Organic silicon modified polyimide binder and preparation method and application thereof

Abstract

The invention discloses an organic silicon modified polyimide binder as well as a preparation method and application thereof, and belongs to the technical field of materials. Aiming at the problem that the mechanical property of a product is influenced by harsh reaction conditions in the existing method for preparing polyimide, the method comprises the following steps: stirring a purified DABA monomer and an NMP solvent in an ice bath until the DABA monomer and the NMP solvent are completely dissolved; then dropwise adding a diamino silane coupling agent NH2-DS, after stirring, dissolving and uniformly dispersing, adding purified 6FDA in four times, and continuously stirring for 24 hours; slowly adding a 3-methylpyridine / acetic anhydride cyclizing agent into the solution, continuously stirring for 24 hours to obtain a viscous solution, quickly stirring to obtain a solid, pressurizing, performing suction filtration, cleaning, and soaking in deionized water overnight; and removing residual reactants, and drying to obtain the organic silicon modified polyimide binder. The binder prepared by the invention has good cycling stability and rate capability, and has low interface and charge transfer resistance.

Description

Technical Field

[0001] The present invention belongs to the technical field of materials, and particularly relates to a silicone-modified polyimide binder, a preparation method and an application thereof. Background Art

[0002] Polyimide is a kind of cyclic chain compound. According to its main chain structure, it can be divided into polyimide containing aliphatic chains and polyimide containing aromatic ring chains. The main synthesis methods include melt polycondensation method, interfacial polycondensation method, vapor deposition method and solution polycondensation method. Among them, the solution polycondensation method is a typical method for preparing polyimide, which refers to the method of polymerizing reactants in a solvent, and generally can be divided into a one-step method and a two-step method. The one-step method is a reaction in which dianhydride and diamine monomers carry out polycondensation and dehydration cyclization simultaneously, and directly synthesize polyimide in one step. This method is often used in the synthesis of polyimide containing aliphatic chains. The two-step method divides the polycondensation and dehydration cyclization processes into two steps. The first step is to carry out a polycondensation reaction of dianhydride and diamine monomers in a solvent to obtain a polyamic acid precursor; the second step is to dehydrate and close the ring of the polyamic acid precursor by heating or using a chemical cyclizing agent to complete imidization and thus generate polyimide. The two-step method is often used in the preparation of polyimide containing aromatic ring chains.

[0003] The thermal imidization method refers to first dehydrating at a temperature below 100°C, and then carrying out a ring closure reaction of polyamic acid molecules at a high temperature of 250°C - 400°C to obtain polyimide. The advantage of the thermal imidization method is that it has simple operation, short reaction time and can obtain polyimide with a high degree of imidization. However, this method requires a very high reaction temperature, and bubbles will be generated during the reaction, thus affecting the mechanical properties of polyimide. The chemical imidization method refers to using an acid anhydride as a dehydrating agent and pyridine or triethylamine as a catalyst to carry out an imidization reaction with a polyamic acid precursor in a solution to generate polyimide. Although the chemical imidization method requires a longer reaction time, usually the reaction process needs to be more than 4 hours, but the advantage of chemical imidization is that it can carry out an imidization reaction at a low temperature, avoiding phenomena such as brittleness and fragility of the material, and can more accurately control the degree of imidization. During the process of preparing polyimide by the two-step method, the cyclization step of polyamic acid is very crucial and can affect the quality of the properties of polyimide itself. Summary of the Invention

[0004] Aiming at the problem that the reaction conditions in the current method for preparing polyimide are harsh and affect the mechanical properties of the product, the present invention provides a silicone-modified polyimide binder, a preparation method and an application thereof.

[0005] To solve the above technical problems, the present invention adopts the following technical solutions:

[0006] A preparation method of a silicone-modified polyimide binder, comprising the following steps:

[0007] Step 1: Vacuum dry and purify 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 3,5-diaminobenzoic acid (DABA) monomers.

[0008] Step 2: Stir the purified 3,5-diaminobenzoic acid (DABA) monomer with N-methylpyrrolidone (NMP) solvent in an ice bath until completely dissolved.

[0009] Step 3: Dropwise add the diamino silane coupling agent 1,3-bis(aminopropyl)tetramethyldisiloxane (NH 2 -DS). After stirring until dissolved and evenly dispersed, add the purified 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) in four portions and continuously stir for 24 h.

[0010] Step 4: Slowly add the 3-methylpyridine / acetic anhydride cyclizing agent to the solution and continue stirring for 24 h to obtain a viscous solution.

[0011] Step 5: Slowly pour the obtained viscous solution in a thin stream into deionized water and stir rapidly to obtain a solid. Cut the obtained solid into pieces, subject it to pressure filtration and washing, and then soak it in deionized water overnight.

[0012] Step 6: Filter with suction and rinse multiple times to remove residual reactants, and vacuum dry at 50 °C for 12 h to obtain an organosilicon-modified polyimide binder.

[0013] Furthermore, the molar ratio of 3,5-diaminobenzoic acid (DABA) to the diamino silane coupling agent is 8:2.

[0014] Furthermore, the molar ratio of 3,5-diaminobenzoic acid (DABA) and the diamino silane coupling agent to 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) is 1:1.02.

[0015] Furthermore, the solute content in Step 3 is 20 wt%.

[0016] Furthermore, Steps 1 to 6 are all carried out under nitrogen protection.

[0017] Furthermore, the molar ratio of 3-methylpyridine / acetic anhydride to 4,4′-(hexafluoroisopropylidene)diphthalic anhydride is 1:2, 1:1, and 10:1.

[0018] Furthermore, the vacuum drying and purification in Step 1 is specifically carried out by vacuum drying at 150 °C for 24 h for purification.

[0019] Further, in step 3, an appropriate amount of NMP is used each time to rinse the small amount of monomers adhering to the bottle wall, and wait for the monomers to be fully dissolved and dispersed before proceeding to the next step.

[0020] An organosilicon-modified polyimide binder prepared by the above preparation method.

[0021] Application of an organosilicon-modified polyimide binder prepared by the above preparation method in a silicon negative electrode.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] The present invention uses an improved chemical imidization method to synthesize four groups of organosilicon-modified polyimide binders with theoretical cyclization degrees of 0%, 25%, 50%, and 100% respectively by using different proportions of cyclizing agents. The prepared SP-25, SP-50, and SP-100 binders exhibit higher mechanical strength, modulus, and adhesion strength compared to the uncyclized SP-0 binder, and the electrochemical cycle stability, initial Coulombic efficiency, and rate performance are all significantly improved.

[0024] Among them, the SP-50 binder prepared by the present invention performs optimally in the silicon negative electrode. At a current density of 0.2C, the initial discharge specific capacity of the SP-50 electrode is 2877.9 mAh g-1, and the specific capacity after 100 cycles is 2591.6 mAh g-1, showing good cycle stability during charge and discharge. Under the same conditions, the capacity retention rate of the SP-0 electrode is only 48%. The initial Coulombic efficiency of the SP-50 electrode is 86%, and the Coulombic efficiency quickly stabilizes above 98% in subsequent cycles. By comparing the rate performance and electrochemical impedance spectrum, it can be seen that the SP-50 electrode maintains good cycle stability and rate performance, and has a lower interfacial and charge transfer resistance. At the same time, the overall electrochemical performance of the SP-50 electrode is better than that of SP-100, indicating that the improvement of electrochemical performance is not completely related to the increase in the degree of imidization. Description of the Drawings

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0026] Figure 1 It is a schematic flow chart of the reaction synthesis of the binder;

[0027] Figure 2 It is the 1H NMR spectra of two groups of modified polyimide polymers with a theoretical cyclization degree of 50% and full cyclization;

[0028] Figure 3 are the FTIR spectra of SP-0, SP-25, SP-50, and SP-100;

[0029] Figure 4 are the TGA curves of the SP-0, SP-25, SP-50, and SP-100 binders;

[0030] Figure 5 are the full XPS scan spectra of the SP-0, SP-25, SP-50, and SP-100 binders;

[0031] Figure 6 are the C1s spectral fitting curves of the SP-0, SP-25, SP-50, and SP-100 binders;

[0032] Figure 7 are the O1s spectral fitting curves of the SP-0, SP-25, SP-50, and SP-100 binders;

[0033] Figure 8 are the nanoindentation load-depth test curves of the SP-0, SP-25, SP-50, and SP-100 binders;

[0034] Figure 9 is a schematic diagram of the electrochemical performance characterization results of silicone-modified polyimide binders with different degrees of cyclization in the silicon anode of lithium-ion batteries;

[0035] Figure 10 are the SEM images of the SP-0, SP-25, SP-50, and SP-100 electrodes before cycling and after 100 cycles at a current density of 0.2C;

[0036] Figure 11 are the electrochemical impedance spectra of the SP-0, SP-25, SP-50, and SP-100 electrodes. Detailed implementation mode

[0037] To gain a deep understanding of the present invention, we will describe it comprehensively and meticulously. However, the present invention has multiple implementation manners and is not limited to the specific examples listed herein. The presentation of these examples aims to deepen the comprehensive understanding of the disclosed content of the present invention.

[0038] A preparation method of a silicone-modified polyimide binder includes the following steps:

[0039] Step 1, vacuum dry and purify 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) and 3,5-diaminobenzoic acid (DABA) monomers at 150°C for 24 h;

[0040] Step 2: Stir the purified 3,5-diaminobenzoic acid (DABA) monomer with N-methylpyrrolidone (NMP) solvent in an ice bath until it is completely dissolved.

[0041] Step 3: Dropwise add the diamino silane coupling agent 1,3-bis(aminopropyl)tetramethyldisiloxane (NH 2 -DS). After stirring until it is dissolved and evenly dispersed, add the purified 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) in four portions. Each time, rinse the small amount of monomer sticking to the bottle wall with an appropriate amount of NMP, and wait until the monomer is fully dissolved and dispersed before the next addition. Continuously stir for 24 h; the solute content is 20 wt%.

[0042] Step 4: Slowly add the 3-methylpyridine / acetic anhydride cyclizing agent to the solution and continue stirring for 24 h to obtain a viscous solution.

[0043] Step 5: Slowly pour the obtained viscous solution into deionized water in a thin stream while rapidly stirring to obtain a solid; cut the obtained solid into pieces, subject it to pressure filtration and washing, and then soak it in deionized water overnight.

[0044] Step 6: Perform suction filtration and washing multiple times to remove the residual reactants, and vacuum dry at 50 °C for 12 h to obtain the organosilicon-modified polyimide binder. Steps 1 to 6 are all carried out under nitrogen protection.

[0045] Furthermore, the molar ratio of 3,5-diaminobenzoic acid (DABA) to the diamino silane coupling agent is 8:2.

[0046] Furthermore, the molar ratio of 3,5-diaminobenzoic acid (DABA) and the diamino silane coupling agent to 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) is 1:1.02.

[0047] The molar ratio of 3-methylpyridine / acetic anhydride to 4,4′-(hexafluoroisopropylidene)diphthalic anhydride is 1:2, 1:1, and 10:1.

[0048] Synthesis of organosilicon-modified polyimide binders with different degrees of cyclization in Example 1

[0049] The reaction synthesis process of the organosilicon-modified polyimide binder with different degrees of cyclization is as Figure 1 shown.

[0050] First, vacuum dry the 4,4′-(hexafluoroisopropylidene)diphthalic anhydride and 3,5-diaminobenzoic acid monomer at 150 °C for 24 h for purification.

[0051] Then, pour the weighed DABA into a three-necked flask containing NMP solvent and stir in an ice bath until it is completely dissolved.

[0052] Next, dropwise add the diamino silane coupling agent 1,3-bis(aminopropyl)tetramethyldisiloxane (NH 2 -DS). After stirring until it is dissolved and evenly dispersed, add 6FDA in four portions. Each time, an appropriate amount of NMP can be used to rinse the small amount of monomers sticking to the wall of the three-necked flask, and wait until the monomers are fully dissolved and dispersed before adding the next batch. After all the monomers are added and stirred until completely dissolved, remove the ice bath and continue stirring. Among them, the total molar ratio of DABA and silicone to 6FDA is 1:1.02, the molar ratio of DABA to silicone is 8:2, and the solute content is 20 wt%.

[0053] After continuously stirring for 24 h, slowly add four groups of 3-methylpyridine / acetic anhydride cyclizing agents with different contents to the solution, and continue stirring for 24 h. Among them, the molar ratios of acetic anhydride to 6FDA are 0, 1:2, 1:1, and 10:1 respectively. The above processes are all carried out under nitrogen protection.

[0054] Slowly pour the obtained viscous solution into deionized water in a thin stream, and stir rapidly. Cut the obtained solid into pieces, filter it under pressure and wash it, and then soak the obtained sample in deionized water overnight.

[0055] Filter and wash multiple times to remove the residual reactants, and finally dry it in vacuo at 50 °C for 12 h to obtain the organosilicon-modified polyimide copolymer binders with different degrees of cyclization.

[0056] Theoretically, according to the addition amount of the chemical cyclizing agent, the degrees of cyclization of the synthesized polymers should be 0%, 25%, 50%, and 100% respectively, and they are named SP-0, SP-25, SP-50, and SP-100 according to the theoretical degrees of cyclization. For convenient use, the four groups of binder samples can be dissolved in N-methylpyrrolidone (NMP) at a ratio of 0.05 mg per milliliter and sealed for use.

[0057] Preparation, assembly and battery testing of the electrode in Example 2

[0058] Preparation of the electrode: A coin cell was used for half-cell testing. Silicon nanoparticles were used as the active material, SuperP conductive carbon black was used as the conductive agent, and the synthesized organosilicon-modified polyimide polymer was used as the binder. They were dispersed in NMP solvent at a mass ratio of 8:1:1, and the mixture was ground and mixed into a homogeneous slurry with good dispersibility. Then, the slurry was coated on copper foil using a film coater and dried in vacuo at 60 °C for 12 h. The dried copper foil electrode was cut into several discs with a diameter of 10 mm to form four groups of electrodes to be tested. For convenient recording, the names of the four groups of electrodes are the same as those of the corresponding binders.

[0059] Assembly of the electrode: All the electrochemical properties of the silicon negative electrode were tested using a 2032 coin cell with a lithium foil as the counter electrode, a Celgard 2400 sheet as the separator, and the prepared silicon electrode as the working electrode. The assembly of the coin cell was completed in a glove box under an argon atmosphere protection, with the water content and oxygen content both less than 0.1 ppm. The electrolyte was a mixed solution of 1 M lithium hexafluorophosphate (LiPF 6 ) dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of 1:1 and 5% fluoroethylene carbonate (FEC) additive.

[0060] Battery testing: To characterize the electrochemical properties of the battery, the constant current charge-discharge cycle test of the coin cell was carried out using a BlueTEC battery analysis system, with a voltage window of 0.01 V - 1.5 V. The rate performance was tested by cycling 5 laps at 0.1 C (420 mAh g -1 ), 0.2 C, 0.5 C, 1 C, and 2 C respectively. The cyclic voltammetry curve and electrochemical impedance spectroscopy were tested using an electrochemical workstation, with a voltage window of 0.01 V - 1.5 V and a scan rate of 0.1 mV s -1 , and the frequency range was 10 -2 -10 5 Hz. All the electrochemical characterizations were carried out in an air-conditioned room environment at 25 °C.

[0061] In summary, in this example, through an improved chemical cyclization method, the ratio of the dehydrating agent and the catalyst was regulated, and organosilicon-modified polyimide polymers with theoretical cyclization degrees of 0%, 25%, 50%, and 100% were synthesized. Their structures, thermal stabilities, mechanical properties, and electrochemical properties as the silicon negative electrode binder were characterized, and an attempt was made to discuss the influence of different cyclization degrees on the polymer characteristics and the binder performance.

[0062] SP-0, SP-25, SP-50, and SP-100 are four groups of polymers with different cyclization degrees synthesized by adding different proportions of acetic anhydride dehydrating agent in the same batch of reactions. Since the reaction conditions such as synthesis temperature, time, and solvent are completely the same, the molecular weights of the obtained polymers should be approximately the same.

[0063] Table 1 shows the table of the molecular weights and their dispersion characteristics of SP-0, SP-25, SP-50, and SP-100 polymers obtained by GPC testing

[0064]

[0065] It can be seen that the four groups of polymers have relatively high and similar molecular weights and low polydispersity. The high molecular weight components can provide excellent mechanical properties, which is beneficial to their application as binders in the anode materials of lithium-ion batteries. Since the relative molecular weights of SP-0, SP-25, SP-50, and SP-100 are similar and they are synthesized under the same reaction conditions in the same batch, it indicates that the self-properties of the four groups of polymers are related to their structures.

[0066] Example 3 characterized the chemical structure and component composition by performing 1 1H-NMR tests on organosilicon-modified polyimide polymers with different degrees of cyclization.

[0067] Since the chemical structures of the incompletely cyclized organosilicon-modified polyimide polymers are similar, in this example, two groups of samples, SP-50 and SP-100, were used for structural comparison.

[0068] Figure 2 1H-NMR spectra of two modified polyimide polymers with a theoretical degree of cyclization of 50% and complete cyclization. It can be seen from the figure that there are six vibration peaks common to all polymers. Among them, the two proton peaks of 1 (-CH 3 ) and 2 (-CH 2 ) connected to silicon are located at 0 ppm and 0.46 ppm respectively, the proton peak of 3 (-CH 2 ) connected to silicon appears at 1.55 ppm, and the proton peak of 5 (-CH 2 ) connected to the amino group appears at 3.48 ppm. These are all characteristic peaks of the bis-amino siloxane segment introduced by organosilicon, indicating that organosilicon copolymerizes with the polyimide main chain. The broad proton peak at 6 in the figure is caused by the proton vibration of the benzene ring in the monomer, and it can be seen that the spectra of SP-50 and SP-100 polymers show split peaks of varying degrees at this position. This is mainly due to the different imidization structures of the two groups of polymers and the embedding of the siloxane segment, resulting in different vibrations caused by the different chemical environments of the H atoms on the benzene ring. At the same time, the broad proton peak at 8 in the figure is caused by the carboxyl group in the diamine monomer. In addition, by comparison, the 1H-NMR spectrum of the SP-50 polymer has two more vibration peaks than that of the SP-100. Among them, the 4 (-NH) at 2.27 ppm is the proton peak connected to N, and the 7 (-COOH) at 10.74 is the proton peak on the carboxyl group. This is mainly because there is an amic acid structure in the SP-50 polymer, which further indicates that the amic acid group in the SP-100 molecule disappears and the imidization is completed in terms of structure. This means that there are differences in the structures of organosilicon-modified polyimide polymers with different degrees of cyclization.

[0069] The imidization degrees of four groups of polymers, namely SP-0, SP-25, SP-50, and SP-100, were characterized by FTIR testing. Figure 3 It is the infrared spectrum of organosilicon-modified polyimide polymers with different degrees of cyclization. Generally, polyimide has an asymmetric stretching vibration peak of C=O at 1780 cm -1 and a symmetric stretching vibration peak of C=O bond at 1725 cm -1 There is a C-N stretching vibration peak at 1360 cm -1 and a C-O bending vibration peak at 723 cm -1 They are all characteristic absorption bands of polyimide and are manifestations of the presence of imide groups. The absorption peaks at 1661 cm -1 and 1556 cm -1 are attributed to the amide I band and amide II band of polyamic acid respectively, which are characteristic absorptions of polyamic acid. As can be seen from Figure 3 , with the increase of the chemical cyclization degree, the characteristic peaks of polyimide gradually become stronger, and the characteristic peaks of polyamic acid gradually become weaker, indicating that the amide groups are gradually converted into imide rings, and the organosilicon-modified polyimide polymers have completed different degrees of cyclization. The absorption peak at 1260 cm -1 comes from the stretching vibration of C-F in the dianhydride monomer. Since its symmetry does not change during the amide-imideization process and the peak value changes little, it can be used as an internal standard for calculating the cyclization degree. Therefore, according to the infrared spectrum, the imidization degree (ID) can be calculated using the following formula:

[0070]

[0071] where S 1360 is the stretching vibration peak intensity of the imide C-N bond, S 1260 is the stretching vibration peak intensity of the C-F bond in the dianhydride monomer, i is the polymer with different cyclization degrees, and ∞ is the polymer after complete cyclization. The actual cyclization degrees of the four groups of polymers, SP-0, SP-25, SP-50, and SP-100, can be calculated using the formula, as shown in Table 2. By comparison, it is found that the actual values of the imidization degrees of the polymers synthesized by the chemical cyclization method are consistent with the theoretical values.

[0072] Table 2 shows the imide degrees of the SP-0, SP-25, SP-50, and SP-100 binders

[0073]

[0074] To further illustrate the influence of the cyclization degree on the structure and properties, TGA characterization was carried out on organosilicon-modified polyimide polymers with different cyclization degrees, as shown in Figure 4As shown. Through thermogravimetric curve analysis, it can be seen that in the range of 30°C to 100°C, SP-0, SP-25, and SP-50 exhibit the first thermal weight loss, which is mainly caused by the volatilization of free water molecules contained in them. In the range of 100°C to 300°C, SP-0, SP-25, and SP-50 exhibit the second thermal weight loss, which is mainly due to the dehydration of polyamic acid during the imidization reaction and the volatilization of the solvent remaining in the polymer. Therefore, the SP-0, SP-25, and SP-50 polymers all have incompletely cyclized structures. Starting from around 450°C, the four groups of polymers, namely SP-0, SP-25, SP-50, and SP-100, all exhibit thermal weight loss. This stage is caused by the decomposition and fracture of the ether bond structure and imide ring structure in the imidized polymer during heating. It can be seen that the thermal stabilities exhibited by fully cyclized polyimide and partially cyclized polyimide are different. The initial decomposition temperature of SP-100 is 448°C, and those of SP-0, SP-25, and SP-50 are 437°C, 441°C, and 445°C respectively, and with the increase in the degree of cyclization, the proportion of thermal weight loss decreases.

[0075] To characterize the structural features of silicone-modified polyimide binders with different degrees of cyclization, the XPS full-spectrum scanning spectra of SP-0, SP-25, SP-50, and SP-100 are as Figure 5 shown. As can be seen from the figure, the full-spectrum scanning diagrams of the four groups of polymers all contain five elements: C, N, F, O, and Si.

[0076] The high-resolution fine scans of the C and O elements in the four groups of polymers with different degrees of cyclization are carried out respectively. Figure 6Figures a - d are the C elemental peak fitting diagrams of SP - 0, SP - 25, SP - 50, and SP - 100 polymers respectively. After fitting, 6 peaks are obtained for each. Respectively, the fitting peaks at binding energy positions 284.02 eV, 283.94 eV, 283.92 eV, and 283.53 eV correspond to the C - Si group; the fitting peaks at binding energy positions 284.61 eV, 284.57 eV, 284.48 eV, and 284.33 eV correspond to the C - C and C - H groups. The benzene ring and diamino siloxane in 3,5 - diaminobenzoic acid (DABA) introduce the C - C and C - H groups; the fitting peaks at binding energy positions 285.21 eV, 285.19 eV, 285.10 eV, and 285.02 eV correspond to the C - N and C - C groups. The C - C group here is introduced by the benzene ring in 6FDA and is shifted due to the influence of the imide group; the fitting peaks at binding energy positions 286.09 eV, 286.07 eV, 286.04 eV, and 285.74 eV correspond to the C - O group, and the fitting peaks at binding energy positions 288.15 eV, 288.16 eV, 288.19 eV, and 288.21 eV correspond to the C═O group. The fitting peaks at binding energy positions 292.59 eV, 292.56 eV, 292.55 eV, and 292.47 eV correspond to the C - F group. The content of the C - N group increases with the increase in the proportion of the cyclizing agent, indicating an increase in the imidization degree of the silicone - modified polyimide binder. The peak positions of the carbon - related groups in the binders with different cyclization degrees show different degrees of shift, indicating that different cyclization degrees cause changes in the atomic binding energy environment.

[0077] Figure 7 Figures a - d are the O elemental peak fitting diagrams of SP - 0, SP - 25, SP - 50, and SP - 100 polymers respectively. Similar to the C element, 3 peaks are obtained after fitting the O1s spectra of the four groups of polymers. Different binding energy positions are attributed to three groups: Si - O (530.74 eV, 530.68 eV, 530.65 eV, and 530.58 eV), C═O (531.58 eV, 531.53 eV, 531.48 eV, and 531.39 eV), and C - O (532.77 eV, 532.65 eV, 532.56 eV, and 532.27 eV). The presence of the Si - O group indicates that the siloxane segment has successfully copolymerized with the polyimide monomer.

[0078] Mechanical Property Tests of Silicone - Modified Polyimide Polymer Binders with Different Cyclization Degrees in Example 4

[0079] To study the mechanical properties of silicone-modified polyimide polymer binders with different degrees of cyclization, nanoindentation tests were performed on SP-0, SP-25, SP-50, and SP-100, and the corresponding data were analyzed and calculated using software. Figure 8 Figure 2 shows the nanoindentation load-depth test curves of SP-0, SP-25, SP-50, and SP-100. It can be seen from the comparison in the figure that under the same external force, the deformation of SP-100 is the smallest, followed by SP-50 and SP-25, and the deformation of SP-0 is the largest. When the external force is withdrawn, the resilience performance of SP-100 is also the best, with less irreversible deformation than the other three groups. This indicates that with the decrease in the degree of cyclization, the silicone-modified polyimide becomes more flexible. The mechanical property parameters obtained from the nanoindentation test are shown in Table 3.

[0080] Table 3 Er, H, and h of the binders of SP-0, SP-25, SP-50, and SP-100 f Value

[0081]

[0082] Among them, Er is the elastic modulus, H is the hardness, and h f is the deformation after the external force is withdrawn. It can be seen that SP-100 has higher hardness and elastic modulus compared to SP-0, SP-25, and SP-50. Based on the above analysis, as the degree of cyclization increases, the molecules of the silicone-modified polyimide polymer become more rigid, and its hardness and elastic modulus increase accordingly; but as the degree of cyclization decreases, the incompletely cyclized polyamic acid molecules contain acyl amino acid groups, making the polymer more flexible. Higher mechanical strength is beneficial for inhibiting the volume expansion of the silicon active material and maintaining the stability of the electrode structure during charge and discharge after the polymer binder is mixed with silicon nanoparticles to form the electrode material. And the more flexible polymer binder will be more adaptable to the volume change of silicon during the cycle, which is beneficial for maintaining the integrity of the electrode.

[0083] Silicone-modified polyimide binders with different degrees of cyclization were mixed with silicon nanoparticles and then coated on copper foils to make electrode samples of a certain specification, and 180° peel strength tests were carried out to investigate the interaction between the binder and the active material and the current collector, and to determine its bonding ability in the electrode. The adhesion strength is shown in Table 4.

[0084] Table 4 Electrode adhesion strength and corresponding swelling rate of the binders of SP-0, SP-25, SP-50, and SP-100

[0085]

[0086] The peel strengths of the silicon electrodes prepared with SP-0, SP-25, SP-50, and SP-100 binders are 0.68 N cm -1 , 0.84 N cm -1 , 1.57 N cm -1 , and 1.18 N cm -1 respectively. Among them, the SP-50 electrode has the highest peel strength, while the SP-0 has the lowest. This means that as the degree of cyclization increases, the interaction between the silicone-modified polyimide binder and the active material and current collector becomes stronger, which is beneficial to maintaining the stability of the electrode structure, maintaining the integrity of the silicon negative electrode during the cycling of lithium-ion batteries, reducing the shedding of electrode materials, and thus improving the cycling stability of the silicon electrode. However, theoretically speaking, with the increase in the degree of cyclization, the proportion of imide structures in the polymer molecules increases, the amic acid groups gradually disappear, and the rigidity of the polymer molecules increases accordingly, which should be more conducive to the increase in viscosity. In this group of polymers, the peel strength of the SP-50 electrode with a cyclization degree of 50% is higher than that of the fully cyclized S-100 electrode, which shows that the adhesion effect is closely related to the groups carried by the binder molecules in addition to the mechanical properties of the binder itself. The SP-50 binder has more polar groups such as -NH and -COOH and an open-ring structure, which can form strong intermolecular hydrogen bond interactions with silicon nanoparticles and copper foils, improving the adhesion strength of the electrode. At the same time, SP-0, SP-25, SP-50, and SP-100 binders were immersed in the electrolyte for 48 h respectively, and the swelling rate was tested. The statistical results are shown in Table 4. Generally speaking, the fully cyclized SP-100 binder has the smallest swelling rate, and the SP-50 is the second and the difference is not large, which means that the binder has strong intermolecular forces and stability, can effectively maintain the integrity of the electrode structure, reduce side reactions with the electrolyte, and thus ensure the long cycling stability of the silicon negative electrode.

[0087] Example 5 Electrochemical Performance Test of Silicone-Modified Polyimide Binders with Different Degrees of Cyclization in Silicon Anodes of Lithium-Ion Batteries

[0088] The electrochemical performance of silicone-modified polyimide binders with different degrees of cyclization in silicon anodes of lithium-ion batteries was characterized by a Blue Power battery test system and an electrochemical workstation.

[0089] Figure 9 In a, the cycling performance curves of silicon electrodes with binders of different degrees of cyclization under constant current charge and discharge are shown. The current density is 0.2 C, and the voltage window is 0.01 V - 1.5 V. As can be seen from the figure, the SP-0, SP-25, SP-50, and SP-100 silicon anodes all show good initial discharge specific capacities, which are 2762.5 mAh g -1 , 2782.1 mAh g -1, 2877.9 mAh g -1 and 2794.6 mAh g -1 , the discharge specific capacities of the electrodes after 100 cycles are 1312.9 mAh g -1 , 1877.6 mAh g -1 , 2591.6 mAh g -1 and 2435.5 mAh g -1 , and the corresponding capacity retention rates are 48%, 67%, 90% and 87% respectively. It can be observed that the SP-50 electrode has excellent cycle stability, while the SP-0 electrode shows rapid specific capacity decay. Overall, the higher the degree of polymer cyclization, the better the cycle stability as the binder for silicon anodes. At the same time, compared with the fully cyclized binder SP-100, the SP-50 electrode shows more excellent cycle stability in terms of cycling performance. This is because SP-50 has similar strength and modulus to SP-100, while having better flexibility and more abundant functional groups such as carboxyl, hydroxyl and amino groups, which can form stronger polar interactions with silicon nanoparticles, facilitating the binder to have a greater degree of deformation to adapt to the volume expansion of silicon nanoparticles during charge and discharge, maintaining the integrity of the electrode structure and improving the cycle stability of the electrode.

[0090] Figure 9 In [figure reference], b shows the first Coulombic efficiency and cycle Coulombic efficiency curves of silicon anodes with organosilicon-modified polyimide binders with different degrees of cyclization at a current density of 0.2 C, and the voltage window is 0.01 V - 1.5 V. It can be seen from the figure that the silicon anodes with SP-0, SP-25, SP-50 and SP-100 binders all have good first Coulombic efficiencies, all above 80%, and the first Coulombic efficiency of the SP-50 electrode is 86%. In the subsequent cycles, the Coulombic efficiency of the lithium-ion battery is quickly stable. It can be seen from the cycle Coulombic efficiency curve that during the entire charge and discharge process, the Coulombic efficiencies of the SP-0, SP-25, SP-50 and SP-100 silicon anodes are all maintained above 98%, showing good cycle stability. It can be seen that with the increase in the degree of cyclization, the first Coulombic efficiency of the silicon anodes with organosilicon-modified polyimide binders increases accordingly. However, relatively speaking, the Coulombic efficiency of the SP-50 electrode is significantly higher than that of the SP-100 electrode, indicating that SP-50 as the binder for silicon anodes in lithium-ion batteries can have good binding performance and more excellent interfacial stability during the first cycle, which is conducive to fully activating and forming a stable SEI film on the electrode surface, providing a reversible transmission channel for lithium ions. This is mainly because the SP-50 binder has better adhesion strength and elasticity compared with the SP-100 binder, which helps to relieve the electrode structure expansion during cycling and maintain better stability, thus improving the cycle Coulombic efficiency.

[0091] To further investigate the influence of different degrees of cyclization on the electrochemical performance of the polymer binder, the rate performance of four groups of binder electrodes was characterized at current densities of 0.1C, 0.2C, 0.5C, 1C, and 2C, respectively, with a voltage window of 0.01V - 1.5V. Figure 9 c shows the rate performance curves of SP-0, SP-25, SP-50, and SP-100 electrodes. As can be seen from the figure, the SP-50 silicon anode exhibits higher specific capacities at different current densities. Especially at high current densities, the SP-50 anode still maintains a relatively high specific capacity, showing more advantages than other binders. To exclude the influence of cycling performance, by comparing the change in specific capacity between two adjacent cycles at different rates, it can be known that the specific capacity retention rates of the SP-50 electrode at 0.2C, 0.5C, 1C, and 2C rates are 91%, 89%, 87%, and 67%, respectively, showing more stable rate performance than the SP-0, SP-25, and SP-100 electrodes. When the current density changes from 2C to 0.1C, the specific capacities of the SP-0, SP-25, SP-50, and SP-100 electrodes can all recover well to a specific capacity close to that at the initial current density. At the same time, it can be observed that the uncyclized SP-0 electrode has poor rate performance, with a large decrease in specific capacity at different current densities and a rapid performance decay. However, the rate performance and specific capacity retention rate of the cyclized SP-25, SP-50, and SP-100 binder electrodes have been improved, and SP-50 is better than SP-100. From the above results, it can be known that the SP-0, SP-25, SP-50, and SP-100 binders all provide good charge-discharge cycle reversibility for the silicon anode, and the imidization process improves the rate performance of the organosilicon-modified polyimide binder. SP-50 has more excellent rate performance than SP-100 at different current densities, indicating that the SP-50 binder can form better interactions with silicon nanoparticles, endowing the electrode with higher mechanical stability, maintaining electrode integrity, and having excellent electrolyte absorption, which can enhance the SEI film on the electrode surface, promote interfacial ion transport, and thus obtain better cycle performance.

[0092] Figure 9 d shows the CV curve of the SP-50 electrode at a scan rate of 0.1mV s -1 The scan rate shows the typical electrochemical behavior of the SP-50 silicon anode during reversible charge-discharge cycling. In the first anodic scan, a broad reduction peak appears at 0.19V, which is attributed to the lithiation process of the silicon active material, forming Li xSi alloy. At the same time, two broad oxidation peaks appeared at 0.34V and 0.5V, which represent the dealloying process of lithium-silicon. With the increase of the number of cycles, the peak intensity increases, indicating that the silicon nanoparticles on the electrode are constantly undergoing activation during the lithiation and delithiation process. The position of the reduction peak shifted slightly negatively, and the position of the oxidation peak remained almost unchanged, which shows that the SP-50 binder produced a reversible capacity in the silicon negative electrode and a reversible reaction occurred. The lithium-silicon alloying and dealloying processes can be well reversibly transformed during the charge and discharge process. In addition, it can be observed from the curve that there are no other side reactions in the SP-50 electrode, which means that the SP-50 binder has good electrochemical stability in the silicon negative electrode and has no effect on the electrochemical properties of the electrode.

[0093] Example 6 Effect of different cyclization degrees on the modified binder on the electrode structure

[0094] The surface morphology changes of electrodes prepared by silicone-modified polyimide binders with different cyclization degrees before and after 100 cycles were characterized by scanning electron microscopy. The test results are shown in Figure 2. Figure 10 As shown in a~d, the initial surfaces of SP-0, SP-25, SP-50 and SP-100 electrodes all exhibit a uniform and flat surface morphology and have a certain pore structure. Figure 10 In the low magnification SEM images of the surfaces of the four groups of electrodes after 100 cycles, it can be observed that there are dense thin layers on the electrode surfaces, which indicates that a solid electrolyte interface layer has been formed. At the same time, cracks of varying degrees were observed on the surfaces of the SP-0, SP-25, SP-50 and SP-100 electrodes after cycling, which is caused by the rupture of the SEI film during the charge and discharge process. 2 ) and their average crack width were statistically analyzed, such as Figure 10 As shown. It can be observed that the number of cracks in the electrode using the uncyclized SP-0 binder is large and the cracks are wide, with an average width of 4.5um, and the electrode surface has been split. With the appearance of the imide group, the cracks in the organosilicon-modified polyimide binder electrode gradually decreased and became narrower, and the electrode structure was more complete. Among them, the SP-50 electrode has the least number of cracks and the cracks are finer, indicating that as a silicon negative electrode binder, it can better adapt to the huge volume changes of silicon nanoparticles, which is beneficial to stabilize the electrode structure and inhibit electrode pulverization. This is mainly due to its good mechanical strength and flexibility as well as the presence of rich polar functional groups, which can effectively inhibit the volume expansion of silicon and form a stable SEI film.

[0095] Figure 11Electrochemical impedance spectra of silicon anodes prepared with silicone-modified polyimide binders of different degrees of cyclization, used to characterize the resistance of SP-0, SP-25, SP-50, and SP-100 electrodes. As can be seen from the obtained Nyquist plots, the four groups of electrodes are all composed of two semicircles in the high-frequency region and the middle-frequency region and a slanting line in the low-frequency region. The EIS curves were subjected to fitting analysis, and the statistical values of the impedance parameters of the four groups of electrodes are shown in Table 5. By comparison, it can be seen that the R c values of each electrode are similar and not very different. With the appearance of the cyclized structure, the R sei and R ct resistance values of the silicone-modified polyimide binder electrodes are decreasing, which means that the cyclized binder improves the electron transfer efficiency of the electrode. Among them, the R sei and R ct resistance values of the SP-50 electrode are smaller than those of the SP-0, SP-25, and SP-100 electrodes, indicating that the SP-50 electrode has a smaller interfacial resistance and charge transfer resistance. Therefore, a certain degree of cyclization is beneficial for the silicone-modified polyimide binder to provide an improved electrolyte-silicon nanoparticle interface for the silicon electrode, which is more conducive to the diffusion and transport of ions, which is consistent with the better specific capacity and rate performance of the SP-50 electrode.

[0096] In summary, the excellent electrochemical performance of the SP-50 electrode is mainly attributed to two points: one is the mechanical properties. The SP-50 binder has a strength and modulus similar to that of SP-100, and the presence of the acylamino group improves the flexibility of the polymer; the other is the intermolecular interaction. Since the SP-50 binder is not fully cyclized, there are relatively more polar groups such as carboxyl groups and amino groups, which are conducive to forming interactions with the silicon dioxide oxide layer on the surface of the silicon nanoparticles. The existence of these characteristics makes the SP-50 binder have higher mechanical strength and adhesion strength, which is beneficial to better maintaining the integrity of the electrode structure. At the same time, the flexibility of the polymer molecules can better adapt to the volume expansion of the silicon nanoparticles during charge and discharge, inhibiting the pulverization of the electrode. At the same time, SP-50 has a lower swelling rate and electrochemical impedance value. The good chemical stability reduces the generation of side reactions during lithiation and delithiation of the electrode, thus forming a stable SEI film. The good conductivity provides support for the transport and diffusion of lithium ions, thus ensuring excellent rate performance.

[0097] Therefore, a certain degree of cyclization can improve the electrochemical performance of the silicone-modified polyimide binder in the silicon anode of lithium-ion batteries, and can better maintain the integrity of the electrode structure under long cycle periods and high current densities. At an incompletely cyclized level, the mechanical properties provided by the imide structure and the abundant functional groups provided by the amic acid structure work together for the silicone-modified polyimide binder to play a role in the silicon anode. The silicone-modified polyimide polymer with a certain degree of cyclization has certain application potential as a binder for the silicon anode of lithium-ion batteries.

[0098] The content not detailed in the description of the present invention belongs to the prior art well known to those skilled in the art. Although the illustrative specific embodiments of the present invention are described above for the understanding of those skilled in the art of the present technology, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those of ordinary skill in the art of the present technology, as long as various changes are within the spirit and scope of the present invention defined and determined by the appended claims, these changes are obvious, and all inventions made using the concept of the present invention are within the scope of protection.

Claims

1. A method for preparing a silicone-modified polyimide adhesive, characterized in that: The following steps are involved: Step 1, vacuum drying and purifying 4,4′-(hexafluoroisopropylene) diphthalic anhydride and 3,5-diaminobenzoic acid monomers; Step 2, stirring the purified 3,5-diaminobenzoic acid monomer and N-methylpyrrolidone solvent in an ice bath until completely dissolved; Step 3, add bisaminosilane coupling agent 1,3-bis(aminopropyl)tetramethyldisiloxane dropwise, and after stirring and dissolving to be uniformly dispersed, add purified 4,4′-(hexafluoroisopropylene) diphthalic anhydride in four portions, and stir continuously for 24 hours; Step 4, slowly add 3-methylpyridine / acetic anhydride cyclizing agent to the solution and continue stirring for 24 hours to obtain a viscous solution; Step 5, slowly pouring the obtained viscous solution into deionized water with rapid stirring to obtain a solid; chopping the obtained solid, filtering and cleaning it under pressure, and then soaking it in deionized water overnight; Step 6, filtering and rinsing for multiple times to remove residual reactants, and vacuum drying at 50° C. for 12 hours to obtain an organosilicon-modified polyimide adhesive.

2. The method for preparing a silicone-modified polyimide adhesive according to claim 1, characterized in that: The molar ratio of the 3,5-diaminobenzoic acid to the bisaminosilane coupling agent is 8:

2.

3. The method for preparing a silicone-modified polyimide adhesive according to claim 1, characterized in that: The molar ratio of the 3,5-diaminobenzoic acid and the bisaminosilane coupling agent to the 4,4′-(hexafluoroisopropylene) diphthalic anhydride is 1:1.

02.

4. The method for preparing a silicone-modified polyimide adhesive according to claim 1, characterized in that: The solute content in step 3 is 20 wt%.

5. The method for preparing a silicone-modified polyimide adhesive according to claim 1, characterized in that: The steps 1 to 4 are all carried out under nitrogen protection.

6. The method for preparing a silicone-modified polyimide adhesive according to claim 1, characterized in that: The molar ratios of the 3-methylpyridine / acetic anhydride to 4,4′-(hexafluoroisopropylene) diphthalic anhydride are 1:2, 1:1 and 10:

1.

7. The method for preparing a silicone-modified polyimide adhesive according to claim 1, characterized in that: The vacuum drying and purification in step 1 is specifically performed by vacuum drying at 150° C. for 24 hours for purification.

8. The method for preparing a silicone-modified polyimide adhesive according to claim 1, characterized in that: In the step 3, a small amount of monomer adhering to the bottle wall is washed away each time with an appropriate amount of NMP, and the next step is performed after the monomer is fully dissolved and dispersed.

9. An organosilicon-modified polyimide adhesive prepared according to the preparation method according to any one of claims 1 to 8.

10. Use of an organosilicon-modified polyimide binder prepared according to the preparation method according to any one of claims 1 to 8 in a silicon negative electrode.

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