A polyacrylonitrile-based activated carbon fiber, its preparation method and application

Abstract

This invention relates to the field of materials technology, specifically disclosing a polyacrylonitrile-based activated carbon fiber, its preparation method, and its application. The preparation method involves: crushing polyacrylonitrile to 140-200 mesh, immersing it in a 5-15 mM phosphoric acid solution, and then ultrasonically treating it for 10-20 minutes to obtain polyacrylonitrile flocculents; under an inert atmosphere, placing the polyacrylonitrile flocculents in a microwave tube furnace and activating them at 400-600°C for 10-30 minutes; after high-temperature activation, subjecting them to multiple gas bath washings and finally vacuum drying to obtain the polyacrylonitrile-based activated carbon fiber. The preparation process of this invention is simple, has a short production cycle, and low cost. The resulting polyacrylonitrile-based activated carbon fiber has a relatively uniform aspect ratio and a surface distribution of microporous structures. When used in conjunction with MXene materials, it can effectively improve the stability of the layered structure of MXene materials, thereby enhancing the rate performance and cycle stability of sodium-ion batteries.

Description

Technical Field

[0001] This invention belongs to the field of materials technology, and relates to carbon fiber, and particularly to a polyacrylonitrile-based activated carbon fiber, its preparation method and application. Background Technology

[0002] Energy is one of the major challenges facing social development. With the depletion of fossil fuels and the gradual deterioration of the environment, there is an urgent need to develop efficient, clean, and sustainable new energy sources and matching energy storage materials. Activated carbon possesses a highly developed pore structure and specific surface area, primarily composed of carbon. However, differences in raw materials often result in the presence of small amounts of non-carbon elements such as hydrogen, oxygen, and nitrogen, leading to the possibility of localized segregation and defects in the carbon material. Activated carbon fibers, using organic fibers as precursors, exhibit excellent acid and alkali resistance, mechanical properties, electrical conductivity, and chemical stability. Carbon fibers have limited applications in energy storage. Directly using them as anode materials for sodium-ion batteries also presents some drawbacks. For example, the excessively large pore structure and specific surface area of ​​activated carbon fibers may increase irreversible capacity loss during the initial charge-discharge process, reducing the battery's coulombic efficiency. Furthermore, surface defects in activated carbon fibers may become unstable sites during sodium-ion insertion / extraction, affecting the battery's cycle stability.

[0003] MXene is an emerging two-dimensional layered material with unique physicochemical properties and good electrical conductivity. Its structure is similar to graphite, and it has already shown great application potential in lithium-ion battery anode materials. (SEM image shown...) Figure 1 As shown. However, sodium-ion battery anode materials have high requirements for anode materials. The characteristic that the ionic radius is larger than that of lithium ions makes it impossible to directly use either graphite or MXene. This is because the layers of MXene material are prone to stacking, which obstructs the ion transport channels and affects the rate performance and cycle stability of the battery. At the same time, the high surface energy of MXene material makes it easy to have side reactions with the electrolyte, which further affects the cycle life and safety of the battery. Summary of the Invention

[0004] In view of the above-mentioned shortcomings of the existing technology, the purpose of this invention is to provide a polyacrylonitrile-based activated carbon fiber, its preparation method and application. The preparation process of this invention is simple, the production cycle is short and the cost is low. The obtained polyacrylonitrile-based activated carbon fiber is uniform and has a smooth surface. When used in combination with MXene material, it can effectively improve the stability of the layered structure of MXene material, thereby improving the rate performance and cycle stability of sodium-ion batteries.

[0005] The technical solution of this invention is implemented as follows:

[0006] A method for preparing polyacrylonitrile-based activated carbon fibers specifically includes the following steps:

[0007] Step 1: Crush the polyacrylonitrile to 140-200 mesh, then immerse it in a 5-15 mM phosphoric acid solution, and then ultrasonically treat it for 10-20 min to obtain polyacrylonitrile flocculents;

[0008] Step 2: Under an inert atmosphere, place the polyacrylonitrile flocculent from Step 1 into a microwave tube furnace and activate it at 400~600℃ for 10~30 min.

[0009] Step 3: After high-temperature activation, the carbon fiber is washed multiple times by air bath and finally dried under vacuum to obtain the polyacrylonitrile-based activated carbon fiber.

[0010] Furthermore, during ultrasonic treatment, the power is 100~300W.

[0011] Furthermore, the high-temperature activation temperature in step 2 is 450~550℃. Studies have shown that high-temperature activation at 450~550℃ results in better carbonization of the polyacrylonitrile-based active fibers, maintaining excellent fiber morphology, consistent aspect ratio, and a dispersed structure without agglomeration. In particular, the best carbonization effect is achieved when high-temperature activation is performed at 500℃.

[0012] Furthermore, in step 3, during vacuum drying, the vacuum degree is 0.005~0.01MPa.

[0013] The aforementioned application of a polyacrylonitrile-based activated carbon fiber in sodium ion anode materials.

[0014] Furthermore, electrodes are prepared by coating polyacrylonitrile-based activated carbon fibers, MXene materials, conductive carbon black, and PVDF together, and then sodium-ion batteries are assembled, wherein the mass ratio of polyacrylonitrile-based activated carbon fibers, MXene materials, conductive carbon black, and PVDF is 2~3:4~5:2:1.

[0015] Furthermore, the mass ratio of polyacrylonitrile-based activated carbon fiber, MXene material, conductive carbon black, and PVDF is 1:6:2:1.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] 1. This invention uses polyacrylonitrile (PA) as a low-cost raw material to prepare activated carbon fibers, effectively reducing production costs. Simultaneously, a low-concentration, highly stable phosphoric acid solution is used as an impurity pretreatment agent. Phosphoric acid has strong water absorption properties, and phosphate ions have a strong binding affinity to organic matter, effectively removing impurities and organic matter from the PA raw material. Ultrasonic treatment is employed, utilizing the strong penetrating power and concentrated energy of ultrasound to allow the phosphoric acid solution to penetrate the PA more effectively, accelerating impurity removal and shortening impregnation time. Furthermore, the rapid, uniform, selective, and energy-efficient characteristics of microwave heating achieve good environmental and energy benefits. In addition, the entire reaction process is carried out under an inert atmosphere, effectively reducing material oxidation loss and further improving the yield of activated carbon fibers. The yield of activated carbon fibers in this invention can reach 89%.

[0018] 2. The polyacrylonitrile-based activated carbon fibers prepared by this invention are uniform and structurally complete. Through grinding, the polyacrylonitrile-based activated carbon fibers are mixed with two-dimensional layered MXene in slurry form, allowing the polyacrylonitrile-based activated carbon fibers to be embedded into the MXene material in a thread-spun form to adjust the interlayer spacing, thereby ensuring good electrochemical performance of the sodium-ion battery. MXene material, as the main body of the two-dimensional layered structure, has a large interlayer spacing and excellent conductivity. The addition of polyacrylonitrile-based activated carbon fibers not only increases the specific capacity and cycle stability of the negative electrode material but also provides a suitable two-dimensional layered structure, which is beneficial for the stable insertion and extraction of sodium ions during charge and discharge.

[0019] Furthermore, by optimizing the ratio of activated carbon fibers and MXene materials, the electrochemical performance of the anode material can be further controlled, thereby meeting the fabrication requirements of sodium-ion batteries with different performance specifications. The construction of this novel anode material provides new ideas and directions for the development of sodium-ion batteries. Attached Figure Description

[0020] Figure 1 - SEM image of MXene material.

[0021] Figure 2 - SEM image of polyacrylonitrile-based activated carbon fibers prepared in Example 1, magnified 500 times.

[0022] Figure 3 - SEM images of polyacrylonitrile-based activated carbon fibers prepared in Examples 1, 4 and 5, magnified 50 times.

[0023] Figure 4 -IR spectra of polyacrylonitrile-based activated carbon fibers prepared in Examples 1 and 4.

[0024] Figure 5 - Rate cycling diagram of the assembled sodium-ion full cell.

[0025] Figure 6 - Charge-discharge curves of PAMX-0(a) and PAMX-3(b) anode materials.

[0026] Figure 7 - Impedance diagrams of the assembled sodium-ion full cells PAMX-0 and PAMX-3. Detailed Implementation

[0027] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0028] The scanning electron microscope used in the following examples was a Sigma 300 (made by Carl Zeiss Microscopy Limited), the integrated constant temperature battery test chamber was a MIHW-200-160CH (made by Shenzhen Xinwei Electronics Co., Ltd.), the ultrasonic breaker was a ZOLLO-650Y (made by Shanghai Zuole Instrument Co., Ltd.), and the water used in the experiments was distilled water.

[0029] Example 1

[0030] A method for preparing polyacrylonitrile-based activated carbon fibers specifically includes the following steps:

[0031] (1) The polyacrylonitrile was crushed to 180 mesh and then immersed in a 10 mM dilute phosphoric acid solution for ultrasonic treatment for 20 min to obtain polyacrylonitrile flocculents; the power of ultrasonic treatment was 100 W.

[0032] (2) Under inert atmosphere protection (flow rate of 0.05 m³ / h) 3 / h), weigh 5g of polyacrylonitrile flocculent material and put it into a microwave tube furnace for high-temperature activation. Set the temperature to 500℃ and the activation time to 20min.

[0033] (3) After activation, the carbon fiber is cooled and then placed in a steam circulation system for multiple gas bath washing to remove phosphoric acid. Then it is vacuum dried in a vacuum chamber with a vacuum degree of 0.005 MPa to obtain polyacrylonitrile-based activated carbon fiber.

[0034] Example 2

[0035] This embodiment is the same as Embodiment 1, except that the temperature during high-temperature activation in this embodiment is 450°C.

[0036] Example 3

[0037] This embodiment is the same as Embodiment 1, except that the temperature during high-temperature activation in this embodiment is 550°C.

[0038] Example 4

[0039] This embodiment is the same as Embodiment 1, except that the temperature during high-temperature activation in this embodiment is 400°C.

[0040] Example 5

[0041] This embodiment is the same as Embodiment 1, except that the temperature during high-temperature activation in this embodiment is 600°C.

[0042] 1. In Example 1, the yield of polyacrylonitrile-based activated carbon fiber was 89% (yield = mass of polyacrylonitrile-based activated carbon fiber / mass of polyacrylonitrile flocculents added to the microwave tube furnace × 100%). The polyacrylonitrile activated carbon fibers obtained in Examples 1 to 3 had consistent aspect ratios and exhibited dispersed structures without agglomeration. A 500x magnified SEM image of the polyacrylonitrile-based activated carbon fiber obtained in Example 1 is shown below. Figure 2 As shown, by Figure 2 It can be seen that the polyacrylonitrile-based activated carbon fibers have a consistent aspect ratio and a complete structure, indicating that the internal structure of the polyacrylonitrile-based activated carbon fibers is completely transformed at this temperature.

[0043] SEM images of the polyacrylonitrile-based activated carbon fibers prepared in Examples 1, 4, and 5, magnified 50 times, are shown below. Figure 3 As shown in (a), (b), and (c), by Figure 3 It can be seen that the carbonization effect of polyacrylonitrile-based active fibers is the best at 500˚C, maintaining excellent fiber morphology and consistent aspect ratio; the carbonization of polyacrylonitrile-based active fibers is incomplete at 400˚C, and some fibers form agglomerates; the carbonization of polyacrylonitrile-based active fibers is excessive at 600˚C, resulting in fiber bending and inconsistent size.

[0044] 2. The IR spectra of the polyacrylonitrile-based activated carbon fibers prepared in Examples 1 and 4 are shown below. Figure 4 As shown in the figure, the red line corresponds to Example 1, and the black line corresponds to Example 4. It can be seen from the figure that after high-temperature treatment, the infrared spectrum is at 2354 cm⁻¹. -1 There is a distinct absorption peak for the cyano group (C≡N), at 1685 cm⁻¹. -1 The peak value is the stretching vibration peak at 1260 cm⁻¹, corresponding to C=C and C=N. -1 The peak at 1260 cm⁻¹ represents the vibrational peak of the COH structure. As temperature increases, the absorption peak of the cyano structure in polyacrylonitrile decreases, almost disappearing at 500℃, while the peak at 1260 cm⁻¹... -1 The absorption peak weakens. After carbonization at 600℃, the content of carbon-based planar structures is relatively large, and no obvious absorption peak is found in the spectrum.

[0045] 3. The polyacrylonitrile-based activated carbon fiber obtained in Example 1 was mixed with MXene material, conductive carbon black, and PVDF and coated to prepare an electrode. A sodium-ion full cell was then assembled and subjected to rate cycling tests. The mass ratio of polyacrylonitrile-based activated carbon fiber, MXene material, conductive carbon black, and PVDF was 0:7:2:1 (PAMX-0), 1:6:2:1 (PAMX-3), 2:5:2:1 (PAMX-4), 3:4:2:1 (PAMX-1), and 3.5:3.5:2:1 (PAMX-2). Specifically, the raw materials were mixed according to the above ratios, and an appropriate amount of N-methylpyrrolidone was added and ground until a glossy black slurry without obvious particles was formed. Copper foil was selected as the current collector for coating, and after drying, it was cut into sheets with a diameter of 15 mm. When assembling the battery, Prussian blue NaFe4[Fe(CN)6]3 was used as the positive electrode material. The CR2025 sodium-ion full cell was assembled and tested in the following order: positive electrode shell, gasket, positive electrode material, separator, negative electrode material, gasket, and negative electrode shell.

[0046] The results of the multiplier cycle test are as follows: Figure 5 As shown in the figure, the PAMX-3 and PAMX-4 assembled batteries exhibit better performance, achieving initial capacities of 452 mAh / g and 473 mAh / g, respectively, at a current density of 0.1 A / g. In contrast, the PAMX-0 group, without the addition of polyacrylonitrile-based activated carbon fiber, only achieves an initial capacity of 207 mAh / g at the same current density. This demonstrates that the polyacrylonitrile-based activated carbon fiber does indeed contribute to improving battery performance. Furthermore, when the current density increases to 1 A / g, the reversible capacity of PAMX-0 is 2.9 mAh / g, while the capacities of PAMX-3 and PAMX-4 groups are 14.9 Ah / g and 8.1 Ah / g, respectively, showing a significant increase in rate performance compared to PAMX-0. Regardless of whether the current is high or low, the charge / discharge capability mainly depends on the electron transport efficiency and ion transport capability of the material. MXene is obtained by removing Al from MAX-Ti3AlC2 through HF etching, and it has good conductivity. However, as cycling progresses, its accordion-like structure becomes unstable, thus reducing its ion transport capability. Therefore, MXene anodes exhibit poor rate performance. PAMX-3 and PAMX-4 anode materials show significantly improved rate performance due to the dispersion effect of polyacrylonitrile-based activated carbon fibers. However, polyacrylonitrile-based activated carbon fibers do not have sufficient ion transport channels. Therefore, when too much polyacrylonitrile-based activated carbon fiber is added to the PAMX-1 group of anode materials, its rate performance is poor.

[0047] To further investigate the structural stability of PAMX-3, PAMX-0 was used as a control, and the two were combined into a battery and subjected to charge-discharge cycle tests at a current density of 0.2 A / g. Figure 6 -a shows that the PAMX-0 first charge-discharge specific capacity reached 5 / 24 mAh / g, and the second to fifth charges showed a significant decrease compared to the first. This is because although the MXene material has a rich three-dimensional accordion structure that can provide a large number of sodium ion transport channels, its structure is unstable under electrolyte immersion and during charge-discharge processes. Figure 6 The charge-discharge curves of -b PAMX-3 clearly show that its initial charge-discharge capacity reached 60 / 154 mAh / g. Although the specific capacity decreased somewhat in subsequent charge-discharge processes, it remained significantly higher than that of PAMX-0, and remained relatively stable throughout the subsequent charge-discharge cycles with minimal fluctuations. This indicates that the addition of polyacrylonitrile-based activated carbon fibers increased the structural stability of MXene, enabling it to possess greater specific capacity and higher activity during charge-discharge processes. The embedding of polyacrylonitrile-based activated carbon fibers indeed increases the material's capacitive sodium storage capacity, inhibiting SEI film formation and slowing down the material's deactivation process.

[0048] Meanwhile, the impedance diagrams for PAMX-0 and PAMX-3 are as follows: Figure 7 As shown, the obtained curves are typical Nyquist curves, consisting of a semicircle in the high-frequency region and a small-radius micro-curve in the low-frequency region. The figure shows that the slope of the low-frequency region of the PAMX-3 electrode with added polyacrylonitrile-based activated carbon fiber is larger than that of the PAMX-0 without activated carbon fiber, indicating the presence of capacitive behavior. Furthermore, the magnified view of the high-frequency region shows that the semicircle diameter of the electrode with added polyacrylonitrile-based activated carbon fiber is smaller, indicating that the addition of polyacrylonitrile-based activated carbon fiber improves the performance of MXene, resulting in a relatively lower charge transfer impedance.

[0049] Finally, it should be noted that the above embodiments of the present invention are merely illustrative examples and not intended to limit the implementation of the invention. Those skilled in the art can make other variations and modifications based on the above description. It is impossible to exhaustively list all possible implementations here. All obvious variations or modifications derived from the technical solutions of this invention are still within the scope of protection of this invention.

Claims

1. A method for preparing polyacrylonitrile-based activated carbon fibers, characterized in that, Specifically, the following steps are included: Step 1: Crush the polyacrylonitrile to 140-200 mesh, then immerse it in a 5-15 mM phosphoric acid solution, and then treat it with ultrasound for 10-20 min to obtain polyacrylonitrile flocculents. Step 2: Under an inert atmosphere, place the polyacrylonitrile flocculent from Step 1 into a microwave tube furnace and activate it at 450~550℃ for 10~30 min. Step 3: After high-temperature activation, the carbon fiber is washed multiple times by air bath and finally dried under vacuum to obtain the polyacrylonitrile-based activated carbon fiber.

2. The method for preparing polyacrylonitrile-based activated carbon fiber according to claim 1, characterized in that, During ultrasonic treatment, the power is 100~300W.

3. The method for preparing polyacrylonitrile-based activated carbon fiber according to claim 1, characterized in that, The high-temperature activation temperature in step 2 is 500℃.

4. The method for preparing polyacrylonitrile-based activated carbon fiber according to claim 1, characterized in that, In step 3, during vacuum drying, the vacuum degree is 0.005~0.01MPa.

5. A polyacrylonitrile-based activated carbon fiber, characterized in that, It is prepared using the preparation method of polyacrylonitrile-based activated carbon fiber according to any one of claims 1 to 4.

6. The application of the polyacrylonitrile-based activated carbon fiber as described in claim 5 in sodium ion anode materials.

7. The application of the polyacrylonitrile-based activated carbon fiber according to claim 6, characterized in that, Electrodes were prepared by coating a mixture of polyacrylonitrile-based activated carbon fibers, MXene material, conductive carbon black, and PVDF, and then assembling sodium-ion batteries. The mass ratio of polyacrylonitrile-based activated carbon fibers, MXene material, conductive carbon black, and PVDF was 1~2:5~6:2:

1.

8. The application of the polyacrylonitrile-based activated carbon fiber according to claim 7, characterized in that, The mass ratio of polyacrylonitrile-based activated carbon fiber, MXene material, conductive carbon black and PVDF is 1:6:2:1.

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