阅读说明：本技术 多孔氮化硼纤维/还原氧化石墨烯复合型锂硫电池隔膜材料 (Porous boron nitride fiber/reduced graphene oxide composite lithium-sulfur battery diaphragm material ) 是由 薛彦明 李梦圆 唐成春 于 2020-12-14 设计创作，主要内容包括：本发明为多孔氮化硼纤维/还原氧化石墨烯复合型锂硫电池隔膜材料,该隔膜材料包括还原氧化石墨烯包覆多孔氮化硼纤维的结构,这种包覆的结构能够在缓解多硫化锂穿梭效应的同时为正极提供有效的导电通路。该复合型锂硫电池隔膜材料的制备方法包括以下步骤：将多孔氮化硼纤维与氧化石墨烯水溶液混合搅拌处理,至混合均匀得到多孔氮化硼纤维/氧化石墨烯复合材料；再在氮气条件下高温处理,高温条件为800-1500℃,得到多孔氮化硼纤维/还原氧化石墨烯复合材料；再将多孔氮化硼纤维/还原氧化石墨烯复合材料与粘结剂混合研磨后,用刮刀涂在隔膜主体上烘干,得到。在锂硫电池中表现出优异的倍率、容量和循环性能。(The invention relates to a porous boron nitride fiber/reduced graphene oxide composite lithium-sulfur battery diaphragm material, which comprises a structure that the reduced graphene oxide coats porous boron nitride fibers, and the coated structure can provide an effective conductive path for a positive electrode while relieving a lithium polysulfide shuttle effect. The preparation method of the composite lithium-sulfur battery diaphragm material comprises the following steps: mixing and stirring porous boron nitride fibers and a graphene oxide aqueous solution until the porous boron nitride fibers and the graphene oxide aqueous solution are uniformly mixed to obtain a porous boron nitride fiber/graphene oxide composite material; then carrying out high-temperature treatment under the nitrogen condition, wherein the high-temperature condition is 800-1500 ℃, and obtaining the porous boron nitride fiber/reduced graphene oxide composite material; and mixing and grinding the porous boron nitride fiber/reduced graphene oxide composite material and a binder, coating the mixture on the diaphragm main body by using a scraper, and drying to obtain the porous boron nitride/reduced graphene oxide composite material. The lithium-sulfur battery has excellent rate, capacity and cycle performance.)

1. The porous boron nitride fiber/reduced graphene oxide composite lithium-sulfur battery diaphragm material is characterized by comprising a structure that the reduced graphene oxide coats porous boron nitride fibers, and the coated structure can provide an effective conductive path for a positive electrode while relieving a lithium polysulfide shuttle effect.

2. The membrane material according to claim 1, wherein the diameter and length of the porous boron nitride fiber are 50 to 100nm and 10 to 20 μm, respectively, and the diameter of the surface pores of the porous boron nitride fiber is less than 10 nm.

3. The membrane material as claimed in claim 1, wherein the reduced graphene oxide is obtained by treating graphene oxide at a high temperature of 800-1500 ℃ under an oxygen-free condition.

4. The membrane material according to claim 3, wherein the mass ratio of the porous boron nitride fibers to the graphene oxide is 1/1-1/3.

5. A preparation method of a porous boron nitride fiber/reduced graphene oxide composite lithium-sulfur battery diaphragm material comprises the following steps: (1) preparing porous boron nitride fibers; (2) mixing and stirring porous boron nitride fibers and a graphene oxide aqueous solution until the porous boron nitride fibers and the graphene oxide aqueous solution are uniformly mixed to obtain a porous boron nitride fiber/graphene oxide composite material; (3) carrying out high-temperature treatment on the porous boron nitride fiber/graphene oxide composite material under the nitrogen condition, wherein the high-temperature condition is 800-1500 ℃, and obtaining the porous boron nitride fiber/reduced graphene oxide composite material; (4) and mixing and grinding the porous boron nitride fiber/reduced graphene oxide composite material and a binder, coating the mixture on a diaphragm main body by using a scraper, and drying to obtain the porous boron nitride fiber/reduced graphene oxide composite lithium-sulfur battery diaphragm material.

6. The preparation method according to claim 5, wherein the mixing and stirring treatment in the step (2) is carried out by: putting the porous boron nitride fiber into a graphene oxide aqueous solution, stirring for 2-5 hours, performing ultrasonic treatment for 10-30 minutes to uniformly mix the porous boron nitride fiber and the graphene oxide, and performing freeze drying to obtain a black light sample, namely the porous boron nitride fiber/graphene oxide composite material; the mass ratio of the porous boron nitride fibers to the graphene oxide is 1/1-1/3.

7. The preparation method according to claim 5, wherein the specific process of the step (3) is as follows: and (3) firing the porous boron nitride fiber/graphene oxide composite material obtained in the step (2) at a high temperature under the nitrogen condition, maintaining the nitrogen flow at 30-300mL/min, heating at 800-1000 ℃, heating for 2-4 hours, and then cooling to room temperature to obtain the porous boron nitride fiber/reduced graphene oxide composite material.

8. The preparation method according to claim 5, wherein the specific process of the step (4) is as follows: mixing and grinding the porous boron nitride fiber/reduced graphene oxide composite material obtained in the step (3) and polytetrafluoroethylene, uniformly coating the mixture on a polypropylene diaphragm by using a scraper, wherein the thickness of the scraper is 50-200 microns, and drying the mixture at 50-60 ℃ to obtain the porous boron nitride fiber/reduced graphene oxide composite lithium-sulfur battery diaphragm material; the mass ratio of the porous boron nitride fiber/reduced graphene oxide composite material to the polytetrafluoroethylene is 4/1-9/1.

9. The production method according to claim 5, wherein the porous boron nitride fiber is produced by: putting melamine and boric acid into a big beaker filled with deionized water, keeping the molar ratio of the boric acid to the melamine between 1/1 and 1/3, then putting the big beaker into a water bath kettle, wherein the water bath temperature range is 70-95 ℃, and continuously stirring to obtain a clear and transparent solution;

then putting the clear transparent solution into liquid nitrogen for rapid cooling to obtain white flocculent precipitate; then freezing and drying to obtain a white light sample, namely a porous boron nitride fiber precursor;

and finally, carrying out heat treatment on the porous boron nitride fiber precursor in nitrogen flow, wherein the heat treatment temperature range is 1000-1100 ℃, and obtaining a white light sample, namely the porous boron nitride fiber BNFs.

10. A lithium sulfur battery, characterized in that the battery comprises: the composite lithium-sulfur battery diaphragm material is provided with a high-potential positive active electrode piece, a low-potential negative lithium material and the composite lithium-sulfur battery diaphragm material which is arranged between the negative electrode piece and the positive electrode piece and is disclosed in any one of claims 1 to 9; the positive pole piece is composed of sublimed sulfur, carbon black and polytetrafluoroethylene, the carbon black is used as a conductive agent, the mass content is 15% -30%, the polytetrafluoroethylene is used as a binder, and the mass content is 5-10%; the balance being sublimed sulfur.

Technical Field

The invention relates to a lithium-sulfur battery diaphragm material, in particular to a porous boron nitride fiber/reduced graphene oxide composite lithium-sulfur battery diaphragm material.

Background

Lithium ion batteries are energy storage devices based on lithium intercalation electrochemistry. At present, the highest energy density of lithium ion batteries is close to the limit, but the lithium ion batteries cannot meet the urgent requirements of emerging electric vehicles, hybrid electric vehicles and next-generation portable electronic devices, and an advanced battery system with higher energy density is needed. Among the alternative battery technologies, lithium sulfur batteries are due to their high theoretical energy density (2500Wh kg)^-1) Low cost, small environmental impact, etc., and is considered to be one of the most promising next-generation energy storage systems.

Despite these advantages, practical application of lithium sulfur batteries is hampered by problems including low conductivity of sulfur and lithium sulfide, shuttling effect of polysulfides (LiPSs), and large volume change of sulfur during cycling. Among these obstacles, the shuttle effect is considered to be a major obstacle limiting the commercial application of lithium sulfur batteries. Therefore, in the last decade, many efforts have been made to solve this complex problem. Due to the high conductivity and large specific surface area of sulfur, infiltration of sulfur into various carbon nanostructures is one of the most common strategies. However, since the physical entrapment of polar LiPSs by non-polar carbon is weak, the shuttle motion of polysulfides is difficult to suppress, especially over long cycle times.

Disclosure of Invention

In view of the above problems in the prior art, the present invention aims to provide a porous boron nitride fiber/reduced graphene oxide composite lithium-sulfur battery separator material by combining the advantages of porous Boron Nitride Fibers (BNFs) and reduced graphene oxide (rGO). This coated porous structure helps to immobilize soluble polysulfides in the positive region, preventing their diffusion to the negative electrode, reducing the loss of active sulfur, and thus improving cycling stability. Meanwhile, compared with boron nitride nanosheets, the porous boron nitride fibers have a large number of active functional groups, have extremely strong adsorption capacity on polysulfide, and can form a wrapping structure with reduced graphene oxide, so that the shuttle effect of polysulfide can be effectively inhibited. And the reduced graphene oxide can construct an efficient and stable electronic conducting channel, so that the conductivity of the anode is obviously improved, and the structural integrity is facilitated. The coated porous diaphragm material can improve the charge-discharge specific capacity, the thermal stability and the cycle life of the lithium-sulfur battery.

In order to achieve the purpose, the technical scheme of the invention is as follows:

the porous boron nitride fiber/reduced graphene oxide composite lithium-sulfur battery diaphragm material is characterized by comprising a structure that the reduced graphene oxide coats porous boron nitride fibers, and the coated structure can provide an effective conductive path for a positive electrode while relieving a lithium polysulfide shuttle effect.

The diameter and the length of the porous boron nitride fiber are respectively 50-100 nm and 10-20 mu m, and the diameter of the surface pore of the porous boron nitride fiber is less than 10 nm. The reduced graphene oxide is obtained by processing graphene oxide at a high temperature of 800-1500 ℃ under an oxygen-free condition, has high crystallinity, can coat porous boron nitride fibers, and does not cause activity reduction of the porous boron nitride fibers.

The mass ratio of the porous boron nitride fibers to the graphene oxide is 1/1-1/3, the completely reduced graphene oxide is creatively selected and prepared, the conductivity of the graphene can be improved, the using amount of the graphene is increased, the using amount of the porous boron nitride fibers is reduced, and the improvement of the overall conductivity is guaranteed.

The preparation method of the composite lithium-sulfur battery diaphragm material comprises the following steps: (1) preparing porous boron nitride fibers; (2) mixing and stirring porous boron nitride fibers and a graphene oxide aqueous solution until the porous boron nitride fibers and the graphene oxide aqueous solution are uniformly mixed to obtain a porous boron nitride fiber/graphene oxide composite material; (3) carrying out high-temperature treatment on the porous boron nitride fiber/graphene oxide composite material under the nitrogen condition, wherein the high-temperature condition is 800-1500 ℃, and obtaining the porous boron nitride fiber/reduced graphene oxide composite material; (4) and mixing and grinding the porous boron nitride fiber/reduced graphene oxide composite material and a binder, coating the mixture on a polypropylene diaphragm by using a scraper, and drying to obtain the porous boron nitride fiber/reduced graphene oxide composite lithium-sulfur battery diaphragm material.

The concrete process of the mixing and stirring treatment in the step (2) is as follows: putting the porous boron nitride fiber into a graphene oxide aqueous solution, stirring for 2-5 hours, performing ultrasonic treatment for 10-30 minutes to uniformly mix the porous boron nitride fiber and the graphene oxide, and performing freeze drying to obtain a black light sample, namely the porous boron nitride fiber/graphene oxide composite material; the mass ratio of the porous boron nitride fibers to the graphene oxide is 1/1-1/3.

The specific process of the step (3) is as follows: and (3) firing the porous boron nitride fiber/graphene oxide composite material obtained in the step (2) at a high temperature of 800-1000 ℃ under the condition of nitrogen (the nitrogen flow is maintained at 30-300mL/min), heating for 2-4 hours, and then cooling to room temperature to obtain the porous boron nitride fiber/reduced graphene oxide composite material.

The specific process of the step (4) is as follows: mixing and grinding the porous boron nitride fiber/reduced graphene oxide composite material obtained in the step (3) and polytetrafluoroethylene, uniformly coating the mixture on a polypropylene diaphragm by using a scraper, wherein the thickness of the scraper is 50-200 microns, and drying the mixture at 50-60 ℃ to obtain the porous boron nitride fiber/reduced graphene oxide composite lithium-sulfur battery diaphragm material; the mass ratio of the porous boron nitride fiber/reduced graphene oxide composite material to polytetrafluoroethylene (binder) is 4/1-9/1, and the lithium-sulfur battery obtained under the condition has the best comprehensive performance including specific capacity and cycling stability.

The preparation steps of the porous boron nitride fiber are as follows: putting melamine (melamine) and boric acid into a big beaker filled with deionized water, keeping the molar ratio of the boric acid to the melamine between 1/1 and 1/3, then putting the beaker into a water bath kettle, wherein the water bath temperature interval is 70-95 ℃, and continuously stirring to obtain a clear and transparent solution;

then putting the clear transparent solution into liquid nitrogen for rapid cooling to obtain white flocculent precipitate; then freezing and drying to obtain a white light sample, namely a porous boron nitride fiber precursor;

and finally, carrying out heat treatment on the porous boron nitride fiber precursor in nitrogen flow (the nitrogen flow is maintained at 30-300mL/min), wherein the heat treatment temperature range is 1000-1100 ℃, and obtaining a white light sample, namely the porous Boron Nitride Fibers (BNFs).

And (2) assembling the prepared composite lithium-sulfur battery diaphragm material into a lithium-sulfur battery, wherein the assembled battery comprises: the lithium-sulfur battery diaphragm material comprises a high-potential positive active electrode piece, a low-potential negative lithium material and a composite lithium-sulfur battery diaphragm material which is arranged between the negative electrode piece and the positive electrode piece; the positive pole piece is composed of sublimed sulfur, carbon black and polytetrafluoroethylene, the carbon black is used as a conductive agent, the mass content is 15% -30%, the polytetrafluoroethylene is used as a binder, and the mass content is 5-10%; the balance being sublimed sulfur.

Compared with the prior art, the invention has the beneficial effects that:

the lithium-sulfur battery obtained by the invention has the following characteristics:

the porous structure of the porous boron nitride fiber in the composite lithium-sulfur battery diaphragm material is beneficial to fixing soluble polysulfide in a positive electrode area, preventing the soluble polysulfide from diffusing to a negative electrode and reducing the loss of active sulfur; the porous boron nitride fiber is wrapped by the reduced graphene oxide with high crystallinity, so that the stability of an electronic conductive channel constructed by the reduced graphene oxide can be effectively kept, meanwhile, the reduced graphene oxide constructs a high-efficiency and stable electronic conductive channel by using the excellent conductivity of the reduced graphene oxide, the conductivity of the anode is obviously improved on the whole, the structural integrity is facilitated, the adsorption performance of the porous boron nitride fiber is good, the combination of the porous boron nitride fiber and the reduced graphene oxide can obviously improve the comprehensive performance of the battery, and the shuttle effect of polysulfide is effectively inhibited.

Porous boron nitride fibers/reduced graphene oxide in the composite lithium-sulfur battery diaphragm material are in a coated structure, and by utilizing the advantages of strong adsorption effect of the porous boron nitride fibers, good conductivity of the reduced graphene oxide and the like, the porous boron nitride fibers and the reduced graphene oxide are compounded to generate a synergistic effect, so that soluble polysulfide can be favorably fixed in an anode region, the soluble polysulfide can be prevented from being diffused to a cathode, the loss of active sulfur is reduced, and the cycling stability is improved.

The invention has the remarkable advantages that:

1. according to the invention, the porous boron nitride fiber and the reduced graphene oxide are innovatively compounded, and the synergistic property of the porous boron nitride fiber and the reduced graphene oxide shows excellent multiplying power, capacity and cycle performance in the lithium-sulfur battery.

2. The product obtained by the preparation method is a porous boron nitride fiber/reduced graphene oxide composite lithium-sulfur battery diaphragm material; the diffraction peak of the XRD spectrogram (figure 1) is clear, the XRD spectrogram is staggered boron nitride, and as seen from an SEM (figure 2), the porous boron nitride fiber/reduced graphene oxide composite material is obtained by high-temperature reduction under the condition of nitrogen after mixing and stirring, and shows a structure that the porous boron nitride fiber is coated by the reduced graphene oxide, and the porous boron nitride fiber/reduced graphene oxide structure can promote the conversion of lithium polysulfide, so that the loss of polysulfide is reduced more effectively. As seen from the TEM image in fig. 3, the porous boron nitride fiber/reduced graphene oxide shows a coated structure, and the coated structure formed by the high-temperature compounding of the porous boron nitride fiber and the reduced graphene oxide has the advantages of high specific surface area and strong adsorption of the porous boron nitride fiber and good conductivity of the reduced graphene oxide, so that a large amount of electrolyte is absorbed by the coated structure, and polysulfide dissolved in the electrolyte is positioned in the positive electrode region and prevented from diffusing to the lithium negative electrode, and the loss of active sulfur is reduced, thereby improving the cycling stability of the battery and increasing the specific capacity of the battery.

3. The product obtained by the method of the invention has high specific capacity and good cyclic stability, and as shown by a cyclic voltammetry curve chart (figure 4), two reduction peaks of the porous boron nitride fiber/reduced graphene oxide are about 2.3V and 2.0V, which are long-chain Li respectively₂S_x(4. ltoreq. x. ltoreq.8) and L which is subsequently converted into the solid state_i2S₂/Li₂And S. The oxidation peak is about 2.41V, corresponding to polysulfide and last S₈The reaction of (1). The two reduction peaks of the voltammograms of the second and third cycles are absent compared to the voltammogram of the first cycleA significant displacement. While the small shift of the oxidation peak from 2.4 to 2.38 is due to the activation process caused by the first-cycle redox reaction. In the subsequent cycle, the peak position of the porous boron nitride fiber/reduced graphene oxide is kept unchanged, and no obvious position change is generated, so that good reversibility is shown. From the first charge-discharge curve (fig. 5), the porous boron nitride fiber/reduced graphene oxide battery still maintains a stable charge-discharge platform at a high rate of 4C, and has very excellent capacity at different rates. From the charge-discharge cycle performance diagram (fig. 6), the current density is from 0.1 to 4C, and after multiple cycles, the stable specific discharge capacity is still maintained, so that the good stability of the porous boron nitride fiber/reduced graphene oxide battery under different multiplying powers is reflected.

4. According to the preparation method, the stirring and ultrasonic time is controlled according to the structure of the porous boron nitride fiber/graphene oxide, so that the materials can be uniformly mixed under the condition of not damaging the structure of the materials. Researches show that the diameter, the length and the pore size of the porous boron nitride fiber not only influence the adsorption performance of the porous boron nitride fiber on polysulfide, but also influence the effective coating of the porous boron nitride fiber by reduced graphene oxide. Researches find that the coating structure of the porous boron nitride fiber/reduced graphene oxide is matched to adjust different proportions of the porous boron nitride fiber and the reduced graphene oxide, so that the subsequent lithium-sulfur battery diaphragm material with excellent performance can be further prepared.

5. The raw materials adopted by the invention are boric acid, melamine and graphene oxide, which belong to common chemical raw materials in industrial production, are cheap, easily available, nontoxic, green and environment-friendly, and reduce the cost of the product; the production process is simple, and is a process technology of the lithium-sulfur battery which can be produced in a large scale and has good cycling stability; will help to further development of lithium sulfur batteries.

Drawings

The invention is further described with reference to the following figures and detailed description.

Fig. 1 is an X-ray diffraction pattern of the porous boron nitride fiber/reduced graphene oxide of example 1.

Fig. 2 is a scanning electron microscope image of the porous boron nitride fiber/reduced graphene oxide of example 1.

Fig. 3 is a transmission electron microscope image of the porous boron nitride fiber/reduced graphene oxide of example 1.

Fig. 4 is a cyclic voltammogram of the porous boron nitride fiber/reduced graphene oxide of example 1.

Fig. 5 is a first charge-discharge curve diagram of the porous boron nitride fiber/reduced graphene oxide in example 1 at different rates.

Fig. 6 is a graph of charge and discharge cycle performance of the porous boron nitride fiber/reduced graphene oxide of example 1 at different rates.

Detailed Description

The present invention is further explained with reference to the following examples and drawings, but the scope of the present invention is not limited thereto.

Example 1.

Firstly, preparing porous boron nitride fiber:

(1) putting 3.78g of melamine (melamine) and 3.71g of boric acid into a large beaker filled with 200 ml of deionized water, then putting the beaker into a water bath kettle, wherein the water bath temperature is 90 ℃, and continuously stirring to obtain a clear and transparent solution;

(2) putting the clear and transparent solution obtained by the reaction in the step (1) into liquid nitrogen for rapid cooling to obtain white flocculent precipitate;

(3) freezing and drying the white flocculent precipitate obtained in the step (2) to obtain a white light sample, namely a porous boron nitride fiber precursor;

(4) carrying out heat treatment on the sample obtained in the step (3) in nitrogen flow (the nitrogen flow is maintained at 60ml/min), wherein the heat treatment temperature interval is 1050 ℃, and obtaining a white light sample, namely the porous boron nitride fiber;

the average values of the diameter and the length of the porous boron nitride fiber are 60nm and 10 mu m respectively, and the diameter of the surface pores of the porous boron nitride fiber is less than 10 nm.

Secondly, putting the porous boron nitride fiber obtained in the first step into a graphene oxide aqueous solution, stirring for 3 hours, performing ultrasonic treatment for 30 minutes to uniformly mix the porous boron nitride fiber and the graphene oxide, adding 80mL of graphene oxide aqueous solution (5mg/mL) into every 200mg of the porous boron nitride fiber, wherein the mass ratio of the porous boron nitride fiber to the graphene oxide is 1/2, and freezing and drying to obtain a black light sample, namely the porous boron nitride fiber/graphene oxide composite material;

and thirdly, firing the porous boron nitride fiber/graphene oxide composite material obtained in the second step at a high temperature under the condition of nitrogen (the nitrogen flow is maintained at 60ml/min), heating for 2 hours at the temperature interval of 900 ℃, and cooling to room temperature to obtain the porous boron nitride fiber/reduced graphene oxide composite material.

And step four, mixing and grinding the porous boron nitride fiber/reduced graphene oxide composite material obtained in the step three and polytetrafluoroethylene in a mass ratio of 9: uniformly coating the porous boron nitride fiber/reduced graphene oxide composite lithium-sulfur battery diaphragm material on a polypropylene diaphragm by using a scraper, wherein the thickness of the polypropylene diaphragm is 200 microns, and drying the polypropylene diaphragm in a drying oven at 60 ℃ to obtain the porous boron nitride fiber/reduced graphene oxide composite lithium-sulfur battery diaphragm material.

The lithium-sulfur battery assembled by using the porous boron nitride fiber/reduced graphene oxide composite lithium-sulfur battery diaphragm material comprises: the active positive pole piece of the positive pole with high potential (composed of sublimed sulfur, carbon black and polytetrafluoroethylene, the proportion of the sublimed sulfur, the carbon black and the polytetrafluoroethylene is 6: 3: 1); a low potential negative electrode lithium material; and the porous boron nitride fiber/reduced graphene oxide composite lithium-sulfur battery diaphragm is arranged between the negative pole piece and the positive pole piece.

The specific capacity results of charging and discharging obtained by the first charging and discharging experiment under different multiplying powers are shown in fig. 6, and the first charging and discharging curve chart shows that the porous boron nitride fiber/reduced graphene oxide battery still maintains a stable charging and discharging platform under the high multiplying power of 4C, and has very excellent capacity under different multiplying powers.

Example 2.

The steps of this example are the same as example 1, except that the thickness of the porous boron nitride fiber/reduced graphene oxide composite material coated on the polypropylene diaphragm is 50 μm. The lithium sulfur battery was charged and discharged at a rate of 1C.

Example 3.

The steps of this example are the same as example 2, except that the mass ratio of the porous boron nitride fibers to the graphene oxide is 1/3. And the prepared lithium-sulfur battery is charged and discharged at the rate of 1C.

Example 4.

The steps of this example are the same as example 1, except that the lithium sulfur battery was charged and discharged at a rate of 1C.

Example 5.

The procedure of this example was the same as example 3, except that the lithium sulfur battery was charged and discharged at a rate of 4C.

Example 6.

The steps of this example are the same as example 1, except that the thickness of the porous boron nitride fiber/reduced graphene oxide composite material coated on the polypropylene diaphragm is 100 μm. The lithium sulfur battery was charged and discharged at a rate of 1C.

Example 7.

The diameter and length of the porous boron nitride crude fiber (h-BNFs) used in this example were 2.5 μm and 100 μm, respectively, and the diameter of the surface pores of the porous boron nitride crude fiber was greater than 20 nm. The other process parameters were the same as in example 1.

The lithium sulfur battery is charged and discharged at a rate of 1C.

Compared with example 1, the diameter of the porous boron nitride crude fiber is too large, so that the reduced graphene oxide can not wrap the porous boron nitride crude fiber, and the overall conductivity and the adsorption capacity to polysulfide of the material are reduced. Under the multiplying power of 1C, the overall capacity of the material is not high, the attenuation is obvious along with the increase of the cycle number, and the effect is poorer on fine fibers.

Example 8.

The steps of this example are the same as example 7, except that the mass ratio of the porous boron nitride crude fiber to the graphene oxide is 1/1. Due to the increase of the content of the porous boron nitride crude fibers, the reduced graphene oxide cannot effectively wrap the porous crude boron nitride fibers, a large amount of porous boron nitride crude fibers are independently attached to the diaphragm, the alternating current impedance of the battery is obviously increased, and the ionic conductivity of the battery is reduced. Under the multiplying power of 1C, the capacity of the battery is obviously reduced, and the attenuation is obvious along with the increase of the cycle number. But the performance is better compared with the boron nitride sheet structure under the same condition.

Example 9.

The steps of this example are the same as example 1, except that the mass ratio of the porous boron nitride fibers to the graphene oxide is 2/1. And the prepared lithium-sulfur battery is charged and discharged at the rate of 1C.

Compared with example 1, the addition of too much boron nitride fiber results in the decrease of conductivity, and the capacity and the cycle stability of the battery are obviously reduced, but the boron nitride fiber is superior to the separator material prepared by using graphene oxide without reducing the graphene oxide under the same conditions. The reduced graphene oxide has the function of providing a channel for electron movement in the electrode, the appropriate content of the graphene oxide can obtain higher discharge capacity and better cycle performance, and if the content is too low, the number of electron conductive channels is small, so that the large-current charge and discharge are not facilitated; too high reduces the relative content of boron nitride fibers, which cannot effectively inhibit the shuttling effect of lithium polysulfide and reduces the battery capacity.

Fig. 6 is a comparison graph of charge and discharge cycle performance of porous boron nitride fibers/reduced graphene oxide at different rates. In fig. 6, a is a comparative graph of the charge and discharge performance of the porous boron nitride fiber/reduced graphene oxide prepared in example 1 after 320 discharge cycles at 1C, in which a white circle curve is a coulomb efficiency and cycle number curve, and a black circle curve is a battery capacity and cycle number curve. In fig. 6, b is a comparison graph of the charge and discharge performance of the porous boron nitride fiber/reduced graphene oxide in example 1 after 400 discharge cycles at 4C. The current density is 1-4C, and after multiple cycles, the stable specific discharge capacity is still maintained, so that the good stability of the porous boron nitride fiber/reduced graphene oxide battery under different multiplying powers is reflected.

The foregoing is merely illustrative of the principles and features of the invention and is not intended to limit the scope of the claims which follow.

Nothing in this specification is said to apply to the prior art.