Method for preparing flexible negative electrode sheet for sodium battery

Abstract

The application discloses a preparation method of a flexible negative electrode sheet for a sodium battery, which comprises the following main steps: raw material pretreatment, electrode slurry preparation, gradient coating, vacuum drying, low-temperature gradient rolling, and sheet making, so as to obtain the flexible negative electrode sheet for the sodium battery. By adopting two kinds of crosslinking agents, crosslinking components with at least two different functional groups are formed, chemical crosslinking and physical entanglement are cooperated in the preparation process, a three-dimensional interpenetrating network is constructed, the electrode sheet is endowed with excellent flexibility and high mechanical strength, the volume stress during charging and discharging is effectively buffered, the structural integrity is ensured, cracks are prevented, the active sites on the electrode surface are passivated, the irreversible decomposition of the electrolyte is inhibited, a thin and stable SEI film is induced to be formed, the first sodium loss is significantly reduced, the initial coulombic efficiency is improved, the interface bonding between the active material, the conductive agent and the current collector is enhanced, the interface impedance is reduced, and the conduction of ions and electrons is optimized.

Description

Technical Field

[0001] This invention relates to the field of sodium batteries, and in particular to a method for preparing a flexible negative electrode sheet for sodium batteries. Background Technology

[0002] Sodium-ion batteries, due to their abundant sodium resources, low cost, and excellent electrochemical performance, have become an important alternative to lithium-ion batteries. Flexible sodium-ion batteries, as the core energy storage unit for flexible electronic devices, have become a research hotspot in recent years. The core of flexible sodium-ion batteries is the flexible anode sheet, which not only needs excellent electrochemical performance, such as high initial coulombic efficiency, high specific capacity, and long cycle stability, but also needs to meet good flexible mechanical properties, able to withstand repeated bending and rolling without damage, and with no significant degradation in electrochemical performance. Hard carbon, as the mainstream active material for sodium-ion battery anodes, has large interlayer spacing and good sodium intercalation reversibility. However, pure hard carbon suffers from low initial coulombic efficiency and poor conductivity. Furthermore, conventional hard carbon anode sheets use rigid current collectors such as copper or aluminum foil, combined with a single binder and crosslinking agent, resulting in high electrode brittleness and poor flexibility, which cannot meet the requirements of flexible batteries.

[0003] Current improvements to flexible sodium-ion battery anode sheets mainly focus on simply replacing the flexible current collector, such as using carbon fiber cloth or metal foil mesh. However, the following core problems remain: First, the active material still uses unmodified or simply modified hard carbon, resulting in low initial coulombic efficiency and significant sodium ion loss during the first charge-discharge cycle. Second, the binders are mostly single sodium alginate or sodium carboxymethyl cellulose, which have weak adhesion to the flexible current collector, making the electrode layer easily peel off from the current collector during cycling or bending. Third, there is a lack of targeted flexible electrode structure design; the slurry coating, drying, and rolling processes all follow the preparation methods of rigid electrodes, leading to high internal stress and easy cracking and detachment during bending. Fourth, the synergistic improvement of electrochemical performance and flexible mechanical properties is not considered; although some flexible anode sheets have certain bending performance, their electrochemical performance is significantly degraded, with initial coulombic efficiency often below 90% and poor cycle stability. Therefore, it is necessary to propose a new solution to address these problems. Summary of the Invention

[0004] In view of this, the present invention addresses the deficiencies of the existing technology, and its main objective is to provide a method for preparing a flexible negative electrode sheet for sodium batteries. This method can effectively solve the problems of low initial coulombic efficiency, severe sodium ion loss during the first charge and discharge, easy peeling of the electrode layer from the current collector during cycling or bending, and high internal stress of the electrode sheet, which easily leads to cracks and detachment during bending.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a flexible negative electrode sheet for sodium batteries includes the following main steps: (1) Raw material pretreatment: The modified hard carbon, conductive agent and first crosslinking agent were placed in a vacuum drying oven and dried at 85°C for 3.5h for later use; the carbon fibers were arranged in anhydrous ethanol for ultrasonic cleaning with an ultrasonic power of 300W and a cleaning time of 25min, and then dried at 65°C for 3.5h for later use. (2) Preparation of negative electrode slurry: By weight, take 90-96 parts of modified hard carbon and 1-3 parts of conductive agent and add them to 200-300 parts of deionized water. Disperse ultrasonically for 25 min at an ultrasonic power of 350 W and stir at 250 r / min for 18 min to obtain a pre-dispersion liquid. Add 2-4 parts of composite binder, 0.1-0.3 parts of first crosslinking agent, 0.7-0.9 parts of second crosslinking agent, and 0.3-0.5 parts of sodium citrate to the pre-dispersion liquid. Then stir for 35 min at a stirring speed of 400 r / min. The composite binder is composed of sodium alginate and carboxymethyl cellulose, and the mass ratio of sodium alginate to carboxymethyl cellulose is 1:(0.8-1) to obtain a negative electrode slurry. The solid content of the negative electrode slurry is 28%, and the viscosity of the negative electrode slurry is 4000 mPa. s, the pH of the negative electrode slurry is 7.0; (3) Gradient coating: The negative electrode slurry obtained in step (2) is coated onto the surface of carbon fiber cloth by step-by-step scraping method. The slurry coating thickness of the bottom layer is 40-50μm, the slurry coating thickness of the surface layer is 60-70μm, and the total coating thickness is 100-120μm to obtain the electrode blank. (4) Vacuum drying: The electrode blank obtained in step (3) is dried. First, it is placed at a low temperature of 65°C for 2.5 hours for pre-drying, and then vacuum dried at a temperature of 95°C, a drying pressure of -0.09MPa, and a drying time of 4.5 hours to obtain the initial flexible electrode. (5) Low temperature gradient rolling: The initial flexible electrode obtained in step (4) is rolled at a low temperature of 8℃ with a pressure of 5-8MPa 1-2 times, and then rolled at a pressure of 10-15MPa 1-2 times with a roller speed of 0.8m / min to obtain the pressed initial flexible electrode. (6) Sheet making: The pressed initial flexible electrode obtained in step (5) is cut and punched to obtain a flexible negative electrode sheet for sodium battery. The flexible negative electrode sheet for sodium battery is a circular sheet with a diameter of 12mm.

[0006] As a preferred embodiment, the modified hard carbon is a modified hard carbon with a composite coating layer.

[0007] As a preferred embodiment, the composite coating layer is made of polypyrrole and carbon nanotubes.

[0008] As a preferred embodiment, the conductive agent is conductive carbon black, the first crosslinking agent is organically modified montmorillonite, and the second crosslinking agent is aldehyde-terminated hyperbranched polyester.

[0009] As a preferred embodiment, the areal density of the pre-pressed flexible electrode is 15-20 mg / cm², and the density of the pre-pressed flexible electrode is 0.8-0.9 g / cm³.

[0010] Compared with the prior art, the present invention has obvious advantages and beneficial effects. Specifically, as can be seen from the above technical solution: By employing a compound of two crosslinking agents, consisting of crosslinking components with at least two different functional groups, a synergistic effect of chemical crosslinking and physical entanglement occurs during the preparation process, thereby constructing a three-dimensional interpenetrating network. This imparts excellent flexibility and high mechanical strength to the electrode, effectively buffers charge-discharge volume stress, ensures structural integrity, prevents cracking, and passivates active sites on the electrode surface, inhibiting irreversible electrolyte decomposition and inducing the formation of a thin and stable SEI film. This significantly reduces initial sodium loss and improves initial coulombic efficiency. Simultaneously, it enhances the interfacial bonding between the active material, conductive agent, and current collector, making them less prone to peeling, reducing interfacial impedance, and optimizing ion and electron conduction.

[0011] To more clearly illustrate the structural features and effects of the present invention, the present invention will be described in detail below with reference to several specific embodiments. Detailed Implementation

[0012] This invention discloses a method for preparing a flexible negative electrode sheet for sodium batteries, which includes the following main steps: (1) Raw material pretreatment: The modified hard carbon, conductive agent and first crosslinking agent are placed in a vacuum drying oven and dried at 85°C for 3.5h for later use; the carbon fiber is placed in anhydrous ethanol for ultrasonic cleaning with an ultrasonic power of 300W and a cleaning time of 25min, and then dried at 65°C for 3.5h for later use; the modified hard carbon is a modified hard carbon with a composite coating layer. Specifically, the composite coating layer is made of polypyrrole and carbon nanotubes, and the conductive agent is conductive carbon black, and the first crosslinking agent is organic modified montmorillonite.

[0013] (2) Preparation of negative electrode slurry: By weight, take 90-96 parts of modified hard carbon and 1-3 parts of conductive agent and add them to 200-300 parts of deionized water. Disperse ultrasonically for 25 min at an ultrasonic power of 350 W and stir at 250 r / min for 18 min to obtain a pre-dispersion liquid. Add 2-4 parts of composite binder, 0.1-0.3 parts of first crosslinking agent, 0.7-0.9 parts of second crosslinking agent, and 0.3-0.5 parts of sodium citrate to the pre-dispersion liquid. Then stir for 35 min at a stirring speed of 400 r / min. The composite binder is composed of sodium alginate and carboxymethyl cellulose, and the mass ratio of sodium alginate to carboxymethyl cellulose is 1:(0.8-1) to obtain a negative electrode slurry. The solid content of the negative electrode slurry is 28%, and the viscosity of the negative electrode slurry is 4000 mPa. s, the pH of the negative electrode slurry is 7.0; the second crosslinking agent is a terminal aldehyde hyperbranched polyester.

[0014] (3) Gradient coating: The negative electrode slurry obtained in step (2) is coated onto the surface of carbon fiber cloth by step-by-step scraping method. The slurry coating thickness of the bottom layer is 40-50μm, the slurry coating thickness of the surface layer is 60-70μm, and the total coating thickness is 100-120μm to obtain the electrode blank.

[0015] (4) Vacuum drying: The electrode blank obtained in step (3) is dried. First, it is placed at a low temperature of 65°C for 2.5 hours for pre-drying, and then vacuum dried at a temperature of 95°C, a drying pressure of -0.09MPa, and a drying time of 4.5 hours to obtain the initial flexible electrode.

[0016] (5) Low temperature gradient rolling: The initial flexible electrode obtained in step (4) is rolled at a low temperature of 8°C with a pressure of 5-8 MPa 1-2 times, and then rolled at a pressure of 10-15 MPa 1-2 times, with a roller speed of 0.8 m / min, to obtain the pressed initial flexible electrode; the electrode surface density of the pressed initial flexible electrode is 15-20 mg / cm², and the density of the pressed initial flexible electrode is 0.8-0.9 g / cm³.

[0017] (6) Sheet making: The pressed initial flexible electrode obtained in step (5) is cut and punched to obtain a flexible negative electrode sheet for sodium battery. The flexible negative electrode sheet for sodium battery is a circular sheet with a diameter of 12mm.

[0018] The following is a detailed analysis using multiple embodiments and comparative examples.

[0019] Example 1 (1) Raw material pretreatment: The modified hard carbon, conductive agent and first crosslinking agent are placed in a vacuum drying oven and dried at 85°C for 3.5h for later use; the carbon fiber is placed in anhydrous ethanol for ultrasonic cleaning with an ultrasonic power of 300W and a cleaning time of 25min, and then dried at 65°C for 3.5h for later use; the modified hard carbon is a modified hard carbon with a composite coating layer. Specifically, the composite coating layer is made of polypyrrole and carbon nanotubes, and the conductive agent is conductive carbon black, and the first crosslinking agent is organic modified montmorillonite.

[0020] (2) Preparation of negative electrode slurry: 93 parts by weight of modified hard carbon and 2 parts by weight of conductive agent were added to 250 parts by weight of deionized water, ultrasonically dispersed for 25 min at an ultrasonic power of 350 W, and stirred at 250 r / min for 18 min to obtain a pre-dispersion liquid; 3 parts by weight of composite binder, 0.2 parts by weight of first crosslinking agent, 0.8 parts by weight of second crosslinking agent, and 0.4 parts by weight of sodium citrate were added to the pre-dispersion liquid, and then stirred for 35 min at a stirring speed of 400 r / min. The composite binder is composed of sodium alginate and carboxymethyl cellulose, and the mass ratio of sodium alginate to carboxymethyl cellulose is 1:1 to obtain the negative electrode slurry. The solid content of the negative electrode slurry is 28%, and the viscosity of the negative electrode slurry is 4000 mPa. s, the pH of the negative electrode slurry is 7.0; the second crosslinking agent is a terminal aldehyde hyperbranched polyester.

[0021] (3) Gradient coating: The negative electrode slurry obtained in step (2) is coated onto the surface of carbon fiber cloth using a step-by-step scraping method. The slurry coating thickness of the bottom layer is 45 μm, the slurry coating thickness of the surface layer is 65 μm, and the total coating thickness is 110 μm, thus obtaining the electrode blank.

[0022] (4) Vacuum drying: The electrode blank obtained in step (3) is dried. First, it is placed at a low temperature of 65°C for 2.5 hours for pre-drying, and then vacuum dried at a temperature of 95°C, a drying pressure of -0.09MPa, and a drying time of 4.5 hours to obtain the initial flexible electrode.

[0023] (5) Low temperature gradient rolling: The initial flexible electrode obtained in step (4) is rolled once at a low temperature of 8°C with a pressure of 6MPa, and then rolled once at a pressure of 12MPa with a roller speed of 0.8m / min to obtain the pressed initial flexible electrode; the electrode surface density of the pressed initial flexible electrode is 18mg / cm², and the density of the pressed initial flexible electrode is 0.85g / cm³.

[0024] (6) Sheet making: The pressed initial flexible electrode obtained in step (5) is cut and punched to obtain a flexible negative electrode sheet for sodium battery. The flexible negative electrode sheet for sodium battery is a circular sheet with a diameter of 12mm.

[0025] Example 2 (1) Raw material pretreatment: The modified hard carbon, conductive agent and first crosslinking agent are placed in a vacuum drying oven and dried at 85°C for 3.5h for later use; the carbon fiber is placed in anhydrous ethanol for ultrasonic cleaning with an ultrasonic power of 300W and a cleaning time of 25min, and then dried at 65°C for 3.5h for later use; the modified hard carbon is a modified hard carbon with a composite coating layer. Specifically, the composite coating layer is made of polypyrrole and carbon nanotubes, and the conductive agent is conductive carbon black, and the first crosslinking agent is organic modified montmorillonite.

[0026] (2) Preparation of negative electrode slurry: 90 parts by weight of modified hard carbon and 1 part by weight of conductive agent were added to 200 parts by weight of deionized water, ultrasonically dispersed for 25 min at an ultrasonic power of 350 W, and stirred at 250 r / min for 18 min to obtain a pre-dispersion liquid; 2 parts by weight of composite binder, 0.1 parts by weight of first crosslinking agent, 0.7 parts by weight of second crosslinking agent, and 0.3 parts by weight of sodium citrate were added to the pre-dispersion liquid, and then stirred for 35 min at a stirring speed of 400 r / min. The composite binder is composed of sodium alginate and carboxymethyl cellulose, and the mass ratio of sodium alginate to carboxymethyl cellulose is 1:0.8 to obtain the negative electrode slurry. The solid content of the negative electrode slurry is 28%, and the viscosity of the negative electrode slurry is 4000 mPa. s, the pH of the negative electrode slurry is 7.0; the second crosslinking agent is a terminal aldehyde hyperbranched polyester.

[0027] (3) Gradient coating: The negative electrode slurry obtained in step (2) is coated onto the surface of carbon fiber cloth using a step-by-step scraping method. The slurry coating thickness of the bottom layer is 40 μm, the slurry coating thickness of the top layer is 60 μm, and the total coating thickness is 100 μm, thus obtaining the electrode blank.

[0028] (4) Vacuum drying: The electrode blank obtained in step (3) is dried. First, it is placed at a low temperature of 65°C for 2.5 hours for pre-drying, and then vacuum dried at a temperature of 95°C, a drying pressure of -0.09MPa, and a drying time of 4.5 hours to obtain the initial flexible electrode.

[0029] (5) Low temperature gradient rolling: The initial flexible electrode obtained in step (4) is rolled twice at a low temperature of 8°C with a pressure of 5MPa, and then rolled twice at a pressure of 10MPa with a roller speed of 0.8m / min to obtain the pressed initial flexible electrode; the electrode surface density of the pressed initial flexible electrode is 15mg / cm², and the density of the pressed initial flexible electrode is 0.8g / cm³.

[0030] (6) Sheet making: The pressed initial flexible electrode obtained in step (5) is cut and punched to obtain a flexible negative electrode sheet for sodium battery. The flexible negative electrode sheet for sodium battery is a circular sheet with a diameter of 12mm.

[0031] Example 3 (1) Raw material pretreatment: The modified hard carbon, conductive agent and first crosslinking agent are placed in a vacuum drying oven and dried at 85°C for 3.5h for later use; the carbon fiber is placed in anhydrous ethanol for ultrasonic cleaning with an ultrasonic power of 300W and a cleaning time of 25min, and then dried at 65°C for 3.5h for later use; the modified hard carbon is a modified hard carbon with a composite coating layer. Specifically, the composite coating layer is made of polypyrrole and carbon nanotubes, and the conductive agent is conductive carbon black, and the first crosslinking agent is organic modified montmorillonite.

[0032] (2) Preparation of negative electrode slurry: 96 parts by weight of modified hard carbon and 3 parts by weight of conductive agent were added to 300 parts by weight of deionized water, ultrasonically dispersed for 25 min at an ultrasonic power of 350 W, and stirred at 250 r / min for 18 min to obtain a pre-dispersion liquid; 4 parts by weight of composite binder, 0.3 parts by weight of first crosslinking agent, 0.9 parts by weight of second crosslinking agent, and 0.5 parts by weight of sodium citrate were added to the pre-dispersion liquid, and then stirred for 35 min at a stirring speed of 400 r / min. The composite binder is composed of sodium alginate and carboxymethyl cellulose, and the mass ratio of sodium alginate to carboxymethyl cellulose is 1:1 to obtain the negative electrode slurry. The solid content of the negative electrode slurry is 28%, and the viscosity of the negative electrode slurry is 4000 mPa. s, the pH of the negative electrode slurry is 7.0; the second crosslinking agent is a terminal aldehyde hyperbranched polyester.

[0033] (3) Gradient coating: The negative electrode slurry obtained in step (2) is coated onto the surface of carbon fiber cloth using a step-by-step scraping method. The slurry coating thickness of the bottom layer is 50 μm, the slurry coating thickness of the surface layer is 70 μm, and the total coating thickness is 20 μm, thus obtaining the electrode blank.

[0034] (4) Vacuum drying: The electrode blank obtained in step (3) is dried. First, it is placed at a low temperature of 65°C for 2.5 hours for pre-drying, and then vacuum dried at a temperature of 95°C, a drying pressure of -0.09MPa, and a drying time of 4.5 hours to obtain the initial flexible electrode.

[0035] (5) Low temperature gradient rolling: The initial flexible electrode obtained in step (4) is rolled twice at a low temperature of 8℃ with a pressure of 8MPa, and then rolled twice at a pressure of 15MPa with a roller speed of 0.8m / min to obtain the pressed initial flexible electrode; the electrode surface density of the pressed initial flexible electrode is 20mg / cm², and the density of the pressed initial flexible electrode is 0.9g / cm³.

[0036] (6) Sheet making: The pressed initial flexible electrode obtained in step (5) is cut and punched to obtain a flexible negative electrode sheet for sodium battery. The flexible negative electrode sheet for sodium battery is a circular sheet with a diameter of 12mm.

[0037] Example 4 (1) Raw material pretreatment: The modified hard carbon, conductive agent and first crosslinking agent are placed in a vacuum drying oven and dried at 85°C for 3.5h for later use; the carbon fiber is placed in anhydrous ethanol for ultrasonic cleaning with an ultrasonic power of 300W and a cleaning time of 25min, and then dried at 65°C for 3.5h for later use; the modified hard carbon is a modified hard carbon with a composite coating layer. Specifically, the composite coating layer is made of polypyrrole and carbon nanotubes, and the conductive agent is conductive carbon black, and the first crosslinking agent is organic modified montmorillonite.

[0038] (2) Preparation of negative electrode slurry: 93 parts by weight of modified hard carbon and 2 parts by weight of conductive agent were added to 250 parts by weight of deionized water, ultrasonically dispersed for 25 min at an ultrasonic power of 350 W, and stirred at 250 r / min for 18 min to obtain a pre-dispersion liquid; 3 parts by weight of composite binder, 0.1 parts by weight of first crosslinking agent, 0.9 parts by weight of second crosslinking agent, and 0.4 parts by weight of sodium citrate were added to the pre-dispersion liquid, and then stirred for 35 min at a stirring speed of 400 r / min. The composite binder is composed of sodium alginate and carboxymethyl cellulose, and the mass ratio of sodium alginate to carboxymethyl cellulose is 1:0.9 to obtain the negative electrode slurry. The solid content of the negative electrode slurry is 28%, and the viscosity of the negative electrode slurry is 4000 mPa. s, the pH of the negative electrode slurry is 7.0; the second crosslinking agent is a terminal aldehyde hyperbranched polyester.

[0039] (3) Gradient coating: The negative electrode slurry obtained in step (2) is coated onto the surface of carbon fiber cloth using a step-by-step scraping method. The slurry coating thickness of the bottom layer is 45 μm, the slurry coating thickness of the surface layer is 65 μm, and the total coating thickness is 110 μm, thus obtaining the electrode blank.

[0040] (4) Vacuum drying: The electrode blank obtained in step (3) is dried. First, it is placed at a low temperature of 65°C for 2.5 hours for pre-drying, and then vacuum dried at a temperature of 95°C, a drying pressure of -0.09MPa, and a drying time of 4.5 hours to obtain the initial flexible electrode.

[0041] (5) Low temperature gradient rolling: The initial flexible electrode obtained in step (4) is rolled once at a low temperature of 8°C with a pressure of 6MPa, and then rolled once at a pressure of 12MPa with a roller speed of 0.8m / min to obtain the pressed initial flexible electrode; the electrode surface density of the pressed initial flexible electrode is 18mg / cm², and the density of the pressed initial flexible electrode is 0.85g / cm³.

[0042] (6) Sheet making: The pressed initial flexible electrode obtained in step (5) is cut and punched to obtain a flexible negative electrode sheet for sodium battery. The flexible negative electrode sheet for sodium battery is a circular sheet with a diameter of 12mm.

[0043] Example 5 (1) Raw material pretreatment: The modified hard carbon, conductive agent and first crosslinking agent are placed in a vacuum drying oven and dried at 85°C for 3.5h for later use; the carbon fiber is placed in anhydrous ethanol for ultrasonic cleaning with an ultrasonic power of 300W and a cleaning time of 25min, and then dried at 65°C for 3.5h for later use; the modified hard carbon is a modified hard carbon with a composite coating layer. Specifically, the composite coating layer is made of polypyrrole and carbon nanotubes, and the conductive agent is conductive carbon black, and the first crosslinking agent is organic modified montmorillonite.

[0044] (2) Preparation of negative electrode slurry: 90 parts by weight of modified hard carbon and 1 part by weight of conductive agent were added to 200 parts of deionized water, ultrasonically dispersed for 25 min at an ultrasonic power of 350 W, and stirred at 250 r / min for 18 min to obtain a pre-dispersion liquid; 2 parts by weight of composite binder, 0.2 parts by weight of first crosslinking agent, 0.9 parts by weight of second crosslinking agent, and 0.2 parts by weight of sodium citrate were added to the pre-dispersion liquid, and then stirred for 35 min at a stirring speed of 400 r / min. The composite binder is composed of sodium alginate and carboxymethyl cellulose, and the mass ratio of sodium alginate to carboxymethyl cellulose is 1:1 to obtain the negative electrode slurry. The solid content of the negative electrode slurry is 28%, and the viscosity of the negative electrode slurry is 4000 mPa. s, the pH of the negative electrode slurry is 7.0; the second crosslinking agent is a terminal aldehyde hyperbranched polyester.

[0045] (3) Gradient coating: The negative electrode slurry obtained in step (2) is coated onto the surface of carbon fiber cloth using a step-by-step scraping method. The slurry coating thickness of the bottom layer is 40 μm, the slurry coating thickness of the top layer is 60 μm, and the total coating thickness is 100 μm, thus obtaining the electrode blank.

[0046] (4) Vacuum drying: The electrode blank obtained in step (3) is dried. First, it is placed at a low temperature of 65°C for 2.5 hours for pre-drying, and then vacuum dried at a temperature of 95°C, a drying pressure of -0.09MPa, and a drying time of 4.5 hours to obtain the initial flexible electrode.

[0047] (5) Low temperature gradient rolling: The initial flexible electrode obtained in step (4) is rolled twice at a low temperature of 8°C with a pressure of 5MPa, and then rolled twice at a pressure of 10MPa with a roller speed of 0.8m / min to obtain the pressed initial flexible electrode; the electrode surface density of the pressed initial flexible electrode is 15mg / cm², and the density of the pressed initial flexible electrode is 0.8g / cm³.

[0048] (6) Sheet making: The pressed initial flexible electrode obtained in step (5) is cut and punched to obtain a flexible negative electrode sheet for sodium battery. The flexible negative electrode sheet for sodium battery is a circular sheet with a diameter of 12mm.

[0049] Example 6 (1) Raw material pretreatment: The modified hard carbon, conductive agent and first crosslinking agent are placed in a vacuum drying oven and dried at 85°C for 3.5h for later use; the carbon fiber is placed in anhydrous ethanol for ultrasonic cleaning with an ultrasonic power of 300W and a cleaning time of 25min, and then dried at 65°C for 3.5h for later use; the modified hard carbon is a modified hard carbon with a composite coating layer. Specifically, the composite coating layer is made of polypyrrole and carbon nanotubes, and the conductive agent is conductive carbon black, and the first crosslinking agent is organic modified montmorillonite.

[0050] (2) Preparation of negative electrode slurry: 96 parts by weight of modified hard carbon and 3 parts by weight of conductive agent were added to 300 parts by weight of deionized water, ultrasonically dispersed for 25 min at an ultrasonic power of 350 W, and stirred at 250 r / min for 18 min to obtain a pre-dispersion liquid; 4 parts by weight of composite binder, 0.3 parts by weight of first crosslinking agent, 0.8 parts by weight of second crosslinking agent, and 0.5 parts by weight of sodium citrate were added to the pre-dispersion liquid, and then stirred for 35 min at a stirring speed of 400 r / min. The composite binder is composed of sodium alginate and carboxymethyl cellulose, and the mass ratio of sodium alginate to carboxymethyl cellulose is 1:0.95 to obtain the negative electrode slurry. The solid content of the negative electrode slurry is 28%, and the viscosity of the negative electrode slurry is 4000 mPa. s, the pH of the negative electrode slurry is 7.0; the second crosslinking agent is a terminal aldehyde hyperbranched polyester.

[0051] (3) Gradient coating: The negative electrode slurry obtained in step (2) is coated onto the surface of carbon fiber cloth using a step-by-step scraping method. The slurry coating thickness of the bottom layer is 50 μm, the slurry coating thickness of the surface layer is 70 μm, and the total coating thickness is 20 μm, thus obtaining the electrode blank.

[0052] (4) Vacuum drying: The electrode blank obtained in step (3) is dried. First, it is placed at a low temperature of 65°C for 2.5 hours for pre-drying, and then vacuum dried at a temperature of 95°C, a drying pressure of -0.09MPa, and a drying time of 4.5 hours to obtain the initial flexible electrode.

[0053] (5) Low temperature gradient rolling: The initial flexible electrode obtained in step (4) is rolled twice at a low temperature of 8℃ with a pressure of 8MPa, and then rolled twice at a pressure of 15MPa with a roller speed of 0.8m / min to obtain the pressed initial flexible electrode; the electrode surface density of the pressed initial flexible electrode is 20mg / cm², and the density of the pressed initial flexible electrode is 0.9g / cm³.

[0054] (6) Sheet making: The pressed initial flexible electrode obtained in step (5) is cut and punched to obtain a flexible negative electrode sheet for sodium battery. The flexible negative electrode sheet for sodium battery is a circular sheet with a diameter of 12mm.

[0055] Comparative Example 1 (1) Raw material pretreatment: The modified hard carbon and conductive agent are placed in a vacuum drying oven and dried at 85°C for 3.5h for later use; the carbon fiber is placed in anhydrous ethanol for ultrasonic cleaning with an ultrasonic power of 300W and a cleaning time of 25min, and then dried at 65°C for 3.5h for later use; the modified hard carbon is a modified hard carbon with a composite coating layer. Specifically, the composite coating layer is made of polypyrrole and carbon nanotubes, and the conductive agent is conductive carbon black.

[0056] (2) Preparation of negative electrode slurry: 93 parts by weight of modified hard carbon and 2 parts by weight of conductive agent were added to 250 parts by weight of deionized water, ultrasonically dispersed for 25 min at an ultrasonic power of 350 W, and stirred at 250 r / min for 18 min to obtain a pre-dispersion liquid; 3 parts by weight of sodium alginate, 1 part by weight of aldehyde-terminated hyperbranched polyester, and 0.4 parts by weight of sodium citrate were added to the pre-dispersion liquid, and then stirred for 35 min at a stirring speed of 400 r / min to obtain a negative electrode slurry. The solid content of the negative electrode slurry was 28%, and the viscosity of the negative electrode slurry was 4000 mPa. s, the pH of the negative electrode slurry is 7.0; the second crosslinking agent is.

[0057] (3) Gradient coating: The negative electrode slurry obtained in step (2) is coated onto the surface of carbon fiber cloth using a step-by-step scraping method. The slurry coating thickness of the bottom layer is 45 μm, the slurry coating thickness of the surface layer is 65 μm, and the total coating thickness is 110 μm, thus obtaining the electrode blank.

[0058] (4) Vacuum drying: The electrode blank obtained in step (3) is dried. First, it is placed at a low temperature of 65°C for 2.5 hours for pre-drying, and then vacuum dried at a temperature of 95°C, a drying pressure of -0.09MPa, and a drying time of 4.5 hours to obtain the initial flexible electrode.

[0059] (5) Low temperature gradient rolling: The initial flexible electrode obtained in step (4) is rolled once at a low temperature of 8°C with a pressure of 6MPa, and then rolled once at a pressure of 12MPa with a roller speed of 0.8m / min to obtain the pressed initial flexible electrode; the electrode surface density of the pressed initial flexible electrode is 18mg / cm², and the density of the pressed initial flexible electrode is 0.85g / cm³.

[0060] (6) Sheet making: The pressed initial flexible electrode obtained in step (5) is cut and punched to obtain a flexible negative electrode sheet for sodium battery. The flexible negative electrode sheet for sodium battery is a circular sheet with a diameter of 12mm.

[0061] Performance tests were conducted on the above embodiments and comparative examples. The test results are shown in Table 1. The test methods are as follows: The testing method is as follows: 1. Electrochemical performance: The Blue Battery Tester was used for constant current charge-discharge testing with a voltage window of 0.01~2.5V to test the 0.1C discharge specific capacity, initial coulombic efficiency, 1C discharge specific capacity, and capacity retention rate after 200 cycles at 0.1C. 2. Flexible mechanical properties: The negative electrode sheet was subjected to repeated 180° bending tests at a bending rate of 10 times / minute. After 500 bending cycles, the damage of the electrode and the retention rate of the 0.1C discharge specific capacity of the assembled battery were tested. The bonding strength between the electrode layer and the carbon fiber cloth was tested using the 90° peel method.

[0062]

[0063] Table 1 A detailed analysis of the above data shows that the flexible negative electrode sheet prepared by the method of this invention is superior to Comparative Example 1 in all electrochemical and mechanical properties. Comparing Example 1 and Comparative Example 1, the difference lies in that Comparative Example 1 did not use a composite binder and crosslinking agent; all other parameters are the same. However, Example 1 exhibits a 0.1C discharge specific capacity range of 342.6 mAh / g and a 1C discharge specific capacity range of 315.2 mAh / g, significantly higher than Comparative Example 1's 285.3 mAh / g and 262.4 mAh / g. The specific capacity of Example 1 is 93.8%, significantly higher than that of Comparative Example 1 (87.5%). Regarding cycle stability, Example 1 achieves 99.3%, and after 500 bends, the capacity retention remains at 98.2%, while Comparative Example 1 only achieves 92.1% and 85.3% respectively, indicating poor bending resistance and a much shorter lifespan. Most importantly, in terms of mechanical bonding performance, the bonding strength of Example 1 is nearly twice that of Comparative Example 1, and the electrode sheet shows no damage or peeling after 500 bends, while Comparative Example 1 exhibits microcracks and localized peeling in the electrode layer. This fully demonstrates that the preparation method of this invention can simultaneously improve the specific capacity, charge-discharge reversibility, cycle life, and flexible mechanical stability of the electrode sheet.

[0064] The above description is merely a preferred embodiment of the present invention and does not constitute any limitation on the technical scope of the present invention. Therefore, any minor modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a flexible negative electrode sheet for sodium batteries, characterized in that: It includes the following main steps: (1) Raw material pretreatment: The modified hard carbon, conductive agent and first crosslinking agent were placed in a vacuum drying oven and dried at 85°C for 3.5h for later use; the carbon fibers were arranged in anhydrous ethanol for ultrasonic cleaning with an ultrasonic power of 300W and a cleaning time of 25min, and then dried at 65°C for 3.5h for later use. (2) Preparation of negative electrode slurry: By weight, take 90-96 parts of modified hard carbon and 1-3 parts of conductive agent and add them to 200-300 parts of deionized water. Disperse ultrasonically for 25 min at an ultrasonic power of 350 W and stir at 250 r / min for 18 min to obtain a pre-dispersion liquid. Add 2-4 parts of composite binder, 0.1-0.3 parts of first crosslinking agent, 0.7-0.9 parts of second crosslinking agent, and 0.3-0.5 parts of sodium citrate to the pre-dispersion liquid. Then stir for 35 min at a stirring speed of 400 r / min. The composite binder is composed of sodium alginate and carboxymethyl cellulose, and the mass ratio of sodium alginate to carboxymethyl cellulose is 1:(0.8-1) to obtain a negative electrode slurry. The solid content of the negative electrode slurry is 28%, and the viscosity of the negative electrode slurry is 4000 mPa. s, the pH of the negative electrode slurry is 7.0; (3) Gradient coating: The negative electrode slurry obtained in step (2) is coated onto the surface of carbon fiber cloth by step-by-step scraping method. The slurry coating thickness of the bottom layer is 40-50μm, the slurry coating thickness of the surface layer is 60-70μm, and the total coating thickness is 100-120μm to obtain the electrode blank. (4) Vacuum drying: The electrode blank obtained in step (3) is dried. First, it is placed at a low temperature of 65°C for 2.5 hours for pre-drying, and then vacuum dried at a temperature of 95°C, a drying pressure of -0.09MPa, and a drying time of 4.5 hours to obtain the initial flexible electrode. (5) Low temperature gradient rolling: The initial flexible electrode obtained in step (4) is rolled at a low temperature of 8℃ with a pressure of 5-8MPa 1-2 times, and then rolled at a pressure of 10-15MPa 1-2 times with a roller speed of 0.8m / min to obtain the pressed initial flexible electrode. (6) Sheet making: The pressed initial flexible electrode obtained in step (5) is cut and punched to obtain a flexible negative electrode sheet for sodium battery.

2. The method for preparing a flexible negative electrode sheet for a sodium battery according to claim 1, characterized in that: The modified hard carbon is a modified hard carbon with a composite coating layer.

3. The method for preparing a flexible negative electrode sheet for a sodium battery according to claim 2, characterized in that: The composite coating layer is made of polypyrrole and carbon nanotubes.

4. The method for preparing a flexible negative electrode sheet for a sodium battery according to claim 1, characterized in that: The conductive agent is conductive carbon black, the first crosslinking agent is organically modified montmorillonite, and the second crosslinking agent is aldehyde-terminated hyperbranched polyester.

5. The method for preparing a flexible negative electrode sheet for a sodium battery according to claim 1, characterized in that: The areal density of the pre-fabricated flexible electrode after pressing is 15-20 mg / cm², and the density of the pre-fabricated flexible electrode after pressing is 0.8-0.9 g / cm³.

6. The method for preparing a flexible negative electrode sheet for a sodium battery according to claim 1, characterized in that: The flexible negative electrode sheet used in the sodium battery is a circular sheet with a diameter of 12 mm.

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