Method for detecting lithium ions in ion exchange membrane, application

Abstract

This invention belongs to the field of nuclear magnetic resonance (NMR) detection technology, specifically relating to a method and application for detecting lithium ions in an ion exchange membrane. The detection method includes (1) preparing a sample from an ion exchange membrane after soaking, winding the sample onto a uniform rod, and inserting the sample into the bottom of an NMR tube using a screw-in method; the sample shape includes a polygon with a symmetrical structure and at least 4 sides; (2) placing the NMR tube obtained in step (1) into an NMR spectrometer, selecting a pulse sequence, tuning and shimming, adjusting the NMR test parameters, and acquiring the signal; the NMR test parameters include: a 90° pulse width of 10–14 μs; and a power of 30–55 W. The NMR sample obtained by this invention using a specific sample preparation method can detect 7Li inside the ion exchange membrane bulk phase at a 90° pulse width of 10–14 μs and a power of 30–55 W, providing strong support for the detection and analysis of structural information within the ion exchange membrane bulk phase.

Description

Technical Field

[0001] This invention belongs to the field of nuclear magnetic resonance technology detection, specifically relating to a method and application for detecting lithium ions in an ion exchange membrane. Background Technology

[0002] Ion exchange membranes are thin films prepared using polymer materials as the substrate, typically made of plastics, adhesives, rubber, or other organic polymers. Ion exchange membranes include various functional films, such as optical films, water treatment films, battery separators, and conductive films, and are widely used in optical devices, photovoltaic power generation, wastewater treatment, element enrichment, seawater desalination, lithium-ion batteries, and other fields in medicine, food, agriculture, and chemicals, showing broad application prospects. The detection of the internal structure, surface morphology, mechanical properties, and electrochemical properties of ion exchange membranes has always been one of the main research hotspots in the field. Currently, the main characterization techniques for ion exchange membranes include thermogravimetric analysis, infrared spectroscopy, Raman spectroscopy, atomic force microscopy, and electron microscopy.

[0003] Liquid-state nuclear magnetic resonance (Liquid-State NMR) has become a core tool for chemical structure characterization and dynamic process research due to its excellent resolution (δ≤0.1ppm), non-destructive detection characteristics, and multidimensional spectral resolution capabilities. However, ion exchange membranes, as key components of energy conversion and storage systems (such as fuel cells and flow batteries), are functional materials with special microphase separation structures. Their performance is highly dependent on the distribution of ion transport channel networks and hydrophilic and hydrophobic water regions in the solid state. Using traditional dissolved liquid NMR detection methods would destroy their intrinsic structure, leading to the loss of key microstructural information (such as ion cluster size, connectivity, and dynamic behavior). Therefore, how to efficiently characterize the microenvironment and ion transport mechanisms of ion exchange membranes using liquid NMR while maintaining the structural integrity of the membrane material remains a challenge in the current technological field.

[0004] In addition, lithium ions (Li + Due to its unique physicochemical properties, lithium ions are widely used in energy storage and battery technology (lithium-ion batteries, solid-state lithium batteries, lithium-sulfur batteries), functional materials and chemical engineering (ion exchange membranes, solid electrolytes, polymer electrolytes), biomedicine and neuroscience (lithium salt drugs for treating mental illnesses, etc.), nuclear energy and isotope separation, and lithium-ion sensors. Liquid nuclear magnetic resonance technology is used for the detection of lithium ions (Li... + The detection of Li nuclei plays a crucial role in energy storage and materials science; however, the quadrupole moment broadening effect of Li nuclei spin (I = 3 / 2) limits its low sensitivity (the gyromagnetic ratio is only 1 / 3). 1The spectral peaks broaden significantly in asymmetric electric field gradient environments, such as binding sites in ion exchange membranes or solid electrode interfaces, resulting in a decrease in resolution and severely limiting their practical applications. Therefore, there is an urgent need to develop an NMR detection method that can maintain the intrinsic microstructure of ion exchange membrane materials. (Note: The text also mentions a 38.6% reduction in H and a decrease in signal-to-noise ratio (SNR) of approximately three orders of magnitude, but this seems unrelated to the main point about NMR detection.) Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is how to detect the nuclear magnetic resonance signal of lithium ions in ion exchange membranes while maintaining the intrinsic microstructure of ion exchange membrane materials, thereby providing a method for detecting lithium ions in ion exchange membranes.

[0006] This invention innovatively provides a method for detecting lithium ion signals in ion exchange membranes using liquid nuclear magnetic resonance technology. By optimizing sample processing techniques and pulse sequence design, it achieves simultaneous and efficient detection of lithium ion and free-state lithium ion signals in ion exchange membranes for the first time, providing a new method for analyzing the structural performance of ion exchange membranes.

[0007] This invention provides a method for detecting lithium ions in an ion exchange membrane, comprising the following steps:

[0008] (1) The soaked ion exchange membrane is prepared into a sample, which is then wound around a uniform rod. The sample is inserted into the bottom of the NMR tube by screwing it in, and the rod is either removed or left in the NMR tube. The shape of the sample includes polygons with a symmetrical structure and at least 4 sides. It should be noted that soaking here refers to immersing the ion exchange membrane in a solution; or, for ion exchange membranes used in a liquid environment, this process also falls under soaking. For example, polygons include quadrilaterals, pentagons, hexagons, etc., and the polygons satisfy the condition that the number of sides is at least 4 and that they have a symmetrical structure. Furthermore, polygons also include chamfers and / or rounded corners. This invention has the advantages of simple NMR sample preparation method, good shimming effect, high detection sensitivity, and high spectral resolution during detection. When detecting lithium ions, this invention requires that the ion exchange membrane does not bend or break when wound around the glass rod, and requires that the ion exchange membrane be placed in a regular and orderly manner inside the NMR tube.

[0009] (2) Place the nuclear magnetic tube obtained in step (1) into the nuclear magnetic resonance spectrometer, select the pulse sequence, tune and homogenize, adjust the nuclear magnetic resonance test parameters, and acquire the signal; the nuclear magnetic resonance test parameters include: 90° pulse width of 10-14 μs; power of 30-55 W.

[0010] The principle of using liquid NMR to collect lithium ion signals in ion exchange membranes provided by this invention is as follows: After the ion exchange membrane is soaked in the solution, it swells. Since the ion exchange membrane can selectively transmit ions (including chlorine, sodium, lithium, etc.), ions and water molecules exist simultaneously inside the bulk phase of the ion exchange membrane. That is, ions and water molecules form countless microscopic liquid environments in the ion exchange membrane. Therefore, the NMR signal of lithium atoms in the ion exchange membrane can be successfully detected by liquid NMR.

[0011] Cation exchange membranes possess cation-selective transport properties, allowing Li to be transported selectively during the immersion process. + Lithium ions remain inside the membrane phase. The low lithium ion concentration within the ion exchange membrane limits the detection capabilities of current liquid NMR techniques due to inherent limitations in liquid NMR sample preparation and the significantly lower lithium ion concentration inside the membrane phase compared to the liquid itself. + Accurate analysis is achieved. This invention uses a specially prepared sample to obtain NMR detection samples, and tests are conducted under specific NMR test parameters (90° pulse width of 10–14 μs; power of 30–55 W). Without damaging the ion exchange membrane structure, it can acquire the NMR signal of lithium ions inside the ion exchange membrane phase, providing strong support for the detection and analysis of structural information within the ion exchange membrane phase. Conventional liquid NMR sample preparation methods mainly acquire signals from free-state atoms, i.e., lithium ions in the NMR tube solution (located outside the membrane), and cannot detect lithium ion signals on the ion exchange membrane. Using the sample preparation method of this invention combined with the detection method of this invention, the NMR signal of lithium atoms within the ion exchange membrane phase can be detected.

[0012] As an optional implementation, the nuclear magnetic resonance test parameters further include: pulse sequence of zg; relaxation time: 1-5s; number of sampling points: 16k-32k, where k is 1024; sampling time: 1-5s; spectral width: 60-120ppm; spectral range: -60ppm to +60ppm; number of accumulations: 4-256.

[0013] Preferably, the spectral width is 40 ppm and the spectral range is -20 ppm to +20 ppm.

[0014] As an optional implementation, the 90° pulse width is 12μs; the power is 40W.

[0015] For example, the 90° pulse width is 10μs, 11μs, 12μs, 13μs or 14μs; the power is 30W, 35W, 40W, 45W or 55W.

[0016] It should be noted that after selecting the pulse sequence, the 7Li tuning channel is also selected. Different nuclides have different resonance frequencies, therefore they need to be independently tuned and matched. The 7Li tuning channel is in the X channel. Taking a 600MHz resonance spectrometer as an example, the resonance frequency is 233.2MHz. The resonance frequency of each nuclide in the detection instrument is related to the field strength of the detection instrument and the type of nuclide. Those skilled in the art can determine the resonance frequency based on the specific nuclide and the field strength of the detection instrument.

[0017] As an optional implementation, the lithium is 7Li; and / or,

[0018] The ion exchange membrane is a cation exchange membrane; preferably, the ion exchange membrane is derived from a battery.

[0019] Ion exchange membranes can also come from devices equipped with ion exchange membranes, such as diaphragms in fuel cells, electrodialysis devices, or electrolysis devices.

[0020] As an optional implementation, step (2) further includes adding 5 to 20 μL of a lithium-ion-containing solution before placing the NMR tube into the NMR spectrometer;

[0021] Preferably, the concentration of the lithium-ion-containing solution is 0.2–2.0 mol / L;

[0022] Preferably, the concentration of the lithium-ion-containing solution is 1.0 mol / L.

[0023] The method of this invention can simultaneously detect the NMR signals of lithium ions inside the ion exchange membrane and lithium ions outside the membrane (in a free state). Based on the difference in chemical shift of the NMR signals of lithium atoms inside and outside the cation exchange membrane, the transport performance of the cation exchange membrane for lithium ions can be determined, providing strong support for the structural performance analysis of cation exchange membranes. Furthermore, it provides a new approach for screening the special functions of cation exchange membranes used in different fields.

[0024] As an optional implementation, the lithium-ion-containing solution includes a soluble lithium salt;

[0025] Preferably, the soluble lithium salt includes at least one of LiCl, Li2SO4, LiNO3, and CH COOLi.

[0026] As an optional implementation, the solvent in the lithium-ion-containing solution includes a deuterated solvent or a non-deuterated solvent;

[0027] Preferably, the solvent includes at least one selected from water, chloroform (CHCl3), dimethyl sulfoxide (DMSO), dichloromethane (CH2Cl2), methanol, and ethanol;

[0028] Preferably, when the solvent in the lithium-ion-containing solution includes a deuterated solvent, a field locking process is also included before shimming.

[0029] As an optional implementation, in step (1), the soaking solution is capable of dissociating lithium ions. It should also be noted that, during sample preparation, the soaking solution of the ion exchange membrane includes components capable of dissociating Li... + Compounds that can dissociate Li + The solution of the compound is selected from any of the lithium-ion-containing solutions mentioned above. The soaking solution for the ion exchange membrane can be the same as or different from the lithium-ion-containing solution, but preferably the same. The present invention does not require a soaking time, and allows the ion exchange membrane to swell and contain ions and water molecules inside, for example, 1h, 3h, 8h, 11h, 14h, 17h, 20h, etc.

[0030] As an optional implementation, in step (1), the sample is square in shape; and / or,

[0031] The diameter of the rod is 2-3 mm; and / or,

[0032] The rod material includes a rubber rod, a glass rod, or a metal rod; and / or,

[0033] The rod is a hollow cylinder. The rod being a hollow cylinder means it can be a liner tube with a hollow interior to accommodate liquid. In this case, a deuterated reagent, such as deuterated water, deuterated DMSO, or deuterated chloroform, can be added into the hollow rod. This allows for precise field locking to pinpoint the chemical shift of the analyte nucleus while avoiding the influence of the deuterated reagent on the ion exchange membrane structure. Furthermore, since deuterated reagents are expensive, using a liner tube to achieve field locking also reduces the amount of deuterated reagent used.

[0034] When the sample is square, its length is 4-5 cm. It should be noted that the sample size can be selected based on the size of the rod, as long as it can be wound around the rod. Square includes rectangular, rectangular, etc.

[0035] As an optional implementation, the ion exchange membrane includes a cation exchange membrane. Several commercial models are listed here, including ASTOM Neosepta CXP-S, Fumaep-E-620(K), Fumasep FKS-30, and CMI-7000S.

[0036] This invention provides the application of the above-described detection method in the detection of membranes in fuel cells, electrodialysis devices, or electrolysis devices. This application can detect lithium-ion signals in used ion exchange membranes, providing strong support for the structural performance analysis of ion exchange membranes.

[0037] The technical solution of this invention has the following advantages:

[0038] 1. The method for detecting lithium ions in an ion exchange membrane provided by the present invention includes (1) preparing a sample from an ion exchange membrane after soaking, winding the sample onto a uniform rod, inserting the sample into the bottom of a nuclear magnetic resonance (NMR) tube by a screw-in method, and removing the rod or retaining it in the NMR tube; the shape of the sample includes a polygon with a symmetrical structure and a number of sides not less than 4; (2) placing the NMR tube obtained in step (1) into a nuclear magnetic resonance spectrometer, selecting a pulse sequence, tuning and shimming, adjusting the NMR test parameters, and acquiring the signal; the NMR test parameters include: a 90° pulse width of 10–14 μs; and a power of 30–55 W. The NMR sample obtained by the specific sample preparation method of the present invention has a good shimming effect, high detection sensitivity and spectral resolution. At a 90° pulse width of 10–14 μs and a power of 30–55 W, it can detect 7Li inside the bulk phase of the ion exchange membrane, providing strong support for the detection and analysis of the structural information inside the bulk phase of the ion exchange membrane.

[0039] Furthermore, the method of this invention can simultaneously detect the NMR signals of lithium ions inside the ion exchange membrane and lithium ions outside the membrane (in a free state). Based on the difference in chemical shift of the NMR signals of lithium atoms inside and outside the cation exchange membrane, the transport performance of the cation exchange membrane for lithium ions can be determined, and the membrane structure performance can be analyzed. This provides a new approach for screening the special functions of cation exchange membranes used in different fields. Attached Figure Description

[0040] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0041] Figure 1 This is the NMR spectrum obtained from Example 1 of the present invention;

[0042] Figure 2 This is the NMR spectrum obtained from the test in Example 2 of the present invention.

[0043] Figure 3 This is the NMR spectrum obtained from Comparative Example 1 of this invention.

[0044] Figure 4 This is a schematic diagram of the ion exchange membrane in the NMR tube of Embodiment 1 of the present invention, referred to as the sample diagram;

[0045] Figure 5 This is a schematic diagram of the ion exchange membrane of Comparative Example 1 of the present invention inside an NMR tube, referred to as the sample diagram. Detailed Implementation

[0046] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0047] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0048] Example 1

[0049] This embodiment provides a method for detecting lithium ions in an ion exchange membrane, comprising the following steps:

[0050] (1) The ion exchange membrane (Fumasep FKS-30) was immersed in a 1 mol / L LiCl aqueous solution (non-deuterated reagent) for 24 h. After removal and drying, the ion exchange membrane was cut into regular square samples with a length of 4 cm. These samples were then wound around a glass rod with a diameter of 3 mm. The samples were then evenly wrapped around the bottom of the NMR tube using a screw-in method. The glass rod was then removed. The state of the samples inside the NMR tube is shown in the figure. Figure 4 .

[0051] (2) Place the NMR tube into the NMR spectrometer, tune and shim it. The instrument is a 600MHz liquid NMR spectrometer with a resonance frequency of 233.2MHz. The NMR test parameters include: 90° pulse width of 12μs; power of 40W; pulse sequence of zg; relaxation time of 1s; number of sampling points of 32k, k of 1024; sampling time of 1.7s; spectral width of 40ppm; spectrum range of -20ppm to +20ppm; number of accumulations of 128.

[0052] See NMR spectrum Figure 1 As can be seen from the figure, the chemical shift of 7Li is -0.23 ppm. The sample preparation method and testing method of the present invention can obtain the NMR signal of 7Li in the ion exchange membrane with high resolution and high signal-to-noise ratio.

[0053] Example 2

[0054] This embodiment provides a method for detecting lithium ions in an ion exchange membrane, comprising the following steps:

[0055] (1) Immerse the ion exchange membrane (same as in Example 1) in a 1 mol / L LiCl aqueous solution (the solvent is a non-deuterated reagent), take it out and wipe it dry. Cut the ion exchange membrane into a regular square sample with a length of 4 cm, and then wrap it around a glass rod with a diameter of 3 mm. Use a screw-in method to evenly wrap the sample around the bottom of the NMR tube and remove the glass rod. Add 10 μL of 1 mol / L LiCl aqueous solution to the NMR tube.

[0056] (2) Place the NMR tube into the NMR spectrometer, tune and shim it. The instrument is a 600MHz liquid NMR spectrometer with a resonance frequency of 233.2MHz. The NMR test parameters include: 90° pulse width of 12μs; power of 40W; pulse sequence of zg; relaxation time of 1s; number of sampling points of 32k, k of 1024; sampling time of 1.7s; spectral width of 40ppm; spectrum range of -20ppm to +20ppm; number of accumulations of 128.

[0057] See NMR spectrum Figure 2 The chemical shift of 7Li inside the membrane is -0.23 ppm, and the chemical shift of 7Li outside the membrane is 0.1 ppm. In addition, due to the restricted movement of 7Li atoms inside the ion exchange membrane phase, the transverse relaxation time (T2) of 7Li is shortened, and the spectral peak is broadened relative to the outside of the membrane. This indicates that the present invention can detect the NMR signal of 7Li inside and outside the ion exchange membrane phase, and has high resolution and signal-to-noise ratio.

[0058] Comparative Example 1

[0059] This comparative example provides a method for detecting lithium ions in an ion exchange membrane, comprising the following steps:

[0060] (1) The ion exchange membrane (same as in Example 1) was immersed in a 1 mol / L LiCl aqueous solution, removed, dried, rolled up, cut into small segments (about 1 cm long), and inserted into the NMR tube. The ion exchange membrane formed an irregular clump inside the NMR tube. See the schematic diagram of the sample. Figure 5 .

[0061] (2) Place the NMR tube into the NMR spectrometer, tune and shim it. The instrument is a 600MHz liquid NMR spectrometer with a resonance frequency of 233.2MHz. The NMR test parameters include: 90° pulse width of 12μs; power of 40W; pulse sequence of zg; relaxation time of 1s; number of sampling points of 32k, k of 1024; sampling time of 1.7s; spectral width of 40ppm; spectrum range of -20ppm to +20ppm; number of accumulations of 128.

[0062] See NMR spectrum Figure 3 ,Depend on Figure 3It can be seen that although the NMR signal of 7Li in the membrane phase can be detected by using ordinary sample preparation methods, the shimming effect is worse, the spectrum is broadened and the spectrum resolution is reduced, which is not helpful for judging the structure and performance of ion exchange membranes.

[0063] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for detecting lithium ions in an ion exchange membrane, characterized in that, Includes the following steps: (1) Prepare a sample from the soaked ion exchange membrane, wind the sample onto a uniform rod, insert the sample into the bottom of the NMR tube by screwing it in, and remove the rod or leave it in the NMR tube; the shape of the sample includes a polygon with a symmetrical structure and a number of sides of not less than 4. (2) Place the nuclear magnetic tube obtained in step (1) into the nuclear magnetic resonance spectrometer, select the pulse sequence, tune and shim, adjust the nuclear magnetic resonance test parameters, and acquire the signal; the nuclear magnetic resonance test parameters include: 90° pulse width of 10~14μs; The power is 30~55W.

2. The detection method according to claim 1, characterized in that, The nuclear magnetic resonance test parameters also include: pulse sequence of zg; relaxation time: 1~5s; number of sampling points: 16k~32k, k is 1024; sampling time: 1~5s; spectral width: 60~120ppm; spectrum range: -60ppm~+60ppm; number of accumulations: 4~256.

3. The detection method according to claim 2, characterized in that, The spectral width is 40 ppm, and the spectral range is -20 ppm to +20 ppm.

4. The detection method according to claim 1, characterized in that, The 90° pulse width is 12μs; the power is 40W.

5. The detection method according to any one of claims 1 to 4, characterized in that, The lithium is 7Li; and / or the ion exchange membrane is a cation exchange membrane.

6. The detection method according to claim 5, characterized in that, The ion exchange membrane is derived from a battery.

7. The detection method according to claim 1, characterized in that, Step (2) further includes adding 5-20 μL of lithium-ion-containing solution before placing the NMR tube into the NMR spectrometer.

8. The detection method according to claim 7, characterized in that, The concentration of the lithium-ion-containing solution is 0.2~2.0 mol / L.

9. The detection method according to claim 8, characterized in that, The concentration of the lithium-ion-containing solution is 1.0 mol / L.

10. The detection method according to any one of claims 7-9, characterized in that, The lithium-ion-containing solution includes soluble lithium salts.

11. The detection method according to claim 10, characterized in that, The soluble lithium salt includes at least one of LiCl, Li2SO4, LiNO3, and CH3COOLi.

12. The detection method according to claim 11, characterized in that, The solvent in the lithium-ion-containing solution includes deuterated solvents or non-deuterated solvents.

13. The detection method according to claim 12, characterized in that, The solvent includes at least one of water, chloroform, dimethyl sulfoxide, dichloromethane, methanol, and ethanol.

14. The detection method according to claim 13, characterized in that, When the solvent in the lithium-ion-containing solution includes a deuterated solvent, a field locking process is also included before shimming.

15. The detection method according to claim 1, characterized in that, In step (1), the soaking solution can dissociate lithium ions.

16. The detection method according to claim 1, characterized in that, In step (1), the sample is square in shape; and / or, The diameter of the rod is 2-3 mm; and / or, The rod is a rubber rod, a glass rod, or a metal rod; and / or, The rod is a hollow cylinder.

17. The application of the detection method according to any one of claims 1 to 16 in detecting the membrane in a fuel cell, an electrodialysis device, or an electrolysis device.

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