Preparation method of lithium salt-grafted carbon nanotubes, carbon nanotubes and carbon nanotube suspension

Abstract

The present invention discloses a method for preparing lithium-salt-grafted carbon nanotubes, comprising the following steps: adding fully carboxylated carbon nanotubes and lithium salts to a pure water solvent to form a first suspension, ultrasonically dispersing the carbon nanotubes, stirring and heating the suspension to obtain a precipitate, washing the suspension with pure water until the pH reaches 5-6; drying the suspension in an oven, and grinding the suspension into a powder to obtain the lithium-salt-grafted carbon nanotubes. The one-dimensional structure of the carbon nanotubes is fully carboxylated by oxidation, with carboxyl functional groups covering the inner walls and ports of the carbon nanotubes. Lithium salts are then grafted onto the inner walls and ports of the carbon nanotubes, ensuring that electrons cannot pass through the nanotubes but ions can. This material can be used in various ion transport media. The lithium-ion-grafted carbon nanotubes significantly shorten the transmission distance of lithium ions, replacing the existing electrolyte solution for lithium ion transport. This opens the possibility of manufacturing solid-state batteries, significantly improving the battery life, charge and discharge speed, and safety.

Description

Technical Field

[0001] The present invention relates to the technical field of solid-state batteries, and in particular to a preparation method of lithium salt-grafted carbon nanotubes and applications thereof. Background Art

[0002] At present, electric vehicles are developing rapidly, with the annual growth rate increasing year by year. Various countries have also announced the time for banning the sale of fuel vehicles. The battery system of electric vehicles is particularly important. Currently, electric vehicles on the market are mainly lithium batteries, but lithium batteries currently have problems such as low energy density, large volume, low safety factor, and slow charging and discharging speed. Existing solid-state batteries use binders such as PVDF as substitutes for diaphragms, but the heat resistance and ionic conductivity of the simple binder layer are low, which will reduce the passage rate of ions in the binder.

[0003] Since carbon nanotubes themselves have high temperature resistance, electrical conductivity and agglomeration properties, if products purchased on the market are directly used as additives in batteries, problems with electronic conductivity will arise, posing a safety hazard. Summary of the Invention

[0004] The object of the present invention is to provide a method for preparing lithium salt-grafted carbon nanotubes, carbon nanotubes and a carbon nanotube suspension, so as to solve one or more of the above-mentioned problems in the prior art.

[0005] In one aspect, the present invention provides a method for preparing lithium salt-grafted carbon nanotubes, comprising the following steps:

[0006] adding completely carboxylated carbon nanotubes and lithium salt into a pure water solvent to form a first suspension, and performing ultrasonic dispersion;

[0007] The lithium salt-grafted carbon nanotubes are obtained by drying in an oven and grinding into powder.

[0008] Preferably, the mass ratio of the fully carboxylated carbon nanotubes to the lithium salt is 1:16 to 1:1.

[0009] It can be understood that in other embodiments, the mass ratio of fully carboxylated carbon nanotubes to lithium salts is not in the range of 1:16 to 1:1, and lithium salts can be grafted onto carbon nanotubes. However, when the mass ratio of the two components is in the range of 1:16 to 1:1, the compatibility of the two components is better, which can reduce the waste of raw materials in the preparation and improve the economy of the preparation process of lithium salt grafted carbon nanotubes.

[0010] In some embodiments, after the step of adding the completely carboxylated carbon nanotubes and lithium salt into a pure water solvent to form a first suspension and performing ultrasonic dispersion, the method further includes stirring and heating the first suspension.

[0011] In some embodiments, the steps of ultrasonically dispersing the first suspension and stirring and heating the first suspension include at least one of the following three steps:

[0012] 1) Adjust the stirring speed to 30-50 rmb / min and the stirring time to 5-10 h.

[0013] 2) Adjust the heating temperature to 20-60°C

[0014] 3) adjusting the ultrasonic dispersion time to 60-180 min and the ultrasonic frequency to 49 Hz.

[0015] It can be understood that the three steps of treating the first suspension are all for providing a suitable reaction environment for the lithium salt grafting reaction and increasing the yield of lithium salt grafted carbon nanotubes.

[0016] In some embodiments, after the step of stirring and heating the first suspension, a filtration and washing step is further included: filtering the first suspension to obtain a precipitate, and washing the precipitate with pure water 2 to 3 times. Preferably, the precipitate is washed with pure water until the pH reaches 5 to 6.

[0017] It is understood that impurities such as ungrafted lithium salt remain on the surface of the precipitate obtained after filtration, and these impurities can be washed away with pure water.

[0018] In some embodiments, the length of the lithium salt grafted carbon nanotubes is 1 to 10 μm. It is understood that when the lithium salt grafted carbon nanotubes are ground into powder and have a length of 1 to 10 μm, they are more conducive to being used in batteries in a lightweight and thin manner.

[0019] In some embodiments, the fully carboxylated carbon nanotubes are prepared by the following steps:

[0020] Adding carbon nanotubes to a mixed acid comprising concentrated sulfuric acid and concentrated nitric acid in a molar ratio of 1:1 to 4:1, heating the mixture to 60 to 100° C. and performing a reflux stirring reaction;

[0021] The intermediate product is obtained by filtration;

[0022] The intermediate product is washed multiple times with pure water until the pH of the washed water is 3-4, and then dried in an oven to obtain fully carboxylated carbon nanotubes. It is understood that the intermediate product is carboxylated carbon nanotubes with excess acid attached to their surfaces. The pure water washing removes excess acid from the carboxylated carbon nanotubes, thereby reducing interference with the subsequent lithium salt grafting reaction.

[0023] In some embodiments, in the step of adding the carbon nanotubes to the mixed acid, the solid-to-liquid ratio of the carbon nanotubes to the mixed acid is 1:150-1200 (g / mL). It is understood that the solid-to-liquid ratio here means that 1g of carbon nanotubes requires 150-1200mL of mixed acid.

[0024] In some embodiments, the step of drying in an oven and grinding into a powder to obtain the lithium salt-grafted carbon nanotubes includes adjusting the drying temperature to 80-120° C. and the drying time to 12-24 hours. Preferably, the drying temperature is adjusted to 120° C. and the drying time is 12 hours.

[0025] In some embodiments, the fully carboxylated carbon nanotubes and lithium salt are dissolved in a pure water solvent to form a first suspension, and in the ultrasonic dispersion step, the lithium salt is one or more of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium cobaltate, or lithium hexafluorophosphate.

[0026] In some embodiments, the mass ratio of the fully carboxylated carbon nanotubes to the lithium salt is 1:16 to 1:1.

[0027] It is understood that the various reaction conditions and steps defined in this application are intended to ensure that appropriate reaction conditions promote the full progress of the carboxylation reaction. Exceeding the reaction conditions may result in incomplete carboxylation or residual reactants in the system or the production of other by-products, so that the desired product cannot be obtained, resulting in unnecessary waste.

[0028] In another aspect, the present invention provides a carbon nanotube prepared by the above method.

[0029] In another aspect, the present invention provides a carbon nanotube suspension for solid-state batteries, the suspension comprising

[0030] 5 to 10 parts of carbon nanotubes prepared by the above method;

[0031] 0.5-2 parts dispersant;

[0032] and 100 parts of organic solvent

[0033] Beneficial effects of the present invention:

[0034] The present application uses a strong acid oxidation method to fully oxidize the carbon nanotubes, completely carboxylating the inner walls and ports, adding lithium salts to the carbon nanotubes, and after sufficient reaction, washing the carbon nanotubes with water to clean them, and drying them at high temperature to obtain lithium salt-grafted carbon nanotubes.

[0035] The one-dimensional structure of the carbon nanotubes in this application is fully carboxylated by oxidation, with carboxyl functional groups covering the inner wall and ports of the carbon nanotubes. Lithium salts are then grafted into the tube walls and ports to ensure that electrons cannot pass through the interior of the nanotubes, but ions can be transferred. This material can be used in various ion transmission media, such as semi-solid-state batteries and quasi-solid-state batteries. The lithium-ion-grafted carbon nanotubes greatly shorten the transmission distance of lithium ions, replacing the lithium ion transmission in existing electrolyte solutions, providing the possibility of making solid-state batteries, greatly improving the battery life, charge and discharge speed, and safety characteristics. The one-dimensional structure of the carbon nanotubes is disordered when filled with a binder. By grafting lithium salts onto the carbon nanotubes, lithium ions can be transmitted through the inner wall of the carbon nanotubes, greatly improving the lithium ion transmission speed, which can help the battery achieve fast charging characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 Comparative infrared spectra of carbon nanotubes, carboxylated carbon nanotubes and lithium salt grafted carbon nanotubes. DETAILED DESCRIPTION

[0037] The present invention will be further described below with reference to the following examples. The following examples are only used to more clearly illustrate the performance of the present invention and are not intended to be limiting.

[0038] Example 1

[0039] Weigh 0.5g of carbon nanotubes and put them into a three-necked flask, add 200ml of concentrated sulfuric acid and 100ml of concentrated nitric acid, heat to 60℃ for reflux stirring reaction, stirring speed 30rmb / min, stirring time 5h, then wash with pure water 3 times to pH 3-4, then put it into a drying oven at 80℃ and dry it for 24h to obtain carboxylated carbon nanotubes.

[0040] Then weigh 0.5g of carboxylated carbon nanotubes and put them into a beaker, add 2g of lithium perchlorate (LiClO4) into the beaker, pour in 300g of pure water and stir evenly, heat to 20°C, ultrasonically disperse for 180min, stir and heat to react, obtain a precipitate, then wash with pure water until the pH reaches 5-6, then put it into a 120°C oven and dry it for 12h, then grind the dried material in a mortar for 10min to powder to obtain lithium-grafted carbon nanotubes.

[0041] Example 2

[0042] Weigh 2g of carbon nanotubes and place them in a three-necked flask, add 600ml of concentrated sulfuric acid and 200ml of concentrated nitric acid, heat to 100℃ and reflux with stirring, stirring at a speed of 50rmb / min, stirring for 5h, then wash with pure water 3 times to a pH of 3-4, then place in a drying oven at 120℃ and dry for 12h to obtain carboxylated carbon nanotubes.

[0043] Then weigh 2g of carboxylated carbon nanotubes and put them into a beaker, add 8g of lithium tetrafluoroborate (LiBF4) into the beaker, pour in 500g of pure water and stir evenly, heat to 60°C, ultrasonically disperse for 60 minutes, stir and heat to react, obtain a precipitate, then wash with pure water until the pH reaches 5-6, then put it into a 120°C oven and dry it for 24 hours, then grind the dried material with a mortar for 10 minutes to powder to obtain lithium-grafted carbon nanotubes.

[0044] Example 3

[0045] Weigh 2g of carbon nanotubes and place them in a three-necked flask, add 1500ml of concentrated sulfuric acid and 500ml of concentrated nitric acid, heat to 80℃ for reflux stirring reaction, stirring speed 40rmb / min, stirring time 8h, then wash with pure water 3 times to pH 3-4, then place in a drying oven at 100℃ and dry for 18h to obtain carboxylated carbon nanotubes.

[0046] Then weigh 1g of carboxylated carbon nanotubes and put them into a beaker, add 6g of lithium hexafluorophosphate (LiPF6) into the beaker, pour in 400g of pure water and stir evenly, heat to 40°C, ultrasonically disperse for 120 minutes, stir and heat to react, obtain a precipitate, then wash with pure water until the pH reaches 5-6, then put it into a 120°C oven and dry it for 18 hours, then grind the dried material in a mortar for 10 minutes to powder to obtain lithium-grafted carbon nanotubes.

[0047] Comparative Example 1: 0.5 g unmodified carbon nanotubes

[0048] Comparative Example 2: Weigh 0.5 g of carbon nanotubes and place them in a three-necked flask. Add 200 ml of concentrated sulfuric acid and 100 ml of concentrated nitric acid. Heat to 60°C for reflux stirring reaction at a stirring speed of 30 rmb / min for 10 h. Then wash with pure water three times to a pH of 3-4. Then place in a drying oven at 80°C and dry for 24 h to obtain carboxylated carbon nanotubes.

[0049] Comparative Example 3: 0.5g of carbon nanotubes were weighed and placed in a beaker. 2g of lithium perchlorate (LiClO4) was added to the beaker, and 300g of pure water was poured in and stirred evenly. At this point, the carbon nanotubes and lithium salt solution were mixed together, but no grafting reaction occurred. Performance Testing: Infrared spectroscopy was performed on Example 1, Comparative Example 1, and Comparative Example 2.

[0050] from Figure 1 It can be seen that the carbon nanotubes after carboxylation in Comparative Example 2 have strong absorption peaks near 3300-3600 cm-1 and 1700 cm-1, which are the characteristic peaks of -OH and C=O bonds of free carboxyl groups, proving that the carbon nanotubes have been carboxylated after being subjected to strong oxygen. In Example 1, when COOH becomes a carboxyl salt, the double bond characteristics of the C=O bond are reduced, the absorption peak shifts to a low frequency, the original single bond characteristics of the CO bond increase, the absorption frequency increases, and two absorption peaks appear. The symmetric stretching vibration is located near 1400cm-1, and the asymmetric stretching vibration is at 1550~1650cm-1. Both vibration absorption peaks are relatively strong, and the corresponding C=O peak near 1700cm-1 disappears, proving that the carboxyl group has completely become a carboxyl anion. Compared with the carboxylated carbon nanotubes, a new absorption peak of 685cm-1 is added at a low frequency position. This is the characteristic peak of Li and the oxygen in the hydroxyl group or the oxygen atoms of other groups to produce lithium bonds, thereby proving the presence of lithium in the carbon nanotubes. The infrared spectrum can prove that the carboxyl group has become carboxyl lithium.

[0051] After the lithium salt grafting modification of the carbon nanotube suspension is mixed with other adhesives, it can be used in solid-state batteries to replace existing solid-state battery separators, effectively improving ionic conductivity while reducing electronic conductivity. The electronic and ionic conductivity tests were conducted on unmodified carbon nanotubes, the carbon nanotubes grafted and modified in Examples 1 to 3, and the mixture of lithium salt and unmodified carbon nanotubes in Comparative Example 1:

[0052] Test method: The carbon nanotubes of Examples 1 to 3, Comparative Examples 1 and 3 were mixed with PVDF at a ratio of 1:20, and then pressed into sheets using a tablet press. The sheets were sprayed with gold and polished. The electronic and ionic conductivities were measured using an electrochemical workstation. The results are shown in the following table:

[0053]

[0054] From the test results we can see that:

[0055] In Comparative Example 1, the carbon nanotubes are conductive, but the lithium ion conductivity is very low; in Comparative Example 3, after mixing the lithium salt and the unmodified carbon nanotubes, the lithium ion conductivity is improved to a certain extent, but the mixture is still conductive; the lithium salt-grafted carbon nanotubes of Examples 1-3 are basically non-conductive, and the lithium ion conductivity is greatly improved.

[0056] Therefore, the one-dimensional structure of the carbon nanotubes in the embodiment of the present application is fully carboxylated by oxidation, and the carboxyl functional groups cover the inner wall and port of the carbon nanotubes. Then, lithium salts are grafted into the tube wall and port to ensure that electrons cannot pass through the inside of the nanotubes, but ions can be transmitted. This material can be used in various ion transmission media, such as semi-solid batteries and quasi-solid-state batteries. The lithium ion grafted carbon nanotubes greatly shorten the transmission distance of lithium ions, replacing the lithium ions in the existing electrolyte solution for transmission, providing the possibility of making solid-state batteries, greatly improving the battery life and charging and discharging speed, as well as safety characteristics.

[0057] The above description is only a preferred embodiment of the present invention. It should be pointed out that a person skilled in the art can make several modifications and improvements without departing from the inventive concept of the present invention, and these should also be considered to be within the scope of protection of the present invention.

Claims

1. A method for preparing lithium salt-grafted carbon nanotubes, characterized in that: The following steps are involved: Adding fully carboxylated carbon nanotubes and lithium salt into a pure water solvent to form a first suspension, and performing ultrasonic dispersion, wherein the mass ratio of the fully carboxylated carbon nanotubes to the lithium salt is 1:16 to 1:1, and the fully carboxylated carbon nanotubes are prepared by the following steps: Adding carbon nanotubes to mixed acid at a solid-liquid ratio of 1:150-1200 g / mL, wherein the mixed acid comprises concentrated sulfuric acid and concentrated nitric acid at a molar ratio of 1:1-4:1, and heating to 60-100° C. for reflux stirring reaction; Filtration to obtain the intermediate product; The intermediate product is washed with pure water for multiple times until the pH of the washed pure water is 3-4, and then dried in an oven to obtain completely carboxylated carbon nanotubes; The lithium salt-grafted carbon nanotubes are obtained by drying in an oven and grinding into powder.

2. The method for preparing lithium salt-grafted carbon nanotubes according to claim 1, characterized in that: After the step of adding the completely carboxylated carbon nanotubes and lithium salt into a pure water solvent to form a first suspension and performing ultrasonic dispersion, the method further includes stirring and heating the first suspension.

3. The method for preparing lithium salt-grafted carbon nanotubes according to claim 2, characterized in that: The step of ultrasonically dispersing the first suspension and stirring and heating the first suspension comprises at least one of the following three steps: 1) Adjust the stirring speed to 30-50 rpm and the stirring time to 5-10 hours; 2) adjusting the heating temperature to 20-60° C.; 3) Adjusting the ultrasonic dispersion time to 60-180 min and the ultrasonic frequency to 49 Hz.

4. The method for preparing lithium salt-grafted carbon nanotubes according to claim 2, wherein: After the step of stirring and heating the first suspension, the method further includes filtering and washing: filtering the first suspension to obtain a precipitate, and washing the precipitate with pure water for 2 to 3 times.

5. The method for preparing lithium salt-grafted carbon nanotubes according to claim 4, characterized in that: Wash with pure water until the pH reaches 5~6.

6. The method for preparing lithium salt-grafted carbon nanotubes according to claim 1, characterized in that: The length of the powdered lithium salt grafted carbon nanotubes is 1-10 μm.

7. The method for preparing lithium salt-grafted carbon nanotubes according to claim 1, characterized in that: The step of drying in an oven and grinding into powder to obtain the lithium salt-grafted carbon nanotubes comprises: adjusting the drying temperature to 80-120° C. and the drying time to 12-24 hours.

8. The method for preparing lithium salt-grafted carbon nanotubes according to claim 7, characterized in that: The drying temperature was adjusted to 120° C. and the drying time was adjusted to 12 h.

9. The method for preparing lithium salt-grafted carbon nanotubes according to claims 1 to 8, characterized in that: The fully carboxylated carbon nanotubes and lithium salt are dissolved in a pure water solvent to form a first suspension, and in the step of ultrasonic dispersion, the lithium salt is one or more of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium cobaltate or lithium hexafluorophosphate.

10. A carbon nanotube, characterized in that: The method is prepared by any one of claims 1 to 9.

11. A carbon nanotube suspension for solid-state batteries, characterized in that: The suspension comprises 5-10 parts of carbon nanotubes prepared by the method according to any one of claims 1 to 9; 0.5~2 parts of dispersant; and 100 parts of organic solvent.

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