CN108598415B

CN108598415B - Composite material for lithium-sulfur battery positive electrode and preparation method thereof

Info

Publication number: CN108598415B
Application number: CN201810370105.4A
Authority: CN
Inventors: 李忠涛; 闫迎春; 邓深圳; 冯建泽; 苏鑫; 吴明铂
Original assignee: China University of Petroleum East China
Current assignee: China University of Petroleum East China
Priority date: 2018-04-24
Filing date: 2018-04-24
Publication date: 2020-05-29
Anticipated expiration: 2038-04-24
Also published as: CN108598415A

Abstract

The invention relates to a composite material for a lithium-sulfur battery anode and a preparation method thereof. The covalent triazine framework doped with nitrogen atoms has good conductivity, and can effectively improve the conductivity of the sulfur anode; the stable covalent triazine framework structure can relieve the damage of volume expansion and contraction of the sulfur positive electrode to the positive electrode in the charge-discharge process; the sulfur fixed in the material in the form of chemical bonds can effectively inhibit the shuttle effect of lithium polysulfide. The composite material shows excellent cycle performance and rate performance when used for a lithium-sulfur battery.

Description

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

Technical Field

The invention relates to the field of chemical power sources, in particular to a composite material for a lithium-sulfur battery anode and a preparation method thereof.

Background

With the rapid development of modern science and technology, lithium ion batteries are also gradually applied to large-sized devices (such as electric vehicles) from small portable devices. The traditional lithium ion battery can not meet the requirement of people on a high-endurance chemical energy storage device. The theoretical specific capacity of elemental sulfur is 1675mAh g^-1The energy density is as high as 2600 Wh.kg^-1And it has the advantages of abundant reserves, environmental protection, low price and the like. Therefore, lithium-sulfur batteries are in the favor of researchers and are considered to be currently commercially available lithiumOne of the viable alternatives to ion batteries.

Despite the many advantages of lithium-sulfur batteries, several significant technical hurdles are not negligible, such as sulfur and its solid products (Li)₂S₂/Li₂S), bulk expansion during lithiation (79%), more importantly dissolved lithium polysulphides (Li)₂S_nAnd n is more than or equal to 4 and less than or equal to 8) into the electrolyte, so that a shuttle effect is generated in the charging and discharging processes. On the one hand, the shuttling effect leads to a loss of capacity of the sulfur positive electrode and poor cycle life of lithium sulfur batteries; on the other hand, passivating the lithium negative electrode limits the rate capability and the utilization rate of active sulfur of the lithium sulfur battery.

In order to solve the above problem, it is necessary to select a suitable sulfur carrier that performs three functions: the method is characterized in that firstly, the method provides electric conductivity for insulating elemental sulfur, secondly, the method provides a stable pore structure for loading sulfur, and thirdly, the shuttle effect of soluble lithium polysulfide can be inhibited. In recent years, researchers have developed a number of strategies for improving the electrochemical performance of lithium sulfur batteries. Among them, one of the most effective ways is to fix sulfur inside the support by means of physical confinement or chemical bonding energy. The physical confinement is to diffuse sulfur into a porous material with high specific surface area and pore volume by a melting method, such as mesoporous carbon, graphene oxide, carbon nanotubes, conductive polymers, Metal Organic Frameworks (MOFs), Covalent Organic Frameworks (COFs), porous organic polymers and the like. Although these composites have improved cycling performance to some extent, these materials have a weak binding force to sulfur or lithium polysulfide which still generates a shuttling effect after longer cycling. The above problems can be effectively solved by binding sulfur by chemical force. According to previous studies, elemental sulfur is opened at high temperatures to form a linear diradical form, which can react with organic monomers or polymers through radical insertion reactions to form stable C — S bonds.

In addition, the conjugated organic framework has the advantages of large specific surface area and pore volume, high conductivity, adjustable pore diameter, good stability and the like, so that the conjugated organic porous material has outstanding advantages when being used as the sulfur carrier of the lithium-sulfur battery.

Based on the research, the invention takes the positive electrode material for the lithium-sulfur battery and the preparation method thereof as research objects, diaminomaleonitrile and sulfur are used for reacting at high temperature to carry out prevulcanization, covalent triazine framework is synthesized through the catalysis of free radicals formed at high temperature of sulfur, meanwhile, sulfur is fixed on a polymer framework in the form of C-S bond, and then, a melting method is carried out to load more sulfur on the covalent triazine framework. The prepared composite material not only has good conductivity, but also can effectively inhibit the dissolution and diffusion of polysulfide. The lithium-sulfur battery positive electrode shows excellent cycle stability and rate capability when used for the lithium-sulfur battery positive electrode.

Disclosure of Invention

In view of the challenges faced by lithium-sulfur batteries, it is an object of the present invention to provide a composite material for a positive electrode of a lithium-sulfur battery and a method for preparing the same, thereby improving cycle performance and rate performance of the lithium-sulfur battery.

The invention is realized by the following technical scheme:

a composite material for a lithium-sulfur battery positive electrode and a preparation method thereof are disclosed, and the preparation method comprises the following steps: first 3.5g of sublimed sulphur was dissolved in 7.6g of CS₂Performing ultrasonic dispersion for 5 min; then adding 5.4g of diaminomaleonitrile, and carrying out ultrasonic dispersion for 30 min; then putting the mixed solution into a high-pressure kettle, repeatedly vacuumizing and supplementing nitrogen for three times, sealing, and heating for 12 hours at 400 ℃; grinding the obtained product and sublimed sulfur uniformly according to the mass ratio of 40:60, putting the ground product and the sublimed sulfur into a glass tube, vacuumizing and sealing the glass tube; the glass tube is put into a tube furnace to be heated for 12 hours at the temperature of 155 ℃, and then the temperature is raised to 200 ℃ to be heated for 30 min. Finally, preparing the composite material S @ CTF-S.

A button type lithium-sulfur battery is assembled by the composite material. The specific assembling method comprises the following steps: according to the following steps: 2: 1, weighing the composite material, acetylene black and polyvinylidene fluoride (PVDF) according to the mass ratio, and uniformly mixing the composite material, the acetylene black and the PVDF to form slurry; uniformly coating the slurry on an aluminum foil, and drying for 12 hours in a vacuum drying oven at 60 ℃; and cutting the electrode plate into a circular electrode plate with the diameter of 12mm, taking metal lithium as a counter electrode, and manufacturing the CR2032 button type lithium-sulfur battery in a glove box filled with argon.

The effective gains of the present invention are:

according to the invention, diaminomaleonitrile and sulfur are used for carrying out a prevulcanization polymerization reaction at a high temperature, the sulfur is used for catalyzing the trimerization of cyano groups, and meanwhile, the sulfur is fixed on a polymer framework in a C-S bond form, so that the synthesis efficiency of the composite material can be effectively improved, and the loading capacity of the sulfur in the composite material can be improved. The nitrogen atom doped covalent triazine framework has good conductivity, and the conductivity of the framework is further improved through the mutual crosslinking effect of sulfur, so that the conductivity of the sulfur anode can be effectively improved. The stable covalent triazine framework structure can relieve the damage of volume expansion and shrinkage of a sulfur positive electrode to the positive electrode in the charging and discharging process, and sulfur fixed in the material in a chemical bond form can effectively inhibit the shuttle effect of lithium polysulfide.

Drawings

The invention has the following drawings:

FIG. 1 is a thermogravimetric plot of S @ CTF-S;

FIG. 2 is an X-ray diffraction diagram of S @ CTF-S and elemental sulfur;

FIG. 3 is an X-ray photoelectron spectrum of S @ CTF-S;

FIG. 4(a) a plot of the capacity voltage at 0.5C magnification for S @ CTF-S, (b) as CS₂A capacity voltage diagram of 0.5C multiplying power of a lithium sulfur battery prepared by using CTF-S as a cathode material, (C) a capacity voltage diagram of 0.5C multiplying power of a lithium sulfur battery prepared by using CTF-S as a cathode material, and (d) a capacity voltage diagram of 0.1mV S of a lithium sulfur battery prepared by using S @ CTF-S as a cathode material^-1Cyclic voltammogram at scanning rate, (e) with CS₂0.1mVs of lithium-sulfur battery prepared by taking CTF-S as cathode material^-1Cyclic voltammogram at scanning rate, (f) 0.1mV S for lithium-sulfur battery prepared by using CTF-S as anode material^-1Cyclic voltammograms at scan rate;

FIG. 5 shows S @ CTF-S, CS, respectively₂CTF-S, wherein the CTF-S is a cycle stability performance diagram and a coulombic efficiency diagram of the lithium-sulfur battery made of the cathode material under the multiplying power of 0.5C;

FIG. 6 shows a cycle stability diagram and a coulombic efficiency diagram of a lithium-sulfur battery manufactured by using S @ CTF-S as a positive electrode material at a magnification of 1C;

FIG. 7 shows S @ CTF-S, CS, respectively₂-CTF-S, wherein the CTF-S is made of anode materialAn alternating current impedance plot for a lithium sulfur battery;

FIG. 8 is a graph showing rate performance of a lithium-sulfur battery using S @ CTF-S as a cathode material.

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings.

Detailed description of the invention

(1) First 3.5g of sublimed sulphur was dissolved in 7.6g of CS₂Performing ultrasonic dispersion for 5 min; then adding 5.4g of diaminomaleonitrile, and carrying out ultrasonic dispersion for 30 min; then putting the mixed solution into a high-pressure kettle, repeatedly vacuumizing and supplementing nitrogen for three times, sealing, and heating for 12 hours at 400 ℃; grinding the obtained product and sublimed sulfur uniformly according to the mass ratio of 40:60, putting the ground product and sublimed sulfur into a glass tube, and vacuumizing and sealing the glass tube; the glass tube is put into a tube furnace to be heated for 12 hours at the temperature of 155 ℃, and then the temperature is raised to 200 ℃ to be heated for 30 min. Finally, preparing the composite material S @ CTF-S. The same method is used for preparing the pre-vulcanization of the sulfur without adding sublimed sulfur and the CS with 66 percent of sulfur content₂Prevulcanised composite materials, respectively designated CS₂-CTF-S and CTF-S.

(2) According to the following steps: 2: 1, weighing the composite material, acetylene black and polyvinylidene fluoride (PVDF) according to the mass ratio, and uniformly mixing the composite material, the acetylene black and the PVDF to form slurry; and uniformly coating the slurry on an aluminum foil, and drying in a vacuum drying oven at 60 ℃ for 12h to obtain the lithium-sulfur battery anode.

(3) And cutting the positive electrode material into sheets with the diameter of 12mm to prepare the pole pieces. Metal lithium was used as a counter electrode, and a CR2032 type button cell was fabricated in a glove box filled with argon, the separator was a microporous polypropylene separator (Celgard2400, usa), and the electrolyte was l.0mol L^-1The lithium bis (trifluoromethanesulfonyl) imide is dissolved in a mixed solution prepared from 1, 3-dioxolane and glycol dimethyl ether in a volume ratio of 1:1, and the additive is anhydrous lithium nitrate with the mass fraction of 1 wt%. And the LandCT2001A battery test system is adopted to test the charge and discharge performance of the sample, and the charge and discharge termination voltage is 1.7-2.8V. The cyclic voltammetry test adopts the electrochemical workstation of Shanghai Chenghua CHI760E to test, and the scanning rate is 0.1 mV.s^-1And the voltage range is 1.7-2.8V.

Analytical characterisation

the samples were analyzed for structure and phase by X' Pert PRO MPD X-ray diffractometer (XRD, CuK α, λ 0.15406nm) in the netherlands.german STA 409PC Luxx thermogravimetric analyzer (TGA) was used to measure the sulfur content of the samples under nitrogen atmosphere.

Results and analysis:

to obtain the sulfur content of the S @ CTF-S composite, the sample was subjected to a thermogravimetric test (see FIG. 1). As can be seen from FIG. 1, the sulfur loss mainly occurs between 200-400 ℃, and the sulfur content in the composite material is 66%. Between 200 and 350 ℃ there is a loss of about 58% of the sulfur, the portion being mainly sulfur that is melt diffused into the composite material, and between 350 and 400 ℃ there is a loss of about 8% of the sulfur, the portion being mainly sulfur that is chemically fixed into the composite material.

FIG. 2 is an X-ray diffraction pattern of the S @ CTF-S composite and sublimed sulfur. Fig. 2(a) shows a significantly reduced diffraction peak for the sulfur crystals compared to fig. 2(b), indicating that most of the sulfur has entered the interior of the material.

To further verify the chemical composition of the composite, the S @ CTF-S composite was subjected to XPS testing. C1S can be divided into 3 peaks as shown in fig. 3(a), corresponding to C-N ═ C (288.6ev), C-S (286.1ev), C-C (284.6ev), respectively. As shown in FIG. 3(b), the peak N1s can be divided into two peaks, corresponding to pyridine type N (398.3eV) and pyrrole type N (399.5eV) with high conjugation. Theoretical research shows that the hole and electron defect sites on the pyridine N are beneficial to the rapid transmission of lithium ions, so that the conductivity of the material is improved. As shown in FIG. 4(C), S2p can be divided into four peaks corresponding to C-S (168ev), C-S_n-C(164.8ev)，C-S_n-C (163.6ev), S-S (161.7ev), the formation of C-S in the composite material indicates that sulfur is fixed inside the CTF material in the form of chemical bonds. Combining the above analyses, it can be concluded that S @ CTF-S composites have been successfully prepared herein.

FIG. 4 is S @ CTF-S, CS₂CTF-S and CTF-S constant current charging at 0.5C Current Density at

times

1, 2, 100 and 700Discharge curve and at 0.1mV s^-1Cyclic voltammograms at scan rates, as can be seen from fig. 4(a), (b), and (c), Δ E ═ 243mV between the charge and discharge plateaus of S @ CTF-S lithium sulfur cells, while CS₂Delta E between charge and discharge plateaus for CTF-S and CTF-S lithium sulfur cells was 317mV and 292mV, respectively. The S @ CTF-S lithium sulfur battery has lower electrode polarizability, smaller kinetic reaction barrier and higher reversibility. FIGS. 4(d), (e), (f) all have two distinct reduction peaks and one oxidation peak, with the higher reduction peak corresponding to the conversion of elemental sulfur to soluble lithium polysulfide (LiS) during discharge_xX is more than or equal to 4 and less than or equal to 8), the lower reduction peak corresponds to the conversion of soluble lithium polysulphide into insoluble Li during the discharge process₂S₂And Li₂S process, one oxidation peak corresponding to Li in the charging process₂S₂/Li₂Conversion of S to S₈The process of (1). This is consistent with the charge and discharge plateaus in the constant current charge and discharge diagrams in fig. 4(a), (b), and (c). In addition, as can be seen from the cyclic voltammogram, the distance between the oxidation peak and the reduction peak of S @ CTF-S is reduced from the second circle, which shows that the reaction resistance is reduced and the reversibility and stability are improved after the activation of the first circle.

FIG. 5 is S @ CTF-S, CS₂-CTF-S and CTF-S cycling behavior at a current density of 0.5C and coulombic efficiency plot, S @ CTF-S, CS₂Initial capacities of-CTF-S and CTF-S were 1050.8mA g, respectively^-1，681mAh·g^-1，531.1mAh·g^-1After 700 cycles, the capacity was maintained at 544.5mAh g^-1，343.3mAh·g^-1，316mAh·g^-1. It can be seen that S @ CTF-S exhibits high specific capacity and cycling stability. The S @ CTF-S composite material can effectively inhibit the shuttle effect of lithium polysulfide, improves the cycle stability of the lithium sulfur battery, and shows that sulfur and triazine framework structure are combined in a C-S bond mode after prevulcanization, and the lithium polysulfide is prevented from being dissolved and diffused by strong chemical acting force.

In order to further prove the good cycling stability of the S @ CTF-S composite material, a constant-current charge-discharge curve of the S @ CTF-S composite material under the current density of 1C is tested. As shown in FIG. 6, the initial capacity of S @ CTF-S is 834.1mA · g^-1After 700 cycles, the capacity retention rate was maintained at 447.9mA · g^-1The coulombic efficiency is still as high as 97.5%, which shows that S @ CTF-S has good circulation stability.

In order to further analyze the reason that the S @ CTF-S composite material battery has good performance, alternating current impedance tests are respectively carried out on the lithium-sulfur batteries of the three composite materials. As can be seen from fig. 7, the ac impedance curve is composed of a circular arc in the high frequency region and a straight line in the low frequency region. The smaller the semi-circle diameter of the high frequency region, the smaller the resistance. And CS₂The reduced impedance of the S @ CTF-S composite compared to the CTF-S composite indicates that the presence of sulfur promotes the formation of the covalent triazine framework and that the conductivity of the covalent triazine framework is improved by the cross-linking interaction of sulfur.

In order to further illustrate the good electrochemical performance of the S @ CTF-S composite material, a rate capability test is carried out on the S @ CTF-S composite material. FIG. 8 is a graph showing the rate capability of batteries assembled with S @ CTF-S composite material, and it can be seen that the specific capacity of the batteries with S @ CTF-S composite material can be maintained at 830.9mAh g at 0.1C, 0.2C, 0.5C, 1C and 2C, respectively^-1、783.2mAh·g^-1、706.3mAh·g^-1、605.3mAh·g^-1When the specific capacity is returned to 0.5C, the specific capacity can still be returned to 531.9mAh g^-1The S @ CTF-S composite material battery has good rate capability.

The S @ CTF-S composite material is prepared by pre-vulcanizing diaminomaleonitrile and sulfur through reaction at high temperature, synthesizing covalent triazine framework through catalysis of free radicals formed at high temperature of sulfur, simultaneously fixing the sulfur on a polymer framework in a form of C-S bonds, and then carrying out a melting method to load more sulfur on the covalent triazine framework, and shows good electrochemical performance when the composite material is used for a positive electrode of a lithium-sulfur battery: under the current density of 1C, the specific discharge capacity of the lithium ion battery is still as high as 447.9mAh g after 700 times of circulation^-1The coulombic efficiency is kept above 97.5%. This is due to the fact that sulfur promotes the trimerization of cyano groups into triazine framework structures, and the presulfided covalent triazine framework has a larger pore volume that favors the efficient loading of high sulfur content; the nitrogen atom doped covalent triazine framework has good conductivity and is further crosslinked through sulfur interactionThe conductivity of the covalent triazine framework is improved, so that the conductivity of the sulfur anode can be effectively improved; the stable covalent triazine framework structure can relieve the damage of volume expansion and contraction of the sulfur positive electrode to the positive electrode in the charge-discharge process; the sulfur fixed in the material in the form of chemical bonds can effectively inhibit the shuttle effect of lithium polysulfide.

Claims

1. A preparation method of a composite material for a positive electrode of a lithium-sulfur battery is characterized by comprising the following steps: first 3.5g of sublimed sulphur was dissolved in 7.6g of CS₂Performing ultrasonic dispersion for 5 min; then adding 5.4g of diaminomaleonitrile, and carrying out ultrasonic dispersion for 30 min; then putting the mixed solution into a high-pressure kettle, repeatedly vacuumizing and supplementing nitrogen for three times, sealing, and heating at 400 ℃ for 12 hours for prevulcanization; grinding the obtained product and sublimed sulfur uniformly according to the mass ratio of 40:60, putting the ground product and sublimed sulfur into a glass tube, and vacuumizing and sealing the glass tube; putting the glass tube into a tube furnace, heating for 12h at 155 ℃, and then heating for 30min when the temperature is raised to 200 ℃; finally, a composite material S @ CTF-S is prepared and applied to a positive electrode of a button type lithium-sulfur battery.

2. The method for preparing a composite material for a positive electrode of a lithium sulfur battery according to claim 1, characterized in that: according to the following steps: 2: 1, weighing the composite material S @ CTF-S, acetylene black and polyvinylidene fluoride (PVDF) in a mass ratio, and uniformly mixing the three materials to form slurry; uniformly coating the slurry on an aluminum foil, and drying for 12 hours in a vacuum drying oven at 60 ℃; and cutting the electrode plate into a circular electrode plate with the diameter of 12mm, taking metal lithium as a counter electrode, and manufacturing the CR2032 button type lithium-sulfur battery in a glove box filled with argon.