An electrode based on sub-nanometer tellurium@hierarchical porous carbon fiber and a preparation method thereof

Abstract

The present invention belongs to the field of electrochemistry technology, and particularly relates to an electrode based on sub-nanometer tellurium@hierarchical porous carbon fiber and a preparation method thereof. The present invention provides an electrode of nitrogen and phosphorus co-doped porous carbon fiber loaded with sub-nanometer tellurium and a preparation method thereof, characterized in that: the space confinement effect of micron pores in the carbon fiber and the nitrogen and phosphorus co-doping enhance the tellurium immobilization and the adsorption of polytellurides, and inhibit the volume change and the shuttle effect of polytellurides during cycling. Compared with non-doping and single nitrogen doping, the space confinement effect of micron pores in the nitrogen and phosphorus co-doped carbon fiber ensures the formation of sub-nanometer tellurium, effectively improves the tellurium loading amount, slows down the volume expansion and the dissolution of polytellurides, and enables the electrode to exhibit high specific capacity and stable potassium storage performance. The present invention provides new ideas and new ways for the design and preparation of high-performance chalcogen element single substance-based electrodes and the regulation of their electrochemical properties.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrochemistry, and particularly relates to an electrode based on sub-nanometer tellurium@hierarchical porous carbon fiber and a preparation method thereof. Background Art

[0002] In recent years, the continuous maturity of lithium-ion battery technology has promoted the rapid development of industrialization and science and technology, which benefits from the advantages of lithium-ion batteries such as long cycle life, high specific energy, high voltage, no memory effect, and low self-discharge rate. However, due to the limited lithium source reserves (0.0017 wt%) and uneven distribution, the widespread application of lithium-ion batteries will surely accelerate the depletion of lithium resources. Therefore, there is an urgent need to develop a new alternative battery system. Based on a similar working mechanism, due to the higher natural abundance of potassium element (1.5 wt%), metallic potassium (-2.93 V) has a potential closer to that of metallic lithium (-3.04 V), and the research on potassium-ion batteries has received extensive attention. Importantly, compared with Li + / Na + , K + has a weaker Lewis acidity, resulting in a smaller solvated ionic radius and a lower desolvation energy, thus promoting its diffusion at the electrolyte / electrode interface, which means that potassium-ion batteries have potential excellent electrochemical performance.

[0003] Chalcogen element simple substances store alkali metal ions through a two-electron conversion reaction mechanism, showing the advantage of high volume specific capacity, which is beneficial to the efficient packaging and reasonable design of devices, and has become a candidate for potassium-ion battery electrode materials. Tellurium (Te), as a typical chalcogen element, has similar physical and chemical properties to sulfur (S) and selenium (Se). Different from S (5×10 -30 Sm –1 ) and Se (1×10 -3 S m –1 ), Te has semi-metallicity, and its electronic conductivity is as high as 2×10 2 S m –1 . As an electrode active material, it helps to achieve fast charge and discharge, and the relatively large atomic radius of Te limits the formation of poly-telluride intermediates, resulting in a weaker shuttle effect during cycling. Therefore, potassium-tellurium batteries are becoming increasingly popular among researchers due to their high volume specific capacity and excellent fast charge efficiency performance.

[0004] Although loading bulk micron-scale Te on porous carbon by the melt diffusion method is the most widely used method for preparing Te-based electrode materials, the pore structure of the carbon matrix has a great influence on the loading amount and micro-morphology structure of Te. Using a carbon matrix with micropores can achieve a uniform distribution of Te, existing in an amorphous state, but its formation mechanism is still unclear, let alone the detailed electrochemical reaction process of Te during the potassium insertion (extraction) process. In addition, K+ The ionic radius of Te is relatively large, resulting in a volume expansion of up to 420% between Te and the final potassium storage product, K2Te. The dissolution of the polytelluride intermediate in the organic electrolyte during the potassium insertion (extraction) process leads to the loss of the active Te component and the shuttle effect. Due to the high reactivity of the potassium metal anode, potassium dendrite growth and electrolyte decomposition occur, further exacerbating the capacity decay. Therefore, the present invention proposes an electrode of sub-nanoscale Te@hierarchical porous carbon fiber and its preparation method, which utilizes the spatial confinement effect of the micropores in the hierarchical carbon matrix and nitrogen-phosphorus co-doping to enhance the Te immobilization and the adsorption of the polytelluride intermediate during potassium storage, relieve the volume expansion and the shuttle effect, and thus greatly improve its potassium storage capacity and long cycle stability. Summary of the Invention

[0005] The purpose of the present invention is to provide an electrode of sub-nanoscale Te@hierarchical porous carbon fiber and its preparation method. Compared with the undoped and single-nitrogen-doped porous carbon fibers, the spatial confinement effect of the micropores and the nitrogen-phosphorus co-doping in the nitrogen-phosphorus co-doped carbon fibers ensure the formation of single atoms and small molecule Te allotropes and the adsorption of the polytelluride intermediate during potassium storage, effectively increasing the Te loading amount, relieving the volume expansion and the dissolution of the polytelluride during the potassium insertion (extraction) process, reducing the shuttle effect, and enabling the composite electrode to exhibit stable potassium storage performance with high specific capacity and long life, and excellent cycle stability even at low currents.

[0006] The present invention provides an electrode of nitrogen-phosphorus co-doped porous carbon nanofibers loaded with sub-nanoscale Te (Te@N,P-codoped PCNFs) and its preparation method, which is characterized by being realized through the following technical solutions:

[0007] (1) Synthesis of zeolitic imidazolate framework-8 (ZIF-8): Dissolve zinc acetate dihydrate (Zn(CH3COO)2·2H2O) in deionized water to obtain solution A; dissolve cetyltrimethylammonium bromide (CTAB) and 2-methylimidazole (2-MiM) in deionized water to obtain solution B; mix the above two solutions, and after aging the resulting suspension at room temperature for a period of time, vacuum filter to collect the white powder, wash it repeatedly with deionized water several times, and then obtain ZIF-8 through blast drying.

[0008] (2) Preparation of hierarchically porous carbon nanofibers: The ZIF-8 prepared in the above step (1), urea (as a nitrogen source), and triphenylphosphine (TPP) (as a phosphorus source) were co-dispersed in N,N-dimethylformamide (DMF). After stirring evenly, polyacrylonitrile (PAN) was added and stirred evenly to obtain a precursor solution. The solution was loaded into a syringe with a stainless steel nozzle, and a ZIF-8 / urea / TPP@PAN nanofiber film was prepared by electrospinning. Subsequently, the collected film was placed in a corundum boat and loaded into a vacuum tube furnace. It was subjected to a two-stage constant-temperature carbonization process in an air atmosphere at a relatively low temperature (200-280 °C) for pre-oxidation and then in a hydrogen-argon atmosphere at 450-550 °C (the first stage) and 900-1000 °C (the second stage) to prepare nitrogen and phosphorus co-doped hierarchically porous carbon nanofibers, labeled as N,P-codoped PCNFs. In addition, for comparative analysis, no urea and triphenylphosphine (TPP) were added to the precursor solution, or only urea was added, and hierarchically porous carbon nanofibers without doping or nitrogen-doped hierarchically porous carbon nanofibers were prepared using the same electrospinning and high-temperature calcination processes, labeled as PCNFs or N-doped PCNFs, respectively.

[0009] (3) Preparation of Te-loaded hierarchically porous carbon nanofiber composites: Te powder and the above hierarchically porous carbon nanofibers were respectively placed in a tube furnace. Under an argon atmosphere, a Te-loaded hierarchically porous carbon nanofiber composite was prepared by a constant-temperature melting-diffusion method at a certain temperature (480-500 °C). According to whether there are heteroatom dopants and the type of heteroatoms in the hierarchically porous carbon fibers used, they were respectively labeled as Te@PCNFs, Te@N-doped PCNFs, and Te@N,P-codoped PCNFs.

[0010] The advantages and positive effects of the present invention are:

[0011] Through the design of nitrogen and phosphorus co-doping and the hierarchically porous structure of the carbon matrix, the present invention prepares Te-loaded nitrogen and phosphorus co-doped hierarchically porous carbon nanofibers, which have the advantages of relatively simple modification means and low cost. Sub-nanoscale Te composed of single atoms and small molecule allotropes is uniformly distributed in the micropores of N,P co-doped hierarchically porous carbon nanofibers (Te@N,P-codoped PCNFs). As a self-supporting electrode for storing potassium ions, compared with undoped and single-nitrogen-doped carbon matrices, the strong adsorption effect of N,P-codoped PCNFs on K2Te and the spatial confinement of abundant micropores effectively buffer the volume expansion, enabling the Te@N,P-codoped PCNFs electrode to exhibit excellent electrochemical performance, including high reversible capacity and excellent cycle stability at low currents. Description of the Drawings

[0012] Figure 1It is the scanning electron microscope (SEM) image of the Te@N,P-codoped PCNFs sample prepared in Example 1;

[0013] Figure 2 It is the high-angle annular dark-field (HAADF) image and (B) aberration-corrected scanning transmission electron microscope (STEM) image of the Te@N,P-codoped PCNFs sample prepared in Example 1 under (A) transmission electron microscope (TEM);

[0014] Figure 3 It is the X-ray diffraction (XRD) patterns of N,P-codoped PCNFs, Te@N,P-codoped PCNFs, and the initial bulk Te prepared in Example 1;

[0015] Figure 4 It is the X-ray photoelectron spectroscopy (XPS) spectrum of the Te@N,P-codoped PCNFs prepared in Example 1;

[0016] Figure 5 It is the pore size distribution diagram of the Te@N,P-codoped PCNFs prepared in Example 1;

[0017] Figure 6 It is the cyclic voltammetry (CV) curve of the self-supporting electrode of Te@N,P-codoped PCNFs prepared in Example 1 at a scanning rate of 0.1 mV s –1 ;

[0018] Figure 7 It is the cycling performance curves of the self-supporting electrodes of Te@N,P-codoped PCNFs, Te@N-doped PCNFs, and Te@PCNFs prepared in Example 1 at 0.1 A g –1 ; Detailed implementation manners

[0019] The present invention will be further described in detail through specific embodiments below. The following embodiments can enable those skilled in the art to understand the present invention more comprehensively, but do not limit the present invention in any way.

[0020] Example 1:

[0021] (1) First, synthesize ZIF-8. The specific process is as follows: Dissolve 3 g of zinc acetate dihydrate (Zn(CH3COO)2·2H2O) in 50 mL of deionized water to obtain solution A; dissolve 9.11 mg of cetyltrimethylammonium bromide (CTAB) and 11.165 g of 2-methylimidazole (2-MiM) in 50 mL of deionized water, stir for 15 min to get solution B; then, mix the above solution A and solution B, and age the resulting suspension at room temperature for 3 h; collect the white powder by vacuum filtration, wash it with deionized water, and dry it at 70 °C for 24 h to prepare ZIF-8.

[0022] (2) On the premise of obtaining the above ZIF-8 sample, prepare nitrogen and phosphorus co-doped porous carbon nanofibers (N,P-codoped PCNFs) by electrospinning-high temperature calcination method. The specific preparation process is as follows: First, dissolve 500 mg of ZIF-8, 100 mg of urea, and 300 mg of triphenylphosphine (TPP) in 5 ml of DMF, ultrasonically disperse for 30 min, and then add 500 mg of PAN (M w = 150000), stir magnetically for 24 h to obtain the precursor solution; then, load this solution into a syringe (5 mL) with a stainless-steel nozzle, and prepare ZIF-8 / urea / TPP@PAN nanofibers through the electrospinning process, where the applied voltage is 15 kV, the working distance is 20 cm, and the feeding rate is 8 μL min -1 . Subsequently, heat the collected film in an air atmosphere to 250 °C at a heating rate of 1 °C min –1 , keep it at a constant temperature for 5 h to complete pre-oxidation, and then heat it to 500 °C in an Ar / H2 (5%) atmosphere at a heating rate of 3 °C min –1 for 1 h of carbonization, and then heat it to 950 °C in an Ar / H2 (5%) atmosphere at a heating rate of 5 °C min –1 for 2 h of further carbonization to finally obtain N,P-codoped PCNFs. As a control study, undoped porous carbon nanofibers (PCNFs) and nitrogen-doped porous carbon nanofibers (N-doped PCNFs) were also prepared under the same above conditions, except that urea and TPP were not added respectively during the preparation process of the precursor solution, or only urea was added, and other processes were exactly the same.

[0023] (3) Uniformly distribute Te on the N,P-codoped PCNFs prepared in the previous step through a high-temperature melting and diffusion process, thereby obtaining nitrogen and phosphorus co-doped porous carbon nanofibers loaded with sub-nanometer Te. The specific process is as follows: Load 100 mg of bulk Te powder (99.999%) and 25 mg of N,P-codoped PCNFs at both ends of a corundum boat, and place it in a tube furnace. The Te powder is located upstream of the gas flow direction. Under an argon atmosphere, heat it to 480 °C at a heating rate of 5 °C min –1 . Keep the temperature constant for 17 h to complete Te loading and prepare a self-supporting electrode of Te@N,P-codoped PCNFs. In addition, for comparative studies, self-supporting electrodes of Te@PCNFs and Te@N-doped PCNFs are also prepared under the same conditions as above, except that undoped porous carbon nanofibers (PCNFs) and nitrogen-doped porous carbon nanofibers (N-doped PCNFs) are used respectively during the Te loading process, and other processes are exactly the same.

[0024] The samples prepared in the present invention are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), and a fully automatic specific surface area and porosity analyzer (BET) to observe their morphology, structure, and composition. Secondly, assemble a potassium-tellurium battery and test its electrochemical performance. The specific process and steps are as follows: Cut the self-supporting Te@N,P-codoped PCNFs, Te@N-doped PCNFs, and Te@PCNFs films prepared in Example 1 into circular discs with a diameter of 8 mm as the positive electrode, with an average area loading of about 1.6 - 2.0 mg cm –2 . Use K metal as the negative electrode, 5.0 M potassium bis(fluorosulfonyl)imide (KFSI) / ethylene glycol dimethyl ether (DME) as the electrolyte, and Whatman glass fiber as the separator to assemble a potassium-ion battery in an argon-filled glove box. Use a Neware CT-4008 battery tester to perform constant current charge and discharge tests in the voltage range of room temperature, 0.01 - 3.0 V (vs. K / K + ). Perform cyclic voltammetry (CV) tests on an electrochemical workstation.

[0025] Figure 1It is the SEM image of the Te@N,P-codoped PCNFs sample prepared in Example 1. The rough surface of the composite sample indicates that the macroporous structure derived from the high-temperature pyrolysis of ZIF-8 in the carbon matrix is retained after Te loading. With the introduction of nitrogen and phosphorus sources into the electrospinning precursor solution, the fiber diameter of the composite sample is about 800 nm. Due to the introduction of nitrogen and phosphorus sources, the specific surface areas of PCNFs, N-doped PCNFs, and N,P-codpoed PCNFs gradually increase, and the Te loading in the composite sample increases accordingly. Based on the nitrogen and phosphorus co-doped composite sample, the Te loading is as high as 51.8%.

[0026] Figure 2 It is the HAADF and (B) STEM images of the Te@N,P-codoped PCNFs sample prepared in Example 1 under (A) TEM. The distribution of macropores in the composite sample can be observed from the HAADF image, which is beneficial to the penetration of the electrolyte and promotes ion diffusion. The STEM image shows that there are a large number of sub-nanometer Te in the composite sample, mainly in the form of single-atom Te and small-molecule Te.

[0027] Figure 3 It is the XRD patterns of N,P-codoped PCNFs, Te@N,P-codoped PCNFs, and the initial bulk Te prepared in Example 1. As can be seen from the figure, the XRD pattern of bulk Te confirms that it belongs to the hexagonal crystal system; while the XRD spectrum of the Te@N,P-codoped PCNFs composite sample is similar to that of N,P-codoped PCNFs, and no characteristic diffraction peaks attributed to hexagonal crystal system Te are observed, indicating that the sub-nanometer Te in the composite sample presents in an "amorphous state", which is mainly due to the shortening of the molecular chain of crystalline Te during the high-temperature melting process and the spatial confinement effect of the micron-sized pores on the porous carbon fiber, existing in the form of single-atom and small-molecule Te in the composite sample.

[0028] Figure 4 It is the XPS image of Te@N,P-codoped PCNFs prepared in Example 1 to study the surface chemical composition of the composite sample. According to the analysis, the Te 3d 5 / 2 sub-spectrum consists of four peaks located at 576.5, 575.8, 574.4, and 573.5 eV, corresponding to Te-O, Te-C, Te-N, and Te-Te bonds. According to the peak area size, it can be judged that a small amount of TeO appears on the surface of the composite sample x , and the main composition of tellurium element is elemental Te 0 .

[0029] Figure 5BET pore size distribution diagram of Te@N,P-codoped PCNFs prepared in Example 1. Among them, N,P-codoped PCNFs has a three-level pore structure composed of micropores-mesopores-macropores. After loading Te through the melting-diffusion process of tellurium at high temperature, the micropore structure on the porous carbon fiber disappears. This result indicates that the fragmented small-molecule tellurium is effectively confined in the sub-nanometer pores of the porous carbon fiber during the high-temperature treatment.

[0030] Figure 6 Cyclic voltammetry (CV) curves of the Te@N,P-codoped PCNFs electrode prepared in Example 1 at a scan rate of 0.1 mV s –1 The irreversible peak at 0.22 V in the first-cycle discharge curve is due to the formation of an electrode / electrolyte interface film by the decomposition of the electrolyte. In subsequent cycles, the reduction peak at 0.83 V on the discharge curve and the oxidation peak at 1.65 V on the charge curve correspond to the reversible conversion of Te intercalating potassium to form K2Te and K2Te deintercalating potassium to form Te, respectively.

[0031] Figure 7 Cycling performance curves of the self-supporting electrodes of Te@N,P-codoped PCNFs, Te@N-doped PCNFs, and Te@PCNFs prepared in Example 1 at 0.1 A g –1 Compared with the undoped (Te@PCNFs) and nitrogen-doped (Te@N-doped PCNFs) electrodes, the nitrogen-phosphorus co-doped (Te@N,P-codoped PCNFs) electrode has a higher specific capacity and excellent cycling stability. The potassium-tellurium battery based on the Te@N,P-codoped PCNFs self-supporting electrode has a first-cycle reversible specific capacity of 254.2 mAh g –1 at 0.1 A g –1 , the first-cycle Coulombic efficiency is 46.1%, and the capacity remains at 228.3 mAh g –1 after 700 cycles, with a retention rate of 89.8%. The electrochemical test results show that the hierarchical porous carbon matrix can effectively immobilize sub-nanometer Te. Nitrogen-phosphorus co-doping enhances the adsorption of K2Te by the carbon matrix. The spatial confinement effect of the micropores effectively alleviates the volume expansion of the active material in the electrode, inhibits the stripping of polytellurides and their dissolution in the electrolyte, reduces the influence of the shuttle effect, and also indicates that the synergistic strategy of carbon matrix structure design and heteroatom doping is an effective modification method to improve the electrochemical performance of Te-based electrodes, and is expected to be extended to other chalcogen element-based electrode materials to increase the active material loading, change the electrochemical reaction process, thereby improving the cycling stability and promoting its application in the energy storage field.

Claims

1. A method for preparing an electrode based on sub-nanometer tellurium@hierarchical porous carbon fiber, characterized in that It includes the following steps: (1) First, synthesize the zeolitic imidazolate framework material ZIF-8. The specific process is as follows: Dissolve zinc acetate dihydrate in deionized water to obtain solution A; dissolve cetyltrimethylammonium bromide and 2-methylimidazole in deionized water to obtain solution B; mix the above two solutions, and after the obtained suspension is aged at room temperature for a period of time, collect it by vacuum filtration, wash it with deionized water, and dry it to obtain white ZIF-8 powder; (2) Weigh a certain mass of the ZIF-8 sample prepared in the above step (1), disperse it together with urea and triphenylphosphine in N,N-dimethylformamide, and then add polyacrylonitrile to obtain a precursor solution; Subsequently, load the solution into a syringe with a stainless steel nozzle, and prepare a ZIF-8 / urea / triphenylphosphine@polyacrylonitrile composite nanofiber film through an electrospinning process. Put the collected film into a corundum boat and place it in a tube furnace. After pre-oxidation in an air atmosphere for a period of time, perform high-temperature carbonization treatment in two stages under a hydrogen-argon mixed atmosphere to obtain nitrogen and phosphorus co-doped porous carbon fibers; (3) Place the nitrogen and phosphorus co-doped porous carbon fibers prepared in step (2) and bulk Te powder at both ends of a corundum boat and put them into a tube furnace. Through a molten diffusion process and using the space confinement effect of the micropores in the porous carbon fibers, realize sub-nanometer Te loading to obtain a Te-loaded nitrogen and phosphorus co-doped porous carbon fiber self-supporting electrode.

2. The method for preparing an electrode based on sub-nanometer tellurium@hierarchical porous carbon fiber according to claim 1, wherein: In step (1), cetyltrimethylammonium bromide is added as a surfactant to reduce the ZIF-8 particle size, and the aging time is 3-5 h.

3. A method for preparing an electrode based on sub-nanometer tellurium@hierarchical porous carbon fiber according to claim 1, characterized in that: In step (2), the voltage applied during the electrospinning process is 15 - 20 kV, the working distance is 15 - 20 cm, and the advancing speed is 8 - 12 μL / min -1 ; urea is used as the nitrogen source and triphenylphosphine is used as the phosphorus source; the pre-oxidation temperature is 200 - 280 °C, and the heating rate is 1 - 3 °C / min -1 , the pre-oxidation time is 4 - 6 hours; the atmosphere during the high-temperature carbonization process is a mixed atmosphere of Ar and H2, where the volume fraction of H2 in the mixed gas is 5 - 10 vol.%, the carbonization temperature in the first stage is 450 - 550 °C, and the heating rate is 2 - 4 °C / min -1 , the carbonization time in the first stage is 1 - 2 hours, the carbonization temperature in the second stage is 900 - 1000 °C, and the heating rate is 4 - 6 °C / min -1 , and the carbonization time in the second stage is 2 - 4 h.

4. A method for preparing an electrode based on sub-nanometer tellurium@hierarchical porous carbon fiber according to claim 1, characterized in that: In step (3), the bulk Te powder is located upstream of the gas flow direction, and the above-mentioned porous carbon fiber is located downstream of the gas flow direction. Heat it to 480-500 °C in an argon atmosphere, and the heating rate is 4-6 °C / min -1 , and keep it at a constant temperature for 12-24 h to complete Te loading.

5. An electrode prepared by the method for preparing an electrode based on sub-nanometer tellurium@hierarchical porous carbon fiber according to claim 1, characterized in that: Sub-nanometer Te composed of single atoms and small molecule allotropes is uniformly distributed in the micropores of the nitrogen and phosphorus co-doped hierarchical porous carbon nanofibers.

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