Asymmetric hydrophilic and hydrophobic lithium ion screening membrane as well as preparation method and application thereof

Abstract

The invention relates to an asymmetric hydrophilic and hydrophobic lithium ion screening membrane and a preparation method and application thereof, the asymmetric hydrophilic and hydrophobic lithium ion screening membrane comprises a carbon-based lithium manganate composite membrane and a carbon-based polypyrrole composite membrane, and the carbon-based lithium manganate composite membrane and the carbon-based polypyrrole composite membrane are both asymmetric hydrophilic and hydrophobic composite membranes. According to the technical scheme, the asymmetric hydrophilic and hydrophobic interface regulation and control system is constructed, so that directional transmission of lithium ions and dynamic blocking of interfering ions are realized. The carbon-based lithium manganate composite membrane forms a lithium ion specific recognition channel under the regulation and control of an electric field based on the specific selectivity of a spinel structure, and a three-dimensional carbon network skeleton of the carbon-based lithium manganate composite membrane synchronously enhances the charge conduction efficiency; according to the carbon-based polypyrrole composite membrane, a gradient infiltration barrier layer is established through hydrophobic-pi interaction of molecular chain conformation, and the non-selective permeation trend of magnesium ions is effectively weakened. The two membrane groups form a lithium ion recognition-transmission relay mechanism through cooperative matching of hydrophilic and hydrophobic interfaces, and finally, the overall flux stability is improved while the ion screening precision is maintained.

Description

Technical Field

[0001] The present invention belongs to the technical field of the preparation of battery materials, relates to the technical field of extracting lithium from salt lake brine, and particularly relates to an asymmetric hydrophilic-hydrophobic lithium ion sieving membrane, a preparation method thereof, and an application thereof. Background Art

[0002] The booming development of the global new energy industry has driven the demand for lithium resources into a rapid growth cycle. Due to its environmental friendliness and sustainability advantages, the technology of extracting lithium from salt lake brine has gradually become an important direction for the transformation of the lithium industry. Different from the high-energy-consuming treatment links such as high temperature and strong acid in the traditional lithium extraction process from ores, the extraction of lithium from salt lake brine can achieve resource acquisition through selective separation technology, showing significant industrial competitiveness under the background of green development. Although the salt lakes on the Qinghai-Tibet Plateau in China are rich in reserves, their unique characteristics of high magnesium-lithium ratio lead to common problems such as poor selectivity and low recovery efficiency in conventional separation technologies. There is an urgent need to develop a new type of efficient lithium extraction system to meet the industrialization requirements.

[0003] As a breakthrough separation method, the electrochemically controlled ion-selective permeation (ESIP) technology exhibits precise ion recognition characteristics by coupling electrochemical regulation and ion sieving effects. The core of this technology lies in the dynamic response mechanism of the electrochemically controlled ion permeation membrane (ESIPM): using potential changes to reversibly adjust the charge state of electroactive materials in the membrane, thereby constructing an intelligent ion transport channel. The current preparation strategies for ESIP membranes usually blend electroactive materials (such as lithium manganese oxides) with conductive media in a polymer matrix. However, such composite structures have double limitations in practical applications: First, the steric hindrance formed by the polymer chain network significantly extends the transmembrane migration path of lithium ions; Second, the discrete distribution of conductive components and active sites weakens the electric field response sensitivity. Especially when dealing with salt lake brine with a high magnesium background, high-concentration magnesium ions are prone to form non-selective diffusion channels along the matrix polymer phase, resulting in a significant decrease in the interception rate of target lithium ions.

[0004] In response to the above challenges, prior art such as the "lock-gate type" electrochemically controlled ion permeation system proposed in CN117065573A optimized the ion migration kinetics by establishing a cascade potential regulation model. However, this technology shows a problem of selective attenuation when applied to real salt lake brine. The fundamental reason lies in the design limitations of the membrane material system, such as Figure 1 shown: On the one hand, when lithium ions in the actual feed liquid are at trace levels (the lithium ion concentration in the developed brine of the ESIP membrane disclosed by it is 0.03 mol / L and the magnesium ion concentration is 0.2 mol / L), the adsorption power of conventional electroactive sites is not sufficient to drive effective capture; On the other hand, the ultra-high magnesium-lithium ratio environment leads to the formation of a magnesium ion permeation dominant path in the traditional non-directional ion channels in the membrane, seriously interfering with the selective enrichment process of lithium ions. Such key performance defects limit the practical application value of existing ESIP membranes in industrial lithium extraction scenarios.

[0005] In addition, on the one hand, there are differences in the understanding of those skilled in the art; on the other hand, when the inventor made this invention, a large number of documents and patents were studied, but due to space limitations, all details and content were not listed in detail. However, this does not mean that this invention does not possess the features of these prior arts. On the contrary, this invention already possesses all the features of the prior arts, and the applicant reserves the right to add relevant prior arts in the background art. Summary of the Invention

[0006] Based on the above technical problems, the present invention provides an asymmetric hydrophilic-hydrophobic lithium-ion sieving membrane, its preparation method and application to achieve efficient separation of lithium ions.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] An asymmetric hydrophilic-hydrophobic lithium-ion sieving membrane, which comprises a carbon-based lithium manganate composite membrane and a carbon-based polypyrrole composite membrane. Among them, both the carbon-based lithium manganate composite membrane and the carbon-based polypyrrole composite membrane are set as asymmetric hydrophilic-hydrophobic composite membranes.

[0009] According to a preferred embodiment, the carbon-based lithium manganate composite membrane comprises a lithium manganate-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride-polyacrylic acid hydrophilic membrane, a carbon cloth substrate, and a lithium manganate-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride hydrophobic membrane. The lithium manganate-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride-polyacrylic acid hydrophilic membrane and the lithium manganate-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride hydrophobic membrane are respectively arranged on both sides of the carbon cloth substrate.

[0010] According to a preferred embodiment, the carbon-based polypyrrole composite membrane comprises a polypyrrole-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride-polyacrylic acid hydrophilic membrane, a carbon cloth substrate, and a polypyrrole-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride hydrophobic membrane. The polypyrrole-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride-polyacrylic acid hydrophilic membrane and the polypyrrole-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride hydrophobic membrane are respectively arranged on both sides of the carbon cloth substrate.

[0011] According to a preferred embodiment, in the polypyrrole-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride-polyacrylic acid hydrophilic membrane, by weight, polypyrrole: multi-walled carbon nanotube: binder is 14-16: 1-2: 3-5.

[0012] According to a preferred embodiment, in the polypyrrole-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride hydrophobic membrane, by weight, polypyrrole: multi-walled carbon nanotube: polyvinylidene fluoride is 14-16: 1-2: 3-5.

[0013] Preferably, in the polypyrrole-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride-polyacrylic acid hydrophilic membrane, by weight, the ratio of polypyrrole: multi-walled carbon nanotube: binder is 15:1:4.

[0014] Preferably, in the polypyrrole-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride hydrophobic membrane, by weight, the ratio of polypyrrole: multi-walled carbon nanotube: polyvinylidene fluoride is 15:1:4.

[0015] Preferably, the binder comprises polyvinylidene fluoride and polyacrylic acid. The weight ratio of polyvinylidene fluoride to polyacrylic acid is 9:11.

[0016] Preferably, in the polypyrrole-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride-polyacrylic acid hydrophilic membrane, by weight, the ratio of polypyrrole: multi-walled carbon nanotube: polyvinylidene fluoride: polyacrylic acid is 15:1:1.8:2.2.

[0017] According to a preferred embodiment, in the lithium manganate-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride-polyacrylic acid hydrophilic membrane, by weight, the ratio of polypyrrole: multi-walled carbon nanotube: binder containing polyvinylidene fluoride and polyacrylic acid is 14-16:1-2:3-5.

[0018] According to a preferred embodiment, in the lithium manganate-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride hydrophobic membrane, by weight, the ratio of lithium manganate: multi-walled carbon nanotube: polyvinylidene fluoride is 14-16:1-2:3-5.

[0019] Preferably, in the lithium manganate-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride-polyacrylic acid hydrophilic membrane, by weight, the ratio of polypyrrole: multi-walled carbon nanotube: binder is 15:1:4.

[0020] Preferably, the binder comprises polyvinylidene fluoride and polyacrylic acid. The weight ratio of polyvinylidene fluoride to polyacrylic acid is 9:11.

[0021] Preferably, in the lithium manganate-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride-polyacrylic acid hydrophilic membrane, by weight, the ratio of polypyrrole: multi-walled carbon nanotube: polyvinylidene fluoride: polyacrylic acid is 15:1:1.8:2.2.

[0022] Preferably, in the lithium manganate-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride hydrophobic membrane, by weight, the ratio of lithium manganate: multi-walled carbon nanotube: polyvinylidene fluoride is 15:1:4.

[0023] One of the objects of the present invention is also to provide an electro-controlled ion exchange device which uses the above-mentioned asymmetric hydrophilic-hydrophobic lithium ion sieving membrane.

[0024] According to a preferred embodiment, the electro-controlled ion exchange device is used for Li + and Cl- transmission

[0025] According to a preferred embodiment, the electro-controlled ion exchange device can be used for the separation of brines of Li + / Mg 2+ 、Li + / Na + 、Li + / K + and Li + / Ca 2+ brines

[0026] One of the objects of the present invention is also to provide a method for preparing an asymmetric hydrophilic-hydrophobic carbon-based lithium manganate composite membrane, which comprises the following steps:

[0027] S1: Polyacrylic acid and polyvinylidene fluoride are added to an N-methylpyrrolidone solvent and stirred evenly;

[0028] S2: Lithium manganate, multi-walled carbon nanotubes and carbon black are stirred evenly;

[0029] S3: The mixtures obtained in S1 and S2 are mixed and stirred for 24 h to obtain a slurry;

[0030] S4: The slurry of S3 is ultrasonically treated for 30 min and then poured into a groove formed on a glass plate and left standing until there are no bubbles;

[0031] S5: A carbon cloth substrate is laid on the surface of the slurry in the groove to make it completely fit the slurry;

[0032] S6: Polyvinylidene fluoride is added to an N-methylpyrrolidone solvent;

[0033] S7: Lithium manganate, multi-walled carbon nanotubes and carbon black are stirred evenly;

[0034] S8: The mixtures obtained in S6 and S7 are mixed and stirred for 24 h to obtain a slurry;

[0035] S9: The slurry obtained in S8 is ultrasonically treated for 30 minutes and then poured into the groove and left standing until there are no bubbles;

[0036] S10: Drying treatment

[0037] One of the objects of the present invention is also to provide a method for preparing an asymmetric hydrophilic-hydrophobic carbon-based polypyrrole composite membrane, which comprises the following steps:

[0038] S1: Polyacrylic acid and polyvinylidene fluoride are added to an N-methylpyrrolidone solvent and stirred evenly;

[0039] S2: Polypyrrole, multi-walled carbon nanotubes and carbon black are stirred evenly;

[0040] S3: Mix the mixtures obtained in S1 and S2 and stir for 24 h to obtain a slurry.

[0041] S4: After ultrasonic treatment of the slurry in S3 for 30 min, pour it into the groove formed on the glass plate and let it stand until there are no bubbles.

[0042] S5: Lay the carbon cloth substrate on the surface of the slurry in the groove to make it completely fit the slurry.

[0043] S6: Add polyvinylidene fluoride to the N-methylpyrrolidone solvent.

[0044] S7: Stir polyaniline, multi-walled carbon nanotubes and carbon black evenly.

[0045] S8: Mix the mixtures obtained in S6 and S7 and stir for 24 h to obtain a slurry.

[0046] S9: After ultrasonic treatment of the slurry obtained in S8 for 30 minutes, pour it into the groove and let it stand until there are no bubbles.

[0047] S10: Conduct drying treatment.

[0048] Advantages of the present invention:

[0049] Through the construction of an asymmetric hydrophilic-hydrophobic interface regulation system, the present technical solution realizes the directional transport of lithium ions and the dynamic block of interfering ions. Relying on the specific selectivity of the spinel structure, the carbon-based lithium manganate composite membrane forms a lithium-ion specific recognition channel under the regulation of an electric field, and its three-dimensional carbon network skeleton simultaneously enhances the charge conduction efficiency; the carbon-based polypyrrole composite membrane establishes a gradient infiltration barrier layer through the hydrophobic-π interaction of the molecular chain conformation, effectively weakening the non-selective permeation trend of magnesium ions. The two membranes form a relay mechanism of "recognition-transport" of lithium ions through the synergistic matching of the hydrophilic-hydrophobic interface, where the hydrophilic surface accelerates the capture process of target ions, and the hydrophobic surface inhibits the transmembrane migration of non-target ions through the solvation effect, ultimately improving the overall flux stability ( Figure 3 and Figure 5 ) while maintaining the ion sieving accuracy ( Figure 4 ). Description of the drawings

[0050] Figure 1 is a schematic diagram of the use of an electro-controlled ion-selective permeation membrane unit coupled with an auxiliary circulation system;

[0051] Figure 2 is a structural diagram of the composite membrane involved in the present invention. Among them, the left figure is the carbon-based lithium manganate composite membrane; the right figure is the carbon-based polypyrrole composite membrane;

[0052] Figure 3 is a continuous concentration test result diagram of the present invention. Among them, a is the concentration change curve of Li + and b is the concentration change curve of Mg2+ Concentration change curve;

[0053] Figure 4 It is the data graph of the cycle stability of the composite membrane according to the present invention;

[0054] Figure 5 It is the test result graph of the separation performance of the composite membrane in simulated salt lake brine according to the present invention;

[0055] Figure 6 It is the influence of membranes with different properties (hydrophilicity and hydrophobicity) involved in the present invention on ion flux and selectivity.

[0056] Reference numerals

[0057] 100: Lithium manganese oxide composite membrane based on carbon; 110: Lithium manganese oxide - multi - walled carbon nanotube - carbon black - polyvinylidene fluoride - polyacrylic acid hydrophilic membrane; 120: First carbon cloth substrate; 130: Lithium manganese oxide - multi - walled carbon nanotube - carbon black - polyvinylidene fluoride hydrophobic membrane; 200: Carbon - based polypyrrole composite membrane; 210: Polypyrrole - multi - walled carbon nanotube - carbon black - polyvinylidene fluoride hydrophilic membrane; 220: Second carbon cloth substrate; 230: Polypyrrole - multi - walled carbon nanotube - carbon black - polyvinylidene fluoride - polyacrylic acid hydrophobic membrane. Detailed implementation manners

[0058] In the description of the present invention, the terms are only for the purpose of description and cannot be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. In the description of the present invention, "a plurality of" means two or more unless otherwise specifically defined.

[0059] As Figure 2 shown, the asymmetric hydrophilic - hydrophobic lithium ion sieving membrane group includes a lithium manganese oxide composite membrane based on carbon 100 and a polypyrrole composite membrane based on carbon 200. The lithium manganese oxide composite membrane based on carbon 100 is composed of a lithium manganese oxide - multi - walled carbon nanotube - carbon black - polyvinylidene fluoride - polyacrylic acid hydrophilic membrane 110 (LiMn2O4 / MWCNTs / CB / PVDF / PAA), a first carbon cloth substrate 120, and a lithium manganese oxide - multi - walled carbon nanotube - carbon black - polyvinylidene fluoride hydrophobic membrane 130 (LiMn2O4 / MWCNTs / CB / PVDF); the polypyrrole composite membrane based on carbon 200 is composed of a polypyrrole - multi - walled carbon nanotube - carbon black - polyvinylidene fluoride hydrophobic membrane 210 (PPy / MWCNTs / CB / PVDF), a second carbon cloth substrate 220, and a polypyrrole - multi - walled carbon nanotube - carbon black - polyvinylidene fluoride - polyacrylic acid hydrophilic membrane 230 (PPy / MWCNTs / CB / PVDF / PAA). The lithium manganese oxide composite membrane based on carbon 100 is used as a cation - exchange membrane for the transmission of Li + and the polypyrrole composite membrane based on carbon 200 is used as an anion - exchange membrane for the transmission of Cl -The transmission. The composite membrane is attached to the conductive substrate by waterproof tape, and the effective area is 5×10 cm 2 .

[0060] In the ion membrane separation process cycle, each cycle is divided into two stages. In the first stage: the feed chamber is filled with the raw material liquid, and at the same time, an adsorption potential is applied to the lithium manganese oxide / carbon composite membrane 100 and the polypyrrole / carbon composite membrane 200. Affected by the reduction potential (negative potential), the electroactive material Li 1-x Mn2O4 (0 < x < 1) on the lithium manganese oxide / carbon composite membrane 100 gains electrons, and Mn 4+ is reduced to Mn 3+ . To maintain the electrical neutrality of the membrane, Li+ in the solution will selectively embed into the lattice vacancies of Li 1-x Mn2O4 based on the lattice confinement effect. At the same time, the polypyrrole / carbon composite membrane loses electrons and shows a positive charge. Due to the ion imprinting effect, Cl - in the solution is selectively adsorbed into the composite membrane, completing the synchronous separation of anions and cations. In the second stage: the solution in the feed chamber is discharged, and the receiving liquid is introduced into the two receiving chambers for circulating flow. A deintercalation potential with the positive and negative poles exchanged with the adsorption potential is applied to the lithium manganese oxide / carbon composite membrane 100 and the polypyrrole / carbon composite membrane 200 respectively. Cl - and Li + are deintercalated into the receiving liquids on both sides under the action of charge. After multiple cycles, the separation and enrichment of Cl - and Li + are achieved.

[0061] Using lithium manganese oxide (LiMn2O4) or polypyrrole (PPy) as the electroactive material, multi-walled carbon nanotubes (MWCNTs) and carbon black (CB) as the conductive material, the hydrophilic membrane uses polyvinylidene fluoride (PVDF) and polyacrylic acid (PAA) as the binder, and the hydrophobic membrane uses only polyvinylidene fluoride as the binder. The ratio of the electroactive material, conductive material, and binder is 15:1:4.

[0062] Example 1

[0063] This example relates to a preparation method of a composite membrane.

[0064] I. Preparation of hydrophilic composite membrane slurry

[0065] Weigh 0.44 g of PAA and 0.36 g of PVDF and add them to 40 mL of N-methylpyrrolidone (NMP) solvent, and stir well for 12 hours to form a composite binder.

[0066] Weigh 3 g of LiMn2O4, 0.1 g of MWCNTs, and 0.1 g of CB, mix and grind them evenly in a mortar, then add them to the prepared composite binder and stir at room temperature for 24 hours to form a uniformly dispersed LiMn2O4 / MWCNTs / CB / PVDF / PAA composite membrane slurry. The mass ratio of PAA, PVDF, NMP, LiMn2O4, MWCNTs, and CB is 22:18:2000:150:5:5.

[0067] Using the same procedure, replace 3 g of LiMn2O4 with 3 g of PPy to prepare a PPy / MWCNTs / CB / PVDF / PAA composite membrane slurry. The mass ratio of PAA, PVDF, NMP, PPy, MWCNTs, and CB is 22:18:2000:150:5:5.

[0068] II. Preparation of Hydrophobic Composite Membrane Slurry

[0069] Weigh 0.8 g of PVDF and add it to 40 mL of NMP solvent, stir well for 12 hours to form a binder. Weigh 3 g of LiMn2O4, 0.1 g of MWCNTs, and 0.1 g of CB, mix and grind them evenly in a mortar, then add them to the prepared binder and stir at room temperature for 24 hours to form a uniformly dispersed LiMn2O4 / MWCNTs / CB / PVDF composite membrane slurry. The mass ratio of PVDF, NMP, LiMn2O4, MWCNTs, and CB is 8:400:30:1:1. Using the same procedure, replace 3 g of LiMn2O4 with 3 g of PPy to prepare a PPy / MWCNTs / CB / PVDF composite membrane slurry. The mass ratio of PVDF, NMP, PPy, MWCNTs, and CB is 8:400:30:1:1.

[0070] III. Preparation of Asymmetric Hydrophilic-Hydrophobic Lithium Ion Sieve Membrane

[0071] Prepare an asymmetric hydrophilic-hydrophobic carbon-based lithium manganate composite membrane 100 on a carbon cloth substrate using the "double-sided casting method".

[0072] First step, two grooves of 12 cm×7 cm are pasted on a smooth glass plate with waterproof tape. The prepared LiMn2O4 / MWCNTs / CB / PVDF composite film slurry is ultrasonically treated for 30 minutes, and then the slurry is poured into the grooves and left at room temperature for 24 hours to remove bubbles. Second step, the carbon cloth substrate is carefully laid on the surface of the LiMn2O4 / MWCNTs / CB / PVDF composite film slurry in the groove, and pressure is applied to ensure its complete adhesion to the slurry. The prepared LiMn2O4 / MWCNTs / CB / PVDF / PAA composite film slurry is ultrasonically treated for 30 minutes, then the slurry is poured into the groove, the surface is scraped flat with a glass rod, and it is left at room temperature for 24 hours to remove bubbles. Finally, it is dried in a forced-air drying oven at 35 °C for 36 hours to obtain the asymmetric hydrophilic-hydrophobic lithium manganate composite film 100.

[0073] Example 2

[0074] This example involves performing relevant performance tests on the asymmetric hydrophilic-hydrophobic lithium manganate composite film 100 and the carbon-based polypyrrole composite film 200 obtained based on Example 1.

[0075] I. Continuous concentration test

[0076] A Mg / Li mixed solution (Mg 2+ / Li + = 226 ppm: 195 ppm) is used as the feed liquid, and 20 cycles of continuous concentration tests (adsorption for 10 min / desorption for 20 min) are carried out in the SL-ESIP system.

[0077] As Figure 3 shown, after 20 cycles, the system exhibits good Li + selective separation performance. The Li + concentration in the feed liquid decreases significantly from the initial 195.221 mg·L -1 to 18.291 mg·L -1 , and the decline rate reaches 90.6%. The cumulative increase in the Li+ concentration in the receiving liquid is 179.321 mg·L -1 . It is calculated that the permeability of Li + is as high as 98%. In contrast, the permeability of Mg 2+ is 36% (the feed liquid decreases by 117.791 mg·L -1 , and the receiving liquid increases by 41.834 mg·L -1 ), and the Mg 2+ / Li + separation coefficient in the final receiving liquid reaches 5.1. These results fully confirm the significant selective permeation advantage of the composite film for Li + .

[0078] II. Permeation Test

[0079] Using a mixed solution of magnesium and lithium (Mg 2+ :Li + = 4000 ppm:200 ppm) and 0.05 mol·L -1 Na2SO4 solution as the feed solution and the receiving solution respectively. The permeation test was repeated 10 times, and the ion flux was measured at each stage. After each cycle, fresh solutions were added to the feed chamber and the receiving chamber respectively. As Figure 4 shown, as the number of cycles increased, the Li + permeation flux gradually decreased. This is because after the repeated application of the oxidation / reduction potential to the lithium manganese oxide composite film 100, manganese dissolution occurred in the electroactive material lithium manganese oxide, resulting in the collapse of the spinel structure, reducing the adsorption active sites for Li + , thus leading to a decrease in the Li + permeation flux. However, the selectivity remained basically stable. Finally, the Li + flux reached 0.055 mol·h -1 ·m -2 and the magnesium-lithium separation factor was stable at 18.03. The asymmetric hydrophilic-hydrophobic lithium manganese oxide composite film 100 and the carbon-based polypyrrole composite film 200 have a dual-selective adsorption effect on Li + ions. This synergistic effect makes the SL-ESIP system exhibit good selection stability.

[0080] III. Separation Performance Test

[0081] The separation performance of the lithium manganese oxide composite film 100 and the carbon-based polypyrrole composite film 200 in simulated salt lake brine was tested. The composition of the simulated salt lake brine is shown in Table 1. Due to the high ion concentration, after the adsorption test in the first stage, the feed chamber was continuously rinsed with deionized water for 10 minutes. This can remove the ions adsorbed on the surface of the composite film in the double layer, effectively reducing the permeability of competing ions, thereby improving the overall selectivity. As Figure 5 shown, after three repeated washing tests, the separation factors of Li + / Mg 2+ , Li + / Na + , Li + / K + and Li + / Ca 2+ reached 120.21, 89.78, 53.64 and 8.76 respectively. The asymmetric hydrophilic-hydrophobic lithium manganese oxide composite film 100 and the carbon-based polypyrrole composite film 200 have good selectivity for Li + in the simulated salt lake brine. This is due to the Li +The imprinted vacancy structure is selective and can recognize target ions.

[0082] Table 1

[0083]

[0084] IV. Influence of Different Membrane Systems on Permeation Performance and Ion Selectivity

[0085] As Figure 6 shown, multiple membrane electrode systems are constructed by matching composite membranes with different hydrophilicities. R 亲 represents that the hydrophilic side of the composite membrane faces the receiving liquid, and R 疏 represents that the hydrophobic side of the composite membrane faces the receiving liquid; F 亲 represents that the hydrophilic side of the composite membrane faces the feeding liquid, and F 疏 represents that the hydrophobic side of the composite membrane faces the feeding liquid. Since the composite membrane has a bilayer structure, R 亲 ||F 亲 means that both sides of the composite membrane are hydrophilic; similarly, R 疏 ||F 疏 and R 亲 ||F 疏 (R 疏 ||F 亲 ) represent hydrophobic on both sides and hydrophilic on one side and hydrophobic on the other side, respectively. Using a mixed solution of magnesium and lithium (Mg 2+ : Li + = 200 ppm: 200 ppm) as the raw material solution, the ion flux and selectivity under different composite systems were studied respectively.

[0086] As Figure 6 shown in a, there is a relatively high ion flux in the R 亲 ||F 亲 system, but the selectivity is relatively low. This is because the good hydrophilicity promotes full contact between the solution and the membrane surface, reducing the ion transfer resistance. However, under the action of the electric field, Mg 2+ will also occupy the surface of the composite membrane faster and permeate into the receiving liquid, resulting in a decrease in selectivity. On the contrary, a higher selectivity will be shown in the R 疏 ||F 疏 system.

[0087] As Figure 6 shown in b, the permeation performance of the asymmetric structure (hydrophilic on one side and hydrophobic on the other side) membrane system was investigated. The SL-ESIP system shows better performance in the R 疏 ||F 亲 system. Compared with R 亲 ||F 亲The system can maintain a high selectivity even when the ion flux does not decrease, which is attributed to the bilayer membrane structure of the "sandwich-type" composite membrane. When the feed chamber is filled with liquid for ion extraction, the hydrophilic surface faces the feed liquid, which promotes more ions to penetrate into the membrane. When the receiving liquid is filled with liquid for the ion separation stage, the ions need to pass through the hydrophobic thin film on the other side. Under the action of the potential, Li in the composite membrane + will preferentially penetrate into the receiving liquid, thus maintaining a high ion selectivity.

[0088] In this invention, the LiMn2O4 / MWCNTS / CB / PVDF / PAA composite membrane and the PPy / MWCNTS / CB / PVDF composite membrane were respectively prepared by the double-sided coating method. Through assembly in the SL-ESIP system, the Li in the simulated brine + was subjected to extraction experiments, showing good ion selectivity, but its ion flux was relatively low. The binder PVDF in the composite membrane is an extremely hydrophobic polymer material, resulting in a relatively large hydrophobicity of the prepared membrane. The hydrophobic ion-permeable composite membrane helps to improve the selectivity of target ions, but due to relatively less contact with the solution, it indirectly leads to a decrease in the permeation flux. PAA, as a hydrophilic binder, can regulate the hydrophilicity of the composite membrane surface by adjusting the ratio of PAA to PVDF. A membrane surface with strong hydrophilicity is easy to form a stable hydration layer, reducing the resistance of ion transport. The strong interaction between water molecules and the membrane surface can promote the dissolution and diffusion of ions, thus increasing the ion flux.

[0089] In this invention, the PVDF hydrophobic binder and the PVDF-PAA composite binder were respectively prepared, and the double-sided asymmetric heterostructure composite membrane was prepared by the "two-step casting method", including the LiMn2O4 / MWCNTS / CB / PVDF / PAA composite membrane and the PPy / MWCNTS / CB / PVDF / PAA composite membrane. By adjusting the different contents of PAA, composite membrane surfaces with different hydrophilic degrees were prepared; the bottom surface used pure PVDF as the binder, and due to direct contact with the smooth glass plate, a hydrophobic composite membrane bottom surface was prepared. Thus, an asymmetric double-sided composite membrane with different hydrophilic effects on both sides was successfully prepared. The relevant properties of the composite membrane were confirmed by SEM contact angle, electrochemical CV test, EIS test, etc. Further, the influence of membranes with different properties (hydrophilic and hydrophobic), different numbers of conductive connections, different membrane spacings, etc. on the permeation performance was studied.

[0090] It should be noted that the above specific embodiments are exemplary. Those skilled in the art can come up with various solutions inspired by the disclosure of the present invention, and these solutions also fall within the scope of the disclosure of the present invention and within the protection scope of the present invention. Those skilled in the art should understand that the specification and drawings of the present invention are illustrative and do not constitute a limitation on the claims. The protection scope of the present invention is defined by the claims and their equivalents.

Claims

1. An asymmetric hydrophilic-hydrophobic lithium-ion sieving membrane, characterized in that, It includes a carbon-based lithium manganese oxide composite membrane (100) and a carbon-based polypyrrole composite membrane (200), wherein both the carbon-based lithium manganese oxide composite membrane (100) and the carbon-based polypyrrole composite membrane (200) are set as asymmetric hydrophilic-hydrophobic composite membranes.

2. The asymmetric hydrophilic-hydrophobic lithium-ion sieving membrane according to claim 1, wherein The carbon-based lithium manganese oxide composite membrane (100) includes a lithium manganese oxide-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride-polyacrylic acid hydrophilic membrane (110), a first carbon cloth substrate (120), and a lithium manganese oxide-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride hydrophobic membrane (130). The lithium manganese oxide-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride-polyacrylic acid hydrophilic membrane (110) and the lithium manganese oxide-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride hydrophobic membrane (130) are respectively disposed on both sides of the first carbon cloth substrate (120).

3. The asymmetric hydrophilic-hydrophobic lithium ion sieving membrane according to claim 1 or 2, characterized in that, The carbon-based polypyrrole composite membrane (200) includes a polypyrrole-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride-polyacrylic acid hydrophilic membrane (210), a second carbon cloth substrate (220), and a polypyrrole-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride hydrophobic membrane (230). The polypyrrole-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride-polyacrylic acid hydrophilic membrane (210) and the polypyrrole-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride hydrophobic membrane (230) are respectively disposed on both sides of the second carbon cloth substrate (220).

4. The asymmetric hydrophilic-hydrophobic lithium ion sieving membrane according to claim 3, wherein In the polypyrrole-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride-polyacrylic acid hydrophilic membrane (210), by weight, polypyrrole: multi-walled carbon nanotube: binder containing polyvinylidene fluoride and polyacrylic acid is 14-16: 1-2: 3-5; In the polypyrrole-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride hydrophobic membrane (230), by weight, polypyrrole: multi-walled carbon nanotube: polyvinylidene fluoride is 14-16: 1-2: 3-5.

5. The asymmetric hydrophilic-hydrophobic lithium-ion sieving membrane according to claim 2, characterized in that, In the lithium manganese oxide-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride-polyacrylic acid hydrophilic membrane (110), by weight, polypyrrole: multi-walled carbon nanotube: binder containing polyvinylidene fluoride and polyacrylic acid is 14-16: 1-2: 3-5; In the lithium manganese oxide-multi-walled carbon nanotube-carbon black-polyvinylidene fluoride hydrophobic membrane (120), by weight, lithium manganese oxide: multi-walled carbon nanotube: polyvinylidene fluoride is 14-16: 1-2: 3-5.

6. An electro-controlled ion exchange device, characterized in that, Use the asymmetric hydrophilic-hydrophobic lithium ion sieving membrane according to any one of claims 1 to 5.

7. The electro-controlled ion exchange device according to claim 6, characterized in that, The electrochemically controlled ion exchange device is used for the transport of Li + and Cl - .

8. The electro-controlled ion exchange device according to claim 6, characterized in that The described electrochemically controlled ion exchange device can be used for the separation of brines containing Li + / Mg 2+ 、Li + / Na + 、Li + / K + and Li + / Ca 2+ .

9. A preparation method of an asymmetric hydrophilic-hydrophobic carbon-based lithium manganate composite membrane, characterized in that, It includes the following steps: S1: Add polyacrylic acid and polyvinylidene fluoride to an N-methylpyrrolidone solvent and stir evenly. S2: Stir lithium manganese oxide, multi-walled carbon nanotubes, and carbon black evenly. S3: Mix the mixtures obtained in S1 and S2 and stir for 24 h to obtain a slurry. S4: Ultrasonically treat the slurry in S3 for 30 min and then pour it into a groove formed on a glass plate and let it stand until there are no bubbles. S5: Lay a carbon cloth substrate on the surface of the slurry in the groove to make it completely fit the slurry. S6: Add polyvinylidene fluoride to an N-methylpyrrolidone solvent. S7: Stir lithium manganese oxide, multi-walled carbon nanotubes, and carbon black evenly. S8: Mix the mixtures obtained in S6 and S7 and stir for 24 h to obtain a slurry. S9: The slurry obtained in S8 is poured into the groove after ultrasonic treatment for 30 minutes and left to stand until there are no bubbles; S10: Drying treatment.

10. A preparation method of an asymmetric hydrophilic-hydrophobic carbon-based polypyrrole composite membrane (200), characterized in that, Including: S1: Polyacrylic acid and polyvinylidene fluoride are added to the N-methylpyrrolidone solvent and stirred evenly; S2: Polypyrrole, multi-walled carbon nanotubes and carbon black are stirred evenly; S3: The mixtures obtained in S1 and S2 are mixed and stirred for 24 h to obtain a slurry; S4: The slurry in S3 is poured into the groove formed on the glass plate after ultrasonic treatment for 30 min and left to stand until there are no bubbles; S5: The carbon cloth substrate is laid on the surface of the slurry in the groove to make it completely fit the slurry; S6: Polyvinylidene fluoride is added to the N-methylpyrrolidone solvent; S7: Polypyrrole, multi-walled carbon nanotubes and carbon black are stirred evenly; S8: The mixtures obtained in S6 and S7 are mixed and stirred for 24 h to obtain a slurry; S9: The slurry obtained in S8 is poured into the groove after ultrasonic treatment for 30 minutes and left to stand until there are no bubbles; S10: Drying treatment.

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