Poplar catkin-based nitrogen-doped carbon microtube material for negative electrode of potassium ion battery and preparation method

Abstract

The invention belongs to the technical field of negative electrode materials of potassium ion batteries, and particularly relates to a poplar catkin-based nitrogen-doped carbon microtube material of a negative electrode of a potassium ion battery and a preparation method of the poplar catkin-based nitrogen-doped carbon microtube material of the negative electrode of the potassium ion battery. The activated poplar catkins are subjected to two-step carbonization in an inert atmosphere to obtain carbonized poplar catkins, the two-step carbonization comprises low-temperature pre-carbonization and high-temperature carbonization, the temperature of the low-temperature pre-carbonization is 200-300 DEG C, and the temperature of the high-temperature carbonization is 800-1000 DEG C; and washing the carbonized poplar catkin with an acid solution, washing with water until the filtrate is neutral, and drying to obtain the poplar catkin-based nitrogen-doped carbon microtube material for the negative electrode of the potassium ion battery. The material provided by the invention has high reversibility, high specific capacity, high cycling stability and excellent rate capability.

Description

Technical Field

[0001] The present invention belongs to the technical field of potassium ion battery negative electrode materials, and in particular relates to a potassium ion battery negative electrode poplar-based nitrogen-doped carbon microtube material and a preparation method thereof. Background Art

[0002] Potassium-ion batteries (KIBs) are expected to replace lithium-ion batteries and become the mainstream choice for commercial electrochemical energy storage technology. However, the anode materials of KIBs have limited their further development.

[0003] Carbonaceous materials are important candidate materials for negative electrodes of energy storage systems. For example, the theoretical potassium ion storage capacity of commercial graphite can reach 279 mAh·g -1 However, the instability of the carbon material structure causes low rate capability, poor cycle stability and low capacity. Nitrogen doping is one of the common methods to improve the structural stability of carbon materials. First, nitrogen doping can regulate the interlayer spacing of carbon materials. Compared with ordinary carbon materials (about 0.34 nm), it is more suitable for large-radius K + (1.38 Å) intercalation / deintercalation mitigates volume expansion and improves cycling stability. Second, the introduction of nitrogen atoms (e.g., pyridinic and pyrrolic nitrogen) not only enhances the electronic conductivity of the carbon material but also forms more active sites for potassium storage. Biomass-derived carbon materials have attracted considerable attention due to their advantages of waste resource utilization, high natural nitrogen content, and rational resource utilization. Using rational methods, they can be prepared as anode carbon materials for potassium-ion batteries while preserving their inherent nitrogen, resulting in carbon materials with high structural stability, high electronic conductivity, and multiple active sites for potassium storage.

[0004] Carbon microtube materials have the following significant advantages as negative electrode materials for potassium ion batteries: First, high conductivity and rapid ion / electron transport. Carbon microtubes have excellent electronic conductivity and can construct efficient electron transport channels, improving the overall conductivity of the electrode. At the same time, the one-dimensional tubular structure provides open pores, shortening the diffusion path of potassium ions, which is especially suitable for large-sized potassium ions, thereby supporting high-rate charge and discharge; second, high specific surface area and increased capacity. Carbon microtubes have a high specific surface area and can provide more potassium storage sites. The theoretical capacity is significantly improved compared to traditional graphite. At the same time, surface defects or heteroatom doping (such as N, S) can be used to enhance the adsorption capacity of potassium ions and improve the reversible capacity, so they have commercial prospects.

[0005] CN115763792A discloses a potassium ion battery negative electrode material, its preparation method, and application, comprising the following steps: dispersing a nitrogen source in water to obtain a nitrogen source solution; using a biomass material as a carbon source, pre-treating it, and then immersing it in the nitrogen source solution, followed by drying to obtain a precursor material; wherein the biomass material is any one of fallen leaves, weeds, and bacterial cellulose; and pyrolyzing the precursor material at 750-850°C in an inert atmosphere to obtain the potassium ion battery negative electrode material. Urea is additionally added as a nitrogen source, and fallen leaves, weeds, and bacterial cellulose are used as carbon sources.

[0006] CN107681133A discloses a lithium-ion battery negative electrode material and a preparation method, comprising: placing catkins in acetone and ultrasonically treating the catkins; placing the catkins in deionized water, vigorously stirring and washing the catkins; and then drying the catkins under vacuum conditions; placing the pretreated catkins in a tubular furnace for heat treatment to obtain catkin-derived hollow carbon microtubes; dispersing the catkin-derived hollow carbon microtubes in a clarified solution containing a molybdenum source and a sulfur source, hydrothermally treating the catkins, washing the catkins, and vacuum drying the catkins to obtain a precursor powder; placing the precursor powder in a tubular furnace for heat treatment in a protective atmosphere for a period of time, and cooling the precursor powder to room temperature in the furnace to obtain a molybdenum disulfide composite hollow carbon microtube lithium-ion battery negative electrode material. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a potassium ion battery negative electrode poplar-based nitrogen-doped carbon microtube material and a preparation method. The material of the present invention has high reversibility, high specific capacity, high cycle stability and excellent rate performance.

[0008] The embodiment of the present invention provides a method for preparing a potassium ion battery negative electrode poplar-based nitrogen-doped carbon microtube material, comprising the following steps: The poplar fluff after impurities are removed is immersed in alkali solution and kept warm to obtain activated poplar fluff; The activated poplar fluff is subjected to a two-step carbonization in an inert atmosphere to obtain carbonized poplar fluff, wherein the two-step carbonization includes low-temperature pre-carbonization and high-temperature carbonization, wherein the temperature of the low-temperature pre-carbonization is 200-300° C. and the temperature of the high-temperature carbonization is 800-1000° C.; The carbonized poplar fluff is washed with acid solution, rinsed with water until the filtrate is neutral, and dried to obtain a potassium ion battery negative electrode poplar fluff-based nitrogen-doped carbon microtube material.

[0009] Preferably, the alkali solution is one or more of sodium hydroxide and potassium hydroxide solution, with a concentration of 1-3 mol / L, the insulation temperature is 60-100° C., and the treatment time is 8-24 h.

[0010] Preferably, when performing low-temperature pre-carbonization, the heating rate is 1-3°C / min, and the low-temperature pre-carbonization time is 1-3 hours.

[0011] Preferably, when high-temperature carbonization is performed, the heating rate is 5-10° C. / min, and the high-temperature carbonization time is 1-3 h.

[0012] Preferably, the gas used for the inert atmosphere is argon.

[0013] Preferably, the temperature of the low-temperature pre-carbonization is 250°C, and the temperature of the high-temperature carbonization is 800°C.

[0014] Preferably, the acid solution is one or more of hydrochloric acid, sulfuric acid, nitric acid, perchloric acid, and citric acid, with a concentration of 1-3 mol / L.

[0015] Preferably, the method for removing impurities from the poplar fluff is to sieve out the seeds mixed in the poplar fluff; use an ethanol-water solution to clean the surface impurities, and ultrasonically clean to remove organic matter and dust; The drying temperature is 80° C. and the drying time is 12 hours.

[0016] Preferably, in the ethanol-water solution, the volume ratio of ethanol to water is 1:2-5, and the ultrasonic cleaning time is 3-12 hours.

[0017] An embodiment of the present invention provides a potassium ion battery negative electrode poplar-based nitrogen-doped carbon microtube material, which is prepared using the preparation method.

[0018] The beneficial effect of the present invention is that the present invention discovered that poplar fluff is a biomass material with a one-dimensional fiber structure. The present invention prepares it into a carbon microtube material and uses a reasonable preparation method to achieve doping, thereby obtaining a carbon material with high structural stability, high electronic conductivity, and multiple potassium storage active sites. The carbon material has a stable structure, good cycle performance, and is green and pollution-free.

[0019] The present invention involves immersing poplar fluff in a high-temperature alkali solution for activation and a step-by-step carbonization process, effectively retaining the naturally occurring nitrogen in the fluff and converting it into nitrogen-doped carbon microtubes with potassium storage active sites. Specifically, the process is as follows: First, chemical activation fixes nitrogen: Activating the poplar fluff with a high-temperature alkaline solution forms a stable chemical bond between the cellulose and nitrogen, preventing nitrogen loss during the subsequent high-temperature carbonization process. Furthermore, the alkaline activator modulates the pore structure of the carbon skeleton, providing anchoring sites for nitrogen atoms and forming active doping forms such as pyridinic and pyrrolic nitrogen.

[0020] Second, step-by-step carbonization optimizes the nitrogen-doped structure: A low-temperature pre-carbonization stage, controlled at 200-300°C, achieves directional decomposition of cellulose, preventing the formation of a disordered carbon structure while retaining the one-dimensional tubular skeleton and nitrogen. A high-temperature carbonization stage, controlled at 800-1000°C, promotes the embedding of nitrogen atoms into the carbon layer to form graphitic nitrogen while preventing nitrogen loss due to excessive carbonization. These two carbonization steps, performed sequentially without any other intermediate steps, work synergistically to optimize pore distribution, increase the material's specific surface area, and achieve efficient nitrogen doping and structural stabilization.

[0021] Third, the regulation of nitrogen-doped surface by pickling treatment: pickling after carbonization removes residual ash while exposing the nitrogen-doped active sites on the surface of the carbon material, avoiding the failure of active sites caused by impurity coverage, and further improving the utilization rate of nitrogen doping.

[0022] Fourth, the two-step heating method is conducive to maintaining the one-dimensional tubular structure of poplar fluff, thereby obtaining carbon microtube materials with high specific surface area, high conductivity, fast ion / electron transport and stable structure.

[0023] The potassium ion battery negative electrode poplar-based nitrogen-doped carbon microtube material prepared by the present invention has the following characteristics: First, the natural hollow fiber structure of carbon microtubes. After carbonization, poplar fluff forms hollow carbon microtubes, which retain the topology of natural fibers. This hollow structure not only provides a high specific surface area, but also shortens the K + diffusion path, while buffering the volume expansion stress through internal pores to improve cycle stability.

[0024] Second, the nitrogen-containing synergistic effect forms a composite structure of pyridinic nitrogen, pyrrolic nitrogen and graphitic nitrogen on the surface of the material through KOH activation and carbonization. Among them: pyridinic / pyrrolic nitrogen: increases active sites and enhances K + Adsorption capacity; graphitic nitrogen: improves electron conduction efficiency and reduces charge transfer impedance.

[0025] Third, defect and functional group regulation: The carbon material prepared by this method contains abundant structural defects, which further increase the active sites for potassium storage.

[0026] Fourth, waste resource utilization. Poplar fluff, a seasonal environmental pollutant, can be carbonized to solve environmental pollution problems while also reducing raw material costs, in line with the concept of a circular economy. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 The structure and morphology of carbon microtubes (CMTs) prepared in Example 1. (a) Macromorphology of poplar fluff and CMTs, (b) and (c) SEM images of CMTs, (d) TEM image of CMTs, (e) XRD pattern of CMTs, and (f) Raman spectrum of CMTs.

[0028] Figure 2 These are XPS spectra of carbon microtubes (CMTs) from Example 1. (a) is the full spectrum of the XPS spectra, (b) is the C 1s spectrum of the XPS spectra, (c) is the N 1s spectrum of the XPS spectra, and (d) is the O 1s spectrum of the XPS spectra.

[0029] Figure 3 The electrochemical performance diagram of the lithium-ion battery prepared using the carbon microtubes (CMTs) in Example 1 as the negative electrode material. (a) is the cyclic voltammetry curve, and (b) is the cyclic voltammetry curve at 0.1 A·g -1 Long cycle performance diagram under current density, (c) is the constant current charge and discharge curve diagram, (d) is the rate performance diagram, and (e) is the long cycle performance diagram.

[0030] Figure 4 (a) is 0.1 A·g of the material in Comparative Example 1 -1 Long cycle performance diagram of current density, Figure 4 (b) is the specific capacity voltage diagram of the material of Comparative Example 1.

[0031] Figure 5 (a) is 0.1 A·g of the material in Comparative Example 2 -1 Long cycle performance diagram of current density, Figure 5 (b) is the specific capacity voltage diagram of the material of comparative example 2.

[0032] Figure 6 (a) is 0.1 A·g of the material in Comparative Example 3 -1 Long cycle performance diagram of current density, Figure 6 (b) is the specific capacity voltage diagram of the material of comparative example 3.

[0033] Figure 7 (a) is 0.1 A·g of the material in Comparative Example 4 -1 Long cycle performance diagram of current density, Figure 7 (b) is the specific capacity voltage diagram of the material of comparative example 4.

[0034] Figure 8 (a) is 0.1 A·g of the material in Comparative Example 5 -1 Long cycle performance diagram of current density, Figure 8 (b) is the specific capacity voltage diagram of the material of comparative example 5.

[0035] Figure 9 (a) is 0.1 A·g of the material in Comparative Example 6 -1 Long cycle performance diagram of current density, Figure 9 (b) is the specific capacity-voltage diagram of the material of Comparative Example 6. DETAILED DESCRIPTION

[0036] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. The following embodiments further illustrate the content of the present invention in detail, but the scope of protection of the claims of the present invention is not limited by the embodiments.

[0037] In the present invention, the potassium ion battery performance test method is as follows: manganese dioxide active material, conductive agent Ketjen black, and binder carboxymethyl cellulose (CMC) are mixed in a mass ratio of 8:1:1, and after sufficient grinding, N-methyl-2-pyrrolidone (NMP) is added to form a uniform black paste slurry, which is then coated on copper foil and dried at 100°C in vacuum for 12 hours. The slurry is used as the positive electrode, wherein the active material loading is 0.8-1.2 mg / cm 2 , using potassium sheet as the negative electrode, glass fiber as the separator, 1 M KPF6 as the solute, and ethylene carbonate / dimethyl carbonate (EC / DMC) (volume ratio of 1:1) as the electrolyte, and assembled into CR2016 button batteries in a glove box. The charge and discharge current density used to test the cycle performance was 100 mA / g~1000 mA / g, and the test temperature was room temperature.

[0038] Example 1 A method for preparing a potassium ion battery negative electrode poplar-based nitrogen-doped carbon microtube material comprises the following steps: S1. Raw material pretreatment: Screening to remove seeds mixed in the poplar fluff; repeatedly clean the surface impurities with an ethanol-water mixture (ethanol:water volume ratio of 1:3), and ultrasonically clean for 6 hours to completely remove organic matter and dust.

[0039] S2. Chemical activation: immerse the poplar fluff treated in step S1 in a 2 mol / L KOH solution and treat at a constant temperature of 80° C. for 24 hours.

[0040] S3. Two-step carbonization: The poplar fluff treated in step S2 is subjected to two-step carbonization in a tubular furnace. Low-temperature pre-carbonization: heating to 250°C at 2°C / min under argon atmosphere and maintaining the temperature for 1 hour; high-temperature carbonization: heating to 800°C at 5°C / min and maintaining the temperature for 1 hour.

[0041] S4. Post-treatment: The material that has undergone two-step carbonization is washed with a 2 mol / L hydrochloric acid solution to remove residual ash and improve the purity of the material, and then repeatedly rinsed with deionized water until the filtrate is neutral; then it is vacuum dried at 80°C for 12 hours to obtain the potassium ion battery negative electrode poplar-based nitrogen-doped carbon microtube material.

[0042] Figure 1 a shows the morphology of poplar fluff and its carbonized samples. Figure 1 b. Figure 1 c shows that CMTs have a complete hollow tubular structure; Figure 1 As shown in (d), the TEM image further confirmed the hollow structure of CMTs, with a tube diameter of approximately 10-15 μm, proving the successful synthesis of the microtubule material. Figure 1 e indicates the successful synthesis of carbon materials, Figure 1 f at 1350 cm -1 and 1590 cm -1 The two characteristic peaks at represent defective carbon crystallites and crystalline graphite, respectively.

[0043] Figure 2 The XPS spectrum further illustrates the successful synthesis of nitrogen-containing carbon micromaterials, where Figure 2 The N 1s spectrum of c contains three peaks: 398.5 eV (pyridinic nitrogen), 399.4 eV (pyrrolic nitrogen), and 400.6 eV (graphitic nitrogen). Pyridinic and pyrrolic nitrogen can create defects and active sites, while graphitic nitrogen promotes internal electron transport.

[0044] Figure 3 a is the cyclic voltammetry (CV) curve of CMTs. A clear redox peak was observed in the first cycle, corresponding to K + reversible insertion / extraction process.

[0045] Figure 3 b shows that the first discharge capacity of CMTs reaches 474 mAh·g -1 The capacity decayed significantly in the first ten weeks and stabilized at 231 mAh g after 500 cycles. -1 .

[0046] The first charge and discharge capacities were 270 mAh g -1 and 474 mAh·g -1 , the Coulombic efficiency is only 57% ( Figure 3 c). The charge-discharge curves at different cycles are similar in shape, indicating that the material has excellent cycling stability.

[0047] Figure 3 d shows the rate performance: when the current density increases to 5 A·g -1 The capacity is maintained at 70 mAh g -1 , and the current returned to 0.1 A·g -1 The capacity returned to 232 mAh g -1 , showing good rate characteristics.

[0048] At 1 A·g -1 After 1000 cycles at high current density, the capacity still remains at 101 mAh g -1 ( Figure 3 e), confirming the excellent long-cycle performance.

[0049] Example 1 illustrates the successful preparation of nitrogen-containing carbon microtube materials, which are applied to potassium ion batteries and have high reversibility, high specific capacity, high cycle stability and excellent rate performance.

[0050] Comparative Example 1 S1. Raw material pretreatment: Screening to remove seeds mixed in the poplar fluff; repeatedly clean the surface impurities with an ethanol-water mixture (ethanol:water volume ratio of 1:3), and ultrasonically clean for 6 hours to completely remove organic matter and dust.

[0051] S2. Chemical activation: immerse the poplar fluff treated in step S1 in a 2 mol / L KOH solution and treat at a constant temperature of 80° C. for 24 hours.

[0052] S3. One-step carbonization: The poplar fluff treated in step S2 is carbonized in a tube furnace by heating the mixture at a rate of 2° C. / min to 250° C. in an argon atmosphere and maintaining the temperature for 2 hours.

[0053] S4. Post-treatment: The carbonized material was washed with a 2 mol / L hydrochloric acid solution to remove residual ash and improve the purity of the material, and then repeatedly rinsed with deionized water until the filtrate was neutral; then vacuum dried at 80°C for 12 hours to obtain a poplar-based nitrogen-containing carbon microtube material for potassium ion batteries.

[0054] Figure 4 The long cycle performance diagram and specific capacity voltage diagram of the carbon material prepared in Comparative Example 1 show that the initial charge and discharge capacities are 26 mAh·g - ¹ and 148 mAh g - ¹, the Coulombic efficiency is only 17%, and the specific capacity is very low during the cycle, indicating that the carbon material formed only by low-temperature carbonization can hardly store potassium ions.

[0055] Comparative Example 2 S1. Raw material pretreatment: Screening to remove seeds mixed in the poplar fluff; repeatedly clean the surface impurities with an ethanol-water mixture (ethanol:water volume ratio of 1:3), and ultrasonically clean for 6 hours to completely remove organic matter and dust.

[0056] S2. Chemical activation: immerse the poplar fluff treated in step S1 in a 2 mol / L KOH solution and treat at a constant temperature of 80° C. for 24 hours.

[0057] S3, one-step carbonization: carbonize the poplar fluff treated in step S2 using a tube furnace, heating it to 800° C. at a rate of 5° C. / min under an argon atmosphere and maintaining the temperature for 2 hours.

[0058] S4. Post-treatment: The carbonized material was washed with a 2 mol / L hydrochloric acid solution to remove residual ash and improve the purity of the material, and then repeatedly rinsed with deionized water until the filtrate was neutral; then vacuum dried at 80°C for 12 hours to obtain a poplar-based nitrogen-containing carbon microtube material for potassium ion batteries.

[0059] Figure 5 The long cycle performance diagram and specific capacity voltage diagram of the carbon material prepared in Comparative Example 2 show that the initial charge and discharge capacities are 71 mAh·g - ¹ and 328 mAh g - ¹, the coulombic efficiency is only 21%, and the capacity is only 28 mAh g after 500 cycles. - ¹, indicating that low-temperature pre-carbonization is a key step in retaining nitrogen by gently decomposing cellulose and avoiding high temperature direct destruction of the nitrogen-doped structure.

[0060] Comparative Example 3 S1. Raw material pretreatment: Screening to remove seeds mixed in the poplar fluff; repeatedly clean the surface impurities with an ethanol-water mixture (ethanol:water volume ratio of 1:3), and ultrasonically clean for 6 hours to completely remove organic matter and dust.

[0061] S2. Chemical activation: immerse the poplar fluff treated in step S1 in a 2 mol / L KOH solution and treat at a constant temperature of 80° C. for 24 hours.

[0062] S3. Two-step carbonization: The poplar fluff treated in step S2 is subjected to two-step carbonization in a tubular furnace. Low-temperature pre-carbonization: heating to 250°C at 2°C / min under argon atmosphere and maintaining the temperature for 1 hour; high-temperature carbonization: heating to 800°C at 5°C / min and maintaining the temperature for 1 hour.

[0063] S4. Post-treatment: repeatedly rinse with deionized water until the filtrate is neutral; then vacuum dry it at 80° C. for 12 hours to obtain a poplar-based nitrogen-containing carbon microtube material for potassium ion batteries.

[0064] Figure 6 The long cycle performance diagram and specific capacity voltage diagram of the carbon material prepared in Comparative Example 3 show that the initial charge and discharge capacities are 183 mAh·g - ¹ and 523 mAh g - ¹, the coulombic efficiency is only 34%, and the capacity is only 57 mAh·g after 500 cycles. - ¹, indicating that acid washing not only removes ash but also exposes nitrogen-doped active sites. Without acid washing, the residual ash would greatly affect the potassium storage efficiency of the prepared carbon material.

[0065] Comparative Example 4 Comparative Example 4 is the same as Example 1 in that step S2 (chemical activation) is omitted and two-step carbonization is performed directly. Other differences are the same as Example 1.

[0066] Figure 7 The long cycle performance diagram and specific capacity voltage diagram of the carbon material prepared in Comparative Example 4 show that the initial charge and discharge capacities are 826 mAh·g - ¹ and 215 mAh g - ¹, the coulombic efficiency is only 26%, and the capacity continues to decay during the cycle. After 100 cycles, the capacity is only 89 mAh·g - ¹, the reason is the lack of an alkaline activation step. A large amount of nitrogen volatilizes during the carbonization process, which reduces the potassium storage active sites and reduces the electron conduction efficiency. However, an alkaline environment can promote the stable combination of nitrogen and the carbon skeleton.

[0067] Comparative Example 5 S1. Raw material pretreatment: Screening to remove seeds mixed in the poplar fluff; repeatedly clean the surface impurities with an ethanol-water mixture (ethanol:water volume ratio of 1:3), and ultrasonically clean for 6 hours to completely remove organic matter and dust.

[0068] S2. Chemical activation: immerse the poplar fluff treated in step S1 in a 2 mol / L KOH solution and treat at a constant temperature of 80° C. for 24 hours.

[0069] S3. Low-temperature pre-carbonization: The poplar fluff treated in step S2 is subjected to low-temperature pre-carbonization in a tube furnace. The temperature is raised at 2°C / min to 250°C under an argon atmosphere and kept constant for 1 hour.

[0070] S4. Pickling: The low-temperature pre-carbonized material is washed with a 2 mol / L hydrochloric acid solution to remove residual ash and improve the purity of the material, and then repeatedly rinsed with deionized water until the filtrate is neutral; then vacuum dried at 80°C for 12 hours.

[0071] S5. High-temperature carbonization: The poplar fluff, after acid washing in step S4, is subjected to high-temperature carbonization in a tube furnace. The temperature is raised at 5°C / min to 800°C under an argon atmosphere and held at this temperature for 1 hour. This yields a potassium ion battery negative electrode poplar fluff-based nitrogen-doped carbon microtube material.

[0072] Figure 8 The long cycle performance diagram and specific capacity voltage diagram of the carbon material prepared in Comparative Example 5 show that the initial charge and discharge capacities are 738 mAh·g - ¹ and 179 mAh·g - ¹, the coulombic efficiency is only 24%, and the capacity continues to decay during the cycle, with the capacity remaining at only 110 mAh·g after 300 cycles. -There are two reasons for this: first, the nitrogen is lost prematurely. The nitrogen in the poplar fluff needs to be activated with alkaline (KOH) to form a stable chemical bond with the carbon skeleton. In Comparative Example 5, acid washing in advance after pre-carbonization will neutralize the residual alkali solution and destroy the bond between the nitrogen and the carbon skeleton, resulting in a large amount of nitrogen volatilization during subsequent high-temperature carbonization and a reduction in active sites. Second, although acid washing can remove the ash after pre-carbonization, the carbon structure has not yet been fully formed at this time. The exposed surface in advance is easily destroyed during high-temperature carbonization, and the nitrogen-doped active sites (such as pyridinic nitrogen and pyrrolic nitrogen) are reduced.

[0073] Comparative Example 6 The difference between Comparative Example 6 and Example 1 is that the carbonization temperature of the low-temperature pre-carbonization is 400° C., and the rest is the same as Example 1.

[0074] Figure 9 The long cycle performance diagram and specific capacity voltage diagram of the carbon material prepared in Comparative Example 6 show that the initial charge and discharge capacities are 1183 mAh·g - ¹ and 223 mAh g - ¹, the coulombic efficiency is only 19%, and the capacity continues to decay during the cycle. After 149 cycles, the capacity is 185 mAh g - ¹ However, overcharging and battery failure subsequently occurred for two reasons. First, uncontrolled cellulose decomposition. The core function of low-temperature pre-carbonization is to gently decompose cellulose while preserving its one-dimensional tubular skeleton. However, 400°C, which is close to the rapid pyrolysis temperature of some biomasses, leads to disordered cellulose decomposition, forming numerous fragments rather than complete microtubules, reducing specific surface area and potassium storage sites. Second, thermal volatilization of nitrogen. High-temperature pre-carbonization accelerates the pyrolysis of nitrogen-containing organic matter, leading to premature volatilization of nitrogen in the form of NH3 and HCN, reducing nitrogen doping efficiency and insufficient graphitization.

[0075] Those skilled in the art should understand that the discussion of any of the above embodiments is merely illustrative and is not intended to imply that the scope of protection of the present application is limited to these examples. In line with the present application, the technical features in the above embodiments or different embodiments may be combined, the steps may be implemented in any order, and there are many other variations of different aspects of one or more embodiments of the present application as described above, which are not provided in detail for the sake of simplicity.

[0076] The one or more embodiments of this application are intended to encompass all such substitutions, modifications, and variations that fall within the broad scope of this application. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of one or more embodiments of this application should be included in the scope of protection of this application.

Claims

1. A method for preparing a potassium ion battery negative electrode poplar-based nitrogen-doped carbon microtube material, characterized in that: The following steps are included: The poplar fluff after impurities are removed is immersed in alkali solution and kept warm to obtain activated poplar fluff; The activated poplar fluff is subjected to a two-step carbonization in an inert atmosphere to obtain carbonized poplar fluff, wherein the two-step carbonization includes low-temperature pre-carbonization and high-temperature carbonization, wherein the temperature of the low-temperature pre-carbonization is 200-300° C. and the temperature of the high-temperature carbonization is 800-1000° C.; The carbonized poplar fluff is washed with acid solution, rinsed with water until the filtrate is neutral, and dried to obtain a potassium ion battery negative electrode poplar fluff-based nitrogen-doped carbon microtube material.

2. The preparation method according to claim 1, wherein The alkali solution is one or more of sodium hydroxide and potassium hydroxide solutions, with a concentration of 1-3 mol / L, a heat preservation temperature of 60-100° C., and a treatment time of 8-24 h.

3. The preparation method according to claim 1, wherein When performing low-temperature pre-carbonization, the heating rate is 1~3℃ / min, and the low-temperature pre-carbonization time is 1~3h.

4. The preparation method according to claim 1, wherein When high-temperature carbonization is performed, the heating rate is 5~10℃ / min, and the high-temperature carbonization time is 1~3h.

5. The preparation method according to claim 1, wherein The gas used for the inert atmosphere is argon.

6. The preparation method according to claim 1, wherein: The temperature of the low-temperature pre-carbonization is 250°C, and the temperature of the high-temperature carbonization is 800°C.

7. The preparation method according to claim 1, wherein: The acid solution is one or more of hydrochloric acid, sulfuric acid, nitric acid, perchloric acid, and citric acid, and the concentration is 1-3 mol / L.

8. The preparation method according to any one of claims 1 to 7, wherein: The method for removing impurities from the poplar fluff is to screen and remove the seeds mixed in the poplar fluff; Use ethanol-water solution to clean surface impurities and ultrasonic cleaning to remove organic matter and dust; The drying temperature is 80° C. and the drying time is 12 hours.

9. The preparation method according to claim 8, characterized in that: In the ethanol aqueous solution, the volume ratio of ethanol to water is 1:2-5, and the ultrasonic cleaning time is 3-12 hours.

10. A poplar-based nitrogen-doped carbon microtube material for a negative electrode of a potassium ion battery, characterized in that: The method is described in any one of claims 1 to 9.

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