Cobalt-doped defective carbon-cobalt-containing polyimide composite separator and its preparation method and application

Abstract

The present invention provides a cobalt-doped defective carbon-cobalt-containing polyimide composite separator and its preparation method and application, specifically relating to the technical field of lithium-sulfur batteries. The preparation method performs first electrospinning on a cobalt-containing polyamic acid spinning solution to obtain a cobalt-containing polyamic acid fiber membrane; then a layer of polyamic acid fiber membrane is obtained by second electrospinning on its surface, and the obtained polyamic acid / cobalt-containing polyamic acid fiber membrane is subjected to thermal imidization treatment and laser irradiation to obtain the cobalt-doped defective carbon-cobalt-containing polyimide composite separator. By combining electrospinning technology with laser irradiation-induced carbonization technology, the preparation method realizes the construction of a separator for a lithium-sulfur battery with a composite functional layer and insulating layer, and has the advantages of simple process, low cost, high efficiency and controllability.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-sulfur batteries, and more particularly to a cobalt-doped defective carbon-cobalt-containing polyimide composite separator and a preparation method and application thereof. Background Art

[0002] Lithium-sulfur batteries are considered to be one of the most promising next-generation energy storage devices due to their advantages such as high energy density, low cost, and pollution-free. However, soluble polysulfides in lithium-sulfur batteries have a serious shuttle effect and the reaction kinetics of sulfur species is slow, resulting in poor actual rate performance and cycling performance of lithium-sulfur batteries, which hinders their commercial development.

[0003] The separator plays a role in separating the positive and negative electrodes to prevent internal short circuit of the battery. Adding an interlayer with both adsorption and catalytic capabilities between the positive electrode and the separator of the lithium-sulfur battery can effectively solve the above problems. The interlayer between the positive electrode and the separator of the lithium-sulfur battery mainly exists in two forms: self-supporting film or coating layer on the separator surface, which has disadvantages such as high cost, cumbersome preparation, long steps, and large interfacial impedance.

[0004] In view of this, the present invention is specifically proposed. Summary of the Invention

[0005] One object of the present invention is to provide a preparation method of a cobalt-doped defective carbon-cobalt-containing polyimide composite separator to alleviate the technical problems of high preparation cost, cumbersome preparation, long steps, and large interfacial impedance of the interlayer between the positive electrode and the separator in the prior art.

[0006] Another object of the present invention is to provide a cobalt-doped defective carbon-cobalt-containing polyimide composite separator to integrate the functions of the separator and the interlayer.

[0007] To achieve the above objects of the present invention, the following technical solutions are specifically adopted:

[0008] The first aspect of the present invention provides a preparation method of a cobalt-doped defective carbon-cobalt-containing polyimide composite separator, including the following steps:

[0009] (a) Adding a soluble cobalt salt to a polyamic acid spinning solution to obtain a cobalt-containing polyamic acid spinning solution; performing first electrospinning on the cobalt-containing polyamic acid spinning solution to obtain a cobalt-containing polyamic acid fiber membrane;

[0010] (b) Performing second electrospinning on the surface of the cobalt-containing polyamic acid fiber membrane using the polyamic acid spinning solution to obtain a layer of polyamic acid fiber membrane, that is, obtaining a polyamic acid / cobalt-containing polyamic acid fiber membrane;

[0011] (c) Performing thermal imidization treatment on the polyamic acid / cobalt-containing polyamic acid fiber membrane to obtain a polyimide / cobalt-containing polyimide fiber membrane;

[0012] (d) Laser irradiate the polyimide / cobalt-containing polyimide fiber membrane to obtain a cobalt-doped defective carbon-cobalt-containing polyimide composite separator.

[0013] Optionally, in the cobalt-containing polyamic acid spinning solution, the mass concentration of polyamic acid is 16.9-17.5%.

[0014] Preferably, in the cobalt-containing polyamic acid spinning solution, the mass concentration of cobalt ions is 2.5%-3.2%.

[0015] Optionally, a CO2 laser is used for the laser irradiation.

[0016] Preferably, the power of the laser irradiation is 2.5W-5.8W.

[0017] Optionally, the thermal imidization treatment includes a step of gradually heating the polyamic acid / cobalt-containing polyamic acid fiber membrane in air, and the heating rate is 1°C / min-3°C / min.

[0018] Optionally, the stepwise heating includes first heating to 70°C-90°C and holding for 1h-2h, then heating to 150°C-170°C and holding for 1h-2h, then heating to 240°C-260°C and holding for 1h-2h, then heating to 290°C-310°C and holding for 0.5h-1h, and finally heating to 350°C and holding for 0.5h-1h.

[0019] Optionally, the soluble cobalt salt is selected from at least one of cobalt nitrate, cobalt chloride and cobalt sulfate.

[0020] Optionally, the process parameters of the first electrospinning and the second electrospinning each independently include:

[0021] The voltage of electrospinning is 16kV-20kV, the liquid feeding rate of the syringe is 3μL / min-18μL / min, the rotating speed of the receiver is 1400r / min-1600r / min, and the electrospinning time is 1h-3h.

[0022] The second aspect of the present invention provides a cobalt-doped defective carbon-cobalt-containing polyimide composite separator.

[0023] The third aspect of the present invention provides the application of the cobalt-doped defective carbon-cobalt-containing polyimide composite separator in a lithium-sulfur battery.

[0024] Optionally, the lithium-sulfur battery is mainly assembled from a carbon nanotube / sulfur cathode, a metallic lithium anode, a cobalt-doped defective carbon-cobalt-containing polyimide composite separator and a lithium thioether-based electrolyte.

[0025] Compared with the prior art, the present invention has at least the following beneficial effects:

[0026] The preparation method provided by the present invention combines electrospinning technology with laser-induced carbonization technology to achieve the construction of a lithium-sulfur battery separator with a composite functional layer and insulating layer, and has the advantages of simple process, low cost, high efficiency and controllability. The carbon-cobalt single-element composite material is prepared by laser-induced carbonization technology, which subverts the necessary requirement of high-temperature preparation technology for inert atmosphere protection, and provides a new technical route for the controllable preparation of such materials. In addition, the instantaneous high temperature and rapid cooling characteristics of the laser, on the one hand, cause a large number of topological defects and vacancy defects to form during the surface carbonization of the polymer fiber membrane, thereby constructing a three-dimensional foam structure and a porous structure; on the other hand, it avoids the problem that cobalt particles are prone to migrate and agglomerate during the traditional carbonization process, ensuring the full exposure of active sites and the smoothness of the carbon substrate pores.

[0027] The cobalt-doped defective carbon-cobalt-containing polyimide composite separator prepared by the present invention seamlessly connects the functional layer and the insulating layer, and has the advantages of small interfacial impedance and stable structure and properties of the functional layer compared with other modified separators. In this functional layer, the carbon intrinsic defect structure and the cobalt nanoparticles uniformly embedded in the carbon substrate synergistically regulate the charge density around the carbon atoms, significantly enhancing the adsorption and catalytic conversion ability of the functional layer for polysulfide lithium, thereby inhibiting the shuttle effect caused by the penetration of polysulfide lithium through the separator.

[0028] The application of the composite separator provided by the present invention in lithium-sulfur batteries can not only effectively improve the electrochemical performance of lithium-sulfur batteries, but also reduce the risk of battery fire and explosion caused by overcharging heat generation or collision extrusion, and has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0030] Figure 1 It is a photograph of the material obtained after laser direct writing provided for Comparative Example 1 of the present invention;

[0031] Figure 2 It is an external view of the separator provided for Example 1 of the present invention;

[0032] Figure 3 It is an external view of the separator provided for Comparative Example 2 of the present invention;

[0033] Figure 4 It is an external view of the separator provided for Comparative Example 3 of the present invention;

[0034] Figure 5 Appearance diagram of the separator provided for Comparative Example 4 of the present invention;

[0035] Figure 6 XRD diagram of the carbon layer provided for Example 1 and Comparative Example 2 of the present invention;

[0036] Figure 7 SEM diagram of the positive electrode side of the cobalt-doped defective carbon-cobalt-containing polyimide composite separator provided for Example 1 of the present invention;

[0037] Figure 8 SEM diagram of the negative electrode side of the cobalt-doped defective carbon-cobalt-containing polyimide composite separator provided for Example 1 of the present invention;

[0038] Figure 9 SEM diagram of the cross-section of the cobalt-doped defective carbon-cobalt-containing polyimide composite separator provided for Example 1 of the present invention;

[0039] Figure 10 TEM diagram of the cobalt-doped defective carbon provided for Example 1 of the present invention;

[0040] Figure 11 SEM diagram of the cross-section of the defective carbon-polyimide composite separator provided for Comparative Example 2 of the present invention;

[0041] Figure 12 TEM diagram of the defective carbon provided for Comparative Example 2 of the present invention;

[0042] Figure 13 SEM diagram of the cobalt-containing polyamic acid fiber membrane provided for Comparative Example 3 of the present invention;

[0043] Figure 14 SEM diagram of the polyamic acid fiber membrane provided for Comparative Example 4 of the present invention. Specific embodiments

[0044] The embodiments of the present invention will be described in detail below in conjunction with the embodiments and examples. However, those skilled in the art will understand that the following embodiments and examples are only used to illustrate the present invention and should not be regarded as limiting the scope of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0045] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or elements inherent to such process, method, article or device.

[0046] The first aspect of the present invention provides a method for preparing a cobalt-doped defective carbon-cobalt-containing polyimide composite separator, comprising the following steps:

[0047] (a) Adding a soluble cobalt salt to a polyamic acid spinning solution to obtain a cobalt-containing polyamic acid spinning solution; performing first electrospinning on the cobalt-containing polyamic acid spinning solution to obtain a cobalt-containing polyamic acid fiber membrane;

[0048] (b) Performing second electrospinning on the surface of the cobalt-containing polyamic acid fiber membrane using the polyamic acid spinning solution to obtain a layer of polyamic acid fiber membrane, that is, obtaining a polyamic acid / cobalt-containing polyamic acid fiber membrane;

[0049] (c) Performing thermal imidization treatment on the polyamic acid / cobalt-containing polyamic acid fiber membrane to obtain a polyimide / cobalt-containing polyimide fiber membrane;

[0050] (d) Performing laser irradiation on the polyimide / cobalt-containing polyimide fiber membrane to obtain a cobalt-doped defective carbon-cobalt-containing polyimide composite separator.

[0051] The preparation method provided by the present invention combines electrospinning technology with laser-induced carbonization technology to realize the construction of a separator for a lithium-sulfur battery with a composite functional layer and insulating layer, and has the advantages of simple process, low cost, high efficiency and controllability. The use of laser-induced carbonization technology to prepare carbon-cobalt elemental composites subverts the necessary requirement of high-temperature preparation technology for inert atmosphere protection, and provides a new technical route for the controllable preparation of such materials. In addition, the instantaneous high temperature and rapid cooling characteristics of the laser on the one hand cause a large number of topological defects and vacancy defects to form during the surface carbonization process of the polymer fiber membrane, thereby constructing a three-dimensional foam structure and a porous structure; on the other hand, it avoids the problem that cobalt particles are prone to migrate and agglomerate during the traditional carbonization process, ensuring the full exposure of active sites and the smoothness of the pore channels of the carbon substrate.

[0052] The inventive concept of the present invention is to optimize the carbon surface charge density of the carbon material rich in intrinsic defects through intrinsic defects, enhance the carbon surface polarity, thereby effectively adsorbing polar lithium polysulfide and further promoting its catalytic conversion. At the same time, cobalt element with high conductivity and strong sulfur affinity is an excellent lithium polysulfide adsorbent and catalyst. Therefore, the cobalt element is loaded on the defect-rich carbon substrate to synergistically exert the advantages of both, realize the efficient adsorption and catalytic conversion of lithium polysulfide, and then realize the excellent electrochemical performance of lithium-sulfur batteries.

[0053] In some embodiments of the present invention, the polyamic acid spinning solution is obtained by polymerization of 4,4'-diaminodiphenyl ether (ODA) and pyromellitic anhydride (PMDA) in N,N-dimethylformamide (DMF).

[0054] It should be noted that when the second electrospinning is performed on the surface of the cobalt-containing polyamic acid fiber membrane, it only needs to be performed on one side of the surface of the cobalt-containing polyamic acid fiber membrane.

[0055] Optionally, in the cobalt-containing polyamic acid spinning solution, the mass concentration of polyamic acid is 16.9%-17.5%.

[0056] When the mass concentration of polyamic acid is lower than 16.9%, the spinning solution will be too thin, causing splashing during the spinning process; when the mass concentration of polyamic acid is higher than 17.5%, the spinning solution will be too thick, blocking the spinning needle during the spinning process.

[0057] In some embodiments of the present invention, the mass concentration of polyamic acid is typically but not limited to 16.9%, 17.1%, 17.3% or 17.5%.

[0058] Preferably, the mass concentration of cobalt ions in the cobalt-containing polyamic acid spinning solution is 2.5%-3.2%.

[0059] When the mass concentration of cobalt ions is lower than 2.5%, the ion content in the solution is too low, and it is difficult to confirm whether cobalt ions exist after spinning into a film; when the mass concentration of cobalt ions is higher than 3.2%, the solution reaches saturation and crystallization occurs, which is not conducive to spinning.

[0060] In some embodiments of the present invention, the mass concentration of cobalt ions is typically but not limited to 2.6%, 2.8%, 2.9%, 3.1%.

[0061] Optionally, the laser irradiation is performed using a CO2 laser.

[0062] The carbon-cobalt composite material is prepared by laser-induced carbonization technology, which subverts the necessary requirement of inert atmosphere protection in high-temperature preparation technology and provides a new technical route for the controllable preparation of such materials. On the one hand, the instantaneous high temperature and rapid cooling characteristics of the laser lead to the formation of a large number of topological defects and vacancy defects on the surface of the polymer fiber membrane during the carbonization process, thus constructing a three-dimensional foam structure and a porous structure; on the other hand, it avoids the problem that cobalt particles are prone to migrate and agglomerate during the traditional carbonization process, ensuring the full exposure of active sites and the smoothness of the pore channels of the carbon substrate.

[0063] Preferably, the power of the laser irradiation is 2.5W - 5.8W.

[0064] When the power of the laser irradiation is less than 2.5W, during the laser-induced carbonization process, the carbonization on the surface of the separator is uneven, and carbonization cannot be successfully carried out at some positions; when the power of the laser irradiation is greater than 5.8W, the whole separator is carbonized and cannot function.

[0065] In some embodiments of the present invention, the power of the laser irradiation is typically but not limited to 2.5W, 3W, 3.5W, 4W, 4.5W, 5W, 5.5W or 5.8W.

[0066] Optionally, the thermal imidization treatment includes the step of gradually heating the polyamic acid / cobalt-containing polyamic acid fiber membrane in air, and the heating rate is 1℃ / min - 3℃ / min.

[0067] Optionally, the step of gradually heating includes first heating to 70℃ - 90℃ and holding for 1h - 2h, then heating to 150℃ - 170℃ and holding for 1h - 2h, then heating to 240℃ - 260℃ and holding for 1h - 2h, then heating to 290℃ - 310℃ and holding for 0.5h - 1h, and finally heating to 350℃ and holding for 0.5h - 1h.

[0068] Optionally, the soluble cobalt salt is selected from at least one of cobalt nitrate, cobalt chloride and cobalt sulfate.

[0069] Optionally, the process parameters of the first electrospinning and the second electrospinning each independently include:

[0070] The voltage of the electrospinning is 16kV - 20kV, the liquid feeding rate of the syringe is 3μL / min - 18μL / min, the rotation speed of the receiver is 1400r / min - 1600r / min, and the electrospinning time is 1h - 3h.

[0071] The second aspect of the present invention provides a cobalt-doped defective carbon-cobalt-containing polyimide composite separator.

[0072] The prepared cobalt-doped defective carbon-cobalt-containing polyimide composite separator provided by the present invention seamlessly connects the functional layer and the insulating layer. Compared with other modified separators, it has the advantages of small interfacial impedance and stable structure and properties of the functional layer. In this functional layer, the carbon intrinsic defect structure and the cobalt nanoparticles uniformly embedded in the carbon substrate synergistically regulate the charge density around carbon atoms, significantly enhancing the adsorption and catalytic conversion ability of the functional layer for polysulfide lithium, thereby inhibiting the penetration of polysulfide lithium through the separator to generate a shuttle effect.

[0073] The third aspect of the present invention provides the application of the cobalt-doped defective carbon-cobalt-containing polyimide composite separator in a lithium-sulfur battery.

[0074] The application of the composite separator provided by the present invention in a lithium-sulfur battery can not only effectively improve the electrochemical performance of the lithium-sulfur battery, but also reduce the risk of battery fire and explosion caused by overcharging heat generation or collision extrusion, and has good application prospects.

[0075] Optionally, the lithium-sulfur battery is mainly assembled from a carbon nanotube / sulfur cathode, a metallic lithium anode, a cobalt-doped defective carbon-cobalt-containing polyimide composite separator, and a lithium sulfide ether electrolyte.

[0076] The present invention will be further illustrated below by specific examples and comparative examples. However, it should be understood that these examples are only for more detailed illustration and should not be construed as limiting the present invention in any way. For the raw materials used in the examples and comparative examples of the present invention, unless otherwise specified, they are carried out under conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments used without indicating the manufacturer are all conventional products that can be obtained through commercial purchase.

[0077] Example 1

[0078] This example provides a preparation method for a cobalt-doped defective carbon-cobalt-containing polyimide composite separator, and the specific steps are as follows:

[0079] 1. Dissolve 1.5 g of 4,4'-diaminodiphenyl ether in N,N-dimethylformamide. In an ice-water bath, add 1.6 g of pyromellitic dianhydride to the 4,4'-diaminodiphenyl ether solution. After reacting for 6 h, a polyamic acid spinning solution is formed. Then add 531.4 mg of anhydrous cobalt chloride to the polyamic acid spinning solution and stir for 4 h to fully mix it to obtain a cobalt-containing polyamic acid spinning solution.

[0080] 2. Load the obtained cobalt-containing polyamic acid spinning solution into a 5 mL syringe, assemble a metal needle, and fix the syringe on a high-voltage electrospinning machine. The metal needle is connected to a high-voltage power supply. Cover a layer of siliconized paper on the receiver. The voltage of the high-voltage electrospinning machine is 18 kV, the rotational speed of the receiver is 1500 r / min, the injection speed is 5 μL / min, and the spinning time is 2 h to obtain a cobalt-containing polyamic acid fiber membrane.

[0081] 3. Continue to perform electrospinning on the surface of the cobalt-containing polyamic acid fiber membrane using the polyamic acid spinning solution. Set the voltage to 18 kV, the rotational speed of the receiver to 1500 r / min, the injection speed to 15 μL / min, and the spinning time to about 1 h to obtain a polyamic acid / cobalt-containing polyamic acid fiber membrane.

[0082] 4. Perform thermal imidization treatment on the polyamic acid / cobalt-containing polyamic acid fiber membrane in air, that is, use a stepwise temperature increase with a heating rate of 2 °C / min. The specific steps are to hold at 80 °C for 1 h, then increase the temperature to 160 °C and hold for 1 h, then increase the temperature to 250 °C and hold for 1 h, then to 300 °C and hold for 30 min, and finally heat to 350 °C and hold for 30 min to obtain a polyimide / cobalt-containing polyimide fiber membrane.

[0083] 5. Irradiate the polyimide / cobalt-containing polyimide fiber membrane with a CO2 laser to induce carbonization of the polyimide layer and the cobalt-containing polyimide surface layer. The wavelength of the CO2 laser is 10.6 μm, the laser power is 4.5 W, the laser scanning speed is 312.5 mm / s, the pattern design is a 6 cm × 6 cm square, and the working mode is continuous laser output; Cut the membrane material obtained after laser irradiation into a circle with a diameter of 16 mm to obtain a cobalt-doped defective carbon / cobalt-containing polyimide composite separator.

[0084] Example 2

[0085] This example provides a preparation method of a cobalt-doped defective carbon / cobalt-containing polyimide composite separator. Different from Example 1, the laser power in step 5 is 4.2 W, and the other steps are the same as those in Example 1 and will not be elaborated here.

[0086] Example 3

[0087] This example provides a preparation method of a cobalt-doped defective carbon / cobalt-containing polyimide composite separator. Different from Example 1, the laser power in step 5 is 3.6 W, and the other steps are the same as those in Example 1 and will not be elaborated here.

[0088] Example 4

[0089] This example provides a method for preparing a cobalt-doped defective carbon-cobalt-containing polyimide composite separator. Different from Example 1, the amount of anhydrous cobalt chloride used in Step 1 is 177.1 mg, and the remaining steps are the same as those in Example 1, which will not be elaborated here.

[0090] Comparative Example 1

[0091] This comparative example provides a method for preparing a composite separator. Different from Example 1, Step 3 is not included, and the remaining steps are the same as those in Example 1, which will not be elaborated here.

[0092] During the laser irradiation of the cobalt-containing polyimide fiber membrane with a CO2 laser, a burning phenomenon will occur, and the obtained material is as Figure 1 shown.

[0093] Comparative Example 2

[0094] This comparative example provides a method for preparing a composite separator. Different from Example 1, anhydrous cobalt chloride is not added in Step 1, Step 2 is not included, and the polyamic acid fiber membrane is directly prepared according to the electrospinning parameters of Step 3. The remaining steps are the same as those in Example 1, which will not be elaborated here.

[0095] Comparative Example 3

[0096] This comparative example provides a method for preparing a polyimide / cobalt-containing polyimide fiber membrane. Different from Example 1, Step 5 is not included, and the remaining steps are the same as those in Example 1, which will not be elaborated here.

[0097] Comparative Example 4

[0098] This comparative example provides a method for preparing a polyimide fiber membrane. Different from Comparative Example 2, Step 5 is not included, and the remaining steps are the same as those in Comparative Example 2, which will not be elaborated here.

[0099] Comparative Example 5

[0100] This comparative example provides a separator with the model number Celgard 2500, which is obtained by commercial purchase.

[0101] Test Example 1

[0102] The surface morphologies of the separators obtained in Example 1 and Comparative Examples 2-4 were observed, and the results are as Figure 2 , Figure 3 , Figure 4 and Figure 5 shown.

[0103] From Figure 2 , Figure 3 , Figure 4 and Figure 5It can be seen that Examples 1 and Comparative Examples 2-4 can all be cut into flexible diaphragms with a diameter of 16 mm. Among them, the surfaces of Example 1 and Comparative Example 2 obtained by laser irradiation were completely carbonized and met the requirements.

[0104] Test Example 2

[0105] The cobalt-doped defective carbon-cobalt-containing polyimide composite diaphragm provided in Example 1 was respectively subjected to phase composition detection and microstructure detection, and the results are as Figures 6 - 10 shown.

[0106] From Figure 6 it can be seen that the XRD pattern of the cobalt-doped defective carbon on the positive electrode side of the cobalt-doped defective carbon-cobalt-containing polyimide composite diaphragm has diffraction peaks at about 25.8° and 42.8°, corresponding to the diffraction characteristic peaks of the (002) and (100) crystal planes of carbon respectively, and has diffraction peaks at about 44.1° and 51.5°, corresponding to the diffraction characteristic peaks of the (111) and (200) crystal planes of cobalt metal respectively, indicating that the surface of the polyimide / cobalt-containing polyimide fiber membrane was successfully carbonized by the combination of electrospinning and CO2 laser direct writing technology, and cobalt metal was successfully formed and embedded in the carbon material; while the XRD pattern of the defective carbon on the positive electrode side of the composite diaphragm of Comparative Example 2 only has diffraction peaks at about 25.8° and 42.8°, corresponding to the diffraction characteristic peaks of the (002) and (100) crystal planes of carbon respectively, and there are no diffraction peaks at other positions.

[0107] From Figure 7 it can be seen that the cobalt-doped defective carbon on the positive electrode side of the cobalt-doped defective carbon-cobalt-containing polyimide composite diaphragm presents a three-dimensional fluffy foam structure composed of nanosheets. This structure is beneficial to exposing a large number of active sites, providing guarantee for the adsorption and catalysis of sulfur species, and is beneficial to accelerating the transport of electrons and ions in the composite diaphragm.

[0108] From Figure 8 it can be seen that the cobalt-doped polyimide fibers on the negative electrode side of the cobalt-doped defective carbon-cobalt-containing polyimide composite diaphragm are still fiber structures that have not been induced to carbonize by laser, providing insulation conditions for the composite diaphragm.

[0109] From Figure 9 it can be seen that the upper layer of the cobalt-doped defective carbon-cobalt-containing polyimide composite diaphragm is a fluffy cobalt-doped porous carbon layer, and the lower layer is a dense cobalt-containing polyimide fiber. These two layers have a synergistic effect on the adsorption and catalysis of polysulfides, and thus the lithium-sulfur battery based on this composite diaphragm exhibits excellent electrochemical performance.

[0110] From Figure 10It can be seen that there are a large number of broken and bent graphite lattice stripes in the cobalt-doped porous carbon layer on the positive electrode side of the cobalt-doped defective carbon-containing cobalt polyimide composite separator, indicating the existence of a rich intrinsic defect structure. This defect structure can induce the local electron rearrangement of carbon atoms, enhance the polarity of the carbon surface, facilitate the anchoring of polysulfides and accelerate their reaction kinetics. Moreover, it can be seen that nanoparticles are embedded in the carbon layer, and the lattice diffraction stripes thereof correspond to the (111) crystal plane of elemental cobalt, indicating the successful embedding of elemental cobalt. Elemental cobalt can further improve the chemisorption performance of the composite separator and become an active site for efficiently catalyzing the conversion of sulfur species.

[0111] Test Example 3

[0112] The composite separator provided in Comparative Example 2 was characterized by scanning electron microscopy and transmission electron microscopy, and the results are as Figure 11 and Figure 12 shown.

[0113] From Figure 11 and Figure 12 it can be seen that the upper layer of the composite separator is a fluffy defective carbon-rich porous layer, and the lower layer is a dense polyimide fiber. At the same time, it can be seen that there are no cobalt nanoparticles in the carbon layer of the composite separator. Although the defective carbon layer of the separator has a certain polarity, its ability to adsorb and catalytically convert polysulfides is limited, which will cause the polysulfides to accumulate and saturate on the positive electrode side, thereby causing the shuttle effect.

[0114] Test Example 4

[0115] The composite separators provided in Comparative Example 3 and Comparative Example 4 were characterized by scanning electron microscopy, and the results are as Figure 13 and Figure 14 shown.

[0116] From Figure 13 and Figure 14 it can be seen that both of these separators exhibit a fibrous structure. This type of polymer separator can only anchor polysulfides through electrostatic adsorption, with limited effectiveness, so it cannot inhibit the shuttle effect of polysulfides.

[0117] Test Example 5

[0118] The separators provided in Examples 1-4 and Comparative Examples 2-5 were subjected to electrochemical performance tests.

[0119] The separator, carbon nanotube / sulfur positive electrode, lithium metal negative electrode, and 1,3-dioxolane (DOL) / ethylene glycol dimethyl ether (DME)-based lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) electrolyte (containing 2 wt.% LiNO3 additive) were assembled into a button battery and its electrochemical performance was tested.

[0120] Electrochemical test: The assembled battery was subjected to constant current charge-discharge test, and the voltage test range was 1.7 - 2.8V.

[0121] The rate performance test was carried out under the conditions of current densities of 0.2C, 0.5C, 1C, and 2C (1C = 1675 mA / g). The specific electrochemical detection data are shown in Table 1.

[0122] The cycle performance test was carried out under the condition of current density of 0.5C. The specific electrochemical detection data are shown in Table 2.

[0123] Table 1 Rate performance data of lithium-sulfur battery

[0124]

[0125] Table 2 Cycle performance data of lithium-sulfur battery

[0126]

[0127]

[0128] It can be seen from Table 1 and Table 2 that compared with Comparative Examples 2 - 5, Examples 1 - 4 show better rate performance, initial discharge specific capacity, and better cycle stability, indicating that the cobalt-doped defective carbon-cobalt-containing polyimide composite separator prepared by the present invention can effectively inhibit the shuttle effect when used as a lithium-sulfur battery separator, thereby improving the performance and service life of the battery.

[0129] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A preparation method of a cobalt-doped defective carbon-cobalt-containing polyimide composite separator, characterized in that, It includes the following steps: (a) Add a soluble cobalt salt to the polyamic acid spinning solution to obtain a cobalt-containing polyamic acid spinning solution; perform first electrospinning on the cobalt-containing polyamic acid spinning solution to obtain a cobalt-containing polyamic acid fiber membrane; In the cobalt-containing polyamic acid spinning solution, the mass concentration of cobalt ions is 2.5% - 3.2%; (b) Perform second electrospinning on the surface of the cobalt-containing polyamic acid fiber membrane using the polyamic acid spinning solution to obtain a layer of polyamic acid fiber membrane, that is, obtain a polyamic acid / cobalt-containing polyamic acid fiber membrane; (c) Perform thermal imidization treatment on the polyamic acid / cobalt-containing polyamic acid fiber membrane to obtain a polyimide / cobalt-containing polyimide fiber membrane; (d) Perform laser irradiation on the polyimide / cobalt-containing polyimide fiber membrane, and the power of the laser irradiation is 2.5 W - 5.8 W; obtain a cobalt-doped defective carbon / cobalt-containing polyimide composite separator.

2. The preparation method according to claim 1, wherein In the cobalt-containing polyamic acid spinning solution, the mass concentration of polyamic acid is 16.9 - 17.5%.

3. The preparation method according to claim 1, characterized in that, Use a CO2 laser for the laser irradiation.

4. The preparation method according to claim 1, characterized in that, The thermal imidization treatment includes a step of gradually raising the temperature of the polyamic acid / cobalt-containing polyamic acid fiber film in air, and the heating rate is 1 o C / min - 3 o C / min.

5. The preparation method according to claim 4, characterized in that, The stepwise temperature increase includes first heating to 70 o °C - 90 o °C for heat preservation for 1 h - 2 h, then heating to 150 o °C - 170 o °C for heat preservation for 1 h - 2 h, then heating to 240 o °C - 260 o °C for heat preservation for 1 h - 2 h, then heating to 290 o °C - 310 o °C for heat preservation for 0.5 h - 1 h, and finally heating to 350 o °C for heat preservation for 0.5 h - 1 h.

6. The preparation method according to claim 1, characterized in that, The cobalt salt includes a soluble cobalt salt.

7. The preparation method according to claim 6, characterized in that, The soluble cobalt salt includes at least one of cobalt nitrate, cobalt chloride, and cobalt sulfate.

8. The preparation method according to any one of claims 1-7, characterized in that, The process parameters of the first electrospinning and the second electrospinning each independently include: The voltage of electrospinning is 16 kV - 20 kV, the liquid feeding rate of the syringe is 3 μL / min - 18 μL / min, the rotating speed of the receiver is 1400 r / min - 1600 r / min, and the electrospinning time is 1 h - 3 h.

9. A cobalt-doped defective carbon / cobalt-containing polyimide composite separator prepared by the preparation method according to any one of claims 1 - 8.

10. An application of the cobalt-doped defective carbon / cobalt-containing polyimide composite separator according to claim 9 in a lithium-sulfur battery.

11. The application according to claim 10, wherein, The lithium-sulfur battery is mainly assembled from a carbon nanotube / sulfur cathode, a metallic lithium anode, a cobalt-doped defective carbon / cobalt-containing polyimide composite separator, and a lithium sulfide-based electrolyte.

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