Diaphragm modified material, preparation method of diaphragm modified material, modified diaphragm and preparation method of modified diaphragm

Abstract

The invention discloses a diaphragm modified material and a preparation method thereof, and a modified diaphragm and a preparation method thereof, and belongs to the technical field of new energy. The preparation method comprises the following steps: (1) dissolving cobalt nitrate hexahydrate and p-phenylenediamine in a molar ratio of 1: 3 in methanol, and carrying out hydrothermal reaction at 100-140 DEG C to obtain a Co-MOFs precursor; (2) the Co-MOFs precursor is placed in an argon tube furnace for high-temperature calcination, and Co-coated NPC is obtained; (3) carrying out acid pickling and drying on the Co-coated NPC by using a diluted hydrochloric acid solution with a certain concentration to obtain Co-coated NPC-A, and respectively placing the Co-coated NPC-A and a phosphorus source in the upstream and downstream of a tube furnace for pyrolysis to obtain a Co / CoP-coated NPC diaphragm modified material; and (4) mixing the Co / CoP-coated NPC, a PVDF binder and an NMP solution to prepare a uniform dispersion liquid, taking a certain volume, and performing suction filtration by taking a PP diaphragm as filter paper to prepare the modified diaphragm. By means of the synergistic effect of cobalt monatomic and cobalt phosphide, the electrochemical performance of the lithium-sulfur battery is improved, and a theoretical basis is provided for preparation of the modified diaphragm of the lithium-sulfur battery.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, in particular to a separator modification material and a preparation method thereof, and a modified separator for a lithium-sulfur battery prepared by using the material. Background Art

[0002] Lithium-sulfur batteries (Li-S) use metallic lithium with a high theoretical specific capacity (3860 mAh / g) and a low electrode potential as the negative electrode, and elemental sulfur, which is inexpensive and abundant in reserves, as the positive electrode, and have a high theoretical energy density (2600 Wh / kg), and are regarded as a next-generation energy storage system with great potential. However, due to its own characteristics and reaction mechanism, there are problems such as the shuttle effect of soluble polysulfides in the positive electrode and uneven deposition / stripping of metallic lithium in the negative electrode, which seriously hinder its commercialization. For this reason, researchers have invested a lot of energy. After decades of exploration, separator modification is one of the effective strategies to solve the above problems. For the positive electrode, the modified separator can prevent the shuttle of polysulfides, and if a catalytic material is used for modification, it can effectively adsorb polysulfides and catalyze their rapid conversion; for the negative electrode, the modified separator can uniform the lithium ion flux, regulate the deposition / stripping behavior of lithium ions at the negative electrode, and then avoid the deposition of solid lithium sulfide on the surface of lithium metal, effectively improving the rate performance and cycle life of Li-S batteries and promoting their industrial application.

[0003] According to literature reports, porous carbon materials and nitrogen-doped carbon materials are commonly used to modify Li-S battery separators. However, due to their large specific surface area, more electrolyte is required to be fully infiltrated, and the doping amount of heteroatoms is limited, making it difficult to fully adsorb soluble polysulfides. In addition, compounds such as metal oxides, metal sulfides, and metal nitrides have also been used to modify separators, but there are often dilemmas such as insufficient conductivity or limited catalytic ability. In addition, single-metal catalysts are also difficult to simultaneously stabilize all intermediates at the active sites with the optimal binding energy due to their insufficient adsorption ability for polysulfides, resulting in the inability to efficiently block the shuttle of long-chain polysulfides to the lithium metal positive electrode. In contrast, metal phosphides have significant advantages in modifying separators. On the one hand, metal phosphides have good conductivity and can effectively improve the overall electron transport efficiency of the battery; on the other hand, they have a unique crystal structure and electronic properties, with abundant and suitable active sites, which can strongly chemisorb and catalyze polysulfides, accelerate the kinetic process of the conversion reaction of polysulfides, promote the transformation of soluble polysulfides into insoluble sulfides, and efficiently inhibit the shuttle phenomenon of long-chain polysulfides. In addition, metal phosphides have a lithium-philic property, which can, to a certain extent, regulate the deposition behavior of lithium metal on the surface of the negative electrode, promote uniform deposition of lithium, reduce the generation of lithium dendrites, and reduce the risk of battery short circuit. Therefore, metal phosphides have great potential as lithium-sulfur battery separator modification materials. Summary of the Invention

[0004] To solve at least one of the above problems, the present invention provides a method for preparing a separator modification material, which can improve the electro-chemical reaction kinetics of lithium-sulfur batteries and inhibit the growth of lithium dendrites on the negative electrode, and has broad application prospects.

[0005] The present invention provides a separator modification material and a preparation method thereof, comprising the following steps:

[0006] Take cobalt nitrate hexahydrate and p-phenylenediamine with a molar ratio of 1:3 and dissolve them in methanol respectively. Mix the two and react at 100-140 °C for 6-10 h to obtain a MOFs precursor after the hydrothermal reaction.

[0007] Place the Co-MOFs precursor in a tubular furnace under a helium atmosphere and calcine it at high temperature to obtain Co@NPC after carbonization.

[0008] Pickle the Co@NPC with a dilute hydrochloric acid solution of a certain concentration and dry it to obtain Co@NPC-A.

[0009] Place Co@NPC-A and a phosphorus source at the upstream and downstream of the tubular furnace respectively and then pyrolyze them. After cooling, the separator modification material Co / CoP@NPC is obtained downstream.

[0010] Take the separator modification material Co / CoP@NPC and PVDF with a mass ratio of 9:1, add them to a certain amount of NMP solution, and configure a uniformly dispersed solution with a certain concentration after 20-40 min of ultrasonic treatment. Then, using a commercial PP separator as a filter paper, take 1-3 mL of the dispersed solution and vacuum filter it on the ethanol-wetted separator for 1-3 min, and dry it at 50-70 °C for 10-24 h to obtain the product.

[0011] As a possible design, the concentration of the p-phenylenediamine solution is 0.3-0.5 mmol / mL, and the concentration of the cobalt nitrate hexahydrate solution is 0.05-0.15 mmol / mL; after the reaction of cobalt nitrate hexahydrate and p-phenylenediamine, the Co-MOFs precursor is obtained through filtration, washing, and drying.

[0012] As a possible design, the high-temperature carbonization conditions are: under an argon atmosphere, carbonize at a temperature of 800 °C for 1-3 h.

[0013] As a possible design, the precursor is coated with copper foil before carbonization.

[0014] As a possible design, the concentration of the dilute hydrochloric acid solution is 0.3-0.8 mol / L.

[0015] As a possible design, the phosphorus source is sodium dihydrogen hypophosphite, and the mass ratio of the phosphorus source to Co@NPC-A is 1:10-20.

[0016] As a possible design, the pyrolysis phosphating conditions are: pyrolysis at a temperature of 350° C. for 1 to 3 hours in an argon atmosphere.

[0017] As a possible design, the concentration of the successfully prepared uniform dispersion is 2 mg / mL, and the volume taken for each filtration is 2 mL at this concentration of dispersion.

[0018] As a possible design, the specific model of the commercial PP separator is Celgard 2500.

[0019] Beneficial effects of the present invention:

[0020] The diaphragm modified material prepared by the present invention has a uniform morphology. In the process of carbonization followed by pickling, a part of the cobalt aggregates to form a larger elemental cobalt, and the other part forms a Co-N-C structure in a highly graphitized carbon layer, thereby fully exposing the active sites. After a subsequent phosphating process, the elemental cobalt can be converted into cobalt phosphide for adsorbing lithium polysulfide, and is helpful in forming a stable SEI film on the surface of the lithium negative electrode, and improving the electrochemical reaction kinetics of the Li-S battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The XRD spectrum of the materials in the examples of this application;

[0022] Figure 2 This is a SEM image of the material in the examples of this application;

[0023] Figure 3 This is a comparison chart of the coulombic efficiency of Li / / Cu half-cells assembled with the membranes of Example 1, Example 2, Example 3 and the comparative example in the present application;

[0024] Figure 4 This is a comparison chart of the constant current cycle performance of Li / / Li symmetrical batteries assembled with the diaphragms of Example 2 and the comparative example in this application;

[0025] Figure 5 Comparison diagram of cyclic voltammetry and AC impedance of Li-S batteries assembled with the membranes of Example 2 and the comparative example in this application;

[0026] Figure 6 This is a comparison chart of the rate performance of Li-S batteries assembled with the separators of Example 2 and the comparative example in this application;

[0027] Figure 7 This is a comparison chart of the long cycle performance of Li-S batteries assembled with the comparative example membranes of Example 2 in this application. DETAILED DESCRIPTION

[0028] The specific implementation manners of the present invention will be clearly and completely described below in conjunction with examples. Obviously, the described examples are only a part of the embodiments of the present invention, rather than all of the embodiments.

[0029] The present invention will be further described below in conjunction with embodiments:

[0030] In the following embodiments, unless otherwise specified, the operations are conventional operations in the art.

[0031] In the following embodiments, unless otherwise specified, the raw materials used can all be obtained through conventional commercial channels.

[0032] Embodiment 1

[0033] This embodiment discloses a preparation method of a modified separator, which specifically includes the following steps:

[0034] (1) Dissolve 6.2 mmol of cobalt nitrate hexahydrate in 20 mL of methanol, and dissolve 18.6 mmol of p-phenylenediamine in 40 mL of methanol. Then, slowly pour the cobalt nitrate solution into the p-phenylenediamine solution, stir for 30 min, and carry out a hydrothermal reaction at 100 - 140 °C for 6 - 10 h. The obtained black product is filtered by suction, washed 2 - 3 times with methanol, and finally dried at 60 - 80 °C for 8 - 12 h to obtain the MOFs precursor.

[0035] (2) Grind the dried precursor and load it into a porcelain boat, and then place it in a tube furnace. Under an argon atmosphere, carbonize at a high temperature of 800 °C for 1 - 3 h to obtain the carbonized product Co@NPC.

[0036] (3) Disperse Co@NPC in 50 mL of 0.3 - 0.8 mol / L hydrochloric acid solution, stir and pickle at 50 - 70 °C for 2 - 3 h, and then dry to finally obtain Co@NPC-A.

[0037] (4) Put Co@NPC-A and sodium hypophosphite with a mass ratio of 1:10 - 20 into a tube furnace. Under an argon atmosphere, phosphorate at 350 °C for 1 - 3 h to obtain the phosphorated product Co / CoP@NPC.

[0038] (5) 36 mg of Co / CoP@NPC and 4 mg of PVDF were added to 20 mL of NMP, and after 20 - 40 min of ultrasonic treatment, a homogeneous dispersion was formed. A conventional polypropylene separator (PP) was used as a filter paper, and 1 mL of the dispersion was sucked with a dropper and dropped onto the PP filter paper for vacuum filtration to obtain a modified separator of Co / CoP@NPC. Then, under vacuum conditions at 50 - 70 °C, the modified separator was dried for 10 - 24 h and then cut into a circle with a diameter of 19 mm using a tablet press. The obtained modified separator was labeled as Co / CoP-NPC@PP-1.

[0039] Example 2

[0040] (1) 6.2 mmol of cobalt nitrate hexahydrate was dissolved in 20 mL of methanol, and 18.6 mmol of p-phenylenediamine was dissolved in 40 mL of methanol. Then, the cobalt nitrate solution was slowly poured into the p-phenylenediamine solution, and after stirring for 30 min, a hydrothermal reaction was carried out at 100 - 140 °C for 6 - 10 h. The obtained black product was filtered by suction, washed 2 - 3 times with methanol, and finally dried at 60 - 80 °C for 8 - 12 h to prepare the MOFs precursor.

[0041] (2) The dried precursor was ground and filled into a porcelain boat, and then placed into a tube furnace. Under an argon atmosphere, it was carbonized at 800 °C for 1 - 3 h to obtain the carbonized product Co@NPC.

[0042] (3) Co@NPC was dispersed in 50 mL of 0.3 - 0.8 mol / L hydrochloric acid solution, stirred and pickled at 50 - 70 °C for 2 - 3 h and then dried to finally obtain Co@NPC-A.

[0043] (4) Co@NPC-A and sodium hypophosphite with a mass ratio of 1:10 - 20 were put into a tube furnace. Under an argon atmosphere, it was phosphated at 350 °C for 1 - 3 h to obtain the phosphated product Co / CoP@NPC.

[0044] (5) 36 mg of Co / CoP@NPC and 4 mg of PVDF were added to 20 mL of NMP, and after 20 - 40 min of ultrasonic treatment, a homogeneous dispersion was formed. A conventional polypropylene separator (PP) was used as a filter paper, and 2 mL of the dispersion was sucked with a dropper and dropped onto the PP filter paper for vacuum filtration to obtain a modified separator of Co / CoP@NPC. Then, under vacuum conditions at 50 - 70 °C, the modified separator was dried for 10 - 24 h and then cut into a circle with a diameter of 19 mm using a tablet press. The obtained modified separator was labeled as Co / CoP-NPC@PP-2.

[0045] Example 3

[0046] (1) Dissolve 6.2 mmol of cobalt nitrate hexahydrate in 20 mL of methanol, and dissolve 18.6 mmol of p-phenylenediamine in 40 mL of methanol. Then, slowly pour the cobalt nitrate solution into the p-phenylenediamine solution, stir for 30 min, and carry out a hydrothermal reaction at 100 - 140 °C for 6 - 10 h. The obtained black product is filtered by suction, washed with methanol 2 - 3 times, and finally dried at 60 - 80 °C for 8 - 12 h to obtain the MOFs precursor.

[0047] (2) Grind the dried precursor and load it into a porcelain boat, then place it into a tube furnace. Under an argon atmosphere, carbonize it at 800 °C for 1 - 3 h to obtain the carbonized product Co@NPC.

[0048] (3) Disperse Co@NPC in 50 mL of 0.3 - 0.8 mol / L hydrochloric acid solution, stir and pickling at 50 - 70 °C for 2 - 3 h, then dry it to finally obtain Co@NPC-A.

[0049] (4) Put Co@NPC-A and sodium hypophosphite with a mass ratio of 1:10 - 20 into a tube furnace. Under an argon atmosphere, phosphorate it at 350 °C for 1 - 3 h to obtain the phosphorated product Co / CoP@NPC.

[0050] (5) Take 36 mg of Co / CoP@NPC and 4 mg of PVDF and add them to 20 mL of NMP. After 20 - 40 min of ultrasonic treatment, a homogeneous dispersion is formed. Use a conventional polypropylene separator (PP) as the filter paper, suck 3 mL of the dispersion with a dropper and drop it on the PP filter paper for vacuum filtration to obtain the modified separator of Co / CoP@NPC. Then, under a vacuum condition at 50 - 70 °C, dry the modified separator for 10 - 24 h, and then use a tablet press to cut it into a circular shape with a diameter of 19 mm. The obtained modified separator is marked as Co / CoP-NPC@PP-3.

[0051] Comparative example

[0052] Cut the commercial separator into a circular sheet with a diameter of 19 mm using a tablet press. The obtained separator is marked as PP.

[0053] To further illustrate the effects of the embodiments of the present invention, its performance is tested below.

[0054] Since a complete battery model is required for the test, but only the separator is given in the above embodiments, therefore, the positive electrode, negative electrode, and battery model are given here.

[0055] In the glove box, the diaphragms prepared in Example 1, Example 2, Example 3, and the comparative example were respectively taken. Using a metal copper sheet as the positive electrode and a commercial lithium sheet as the negative electrode, a CR 2032 type coin-shaped Li / / Cu half-cell was assembled. The solute of the electrolyte used was lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and the solvent was 1,3-dioxolane (DOL) and dimethoxymethane (DME) with a volume ratio of 1:1, containing 2 wt% of LiNO 3 additive, the electrolyte concentration was 1 mol / L, and the dosage was 60 μL.

[0056] The diaphragms prepared in Example 2 and the comparative example were taken, and a commercial lithium sheet was used as both the positive and negative electrodes. A CR2032 type coin-shaped Li / / Li symmetric cell was assembled in the glove box.

[0057] The diaphragms prepared in Example 2 and the comparative example were taken, a commercial lithium sheet was used as the negative electrode, and sulfur-carbon (sulfur content was 70 wt%) was used as the positive electrode. A CR 2032 type coin-shaped Li-S cell was assembled in the glove box.

[0058] 1. X-ray diffraction analysis (XRD)

[0059] Figure 1 is the XRD pattern of the diaphragm modification material in the example. It can be seen from Figure 1 a that after the precursor was carbonized, three characteristic peaks appeared at 2θ≈44.23°, 51.53°, and 75.87°, corresponding to the (111), (200), and (220) crystal planes of Co, respectively. Secondly, a weak and broad diffraction peak near 2θ≈26° was classified as the (002) crystal plane of graphitized carbon, proving that the precursor material was successfully carbonized. Therefore, the material was denoted as Co@NPC. Figure 1 b is the XRD pattern of the material after Co@NPC was pickled and phosphated. It can be seen that a new diffraction peak appeared at 2θ≈48.12°, corresponding to the (211) crystal plane of CoP. At the same time, a characteristic diffraction peak of Co appeared at 2θ≈44.23°, indicating that some cobalt particles on the surface of the material had been successfully phosphated, while the Co in the internal pores still existed in the form of elemental substances. Therefore, the material after phosphating was denoted as Co / CoP@NPC.

[0060] 2. Scanning electron microscopy characterization (SEM)

[0061] The SEM images of the MOFs precursor, Co@NPC, and Co / CoP@NPC samples in the example are as Figure 2 shown. It can be seen from Figure 2 a and 2b that the MOFs precursor presented a flower-like sphere formed by the cross-linking of nanosheets, and the particle size was in the range of 1.5 - 2 μm. As Figure 2As shown in Figures 2c and 2d, after high-temperature pyrolysis, p-phenylenediamine is carbonized and decomposed, and the Co@NPC sample can still maintain its spherical flower-like structure. However, particles with a size of 50-100 nm are distributed on its surface, corresponding to elemental cobalt. Figure 2 Figures 2e and 2f are SEM images of the Co / CoP@NPC sample after phosphidation. It is found that the flower-like structure has basically disappeared. The reason may be that some small particles of elemental cobalt are embedded in the petals, resulting in volume expansion during the phosphidation process. At the same time, the large particles of cobalt on its surface also undergo greater volume changes during the phosphidation process. In addition, a part of cobalt is distributed in the highly graphitized carbon layer and combines with nitrogen and carbon atoms therein to jointly form a Co–N–C structure, which can exist stably in acid solutions.

[0062] 3. Cycle number-Coulomb efficiency test analysis of the Li / / Cu half-cell in the examples

[0063] Constant current deposition / stripping tests were carried out on the Li / / Cu half-cells with different separators in Example 1, Example 2, Example 3, and the comparative example to evaluate the cycle performance of the Coulomb efficiency. Figure 3 Figure 10 is a comparison chart of the Coulomb efficiency of the lithium-copper half-cells assembled with unmodified and modified separators at different current densities. It is found that the half-cell assembled with the Co / CoP-NPC@PP separator in Example 2 has the best Coulomb efficiency performance. It can maintain a high Coulomb efficiency of 97.5% after 20 cycles at 1 mA·cm -2 and remains above 96.4% in more than 115 cycle periods. In contrast, the Coulomb efficiency of the half-cell assembled with the PP separator decreases significantly after 50 cycles, and there is no obvious improvement compared with the half-cells assembled in Example 1 and Example 3. The above conclusions show that the functional separator modification material in Example 2 can ensure the uniform distribution of lithium ions and current on the electrode surface, promote the uniform deposition of lithium, effectively control the lithium deposition process, and thus reduce the generation of lithium dendrites.

[0064] 4. Time-voltage curve analysis of the Li / / Li symmetric cell in the examples

[0065] The Li / / Li symmetric cells were assembled using the separators in Example 2 and the comparative example respectively for constant current cycling tests, and then the long-term cycling performance during the lithium deposition / stripping process in the cells was evaluated. Figure 4 Figure 20a shows the comparison of the constant current cycling performance of the Li / / Li symmetric cells assembled in Example 2 and the comparative example at a current density of 0.5 mA·cm -2 It can be seen that the polarization voltage of the former is significantly lower, about 12 mV, and the cycle life exceeds 1000 h. In contrast, the polarization voltage of the symmetric cell assembled with the PP separator is about 20 mV, and the polarization voltage fluctuates greatly during the period from 20 h to 200 h. Figure 4b It can be found that when the current density increases to 5 mA·cm -2 ², the symmetric cell assembled with Example 2 also exhibits a stable voltage distribution and can cycle for more than 200 h at a polarization voltage of 20 mV. However, the polarization voltage of the symmetric cell assembled with the comparative example suddenly increases to more than 100 mV after 138 h. The above results indicate that the Co / CoP-NPC@PP separator in Example 2 can remain stable during cycling and guide the uniform deposition of lithium ions, thereby reducing the polarization voltage.

[0066] 5. Cyclic voltammetry curve and AC impedance of the Example Li-S full cell

[0067] Figure 5 a and b are the CV curves measured at 30 °C for the lithium-sulfur full cells assembled with the Co / CoP@NPC functional separator in Example 2 and the PP separator in the comparative example, respectively. The voltage window is 1.7 - 2.8 V (vs Li + ⁺ / Li), and the scanning rate is 0.1 mV / s. The reduction peaks that appear at medium-high voltage and low voltage in the CV curve correspond to the oxidation of elemental sulfur to long-chain polysulfides and its further conversion to solid lithium sulfide, respectively; the corresponding oxidation peak is the reaction of lithium sulfide being oxidized to elemental sulfur through multiple steps. By comparison, it can be seen that the peak potential difference of the oxidation-reduction peaks of the lithium-sulfur battery in Example 2 is smaller than that of the comparative example, further indicating that the polarization of the battery is smaller. Figure 5 c is the EIS diagram of the Li-S batteries using Example 2 and the comparative example, respectively. It can be clearly seen that the diameters of the two semicircles of the Li-S battery assembled with Example 2 in the low-frequency region and the middle frequency are significantly smaller, and the straight-line slope in the high-frequency region is larger, indicating that Example 2 has a smaller solid electrolyte resistance, charge transfer resistance, and lithium ion diffusion rate compared with the comparative example.

[0068] 6. Rate performance and cycling performance of the Example Li-S full cell

[0069] The Li-S coin cells assembled with Example 2 and the comparative example were subjected to different-rate constant current charge-discharge and long-cycle performance tests at 30 °C using a Neware battery test system, and the charge-discharge voltage range was 1.7 - 2.8 V. The results of the constant current charge-discharge ( Figure 6 ) show that the average discharge capacities of the full cells assembled with Example 2 are 1082.75, 923.89, 779.06, and 615.22 mAh·g -1 ⁻¹ at 0.1C, 0.2C, 0.3C, and 0.5C, respectively. When the current drops from 0.5C to 0.1C, it rebounds to 976.26 mAh·g -1 ⁻¹, and the Coulomb efficiency exceeds 94%; the discharge capacities of the comparative example cells are only 179.66, 109.35, 92.97, and 74.1 mAh·g -1, indicating that the battery rate performance of Example 2 is excellent. The long-cycle test results ( Figure 7 ) show that the initial specific capacity of the Li-S battery assembled in Example 2 is 550.8 mAh·g -1 . At 1.0 C, the capacity decay rate is about 0.2% per cycle after 330 cycles, and the average Coulombic efficiency is 100%. For the comparative battery, the average specific capacity is 80.8 mAh·g after 350 cycles -1 , and the average Coulombic efficiency is 98.6%. It shows that the Co / CoP-NPC@PP separator of Example 2 can inhibit the polysulfide shuttle, regulate lithium deposition / stripping, and improve the battery stability, reversibility and cycle performance.

[0070] The above are only the preferred embodiments of the present invention, and do not impose any form of limitation on the present invention. Although the present invention is disclosed in the preferred embodiments, it is not intended to limit the present invention. Those skilled in the art can make changes and modifications using the above technical content within the scope of the technical solution of the present invention to form equivalent embodiments with equivalent changes. Any simple modification, equivalent change and modification made to the above embodiments based on the technical essence without departing from the technical solution content of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A membrane modified material and a preparation method thereof, characterized in that: The following steps are involved: Cobalt nitrate hexahydrate and p-phenylenediamine in a molar ratio of 1:3 are dissolved in methanol respectively, the two are mixed and reacted at 100-140° C. for 6-10 hours, and a MOFs precursor is obtained after the reaction is completed; The Co-MOFs precursor was placed in a tube furnace and carbonized at high temperature to obtain Co@NPC; The Co@NPC was acid-washed with a dilute hydrochloric acid solution and dried to obtain Co@NPC-A; Co@NPC-A and phosphorus source were placed in the upstream and downstream of a tube furnace respectively and pyrolyzed to obtain the diaphragm modified material Co / CoP@NPC.

2. The preparation method according to claim 1, characterized in that: The concentration of the p-phenylenediamine solution is 0.3-0.5 mmol / mL, and the concentration of the cobalt nitrate hexahydrate solution is 0.05-0.15 mmol / mL.

3. The preparation method according to claim 1, characterized in that: The high temperature carbonization conditions are: carbonization at a temperature of 800° C. for 1 to 3 hours in an argon atmosphere.

4. The preparation method according to claim 1, characterized in that: The concentration of the dilute hydrochloric acid solution is 0.3-0.8 mol / L.

5. The preparation method according to claim 1, characterized in that: The phosphorus source is sodium dihydrogen hypophosphite, and the mass ratio of Co@NPC-A to the phosphorus source is 1:10-20.

6. The preparation method according to claim 1, characterized in that: The pyrolysis phosphating conditions are: pyrolysis at 350° C. for 1 to 3 hours in an argon atmosphere.

7. A diaphragm modified material, prepared by the method according to any one of claims 1 to 6.

8. A modified diaphragm and a preparation method thereof, a modified diaphragm preparation method, comprising the following steps: adding a certain amount of NMP to the diaphragm modified material of claim 7 and PVDF at a mass ratio of 9:1, and ultrasonically treating for 20-40 minutes to prepare a uniform dispersion. Using a commercial PP diaphragm as filter paper, taking 1-3 mL of the dispersion and dropping it on the diaphragm soaked with ethanol, vacuum filtering for 1-3 minutes, and then drying at 50-70°C for 10-24 hours to obtain a modified diaphragm.

9. The preparation method according to claim 8, characterized in that: The specific model of the commercial PP diaphragm is Celgard2500.

10. A modified diaphragm prepared by the method according to claim 8.

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