Three-dimensional pore gradient distribution thick pole piece, preparation method thereof and electrochemical lithium extraction device

Abstract

The invention discloses a three-dimensional porosity gradient distribution thick pole piece and a preparation method thereof and an electrochemical lithium extraction device.The three-dimensional porosity gradient distribution thick pole piece comprises a current collector and a coating located on at least one side of the current collector, the coating comprises a first coating and a second coating located on the first coating, and the first coating and the second coating are arranged on the current collector; the first coating and the second coating are coated on the current collector, the pore structures of the coatings are gradually increased in a gradient manner along the direction far away from the current collector, the first coating is provided with a first pore structure, the second coating is provided with a second pore structure, the first pore structure is provided with small pores which are communicated in a penetrating manner and have the pore diameter of 1.5-2.5 microns, and the second pore structure is provided with small pores with the pore diameter of 6.5-10.5 microns and large pores with the pore diameter of 0.2-1.0 mm, which are interspersed with each other. According to the gradient aperture coating, the ion transmission distance of the thick electrode in the electrochemical reaction process is shortened, the rate capability of the thick electrode is remarkably enhanced, and the energy density and the lithium extraction efficiency of the electrode plate are improved.

Description

Technical Field

[0001] This invention relates to the field of thick electrode preparation technology, and more specifically, to a three-dimensional pore gradient distribution thick electrode, its preparation method, and an electrochemical lithium extraction device. Background Technology

[0002] Salt lake brine is a substance containing lithium ions and a large number of other metal cations (Na+). + K + Mg 2+ Concentrated salt solutions (such as those from ore) are used. Although lithium extraction from ore is a relatively mature technology, its production costs are high and it causes serious environmental pollution. Therefore, all sectors of society are paying increasing attention to the development of lithium extraction technology from salt lakes.

[0003] The electrochemical method based on rocking chair batteries (LiFePO4 / LiMn2O4) provides a new research direction for lithium extraction from salt lakes. This electrochemical method controls the operating potential to extract lithium from salt lakes. + Selective extraction / intercalation of lithium into electrodes enables the extraction of lithium from salt lake brine. To achieve higher lithium capacity in electrochemical lithium extraction systems, increasing the active material loading in the electrode (thick electrode) is considered the most direct and economical way to improve lithium capacity. For example, CN118389851A discloses a sandwich-structured electrode and its application in electrochemical lithium extraction. The sandwich-structured lithium extraction electrode, prepared by designing a double-layer current collector, can effectively reduce the lithium-ion transport distance and improve the electrochemical lithium extraction rate while maintaining a large loading capacity; however, it suffers from the problem of large current collector usage and the ineffective utilization of the electrochemical performance of the intermediate coating (current collector-coating-current collector). Currently, the lithium recovery rate of electrochemical lithium extraction technology is affected by the active material loading, electrode stability, and ion / electron conduction, resulting in low lithium extraction efficiency and poor stability, which greatly limits the application and development of electro-extraction / intercalation lithium extraction technology. Therefore, it is necessary to develop a simple and reliable method to prepare high-performance electrochemical lithium extraction electrodes to improve the problems of poor electrode stability and poor ion / electron conduction during the lithium extraction process, thereby improving the lithium extraction performance of the electrode. Summary of the Invention

[0004] The purpose of this invention is to overcome the above-mentioned shortcomings of the prior art and provide a three-dimensional pore gradient distribution thick electrode, its preparation method, and an electrochemical lithium extraction device.

[0005] The technical problem solved by this invention is achieved by the following technical solution.

[0006] This invention provides a three-dimensional pore gradient distribution thick electrode sheet, comprising a current collector and a coating located on at least one side of the current collector. The coating includes a first coating and a second coating located on the first coating, and the pore structure of the coating increases in a gradient along the direction away from the current collector. The first coating has a first pore structure, and the second coating has a second pore structure. The first pore structure has through-connected small pores with a diameter of 1.5-2.5 μm, and the second pore structure has interpenetrating small pores with a diameter of 6.5-10.5 μm and large pores with a diameter of 0.2-1.0 mm.

[0007] The present invention also provides a method for preparing the above-mentioned three-dimensional pore gradient distribution thick electrode sheet, which includes: preparing electrode slurry containing different pore-forming agents; coating the electrode slurry onto the current collector and drying and shaping it; immersing the dried electrode sheet in pure water and soaking and rinsing it to completely remove the pore-forming agents.

[0008] The present invention also provides an electrochemical lithium extraction device, comprising the above-mentioned thick electrode with a three-dimensional pore gradient distribution.

[0009] The present invention has the following beneficial effects: This invention provides a three-dimensional pore gradient distribution thick electrode, its preparation method, and an electrochemical lithium extraction device. The three-dimensional pore gradient distribution thick electrode provided by this invention has a coating with a gradient pore size on the surface of the current collector. By constructing a porous structure inside the electrode to increase porosity, on the one hand, it increases the electrode thickness and active material loading, while shortening the ion transport distance during the electrochemical reaction, significantly enhancing the rate performance of the thick electrode. On the other hand, the gradient pore size in the coating can alleviate the volume change stress changes during electrode fabrication / charge-discharge processes, reduce tortuosity, reduce electrode cracking and pulverization, and improve the cycle stability of the electrode. This, in turn, improves the energy density and lithium extraction efficiency of the electrode. Attached Figure Description

[0010] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0011] Figure 1 A cross-sectional schematic diagram of a thick electrode sheet with a three-dimensional pore gradient distribution prepared according to an embodiment of the present invention; Figure 2 This is one of the structural schematic diagrams of a three-dimensional pore gradient distribution thick electrode sheet prepared according to an embodiment of the present invention; Figure 3This is the second schematic diagram of the structure of the three-dimensional pore gradient distribution thick electrode sheet prepared according to an embodiment of the present invention; Figure 4 The graph shows the long-cycle performance of the thick electrode sheets prepared in Example 1 and Comparative Examples 1-2 of this invention at a charge-discharge rate of 0.1C. Figure 5 The rate performance diagrams are for the thick electrodes prepared in Example 1 and Comparative Examples 1-2 of this invention. Figure numbers: 1-Second coating; 2-First coating; 3-Current collector. Detailed Implementation

[0012] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0013] The following is a detailed description of a three-dimensional pore gradient distribution thick electrode, its preparation method, and an electrochemical lithium extraction device provided by embodiments of the present invention.

[0014] In a first aspect, embodiments of the present invention provide a thick electrode sheet with a three-dimensional pore gradient distribution, see [link to previous document]. Figure 1 The coating includes a current collector 3 and a coating located on at least one side of the current collector. The coating includes a first coating 2 and a second coating 1 located on the first coating 2. The pore structure of the coating increases in a gradient away from the current collector. The first coating 2 has a first pore structure, and the second coating 1 has a second pore structure. The first pore structure has through-connected pores with a diameter of 1.5-2.5 μm, and the second pore structure has interpenetrating pores with a diameter of 6.5-10.5 μm and macropores with a diameter of 0.2-1.0 mm.

[0015] The three-dimensional pore gradient distribution thick electrode provided by this invention increases porosity by constructing a porous structure inside the electrode. On the one hand, this increases electrode thickness and active material loading while shortening the ion transport distance during electrochemical reactions, significantly enhancing the rate performance of the thick electrode. On the other hand, the gradient change in the pore structure of the coating along the direction away from the current collector alleviates volume stress changes during electrode fabrication / charge-discharge processes, reduces tortuosity, minimizes electrode cracking and pulverization, and improves electrode cycle stability. This, in turn, improves the energy density and lithium extraction efficiency of the electrode.

[0016] In some alternative embodiments, the ratio of the sum of the volumes of the first pore structures to the sum of the volumes of the second pore structures is 10-30:65-85.

[0017] The three-dimensional pore gradient distribution thick electrode sheet provided by this invention can further optimize the transport of lithium ions and electrons by designing a gradient pore structure. As described, the pore structure of the coating is arranged in a gradient increasing structure in terms of the number or size of pores along the direction away from the current collector. The increase in porosity and pore size of the coating on the side away from the current collector can improve the wettability of the electrolyte, promote the rapid penetration of the electrolyte and ion transport, thereby enhancing the lithium ion transport efficiency, improving the liquid phase transport process of the battery, and allowing the active material to fully react during charging and discharging.

[0018] In some alternative embodiments, a thick electrode with a three-dimensional pore gradient distribution is described, see [link to previous document]. Figure 2 This includes a current collector 3 and a coating located on one side of the current collector, the coating comprising a first coating 2 and a second coating 1 located on the first coating 2; or, a thick electrode with a three-dimensional pore gradient distribution, see [reference needed]. Figure 3 It includes a current collector 3 and a coating located on one side of the current collector, the coating including a first coating 2 and a second coating 1 located on the first coating 2.

[0019] In some alternative embodiments, the thickness of the first coating 2 is 0.2-0.8 mm and the porosity of the first coating 2 is 10-30%, the thickness of the second coating 1 is 1.5-3 mm, and the porosity of the second coating 1 is 65-85%.

[0020] In some alternative embodiments, the current collector 3 is at least one of titanium mesh, graphite felt, carbon cloth and graphite plate, and the thickness of the current collector is 1-3 mm.

[0021] In some alternative implementations, the thickness of the three-dimensional pore gradient distribution thick electrode sheet is 2-4 mm.

[0022] Secondly, the present invention provides a method for preparing the above-mentioned three-dimensional pore gradient distribution thick electrode sheet, comprising the following steps: S1. A mixture including positive electrode active material, binder, reinforcing agent and hydrophilic agent and a first pore-forming agent are added to a solvent in sequence and mixed evenly under high-speed stirring to prepare a first electrode slurry. A mixture including positive electrode active material, binder and additives and a second pore-forming agent are added to a solvent in sequence and mixed evenly under high-speed stirring to prepare a second electrode slurry. S2. The first electrode paste is coated onto the current collector, and the second electrode paste is coated on. After drying and shaping, a dry electrode sheet is obtained. S3. Immerse the dried electrode sheet in pure water, soak and rinse to thoroughly remove the first pore-forming agent and the second pore-forming agent, and obtain a thick electrode sheet with a three-dimensional pore gradient distribution.

[0023] in: S1 includes: mixing a hydrophilic agent and a binder with a solvent to obtain a slurry, then adding a conductive agent, a positive electrode active material, a reinforcing agent, and a first pore-forming agent to the slurry and mixing them to obtain a first electrode slurry; mixing a hydrophilic agent and a binder with a solvent to obtain a slurry, then adding a conductive agent, a positive electrode active material, a reinforcing agent, and a second pore-forming agent to the slurry and mixing them to obtain a second electrode slurry.

[0024] In some alternative embodiments, the positive electrode active material includes at least one of lithium iron phosphate, lithium manganese oxide, lithium titanate, lithium manganese iron phosphate, and lithium nickel cobalt manganese oxide. The conductive agent includes at least one of carbon black, acetylene black, superconducting carbon, graphene, and carbon nanotubes. The adhesive includes at least one of polytetrafluoroethylene, polyvinylidene fluoride, and polyvinyl chloride; The reinforcing agent includes at least one of carbon fiber, graphite fiber, and silver nanofiber. The hydrophilic agent includes at least one of polyvinyl alcohol, polyethylene glycol, polyacrylic acid, and polyvinylpyrrolidone; The solvent includes at least one of NMP, DMF, and DMA.

[0025] In the preparation of thick electrodes with three-dimensional pore gradient distribution, the addition of binders can enhance the adhesion force inside the electrode and improve its crack resistance; the addition of conductive agents can ensure the continuity of the electron conduction path by optimizing the distribution of the conductive agents; the addition of reinforcing agents can enhance the mechanical strength of the electrode; and the addition of hydrophilic agents can enhance the hydrophilicity of the coating, thereby improving the wettability and adhesion of the coating.

[0026] In some optional embodiments, the components in the mixture, by weight percentage, are: 65wt%-85wt% positive electrode active material, for example, 65wt%, 75wt%, 80wt%, or 85wt%, and 2wt% hydrophilic agent. 10wt%, for example, it could be 2wt%, 4wt%, 6wt%, 8wt%, or 10wt%, with 4wt% conductive agent. 10 wt%, for example, it could be 4 wt%, 6 wt%, 8 wt%, or 10 wt%, with 4 wt% of adhesive. 10 wt%, for example, it could be 4 wt%, 6 wt%, 8 wt%, or 10 wt%, with 1 wt% reinforcing agent. 5wt%, for example, can be 1wt%, 2wt%, 3wt%, 4wt% or 5wt%, but is not limited to the listed values; other unlisted values ​​within the above range also apply.

[0027] In some alternative embodiments, the solvent content is 100 wt% of the mixture content. 120wt%, for example, can be 100wt%, 105wt%, 110wt%, 115wt% or 120wt%, but is not limited to the listed values, other unlisted values ​​within the above range also apply; In some alternative embodiments, the first pore-forming agent comprises a soluble substance, and the second pore-forming agent comprises a soluble substance and a thermally decomposable substance, wherein the mass ratio of the soluble substance to the thermally decomposable substance is 1:(0) 2) (For example, it can be 1:0, 1:1, 1:1.5 or 1:2, but not limited to the listed values, other unlisted values ​​within the above range are also applicable), the soluble substance includes at least one of sodium chloride, potassium chloride, sodium carbonate and glucose; the thermally decomposable substance includes at least one of ammonium bicarbonate and sodium bicarbonate.

[0028] In some alternative embodiments, the pore-forming agent is present in an amount of 10 wt% of the active material composition. 30wt%; for example, it can be 10wt%, 15wt%, 20wt% or 30wt%, but is not limited to the listed values, and other unlisted values ​​within the above range are also applicable.

[0029] This invention improves electrode performance by controlling the mass ratio of soluble and thermally decomposable substances. The thermally decomposed substances create three-dimensional macropores that provide excellent pathways for ion transport, while the soluble substances form micropores within the coating that enhance ion transport, thus improving the overall performance. If the mass ratio is too high, it fails to provide a good three-dimensional network transport pathway for ions within the electrode, leading to poor electrochemical performance. Conversely, if the mass ratio is too low, there are too many large (millimeter-scale) cavities within the electrode, resulting in poor electron / ion conductivity and insufficient bonding between coating layers (leading to easy detachment).

[0030] S2 includes: coating the first electrode paste onto the current collector, drying and shaping it, coating the second electrode paste, drying and shaping it again to obtain a dried electrode sheet.

[0031] In some alternative embodiments, the drying includes a pre-drying process of 30°C. Insulate at 50℃ for 5 days 9 hours, for example, could be 30℃, 40℃, 45℃ or 50℃, 5 hours, 6 hours, 7 hours, 8 hours or 9 hours, and then at 70℃. Insulate at 100℃ for 10 15h, for example, can be 70℃, 80℃, 90℃ or 100℃, 10h, 11h, 12h, 13h, 14h or 15h, but is not limited to the listed values. Other unlisted values ​​within the above range also apply.

[0032] S3 includes: immersing the dried electrode sheet in pure water, soaking and rinsing to thoroughly remove the first pore-forming agent and the second pore-forming agent, to obtain a thick electrode sheet with a three-dimensional pore gradient distribution.

[0033] In some alternative embodiments, the soaking time is 10 minutes. 20h, for example, it could be 10h, 15h or 20h, with a rinsing time of 5 hours. 10 minutes, for example, could be 5 minutes, 7 minutes or 10 minutes, but is not limited to the listed values; other unlisted values ​​within the above range also apply.

[0034] The present invention provides a method for preparing a thick electrode with a three-dimensional pore gradient distribution. By constructing a coating with a gradient change in pore structure on the surface of the current collector, the problem of poor ion channel connectivity is solved. This makes the degree of cracking inside the electrode controllable, allowing lithium ions to have a good path during transport, making ion conduction smoother during electro-intercalation and deintercalation, accelerating brine circulation, reducing concentration polarization, enhancing ion conduction during electrode operation, and improving the lithium extraction performance of the electrode.

[0035] The present invention will be further described below with reference to embodiments.

[0036] Example 1 This embodiment provides a method for preparing a thick electrode sheet with a three-dimensional pore gradient distribution, the method comprising the following steps: S1. Slurry preparation: Based on the percentage of each material by mass, first add 5.4 wt% polyethylene glycol (PEG) and 6.5 wt% polyvinylidene fluoride (PVDF) to 100 wt% N. A gel solution was prepared by stirring and dissolving methylpyrrolidone (NMP) solvent. Then, 10.2 wt% superconducting carbon (SP), 75.5 wt% lithium iron phosphate (LFP) and 2.4 wt% carbon fiber (CF) were slowly added to the prepared gel solution and stirred until homogeneous. Finally, 10 wt% sodium chloride (NaCl) was added and stirred until homogeneous to obtain the electrode slurry of the first layer in the thick electrode structure.

[0037] Based on the percentage of each material by mass, first add 5.4 wt% polyethylene glycol (PEG) and 6.5 wt% polyvinylidene fluoride (PVDF) to 100 wt% N. A gel solution was prepared by stirring and dissolving methylpyrrolidone (NMP) solvent. Then, 10.2 wt% superconducting carbon (SP), 75.5 wt% lithium iron phosphate (LFP), and 2.4 wt% carbon fiber (CF) were slowly added to the prepared gel solution and stirred until homogeneous. Finally, 20 wt% sodium chloride (NaCl) and 10 wt% ammonium bicarbonate (NH4HCO3) were added and stirred until homogeneous, with a mass ratio of 2:1, to obtain the electrode paste for the second layer in the thick electrode structure.

[0038] S2. Thick electrode coating and drying: First, the first layer of electrode paste prepared by S1 is coated onto both sides of the current collector titanium mesh using a 0.5mm coating mold. Then, a 2.5mm coating mold is superimposed on the 0.5mm coating mold to coat the second layer of electrode paste prepared by S1 onto the first coating layer using a scraping method. The thickness of the coating layer is consistent with that of the coating mold. After coating, the electrode plate is first baked at 40℃ for 8 hours and then at 90℃ for 12 hours to obtain the coated electrode plate.

[0039] S3. Immerse and rinse the thick electrode: First, place the dried coated electrode in pure water and stir and soak for 10 hours. Then, rinse the electrode under running water for 5 minutes to ensure that the soluble substances in the electrode are rinsed off, thus obtaining an electrode for electrochemical lithium extraction.

[0040] Example 2 This embodiment provides a method for preparing a thick electrode sheet with a three-dimensional pore gradient distribution, the method comprising the following steps: S1. The steps are the same as in Example 1, except that the pore-forming agents sodium chloride (NaCl) and ammonium bicarbonate (NH4HCO3) in the second layer of electrode slurry in the thick electrode structure are in a mass ratio of 1:1, that is, 15wt% NaCl and 15wt% NH4HCO3.

[0041] S2-S3 are the same as the steps in Example 1.

[0042] Example 3 This embodiment provides a method for preparing a thick electrode sheet with a three-dimensional pore gradient distribution, the method comprising the following steps: S1. Slurry preparation: The steps are the same as in Example 1, except that the pore-forming agents sodium chloride (NaCl) and ammonium bicarbonate (NH4HCO3) in the second layer of electrode slurry in the thick electrode structure are in a mass ratio of 1:2, that is, 10wt% NaCl and 20wt% NH4HCO3.

[0043] S2-S3 are the same as the steps in Example 1.

[0044] Comparative Example 1 This comparative example provides a method for preparing a thick electrode sheet with a three-dimensional pore gradient distribution, the method comprising the following steps: S1. Slurry preparation: Based on the percentage of each material by mass, first add 5.4 wt% polyethylene glycol (PEG) and 6.5 wt% polyvinylidene fluoride (PVDF) to 100 wt% N. A gel solution was prepared by stirring and dissolving methylpyrrolidone (NMP) solvent. Then, 10.2 wt% superconducting carbon (SP), 75.5 wt% lithium iron phosphate (LFP), and 2.4 wt% carbon fiber (CF) were slowly added to the prepared gel solution and stirred until homogeneous. Finally, 20 wt% sodium chloride (NaCl) and 10 wt% ammonium bicarbonate (NH4HCO3) were added and stirred until homogeneous, with a mass ratio of 2:1, to obtain the electrode paste for the first layer of the thick electrode structure.

[0045] Based on the percentage of each material by mass, first add 5.4 wt% polyethylene glycol (PEG) and 6.5 wt% polyvinylidene fluoride (PVDF) to 100 wt% N. A gel solution was prepared by stirring and dissolving methylpyrrolidone (NMP) solvent. Then, 10.2 wt% superconducting carbon (SP), 75.5 wt% lithium iron phosphate (LFP) and 2.4 wt% carbon fiber (CF) were slowly added to the prepared gel solution and stirred until homogeneous. Finally, 10 wt% sodium chloride (NaCl) was added and stirred until homogeneous to obtain the electrode paste for the second layer in the thick electrode structure.

[0046] S2. Thick electrode coating and drying: First, the first layer of electrode paste prepared by S1 is coated onto both sides of the current collector titanium mesh using a 0.5mm coating mold. Then, a 2.5mm coating mold is superimposed on the 0.5mm coating mold to coat the second layer of electrode paste prepared by S1 onto the first coating layer using a scraping method. The thickness of the coating layer is consistent with that of the coating mold. After coating, the electrode plate is first baked at 40℃ for 8 hours and then at 90℃ for 12 hours to obtain the coated electrode plate.

[0047] S3. Immerse and rinse the thick electrode: First, place the dried coated electrode in pure water and stir and soak for 10 hours. Then, rinse the electrode under running water for 5 minutes to ensure that the soluble substances in the electrode are rinsed off, thus obtaining an electrode for electrochemical lithium extraction.

[0048] Comparative Example 2 This comparative example provides a method for preparing a thick electrode sheet for electrochemical lithium extraction, the method comprising the following steps: S1. Slurry preparation: Based on the percentage of each material by mass, first add 5.4 wt% polyethylene glycol (PEG) and 6.5 wt% polyvinylidene fluoride (PVDF) to 100 wt% N. The electrode slurry is prepared by stirring and dissolving the methylpyrrolidone (NMP) solvent. Then, 10.2 wt% superconducting carbon (SP), 75.5 wt% lithium iron phosphate (LFP) and 2.4 wt% carbon fiber (CF) are added to the prepared slurry in sequence and stirred until homogeneous. Finally, 10 wt% sodium chloride (NaCl) is added and stirred until homogeneous to obtain the electrode slurry.

[0049] S2. Thick electrode coating and drying: First, the electrode paste prepared by S1 is coated onto both sides of the current collector titanium mesh using a 3mm coating mold. The thickness of the coating is consistent with that of the coating mold. After coating, the electrode is baked at 90℃ for 14 hours to obtain the coated electrode plate.

[0050] S3. Immerse and rinse the thick electrode: First, the dried coated electrode is placed in pure water and stirred for 20 hours. Then, the electrode is rinsed under running water for 10 minutes to ensure that the soluble substances in the electrode are rinsed off, thus obtaining an electrode for electrochemical lithium extraction.

[0051] Comparative Example 3 This comparative example provides a method for preparing a thick electrode sheet for electrochemical lithium extraction, the method comprising the following steps: S1. Slurry preparation: Based on the percentage of each material by mass, first add 5.4 wt% polyethylene glycol (PEG) and 6.5 wt% polyvinylidene fluoride (PVDF) to 100 wt% N. A gel solution was prepared by stirring and dissolving methylpyrrolidone (NMP) in solvent. Then, 10.2 wt% superconducting carbon (SP), 75.5 wt% lithium iron phosphate (LFP), and 2.4 wt% carbon fiber (CF) were slowly added to the prepared gel solution and stirred until homogeneous. Finally, 20 wt% sodium chloride (NaCl) and 10 wt% ammonium bicarbonate (NH4HCO3) were added and stirred until homogeneous, with a mass ratio of 2:1, to obtain the electrode slurry.

[0052] S2. Thick electrode coating and drying: First, the electrode paste prepared by S1 is coated onto both sides of the current collector titanium mesh using a 3mm coating mold. The thickness of the coating is consistent with that of the coating mold. The coating is then baked at 40℃ for 8 hours and then at 90℃ for 12 hours to obtain the coated electrode plate.

[0053] S3. Immerse and rinse the thick electrode: First, place the dried coated electrode in pure water and stir and soak for 10 hours. Then, rinse the electrode under running water for 5 minutes to ensure that the soluble substances in the electrode are rinsed off, thus obtaining an electrode for electrochemical lithium extraction.

[0054] Comparative Example 4 This comparative example provides a method for preparing a thick electrode sheet for electrochemical lithium extraction, the method comprising the following steps: S1. Slurry preparation: Based on the percentage of each material by mass, first add 5.4 wt% polyethylene glycol (PEG) and 6.5 wt% polyvinylidene fluoride (PVDF) to 100 wt% N. The electrode slurry is prepared by stirring and dissolving the methylpyrrolidone (NMP) solvent. Then, 10.2 wt% superconducting carbon (SP), 75.5 wt% lithium iron phosphate (LFP) and 2.4 wt% carbon fiber (CF) are slowly added to the prepared slurry and stirred until homogeneous.

[0055] S2. Thick electrode coating and drying: First, the electrode paste prepared by S1 is coated onto both sides of the current collector titanium mesh using a 3mm coating mold by scraping. The thickness of the coating is consistent with that of the coating mold. The coated electrode plate is then baked at 90℃ for 14 hours.

[0056] S3. Immerse and rinse the thick electrode: First, the dried coated electrode is placed in pure water and stirred for 20 hours. Then, the electrode is rinsed under running water for 5 minutes to ensure that the soluble substances in the electrode are rinsed off, thus obtaining an electrode for electrochemical lithium extraction.

[0057] Comparative Example 5 This comparative example provides a method for preparing a thick electrode sheet for electrochemical lithium extraction, the method comprising the following steps: S1 and S3 are the same as the steps in Example 1.

[0058] S2. Thick electrode coating and drying: The steps are the same as in Example 1, except that the thick electrode is first baked at 25°C for 8 hours and then baked at 90°C for 12 hours to obtain the coated electrode plate.

[0059] Comparative Example 6 This comparative example provides a method for preparing a thick electrode sheet for electrochemical lithium extraction, the method comprising the following steps: S1 and S3 are the same as the steps in Example 1.

[0060] S2. Thick electrode coating and drying: The steps are the same as in Example 1, except that the thick electrode is first baked at 60°C for 8 hours and then baked at 90°C for 12 hours to obtain the coated electrode plate.

[0061] Performance testing Example 1 3 and Comparative Example 1 The prepared electrochemical lithium extraction electrode sheet is used as the positive electrode, and the negative electrode sheet is an electrode sheet after partial lithium ion removal by chemical oxidation. Simultaneously, an anion exchange membrane is combined and assembled into an electrochemical lithium extraction device according to existing technology. The lithium extraction performance and cycle performance of the electrode sheet are tested, specifically including the following steps: 1) Lithium extraction performance test The average specific capacity of the electrode was measured using an electrochemical workstation (after 10 lithium extraction cycles). The theoretical lithium extraction amount was calculated based on the average specific capacity of the electrode. The test conditions were: 25℃, 0.1C (constant current) + 0.35V (cutoff voltage).

[0062] 2) Cyclic performance test The capacity retention of the electrode after 50 lithium extraction cycles was measured using an electrochemical workstation. Test conditions: 25℃, 0.1C (constant current) + 0.35V (cutoff voltage).

[0063] The test results are shown in Table 1:

[0064] See Table 1 and Figure 4-5 From Example 1 3. As can be seen, the embodiments of the present invention construct a coating with a gradient distribution of pore structure on the surface of the current collector. For example, the first coating in contact with the surface of the current collector has small pores formed by the dissolution of soluble salts, and the second coating located on the surface of the first coating has macropores formed by the thermal decomposition of ammonium bicarbonate and small pores formed by the dissolution of soluble salts. Among them, macropores can provide a good path for ion transport, and micropores provide a large number of active sites for lithium ion insertion / extraction, reducing the migration distance of lithium ions at the "pore-active material interface", thereby reducing electrode impedance, enhancing ion diffusion ability, and enabling the prepared thick electrode plate to have excellent rate performance and cycle performance, thus enhancing the lithium extraction capability. When the three-dimensional pore gradient distribution thick electrode plate prepared in the above embodiments is used for lithium extraction, the high porosity of the outer layer can promote the migration of lithium ions in the brine into the electrode interior, and the low porosity of the inner layer can ensure that its electronic and ion conduction capabilities can be effectively improved, thereby enhancing the specific capacity and cycle stability of the electrode plate.

[0065] In Comparative Example 1, the slurry formulations of the first and second coatings were interchanged. Specifically, the first coating added 20 wt% sodium chloride and 10 wt% ammonium bicarbonate, and the second coating added 10 wt% sodium chloride. Due to the thermal decomposition of ammonium bicarbonate, cracks and pores were generated, which reduced the contact area between the current collector and the coating. This hindered the conduction of electrons inside the electrode.

[0066] In Comparative Examples 2-4, no gradient pore distribution coating was set. For example, Comparative Example 2 had only one coating layer, and the coating slurry contained only 10 wt% sodium chloride. Comparative Example 3 also had only one coating layer, and the coating slurry contained only 20 wt% sodium chloride and 10 wt% ammonium bicarbonate. The coating slurry of Comparative Example 4 did not contain any pore-forming agent. The thick electrode sheets prepared using the above comparative examples had their electrode impedance, electro-ionic conduction, and brine wettability greatly affected due to the excessive or insufficient pore structure inside the electrode.

[0067] In Comparative Examples 5-6, when the baking temperature of the electrode exceeds the specified range, the pore structure within the coating changes, reducing electronic and ion conduction capabilities, increasing electrode impedance and resistance, and thus reducing lithium extraction efficiency.

[0068] In summary, this invention, in the process of preparing electrochemical lithium-extraction thick electrode slurry, precisely controls the mass ratio of thermally decomposed substances to soluble substances between different coatings. Pore formation is achieved through heating decomposition and immersion dissolution. The resulting three-dimensional pore gradient distribution thick electrode effectively solves the problem of poor ion channel connectivity at the brine-electrode interface, improves electron conduction between the current collector and the inner electrode layer during electrochemical processes, and significantly enhances the lithium extraction performance of the electrode. Furthermore, the pore gradient distribution transition layer incorporated during electrode preparation alleviates volumetric stress changes during processing / charge-discharge, reduces tortuosity, minimizes electrode cracking and pulverization, and improves electrode cycle stability. It also significantly increases the electrode's energy density and lithium extraction efficiency.

[0069] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A thick electrode sheet with a three-dimensional pore gradient distribution, characterized in that, The device includes a current collector and a coating located on at least one side of the current collector. The coating includes a first coating and a second coating located on the first coating. The pore structure of the coating increases in a gradient direction away from the current collector. The first coating has a first pore structure, and the second coating has a second pore structure. The first pore structure has through-connected pores with a diameter of 1.5-2.5 μm, and the second pore structure has interpenetrating pores with a diameter of 6.5-10.5 μm and macropores with a diameter of 0.2-1.0 mm.

2. The three-dimensional pore gradient distribution thick electrode sheet according to claim 1, characterized in that, The ratio of the sum of the volumes of the first pore structures to the sum of the volumes of the second pore structures is 10-30:65-85.

3. The three-dimensional pore gradient distribution thick electrode sheet according to claim 1 or 2, characterized in that, It includes a current collector and a coating located on one or both sides of the current collector, the coating including a first coating and a second coating located on the first coating.

4. The three-dimensional pore gradient distribution thick electrode sheet according to claim 1 or 2, characterized in that, The thickness of the first coating is 0.2-0.8 mm, and the porosity of the first coating is 10-30%. The thickness of the second coating is 1.5-3 mm, and the porosity of the second coating is 65-85%.

5. The three-dimensional pore gradient distribution thick electrode sheet according to claim 1 or 2, characterized in that, The current collector is at least one of titanium mesh, graphite felt, carbon cloth and graphite plate, and the thickness of the current collector is 1-3 mm. The thickness of the three-dimensional pore gradient distribution thick electrode sheet is 2-4 mm.

6. A method for preparing a thick electrode sheet with a three-dimensional pore gradient distribution according to any one of claims 1-5, characterized in that, It includes: Prepare electrode slurries containing different pore-forming agents; The electrode paste is coated onto the current collector and then dried and shaped. Immerse the dried electrode in pure water and soak and rinse to thoroughly remove the pore-forming agent.

7. The method for preparing a three-dimensional pore gradient distribution thick electrode sheet according to claim 6, characterized in that, Includes the following steps: A mixture including positive electrode active material, binder, reinforcing agent and hydrophilic agent and a first pore-forming agent are added to a solvent in sequence and mixed evenly under high-speed stirring to prepare a first electrode slurry. A mixture including positive electrode active material, binder and additives and a second pore-forming agent are added to a solvent in sequence and mixed evenly under high-speed stirring to prepare a second electrode slurry. The first electrode paste is coated onto the current collector, then dried and shaped. The second electrode paste is then coated onto the current collector and dried and shaped again to obtain a dry electrode sheet. The dried electrode sheet is immersed in pure water and soaked and rinsed to completely remove the first pore-forming agent and the second pore-forming agent, thereby obtaining a thick electrode sheet with a three-dimensional pore gradient distribution.

8. The method for preparing a three-dimensional pore gradient distribution thick electrode sheet according to claim 7, characterized in that, The preparation of the electrode slurry includes: mixing a hydrophilic agent and a binder with a solvent to obtain a colloid, then adding a conductive agent, a positive electrode active material and a reinforcing agent and a first pore-forming agent to the colloid and mixing them to obtain a first electrode slurry; mixing a hydrophilic agent and a binder with a solvent to obtain a colloid, then adding a conductive agent, a positive electrode active material and a reinforcing agent and a second pore-forming agent to the colloid and mixing them to obtain a second electrode slurry. Preferably, the content of each component in the mixture, calculated by weight percentage, is: 65 wt% positive electrode active material. 85 wt%, hydrophilic agent 2 wt%. 10wt%, conductive agent 4wt% 10 wt%, adhesive 4 wt% 10 wt%, reinforcing agent 1 wt% 5wt%; Preferably, the first pore-forming agent comprises a soluble substance, and the second pore-forming agent comprises a soluble substance and a thermally decomposable substance, wherein the mass ratio of the soluble substance to the thermally decomposable substance is 1:(0) 2) The soluble substance includes at least one of sodium chloride, potassium chloride, sodium carbonate, and glucose; the thermally decomposable substance includes at least one of ammonium bicarbonate and sodium bicarbonate. Preferably, the pore-forming agent content is 10 wt% of the mixture. 30wt%; Preferably, the solvent content is 100 wt% of the mixture. 120wt%; Preferably, the positive electrode active material includes at least one of lithium iron phosphate, lithium manganese oxide, lithium titanate, lithium manganese iron phosphate, and lithium nickel cobalt manganese oxide; Preferably, the conductive agent includes at least one of carbon black, acetylene black, superconducting carbon, graphene, and carbon nanotubes; Preferably, the adhesive comprises at least one of polytetrafluoroethylene, polyvinylidene fluoride, and polyvinyl chloride; Preferably, the reinforcing agent includes at least one of carbon fiber, graphite fiber, and silver nanofiber; Preferably, the hydrophilic agent includes at least one of polyvinyl alcohol, polyethylene glycol, polyacrylic acid, and polyvinylpyrrolidone; Preferably, the solvent includes at least one of NMP, DMF, and DMA.

9. The method for preparing a three-dimensional pore gradient distribution thick electrode sheet according to claim 7, characterized in that, The preparation of the three-dimensional pore gradient distribution thick electrode sheet includes: the electrode slurry is coated by at least one of scraping, spraying, and dipping; during coating, different groups of electrode slurry are coated layer by layer from both sides of the current collector in a gradient distribution according to the mass of the pore-forming agent; the drying and shaping includes: first, before 30 Insulate at 50℃ for 5 days 9h, then at 70 Insulate at 100℃ for 10 15h; the soaking and rinsing times for the dried electrode sheets are 10h and 15h respectively. 20h and 5-10min to ensure that the pore-forming agent in the electrode is fully dissolved.

10. An electrochemical lithium extraction device, characterized in that, The three-dimensional pore gradient distribution thick electrode sheet as described in any one of claims 1-5, or the three-dimensional pore gradient distribution thick electrode sheet prepared by the preparation method described in any one of claims 6-9.

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