A method for preparing Dawson-type phosphomolybdate-modified PP membrane and its application

Abstract

This invention provides a method for preparing a Dawson-type phosphomolybdate-modified PP separator, characterized in that: the preparation method includes the preparation and coating of octadecylmolybdate ammonium phosphate; the coating method involves mixing octadecylmolybdate ammonium phosphate and perfluorosulfonic acid resin solution to obtain a mixed slurry, coating it on one side of a polypropylene separator, and drying it to obtain the modified PP separator; the mass ratio of octadecylmolybdate ammonium phosphate to perfluorosulfonic acid resin solution is 1:8.5-9.5; the modified PP separator prepared by this invention significantly improves the wettability of the electrolyte, greatly promoting the migration rate of lithium ions, and the contact angle between the modified separator and the electrolyte is only 20°; after the modified PP separator prepared by this invention is assembled into a lithium symmetric battery, it can inhibit the growth of lithium dendrites, and while inhibiting lithium dendrites, it also converts harmful lithium dendrites into usable lithium ions, thereby greatly improving the service life and safety performance of lithium metal batteries.

Description

Technical Field

[0001] This invention relates to the preparation and application of a PP separator based on Dawson-type phosphomolybdate modification, belonging to the technical field of lithium metal battery separator materials. Background Technology

[0002] Lithium metal batteries are considered one of the most promising next-generation energy storage systems, featuring high theoretical specific capacity (3860 mAh / g), low reduction potential (-3.04 V vs. standard hydrogen electrode), and low weight density (0.53 g / cm³). 3 Lithium metal anodes are currently a hot topic in the field of electrochemical energy storage. However, during battery cycling, uneven lithium deposition / stripping processes can lead to the growth of lithium dendrites, which accelerates the consumption of the lithium anode and electrolyte, forming "dead lithium" and causing a decline in coulombic efficiency and cycle performance. More seriously, lithium dendrites can pierce the separator, causing a short circuit, battery explosion, and spontaneous combustion, which greatly limits the commercial application of lithium metal batteries.

[0003] To address the lithium dendrite problem, researchers typically focus on modifying the lithium metal anode, such as modifying the lithium metal surface with a solid electrolyte interface (SEI) or embedding lithiophilic active sites into the lithium metal. However, these methods inevitably reduce the areal capacity utilization and overall energy density of lithium. Furthermore, using solid electrolytes can also suppress lithium dendrite formation, but their lower ionic conductivity also affects battery performance. In recent years, researchers have discovered that membrane modification is also an effective method for solving the lithium dendrite problem. Modifying the membrane with functional materials can effectively regulate ion transport and achieve uniform lithium deposition. Moreover, a well-designed modified membrane has minimal impact on the battery's weight and volume, thus its effect on the battery's mass energy density and volumetric energy density is negligible.

[0004] Currently, commercially available lithium-ion battery separators are mainly polyolefin membranes, primarily including polyethylene (PE) and polypropylene (PP) microporous membranes. However, these membrane materials have low mechanical strength and are easily punctured by lithium dendrites when used in lithium batteries. These dendrites can penetrate the separator and reach the positive electrode, causing electrical contact and short circuits in the working battery, leading to thermal runaway. This can result in the combustion of the organic electrolyte or even a battery explosion. Furthermore, existing separators have poor electrolyte wettability, which is not conducive to the rapid transport of lithium ions; in addition, their cycle stability and rate performance are poor.

[0005] Modified separators are stable in both air and humid atmospheres, allowing for flexible modification operations without the need for inert gas protection. Furthermore, the most common process for separator modification is coating, which is simple to prepare and has low industrialization costs. These characteristics will greatly promote the commercial application of modified separators in batteries.

[0006] Polyoxometalates (POMs) are composed of structurally diverse polyanionic oxygen clusters and countercations. They can act as "electron sponges" to undergo reversible multi-electron redox reactions while maintaining their structural integrity. Therefore, POMs have become a research hotspot in the field of electrochemical energy storage due to their low cost and excellent electrochemical activity.

[0007] There are no reports in the current technology of using polyoxometalate-modified PP separators for the protection of the negative electrode in lithium-ion batteries. In addition, the existing polyoxometalate-modified separators are relatively thick, generally between 4-10 μm, which not only reduces the volumetric energy density of the battery, but also increases the battery cost. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention provides a method for preparing a Dawson-type phosphomolybdate-modified PP separator and its application, achieving the following objectives: when used in lithium-ion batteries, it can suppress the growth of lithium dendrites; improve the wettability of the electrolyte; enhance cycle stability and rate performance; and reduce the thickness of the separator.

[0009] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0010] A method for preparing a Dawson-type phosphomolybdate-modified PP membrane, the method comprising the preparation and coating of octadecylmolybdate ammonium phosphate; the coating method comprising mixing octadecylmolybdate ammonium phosphate and perfluorosulfonic acid resin solution to obtain a mixed slurry, coating it on one side of a polypropylene membrane, and drying it to obtain a modified PP membrane; the mass ratio of octadecylmolybdate ammonium phosphate to perfluorosulfonic acid resin solution is 1:8.5-9.5.

[0011] The perfluorosulfonic acid resin solution is designated as type D520.

[0012] The method for preparing the mixed slurry is as follows: after grinding octadecylmolybdenum ammonium phosphate for 28-32 minutes, a perfluorosulfonic acid resin solution is added, and grinding is continued for another 28-32 minutes to obtain the mixed slurry.

[0013] The coating dosage is 1.4-1.6 mg / cm³. 2 .

[0014] The drying method is to dry at 58-52℃ for 11.5-12.5 hours.

[0015] The preparation method of the aforementioned octadecylmolybdate ammonium phosphate is as follows: under stirring, sodium molybdate dihydrate is dissolved in deionized water, phosphoric acid solution and hydrochloric acid solution are added to obtain a mixed solution, which is then boiled under reflux for 7.8-8.2 h. After cooling to room temperature, a portion of solid ammonium chloride is added, and the mixture is stirred vigorously until no more precipitate is formed. The precipitate is collected, dissolved in distilled water, and the remaining solid ammonium chloride is added. The mixture is then allowed to stand at room temperature for 7.8-8.2 h to obtain green crystals, which are then filtered and vacuum dried to obtain octadecylmolybdate ammonium phosphate.

[0016] The mass-to-volume ratio of sodium molybdate dihydrate to deionized water is 1 g: 4-5 mL; the volume ratio of deionized water to phosphoric acid solution is 28-32:1; the volume ratio of deionized water to hydrochloric acid solution is 5.5-5.7:1; the concentration of phosphoric acid solution is 84-86 wt%; and the concentration of hydrochloric acid solution is 35-37 wt%.

[0017] The mass ratio of sodium molybdate dihydrate to the first added solid ammonium chloride is 1:0.9-1.1; the mass ratio of sodium molybdate dihydrate to the second added solid ammonium chloride is 1.3-1.5:1; and the volume ratio of hydrochloric acid solution to distilled water is 1:3.7-3.8.

[0018] The stirring speed is 1400-1600 r / min.

[0019] Application of the modified PP separator prepared by the above method in lithium metal batteries.

[0020] Compared with the prior art, the present invention achieves the following beneficial effects:

[0021] 1. The modified PP membrane prepared by this invention has a uniform and smooth surface, with no agglomerated particles found, indicating that (NH4)6[P2Mo] 18 O 62 The modified layer is fully dispersed in the modified layer; and the modified layer adheres tightly and firmly to the membrane, with a thickness of 2 μm; in the prepared PP membrane, (NH4)6[P2Mo] 18 O 62 The structure remains unchanged.

[0022] 2. Compared with currently commercially available PP separators, the modified PP separator prepared in this invention has significantly improved wettability to electrolytes, which can greatly promote the migration rate of lithium ions, and the contact angle between the modified separator and the electrolyte is only 20°.

[0023] 3. The modified PP separator prepared in this invention, when assembled into a lithium-ion symmetric battery, exhibits excellent cycle stability at a current density of 1 mA / cm². 2 The dough mixing capacity is 1 mAh / cm³.2 Under the test conditions, it can cycle stably for 2000 hours, and even at high current density or high areal capacity, it still exhibits excellent cycle stability (at 3 mA / cm²). 2 and 5 mAh / cm 2 Under certain conditions, it can be stably cycled for 1000 h); the half-cell using the modified PP separator can be stably charged / discharged for up to 150 cycles with an average coulombic efficiency of 96.5%.

[0024] 4. The modified PP separator prepared in this invention, when assembled into a lithium-ion symmetric battery, exhibits a smaller overpotential at different current densities, 1 mA / cm². 2 At a current density of 40 mV, the overpotential is 2 mA / cm². 2 At a current density of 3 mA / cm², the overpotential is 65 mV; at 3 mA / cm², the overpotential is 65 mV. 2 At a current density of 96 mV, the overpotential is 96 mV; at 5 mA / cm², the overpotential is 96 mV. 2 At the given current density, the overpotential is 119 mV.

[0025] 5. The modified PP separator prepared by this invention, when assembled into a lithium symmetric battery, can inhibit the growth of lithium dendrites. While inhibiting lithium dendrites, it also converts harmful lithium dendrites into usable lithium ions, thereby greatly improving the service life and safety performance of lithium metal batteries.

[0026] 6. The modified PP membrane is modified by coating method. Compared with other modification methods to suppress lithium dendrites, the preparation process is simple and easy to industrialize. Attached Figure Description

[0027] Figure 1 (NH4)6[P2Mo 18 O 62 FTIR and Raman spectra of [the image / data].

[0028] 1(a) is the FTIR spectrum; 1(b) is the Raman spectrum;

[0029] Figure 2 This is a schematic diagram of the preparation process of the modified PP membrane;

[0030] Figure 3 SEM images of the surface and cross-section of the modified PP membrane, as well as images of the folding process;

[0031] 3(a) is a SEM image of the surface of the modified PP diaphragm; 3(b) is an image of the diaphragm folding process; 3(c) is a SEM image of the cross section of the modified PP diaphragm.

[0032] Figure 4Mapping diagram of the modified PP membrane surface;

[0033] Image 4(a) is a scanning electron microscope image of the mapping region on the surface of the modified PP membrane; image 4(b) is a layered image of the mapping region on the surface of the modified PP membrane; image 4(c) is a C element distribution image; image 4(d) is an O element distribution image; image 4(e) is a P element distribution image; image 4(f) is a N element distribution image; and image 4(g) is a Mo element distribution image.

[0034] Figure 5 XPS analysis of modified PP membrane

[0035] 5(a) is the P 2p spectrum; 5(b) is the Mo 3d spectrum; 5(c) is the O 1s spectrum;

[0036] Figure 6 Electrolyte contact angle test results for PP diaphragm and modified PP diaphragm;

[0037] Figure 6(a) shows the contact angle test results between the PP diaphragm and the electrolyte; Figure 6(b) shows the contact angle test results between the modified PP diaphragm and the electrolyte.

[0038] Figure 7 Figure 1 shows the cycle performance test results of lithium-ion symmetric batteries with PP membrane and modified PP membrane.

[0039] Where 7(a) is 1 mA / cm 2 and 1 mAh / cm 2 The following is a graph showing the cycle performance test results; 7(b) is 3 mA / cm². 2 and 5 mAh / cm 2 The following is a graph showing the cycle performance test results; 7(c) is 5 mA / cm. 2 and 1 mAh / cm 2 The following is a graph showing the cyclic performance test results;

[0040] Figure 8 Rate performance and coulombic efficiency tests of lithium-ion symmetric batteries with PP separators and modified PP separators are shown in the graphs.

[0041] 8(a) is the rate performance test chart; 8(b) is the coulomb efficiency test chart.

[0042] Figure 9 The lithium dendrite morphology of PP membrane and modified PP membrane in in-situ lithium symmetric cells at different discharge times is shown.

[0043] Image 9(a) shows the lithium dendrite morphology of the PP separator after 0 min of discharge in an in-situ lithium symmetric battery; image 9(b) shows the lithium dendrite morphology of the PP separator after 15 min of discharge in an in-situ lithium symmetric battery; image 9(c) shows the lithium dendrite morphology of the PP separator after 30 min of discharge in an in-situ lithium symmetric battery; image 9(d) shows the lithium dendrite morphology of the modified PP separator after 0 min of discharge in an in-situ lithium symmetric battery; image 9(e) shows the lithium dendrite morphology of the modified PP separator after 15 min of discharge in an in-situ lithium symmetric battery; image 9(f) shows the lithium dendrite morphology of the modified PP separator after 30 min of discharge in an in-situ lithium symmetric battery.

[0044] Figure 10 XPS spectra of Mo on the surface of the modified PP separator before and after cycling in a lithium-ion symmetric battery;

[0045] 10(a) is the XPS spectrum of Mo on the surface of the modified PP separator before cycling in a lithium-symmetric battery; 10(b) is the XPS spectrum of Mo on the surface of the modified separator after cycling. Detailed Implementation

[0046] The molecular formula of octadecylmolybdenum ammonium phosphate is (NH4)6[P2Mo 18 O 62 ].

[0047] Example 1 (NH4)6[P2Mo 18 O 62 Preparation and characterization of [ ] (refer to the following literature: RSC Adv. October 2020 (40005)

[0048] Sodium molybdate dihydrate (50 g) was dissolved in 225 mL of deionized water under continuous stirring (150 r / min). Then, while stirring (150 r / min), 7.5 mL of phosphoric acid solution (85 wt%) and 40 mL of hydrochloric acid solution (36 wt%) were slowly added to the solution sequentially. The resulting mixture was refluxed under boiling conditions for 8 hours. After cooling to room temperature (25°C), 50 g of solid ammonium chloride was added, and the mixture was stirred vigorously (1500 r / min) until no more precipitate was formed. The precipitate was collected by filtration and dissolved in 150 mL of distilled water. Then, 35 g of solid ammonium chloride was added, and the solution was allowed to stand at 25°C for 8 hours to obtain green crystals (NH4)6[P2Mo 18 O 62 The crystals obtained by filtration are vacuum dried at 25°C for 12 hours and then set aside for later use.

[0049] The obtained crystal was characterized by infrared and Raman spectroscopy. Figure 1 As shown in (a), (NH4)6[P2Mo 18 O 62At 1077.5 and 1002.6 cm -1 The infrared characteristic peak at that location is v as (P–O a The stretching and contracting vibrations of ) are generated; and v as (Mo=O d ), v as (Mo–O b –Mo) and v as (Mo–O c The characteristic peaks of –Mo) appeared at 940.2, 903.3 and 781.1 cm⁻¹, respectively. -1 Location. Raman characterization such as Figure 1 As shown in b, at 972 and 711 cm -1 The peaks at these locations correspond to the terminal oxygen Mo–O, respectively. d The stretching vibrations of the bridging oxygen Mo–O–Mo. Infrared and Raman analyses indicate that (NH4)6[P2Mo]... 18 O 62 It was successfully prepared.

[0050] Example 2: Preparation of modified PP membrane

[0051] PP membrane modification process as follows Figure 2 As shown, the dried (NH4)6[P2Mo] from Example 1 was... 18 O 62 The mixture was finely ground in an agate mortar for 30 minutes, then a perfluorosulfonic acid resin solution (Nafion solution, 5 wt%, D520, DuPont) was added and grinding continued for another 30 minutes to obtain a mixed slurry; the dried (NH4)6[P2Mo] 18 O 62 The weight ratio of the mixture to the Nafion solution was 1:9. The mixed slurry was applied to one side of a polypropylene (PP Celgard 2500) diaphragm using a four-sided coater, with a coating amount of 1.5 mg / cm². 2 The modified PP diaphragm is then placed in a forced-air drying oven and dried at 50°C for 12 hours. Finally, the modified diaphragm is punched into a circular membrane with a diameter of 20 mm using a slicing machine for later use.

[0052] Example 3 Characterization of modified PP membrane

[0053] The surface and cross-section of the modified PP membrane were tested by scanning electron microscopy (SEM), such as... Figure 3 .in Figure 3 (a) shows that the surface of the modified membrane is uniform and smooth, with no agglomerated particles found, indicating that (NH4)6[P2Mo]18 O 62 The coating is fully dispersed in the modified layer. Repeated folding experiments on the modified diaphragm showed that the modified layer of the diaphragm maintained its structural integrity, with no visible cracks or peeling, indicating that the coating adheres very tightly to the diaphragm (see...). Figure 3 (b)).

[0054] Furthermore, the SEM image of the cross-section of the diaphragm coating clearly shows (NH4)6[P2Mo 18 O 62 The thickness of the modified layer is approximately 2 μm (see...). Figure 3 (c)). Surface scanning of the diaphragm coating was performed (see...). Figure 4 ), Figure 4 (This is a scanning electron microscope image of the mapping region on the surface of the modified PP membrane.) Figure 4 (b) is a layered image of the mapping region on the surface of the modified PP diaphragm; Figure 4 (c) shows the distribution of C element in the modified layer, and similarly, Figure 4 (d), 4(e), 4(f) and 4(g) show the uniform distribution of O, P, N and Mo elements in the modified layers, respectively.

[0055] Furthermore, XPS analysis was performed on the surface of the modified diaphragm, such as... Figure 5 As shown in (a), the peak at 133.4 eV in the P 2p spectrum belongs to the PO bond; Figure 5 (b) The spectrum of Mo 3d is shown, with peaks at 235.8 and 232.7 eV corresponding to the Mo 3d spectrum of Mo. 3 / 2 and Mo3d 5 / 2 Consistent; Figure 5 (c) The O 1s spectrum peaks are shown, with peaks at 532.2 and 530.8 eV corresponding to surface-adsorbed oxygen and Mo-O bonds, respectively. This indicates that (NH4)6[P2Mo] 18 O 62 The structure did not change during the modification process.

[0056] To understand the changes in the wettability of the modified diaphragm with the electrolyte, a contact angle experiment was conducted. The electrolyte used in the experiment was a mixed solution containing 1 M LiTFSI (lithium bis(trifluoromethanesulfonylimide)) and 1 wt% LiNO3 in DOL (1,3-dioxolane) and DME (ethylene glycol dimethyl ether).

[0057] The volume ratio of 1,3-dioxolane to ethylene glycol dimethyl ether is 1:1;

[0058] The commercial PP mentioned in the following experiments is polypropylene (PP Celgard 2500).

[0059] Figure 6 (a) shows that the contact angle between the commercial PP separator and the electrolyte is 47°, while the contact angle between the modified separator and the electrolyte is reduced to 20° (see Figure 6 (b) Therefore, the modified PP membrane greatly improves the wettability with the electrolyte, which helps to reduce the interfacial concentration gradient of lithium ions during transport, thereby promoting the migration rate of lithium ions.

[0060] Example 4: Cycle Performance Test of Lithium Metal Symmetric Battery

[0061] The modified PP separator was assembled into a lithium-ion symmetric battery, and charge-discharge cycle tests were performed. The results are as follows: Figure 7 As shown in (a), the modified PP membrane operates at a current density of 1 mA / cm². 2 The dough mixing capacity is 1 mAh / cm³. 2 (at 1 mA / cm) 2 Discharging or charging at a current density for 1 hour yields a surface capacity of 1 mAh / cm³. 2 Under the test conditions, it can stably cycle for 2000 hours, while commercial PP membranes can only maintain a cycle of 170 hours;

[0062] At 3 mA / cm 2 5 mAh / cm 2 and 5 mA / cm 2 1 mAh / cm 2 Under the test conditions, the battery assembled with the modified separator can still cycle stably for 1000 hours (see...). Figure 7 (b), and Figure 7 (c)).

[0063] Furthermore, rate performance tests showed that, at different current densities, the symmetric cell using the modified separator exhibited a smaller overpotential than the symmetric cell using the PP separator (see...). Figure 8 (a)), the specific overpotential results are as follows:

[0064] For symmetrical cells using common commercial PP separators, at 1 mA / cm 2 At a current density of 100 mV, the overpotential is 100 mV; at 2 mA / cm 2 At a current density of 170 mV, the overpotential is 170 mV; at 3 mA / cm 2 At a current density of 231 mV, the overpotential is 231 mV; at 5 mA / cm², the overpotential is 231 mV. 2 At the current density, the overpotential is 334 mV.

[0065] For symmetric cells with modified separators at 1 mA / cm 2At a current density of 40 mV, the overpotential is 2 mA / cm². 2 At a current density of 3 mA / cm², the overpotential is 65 mV; at 3 mA / cm², the overpotential is 65 mV. 2 At a current density of 96 mV, the overpotential is 96 mV; at 5 mA / cm², the overpotential is 96 mV. 2 At the given current density, the overpotential is 119 mV.

[0066] Finally, lithium-copper half-cell tests were conducted, such as... Figure 8 b. Half-cells using modified PP separators can be stably charged / discharged for up to 150 cycles with an average coulombic efficiency of 96.5%, while half-cells using commercial PP separators show a sudden drop in coulombic efficiency after 30 cycles.

[0067] Example 5: In-situ testing with an optical microscope

[0068] To more intuitively verify the modified separator's ability to suppress lithium dendrites, the growth of lithium dendrites in a lithium-symmetric battery was observed in situ using an optical microscope. Figure 9 As shown in (a)-(c), batteries assembled directly using PP separators exhibit typical dendritic lithium growth after 15 minutes (solid circles in the figures), and even more abundant dendritic lithium clusters appear after 30 minutes of discharge (solid circles in the figures). In contrast, modified separators (see 9(d)) effectively suppress lithium dendrite growth, and after 15 minutes of discharge in symmetrical batteries (as shown in (a)-(c)), the growth of lithium dendrites is significantly reduced. Figure 9 (e) As shown, no lithium dendrites appeared in the areas covered by the modified separator (dashed circles in the figure), indicating that dendrite growth was significantly suppressed. However, lithium dendrites appeared in the areas not covered by the modified separator (solid circles in the figure). After the symmetrical battery discharged for 30 minutes, as... Figure 9 As shown in (f), lithium dendrites continue to grow uncontrollably in areas not covered by the modified membrane (solid circle in the figure), in stark contrast to the areas covered by the modified membrane where lithium dendrites are significantly suppressed (dashed circle in the figure).

[0069] Figure 9 The discharge current density is 5 mA / cm². 2 ;

[0070] Figure 9 In (d)-(f), during battery assembly, the modified separator partially covered the lithium sheet to observe the growth state of lithium dendrites in the covered and uncovered areas.

[0071] Example 6: XPS characterization of the surface of the modified PP membrane

[0072] Figure 10 (a) shows that the peaks at 235.8 and 232.7 eV in the Mo 3d XPS spectrum of the modified separator surface before cycling of the lithium metal symmetric battery are respectively related to Mo6+ Mo 3d 3 / 2 and Mo 3d 5 / 2 Consistent; while conducting lithium metal symmetric battery tests (at a current density of 1 mA / cm²) 2 The dough mixing capacity is 1 mAh / cm³. 2 After 100 cycles under the specified conditions, the Mo 3d spectrum on the modified membrane surface showed a new Mo valence state. 5+ ( Figure 10 (b) indicates that (NH4)6[P2Mo] in the modified layer 18 O 62 An effective redox reaction occurred with the lithium dendrites, and the metallic lithium dendrites were oxidized to Li. + And (NH4)6[P2Mo 18 O 62 ] in Mo 6+ Reduced to Mo 5+ This redox reaction prevents lithium dendrites from continuing to grow, thus achieving a significant effect in inhibiting lithium dendrite growth.

Claims

1. A method for preparing a Dawson-type phosphomolybdate-modified PP membrane, characterized in that: The preparation method includes the preparation and coating of octadecylmolybdenum ammonium phosphate; the coating method is as follows: after mixing octadecylmolybdenum ammonium phosphate and perfluorosulfonic acid resin solution to obtain a mixed slurry, the slurry is coated on one side of a polypropylene diaphragm and dried to obtain a modified PP diaphragm. The mass ratio of the octadecylmolybdate ammonium phosphate to the perfluorosulfonic acid resin solution is 1:8.5-9.5; The perfluorosulfonic acid resin solution is designated as D520. The method for preparing the mixed slurry is as follows: after grinding octadecylmolybdenum ammonium phosphate for 28-32 minutes, a perfluorosulfonic acid resin solution is added, and grinding is continued for 28-32 minutes to obtain the mixed slurry; The coating dosage is 1.4-1.6 mg / cm³. 2 .

2. The method for preparing a Dawson-type phosphomolybdate-modified PP membrane according to claim 1, characterized in that: The drying method is as follows: drying at 52-58℃ for 11.5-12.5 hours.

3. The method for preparing a Dawson-type phosphomolybdate-modified PP membrane according to claim 1, characterized in that: The preparation method of the octadecylmolybdate ammonium phosphate is as follows: under stirring, sodium molybdate dihydrate is dissolved in deionized water, phosphoric acid solution and hydrochloric acid solution are added to obtain a mixed solution, and then the solution is boiled under reflux for 7.8-8.2 hours. After cooling to room temperature, a portion of solid ammonium chloride is added, and the solution is stirred vigorously until no more precipitate is formed. The precipitate is collected, dissolved in distilled water, and the remaining solid ammonium chloride is added. The solution is then allowed to stand at room temperature for 7.8-8.2 hours to obtain green crystals. After filtration and vacuum drying, octadecylmolybdate ammonium phosphate is obtained.

4. The method for preparing a Dawson-type phosphomolybdate-modified PP membrane according to claim 3, characterized in that: The mass-to-volume ratio of sodium molybdate dihydrate to deionized water is 1 g: 4-5 mL; the volume ratio of deionized water to phosphoric acid solution is 28-32:1; the volume ratio of deionized water to hydrochloric acid solution is 5.5-5.7:1; the concentration of phosphoric acid solution is 84-86 wt%; the concentration of hydrochloric acid solution is 35-37 wt%; the mass ratio of sodium molybdate dihydrate to the first added solid ammonium chloride is 1:0.9-1.1; the mass ratio of sodium molybdate dihydrate to the second added solid ammonium chloride is 1.3-1.5:1; and the volume ratio of hydrochloric acid solution to distilled water is 1:3.7-3.

8.

5. The method for preparing a Dawson-type phosphomolybdate-modified PP separator according to claim 3, characterized in that: The stirring speed is 1400-1600 r / min.

6. The application of the modified PP separator prepared by the method of claim 1 in lithium metal batteries.

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