the embodiment in the voltage range of 0.01-1.5V and the first three weeks of 0.1C cycling.

[0031] Figure 14 This is the SEM top view of PAA-Li in Comparative Example 1 in the voltage range of 0.01-1.5 V and the first three weeks of 0.1C cycling.

[0032] Figure 15 This is the SEM image of PAA / SA / PB in Example 1 after 100 cycles at 0.1C in the voltage range of 0.01-1.5V.

[0033] Figure 16 This is the SEM image of PAA-Li in Comparative Example 1 after 100 cycles at 0.1C in the voltage range of 0.01-1.5V.

[0034] Figure 17 This is an electron micrograph of the Si-PAA / SA / PB electrode in Example 1 after 100 cycles at 0.3C in the voltage range of 0.01-1.5 V. DETAILED DESCRIPTION

[0035] The present invention will be further described in detail below with reference to specific embodiments.

[0036] In the following embodiments:

[0037] Fourier transform infrared spectroscopy (FTIR) test: Nicolet Magna AVATAR-360 infrared spectrometer was used in the range of 500-4000 cm -1 The test was conducted within the wave number range.

[0038] Scanning electron microscope test: A JSM-6360LV scanning electron microscope was used to test the morphology of the material.

[0039] Assembly and testing of CR2032 button batteries: Microsilicon with a particle size of 5-10 μm, acetylene black, and the binders of the comparative example and the embodiment are prepared into a slurry in a mass ratio of 8:1:1 and coated on aluminum foil. The dried aluminum foil loaded with the slurry is cut into small discs with a diameter of approximately 1 cm using a cutting machine for use as the positive electrode. A metallic lithium sheet is used as the negative electrode, Celgard 2300 is used as the separator, and a 1M carbonate solution is used as the electrolyte (wherein the solvent is a mixed solution of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1, and the solute is LiPF6). CR2032 button batteries are assembled in an argon glove box (water pressure ≤0.01 ppm, oxygen pressure ≤0.01 ppm); the assembled CR2032 button batteries are subjected to constant current charge and discharge tests at different current densities using a CT2001ALand battery tester, and the test temperature is 25°C.

[0040] Comparative Example 1

[0041] To prepare the PAA-Li binder, first weigh 2 g of a PAA aqueous solution (molecular weight: 450,000, mass fraction: 25%), then add 3 g of deionized water, stir at 250 rpm for 30 min, then add 1.16 g of LiOH (10%) solution to stabilize the pH at 7, and finally add 3.84 g of deionized water, stir at 250 rpm for 12 h to obtain PAA-Li with a mass fraction of 10%, which is Comparative Example 1.

[0042] Comparative Example 2

[0043] 0.5 g of PAA (molecular weight: 450,000) was dissolved in 9.5 g of deionized water, and then stirred in a water bath at 50° C. until the PAA was completely dissolved. The mixture was then stirred at room temperature for 12 h to obtain a PAA binder with a mass fraction of 5%, which was recorded as Comparative Example 2.

[0044] Comparative Example 3

[0045] A 5% by mass sodium carboxymethyl cellulose (CMC) binder was prepared using the same method, with deionized water as the solvent, which is recorded as Comparative Example 3.

[0046] Comparative Example 4

[0047] 0.5 g of PVDF was dissolved in 9.5 g of 1-methyl-2-pyrrolidone (NMP), and the mixture was stirred at room temperature for 48 h to obtain a PVDF binder with a mass fraction of 5%, which was recorded as Comparative Example 4.

[0048] Micron silicon, acetylene black and comparative examples 1-4 were mixed into a slurry and coated on copper foil to make electrodes denoted as Si-PAA-Li, Si-PAA, Si-CMC / SBR, and Si-PVDF. Figure 2 As shown in Figure 2, exposed micron silicon particles were observed on the surface of the electrode, indicating that the micron silicon particles were not tightly coated by the binder and the structure was relatively loose. Figure 3-5 As shown in the figure, there are more exposed micron silicon particles on the surface of the electrode, which means that the electrode structure is looser and the active material is likely to fall off from the electrode due to volume expansion during the cycle.

[0049] like Figure 7-10 As shown in the figure, in the voltage range of 0.01-1.5 V, the charge and discharge capacities of Si-PAA-Li, Si-PAA, Si-CMC / SBR and Si-PVDF electrodes at 0.3C cycle in the first cycle were 2483.8 mAh g -1 / 2050.3mAh g -1 、2911.8mAhg -1 / 2543.1mAh g -1 、3097.5mAh g -1 / 1827.6mAh g -1 、3140.5mAh g -1 / 2041.9mAh g -1 The first-cycle Coulombic efficiency is 82.55%, 87.34%, 59.00% and 65.01% respectively. It can be seen that the first-cycle Coulombic efficiency of Si-CMC / SBR and Si-PVDF electrodes is lower than that of PAA-based binder electrodes. This is mainly due to the weak covalent bond between the binder and the micron silicon, which is insufficient to accommodate the huge volume expansion during the charge and discharge process, resulting in the pulverization of the micron silicon into "dead silicon", which in turn leads to low first-cycle Coulombic efficiency. In addition, from the charge and discharge curves of the second and third cycles, the Si-CMC / SBR and Si-PVDF electrodes also experienced obvious capacity decay.

[0050] The cycling performance curves of Si-PAA-Li, Si-PAA, Si-CMC / SBR, and Si-PVDF electrodes in the voltage range of 0.01-1.5V and 0.3C are as follows: Figure 11As shown in the figure, it can be seen that all four electrodes show a trend of capacity decay, among which Si-CMC / SBR and Si-PVDF show the largest decay. After 100 cycles, the discharge specific capacities of Si-PAA-Li, Si-PAA, Si-CMC / SBR, and Si-PVDF electrodes are 1099.6 mAh g -1 , 504mAh g -1 , 17.9mAh g -1 , 138.2mAh g -1 The capacity decay is mainly attributed to the huge volume effect of silicon, which causes the electrode to pulverize and then the active material to fall off from the electrode surface, thus resulting in poor cycle stability.

[0051] The performance curves of Si-PAA-Li, Si-PAA, Si-CMC / SBR and Si-PVDF electrodes in the voltage range of 0.01-1.5V and 0.5C long cycle performance are as follows: Figure 12 As shown in Figure 2, the capacity of the four electrodes decayed rapidly at high current density. After 200 cycles, the discharge specific capacities of the Si-PAA-Li, Si-PAA, Si-CMC / SBR, and Si-PVDF electrodes were 319.5 mAh g -1 , 83.4mAh g -1 , 5.2mAh g -1 , 1.2mAh g -1 It can be seen that PAA-Li, PAA, CMC / SBR, and PVDF binders cannot withstand long-term cycling at high current density, and the active material falls off from the electrode to a large extent.

[0052] The electron microscopy images of the Si-PAA-Li electrode in the voltage range of 0.01-1.5V and the first three cycles at 0.3C are as follows Figure 13 As shown in the figure, after the first cycle, a few cracks appeared on the electrode surface. Electron micrographs after the second and third discharge cycles showed that severe cracks of 10μm to 20μm had formed on the electrode surface. Such cracks can cause the active materials to lose close contact with each other and then fall off from the current collector, resulting in irreversible capacity loss.

[0053] The electron microscope images of the Si-PAA electrode in the voltage range of 0.01-1.5V and the first three cycles at 0.3C are as follows Figure 14As shown in the figure, after the first discharge cycle, obvious cracks appeared on the electrode surface. After charging, the electrode surface became rough and uneven as the lithium ions de-alloyed. Electron microscopy images after the second and third discharge cycles show that the electrode has undergone a dramatic volume expansion, with an uneven surface. The electrode structure has become loose, and the active material easily falls off the electrode sheet, resulting in capacity loss.

[0054] The electron microscopy images of the Si-PAA-Li electrode after 100 cycles at 0.3C in the voltage range of 0.01-1.5V are shown in Figure 2. Figure 16 As shown in the figure, it can be clearly seen that there is a wide crack on the electrode surface, and the electrode surface is uneven, which further shows that the PAA-Li binder cannot adapt to the volume effect during the cycle, which is also the reason for the low capacity retention rate and capacity attenuation during the cycle.

[0055] Example 1

[0056] (1) 1.6 g of PAA was dissolved in 18.4 g of deionized water and stirred in a water bath at 80°C for 1.5 h at a stirring rate of 200 rpm. After complete dissolution, the heat was turned off and the mixture was cooled to room temperature while stirring to obtain a PAA aqueous solution.

[0057] (2) Dissolve 0.4 g of sodium alginate in 19.6 g of deionized water, stir for 1.5 h in a 45 °C water bath at a stirring rate of 200 rpm. After complete dissolution, turn off the heat and cool to room temperature while stirring to obtain a sodium alginate aqueous solution.

[0058] (3) The PAA aqueous solution and the sodium alginate aqueous solution were mixed and stirred for 1.5 h at a stirring rate of 200 rpm to obtain a mixed solution.

[0059] (4) 0.1 g of sodium borate was dissolved in 9.9 g of deionized water and stirred to dissolve, and then the mixed solution was added and stirred for 12 h at a stirring rate of 200 rpm. After the stirring was completed, a polyacrylic acid-based adhesive with self-healing function was obtained, which was recorded as PAA / SA / PB adhesive.

[0060] Micron silicon, acetylene black and PAA / SA / PB binder were mixed into a slurry and then coated on copper foil to make an electrode denoted as Si-PAA / SA / PB. Figure 1 As shown, it can be seen that compared with comparative examples 1-4, the surface of the Si-PAA / SA / PB electrode is tightly coated with the binder and connected to each other to form a relatively dense structure, and no exposed silicon particles are observed, indicating that the PAA / SA / PB binder has excellent adhesion to the active material and copper foil, which helps to maintain the stability of the electrode during the cycle.

[0061] like Figure 6 As shown in the figure, in the voltage range of 0.01-1.5 V, the charge and discharge capacity of the Si-PAA / SA / PB electrode in the first cycle of 0.3C cycle is as high as 3473.8 mAh g -1 / 3035.6mAh g -1 , the first-cycle coulombic efficiency is 87.3%. It can be seen that the charge and discharge specific capacity and the first-cycle coulombic efficiency of the Si-PAA / SA / PB electrode are significantly improved compared with comparative examples 1-4. This is mainly due to the self-healing properties of PAA / SA / PB, which inhibit the cracking and pulverization of micron silicon during the first-cycle charge and discharge process, placing the entire electrode in a three-dimensional network structure, avoiding the formation of "dead silicon", and thereby improving the first-cycle coulombic efficiency and charge and discharge specific capacity.

[0062] The cycling performance curve of Si-PAA / SA / PB electrode in the voltage range of 0.01-1.5V and 0.3C cycle is as follows Figure 11 As shown in the figure, it can be seen that the Si-PAA / SA / PB electrode has better cycle stability and the attenuation amplitude is the smallest. After 100 cycles, the discharge specific capacity of the Si-PAA / SA / PB electrode is 1863.5 mAh g -1 The long cycle performance curve of Si-PAA / SA / PB electrode in the voltage range of 0.01-1.5V and 0.5C cycle is as follows: Figure 12 As shown in Figure 2, the Si-PAA / SA / PB electrode exhibits excellent cycle stability and small capacity decay. After 200 cycles, the capacity of the Si-PAA / SA / PB electrode is stable at 1674.5 mAh g -1 . However, the capacity of the four electrodes in the comparative example showed a rapid decay at high current density, almost close to 0. It can be seen that the PAA / SA / PB binder has a significant effect on improving the cycle stability of the micron silicon negative electrode. This is mainly because in the process of silicon volume expansion and cracks, the large number of hydrogen bonds inside PAA / SA / PB have a self-repairing effect on the cracks, maintaining the three-dimensional cross-linked structure of the entire electrode, maintaining good adhesion between the active material and the electrode, and preventing the active material from falling off the copper foil surface.

[0063] The electron microscopy images of the Si-PAA / SA / PB electrode in the voltage range of 0.01-1.5V and the first three cycles at 0.3C are shown in Figure 2. Figure 15 As shown, it can be seen in the electron microscope image of the first cycle of discharge that a few cracks were generated, and this crack disappeared in the subsequent cycles, proving the self-healing effect of the PAA / SA / PB electrode.

[0064] The electron microscopy images of the Si-PAA / SA / PB electrode after 100 cycles at 0.3C in the voltage range of 0.01-1.5V are shown in Figure 2. Figure 17 As shown, it can be seen that after cycling, only tiny cracks appeared on the surface of PAA / SA / PB, and the surface was relatively flat. There were no obvious micron-level cracks and uneven electrode surfaces as shown in the electron microscope image of Comparative Example 1. This once again proves that PAA / SA / PB can effectively maintain the structural stability of the electrode during long-term cycling, thereby improving the cycle life.

[0065] In summary, the invention includes but is not limited to the above embodiments. Any equivalent replacement or partial improvement made under the spirit and principle of the present invention shall be deemed to be within the scope of protection of the present invention.

Claims

1. Application of a polyacrylic acid-based binder with self-healing function as a binder for micron silicon negative electrodes of lithium-ion batteries, characterized in that: The raw materials of the self-healing polyacrylic acid-based binder are polyacrylic acid, sodium alginate and sodium borate. The carboxyl groups of the polyacrylic acid in the self-healing polyacrylic acid-based binder are combined with the hydroxyl groups of the sodium alginate to form hydrogen bonds, and the sodium borate is combined with the polyacrylic acid and the sodium alginate respectively through boron ester bonds, thereby forming a three-dimensional cross-linked network structure. The mass ratio of polyacrylic acid, sodium alginate and sodium borate is 16:1-4:1-4; The molecular weight of polyacrylic acid is 240,000 to 1,000,000.

2. The use according to claim 1, characterized in that: The polyacrylic acid-based adhesive with self-healing function is prepared by the following method, which comprises the following steps: The polyacrylic acid aqueous solution and the sodium alginate aqueous solution are mixed and stirred at a stirring rate of 100 to 300 rpm at 40 to 60° C. for 8 to 20 hours, and then the sodium borate aqueous solution is added. The above conditions are maintained and stirring is continued for 8 to 20 hours. After the stirring is completed, a polyacrylic acid-based adhesive with self-healing function is obtained.

3. The use according to claim 2, characterized in that: The mass fraction of the polyacrylic acid aqueous solution is 8-10%, the mass fraction of the sodium alginate aqueous solution is 1-3%, and the mass fraction of the sodium borate aqueous solution is 1-3%.

4. The use according to claim 2, wherein: Add polyacrylic acid into deionized water, heat to 70-90° C., and stir for 1-3 hours at a stirring rate of 100-300 rpm until the polyacrylic acid is completely dissolved to obtain a polyacrylic acid aqueous solution.

5. The use according to claim 2, characterized in that: Sodium alginate is added into deionized water, heated to 40-60° C. and stirred for 1-3 hours at a stirring rate of 100-300 rpm until the water is completely dissolved to obtain a sodium alginate aqueous solution.

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