CN115911308B

CN115911308B - Composite sulfur positive electrode material and preparation method thereof, and composite sulfur positive electrode and preparation method thereof

Info

Publication number: CN115911308B
Application number: CN202211400156.XA
Authority: CN
Inventors: 郭秉淑; 陈蜜; 宋贵浩; 余皓; 金彦阳; 邓炜一; 陈漠夷; 冉孟元; 李星
Original assignee: Southwest Petroleum University
Current assignee: Southwest Petroleum University
Priority date: 2022-11-09
Filing date: 2022-11-09
Publication date: 2023-09-22
Anticipated expiration: 2042-11-09
Also published as: CN115911308A

Abstract

The invention provides a composite sulfur positive electrode material and a preparation method thereof, a composite sulfur positive electrode and a preparation method thereof, and belongs to the technical field of new energy. The composite sulfur positive electrode is prepared by the following method: respectively dissolving inorganic zinc salt and 2-methylimidazole in methanol to react, and obtaining a ZIF-8 precursor after the reaction is finished; taking a ZIF-8 precursor, dispersing the ZIF-8 precursor in a buffer solution, adding dopamine hydrochloride, inorganic ferric salt and inorganic cobalt salt for reaction, and obtaining CoFe-PDA@ZIF-8 after the reaction is finished; mixing CoFe-PDA@ZIF-8 with a nitrogen source, performing pyrolysis, and purifying a pyrolysis product to obtain an intermediate; uniformly mixing the intermediate with sublimed sulfur, and preserving heat of the mixed product at a first temperature under the condition of inert atmosphere to obtain the catalyst; the composite sulfur positive electrode is prepared from composite sulfur positive electrode material, acetylene black and polyvinylidene fluoride. The invention improves the electrochemical performance of the lithium-sulfur battery by utilizing the synergistic catalysis of the single atoms of iron and cobalt and the iron-cobalt alloy, and provides a theoretical basis for reasonably selecting metal catalysis in the field of lithium-sulfur batteries.

Description

Composite sulfur positive electrode material and preparation method thereof, and composite sulfur positive electrode and preparation method thereof

Technical Field

The invention relates to the technical field of batteries, in particular to a composite sulfur positive electrode material and a preparation method thereof, and a lithium sulfur battery positive electrode prepared by adopting the composite sulfur positive electrode material.

Background

The lithium sulfur battery has high theoretical energy density (2600 Whkg ^-1 ) The positive electrode material has the characteristics of abundant reserves, low price, no toxicity and easy recovery, and is one of the most promising energy storage systems. However, due to the characteristics and reaction mechanism of the lithium-sulfur battery, the problems of poor conductivity of sulfur and discharge products thereof, serious shuttle effect of polysulfide, large volume expansion (80 percent) of the positive electrode in the charge-discharge process, high reactivity of the lithium negative electrode and an intermediate, slow electrochemical reaction kinetics and the like exist, and the large-scale application of the lithium-sulfur battery is limited. Therefore, in order to accelerate the industrial application of the lithium-sulfur battery, scientific researchers invest a lot of time and effort, and through continuous exploration for decades, the effective strategies of introducing a metal catalyst with high conductivity to improve the rate capability, the cycle life and efficiently catalyze the reversible conversion of lithium polysulfide from a sulfur positive electrode material are discovered.

Although there have been many reports so far on the use of a single metal catalyst with a porous carbon composite for a lithium sulfur battery carrier, in view of the multi-step conversion reactions that occur during charge and discharge of a lithium sulfur battery, it is necessary to stabilize a plurality of reaction intermediates in the process, and for a single metal catalyst, the number of active sites is limited, and it is impossible to stabilize all intermediates at the active sites having the optimal binding energy at the same time.

Disclosure of Invention

In order to solve at least one of the problems, the invention provides a preparation method of a composite sulfur positive electrode material, which has a large number of active sites, can improve electrochemical reaction kinetics of a lithium sulfur battery, and has wide application prospect.

In order to achieve the above object, the technical scheme of the present invention is as follows: a composite sulfur positive electrode material and a preparation method thereof comprise the following steps:

the weight ratio is 0.4-0.6: 1 and 2-methylimidazole are respectively dissolved in methanol, the inorganic zinc salt and the 2-methylimidazole are mixed and react for 20 to 30 hours at the temperature of 15 to 35 ℃, and a ZIF-8 precursor is obtained after the reaction is finished;

taking ZIF-8 precursor, dispersing the precursor in buffer solution, adding dopamine hydrochloride, inorganic ferric salt and inorganic cobalt salt, and reacting for 2-5 h under the stirring condition of 15-35 ℃ to obtain CoFe-PDA@ZIF-8 after the reaction is finished;

mixing CoFe-PDA@ZIF-8 and a nitrogen source in a mass ratio of 1:1.1-1.3, performing pyrolysis, and purifying a pyrolysis product to obtain an intermediate;

uniformly mixing an intermediate with the mass ratio of 1:2.5-3.5 with sublimed sulfur, and preserving the temperature of the mixed product for 10-15 hours under the condition of a first temperature and inert atmosphere to obtain the catalyst;

wherein, 0.1 to 0.2mmol of dopamine hydrochloride, 0.04 to 0.12mmol of inorganic ferric salt and 0.1 to 0.2mmol of inorganic cobalt salt are added into each 0.2g of ZIF-8 precursor, and the first temperature is 145 to 180 ℃.

One embodiment of the invention is that the concentration of the 2-methylimidazole is 10 to 12 percent, and the concentration of the inorganic zinc salt is 4.7 to 5.2 percent; and after the reaction of the inorganic zinc salt and the 2-methylimidazole is finished, filtering, washing and drying to obtain the ZIF-8 precursor.

One embodiment of the invention is that the inorganic zinc salt is one of zinc nitrate and zinc chloride, the inorganic ferric salt is one of ferric nitrate and ferric chloride, and the inorganic cobalt salt is one of cobalt nitrate and cobalt chloride.

In one embodiment of the present invention, the buffer is tris (hydroxymethyl) aminomethane buffer having a pH of 8.0 to 9.0, and the solvent of the buffer is a 1:1 aqueous methanol solution.

One embodiment of the invention is that after 2-5 hours of reaction at room temperature under stirring, coFe-PDA@ZIF-8 is obtained by centrifugation and drying.

One embodiment of the invention is that the nitrogen source is one of melamine, urea or an amino acid.

One embodiment of the present invention is that the pyrolysis conditions are: pyrolyzing for 2 hours at 900-950 ℃ in a second inert atmosphere comprising helium and argon; after the pyrolysis operation is finished, the product is cooled and washed by sulfuric acid solution with the concentration of 2-6 mol/L, and then the intermediate is obtained after filtration and drying.

The invention also aims to disclose a composite sulfur positive electrode material which is prepared by adopting any one of the methods.

Another object of the present invention is to disclose a method for preparing a composite sulfur positive electrode, comprising the steps of: taking the composite sulfur cathode material, acetylene black and polyvinylidene fluoride according to the mass ratio of 7-8:1-2:1, mixing and grinding, adding N-methyl pyrrolidone after grinding, continuously grinding until uniform slurry is formed, then scraping the slurry on an aluminum foil, scraping the aluminum foil to a thickness of 100 mu m, and drying at 60 ℃ for at least 10 hours to obtain the composite sulfur cathode material.

The invention also aims to disclose a composite sulfur positive electrode prepared by the method.

The beneficial effects are that: the invention provides a preparation method of a composite sulfur anode, which comprises the steps of firstly dissolving 2-methylimidazole in methanol, then mixing the methanol solution with a methanol solution containing zinc nitrate hexahydrate, and complexing zinc ions with the 2-methylimidazole in a standing process to synthesize ZIF-8. Uniformly mixing ZIF-8 serving as a precursor with ferric nitrate nonahydrate, cobalt nitrate hexahydrate and dopamine hydrochloride, coating polydopamine on the surface of the ZIF-8 under an alkaline condition, and combining an ion exchange process to dissolve zinc ions from the interior of the ZIF-8 and dope iron and cobalt ions into the ZIF-8 to synthesize the CoFe-PDA@ZIF-8 with a hollow structure. And fully grinding and mixing CoFe-PDA@ZIF-8 powder and melamine according to the mass ratio of 1:1.1-1.3, placing in an argon atmosphere, and pyrolyzing at 920 ℃ for 2 hours, wherein 2-methylimidazole and polydopamine are decomposed and converted into nitrogen-doped carbon, one part of iron and cobalt ions are coordinated with nitrogen atoms in a carbon material, exist in the form of single atoms and alloy, one part of iron and cobalt ions are reduced at a high temperature and converted into alloy nano particles, and residual zinc is converted into gas under a high-temperature condition and escapes. Dispersing the calcined product into 2M sulfuric acid solution, heating and stirring for 12 hours, removing metal particles on the surface of the material to obtain an intermediate, fully drying, and loading sulfur into the intermediate by a melt diffusion method at 155 ℃ to finally prepare the composite sulfur anode material.

In the whole preparation process, ZIF-8 is a good precursor with uniform morphology, rich pores and stable hollow structure; in addition, in the process of coating polydopamine, the doping amounts of Fe and Co can be regulated and controlled by changing the adding proportion of Fe and Co, and Fe and Co are well dispersed in an ion exchange mode, so that the exposed active sites are maximized, thereby realizing the adsorption of polysulfide and improving the electrochemical reaction kinetics of the lithium-sulfur battery. The invention improves the electrochemical performance of the lithium-sulfur battery by utilizing the synergistic catalysis of the single atoms of iron and cobalt and the iron-cobalt alloy, and provides a theoretical basis for reasonably selecting metal catalysis in the field of lithium-sulfur batteries.

Drawings

FIG. 1 is an XRD pattern of the materials of example 1 and example 2;

FIG. 2 is a FESEM image of the materials of example 1 and example 2;

FIG. 3 is a Raman spectrum of the material of example 1;

FIG. 4 is a graph showing adsorption effects of the intermediates of example 1 and example 2;

FIG. 5 is a graph of the catalytic effect of the intermediates of examples 1 and 2 on lithium polysulfide;

FIG. 6 is a cyclic voltammogram and AC impedance plot of the positive electrode of examples 3 and 4;

fig. 7 is a graph of the rate performance and charge-discharge curves of the positive electrodes of examples 3 and 4;

fig. 8 is a long cycle performance graph of the positive electrode of example 3 and example 4;

fig. 9 is a graph of the rate performance of the positive electrodes of comparative examples 1 and 2.

Detailed Description

The following detailed description of the invention will be clearly and fully described in connection with the examples which are set forth to illustrate, but are not necessarily all embodiments of the invention.

The invention is further described below with reference to examples:

in the following examples, unless otherwise specified, the operations described are conventional in the art.

In the examples described below, the starting materials employed are all commercially available, unless otherwise specified.

Example 1

10g of 2-methylimidazole was dissolved in 100mL of methanol, 4.7g of zinc nitrate hexahydrate was dissolved in 100mL of methanol, and then both were mixed, and allowed to stand at room temperature for 24 hours. The obtained product is collected by centrifugation, washed 2 to 3 times by methanol and dried overnight in an oven at 70 ℃ to prepare ZIF-8.

After 0.2g ZIF-8 and 0.12g tris (hydroxymethyl) aminomethane were placed in 100mL of a mixture (volume ratio=1:1 aqueous methanol solution), 100mL of an aqueous solution containing 0.15mmol of dopamine hydrochloride, 0.075mmol of ferric nitrate nonahydrate and 0.15mmol of cobalt nitrate hexahydrate was poured thereinto after ultrasonic dispersion, magnetically stirred at room temperature for 4 hours, and then the gray powder was collected by centrifugation and dried under vacuum at 70℃to prepare CoFe-PDA@ZIF-8.

Grinding and mixing CoFe-PDA@ZIF-8 and melamine at a mass ratio of 1:1, transferring into a tube furnace, pyrolyzing at 920 ℃ for 2h under argon atmosphere, cooling to room temperature, and dispersing the obtained black powder to 2M H at 80 DEG C ₂ SO ₄ Stirring the solution for 12h by magnetic force, washing, filtering and drying at 70 ℃ to obtain the intermediate.

Mixing and grinding the intermediate and sublimed sulfur according to the mass ratio of 3:7, placing the mixture into a tube furnace, and preserving the heat for 12 hours under the condition of argon atmosphere and 155 ℃, and marking the obtained material as Fe _0.5 Co ₁ NPCs@S composite positive electrode material.

Example 2

0.2g ZIF-8 and 0.12g tris (hydroxymethyl) aminomethane were dispersed in 100mL of the mixture (volume ratio=1:1), and 100mL of an aqueous solution containing 0.15mmol of dopamine hydrochloride, 0.15mmol of ferric nitrate nonahydrate and 0.075mmol of cobalt nitrate hexahydrate was poured thereinto after ultrasonic dispersion, magnetically stirred at room temperature for 4 hours, and then the gray powder was collected by centrifugation, and dried under vacuum at 70℃to prepare CoFe-PDA@ZIF-8.

Mixing and grinding the intermediate and sublimed sulfur according to the mass ratio of 3:7, placing the mixture into a tube furnace, and preserving the heat for 12 hours under the condition of argon atmosphere and 155 ℃, and marking the obtained material as Fe ₁ Co _0.5 NPCs@S composite positive electrode material.

Example 3

Fe is added to _0.5 Co ₁ Mixing and grinding NPCs@S, acetylene black and PVDF according to the mass ratio of 8:1:1, adding 150 mu LNMP after grinding uniformly, continuing grinding until uniform slurry is formed, then scraping the slurry onto a smooth aluminum foil by using a scraper, wherein the thickness of the scraping is 100 mu m, and drying overnight in an oven at 60 ℃. Cutting the scraped sample into small discs with the diameter of 14mm by using a slicer to obtain Fe _0.5 Co ₁ NPCs@S positive electrode.

Example 4

Fe is added to ₁ Co _0.5 Mixing and grinding/NPCs@S, acetylene black and PVDF according to the mass ratio of 8:1:1, adding 150 mu LNMP after uniform grinding, and continuously grindingGrinding to form uniform slurry, then scraping the slurry onto a smooth aluminum foil with a scraper to a thickness of 100 μm, and oven drying at 60deg.C overnight. Then, the scraped sample is cut into small discs with the diameter of 14mm by a slicer, and the Fe1Co0.5/NPCs@S anode is obtained.

Comparative example 1

The difference from example 1 is that cobalt nitrate hexahydrate was not added, and the rest was the same, to finally prepare an Fe/NPCs@S cathode material, which was then prepared into an Fe/NPCs@S cathode by the method of example 3.

Comparative example 2

The difference from example 1 is that no ferric nitrate nonahydrate was added, and the rest was the same, to finally prepare a Co/NPCs@S cathode material, which was then prepared into a Co/NPCs@S cathode by the method of example 3.

To further illustrate the effects of embodiments of the present invention, performance tests were performed below.

Since a battery model is required for the test, and only the positive electrode is given in the above-described embodiment, the negative electrode and the battery model are given here.

The positive electrodes prepared in example 3, example 4, comparative example 1 and comparative example 2 were prepared, and a CR2032 type lithium sulfur battery was assembled in a glove box having a water oxygen content of less than 0.1ppm using a commercial lithium sheet as a negative electrode and polypropylene as a separator. Wherein the electrolyte solute is lithium trifluoromethylsulfonyl imide (LiTFSI), the solvent adopts 1, 3-Dioxolane (DOL) and Dimethoxymethane (DME) with volume ratio of 1:1, and the additive is LiNO with the concentration of 0.1mol/L ₃ The concentration of the electrolyte is 1mol/L, and the dosage is 15 mu L.

The CR2032 type lithium sulfur battery was assembled in a glove box having a water oxygen content of less than 0.1ppm using the electrodes of example 3 and example 4 as both positive and negative electrodes and polypropylene as a separator. Wherein the electrolyte solute is lithium trifluoromethylsulfonyl imide (LiTFSI), the solvent adopts 1, 3-Dioxolane (DOL) and Dimethoxymethane (DME) with volume ratio of 1:1, and the additive is LiNO with the concentration of 0.1mol/L ₃ The concentration of the electrolyte is 1mol/L, the dosage is 15 mu L, and 15 mu LLi is added in the process of assembling the battery ₂ S ₆ A solution. Shape of a Chinese characterThe cells were compared and their catalytic performance was tested.

1. XRD spectrum analysis

Fig. 1 is XRD patterns of the intermediate and composite cathode materials of examples 1 and 2, ranging from 5 ° to 70 °, with fig. 1a being CRD patterns of the intermediate of examples 1 and 2 and fig. 1b being XRD patterns of the composite cathode materials of examples 1 and 2.

From FIG. 1a, fe can be seen _0.5 Co ₁ The NPCs have three diffraction peaks at 2 theta (26 DEG), 45 DEG and 60 DEG, wherein the diffraction peaks at 26 DEG are characteristic peaks of carbon materials, and the diffraction peaks at 45 DEG and 60 DEG correspond to iron-cobalt alloys, indicating Fe _0.5 Co ₁ The NPCs sample is composed of Co ₃ Fe ₇ Alloy and nitrogen doped carbon. In contrast, fe1Co _0.5 Besides the characteristic peak of the carbon material at 26 degrees, the NPCs also show three diffraction peaks corresponding to PDF standard cards of the iron-cobalt alloy at 2 theta (approximately 43 degrees), 45 degrees and 51 degrees, which shows that as the proportion of cobalt element is reduced, the proportion of iron element is increased, and Fe _0.5 Co ₁ Conversion of NPCs sample to Co ₃ Fe ₇ And Co _0.72 Fe0.28 alloy and nitrogen-doped carbon. FIG. 1b shows Fe prepared in example 1 and example 2 _0.5 Co ₁ NPCs@S and Fe ₁ Co _0.5 XRD spectra of the NPCs@S samples show that after sulfur is evaporated by melting and diffusion, strong sulfur characteristic diffraction peaks appear in the two samples, and the fact that the sulfur enters the pore structure of the carrier material through capillary effect under the condition of 155 ℃ is proved.

2. FESEM image

Samples prepared in examples 1 and 2 were taken and subjected to FESEM characterization with the final results shown in FIG. 2.

FIG. 2A is a FESEM of the intermediate of example 1, FIG. 2b is a FESEM of the intermediate of example 2, and FIG. 2c is a Fe of example 1 _0.5 Co ₁ FIG. 2d is a FESEM photograph of NPCs@S showing Fe in example 2 _0.5 Co ₁ FESEM of NPCs@S.

As can be seen from FIG. 2a, the sample consists of a hollow core with a diameter of about 200nmPolyhedron and beaded nanofibers. Wherein the polyhedral morphology inherits a ZIF-8 precursor, and the hollow structure is attributed to Fe in the poly-dopamine coating process ³⁺ 、Co ²⁺ With Zn in ZIF-8 ²⁺ The exchange between the poly-dopamine layer and the residual 2-methylimidazole is converted into nitrogen doped hollow carbon in the figure 2a after high-temperature pyrolysis; secondly, the nanofiber in 2a is derived from melamine carbon source and is formed by the catalysis of Fe and Co metal alloy in the pyrolysis process, and the nanofiber is in a hollow structure by combining reference data.

As can be seen from fig. 2b, the amount of nanofibers is gradually increased due to the decrease in the proportion of cobalt element and the increase in the proportion of iron element, and a large number of hollow polyhedral structures are destroyed.

As can be seen from FIGS. 2c and 2d, fe after being steamed with sulfur _0.5 Co ₁ The microscopic morphology of the sample/NPCs@S is not greatly changed compared with that of the carrier material, and a hollow structure can still be seen, which shows that enough space is provided to relieve the volume effect of sulfur in the charge and discharge process. However, fe ₁ Co _0.5 The surface of the sample of NPCs@S is agglomerated after sulfur steaming, the structure is not as loose as that of a carrier material, and the surface Fe ₁ Co _0.5 The NPCs carrier has limited specific surface area and pore structure, cannot bear a large amount of elemental sulfur, and has poor limiting capability on sulfur volume effect in the charge and discharge process.

3. Raman analysis

FIG. 3 is Fe in example 1 _0.5 Co ₁ NPCs and Fe _0.5 Co ₁ Raman spectrum of sample/NPCs@S, test wavenumber range of 200-3000 cm ^-1 . As can be seen from the figure, at 1340 and 1574cm ^-1 Two distinct peaks appear respectively, corresponding to sp in turn ³ Hybrid carbons (D peak) and sp ² Vibration of carbon-carbon bonds in carbon (G peak). Wherein Fe is _0.5 Co ₁ The G peak intensity of the NPCs material is obviously higher than that of the D peak, which shows that the graphitization degree of the carbon component is higher, and the conductivity of the material is better. Secondly, fe _0.5 Co ₁ In addition to the D and G peaks, the NPCs@S was at 464cm ^-1 A peak appears nearbyCorresponds to S ₈ Indicating that sulfur was successfully distilled into the base material.

4. Adsorption effect

306mg of S and 156mg of Li were weighed out respectively in a glove box ₂ S, mixing and adding the mixture into a 20mL glass bottle, taking 10mL of Dimethoxymethane (DME) solution into the glass bottle through a pipette, sealing a bottle opening, and stirring the bottle opening in a glove box for 20h until powder in the bottle completely reacts to obtain Li with the concentration of 0.2mol mL-1 ₂ S ₆ A solution. Fe prepared in example 1 and example 2 _0.5 Co ₁ NPCs and Fe ₁ Co _0.5 The NPCs sample is taken to have the same mass and is respectively added into two clean glass bottles, and then Li diluted by DME is sequentially added into the glass bottles ₂ S ₆ The solution was allowed to stand for 10 minutes, and then the color change was observed. The results are shown in FIG. 4, wherein (1) is a comparative graph, (2) is a graph of the adsorption effect of the intermediate material of example 1, and (3) is a graph of the adsorption effect of the intermediate material of example 2, fe _0.5 Co ₁ NPCs and Fe ₁ Co _0.5 NPCs can change the color of the solution into bright yellow and non-color, which indicates that the NPCs are used as sulfur carriers and are used for Li ₂ S ₆ Has adsorption capacity mainly due to Fe _0.5 Co ₁ NPCs and Fe ₁ Co _0.5 Alloy nanoparticles and nitrogen doped carbon and Li in NPCs composite materials ₂ S ₆ Synergistic chemisorption therebetween.

5. Catalytic performance

The comparison battery is respectively clamped between three stainless steel electrodes, CV test is carried out by using an electrochemical workstation at 30 ℃, the voltage range is-0.8V, and the sweeping speed is 20mVs ^-1 . The final results are shown in FIG. 5.

As can be seen from the figure, a pair of redox peaks appear in both cells, corresponding to Li ₂ S ₆ Indicating the reversible reaction of Fe _0.5 Co ₁ NPCs and Fe ₁ Co _0.5 Two material pairs Li of NPCs ₂ S ₆ Has catalytic action on the oxidation-reduction peaks. However, fe _0.5 Co ₁ Current response of the NPCs electrode is compared with Fe ₁ Co _0.5 Higher NPCs electrode, indicating Fe _0.5 Co ₁ Co in pure phase in NPCs ₃ Fe ₇ For Li ₂ S ₆ The catalytic conversion effect of (2) is stronger.

6. Cyclic voltammogram and ac impedance

The CR2032 type button cells assembled in example 3 and example 4 were sandwiched between three stainless steel electrodes, respectively, and CV and impedance tests were performed using an electrochemical workstation at 30 ℃. The final results are shown in FIG. 6.

FIG. 6a is Fe _0.5 Co ₁ NPCs@S and Fe ₁ Co _0.5 The Cyclic Voltammetry (CV) curve of the NPCs@S composite positive electrode assembled battery can show that two reduction peaks and a split oxidation peak appear on two electrodes, wherein the reduction peaks are gradually oxidized into long-chain Li from high voltage to low voltage corresponding to elemental sulfur in sequence ₂ S _n (n=6-8) and medium-long chain Li ₂ S _n (n=3-5), finally to Li ₂ S, oxidation peak corresponds to Li ₂ S experience Li ₂ The Sn is gradually reduced to elemental sulfur in a multi-step reaction. Next, as can also be seen in FIG. 6a, fe _0.5 Co ₁ The potential difference between the oxidation peak and the reduction peak of the NPCs@S electrode is smaller, which indicates that Fe _0.5 Co ₁ Polarization of/NPCs@S is less than Fe ₁ Co _0.5 The area of the reduction peak and the oxidation peak at the high potential in the CV curve can be deduced by combining the NPCs@S electrode, and the single-phase Fe ₃ Co ₇ Alloy pair Li ₂ The catalytic effect of reversible transformation of Sn is more remarkable. FIG. 6b shows Fe as example 1 and Fe as example 2 _0.5 Co ₁ NPCs@S and Fe ₁ Co _0.5 The AC impedance test of the/NPCs@S composite positive electrode assembled battery can obviously show that Fe _0.5 Co ₁ The semicircle diameter of the NPCs@S electrode in the low frequency region is obviously smaller, and the linear slope of the high frequency region is also larger, which indicates that Fe _0.5 Co ₁ The NPCs@S electrode has smaller charge transfer resistance and lithium ion diffusion rate.

7. Rate capability of battery

The CR2032 type button cells assembled in example 3 and example 4 were respectively sandwiched between three stainless steel electrodes, and the rate performance test was performed at 30 ℃ using a new wilt cell test system, with a test voltage range of 1.7 to 2.8V. The final results are shown in FIG. 7.

FIG. 7a is a graph showing the rate performance at different current densities, and it can be seen that Fe _0.5 Co ₁ Specific discharge capacities of the/NPCs@S electrodes at 0.1, 0.2, 0.3, 0.5, 1.0 and 2.0C current densities were 982, 846, 786, 727, 650 and 560mAh g-1, and Fe ₁ Co _0.5 The specific discharge capacities of the NPCs@S under the same conditions are 758, 704, 660, 607, 532 and 430mAh g-1 which are respectively lower than Fe _0.5 Co ₁ NPCs@S electrode, further indicating single phase Co ₃ Fe ₇ The catalysis effect of the alloy on Li2Sn is better than that of adding Co _0.72 Fe _0.28 So that Fe is _0.5 Co ₁ the/NPCs@S electrode still has higher discharge capacity at high magnification. When the current density is suddenly switched from 2.0C to 0.1C, the discharge capacity of both electrodes can be recovered to 800mAh g ^-1 The above. FIG. 7b is Fe _0.5 Co ₁ NPCs@S and Fe ₁ Co _0.5 The charge-discharge curves of the NPCs@S electrode after 5 circles of circulation all show two discharge platforms and one charge platform under the current density of 0.1C, but Fe _0.5 Co ₁ All platforms of the NPCs@S are longer, the voltage difference (electrode polarization) between the charge and discharge platforms is smaller, and the single-phase Co is fully shown by being consistent with CV curve analysis results ₃ Fe ₇ Excellent catalysis of the alloy.

8. Battery cycle performance

The CR2032 type button cells assembled in example 3 and example 4 were respectively sandwiched between three stainless steel electrodes, and long cycle performance test was performed at 30 ℃ using a new wilcell test system with a test voltage range of 1.7 to 2.8V and a test charge/discharge rate of 1.0C, and the test results are shown in fig. 8.

It can be seen that Fe _0.5 Co ₁ The initial specific capacity of the/NPCs@S electrode is 943mAh g-1, and the capacity retention rate after 500 cycles is 42%. Fe (Fe) ₁ Co _0.5 The initial capacity of the NPCs@S electrode is 199mAh g-1, and the NPCs@S electrode is subjected to 50 cyclesThe ring was then raised to 601mAh g-1, indicating that Fe was responsible for ₁ Co _0.5 The NPCs@S carrier has the advantages that a large number of hollow polyhedral structures are destroyed, the pore structures in the material are destroyed, the porosity is reduced, and electrolyte can fully infiltrate Fe after one-stage circulation ₁ Co _0.5 and/NPCs@S electrode, so that the composite sulfur anode needs to be activated in the initial cycle process to normally exert capacity. In contrast, fe _0.5 Co ₁ The polyhedral structure of the carrier material in the NPCs@S composite anode is kept complete, the pore structure is rich, and the high capacity can be released without activation in the initial cycle process. Fe (Fe) ₁ Co _0.5 After 500 cycles of NPCs@S electrode, the capacity is only 172mAh g-1, the retention rate is 28.6%, and the retention rate is far lower than Fe ₁ Co _0.5 NPCs@S electrode. The above results demonstrate that Fe _0.5 Co ₁ The NPCs@S carrier material has stable microscopic results and excellent catalytic performance, can fully inhibit volume expansion of a sulfur anode and shuttle effect of Li2Sn (n=4-8), and effectively reduces attenuation of battery capacity.

The CR2032 type button cells assembled in comparative examples 1 and 2 were respectively sandwiched between three stainless steel electrodes, and a rate performance test was performed at 30 ℃ using a new wilcell test system with a test voltage range of 1.7 to 2.8V, and the test results are shown in fig. 9.

As can be seen from FIG. 9, the discharge capacities of the Fe/NPCs@S electrodes at current densities of 0.1, 0.2, 0.3, 0.5, 1.0 and 2.0C are 1085, 935, 859, 741, 208 and 84mAh g, respectively ^-1 The discharge capacities of the Co/NPCs@S electrode under the same conditions are 1052, 922, 874, 813, 201 and 125mAh g ^-1 The capacity of the two electrodes decays rapidly under high current density, which shows that pure iron and pure cobalt materials are used for Li ₂ S _n Has limited adsorption and catalytic capability, and has poor shuttle effect and electrochemical reaction kinetics at high multiplying power.

The present invention is not limited to the above-mentioned embodiments, but is intended to be limited to the following embodiments, and any modifications, equivalents and modifications can be made to the above-mentioned embodiments without departing from the scope of the invention.

Claims

1. The preparation method of the composite sulfur positive electrode material of the lithium sulfur battery is characterized by comprising the following steps of:

taking ZIF-8 precursor, dispersing the precursor in buffer solution, adding dopamine hydrochloride, inorganic ferric salt and inorganic cobalt salt, reacting for 2-5 h under the stirring condition of 15-35 ℃, and obtaining CoFe-PDA@ZIF-8 after the reaction is finished;

mixing CoFe-PDA@ZIF-8 and a nitrogen source in a mass ratio of 1:1.1-1.3, performing pyrolysis, and purifying a pyrolysis product to obtain an intermediate; the pyrolysis conditions are as follows: pyrolyzing for 2 hours at 900-950 ℃ in a second inert atmosphere comprising helium and argon;

uniformly mixing the intermediate with the mass ratio of 1:2.5-3.5 with sublimed sulfur, and preserving the temperature of the mixed product for 10-15 hours under the condition of a first temperature and an inert atmosphere to obtain the catalyst;

wherein, 0.1 to 0.2mmol of dopamine hydrochloride, 0.06 to 0.16mmol of inorganic ferric salt and 0.06 to 0.16mmol of inorganic cobalt salt are added into each 0.2g of ZIF-8 precursor, and the first temperature is 145 to 180 ℃.

2. The preparation method according to claim 1, wherein the concentration of the 2-methylimidazole is 10% -12%, and the concentration of the inorganic zinc salt is 4.7% -5.2%; and after the reaction of the inorganic zinc salt and the 2-methylimidazole is finished, filtering, washing and drying to obtain the ZIF-8 precursor.

3. The preparation method according to claim 1, wherein the inorganic zinc salt is one of zinc nitrate and zinc chloride, the inorganic iron salt is one of ferric nitrate and ferric chloride, and the inorganic cobalt salt is one of cobalt nitrate and cobalt chloride.

4. The method according to claim 1, wherein the buffer is tris (hydroxymethyl) aminomethane buffer having a pH of 8.0 to 9.0, and the solvent of the buffer is a 1:1 aqueous methanol solution.

5. The preparation method according to claim 1, wherein the CoFe-PDA@ZIF-8 is obtained by centrifugation and drying after reacting for 2-5 hours at room temperature under stirring.

6. The method of claim 1, wherein the nitrogen source is one of melamine, urea, or an amino acid.

7. The method of claim 1, wherein the purification of the pyrolysis product is performed by: after the pyrolysis operation is finished, the product is cooled and washed by sulfuric acid solution with the concentration of 2-6 mol/L, and then the intermediate is obtained after filtration and drying.

8. A composite sulfur cathode material for lithium-sulfur batteries, which is prepared by the method of any one of claims 1 to 7.

9. The preparation method of the composite sulfur positive electrode of the lithium sulfur battery is characterized by comprising the following steps of: taking the composite sulfur cathode material, acetylene black and polyvinylidene fluoride according to the mass ratio of 7-8:1-2:1, mixing and grinding, adding N-methyl pyrrolidone after grinding, continuously grinding until uniform slurry is formed, then scraping the slurry on an aluminum foil, scraping the aluminum foil to a thickness of 100 mu m, and drying at 60 ℃ for at least 10 hours to obtain the composite sulfur cathode material.

10. A composite sulfur positive electrode for a lithium sulfur battery, prepared by the method of claim 9.