Conductive-bonding difunctional polymer binder, preparation method thereof and application of conductive-bonding difunctional polymer binder in sodium ion battery

Abstract

The invention belongs to the field of sodium ion battery binders, and discloses a conductive-bonding difunctional polymer binder, a preparation method thereof and an application of the conductive-bonding difunctional polymer binder in a sodium ion battery. According to the invention, a strong reducing solution is prepared from an ether solvent, an aromatic hydrocarbon compound and metal sodium to carry out controllable defluorination on polyvinylidene fluoride, and the C = C-containing conjugated polymer is prepared and endowed with good electronic conductivity. According to an electrode prepared by adopting the conductive binder, the proportion of traditional inactive components can be effectively reduced, the rate capability and the energy density of the electrode are remarkably improved, and wide research prospects and application values are provided for design and development of a high-performance sodium ion battery.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, specifically relating to a conductive-adhesive bifunctional polymer binder, its preparation method, and its application in sodium-ion batteries. Background Technology

[0002] Sodium resources are abundant, evenly distributed, and inexpensive in the Earth's crust, which is expected to drive the application of sodium-ion batteries in large-scale energy storage. Energy density, as a core indicator for improving battery performance and economics, directly determines the application value and industrialization potential of the battery. The energy density of sodium-ion batteries is limited by the large intrinsic mass and radius of sodium ions, resulting in limited electrode material capacity and low battery operating voltage. In recent years, improving the energy density of sodium-ion batteries has become a research focus, with the main strategy being the optimization of electrode materials and electrolytes. Developing high-voltage / high-capacity cathode materials (such as elemental doping and anion redox strategies), improving the performance of hard carbon materials, and exploring higher-capacity anode systems (such as alloy materials) can enhance the battery's energy storage capacity. Expanding the electrochemical window of the electrolyte and constructing a stable solid-state electrolyte interface helps ensure stable battery operation at high voltages. However, the energy density of sodium-ion batteries is generally 80-160 Wh / kg, and traditional modification methods are unlikely to bring significant breakthroughs; their energy density is approaching its limit.

[0003] Energy density depends not only on the capacity of the electrode materials and the battery's operating voltage, but also on its overall mass. Besides the active material, electrodes also contain inactive components such as binders and conductive agents, accounting for approximately 5%-20% of their mass. Therefore, increasing the active material loading by reducing the proportion of inactive components can further improve the battery's energy density. Binders are key materials with a large proportion of inactive components, mainly including polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR). Currently, research in this field mainly focuses on improving their adhesion and elasticity by regulating their molecular structure. However, these binders generally have poor conductivity, requiring the addition of large amounts of conductive agents (such as carbon black and Super P) to ensure efficient electron transport. The introduction of a large number of inactive components, however, limits the improvement of battery energy density.

[0004] To reduce the content of inactive components, researchers have attempted to develop conductive binders. Existing technologies mainly employ physical blending strategies, which involve mechanically mixing traditional binders such as PVDF with externally added conductive polymers such as polyaniline, polypyrrole, polythiophene, or poly(p-phenylenevinylene) (e.g., prior art documents CN104282912A, CN104861897A, CN115216245A). However, this method has inherent drawbacks: (1) the physical mixing is uneven, which easily leads to phase separation and affects the stability of the electrode slurry; (2) the interfacial resistance between the insulating binder matrix and the conductive filler phase is high, limiting the electronic conduction efficiency; and (3) the functions of the binder and the conductive agent are independent of each other, making it impossible to fundamentally reduce the total content of inactive substances.

[0005] Other technologies (such as CN114824258A) attempt to improve the mechanical properties of PVDF through cross-linking and then combine it with conductive polymers, but this does not change the fundamental nature of physical composite conductivity. The process is complex and the uniformity of the conductive network needs to be improved.

[0006] Therefore, developing a single-component material that combines good adhesion and intrinsic high electronic conductivity through molecular design to fundamentally replace the traditional binder + conductive agent system is of great significance for realizing high-energy-density sodium-ion batteries. Summary of the Invention

[0007] To address the shortcomings of existing conductive binders, which are mostly physically blended composite materials, such as interface problems, low electronic conductivity, and a high proportion of inactive components, this invention aims to provide a novel conductive-adhesive bifunctional polymer binder. This binder achieves excellent adhesion and intrinsic electronic conductivity simultaneously through controllable chemical structural modification of PVDF, constructing a conjugated structure in situ on its molecular chain. Another objective of this invention is to provide a simple and controllable preparation method for this binder. A further objective is to provide the application of this binder in sodium-ion battery electrodes to significantly reduce the proportion of inactive materials in the electrode, thereby improving the battery's energy density and rate performance.

[0008] In a first aspect, the present invention provides a method for preparing a conductive-adhesive bifunctional polymer adhesive, the core of which lies in using a strongly reducing sodium-aromatic hydrocarbon complex solution to selectively defluorinate PVDF, thereby introducing a conductive carbon-carbon double bond conjugated structure while retaining its adhesive framework. Specific steps include preparing a strongly reducing solution, preparing a PVDF solution, mixing and reacting, and post-treatment.

[0009] The preparation method includes the following steps:

[0010] S1. Mix metallic sodium, aromatic hydrocarbons and ether solvents to form a strongly reducing solution.

[0011] S2. Dissolve polyvinylidene fluoride in an organic solvent to form a polyvinylidene fluoride solution;

[0012] S3. A strong reducing solution is mixed with a polyvinylidene fluoride solution, and a defluorination reaction is carried out under stirring conditions. The defluorination reaction introduces a carbon-carbon double bond conjugated structure in situ on the polyvinylidene fluoride molecular chain.

[0013] S4. The solution after the reaction is recrystallized, washed, and dried to obtain modified polyvinylidene fluoride powder, which is the conductive-adhesive bifunctional polymer binder.

[0014] Further, in step S1, the aromatic hydrocarbon compound is one or more selected from 2,2-bipyridine, biphenyl, naphthalene, anthracene, and benzophenone; the ether solvent is one or more selected from ethylene glycol diethyl ether, ethylene glycol dimethyl ether, tetrahydrofuran, and 2-methyltetrahydrofuran; and the concentration of the strong reducing solution is 0.01 mol / L. -1 ~1 mol L -1 .

[0015] Further, in step S2, the organic solvent is one or more of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide; the molecular weight of the polyvinylidene fluoride is 400,000 to 2,000,000; and the mass ratio of the polyvinylidene fluoride to the organic solvent is 1:10 to 1:100.

[0016] Furthermore, the volume ratio of the strong reducing solution to the polyvinylidene fluoride solution is 0.2:1 to 0.3:1; the reaction temperature is 25 to 60°C; and the reaction time is 0.5 to 3 hours.

[0017] Furthermore, in step S4, the recrystallization is achieved by adding the reaction mixture dropwise into water or ethanol to produce a precipitate; the drying is carried out under vacuum conditions at 60~80℃.

[0018] Secondly, the present invention provides a conductive-adhesive bifunctional polymer adhesive prepared by the above method. Its molecular structure is characterized by the removal of some fluorine atoms from the PVDF backbone, forming carbon-carbon double bonds between adjacent carbon atoms; the structure can be represented as follows:

[0019]

[0020] Where x+y is 1, and x:y is 1:0 to 0:1.

[0021] Thirdly, the present invention provides a sodium-ion battery electrode sheet comprising the above-mentioned conductive-adhesive bifunctional polymer binder. Because the binder itself is conductive, it can significantly reduce or even completely replace the use of traditional conductive agents such as Super P. Preferably, the mass ratio of the active material, the binder, and the conductive agent in the electrode sheet is (93-98):(2-7):(0-2).

[0022] Fourthly, the present invention provides a sodium-ion battery comprising the above-mentioned electrode sheet, including a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, characterized in that the positive electrode sheet and / or the negative electrode sheet are the above-mentioned sodium-ion battery electrode sheets.

[0023] Furthermore, the active material of the positive electrode is at least one of layered oxide, polyanionic compound, or Prussian blue material; the active material of the negative electrode is at least one of hard carbon or metallic sodium sheet.

[0024] Advantages and beneficial effects of the present invention:

[0025] 1. This invention provides a conductive-adhesive bifunctional polymer additive achieved through molecular structure regulation, overcoming the limitations of traditional binders with only adhesive function. By introducing a conductive conjugated structure into the PVDF backbone, synergistic regulation of electronic conduction and adhesion is achieved, effectively reducing the proportion of inactive components in the electrode, thereby improving the overall energy density and cycle stability of sodium-ion batteries.

[0026] 2. The modification method employed in this invention is simple, rapid, safe, and reliable, exhibiting good repeatability and controllability. The overall process is short, with mild operating conditions, facilitating large-scale preparation and industrial application. This method has strong versatility and can be applied to various positive and negative electrode systems, providing material design concepts and process pathways for constructing novel energy storage batteries with high energy density, long lifespan, and low cost. Attached Figure Description

[0027] Figure 1 X-ray diffraction patterns of the original PVDF binder in Comparative Example 1, and the modified PVDF binders in Examples 1 and 2-6.

[0028] Figure 2 The conductivity test graphs are for the original PVDF adhesive in Comparative Example 1, and the modified PVDF adhesives in Examples 1 and 2-6.

[0029] Figure 3 Fourier transform infrared (FTIR) images of the modified PVDF adhesives of Example 1 and Comparative Examples 2-6;

[0030] Figure 4The images show scanning electron microscope (SEM) images of electrode sheets using sample 2.5 conductive adhesive and electrode sheets using the original PVDF adhesive, where a represents PVDF and b represents 2.5 conductive adhesive.

[0031] Figure 5 The first charge-discharge curves are shown for the electrode sheet using the conductive binder of sample 2.5 and the electrode sheet using the original PVDF binder. Detailed Implementation

[0032] The present invention will now be described in detail and completely with reference to the accompanying drawings, but these embodiments are not intended to limit the scope of protection of the present invention. Unless otherwise specified, the reagents used in the embodiments are commercially available or obtained through conventional synthesis methods in the art.

[0033] Example 1

[0034] Using 10 mL of ethylene glycol diethyl ether (DEE) as solvent, add 0.78 g of 2,2'-bipyridine (Bpy) and stir magnetically for 30 min to obtain a homogeneous, clear solution. Add 0.046 g of metallic sodium to this solution and continue stirring magnetically for 120 min to obtain a homogeneous, deep purple, strongly reducing solution with a concentration of 0.5 mol / L. -1 Weigh 0.1 g of polyvinylidene fluoride (PVDF) powder and add it to 10 mL of N-methylpyrrolidone (NMP) solvent. Stir magnetically for 1 h until completely dissolved to obtain the adhesive solution.

[0035] 2.5 mL of a strong reducing solution was added dropwise to the binder solution, and the system was stirred to ensure complete reaction and homogeneity. The resulting homogeneous solution was then added dropwise to excess deionized water, and precipitate formation was observed. The precipitate was separated by vacuum filtration, washed three times successively with water and ethanol, and dried at 60 °C for 2 h to obtain the modified PVDF binder powder, designated as sample 2.5.

[0036] Comparative Example 1

[0037] Weigh 0.1 g of PVDF powder and add it to 10 mL of N-methylpyrrolidone (NMP) solvent. Stir magnetically for 1 h until completely dissolved to obtain an adhesive solution. Add the resulting homogeneous solution dropwise to excess deionized water, and precipitate formation can be observed. Separate the precipitate by vacuum filtration, wash it three times with water and ethanol respectively, and dry it at 60 °C for 2 h to obtain PVDF adhesive powder, which is denoted as sample PVDF.

[0038] Comparative Examples 2-6

[0039] The preparation process was the same as in Example 1, except that the volume of the strong reducing solution added was changed to 0.5 mL, 1.0 mL, 1.5 mL, 2.0 mL and 3.0 mL, respectively. The resulting products were recorded as Sample 0.5, Sample 1.0, Sample 1.5, Sample 2.0 and Sample 3.0, respectively.

[0040] Performance testing and characterization

[0041] 1. Structural characterization (XRD): X-ray diffraction tests were performed on the products of Example 1 and Comparative Examples 1-6. The results are shown in the figure. Figure 1 The diffraction peak positions of all modified samples were basically consistent with those of the original PVDF, and no new diffraction peaks were observed, indicating that the modification process did not destroy the main crystal structure of PVDF.

[0042] 2. Conductivity test: The conductivity of the products of Example 1 and Comparative Examples 1-6 was tested, and the results are shown in the figure. Figure 2 The average conductivity of the PVDF samples under different pressure conditions is approximately 1 × 10⁻⁶. -11.9 S·m -1 It exhibits extremely low electronic conductivity, classifying it as a typical insulating polymer. The average conductivity of sample 0.5 is approximately 1 × 10⁻⁵. -9.3 S·m -1 The average conductivity of sample 1.0 is approximately 1 × 10⁻⁶. -8.2 S·m -1 The average conductivity of sample 1.5 is approximately 1 × 10⁻⁶. -6.9 S·m -1 The average conductivity of sample 2.0 is approximately 1 × 10⁻⁶. -5.7 S·m -1 The average conductivity of sample 2.5 is approximately 1 × 10⁻⁶. -4.5 S·m -1 The average conductivity of sample 3.0 is approximately 1 × 10⁻⁶. -4.1 S·m -1 This indicates that the conductivity of the modified binder is significantly improved. When the amount of strong reducing solution added is 2.5 mL, the conductivity of the system has reached its maximum value, and further increasing the amount added has limited effect on improving conductivity.

[0043] Some fluorine atoms are removed from the PVDF backbone, forming carbon-carbon double bonds between adjacent carbon atoms. The structure can be represented as follows:

[0044]

[0045] Characterized by Fourier transform infrared spectroscopy (FTIR), at 1673 cm⁻¹ -1 A characteristic absorption peak of C=C was observed at [location], and its peak intensity gradually increased with the increase of the degree of reaction. The results are shown in [figure]. Figure 3 .

[0046] Adhesive application performance test

[0047] ① Sample 2.5, with the best conductivity, was selected as the conductive binder. Na 0.9 Ni 0.32 Zn 0.08 Fe 0.1 Mn 0.3 Ti 0.2 O2 (ZT-NFM) oxide material was used as the positive electrode active material. The active material, conductive binder, and Super P were mixed in a mass ratio of 8:1:1. An appropriate amount of NMP solvent was added to prepare the slurry. After thorough mixing, the slurry was coated onto the surface of an aluminum foil current collector and dried at 80°C for 6 hours to obtain the composite electrode sheet. Figure 4 Figure b shows a scanning electron microscope (SEM) image of the electrode sheet using 2.5 conductive binder. It can be seen that the electrode material is uniformly distributed, and no obvious agglomeration or clumping was observed.

[0048] The above-mentioned positive electrode and sodium plate were assembled into a half-cell, with a glass fiber separator; the solute in the electrolyte was 1M NaPF6, and the solvents were EMC, EC, and DMC, with a volume ratio of EMC, EC, and DMC of 1:1:1; the positive electrode active material loading was ~1.5 mg / cm³. -2 , at 10 mA g -1 Constant current charge-discharge tests were conducted within the voltage range of 1.5 V to 4.0 V at current density. Figure 5 The image shows the first-cycle charge-discharge curves of the ZT-NFM electrode using a 2.5 conductive binder. Electrochemical test results show that the charging and discharging capacities of sample 2.5 are 145.1 mAh g⁻¹. -1 and 135.5 mAh g -1 .

[0049] ② Select raw PVDF as the binder, Na 0.9 Ni 0.32 Zn 0.08 Fe 0.1 Mn 0.3 Ti 0.2 O2 (ZT-NFM) oxide material was used as the positive electrode active material. The active material, conductive binder, and Super P were mixed in a mass ratio of 8:1:1. An appropriate amount of NMP solvent was added to prepare the slurry. After thorough mixing, the slurry was coated onto the surface of an aluminum foil current collector and dried at 80°C for 6 hours to obtain the composite electrode sheet. Figure 4 Image 'a' in the figure is a scanning electron microscope (SEM) image of an electrode sheet using the original PVDF binder. It can be seen that the electrode material is unevenly distributed, with obvious agglomeration or clumping observed.

[0050] The above-mentioned positive electrode sheet and sodium sheet were assembled into a half-cell, with a positive electrode active material loading of ~1.5 mg cm⁻¹. -2 , at 10 mA g -1 Constant current charge-discharge tests were conducted within the voltage range of 1.5 V to 4.0 V at current density. Figure 5 The image shows the first charge-discharge curves of the ZT-NFM electrode using the original PVDF binder. Electrochemical test results show that the charging capacity and discharging capacity of the PVDF sample are 146.4 mAh g⁻¹. -1 and 135.4 mAh g -1 The performance of the conductive binder is basically consistent with that of sample 2.5, indicating that the introduction of the conductive binder will not affect the electrochemical behavior of the electrode material or the stable operation of the battery.

[0051] In summary, this invention has developed a binder with both conductive and adhesive functions through molecular structure design. Controllable defluorination of PVDF was performed using a sodium-bipyridine / ethylene glycol diethyl ether solution (Na-Bpy / DEE) to prepare a polymer additive containing a C=C conjugated structure. Batteries assembled using this conductive binder exhibit excellent rate performance, providing broad research prospects and application value for the design and development of high-performance sodium-ion batteries.

[0052] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a conductive-adhesive bifunctional polymer adhesive, characterized in that, Includes the following steps: S1. Mix metallic sodium, aromatic hydrocarbons and ether solvents to form a strongly reducing solution. S2. Dissolve polyvinylidene fluoride in an organic solvent to form a polyvinylidene fluoride solution; S3. A strong reducing solution is mixed with a polyvinylidene fluoride solution, and a defluorination reaction is carried out under stirring conditions. The defluorination reaction introduces a carbon-carbon double bond conjugated structure in situ on the polyvinylidene fluoride molecular chain. S4. The solution after the reaction is recrystallized, washed, and dried to obtain modified polyvinylidene fluoride powder, which is the conductive-adhesive bifunctional polymer binder.

2. The preparation method according to claim 1, characterized in that, In step S1, the aromatic hydrocarbon compound is one or more selected from 2,2-bipyridine, biphenyl, naphthalene, anthracene, and benzophenone; the ether solvent is one or more selected from ethylene glycol diethyl ether, ethylene glycol dimethyl ether, tetrahydrofuran, and 2-methyltetrahydrofuran; and the concentration of the strong reducing solution is 0.01 mol / L. -1 ~1 mol L -1 .

3. The preparation method according to claim 1, characterized in that, In step S2, the organic solvent is one or more of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide; the molecular weight of the polyvinylidene fluoride is 400,000 to 2,000,000; and the mass ratio of the polyvinylidene fluoride to the organic solvent is 1:10 to 1:

100.

4. The preparation method according to claim 1, characterized in that, The volume ratio of the strong reducing solution to the polyvinylidene fluoride solution is 0.2:1 to 0.3:1; the reaction temperature is 25 to 60°C; and the reaction time is 0.5 to 3 hours.

5. The preparation method according to claim 1, characterized in that, In step S4, the recrystallization is achieved by adding the reaction mixture dropwise into water or ethanol to produce a precipitate; the drying is carried out under vacuum conditions at 60~80℃.

6. A conductive-adhesive bifunctional polymer adhesive prepared by the method according to any one of claims 1 to 5, characterized in that, Its structural formula is: Where x+y is 1, and x:y is 1:0 to 0:

1.

7. A sodium-ion battery electrode sheet, comprising a current collector and an active material layer disposed on the surface of the current collector, characterized in that, The active material layer comprises an active material, a conductive agent, and the conductive-adhesive bifunctional polymer adhesive as described in claim 6.

8. The sodium-ion battery electrode sheet according to claim 7, characterized in that, Based on the total mass of the active material layer being 100%, the active material accounts for 93% to 98% of the mass, the conductive-adhesive bifunctional polymer adhesive accounts for 2% to 7% of the mass, and the conductive agent accounts for 0% to 2% of the mass.

9. A sodium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The positive electrode and / or the negative electrode are sodium-ion battery electrode sheets as described in claim 7 or 8.

10. The sodium-ion battery according to claim 9, characterized in that, The active material of the positive electrode is at least one of layered oxides, polyanionic compounds, or Prussian blue materials; the active material of the negative electrode is hard carbon or metallic sodium sheet.

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