Composite negative electrode material for aqueous zinc metal batteries and preparation method thereof

Abstract

The application belongs to the technical field of aqueous zinc metal batteries, and provides a composite negative electrode material for aqueous zinc metal batteries and a preparation method thereof, so as to improve the cycle performance of the aqueous zinc metal batteries and promote commercial application thereof. The composite negative electrode material in the application has a three-layer coating structure and is composed of zinc powder, carbon spheres and bismuth nanosheets. The carbon spheres have an egg yolk shell structure, the zinc powder is coated inside the carbon spheres, and the bismuth nanosheets are coated on the surface of the carbon spheres. The unique structure of the composite negative electrode can not only provide a large number of continuous interfaces to promote efficient transfer of electrons, but also reduce the volume expansion and shrinkage of zinc metal generated in the repeated electroplating / detaching process, thereby effectively inhibiting the electrical contact loss of the zinc negative electrode. The outer layer of bismuth nanosheets can not only inhibit the hydrogen evolution reaction at the zinc metal / electrolyte interface, but also enhance the ion transport in the electrochemical process, and induce the Zn(002) crystal surface to deposit on the electrode surface. Through this plane deposition mechanism, the formation of zinc dendrites can be effectively inhibited.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of aqueous zinc metal batteries, and relates to a negative electrode material, and particularly provides a composite negative electrode material for an aqueous zinc metal battery and a preparation method thereof. BACKGROUND

[0002] At present, most aqueous zinc metal batteries use commercial zinc foil as the negative electrode material, but the utilization efficiency of zinc foil is low, and the preparation process is relatively complex, which greatly limits its practical application in industry. In comparison, when zinc powder is used as the negative electrode material of the aqueous zinc metal battery, the cost-effectiveness is high, and the zinc powder has excellent processability and adjustability. However, compared with the zinc foil, the specific surface area of the zinc powder is significantly increased, and the solid spherical structure makes the zinc negative electrode more complex and difficult to control in the electrochemical process. In addition, due to the long and uneven stripping time, the zinc powder negative electrode is more prone to electrical contact failure, which leads to the separation of active substances and the electrode, thereby increasing the polarization of the battery and resulting in poor coulombic efficiency. Therefore, the development of stable and durable zinc powder-based negative electrode materials is crucial for improving the cycle performance of the aqueous zinc metal battery.

[0003] Unlike the two-dimensional zinc foil negative electrode, the monodisperse three-dimensional zinc powder negative electrode makes the redistribution path of zinc ions longer, the nucleation range wider, and the electronic transition discontinuous. Therefore, the top region of the zinc powder is more prone to uneven deposition, and the zinc dendrites continuously accumulate during the electroplating / stripping process, eventually penetrating into the separator, causing internal short circuit and "dead zinc". In addition, the volume shrinkage caused by zinc dissolution during the discharge stage leads to electrical contact failure between the powders. In order to solve these problems, classical conductive materials such as MXene, graphene and carbon nanotubes are often combined with zinc powder to form a composite zinc negative electrode, which can provide sufficient electrons for the zinc powder negative electrode and inhibit the electrical contact failure of the electrode material and the growth of zinc dendrites. Although significant progress has been made in the research of composite zinc negative electrodes, the enhanced conductivity of the electrode will lead to the easy occurrence of hydrogen evolution reaction between the zinc negative electrode and the electrolyte interface, thereby increasing the gas production of the aqueous zinc metal battery and the potential risk of failure. Therefore, it is crucial to develop a zinc negative electrode material that can enhance the electrical conductivity of the material while inhibiting the hydrogen evolution reaction. SUMMARY

[0004] The application aims to provide a composite negative electrode material for aqueous zinc metal batteries and a preparation method thereof, so as to improve the cycle performance of the aqueous zinc metal batteries and promote the commercial application thereof.

[0005] To achieve the above-mentioned purpose, the technical scheme adopted by the application is as follows:

[0006] A composite negative electrode material for aqueous zinc metal batteries, characterized in that the composite negative electrode material is a three-layer coated structure and is composed of zinc powder, carbon spheres and bismuth nanosheets; the carbon spheres have an egg yolk shell structure, the zinc powder is coated inside the carbon spheres, and the bismuth nanosheets are coated on the surface of the carbon spheres.

[0007] Further, the composite negative electrode material has a spherical structure and a diameter of 0.5-1.5 μm.

[0008] Further, in the composite negative electrode material, the content of the zinc powder is 70-85 wt%, the content of the carbon spheres is 5-10 wt%, and the content of the bismuth nanosheets is 5-20 wt%.

[0009] Further, the composite negative electrode material is applied to conventional voltage aqueous zinc metal batteries and high-voltage aqueous zinc metal batteries as a negative electrode.

[0010] Further, the preparation method of the composite negative electrode material for aqueous zinc metal batteries comprises the following steps:

[0011] The carbon powder with an egg yolk shell structure and the zinc powder are mixed uniformly in proportion, and the mixture is subjected to high-temperature heat treatment to obtain a zinc-carbon composite powder material;

[0012] The zinc-carbon composite powder material is immersed in a bismuth nitrate solution in proportion, a bismuth nanosheet coating is formed on the surface of the zinc-carbon composite powder material through liquid deposition, and a composite negative electrode material is obtained.

[0013] Further, the specific process of the heat treatment is as follows:

[0014] The mixture is loaded into a container and sealed, the container is heated to 300-600 DEG C at a heating rate of 2-5 DEG C / min under an argon atmosphere, and then heat-treated for 5-15 hours, and a zinc-carbon composite powder material is obtained after cooling; wherein the mass ratio of the carbon powder to the zinc powder is 1:20-1:5 or 1:(5-20).

[0015] Further, the specific process of the liquid phase deposition is as follows:

[0016] The bismuth nitrate pentahydrate is mixed with ethylene glycol and stirred for 10-30 minutes, then ethanol is added, and the mixture is ultrasonically treated for 10-30 minutes and stirred for 10-30 minutes in sequence to obtain a uniform bismuth nitrate solution, wherein the mass ratio of the bismuth nitrate, the ethylene glycol and the ethanol is 1:(5-15):(5-20); the zinc-carbon composite powder material is immersed in the uniform bismuth nitrate solution for 5-30 minutes to obtain a suspension containing a zinc-carbon-bismuth composite powder material, wherein the mass ratio of the zinc-carbon composite powder material to the bismuth nitrate is 1:(0.5-2.5); and the suspension is centrifuged, washed and dried to obtain the composite negative electrode material.

[0017] Further, the process for preparing the carbon powder is as follows:

[0018] The zinc nitrate, the hexamethylenetetramine and the potassium citrate are dissolved in distilled water at a molar ratio of 4:2:1, and the mixture is stirred uniformly to obtain a mixed aqueous solution;

[0019] The mixed aqueous solution is heated in a water bath at 60-90 DEG C for 10-30 minutes, and then naturally aged at room temperature for 1-12 hours to obtain the zinc citrate microspheres with a yolk-shell structure;

[0020] The zinc citrate microspheres are calcined in an inert atmosphere at 500-900 DEG C for 1-5 hours, the product is stirred in a 0.05-0.5 mol / L KOH solution for 10-60 minutes, and then washed, centrifuged and dried to obtain the carbon powder with a yolk-shell structure.

[0021] Based on the above technical solutions, the present application has the following advantages:

[0022] This invention provides a composite anode material for aqueous zinc metal batteries, creatively constructing a three-layer zinc composite anode consisting of zinc powder, eggshell-structured carbon, and bismuth metal nanosheets from the inside out. First, the eggshell-structured carbon and the zinc powder inside form a zinc-carbon composite material. Compared with existing commercial zinc powder, the size of this zinc-carbon composite material is significantly reduced, and its conductivity increases after being fabricated into an electrode. Furthermore, the eggshell-structured carbon can buffer the volume changes caused by zinc metal electroplating / stripping during electrode cycling. Second, the bismuth metal nanosheets coated on the zinc-carbon composite material not only suppress the hydrogen evolution reaction at the electrolyte / electrode interface but also simultaneously induce zinc metal to be deposited oriented along the Zn(002) crystal plane, achieving a planar zinc deposition method. Based on this, the composite negative electrode material for aqueous zinc metal batteries proposed in this invention can effectively suppress electrical contact loss, zinc dendrite growth, side reactions, and hydrogen evolution reactions that occur inside the battery during the electrochemical reaction process. Among them, the zinc-carbon composite material can induce the generation of zinc ion diffusion energy barrier with low zinc ion diffusion energy barrier and accelerate the transport of zinc ions at the electrode / electrolyte interface, so that the deposited zinc metal is mainly distributed inside the yolk shell carbon spheres. Bismuth metal nanosheets are coated on the outer surface layer, which have high Gibbs free energy, can suppress the hydrogen evolution reaction of zinc-carbon composite material, and can also induce a portion of zinc metal to be uniformly deposited on the bismuth surface.

[0023] In summary, this invention synthesizes a zinc-carbon-bismuth composite microsphere material with a three-layer structure by combining high-temperature zinc infiltration and room-temperature liquid-phase reduction strategies. This effectively constructs an ideal zinc metal battery anode material with high electron / ion transport capability, low resistivity, and high Young's modulus, demonstrating ideal synergistic effects. Finally, the composite anode material for aqueous zinc metal batteries in this invention achieves a performance of 1 mA / cm². 2 At a current density of 0.5 mAh / cm 2 An overpotential of less than 35 mV was achieved at a high areal capacity, providing a novel perspective for the study of stabilizing the zinc / electrolyte interface. Furthermore, a high-performance zinc metal secondary battery was successfully obtained by using high areal capacity CNT@MnO2 as the positive electrode, 3 mol / L ZnSO4 + 0.2 mol / L MnSO4 electrolyte as the electrolyte, and matching it with a composite negative electrode material for zinc metal batteries.

[0024] In addition, the preparation process of the zinc-carbon-bismuth composite anode material for aqueous zinc metal batteries in this invention is novel and simple, with advantages such as low cost and good synthesis consistency. It is fundamentally different from the preparation process of similar zinc anodes in the prior art. Furthermore, the prepared composite anode material can not only reduce the volume expansion and contraction of zinc metal during repeated electroplating / stripping, but also reduce the transport barrier of zinc ions at the electrolysis interface and inhibit the hydrogen evolution reaction. Attached Figure Description

[0025] Figure 1 The structure and working principle comparison chart of the composite negative electrode material for the aqueous zinc metal battery in the application and the traditional zinc powder negative electrode.

[0026] Figure 2 The SEM and TEM charts of the composite negative electrode material for the aqueous zinc metal battery in Example 1 of the application.

[0027] Figure 3 The SEM morphology chart of the zinc metal symmetrical battery in Example 1 and Comparative Example 1 of the application after electroplating for 80 min under the current density of 5 mA / cm 2 .

[0028] Figure 4 The cycle performance chart of the zinc metal symmetrical battery in Example 1 and Comparative Example 1 of the application under the current density of 1 mA / cm 2 and the surface capacity of 0.5 mAh / cm 2 .

[0029] Figure 5 The overpotential chart of the zinc metal symmetrical battery in Example 2 and Comparative Example 2 of the application under the current density of 1 mA / cm 2 and the surface capacity of 1 mAh / cm 2 .

[0030] Figure 6 The hydrogen evolution reaction comparison chart of the three-electrode test system in 3 mol / L ZnSO4 electrolyte in Example 2 and Comparative Example 2 of the application.

[0031] Figure 7 The cycle performance chart of the aqueous zinc metal battery (zinc-carbon-bismuth composite negative electrode / CNT@MnO2 positive electrode) in Example 3 and Comparative Example 3 of the application. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical scheme and technical effect of the application more clear and complete, the application is further described in detail below in combination with the drawings and examples.

[0033] Example 1

[0034] The present embodiment provides a composite negative electrode material for an aqueous zinc metal battery and a preparation method thereof, as shown in the formula (I), a three-layer coating structure composed of zinc powder, carbon spheres and bismuth nanosheets is adopted, the carbon spheres have an egg yolk shell structure, the zinc powder is coated inside the carbon spheres, and the bismuth nanosheets are coated on the surface of the carbon spheres. Figure 1 The composite negative electrode material for the aqueous zinc metal battery is prepared by the following steps:

[0035]

[0036] ​0.48 mmol of zinc nitrate hexahydrate (Zn(NO3)2·6H2O), 0.26 mmol of hexamethylenetetramine ((CH2)6N4) and 0.13 mmol of potassium citrate monohydrate (K3C6H5O7·H2O) were dissolved in 50 mL of distilled water and stirred uniformly;

[0037] The mixed aqueous solution was heated in a water bath at 90°C for 20 minutes, and then naturally aged at room temperature. As the aging time increased, the growth of the shell and the shrinkage of the core continued to proceed. When the aging time increased to 8 hours, the core disappeared, leaving a spherical yolk-shell structure of zinc citrate microspheres;

[0038] The spherical yolk-shell structure of zinc citrate microspheres was calcined at 800°C for 2 hours in an argon / hydrogen (9:1) atmosphere. The product was placed in a 0.1 mol / L KOH solution and stirred for 20 minutes to remove excess zinc oxide. Then, the carbon powder material was obtained by washing, centrifugation and drying;

[0039] After 5 g of carbon powder and 25 g of zinc powder were ground and mixed for 3 hours, the mixture was placed in a ceramic container and sealed, and then heated to 500°C at a heating rate of 5°C / min in an argon atmosphere and heat-treated for 12 hours to obtain a zinc-carbon composite powder material;

[0040] 3 mmol of bismuth nitrate pentahydrate (Bi(NO3)2·5H2O) was magnetically stirred in a mixed solution of 20 mL of ethylene glycol for 30 minutes, and then 40 mL of ethanol was added and ultrasonically treated for 30 min, and then stirred for another 30 min to obtain a uniform solution. 10 g of zinc-carbon composite powder was immersed in the uniform solution for 5 min. At room temperature, zinc atoms reduced bismuth ions to bismuth metal, forming bismuth metal nanosheets, thereby forming a black coating of bismuth metal nanosheets on the surface of the zinc-carbon composite powder material, obtaining a suspension containing zinc-carbon-bismuth composite powder material;

[0041] Finally, the obtained suspension containing zinc-carbon-bismuth composite powder material was centrifuged, washed and dried to obtain a zinc-carbon-bismuth composite powder material, which is the composite negative electrode material for the aqueous zinc metal battery.

[0042] In order to test the electrochemical performance of the composite negative electrode material for the aqueous zinc metal battery, a zinc metal symmetric battery was constructed based on the above-mentioned composite negative electrode material. Both sides of the electrode were mainly composed of zinc-carbon-bismuth composite material, and the electrolyte was composed of zinc sulfate and solvent (3 mol / L zinc sulfate electrolyte was prepared by dissolving ZnSO4·7H2O in distilled water and then diluting to volume);

[0043] Meanwhile, the present embodiment also provides Comparative Example 1: both sides of the electrode are commercial pure zinc powder, matched with the same concentration of zinc sulfate electrolyte, and a zinc metal symmetric battery is obtained by assembling under the same process of the present embodiment.

[0044] Example 2

[0045] The present example provides a three-electrode test system, the working electrode, the counter electrode and the reference electrode are the zinc-carbon-bismuth composite negative electrode material prepared in Example 1, a commercial platinum sheet and Ag / AgCl respectively, and the electrolyte is the same as that in Example 1.

[0046] Meanwhile, the present example also provides Comparative Example 2: the working electrode is replaced with pure zinc material or zinc-carbon composite material prepared in Example 1, and the others remain unchanged, and the hydrogen evolution reaction is tested under the same process as in the present example.

[0047] Example 3

[0048] The present example provides a water-based zinc metal battery, the positive electrode is a CNT@MnO2 positive electrode sheet, the negative electrode is the zinc-carbon-bismuth composite negative electrode material prepared in Example 1, and the electrolyte is 3 mol / L ZnSO4+0.2 mol / L MnSO4.

[0049] Meanwhile, the present example also provides Comparative Example 3: the positive electrode is a CNT@MnO2 positive electrode sheet, the negative electrode is a commercial pure zinc powder, and the same electrolyte as in the present example is matched, and a water-based zinc metal battery is assembled under the same process as in the present example.

[0050] The beneficial effects of the present application will be described in detail below in combination with working principles and test results.

[0051] As Figure 1 shown in the figure is the physical and chemical properties of the composite negative electrode material and the commercial pure zinc powder material for the water-based zinc metal battery in the present application and the schematic diagram of the negative electrode surface morphology structure after battery cycling; as can be seen from the figure, due to the long and uneven peeling time of the battery negative electrode, the pure zinc powder is prone to uncontrolled electrical contact loss, thereby causing the active material to separate from the electrode, thereby increasing the battery polarization and deteriorating the coulombic efficiency; in addition, due to the defects of the pure zinc powder negative electrode such as low Young's modulus, high ion transport barrier and easy corrosion by water-based electrolyte, the pure zinc powder negative electrode is prone to dendrite growth, hydrogen evolution reaction and heterogeneous metal deposition during battery cycling. Compared with the pure zinc powder negative electrode, the zinc-carbon-bismuth composite negative electrode with a three-layer coating structure provided by the present application can provide high electrical conductivity, firm electrical contact, enhanced Gibbs free energy, can effectively inhibit the hydrogen evolution reaction and adjust the reversibility of metal deposition; and also has a low ion transport barrier, can induce the preferential deposition of Zn(002) crystal plane, thereby realizing the planar deposition of zinc metal to achieve the purpose of inhibiting zinc dendrites. As can be seen, the present application combines high-temperature zinc infiltration and room-temperature liquid-phase reduction strategy to synthesize a zinc-carbon-bismuth composite microsphere material with a three-layer coating structure, effectively constructs an ideal zinc metal battery negative electrode material with ideal high electron / ion transport capacity, low resistivity and high Young's modulus, and plays an ideal synergistic effect.

[0052] like Figure 2 The figures show SEM and TEM images of the zinc-carbon-bismuth composite anode material prepared in Example 1. As can be seen from the figures, the prepared zinc-carbon-bismuth composite material has a spherical structure with a diameter of approximately 0.5–1.5 μm. Figure 2 As shown in (a), bismuth metal nanosheets are coated on the surface of zinc-carbon microspheres, as... Figure 2 As shown in (b) and (c).

[0053] like Figure 3 The image shows the zinc symmetric cells in Example 1 and Comparative Example 1 at 5 mA / cm². 2 The SEM images after electroplating at the specified current density for 20 min and 80 min show that, as can be seen from the figures, the zinc deposition corresponding to Comparative Example 1 shows obvious zinc dendrite growth and uneven zinc deposition, and the zinc deposition layer is not dense. In contrast, the zinc deposition corresponding to Example 1 did not show obvious zinc dendrites and the zinc deposition was dense and uniform. This indicates that the zinc-carbon-bismuth composite anode in this invention can effectively induce zinc metal to deposit along the Zn(002) crystal plane, thereby achieving planar deposition and suppressing the growth of zinc dendrites.

[0054] like Figure 4 The image shows the zinc metal symmetric cells in Example 1 and Comparative Example 1 at 1 mA / cm². 2 The current density and 0.5 mAh / cm 2 The cycling performance diagram under the areal capacity is shown in the figure. As can be seen from the figure, Comparative Example 1 experienced severe side reactions and the polarization voltage increased sharply after 4 hours. In contrast, Example 1 showed excellent cycling stability and the side reactions were significantly suppressed. It was still able to provide stable cycling after 60 hours and the polarization voltage value remained basically unchanged. This indicates that the zinc-carbon-bismuth composite negative electrode in this invention can significantly suppress the electrical contact loss of the electrode and the side reactions between the electrode and electrolyte interface.

[0055] like Figure 5 The image shows the zinc metal symmetric cells in Example 1 and Comparative Example 1 at 1 mA / cm. 2 Current density and 1mAh / cm 2 The overpotential diagram under the areal capacity shows that the overpotentials of the pure zinc anode and the zinc-carbon composite anode are higher than those of the zinc-carbon-bismuth composite electrode. This is mainly due to the high conductivity provided by the composite carbon material and the low ion transport barrier provided by the bismuth metal nanosheets, which leads to the suppression of interfacial side reactions and the promotion of rapid ion dynamics.

[0056] like Figure 6The figure shows the hydrogen evolution ability of three electrodes in the three-electrode test system in Example 2 and Comparative Example 2, and it can be seen from the figure that the zinc-carbon composite negative electrode is most prone to hydrogen evolution reaction, followed by the pure zinc negative electrode and the zinc-carbon-bismuth composite negative electrode, which is mainly due to the increase of the conductivity and specific surface area of the zinc-carbon composite material electrode, which promotes the hydrogen evolution reaction of the negative electrode interface more easily; after the bismuth metal nanosheet is coated on the outer layer of the zinc-carbon composite material, since it has high Gibbs free energy, it can inhibit the hydrogen evolution reaction of the zinc-carbon composite material.

[0057] As shown in Figure 7 The figure shows the cycle performance of the aqueous zinc metal battery (zinc-carbon-bismuth composite negative electrode / CNT@MnO2 positive electrode) in Example 3 and Comparative Example 3 at a current density of 0.2 A g -1 -1 and at 25℃, and it can be seen from the figure that Comparative Example 3 shows poor cycle stability, and the battery fails after 54 cycles, while Example 3 shows more excellent cycle stability, and the capacity retention rate is as high as 97% after 100 cycles, indicating that the zinc-carbon-bismuth composite negative electrode material with a three-layer coating structure in the present application has extremely strong practical value.

[0058] The above is only a specific embodiment of the present application, any feature disclosed in the specification can be replaced by other equivalent or similar purpose alternative features unless specifically stated; all features disclosed, or steps in all methods or processes, except for mutually exclusive features and / or steps, can be combined in any way.

Claims

1. A composite anode material for aqueous zinc metal batteries, characterized by, The composite negative electrode material is a three-layer coating structure, which is composed of zinc powder, carbon spheres and bismuth nanosheets; the carbon spheres have an egg yolk shell structure, the zinc powder is coated inside the carbon spheres, and the bismuth nanosheets are coated on the surface of the carbon spheres. 2.The composite anode material for aqueous zinc metal batteries according to claim 1, characterized in that, The composite negative electrode material has a spherical structure with a diameter of 0.5-1.5 microns. 3.The composite anode material for aqueous zinc metal batteries according to claim 1, characterized in that, In the composite negative electrode material, the content of zinc powder is 70-85 wt%, the content of carbon spheres is 5-10 wt%, and the content of bismuth nanosheets is 5-20 wt%. 4.The composite anode material for aqueous zinc metal batteries of claim 1, wherein The composite negative electrode material is applied to conventional voltage aqueous zinc metal batteries and high voltage aqueous zinc metal batteries as a negative electrode.

5. The method of claim 1, wherein the method is characterized by: The method comprises the following steps: The carbon powder with an egg yolk shell structure and zinc powder are uniformly mixed in a certain proportion, and the mixture is subjected to high-temperature heat treatment to obtain a zinc-carbon composite powder material; The zinc-carbon composite powder material is immersed in a bismuth nitrate solution in a certain proportion, a bismuth nanosheet coating is formed on the surface of the zinc-carbon composite powder material by liquid deposition, and a composite negative electrode material is obtained.

6. The method of claim 5, wherein the method is characterized by, The specific process of heat treatment is as follows: The mixture is loaded into a container and sealed, the container is placed in an argon atmosphere, heated to 300-600℃ at a heating rate of 2-5℃ / min, and then heat treated for 5-15 hours, and the zinc-carbon composite powder material is obtained after cooling; wherein the mass ratio of carbon powder to zinc powder is 1:(5-20).

7. The method of claim 5, wherein the method is characterized by, The specific process of liquid deposition is as follows: Bismuth nitrate pentahydrate and ethylene glycol are mixed and stirred for 10-30 minutes, then ethanol is added, and the mixture is ultrasonically treated for 10-30 minutes, stirred for 10-30 minutes, and then ultrasonically treated for 10-30 minutes, to obtain a uniform bismuth nitrate solution; wherein the mass ratio of bismuth nitrate, ethylene glycol and ethanol is 1:(5-15):(5-20); the zinc-carbon composite powder material is immersed in the uniform bismuth nitrate solution for 5-30 minutes to obtain a suspension containing zinc-carbon-bismuth composite powder material, and the mass ratio of zinc-carbon composite powder material to bismuth nitrate is 1:(0.5-2.5); the suspension is centrifuged, washed and dried to obtain the composite negative electrode material.

8. The method of claim 5, wherein the method is characterized by, The preparation process of the carbon powder is as follows: Zinc nitrate, hexamethylenetetramine and potassium citrate are dissolved in distilled water in a molar ratio of 4:2:1, and the mixture is stirred uniformly to obtain a mixed aqueous solution; The mixed aqueous solution is heated in a water bath at 60-90℃ for 10-30 minutes, and then naturally aged at room temperature for 1-12 hours to obtain zinc citrate microspheres with an egg yolk shell structure; The zinc citrate microspheres are calcined at 500-900℃ in an inert atmosphere for 1-5 hours, the product is stirred in a 0.05-0.5 mol / L KOH solution for 10-60 minutes, and then washed, centrifuged and dried to obtain the carbon powder with an egg yolk shell structure.

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