Lithium-sulfur battery positive electrode material based on modified carbon nanotubes and preparation method thereof

Abstract

The present invention discloses a lithium-sulfur battery positive electrode material based on modified carbon nanotubes and a preparation method thereof, belonging to the technical field of lithium-sulfur battery positive electrode materials. The method comprises the following steps: acid-treating carbon nanotubes to obtain pretreated carbon nanotubes; treating the pretreated carbon nanotubes with hydrogen peroxide to obtain a filter residue; preparing a carbon nanotube-sulfur material using the filter residue and sulfur; sequentially reacting the carbon nanotube-sulfur material in an acid solution containing stannous chloride and then in an acid solution containing palladium chloride, followed by washing and drying to obtain an activated carbon nanotube-sulfur material; and dispersing the activated carbon nanotube-sulfur material in a nickel-containing plating solution, followed by washing and drying to obtain a nickel-carbon nanotube-sulfur material. The modification of the carbon nanotubes in the present invention increases the content of oxygen-containing functional groups in the carbon nanotubes, thereby enhancing the carbon nanotubes' adsorption capacity for polysulfides and significantly inhibiting the dissolution and shuttling of polysulfides.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-sulfur battery positive electrode materials, and particularly relates to a lithium-sulfur battery positive electrode material based on modified carbon nanotubes and a preparation method thereof. Background Art

[0002] In order to solve the "shuttle phenomenon" of polysulfide polymer examples during the charging and discharging process of lithium-sulfur batteries, researchers use carbon nanotubes and sulfur composites to improve the electrochemical capacity of sulfur.

[0003] However, after many experiments, it was found that the lithium-sulfur battery using carbon nanotube-sulfur composite materials as the positive electrode has poor cycle performance during the charge and discharge process. The main reason is that carbon nanotubes cannot effectively prevent the shuttle of polysulfide polymers, which in turn leads to a decrease in the cycle performance of the lithium-sulfur battery. Summary of the Invention

[0004] Purpose of the invention: In order to solve the above problems, the present invention provides a lithium-sulfur battery positive electrode material based on modified carbon nanotubes and a preparation method thereof.

[0005] Technical solution: A method for preparing a lithium-sulfur battery cathode material based on modified carbon nanotubes, comprising the following steps:

[0006] Step 1: adding carbon nanotubes to an acid solution, and heating in a water bath for 7 to 8 hours while ultrasonically dispersing the carbon nanotubes, and then washing the carbon nanotubes to neutrality to obtain pretreated carbon nanotubes;

[0007] Step 2: first heat the pretreated carbon nanotubes, then add them to a hydrogen peroxide solution to obtain a solid-liquid mixture; after the solid-liquid mixture reacts for 21 to 25 hours, filter the solution to obtain a filter residue; and dry the filter residue;

[0008] Step 3: Grind and mix the dried filter residue and sulfur to obtain a solid mixture; heat the solid mixture in an oxygen-free environment to obtain a carbon nanotube@sulfur material;

[0009] Step 4: placing the carbon nanotube@sulfur material in an acid solution containing stannous chloride for reaction for 50 to 60 minutes, followed by washing and drying; then placing the washed and dried carbon nanotube@sulfur material in an acid solution containing palladium chloride for reaction for 50 to 60 minutes, followed by washing and drying to obtain an activated carbon nanotube@sulfur material;

[0010] Step 5: Disperse the activated carbon nanotube@sulfur material in a nickel-containing plating solution, heat in vacuum under electromagnetic stirring for 3 to 4 hours, and then wash and dry to obtain the nickel@carbon nanotube@sulfur material.

[0011] After numerous tests, it was concluded that existing lithium-sulfur batteries using carbon nanotube-sulfur composites as cathode materials, while showing improvements in initial discharge capacity and residual capacity after 100 cycles compared to sulfur cathode materials, still fall short of ideal performance. A certain percentage of polysulfides generated during the charge and discharge process of lithium-sulfur batteries still circulate, and the carbon nanotubes have limited adsorption capacity for these compounds.

[0012] Research has found that the reasons for the adsorption of polysulfides by carbon nanotubes are as follows: (1) The purity of carbon nanotubes is low, and they contain a large amount of impurities such as carbon nanoparticles, metal catalysts and amorphous carbon, which directly affects the physical properties of carbon nanotubes; (2) Due to their own reasons, carbon nanotubes have strong van der Waals forces inside and are difficult to disperse; (3) The surface of carbon nanotubes lacks active reaction functional groups, and the interfacial bonding force between carbon nanotubes and the matrix is ​​weak.

[0013] To address the above-mentioned issues, the present method modifies carbon nanotubes. First, in step 1, the carbon nanotubes are treated with acid to remove impurities from the carbon nanotubes. -OH and -COOH functional groups are then grafted onto the surface of the carbon nanotubes to increase their surface activity. Next, the carbon nanotubes are treated with hydrogen peroxide. The hydrogen peroxide reacts with the surface of the carbon nanotubes, changing their microscopic pore structure and disrupting their surface order. Furthermore, the presence of the -OH and -COOH functional groups increases the negative charge on the surface of the carbon nanotubes, enhancing the electrostatic repulsion between the carbon nanotubes and reducing entanglement between the carbon nanotubes, thereby improving their dispersion properties. The reaction between the carbon nanotubes and hydrogen peroxide increases the content of oxygen, -COOH, C=O, and other oxygen-containing functional groups on the carbon nanotubes. Under the action of these functional groups, the carbon nanotubes' adsorption capacity for polysulfides increases.

[0014] The carbon nanotubes treated in steps 1 and 2 are then mixed with sulfur in step 3 to produce a carbon nanotube@sulfur material. Due to the increased surface adsorption of the carbon nanotubes, in step 4, tin ions from a stannous chloride-containing acid solution are first adsorbed onto the surface. The tin ions then reduce palladium ions into palladium atoms, which are then adsorbed onto the surface of the carbon nanotubes. The palladium atoms, with their strong catalytic activity, reduce nickel ions in the plating solution into nickel atoms, which are then adsorbed onto the surface of the carbon nanotubes. Nickel atoms have a strong chemical interaction and electrocatalytic effect on polysulfides, significantly inhibiting their dissolution and shuttling.

[0015] Furthermore, the heating temperature of the heating treatment in step 2 is 90-100° C., and the heating time is 2-4 hours.

[0016] Furthermore, the heating temperature of the heating treatment in step three is 130-150° C., and the heating time is 15-18 hours.

[0017] Furthermore, the acid solution containing stannous chloride in step 4 comprises: 6-8 g / L SnCl2 and 8-12 ml / L HCl solution.

[0018] Furthermore, the acid solution containing palladium chloride in step 4 comprises: 0.6-1 g / L PdCl2 and 0.35-0.50 ml / L HCl solution.

[0019] Furthermore, the nickel-containing plating solution in step five comprises nickel sulfate, sodium hypophosphite, sodium citrate, ammonium chloride, and sodium dodecylsulfonate.

[0020] Furthermore, the acid solution in step 1 includes: one or a combination of concentrated nitric acid, concentrated sulfuric acid or concentrated hydrochloric acid.

[0021] In another technical solution, a lithium-sulfur battery cathode material based on modified carbon nanotubes is provided, which is prepared using the above-mentioned method for preparing a lithium-sulfur battery cathode material based on modified carbon nanotubes.

[0022] Beneficial effects: The carbon nanotubes are modified by acid treatment, hydrogen peroxide treatment, and nickel plating treatment. After the modification of the carbon nanotubes, the microscopic pore structure of the carbon nanotubes is changed, the order of the carbon nanotube surface is destroyed, the negative charge on the carbon nanotube surface is increased, the electrostatic repulsion between the carbon nanotubes is enhanced, the entanglement between the carbon nanotubes is weakened, and the dispersion performance is improved; the content of oxygen elements, -COOH, C=O and other oxygen-containing functional groups on the carbon nanotubes is increased. Under the action of various functional groups, the adsorption capacity of the carbon nanotubes to polysulfides is increased; nickel atoms have strong chemical interactions and electrode catalysis on polysulfides, which greatly inhibit the dissolution and shuttling of polysulfides. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a comparison chart of the first discharge capacity;

[0024] Figure 2 This is a comparison chart of capacity retention after 100 cycles. DETAILED DESCRIPTION

[0025] Example 1

[0026] The carbon nanotubes were added to an acid solution of concentrated nitric acid and concentrated hydrochloric acid in a volume ratio of 1:3, and heated in a water bath for 7 hours while ultrasonically dispersing the solution at a water bath temperature of 50° C. The carbon nanotubes were then washed with deionized water until neutral to obtain pretreated carbon nanotubes.

[0027] The pretreated carbon nanotubes were first heated at 100°C for 2 hours. The pretreated carbon nanotubes were then added to a 10% by volume hydrogen peroxide solution, which submerged the carbon nanotubes to obtain a solid-liquid mixture. After the solid-liquid mixture reacted at room temperature for 21 hours, the solution was filtered to obtain a filter residue. The filter residue was then dried in a drying oven.

[0028] The dried filter residue and sulfur are ground and mixed evenly, with the volume ratio of the filter residue to the sulfur being 2:1, to obtain a solid mixture; the solid mixture is placed in a reactor, and nitrogen is continuously passed through to exhaust the air in the reactor. The solid mixture is heated in an oxygen-free environment at a heating temperature of 150°C for 18 hours to obtain a carbon nanotube@sulfur material.

[0029] The carbon nanotube@sulfur material was placed in a mixed solution of 6 g / L SnCl2 and 12 ml / L HCl and reacted for 50 minutes, then washed with deionized water and dried; the washed and dried carbon nanotube@sulfur material was then placed in a mixed solution of 0.6 g / L PdCl2, an acid solution containing palladium chloride, and 0.50 ml / L HCl and reacted for 50 minutes, then washed with deionized water and dried to obtain an activated carbon nanotube@sulfur material.

[0030] The activated carbon nanotube@sulfur material was dispersed in a nickel-containing plating solution (the nickel-containing plating solution was a mixed solution of nickel sulfate, sodium hypophosphite, sodium citrate, ammonium chloride, and sodium dodecylsulfonate), and vacuum heated for 3 hours under electromagnetic stirring at a heating temperature of 85°C. The material was washed with deionized water and dried to obtain the nickel@carbon nanotube@sulfur material.

[0031] Example 2

[0032] The carbon nanotubes were added to concentrated nitric acid in an acid solution, and heated in a water bath for 8 hours while being ultrasonically dispersed at a temperature of 40° C. The carbon nanotubes were then washed with deionized water until neutral, thereby obtaining pretreated carbon nanotubes.

[0033] The pretreated carbon nanotubes were first heated at 90°C for 4 hours; then added to a 20% by volume hydrogen peroxide solution, which submerged the carbon nanotubes to obtain a solid-liquid mixture; after the solid-liquid mixture reacted at room temperature for 25 hours, the solution was filtered to obtain a filter residue; and the filter residue was dried in a drying oven.

[0034] The dried filter residue and sulfur are ground and mixed evenly, with the volume ratio of the filter residue to the sulfur being 3:1, to obtain a solid mixture; the solid mixture is placed in a reactor, and nitrogen is continuously passed through it to exhaust the air in the reactor. The solid mixture is heated in an oxygen-free environment at a heating temperature of 130°C for 15 hours to obtain a carbon nanotube@sulfur material.

[0035] The carbon nanotube@sulfur material was placed in a mixed solution of 8g / L SnCl2 and 8ml / L HCl solution to react for 60 minutes, and then washed with deionized water and dried; then the washed and dried carbon nanotube@sulfur material was placed in a mixed solution of 1g / L PdCl2, an acid solution containing palladium chloride, and 0.35ml / L HCl to react with a nickel plating solution for 60 minutes, and then washed with deionized water and dried to obtain an activated carbon nanotube@sulfur material.

[0036] The activated carbon nanotube@sulfur material was dispersed in a nickel-containing plating solution (a mixed solution of nickel sulfate, sodium hypophosphite, sodium citrate, ammonium chloride, and sodium dodecylsulfonate), heated in vacuum under electromagnetic stirring for 4 hours at a heating temperature of 80°C, washed with deionized water, and dried to obtain the nickel@carbon nanotube@sulfur material.

[0037] Example 3

[0038] The carbon nanotubes were added to an acid solution having a volume ratio of concentrated nitric acid to concentrated hydrochloric acid of 1:3, and heated in a water bath for 7.5 hours while ultrasonically dispersing the carbon nanotubes at a water bath temperature of 45° C. The carbon nanotubes were then washed with deionized water until neutral to obtain pretreated carbon nanotubes.

[0039] The pretreated carbon nanotubes were first heated at 95°C for 3 hours; then added to a 15% by volume hydrogen peroxide solution, which submerged the carbon nanotubes to obtain a solid-liquid mixture; after the solid-liquid mixture reacted at room temperature for 23 hours, the solution was filtered to obtain a filter residue; and the filter residue was dried in a drying oven.

[0040] The dried filter residue and sulfur are ground and mixed evenly, with the volume ratio of the filter residue to sulfur being 2:1, to obtain a solid mixture; the solid mixture is placed in a reactor, and nitrogen is continuously passed through it to exhaust the air in the reactor. The solid mixture is heated in an oxygen-free environment at a heating temperature of 140°C for 16 hours to obtain a carbon nanotube@sulfur material.

[0041] The carbon nanotube@sulfur material was placed in a mixed solution of 7g / L SnCl2 and 10ml / L HCl solution and reacted for 55 minutes, and then washed with deionized water and dried. The washed and dried carbon nanotube@sulfur material was then placed in a mixed solution of 0.8g / L PdCl2, an acid solution containing palladium chloride, and 0.4ml / L HCl and reacted for 55 minutes, and then washed with deionized water and dried to obtain an activated carbon nanotube@sulfur material.

[0042] The activated carbon nanotube@sulfur material was dispersed in a nickel-containing plating solution (the nickel-containing plating solution was a mixed solution of nickel sulfate, sodium hypophosphite, sodium citrate, ammonium chloride, and sodium dodecylsulfonate), and vacuum heated for 3.5 hours under electromagnetic stirring at a heating temperature of 82°C. The material was washed with deionized water and dried to obtain the nickel@carbon nanotube@sulfur material.

[0043] Example 4

[0044] The carbon nanotubes were added to concentrated sulfuric acid in an acid solution, and heated in a water bath for 7 hours while being ultrasonically dispersed at a temperature of 50° C. The carbon nanotubes were then washed with deionized water until neutral, thereby obtaining pretreated carbon nanotubes.

[0045] The pretreated carbon nanotubes were first heated at 97°C for 3.5 hours. The pretreated carbon nanotubes were then added to a 16% by volume hydrogen peroxide solution, which submerged the carbon nanotubes to obtain a solid-liquid mixture. The solid-liquid mixture reacted at room temperature for 24 hours, and the solution was filtered to obtain a filter residue. The filter residue was then dried in a drying oven.

[0046] The dried filter residue and sulfur are ground and mixed evenly, with the volume ratio of the filter residue to sulfur being 3:1, to obtain a solid mixture; the solid mixture is placed in a reactor, and nitrogen is continuously passed through it to exhaust the air in the reactor. The solid mixture is heated in an oxygen-free environment at a heating temperature of 146°C for 16 hours to obtain a carbon nanotube@sulfur material.

[0047] The carbon nanotube@sulfur material was placed in a mixed solution of 7.5g / L SnCl2, an acid solution containing stannous chloride, and 11ml / L HCl, and reacted for 50 minutes, and then washed with deionized water and dried. The washed and dried carbon nanotube@sulfur material was then placed in a mixed solution of 0.7g / L PdCl2, an acid solution containing palladium chloride, and 0.45ml / L HCl, and reacted for 60 minutes, and then washed with deionized water and dried to obtain an activated carbon nanotube@sulfur material.

[0048] The activated carbon nanotube@sulfur material was dispersed in a mixed solution of nickel plating solution (the nickel plating solution was nickel sulfate, sodium hypophosphite, sodium citrate, ammonium chloride, and sodium dodecylsulfonate). After vacuum heating for 4 hours under electromagnetic stirring at a heating temperature of 83°C, the material was washed with deionized water and dried to obtain the nickel@carbon nanotube@sulfur material.

[0049] Comparative Example 1

[0050] The carbon nanotubes and sulfur are ground and mixed evenly, with a volume ratio of carbon nanotubes to sulfur of 2:1, to obtain a solid mixture; the solid mixture is placed in a reactor, and nitrogen is continuously passed through it to exhaust the air in the reactor. The solid mixture is heated in an oxygen-free environment at a heating temperature of 150°C for 18 hours to obtain a carbon nanotube@sulfur material.

[0051] Comparative Example 2

[0052] The carbon nanotubes were added to an acid solution of concentrated nitric acid and concentrated hydrochloric acid in a volume ratio of 1:3, and heated in a water bath for 7 hours while ultrasonically dispersing the solution at a water bath temperature of 50° C. The carbon nanotubes were then washed with deionized water until neutral to obtain pretreated carbon nanotubes.

[0053] The pretreated carbon nanotubes and sulfur are ground and mixed evenly, with the volume ratio of the pretreated carbon nanotubes to sulfur being 2:1, to obtain a solid mixture; the solid mixture is placed in a reactor, and nitrogen is continuously passed through it to exhaust the air in the reactor. The solid mixture is heated in an oxygen-free environment at a heating temperature of 150°C for 18 hours to obtain a carbon nanotube@sulfur material.

[0054] The carbon nanotube@sulfur material was placed in a mixed solution of 6 g / L SnCl2 and 12 ml / L HCl and reacted for 50 minutes, then washed with deionized water and dried; the washed and dried carbon nanotube@sulfur material was then placed in a mixed solution of 0.6 g / L PdCl2, an acid solution containing palladium chloride, and 0.50 ml / L HCl and reacted for 50 minutes, then washed with deionized water and dried to obtain an activated carbon nanotube@sulfur material.

[0055] The activated carbon nanotube@sulfur material was dispersed in a nickel-containing plating solution (the nickel-containing plating solution was a mixed solution of nickel sulfate, sodium hypophosphite, sodium citrate, ammonium chloride, and sodium dodecylsulfonate), and vacuum heated for 3 hours under electromagnetic stirring at a heating temperature of 85°C. The material was washed with deionized water and dried to obtain the nickel@carbon nanotube@sulfur material.

[0056] The performance of the materials obtained in Examples 1 to 4, Comparative Examples 1, and 2 was measured using the following method: The positive electrode materials, conductive agents, and binders obtained in each of the above experiments were used to prepare a slurry. The slurry was then applied to aluminum foil using a doctor blade and dried at 50°C for 14 hours to prepare a sulfur positive electrode. The binder and conductive agent were conventional materials used in lithium-sulfur battery systems. The prepared sulfur electrode, conventional lithium negative electrode, separator, and electrolyte were assembled into a battery. The battery was subjected to electrochemical performance testing using a standard test environment, which will not be described in detail here. The test results are shown in Table 1:

[0057] Table 1

[0058] experimental group First discharge capacity (mAh / g) Capacity retention after 100 cycles (%) Example 1 1585.7 85.6 Example 2 1580.6 86.9 Example 3 1573.2 88.2 Example 4 1597.7 85.7 Comparative Example 1 1356.5 70.1 Comparative Example 2 1450.3 75.4

[0059] Comparative Example 1 is a preparation method of carbon nanotube@sulfur material in the prior art, and Comparative Example 2 is a preparation method without step 2 of hydrogen peroxide treatment in the present method.

[0060] As shown in the table above, compared to Comparative Examples 1 and 2, the batteries prepared using the nickel@carbon nanotube@sulfur materials obtained in Examples 1 to 4 as the positive electrode material all exhibited initial discharge capacities exceeding 1550 mAh / g, and capacity retention rates exceeding 85% after 100 cycles, both of which were higher than those of the corresponding batteries prepared in Comparative Examples 1 and 2. This is because the acid-modification, hydrogen peroxide-modification, and nickel-plating of the carbon nanotubes not only altered their microstructure but also added oxygen-containing functional groups such as -COOH and C=O, which enhanced the carbon nanotubes' adsorption of polysulfides and significantly reduced polysulfide shuttling.

[0061] The preparation method of Comparative Example 2 lacks the hydrogen peroxide treatment in step 2. The performance of the battery prepared using the resulting nickel@carbon nanotube@sulfur material as the positive electrode material is somewhat lower than that of Examples 1 to 4. This is because, although the acid treatment and nickel plating of the carbon nanotubes in Comparative Example 2 improve their adsorption capacity to a certain extent, the lack of hydrogen peroxide treatment does not modify the microscopic pore structure of the carbon nanotubes, resulting in relatively weak adsorption and dispersion properties. This shows that in this method, the acid treatment, hydrogen peroxide treatment, and nickel plating are all essential for modifying the carbon nanotubes. The modification of the carbon nanotubes alters their microscopic pore structure, disrupts the order of their surface, and increases the negative charge on their surface, enhancing electrostatic repulsion between the carbon nanotubes and reducing entanglement between them, thereby improving dispersion properties. Furthermore, the content of oxygen, -COOH, and C=O oxygen-containing functional groups on the carbon nanotubes is increased. These functional groups enhance the adsorption capacity of the carbon nanotubes for polysulfides. Nickel atoms have strong chemical interactions and electrode catalysis on polysulfides, greatly inhibiting the dissolution and shuttling of polysulfides.

Claims

1. A method for preparing a lithium-sulfur battery cathode material based on modified carbon nanotubes, characterized in that: The following steps are involved: Step 1: adding carbon nanotubes to an acid solution, and heating in a water bath for 7 to 8 hours while ultrasonically dispersing the carbon nanotubes, and then washing the carbon nanotubes to neutrality to obtain pretreated carbon nanotubes; Step 2: first heat the pretreated carbon nanotubes, then add them to a hydrogen peroxide solution to obtain a solid-liquid mixture; after the solid-liquid mixture reacts for 21 to 25 hours, filter the solution to obtain a filter residue; and dry the filter residue; Step 3: Grind and mix the dried filter residue and sulfur to obtain a solid mixture; heat the solid mixture in an oxygen-free environment to obtain a carbon nanotube@sulfur material; Step 4: placing the carbon nanotube@sulfur material in an acid solution containing stannous chloride for reaction for 50 to 60 minutes, followed by washing and drying; then placing the washed and dried carbon nanotube@sulfur material in an acid solution containing palladium chloride for reaction for 50 to 60 minutes, followed by washing and drying to obtain an activated carbon nanotube@sulfur material; Step 5: Disperse the activated carbon nanotube@sulfur material in a nickel-containing plating solution, heat in vacuum under electromagnetic stirring for 3 to 4 hours, and then wash and dry to obtain the nickel@carbon nanotube@sulfur material.

2. The method for preparing a lithium-sulfur battery cathode material based on modified carbon nanotubes according to claim 1, wherein: The heating temperature of the heating treatment in the step 2 is 90-100° C., and the heating time is 2-4 hours.

3. The method for preparing a lithium-sulfur battery cathode material based on modified carbon nanotubes according to claim 1, wherein: The heating temperature of the heating treatment in step 3 is 130-150° C., and the heating time is 15-18 hours.

4. The method for preparing a lithium-sulfur battery cathode material based on modified carbon nanotubes according to claim 1, wherein: The acid solution containing stannous chloride in step 4 comprises 6-8 g / L SnCl2 and 8-12 ml / L HCl solution.

5. The method for preparing a lithium-sulfur battery cathode material based on modified carbon nanotubes according to claim 1, wherein: The acid solution containing palladium chloride in step 4 comprises: 0.6-1 g / L PdCl2 and 0.35-0.50 ml / L HCl solution.

6. The method for preparing a lithium-sulfur battery cathode material based on modified carbon nanotubes according to claim 1, wherein: The nickel-containing plating solution in step five comprises nickel sulfate, sodium hypophosphite, sodium citrate, ammonium chloride, and sodium dodecylsulfonate.

7. The method for preparing a lithium-sulfur battery cathode material based on modified carbon nanotubes according to claim 1, wherein: The acid solution in step 1 includes: one or a combination of concentrated nitric acid, concentrated sulfuric acid or concentrated hydrochloric acid.

8. A lithium-sulfur battery cathode material based on modified carbon nanotubes, characterized in that: The modified carbon nanotube-based lithium-sulfur battery cathode material is prepared by the method for preparing the modified carbon nanotube-based lithium-sulfur battery cathode material according to any one of claims 1 to 7.

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