Lithium-sulfur battery positive electrode material as well as preparation method and application thereof

Abstract

The invention belongs to the field of electrochemical energy storage, and discloses a lithium-sulfur battery positive electrode material and a preparation method and application thereof.The lithium-sulfur battery positive electrode material is prepared from graphene oxide, aniline, boric acid, ammonium pentaborate, urea, diammonium hydrogen phosphate and sodium chloride through a simple heat treatment method. The method is simple and low in cost, the prepared high-performance three-dimensional porous phosphorus-doped boron nitride / multi-doped graphene composite material is used as a sulfur carrier, the conductivity of an active substance sulfur can be effectively improved, the huge volume expansion effect of the active substance sulfur in the charging and discharging process can be relieved, and the purpose of improving the electrochemical performance of a lithium-sulfur battery can be achieved.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy storage, specifically relating to a lithium-sulfur battery cathode material, its preparation method, and its application. Background Technology

[0002] Currently, the energy storage mechanism of lithium-sulfur batteries is based on the redox reaction between sulfur and lithium: during charging, lithium sulfide decomposes into elemental sulfur and lithium ions; during discharging, lithium ions react with elemental sulfur in multiple steps to gradually generate a series of lithium polysulfides (Li2S). n The lithium-sulfur battery, with a charge-to-discharge ratio of 4 ≤ n ≤ 8, is ultimately converted into Li₂S (lithium sulfide). This unique reaction mechanism enables the storage and release of charge through the charging and discharging processes. This unique reaction mechanism endows lithium-sulfur batteries with energy density potential far exceeding that of traditional lithium-ion batteries. Furthermore, sulfur, as a positive electrode active material, has the advantages of being non-toxic and low-cost, aligning with the green and low-cost development needs of new energy storage devices. However, the commercialization of lithium-sulfur batteries is still constrained by three core issues: First, sulfur and its discharge products (Li₂S) have poor conductivity, leading to slow electrode reaction kinetics; second, during charging and discharging, lithium polysulfides easily dissolve in the electrolyte and shuttle between the positive and negative electrodes, resulting in a shuttle effect that causes loss of active material and rapid capacity decay; third, the volume change between sulfur and Li₂S is as high as 80%, which can easily lead to electrode structure collapse during long-term cycling, affecting cycle stability. Summary of the Invention

[0003] The purpose of this invention is to provide a lithium-sulfur battery cathode material, its preparation method, and its application, so as to solve the three core problems faced by lithium-sulfur batteries mentioned in the background art.

[0004] To achieve the above objectives, the present invention adopts the following technical solution: This invention provides a method for preparing a lithium-sulfur battery cathode material, comprising the following steps: Porous phosphorus-doped boron nitride is mixed with sulfur and carbon disulfide to obtain a mixed slurry, and then the mixed slurry is evaporated to dryness to obtain sulfur / porous phosphorus-doped boron nitride; wherein the ratio of porous phosphorus-doped boron nitride, sulfur and carbon disulfide is: (50~200mg):(200~800mg):(5~10mL); Nitrogen-boron-oxygen co-doped graphene powder was added to a distilled water / ethanol mixed solution containing sulfur / porous phosphorus-doped boron nitride. After thorough stirring and uniform dispersion, the mixture was centrifuged and dried. The dried product was placed in a protective atmosphere and kept at 152-158°C for 6-8 hours to obtain the lithium-sulfur battery cathode material. The lithium-sulfur battery cathode material is a sandwich-structured nitrogen-boron-oxygen co-doped graphene / sulfur / porous phosphorus-doped boron nitride lithium-sulfur battery cathode material. The ratio of nitrogen-boron-oxygen co-doped graphene, sulfur / porous phosphorus-doped boron nitride, ethanol, and distilled water was (50-200 mg):(50-200 mg):(20-30 mL):(20-30 mL). Preferably, the preparation process of the porous phosphorus-doped boron nitride includes: Boric acid, urea, diammonium hydrogen phosphate, and sodium chloride are dissolved in distilled water and stirred until completely dissolved to form solution A. The ratio of boric acid, urea, diammonium hydrogen phosphate, sodium chloride, and distilled water is (200~500 mg): (2.4-7.2 g): (200~600 mg): (8~15 g): (50~90 mL). Solution A was stirred and evaporated to dryness to obtain the first precursor material; The first precursor material was then placed in a protective atmosphere and calcined at 700-800°C for 2-3 hours. After the product cooled naturally to room temperature, the first calcined product was obtained. The first calcined product was sequentially washed with distilled water, centrifuged, and dried to obtain porous phosphorus-doped boron nitride.

[0005] Preferably, the solution A is stirred and evaporated to dryness at 70-80°C to obtain the first precursor material.

[0006] Preferably, the first calcined product is washed with water 3 to 5 times, and then dried at 30 to 40°C to remove moisture, thereby obtaining porous phosphorus-doped boron nitride.

[0007] Preferably, the preparation process of the nitrogen-boron-oxygen co-doped graphene includes: Graphene oxide was dispersed in a solution containing aniline, ammonium persulfate and water, and stirred to induce a polymerization reaction. After centrifugation, washing and freeze-drying, a graphene oxide-polyaniline precursor was obtained. The ratio of graphene oxide, ammonium persulfate, aniline and water was (50~80mg): (400~700mg): (10~15 mL): (10~70mL). The graphene oxide-polyaniline precursor was ground and mixed with ammonium pentaborate and calcined at 600-800℃ for 2-3 hours under a protective atmosphere. After the product was naturally cooled to room temperature, a second calcined product was obtained. The second calcined product was then washed, centrifuged, and dried to finally obtain nitrogen-boron-oxygen co-doped graphene. The mass ratio of graphene oxide-polyaniline precursor to ammonium pentaborate was (50-80 mg): (150-190 mg). Preferably, when the graphene oxide-polyaniline precursor and ammonium pentaborate are calcined in a protective atmosphere at 600–800°C for 2–3 hours to obtain the first calcined product, the gas flow rate in the protective atmosphere is 28–32 cm⁻¹. 3 / min.

[0008] The present invention also provides a lithium-sulfur battery cathode material, which is obtained by the preparation method of the lithium-sulfur battery cathode material described above.

[0009] The present invention also provides an application of the lithium-sulfur battery cathode material as described above, wherein the lithium-sulfur battery cathode material is used to prepare the cathode of a lithium-sulfur battery.

[0010] The present invention has the following beneficial effects: The method for preparing the lithium-sulfur battery cathode material of the present invention can achieve uniform loading and interfacial anchoring of sulfur in a porous phosphorus-doped boron nitride (P-BN) and nitrogen-boron-oxygen co-doped graphene (N / B / OG) composite carrier. The reaction mechanism and structural evolution logic are as follows: When porous phosphorus-doped boron nitride, sulfur and carbon disulfide are mixed in the ratio of (50~200mg):(200~800mg):(5~10mL), carbon disulfide, as a good solvent for sulfur, can dissolve elemental sulfur into a molecular-level dispersion, thereby forming a uniform mixed slurry. This ratio range ensures a high sulfur loading (the sulfur content can reach more than 75%, see the sulfur content ratio in Example 2 of the present invention) and allows the porous phosphorus-doped boron nitride to be fully dispersed to exert the channel confinement effect. This invention rationally configures the ratio of nitrogen-boron-oxygen co-doped graphene powder and sulfur / porous phosphorus-doped boron nitride distilled water / ethanol dispersion, specifically, the ratio of nitrogen-boron-oxygen co-doped graphene, sulfur / porous phosphorus-doped boron nitride, ethanol, and water is (50~200mg):(50~200mg):(20~30mL):(20~30mL). Under the above ratio, nitrogen-boron-oxygen co-doped graphene sheets can be intercalated into the porous framework gaps of porous phosphorus-doped boron nitride to form a three-dimensional cross-linked structure. Since sulfur is insoluble in the water / ethanol system, lithium can be ensured to fill the pores of porous phosphorus-doped boron nitride. In this invention, nitrogen-boron-oxygen co-doped graphene powder / porous phosphorus-doped boron nitride is heat-treated (i.e., held at temperature) at 152~158℃ for 6~8h. This low-temperature condition promotes the formation of weak coordination bonds between sulfur molecules and the phosphorus and nitrogen doping sites of porous phosphorus-doped boron nitride and the nitrogen, boron, and oxygen doping sites of nitrogen-boron-oxygen co-doped graphene, thereby achieving sulfur interface anchoring. At the same time, this temperature range does not destroy the porous structure of porous phosphorus-doped boron nitride and the conductive network of nitrogen-boron-oxygen co-doped graphene, ultimately forming a composite cathode structure with porous phosphorus-doped boron nitride channel confinement, nitrogen-boron-oxygen co-doped graphene network support, and sulfur interface anchoring (i.e., the lithium-sulfur battery cathode material of this invention).

[0011] In the lithium-sulfur battery cathode material prepared by this invention, the nitrogen, boron, and oxygen co-doping of nitrogen-boron-oxygen co-doped graphene introduces a large number of electron transport sites into the graphene lattice, building a three-dimensional conductive network. Furthermore, the mass ratio of porous phosphorus-doped boron nitride to nitrogen-boron-oxygen co-doped graphene is in the range of (50~200mg):(50~200mg). This material ratio ensures the formation of a sandwich structure in the conductive network, allowing both sulfur and the discharge product lithium sulfide to contact the conductive pathway. Experimental data from the embodiments of this invention show that the lithium-sulfur battery cathode material obtained by this invention achieves a discharge specific capacity of 750mAh / g at a current density of 0.2C, demonstrating a significant improvement in its conductivity and reaction kinetics. It can be seen that this invention solves the current defects of poor conductivity and slow electrode reaction kinetics in sulfur and lithium sulfide. Furthermore, the porous structure in the porous phosphorus-doped boron nitride of this invention can physically confine lithium polysulfides within the channels, and the phosphorus doping sites in the porous phosphorus-doped boron nitride and the multi-element doping sites of the nitrogen-boron-oxygen co-doped graphene can chemically adsorb lithium polysulfides through polar interactions, forming a dual anti-shuttle barrier of physical confinement and chemical adsorption. The experimental results in Example 2 of this invention confirm that the capacity of the lithium-sulfur battery cathode material of this invention still remains at 750 mAh / g after 120 cycles, which proves the superior anti-shuttle effect of the lithium-sulfur battery cathode material of this invention. It can be seen that this invention solves the defects of lithium polysulfide shuttle effect and active material loss in the prior art. Furthermore, the porous channels of the porous phosphorus-doped boron nitride of this invention provide a buffer space for the volume expansion of sulfur, and the flexible sheets of nitrogen-boron-oxygen co-doped graphene can disperse the stress generated by the volume expansion. At the same time, the ratio range of porous phosphorus-doped boron nitride and nitrogen-boron-oxygen co-doped graphene ensures the synergy between the rigid porous framework of porous phosphorus-doped boron nitride and the flexible conductive network of nitrogen-boron-oxygen co-doped graphene, avoiding electrode structure cracking. As can be seen from the experimental results of the embodiments of this invention, the discharge area specific capacity of the soft-pack battery is still 4.7 mAh / cm² after 100 cycles, demonstrating excellent structural stability. Therefore, this invention solves the defects of large volume changes of sulfur and lithium sulfide in the prior art, which lead to electrode structure collapse. Attached Figure Description

[0012] Figure 1 The X-ray powder diffraction pattern is that of the lithium-sulfur battery cathode material prepared in Example 3 of this invention, and phosphorus-doped boron nitride.

[0013] Figure 2 This is a SEM image of the porous phosphorus-doped boron nitride prepared in Example 3 of the present invention. Figure 3 This is a SEM image of the nitrogen-boron-oxygen co-doped graphene prepared in Example 3 of this invention. Figure 4 This is a SEM image of the lithium-sulfur battery cathode material prepared in Example 3 of the present invention. Figure 5This is a TEM image of the lithium-sulfur battery cathode material prepared in Example 3 of the present invention.

[0014] Figure 6 This is a cycle performance diagram of the lithium-sulfur battery cathode material, phosphorus-doped boron nitride / S, and multi-doped graphene / S composite material in Example 3 of the present invention at 0.2C in a button lithium-sulfur battery.

[0015] Figure 7 This is a graph showing the lithium-sulfur battery cycle performance of the lithium-sulfur battery cathode material in Example 3 of the present invention at 0.15C in a soft-pack battery. Detailed Implementation

[0016] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0017] The preparation method of the lithium-sulfur battery cathode material of the present invention includes the following steps: Step 1: Dissolve boric acid, urea, diammonium hydrogen phosphate, and sodium chloride in distilled water and stir until completely dissolved to form solution A. Evaporate solution A at 70–90 °C with stirring until dry. This stirring-evaporation process achieves uniform molecular-level dispersion of the components, yielding the first precursor. Then, place the first precursor material in a protective atmosphere (such as Ar atmosphere) and calcine it at 700–800 °C for 2–3 hours. After the product naturally cools to room temperature, wash the calcined product 3–5 times with distilled water, centrifuge, and dry to obtain porous phosphorus-doped boron nitride. The ratio of boric acid, urea, diammonium hydrogen phosphate, sodium chloride, and distilled water is (200–500 mg): (2.4–7.2 g): (200–600 mg): (8–15 g): (50–90 mL). The gas flow rate of the protective atmosphere is 28–32 cm⁻¹. 3 / min; Step 2: Mix porous phosphorus-doped boron nitride with sulfur and carbon disulfide to obtain a mixed slurry. Then, evaporate the mixed slurry to dryness at 30-50℃ to obtain sulfur / porous phosphorus-doped boron nitride. The ratio of porous phosphorus-doped boron nitride, sulfur and carbon disulfide is (50~200mg): (200~600mg): (5~10mL). Step 3: Graphene oxide is dispersed in a solution containing aniline, ammonium persulfate and water, stirred to induce polymerization, washed by centrifugation and freeze-dried to obtain graphene oxide-polyaniline precursor; wherein the ratio of graphene oxide, ammonium persulfate, aniline and water is (50~80mg): (400~700mg): (10~15 mL): (10~70mL); The graphene oxide-polyaniline precursor was ground and mixed with ammonium pentaborate and calcined at 600–800 °C for 2–3 h in a protective atmosphere (such as Ar). After the product cooled naturally to room temperature, a second calcined product was obtained. The second calcined product was washed 3–5 times, centrifuged, and dried to finally obtain nitrogen-boron-oxygen co-doped graphene (also referred to as multi-doped graphene). The mass ratio of graphene oxide-polyaniline precursor to ammonium pentaborate was (50–80 mg): (150–190 mg); the gas flow rate of the protective atmosphere was 28–32 cm⁻¹. 3 / min.

[0018] Step 4: Add the multi-co-doped graphene powder to a distilled water / ethanol mixed solution containing sulfur / porous phosphorus-doped boron nitride. After thorough dispersion by stirring, centrifuge and dry the solution. Place the dried product in a protective atmosphere (such as Ar atmosphere) and keep it at 152~158℃ for 6-8 hours to obtain a sandwich-structured nitrogen-boron-oxygen co-doped graphene / sulfur / porous phosphorus-doped boron nitride (i.e., the lithium-sulfur battery cathode material of this invention). The ratio of nitrogen-boron-oxygen co-doped graphene, sulfur / porous phosphorus-doped boron nitride, ethanol, and water is (50~200mg):(50~200mg):(20~30mL):(20~30mL). When the lithium-sulfur battery cathode material prepared by the above-described scheme of the present invention is used to prepare the cathode of a lithium-sulfur battery, the lithium-sulfur battery cathode material is mixed with a conductive agent (such as acetylene black) and a binder (such as polyvinylidene fluoride), then N-methylpyrrolidone is added dropwise and mixed evenly. Afterwards, it is coated onto a conductive substrate and dried to obtain the lithium-sulfur battery cathode. The lithium-sulfur battery cathode material prepared by the present invention is mainly used as the active material of the lithium-sulfur battery cathode. Other materials for the lithium-sulfur battery cathode use currently conventional materials, such as the aforementioned acetylene black, polyvinylidene fluoride, and N-methylpyrrolidone, and may also contain other additives. The following embodiments of the present invention only provide an exemplary preparation process for a lithium-sulfur battery cathode to demonstrate the feasibility of the technical solution of the present invention.

[0019] Example 1 The preparation method of the lithium-sulfur battery cathode material in this embodiment includes the following steps: Step 1: Dissolve 200 mg boric acid, 2.4 g urea, 200 mg diammonium hydrogen phosphate, and 8 g sodium chloride in 50 mL distilled water and stir until completely dissolved to form solution A; evaporate solution A to dryness at 70 °C to obtain the first precursor; then place the first precursor material in an Ar atmosphere and calcine it at 700 °C for 2 hours, wherein the Ar gas flow rate is 30 cm⁻¹. 3 / min; After the product cools naturally to room temperature, the calcined product is washed with water three times, centrifuged and dried to obtain porous phosphorus-doped boron nitride.

[0020] Step 2: Mix 50 mg of porous phosphorus-doped boron nitride with 200 mg of sulfur and 5 mL of carbon disulfide to obtain a mixed slurry. Then, evaporate the mixed slurry to dryness at 50 °C to obtain sulfur / porous phosphorus-doped boron nitride.

[0021] Step 3: Disperse 50 mg of graphene oxide powder in a solution containing 10 mL of aniline, 400 mg of ammonium persulfate and 10 mL of distilled water, stir to induce polymerization, centrifuge, wash and freeze dry to obtain graphene oxide-polyaniline precursor; then grind and mix 50 mg of graphene oxide-polyaniline precursor with 150 mg of ammonium pentaborate and calcine in Ar at 800 °C for 2 h. After the product cools naturally to room temperature, wash three times, centrifuge and dry in sequence to finally obtain nitrogen-boron-oxygen co-doped graphene.

[0022] Step 4: 50 mg of multi-co-doped graphene and 50 mg of sulfur / porous phosphorus-doped boron nitride were uniformly dispersed in a mixed solution of 20 mL of distilled water and 20 mL of ethanol. After being stirred and fully dispersed, the mixture was centrifuged and dried. The dried product was placed in an Ar atmosphere and kept at 152 °C for 6 hours to obtain a sandwich structure of nitrogen-boron-oxygen co-doped graphene / sulfur / porous phosphorus-doped boron nitride (i.e., the lithium-sulfur battery cathode material in this embodiment).

[0023] Example 2 The preparation method of the lithium-sulfur battery cathode material in this embodiment includes the following steps: Step 1: Dissolve 300 mg boric acid, 3.6 g urea, 300 mg diammonium hydrogen phosphate, and 10 g sodium chloride in 70 mL distilled water and stir until completely dissolved to form solution A; evaporate solution A to dryness at 80 °C to obtain the first precursor; then place the first precursor material in an Ar atmosphere and calcine it at 730 °C for 2.2 hours, wherein the Ar gas flow rate is 28 cm⁻¹. 3 / min; After the product cools naturally to room temperature, the calcined product is washed with water 4 times, centrifuged and dried to obtain porous phosphorus-doped boron nitride.

[0024] Step 2: Mix 100 mg of porous phosphorus-doped boron nitride with 400 mg of sulfur and 7 mL of carbon disulfide to obtain a mixed slurry. Then, evaporate the mixed slurry to dryness at 40 °C to obtain sulfur / porous phosphorus-doped boron nitride.

[0025] Step 3: 60 mg of graphene oxide powder was dispersed in a solution containing 12 mL of aniline, 500 mg of ammonium persulfate and 50 mL of distilled water. The mixture was stirred to induce a polymerization reaction. After centrifugation, washing and freeze-drying, a graphene oxide-polyaniline precursor was obtained. Then, 60 mg of the graphene oxide-polyaniline precursor was ground and mixed with 160 mg of ammonium pentaborate and calcined in Ar at 700 °C for 2.5 h. After the product was naturally cooled to room temperature, it was washed 4 times, centrifuged and dried in sequence to finally obtain nitrogen-boron-oxygen co-doped graphene.

[0026] Step 4: 100 mg of multi-co-doped graphene and 100 mg of sulfur / porous phosphorus-doped boron nitride were uniformly dispersed in a mixed solution of 23 mL of distilled water and 23 mL of ethanol. After being stirred and fully dispersed, the mixture was centrifuged and dried. The dried product was placed in an Ar atmosphere and kept at 155 °C for 7 hours to obtain a sandwich-structured nitrogen-boron-oxygen co-doped graphene / sulfur / porous phosphorus-doped boron nitride (i.e., the lithium-sulfur battery cathode material in this embodiment).

[0027] Example 3 The preparation method of the lithium-sulfur battery cathode material in this embodiment includes the following steps: Step 1: Dissolve 400 mg boric acid, 5 g urea, 450 mg diammonium hydrogen phosphate, and 12 g sodium chloride in 80 mL of distilled water and stir until completely dissolved to form solution A; evaporate solution A to dryness at 80 °C to obtain the first precursor; then place the first precursor material in an Ar atmosphere and calcine it at 750 °C for 3 hours, wherein the Ar gas flow rate is 30 cm⁻¹. 3 / min; After the product cools naturally to room temperature, the calcined product is washed with water 5 times, centrifuged and dried to obtain porous phosphorus-doped boron nitride.

[0028] Step 2: Mix 150 mg of porous phosphorus-doped boron nitride with 600 mg of sulfur and 9 mL of carbon disulfide to obtain a mixed slurry. Then, evaporate the mixed slurry to dryness at 30°C to obtain sulfur / porous phosphorus-doped boron nitride.

[0029] Step 3: 70 mg of graphene oxide powder was dispersed in a solution containing 14 mL of aniline, 600 mg of ammonium persulfate and 60 mL of distilled water. The mixture was stirred to induce a polymerization reaction. After centrifugation, washing and freeze-drying, a graphene oxide-polyaniline precursor was obtained. Then, 70 mg of the graphene oxide-polyaniline precursor was ground and mixed with 180 mg of ammonium pentaborate and calcined in Ar at 600 °C for 3 h. After the product was naturally cooled to room temperature, it was washed 5 times, centrifuged and dried in sequence to finally obtain nitrogen-boron-oxygen co-doped graphene.

[0030] Step 4: 150 mg of co-doped graphene and 150 mg of sulfur / porous phosphorus-doped boron nitride were uniformly dispersed in a mixed solution of 28 mL of distilled water and 28 mL of ethanol. After being stirred and fully dispersed, the mixture was centrifuged and dried. The dried product was placed in an Ar atmosphere and kept at 156 °C for 7.5 hours to obtain a sandwich-structured nitrogen-boron-oxygen co-doped graphene / sulfur / porous phosphorus-doped boron nitride (i.e., the lithium-sulfur battery cathode material in this embodiment).

[0031] from Figure 1 It can be seen that the pure phosphorus-doped boron nitride prepared in this embodiment exhibits a relatively weak diffraction peak near 2θ≈26°; while the phosphorus-doped boron nitride / multi-doped graphene composite material shows a significantly stronger diffraction peak at the same angle. This is due to the superposition of the characteristic peaks of boron nitride and graphene (whose (002) crystal plane diffraction peak is also located at 2θ≈26°), which not only reflects the composite structure of the two, but also shows that the introduction of graphene did not destroy the crystal structure of boron nitride and did not generate a new crystal phase. As can be seen from Figure 2, the porous phosphorus-doped boron nitride exhibits a porous layered structure, which lays the structural foundation for the synergistic suppression of the lithium polysulfide shuttle effect by "physical confinement + chemical adsorption"; Figure 3 shows that the nitrogen-boron-oxygen co-doped graphene has a typical two-dimensional layered morphology, which provides structural compatibility for constructing a sandwich structure with porous phosphorus-doped boron nitride. Combining the characterization results in Figures 4 and 5, no sulfur powder particles were observed on the surface of the phosphorus-doped boron nitride / multi-doped graphene, indicating that the active material sulfur has been completely filled into the porous structure of the composite material. This state not only allows the phosphorus doping sites of porous phosphorus-doped boron nitride and the multi-element doping sites of nitrogen-boron-oxygen co-doped graphene to achieve chemisorption of lithium polysulfides through polar interactions, thus forming a dual anti-shuttle barrier of "physical confinement + chemisorption", but also provides a reliable guarantee for effectively mitigating the volume expansion effect of sulfur during charging and discharging.

[0032] Example 4 The preparation method of the lithium-sulfur battery cathode material in this embodiment includes the following steps: Step 1: Dissolve 500 mg boric acid, 7.2 g urea, 600 mg diammonium hydrogen phosphate, and 15 g sodium chloride in 90 mL of distilled water and stir until completely dissolved to form solution A; evaporate solution A to dryness at 70 °C to obtain the first precursor; then place the first precursor material in an Ar atmosphere and calcine it at 800 °C for 3 hours, wherein the Ar gas flow rate is 32 cm⁻¹. 3 / min; After the product cools naturally to room temperature, the calcined product is washed with water three times, centrifuged, and dried to obtain porous phosphorus-doped boron nitride.

[0033] Step 2: Mix 200 mg of porous phosphorus-doped boron nitride with 800 mg of sulfur and 10 mL of carbon disulfide to obtain a mixed slurry. Then, evaporate the mixed slurry to dryness at 40 °C to obtain sulfur / porous phosphorus-doped boron nitride.

[0034] Step 3: 80 mg of graphene oxide powder was dispersed in a solution containing 15 mL of aniline, 700 mg of ammonium persulfate and 70 mL of distilled water. The mixture was stirred to induce a polymerization reaction. After centrifugation, washing and freeze-drying, a graphene oxide-polyaniline precursor was obtained. Then, 80 mg of the graphene oxide-polyaniline precursor was ground and mixed with 190 mg of ammonium pentaborate and calcined in Ar at 750 °C for 2 h. After the product was naturally cooled to room temperature, it was washed three times, centrifuged and dried in sequence to finally obtain nitrogen-boron-oxygen co-doped graphene.

[0035] Step 4: 200 mg of multi-co-doped graphene and 200 mg of sulfur / porous phosphorus-doped boron nitride were uniformly dispersed in a mixed solution of 30 mL of distilled water and 30 mL of ethanol. After being stirred and fully dispersed, the mixture was centrifuged and dried. The dried product was placed in an Ar atmosphere and kept at 158 ​​°C for 8 hours to obtain a sandwich-structured nitrogen-boron-oxygen co-doped graphene / sulfur / porous phosphorus-doped boron nitride (i.e., the lithium-sulfur battery cathode material in this embodiment).

[0036] To demonstrate the beneficial effects of the present invention, the inventors sampled the composite materials from Examples 1 to 4 above, prepared working electrodes respectively, and then assembled them into lithium-sulfur button batteries and pouch batteries. The electrochemical performance of the batteries was then tested. The specific experimental results are as follows: (1) Preparation of working electrode Using the lithium-sulfur battery cathode material prepared in the above embodiments as the active material, the lithium-sulfur battery cathode material was mixed with acetylene black and polyvinylidene fluoride at a mass ratio of 8:1:1 until homogeneous. Then, N-methylpyrrolidone was added dropwise and stirred until homogeneous to obtain a slurry. The amount of N-methylpyrrolidone added was such that the slurry formed had no obvious particles or lumps. The homogeneous slurry was then uniformly coated onto carbon-coated aluminum foil and dried in a vacuum drying oven at 45 °C. The sulfur content was 1.3 ± 0.1 mg / cm³. -2 .

[0037] (2) Lithium-ion battery assembly The electrode sheet prepared in step (1) above is used as the working electrode. For coin cells, the electrode sheet is punched into a 12 mm diameter disc, and a pure lithium metal disc is used as the counter / reference electrode to assemble a pouch cell; for pouch cells, the electrode sheet is cut into 60 mm × 40 mm pieces, and a copper-lithium composite strip is used as the counter / reference electrode to assemble a pouch cell. A polypropylene porous membrane Celgard 2400 is used as the separator, and the electrolyte is a mixed solvent of dioxane and dimethyl ether (v / v 1:1) dissolving 1 mol L -1 The composition consists of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, 99.95%) and 1 wt% LiNO3 as additives.

[0038] (3) Electrochemical performance testing The electrochemical performance of the lithium-sulfur battery was tested using the Wuhan Landian CT2001A battery tester. The test results are shown below. Figure 6 and Figure 7 .

[0039] Figure 6 The lithium-sulfur battery cycle performance of the phosphorus-doped boron nitride / S / multi-doped graphene composite material, phosphorus-doped boron nitride / S, and multi-doped graphene / S composite cathode materials prepared in Example 3 at 1 C is presented in a comparative manner. Figure 6 As observed in Example 2, the phosphorus-doped boron nitride / S / multi-doped graphene composite material, phosphorus-doped boron nitride / S, multi-doped graphene / S composite cathode material, and pure S electrode material exhibit discharge specific capacities of 750, 623, 440, and 95 mAh / g, respectively, after 120 cycles at a current density of 0.2C. These results indicate that the phosphorus-doped boron nitride / S / multi-doped graphene composite material exhibits the best electrical performance and demonstrate the effectiveness of this synthesis method. Furthermore, the phosphorus-doped boron nitride / S / multi-doped graphene composite materials in Examples 1, 2, and 4 exhibited better cycling performance than the phosphorus-doped boron nitride / S and multi-doped graphene / S composite cathode materials at the same current density. Moreover, the discharge specific capacities of the phosphorus-doped boron nitride / S / multi-doped graphene composite materials in Examples 1, 2, and 4 after 120 cycles at a current density of 0.2C were 732, 745, and 720 mAh / g, respectively, with the data differences in cycling performance ranging from 1% to 4%.

[0040] Figure 7 The graph shows the lithium-sulfur battery cycle performance of the phosphorus-doped boron nitride / S / multi-doped graphene composite material prepared in Example 3 at 0.15 C in a pouch cell. It can be seen that after 100 cycles, the discharge areal capacity of this electrode slowly decreases from 5.6 mAh / cm² to 4.7 mAh / cm². -2The phosphorus-doped boron nitride / S / multi-doped graphene composite materials in Examples 1, 2, and 4 exhibited discharge specific capacities of 3.9, 4.2, and 3.5 mAh / g respectively after 100 cycles at a current density of 0.15C. These pouch batteries demonstrate certain practical value. As can be seen from the above scheme, the composite material of the present invention has the characteristics of controllable morphology and structure and hierarchical porous composite structure, achieving the structure-property relationship where structure determines properties and properties determine performance. The phosphorus-doped boron nitride / S / multi-doped graphene composite material provided by the present invention can significantly improve capacity and cycle stability when used as a cathode material for lithium-sulfur batteries.

[0041] As can be seen from the above scheme, the method of the present invention is simple and low in cost. The high-performance three-dimensional porous phosphorus-doped boron nitride / multi-doped graphene composite material prepared can effectively improve the conductivity of active material sulfur and alleviate its volume expansion effect during charging and discharging, thereby achieving the goal of improving the electrochemical performance of lithium-sulfur batteries.

[0042] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a lithium-sulfur battery cathode material, characterized in that, The process includes the following: Porous phosphorus-doped boron nitride is mixed with sulfur and carbon disulfide to obtain a mixed slurry, and then the mixed slurry is evaporated to dryness to obtain sulfur / porous phosphorus-doped boron nitride; wherein the ratio of porous phosphorus-doped boron nitride, sulfur and carbon disulfide is: (50~200mg):(200~600mg):(5~10mL); Nitrogen-boron-oxygen co-doped graphene was dispersed in a sulfur-containing / porous phosphorus-doped boron nitride distilled water / ethanol mixed solution. After uniform dispersion, the solution was centrifuged and dried. The dried product was placed in a protective atmosphere and kept at 152~158℃ for 6-8 hours to obtain the lithium-sulfur battery cathode material. The ratio of nitrogen-boron-oxygen co-doped graphene, sulfur / porous phosphorus-doped boron nitride, ethanol and distilled water was (50~200mg):(50~200mg):(20~30mL):(20~30mL).

2. The method for preparing a lithium-sulfur battery cathode material according to claim 1, characterized in that, The preparation process of the porous phosphorus-doped boron nitride includes: Boric acid, urea, diammonium hydrogen phosphate, and sodium chloride are dissolved in distilled water to obtain solution A; the ratio of boric acid, urea, diammonium hydrogen phosphate, sodium chloride, and distilled water is (200~500 mg): (2.4~7.2 g): (200~600 mg): (8~15 g): (50~90 mL). The solution A was stirred and evaporated to dryness to obtain the first precursor material; The first precursor material was calcined at 700-800°C for 2-3 hours in a protective atmosphere to obtain the first calcined product. The first calcined product was washed with water and dried to obtain porous phosphorus-doped boron nitride.

3. The method for preparing a lithium-sulfur battery cathode material according to claim 2, characterized in that, The solution A was stirred and evaporated to dryness at 70-80°C to obtain the first precursor material.

4. The method for preparing a lithium-sulfur battery cathode material according to claim 2, characterized in that, The first precursor material was calcined at 700–800°C for 2–3 hours in a protective atmosphere to obtain the first calcined product. The gas flow rate of the protective atmosphere was 28–32 cm⁻¹. 3 / min.

5. The method for preparing a lithium-sulfur battery cathode material according to claim 2, characterized in that, The first calcined product was washed with water 3 to 5 times, and then dried at 80 to 100°C to remove moisture, thus obtaining porous phosphorus-doped boron nitride.

6. The method for preparing a lithium-sulfur battery cathode material according to claim 1, characterized in that, The preparation process of the nitrogen-boron-oxygen co-doped graphene includes: Graphene oxide was dispersed in a solution containing aniline, ammonium persulfate and water, and stirred to induce a polymerization reaction. After centrifugation, washing and freeze-drying, a graphene oxide-polyaniline precursor was obtained. The ratio of graphene oxide, ammonium persulfate, aniline and water was (50~80mg): (400~700mg): (10~15 mL): (10~70mL). The graphene oxide-polyaniline precursor was ground and mixed with ammonium pentaborate and calcined at 600-800℃ for 2-3 hours under a protective atmosphere. After the product was naturally cooled to room temperature, a second calcined product was obtained. The second calcined product was then washed, centrifuged and dried to finally obtain nitrogen-boron-oxygen co-doped graphene. The mass ratio of graphene oxide-polyaniline precursor to ammonium pentaborate was (50-80 mg): (150-190 mg).

7. The method for preparing a lithium-sulfur battery cathode material according to claim 6, characterized in that, The second precursor was calcined in a protective atmosphere at 600–800°C for 2–3 hours to obtain the second calcined product. The gas flow rate of the protective atmosphere was 28–32 cm⁻¹. 3 / min.

8. A method for preparing a lithium-sulfur battery cathode material according to claim 2, 6, or 7, characterized in that, The protective atmosphere is argon.

9. A lithium-sulfur battery cathode material, characterized in that, The lithium-sulfur battery cathode material is obtained by the preparation method of the lithium-sulfur battery cathode material according to any one of claims 1-8.

10. The application of the lithium-sulfur battery cathode material according to claim 9, characterized in that, The aforementioned lithium-sulfur battery cathode material is used to prepare the cathode of a lithium-sulfur battery.

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