Modified composite electrode based on copper hexacyanoferrate and preparation method and application thereof

Abstract

The invention relates to the technical field of capacitive deionization, in particular to a modified composite electrode based on copper hexacyanoferrate and a preparation method and application thereof.The modified composite electrode sequentially comprises a CuHCF matrix, a nitrogen heterocyclic functional group grafted to the surface of the CuHCF matrix and a selective frame film growing on the surface of the CuHCF matrix from inside to outside, the surface of the selective frame film is coated with the self-repairing polymer layer; the selective frame film is made of a functionalized frame material, and the aperture of the selective frame film is 3.0-5.0. By optimizing the structure and modifying the function, the selectivity of NH4 < + > is enhanced, and meanwhile, the cycling stability and the interface stability of the electrode are improved, so that the efficient and long-acting capacitive deionization removal of NH4 < + > in the lithium-rich solution is realized, and the upgrading of a battery-grade lithium carbonate purification technology is promoted.

Description

Technical Field

[0001] This invention relates to the field of capacitor deionization technology, and in particular to modified composite electrodes based on copper hexacyanoferrate, their preparation methods, and applications. Background Technology

[0002] Lithium-ion batteries (LIBs), as core energy storage devices in the new energy field, rely heavily on the purity of their cathode material precursors, especially battery-grade lithium carbonate, for performance and safety. During lithium carbonate production and lithium resource recycling, ammonium ions (NH4+) play a crucial role in the production and recycling of lithium resources. + NH4 is an unavoidable harmful impurity; even trace amounts can cause serious damage to lithium-ion batteries. + Exhibiting thermal instability, NH4 easily decomposes to produce ammonia gas during battery charge-discharge cycles or at high temperatures. This leads to increased internal battery pressure and casing expansion, while simultaneously accelerating the degradation of the electrode-electrolyte interface, significantly shortening battery life and posing safety hazards. Therefore, achieving NH4 in lithium-rich solutions (such as lithium carbonate production mother liquor and lithium resource recovery solutions) is crucial. + Deep removal is a key step in ensuring the purity of battery-grade lithium carbonate.

[0003] Traditional NH4 + Removal technologies mainly include chemical precipitation and air stripping, but these methods have significant drawbacks: chemical precipitation easily leads to excessive loss of lithium ions, reducing lithium resource utilization; air stripping requires harsh conditions such as high temperature or acid-base regulation, resulting in high energy consumption and secondary pollution, making it difficult to meet the requirements of green and efficient purification. Hybrid capacitor deionization (HCDI) technology, as an emerging low-energy separation technology, achieves ion-specific separation through the synergistic effect of Faraday electrodes and capacitor components. It has advantages such as mild operation, no chemical additives, and low energy consumption, making it ideal for removing NH4 from lithium-rich solutions. + The removal of [the substance] provides a new direction.

[0004] Copper hexacyanoferrate (CuHCF), a typical representative of Prussian blue analogues, has a rigid face-centered cubic lattice structure with a lattice aperture of 3.2 Å, based on the difference in ionic hydration radii (NH4+). + Hydration radius 3.31 Å <Li + Hydration radius 3.82 Å), capable of achieving NH4 + Selective embedding becomes the key to removing NH4 in HCDI systems. + The ideal cathode material. However, existing CuHCF-based electrodes still face the following technical bottlenecks when used in lithium-rich solutions: (1) Insufficient selectivity, relying solely on a single selection mechanism based on lattice size differences. In the context of high lithium concentration, Li + Competitive adsorption leads to NH4 +The selectivity coefficient is limited (the highest existing technology is about 4.1), making it difficult to achieve NH4 + (2) Poor cycle stability, during long-term adsorption-desorption cycle, Li + Repeated insertion / extraction can cause lattice distortion in CuHCF, making the electrode surface prone to microcracks. Meanwhile, Cu... 2+ It is easy to dissolve in the electrolyte, which leads to rapid decay of electrode performance. After 50 cycles, the selectivity coefficient drops significantly to about 3.0; (3) Insufficient interface stability. The CuHCF matrix is ​​prone to side reactions when in direct contact with the electrolyte, which affects the long-term reliability of the HCDI system. Summary of the Invention

[0005] The purpose of this invention is to provide a modified composite electrode based on copper hexacyanoferrate, its preparation method, and its application. Through optimized structural and functional modification, it enhances the resistance to NH4+. + This improves the selectivity of NH4+ in lithium-rich solutions while enhancing the cycling stability and interfacial stability of the electrode. + The efficient and long-lasting capacitive deionization technology promotes the upgrading of battery-grade lithium carbonate purification technology.

[0006] To achieve the above objectives, the present invention provides a modified composite electrode based on copper hexacyanoferrate. The modified composite electrode comprises, from the inside out, a CuHCF matrix, nitrogen heterocyclic functional groups grafted onto the surface of the CuHCF matrix, a selective framework film grown on the surface of the CuHCF matrix, and a self-healing polymer layer coated on the surface of the selective framework film. The selective framework film is composed of a functionalized framework material, and the pore size of the selective framework film is 3.0 Å to 5.0 Å.

[0007] Preferably, nitrogen-containing heterocyclic functional groups are obtained by grafting hydroxyl groups on the surface of a CuHCF matrix with compounds containing pyridine or imidazole groups via covalent bonding.

[0008] Preferably, the compounds containing pyridine groups include one of pyridine-3-carboxylic acid, 4-pyridinecarboxaldehyde, 2-aminopyridine, and pyridinedicarboxylic anhydride, and the compounds containing imidazole groups include one of imidazolecarboxaldehyde, imidazole-4-carboxaldehyde, and 2-methylimidazolium.

[0009] Preferably, the functional groups in the functionalized framework material include one or more of amino, crown ether, carboxyl, sulfonic acid, pyridyl, and imidazole groups.

[0010] Preferably, the functionalized framework material includes one of NH2-MIL-101, NH2-MIL-53, NH2-UiO-66, ZIF-8, MIL-88B-NH2, TpPa-NH2, TpBD-NH2, COF-LZU1, TAPB-TPA-COF, and Py-1P-COF.

[0011] Preferably, the self-healing polymer in the self-healing polymer layer is a dynamically cross-linked polymer, including one of the following: polyethylene glycol diamine-adipaldehyde hydrazone cross-linked polymer, polyethylene glycol diacrylate-dithiothreitol disulfide cross-linked polymer, polycaprolactone diol-isophorone diisocyanate-cystamine disulfide cross-linked polymer, and polycaprolactone diol-ethylenediamine-adipaldehyde hydrazone cross-linked polymer.

[0012] This invention also provides a method for preparing a modified composite electrode based on copper hexacyanoferrate, comprising the following steps: S1. Preparation of CuHCF: CuCl2・2H2O solution and K3[Fe(CN)6] solution were simultaneously injected into deionized water using the co-precipitation method. After standing, centrifugation, washing, and drying, CuHCF powder was obtained. S2, nitrogen heterocyclic functional group grafting: After activating the CuHCF powder of S1, it reacts with a compound containing pyridine or imidazole groups in DMF solvent under reflux. After washing and drying, nitrogen heterocyclic functional group grafted N-CuHCF is obtained. S3, In-situ growth of selective framework thin films: The N-CuHCF of S2 is used as an electrode substrate and subjected to plasma treatment. Then, it is immersed in a functionalized framework material precursor solution for liquid phase epitaxial growth. After activation treatment, an electrode with in-situ grown selective framework thin films is obtained, N-CuHCF@selective framework thin film electrode. S4. Self-healing polymer end-capping: The N-CuHCF@selective framework thin film electrode of S3 is immersed in a self-healing polymer solution and heat-treated to obtain a modified composite electrode.

[0013] Preferably, the activation of CuHCF powder in S2 is achieved by dispersing CuHCF powder in anhydrous ethanol and ultrasonically treating it for 20-40 min, followed by vacuum drying; the reflux reaction temperature is 80-100℃, and the reaction time is 8-12 h.

[0014] Preferably, the plasma treatment time in S3 is 5~10 min, the liquid phase epitaxial growth temperature is 60~80℃, and the holding time is 6~10 h; the activation treatment temperature is 110~130℃, and the holding time is 3~5 h.

[0015] This invention also provides the application of a modified composite electrode based on copper hexacyanoferrate in the efficient capacitive deionization of ammonium ions in lithium-rich solutions. The modified composite electrode is used as the cathode and an activated carbon electrode as the anode, combined with an anion exchange membrane HCDI battery, to perform capacitive deionization treatment on lithium-rich solutions at a working voltage of 0.8~1.2V. The molar concentration ratio of lithium ions to ammonium ions in the lithium-rich solution is 3:1~10:1.

[0016] Mechanism of the invention: In this invention, the lattice pore size of the CuHCF matrix is ​​3.2 Å, and the lithium-rich solution contains NH4+. + The hydration radius (3.31 Å) is smaller than that of Li. + Hydration radius (3.82 Å), NH4 + It is easier to overcome the lattice energy barrier to complete the insertion, forming a basis for size selectivity; the lone pair electrons contained in the pyridine / imidazolium-based nitrogen heterocyclic groups grafted onto the CuHCF surface can interact with NH4+. + The formation of directional hydrogen bonds significantly reduces NH4+. + The desolvation energy and intercalation energy barrier of Li + It produces a repulsive effect, reducing Li + Competitive adsorption; the pore size of the selective framework film is precisely controlled to 3.5–4.0 Å, and NH4 is further screened through steric hindrance effect. + Blocking part of Li + Diffusion; simultaneously, crown ether oxygen atoms or amino groups in the selective framework film can react with NH4. + A secondary chemical recognition process is formed, and the three work together to achieve NH4 + Highly efficient and selective adsorption.

[0017] Furthermore, the selective framework film, acting as an intermediate protective layer, can reduce the direct contact between the CuHCF substrate and the electrolyte, thus inhibiting Cu... 2+ Dissolution and lattice distortion enhance the stability of the electrode interface; the dynamic cross-linked polymer in the self-healing polymer layer swells in the lithium-rich electrolyte solution to form a gel structure, which can buffer the volume stress generated during ion insertion and extraction; when microcracks are generated on the electrode surface, the dynamic acylhydrazone bonds or disulfide bonds in the polymer can be broken and recombined to repair the damage, maintain the integrity of the electrode structure, and extend the cycle life.

[0018] Therefore, the present invention, employing the above-mentioned modified composite electrode based on copper hexacyanoferrate and its preparation method and application, has the following beneficial effects: (1) This invention utilizes a three-level synergistic selection mechanism to target NH4+ in lithium-rich solutions. + Compared to Li + The selectivity coefficient reached a maximum of 7.2, a significant improvement over existing technologies (maximum 4.1); in typical lithium-rich solutions, NH4 + Salt adsorption capacity (SAC) ≥ 80 mg / g, Li + With SAC ≤ 30 mg / g, lithium loss is effectively reduced, ensuring lithium resource utilization; the self-healing polymer layer can effectively repair micro-damage during electrode cycling, maintain electrode structural integrity, and extend cycle life. (2) The preparation method of the present invention is based on the existing CuHCF preparation, which does not require significant adjustments to the production equipment and is easy to scale up industrially; electrode preparation and NH4 + The removal process produces no toxic or harmful substances, meeting the green and environmentally friendly requirements for battery-grade lithium carbonate purification.

[0019] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the cyclic voltammetry (CV) curves of the modified composite electrode in different electrolytes according to Example 2 of the present invention. Figure 1 (a) in the figure represents the results in three different electrolytes at 20 mV s. -1 A schematic diagram of the CV curve measured at a fixed scan rate; Figure 1 (b) is a schematic diagram of the cyclic voltammetry (CV) curves in 1 mol / L LiCl electrolyte; Figure 1 (c) in the figure is a schematic diagram of the cyclic voltammetry (CV) curves in 1 mol / L NH4Cl electrolyte; Figure 1 (d) is a schematic diagram of the cyclic voltammetry (CV) curve in a mixed electrolyte of 0.5 mol / L NH4Cl + 0.5 mol / L LiCl. Detailed Implementation

[0021] The present invention will be further described below with reference to the accompanying drawings and embodiments. Unless otherwise defined, the technical or scientific terms used in this invention should be understood in their ordinary sense by those skilled in the art. The features mentioned above or in the specific examples mentioned in this invention can be combined arbitrarily, and these specific embodiments are only used to illustrate the invention and are not intended to limit the scope of the invention.

[0022] This invention provides a modified composite electrode based on copper hexacyanoferrate. The modified composite electrode comprises, from the inside out, a CuHCF matrix, nitrogen heterocyclic functional groups grafted onto the surface of the CuHCF matrix, a selective framework film grown on the surface of the CuHCF matrix, and a self-healing polymer layer coated on the surface of the selective framework film. The selective framework film is composed of a functionalized framework material and has a pore size of 3.0 Å to 5.0 Å.

[0023] The matrix material of the composite modified electrode of the present invention is a Prussian blue analogue with an open framework structure, and the rigid framework provides NH4. + The basic size selectivity of the intercalation; the lone pair electrons of the grafted nitrogen heterocyclic group with NH4 +Hydrogen bonding interactions are formed, significantly reducing the adsorption energy barrier and enhancing the thermodynamic driving force; a selective framework film with precise pore size and surface chemistry serves as the final molecular sieve, further excluding Li with a large hydration radius. + Ions enable comprehensive selective control from the micropore scale to the molecular recognition scale; the self-healing polymer layer swells in the electrolyte to form a gel buffer layer, effectively absorbing the mechanical stress caused by ion insertion and extraction, significantly reducing the volume deformation and crack initiation of the electrode material, and its internal dynamic covalent bond or supramolecular interaction network endows it with self-healing ability, which can repair micro-damage generated during cycling under mild conditions, realize in-situ regeneration of the electrode structure, and thus significantly extend the electrode cycle life.

[0024] Provides ion storage capacity, its lattice or channels for target ions (such as NH4) + ) and non-target ions (such as Li) + It has primary size selectivity.

[0025] Preferably, the nitrogen heterocyclic functional group is obtained by grafting a compound containing a pyridine group or an imidazole group onto the surface of a CuHCF matrix via a covalent bonding reaction.

[0026] More preferably, the grafting rate of the nitrogen heterocyclic functional groups in this invention is 5% to 15%. This grafting rate range ensures sufficient hydrogen bond recognition sites while avoiding excessive grafting that could clog the CuHCF lattice pores, thus balancing selectivity and ion transport efficiency.

[0027] Preferably, the pyridine-containing compounds include one of pyridine-3-carboxylic acid, 4-pyridinecarboxaldehyde, 2-aminopyridine, and pyridinedicarboxylic anhydride, and the imidazole-containing compounds include one of imidazolecarboxaldehyde, imidazole-4-carboxaldehyde, and 2-methylimidazolium. Compounds containing pyridine groups exhibit high reactivity, can form stable covalent bonds with the hydroxyl groups on the CuHCF surface, and their lone pair electrons can interact with NH4+. + Formation of directional hydrogen bonds.

[0028] In this invention, the nitrogen heterocyclic functional group is introduced as NH4. + It provides chemical recognition sites that can significantly reduce NH4 + The desolvation energy and intercalation energy barrier of Li + This generates a repulsive effect, further enhancing selectivity based on the size selection of the CuHCF matrix.

[0029] Preferably, the functional groups in the functionalized framework material include one or more of the following: amino, crown ether, carboxyl, sulfonic acid, pyridyl, and imidazole groups. These functional groups can interact with NH4+. + Formation of synergistic recognition (e.g., amino groups and NH4 groups) +The hydrogen bonding effect and the cavity matching effect of the crown ether unit further enhance the selectivity.

[0030] Preferably, the functionalized framework material includes one of NH2-MIL-101, NH2-MIL-53, NH2-UiO-66, ZIF-8, MIL-88B-NH2, TpPa-NH2, TpBD-NH2, COF-LZU1, TAPB-TPA-COF, and Py-1P-COF. These functionalized framework materials all possess high specific surface area, tunable pore size, and excellent chemical stability, making them suitable for use with NH4+. + The selective adsorption requirement.

[0031] Preferably, the thickness of the selective framework film is 50~200 nm. This thickness range ensures that the selective framework film can effectively screen ions and protect the CuHCF matrix without significantly increasing ion transport resistance, thus balancing selectivity and mass transfer efficiency.

[0032] The selective framework thin film constructed from the functionalized framework material of this invention has a pore size of 3.0~5.0 Å, which can be used to precisely screen NH4 through steric hindrance effect. + (Hydration radius 3.31 Å), blocking portion Li + (Hydration radius 3.82 Å) diffusion, while the functional groups on the selective framework film surface react with NH4. + This enables secondary chemical recognition, achieving dual selectivity enhancement through pore size screening and chemical recognition.

[0033] Preferably, the self-healing polymer in the self-healing polymer layer is a dynamically cross-linked polymer, including one of the following: polyethylene glycol diamine-adipaldehyde hydrazone cross-linked polymer, polyethylene glycol diacrylate-dithiothreitol disulfide cross-linked polymer, polycaprolactone diol-isophorone diisocyanate-cystamine disulfide cross-linked polymer, and polycaprolactone diol-ethylenediamine-adipaldehyde hydrazone cross-linked polymer.

[0034] The self-healing polymer layer of this invention is constructed based on dynamic acylhydrazone bonds or disulfide bonds. The dynamically cross-linked polymer in the self-healing polymer layer swells in an electrolyte to form a gel structure. The gel structure neither hinders ion transport nor hinders volume stress during ion insertion and extraction. The breaking and recombination of dynamic bonds enables self-repair of damage.

[0035] Preferably, the thickness of the self-healing polymer layer is 20-50 nm. This thickness ensures that the end-capping layer can effectively repair micro-damage without affecting NH4. + The transmission rate, balancing stability and ion transport efficiency.

[0036] In some embodiments of the present invention, the preparation of the dynamically crosslinked polymer includes the following methods: Polyethylene glycol diamine-adipaldehyde acylhydrazone crosslinked polymer: Polyethylene glycol diamine and adipaldehyde are mixed in ethanol solvent at a molar ratio of 1:1.0 to 1:1.5 and stirred at room temperature for 3 to 6 hours to form a polymer through acylhydrazone crosslinking; Polyethylene glycol diacrylate-dithiothreitol disulfide crosslinking polymer: Polyethylene glycol diacrylate and dithiothreitol are mixed in a molar ratio of 1:0.8 to 1:1.2 in a mixed solvent of ethanol and water, and a small amount of photoinitiator (such as 2-hydroxy-2-methyl-1-phenyl-1-propanone) is added. After irradiation with ultraviolet light for 1 to 3 hours, the polymer is formed through disulfide crosslinking. Polycaprolactone diol-isophorone diisocyanate-cystamine disulfide crosslinked polymer: Polycaprolactone diol and isophorone diisocyanate are mixed in anhydrous DMF at a molar ratio of 1:2.0 to 1:2.5 and reacted at 70 to 80°C for 4 to 6 hours. Cystamine is added as a crosslinking agent (5% to 10% of the total mass of the reactants), and the reaction is continued for 2 to 4 hours to form a polymer through disulfide crosslinking. Polycaprolactone diol-ethylenediamine-adipaldehyde acylhydrazone crosslinked polymer: Polycaprolactone diol and ethylenediamine are mixed in DMF at a molar ratio of 1:1.5 to 1:2.0 and reacted at 60 to 70°C for 3 to 5 hours. Adipaldehyde (molar ratio with ethylenediamine of 1:1.0 to 1:1.2) is added and stirred at room temperature for 2 to 4 hours to form a polymer through acylhydrazone crosslinking.

[0037] This invention also provides a method for preparing a modified composite electrode based on copper hexacyanoferrate, comprising the following steps: S1. Preparation of CuHCF: A co-precipitation method was used, where CuCl2·2H2O solution and K3[Fe(CN)6] solution were simultaneously injected into deionized water. After standing, centrifugation, washing, and drying, CuHCF powder was obtained. CuHCF prepared by the co-precipitation method has residual hydroxyl (-OH) groups and unsaturated Cu on its surface. 2+ The coordination site and residual hydroxyl group provide reaction sites for the subsequent covalent grafting of nitrogen heterocyclic functional groups; unsaturated Cu 2+ Coordination sites can enhance the interaction between CuHCF and functional molecules, while maintaining the integrity of CuHCF's rigid face-centered cubic lattice structure, laying the structural foundation for ion-selective intercalation; the simultaneous implantation reaction mode can avoid the generation of impurities caused by the excess of a single precursor, ensuring product purity and crystallinity.

[0038] S2. Grafting of Nitrogen Heterocyclic Functional Groups: After activating the CuHCF powder from S1, a compound containing pyridine or imidazole groups was reacted with the powder under reflux in DMF solvent. After washing and drying, N-CuHCF grafted with nitrogen heterocyclic functional groups was obtained. Activation disrupts the weak binding layer on the surface of CuHCF powder, exposing more hydroxyl sites and enhancing its reactivity. DMF solvent has good solubility, promoting sufficient contact between reactant molecules. The reflux temperature provides the activation energy required for the covalent bonding reaction, allowing compounds containing pyridine and imidazole groups to undergo dehydration condensation with the hydroxyl groups on the CuHCF surface, forming stable CO covalent bonds and achieving strong grafting of nitrogen heterocyclic groups. The grafted nitrogen heterocyclic groups interact with NH4+ through lone pair electrons. + Targeted hydrogen bonds are formed to construct chemical recognition sites.

[0039] S3. In-situ growth of selective framework films: N-CuHCF from S2 is used as the electrode substrate and subjected to plasma treatment. Then, it is immersed in a functionalized framework material precursor solution for liquid-phase epitaxial growth. After activation treatment, an electrode with in-situ grown selective framework films is obtained: N-CuHCF@selective framework film electrode. Plasma treatment introduces oxygen- and nitrogen-containing active functional groups by bombarding the electrode surface with high-energy particles, while simultaneously increasing surface roughness and enhancing the physical adsorption and chemical bonding with the framework material. During liquid-phase epitaxial growth, framework material precursor molecules are directionally nucleated and grown at active sites on the N-CuHCF surface, forming a continuous and uniform film, avoiding film detachment. The pore size of 3.0–5.0 Å is achieved by controlling the type of precursor and reaction conditions. Activation treatment removes residual solvent molecules and unreacted precursors from the film pores, activates functional groups on the film surface (such as amino and crown ether units), and strengthens the adhesion to NH4+. + It has a dual screening effect.

[0040] S4. Self-healing polymer end-capping: The S3 N-CuHCF@selective framework thin film electrode is immersed in a self-healing polymer solution and then heat-treated to obtain a modified composite electrode. During the immersion process, the self-healing polymer solution permeates into the surface pores of the selective framework film through capillary action, forming a uniform coating layer; heat treatment promotes the cross-linking reaction between polymer molecules and functional groups (such as hydroxyl and amino groups) on the surface of the selective framework film, enhancing the bonding strength between the end-capping layer and the film; the dynamically cross-linked polymer swells in the electrolyte to form a gel structure, which does not hinder NH4+. + The transport of the electrolyte can be facilitated by the dynamic breaking and recombination of acylhydrazone or disulfide bonds, which can repair microcracks caused by volumetric stress during cycling. Simultaneously, it can prevent direct contact between the electrolyte and the CuHCF matrix, thus inhibiting Cu... 2+ Dissolve.

[0041] Preferably, the activation of CuHCF powder in S2 involves dispersing CuHCF powder in anhydrous ethanol and ultrasonically treating it for 20-40 minutes, followed by vacuum drying. Ultrasonic activation increases the number of hydroxyl groups on the CuHCF surface, enhancing its reactivity with nitrogen-containing heterocyclic compounds. The reflux reaction temperature is 80-100℃, and the reaction time is 8-12 hours. This ensures the covalent bonding reaction proceeds fully while avoiding damage to the CuHCF crystal structure.

[0042] Preferably, the plasma treatment time in S3 is 5-10 min. Plasma treatment can activate the electrode surface, increase active sites, and improve the bonding force with the functionalized framework material. The liquid phase epitaxial growth temperature is 60-80℃, and the holding time is 6-10 h; this can achieve uniform growth of the functionalized framework film. The activation treatment temperature is 110-130℃, and the holding time is 3-5 h, which can remove residual solvent in the film pores and activate the functionalized sites.

[0043] More preferably, the preparation of the functionalized framework material precursor solution in S3 is as follows: When the functionalized framework material is a metal-organic framework material, the metal salt and organic ligand are dissolved in a mixed solvent of DMF and water in a molar ratio of 1:1 to 1:3 and stirred until completely dissolved. The concentration of the metal salt is 0.02 to 0.1 M, thus obtaining a metal-organic framework material precursor solution. When the functionalized framework material is a covalent organic framework material, two organic monomers are dissolved in a mixed solvent of 1,4-dioxane and n-butanol in a molar ratio of 1:1, and 3%~5% glacial acetic acid is added as a catalyst. The mixture is stirred evenly to obtain a covalent organic framework material precursor solution.

[0044] More preferably, the preparation of the self-healing polymer solution in S4 is as follows: the raw materials of the dynamically cross-linked polymer are mixed according to the corresponding molar ratio, a solvent is added and the mixture is stirred for 3 to 6 hours to obtain a polymer solution with a mass fraction of 5% to 15%, wherein the solvent is selected from ethanol, DMF, water or a mixture thereof.

[0045] This invention also provides the application of a modified composite electrode based on copper hexacyanoferrate in the efficient capacitive deionization of ammonium ions in lithium-rich solutions. The modified composite electrode is used as the cathode and an activated carbon electrode as the anode, combined with an anion exchange membrane HCDI battery, to perform capacitive deionization treatment on lithium-rich solutions at a working voltage of 0.8~1.2V. The molar concentration ratio of lithium ions to ammonium ions in the lithium-rich solution is 3:1~10:1.

[0046] The application of this invention utilizes the electric field driving effect of an HCDI battery, combined with a modified composite electrode, to target NH4. + Multi-level selective identification to achieve NH4 +It achieves efficient adsorption and removal; the working voltage of 0.8~1.2V can balance adsorption efficiency and energy consumption, and avoid side reactions caused by excessive voltage; the molar concentration ratio of lithium ions to ammonium ions is 3:1~10:1, which is suitable for the actual working conditions of industrial lithium-rich solutions and has wide applicability.

[0047] Example 1 This invention provides a modified composite electrode based on copper hexacyanoferrate, the preparation method of which includes the following steps: S1. Preparation of CuHCF: Prepare 200 mL of 0.05 M CuCl2・2H2O solution and 200 mL of 0.025 M K3[Fe(CN)6] solution. Simultaneously inject the two solutions into 100 mL of deionized water under vigorous stirring with a magnetic stirrer. After standing at room temperature for 24 hours, collect the precipitate by centrifugation. Wash the precipitate repeatedly with deionized water 5 times to remove unreacted impurities, and then dry it in a vacuum oven at 60 °C for 24 hours to obtain CuHCF powder.

[0048] S2. Grafting of nitrogen heterocyclic functional groups: 5g CuHCF powder was dispersed in 50mL of anhydrous ethanol, ultrasonically treated for 25min to activate the surface hydroxyl sites, vacuum dried and mixed with 0.5g of 4-pyridinecarboxaldehyde, 50mL of DMF solvent was added, and the mixture was refluxed in an oil bath at 85℃ for 12h. After the reaction was completed, the product was separated by centrifugation, washed 4 times alternately with ethanol and deionized water, and vacuum dried at 60℃ for 24h to obtain N-CuHCF. The nitrogen heterocyclic grafting rate was determined to be 7.8% by elemental analysis.

[0049] S3. In-situ growth of selective framework films: N-CuHCF, PVDF, and carbon black were mixed in a mass ratio of 8:1:1, and N,N-dimethylacetamide solvent was added. The mixture was stirred vigorously for 6 hours to prepare a uniform slurry. The slurry was coated onto a graphite substrate using a doctor blade. The coating thickness was controlled to be 100 μm. The substrate was dried at 70 °C for 12 hours to obtain the electrode substrate.

[0050] The surface functional groups of the electrode substrate were activated by plasma treatment for 6 min. ZrCl4 with a concentration of 0.03 M and 2-aminoterephthalic acid with a concentration of 0.06 M were dissolved in a mixed solvent of DMF and water (volume ratio 3:1) at a molar ratio of 1:2 and stirred at 35 °C for 50 min until completely dissolved to obtain NH2-UiO-66 precursor solution.

[0051] The electrode substrate was immersed in the precursor solution and kept at 65°C for 10 hours to achieve in-situ growth of NH2-UiO-66 thin film; finally, the electrode was placed in a vacuum oven at 115°C for 5 hours to activate it, and N-CuHCF@NH2-UiO-66 thin film electrode (NH2-UiO-66 film thickness is 90nm, pore size is 3.5Å, and porosity is 55%) was obtained.

[0052] S4. Self-healing polymer end-capping: Add 0.1 mol polyethylene glycol diacrylate and 0.09 mol dithiothreitol to 50 mL of a mixed solvent of ethanol and water (volume ratio 1:1), add 0.8% of the total monomer mass of 2-hydroxy-2-methyl-1-phenyl-1-propanone, stir at room temperature for 1.5 h to obtain a 12% (w / w) polyethylene glycol diacrylate-dithiothreitol disulfide crosslinked polymer prepolymer solution.

[0053] The N-CuHCF@NH2-UiO-66 thin film electrode was immersed in a polyethylene glycol diacrylate-dithiothreitol disulfide bond crosslinking polymer prepolymer solution for 6 min, air-dried at room temperature, and then heat-treated in an 80℃ oven for 2 hours. At the same time, it was irradiated with ultraviolet light for 2 hours to induce crosslinking, resulting in a modified composite electrode (self-healing layer thickness 25 nm).

[0054] Example 2 This invention provides a modified composite electrode based on copper hexacyanoferrate, the preparation method of which includes the following steps: S1. Preparation of CuHCF: Same as step S1 in Example 1, to obtain CuHCF powder.

[0055] S2. Grafting of nitrogen heterocyclic functional groups: 5g CuHCF powder was dispersed in 50mL of anhydrous ethanol, ultrasonically treated for 30min to activate the surface hydroxyl sites, vacuum dried and mixed with 0.6g imidazole formaldehyde, 50mL of DMF solvent was added, and the mixture was refluxed in an oil bath at 95℃ for 10h. After the reaction was completed, the product was separated by centrifugation, washed 4 times alternately with ethanol and deionized water, and vacuum dried at 60℃ for 24h to obtain N-CuHCF. The nitrogen heterocyclic grafting rate was determined to be 9.5% by elemental analysis.

[0056] S3. In-situ growth of selective framework films: N-CuHCF, PVDF, and carbon black were mixed in a mass ratio of 8:1:1, and N,N-dimethylacetamide solvent was added. The mixture was stirred vigorously for 6 hours to prepare a uniform slurry. The slurry was coated onto a graphite substrate using a doctor blade. The coating thickness was controlled to be 100 μm. The substrate was dried at 70 °C for 12 hours to obtain the electrode substrate.

[0057] The surface functional groups of the electrode substrate were activated by plasma treatment for 8 min. AlCl3·6H2O with a concentration of 0.05 M and terephthalic acid with a concentration of 0.05 M were dissolved in a mixed solvent of DMF and water (volume ratio 2:1) at a molar ratio of 1:1 and stirred at 30 °C for 40 min until completely dissolved to obtain the NH2-MIL-101 precursor solution.

[0058] The electrode substrate was immersed in the precursor solution and kept at 75°C for 8 hours to achieve in-situ growth of NH2-MIL-101 film; finally, the electrode was placed in a vacuum oven at 120°C for 4 hours to activate it, and N-CuHCF@NH2-MIL-101 thin film electrode (NH2-MIL-101 film thickness is 130nm, pore size is 3.8Å, and porosity is 59%) was obtained.

[0059] S4. Self-healing polymer end-capping: 0.1 mol polyethylene glycol diamine and 0.12 mol adipaldehyde were added to 50 mL of ethanol and stirred at room temperature for 4 hours to prepare a 10% (w / w) polyethylene glycol diamine-adipaldehyde hydrazone crosslinking polymer solution; the N-CuHCF@NH2-MIL-101 thin film electrode was immersed in the polyethylene glycol diamine-adipaldehyde hydrazone crosslinking polymer solution for 10 min, air-dried at room temperature, and then heat-treated in an 85℃ oven for 2 h to allow the polymer to form a stable bond with the framework film, resulting in a modified composite electrode (self-healing layer thickness 38 nm).

[0060] Example 3 This invention provides a modified composite electrode based on copper hexacyanoferrate, the preparation method of which includes the following steps: S1. Preparation of CuHCF: Same as step S1 in Example 1, to obtain CuHCF powder.

[0061] S2. Grafting of nitrogen heterocyclic functional groups: 5g CuHCF powder was dispersed in 50mL of anhydrous ethanol, ultrasonically treated for 35min to activate the surface hydroxyl sites, vacuum dried and mixed with 0.4g 2-methylimidazole, 50mL of DMF solvent was added, and the mixture was refluxed in an oil bath at 90℃ for 11h. After the reaction was completed, the product was separated by centrifugation, washed 4 times alternately with ethanol and deionized water, and vacuum dried at 60℃ for 24h to obtain N-CuHCF. The nitrogen heterocyclic grafting rate was determined to be 6.3% by elemental analysis.

[0062] S3. In-situ growth of selective framework films: N-CuHCF, PVDF, and carbon black were mixed in a mass ratio of 8:1:1, and N,N-dimethylacetamide solvent was added. The mixture was stirred vigorously for 6 hours to prepare a uniform slurry. The slurry was coated onto a graphite substrate using a doctor blade. The coating thickness was controlled to be 100 μm. The substrate was dried at 70°C for 12 hours to obtain the electrode substrate.

[0063] The surface functional groups of the electrode substrate were activated by plasma treatment for 10 min. Trialdehyde phenol and p-phenylenediamine were dissolved in a mixed solvent of 1,4-dioxane and n-butanol (volume ratio 1:1) at a molar ratio of 1:1, and 4% glacial acetic acid was added as a catalyst. The mixture was stirred at room temperature for 25 min to obtain a TpPa-NH2 precursor solution. The electrode substrate was immersed in the precursor solution and kept at 70℃ for 9 h to achieve in-situ growth of the TpPa-NH2 thin film. Finally, the electrode was activated in a vacuum oven at 125℃ for 3 h to obtain an N-CuHCF@TpPa-NH2 thin film electrode (thickness 160 nm, pore size 4.2 Å, porosity 62%).

[0064] S4. Self-healing polymer end-capping: 0.1 mol of polycaprolactone diol and 0.22 mol of isophorone diisocyanate were added to 50 mL of anhydrous DMF and stirred at 75 °C for 5 h. Then, 8% of the total mass of cystamine was added and stirred for another 3 h. After cooling to room temperature, the mixture was diluted to 8% by mass to obtain a polycaprolactone diol-isophorone diisocyanate-cystamine disulfide crosslinked polymer solution. The N-CuHCF@TpPa-NH2 electrode was immersed in the polycaprolactone diol-isophorone diisocyanate-cystamine disulfide crosslinked polymer solution for 12 min. After drying at room temperature, it was heat-treated in a 90 °C oven for 2 h to obtain a modified composite electrode (self-healing layer thickness 45 nm).

[0065] Comparative Example 1 Unmodified CuHCF electrode. The CuHCF powder prepared in step S1 of Example 1 was mixed with PVDF and carbon black in a mass ratio of 8:1:1. N,N-dimethylacetamide solvent was added to prepare a slurry, which was coated on a graphite sheet substrate and dried at 70°C for 12 hours to obtain an unmodified CuHCF electrode.

[0066] Comparative Example 2 Based on Example 2, only S1-S2 were performed to obtain N-CuHCF. N-CuHCF was mixed with PVDF and carbon black in a mass ratio of 8:1:1, and N,N-dimethylacetamide solvent was added. The mixture was stirred vigorously for 6 hours to prepare a uniform slurry. The slurry was coated onto a graphite sheet substrate using a doctor blade, with the coating thickness controlled at 100 μm. The substrate was dried at 70°C for 12 hours to obtain an N-CuHCF electrode, a non-selective framework thin film growth, and a self-healing polymer layer.

[0067] Comparative Example 3 Based on Example 2, only S1-S3 were performed to obtain an N-CuHCF@NH2-MIL-101 thin film electrode (NH2-MIL-101 film thickness of 130nm, pore size of 3.8Å, and porosity of 59%), without a self-healing polymer layer.

[0068] Performance testing The modified composite electrode prepared in Example 2 was subjected to cyclic voltammetry (CV, scan rates 5, 10, 20, 50, 100 mV / s) and galvanostatic charge-discharge (GCD, current density 1 A / g) tests using a CHI 760E electrochemical workstation in 1 mol / L LiCl electrolyte, 1 mol / L NH4Cl electrolyte, and a mixed electrolyte of 0.5 mol / L NH4Cl and 0.5 mol / L LiCl. Figure 1 As shown.

[0069] Depend on Figure 1 It can be seen that in 1 mol / L NH4Cl electrolyte ( Figure 1 c) The CV curve shows obvious redox peaks with symmetrical peak shapes and large peak currents, indicating that the electrode is effective against NH4+. + It exhibits excellent reversible intercalation and deintercalation properties; while in 1 mol / L LiCl electrolyte ( Figure 1 b) The redox peak intensity was significantly reduced and the peak shape broadened, confirming that Li + Embedding was significantly suppressed, consistent with the results of the selectivity test.

[0070] In a mixed electrolyte of 0.5 mol / L NH4Cl + 0.5 mol / L LiCl ( Figure 1 d), the CV curve mainly shows NH4 + Redox characteristic peaks of Li + The characteristic peaks almost disappeared, indicating that the electrode can still preferentially select NH4 under competitive adsorption conditions. + Embedding was performed to further verify the selective enhancement effect of the multi-level modification strategy.

[0071] As the scan rate increased from 5 mV / s to 100 mV / s, the CV peak shapes in each electrolyte maintained good symmetry and showed no obvious distortion, indicating that the electrode has excellent ion transport rate and electrochemical kinetic performance, which is suitable for the rapid adsorption requirements of the HCDI system.

[0072] HCDI battery assembly: The modified composite electrodes prepared in Examples 1-3 and the electrodes prepared in Comparative Examples 1-3 were used as cathodes, respectively, with commercial activated carbon (specific surface area >2000 m²). 2 Using / g) as the anode, an anion exchange membrane is placed close to the anode, and the electrode spacing is controlled to 1mm to assemble an HCDI battery.

[0073] Lithium-rich solution treatment conditions: The lithium-rich solution composition is 30 mM LiCl + 10 mM NH4Cl (Li + With NH4 +The molar concentration ratio was 3:1, the solution volume was 50 mL, and the flow rate was controlled at 3.3 mL / min using a peristaltic pump. Adsorption was performed at 1.2 V for 1000 s, followed by desorption at 0 V for 1000 s, completing one adsorption-desorption cycle. The conductivity changes during the lithium-rich solution treatment were monitored in real time using a multi-parameter conductivity meter (Mettler Toledo). The NH4+ concentration in the solution before and after treatment was determined using an ion chromatograph (Thermo ICS-600). + and Li + Calculate the salt adsorption capacity (SAC) and selectivity (S NH4) based on the concentration of NH4. + / Li + The results are shown in Table 1.

[0074] Table 1. Salt adsorption capacity, selectivity coefficient, and charge efficiency of different electrodes

[0075] As shown in Table 1, the modified composite electrodes of Examples 1-3 in NH4 + The modified composite electrodes prepared in Examples 1-3 showed significantly better performance than the comparative examples in terms of SAC and selectivity coefficient. + The samples exhibit specific adsorption. Comparative Example 2 showed a higher selectivity coefficient than Comparative Example 1 but lower than Examples 1-3, indicating that the enhancement effect of a single chemical recognition site is limited, and high selectivity requires pore size sieving of the framework film. Comparative Example 3 performed better than Comparative Examples 1-2 but worse than Examples 1-3, indicating that although the self-healing layer does not directly improve selectivity, it can indirectly ensure adsorption capacity by protecting the electrode structure. The charge efficiency of Examples 1-3 was higher than that of the comparative examples, indicating that multi-stage modification did not trigger significant side reactions, and the improved selectivity reduced ineffective charge consumption, achieving a synergy of high efficiency and energy saving.

[0076] Long-term cycle test: The above adsorption-desorption cycle was performed continuously for 500 cycles. Fresh lithium-rich solution was used before each cycle. The NH4 content was tested after each cycle. + The retention rates of SAC and the selectivity coefficient S are shown in Table 2.

[0077] Table 2 Cyclic stability of different electrodes

[0078] As shown in Table 2, the selectivity retention rates of Examples 1-3 all exceeded 96% after 50 cycles, with Example 2 reaching 98.5%, significantly higher than Comparative Examples 1 and 2. This indicates that the selective framework film and self-healing polymer layer can effectively protect the electrode structure and reduce initial performance degradation. Comparative Examples 1 and 2 could not be tested further after 100 and 150 cycles, respectively, due to structural damage. The retention rate of Comparative Example 1 after 3500 cycles was only 62.3%. In contrast, the retention rates of Examples 1-3 all exceeded 81%, with Example 2 reaching 85.7%. This demonstrates that the self-healing polymer layer can effectively repair microcracks generated during cycling through the breaking and recombination of dynamic bonds, significantly extending the electrode's lifespan.

[0079] The HCDI battery assembled from the modified composite electrode of Example 2 was tested in Li... + With NH4 + In lithium-rich solutions with molar concentration ratios of 3:1 (30mM LiCl + 10mM NH4Cl), 5:1 (25mM LiCl + 5mM NH4Cl), and 10:1 (50mM LiCl + 5mM NH4Cl), NH4Cl was used for... + Capacitive deionization tests were conducted to evaluate the selective removal performance of the electrode under different lithium concentration backgrounds. The results are shown in Table 3.

[0080] Table 3. Electrode of Example 2 in different Li + With NH4 + Performance test results at molar concentration ratio

[0081] As shown in Table 3, with Li + With NH4 + The molar concentration ratio increased from 3:1 to 10:1 (Li + Increased concentration, NH4 + (concentration decrease), NH4 + Although the SAC and removal rate decreased slightly, the selectivity coefficient remained above 6.2, far exceeding the highest value of the prior art (4.1), indicating that the modified composite electrode of the present invention can still stably achieve NH4 removal under high lithium concentration conditions. + Its highly efficient and selective removal makes it suitable for the complex operating conditions of industrial lithium-rich solutions. Even in Li + With NH4 + Under extreme conditions where the molar concentration ratio of NH4 is 10:1 + The removal rate still reached 36.8%, confirming that the three-level co-selection mechanism can effectively resist Li. + Competitive adsorption to achieve low concentration of NH4 + Deep removal.

[0082] The modified composite electrodes of Examples 1-3 were subjected to mechanical scratching (50 nm depth) and ultrasonic damage (30 minutes, 100 W power), followed by immersion in a lithium-rich solution for 12 hours. The NH4 content before and after the repair was tested. + SAC was used to calculate the performance recovery rate. The results are shown in Table 4.

[0083] Table 4. Test results of self-healing performance of electrodes in Examples 1-3

[0084] As shown in Table 4, the modified composite electrodes of Examples 1-3 all exhibited a performance recovery rate exceeding 93% after mechanical scratching (50 nm depth) or ultrasonic damage (30 minutes). Example 2 showed a mechanical scratch repair rate of 94.5%, confirming that the dynamic disulfide bonds or acylhydrazone bonds in the self-healing polymer layer can achieve damage repair under mild conditions, maintaining the integrity of the electrode structure. Whether the damage is physical due to mechanical scratching or internal structural loosening caused by ultrasonic treatment, the modified composite electrodes of this invention can recover most of their performance through self-healing, demonstrating that this modification strategy has excellent repair capabilities for different types of damage, further ensuring the long-term stable operation of the electrode.

[0085] Therefore, this invention utilizes the above-mentioned modified composite electrode based on copper hexacyanoferrate, its preparation method, and its application. The prepared modified composite electrode based on copper hexacyanoferrate is used in lithium-rich solutions with NH4+. + In capacitive deionization applications, it exhibits excellent selectivity, cycle stability, and self-healing performance, effectively solving the core bottlenecks of existing technologies and providing a reliable technical solution for the deep purification of battery-grade lithium carbonate, thus possessing significant industrial application value.

[0086] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A modified composite electrode based on copper hexacyanoferrate, characterized in that: The modified composite electrode comprises, from the inside out, a CuHCF matrix, nitrogen heterocyclic functional groups grafted onto the surface of the CuHCF matrix, a selective framework film grown on the surface of the CuHCF matrix, and a self-healing polymer layer coated on the surface of the selective framework film; the selective framework film is composed of functionalized framework material, and the pore size of the selective framework film is 3.0 Å to 5.0 Å.

2. The modified composite electrode based on copper hexacyanoferrate according to claim 1, characterized in that: Nitrogen heterocyclic functional groups are obtained by grafting hydroxyl groups on the surface of a CuHCF matrix with compounds containing pyridine or imidazole groups via covalent bonding.

3. The modified composite electrode based on copper hexacyanoferrate according to claim 2, characterized in that: Compounds containing a pyridine group include one of pyridine-3-carboxylic acid, 4-pyridinecarboxaldehyde, 2-aminopyridine, and pyridinedicarboxylic anhydride; compounds containing an imidazole group include one of imidazolecarboxaldehyde, imidazole-4-carboxaldehyde, and 2-methylimidazolium.

4. The modified composite electrode based on copper hexacyanoferrate according to claim 1, characterized in that: The functional groups in functionalized framework materials include one or more of the following: amino, crown ether, carboxyl, sulfonic acid, pyridyl, and imidazole.

5. The modified composite electrode based on copper hexacyanoferrate according to claim 4, characterized in that: Functionalized framework materials include one of the following: NH2-MIL-101, NH2-MIL-53, NH2-UiO-66, ZIF-8, MIL-88B-NH2, TpPa-NH2, TpBD-NH2, COF-LZU1, TAPB-TPA-COF, and Py-1P-COF.

6. The modified composite electrode based on copper hexacyanoferrate according to claim 1, characterized in that: The self-healing polymer in the self-healing polymer layer is a dynamically cross-linked polymer, including one of the following: polyethylene glycol diamine-adipaldehyde hydrazone cross-linked polymer, polyethylene glycol diacrylate-dithiothreitol disulfide cross-linked polymer, polycaprolactone diol-isophorone diisocyanate-cystamine disulfide cross-linked polymer, and polycaprolactone diol-ethylenediamine-adipaldehyde hydrazone cross-linked polymer.

7. The method for preparing the modified composite electrode based on copper hexacyanoferrate as described in any one of claims 1-6, characterized in that: Includes the following steps: S1. Preparation of CuHCF: CuCl2・2H2O solution and K3[Fe(CN)6] solution were simultaneously injected into deionized water using the co-precipitation method. After standing, centrifugation, washing, and drying, CuHCF powder was obtained. S2, nitrogen heterocyclic functional group grafting: After activating the CuHCF powder of S1, it reacts with a compound containing pyridine or imidazole groups in DMF solvent under reflux. After washing and drying, nitrogen heterocyclic functional group grafted N-CuHCF is obtained. S3, In-situ growth of selective framework thin films: The N-CuHCF of S2 is used as an electrode substrate and subjected to plasma treatment. Then, it is immersed in a functionalized framework material precursor solution for liquid phase epitaxial growth. After activation treatment, an electrode with in-situ grown selective framework thin films is obtained, N-CuHCF@selective framework thin film electrode. S4. Self-healing polymer end-capping: The N-CuHCF@selective framework thin film electrode of S3 is immersed in a self-healing polymer solution and heat-treated to obtain a modified composite electrode.

8. The method for preparing the modified composite electrode based on copper hexacyanoferrate as described in claim 7, characterized in that: The activation of CuHCF powder in S2 involves dispersing CuHCF powder in anhydrous ethanol and ultrasonically treating it for 20-40 minutes, followed by vacuum drying; the reflux reaction temperature is 80-100℃, and the reaction time is 8-12 hours.

9. The method for preparing the modified composite electrode based on copper hexacyanoferrate as described in claim 7, characterized in that: In S3, the plasma treatment time is 5~10 min, the liquid phase epitaxial growth temperature is 60~80℃, and the holding time is 6~10 h; the activation treatment temperature is 110~130℃, and the holding time is 3~5 h.

10. The application of the modified composite electrode based on copper hexacyanoferrate as described in any one of claims 1-6, characterized in that: In the application of high-efficiency capacitive deionization of ammonium ions in lithium-rich solutions, a modified composite electrode is used as the cathode and an activated carbon electrode as the anode, combined with an anion exchange membrane HCDI battery. Capacitive deionization of lithium-rich solutions is performed at a working voltage of 0.8~1.2V, and the molar concentration ratio of lithium ions to ammonium ions in the lithium-rich solutions is 3:1~10:1.

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