A quantitative method for measuring spin concentration in carbon-based materials based on electron paramagnetic resonance.

Abstract

This invention belongs to the field of analytical testing technology, specifically relating to a quantitative method for measuring the spin concentration of carbon-based materials based on electron paramagnetic resonance (EPR). The method involves using a metal oxide as a standard sample, detecting the signal integration area of ​​the standard sample using an EPR meter, mixing the standard sample and the carbon-based material to be tested to obtain the signal integration area of ​​the standard sample in the mixed test, calculating a calibration factor, testing the carbon-based material separately using an EPR meter to obtain the integration area of ​​the carbon-based material to be tested, providing the corrected integration area, calculating the spin concentration of DPPH powder, obtaining the EPR integration area of ​​the DPPH powder, calculating the free radical concentration per unit area, and finally calculating the spin concentration of the carbon-based material.

Description

Technical Field

[0001] This invention belongs to the field of analytical testing technology, specifically relating to a quantitative method for measuring the spin concentration of carbon-based materials based on electron paramagnetic resonance. Background Technology

[0002] Electron paramagnetic resonance (EPR) spectroscopy, as a highly sensitive and non-destructive single-electron detection method, has irreplaceable advantages in the characterization of defects in carbon-based materials. Carbon-based materials (such as graphene, carbon nanotubes, and hard carbon) exhibit significant application value in new energy (lithium / sodium-ion battery anodes), catalysis (hydrogen evolution / oxygen evolution reaction), and functional composite materials due to their unique sp² hybrid structure and tunable defect states. The defect concentration (such as edge defects and vacancy defects) of materials shows a strong correlation with their performance: for example, in energy storage, defect sites can serve as preferential adsorption channels for lithium ions, directly affecting the battery's specific capacity and initial efficiency; in catalysis, specific topological defects can significantly reduce the activation energy of the reaction.

[0003] Traditional EPR quantitative analysis employs a standardized method based on sample quality. While this method is reliable in insulating materials, it overlooks the skin effect interference caused by the high conductivity of carbon-based materials. When testing highly conductive carbon-based materials, microwaves can only penetrate the surface layer at the micrometer level (typically 1-10 micrometers), resulting in internal defects not being effectively excited, leading to a significant deviation between the measured signal intensity and the true defect concentration.

[0004] The limitations of existing mass-based quantitative techniques are mainly reflected in the following aspects: (1) neglecting the dynamic modulation effect of material conductivity on microwave fields, resulting in defect quantitative errors exceeding the order of magnitude; (2) being unable to distinguish the difference in the contribution of surface defects and bulk defects to the signal; and (3) when the material has a conductivity gradient distribution (such as core-shell structured carbon materials), traditional methods completely fail. These problems seriously restrict the application of EPR technology in the research and development of new carbon materials, especially in the field of new energy, where the accurate determination of defect concentration is directly related to the optimization of electrode material performance and process control. Summary of the Invention

[0005] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments.

[0006] As one aspect of the present invention, the present invention provides a method for quantitatively measuring the spin concentration of carbon-based materials based on electron paramagnetic resonance, which includes the following steps:

[0007] (1) Using metal oxide NMC as a standard, the signal integration area A of the standard was detected by an electron paramagnetic resonance spectrometer;

[0008] (2) The standard sample and the carbon-based material to be tested are mixed and tested. The electron paramagnetic resonance parameters are the same as in step (1), and the signal integration area B of the standard sample in the mixed test is obtained.

[0009] (3) Calculate the calibration factor Q = A / B;

[0010] (4) The carbon-based material to be tested was tested separately using an electron paramagnetic resonance spectrometer to obtain the integral area C of the carbon-based material to be tested;

[0011] The corrected integral area is D = C × Q;

[0012] (5) Calculate the spin concentration F of the DPPH powder and obtain the electron paramagnetic resonance integral area G of the DPPH powder. Calculate the free radical concentration H per unit area = F / G.

[0013] (6) Calculate the spin concentration of the carbon-based material J = (D×H) / I; where I is the mass of the carbon-based material to be tested.

[0014] As a preferred embodiment of the quantitative testing method for spin concentration of carbon-based materials based on electron paramagnetic resonance described in this invention: In step (1), the metal oxide NMC includes LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2.

[0015] As a preferred embodiment of the quantitative testing method for spin concentration of carbon-based materials based on electron paramagnetic resonance described in this invention: in step (1), the electron paramagnetic resonance parameters are set as follows: mode is Transition Metal, central magnetic field is 3300-3500 G, scan width is 4000-5000 G, and temperature is room temperature.

[0016] As a preferred embodiment of the quantitative testing method for spin concentration of carbon-based materials based on electron paramagnetic resonance described in this invention: step (2) includes extracting the signal integration area B of the standard sample in the mixed test by Lorentzian linear fitting.

[0017] As a preferred embodiment of the quantitative testing method for spin concentration of carbon-based materials based on electron paramagnetic resonance described in this invention: in step (4), the electron paramagnetic resonance parameters are set as follows: mode is Organic Radical, central magnetic field is 3300-3400 G, scan width is 200-500 G, and temperature is room temperature.

[0018] As a preferred embodiment of the quantitative testing method for spin concentration of carbon-based materials based on electron paramagnetic resonance described in this invention: In step (5), the spin concentration of DPPH powder is F = E / 394.078766 × 6.02 × 10 23Where E is the mass of DPPH powder.

[0019] As a preferred embodiment of the quantitative testing method for spin concentration of carbon-based materials based on electron paramagnetic resonance described in this invention: in step (5), the electron paramagnetic resonance parameters are set as follows: mode is Organic Radical, central magnetic field is 3300-3400 G, scan width is 200-500 G, and temperature is room temperature.

[0020] As a preferred embodiment of the quantitative testing method for spin concentration of carbon-based materials based on electron paramagnetic resonance described in this invention: in step (1), the standard sample powder is filled into a quartz capillary with an inner diameter of <1 mm, and the signal integration area A of the standard sample is detected by an electron paramagnetic resonance instrument.

[0021] As a preferred embodiment of the quantitative testing method for spin concentration of carbon-based materials based on electron paramagnetic resonance described in this invention: In step (2), the mixing test includes loading the carbon-based material to be tested into a quartz tube, evacuating to remove adsorbed oxygen and sealing it, binding it with a quartz capillary tube containing a standard sample, aligning the bottoms, and performing a mixing test using electron paramagnetic resonance.

[0022] As a preferred embodiment of the quantitative testing method for spin concentration of carbon-based materials based on electron paramagnetic resonance described in this invention, the carbon-based material includes hard carbon anode powder or carbon-based materials with a core-shell structure.

[0023] The beneficial effects of this invention are: The method of this invention, through a dual-standard calibration system, reduces quantitative errors caused by conductivity in traditional methods, resulting in more accurate detection results. This invention is applicable to carbon-based materials with different conductivity levels (conductivity 10...). -2 -10 4 (S / m) and complex structures (such as core-shell and porous materials). Through standard sample encapsulation, modal testing, and vacuum pretreatment processes, experimental repeatability and cross-platform compatibility are ensured. Attached Figure Description

[0024] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below, wherein:

[0025] Figure 1 EPR signals for standard NMC111 and NMC111 mixed with hard carbon anode powder.

[0026] Figure 2 The EPR signals are those of the two carbon materials selected in Example 2 and the EPR signals of the mixed sample of carbon materials C-1#, DPPH, and NMC.

[0027] Figure 3 This is a schematic diagram of the testing process. Detailed Implementation

[0028] To make the above-mentioned objectives, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to specific examples.

[0029] Dual-standard collaborative calibration system:

[0030] Conductivity standard (NMC): Used to correct signal attenuation caused by skin effect. The calibration factor Q is calculated by the change in standard signal intensity (A / B), eliminating the attenuation of signal on carbon-based materials by microwave penetration depth.

[0031] Spin concentration standard (DPPH): used to establish a direct correlation between the integral area of ​​the EPR signal and the spin concentration (H=F / G), converting the calibrated carbon-based signal intensity (D) into a unit mass spin concentration (spin / g).

[0032] Separate mode testing and signal separation: The "Transition Metal" mode was used to test the broad peak signal of the NMC standard, and the "Organic Radical" mode was used to test the narrow peak signal of the carbon-based material. The two signals were separated by line fitting.

[0033] Vacuum pretreatment and standardized packaging: Vacuum treatment (24h) removes adsorbed oxygen molecules from the sample to reduce paramagnetic interference; standard samples and test samples are packaged in quartz capillaries and quartz tubes respectively to ensure geometric consistency (flat bottom) and avoid differences in test cavity field distribution.

[0034] Example 1:

[0035] Acquisition of calibration factor Q and signal correction:

[0036] (1) Standard sample test: The standard sample NMC111 (LiNi) was tested. 1 / 3 Mn 1 / 3 Co 1 / 3 O2 (item number 761001, Sigma-Aldrich) powder was filled into a quartz capillary with an inner diameter of <1 mm (filling length 2-3 cm), and sealed for storage; the carbon-based material to be tested (hard carbon anode powder, Shanghai Wuyu Chemical Co., Ltd.) was loaded into a quartz tube with an inner diameter of 3 mm, vacuumed for 24 h to remove adsorbed oxygen and sealed, and the mass of the hard carbon anode powder was determined to be I (unit: g);

[0037] The NMC111 standard sample was tested separately using an electron paramagnetic resonance (EPR) spectrometer. The EPR parameters were set as follows: mode = Transition Metal, central magnetic field = 3300 G, scan width = 4000 G, temperature = room temperature, and the signal integration area of ​​the NMC111 standard sample was recorded as A.

[0038] (2) Mixed test: The quartz capillary tube containing the standard sample NMC111 was bundled with the carbon-based material (hard carbon anode powder) quartz tube, with the bottom aligned, and tested under the same parameters to obtain the mixed signal. The EPR parameters were set as follows: mode = Transition Metal, central magnetic field = 3300 G, scan width = 4000 G, temperature = room temperature. The broad peak of NMC (peak width 500 G) and the narrow peak of carbon-based material (peak width < 50 G) were separated by Lorentzian line fitting. The signal integration area B of the NMC111 standard sample after the mixed test was extracted.

[0039] (3) Calculate the calibration factor Q = A / B.

[0040] (4) Independent testing of carbon-based materials: Switch to Organic Radical mode to test the EPR signal of individual carbon-based materials. EPR parameters: mode = Organic Radical, central magnetic field = 3300 G, scan width = 500 G, temperature = room temperature; obtain the integral area C according to the mass of the loaded sample.

[0041] The corrected true integral area is D = C × Q.

[0042] Spin concentration calculation:

[0043] (5) Spin concentration DPPH standard sample: Weigh DPPH powder, its mass is E, based on its relative molecular mass (394.078766 g / mol) and Avogadro's constant (6.02×10). 23 Calculate the spin concentration of DPPH powder: F = E / 394.078766 × 6.02 × 10⁻⁶ 23 ;

[0044] Based on the integrated area G of the EPR signal of the tested DPPH, and the EPR parameters: mode = Organic Radical, central magnetic field = 3300 G, scan width = 500 G, temperature = room temperature; calculate the free radical concentration H = F / G per unit area in the EPR spectrum.

[0045] (6) Calculation of spin concentration of carbon-based material: The loaded mass of carbon-based material (hard carbon anode powder) is I (unit: g). Combined with the corrected integral area D, the spin concentration of carbon-based material is calculated as J = (D×H) / I (unit: spin / g).

[0046] Figure 1 The EPR signals are for the standard NMC111 and the mixture of NMC111 and hard carbon anode powder. Figure 1 In the image, (a) is the standard sample NMC111 (LiNi). 1 / 3 Mn 1 / 3 Co1 / 3 (a) EPR signal of O2); (b) EPR signal of NMC111+ hard carbon anode powder mixture test. Figure 1 (a) and Figure 1 In test (b), the NMC111 used was the same sample with the same mass; the comparison shows that the signal of NMC111 weakens under the influence of the skin depth of conductive carbon. The standard sample NMC111 (with a conductivity of approximately 10) was used. -6 S / m) and carbon-based materials (conductivity > 10) 3 During the S / m mixed test, under the same instrument parameters, the intensity of the characteristic peak of NMC111 decreased by 40-70%, demonstrating the shielding effect of the conductive matrix on the microwave field. Conductivity affects the signal intensity of the tested material, thus affecting its quantitative analysis. Figure 1 As can be seen in (b), the standard sample NMC111 and the test carbon material EPR signals are clearly distinguishable. They can be easily distinguished by fitting, that is, the addition of the standard sample will not affect the signal quality of the test sample.

[0047] Comparative Example 1:

[0048] According to existing technical methods, the carbon-based material to be tested (hard carbon anode powder) is loaded into a quartz tube with an inner diameter of 3 mm, vacuumed for 24 h to remove adsorbed oxygen, and then sealed. The mass of the hard carbon anode powder is determined as I (unit: g). The EPR signal of the carbon-based material is tested. The EPR parameters are: mode = Organic Radical, central magnetic field = 3300 G, scan width = 500 G, and temperature = room temperature. The integral area C is obtained based on the mass of the loaded sample.

[0049] Spin concentration DPPH standard sample: Weigh DPPH powder, its mass is E, based on its relative molecular mass (394.078766 g / mol) and Avogadro's constant (6.02 × 10⁻⁶). 23 Calculate the spin concentration of DPPH powder: F = E / 394.078766 × 6.02 × 10⁻⁶ 23 ;

[0050] Based on the integrated area G of the EPR signal of the tested DPPH, and the EPR parameters: mode = Organic Radical, central magnetic field = 3300 G, scan width = 500 G, temperature = room temperature; calculate the free radical concentration H = F / G per unit area in the EPR spectrum.

[0051] Calculation of spin concentration of carbon-based material: The loaded mass of carbon-based material (hard carbon anode powder) is I (unit: g). Calculate the spin concentration of carbon-based material J = (C×H) / I (unit: spin / g).

[0052] The experimental test results of Example 1 and Comparative Example 1 are shown in Table 1;

[0053] Table 1

[0054] .

[0055] Example 2:

[0056] Due to the complexity of thermochemistry, the derived carbon materials may not show a corresponding spectrum in the EPR response, indicating that no single electrons are detected in these materials; therefore, their EPR spectral response is a straight line (e.g., ...). Figure 2 (a)), but its conductivity still exists, that is, the skin effect of microwaves exists, as in Example 1, which will affect the accuracy of quantitative testing. We selected two such samples, and used the prepared DPPH as a standard sample. The corresponding free radical concentration value can be known by mass calculation. The specific test process is briefly described as follows: Encapsulate carbon-based materials, prepare DPPH and place it in a capillary, and calculate the free radical concentration according to the mass. At the same time, different reference samples were selected, including NMC111 and MnO. Test the signals of DPPH, NMC111 / MnO alone, and the combined sample of DPPH + carbon + NMC111 / MnO. Compare the results before and after calibration. The reason for not using ruby ​​is that it is composed of C / H elements, and its EPR signal also appears at the EPR signal position of the carbon-based sample to be tested, which is very easy to cause interference and affect the test results.

[0057] The specific calculation process is described in the above embodiments.

[0058] (1) Preparation of spin concentration DPPH standard sample: A certain mass of DPPH was packaged in a capillary tube. According to the EPR signal integration area of ​​the tested DPPH of 13.26 (see Table 2), the EPR parameters are: mode = Organic Radical, central magnetic field = 3300 G, scan width = 500 G, temperature = room temperature; (2) Standard sample test: The standard sample NMC111 (LiNi 1 / 3 Mn 1 / 3 Co 1 / 3O2) and MnO powder were filled into a quartz capillary with an inner diameter of <1 mm (filling length 2-3 cm) and sealed for preservation; the selected carbon-based materials without EPR signal (amorphous carbon powder C-1# (item number 699640, sigma-Aldrich) and C-2# (item number 702110, sigma-Aldrich)) were loaded into a quartz tube with an inner diameter of 3 mm, vacuumed for 24 h to remove adsorbed oxygen and sealed, and the mass of the hard carbon anode powder was determined, as shown in Table 2. (3) Electron paramagnetic resonance (EPR) spectrometer was used to test NMC111 and MnO standard samples separately. The EPR parameters were set as follows: mode = Transition Metal, central magnetic field = 3300 G, scan width = 4000 G, temperature = room temperature, and the signal integration area of ​​NMC111 or MnO was recorded.

[0059] (4) Mixing test: The above-mentioned quartz capillary containing the standard sample NMC111 was bundled with the carbon-based material (hard carbon anode powder) quartz tube and DPPH capillary, with the bottoms aligned, and tested under the same parameter conditions to obtain the mixed signal. Figure 2 In (b) of the example, the EPR parameters are set as follows: mode = Transition Metal, central magnetic field = 3300 G, scan width = 4000 G, temperature = room temperature. The broad peaks (peak width 500 G) of NMC111 and MnO and the narrow peaks (peak width < 50 G) of DPPH are separated by Lorentzian line fitting. The signal integration area of ​​NMC111 or MnO standard sample after mixed testing is extracted.

[0060] (5) Calculate the calibration factors of NMC111 and MnO standard samples in C-1# and C-2# carbon materials respectively.

[0061] (6) DPPH signal test in carbon-based material + DPPH + standard sample: switch to Organic Radical mode and test the EPR signal of DPPH in the mixed sample. Keep the test parameters consistent with those in step (1). EPR parameters: mode = Organic Radical, central magnetic field = 3300 G, scan width = 500 G, temperature = room temperature; obtain the integral area C according to the packing.

[0062] (7) Calculate the calibrated DPPH integral area in the carbon-based material + DPPH + standard sample.

[0063] Table 2

[0064] .

[0065] Stability test:

[0066] We further tested the stability of MnO as a standard. Taking samples 5# and 6# in Table 2 as examples, we tested the EPR signal of the C-2#+DPPH+NMC111 / MnO mixed sample after different placement times, and calculated the integral area of ​​DPPH. By comparing the stability, we found that the results were more stable when NMC111 was used as a standard.

[0067] Table 3

[0068] .

[0069] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. 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 be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for quantitatively determining the spin concentration of carbon-based materials based on electron paramagnetic resonance, characterized in that: Includes the following steps, (1) Using metal oxide NMC as a standard, the signal integration area A of the standard was detected by an electron paramagnetic resonance spectrometer; (2) The standard sample and the carbon-based material to be tested are mixed and tested. The electron paramagnetic resonance parameters are the same as in step (1), and the signal integration area B of the standard sample in the mixed test is obtained. (3) Calculate the calibration factor Q = A / B; (4) The carbon-based material to be tested was tested separately using an electron paramagnetic resonance spectrometer to obtain the integral area C of the carbon-based material to be tested; The corrected integral area is D = C × Q; (5) Calculate the spin concentration F of the DPPH powder and obtain the electron paramagnetic resonance integral area G of the DPPH powder. Calculate the free radical concentration H per unit area = F / G. (6) Calculate the spin concentration of the carbon-based material J = (D×H) / I; where I is the mass of the carbon-based material to be tested.

2. The method for quantitatively measuring the spin concentration of carbon-based materials based on electron paramagnetic resonance according to claim 1, characterized in that: In step (1), the metal oxide NMC includes LiNi 1 / 3 Mr 1 / 3 Co 1 / 3 O2。 3. The method for quantitatively determining the spin concentration of carbon-based materials based on electron paramagnetic resonance according to claim 2, characterized in that: In step (1), the electron paramagnetic resonance parameters are set as follows: mode is Transition Metal, central magnetic field is 3300-3500 G, scan width is 4000-5000 G, and temperature is room temperature.

4. The method for quantitative determination of spin concentration of carbon-based materials based on electron paramagnetic resonance according to any one of claims 1-3, characterized in that: Step (2) includes extracting the signal integration area B of the standard sample in the mixed test by Lorentzian linear fitting.

5. The method for quantitative determination of spin concentration of carbon-based materials based on electron paramagnetic resonance according to any one of claims 1-3, characterized in that: In step (4), the electron paramagnetic resonance parameters are set as follows: mode is Organic Radical, central magnetic field is 3300-3400 G, scan width is 200-500 G, and temperature is room temperature.

6. The method for quantitative determination of spin concentration of carbon-based materials based on electron paramagnetic resonance according to any one of claims 1-3, characterized in that: In step (5), the spin concentration of the DPPH powder is F = E / 394.078766 × 6.02 × 10 23 Where E is the mass of DPPH powder.

7. The method for quantitative determination of spin concentration of carbon-based materials based on electron paramagnetic resonance according to any one of claims 1-3, characterized in that: In step (5), the electron paramagnetic resonance parameters are set as follows: mode is Organic Radical, central magnetic field is 3300-3400 G, scan width is 200-500 G, and temperature is room temperature.

8. The method for quantitative determination of spin concentration of carbon-based materials based on electron paramagnetic resonance according to any one of claims 1-3, characterized in that: In step (1), the standard powder is filled into a quartz capillary with an inner diameter of <1 mm, and the signal integration area A of the standard is detected by an electron paramagnetic resonance spectrometer.

9. The method for quantitatively determining the spin concentration of carbon-based materials based on electron paramagnetic resonance according to claim 8, characterized in that: In step (2), the mixing test includes loading the carbon-based material to be tested into a quartz tube, evacuating to remove adsorbed oxygen and sealing it, binding it with a quartz capillary tube containing a standard sample, aligning the bottoms, and using electron paramagnetic resonance to perform the mixing test.

10. The method for quantitative determination of spin concentration of carbon-based materials based on electron paramagnetic resonance according to any one of claims 1-3, characterized in that: The carbon-based material includes hard carbon anode powder or carbon-based material with a core-shell structure.

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