A method for preparing a composite electrode for lithium extraction from salt lakes

Abstract

This application discloses a method for preparing a composite electrode for lithium extraction from salt lakes, belonging to the field of salt lake lithium extraction technology. The method involves first preparing nitrogen-doped carbon nanotubes, then mixing nitrogen-doped carbon nanotubes of different masses with the same mass of electrode active material to obtain an electrode active material coated with a nitrogen-doped carbon nanotube layer. These layers are then modified with dopamine to prepare electrode slurries, which are then coated onto a current collector. The mass of the pore-forming agent in each active coating layer decreases in a gradient along the direction away from the current collector, and the thickness of the modified nitrogen-doped carbon coating layer in the electrode active material also decreases in a gradient. By using active materials with different carbon coating layer thicknesses to prepare the lithium extraction electrode, with the bottom carbon coating layer being thicker closer to the current collector, the electrochemical reaction rate of lithium ions is suppressed to a certain extent, thereby reducing cathode polarization, preventing impurity cations from embedding into the crystal lattice, and improving the purity of lithium ions in the salt lake lithium extraction recovery solution.

Description

Technical Field

[0001] This application relates to the field of lithium extraction technology from salt lakes, specifically to a method for preparing a composite electrode for lithium extraction from salt lakes. Background Technology

[0002] In the process of lithium extraction from salt lakes using electrochemical methods, the diffusion of the solution inside the electrode is slow. The electrode reaction proceeds sequentially from the electrode surface to the electrode interior. During production, the reaction rate is usually accelerated by increasing the cell voltage. However, when a large voltage is applied, the electrochemical reaction rate is much faster than the diffusion rate of the solution inside the electrode. The reaction is easily controlled by external diffusion. When the electrode is controlled by external diffusion, the lithium ion reaction rate inside the electrode accelerates, and unreacted lithium ions are rapidly consumed. Lithium ions in the solution cannot be replenished to the electrode interior in time. At this time, coexisting impurity cations in the solution are easily embedded in the internal electrode material and released into the solution, thereby reducing the purity of lithium ions in the recovered solution.

[0003] The related technology discloses a method for preparing an electrochemical lithium extraction electrode, characterized by the following steps: mixing lithium extraction material with a conductive agent, a pore-forming agent, inorganic hydrophilic nanomaterials, a binder, and N-methylpyrrolidone to form a slurry; then uniformly coating the slurry onto a current collector; and finally drying and immersing in water to obtain the electrochemical lithium extraction electrode. This electrode improves the selectivity of the active material for lithium ions by coating the surface of the active material with a hydrophilic graphene / manganese ion sieve composite that selectively transports lithium ions; however, the lithium content in the brine after treatment remains relatively high.

[0004] Therefore, how to reduce the lithium content in brine has become a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] The purpose of this application is to overcome the shortcomings of the existing technology and provide a method for preparing a composite electrode for lithium extraction from salt lakes. The composite electrode reduces the reaction rate of lithium ions inside the electrode while increasing the diffusion rate of ions, so that the lithium extraction process is under electrochemical control, thereby avoiding the embedding of impurity ions inside the electrode and improving the purity of lithium ions in the lithium recovery solution from salt lakes.

[0006] To achieve the above objectives, the technical solution adopted in this application is as follows:

[0007] A method for preparing a composite electrode for lithium extraction from salt lakes includes the following steps:

[0008] Nitrogen-containing organic matter and carbon nanotubes are dispersed in a first solvent, heated under reflux, filtered, dried, washed with water, and dried again to obtain nitrogen-doped carbon nanotubes.

[0009] Take n portions of nitrogen-doped carbon nanotubes of different masses, disperse the nitrogen-doped carbon nanotubes and electrode active materials in a second solvent, ball mill, and dry to obtain n electrode active materials coated with nitrogen-doped carbon nanotube layers of different thicknesses; wherein, the mass of the electrode active material corresponding to each portion of nitrogen-doped carbon nanotubes is the same.

[0010] The prepared nitrogen-doped carbon nanotube-coated electrode active materials were added to dopamine solution for modification, filtered, washed, and dried to obtain n kinds of modified nitrogen-doped carbon-coated electrode active materials with different thicknesses.

[0011] The modified nitrogen-doped carbon coated electrode active material was prepared into n parts of electrode slurry with pore-forming agent, reinforcing agent, conductive agent, binder and third solvent. The electrode slurry was then sequentially coated onto the current collector, dried, rolled and water-immersed to form n active coating layers. The mass of the pore-forming agent in each active coating layer along the direction away from the current collector decreased in a gradient, and the thickness of the modified nitrogen-doped carbon coated layer in the modified nitrogen-doped carbon coated electrode active material decreased in a gradient.

[0012] Where n is a positive integer ≥ 2.

[0013] This application first prepares nitrogen-doped carbon nanotubes, then mixes nitrogen-doped carbon nanotubes of different masses with the same mass of electrode active material to obtain n electrode active materials coated with nitrogen-doped carbon nanotube layers of different thicknesses; then modifies them with dopamine, and then prepares n electrode slurries by mixing them with a pore-forming agent, a reinforcing agent, a conductive agent, a binder and a third solvent, and applies them to a current collector, so that the mass of the pore-forming agent in each active coating along the direction away from the current collector decreases in a gradient, and the thickness of the modified nitrogen-doped carbon coating layer in the modified nitrogen-doped carbon coating layer electrode active material decreases in a gradient.

[0014] During the lithium extraction process, the composite electrode at the bottom (near the current collector) experiences rapid lithium ion consumption due to cathode polarization. The lithium extraction process is diffusion-controlled, and cationic impurity metals such as sodium and magnesium ions enter the material's crystal lattice through electrolysis. This application uses active materials with different carbon coating thicknesses to prepare the lithium extraction electrode in layers. The bottom carbon coating layer, closer to the current collector, is thicker. This improves conductivity while reducing the contact area between the electrode material and the electrolyte, which to some extent inhibits the electrochemical reaction rate of lithium ions, thereby reducing cathode polarization, preventing impurity cations from embedding into the crystal lattice, and improving the purity of lithium ions in the salt lake lithium extraction recovery solution.

[0015] The high mineralization and viscosity of the brine hinder mass transfer within the electrode, resulting in a lower lithium-ion transport rate at the electrode bottom. Adding a larger amount of pore-forming agent to the electrode slurry coated near the current collector layer than to the electrode surface layer further away increases porosity at the electrode bottom, enhancing mass transfer. However, excessive porosity at the electrode bottom reduces the stability of the electrode material. Coating the bottom electrode material with a larger amount of carbon as a framework can improve its stability. The synergistic effect of these two methods can increase the lithium-ion mass transfer rate at the electrode bottom while simultaneously enhancing the stability of the electrode material.

[0016] By coating the surface of electrode materials with carbon nanotubes, their stable structure can form stable ion channels, preventing lattice breakage caused by repeated lithium-ion insertion and extraction, and improving the electrode's recyclability. The carbon coating layer forms a conductive network, reducing impedance, and the polydopamine loaded on the carbon nanotubes has a selective effect on lithium ions, improving the electrode's selectivity for lithium ions.

[0017] Nitrogen doping of carbon nanotubes can not only improve their hydrophilicity, but also change the local charge density of carbon nanotubes, improve their electron transport, and reduce their resistivity. At the same time, the nitrogen-containing functional groups introduced by nitrogen doping can significantly improve the specific capacitance of carbon nanotube electrochemical capacitors.

[0018] As a preferred embodiment of this application, the mass of the modified nitrogen-doped carbon coating electrode active material, reinforcing agent, conductive agent, binder, and third solvent in each portion of electrode slurry is the same.

[0019] As a preferred embodiment of this application, the quality of the pore-forming agent in each portion of electrode slurry is different.

[0020] By controlling the mass of the modified nitrogen-doped carbon coating layer of each electrode slurry to be the same, and the mass of the pore-forming agent to be different, the mass of the pore-forming agent in each active coating layer along the direction away from the current collector decreases in a gradient during coating, and the thickness of the modified nitrogen-doped carbon coating layer in the electrode active material of the modified nitrogen-doped carbon coating layer decreases in a gradient.

[0021] As a preferred embodiment of this application, the nitrogen-containing organic compound includes at least one of hydrazine hydrate, pyrimidine, ethylenediamine, and melamine.

[0022] As a preferred embodiment of this application, the first solvent includes at least one of water, methanol, ethanol, n-propanol, isopropanol, n-butanol, and acetone.

[0023] As a preferred embodiment of this application, the mass ratio of the nitrogen-containing organic matter, carbon nanotubes, and the first solvent is (2-10):(10-40):(100-1000).

[0024] As a preferred embodiment of this application, the second solvent includes at least one of water, methanol, ethanol, n-propanol, isopropanol, n-butanol, and acetone.

[0025] As a preferred embodiment of this application, the electrode active material includes LiMn2O4, LiFePO4, and LiNi. x Co y Mn z At least one of O2, where x+y+z=1.

[0026] In a preferred embodiment of this application, the mass ratio of the nitrogen-doped carbon nanotubes, the electrode active material, and the second solvent is (0.001-0.02):1:(1-10).

[0027] As a preferred embodiment of this application, the mass concentration of the dopamine solution is 1 to 6 g / L.

[0028] As a preferred embodiment of this application, the solid-liquid ratio of the carbon nanotube-coated electrode active material and the dopamine solution is 1g:(4-20)mL.

[0029] As a preferred embodiment of this application, the mass ratio of the electrode active material of the modified nitrogen-doped carbon coating layer to the pore-forming agent, reinforcing agent, conductive agent, binder and third solvent is 100:(5-35):(10-20):(5-20):(100-400).

[0030] In a preferred embodiment of this application, the pore-forming agent is at least one of ammonium carbonate and ammonium bicarbonate.

[0031] In a preferred embodiment of this application, the reinforcing agent comprises chopped carbon fibers.

[0032] In a preferred embodiment of this application, the conductive agent includes at least one of acetylene black and carbon black.

[0033] In a preferred embodiment of this application, the adhesive comprises polyvinylidene fluoride.

[0034] In a preferred embodiment of this application, the third solvent comprises N-methylpyrrolidone.

[0035] In a preferred embodiment of this application, the coating amount of each electrode paste is the same, ranging from 10 to 50 mg / cm³. 2 .

[0036] The beneficial effects of this application are as follows: (1) During the lithium extraction process, the bottom of the composite electrode (near the current collector) is subjected to cathode polarization, which causes lithium ions to be consumed too quickly. The lithium extraction process is controlled by diffusion. Sodium ions and magnesium ions and other cationic impurity metals enter the lattice of the material through electrolysis. This application uses active materials with different carbon coating thicknesses to prepare lithium extraction electrodes by layer coating. The bottom carbon coating layer is thicker closer to the current collector. While improving conductivity, it reduces the contact area between the electrode material and the electrolyte, which to a certain extent inhibits the electrochemical reaction rate of lithium ions, thereby reducing cathode polarization, avoiding the insertion of impurity cations into the lattice, and improving the purity of lithium ions in the lithium extraction recovery liquid of the salt lake; (2) By adding a pore-forming agent in the electrode slurry coated close to the current collector layer, the mass of the pores at the bottom of the electrode is greater than that of the electrode surface layer far from the current collector, the pores at the bottom of the electrode are increased, which enhances the mass transfer effect of the solution at the bottom of the electrode. Excessive porosity at the bottom of the electrode leads to reduced stability of the electrode material. Coating the surface of the bottom electrode material with more carbon as a framework can improve the stability of the electrode material. The synergistic effect of the two can increase the mass transfer rate of lithium ions at the bottom of the electrode and increase the stability of the electrode material at the same time. Attached Figure Description

[0037] Figure 1 The composite electrode prepared in Example 1.

[0038] Figure 2 This is a comparison chart showing the rejection rates of impurity ions by the electrodes described in Example 1 and Comparative Example 1. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0040] There are no particular restrictions on the specific methods of dispersion and mixing.

[0041] In this application, unless otherwise stated, all parts are parts by weight.

[0042] Unless otherwise specified, all reagents or instruments used in this application are commercially available products.

[0043] Example 1

[0044] A method for preparing a composite electrode for lithium extraction from salt lakes includes the following steps:

[0045] (1) Disperse 8g of hydrazine hydrate and 5g of carbon nanotubes in 100mL of deionized water, stir evenly, heat under reflux for 6h, filter and separate to obtain solid, and then wash and freeze dry the solid to obtain nitrogen-doped carbon nanotubes.

[0046] (2) Take three nitrogen-doped carbon nanotubes of different masses and label them as the first nitrogen-doped carbon nanotube, the second nitrogen-doped carbon nanotube, and the third nitrogen-doped carbon nanotube.

[0047] Nitrogen-doped carbon nanotubes and lithium iron phosphate were added to anhydrous ethanol and dispersed evenly by ultrasonication at 500W. Then, the mixture was transferred to a ball mill jar for ball milling. After ball milling, the mixture was dried and cooled to room temperature to obtain an electrode active material coated with a nitrogen-doped carbon nanotube layer. The mass ratio of nitrogen-doped carbon nanotubes, lithium iron phosphate, and anhydrous ethanol was 0.015:1:1.

[0048] The second nitrogen-doped carbon nanotubes and lithium iron phosphate were added to anhydrous ethanol and dispersed evenly by ultrasonication at 500W. Then, they were transferred to a ball mill jar for ball milling. After ball milling, they were dried and cooled to room temperature to obtain an electrode active material coated with a second nitrogen-doped carbon nanotube layer. The mass ratio of the second nitrogen-doped carbon nanotubes, lithium iron phosphate and anhydrous ethanol was 0.01:1:1.

[0049] Third nitrogen-doped carbon nanotubes and lithium iron phosphate were added to anhydrous ethanol and dispersed evenly by ultrasonication at 500W. Then, they were transferred to a ball mill jar for ball milling. After ball milling, they were dried and cooled to room temperature to obtain an electrode active material coated with a third nitrogen-doped carbon nanotube layer. The mass ratio of third nitrogen-doped carbon nanotubes, lithium iron phosphate and anhydrous ethanol was 0.05:1:1.

[0050] (3) The electrode active materials coated with the first nitrogen-doped carbon nanotube layer, the second nitrogen-doped carbon nanotube layer, and the third nitrogen-doped carbon nanotube layer were respectively added to a 5 g / L dopamine solution at the same solid-liquid ratio of 1 g: 7.5 mL for coating modification. After filtration and drying, the modified electrode active materials coated with the first nitrogen-doped carbon nanotube layer, the modified electrode active materials coated with the second nitrogen-doped carbon nanotube layer, and the modified electrode active materials coated with the third nitrogen-doped carbon nanotube layer were obtained respectively.

[0051] (4) Add 200g of modified first nitrogen-doped carbon nanotube layer coated electrode active material, 10g of short carbon fiber, 30g of acetylene black, 25g of polyvinylidene fluoride, and 60g of ammonium bicarbonate to 300g of N-methylpyrrolidone, stir evenly to obtain electrode slurry, which is called 3-1 slurry.

[0052] 200g of modified second nitrogen-doped carbon nanotube layer coated electrode active material, 10g of short-cut carbon fiber, 30g of acetylene black, 25g of polyvinylidene fluoride, and 40g of ammonium bicarbonate were added to 300g of N-methylpyrrolidone and stirred evenly to obtain electrode slurry, which is denoted as slurry 3-2.

[0053] 200g of modified third nitrogen-doped carbon nanotube layer coated electrode active material, 10g of short-cut carbon fiber, 30g of acetylene black, 25g of polyvinylidene fluoride, and 20g of ammonium bicarbonate were added to 300g of N-methylpyrrolidone and stirred evenly to obtain electrode slurry, denoted as 3-3 slurry.

[0054] Slurry 3-1, slurry 3-2, and slurry 3-3 were sprayed sequentially onto the surface of the current collector titanium mesh, with each layer having a surface density of 25 mg / cm³. 2 After each layer of slurry is coated, the coating layer is dried at a low temperature of 70℃ for 10 minutes. After the last layer of electrode slurry is coated and dried at a low temperature, the entire electrode is dried at 100℃ for 5 hours. After the electrode is dried, it is rolled to a thickness of 500μm. Then it is soaked in water, and the water is changed every 0.5 hours for a total of 3 times to obtain a composite electrode for lithium extraction from salt lakes.

[0055] The prepared composite electrode for lithium extraction from salt lakes, such as Figure 1 As shown, it includes a current collector titanium mesh 1, an active coating 3-1, an active coating 3-2, and an active coating 3-3 located on the surface of the current collector titanium mesh; the mass of the pore-forming agent in each active coating decreases in a gradient along the direction away from the current collector, and the thickness of the modified nitrogen-doped carbon coating layer in the electrode active material of the modified nitrogen-doped carbon coating layer decreases in a gradient.

[0056] Among them, active coating 3-1, active coating 3-2 and active coating 3-3 respectively contain electrode active material S1 coated with modified first nitrogen-doped carbon nanotube layer, electrode active material S2 coated with modified second nitrogen-doped carbon nanotube layer and electrode active material S3 coated with modified third nitrogen-doped carbon nanotube layer, and the active coating also contains ammonium bicarbonate 2.

[0057] The electrode active material S1 coated with the modified first nitrogen-doped carbon nanotube layer includes lithium iron phosphate S11 and a modified first nitrogen-doped carbon nanotube layer S12 coated on the surface of lithium iron phosphate.

[0058] Lithium extraction experiment: Using the prepared composite electrode as the anode and nickel foam as the cathode, the electrode was placed in a 20 g / L NaCl solution. A voltage of 1 V was applied across the electrode until the current density dropped below 0.5 A / m. 2A lithium-deficient electrode was obtained. An anion exchange membrane was used to divide the electrolysis device into a cathode chamber and an anode chamber. The prepared lithium-rich electrode and the lithium-deficient electrode were placed in the anode and cathode chambers, respectively. 4.5 L of brine with a lithium concentration of 0.68 g / L was injected into the cathode chamber, and 2 L of 5 g / L NaCl solution was injected into the anode chamber as a supporting electrolyte. A voltage of 0.9 V was applied to the anode and cathode, and electrolysis was carried out at 20 °C for 4 h. The lithium ion concentration and brine impurity concentration before and after electrolysis were measured using inductively coupled plasma atomic emission spectrometry (ICP-AES), as shown in Table 1.

[0059] Table 1

[0060]

[0061] Comparative Example 1

[0062] A mixture of 200g lithium iron phosphate, 10g short carbon fiber, 30g acetylene black, 25g PVDF, and 40g ammonium bicarbonate was thoroughly mixed, and then 300g N-methylpyrrolidone was added and stirred until homogeneous. This resulting electrode slurry was coated onto the surface of a titanium mesh current collector, with the coating thickness being the same as in Example 1. The coating was dried at 70°C for 10 minutes, followed by drying at 100°C for 5 hours. After drying, the electrode was immersed in water, with the water changed every 0.5 hours for a total of 3 water changes to obtain a lithium iron phosphate electrode for electrochemical lithium extraction. Under the same technical parameters as in Example 1, the lithium concentration in the brine decreased from 1.75g / L to 0.33g / L, while the lithium concentration in the lithium-rich anode solution increased to 5.21g / L, resulting in a lithium extraction rate of 81%.

[0063] Figure 2 The graph shows the retention rates of impurity ions for the electrodes described in Example 1 and Comparative Example 1. It can be seen that the composite electrode of this application can reduce the adsorption of impurity cations, and the purity of lithium in the recovered solution is significantly improved.

[0064] Example 2

[0065] A method for preparing a composite electrode for lithium extraction from salt lakes includes the following steps:

[0066] (1) Disperse 8g of hydrazine hydrate and 5g of carbon nanotubes in 100mL of deionized water, stir evenly, heat under reflux for 6h, filter and separate to obtain solid, and then wash and freeze dry the solid to obtain nitrogen-doped carbon nanotubes.

[0067] (2) Take two nitrogen-doped carbon nanotubes of different masses and label them as the first nitrogen-doped carbon nanotube, the second nitrogen-doped carbon nanotube, and the third nitrogen-doped carbon nanotube.

[0068] First nitrogen-doped carbon nanotubes, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3O2 was added to anhydrous ethanol and dispersed evenly by ultrasonication at 500W. Then, it was transferred to a ball mill jar for ball milling. After ball milling, it was dried and cooled to room temperature to obtain an electrode active material coated with a first nitrogen-doped carbon nanotube layer. The mass ratio of the first nitrogen-doped carbon nanotube, lithium iron phosphate, and anhydrous ethanol was 0.015:1:1.

[0069] The second nitrogen-doped carbon nanotubes, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 was added to anhydrous ethanol and dispersed evenly by ultrasonication at 500W. Then, it was transferred to a ball mill jar for ball milling. After ball milling, it was dried and cooled to room temperature to obtain an electrode active material coated with a second nitrogen-doped carbon nanotube layer. The mass ratio of the second nitrogen-doped carbon nanotube, lithium iron phosphate, and anhydrous ethanol was 0.01:1:1.

[0070] (3) The electrode active materials coated with the first nitrogen-doped carbon nanotube layer and the electrode active materials coated with the second nitrogen-doped carbon nanotube layer were respectively added to a 5 g / L dopamine solution at the same solid-liquid ratio of 1 g: 7.5 mL for coating modification, filtered, and dried to obtain the modified electrode active materials coated with the first nitrogen-doped carbon nanotube layer and the modified electrode active materials coated with the second nitrogen-doped carbon nanotube layer, respectively.

[0071] (4) Add 200g of modified first nitrogen-doped carbon nanotube layer coated electrode active material, 10g of short carbon fiber, 30g of acetylene black, 25g of polyvinylidene fluoride, and 65g of ammonium bicarbonate to 300g of N-methylpyrrolidone, stir evenly to obtain electrode slurry, which is called 3-1 slurry.

[0072] 200g of modified second nitrogen-doped carbon nanotube layer coated electrode active material, 10g of short-cut carbon fiber, 30g of acetylene black, 25g of polyvinylidene fluoride, and 25g of ammonium bicarbonate were added to 300g of N-methylpyrrolidone and stirred evenly to obtain electrode slurry, which is denoted as slurry 3-2.

[0073] Slurry 3-1 and slurry 3-2 were sprayed sequentially onto the surface of the current collector titanium mesh, with each layer having a surface density of 25 mg / cm². 2 After each layer of slurry is coated, the coating layer is dried at a low temperature of 70℃ for 10 minutes. After the last layer of electrode slurry is coated and dried at a low temperature, the entire electrode is dried at 100℃ for 5 hours. After the electrode is dried, it is rolled to a thickness of 500μm. Then it is soaked in water, and the water is changed every 0.5 hours for a total of 3 times to obtain a composite electrode for lithium extraction from salt lakes.

[0074] The prepared composite electrode for lithium extraction from salt lakes includes a current collector titanium mesh and two active coatings of the same thickness. The mass of the pore-forming agent in each active coating along the direction away from the current collector decreases in a gradient, and the thickness of the modified nitrogen-doped carbon coating in the electrode active material of the modified nitrogen-doped carbon coating decreases in a gradient.

[0075] Each active coating contains an electrode active material S1 coated with a modified first nitrogen-doped carbon nanotube layer, an electrode active material coated with a modified second nitrogen-doped carbon nanotube layer, and an electrode active material coated with a modified third nitrogen-doped carbon nanotube layer, and the active coating also contains ammonium bicarbonate.

[0076] Modified first nitrogen-doped carbon nanotube layer coated electrode active materials include LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 and coatings on LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 Modified first nitrogen-doped carbon nanotube layer on O2 surface.

[0077] Lithium extraction experiment: Using the prepared composite electrode as the anode and nickel foam as the cathode, the electrode was placed in a 20 g / L NaCl solution. A voltage of 1 V was applied across the electrode until the current density dropped below 0.5 A / m. 2 A lithium-deficient electrode was obtained. An anion exchange membrane was used to divide the electrolysis device into a cathode chamber and an anode chamber. The prepared lithium-rich electrode and the lithium-deficient electrode were placed in the anode and cathode chambers, respectively. 4.5 L of brine with a lithium concentration of 0.68 g / L was injected into the cathode chamber, and 2 L of 5 g / L NaCl solution was injected into the anode chamber as a supporting electrolyte. A voltage of 0.9 V was applied to the anode and cathode, and electrolysis was carried out at 20 °C for 4 h. The lithium ion concentration and brine impurity concentration before and after electrolysis were measured using inductively coupled plasma atomic emission spectrometry (ICP-AES), as shown in Table 2.

[0078] Table 2

[0079]

[0080] Comparative Example 2

[0081] LiNi 1 / 3 Co 1 / 3 Mn 1 / 3After mixing 200g of O2, 10g of short carbon fiber, 30g of acetylene black, 25g of PVDF, and 65g of ammonium bicarbonate, 300g of N-methylpyrrolidone was added and stirred until homogeneous. The resulting electrode slurry was then coated onto the surface of a titanium mesh current collector, with the coating thickness being the same as in Example 1. The coating was dried at 70°C for 10 minutes, followed by drying at 100°C for 5 hours. After drying, the electrode was immersed in water, with the water changed every 0.5 hours for a total of 3 water changes to obtain a lithium iron phosphate electrode for electrochemical lithium extraction. Under the same technical parameters as in Example 2, the lithium concentration in the lithium-rich anode solution increased to 5.19g / L, and the lithium extraction rate was 80%.

[0082] Example 3

[0083] A method for preparing a composite electrode for lithium extraction from salt lakes includes the following steps:

[0084] (1) Disperse 8g of hydrazine hydrate and 5g of carbon nanotubes in 100mL of deionized water, stir evenly, heat under reflux for 6h, filter and separate to obtain solid, and then wash and freeze dry the solid to obtain nitrogen-doped carbon nanotubes.

[0085] (2) Take four nitrogen-doped carbon nanotubes of different masses and label them as the first nitrogen-doped carbon nanotube, the second nitrogen-doped carbon nanotube, the third nitrogen-doped carbon nanotube, and the fourth nitrogen-doped carbon nanotube.

[0086] First nitrogen-doped carbon nanotubes and lithium iron phosphate were added to anhydrous ethanol and dispersed evenly by ultrasonication at 500W. Then, they were transferred to a ball mill jar for ball milling. After ball milling, they were dried and cooled to room temperature to obtain an electrode active material coated with a first nitrogen-doped carbon nanotube layer. The mass ratio of first nitrogen-doped carbon nanotubes, lithium iron phosphate and anhydrous ethanol was 0.01:1:1.

[0087] The second nitrogen-doped carbon nanotubes and lithium iron phosphate were added to anhydrous ethanol and dispersed evenly by ultrasonication at 500W. Then, they were transferred to a ball mill jar for ball milling. After ball milling, they were dried and cooled to room temperature to obtain an electrode active material coated with a second nitrogen-doped carbon nanotube layer. The mass ratio of the second nitrogen-doped carbon nanotubes, lithium iron phosphate and anhydrous ethanol was 0.007:1:1.

[0088] Third nitrogen-doped carbon nanotubes and lithium iron phosphate were added to anhydrous ethanol and dispersed evenly by ultrasonication at 500W. Then, they were transferred to a ball mill jar for ball milling. After ball milling, they were dried and cooled to room temperature to obtain an electrode active material coated with a third nitrogen-doped carbon nanotube layer. The mass ratio of third nitrogen-doped carbon nanotubes, lithium iron phosphate and anhydrous ethanol was 0.004:1:1.

[0089] Nitrogen-doped carbon nanotubes and lithium iron phosphate were added to anhydrous ethanol and dispersed uniformly by ultrasonication at 500W. Then, the mixture was transferred to a ball mill jar for ball milling. After ball milling, the mixture was dried and cooled to room temperature to obtain an electrode active material coated with a nitrogen-doped carbon nanotube layer. The mass ratio of nitrogen-doped carbon nanotubes, lithium iron phosphate, and anhydrous ethanol was 0.001:1:1.

[0090] (3) The electrode active materials coated with the first nitrogen-doped carbon nanotube layer, the second nitrogen-doped carbon nanotube layer, the third nitrogen-doped carbon nanotube layer, and the fourth nitrogen-doped carbon nanotube layer were respectively added to a 5 g / L dopamine solution at the same solid-liquid ratio of 1 g: 7.5 mL for coating modification. After filtration and drying, the modified electrode active materials coated with the first nitrogen-doped carbon nanotube layer, the second nitrogen-doped carbon nanotube layer, the third nitrogen-doped carbon nanotube layer, and the fourth nitrogen-doped carbon nanotube layer were obtained respectively.

[0091] (4) Add 200g of modified first nitrogen-doped carbon nanotube layer coated electrode active material, 10g of short carbon fiber, 30g of acetylene black, 25g of polyvinylidene fluoride, and 64g of ammonium bicarbonate to 300g of N-methylpyrrolidone, stir evenly to obtain electrode slurry, which is called 3-1 slurry.

[0092] 200g of modified second nitrogen-doped carbon nanotube layer coated electrode active material, 10g of short-cut carbon fiber, 30g of acetylene black, 25g of polyvinylidene fluoride, and 48g of ammonium bicarbonate were added to 300g of N-methylpyrrolidone and stirred evenly to obtain electrode slurry, which is denoted as slurry 3-2.

[0093] 200g of modified third nitrogen-doped carbon nanotube layer coated electrode active material, 10g of short-cut carbon fiber, 30g of acetylene black, 25g of polyvinylidene fluoride, and 32g of ammonium bicarbonate were added to 300g of N-methylpyrrolidone and stirred evenly to obtain electrode slurry, denoted as 3-3 slurry.

[0094] 200g of modified fourth nitrogen-doped carbon nanotube layer coated electrode active material, 10g of short-cut carbon fiber, 30g of acetylene black, 25g of polyvinylidene fluoride, and 32g of ammonium bicarbonate were added to 300g of N-methylpyrrolidone and stirred evenly to obtain electrode slurry, which is referred to as slurry 3-4.

[0095] Slurry 3-1, slurry 3-2, slurry 3-3, and slurry 3-4 were sprayed sequentially onto the surface of the current collector titanium mesh, with each layer having a surface density of 25 mg / cm³. 2After each layer of slurry is coated, the coating layer is dried at a low temperature of 70℃ for 10 minutes. After the last layer of electrode slurry is coated and dried at a low temperature, the entire electrode is dried at 100℃ for 5 hours. After the electrode is dried, it is rolled to a thickness of 500μm. Then it is soaked in water, and the water is changed every 0.5 hours for a total of 3 times to obtain a composite electrode for lithium extraction from salt lakes.

[0096] The prepared composite electrode for lithium extraction from salt lakes includes a current collector titanium mesh and four active coatings of the same thickness. The mass of the pore-forming agent in each active coating along the direction away from the current collector decreases in a gradient, and the thickness of the modified nitrogen-doped carbon coating in the electrode active material of the modified nitrogen-doped carbon coating decreases in a gradient.

[0097] Each active coating contains an electrode active material coated with a modified first nitrogen-doped carbon nanotube layer, an electrode active material coated with a modified second nitrogen-doped carbon nanotube layer, an electrode active material coated with a modified third nitrogen-doped carbon nanotube layer, and an electrode active material coated with a modified fourth nitrogen-doped carbon nanotube layer, and the active coating also contains ammonium bicarbonate.

[0098] Lithium extraction experiment: Using the prepared composite electrode as the anode and nickel foam as the cathode, the electrode was placed in a 20 g / L NaCl solution. A voltage of 1 V was applied across the electrode until the current density dropped below 0.5 A / m. 2 A lithium-deficient electrode was obtained. An anion exchange membrane was used to divide the electrolysis device into a cathode chamber and an anode chamber. The prepared lithium-rich electrode and the lithium-deficient electrode were placed in the anode and cathode chambers, respectively. 4.5 L of brine with a lithium concentration of 0.68 g / L was injected into the cathode chamber, and 2 L of 5 g / L NaCl solution was injected into the anode chamber as a supporting electrolyte. A voltage of 0.9 V was applied to the anode and cathode, and electrolysis was carried out at 20 °C for 4 h. The lithium ion concentration and brine impurity concentration before and after electrolysis were measured using inductively coupled plasma atomic emission spectrometry (ICP-AES), as shown in Table 3.

[0099] Table 3

[0100]

[0101] Comparative Example 3

[0102] 200g of lithium iron phosphate, 10g of short carbon fibers, 30g of acetylene black, 25g of PVDF, and 32g of ammonium bicarbonate were mixed evenly, and then 300g of N-methylpyrrolidone was added and stirred evenly to obtain an electrode slurry. This slurry was then coated onto the surface of a titanium mesh current collector, with the coating thickness being the same as in Example 3. The coating layer was dried at 70°C for 10 minutes, and then dried at 100°C for 5 hours. After the electrode was dried, it was immersed in water, with the water changed every 0.5 hours for a total of 3 water changes, to obtain a lithium iron phosphate electrode for electrochemical lithium extraction. Under the same technical parameters as in Example 3, the lithium concentration in the lithium-rich anode solution increased to 5.25g / L, and the lithium extraction rate was 82%.

[0103] Example 4

[0104] A method for preparing a composite electrode for lithium extraction from salt lakes includes the following steps:

[0105] (1) Disperse 8g of hydrazine hydrate and 5g of carbon nanotubes in 100mL of deionized water, stir evenly, heat under reflux for 6h, filter and separate to obtain solid, and then wash and freeze dry the solid to obtain nitrogen-doped carbon nanotubes.

[0106] (2) Take 5 nitrogen-doped carbon nanotubes of different masses and label them as the first nitrogen-doped carbon nanotube, the second nitrogen-doped carbon nanotube, the third nitrogen-doped carbon nanotube, the fourth nitrogen-doped carbon nanotube, and the fifth nitrogen-doped carbon nanotube.

[0107] First nitrogen-doped carbon nanotubes and lithium iron phosphate were added to anhydrous ethanol and dispersed evenly by ultrasonication at 500W. Then, they were transferred to a ball mill jar for ball milling. After ball milling, they were dried and cooled to room temperature to obtain an electrode active material coated with a first nitrogen-doped carbon nanotube layer. The mass ratio of first nitrogen-doped carbon nanotubes, lithium iron phosphate and anhydrous ethanol was 0.02:1:1.

[0108] The second nitrogen-doped carbon nanotubes and lithium iron phosphate were added to anhydrous ethanol and dispersed evenly by ultrasonication at 500W. Then, they were transferred to a ball mill jar for ball milling. After ball milling, they were dried and cooled to room temperature to obtain an electrode active material coated with a second nitrogen-doped carbon nanotube layer. The mass ratio of the second nitrogen-doped carbon nanotubes, lithium iron phosphate and anhydrous ethanol was 0.017:1:1.

[0109] Third nitrogen-doped carbon nanotubes and lithium iron phosphate were added to anhydrous ethanol and dispersed evenly by ultrasonication at 500W. Then, the mixture was transferred to a ball mill jar for ball milling. After ball milling, the mixture was dried and cooled to room temperature to obtain an electrode active material coated with a third nitrogen-doped carbon nanotube layer. The mass ratio of third nitrogen-doped carbon nanotubes, lithium iron phosphate, and anhydrous ethanol was 0.014:1:1.

[0110] Nitrogen-doped carbon nanotubes and lithium iron phosphate were added to anhydrous ethanol and dispersed uniformly by ultrasonication at 500W. Then, the mixture was transferred to a ball mill jar for ball milling. After ball milling, the mixture was dried and cooled to room temperature to obtain an electrode active material coated with a nitrogen-doped carbon nanotube layer. The mass ratio of nitrogen-doped carbon nanotubes, lithium iron phosphate, and anhydrous ethanol was 0.011:1:1.

[0111] The fifth nitrogen-doped carbon nanotube and lithium iron phosphate were added to anhydrous ethanol and dispersed evenly by ultrasonication at 500W. Then, the mixture was transferred to a ball mill jar for ball milling. After ball milling, the mixture was dried and cooled to room temperature to obtain an electrode active material coated with the fifth nitrogen-doped carbon nanotube layer. The mass ratio of the fourth nitrogen-doped carbon nanotube, lithium iron phosphate, and anhydrous ethanol was 0.008:1:1.

[0112] (3) The electrode active materials coated with the first nitrogen-doped carbon nanotube layer, the second nitrogen-doped carbon nanotube layer, the third nitrogen-doped carbon nanotube layer, the fourth nitrogen-doped carbon nanotube layer, and the fifth nitrogen-doped carbon nanotube layer were respectively added to a 5 g / L dopamine solution at the same solid-liquid ratio of 1 g: 7.5 mL for coating modification. After filtration and drying, the modified electrode active materials coated with the first nitrogen-doped carbon nanotube layer, the second nitrogen-doped carbon nanotube layer, the third nitrogen-doped carbon nanotube layer, the fourth nitrogen-doped carbon nanotube layer, and the fifth nitrogen-doped carbon nanotube layer were obtained respectively.

[0113] (4) Add 200g of modified first nitrogen-doped carbon nanotube layer coated electrode active material, 10g of short carbon fiber, 30g of acetylene black, 25g of polyvinylidene fluoride, and 56g of ammonium bicarbonate to 300g of N-methylpyrrolidone, stir evenly to obtain electrode slurry, which is called 3-1 slurry.

[0114] 200g of modified second nitrogen-doped carbon nanotube layer coated electrode active material, 10g of short-cut carbon fiber, 30g of acetylene black, 25g of polyvinylidene fluoride, and 48g of ammonium bicarbonate were added to 300g of N-methylpyrrolidone and stirred evenly to obtain electrode slurry, which is denoted as slurry 3-2.

[0115] 200g of modified third nitrogen-doped carbon nanotube layer coated electrode active material, 10g of short-cut carbon fiber, 30g of acetylene black, 25g of polyvinylidene fluoride, and 40g of ammonium bicarbonate were added to 300g of N-methylpyrrolidone and stirred evenly to obtain electrode slurry, which is denoted as slurry 3-3.

[0116] 200g of modified fourth nitrogen-doped carbon nanotube layer coated electrode active material, 10g of short-cut carbon fiber, 30g of acetylene black, 25g of polyvinylidene fluoride, and 32g of ammonium bicarbonate were added to 300g of N-methylpyrrolidone and stirred evenly to obtain electrode slurry, which is referred to as slurry 3-4.

[0117] 200g of modified fifth nitrogen-doped carbon nanotube layer coated electrode active material, 10g of short-cut carbon fiber, 30g of acetylene black, 25g of polyvinylidene fluoride, and 24g of ammonium bicarbonate were added to 300g of N-methylpyrrolidone and stirred evenly to obtain electrode slurry, which is denoted as slurry 3-5.

[0118] Slurry 3-1, slurry 3-2, slurry 3-3, slurry 3-4, and slurry 3-5 were sprayed sequentially onto the surface of the current collector titanium mesh, with each layer having a surface density of 25 mg / cm². 2 After each layer of slurry is coated, the coating layer is dried at a low temperature of 70℃ for 10 minutes. After the last layer of electrode slurry is coated and dried at a low temperature, the entire electrode is dried at 100℃ for 5 hours. After the electrode is dried, it is rolled to a thickness of 500μm. Then it is soaked in water, and the water is changed every 0.5 hours for a total of 3 times to obtain a composite electrode for lithium extraction from salt lakes.

[0119] The prepared composite electrode for lithium extraction from salt lakes includes a current collector titanium mesh and five active coatings of the same thickness. The mass of the pore-forming agent in each active coating along the direction away from the current collector decreases in a gradient, and the thickness of the modified nitrogen-doped carbon coating in the electrode active material of the modified nitrogen-doped carbon coating decreases in a gradient.

[0120] Each active coating contains an electrode active material coated with a modified first nitrogen-doped carbon nanotube layer, an electrode active material coated with a modified second nitrogen-doped carbon nanotube layer, an electrode active material coated with a modified third nitrogen-doped carbon nanotube layer, an electrode active material coated with a modified fourth nitrogen-doped carbon nanotube layer, and an electrode active material coated with a modified fifth nitrogen-doped carbon nanotube layer, and the active coating also contains ammonium bicarbonate.

[0121] Lithium extraction experiment: Using the prepared composite electrode as the anode and nickel foam as the cathode, the electrode was placed in a 20 g / L NaCl solution. A voltage of 1 V was applied across the electrode until the current density dropped below 0.5 A / m. 2A lithium-deficient electrode was obtained. An anion exchange membrane was used to divide the electrolysis device into a cathode chamber and an anode chamber. The prepared lithium-rich electrode and the lithium-deficient electrode were placed in the anode and cathode chambers, respectively. 4.5 L of brine with a lithium concentration of 0.68 g / L was injected into the cathode chamber, and 2 L of 5 g / L NaCl solution was injected into the anode chamber as a supporting electrolyte. A voltage of 0.9 V was applied to the anode and cathode, and electrolysis was carried out at 20 °C for 4 h. The lithium ion concentration and brine impurity concentration before and after electrolysis were measured using inductively coupled plasma atomic emission spectrometry (ICP-AES), as shown in Table 43.

[0122] Table 4

[0123]

[0124] Comparative Example 4

[0125] A mixture of 200g lithium iron phosphate, 10g short carbon fiber, 30g acetylene black, 25g PVDF, and 40g ammonium bicarbonate was thoroughly mixed, and then 300g N-methylpyrrolidone was added and stirred until homogeneous. This resulting electrode slurry was coated onto the surface of a titanium mesh current collector, with the coating thickness being the same as in Example 4. The coating was dried at 70°C for 10 minutes, followed by drying at 100°C for 5 hours. After drying, the electrode was immersed in water, with the water changed every 0.5 hours for a total of 3 water changes, to obtain a lithium iron phosphate electrode for electrochemical lithium extraction. Under the same technical parameters as in Example 4, the lithium concentration in the lithium-rich anode solution increased to 5.25g / L, and the lithium extraction rate was 82%.

[0126] Example 5

[0127] A method for preparing a composite electrode for lithium extraction from salt lakes includes the following steps:

[0128] (1) Disperse 8g of hydrazine hydrate and 5g of carbon nanotubes in 100mL of deionized water, stir evenly, heat under reflux for 6h, filter and separate to obtain solid, and then wash and freeze dry the solid to obtain nitrogen-doped carbon nanotubes.

[0129] (2) Take 6 nitrogen-doped carbon nanotubes of different masses and label them as first nitrogen-doped carbon nanotube, second nitrogen-doped carbon nanotube, third nitrogen-doped carbon nanotube, fourth nitrogen-doped carbon nanotube, fifth nitrogen-doped carbon nanotube, and sixth nitrogen-doped carbon nanotube.

[0130] First nitrogen-doped carbon nanotubes and lithium iron phosphate were added to anhydrous ethanol and dispersed evenly by ultrasonication at 500W. Then, they were transferred to a ball mill jar for ball milling. After ball milling, they were dried and cooled to room temperature to obtain an electrode active material coated with a first nitrogen-doped carbon nanotube layer. The mass ratio of first nitrogen-doped carbon nanotubes, lithium iron phosphate and anhydrous ethanol was 0.012:1:1.

[0131] The second nitrogen-doped carbon nanotubes and lithium iron phosphate were added to anhydrous ethanol and dispersed evenly by ultrasonication at 500W. Then, they were transferred to a ball mill jar for ball milling. After ball milling, they were dried and cooled to room temperature to obtain an electrode active material coated with a second nitrogen-doped carbon nanotube layer. The mass ratio of the second nitrogen-doped carbon nanotubes, lithium iron phosphate and anhydrous ethanol was 0.011:1:1.

[0132] Third nitrogen-doped carbon nanotubes and lithium iron phosphate were added to anhydrous ethanol and dispersed evenly by ultrasonication at 500W. Then, they were transferred to a ball mill jar for ball milling. After ball milling, they were dried and cooled to room temperature to obtain an electrode active material coated with a third nitrogen-doped carbon nanotube layer. The mass ratio of third nitrogen-doped carbon nanotubes, lithium iron phosphate and anhydrous ethanol was 0.010:1:1.

[0133] Nitrogen-doped carbon nanotubes and lithium iron phosphate were added to anhydrous ethanol and dispersed uniformly by ultrasonication at 500W. Then, the mixture was transferred to a ball mill jar for ball milling. After ball milling, the mixture was dried and cooled to room temperature to obtain an electrode active material coated with a nitrogen-doped carbon nanotube layer. The mass ratio of nitrogen-doped carbon nanotubes, lithium iron phosphate, and anhydrous ethanol was 0.009:1:1.

[0134] The fifth nitrogen-doped carbon nanotube and lithium iron phosphate were added to anhydrous ethanol and dispersed evenly by ultrasonication at 500W. Then, the mixture was transferred to a ball mill jar for ball milling. After ball milling, the mixture was dried and cooled to room temperature to obtain an electrode active material coated with the fifth nitrogen-doped carbon nanotube layer. The mass ratio of the fourth nitrogen-doped carbon nanotube, lithium iron phosphate, and anhydrous ethanol was 0.008:1:1.

[0135] The sixth nitrogen-doped carbon nanotube and lithium iron phosphate were added to anhydrous ethanol and dispersed evenly by ultrasonication at 500W. Then, the mixture was transferred to a ball mill jar for ball milling. After ball milling, the mixture was dried and cooled to room temperature to obtain the electrode active material coated with the fifth nitrogen-doped carbon nanotube layer. The mass ratio of the fourth nitrogen-doped carbon nanotube, lithium iron phosphate and anhydrous ethanol was 0.007:1:1.

[0136] (3) Electrode active materials coated with the first nitrogen-doped carbon nanotube layer, the second nitrogen-doped carbon nanotube layer, the third nitrogen-doped carbon nanotube layer, the fourth nitrogen-doped carbon nanotube layer, the fifth nitrogen-doped carbon nanotube layer, and the sixth nitrogen-doped carbon nanotube layer were respectively added to a 5 g / L dopamine solution at the same solid-liquid ratio of 1 g: 7.5 mL for coating modification. After filtration and drying, the modified first nitrogen-doped carbon nanotube layer coated electrode active materials, the modified second nitrogen-doped carbon nanotube layer coated electrode active materials, the modified third nitrogen-doped carbon nanotube layer coated electrode active materials, the modified fourth nitrogen-doped carbon nanotube layer coated electrode active materials, the modified fifth nitrogen-doped carbon nanotube layer coated electrode active materials, and the modified sixth nitrogen-doped carbon nanotube layer coated electrode active materials were obtained.

[0137] (4) Add 200g of modified first nitrogen-doped carbon nanotube layer coated electrode active material, 10g of short carbon fiber, 30g of acetylene black, 25g of polyvinylidene fluoride, and 70g of ammonium bicarbonate to 300g of N-methylpyrrolidone, stir evenly to obtain electrode slurry, which is called 3-1 slurry.

[0138] 200g of modified second nitrogen-doped carbon nanotube layer coated electrode active material, 10g of short-cut carbon fiber, 30g of acetylene black, 25g of polyvinylidene fluoride, and 58g of ammonium bicarbonate were added to 300g of N-methylpyrrolidone and stirred evenly to obtain electrode slurry, which is denoted as slurry 3-2.

[0139] 200g of modified third nitrogen-doped carbon nanotube layer coated electrode active material, 10g of short-cut carbon fiber, 30g of acetylene black, 25g of polyvinylidene fluoride, and 46g of ammonium bicarbonate were added to 300g of N-methylpyrrolidone and stirred evenly to obtain electrode slurry, which is denoted as slurry 3-3.

[0140] 200g of modified fourth nitrogen-doped carbon nanotube layer coated electrode active material, 10g of short-cut carbon fiber, 30g of acetylene black, 25g of polyvinylidene fluoride, and 34g of ammonium bicarbonate were added to 300g of N-methylpyrrolidone and stirred evenly to obtain electrode slurry, which is referred to as slurry 3-4.

[0141] 200g of modified fifth nitrogen-doped carbon nanotube layer coated electrode active material, 10g of short-cut carbon fiber, 30g of acetylene black, 25g of polyvinylidene fluoride, and 22g of ammonium bicarbonate were added to 300g of N-methylpyrrolidone and stirred evenly to obtain electrode slurry, which is denoted as slurry 3-5.

[0142] 200g of modified sixth nitrogen-doped carbon nanotube layer coated electrode active material, 10g of short-cut carbon fiber, 30g of acetylene black, 25g of polyvinylidene fluoride, and 10g of ammonium bicarbonate were added to 300g of N-methylpyrrolidone and stirred evenly to obtain electrode slurry, which is referred to as slurry 3-5.

[0143] Slurries 3-1, 3-2, 3-3, 3-4, 3-5, and 3-6 were sequentially sprayed onto the surface of the current collector titanium mesh, with each layer having a surface density of 25 mg / cm². 2 After each layer of slurry is coated, the coating layer is dried at a low temperature of 70℃ for 10 minutes. After the last layer of electrode slurry is coated and dried at a low temperature, the entire electrode is dried at 100℃ for 5 hours. After the electrode is dried, it is rolled to a thickness of 500μm. Then it is soaked in water, and the water is changed every 0.5 hours for a total of 3 times to obtain a composite electrode for lithium extraction from salt lakes.

[0144] The prepared composite electrode for lithium extraction from salt lakes includes a current collector titanium mesh and six active coatings of equal thickness. The mass of the pore-forming agent in each active coating decreases in a gradient along the direction away from the current collector, and the thickness of the modified nitrogen-doped carbon coating in the electrode active material of the modified nitrogen-doped carbon coating decreases in a gradient.

[0145] Each active coating contains an electrode active material coated with a modified first nitrogen-doped carbon nanotube layer, an electrode active material coated with a modified second nitrogen-doped carbon nanotube layer, an electrode active material coated with a modified third nitrogen-doped carbon nanotube layer, an electrode active material coated with a modified fourth nitrogen-doped carbon nanotube layer, an electrode active material coated with a modified fifth nitrogen-doped carbon nanotube layer, and an electrode active material coated with a modified sixth nitrogen-doped carbon nanotube layer, and the active coating also contains ammonium bicarbonate;

[0146] Lithium extraction experiment: Using the prepared composite electrode as the anode and nickel foam as the cathode, the electrode was placed in a 20 g / L NaCl solution. A voltage of 1 V was applied across the electrode until the current density dropped below 0.5 A / m. 2 A lithium-deficient electrode was obtained. An anion exchange membrane was used to divide the electrolysis device into a cathode chamber and an anode chamber. The prepared lithium-rich electrode and the lithium-deficient electrode were placed in the anode and cathode chambers, respectively. 4.5 L of brine with a lithium concentration of 0.68 g / L was injected into the cathode chamber, and 2 L of 5 g / L NaCl solution was injected into the anode chamber as a supporting electrolyte. A voltage of 0.9 V was applied to the anode and cathode, and electrolysis was carried out at 20 °C for 4 h. The lithium ion concentration and brine impurity concentration before and after electrolysis were measured using inductively coupled plasma atomic emission spectrometry (ICP-AES), as shown in Table 5.

[0147] Table 4

[0148]

[0149] Comparative Example 5

[0150] 200g of lithium iron phosphate, 10g of short carbon fibers, 30g of acetylene black, 25g of PVDF, and 34g of ammonium bicarbonate were mixed evenly, and then 300g of N-methylpyrrolidone was added and stirred evenly to obtain an electrode slurry. This slurry was then coated onto the surface of a titanium mesh current collector, with the coating thickness being the same as in Example 4. The coating layer was dried at 70°C for 10 minutes, and then dried at 100°C for 5 hours. After the electrode was dried, it was immersed in water, with the water changed every 0.5 hours for a total of 3 water changes, to obtain a lithium iron phosphate electrode for electrochemical lithium extraction. Under the same technical parameters as in Example 5, the lithium concentration in the lithium-rich anode solution increased to 4.95g / L, and the lithium extraction rate was 80%.

[0151] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A method for preparing a composite electrode for lithium extraction from salt lakes, characterized in that, The method comprises the following steps: dispersing nitrogen-containing organic matter and carbon nanotubes in a first solvent, heating to reflux, filtering, drying, washing with water, drying, and obtaining nitrogen-doped carbon nanotubes; taking n portions of nitrogen-doped carbon nanotubes with different masses, dispersing the nitrogen-doped carbon nanotubes and electrode active materials in a second solvent respectively, ball milling, drying, and respectively obtaining n kinds of electrode active materials coated with nitrogen-doped carbon nanotube layers with different thicknesses; wherein the mass of the electrode active material corresponding to each portion of the nitrogen-doped carbon nanotubes is the same; respectively adding the prepared electrode active materials coated with nitrogen-doped carbon nanotube layers to a dopamine solution for modification, filtering, washing, and drying, and respectively obtaining n kinds of electrode active materials with modified nitrogen-doped carbon coating layers with different thicknesses; respectively preparing n electrode slurries by mixing the modified electrode active materials with nitrogen-doped carbon coating layers, a pore-forming agent, a reinforcing agent, a conductive agent, a binder, and a third solvent, and coating the electrode slurries on a current collector in sequence, drying, rolling, and immersing in water to form n active coating layers, so that the mass of the pore-forming agent in each active coating layer in the direction away from the current collector decreases in a gradient, and the thickness of the modified nitrogen-doped carbon coating layer in the electrode active material decreases in a gradient; wherein n is a positive integer greater than or equal to 2.

2. The method for preparing a composite electrode for lithium extraction from salt lakes according to claim 1, characterized in that, The mass of the modified electrode active material with nitrogen-doped carbon coating layer, the reinforcing agent, the conductive agent, the binder, and the third solvent in each electrode slurry is the same.

3. The method of claim 1, wherein the method is characterized by, The mass of the pore-forming agent in each electrode slurry is different.

4. The method of claim 1, wherein the method is characterized by, The nitrogen-containing organic matter includes at least one of hydrazine hydrate, pyrimidine, ethylenediamine, and melamine.

5. The method of claim 1, wherein the method is characterized by: The first solvent includes at least one of water, methanol, ethanol, n-propanol, isopropanol, n-butanol, and acetone.

6. The method of claim 1, wherein the method is characterized by: The mass ratio of the nitrogen-containing organic matter, the carbon nanotubes, and the first solvent is (2-10):(10-40):(100-1000).

7. The method of claim 1, wherein the method further comprises the step of: The second solvent includes at least one of water, methanol, ethanol, n-propanol, isopropanol, n-butanol, and acetone. ​ 8. The method of claim 1, wherein the method is characterized by: The electrode active material includes at least one of LiMn2O4, LiFePO4, LiNi x Co y Mn z O2, wherein x+y+z = 1.

9. The method of claim 1, wherein the method is characterized by, The mass ratio of the nitrogen-doped carbon nanotubes, the electrode active material, and the second solvent is (0.001-0.02):1:(1-10).

10. The method of claim 1, wherein the method is characterized by: The mass concentration of the dopamine solution is 1-6 g / L.

11. The method of claim 1, wherein the method is characterized by: The solid-liquid ratio of the carbon nanotube-coated electrode active material and the dopamine solution is 1 g:(4-20) mL.

12. The method of claim 1, wherein the method is characterized by: The mass ratio of the modified electrode active material with nitrogen-doped carbon coating layer, the pore-forming agent, the reinforcing agent, the conductive agent, the binder, and the third solvent is 100:(5-35):(10-20):(5-20):(100-400).

13. The method of claim 1, wherein the method is characterized by: The pore-forming agent is at least one of ammonium carbonate and ammonium bicarbonate; and / or The reinforcing agent includes short carbon fibers.

14. The method of claim 13, wherein the method is characterized by: The conductive agent includes at least one of acetylene black and carbon black; and / or The binder includes polyvinylidene fluoride; and / or The third solvent includes N-methylpyrrolidone.

15. The method of claim 1, wherein the method is characterized by: The coating amount of each electrode slurry is the same, 10-50 mg / cm 2 .

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