High-flexibility nitrogen and phosphorus co-doped carbon nanofiber membrane, preparation method and application thereof

Abstract

The application provides a high-flexibility nitrogen-phosphorus co-doped carbon nanofiber membrane and a preparation method and application thereof, and belongs to the technical field of lithium metal batteries. Melamine polyphosphoric acid nanoparticles are prepared by adopting a mechanical ball milling and differential centrifugal separation combined technology, and a high-flexibility nitrogen-phosphorus co-doped carbon nanofiber membrane is obtained by electrospinning and high-temperature calcination. When used as a lithium metal negative electrode protection layer, the high-flexibility nitrogen-phosphorus co-doped carbon nanofiber membrane can provide abundant lithiumophilic sites to reduce the overpotential of lithium metal nucleation / deposition, improve interface reaction kinetics, and the crosslinked fiber network can homogenize the lithium ion flux and electric field distribution on the surface of the lithium metal electrode. More importantly, the super-high flexibility of the protection layer can effectively relieve the volume change of the lithium metal negative electrode in the cycle process, and improve the cycle stability of the lithium metal battery.

Description

Technical Field

[0001] This invention belongs to the field of lithium metal battery technology, and specifically relates to a highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane, its preparation method, and its application. Background Technology

[0002] Lithium metal anodes possess the lowest electrode potential and extremely high theoretical specific capacity, making them one of the most promising alternative materials for lithium-ion batteries. However, lithium metal itself exhibits extremely high reactivity, readily reacting with the electrolyte to form a brittle solid electrolyte interphase (SEI) film. During deposition, due to its "hostless" nature, lithium metal undergoes unlimited volume expansion, leading to the continuous rupture and regeneration of the original SEI film. Subsequent lithium deposition preferentially accumulates at these ruptured SEI defects, growing into dendritic lithium dendrites. Once dendrite growth becomes uncontrollable, it leads to a rapid depletion of electrolyte and active lithium metal, causing rapid capacity decay and battery failure; it can even cause short circuits due to dendrites piercing the separator, resulting in hazards such as combustion and explosion.

[0003] Constructing a lithium metal protective layer is one of the effective ways to stabilize lithium metal anodes. It can not only mitigate the side reactions between lithium metal and the electrolyte, but also homogenize the ion flow on the electrode surface, induce uniform lithium metal deposition, and suppress lithium dendrite growth and volume expansion. For example, the literature "Fanfan Liu et al. prepared a mixed Li2S / Li2Se artificial protective layer for stable lithium metal anode [J], Advanced Functional Materials 2020, 2001607" describes a mixed Li2S / Li2Se artificial protective layer applied to lithium metal anodes. This layer not only improves the surface ionic conductivity but also passivates the reaction between lithium metal and organic electrolytes, thereby promoting uniform lithium ion deposition and suppressing lithium dendrite growth. However, inorganic protective layers typically exhibit significant brittleness. The huge volume expansion of the lithium metal anode during cycling can cause the protective layer to crack, introducing numerous lithium dendrite nucleation sites and leading to protective layer failure. To improve the interfacial stability of lithium metal anodes, flexible protective layers have received widespread attention. For example, the literature "Caiyun Chang et al. [Self-healing single-ion-conductive artificial polymeric solid electrolyte interphases for stable lithium metal anodes[J], Nano Energy 2022,93,106871]" reports a polymeric artificial protective layer based on dynamically cross-linked polydimethylsiloxane with high self-healing ability and single-ion conductivity. Its ultra-high flexibility not only adapts well to volume changes during lithium metal deposition / deposition but also synergistically promotes the homogenization of lithium metal nucleation / deposition and inhibits lithium dendrite growth. Although the reported highly flexible polymeric protective layer can alleviate the deformation of the lithium metal anode during cycling, the additional interface it introduces significantly increases the interfacial impedance of the battery, leading to slow electrochemical reaction kinetics. Therefore, there is an urgent need for a protective layer with high flexibility and fast ion conductance to meet the requirements of long cycling and high rate capability for lithium metal anodes. Summary of the Invention

[0004] This invention addresses the technical problems existing in the prior art by providing a highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane, its preparation method, and its applications. Melamine polyphosphate (MPP) nanoparticles are prepared using a combined technique of mechanical ball milling and differential centrifugation, followed by electrospinning and high-temperature calcination to obtain the highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane. When used as a protective layer for lithium metal anodes, the highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane provides abundant lithiophilic sites, reducing the overpotential for lithium metal nucleation / deposition and improving interfacial reaction kinetics. Its cross-linked fiber network can homogenize the lithium-ion flux and electric field distribution on the lithium metal electrode surface. More importantly, the ultra-high flexibility of the protective layer can effectively mitigate volume changes during lithium metal anode cycling, improving the cycle stability of lithium metal batteries.

[0005] The technical solution adopted in this invention is: a method for preparing a highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane, comprising the following steps:

[0006] Step 1: Add melamine powder to deionized water, heat and stir until dissolved, then add polyphosphoric acid solution dropwise to obtain a milky white suspension; the molar ratio of melamine to polyphosphoric acid is 1:(0.5~2); then filter and wash with deionized water until the solution is neutral, and dry in a constant temperature oven to obtain the MPP precursor;

[0007] Step 2: Grind the MPP precursor and pour it into a ball mill jar. After thorough mechanical ball milling, obtain MPP powder.

[0008] Step 3: Disperse MPP powder ultrasonically in N,N-dimethylformamide (DMF) solution, and obtain MPP nanoparticles by differential centrifugation;

[0009] Step 4: Add MPP nanoparticles and polymer powder to an organic solvent, then stir magnetically to dissolve the polymer completely before electrospinning. The resulting polymer / MPP composite nanofiber membrane is then vacuum dried for later use.

[0010] Step 5: The composite nanofiber membrane is first pre-oxidized at low temperature in air atmosphere, and then carbonized at high temperature in inert atmosphere to finally obtain a highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane.

[0011] Furthermore, the stirring and heating temperature is 60–100℃, and the stirring time is 1–5 h; the drying temperature of the MPP precursor is 60–100℃, and the drying time is 6–12 h.

[0012] Furthermore, the ball-to-material ratio of the MPP precursor to the milling beads is 1:(5-40); the milling time is 2-10 hours.

[0013] Furthermore, the ultrasonic dispersion time of MPP powder is 30–120 min; the centrifugation speed of differential centrifugation is 2000–15000 rpm; the centrifugation sequence of differential centrifugation is from low speed to high speed. If necessary, step 3 can be repeated multiple times.

[0014] Furthermore, the polymer powder is one of polyacrylonitrile (PAN), polyvinylpyrrolidone, and polyvinyl alcohol; the mass ratio of the MPP nanoparticles to the polymer powder is 1:(0.1-2); the organic solvent is one of N,N-dimethylformamide (DMF), N,N-dimethylacetamide, dimethyl sulfoxide, and propylene carbonate; the vacuum drying temperature of the polymer / MPP composite nanofiber membrane is 60-100℃, and the time is 6-12h.

[0015] Furthermore, the parameters for electrospinning are as follows: the distance between the needle tip and the receiver is 8–20 cm, the applied voltage is 8–15 kV, the injection pump propulsion rate is 0.5–3.0 mL / h, the collector rotation speed is 100–1000 rpm, the chamber temperature is 20–40 °C, and the chamber relative humidity is 30–60 RH.

[0016] Furthermore, the pre-oxidation temperature is 200–300°C, the carbonization temperature is 700–1000°C, and the heating rate is 1–5°C / min.

[0017] Furthermore, the thickness of the highly flexible nitrogen-phosphorus co-doped carbon nanofiber film is 20–100 μm.

[0018] The technical solution adopted in this invention is still: a highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane, which is prepared by the above-mentioned preparation method.

[0019] The technical solution adopted in this invention is: the application of a highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane as a protective layer for lithium metal anodes.

[0020] Working Principle: The highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane uses MPP particles separated by differential centrifugation as an additive. A three-dimensional cross-linked nanofiber network is constructed using electrospinning technology. During the subsequent high-temperature carbonization process, MPP not only self-activates and creates pores in situ within the carbon nanofibers to improve the membrane's flexibility, but also releases a large amount of nitrogen and phosphorus elements that anchor within the fibers, increasing lithiophilicity. Using the prepared highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane as a lithium metal anode protective layer, its cross-linked network structure can homogenize the electric field and lithium-ion flux distribution on the electrode surface during electrochemical cycling, improving the electrode interface reaction kinetics. Its ultra-high flexibility can adapt to the huge volume expansion during lithium metal deposition / deposition, preventing electrode pulverization. Its abundant nitrogen and phosphorus lithiophilic sites can reduce the lithium nucleation overpotential, homogenize the lithium metal deposition morphology, and prevent dendrite growth, thereby significantly improving the cycle life of the lithium metal battery.

[0021] Compared with the prior art, the beneficial effects of this invention are:

[0022] 1. The MPP used in this invention is an additive that is inexpensive and abundant. During the high-temperature carbonization process, it can not only act as an in-situ self-activating pore-forming agent to create multi-level pores inside carbon nanofibers, but also release a large amount of nitrogen and phosphorus elements to pin the carbon fibers and form heteroatom doping.

[0023] 2. This invention uses differential centrifugation technology to prepare MPP nanoparticles with different particle size distributions as additives for electrospun nanofibers, which can precisely control the internal pore structure of carbon nanofibers.

[0024] 3. The preparation method of the present invention adopts electrospinning technology, which has the advantages of good controllability, ease of operation and scalability.

[0025] 4. The highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane of the present invention exhibits a stronger affinity for lithium ions, which can effectively reduce the overpotential of lithium metal nucleation / deposition, induce uniform lithium deposition / extraction, and improve the coulombic efficiency of the battery.

[0026] 5. The highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane of the present invention can well adapt to the volume expansion of the electrode during cycling, thereby improving the cycle life of lithium metal batteries. Attached Figure Description

[0027] Figure 1 This is a flowchart of the preparation method according to an embodiment of the present invention;

[0028] Figure 2 This is a SEM image of the MPP nanoparticles and the highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane of Example 1 of the present invention.

[0029] Figure 3This is a SEM image of the highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane of Example 2 of the present invention;

[0030] Figure 4 This is a test graph of nucleation overpotential and coulombic efficiency of the highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane in Example 2 of the present invention;

[0031] Figure 5 This is a SEM image of the highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane of Example 3 of the present invention;

[0032] Figure 6 This is a test diagram of the symmetrical battery cycle performance of Embodiment 3 of the present invention. Detailed Implementation

[0033] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0034] Example 1

[0035] Embodiments of the present invention provide a method for preparing a highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane, such as... Figure 1 As shown, it includes the following steps:

[0036] Step 1: Add 2.52g of melamine to 200mL of deionized water and stir magnetically in an 80℃ oil bath until completely dissolved. Then, add 2.5mL of polyphosphoric acid dropwise. The solution quickly turns into a milky white suspension, and the mixture is stirred continuously for 2 hours. The obtained MPP suspension is then repeatedly filtered and washed with deionized water until neutral. After drying in a 90℃ oven for 12 hours, the MPP precursor is obtained.

[0037] Step 2: After grinding the MPP precursor in an agate mortar for 30 minutes, take 2g and place it in a ball mill jar. The ball-to-material ratio of the grinding beads to the MPP precursor is 10:1. The ball mill speed is 300 rpm. After ball milling for 5 hours, MPP powder is obtained.

[0038] Step 3: Add the obtained MPP powder to DMF solution and ultrasonically disperse for 30 min. Then, centrifuge at 5000 rpm for 10 min using a high-speed centrifuge. Collect the supernatant and centrifuge again at 15000 rpm for 10 min to obtain MPP precipitate. Add the MPP precipitate to DMF solution and ultrasonically disperse again for 30 min. Centrifuge at 10000 rpm for 10 min. Collect the supernatant and centrifuge at 15000 rpm for 10 min to obtain ultrafine MPP precipitate. Dry the obtained ultrafine MPP precipitate in a constant temperature oven at 60℃ for 12 h for later use.

[0039] Step 4: After thoroughly grinding the ultrafine MPP precipitate, add 0.158g to 3mL of DMF solution, ultrasonically disperse for 30min, then add 0.316g of polyacrylonitrile (PAN, Mw = 130,000) in small amounts several times, and stir at 30℃ for 12h to obtain a milky white organic solution. Then, electrospin the prepared organic solution with the following parameters: feed rate 1.0mL / h, distance between needle tip and receiver 12cm, applied voltage 10.5kV, collector rotation speed 500rpm, chamber temperature controlled at 25-30℃, and chamber relative humidity controlled at 40-50RH%. After spinning for 1h, a PAN / MPP composite nanofiber membrane is obtained and dried in a vacuum oven for 12h to remove residual organic solvent.

[0040] Step 5: Place the PAN / MPP composite nanofiber membrane in a tube furnace, first pre-oxidize it at 250℃ for 2 hours in an air atmosphere, and then carbonize it at 800℃ for 2 hours in an argon atmosphere. The heating rate is 2℃ / min to obtain a highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane.

[0041] Figure 2 This is a SEM image of the MPP nanoparticles and the highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane obtained after differential centrifugation in this embodiment. The SEM results show that in the supernatant after centrifugation at 10000 rpm, the MPP nanoparticles are spindle-shaped, with a size distribution ranging from 10 to 250 nm and an average particle size of 71 nm. Figure 2 a. Using the obtained MPP nanoparticles as an additive, when the mass ratio of MPP to PAN is 1:2, the resulting highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane exhibits a non-woven cross-linked structure with consistent fiber diameter, such as... Figure 2 b. High-resolution SEM images reveal wrinkled textures in localized areas of the fiber surface. This is a result of in-situ self-activation and pore formation of MPP after high-temperature calcination. Figure 2 c. Due to the enrichment of lithiophilic sites and the tip effect, this surface structure can preferentially induce lithium metal nucleation and, to some extent, release the stress concentration generated by the nanofibers during bending deformation.

[0042] Example 2

[0043] An embodiment of the present invention provides a method for preparing a highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane, which includes the following steps:

[0044] Step 1: Add 5.04 g of melamine to 400 mL of deionized water and stir magnetically in an 80°C oil bath until completely dissolved. Then, add 5 mL of polyphosphoric acid dropwise. The solution quickly turns into a milky white suspension, and the mixture is stirred continuously for 2 hours. The obtained MPP suspension is then repeatedly filtered and washed with deionized water until neutral. After drying in a 90°C oven for 12 hours, the MPP precursor is obtained.

[0045] Step 2: After grinding the MPP precursor in an agate mortar for 30 minutes, take 5g and place it in a ball mill jar. The ball-to-material ratio of the grinding beads to the MPP precursor is 20:1. The ball mill speed is 300 rpm. After ball milling for 5 hours, MPP powder is obtained.

[0046] Step 3: Add the obtained MPP powder to DMF solution and ultrasonically disperse for 30 min. Then, centrifuge at 5000 rpm for 10 min using a high-speed centrifuge. Collect the supernatant and centrifuge again at 15000 rpm for 10 min to obtain MPP precipitate. Add the MPP precipitate to DMF solution and ultrasonically disperse again for 30 min. Centrifuge at 10000 rpm for 10 min. Collect the supernatant and centrifuge at 15000 rpm for 10 min to obtain ultrafine MPP precipitate. Dry the obtained ultrafine MPP precipitate in a constant temperature oven at 60℃ for 12 h for later use.

[0047] Step 4: After thoroughly grinding the ultrafine MPP precipitate, add 0.316g to 3mL of DMF solution, ultrasonically disperse for 30min, then add 0.316g of polyacrylonitrile (PAN, Mw = 130,000) in small amounts several times, and stir at 30℃ for 12h to obtain a milky white organic solution. Then, electrospin the prepared organic solution with the following parameters: feed rate 1.0mL / h, distance between needle tip and receiver 12cm, applied voltage 13.5kV, collector rotation speed 500rpm, chamber temperature controlled at 25-30℃, and chamber relative humidity controlled at 40-50RH%. After spinning for 1h, a PAN / MPP composite nanofiber membrane is obtained and dried in a vacuum oven for 12h to remove residual organic solvent.

[0048] Step 5: Place the PAN / MPP composite nanofiber membrane in a tube furnace, first pre-oxidize it at 250℃ for 2 hours in an air atmosphere, and then carbonize it at 900℃ for 2 hours in an argon atmosphere. The heating rate is 2℃ / min to obtain a highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane.

[0049] Figure 3This is a SEM image of the highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane in this embodiment. The SEM results show that when the MPP to PAN ratio is 1:1, the prepared highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane maintains a uniformly cross-linked fiber network structure, and its surface exhibits a uniform wrinkled texture, such as... Figure 3 a. Cross-sectional SEM revealed a large number of hierarchical pore structures within the fibers, such as... Figure 3 b. This structure can not only induce uniform nucleation / deposition of lithium metal, but also greatly release the stress concentration when the fiber is bent and deformed, thereby effectively alleviating the volume expansion of the lithium metal anode during cycling and improving battery stability.

[0050] To evaluate the reversibility of lithium deposition / desorption in the highly flexible nitrogen-phosphorus co-doped carbon nanofiber protective layer, the prepared material was validated in this embodiment, as detailed below:

[0051] The obtained highly flexible nitrogen-phosphorus co-doped carbon nanofiber film was coated on the surface of copper foil, cut into circular electrodes, and then assembled with lithium foil to form a half cell. Figure 4 This embodiment is at 1mA / cm 2 Current density and 1mAh / cm 2 Coulomb efficiency tests were conducted at the cycle capacity. During initial deposition, such as... Figure 4 a. The prepared highly flexible nitrogen-phosphorus co-doped carbon nanofiber film exhibited a nucleation overpotential of only 43 mV, indicating its good lithium affinity. For example... Figure 4 b. After 200 lithium cycles, the electrode exhibited a coulombic efficiency of up to 97.6%, demonstrating its excellent cycle reversibility.

[0052] Example 3

[0053] An embodiment of the present invention provides a method for preparing a highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane, which includes the following steps:

[0054] Step 1: Add 1.25g of melamine to 100mL of deionized water and stir magnetically in an 80℃ oil bath until completely dissolved. Then, add 1.25mL of polyphosphoric acid dropwise. The solution quickly turns into a milky white suspension, and the mixture is stirred continuously for 2 hours. The obtained MPP suspension is then repeatedly filtered and washed with deionized water until neutral. After drying in a 90℃ constant temperature oven for 12 hours, the MPP precursor is obtained.

[0055] Step 2: Grind the MPP precursor in an agate mortar for 30 minutes, then take 1g and place it in a ball mill jar. The ball-to-material ratio of the grinding beads to the MPP is 50:1. The ball mill speed is 300 rpm. After grinding for 5 hours, the MPP powder is obtained.

[0056] Step 3: Add the obtained MPP powder to the DMF solution and ultrasonically disperse for 30 min. Then, centrifuge at 5000 rpm for 10 min using a high-speed centrifuge. Take the supernatant and continue centrifuging at 15000 rpm for 10 min to obtain the MPP precipitate. Dry the obtained MPP precipitate in a constant temperature oven at 60℃ for 12 h for later use.

[0057] Step 4: After thoroughly grinding the ultrafine MPP precipitate, add 0.158g to 3mL of DMF solution, ultrasonically disperse for 30min, then add 0.316g of polyacrylonitrile (PAN, Mw = 130,000) in small amounts several times, and stir at 30℃ for 12h to obtain a milky white organic solution. Then, electrospin the prepared organic solution with the following parameters: feed rate 1.0mL / h, distance between needle tip and receiver 12cm, applied voltage 11.5kV, collector rotation speed 500rpm, chamber temperature controlled at 25-30℃, and chamber relative humidity controlled at 40-50RH%. After spinning for 1h, a PAN / MPP composite nanofiber membrane is obtained and dried in a vacuum oven for 12h to remove residual organic solvent.

[0058] Step 5: Place the PAN / MPP composite nanofiber membrane in a tube furnace, first pre-oxidize it at 250℃ for 2 hours in an air atmosphere, and then carbonize it at 900℃ for 2 hours in an argon atmosphere. The heating rate is 2℃ / min to obtain a highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane.

[0059] Figure 5 This is a SEM image of the highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane in this embodiment. Figure 5 a and Figure 5 As shown in b, SEM results indicate that the carbon nanofiber membrane exhibits a cross-linked structure, but uneven pores exist at certain nodes on the fiber surface. This is because the MPP particles separated at a low rotation speed of 5000 rpm show a wide size distribution, with larger MPP particles causing localized deformation of the carbon nanofibers. Despite the uneven surface structure of these deformed regions, their abundant internal pore structure effectively releases stress concentration during carbon nanofiber deformation, ensuring the overall flexibility of the fiber membrane.

[0060] To evaluate the cycling stability of the lithium metal anode with a highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane as a protective layer, the prepared material was verified in this embodiment, as follows:

[0061] The obtained highly flexible nitrogen-phosphorus co-doped carbon nanofiber film was coated onto the surface of lithium foil, cut into circular electrodes, and two identical electrodes were assembled into a symmetrical battery to test its cycle stability. Figure 6 As shown, at 1mA / cm 2 Current density and 1mAh / cm2 At the specified cycle capacity, the lithium metal electrode with the protective layer exhibited nearly 700 hours of cycle stability, with the polarization voltage consistently maintained at 13 mV. This demonstrates that the highly flexible nitrogen-phosphorus co-doped carbon nanofiber protective layer can induce uniform lithium metal deposition, thus improving the battery's cycle stability. In contrast, the symmetric battery assembled with a pure lithium metal electrode already exhibited a large overpotential during the initial cycle. This is due to the formation of a large number of lithium dendrites and dead lithium, which caused a sharp increase in the electrode interface impedance during the initial cycle.

[0062] The present invention has been described in detail above through embodiments, but the content described is only an exemplary embodiment of the present invention and should not be considered as limiting the scope of the present invention. The scope of protection of the present invention is defined by the claims. Any technical solutions designed by those skilled in the art using the technical solutions described in the present invention, or designed by those skilled in the art under the inspiration of the technical solutions of the present invention, within the substance and protection scope of the present invention, to achieve the above-mentioned technical effects, or any equivalent changes and improvements made to the scope of the application, should still fall within the patent protection scope of the present invention.

Claims

1. A method for preparing a highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane, characterized in that, Includes the following steps: Step 1: Add melamine powder to deionized water, heat and stir until dissolved, then add polyphosphoric acid solution dropwise to obtain a milky white suspension; the molar ratio of melamine to polyphosphoric acid is 1:(0.5~2); then filter and wash with deionized water until the solution is neutral, and dry in a constant temperature oven to obtain the MPP precursor; Step 2: Grind the MPP precursor and pour it into a ball mill jar. After thorough mechanical ball milling, obtain MPP powder. Step 3: Disperse MPP powder ultrasonically in N,N-dimethylformamide solution, and obtain MPP nanoparticles by differential centrifugation; Step 4: Add MPP nanoparticles and polymer powder to an organic solvent, then stir magnetically to dissolve the polymer completely before electrospinning. The resulting polymer / MPP composite nanofiber membrane is then vacuum dried for later use. Step 5: The composite nanofiber membrane is first pre-oxidized at low temperature in air atmosphere, and then carbonized at high temperature in inert atmosphere to finally obtain a highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane. The stirring and heating temperature is 60~100℃, and the stirring time is 1~5h; the drying temperature of the MPP precursor is 60~100℃, and the drying time is 6~12h. The ratio of MPP precursor to grinding beads is 1:(5~40); the grinding time is 2~10h. The ultrasonic dispersion time for MPP powder is 30-120 min; the centrifugation speed for differential centrifugation is 2000-15000 rpm; the centrifugation sequence for differential centrifugation is from low speed to high speed. The polymer powder is one of polyacrylonitrile, polyvinylpyrrolidone, and polyvinyl alcohol; the mass ratio of MPP nanoparticles to polymer powder is 1:(0.1~2); the organic solvent is one of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, and propylene carbonate; the vacuum drying temperature of the polymer / MPP composite nanofiber membrane is 60~100℃, and the time is 6~12h. The electrospinning parameters are as follows: the distance between the needle tip and the receiver is 8~20cm, the applied voltage is 8~15kV, the injection pump propulsion rate is 0.5~3.0mL / h, the collector rotation speed is 100~1000rpm, the chamber temperature is 20~40℃, and the chamber relative humidity is 30~60%RH%. The pre-oxidation temperature is 200~300℃, the carbonization temperature is 700~1000℃, and the heating rate is 1~5℃ / min. The thickness of the highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane is 20~100μm.

2. A highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane, characterized in that, The high-flexibility nitrogen-phosphorus co-doped carbon nanofiber membrane is prepared by the method described in claim 1.

3. An application of the highly flexible nitrogen-phosphorus co-doped carbon nanofiber membrane according to claim 2, characterized in that, Used as a protective layer for lithium metal anodes.

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