A lithium manganese oxide / nitrogen-doped carbon nanotube composite material, its preparation method and applications

Abstract

The application provides a lithium manganate / nitrogen-doped carbon nanotube composite material and a preparation method and application thereof. 2+ The coordination of the folate and the hydrazine hydrate, the hydrogen bond interaction between the folate molecules and the bridging of the hydrazine hydrate molecules enables the three to self-assemble into a nanotube structure Mn-FA / NTs, then the organic matter is carbonized to form nitrogen-doped carbon nanotubes, and free C produced by carbonization is used to react with Mn 2+ and potassium permanganate to enable the manganese element to be converted into Mn 3+ ~ Mn 4+ , and a small amount of Mn-Nx is generated, then a lithium source is mixed to perform a solid-phase reaction to generate a lithium manganate material with a spinel structure and in-situ embedded between nitrogen-doped carbon nanotubes, thereby obtaining the lithium manganate / nitrogen-doped carbon nanotube composite material.

Description

Technical Field

[0001] This invention belongs to the field of lithium resource extraction technology, and relates to a lithium manganese oxide / nitrogen-doped carbon nanotube composite material, its preparation method and application. Background Technology

[0002] With the increasing prominence of resource and environmental issues, lithium-ion batteries have gradually become one of the most important energy storage devices in daily life, and the demand for lithium is growing rapidly. Solid-phase lithium resources are gradually approaching depletion, while technologies for lithium extraction from liquid-phase lithium resources, such as salt lake brines, which account for 80% of global lithium resources, are attracting increasing attention. Traditional liquid-phase lithium extraction technologies mainly include precipitation, solvent extraction, ion exchange, and calcination leaching. Among them, ion exchange technology based on inorganic ion sieves has attracted much attention due to its high extraction efficiency. However, the ion exchange method using inorganic ion sieves requires the use of large amounts of strong acid for washing during the desorption process, causing serious corrosion and pollution to equipment and the environment. Based on this, researchers have proposed using electrochemical methods for the adsorption and desorption of lithium ions. One of the key aspects of electrochemical methods is the selection of lithium extraction electrode materials. Considering the biosafety and lithium extraction efficiency of electrode materials, lithium manganese oxide has become one of the most promising electrode materials in the field of electrochemical lithium extraction.

[0003] The key goal in the development of lithium manganese oxide electrode materials is to enable them to have abundant electroactive sites, high selectivity for Li ions, and shorten the lithium ion transport path in order to optimize lithium extraction capacity and rate.

[0004] Carbon nanotubes are special carbon materials composed of five-, six-, or seven-membered rings linked together by carbon atoms. They can be understood as one or more hollow tubes formed by rolling up a flat, paper-like sheet, with diameters typically in the nanometer range. Because carbon nanotubes have a layered structure similar to graphene, they possess high strength, electrical conductivity, thermal conductivity, and chemical stability. They are often used as conductive agents and functional additives in the preparation of composite electrode materials to improve their overall performance.

[0005] In the field of electrochemical lithium extraction from salt lakes, the addition of graphene-based materials (such as graphene, graphite, acetylene black, carbon nanotubes, etc.) has been proven to improve the lithium extraction capacity and Li-ion selectivity of the electrode. However, due to the poor bonding force between graphene-based materials and electrode materials, in order to avoid detachment, the usual preparation method is to mix the electrode material and graphene-based materials with a binder to prepare a composite electrode material. This mixing method results in poor dispersion of the conductive agent and the electrode active material, which easily leads to agglomeration. Moreover, since the binder is a non-electrochemically active substance, the active adsorption sites of the composite electrode material are very easily covered by the binder, resulting in a decrease in overall performance. Summary of the Invention

[0006] In view of the problems existing in the prior art, the purpose of the present invention is to provide a lithium manganese oxide / nitrogen-doped carbon nanotube composite material, its preparation method and uses, wherein the preparation method utilizes Mn 2+ The coordination with folic acid and hydrazine hydrate, along with the hydrogen bonding between folic acid molecules and the bridging effect of hydrazine hydrate molecules, enables the three to self-assemble into a nanotube-like structure Mn-FA / NTs. Then, carbonization is used to form nitrogen-doped carbon nanotubes from the organic matter, utilizing the free C and Mn generated during carbonization. 2+ A redox reaction occurs between potassium permanganate and manganese, converting manganese into Mn. 3+ ~Mn 4+ A small amount of Mn-Nx is generated, and then a lithium manganese oxide material with a spinel structure is generated by mixing with a lithium source in a solid-state reaction. This material is then in situ embedded between nitrogen-doped carbon nanotubes to obtain a lithium manganese oxide / nitrogen-doped carbon nanotube composite material.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a method for preparing a lithium manganese oxide / nitrogen-doped carbon nanotube composite material, the method comprising:

[0009] Manganese salt, folic acid and hydrazine hydrate were mixed to form a mixed solution, and a hydrothermal reaction was carried out to obtain Mn-FA / NTs;

[0010] Mn-FA / NTs were mixed with potassium permanganate and subjected to carbonization to obtain Mn-NC / NTs;

[0011] Mn-NC / NTs were mixed with lithium salt and subjected to high-temperature treatment to obtain lithium manganese oxide / nitrogen-doped carbon nanotube composite material LMO-NC / NTs.

[0012] In the Mn-FA / NTs, Mn-NC / NTs, and LMO-NC / NTs of this invention, " / " represents the formation of a composite material, FA represents folic acid, NTs represents nanotubes, NC represents nitrogen-doped carbon, and LMO represents lithium manganese oxide. Mn-Nx refers to a substance formed by nitrogen and manganese.

[0013] The lithium manganese oxide / nitrogen-doped carbon nanotube composite material provided by this invention is based on Mn provided by manganese salt. 2+ The nanotube precursor, synthesized via a hydrothermal reaction with folic acid (FA) and hydrazine hydrate, is then subjected to carbonization and redox reactions before being synthesized with a lithium source via a solid-state synthesis method. Specifically, Mn 2+Coordination with folic acid and hydrazine hydrate, along with hydrogen bonding between folic acid molecules and bridging with hydrazine hydrate molecules, enables the three to self-assemble into a nanotubular precursor, Mn-FA / NTs, via a hydrothermal reaction. Mn is distributed both inside and outside the nanotubular structure of the precursor. Carbonization of Mn-FA / NTs in an inert atmosphere leads to the carbonization of organic matter in the precursor, forming nitrogen-doped carbon nanotubes. Simultaneously, free C and Mn are generated during the carbonization process. 2+ It undergoes a redox reaction with potassium permanganate, causing Mn to... 2+ and MnO 4- Transform into Mn 3+ ~Mn 4+ Mn-Nx was generated and loaded between nitrogen-doped carbon nanotubes to obtain Mn-NC / NTs; further, Mn-NC / NTs were mixed with a lithium source for solid-state synthesis to prepare lithium manganese oxide material with spinel structure, wherein the lithium manganese oxide crystals were embedded between nitrogen-doped carbon nanotubes.

[0014] The lithium manganese oxide / nitrogen-doped carbon nanotube composite material obtained by the preparation method of this invention has the following advantages: First, nitrogen-doped carbon nanotubes have better hydrophilicity than ordinary carbon nanotubes, and nitrogen doping atoms can change the local charge density of carbon nanotubes, improve their electron transport, and reduce the resistivity, thereby improving the lithium diffusion rate and lithium extraction capacity. Second, the specific in-situ synthesis method results in better dispersion and bonding between lithium manganese oxide crystals and nitrogen-doped carbon nanotubes, and the composite material has more mesoporous structures and smaller pore size dispersion, which is beneficial to further reduce the resistivity of the electrode and improve the lithium extraction rate and lithium extraction capacity. Furthermore, the presence of Mn-Nx in the intermediate product Mn-FA / NTs makes it easier for manganese to be converted into Mn during the subsequent solid-phase reaction with the Li source. 4+ This, in turn, increases the Mn content in the material. 4+ / Mn 3+ The appropriate ratio helps to suppress the manganese loss problem of lithium manganese oxide, thereby improving the cycle stability of the material.

[0015] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. The technical objectives and beneficial effects of the present invention can be better achieved and realized through the following technical solutions.

[0016] As a preferred technical solution of the present invention, the preparation method includes preparing a suspension of manganese salt and folic acid, and then adding hydrazine hydrate solution dropwise to obtain a mixed solution.

[0017] Preferably, the solvent in the suspension includes ethanol and water in a volume ratio of (0.8 to 1.5):1, such as 0.8:1, 0.9:1, 1:1, 1.2:1, 1.3:1, 1.4:1 or 1.5:1, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0018] Preferably, the mass concentration of the hydrazine hydrate solution is 60% to 70%, such as 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, or 70%, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0019] In the preparation method described in this invention, folic acid is also used as the main carbon source, and the absence of folic acid would prevent the formation of nanotubes, while hydrazine hydrate is used as the main nitrogen source.

[0020] As a preferred technical solution of the present invention, the amounts of manganese salt, folic acid, and hydrazine hydrate are controlled according to a molar ratio of 1:(0.5~1):(120~140), for example 1:0.5:120, 1:0.6:120, 1:0.7:120, 1:0.8:120, 1:0.9:120, 1:1:120, 1:0.5:125, 1:0.6:125, 1:0.7:125, 1:0.8:125, 1:0.9:125, 1:1:125, 1:0.5:130, 1:0.6:1 30, 1:0.7:130, 1:0.8:130, 1:0.9:130, 1:1:130, 1:0.5:135, 1:0.6:135, 1:0.7:135, 1:0.8:135, 1:0.9:135, 1:1:135, 1:0.5:140, 1:0.6:140, 1:0.7:140, 1:0.8:140, 1:0.9:140, or 1:1:140, etc., but not limited to the listed values; other unlisted values ​​within the above range also apply.

[0021] Preferably, the manganese salt comprises a soluble divalent manganese salt, which includes manganese chloride and / or manganese nitrate.

[0022] As a preferred technical solution of the present invention, the temperature of the hydrothermal reaction is 150-170℃, such as 150℃, 155℃, 160℃, 165℃ or 170℃, and the time is 1-3h, such as 1h, 1.5h, 2h, 2.5h or 3h, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0023] As a preferred technical solution of the present invention, the amount of Mn-FA / NTs and potassium permanganate is controlled according to a mass ratio of 1:(0.2~0.5), such as 1:0.2, 1:0.25, 1:0.3, 1:0.35, 1:0.4, 1:0.45 or 1:0.5, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0024] Preferably, the carbonization process is carried out under a protective atmosphere, which includes argon.

[0025] Preferably, the carbonization treatment temperature is 550–650°C, such as 550°C, 560°C, 570°C, 580°C, 590°C, 600°C, 610°C, 620°C, 630°C, 640°C, or 650°C, etc., the time is 1–3 hours, such as 1 hour, 1.5 hours, 2 hours, 2.5 hours, or 3 hours, etc., and the heating rate is 1–5°C / min, such as 1°C / min, 2°C / min, 3°C / min, 4°C / min, or 5°C / min, etc., but is not limited to the listed values, and other unlisted values ​​within the above range are also applicable.

[0026] As a preferred technical solution of the present invention, the preparation method includes dispersing Mn-NC / NTs in a solvent, adding lithium salt to mix, heating to evaporate the solvent, drying and grinding, and then performing the high-temperature treatment.

[0027] Preferably, the solvent includes ethanol.

[0028] Preferably, the heating and evaporation temperature is 40 to 60°C, such as 40°C, 450°C, 50°C, 55°C, or 60°C, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0029] Preferably, the drying temperature is 60-80°C, such as 60°C, 65°C, 70°C, 75°C or 80°C, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0030] Preferably, the amount of lithium salt and Mn-NC / NTs is controlled according to a Li:Mn molar ratio of (1.05 to 1.2):1, such as 1.05:1, 1.08:1, 1.1:1, 1.12:1, 1.14:1, 1.16:1, 1.18:1, or 1.2:1, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0031] Preferably, the lithium salt includes at least one of lithium chloride, lithium nitrate, or lithium acetate, such as typical but non-limiting combinations including combinations of lithium chloride and lithium nitrate, combinations of lithium chloride and lithium acetate, or combinations of lithium acetate and lithium nitrate.

[0032] As a preferred technical solution of the present invention, the high-temperature treatment includes first performing heat treatment in a high-pressure autoclave, and then calcining at a higher temperature.

[0033] Preferably, the heat treatment temperature is 100–200°C, such as 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, 190°C, or 200°C, and the time is 24–60 hours, such as 24 hours, 28 hours, 32 hours, 36 hours, 40 hours, 44 hours, 48 ​​hours, 52 hours, 56 hours, or 60 hours, but is not limited to the listed values; other unlisted values ​​within the above range are also applicable.

[0034] Preferably, the calcination temperature is 400–600°C, such as 400°C, 420°C, 440°C, 460°C, 480°C, 500°C, 520°C, 540°C, 560°C, 580°C, or 600°C, and the time is 2–6 hours, such as 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, or 6 hours, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0035] As a preferred technical solution of the present invention, the preparation method includes:

[0036] Soluble divalent manganese salt and folic acid were dispersed in an ethanol-water solution with a volume ratio of ethanol to water of (0.8–1.5):1. After vigorous stirring, the solution was sonicated until it became a uniform brown suspension. Then, a 60%–70% hydrazine hydrate solution was added dropwise, and the molar ratio of manganese salt, folic acid, and hydrazine hydrate was controlled to be 1:(0.5–1):(120–140). The resulting mixed solution was sonicated and then transferred to a hydrothermal reactor. The reaction was carried out at 150–170°C for 2 hours. The solution was then repeatedly filtered and washed with deionized water and ethanol, and dried to obtain a brown powder, Mn-FA / NTs.

[0037] According to the mass ratio of 1:(0.2~0.5), the powder obtained by mixing and grinding Mn-FA / NTs with potassium permanganate is evenly spread in a ceramic boat. Under Ar gas conditions, the temperature is raised to 550~650℃ at 1~5℃ / min and held for 1~3h for carbonization treatment to obtain Mn-NC / NTs.

[0038] Mn-NC / NTs were ultrasonically dispersed in ethanol, and lithium salt was added. The amount of lithium salt and Mn-NC / NTs was controlled according to the molar ratio of Li:Mn of (1.05-1.2):1. The temperature was raised to 40-60℃, and stirring was continued until the solvent was completely evaporated. The mixture was then dried at 60-80℃ and ground until there was no obvious particle texture. The mixture was then transferred to an autoclave and heat-treated at 100-200℃ for 24-60 h, and then calcined at 400-600℃ for 2-6 h to obtain lithium manganese oxide / nitrogen-doped carbon nanotube composite material LMO-NC / NTs.

[0039] In a second aspect, the present invention provides a lithium manganese oxide / nitrogen-doped carbon nanotube composite material, wherein the composite material is obtained according to the preparation method described in the first aspect.

[0040] Thirdly, the present invention provides an application of a lithium manganese oxide / nitrogen-doped carbon nanotube composite material, the application of which includes lithium extraction from salt lakes.

[0041] Compared with existing technical solutions, the present invention has at least the following beneficial effects:

[0042] This invention provides a novel preparation method for synthesizing lithium manganese oxide / nitrogen-doped carbon nanotube composite materials, utilizing Mn 2+ It forms a nanotube precursor with folic acid and hydrazine hydrate, and then generates nitrogen-doped carbon nanotubes through carbonization and undergoes a redox reaction to generate Mn. 3+ ~Mn 4+ Subsequently, it reacts with lithium salt in solid phase to generate spinel lithium manganese oxide embedded between nitrogen-doped carbon nanotubes in situ; the preparation method can obtain composite materials with better dispersibility and bonding force without the use of binders, and the preparation method is simple and convenient, and easy to scale up production.

[0043] The lithium manganese oxide / nitrogen-doped carbon nanotube composite material obtained in this invention has more mesoporous structures and smaller pore size dispersion, which is beneficial for further reducing the electrode resistivity and improving the lithium extraction rate and capacity. Furthermore, the presence of Mn-Nx in the intermediate product Mn-FA / NTs makes it easier for manganese to be converted into Mn during the subsequent solid-state reaction with the Li source. 4+ This, in turn, increases the Mn content in the material. 4+ / Mn 3+ The appropriate ratio helps to suppress the manganese loss problem of lithium manganese oxide, thereby improving the cycle stability of the material. Attached Figure Description

[0044] Figure 1 This is the XRD test image of LMO-NC / NTs obtained in Example 1.

[0045] Figure 2 This is a SEM image of LMO-NC / NTs obtained in Example 1.

[0046] Figure 3 This is a SEM image of LMO-NC / NTs obtained from Comparative Example 1.

[0047] Figure 4 This is a TEM image of the LMO-NC / NTs obtained in Example 1.

[0048] Figure 5 These are XPS full-scan spectra of Mn-FA / NTs, Mn-NC / NTs, and LMO-NC / NTs obtained in Example 1.

[0049] Figure 6 This is the XPS oxygen element narrow spectrum of Mn-FA / NTs obtained in Example 1.

[0050] Figure 7 This is the XPS nitrogen element narrow spectrum of Mn-FA / NTs obtained in Example 1.

[0051] Figure 8 These are XPS narrow spectrum images of manganese in Mn-NC / NTs and LMO-NC / NTs obtained in Example 1. Detailed Implementation

[0052] The technical solution of the present invention will be further illustrated below through specific embodiments.

[0053] Those skilled in the art will understand that the embodiments described are merely illustrative of the invention and should not be construed as limiting the invention.

[0054] Example 1

[0055] This embodiment provides a method for preparing a lithium manganese oxide / nitrogen-doped carbon nanotube composite material, the method comprising:

[0056] (1) Manganese chloride was used as a soluble divalent manganese salt and dispersed with folic acid in an ethanol aqueous solution (the volume ratio of ethanol to water was 0.8:1). The concentration of folic acid in the ethanol aqueous solution was controlled at 1 g / 40 mL. After vigorous stirring, the solution was sonicated until it became a uniform brown suspension. Then, a 65% hydrazine hydrate solution was added dropwise. The molar ratio of manganese salt, folic acid and hydrazine hydrate was controlled at 1:0.5:120. The resulting mixed solution was sonicated and transferred to a hydrothermal reactor. The hydrothermal reaction was carried out at 170 °C for 2 h. Then, the solution was repeatedly filtered and washed with deionized water and ethanol. After drying, brown powder Mn-FA / NTs was obtained.

[0057] (2) Mn-FA / NTs and potassium permanganate were mixed and ground in a mass ratio of 1:0.2 and the resulting powder was evenly spread in a ceramic boat. Under Ar gas conditions, the temperature was increased to 600℃ at 2℃ / min and held for 2h for carbonization treatment to obtain Mn-NC / NTs.

[0058] (3) Mn-NC / NTs were ultrasonically dispersed in ethanol, and the solid-liquid ratio was controlled at 1g:50mL. Then lithium chloride was added as lithium salt. The amount of lithium salt and Mn-NC / NTs was controlled according to the molar ratio of Li:Mn of 1.05:1. The temperature was raised to 50℃ and stirred until the solvent was completely evaporated. The mixture was then dried at 70℃ and ground until there was no obvious particle feel. Then it was transferred to an autoclave and heat-treated at 100℃ for 48h. Then it was calcined at 500℃ for 4h to obtain lithium manganese oxide / nitrogen-doped carbon nanotube composite material LMO-NC / NTs.

[0059] Example 2

[0060] This embodiment provides a method for preparing a lithium manganese oxide / nitrogen-doped carbon nanotube composite material, the method comprising:

[0061] (1) Manganese nitrate was used as a soluble divalent manganese salt and dispersed with folic acid in an ethanol aqueous solution (the volume ratio of ethanol to water was 1.5:1). The concentration of folic acid in the ethanol aqueous solution was controlled at 1 g / 50 mL. After vigorous stirring, the solution was sonicated until it became a uniform brown suspension. Then, a 65% hydrazine hydrate solution was added dropwise. The molar ratio of manganese salt, folic acid and hydrazine hydrate was controlled at 1:1:140. The resulting mixed solution was sonicated and then transferred to a hydrothermal reactor. The hydrothermal reaction was carried out at 150 °C for 2 h. Then, the solution was repeatedly filtered and washed with deionized water and ethanol. After drying, brown powder Mn-FA / NTs was obtained.

[0062] (2) Mn-FA / NTs and potassium permanganate were mixed and ground in a mass ratio of 1:0.5 and the resulting powder was evenly spread in a ceramic boat. Under Ar gas conditions, the temperature was increased to 600℃ at 2℃ / min and held for 2h for carbonization treatment to obtain Mn-NC / NTs.

[0063] (3) Mn-NC / NTs were ultrasonically dispersed in ethanol, and the solid-liquid ratio was controlled at 1g:50mL. Then lithium chloride was added as lithium salt, and the amount of lithium salt and Mn-NC / NTs was controlled according to the molar ratio of Li:Mn of 1.1:1. The temperature was raised to 50℃, and the mixture was stirred until the solvent was completely evaporated. The mixture was then dried at 70℃ and ground until there was no obvious particle feel. Then it was transferred to an autoclave and heat-treated at 200℃ for 48h, and then calcined at 400℃ for 4h to obtain lithium manganese oxide / nitrogen-doped carbon nanotube composite material LMO-NC / NTs.

[0064] Example 3

[0065] This embodiment provides a method for preparing a lithium manganese oxide / nitrogen-doped carbon nanotube composite material, the method comprising:

[0066] (1) Manganese nitrate was used as a soluble divalent manganese salt and dispersed with folic acid in an ethanol aqueous solution (the volume ratio of ethanol to water was 1:1). The concentration of folic acid in the ethanol aqueous solution was controlled at 1 g / 40 mL. After vigorous stirring, the solution was sonicated until it became a uniform brown suspension. Then, a 65% hydrazine hydrate solution was added dropwise. The molar ratio of manganese salt, folic acid and hydrazine hydrate was controlled at 1:0.8:130. The resulting mixed solution was sonicated and then transferred to a hydrothermal reactor. The hydrothermal reaction was carried out at 160 °C for 2 h. Then, the solution was repeatedly filtered and washed with deionized water and ethanol. After drying, brown powder Mn-FA / NTs was obtained.

[0067] (2) Mn-FA / NTs and potassium permanganate were mixed and ground in a mass ratio of 1:0.3 and the resulting powder was evenly spread in a ceramic boat. Under Ar gas conditions, the temperature was increased to 600℃ at 2℃ / min and held for 2h for carbonization treatment to obtain Mn-NC / NTs.

[0068] (3) Mn-NC / NTs were ultrasonically dispersed in ethanol, and the solid-liquid ratio was controlled at 1g:50mL. Then lithium chloride was added as lithium salt, and the amount of lithium salt and Mn-NC / NTs was controlled according to the molar ratio of Li:Mn of 1.2:1. The temperature was raised to 50℃, and the mixture was stirred until the solvent was completely evaporated. The mixture was then dried at 70℃ and ground until there was no obvious particle feel. Then it was transferred to an autoclave and heat-treated at 200℃ for 48h, and then calcined at 600℃ for 4h to obtain lithium manganese oxide / nitrogen-doped carbon nanotube composite material LMO-NC / NTs.

[0069] Comparative Example 1

[0070] This comparative example provides a method for preparing a lithium manganese oxide / nitrogen-doped carbon nanotube composite material. The preparation method does not use hydrazine hydrate, and all other conditions are exactly the same as in Example 3.

[0071] Comparative Example 2

[0072] This comparative example provides a method for preparing a lithium manganese oxide / nitrogen-doped carbon nanotube composite material. The preparation method does not use potassium permanganate, and all other conditions are exactly the same as in Example 3.

[0073] Comparative Example 3

[0074] This comparative example provides a composite material of nitrogen-doped nanotubes and LiMn2O4, the preparation method of which includes:

[0075] (1) Solid-phase synthesis of LiMn2O4: MnO2 and LiOH were weighed as raw materials according to the Li / Mn molar ratio of 1:2, and after being thoroughly ground and mixed, LiMn2O4 with spinel structure was obtained by high-temperature solid-phase reaction at 700℃.

[0076] (2) Composite material: LiMn2O4 and nitrogen-doped nanotubes were mixed and ground at a mass ratio of 1:0.5 to obtain composite material.

[0077] Characterization and testing:

[0078] I. The microstructure of the material was observed using a JEOL JSM-6490LV scanning electron microscope.

[0079] II. The crystal phase and crystal structure of the material were studied using an X-ray powder diffractometer (XRD, Rigaku D / max-2600PC, Japan). Kα rays of Cu were used for the test, with a wavelength λ of 0.154056 nm, a voltage of 40 kV, a current of 40 mA, and a scanning range of 2θ of 10–80°. The XRD test results were analyzed using Jade 6 software.

[0080] III. The morphology of the samples was tested using a Talos F200S G2 transmission electron microscope.

[0081] IV. The composition and valence state of the material surface were determined using an ESCALAB 250Xi X-ray photoelectron spectrometer.

[0082] V. Nitrogen adsorption-desorption curves of the composite materials obtained in the examples and comparative examples were measured using a high-performance surface area and micropore analyzer BSD-PM1 to obtain information on their internal micropore structure.

[0083] Figure 1 This is the XRD pattern of the LMO-NC / NTs obtained in Example 1. As can be seen from the figure, the diffraction peaks at 26.2° and 42.2° correspond to the (002) and (100) crystal planes of the carbon material (PDF: 89-8487), respectively. These derived peaks are relatively sharp, indicating that LMO-NC / NTs have good crystallinity. Other diffraction peaks are consistent with LiMn2O4 (JCPDS.35-0782). The results show that the material obtained in Example 1 is a composite material of LiMn2O4 and carbon nanotubes. XRD testing of the composite material obtained in Comparative Example 2 shows that its product does not conform to LiMn2O4.

[0084] Figure 2 and Figure 3 The images show SEM images of the composite materials obtained in Example 1 and Comparative Example 1, respectively. As can be seen from the images, a tubular composite material cannot be obtained without the addition of hydrazine hydrate.

[0085] Figure 4 This is a TEM image of the LMO-NC / NTs obtained in Example 1. As can be seen from the image, the spinel lithium manganese oxide particles are embedded in the nanotubes.

[0086] Figure 5 These are XPS full-scan spectra of Mn-FA / NTs, Mn-NC / NTs, and LMO-NC / NTs obtained in Example 1. As can be seen from the figures, Mn-FA / NTs exhibits C1s, N1s, O1s, Mn 2p3 / 2, and Mn 2p1 / 2; Mn-NC / NTs exhibits C1s, N1s, Mn 2p3 / 2, and Mn 2p1 / 2; and LMO-NC / NTs exhibits C1s, N1s, Mn 2p3 / 2, Mn 2p1 / 2, and Li 1s.

[0087] Figure 6 and Figure 7 These are the XPS narrow-spectrum oxygen and nitrogen spectra of the Mn-FA / NTs obtained in Example 1. The narrow-spectrum O spectrum of Mn-FA / NTs shows that the difference in binding energies between C=O (binding energy 532.eV) and CO (binding energy 532.5eV) of the carboxylic acid group is 0.4eV, significantly smaller than the 1.5eV difference in free carboxylic acid. This indicates an increased electron delocalization effect in the carboxyl group of Mn-FA / NTs, confirming that both O atoms in the carboxylic acid group of folic acid participate in coordination with Mn. The other oxygen element in the spectrum originates from coordinated water (binding energy 533.2eV). The narrow-spectrum N spectrum of Mn-FA / NTs shows that the N1s spectrum can be separated into 399.6eV (NH4+). 2- ), 400.6 eV (N2H4-Mn), 401.1 eV, and 401.6 eV (pteroic acid N). Therefore, this indicates that Mn 2+ It coordinated with both folic acid and hydrazine hydrate.

[0088] Figure 8 These are XPS narrow spectrum images of manganese in Mn-NC / NTs and LMO-NC / NTs obtained in Example 1. The narrow spectrum of Mn in Mn-NC / NTs shows binding energies of 653.8 eV and 642.5 eV, 652.6 eV and 641.3 eV, and 651.1 eV and 640 eV, respectively, corresponding to Mn binding energies. 4+ Mn 3+ Mn-Nx indicates that during the carbonization process, Mn... 2+ With MnO 4- A redox reaction occurred, and Mn-Nx was generated.

[0089] Narrow spectrum of Mn element in LMO-NC / NTs 3+ Mn4+ The peak intensity of Mn-Nx changed; a higher peak intensity indicates a higher content. This can be seen from the fact that after the Mn-Nx reaction with the Li source, the peak intensity changed. 4+ The increase in Mn-Nx content and the decrease in Mn-Nx quantity indicate that some of the Mn-Nx is converted into Mn. 4+ Therefore, the preparation method provided by this invention is beneficial for improving Mn 4+ / Mn 3+ The content of Mn is reduced, thereby inhibiting the dissolution of Mn.

[0090] VI. Preparation and Testing of Lithium Extraction Electrode:

[0091] A slurry for the working electrode was prepared by mixing the composite materials obtained in the examples and comparative examples, carbon black, polyvinylidene fluoride, and N-methylpyrrolidone (NMP) in a weight ratio of 7:2:1. The slurry was then loaded onto carbon fiber cloth at a concentration of 6 mg / cm² and dried at 80°C for 10 h to obtain a lithium-rich electrode. Further, using the prepared lithium-rich electrode as the anode and an Ag / AgCl electrode as the cathode, both electrodes were electrochemically treated in a 0.05 M KCl solution at a constant potential of 1.0 V for 2 h to obtain a lithium-depleted electrode.

[0092] This invention employs a lithium-rich electrode / lithium-poor electrode system (electrochemical deintercalation / intercalation method) to investigate the lithium extraction performance of samples. Specifically, a lithium-rich electrode is used as the anode, and a lithium-poor electrode as the cathode, separated by an anion exchange membrane. The electrolyte (recovery solution) in the anode chamber is a 0.05 M KCl solution, while the electrolyte (extraction solution) in the cathode chamber is a mixed solution of 0.05 M LiCl and 1 M MgCl2. The electrode spacing is 4.5 cm, and the voltage is constant at 1 V. Using Ag as the counter electrode, the lithium-poor electrode obtained after delithiation is used as the cathode to extract Li, and then switched to the anode for further delithiation in this system. Samples are taken from the recovery cell after a certain period, and Li is analyzed using ICP-OES. + Mg 2+ Given the concentration of Mn ions, calculate the lithium extraction capacity E. Li (mg / g), lithium extraction rate r E (mg·g -1 ·min -1 ), separation coefficient α Li / Mg Manganese dissolution rate E Mn The calculation formulas for indicators such as (mg / g) are shown below:

[0093]

[0094]

[0095]

[0096]

[0097] In the formula, and These represent the final mass concentration and volume of Li ions in the recovered solution, respectively. and These represent the Li ion mass concentration and volume at a specific time point, respectively; t(min) represents the sampling time. and The Li in the recovered liquid at time t are respectively + Mg 2+ molar concentration; and Li in the recovered liquid at time 0 + Mg 2+ molar concentration; and This represents the final mass concentration and volume of Mn ions in the recovered solution; and These represent the mass concentration and volume of Mn ions sampled at a certain time; m(g) is the mass of the lithium ion sieve precursor.

[0098] The results are recorded in Table 1.

[0099] Table 1

[0100]

[0101] From the above, we can see that:

[0102] The preparation method described in this invention utilizes Mn 2+ The coordination with folic acid and hydrazine hydrate, along with the hydrogen bonding between folic acid molecules and the bridging effect of hydrazine hydrate molecules, enables the three to self-assemble into a nanotube-like structure Mn-FA / NTs. Then, carbonization is used to form nitrogen-doped carbon nanotubes from the organic matter, utilizing the free C and Mn generated during carbonization. 2+ A redox reaction occurs between potassium permanganate and manganese, converting manganese into Mn. 3+ ~Mn 4+ A small amount of Mn-Nx is generated, and then a lithium manganese oxide material with a spinel structure is generated through a solid-state reaction with a lithium source. This material is then in situ embedded between nitrogen-doped carbon nanotubes to obtain a lithium manganese oxide / nitrogen-doped carbon nanotube composite material. Comparative Example 1 did not use hydrazine hydrate, and the lithium extraction rate of the product was significantly reduced. Comparative Example 2 could not synthesize lithium manganese oxide because it did not use potassium permanganate, and therefore could not be used for lithium extraction. It should be noted that potassium permanganate needs to be used separately from manganese salt. If manganese salt and potassium permanganate are mixed and subjected to hydrothermal reaction in step (1), it will affect the coordination of divalent manganese to form nanotubes. The lithium extraction performance of the product obtained in this invention is better than that of the nitrogen-doped nanotube and LiMn2O4 composite material in Comparative Example 3.

[0103] The present invention has been illustrated with the above embodiments to illustrate its detailed structural features. However, the present invention is not limited to the above detailed structural features, that is, it does not mean that the present invention must rely on the above detailed structural features to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for the components used in the present invention, additions of auxiliary components, and selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

[0104] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0105] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0106] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A method for preparing a lithium manganate / nitrogen-doped carbon nanotube composite, characterized by, The preparation method comprises the following steps: The manganese salt, folic acid and hydrazine hydrate are mixed to form a mixed solution, the manganese salt comprises a soluble divalent manganese salt, and a hydrothermal reaction is performed to obtain Mn-FA / NTs; The Mn-FA / NTs is mixed with potassium permanganate, and carbonization treatment is performed to obtain Mn-NC / NTs; The Mn-NC / NTs is mixed with a lithium salt, and high-temperature treatment is performed, the high-temperature treatment comprises the following steps: first, heat treatment is performed in an autoclave, the heat treatment is performed at a temperature of 100-200 DEG C for 24-60 hours, then, calcination is performed by increasing the temperature, the calcination is performed at a temperature of 400-600 DEG C for 2-6 hours, and a lithium manganate / nitrogen-doped carbon nanotube composite material LMO-NC / NTs is obtained.

2. The production method according to claim 1, characterized by, The preparation method comprises the following steps: the manganese salt and folic acid are prepared into a suspension, and then a hydrazine hydrate solution is added dropwise to obtain a mixed solution.

3. The production method according to claim 2, characterized by, The solvent in the suspension comprises ethanol and water in a volume ratio of (0.8-1.5):

1.

4. The production method according to claim 2, characterized by, The mass concentration of the hydrazine hydrate solution is 60%-70%.

5. The preparation method according to claim 1, characterized in that, The use amount of the manganese salt, folic acid and hydrazine hydrate is controlled according to a molar ratio of 1:(0.5-1):(120-140).

6. The method of claim 1, wherein, The soluble divalent manganese salt comprises manganese chloride and / or manganese nitrate.

7. The preparation method according to claim 1, characterized in that, The temperature of the hydrothermal reaction is 150-170 DEG C, and the time is 1-3 hours.

8. The method of claim 1, wherein, The use amount of the Mn-FA / NTs and potassium permanganate is controlled according to a mass ratio of 1:(0.2-0.5).

9. The method of claim 1, wherein, The carbonization treatment is performed in a protective atmosphere, and the protective atmosphere comprises argon.

10. The method of claim 1, wherein, The temperature of the carbonization treatment is 550-650 DEG C, the time is 1-3 hours, and the temperature increasing rate is 1-5 DEG C / min.

11. The method of claim 1, wherein, The preparation method comprises the following steps: the Mn-NC / NTs is dispersed in a solvent, a lithium salt is added, the solvent is evaporated by heating, and then the following steps are performed: drying, grinding and high-temperature treatment.

12. The method of claim 1, wherein, The use amount of the lithium salt and Mn-NC / NTs is controlled according to a molar ratio of (1.05-1.2):

1.

13. The method of claim 1, wherein, The lithium salt comprises at least one of lithium chloride, lithium nitrate or lithium acetate.

14. The method of claim 1, wherein, The preparation method comprises the following steps: The soluble divalent manganese salt and folic acid are dispersed in an ethanol aqueous solution, the volume ratio of ethanol and water is (0.8-1.5):1, the solution is ultrasonically treated until the solution becomes a uniform brown suspension after being stirred intensively, then a hydrazine hydrate solution with a mass concentration of 60%-70% is added dropwise, the molar ratio of the manganese salt, folic acid and hydrazine hydrate is controlled to be 1:(0.5-1):(120-140), the obtained mixed solution is ultrasonically treated and then transferred into a hydrothermal reaction kettle, a hydrothermal reaction is performed at 150-170 DEG C for 2 hours, then the solution is repeatedly filtered and washed with deionized water and ethanol for multiple times, and a brown powder Mn-FA / NTs is obtained after drying; The Mn-FA / NTs is mixed with potassium permanganate according to a mass ratio of 1:(0.2-0.5), the obtained powder is uniformly laid in a porcelain boat, carbonization treatment is performed under Ar atmosphere by increasing the temperature to 550-650 DEG C at a temperature increasing rate of 1-5 DEG C / min and keeping the temperature for 1-3 hours, and Mn-NC / NTs is obtained. Mn-NC / NTs are ultrasonically dispersed in ethanol, lithium salt is added, the amount of the lithium salt and Mn-NC / NTs is controlled according to the molar ratio of Li:Mn being (1.05~1.2):1, the temperature is raised to 40~60℃, the stirring is continued until the solvent is completely volatilized, then the mixture is dried at 60~80℃, followed by grinding until no obvious particle feeling is felt, then it is transferred to an autoclave for heat treatment at 100~200℃ for 24~60h, and then calcination at 400~600℃ for 2~6h, to obtain lithium manganate / nitrogen-doped carbon nanotube composite material LMO-NC / NTs.

15. A lithium manganate / nitrogen-doped carbon nanotube composite material, characterized by, The preparation method according to any one of claims 1-14.

16. Use of the lithium manganate / nitrogen-doped carbon nanotube composite material according to claim 15, characterized in that The use includes use for extracting lithium from salt lakes.

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