A porous carbon nanofiber, its preparation method and application

Abstract

This invention proposes a porous carbon nanofiber, its preparation method, and its application, belonging to the field of sodium metal battery technology. The invention first dissolves polyacrylonitrile in N,N-dimethylformamide, then adds stannous chloride dihydrate to obtain a tin-containing spinning solution. This solution is then spun using a high-voltage electrospinning machine to obtain Sn@PAN. After curing and high-temperature oxidation, SnO2@NC is obtained. Finally, tellurization treatment yields a porous carbon nanofiber material of SnO2 / SnTe@NC. The material exhibits a uniformly distributed SnO2 / SnTe heterostructure. The Na2O and Na2Te generated after electroactivation can participate in the formation of the SEI as inorganic components, enhancing the mechanical strength of the SEI and accelerating sodium ion transport. The uniformly distributed Na... 15 Sn4 can act as a sodium-loving site, inducing uniform sodium deposition, effectively inhibiting the growth of sodium dendrites, and significantly improving the cycle life and stability of sodium metal batteries.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sodium metal batteries, and particularly relates to a porous carbon nanofiber and a preparation method and application thereof. BACKGROUND

[0002] With the acceleration of energy transformation, the proportion of renewable energy continues to increase. The continuous increase in new energy power generation capacity puts higher requirements on energy storage systems. Although lithium battery energy storage currently dominates the market with absolute advantages, the scarcity and uneven distribution of lithium ore make it difficult to meet the growing demand, and a new energy storage system is urgently needed as an alternative. Sodium metal batteries are considered one of the strong candidates for the next generation of energy storage systems due to their high energy density and wide availability.

[0003] However, sodium metal battery anode materials face many challenges in practical applications. For example, the unstable solid electrolyte interface film and dendrite problem have always been two major obstacles to the development of sodium metal battery anode materials. In addition, the host-free deposition of metallic sodium will cause unlimited volume expansion, exacerbate SEI (solid electrolyte interface film formed on the negative electrode surface during the first discharge) structure damage and local current density, and trigger accelerated growth of dendrites and side reactions. These negative factors not only reduce the life of sodium metal batteries, but also cause serious safety problems.

[0004] Three-dimensional conductive current collectors are considered one of the feasible and effective solutions. Its main feature is to reduce local current density by homogenizing electron / ion flux, and combined with its natural high sodium deposition space, it has certain effect on relieving the problems of volume expansion and dendrite growth. However, sodium deposited on the surface of the three-dimensional conductive framework inevitably produces dendrite problems, and metallic sodium has a lower Lumo energy level relative to organic electrolytes and high chemical activity, which will spontaneously generate a large number of side reactions to generate thick and fragile SEI. The resulting SEI is difficult to resist the growth of dendrites, leading to the exposure of fresh sodium and further reaction with the electrolyte, ultimately leading to short circuit and failure of the sodium metal battery. Therefore, it is necessary to optimize the traditional current collector to improve the sodium ion deposition behavior. SUMMARY

[0005] To solve the above technical problems, the application provides a porous carbon nanofiber and a preparation method and application thereof, the application introduces tin source into the three-dimensional porous carbon to reduce the sodium nucleation energy barrier and guide uniform deposition, fully utilizes the internal space of the three-dimensional porous structure as a sodium storage site, and effectively improves the energy density; and after electric activation, inorganic components such as Na2O and Na2Te serve as components of SEI, the increase of the inorganic components strengthens the mechanical properties of SEI, and the polycrystalline boundary SEI formed thereby promotes the migration of sodium ions, thereby effectively solving the problems of the design defects of the traditional three-dimensional conductive current collector and dendrite growth.

[0006] To achieve the above object, the application provides the following technical solutions.

[0007] The application provides a preparation method of a porous carbon nanofiber, polyacrylonitrile is dissolved in N,N-dimethylformamide to obtain a PAN-DMF solution, then stannous chloride dihydrate is added to obtain a spinning solution containing tin, then the spinning solution is electrospun to obtain Sn@PAN fibers, the Sn@PAN fibers are solidified and calcined and oxidized to obtain SnO2@NC precursors, and finally the SnO2@NC precursors are treated by tellurization to obtain the porous carbon nanofiber. More specifically, the method comprises the following steps.

[0008] Polyacrylonitrile (PAN) is dissolved in N,N-dimethylformamide (DMF) to obtain a PAN-DMF solution; stannous chloride dihydrate (SnCl2·2H2O) is added to the PAN-DMF solution to obtain a spinning solution containing tin (Sn); and the spinning solution is electrospun to obtain Sn@PAN fibers.

[0009] The Sn@PAN fibers are solidified and calcined and oxidized to obtain SnO2@NC precursors.

[0010] The SnO2@NC precursors are calcined with tellurium powder (Te) to obtain the porous carbon nanofiber, denoted as SnO2 / SnTe@NC.

[0011] The discharge products Na2O and Na2Te of the SnO2 / SnTe@NC material of the application can participate in the formation of SEI, increase the inorganic components of SEI, improve the mechanical strength of SEI, and the abundant crystal boundaries formed by Na2O and Na2Te can accelerate the transmission of sodium ions; in addition, the uniform and small Sn-based particles distributed in the spinning can reversibly react with metallic sodium to generate Na 15 The high adsorption performance of Sn4 and sodium-tin alloy to sodium can make them serve as sodium-attracting sites, reduce the nucleation deposition potential barrier of metallic sodium, induce uniform deposition of sodium, and reduce the problem of sodium dendrite growth. The strong and tough SEI with multiple inorganic components can prevent the short circuit of the battery caused by the breakthrough of sodium dendrites, and the uniform distribution of Na 15The Sn4 sodium-philic site can make sodium be uniformly deposited, and the porous carbon nanofiber can effectively alleviate the volume effect of sodium deposition / peeling. The multi-cooperative effect lays a good performance of the SnO2 / SnTe@NC material in the sodium metal battery. Therefore, the SnO2 / SnTe@NC material prepared by the above method can significantly improve the cycle life of the sodium metal battery.

[0012] Further, the concentration of the PAN-DMF solution is 0.08-0.1 g·mL −1 ; the mass ratio of the stannous chloride dihydrate to the polyacrylonitrile is (0.8-1.2):(0.8-1.0).

[0013] Further, the working parameters of the electrospinning include: a voltage of 14-16 kV, a pushing speed of 0.008-0.012 mL·min −1 , a temperature of no less than 40 DEG C, and a relative humidity of 30%-40%.

[0014] Further, the conditions of the solidification calcination are calcination at 250-280 DEG C for 1-2 h in an air atmosphere; and the conditions of the oxidation calcination are calcination at 350-400 DEG C for 2-4 h in an air atmosphere.

[0015] Further, the conditions of the calcination of the SnO2@NC precursor and the tellurium powder are as follows: heating to 450-550 DEG C at a heating rate of 3 DEG C·min −1 in a reducing atmosphere, and keeping the temperature for 1-2 h; wherein the reducing atmosphere is an argon-hydrogen mixed atmosphere, the volume ratio of hydrogen to argon in the mixed atmosphere is (0-10):(90-100), and the volume of hydrogen is not 0, and preferably the volume ratio of hydrogen to argon is (5-10):(90-95).

[0016] Further, the mass ratio of the SnO2@NC precursor to the tellurium powder is 1:2.

[0017] The application further provides a porous carbon nanofiber (SnO2 / SnTe@NC material) prepared by the above preparation method, which is loaded with the SnO2 / SnTe heterostructure.

[0018] The application further provides an application of the above porous carbon nanofiber in a sodium metal battery.

[0019] Further, the porous carbon nanofiber is used as a negative electrode host material or a negative electrode material of the sodium metal battery.

[0020] The porous carbon nanofiber loaded with the SnO2 / SnTe heterostructure prepared by the method has the advantages of forming more inorganic component SEI, higher ionic conductivity, mechanical strength and thermal stability when used as a host material of a sodium metal negative electrode, and in-situ generated Na 15 Sn4 has good sodium affinity, and in combination with the structural design of the porous three-dimensional carbon nanofiber with uniformly distributed Sn-based active sites, can effectively induce uniform deposition of metal sodium and the volume effect caused thereby, and effectively relieve the problem of uncontrolled dendrite growth of the sodium metal negative electrode in the battery cycle process.

[0021] Compared with the prior art, the present application has the following advantages and technical effects:

[0022] (1) The present application first dissolves polyacrylonitrile in N,N-dimethylformamide, then adds stannous chloride dihydrate to obtain a spinning solution containing tin, and then obtains Sn@PAN by spinning through a high-voltage electrostatic spinning machine, obtains SnO2@NC precursor after solidification and high-temperature oxidation, and finally obtains the porous carbon nanofiber loaded with SnO2 / SnTe heterostructure after tellurization treatment of the SnO2@NC precursor. The preparation method of the present application is simple, convenient to operate, has low requirements on equipment, moderate cost, and is suitable for large-scale production.

[0023] (2) The porous carbon nanofiber loaded with SnO2 / SnTe heterostructure prepared by the present application has many advantages in inhibiting sodium dendrite growth, which is embodied in the following aspects:

[0024] The material is a three-dimensional porous carbon spinning material, and the three-dimensional conductive network can reduce the local current density and prevent the dendrite problem caused by the rapid nucleation and growth of sodium in the local high current area;

[0025] The porous structure of the material indicates that it has a large specific surface area, and its characteristics can provide more storage space for metal sodium deposition, effectively relieving the volume expansion problem of sodium metal in the deposition / detachment process;

[0026] The uniformly distributed Sn-based particles of the material can reversibly react with sodium metal in-situ during the deposition / detachment process to form Na 15 Sn4 alloy, effectively reducing the nucleation barrier of sodium metal, promoting the diffusion and deposition of sodium ions, and thus realizing uniform deposition of metal sodium;

[0027] The SEI rich in inorganic components such as Na2O and Na2Te generated by the electrically activated SnO2 / SnTe@NC material has higher ionic conductivity and higher mechanical strength, effectively preventing the problems of dendrites and side reactions caused by the rupture of SEI.

[0028] Based on the above advantages, the sodium metal battery assembled by the SnO2 / SnTe@NC material of the application has excellent cycle stability. BRIEF DESCRIPTION OF DRAWINGS

[0029] The accompanying drawings, which form a part of the present application, are used to provide further understanding of the application, and serve as an explanation of the illustrative embodiments of the present application, and are not used to limit the present application. In the drawings:

[0030] Figure 1 Scanning electron microscope image of Sn@PAN fiber prepared for example 1.

[0031] Figure 2 Scanning electron microscope image of SnO2@NC precursor prepared for example 1.

[0032] Figure 3 Scanning electron microscope image of SnO2 / SnTe@NC material prepared for example 1.

[0033] Figure 4 Transmission electron microscope image of SnO2 / SnTe@NC material prepared for example 1.

[0034] Figure 5 XRD pattern of SnO2@NC precursor and SnO2 / SnTe@NC prepared for example 1.

[0035] Figure 6 Specific surface area pore size distribution diagram of SnO2 / SnTe@NC material prepared for example 1, wherein (a) is N2 adsorption-desorption curve, and (b) is pore size distribution diagram.

[0036] Figure 7 Sodium deposition / detachment coulombic efficiency of SnO2 / SnTe@NC prepared for example 1 as negative electrode material of sodium metal battery under current density of 1mA·cm −2 and area capacity of 1mAh·cm −2 .

[0037] Figure 8 Long cycle performance diagram of SnO2@NC precursor and SnO2 / SnTe@NC prepared for example 1 as negative electrode material of sodium metal battery under current density of 1mA·cm −2 and area capacity of 1mAh·cm −2 .

[0038] Figure 9 Long cycle performance diagram of SnO2@NC precursor and SnO2 / SnTe@NC prepared for example 1 as negative electrode material of sodium metal full battery.

[0039] Figure 10Rate capability plot of Sn02@NC precursor prepared for Example 1, Sn02 / SnTe@NC as a sodium metal full cell anode material.

[0040] Figure 11 High resolution transmission electron microscopy image of Sn02 / SnTe@NC electrode prepared with Sn02 / SnTe@NC as Sn02 / SnTe@NC||Na half cell anode material in Example 1 discharged to 0.01 V.

[0041] Figure 12 High resolution spectroscopy of Te 3d (a) and O 1s (b) of Sn02 / SnTe@NC electrode prepared with Sn02 / SnTe@NC as Sn02 / SnTe@NC||Na half cell anode material in Example 1 discharged to 0.01 V.

[0042] Figure 13 Scanning electron microscopy image of Sn02 / SnTe electrode prepared in Comparative Example 1. DETAILED DESCRIPTION

[0043] Various illustrative embodiments of the present application are now described in detail below. The embodiments discussed herein should be understood as merely illustrative of certain aspects of the present application. Accordingly, the present application should not be thought of as limited to the embodiments discussed herein, but rather the embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the application to those skilled in the art.

[0044] It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application. Additionally, for a range of values of a parameter, unless otherwise stated, the inclusion of either extremity of the range is understood to describe the exclusion of that extremity in a claim. Thus, "between 1 and 5" is to be understood as meaning "from 2 to 4", as opposed to "from 1 to 5" or "from 1 to 4".

[0045] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application, the preferred methods and materials are described. All patents, patent applications, publications, and descriptions mentioned herein are incorporated by reference to the extent allowed by law. In the case of conflict between the description herein and the incorporated material, the description herein shall control.

[0046] In the description of the application specific embodiments, numerous specific details are set forth in order to provide a thorough understanding of the application. However, those of ordinary skill in the art realize that the application can be practiced without all these specific details. In other instances, specific embodiments have not been described in detail in order not to unnecessarily obscure the application. Other embodiments of the application will be apparent to those of ordinary skill in the art from a review of the description of the application and practice of the application. The description of the application and examples are merely exemplary.

[0047] As used herein, "comprise", "comprising", "having", "including", "contain", "containing", "include" and the like are open-ended terms that are intended to mean including, but not limited to.

[0048] The embodiment of the present application provides a preparation method of porous carbon nanofiber, comprising the following steps:

[0049] Polyacrylonitrile (PAN) is dissolved in N,N-dimethylformamide (DMF) to obtain a PAN-DMF solution; stannous chloride dihydrate is added to the PAN-DMF solution to obtain a spinning solution containing tin (Sn); and the spinning solution is electrospun to obtain Sn@PAN fiber.

[0050] The Sn@PAN fiber is solidified and calcined and oxidized to obtain a SnO2@NC precursor;

[0051] The SnO2@NC precursor is calcined with tellurium powder to obtain a porous carbon nanofiber, denoted as SnO2 / SnTe@NC.

[0052] The embodiment of the present application constructs a system method for step-by-step preparation of porous carbon nanofiber, realizes uniform loading of SnO2 / SnTe heterostructure in the carbon nanofiber through electrospinning combined with subsequent solidification, oxidation and tellurization treatment, and provides a process basis for subsequent material functionality (such as sodium affinity, porous structure, SEI enhancement and the like).

[0053] The concentration and mass ratio of the solution can ensure the spinnability and uniformity of the spinning solution, affect the morphology, pore structure and Sn-based particle distribution of the final fiber, and thus regulate the specific surface area and electrochemical active site density of the material. Therefore, in the preferred embodiment of the present application, the concentration of the PAN-DMF solution is 0.08-0.1 g·mL −1 ; and the mass ratio of stannous chloride dihydrate to polyacrylonitrile is (0.8-1.2):(0.8-1.0), preferably, the concentration of the PAN-DMF solution is 0.1 g·mL −1 ; and the mass ratio of stannous chloride dihydrate to polyacrylonitrile is (0.8-1.0):1.0.

[0054] The optimized voltage, propulsion speed, temperature and humidity and the like of the electrospinning are the key to ensuring that the fiber morphology is uniform, continuous and defect-free in the spinning process, and provide a good precursor structure basis for subsequent calcination and tellurization. Therefore, in the preferred embodiment of the present application, the working parameters of the electrospinning include: voltage 14-16 kV, propulsion speed 0.008-0.012 mL·min −1 , temperature not less than 40 DEG C, and relative humidity maintained at 30-40%.

[0055] In the preferred embodiment of the present application, the conditions for the solidification calcination are 250-280℃ calcination for 1-2h in an air atmosphere; and the conditions for the oxidation calcination are 350-400℃ calcination for 2-4h in an air atmosphere. In this step, the solidification calcination can pre-oxidize the PAN to form a stable carbon skeleton structure for preservation; and the oxidation calcination can convert the Sn-metal organic complex into SnO2 and further oxidize to obtain a porous nitrogen-doped carbon structure.

[0056] In the preferred embodiment of the present application, the conditions for the calcination of the SnO2@NC precursor with tellurium powder are to heat to 450-550℃ at a heating rate of 3℃·min −1 -1 in a reducing atmosphere, and to keep the temperature for 1-2h; wherein the reducing atmosphere is an argon-hydrogen mixed atmosphere, the volume ratio of hydrogen to argon in the mixed atmosphere is (0-10):(90-100), and the volume of hydrogen is not 0, preferably the volume ratio of hydrogen to argon is (5-10):(90-95). In the reducing atmosphere, SnO2 is partially reduced and reacts with Te to form SnTe, forming a SnO2 / SnTe heterostructure. The heterostructure can be converted into Na2O and Na2Te after the first discharge, participate in the construction of SEI, and enhance the mechanical strength and ion conductivity of SEI; the use of hydrogen and argon mixed atmosphere can control the reduction degree, ensure the controllable formation and structural integrity of the SnO2 / SnTe heterostructure.

[0057] For example, in an embodiment of the present application, a method for preparing a porous carbon nanofiber (SnO2 / SnTe@NC) comprises the following steps:

[0058] (1) Preparation of spinning solution: 1.0g of polyacrylonitrile (PAN) is added to 10.0mL of N,N-dimethylformamide (DMF), and a clear solution is obtained after stirring at 50℃ for 6h. Then 0.8-1.0g of stannous chloride dihydrate is added, and the solution is stirred at room temperature for 12h to obtain a Sn-containing spinning solution;

[0059] (2) Preparation of Sn@PAN fiber: the Sn-containing spinning solution prepared in step (1) is spun through a high-pressure electrostatic spinning machine, with the following specific parameters: voltage 14-16 kV, pushing speed 0.008-0.012mL·min −1 , temperature not lower than 40℃, and relative humidity 30%-40%, and the obtained spinning is kept in a 70℃ oven for 12h to obtain Sn@PAN fiber;

[0060] (3) Preparation of SnO2@NC precursor: the Sn@PAN fiber prepared in step (2) is placed in a muffle furnace, and calcined at a temperature of 250-280℃ at a heating rate of 10℃ min −1 -1 in an air atmosphere for 1-2h, and then calcined at a temperature of 450-550℃ at a heating rate of 5℃ min−1 The temperature rising rate is increased to 350-400 DEG C, and calcination is carried out for 2-4 h, so that the SnO2@NC precursor is obtained;

[0061] (4) Preparation of SnO2 / SnTe@NC: the SnO2@NC precursor prepared in step (3) and Te powder are placed in the two ends of a porcelain boat at a mass ratio of 1:2, and the porcelain boat is placed in an argon-hydrogen atmosphere, the volume ratio of hydrogen to argon is (0-10):(90-100), the end of the SnO2@NC precursor faces the upstream, the temperature rising rate is increased to 450-550 DEG C at a rate of 3 DEG C·min −1 -1 DEG C / min, and the temperature is kept for 1-2 h, and after the reaction is completed, the SnO2 / SnTe@NC porous carbon nanofiber material loaded with SnO2 / SnTe heterostructure is obtained after cooling to room temperature.

[0062] The embodiment of the application also provides a porous carbon nanofiber (SnO2 / SnTe@NC) prepared by the preparation method.

[0063] The embodiment of the application also provides application of the porous carbon nanofiber in a sodium metal battery, and preferably, the porous carbon nanofiber is used as a negative electrode host material or a negative electrode material of the sodium metal battery.

[0064] The raw materials used in the embodiment of the application are all commercially available.

[0065] In the embodiment of the application, room temperature or normal temperature both refer to "25±3 DEG C".

[0066] The technical solutions of the application are further described below through examples.

[0067] Example 1

[0068] A preparation method of a porous carbon nanofiber (SnO2 / SnTe@NC) specifically comprises the following steps:

[0069] (1) Preparation of a spinning solution: 1.0 g of polyacrylonitrile (PAN) is added to 10.0 mL of N,N-dimethylformamide (DMF), and after stirring at 50 DEG C for 6 h, a clear solution is obtained, and then 0.8 g of stannous chloride dihydrate is added, and after stirring at room temperature for 12 h, a Sn-containing spinning solution is obtained;

[0070] (2) Preparation of Sn@PAN fibers: the Sn-containing spinning solution prepared in step (1) is spun through a high-voltage electrostatic spinning machine, and the specific parameters are as follows: voltage 16 kV, pushing speed 0.008 mL·min −1 -1, temperature 40 DEG C, and relative humidity 38%, and the obtained spinning is kept in a 70 DEG C oven for 12 h, so that Sn@PAN fibers are obtained;

[0071] (3) Preparation of SnO2@NC precursor: The Sn@PAN fiber obtained in step (2) was placed in a muffle furnace and heated at 10°C for 10 min in an air atmosphere. −1 The heating rate was increased to 250℃ and calcined for 2 hours, then further increased at 5℃ per minute. −1 The heating rate was increased to 400℃ and calcined for 2 hours to obtain SnO2@NC precursor;

[0072] (4) Preparation of SnO2 / SnTe@NC: The SnO2@NC precursor obtained in step (3) and Te powder were placed at both ends of a ceramic boat at a mass ratio of 1:2. The boat was placed under an argon-hydrogen atmosphere with a hydrogen to argon volume ratio of 5:95. One end of the SnO2@NC precursor faced upstream. The boat was heated at 3℃·min −1 The temperature was increased to 500℃ at a heating rate and held for 2 hours. After the reaction was completed, the mixture was cooled to room temperature to obtain porous carbon nanofiber material (SnO2 / SnTe@NC) with SnO2 / SnTe heterostructure.

[0073] Figure 1 This is a scanning electron microscope image of the Sn@PAN fiber prepared in Example 1. Figure 2 The image shows a scanning electron microscope (SEM) image of the SnO2@NC precursor prepared in Example 1. Figure 2 It can be seen that the surface of the prepared SnO2@NC precursor is covered with oxidized SnO2 particles.

[0074] Figure 3 , 4 The images shown are scanning electron microscope (SEM) and transmission electron microscope (TEM) images of the SnO2 / SnTe@NC material prepared in Example 1. Figure 3 , 4 It can be seen that the porous carbon nanofibers loaded with SnO2 / SnTe have a relatively smooth surface and uniformly fine SnO2 / SnTe particles distributed inside. The porous structure of the spinning can be clearly observed, which can provide abundant volume buffer space for the deposition / stripping of metallic sodium, which is conducive to better cycle stability.

[0075] Figure 5 XRD patterns of the SnO2@NC precursor and SnO2 / SnTe@NC prepared in Example 1. Analysis Figure 5 It can be seen that the standard diffraction peaks of SnO2@NC, SnO2 / SnTe@NC match well with SnO2 or SnTe, and no other impurity phases were found.

[0076] Figure 6 The images show the specific surface area and pore size distribution of the SnO2 / SnTe@NC material prepared in Example 1, where (a) is the N2 adsorption-desorption curve and (b) is the pore size distribution. Figure 6As shown, the specific surface area of the SnO2 / SnTe@NC material is 189.36 m 2 / g, and the pore size is mainly microporous and mesoporous, which provides deposition sites for sodium and can effectively alleviate the volume effect.

[0077] Preparation of sodium metal negative electrode material and performance characterization

[0078] The SnO2 / SnTe@NC material prepared in Example 1 was cut into a circular electrode with a diameter of 12 mm and placed in a vacuum oven at 60°C for 12 h for drying, and was used as a negative electrode material of a sodium metal battery for electrochemical performance testing. The SnO2@NC precursor obtained in Example 1 was used as a comparison.

[0079] In the half battery, the SnO2 / SnTe@NC circular electrode and the SnO2@NC circular electrode were directly used as self-supporting electrodes; in the full battery, the SnO2 / SnTe@NC circular electrode and the SnO2@NC circular electrode were used as composite negative electrodes with sodium metal, wherein the SnO2 / SnTe@NC circular electrode and the SnO2@NC circular electrode were host materials of the sodium metal battery negative electrode.

[0080] Performance test results

[0081] 1. Characterization of half battery cycle performance

[0082] (1) Assembly of half battery: The SnO2 / SnTe@NC circular electrode with a diameter of 12 mm, the SnO2@NC circular electrode, and the metal sodium sheet were paired in a vacuum glove box filled with argon and with a water and oxygen content of less than 0.01 ppm to assemble SnO2 / SnTe@NC||Na and SnO2@NC||Na half batteries. The electrolyte system was selected to be ethylene glycol dimethyl ether (DME) containing 1 mol / L sodium hexafluorophosphate (NaPF6), the electrolyte dosage was 100 μL, and the separator was a commercial polypropylene porous membrane Celgard 2500.

[0083] (2) Electrochemical test: The coulombic efficiency (CE) is the ratio of sodium stripping capacity to sodium deposition capacity in a single battery cycle, which is a key indicator for evaluating the reversibility of the electrode and the performance of the host material. First, the assembled half battery SnO2 / SnTe@NC||Na was cycled between 0.01-0.6 V at a current of 0.05 mA·cm −2 , then 1 mAh·cm −2 of metal sodium was repeatedly deposited / stripped at a current of 1 mA·cm −2 , and the obtained coulombic efficiency is shown in Figure 7 . As can be seen from the figure, at a current of 1 mA·cm −2current density of 1 mA cm −2 At an area capacity of 1 mAh cm

[0084] 2. Characterization of long cycle performance of symmetric cells

[0085] (1) Assembly of symmetric cells: The previously assembled Sn02 / SnTe@NC||Na, Sn02@NC||Na half-cells were subjected to electrodeposition at a current density of 0.5 mA cm −2 to deposit a capacity of 10 mAh cm −2 of metallic sodium to obtain Sn02 / SnTe@NC@Na, Sn02@NC@Na. The composite metallic sodium electrode Sn02 / SnTe@NC@Na, Sn02@NC@Na was then removed from the glove box and washed with DME solvent to remove residual electrolyte. Two identical composite metallic sodium electrodes Sn02 / SnTe@NC@Na, Sn02@NC@Na were used to assemble Sn02 / SnTe@NC@Na||Sn02 / SnTe@NC@Na, Sn02@NC@Na||Sn02@NC@Na symmetric cells. The electrolyte used was 1 mol / L NaPF6 in DME, with an electrolyte dosage of 100 μL, and the separator used was a commercial polypropylene porous membrane Celgard 2500.

[0086] (2) Electrochemical testing of symmetric cells: The Sn02 / SnTe@NC@Na||Sn02 / SnTe@NC@Na, Sn02@NC@Na||Sn02@NC@Na symmetric cells were subjected to repeated deposition / detachment of 1 mAh cm −2 of metallic sodium at a current density of 1 mA cm −2 . The cycle performance is shown in Figure 8 It can be seen that the Sn02 / SnTe@NC@Na||Sn02 / SnTe@NC@Na symmetric cell can be stably cycled for more than 600 h, with only a hysteresis voltage of about 10 mV, and the voltage change after cycling is gentle, with no obvious polarization phenomenon. In contrast, the Sn02@NC@Na||Sn02@NC@Na symmetric cell changed in voltage after about 180 h of cycling, with obvious polarization phenomenon, and eventually short-circuited, causing the cell to fail.

[0087] 3. Characterization of long cycle and rate performance of full cells

[0088] (1) Assembly of the full cell: Electrodeposition was performed on the previously assembled SnO2 / SnTe@NC||Na and SnO2@NC||Na half cells at a current of 0.5 mA·cm⁻¹. −2 Current density deposited on electrode surface has a surface capacity of 10 mAh·cm −2 Sodium metal was extracted to obtain SnO2 / SnTe@NC@Na and SnO2@NC@Na. The half-cell was disassembled in a vacuum glove box, and the composite sodium metal electrodes SnO2 / SnTe@NC@Na and SnO2@NC@Na were removed. The electrodes were then cleaned with DME solvent to remove residual electrolyte. The active material loading was 4.0 mg·cm³. −2 Sodium vanadium phosphate (NVP) was used to assemble SnO2 / SnTe@NC@Na||NVP and SnO2@NC@Na||NVP full cells. The composite sodium metal electrodes SnO2 / SnTe@NC@Na and SnO2@NC@Na served as the negative electrode material, while sodium vanadium phosphate (NVP) served as the positive electrode material. The electrolyte used was a ethylene carbonate (EC) / diethyl carbonate (DEC) (v / v=1:1) electrolyte containing 1 mol / L sodium perchlorate (NaClO4) and 5 wt.% fluoroethylene carbonate (FEC) additive. The electrolyte volume was 100 μL, and the separator was glass fiber filter paper.

[0089] (2) Electrochemical testing: The SnO2 / SnTe@NC@Na||NVP and SnO2@NC@Na||NVP full cells were charged and discharged between 2.6-3.8V. Their long-cycle performance was as follows: Figure 9 As shown, the SnO2 / SnTe@NC@Na||NVP full cell can be stably cycled for 200 cycles at 1C with low capacity decay. In contrast, the SnO2@NC@Na||NVP full cell exhibits SEI instability after 50 cycles, leading to dendrite breakout, SEI regeneration, and dead sodium formation, resulting in gradual capacity decay until the cell fails due to a short circuit. Rate performance is as follows... Figure 10 As shown, the SnO2@NC@Na||NVP full cell provides 107.6 mAh·g at rates of 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, and 10C, respectively. −1 102.0mAh·g −1 96.3 mAh·g −1 91.5 mAh·g −1 86.1mAh·g −1 77.3 mAh·g −1 and 68.5 mAh·g −1of 111.31 mAh·g −1 , 107.81 mAh·g −1 , 105.29 mAh·g −1 , 102.01 mAh·g −1 , 99.42 mAh·g −1 , 97.17 mAh·g −1 and 95.32 mAh·g −1 at 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, 10C, respectively. The more stable cycling and higher rate performance of the full cell fully demonstrate the practicability of SnO2 / SnTe@NC electrode.

[0090] 4. In-situ high resolution transmission electron microscopy test of SnO2 / SnTe@NC electrode

[0091] After disassembling the SnO2 / SnTe@NC||Na half-cell which was discharged to 0.01V in the vacuum glove box, part of the discharge product was taken for high resolution transmission electron microscopy test. The results are shown in Figure 11 , the SEI thickness is about 35 nm, and the Na2Te (111) crystal face with a crystal face spacing of 0.42 nm and the NaF (200) crystal face with a crystal face spacing of 0.24 nm can be clearly seen. It is worth noting that due to the active nature of Na2O, it is easy to react with water in the air and cannot be detected.

[0092] 5. XPS test of SnO2 / SnTe@NC electrode

[0093] After disassembling the SnO2 / SnTe@NC||Na half-cell which was discharged to 0.01V in the vacuum glove box, part of the discharge product was taken for XPS test. The test results are shown in Figure 12 , where (a) is the high resolution spectrum of Te 3d, and (b) is the high resolution spectrum of O 1s. It can be seen that after etching the 20 nm oxide layer, in the high resolution spectrum peak of Te 3d, 583.3eV and 572.9eV are respectively attributed to Te 3d 3 / 2 and Te 3d 5 / 2The characteristic peaks were detected, and the existence of Na-Te bond was detected at 580.55 eV and 570 eV. In the high-resolution energy spectrum peak of O 1s, the characteristic peak of Na-O bond was detected at 530.15 eV, and the remaining 536.75 eV and 532.20 eV were Auger peaks of sodium and C-O bond. The successful detection of Na-Te bond and Na-O bond proves that there are rich inorganic components of Na2Te and Na2O in the SEI.

[0094] Example 2

[0095] A method for preparing a porous carbon nanofiber (SnO2 / SnTe@NC) comprises the following steps:

[0096] (1) Preparation of spinning solution: 1.0 g of polyacrylonitrile (PAN) is added to 10.0 mL of N,N-dimethylformamide (DMF), and a clear solution is obtained after stirring at 50°C for 6 h. Then 1.0 g of stannous chloride dihydrate is added, and the mixture is stirred at room temperature for 12 h to obtain a Sn-containing spinning solution;

[0097] (2) Preparation of Sn@PAN fiber: the Sn-containing spinning solution prepared in step (1) is spun through a high-pressure electrospinning machine, with specific parameters: voltage 15 kV, pushing speed 0.010 mL·min −1 , temperature control at 40°C, air humidity control at 38%, and the obtained spinning is placed in a 70°C oven for 12 h to obtain Sn@PAN fiber;

[0098] (3) Preparation of SnO2@NC precursor: the Sn@PAN fiber prepared in step (2) is placed in a muffle furnace, and calcined at 280°C for 2 h under air atmosphere at a heating rate of 10°C·min −1 , and then calcined at 380°C for 2 h at a heating rate of 5°C·min −1 to obtain SnO2@NC precursor;

[0099] (4) Preparation of SnO2 / SnTe@NC: the SnO2@NC precursor prepared in step (3) and Te powder are placed in the two ends of a porcelain boat with a mass ratio of 1:2, and the porcelain boat is placed in an argon-hydrogen gas atmosphere with a volume ratio of hydrogen to argon of 5:95. The SnO2@NC precursor is directed towards the upstream, and heated to 550°C at a heating rate of 3°C·min −1 , and kept for 2 h. After the reaction is completed, the porous carbon nanofiber material (SnO2 / SnTe@NC) loaded with SnO2 / SnTe heterostructure is obtained after cooling to room temperature.

[0100] According to the method in Example 1, the porous carbon nanofiber material (SnO2 / SnTe@NC) prepared in Example 2 was assembled as a negative electrode material to test the performance of a sodium metal battery, and the results showed that the porous carbon nanofiber material (SnO2 / SnTe@NC) prepared in Example 2 could maintain a very high coulombic efficiency after 190 cycles in half-cell tests at a current density of 1 mA·cm −2 and an area capacity of 1 mAh·cm −2 The average coulombic efficiency reached 99.9%; in the symmetric battery test, the symmetric battery could be stably cycled for more than 580 h at a current density of 1 mA·cm −2 and a metal sodium deposition / detachment of 1 mAh·cm −2 The hysteresis voltage was only about 11 mV, and the voltage change after cycling was gentle; in the full battery test with sodium vanadium phosphate as the positive electrode material, the full battery provided discharge specific capacities of 110.25 mAh·g −1 , 106.32 mAh·g −1 , 104.18 mAh·g −1 , 101.52 mAh·g −1 , 99.02 mAh·g −1 , 96.86 mAh·g −1 and 94.81 mAh·g −1 at 0.1C, 0.2C, 0.5C, 1C, 2C, 5C and 10C rates, respectively, and could be stably cycled for more than 180 cycles at a rate of 1C.

[0101] Comparative Example 1

[0102] Commercial SnO2 nanoparticles and SnTe nanoparticles were simply mechanically mixed with conductive carbon black and a binder to form a conventional slurry electrode coated on a copper foil current collector, wherein the mass ratio of active material: conductive carbon: binder used in the preparation of the conventional slurry electrode was 8:1:1; the active material contained commercial SnO2 nanoparticles and SnTe nanoparticles, and the mass ratio was 1:1; the conductive carbon was Super P; the binder was a water-based binder, which was a sodium carboxymethyl cellulose solution (CMC-Na, concentration of 10 mg / mL) and a butadiene-styrene rubber solution (SBR, concentration of 50 mg / mL) with a mass ratio of 1:1; the coating thickness was 100 μm.

[0103] The specific preparation method is as follows: 8 mg of SnO2 nanoparticles and 8 mg of SnTe nanoparticles are respectively ground, 2 mg of Super P is added, and the mixture is fully ground and uniformly mixed, then 100 μL of sodium carboxymethyl cellulose solution and 20 μL of butadiene rubber solution are added, and the mixture is again ground and uniformly mixed and placed in a copper foil, and a 100 μm doctor blade is used to evenly spread the mixture, thereby preparing a SnO2 / SnTe electrode.

[0104] The scanning electron microscope image of the SnO2 / SnTe electrode prepared in Comparative Example 1 is shown in FIG. 2, and the active material of the electrode sheet is only a physical mixing structure, not an integrated structure as shown in FIG. 1, indicating that even if the active components are the same, a continuous conductive network and a stable porous framework cannot be formed. Figure 13 Figure 3

[0105] Comparative Example 2

[0106] The same as Example 1, the difference is only that the working parameters of electrospinning include: voltage 25 kV, pushing speed 0.02 mL·min −1 In electrospinning, voltage is the core parameter for regulating jet behavior and fiber morphology. Under normal voltage, the polymer solution / melt forms a stable Taylor cone at the nozzle, and the jet is orderly stretched along the electric field direction. Too high voltage in electrospinning will lead to disordered jet and thus uneven spinning diameter, resulting in defects such as beaded structure or fiber adhesion.

[0107] Comparative Example 3

[0108] The same as Example 1, the difference is only that the conditions for oxidation calcination are calcination at 300℃ for 14 h in air atmosphere. The purpose of oxidation calcination in this comparative example is to oxidize Sn-metal complex to SnO2, and at the same time, the fiber will generate oxygen-containing functional groups or carbon dioxide. Too long calcination time will cause loss of carbon in the spinning fiber, leading to destruction of the fiber structure.

[0109] Comparative Example 4

[0110] The same as Example 1, the difference is only that the conditions for tellurization treatment are heating to 380℃ at a heating rate of 3℃·min −1 in a reducing atmosphere, and holding for 4 h. In this comparative example, the temperature condition for tellurization treatment is 380℃, and it is known that the melting point of tellurium powder is 449.51℃, which is much higher than 380℃, so the tellurium powder cannot be melted and form SnO2 / SnTe@NC by reacting with SnO2@NC.

[0111] ​​In summary, the SnO2 / SnTe@NC material loaded with SnO2 / SnTe heterostructure is obtained through solidification, oxidation and tellurization processes by the electrospinning process. The SnO2 / SnTe heterostructure activated by the electric field generates Na2O and Na2Te to participate in the formation of SEI through conversion reaction. The increase of inorganic components can improve the mechanical properties of SEI, and the abundant grain boundaries generated by Na2O and Na2Te can accelerate the migration of Na ions; the three-dimensional porous carbon conductive network not only ensures the rapid migration of electrons, but also can relieve the volume effect caused by the deposition / peeling of metal sodium, and the uniform distribution of Na 15 The Sn4 alloy can induce the uniform deposition of metal sodium as a sodium-philic site, and inhibit the growth of dendrites. Therefore, the material exhibits good performance in the electrochemical tests of half cells, symmetric cells and full cells.

[0112] The above merely describes the preferred embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art can easily think of changes or replacements within the technical scope disclosed by the present application, which should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A method for producing a porous carbon nanofiber, characterized by, The polyacrylonitrile is dissolved in N,N-dimethylformamide to obtain a PAN-DMF solution, then stannous chloride dihydrate is added to obtain a spinning solution containing tin, then the spinning solution is electrospun to obtain Sn@PAN fibers, and then the Sn@PAN fibers are sequentially subjected to solidification calcination and oxidation calcination to obtain a SnO2@NC precursor, and finally the SnO2 / SnTe heterostructure loaded porous carbon nanofiber is obtained after tellurization treatment. The working parameters of the electrostatic spinning include: voltage 14-16 kV, propelling speed 0.008-0.012 mL·min -1 , temperature not less than 40 DEG C, relative humidity 30%-40%. The oxidation calcination condition is calcination at 350-400 DEG C for 2-4 h in an air atmosphere. The conditions of the tellurium treatment are to heat to 450-550℃ at a heating rate of 3℃·min -1 under a reducing atmosphere and to keep the temperature for 1-2h.

2. The method for preparing porous carbon nanofibers according to claim 1, characterized in that, The concentration of the PAN-DMF solution is 0.08-0.1 g·mL -1 ; the mass ratio of the stannous chloride dihydrate to the polyacrylonitrile is (0.8-1.2):(0.8-1.0).

3. The method for preparing porous carbon nanofibers according to claim 1, characterized in that, The solidification calcination condition is calcination at 250-280 DEG C for 1-2 h in an air atmosphere.

4. The method for preparing porous carbon nanofibers according to claim 1, characterized in that, The reducing atmosphere is an argon-hydrogen mixed atmosphere, and the volume ratio of hydrogen to argon in the mixed atmosphere is (0-10):(90-100).

5. The method for preparing porous carbon nanofibers according to claim 1, characterized in that, The mass ratio of the SnO2@NC precursor to the tellurization-treated tellurium powder is 1:

2.

6. The porous carbon nanofiber produced by the production method according to any one of claims 1 to 5, characterized by, The porous carbon nanofiber is loaded with a SnO2 / SnTe heterostructure.

7. Use of the porous carbon nanofiber of claim 6 in the preparation of a sodium metal battery.

8. Use of the porous carbon nanofibers according to claim 7 in a sodium metal battery, characterized in that, The porous carbon nanofiber is used as a negative electrode host material or a negative electrode material of a sodium metal battery.

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