CN112563506A

CN112563506A - Nanoscale composite material for lithium-sulfur battery positive electrode and preparation method thereof

Info

Publication number: CN112563506A
Application number: CN202011426451.3A
Authority: CN
Inventors: 张佳峰; 黄灿灿; 李东民; 欧星
Original assignee: Central South University
Current assignee: Central South University
Priority date: 2020-12-09
Filing date: 2020-12-09
Publication date: 2021-03-26

Abstract

The invention relates to a nano-scale composite material for a lithium-sulfur battery positive electrode and a preparation method thereof. The preparation method is novel, the problem that the loading capacity of sulfur in the common lithium-sulfur battery anode material is low and uneven is solved through rich porous structures, the introduction of the graphene can improve the conductivity of the material, the graphene can also be used for preparing the lithium-sulfur battery anode material with small volume expansion, and the lithium-sulfur battery assembled by adopting the anode material has good cycle performance and capacity retention rate.

Description

Nanoscale composite material for lithium-sulfur battery positive electrode and preparation method thereof

Technical Field

The invention relates to the technical field of lithium-sulfur batteries, in particular to a nano-scale sulfur/pentavanadium octasulfide/graphene composite material and a preparation method thereof.

Background

In the century today, energy and environment continue to be a focus of human focus, and the rapid development of new and renewable energy is the primary task of human current energy work. Because lithium ion batteries have the advantage of high energy density, green and environment-friendly secondary energy storage devices represented by lithium ion batteries are widely concerned by people and widely applied to portable power supplies, wearable equipment, energy storage power stations, transportation and other aspects. However, the conventional lithium ion battery is limited by the specific capacity of the positive electrode material and the negative electrode material, and thus the requirement of people for high energy density energy storage devices cannot be met. Therefore, it is important to develop new types of high-energy lithium battery positive electrode materials or battery systems to meet the demands of people for high energy density.

The lithium-sulfur battery is a rechargeable battery system with elemental sulfur as a positive electrode and metal lithium as a negative electrode. Due to the ultrahigh theoretical specific capacity (1670mAh g)^-1) And theoretical energy density (2600. Wh kg)^-1) Lithium sulfur batteries have attracted much attention. Meanwhile, the positive electrode material sulfur is used as a byproduct of petroleum refining and a direct extract in sulfur ore, has the advantages of rich resources and environmental friendliness, is one of high-energy secondary batteries with the most development potential, and is beneficial to sustainable development.

However, in the process of practical application of lithium-sulfur batteries, the problems of low sulfur utilization rate, low coulombic efficiency, short cycle life and the like exist, and the main reason is that the lithium-sulfur batteries have some inherent defects and basic problems, mainly including the S positive electrode and the low effective utilization rate of active substances caused by the insulation of lithium polysulfide; the shuttle effect of the intermediate product lithium polysulfide seriously causes the loss of active components in the charge-discharge process; the huge volume change of the active substance in the circulation process destroys the electrode structure and the like. Therefore, the improvement of the microstructure of the positive electrode material of the lithium-sulfur battery and the improvement of the loading capacity and the utilization rate of active substances in the positive electrode material are the keys for effectively improving the cycle performance of the lithium-sulfur battery and improving the electrochemical performance of the positive electrode material of the lithium-sulfur battery.

Disclosure of Invention

The technical problem to be solved by the invention is as follows: the sulfur/pentavanadium octasulfide/graphene ternary composite material with the special structure is designed and prepared by utilizing the synergistic effects of chemical adsorption of pentavanadium octasulfide to polysulfide ions, improvement of the conductivity of graphene to active substances and capability of accommodating expansion of a positive electrode material in a circulating process through a three-dimensional porous structure, and is used for improving the electrochemical performance of the lithium-sulfur battery. The preparation method disclosed by the invention is simple, safe in preparation process, low in energy consumption and strong in operability.

The nano-scale composite material is a sulfur-loaded nano-scale sulfur/pentavanadium octasulfide/graphene composite material, is prepared by taking polystyrene spheres as a template, and has a porous three-dimensional layered structure.

Preferably, in the nano-scale sulfur/pentavanadium octasulfide/graphene composite material, the mass percentage of sulfur is 70-80%.

Preferably, the diameter of the polystyrene spheres in the synthesized polystyrene sphere template is 100-500 nm;

preferably, in the nano-scale sulfur/pentavanadium octasulfide/graphene composite material, or the diameter of the sulfur-loaded pentavanadium octasulfide is 100-500 nm;

preferably, the preparation method comprises the following specific steps:

(1) synthesizing polystyrene spheres as a template, adding ammonium metavanadate and graphene oxide, taking the freeze-dried material as a precursor, and carrying out heat treatment together with sulfur to obtain an octa-sulfide penta-vanadium/graphene composite material;

(2) and uniformly mixing the pentavanadium octasulfide/graphene composite material with solid sulfur powder, so that after sulfur is heated to be molten, the molten sulfur is diffused and distributed in mesoporous channels among the pentavanadium octasulfide to obtain the sulfur/pentavanadium octasulfide/graphene nano composite material.

According to the invention, by using the polystyrene spheres as the template, the synthesized spherical pentavanadium octasulfide has rich micropore and mesoporous structures, so that the specific surface area of the material can be increased, more sulfur can be loaded, and the rapid conversion of lithium polysulfide is promoted, thereby improving the electrochemical performance of the lithium-sulfur battery prepared from the composite material.

12 the graphene oxide is added in the invention, and the advantages that the graphene has good conductivity, is beneficial to electron conduction, has excellent elasticity and flexibility, and can accommodate the volume change of partial sulfur in the charging and discharging process are utilized. Thereby further improving the electrochemical performance of the lithium-sulfur battery prepared by the composite material.

Preferably, in the step (1), the temperature of the heat treatment is 600 ℃ to 800 ℃, and more preferably, the temperature of the heat treatment is 700 ℃. If the heat treatment temperature is too low, the polystyrene pellet template cannot be completely removed, and if the heat treatment temperature is too high, the sulfur is easily volatilized, so that the sulfur is difficult to react with ammonium metavanadate to generate pentavanadium octasulfide, and the content of a target product is reduced.

Preferably, in the step (1), the reaction time of the heat treatment is 1-5 h, and more preferably, the time of the heat treatment is 2 h. If the heat treatment time is too short, the polystyrene pellet template cannot be completely removed, and if the heat treatment time is too long, a series of side reactions are easily caused, so that the decomposition of reaction products is caused, the generation of pentavanadium octasulfide is influenced, and the loading capacity of sulfur is reduced.

Preferably, in the step (1), the heat treatment is performed under a reducing gas atmosphere, the reducing gas is argon-hydrogen gas, and the purity of the argon-hydrogen gas is preferably equal to or more than 99.9%.

Preferably, in the step (1), the polystyrene microsphere template is prepared by taking styrene, methyl methacrylate, acrylic acid, ammonium bicarbonate and ammonium persulfate as raw materials through a solvothermal reaction;

preferably, in the step (1), the mass ratio of the polystyrene spheres to the ammonium metavanadate is 5-10: 2, and more preferably, the ratio of the polystyrene spheres to the ammonium metavanadate is 7: 2; the mass ratio of the graphene oxide to the ammonium metavanadate is 1-5: 2, and more preferably, the mass ratio of the graphene to the ammonium metavanadate is 3: 2;

preferably, in the step (1), the mass ratio of the sulfur to the mixed material of polystyrene, ammonium metavanadate and graphene oxide is 20:1, more preferably, the mass ratio of the sulfur to the mixed materials of polystyrene, ammonium metavanadate and graphene oxide is 8: 1;

preferably, in the step (2), the mass ratio of the nano pentavanadium octasulfide/graphene to sulfur is 3: 7;

preferably, in the step (2), the heat treatment temperature is 115 ℃. The melting point of the sulfur is 112.8 ℃, the sulfur is molten at 115 ℃, the viscosity of the sulfur is lowest, and the sulfur can be loaded on the nano pentavanadium octasulfide by a melting diffusion method.

The invention has the beneficial effects that:

(1) the pentavanadium octasulfide prepared from the polystyrene spheres inherits the structure of the nano-scale polystyrene spheres, maintains the spherical appearance, has the diameter of about 100-500 nm, and can be used as a sulfur main body, so that the specific surface area can be increased, and the contact area of the nano pentavanadium octasulfide and sulfur is increased; and the rapid conversion of lithium polysulfide can be promoted, and when the material prepared by compounding with sulfur and graphene oxide is used as a raw material of a positive electrode material of a lithium-sulfur battery, the prepared positive electrode material has excellent chemical lithium storage performance, higher specific discharge capacity and excellent cycle performance, so that the material has good application prospect in the electrochemical field.

(2) The method for preparing the sulfur/silicon dioxide by the melting diffusion method has lower temperature and can reduce energy consumption.

(3) The invention has simple process and low cost, is easy to realize the batch production of the composite material, and can meet the use requirement of the engineering technical field on the high-performance lithium-sulfur battery anode material.

Drawings

FIG. 1 is an SEM image of nano-scale polystyrene spheres prepared in example 1;

FIG. 2 is an XRD pattern of the three-dimensional porous nano-sized sulfur/pentavanadium octasulfide/graphene composite prepared in example 1;

FIG. 3 is an SEM image of a three-dimensional porous nano-scale sulfur/pentavanadium octasulfide/graphene composite prepared in example 1;

fig. 4 is a graph of the first charge and discharge of the battery assembled in example 1;

FIG. 5 is a graph showing the cycle of the assembled battery in example 1;

fig. 6 is a graph of the first charge and discharge of the battery assembled in example 3;

fig. 7 is a graph of the first charge and discharge of the battery assembled in example 4.

Detailed Description

The present invention will be further described with reference to the following examples and the accompanying drawings.

The purity of the high-purity argon or the high-purity nitrogen used in the embodiment of the invention is 99.99 percent; the starting materials or chemicals used in the examples of the present invention are, unless otherwise specified, commercially available in a conventional manner.

Example 1

1. The preparation procedure of this example is as follows:

(1) 21ml of styrene, 1.1ml of methyl methacrylate, 0.92ml of acrylic acid, 0.49g of ammonium bicarbonate and 0.53g of ammonium persulfate are weighed by a measuring cylinder, dissolved in 100ml of deionized water, added into a 500ml round-bottom flask, heated in an oil bath at 80 ℃ for reaction for 12 hours, the cooled suspension is centrifuged at 700r/min for 10 minutes by using a centrifuge, and then the collected sample is freeze-dried at-45 ℃ for 24 hours to obtain the polystyrene sphere template.

(2) Adding 0.7g of polystyrene pellets obtained in the step (1), 0.2g of ammonium metavanadate and 0.3g of graphene oxide into 20ml of deionized water, stirring and mixing at 75 ℃, then ultrasonically dispersing the solution for 1h, and freeze-drying the obtained solution for 24h at-45 ℃ to obtain a mixed material of polystyrene, ammonium metavanadate and graphene oxide as a precursor;

(3) weighing 0.6g of the precursor obtained in the step (2), grinding the precursor into uniform powder in an agate mortar, placing the powder into a ceramic square boat 1, and weighing 4.8g of sublimed sulfur powder into another ceramic square boat 2; transferring the two boats into a tube furnace, placing the boat 2 close to the air inlet end of the tube furnace, placing the boat 1 close to the air outlet end of the tube furnace, and introducing 120min of high-purity argon and hydrogen at the flow rate of 40mL/min to exhaust air in the tube; then adjusting the flow rate of argon and hydrogen to be 100mL/min, heating from room temperature to 700 ℃ according to the heating rate of 5 ℃/min, and calcining for 2 h; after the reaction is finished, taking out a black sample when the temperature of the furnace body is naturally reduced to below 60 ℃; weighing 0.3g of sample and 0.7g of sublimed sulfur powder, grinding and mixing the sample and the sublimed sulfur powder in an agate mortar to obtain uniform powder, placing the powder in a ceramic square boat, placing the square boat in a tubular furnace, heating the square boat from room temperature to 155 ℃ at the heating rate of 5 ℃/min in the argon atmosphere, and carrying out heat treatment for 12 hours to obtain the nano-scale sulfur/octa-sulfide penta-vanadium/graphene composite material.

An SEM image of the polystyrene pellet template prepared in this example is shown in fig. 1, and the obtained polystyrene pellets have a flat morphology formed by bonding a plurality of uniformly sized pellets, and the average particle size of the pellets is about 400 nm.

The XRD pattern of the nano-scale sulfur/pentavanadium octasulfide/graphene composite powder prepared in this example is shown in fig. 2, which can correspond to PDF card of pentavanadium octasulfide, and at the same time, the existence of partial carbon peaks can be observed.

2. Lithium-sulfur battery positive electrode material prepared from nano-scale sulfur/pentavanadium octasulfide/graphene composite material and performance test thereof

The positive electrode material for lithium-sulfur batteries includes: sulfur/pentavanadium octasulfide/graphene composite powder of example 1, acetylene black, and polyvinylidene fluoride.

The preparation method comprises the following steps: 0.08g of the sulfur/vanadyl octasulfide/graphene composite material prepared in example 1 was weighed, 0.01g of acetylene black as a conductive agent and 0.08g of pvdf (polyvinylidene fluoride) as a binder were added, and N-methylpyrrolidone (NMP) as a solvent were mixed and ground to form a positive electrode material.

The process of assembling the battery by adopting the anode material is as follows: coating the anode material on the surface of an aluminum foil to prepare an electrode plate; then, in a sealed glove box filled with argon, the electrode plate is taken as a positive electrode, a metal lithium plate is taken as a negative electrode, a microporous polypropylene membrane is taken as a diaphragm, 1M LITFSI added with 1 wt.% of lithium nitrate and DOL (dimethyl ether) DME (volume ratio 1:1) type solvent are taken as electrolyte, and a CR2025 type button cell is assembled to perform charge and discharge tests.

As shown in fig. 3 and 4, it is understood that the battery has a charge/discharge voltage of 1.7 to 3.0V,

under the multiplying power of 0.2C, the first discharge specific capacity of the assembled battery is 431.9mAh/g, the first charge specific capacity is 427.4mAh/g, and the first efficiency is 98.95%; the first discharge specific capacity under the multiplying power of 1C is 237.1mAh/g, the first charge specific capacity is 235.8mAh/g, the first efficiency is 99.45%, after the cycle is carried out to 250 circles, the discharge specific capacity is still maintained at 212.3mAh/g, and the capacity retention rate is 90.03%. Therefore, the material can keep the stability of the structure, has small volume expansion and good conductivity in the charge and discharge process, and ensures that the charge and discharge reaction is highly reversible.

Example 2

1. The preparation procedure of this example is as follows:

(2) Adding 0.7g of polystyrene pellets obtained in the step (1), 0.2g of ammonium metavanadate and 0.2g of graphene oxide into 20ml of deionized water, stirring and mixing at 75 ℃, then ultrasonically dispersing the solution for 1h, and freeze-drying the obtained solution for 24h at-45 ℃ to obtain a mixed material of polystyrene, ammonium metavanadate and graphene oxide as a precursor;

The cathode material is prepared by adopting the nano-scale sulfur/pentavanadium octasulfide/graphene composite material, and the formula of the cathode material and the assembly process of the battery prepared by the preparation method are the same as those in the embodiment 1.

The first charge and discharge curve of the assembled battery is shown in fig. 6, under the charge and discharge voltage of 1.7-3.0V and the multiplying power of 0.2C, the first discharge specific capacity of the assembled battery is 420.7mAh/g, the first charge specific capacity is 415.6mAh/g, and the first efficiency is 98.79%;

example 3

1. The preparation procedure of this example is as follows:

(2) Adding 0.2g of polystyrene pellets obtained in the step (1), 0.2g of ammonium metavanadate and 0.3g of graphene oxide into 20ml of deionized water, stirring and mixing at 75 ℃, then ultrasonically dispersing the solution for 1h, and freeze-drying the obtained solution for 24h at-45 ℃ to obtain a mixed material of polystyrene, ammonium metavanadate and graphene oxide as a precursor;

The first charge and discharge curve of the assembled battery is shown in fig. 7, and under the charge and discharge voltage of 1.7-3.0V and the multiplying power of 0.2C, the first discharge specific capacity of the assembled battery is 415.3mAh/g, the first charge specific capacity is 407.1mAh/g, and the first efficiency is 98.03%.

Claims

1. The nanoscale composite material for the positive electrode of the lithium-sulfur battery is characterized by being a sulfur-loaded nanoscale sulfur/vanadic octasulfide/graphene composite material, wherein the vanadic octasulfide is a mixture of vanadic octasulfide and vanadium trioxide, the nanoscale composite material is prepared by taking polystyrene spheres as a template, and the nanoscale sulfur/vanadic octasulfide/graphene composite material has a porous three-dimensional layered structure.

2. The composite material of claim 1, wherein the polystyrene spheres in the synthesized polystyrene sphere template have a diameter of 100 to 500 nm;

or the diameter of the sulfur-loaded pentavanadium octasulfide is 100-500 nm, and the mass percentage of the sulfur is 70-85%.

3. The method for preparing the nanocomposite as claimed in claim 1 or 2, comprising the following steps: (1) synthesizing polystyrene spheres as a template, adding ammonium metavanadate and graphene oxide, taking the freeze-dried material as a precursor, and carrying out heat treatment together with sulfur to obtain an octa-sulfide penta-vanadium/graphene composite material;

4. The preparation method according to claim 3, wherein in the step (1), the polystyrene microsphere template is prepared by taking styrene, methyl methacrylate, acrylic acid, ammonium bicarbonate and ammonium persulfate as raw materials and performing solvothermal reaction to obtain the polystyrene microsphere template with uniform appearance;

preferably, the volume ratio of the styrene to the methyl methacrylate to the acrylic acid is 20:1: 1;

preferably, the mass ratio of the ammonium bicarbonate to the ammonium persulfate is 1: 1;

preferably, the temperature of the solvothermal reaction is 80 ℃;

preferably, the solvothermal reaction time is 12 h.

5. The preparation method according to claims 3 to 4, wherein in the step (1), the mass ratio of the polystyrene spheres to the ammonium metavanadate is 5-10: 2, preferably, the ratio of the polystyrene spheres to the ammonium metavanadate is 7: 2; the mass ratio of the graphene oxide to the ammonium metavanadate is 1-5: 2, and preferably the mass ratio of the graphene to the ammonium metavanadate is 3: 2.

6. The preparation method of claims 3 to 5, wherein in the step (1), the polystyrene spheres, ammonium metavanadate and graphene oxide are mixed, and the freeze-dried precursor material is subjected to heat treatment by adding sulfur into a furnace tube under argon-hydrogen atmosphere to obtain the pentavanadium octasulfide/graphene composite material without the polystyrene sphere template;

preferably, the temperature of the heat treatment is 600-800 ℃, the reaction time of the heat treatment is 1-5 h, more preferably, the temperature of the heat treatment is 700 ℃, and the time of the heat treatment is 2 h.

7. The preparation method of claim 3 to 6, wherein in the step (1), the solid sulfur powder is independently placed in a burning boat, and is subjected to a sulfurization reaction with a mixed material of polystyrene, ammonium metavanadate and graphene oxide while the template is removed.

Preferably, the mass ratio of the sulfur to the mixed materials of polystyrene, ammonium metavanadate and graphene oxide is 20: preferably, the mass ratio of the sulfur to the mixed materials of polystyrene, ammonium metavanadate and graphene oxide is 8: 1.

8. the preparation method according to claims 3 to 7, wherein in the step (2), the mass ratio of the nano-pentavanadium octasulfide/graphene oxide to the sulfur is 1:2 to 5, and preferably the mass ratio of the nano-pentavanadium octasulfide/graphene to the sulfur is 3: 7.

9. The preparation method according to claims 3-8, characterized in that in the step (2), after uniformly mixing sulfur, the pentavanadium octasulfide/graphene oxide composite material and solid sulfur powder, heating to not less than 113 ℃ in an inert atmosphere for heat treatment to obtain sulfur-loaded nano pentavanadium octasulfide/graphene;

preferably, the temperature of the heat treatment is 150-160 ℃, and further preferably, the calcining temperature is 155 ℃; the preparation method according to claim 3 to 9, wherein in the step (2), the heat treatment reaction time is 6 to 24 hours, and more preferably, the calcination time is 12 hours.