CN111261837A - Cathode material of pentafluoromagnesium aluminum/nitrogen carbon-doped lithium sulfur battery and preparation method thereof - Google Patents
Cathode material of pentafluoromagnesium aluminum/nitrogen carbon-doped lithium sulfur battery and preparation method thereof Download PDFInfo
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Abstract
The invention provides a pentafluoromagnesium aluminum/nitrogen carbon-doped lithium sulfur battery anode material and a preparation method thereof, wherein the method comprises the steps of modifying attapulgite with acid, mixing the modified attapulgite with glucose and ammonium chloride uniformly, evaporating to dryness, calcining to obtain nitrogen carbon-doped coated attapulgite, treating with hydrofluoric acid to obtain pentafluoromagnesium aluminum/nitrogen carbon-doped, and carrying sulfur to obtain a pentafluoromagnesium aluminum/nitrogen carbon-doped sulfur-loaded composite material; and mixing the obtained composite material, a conductive agent and a binder in a solvent, coating the mixture on a current collector, and drying to obtain the cathode material of the pentafluoromagnesium aluminum/nitrogen carbon-doped lithium sulfur battery. The nitrogen-doped carbon amorphous carbon tube in the obtained cathode material has a limited domain effect on polysulfide, the magnesium aluminum pentafluoride loaded on the inner surface and the outer surface of the amorphous carbon tube has an adsorption effect on polysulfide, and the synergistic effect of the two can effectively inhibit the shuttle effect of polysulfide and improve the electrochemical performance of the lithium-sulfur battery.
Description
Technical Field
The invention relates to the field of new energy materials, in particular to a pentafluoromagnesium aluminum/nitrogen carbon-doped lithium sulfur battery positive electrode material and a preparation method thereof.
Background
With the increasing demand for renewable energy, high energy density batteries are receiving a great deal of attention from both academic and industrial sectors. Among the numerous battery technologies, lithium sulfur batteries have high theoretical specific capacities (up to 1675mAh g-1) The energy density and the volume density of the battery are respectively as high as 2600Wh kg-1And 2800W h L-1The material is more than five times of other embedded anode materials, and the driving range of the material applied to the electric automobile is theoretically more than 400 km. And the sulfur resource on the earth is rich and environment-friendly. However, there are still problems in the practical application of lithium-sulfur batteries: (1) poor conductivity of sulfur and lithium sulfur products; (2) approximately 80% volume expansion during cycling; (3) intermediate polysulfide (Li)2SxX is more than or equal to 4 and less than or equal to 8) dissolution and shuttle effect in the charge-discharge process; (4) soluble Li2S4To solid Li2The liquid-solid phase transition kinetics of S is slow, resulting in low utilization of S. These are the main causes of capacity fade and coulombic efficiency of lithium-sulfur batteries.
In the current research, a large-specific-surface-area and porous carbon material (porous carbon, graphene and carbon nano tube with different pore size distributions) and an active substance sulfur are compounded to serve as a positive electrode material of a lithium sulfur battery, polysulfide is adsorbed by virtue of pore adsorption, and the shuttle effect is inhibited. However, pure carbon materials are electron neutral, nonpolar, and have only weak intermolecular interactions with polar polysulfides, resulting in poor adsorption effects. Therefore, it is generally necessary to assemble a metal compound (e.g., a positive electrode material of a lithium sulfur battery, molybdenum disulfide, tin disulfide, niobium disulfide, or the like) on a carbon material to fix polysulfides by chemisorption.
Chinese patent CN108649194A discloses a graphene-loaded molybdenum disulfide lithium sulfur battery cathode material and a preparation method thereof, the cathode material has low sulfur loading amount of microporous carbon, high cost of graphene and carbon nanotubes, complex process for assembling metal compounds, high cost, difficulty in mass production, and stable assembly of metal compounds on carbon materials (especially graphene and carbon nanotubes with few defects) is also a challenge. Chinese patent CN109546098A discloses a reduced graphene oxide loaded ReS for a lithium-sulfur battery anode material2Preparation method of (1), ReS2The transition metal sulfide is a polar material, wherein S can form a bond with Li in lithium polysulfide, and Re can form a bond with S in the lithium polysulfide, so that the strong chemical adsorption effect on the lithium polysulfide is achieved, the dissolution and diffusion of the lithium polysulfide in an electrolyte are effectively inhibited, and the electrochemical cycling stability is improved.
Therefore, there is a need for a new positive electrode material for lithium-sulfur batteries and a preparation method thereof, which can simplify the process steps, reduce the preparation cost, improve the adsorption capacity to polysulfide, and improve the electrochemical performance of lithium-sulfur batteries. The attapulgite is a water-containing magnesium-aluminum-rich silicate inorganic non-metallic clay mineral, is low in price, has a one-dimensional rod-like shape, large major diameter ratio and specific surface area, rich pore channel structures and rich surface hydroxyls, and shows good application prospects in the fields of adsorption, catalysis and electrochemistry. The Si nano material extracted from the attapulgite is applied to the lithium battery cathode, so that the preparation cost of the Si cathode is greatly reduced. The attapulgite components and the morphological characteristics are expected to be used for preparing the novel lithium-sulfur cathode material with low cost and high performance.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a pentafluoromagnesium aluminum/nitrogen carbon-doped lithium sulfur battery positive electrode material and a preparation method thereof, and aims to utilize the adsorption effect of the pentafluoromagnesium aluminum on polysulfide, inhibit shuttle effect and promote charge transfer so as to improve the specific capacity and the cycle stability of the lithium sulfur battery.
The nitrogen-doped amorphous carbon tube has the characteristics of porosity and communication, and can promote the migration of lithium ions while providing a sulfur-carrying space and relieving the volume expansion of sulfur. In addition, the raw material attapulgite is cheap and large in quantity, the cost is reduced, and the commercialization is facilitated.
In order to achieve the purpose, the invention adopts the following technical scheme:
a positive electrode material of a pentafluoromagnesium aluminum/nitrogen carbon-doped lithium sulfur battery and a preparation method thereof comprise the following steps:
(1) preparation of magnesium aluminum pentafluoride/nitrogen carbon-doped composite material
Sieving natural attapulgite, placing in acid liquor after sieving, heating, stirring, performing suction filtration, washing and drying to obtain modified attapulgite;
mixing glucose, ammonium chloride and the obtained modified attapulgite according to the mass ratio of 20 (15-20) to (9-13.5), adding deionized water, stirring, and then performing hot water bath until water is completely volatilized to obtain a precursor;
calcining the obtained precursor in an inert atmosphere to obtain the nitrogen-doped carbon-coated modified attapulgite;
adding hydrofluoric acid into the obtained nitrogen-doped carbon-coated modified attapulgite, heating, stirring, reacting, and then performing suction filtration, washing and drying to obtain a magnesium aluminum pentafluoride/nitrogen-doped carbon composite material;
wherein the mass ratio of the nitrogen-doped carbon-coated modified attapulgite to the hydrofluoric acid is 1: (6.72-33.6);
(2) carrying sulfur
Mixing and grinding the magnesium aluminum pentafluoride/nitrogen-doped carbon composite material obtained in the step (1) and elemental sulfur according to a mass ratio of 3:7, placing the mixture into a reaction kettle with a polytetrafluoroethylene lining, placing the reaction kettle into an oven, heating the reaction kettle to 155 ℃ at a heating rate of 5-10 ℃/min, preserving the heat for 12-18h, cooling the reaction kettle to room temperature, and grinding the mixture uniformly to obtain the magnesium aluminum pentafluoride/nitrogen-doped carbon-doped sulfur composite material;
(3) preparation of cathode material
And (3) uniformly mixing the magnesium aluminum pentafluoride/nitrogen-doped carbon-loaded sulfur composite material obtained in the step (2), a conductive agent and a binder in a solvent to obtain positive electrode slurry, coating the obtained positive electrode slurry on a current collector, and drying to obtain the magnesium aluminum pentafluoride/nitrogen-doped carbon-loaded lithium sulfur battery positive electrode material.
Preferably, the acid solution in the step (1) is a hydrochloric acid solution with a concentration of 4-6 mol/L.
Preferably, the heating and stirring treatment in the step (1) is specifically heating and stirring treatment at 70-90 ℃ for 2-4 hours.
Preferably, the calcining temperature in the step (1) is 600-900 ℃; the temperature rising speed is 4-6 ℃/min; the calcination time is 3-7 h.
Preferably, the washing in step (1) is specifically to be neutral with deionized water and absolute ethanol.
Preferably, the elemental sulfur in step (2) is sublimed sulfur.
Preferably, the conductive agent in step (3) is conductive carbon black; the binder is polyvinylidene fluoride; the solvent is N-methyl pyrrolidone; the current collector is a carbon-coated aluminum foil.
Preferably, the mass ratio of the magnesium aluminum pentafluoride/nitrogen-doped carbon-sulfur-loaded composite material, the conductive agent and the binder in the step (3) is 7:2: 1.
The invention also provides a positive electrode material of the pentafluoromagnesium aluminum/nitrogen-doped carbon lithium sulfur battery prepared by the method.
Preferably, the nitrogen-doped carbon is an amorphous carbon tube doped with nitrogen, the magnesium aluminum pentafluoride is loaded on the inner and outer surfaces of the amorphous carbon tube, and the particle size of the magnesium aluminum pentafluoride is 4-100 nm.
The scheme of the invention has the following beneficial effects:
(1) the invention takes the clay mineral attapulgite as a template and the raw material to prepare the pentafluoromagnesium aluminum/nitrogen carbon-doped composite material, and no research for applying the pentafluoromagnesium aluminum to the field of batteries exists at home and abroad.
(2) The invention uses the magnesium aluminum pentafluoride/nitrogen carbon-doped composite material as the anode material of the lithium-sulfur battery after carrying sulfur for the first time, and utilizes the magnesium aluminum pentafluoride to adsorb polysulfide and inhibit shuttle effect, thereby improving the specific capacity and the cycling stability of the lithium-sulfur battery.
(3) The raw material used by the preparation method provided by the invention is clay mineral attapulgite with abundant resources and low price, the preparation process is simple and convenient, the cost is lower, and the commercialization of the lithium-sulfur battery is facilitated.
(4) The nitrogen-doped amorphous carbon tube prepared by the method has more defects, so that the in-situ assembled magnesium aluminum pentafluoride stably grows on the nitrogen-doped amorphous carbon tube, compared with a commercial carbon nanotube, the nitrogen-doped amorphous carbon tube has the characteristics of porosity and communication, and can reduce the barrier of lithium ions diffusing between tube walls while providing a sulfur-carrying space and relieving the volume expansion of sulfur.
(5) In example 3, the lithium-sulfur positive electrode made of the pentafluoromagnesium aluminum/nitrogen carbon-doped composite material has a specific first-cycle discharge capacity of 1018.2mAh g at a rate of 0.5C-1And the specific discharge capacity after 500 cycles is 613.2mAh g-1The capacity retention was 60.2%, and the capacity loss per turn was only 0.0796%. In contrast, comparative example 1, the nitrogen-doped carbon-loaded sulfur composite material was used to prepare a lithium-sulfur positive electrode, and the specific first-cycle discharge capacity was 889.2mAh g at a rate of 0.5C-1The discharge specific capacity after 500 cycles is 290.2mAh g-1The capacity retention was as low as 32.6% with a capacity loss per revolution of 0.1348%. The performance comparison result shows that the magnesium-aluminum pentafluoride has a strong adsorption effect on polysulfide, so that the shuttle effect is inhibited, and the performance of the lithium-sulfur battery is improved.
The invention firstly prepares the positive electrode material of the lithium-sulfur battery doped with carbon and pentafluoromagnesium aluminum/nitrogen, finds that the positive electrode material has strong adsorption effect on polysulfide, can obviously inhibit shuttle effect of polysulfide ions, and improves the cycle stability of the positive electrode material of the lithium-sulfur battery.
Drawings
FIG. 1 is an X-ray diffraction pattern of examples 2-3 and comparative example 1;
FIG. 2 is a transmission electron micrograph of example 3;
FIG. 3 is a cycle performance map of example 2;
FIG. 4 is a cycle performance map of example 3;
FIG. 5 is a cycle performance map of comparative example 1;
FIG. 6 is a cycle performance map of comparative example 2;
FIG. 7 is a coulombic efficiency comparison graph of example 3 versus comparative example 1;
FIG. 8 is the first cycle charge and discharge curve of example 2;
fig. 9 is a graph comparing the ac impedance of example 2 and comparative example 1.
Detailed Description
In order to make the technical problems, technical solutions and advantages to be solved by the present invention clearer, the following detailed description is given with reference to specific embodiments.
The elemental sulfur used in the following examples and comparative examples is sublimed sulfur, an orthorhombic phase, a sulfur-carrying composite material of pentafluoromagnesium aluminum/nitrogen-doped carbon, nitrogen-doped carbon or commercial carbon nanotubes in the preparation of a pole piece, the mass ratio of a conductive agent to a binder is 8:1:1, wherein the conductive agent is conductive carbon black, the binder is polyvinylidene fluoride, a solvent is N-methyl pyrrolidone, a current collector is carbon-coated aluminum foil, the thickness of the current collector is 17 micrometers, and the diameter of the current collector is 12cm, and after coating, the current collector is dried in a vacuum drying oven for 12 hours at 60 ℃. Then in a glove box filled with argon, a sheet of lithium metal was used as the counter electrode and the separator was Celgard 2500. 1mol of LiTFSI was dissolved in DME at 1: 1V% and 1% LiNO was added3. The button cell model is CR 2025. Electrochemical testing: at 0.5C (800mA g)-1) The constant current charge and discharge test is carried out under the current density of the voltage, and the voltage range is 1.7-2.8V. The AC impedance test is carried out under open circuit voltage, with frequency ranging from 0.01 to 100000Hz and amplitude of 0.05 mV.
Example 1
(1) Treatment of attapulgite
Screening natural attapulgite with 200 mesh sieve, adding 6g attapulgite into 200mL hydrochloric acid with concentration of 4mol/L, and heating and stirring at 90 deg.C for 2 hr. And then carrying out suction filtration, washing a product obtained after suction filtration to be neutral, and then placing the obtained sample in a vacuum drying oven for drying for 12 h.
(2) Attapulgite-coated nitrogen-doped carbon
Weighing the 2g of modified attapulgite, 1.5g of glucose and 0.9g of ammonium chloride in a 100mL beaker, adding 40mL of deionized water, continuously stirring for 24h, and then carrying out water bath at 80 ℃ until the water is volatilized completely to obtain a precursor. And (3) calcining the precursor in a tubular furnace under the argon atmosphere at the heating rate of 4 ℃/min at the temperature of 600 ℃ for 3h to obtain the nitrogen-doped carbon-coated attapulgite compound.
(3) HF treatment
Treating the obtained nitrogen-doped carbon-coated attapulgite composite with hydrofluoric acid, wherein the mass ratio of the composite to the hydrofluoric acid is 1: 6.72, heating and stirring at 80 ℃ for reaction for 10 hours, then carrying out suction filtration, washing with deionized water and absolute ethyl alcohol for several times until the mixture is neutral, and then placing the obtained sample in a vacuum drying oven for drying for 12 hours to obtain the magnesium aluminum pentafluoride/nitrogen carbon-doped composite material.
(4) Carrying sulfur
The whole process is carried out in a glove box filled with argon, the materials and elemental sulfur are uniformly ground according to the mass ratio of 3:7, the ground materials and the elemental sulfur are poured into a glass bottle with the capacity of 3mL, then the glass bottle is placed into a reaction kettle with a polytetrafluoroethylene lining with the capacity of 25mL for sealing, the reaction kettle is placed into an oven for keeping at 155 ℃ for 18h, and the heating rate is 10 ℃/min. Naturally cooling to room temperature, taking out, fully and uniformly grinding to obtain the magnesium aluminum pentafluoride/nitrogen carbon-doped sulfur-carrying composite material.
(5) Battery performance results
The specific first-cycle discharge capacity of the lithium-sulfur anode at a multiplying power of 0.5C is 992.7mAh g-1The specific discharge capacity after 500 cycles is 632.5mAh g-1The capacity retention was 63.7%, and the capacity loss per turn was only 0.0726%. The specific data are shown in Table 1.
Example 2
(1) Treatment of attapulgite
Screening natural attapulgite with 200 mesh sieve, adding 6g attapulgite into 200mL hydrochloric acid with concentration of 6mol/L, and heating and stirring at 70 deg.C for 4 hr. And then carrying out suction filtration, washing a product obtained after suction filtration to be neutral, and then placing the obtained sample in a vacuum drying oven for drying for 12 h.
(2) Attapulgite-coated nitrogen-doped carbon
Weighing the 2g of modified attapulgite, 1.5g of glucose and 1.35g of ammonium chloride in a 100mL beaker, adding 40mL of deionized water, continuously stirring for 24h, and then carrying out water bath at 80 ℃ until the water is volatilized completely to obtain a precursor. And (3) calcining the precursor in a tubular furnace under the argon atmosphere at the heating rate of 6 ℃/min at 900 ℃ for 7h to obtain the nitrogen-doped carbon-coated attapulgite compound.
(3) HF treatment
Treating the obtained nitrogen-doped carbon-coated attapulgite composite with hydrofluoric acid, wherein the mass ratio of the composite to the hydrofluoric acid is 1: 8.96, heating and stirring at 80 ℃ for reaction for 10h, then carrying out suction filtration, washing with deionized water and absolute ethyl alcohol for several times until the mixture is neutral, and then placing the obtained sample in a vacuum drying oven for drying for 12h to obtain the magnesium aluminum pentafluoride/nitrogen carbon-doped composite material.
(4) Carrying sulfur
The whole process is carried out in a glove box filled with argon, the materials and elemental sulfur are uniformly ground according to the mass ratio of 3:7, the ground materials and the elemental sulfur are poured into a glass bottle with the capacity of 3mL, then the glass bottle is placed into a reaction kettle with a polytetrafluoroethylene lining with the capacity of 25mL for sealing, the reaction kettle is placed into an oven for keeping at 155 ℃ for 12 hours, and the heating rate is 5 ℃/min. Naturally cooling to room temperature, taking out, fully and uniformly grinding to obtain the magnesium aluminum pentafluoride/nitrogen carbon-doped sulfur-carrying composite material.
(5) Battery performance results
The specific first-cycle discharge capacity of the lithium-sulfur anode at a multiplying power of 0.5C is 896.3mAh g-1The specific discharge capacity after 500 cycles is 633.1mAh g-1The capacity retention was 70.6%, and the capacity loss per turn was only 0.0588%. The specific data are shown in Table 1.
FIG. 3 is a graph showing the cycle characteristics of example 2, having a specific first-cycle discharge capacity of 896.3mAh g at a rate of 0.5C-1The specific discharge capacity after 500 cycles is 633.1mAh g-1The capacity retention rate is 70.6%, the capacity loss per circle is only 0.0588%, the coulombic efficiency is always kept above 98%, and the good cycle performance is due to the strong adsorption effect of the magnesium aluminum pentafluoroate on polysulfide, so that the shuttle effect is inhibited.
FIG. 8 is a first cycle charge-discharge curve of example 2, which is a typical positive charge-discharge curve of a lithium sulfur battery, and the discharge process is divided into two platforms, wherein the first platform near 2.3V corresponds to the transformation of elemental sulfur from lithium intercalation into polysulfide, and the second platform at 2.08V corresponds to the transformation of polysulfide into Li2S2And Li2And S is changed. A platform in the charging process corresponds to Li2And the process of gradually converting S delithiation into elemental sulfur.
Example 3
(1) Treatment of attapulgite
Screening natural attapulgite with 200 mesh sieve, adding 6g attapulgite into 200mL hydrochloric acid with concentration of 6mol/L, and heating and stirring at 80 deg.C for 3 hr. And then carrying out suction filtration, washing a product obtained after suction filtration to be neutral, and then placing the obtained sample in a vacuum drying oven for drying for 12 h.
(2) Attapulgite-coated nitrogen-doped carbon
Weighing the above 2g of modified attapulgite, 2g of glucose and 1.2g of ammonium chloride, putting into a 100mL beaker, adding 40mL of deionized water, continuously stirring for 24h, and then carrying out water bath at 80 ℃ until the water is volatilized completely to obtain a precursor. And (3) calcining the precursor in a tubular furnace under the argon atmosphere at the heating rate of 5 ℃/min at the temperature of 800 ℃ for 5h to obtain the nitrogen-doped carbon-coated attapulgite compound.
(3) HF treatment
Treating the obtained nitrogen-doped carbon-coated attapulgite composite with hydrofluoric acid, wherein the mass ratio of the composite to the hydrofluoric acid is 1: 33.6, heating and stirring at 80 ℃ for reaction for 10h, then carrying out suction filtration, washing with deionized water and absolute ethyl alcohol for several times until the mixture is neutral, and then placing the obtained sample in a vacuum drying oven for drying for 12h to obtain the magnesium aluminum pentafluoride/nitrogen carbon-doped composite material.
(4) Carrying sulfur
The whole process is carried out in a glove box filled with argon, the materials and elemental sulfur are uniformly ground according to the mass ratio of 3:7, the ground materials and the elemental sulfur are poured into a glass bottle with the capacity of 3mL, then the glass bottle is placed into a reaction kettle with a polytetrafluoroethylene lining with the capacity of 25mL for sealing, the reaction kettle is placed into an oven for keeping at 155 ℃ for 12 hours, and the heating rate is 5 ℃/min. Naturally cooling to room temperature, taking out, fully and uniformly grinding to obtain the magnesium aluminum pentafluoride/nitrogen carbon-doped sulfur-carrying composite material.
(5) Battery performance results
The specific first-cycle discharge capacity of the lithium-sulfur anode at a multiplying power of 0.5C is 1018.2mAh g-1And the specific discharge capacity after 500 cycles is 613.2mAh g-1The capacity retention rate is 60.2%, and the capacity loss per circle is only 0.0796%, and the specific data are shown in Table 1.
FIG. 2 is a transmission electron micrograph of example 3, which shows that granular magnesium aluminum pentafluoride is successfully loaded on the carbon-doped nitrogen tube, the particle size of the magnesium aluminum pentafluoride is between 15 and 80nm, and the tube diameter of the carbon tube is between 9 and 21 nm.
FIG. 4 is a graph showing the cycle characteristics of example 3, having a specific first-cycle discharge capacity of 1018.2mAh g at a rate of 0.5C-1And the specific discharge capacity after 500 cycles is 613.2mAh g-1The capacity retention was 60.2%, and the capacity loss per turn was only 0.0796%. The good cycle performance is due to the strong adsorption of magnesium aluminum pentafluoride to polysulfides, inhibiting the shuttling effect.
Comparative example 1
(1) Treatment of attapulgite
Screening natural attapulgite with a 200 mesh sieve, adding 6g attapulgite into 200mL hydrochloric acid with concentration of 6mol/L, and heating and stirring at 80 deg.C for 3 hr. And then carrying out suction filtration, washing a product obtained after suction filtration to be neutral, and then placing the obtained sample in a vacuum drying oven for drying for 12 h.
(2) Attapulgite-coated nitrogen-doped carbon
Weighing the 2g of modified attapulgite, 1.5g of glucose and 0.9g of ammonium chloride in a 100mL beaker, adding 40mL of deionized water, continuously stirring for 24h, and then carrying out water bath at 80 ℃ until the water is volatilized completely to obtain a precursor. And (3) calcining the precursor in a tubular furnace under the argon atmosphere at the heating rate of 5 ℃/min at the temperature of 800 ℃ for 5h to obtain the nitrogen-doped carbon-coated attapulgite compound.
(3) Preparation of nitrogen-doped carbon composite material
Treating the obtained nitrogen-doped carbon-coated attapulgite composite with hydrofluoric acid, wherein the mass ratio of the composite to the hydrofluoric acid is 1: 22.4, heating and stirring at 80 ℃ for reaction for 10 hours, then carrying out suction filtration, washing with deionized water and absolute ethyl alcohol for a plurality of times until the solution is neutral, and using Ca (OH) to treat filtrate2And (6) processing. The solid obtained above was then treated with 100mL of concentrated HCl at 80 ℃ for 24h to remove the magnesium aluminum pentafluoride and then treated with 100mL of 4mol/L NaOH solution at 80 ℃ for 24 h. Washing with deionized water and absolute ethyl alcohol for several times until the sample is neutral, and then placing the obtained sample in a vacuum drying oven for drying for 12 hours to obtain the nitrogen-doped carbon nanotube.
(4) Carrying sulfur
The whole process is carried out in a glove box filled with argon, the materials and elemental sulfur are uniformly ground according to the mass ratio of 3:7, the ground materials and the elemental sulfur are poured into a glass bottle with the capacity of 3mL, then the glass bottle is placed into a reaction kettle with a polytetrafluoroethylene lining with the capacity of 25mL for sealing, the reaction kettle is placed into an oven for keeping at 155 ℃ for 12 hours, and the heating rate is 5 ℃/min. Naturally cooling to room temperature, taking out, and fully and uniformly grinding to obtain the nitrogen-doped carbon-sulfur-carrying composite material.
(5) Battery performance results
The specific first-cycle discharge capacity of the lithium-sulfur anode at a multiplying power of 0.5C is 889.2mAh g-1The discharge specific capacity after 500 cycles is 290.2mAh g-1The capacity retention rate is 32.6%, and the capacity loss per circle is only 0.1348%, and the specific data are shown in table 1.
FIG. 5 is a graph showing the cycle characteristics of comparative example 1, in which the specific first-cycle discharge capacity was 889.2mAh g at a rate of 0.5C-1The discharge specific capacity after 500 cycles is 290.2mAh g-1The capacity retention was 32.6%, and the capacity loss per turn was only 0.1348%. After the magnesium aluminum pentafluoride is not loaded, the nitrogen-doped carbon nanotube sulfur-loaded positive electrode has an obvious overcharge phenomenon after being circulated for 100 weeks, and the capacity attenuation trend is obvious, which shows that the magnesium aluminum pentafluoride can effectively inhibit the shuttle effect and improve the capacity and the circulation stability of the lithium-sulfur battery.
Comparative example 2
(1) Carrying sulfur
The whole process is carried out in a glove box filled with argon, the commercial carbon nano tube and elemental sulfur are uniformly ground according to the mass ratio of 3:7, the ground material is poured into a glass bottle with the capacity of 3mL, then the glass bottle is placed into a reaction kettle with a polytetrafluoroethylene lining with the capacity of 25mL for sealing, the reaction kettle is placed into an oven and is kept for 12 hours at the temperature of 155 ℃, and the heating rate is 5 ℃/min. Naturally cooling to room temperature, taking out, fully and uniformly grinding to obtain the commercial carbon nano tube sulfur-carrying composite material.
(2) Battery performance results
The specific first-cycle discharge capacity of the lithium-sulfur anode at a multiplying power of 0.5C is 591.7mAh g-1After 500 cycles, the discharge specific capacity is 134.4mAh g-1The capacity retention rate is 22.7%, and the capacity loss per circle is only 0.1546%, and the specific data are shown in Table 1.
FIG. 6 is a pairThe cycle performance chart of the ratio 2 shows that the specific discharge capacity at the first cycle of 0.5C is 591.7mAh g-1After 500 cycles, the discharge specific capacity is 134.4mAh g-1The capacity retention rate is only 22.7%, and after 50 cycles, an obvious overcharge phenomenon appears, and the attenuation is obvious. The specific capacity and the cycling stability are far lower than those of the example and the comparative example 1. The battery performance of the nitrogen-doped amorphous carbon tube lithium-sulfur positive electrode is superior to that of a commercial carbon nanotube, and the cost is lower.
TABLE 1 results of Performance test of each example and comparative example
Fig. 1 is an X-ray diffraction pattern of example 2, example 3 and comparative example 1, and the XRD pattern proves that the magnesium aluminum/nitrogen pentafluoride-doped amorphous carbon prepared by the method of example 2 and example 3 and successfully carries sulfur. Wherein three strongest diffraction peaks at 15.80 degrees, 29.92 degrees and 51.76 degrees respectively correspond to (101), (121) and (422) crystal faces of the magnesium aluminum pentafluoride, and a broad peak at 20-30 degrees indicates the existence of amorphous nitrogen-doped carbon. In comparative example 1, magnesium aluminum pentafluoride was removed, and nitrogen-doped amorphous carbon was successfully obtained.
Fig. 7 is a comparison graph of the coulombic efficiencies of example 3 and comparative example 1, wherein the coulombic efficiency of the positive electrode of example 3 is always maintained above 98%, while the coulombic efficiency of the positive electrode of comparative example 1 is gradually reduced, and an obvious overcharge phenomenon exists. Also demonstrated is the effective suppression of the shuttle-threading effect by magnesium aluminum pentafluoride.
FIG. 9 is a graph comparing the AC impedance of example 2 with that of comparative example 1, and it can be seen that the diameter of the low-frequency semicircular shape reflects the magnitude of the charge transfer impedance at the solid-liquid interface between the battery material and the electrolyte. It can be seen that the electron conductivity of the sulfur-loaded cathode loaded with magnesium aluminum pentafluoroate is higher than that of the sulfur-loaded cathode loaded with carbon-doped nitrogen nanotubes of comparative example 1, because magnesium aluminum pentafluoroate has proton conductivity.
While the foregoing is directed to the preferred embodiment of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.
Claims (10)
1. A preparation method of a cathode material of a pentafluoromagnesium aluminum/nitrogen carbon-doped lithium sulfur battery is characterized by comprising the following steps:
(1) preparation of magnesium aluminum pentafluoride/nitrogen carbon-doped composite material
Sieving natural attapulgite, placing in acid liquor, heating, stirring, vacuum filtering, washing, and drying to obtain modified attapulgite;
mixing glucose, ammonium chloride and the obtained modified attapulgite according to the mass ratio of 20 (15-20) to (9-13.5), adding deionized water, stirring, and then performing hot water bath until water is completely volatilized to obtain a precursor;
calcining the obtained precursor in an inert atmosphere to obtain the nitrogen-doped carbon-coated modified attapulgite;
adding hydrofluoric acid into the obtained nitrogen-doped carbon-coated modified attapulgite, heating, stirring, reacting, and then performing suction filtration, washing and drying to obtain a magnesium aluminum pentafluoride/nitrogen-doped carbon composite material;
wherein the mass ratio of the nitrogen-doped carbon-coated modified attapulgite to the hydrofluoric acid is 1: (6.72-33.6);
(2) carrying sulfur
Mixing and grinding the magnesium aluminum pentafluoride/nitrogen-doped carbon composite material obtained in the step (1) and elemental sulfur according to a mass ratio of 3:7, placing the mixture into a reaction kettle with a polytetrafluoroethylene lining, placing the reaction kettle into an oven, heating the reaction kettle to 155 ℃ at a heating rate of 5-10 ℃/min, preserving the heat for 12-18h, cooling the reaction kettle to room temperature, and grinding the mixture uniformly to obtain the magnesium aluminum pentafluoride/nitrogen-doped carbon-doped sulfur composite material;
(3) preparation of cathode material
And (3) uniformly mixing the magnesium aluminum pentafluoride/nitrogen-doped carbon-loaded sulfur composite material obtained in the step (2), a conductive agent and a binder in a solvent to obtain positive electrode slurry, coating the obtained positive electrode slurry on a current collector, and drying to obtain the magnesium aluminum pentafluoride/nitrogen-doped carbon-loaded lithium sulfur battery positive electrode material.
2. The preparation method according to claim 1, wherein the acid solution in the step (1) is a hydrochloric acid solution with a concentration of 4-6 mol/L.
3. The preparation method according to claim 1, wherein the heating and stirring treatment in the step (1) is heating and stirring at 70-90 ℃ for 2-4 h.
4. The preparation method according to claim 1, wherein the calcination temperature in the step (1) is 600 to 900 ℃; the temperature rising speed is 4-6 ℃/min; the calcination time is 3-7 h.
5. The method according to claim 1, wherein the washing in step (1) is performed to neutrality by using deionized water and absolute ethanol.
6. The production method according to claim 1, wherein the elemental sulfur in the step (2) is sublimed sulfur.
7. The production method according to claim 1, wherein the conductive agent in the step (3) is conductive carbon black; the binder is polyvinylidene fluoride; the solvent is N-methyl pyrrolidone; the current collector is a carbon-coated aluminum foil.
8. The preparation method according to claim 1, wherein the mass ratio of the magnesium aluminum pentafluoride/nitrogen-doped carbon-sulfur-loaded composite material, the conductive agent and the binder in the step (3) is 7:2: 1.
9. The positive electrode material of the lithium-sulfur battery doped with carbon and magnesium pentafluoride/nitrogen prepared by the method of any one of claims 1 to 8.
10. The positive electrode material of the lithium-sulfur battery containing pentafluoromagnesium aluminum/nitrogen-doped carbon as claimed in claim 9, wherein the nitrogen-doped carbon is an amorphous carbon tube doped with nitrogen, the pentafluoromagnesium aluminum is loaded on the inner and outer surfaces of the amorphous carbon tube, and the particle size of the pentafluoromagnesium aluminum is 4-100 nm.
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