A nitrogen-doped carbon-bonded expanded graphite composite material and its preparation method and application

Abstract

The present invention discloses a nitrogen-doped carbon-connected expanded graphite composite material and a preparation method and application thereof, and belongs to the technical field of negative electrode materials for potassium ion batteries. The present invention uses a simple and mild method to compound nitrogen-containing ionic liquid with expanded graphite, and obtains a nitrogen-doped carbon-connected expanded graphite composite material after drying and high-temperature calcination. The composite material has an enlarged interlayer spacing, abundant active sites and a stable lamellar structure. When used as a negative electrode material for potassium ion batteries, it is beneficial to accelerate the transmission of ions and electrons in the electrode, improve the potassium ion adsorption capacity, and alleviate the volume change during the process of potassium insertion and extraction, effectively improving the electrochemical performance of the battery.

Description

Technical Field

[0001] The invention belongs to the technical field of negative electrode materials for potassium ion batteries, and in particular relates to a nitrogen-doped carbon-connected expanded graphite composite material and a preparation method and application thereof. Background Art

[0002] Due to the limited supply of lithium resources and the growing demand for electric vehicles and large-scale applications, people are exploring alternative energy storage technologies other than lithium-ion batteries. As the most promising candidate, potassium-ion batteries have attracted widespread attention due to their wide distribution and abundant reserves on the earth. Potassium has a low standard redox potential (-2.93 V relative to the standard electrode potential) and has the smallest Stokes radius (K + (3.6 Å) < Na + (4.6Å) < Li + (4.8Å)), so potassium ion batteries have higher operating voltage, energy density and ionic conductivity. Graphite is commonly used as a negative electrode for commercial lithium-ion batteries. + Can be embedded in it to form KC 8 The compound can therefore be directly applied to potassium ion batteries with a reversible capacity of 279 mAh g -1 However, K + The radius (1.36Å) is larger than Li + (0.76Å), which usually leads to severe volume changes (58%) and slow reaction kinetics during potassium insertion / extraction, thus affecting rate performance and cycle stability. The use of oxidation or pillaring effect can expand the interlayer spacing and accelerate ion diffusion. However, this method easily leads to the peeling of the graphite layer, resulting in capacity decay. Therefore, it is necessary to design the graphite interlayer to improve the structural stability and potassium storage capacity of the electrode.

[0003] Computational and experimental studies have shown that edge nitrogen (pyrrolic nitrogen and pyridinic nitrogen) defect sites can significantly increase potassium ion adsorption energy and improve potassium storage performance. Nitrogen atoms usually come from some small molecules and polymers, which can interact with graphite layers, thereby connecting adjacent sheets to build a complete layered structure. However, the nitrogen atom configuration is not easy to be precisely controlled and usually shows a low edge nitrogen doping level (<80%). Simply increasing the doping level will destroy the electronic conductivity of the π-conjugated system. Therefore, graphite materials with high activity and integrated sheet structures are very ideal negative electrode materials for potassium ion batteries. In view of this, the present invention is specially proposed. Summary of the invention

[0004] In view of the problems existing in the prior art, the present invention provides a nitrogen-doped carbon-linked expanded graphite composite material and a preparation method thereof, which proposes a simple and mild new method for preparing a nitrogen-doped carbon-linked expanded graphite composite material based on the π-π interaction between nitrogen-containing ionic liquid and MXene. The composite material has an enlarged interlayer spacing, rich defects, a high edge nitrogen ratio and a stable lamellar structure, which can promote ion and electron conduction, improve potassium ion storage capacity, and achieve high capacity, fast charge and discharge and long service life of potassium ion battery electrochemical performance.

[0005] The present invention adopts the following technical scheme to solve the above technical problems. A preparation method of a nitrogen-doped carbon-connected expanded graphite composite material is characterized in that the specific process is: uniformly mixing a nitrogen-containing ionic liquid and expanded graphite in a solvent, vacuum drying the mixture, heat treating it at 300-800°C for 1-6h in an inert atmosphere, and then naturally cooling it to room temperature to obtain a nitrogen-doped carbon-connected expanded graphite composite material, wherein the cation in the nitrogen-containing ionic liquid is one or more of an imidazolium salt cation, a pyridinium salt cation or a quaternary ammonium salt cation, and the anion in the nitrogen-containing ionic liquid is Cl - Br - , [PF 6 ] - or [HSO 4 ] - One or more of .

[0006] It is further defined that the feed mass ratio of the expanded graphite to the nitrogen-containing ionic liquid is 1:0.2-20.

[0007] It is further defined that the solvent is one or more of water, methanol or ethanol.

[0008] It is further defined that the inert atmosphere is an argon atmosphere, a helium atmosphere or a nitrogen atmosphere.

[0009] It is further defined that the heating rate during the heat treatment process is 1-10°C min -1 .

[0010] The nitrogen-doped carbon-connected expanded graphite composite material of the present invention is characterized in that it is prepared by the above method.

[0011] Application of the nitrogen-doped carbon-connected expanded graphite composite material as a negative electrode material for a potassium ion battery.

[0012] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0013] The present invention is based on the π-π interaction between the abundant π electrons on the surface of graphite sheets and the nitrogen-containing cations in the ionic liquid, to achieve the intercalation of ionic liquid in expanded graphite, and to form a nitrogen-doped carbon-linked expanded graphite composite material in the subsequent calcination process. The special structure of the ionic liquid precursor makes its derived carbon material have abundant defects and a high edge nitrogen ratio (>80%), which improves the potassium storage capacity. In addition, the structure has an enlarged interlayer spacing and a complete sheet structure, which is conducive to fully exposing active sites, accelerating interlayer ion diffusion, and alleviating volume expansion during charging and discharging. The three-dimensional conductive path can also accelerate electron transfer in the electrode material, thereby improving the rate performance and cycle stability of the potassium ion battery. Based on these advantages, the nitrogen-doped carbon-linked expanded graphite composite material prepared by the present invention exhibits high capacity, excellent rate performance and long-term cycle stability when applied to the negative electrode material of potassium ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 The SEM image (a), TEM image (b) and element distribution map (cf) of the nitrogen-doped carbon-bonded expanded graphite composite material prepared in Example 1.

[0015] Figure 2 This is the XPS N 1s spectrum of the nitrogen-doped carbon-bonded expanded graphite composite material prepared in Example 1.

[0016] Figure 3 This is a rate performance comparison curve of the nitrogen-doped carbon-connected expanded graphite composite material prepared in Example 1 and the comparative example used as a negative electrode material for potassium ion batteries.

[0017] Figure 4 This is a comparison curve of the cycle performance of the nitrogen-doped carbon-connected expanded graphite composite material prepared in Example 1 and the comparative example used as a negative electrode material for potassium ion batteries.

[0018] Figure 5 This is a Raman spectrum of the nitrogen-doped carbon-bonded expanded graphite composite material prepared in Example 2. DETAILED DESCRIPTION

[0019] The above contents of the present invention are further described in detail below through examples, but this should not be understood as the scope of the above subject matter of the present invention being limited to the following examples, and all technologies implemented based on the above contents of the present invention belong to the scope of the present invention.

[0020] Example 1

[0021] Expanded graphite and 1-ethyl-3-methylimidazolium bromide were mixed in ethanol at a mass ratio of 1:1 and stirred for 2 h. The mixture was vacuum dried at 60 ° C for 6 h and then placed in a tube furnace at N 2 Heat treatment was carried out at 600℃ for 3h under protective conditions, with a heating rate of 3℃ min-1 , and finally, after naturally cooling to room temperature, a nitrogen-doped carbon-connected expanded graphite composite material was obtained.

[0022] Figure 1 The SEM image, TEM image and element distribution map of the nitrogen-doped carbon-bonded expanded graphite composite material prepared in Example 1 are shown in FIG. Figure 1 It can be seen that compared with the 0.33nm interlayer spacing and regular layer stacking of traditional graphite materials, the prepared nitrogen-doped carbon-linked expanded graphite composite material layers show distortion and turbine layer phenomena, with the maximum interlayer spacing reaching 0.9nm, and N and O elements are evenly distributed in the carbon material, proving that the elements have good dispersion, stable structure, and are not prone to peeling. Figure 2 This is the XPS N 1s spectrum of the nitrogen-doped carbon-bonded expanded graphite composite material prepared in Example 1. It can be seen from the figure that the nitrogen atoms are mainly doped in the form of pyrrolic nitrogen and pyridinic nitrogen, and the active nitrogen accounts for 82%. Figure 3 The rate performance curve of the nitrogen-doped carbon-connected expanded graphite composite material prepared in Example 1 as a negative electrode material for potassium ion batteries is shown in the figure. -1 The reversible specific capacity is as high as 333.1 mAh g -1 , even at 5A g -1 At a high current density, it can still maintain 162.6 mAh g -1 High reversible capacity. Figure 4 The cycle performance curve of the nitrogen-doped carbon-connected expanded graphite composite material prepared in Example 1 as a negative electrode material for potassium ion batteries is shown in the figure. -1 After 2000 cycles at the same current density, the capacity still remained at 181.9 mAh g -1 Relatively stable capacity.

[0023] Example 2

[0024] Expanded graphite and 1-butyl-3-methylimidazolium hexafluorophosphate were mixed in methanol at a mass ratio of 1:3 and stirred for 2 h. The mixture was dried in vacuum at 60 °C for 6 h and then placed in a tube furnace and heated to 500 °C for 3 h under Ar protection at a heating rate of 3 °C min -1 Finally, after naturally cooling to room temperature, a nitrogen-doped carbon-connected expanded graphite composite material was obtained.

[0025] Figure 5 This is the Raman spectrum of the nitrogen-doped carbon-bonded expanded graphite composite material prepared in Example 2, at 1350 cm -1 and 1580cm -1 Two standard peaks (D peak and G peak) appear nearby. The peaks are wide and not sharp. D / I GThe value is 0.91, which proves that the prepared nitrogen-doped carbon-connected expanded graphite composite is rich in defect sites and has a low degree of graphitization.

[0026] Example 3

[0027] Expanded graphite and 1-ethylpyridine hydrobromide were mixed in ethanol at a mass ratio of 1:5 and stirred for 2 h. The mixture was vacuum dried at 60 °C for 6 h and then placed in a tube furnace under N 2 Heat treatment at 700℃ for 2h under protective conditions, heating rate of 3°Cmin -1 Finally, after naturally cooling to room temperature, a nitrogen-doped carbon-connected expanded graphite composite material was obtained.

[0028] Comparative Example

[0029] The difference between the comparative example and Example 1 is that no 1-ethyl-3-methylimidazolium bromide ionic liquid is added.

[0030] The above embodiments describe the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for illustrating the principles of the present invention. Without departing from the scope of the principles of the present invention, the present invention may have various changes and improvements, and these changes and improvements all fall within the scope of protection of the present invention.

Claims

1. Application of nitrogen-doped carbon-linked expanded graphite composite material as negative electrode material of potassium ion battery, wherein the nitrogen-doped carbon-linked expanded graphite composite material has an enlarged interlayer spacing, rich defects, high edge nitrogen ratio and stable sheet structure, can promote ion and electron conduction, improve potassium ion storage capacity, and achieve electrochemical performance of potassium ion battery with high capacity, fast charge and discharge and long service life, based on the π-π interaction between abundant π electrons on the surface of expanded graphite sheets and nitrogen-containing cations in ionic liquid, intercalation of ionic liquid in expanded graphite is achieved, and nitrogen-doped carbon-linked expanded graphite composite material is formed in the subsequent calcination process; the specific preparation process of the nitrogen-doped carbon-linked expanded graphite composite material is: uniformly mixing nitrogen-containing ionic liquid and expanded graphite in a solvent, then vacuum drying the mixture and heat treating it at 300-800° C. for 1-6 hours in an inert atmosphere, and then naturally cooling it to room temperature to obtain the nitrogen-doped carbon-linked expanded graphite composite material, wherein the cation in the nitrogen-containing ionic liquid is one or more of imidazolium salt cations, pyridinium salt cations or quaternary ammonium salt cations, and the anion in the nitrogen-containing ionic liquid is Cl - Br - , [PF6] - or [HSO4] - One or more of the above, wherein the mass ratio of the expanded graphite to the nitrogen-containing ionic liquid is 1:0.2-20.

2. The use according to claim 1, characterized in that: The solvent is one or more of water, methanol or ethanol.

3. The use according to claim 1, characterized in that: The inert atmosphere is an argon atmosphere, a helium atmosphere or a nitrogen atmosphere.

4. The use according to claim 1, characterized in that: The heating rate during the heat treatment is 1-10°C min -1 .

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