Polyimide Schiff base polymer containing thiophene structure and application of polyimide Schiff base polymer in lithium ion battery

Abstract

The invention discloses a polyimide Schiff base polymer containing a thiophene structure and application of the polyimide Schiff base polymer in a lithium ion battery, and belongs to the technical field of organic electrode materials of the lithium ion battery. The structural general formula of the polyimide Schiff base polymer containing the thiophene structure is shown in the specification. By introducing a thiophene ring, a main chain pi conjugated structure is remarkably expanded, the electron delocalization degree is enhanced, and the intrinsic electron conductivity and the interface charge transmission capacity of the material are improved by utilizing the high polarizability of sulfur atoms and the characteristic that p orbits participate in conjugation; meanwhile, imide carbonyl and a Schiff base C = N bond in the material are main lithium storage active sites, and a thiophene conjugated unit is beneficial to electron transmission and structural stability, so that the material is endowed with relatively high specific capacity, excellent rate capability and cycling stability. The material has the advantages of simple synthesis method, easily available raw materials and stable structure, and is suitable for being used as a high-performance lithium ion battery organic negative electrode material.

Description

Technical Field

[0001] This invention belongs to the field of organic electrode materials technology for lithium-ion batteries, specifically relating to a polyimide Schiff base polymer containing a thiophene structure and its application in lithium-ion batteries. Background Technology

[0002] With the rapid development of electric vehicles, portable electronic devices, and large-scale energy storage systems, the demand for high-energy-density, high-power-density, and long-life rechargeable batteries continues to grow. Lithium-ion batteries, due to their advantages such as high operating voltage, long cycle life, and environmental friendliness, have become one of the most mature and widely used rechargeable battery systems. However, the electrode materials of existing commercial lithium-ion batteries are mainly inorganic materials, with graphite anodes having a theoretical specific capacity of only 372 mAh g⁻¹. -1 It has approached its theoretical limit and is difficult to meet the development needs of future high-energy-density energy storage devices.

[0003] Organic electrode materials, composed of light elements such as C, H, N, O, and S, have attracted widespread attention in recent years due to their advantages including wide availability, highly designable molecular structures, high theoretical specific capacity, and environmental friendliness. Organic molecules containing structural units such as carbonyl groups, imine bonds, and conjugated aromatic rings can store lithium ions through reversible redox processes, thus possessing potential as electrode materials for lithium-ion batteries. In particular, polymer-type organic electrode materials are less soluble in electrolytes and exhibit better cycle stability compared to small-molecule organic electrode materials.

[0004] In existing research, polyimide materials, due to their high content of imide carbonyl groups, exhibit structural stability and high thermal stability, and are frequently used to construct organic anode materials. Meanwhile, the Schiff base reaction offers advantages such as mild conditions, simple steps, and ease of constructing conjugated frameworks. By linking polyimide units with dialdehyde units, the π-conjugated system of the polymer can be further expanded, slowing down the dissolution of active materials and enhancing electron transport capabilities. In existing technologies, polyimide Schiff base polymers constructed using aromatic dialdehydes, especially terephthalaldehyde, have demonstrated certain reversible specific capacity and cycling stability.

[0005] However, existing polyimide Schiff base systems with benzene rings as bridging units still have the following problems: First, the electron cloud distribution of the benzene ring bridging structure is relatively symmetrical, resulting in limited improvement in intrinsic conductivity and limited charge transport at high rates; second, under high current density and long cycling conditions, some organic polymer electrodes still suffer from increased interfacial impedance, slow reaction kinetics, and rapid capacity decay; and third, when relying solely on C=O and C=N as the main active centers, there is still room for further improvement in the number of lithium storage active sites per unit structure.

[0006] Therefore, it is necessary to introduce new conjugated bridging units into the polyimide Schiff base backbone to further enhance electron delocalization, improve intrinsic conductivity, optimize interfacial charge transport, and provide more active sites for lithium storage reactions while maintaining structural stability. Thiophene is a typical sulfur-containing five-membered heterocycle. The lone pair electrons of its sulfur atoms can participate in the conjugated system, exhibiting strong electron donor ability and good polarization characteristics. Introducing it into the polyimide Schiff base backbone is expected to significantly extend the conjugated structure, reduce the electron migration barrier, and improve the kinetic performance of the electrode material. Summary of the Invention

[0007] The purpose of this invention is to provide a polyimide Schiff base polymer containing a thiophene structure and its application in lithium-ion batteries, so as to solve the problems of insufficient conductivity, poor rate performance and limited long-cycle stability of existing organic anode materials.

[0008] This invention relates to a polyimide Schiff base polymer containing a thiophene structure, abbreviated as PITD, with the following general structural formula, where n represents the degree of polymerization, ranging from 10 to 200. This invention has revealed that polymer molecular weight has a significant impact on the electrochemical performance of the material. When the molecular weight is low, the material is easily soluble in the electrolyte, leading to decreased cycle stability; when the molecular weight is too high, interchain stacking is enhanced, which is detrimental to lithium-ion diffusion, thereby reducing rate performance. Therefore, a suitable molecular weight helps to achieve a balance between structural stability and kinetic performance, resulting in higher specific capacity and better cycle stability.

[0009] .

[0010] The main chain of the polyimide Schiff base polymer containing a thiophene structure of the present invention simultaneously contains an imide carbonyl group, a Schiff base C=N bond and a thiophene ring structure, forming a continuously extending conjugated backbone.

[0011] The present invention discloses a method for preparing a polyimide Schiff base polymer containing a thiophene structure. First, 1,4,5,8-naphthalenetetracarboxylic dianhydride is used as a starting material and reacted with hydrazine hydrate under nitrogen protection to obtain an N,N′-diamino-1,4,5,8-naphthalenetetracarboxylic bisimide intermediate. Subsequently, this intermediate undergoes a Schiff base polycondensation reaction with thiophene-2,5-dicarboxaldehyde to obtain the polyimide Schiff base polymer PITD containing a thiophene-bridged structure. Specifically, the method includes the following steps:

[0012] Step 1: Under a nitrogen atmosphere, 1,4,5,8-naphthalenetetracarboxylic acid dianhydride was added to anhydrous ethanol, and hydrazine hydrate was slowly added dropwise under ice bath conditions. After stirring and mixing, the mixture was heated to 60-80℃ and refluxed for 0.5-2 h to obtain N,N′-diamino-1,4,5,8-naphthalenetetracarboxylic acid diimide intermediate.

[0013] Step 2: Add the intermediate obtained in Step 1 and thiophene-2,5-dicarboxaldehyde to N-methylpyrrolidone in a molar ratio of 1:1, add 2-3 drops of acetic acid as a catalyst, and carry out polycondensation reaction at 120-180℃ for 8-16 h under nitrogen protection. After the reaction is completed, filter, wash with ethanol and acetone, and purify by DMF Soxhlet extraction. Finally, vacuum dry to obtain the target polymer PITD.

[0014] In step 1, the molar ratio of 1,4,5,8-naphthalenetetracarboxylic dianhydride to hydrazine hydrate is 1:5-1:15, preferably 1:10.

[0015] In step 2, the solvent used for Soxhlet extraction is one or more of ethanol, acetone, and N,N-dimethylformamide, the Soxhlet extraction time is 12-30 h, and the vacuum drying temperature is 60-100℃.

[0016] The reaction route is shown below:

[0017]

[0018] The present invention relates to the application of a thiophene-containing polyimide Schiff base polymer in the preparation of lithium-ion batteries, specifically using the thiophene-containing polyimide Schiff base polymer as the active material of the negative electrode material of lithium-ion batteries.

[0019] The thiophene-containing polyimide Schiff base polymer, conductive agent, and binder are mixed and uniformly coated onto the surface of a copper foil current collector. The mixture is then vacuum dried at 60-100℃ for 8-24 h to obtain a lithium-ion battery negative electrode sheet. The mass ratio of the thiophene-containing polyimide Schiff base polymer, conductive agent, and binder is 40-70:20-50:5-15.

[0020] The conductive agent is one or more of Ketjen Black, acetylene black, conductive carbon black, and Super P; the binder is one or more of polyacrylic acid, sodium alginate, and carboxymethyl cellulose.

[0021] Furthermore, the adhesive is composed of polyacrylic acid and sodium alginate in a mass ratio of 1-4:1.

[0022] Compared with traditional benzene ring-bridged polyimide Schiff base polymers, this invention uses a thiophene ring as the bridging structural unit, which can achieve synergistic optimization in three aspects at the structural level: First, the thiophene ring has stronger electronic delocalization and higher polarizability than the benzene ring, which is beneficial to expanding the π-conjugation length of the main chain and improving the intrinsic electronic conductivity of the polymer; Second, the sulfur atoms in thiophene can participate in electron cloud delocalization and regulate molecular orbital energy levels, making electron migration in the main chain smoother and helping to reduce interfacial charge transfer resistance; Third, the thiophene conjugation unit, together with the imide carbonyl group and the Schiff base C=N bond, constitutes a multi-active-center lithium storage system, which is beneficial to improving the reversible capacity and rate performance of the material.

[0023] When applying:

[0024] The negative electrode of a lithium-ion battery, prepared using the thiophene-containing polyimide Schiff base polymer as the active material, was cut into circular electrodes with a diameter of 10-12 mm. A 16 mm diameter lithium sheet was used as the counter electrode. The separator was a Celgard 2325 microporous three-layer membrane. The electrolyte was a solution of lithium hexafluorophosphate (LiPF6) dissolved in a mixed solvent of dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC), wherein the volume ratio of DMC:DEC:EMC was 1:1:1, and the LiPF6 concentration was 1 mol / L. CR2032 button batteries were assembled in an argon-filled glove box; the oxygen concentration was no higher than 1 ppm, and the water vapor concentration was no higher than 0.5 ppm.

[0025] This invention further reveals that using a composite aqueous binder composed of polyacrylic acid and sodium alginate can more fully leverage the structural advantages of the PITD material itself. This composite system is rich in carboxyl and hydroxyl groups, which can form a relatively stable hydrogen bond network within the electrode, enhancing the bonding between the active material, conductive agent, and current collector, thereby maintaining a more stable interface structure under high current carrying conditions. The results of examples with different PAA / SA ratios show that although the binder system affects electrode processability and cycle stability, all four sets of examples exhibited high capacity and good rate response related to the thiophene structure, indicating that the fundamental source of performance improvement still lies in the enhanced conductivity brought by the thiophene units in the PITD backbone and the synergistic effect of the active sites.

[0026] Compared with the prior art, the present invention has at least the following beneficial effects:

[0027] 1. By replacing the traditional benzene ring bridging unit with a thiophene bridging unit, the conjugated structure is significantly extended, electron delocalization is enhanced, and intrinsic conductivity is improved;

[0028] 2. The sulfur atoms in thiophene enhance the polarization ability of the polymer backbone, reduce charge transfer resistance, and improve high-rate performance;

[0029] 3. The imide C=O and Schiff base C=N are the main lithium storage active sites, and the thiophene conjugated structure helps to enhance electron delocalization and interfacial charge transport, thus giving the material a high reversible capacity.

[0030] 4. The polymer structure is stable and not easily soluble in electrolyte, exhibiting good long-term cycling stability;

[0031] 5. The raw materials are widely available, the synthesis process is simple, and the conditions are mild, making it suitable for large-scale preparation. Attached Figure Description

[0032] Figure 1 This is a high-resolution C1s X-ray photoelectron spectroscopy (XPS) spectrum of PITD material.

[0033] Figure 2 The graph shows the cycling performance and coulombic efficiency of the PITD electrode at a current density of 0.2 A / g under different PAA / SA composite binder ratios, where PS11, PS21, PS31 and PS41 represent composite binder systems with PAA to SA mass ratios of 1:1, 2:1, 3:1 and 4:1, respectively.

[0034] Figure 3 The graph shows the long-cycle performance of PITD electrodes under different PAA / SA composite binder ratios after pre-activation at 1 A / g followed by a transition to 2 A / g conditions.

[0035] Figure 4 The rate performance of the PITD electrode is shown in the figure for different PAA / SA composite binder ratios. Detailed Implementation

[0036] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but the implementation of the present invention is not limited thereto.

[0037] Example 1: Synthesis of PITD polymer and preparation and testing of PS11 electrode

[0038] 1. Under a nitrogen atmosphere, 0.536 g (2 mmol) of 1,4,5,8-naphthalenetetracarboxylic dianhydride was added to 25 mL of anhydrous ethanol and dispersed by stirring in an ice bath at 0 °C. 10 g (20 mmol) of hydrazine hydrate was slowly added dropwise, and stirring was continued for 30 min. The mixture was then brought to room temperature and refluxed in an oil bath at 70 °C for 1 h. After the reaction was complete, the mixture was filtered and washed with ethanol to obtain the yellowish-brown intermediate N,N′-diamino-1,4,5,8-naphthalenetetracarboxylic dianhydride (DNCB).

[0039] 2. Equimolar amounts of DNCB and thiophene-2,5-dicarboxaldehyde were added to a three-necked flask. An appropriate amount of NMP was added as a solvent, and 2-3 drops of acetic acid were added dropwise as a catalyst. The mixture was refluxed at 150 °C for 12 h under nitrogen protection to induce Schiff base polymerization. After the reaction, the mixture was filtered. The resulting solid was washed successively with ethanol and acetone, and then subjected to Soxhlet extraction with DMF for 24 h. Finally, it was dried under vacuum at 80 °C for 12 h to obtain a brown solid, PITD.

[0040] 3. Polyacrylic acid and sodium alginate were mixed at a mass ratio of 1:1 to prepare a composite water-based binder with a total solid content of 3 wt%, denoted as PS11. PITD active material, Ketjen Black, and PS11 binder were weighed and mixed at a mass ratio of 50:40:10. An appropriate amount of deionized water was added, and the mixture was thoroughly ground to obtain a uniform slurry. The slurry was coated onto copper foil, controlling the areal density of the active material to be 0.4~0.6 mg / cm³. 2 The wet film thickness is about 150 μm. After being vacuum dried at 80 °C overnight, it is punched into electrodes with a diameter of 12 mm.

[0041] 4. Using this electrode as the working electrode and a lithium metal sheet as the counter electrode, a CR2032 coin cell was assembled in an electrolyte system of 1 mol / L LiPF6 / DMC:DEC:EMC = 1:1:1. Its cycling performance at a current density of 0.2 A / g is as follows: Figure 2 As shown, after the electrode undergoes activation in the initial few cycles, its capacity tends to stabilize and slowly increase. After 50 cycles, the discharge specific capacity reaches 1375.6 mAh / g, and the coulombic efficiency stabilizes at close to 100%, indicating that the electrode has good reversibility and cycling stability.

[0042] To investigate the stability at higher current densities, the electrode was first pre-activated at 1 A / g, and then subjected to long-cycle testing at 2 A / g. The results are as follows: Figure 3 As shown, the electrode capacity decays slowly with cycling, but remains at 424.9 mAh / g even after long cycling, demonstrating good high-rate cycling stability.

[0043] The results of the rate performance test are as follows: Figure 4 As shown, as the current density increases sequentially from 0.2 A / g to 0.5 A / g, 1 A / g, 3 A / g, and 5 A / g, the electrode specific capacity decreases stepwise. When the current density recovers to 0.2 A / g, the capacity recovers significantly, indicating that PITD has good structural stability and fast kinetic characteristics. Figure 3 and Figure 4 The results shown correspond to the 2 A / g long cycle and the 0.2–5 A / g rate test processes, respectively, and are consistent with the performance descriptions in the abstract and examples.

[0044] Example 2: Preparation and testing of PITD electrode using PS21 composite binder system

[0045] Except for the binder ratio, the remaining PITD synthesis steps, electrode preparation conditions, and battery assembly conditions were the same as in Example 1. Polyacrylic acid and sodium alginate were mixed at a mass ratio of 2:1 to prepare a composite binder with a total solid content of 3 wt%, denoted as PS21. The PITD@PS21 electrode was prepared by mixing the active material, Ketjen Black, and PS21 binder at a mass ratio of 50:40:10.

[0046] The electrode exhibits a discharge specific capacity of 1125.3 mAh / g after 50 cycles at a current density of 0.2 A / g; after pre-activation at 1 A / g followed by long-term cycling at 2 A / g, the capacity remains stable at 297.7 mAh / g. The trends in its cycling and rate performance are shown in [Figure showing trends in cycling and rate performance]. Figures 2-4 .

[0047] Compared with Example 1, the overall capacity and high-rate long-cycle performance of the PS21 system decreased, but it still maintained a high capacity level, indicating that the PITD material can still fully demonstrate the conductivity and multi-active-center lithium storage advantages brought by the thiophene bridging main chain in this system.

[0048] Example 3: Preparation and testing of PITD electrode using PS31 composite binder system

[0049] Except for the different binder ratio, the other conditions were the same as in Example 1. Polyacrylic acid and sodium alginate were mixed at a mass ratio of 3:1 to prepare a composite binder with a total solid content of 3 wt%, denoted as PS31. The PITD@PS31 electrode was prepared by mixing the active material, Ketjen black and PS31 binder at a mass ratio of 50:40:10.

[0050] The electrode exhibited a discharge specific capacity of 925.0 mAh / g after 50 cycles at a current density of 0.2 A / g; after pre-activation at 1 A / g followed by long-term cycling at 2 A / g, the capacity remained at 250.2 mAh / g. The trends in its cycling and rate performance are shown in the figure. Figures 2-4 .

[0051] Compared to the PS11 and PS21 systems, the capacity of the PS31 system is reduced, but it is still significantly higher than that of traditional graphite anodes, indicating that the intrinsic structure of PITD can still provide high reversible lithium storage capacity.

[0052] Example 4: Preparation and testing of PITD electrode using PS41 composite binder system

[0053] Except for the different binder ratio, the other steps are the same as in Example 1. Polyacrylic acid and sodium alginate were mixed at a mass ratio of 4:1 to prepare a composite binder with a total solid content of 3 wt%, denoted as PS41. The PITD@PS41 electrode was prepared by mixing the active material, Ketjen black and PS41 binder at a mass ratio of 50:40:10.

[0054] The electrode is at 0.2 A g -1 The discharge specific capacity after 50 cycles at the current density was 895.6 mAh / g; after pre-activation at 1 A / g followed by long-term cycling at 2 A / g, the capacity remained at 239.7 mAh / g. The trends in its cycling and rate performance are shown in [Figure showing trends in cycle and rate performance]. Figures 2-4 .

[0055] As shown in Examples 1-4, the PITD electrodes under all four composite binder systems exhibited high capacity and good rate response, with the PS11 system showing the best performance. This result indicates that different composite binders mainly affect the electrode interface stability and processability, while the fundamental source of the material's high capacity and good kinetic performance lies in the thiophene-bridged polyimide Schiff base structure of PITD itself.

[0056] Comparative Example 1: Preparation and Testing of PITD Electrode in Pure PAA Binder System

[0057] PITD active material, Ketjen black and pure PAA binder were mixed in a mass ratio of 50:40:10, and an appropriate amount of deionized water was added to prepare a slurry. The remaining coating, drying, stamping and assembly conditions were the same as in Example 1.

[0058] At a current density of 0.2 A / g, the PITD electrode using pure PAA binder showed a high initial capacity, but the capacity subsequently decreased rapidly, with a discharge specific capacity of approximately 1180 mAh / g at the end of the test. After about 250 cycles at a current density of 2 A / g, the capacity was approximately 430 mAh / g, indicating that the single PAA system is insufficient in terms of interface stability and high-rate adaptability.

[0059] Comparative Example 2: Preparation and Testing of PITD Electrode in Pure SA Binder System

[0060] PITD active material, Ketjen black and pure SA binder were mixed in a mass ratio of 50:40:10, and an appropriate amount of deionized water was added to prepare a slurry. The remaining conditions were the same as in Example 1.

[0061] At 0.2 Ag -1At the specified current density, the overall capacity of the PITD electrode using pure SA binder is lower than that of the PS11 composite system, with a discharge specific capacity of approximately 820 mAh / g at the end of the test; after approximately 250 cycles at a current density of 2 A / g, the capacity is approximately 520 mAh / g. This result indicates that a single SA system cannot simultaneously achieve high capacity at low rates and stable long-cycle stability at high rates.

[0062] Comparative Examples 1 and 2 show that the binder system affects electrode processing and interface stability. However, PITD exhibits high lithium storage capacity under different binder systems, further indicating that the thiophene bridged main chain structure is the core factor determining its high performance. The difference in binders is mainly reflected in the adjustment of electrode film quality and cycle retention behavior.

[0063] like Figure 1 As shown, XPS test results demonstrate that the PITD backbone simultaneously contains C=C, CC / CN, C=N, C=O, and CS bonding environments, indicating that the target polymer simultaneously possesses imide, Schiff base, and thiophene structures. These structural units collectively form a continuously extending conjugated backbone, which is beneficial for improving electron delocalization and intrinsic electronic conductivity.

[0064] In the lithium storage process, imide C=O and Schiff base C=N are the main active sites for lithium storage, while thiophene bridging units are mainly used to enhance the delocalization of the main chain electron cloud, improve the interfacial charge transport and enhance the structural stability, thereby helping the material maintain good reaction kinetics at higher current densities.

[0065] The results of the examples and comparative examples show that PITD exhibits high specific capacity and good rate response under different composite binder systems, indicating that the performance improvement is structurally intrinsic. In particular, under the PS11 system, a discharge specific capacity of 1375.6 mAh / g can be obtained at 0.2 A / g and maintained at 424.9 mAh / g at 2 A / g, further proving that the material of the present invention has both high capacity and good cycle stability; while the pure PAA and pure SA systems have their own characteristics in some cycle ranges, their overall performance is still inferior to that of the PS11 system.

[0066] Therefore, this invention successfully obtained an organic polymer anode material that combines high capacity, superior rate performance, and good cycle stability by introducing thiophene bridging units into the polyimide Schiff base framework. This material uses readily available raw materials and is simple to synthesize, making it suitable for widespread application in the field of organic anode materials for lithium-ion batteries.

Claims

1. A polyimide Schiff base polymer containing a thiophene structure, abbreviated as PITD, characterized in that... Its general structural formula is as follows: 。 2. The method for preparing the polyimide Schiff base polymer containing a thiophene structure according to claim 1, characterized in that: First, using 1,4,5,8-naphthalenetetracarboxylic dianhydride as the starting material, an N,N′-diamino-1,4,5,8-naphthalenetetracarboxylic bisimide intermediate was prepared by reacting it with hydrazine hydrate under nitrogen protection. Subsequently, this intermediate was subjected to a Schiff base polycondensation reaction with thiophene-2,5-dicarboxaldehyde to obtain a polyimide Schiff base polymer PITD containing a thiophene-bridged structure.

3. The preparation method according to claim 2, characterized in that... Includes the following steps: Step 1: Under a nitrogen atmosphere, 1,4,5,8-naphthalenetetracarboxylic acid dianhydride was added to anhydrous ethanol, and hydrazine hydrate was slowly added dropwise under ice bath conditions. After stirring and mixing, the mixture was heated to reflux to obtain N,N′-diamino-1,4,5,8-naphthalenetetracarboxylic acid diimide intermediate. Step 2: Add the intermediate obtained in Step 1 and thiophene-2,5-dicarboxaldehyde to N-methylpyrrolidone, add acetic acid as a catalyst, and carry out polycondensation reaction under nitrogen protection. After the reaction is completed, separate and purify to obtain the target polymer PITD. The reaction route is shown below: 。 4. The application of the thiophene-containing polyimide Schiff base polymer of claim 1 in the preparation of lithium-ion batteries.

5. The application according to claim 4, characterized in that: The polyimide Schiff base polymer containing a thiophene structure is used as the active material of the negative electrode material of a lithium-ion battery.

6. The application according to claim 5, characterized in that: The thiophene-containing polyimide Schiff base polymer, conductive agent, and binder are mixed and uniformly coated onto the surface of a copper foil current collector. After vacuum drying, a lithium-ion battery negative electrode sheet is obtained.

7. The application according to claim 6, characterized in that: The mass ratio of the thiophene-containing polyimide Schiff base polymer, the conductive agent, and the binder is 40-70:20-50:5-15.

8. The application according to claim 6 or 7, characterized in that: The conductive agent is one or more of Ketjen Black, acetylene black, conductive carbon black, and Super P; the binder is one or more of polyacrylic acid, sodium alginate, and carboxymethyl cellulose.

9. The application according to claim 8, characterized in that: The adhesive is composed of polyacrylic acid and sodium alginate.

10. The application according to claim 9, characterized in that: The mass ratio of polyacrylic acid to sodium alginate is 1-4:1.

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