CN111653783B - Porous boron nitride fiber/multiwalled carbon nanotube/sulfur composite lithium-sulfur battery positive electrode material - Google Patents

Porous boron nitride fiber/multiwalled carbon nanotube/sulfur composite lithium-sulfur battery positive electrode material Download PDF

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CN111653783B
CN111653783B CN202010537770.5A CN202010537770A CN111653783B CN 111653783 B CN111653783 B CN 111653783B CN 202010537770 A CN202010537770 A CN 202010537770A CN 111653783 B CN111653783 B CN 111653783B
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boron nitride
porous boron
sulfur
walled carbon
nitride fiber
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CN111653783A (en
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唐成春
李梦圆
薛彦明
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Hebei University of Technology
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    • H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/628—Inhibitors, e.g. gassing inhibitors, corrosion inhibitors
    • H—ELECTRICITY
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    • H—ELECTRICITY
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    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
    • H01M2004/028—Positive electrodes
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Abstract

The invention relates to a porous boron nitride fiber/multi-walled carbon nanotube/sulfur composite lithium-sulfur battery anode material, which comprises a structure that porous boron nitride fibers and multi-walled carbon nanotubes are wound mutually, and the wound structure can wrap and wind sublimed sulfur. 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 intertwined porous structure can effectively absorb electrolyte, and is helpful for fixing soluble polysulfide in a positive electrode area, thereby preventing the soluble polysulfide from diffusing to a negative electrode, reducing the loss of active sulfur and improving the cycle stability. Meanwhile, the agglomeration of multi-wall carbon nano tubes can be effectively inhibited, the vacancy defects of the porous boron nitride fibers have great influence on the local electronic structure, atoms near the vacancies are very active, and polysulfide near the vacancies can be effectively catalyzed from Li2S8To Li2S is converted, so that the shuttle effect of polysulfide can be effectively inhibited.

Description

Porous boron nitride fiber/multiwalled carbon nanotube/sulfur composite lithium-sulfur battery positive electrode material
Technical Field
The invention relates to a lithium-sulfur battery positive electrode material, in particular to a porous boron nitride fiber/multi-walled carbon nanotube/sulfur composite lithium-sulfur battery positive electrode material.
Background
The lithium ion battery is an energy storage device based on lithium intercalation electrochemistry, and has always led to the market of portable electronic and electrical equipment energy sources since the successful introduction of the lithium ion battery in the 90 s of the 20 th century. 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. Following the development of the era, the research and development of lithium-sulfur batteries lead people to see the advancing direction of future advanced energy, and are hot topics in the future energy field.
Lithium-sulfur batteries, which use abundant elemental sulfur as the positive electrode material, are distinguished from intercalation electrochemistry of lithium-ion batteries, which involve multiple electron transfer electrochemistry, according to S8+16Li++16e-＝8Li2The conversion reaction of S has a theoretical specific capacity of 1675mAh g-1The average voltage of the battery is 2.2V vs Li+Specific energy of/Li, Li-S batteryTheoretical density value is 2600Wh kg-1This is a commercial lithium ion battery (LiCoO)2Graphite cell 387Wh kg-1) Five times. It is estimated by lithium sulfur battery manufacturers that future lithium sulfur batteries are expected to have practical energy densities of 400--1Which is twice as high as the most advanced lithium ion battery at present.
However, lithium sulfur batteries have problems of volume expansion and so-called "shuttle effect", which hinder practical use of the lithium sulfur batteries. The shuttle effect is generated by diffusion of soluble polysulfide intermediate products to the negative electrode through the film, which destroys the solid electrolyte interface film (SEI) of the negative electrode, resulting in degradation of the electrochemical performance of the battery.
To overcome the above obstacles, fibrous and porous materials have attracted attention. The carbon fiber and porous carbon material anode material can be applied to carbon fiber and porous carbon material anode materials due to the fact that the fiber and porous material can adapt to the volume change of the anode and improve the adsorption performance of polysulfide, and the ZHao et al (ZHao, D.Qin, S.Wang, G.Chen, Z.Li, electrochim. acta 2014,127, 123-131.) and the ZHou et al (G.ZHou, L.Li, C.Ma, S.Wang, Y.Shi, N.Koratkar, W.Ren, F.Li, H.M.Cheng, Nano Energy 2015,11, 356-365) can adapt the carbon fiber and porous carbon material anode materials, but the process is complex, the material cost is high, the shuttle effect of polysulfide is limited, and the carbon material is easy to agglomerate to block the electronic conduction channel.
Disclosure of Invention
In view of the above problems in the prior art, the present invention aims to provide a porous boron nitride fiber/multi-walled carbon nanotube/sulfur composite type positive electrode material for lithium-sulfur batteries, which combines the advantages of porous Boron Nitride Fibers (BNFs) and multi-walled Carbon Nanotubes (CNTs). The intertwined porous structure can effectively absorb electrolyte, and is helpful for fixing soluble polysulfide in a positive electrode area, thereby preventing the soluble polysulfide from diffusing to a negative electrode, reducing the loss of active sulfur and improving the cycle stability. Meanwhile, compared with boron nitride nanosheets, the porous boron nitride fibers have more active functional groups, so that the porous boron nitride fibers have stronger adsorption capacity on polysulfide, and secondly, compared with carbon fibers, the porous boron nitride fibers can effectively inhibit the agglomeration of multi-wall carbon nanotubes, and the vacancy of the porous boron nitride fibers is lackThe trap has great influence on the local electronic structure, atoms near the vacancy are very active, and the polysulfide near the vacancy can be effectively catalyzed from Li2S8To Li2S is converted, so that the shuttle effect of polysulfide can be effectively inhibited. And the multi-walled carbon nano tube can construct an efficient and stable electronic conductive channel, the conductivity of the anode is obviously improved, and the structural integrity is facilitated. The intertwined porous positive electrode 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 positive electrode material is characterized by comprising a structure in which porous boron nitride fibers and multi-walled carbon nanotubes are wound with each other, and the wound structure can wrap sublimed sulfur.
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 mass ratio of the porous boron nitride fibers to the multi-walled carbon nanotubes is 1/1-1/4, and the sulfur-carrying amount of the anode material is 60-70%.
The porous boron nitride fibers are doped with doping substances such as red phosphorus or metal oxide, and the doping substances are ground and mixed with the porous boron nitride fibers and then mixed and ground with the multi-walled carbon nanotubes; the metal oxide is ferric oxide, nickel oxide and the like.
The preparation method of the composite lithium-sulfur battery positive electrode material comprises the following steps: (1) preparing porous boron nitride fibers; (2) mixing and grinding the porous boron nitride fiber and the multi-walled carbon nanotube until the porous boron nitride fiber and the multi-walled carbon nanotube are uniformly mixed to obtain a porous boron nitride fiber/multi-walled carbon nanotube composite material; (3) and mixing and grinding the porous boron nitride fiber/multi-walled carbon nanotube composite material and sublimed sulfur uniformly, and then carrying out high-temperature treatment to obtain the porous boron nitride fiber/multi-walled carbon nanotube/sulfur composite lithium-sulfur battery cathode material.
The specific process of the mixed grinding treatment in the step (2) is as follows: manually grinding the porous boron nitride fiber and the multi-walled carbon nano tube in advance, then placing the ground porous boron nitride fiber and the multi-walled carbon nano tube into a planetary ball mill, adding a proper amount of deionized water, and then carrying out high-speed ball milling for 2-5 hours at the ball milling rotation speed of 200-350rpm so as to uniformly mix the porous boron nitride fiber and the multi-walled carbon nano tube to obtain a black light sample, namely the porous boron nitride fiber/multi-walled carbon nano tube composite material; the mass ratio of the porous boron nitride fibers to the multi-walled carbon nanotubes is 1/1-1/4.
The specific process of the step (3) is as follows: manually mixing and grinding the porous boron nitride fiber/multi-walled carbon nanotube composite material obtained in the step (2) and sublimed sulfur for 0.5-1 hour, wherein the sulfur-carrying amount of the anode material is controlled to be 60% -70%; and then, placing the materials after manual mixing and grinding into a high-pressure kettle for high-temperature treatment, heating at the temperature of 155-160 ℃ for 10-20 hours, and then cooling to room temperature to obtain the porous boron nitride fiber/multi-walled carbon nanotube/sulfur composite lithium-sulfur battery cathode material.
The preparation steps of the porous boron nitride fiber are as follows: putting melamine (melamine) and boric acid into a large beaker containing 200 ml of deionized water and 500 ml of 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, keeping the temperature of the water bath between 70 and 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 cathode material into a lithium-sulfur battery, wherein the assembled battery comprises: the lithium ion battery comprises a high-potential positive active electrode plate, a low-potential negative lithium material and a porous polypropylene diaphragm arranged between the negative electrode plate and the positive electrode plate; the positive pole piece is composed of a porous boron nitride fiber/multi-walled carbon nanotube/sulfur composite positive pole material, carbon black and polytetrafluoroethylene, wherein the carbon black is used as a conductive agent and accounts for 15-30% by mass, and the polytetrafluoroethylene is used as a binder and accounts for 5-10% by mass; the balance is anode material. The battery assembled by the application can use less active anode materials, and more conductive agents are added, so that the conductivity of the material is obviously improved, and the cycling stability and the specific capacity of the battery are improved to a certain extent compared with the battery with the original proportion.
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 anode material is beneficial to the infiltration of electrolyte and relieves the volume expansion of sulfur in the charging and discharging processes. The porous boron nitride fiber can effectively inhibit the agglomeration phenomenon of the multi-wall carbon nano-tube, keep the stability of an electronic conducting channel constructed by the multi-wall carbon nano-tube, and simultaneously, the vacancy defect of the porous boron nitride fiber can effectively catalyze polysulfide near the vacancy from Li2S8To Li2S is converted, so that the shuttle effect of polysulfide can be effectively inhibited; in addition, the multi-walled carbon nanotube in the composite lithium-sulfur battery cathode material utilizes the excellent conductivity of the multi-walled carbon nanotube to construct a high-efficiency and stable electronic conductive channel, so that the conductivity of the cathode is obviously improved on the whole, and the structural integrity is facilitated.
The porous boron nitride fiber/multi-walled carbon nanotube in the composite lithium-sulfur battery positive electrode material presents a mutual winding structure, and by utilizing the advantages of strong adsorption effect of the porous boron nitride fiber, good conductivity of the multi-walled carbon nanotube and the like, the composite porous boron nitride fiber/multi-walled carbon nanotube composite material can effectively absorb electrolyte and is beneficial to fixing soluble polysulfide in a positive electrode area through a synergistic effect generated by compounding the porous boron nitride fiber and the multi-walled carbon nanotube, so that the soluble polysulfide is prevented from being diffused to a negative electrode, the loss of active sulfur is reduced, and the cycling stability is improved.
The invention has the remarkable advantages that:
1. the invention innovatively discovers the adsorption effect of the porous boron nitride fibers on polysulfide and the catalytic effect of vacancy defects of the porous boron nitride fibers on polysulfide conversion, innovatively compounds the porous boron nitride fibers and the multi-wall carbon nano tubes, and the synergistic property of the porous boron nitride fibers and the multi-wall carbon nano tubes shows excellent multiplying power, capacity and cycle performance in lithium-sulfur batteries.
2. The product obtained by the preparation method is a porous boron nitride fiber/multi-walled carbon nanotube/sulfur composite lithium-sulfur battery positive electrode material; the diffraction peak of the XRD spectrogram (figure 1) is clear, the porous boron nitride fiber/multi-wall carbon nano tube is staggered boron nitride, and as seen from the SEM image (figure 2), the porous boron nitride fiber/multi-wall carbon nano tube after mixed grinding presents a mutual winding structure, and the structure of the boron nitride fiber/multi-wall carbon nano tube can wrap and wind sublimed sulfur, so that the combination of the boron nitride fiber/multi-wall carbon nano tube is tighter, and the loss of polysulfide is more effectively reduced. As seen from the TEM image in fig. 3, a large number of macropores and mesopores exist in the porous boron nitride fiber, and the macropores and mesopores are favorable for the infiltration of electrolyte, and the volume expansion of sulfur in the charging and discharging process is relieved; as seen from the TEM image in fig. 4, the porous boron nitride fiber/multi-walled carbon nanotube exhibits an intertwined structure, and the intertwined structure formed by utilizing the advantages of the high specific surface area and strong adsorption of the porous boron nitride fiber and the good conductivity of the multi-walled carbon nanotube and the synergistic effect generated by the combination of the two enables a large amount of electrolyte to be absorbed, so as to position the polysulfide dissolved in the electrolyte in the positive electrode region, prevent the polysulfide from diffusing to the lithium negative electrode, reduce the loss of active sulfur, improve the cycling stability of the battery, and increase 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 5), two reduction peaks of the porous boron nitride fiber/multi-wall carbon nano tube/sulfur are about 2.3 and 2.0V, which are long-chain Li respectively2Sx(4. ltoreq. x. ltoreq.8) and L which is subsequently converted into the solid statei2S2/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 were not significantly shifted compared to the voltammogram of the first cycle. While the small shift of the oxidation peak from 2.4 to 2.38 is due to the first-cycle redox reactionAnd (4) activating. In the subsequent circulation, the peak position of the porous boron nitride fiber/multi-wall carbon nano tube/sulfur is kept unchanged, no obvious position change is generated, and good reversibility is shown. From the first charge-discharge curve (fig. 6), the porous boron nitride fiber/multiwall carbon nanotube/sulfur battery still maintains a stable charge-discharge platform at a high rate of 1C, and has very excellent capacity at different rates. From the charge-discharge cycle performance diagram (fig. 7), the current density is 0.1-1C, and after multiple cycles, the stable specific discharge capacity is still maintained, so that the good stability of the porous boron nitride fiber/multi-walled carbon nanotube/sulfur battery at different multiplying powers is reflected, and compared with the multi-walled carbon nanotube/sulfur battery, the porous boron nitride fiber/multi-walled carbon nanotube/sulfur battery has higher specific capacity and cycle stability at different multiplying powers.
4. The preparation method controls the rotating speed and time of ball milling according to the structure of the porous boron nitride fiber/multi-walled carbon nano tube, and can ensure that the materials are 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 inhibition effect on the agglomeration phenomenon of multi-wall carbon nanotubes. Researches find that the different proportions of the porous boron nitride fiber and the multi-walled carbon nanotube are adjusted by matching with the mutual winding structure of the porous boron nitride fiber and the multi-walled carbon nanotube, so that the subsequent lithium-sulfur battery active positive electrode material with excellent performance is further prepared.
5. The raw materials adopted by the invention are boric acid, melamine and multi-walled carbon nanotubes, 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 porous boron nitride fibers/multi-walled carbon nanotubes and porous boron nitride fibers/multi-walled carbon nanotubes/sulfur in example 1.
FIG. 2 is a scanning electron microscope image of the porous boron nitride fiber/multi-walled carbon nanotube of example 1.
FIG. 3 is a transmission electron microscope photograph of the porous boron nitride fiber of example 1.
FIG. 4 is a TEM image of the porous boron nitride fiber/multi-walled carbon nanotube in example 1.
FIG. 5 is a cyclic voltammogram of porous boron nitride fibers/multi-walled carbon nanotubes/sulfur in example 1.
Fig. 6 is a first charge-discharge curve diagram of the porous boron nitride fiber/multi-walled carbon nanotube/sulfur in example 1 at different rates.
Fig. 7 is a graph of the charge-discharge cycle performance of the porous boron nitride fiber/multi-walled carbon nanotube/sulfur and the porous boron nitride coarse fiber/multi-walled carbon nanotube/sulfur in examples 1, 2, 3, 4, 5, 6, 7, 8 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, manually grinding the porous boron nitride fibers and the multi-walled carbon nanotubes obtained in the first step for half an hour in advance, then putting the porous boron nitride fibers and the multi-walled carbon nanotubes into a planetary ball mill, adding a proper amount of deionized water, and then carrying out high-speed ball milling for 4 hours at the rotating speed of 300rpm, so that the porous boron nitride fibers and the multi-walled carbon nanotubes are uniformly mixed, wherein every 200mg of the porous boron nitride fibers and the multi-walled carbon nanotubes are added with 1mL of deionized water, and the mass ratio of the porous boron nitride fibers to the multi-walled carbon nanotubes is 1/2, so that a black light sample is obtained, namely the porous boron nitride fiber/multi-walled carbon nanotube composite material;
and thirdly, mixing and grinding the porous boron nitride fiber/multi-walled carbon nanotube composite material obtained in the second step and sublimed sulfur, wherein the mass ratio of the porous boron nitride fiber/multi-walled carbon nanotube composite material to the sublimed sulfur is 2/3, then placing the porous boron nitride fiber/multi-walled carbon nanotube composite material into a high-pressure kettle, heating the high-pressure kettle for 12 hours at the temperature of 155 ℃, and cooling the high-pressure kettle to room temperature to obtain the porous boron nitride fiber/multi-walled carbon nanotube/sulfur composite lithium-sulfur battery anode material.
And (3) replacing the porous boron nitride fiber/multi-walled carbon nanotube composite material with the multi-walled carbon nanotube to repeat the third step of operation to obtain the multi-walled carbon nanotube/lithium sulfur battery anode material.
The lithium-sulfur battery assembled by using the porous boron nitride fiber/multi-walled carbon nanotube/sulfur composite lithium-sulfur battery cathode material of the embodiment comprises: the positive pole piece is provided with a high-potential positive active pole (composed of porous boron nitride fiber/multi-walled carbon nanotube/sulfur composite positive material, carbon black and polytetrafluoroethylene in a ratio of 7: 2: 1); a low potential negative electrode lithium material; and a porous polypropylene diaphragm 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 factors are shown in fig. 6, and the first charging and discharging curve chart shows that the porous boron nitride fiber/multi-walled carbon nanotube/sulfur battery still maintains a stable charging and discharging platform under the high multiplying factor of 1C, and has very excellent capacity under different multiplying factors.
Example 2.
In this example, a porous boron nitride fiber doped with red phosphorus was used, and the mass ratio of red phosphorus to the porous boron nitride fiber was 1: 9, red phosphorus and porous boron nitride fibers are uniformly mixed together in a grinding mode, and then the mixture and the multi-wall carbon nano tubes are subjected to the mixed grinding treatment, and the parameters of the other steps are the same as those in the example 1.
Compared with the embodiment 1, the red phosphorus-doped porous boron nitride fiber/multi-walled carbon nanotube/sulfur positive electrode material can effectively catalyze and promote the conversion process of polysulfide due to the addition of red phosphorus, and can relieve the polarization of a lithium sulfur battery to a certain extent, so that the capacity and the cycling stability of the battery are improved.
Example 3.
The steps of this example are the same as example 1, except that the mass ratio of the porous boron nitride fibers to the multi-walled carbon nanotubes is 1/1. And the lithium-sulfur battery prepared by the method is charged and discharged at a rate of 0.1C.
Example 4.
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 1C.
Example 5.
The steps of this example are the same as example 1, except that the mass ratio of the porous boron nitride fibers to the multi-walled carbon nanotubes is 1/4. The lithium sulfur battery was charged and discharged at a rate of 0.1C.
Example 6.
The steps of this example are the same as example 5, except that the mass ratio of the porous boron nitride fibers to the multi-walled carbon nanotubes is 1/4. 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 0.1C.
Compared with example 1, the diameters of the porous boron nitride coarse fibers and the multi-wall carbon nanotubes are different too much, so that the multi-wall carbon nanotubes cannot be intertwined with the porous boron nitride coarse fibers, and the overall conductivity and the adsorption capacity to polysulfide of the material are reduced. The porous boron nitride crude fiber/multi-wall carbon nano tube has low integral capacity under the multiplying power of 0.1C, and the attenuation is obvious along with the increase of the cycle times.
Example 8.
The procedure of this example was the same as example 7, except that the lithium sulfur battery was charged and discharged at a rate of 1C.
Compared with the embodiment 1, the diameters of the porous boron nitride coarse fibers and the multi-wall carbon nanotubes are too different, so that the multi-wall carbon nanotubes cannot be intertwined with the porous boron nitride coarse fibers, the overall conductivity and the adsorption capacity of the material to polysulfide are reduced, the shuttle effect is obviously increased, the overall capacity of the material is not high, the charge-discharge cycle stability is reduced after the current density is increased, and the attenuation is obvious along with the increase of the cycle number.
Example 9.
The steps of this example are the same as example 1, except that the composition ratio of the active positive electrode sheet (which is composed of porous boron nitride fiber/multi-walled carbon nanotube/sulfur composite positive electrode material, carbon black and polytetrafluoroethylene, and the ratio of the three is 8: 1: 1).
The reduction of the conductive agent leads to a decrease in the conductivity of the positive electrode and a significant decrease in the capacity and cycle stability of the battery, as compared to example 1. The conductive agent has the function of providing a channel for the movement of electrons in the electrode, the proper content of the conductive agent can obtain higher discharge capacity and better cycle performance, and if the content is too low, the electronic conductive channel is few, which is not beneficial to large-current charge and discharge; too high reduces the relative content of active material, resulting in a decrease in battery capacity. When the content of the conductive agent reaches a turning point, too much reduces the electrode density to lower the capacity, and too little reduces the utilization rate of active materials in the electrode and the high-rate discharge performance.
FIG. 7 is a comparison graph of charge-discharge cycle performance of porous boron nitride fibers/multi-walled carbon nanotubes/sulfur (CNTs/BNFs/S) and multi-walled carbon nanotubes/sulfur (CNTs/S) and porous boron nitride crude fibers/multi-walled carbon nanotubes/sulfur (CNTs/h-BNFs/S) at different magnifications. Fig. 7 a is a comparison graph of the charge and discharge performance obtained after 50 discharge cycles of 0.1C for the porous boron nitride fiber/multi-walled carbon nanotube/sulfur and the multi-walled carbon nanotube/sulfur prepared in example 1, wherein a white circle curve in the graph is a coulomb efficiency and cycle number curve, and a black circle curve in the graph is a battery capacity and cycle number curve. Compared with porous boron nitride fiber/multi-wall carbon nano tube/sulfur, the multi-wall carbon nano tube/sulfur has small capacity, the single multi-wall carbon nano tube is easy to agglomerate to block an electronic conduction channel, the adsorption capacity to polysulfide is weak, and the capacity attenuation in charge and discharge cycles is obvious. In fig. 7 b is a comparison graph of the charge and discharge performance obtained after the porous boron nitride fiber/multi-walled carbon nanotube/sulfur and the multi-walled carbon nanotube/sulfur are subjected to 100 discharge cycles at 1C in example 1. The current density is 0.1-1C, the stable specific discharge capacity is still maintained after multiple cycles, the good stability of the porous boron nitride fiber/multi-walled carbon nanotube/sulfur battery under different multiplying powers is reflected, and compared with the multi-walled carbon nanotube/sulfur battery, the porous boron nitride fiber/multi-walled carbon nanotube/sulfur battery has higher specific capacity and cycling stability under different multiplying powers.
In fig. 7, C is a comparison graph of the charge and discharge performance obtained after 50 discharge cycles of 0.1C for the porous boron nitride fiber/multi-walled carbon nanotube/sulfur 1/1 prepared in example 3, the porous boron nitride fiber/multi-walled carbon nanotube/sulfur 1/4 prepared in example 5, and the porous boron nitride crude fiber (h-BNFs)/multi-walled carbon nanotube/sulfur prepared in example 7. The capacity of the material using 1/1 mass ratio of porous boron nitride fiber/multi-walled carbon nanotube was reduced compared to example 1, because the increase of porous boron nitride fiber reduced the conductivity of the whole material, but the stability of the charge-discharge cycle was maintained. The capacity of the material adopting the mass ratio of the porous boron nitride fibers to the multi-wall carbon nanotubes of 1/4 is reduced, because the multi-wall carbon nanotubes are increased, the multi-wall carbon nanotubes are easy to partially agglomerate, the electronic conduction channel of the multi-wall carbon nanotubes is blocked, the adsorption capacity to polysulfide is reduced, and the stability of charge-discharge circulation can be still maintained.
In fig. 7 d is a graph comparing the charge and discharge performance obtained after 100 cycles of 1C discharge between the porous boron nitride fiber/multi-walled carbon nanotube/sulfur 1/1 prepared in example 4 and the porous boron nitride fiber/multi-walled carbon nanotube/sulfur 1/4 prepared in example 6, and the porous boron nitride crude fiber/multi-walled carbon nanotube/sulfur prepared in example 8.
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.

Claims (8)

1. The porous boron nitride fiber/multi-walled carbon nanotube/sulfur composite lithium-sulfur battery positive electrode material is characterized by comprising a structure formed by winding porous boron nitride fibers and multi-walled carbon nanotubes, wherein the wound structure can wrap and wind sublimed sulfur;
the preparation method of the cathode material comprises the following steps: (1) preparing porous boron nitride fibers; (2) mixing and grinding the porous boron nitride fiber and the multi-walled carbon nanotube until the porous boron nitride fiber and the multi-walled carbon nanotube are uniformly mixed to obtain a porous boron nitride fiber/multi-walled carbon nanotube composite material; (3) and mixing and grinding the porous boron nitride fiber/multi-walled carbon nanotube composite material and sublimed sulfur uniformly, and then carrying out high-temperature treatment to obtain the porous boron nitride fiber/multi-walled carbon nanotube/sulfur composite lithium-sulfur battery cathode material.
2. The positive electrode 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 cathode material as claimed in claim 1, wherein the mass ratio of the porous boron nitride fibers to the multi-walled carbon nanotubes is 1/1-1/4, and the sulfur loading of the cathode material is 60-70%.
4. The positive electrode material as claimed in claim 1, wherein the porous boron nitride fibers are doped with red phosphorus or a metal oxide.
5. The positive electrode material as claimed in claim 1, wherein the step (2) of mixed grinding treatment comprises the following specific processes: manually grinding the porous boron nitride fiber and the multi-walled carbon nano tube in advance, then placing the ground porous boron nitride fiber and the multi-walled carbon nano tube into a planetary ball mill, adding a proper amount of deionized water, and then carrying out high-speed ball milling for 2-5 hours at the ball milling rotation speed of 200-350rpm so as to uniformly mix the porous boron nitride fiber and the multi-walled carbon nano tube to obtain a black light sample, namely the porous boron nitride fiber/multi-walled carbon nano tube composite material; the mass ratio of the porous boron nitride fibers to the multi-walled carbon nanotubes is 1/1-1/4.
6. The positive electrode material according to claim 1, wherein the specific process of the step (3) is: manually mixing and grinding the porous boron nitride fiber/multi-walled carbon nanotube composite material obtained in the step (2) and sublimed sulfur for 0.5-1 hour, wherein the sulfur-carrying amount of the anode material is controlled to be 60% -70%; and then, placing the materials after manual mixing and grinding into a high-pressure kettle for high-temperature treatment, heating at the temperature of 155-160 ℃ for 10-20 hours, and then cooling to room temperature to obtain the porous boron nitride fiber/multi-walled carbon nanotube/sulfur composite lithium-sulfur battery cathode material.
7. The positive electrode material as claimed in claim 1, wherein the porous boron nitride fiber is prepared by: putting melamine and boric acid into a big beaker containing 200-500 ml 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 nitrogen flow is maintained at 30-300mL/min, and the heat treatment temperature range is 1000-1100 ℃, so as to obtain a white light sample, namely the porous boron nitride fiber.
8. A lithium-sulfur battery, wherein the positive electrode material of the composite lithium-sulfur battery according to any one of claims 1 to 7 is used for lithium-sulfur battery assembly, and the assembled battery comprises: the lithium ion battery comprises a high-potential positive active electrode plate, a low-potential negative lithium material and a porous polypropylene diaphragm arranged between the negative electrode plate and the positive electrode plate; the positive pole piece is composed of a porous boron nitride fiber/multi-walled carbon nanotube/sulfur composite positive pole material, carbon black and polytetrafluoroethylene, wherein the carbon black is used as a conductive agent and accounts for 15-30% by mass, and the polytetrafluoroethylene is used as a binder and accounts for 5-10% by mass; the balance is anode material.
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