Preparation and application of sodium metal battery and its negative electrode and negative electrode active material

Abstract

The application relates to the field of sodium metal batteries, in particular to a preparation method of a negative electrode active material of a sodium metal battery, and the preparation of ZnO particles; surface reaction of ZnO particles and ligands with a molar ratio of 1:15-25 is carried out, a Zn-based framework material is in-situ coated on the surface of zinc oxide to obtain a Zn framework@ZnO composite precursor with a core-shell structure; the ligand is a compound capable of carrying out a coordination reaction with Zn; the Zn framework@ZnO composite precursor is heat-treated at a temperature of 900-1250 DEG C to prepare the negative electrode active material. The application also comprises the material prepared by the preparation method and the application of the material in sodium metal batteries. The material prepared by the method has excellent sodium metal adaptation effect, and better performance can be obtained.

Description

Technical Field

[0001] This invention belongs to the field of battery materials, specifically relating to the field of sodium metal battery anode materials. Background Technology

[0002] In recent years, to address the fossil fuel crisis and alleviate carbon emissions, clean energy, primarily lithium batteries, has developed rapidly. However, with the widespread adoption of electronic products and the promotion of electric vehicles, lithium batteries still face challenges such as scarce lithium resources (0.0017 wt% in the Earth's crust) and limited energy density. Sodium, as a group element with lithium, has abundant resources (2.74 wt% in the Earth's crust), and sodium metal anodes possess a high theoretical specific capacity (1166 mA h·g). -1 Sodium metal batteries offer advantages such as a lower standard potential (-2.71V vs. standard hydrogen electrode). They hold promise as a commercially viable high-energy-density battery.

[0003] However, sodium's high reactivity can lead to side reactions and the growth of sodium dendrites, posing safety hazards. During the operation of sodium metal batteries, sodium is deposited / stripped onto the surface of the negative electrode current collector in the form of pure metal. Uneven deposition of sodium metal can cause the continuous growth of sodium dendrites, which can then pierce the separator and contact the positive electrode, causing an internal short circuit, resulting in thermal runaway, fire, or even explosion. Summary of the Invention

[0004] To address the problem that existing sodium metal battery anodes are more prone to dendrite formation and have unsatisfactory electrochemical performance, the primary objective of this invention is to provide a method for preparing anode active materials suitable for sodium metal batteries, aiming to obtain anode materials that can effectively induce sodium metal deposition, reduce dendrite formation, and improve electrochemical performance.

[0005] The second objective of this invention is to provide the negative electrode active material prepared by the aforementioned method and its application in sodium metal batteries.

[0006] A third objective of this invention is to provide a sodium metal battery comprising the aforementioned negative electrode active material, and its negative electrode and negative electrode material.

[0007] Sodium ions are larger than lithium ions, making it difficult for materials suitable for lithium-ion batteries to accommodate their insertion and extraction. Furthermore, sodium batteries primarily include sodium-ion batteries and sodium metal batteries. Sodium-ion batteries achieve charge-discharge cycles mainly through sodium ion insertion and extraction, while sodium metal batteries rely on the deposition and dissolution of sodium metal. These different principles lead to different problems. For example, sodium metal batteries are more prone to dendrite formation and less than ideal cycle performance due to the higher reactivity of sodium. To address these issues, this invention provides the following solutions:

[0008] A method for preparing a negative electrode active material for a sodium metal battery, comprising the following steps:

[0009] Step (1):

[0010] Preparation of ZnO particles;

[0011] Step (2):

[0012] ZnO particles and ligands with a molar ratio of 1:15 to 25 were subjected to surface reaction to in situ coat Zn-based framework materials on the surface of zinc oxide, resulting in a core-shell structured Zn framework@ZnO composite precursor.

[0013] The ligands are compounds that can undergo coordination reactions with Zn;

[0014] Step (3):

[0015] The negative electrode active material was prepared by heat-treating the Zn framework@ZnO composite precursor at a temperature of 900–1250 °C.

[0016] This invention pre-forms ZnO particles, which then undergo a surface reaction with a ligand of a required molar amount to form a Zn-based framework shell. This is followed by heat treatment at a specified temperature. Based on the combined control of the molar ratio in step 2 and the temperature in step 3, in-situ carbonization of the framework material within the shell and simultaneous embedding of zinc can be achieved, forming a carbon shell with embedded zinc particles. Furthermore, it simultaneously promotes the thermal reduction and ablation-controlled volatilization of zinc oxide in the core, creating a hollow structure. This also forms zinc volatilization pores in the carbon shell and allows for controlled loading on the inner wall and cavity of the carbon shell. This synergistic effect reduces local current density, induces uniform sodium metal deposition, and prevents sodium dendrite growth. Moreover, it reduces side reactions between metallic sodium and the electrolyte, thereby improving the coulombic efficiency and cycle performance of sodium metal batteries.

[0017] In this invention, the preparation method and the combined control of the molar ratio of ZnO particles and ligands and the heat treatment temperature during the preparation process are the key to achieving in-situ embedding of zinc in the carbon shell, in-situ dissolution of zinc oxide and formation of volatilization pores in the carbon shell, controllable loading of the inner wall and inner cavity, and thus synergistic improvement of electrochemical performance.

[0018] In this invention, the zinc oxide can be prepared using existing methods. Preferably, the zinc source is reacted in a polyol to obtain the ZnO particles. This invention has found that the zinc oxide prepared using this method can be synergistically combined with subsequent processes to further improve the electrochemical performance of the prepared material in sodium metal batteries.

[0019] Preferably, the zinc source is at least one of zinc ion sulfate, nitrate, hydrochloride, and carboxylate;

[0020] Preferably, the polyol is a compound having the structure of Formula 1;

[0021]

[0022] Preferably, the liquid-to-solid ratio (volume-to-weight ratio) of the zinc source and the polyol is 10-100 ml / g, and more preferably 30-60 ml / g;

[0023] Preferably, the reaction process is an atmospheric pressure reaction process or a solvothermal reaction process. This invention has found that using an atmospheric pressure reaction helps to further improve the synergistic effect with subsequent processes and helps to further improve the electrochemical performance of the prepared material in sodium metal batteries.

[0024] Preferably, the temperature during the reaction stage is above 140°C, and considering energy consumption and preparation effect, it is preferably 150-190°C;

[0025] Preferably, the reaction time is more than 1 hour; considering the preparation efficiency, it is preferably 1 to 5 hours, and more preferably 2 to 3 hours.

[0026] Preferably, the D50 particle size of the ZnO particles is 250–550 nm.

[0027] In this invention, zinc oxide and ligands are innovatively reacted on the surface. By controlling the ratio of the two, a synergistic effect can be achieved, which can facilitate the subsequent heat treatment to prepare the special structure and the material with good sodium deposition and electrochemical performance.

[0028] In this invention, the ligand can be any component that can form a coordination framework material with zinc ions. Preferably, the ligand is a compound having the structural formula 2.

[0029]

[0030] In this invention, controlling the ratio of ZnO particles to ligands within the range required by the invention facilitates the successful preparation of the desired material. Furthermore, this invention has found that controlling the molar ratio of ZnO particles to ligands at 1:19–21 can further improve the performance of the prepared material.

[0031] Preferably, the surface reaction temperature is 40–80°C, and more preferably 50–70°C;

[0032] Preferably, the surface reaction time is 1 hour to 3 hours.

[0033] In this invention, the material prepared by surface reaction is innovatively subjected to heat treatment. Based on the combined control of the heat treatment temperature, the shell is carbonized and a zinc-dispersed embedded structure is formed in situ. Furthermore, the core is reduced, dissolved, and volatilized to form a hollow structure, and volatilization pores are formed from the inside out on the carbon shell. Moreover, zinc particles are controllably loaded onto the inner wall of the carbon shell and within the chambers. Based on the combined control of the aforementioned parameters, this invention can construct the material with the special structure in one step through the simultaneous synergy of shell carbonization and embedded structure, and core reduction, dissolution, and volatilization. The material prepared by this approach exhibits superior sodium metal-induced deposition effects, further reducing sodium dendrite formation and improving the electrochemical performance of sodium metal batteries.

[0034] In step (3), the heat treatment atmosphere is a protective atmosphere;

[0035] Preferably, the protective atmosphere is at least one of nitrogen and an inert gas;

[0036] Preferably, in step (3), an auxiliary gas is added to the atmosphere during the heat treatment stage. The auxiliary gas is at least one of hydrogen, CO, and CH4. This invention also found that, at the calcination temperature, further combining the auxiliary gas helps to further adjust the microstructure of the material, giving it better compatibility with sodium metal and induced deposition effects, and further improving the electrochemical performance of sodium metal.

[0037] In a preferred embodiment of this invention, the Zn framework@ZnO composite precursor is preheated at a temperature of 400–600°C before undergoing subsequent heat treatment. This invention also found that the two-stage processing mechanism helps to further synergistically improve the sodium compatibility of the prepared material, thereby further improving its electrochemical performance.

[0038] Preferably, the preheating time is 0.5 to 1.5 hours;

[0039] Preferably, the heat treatment stage lasts for 1 to 3 hours.

[0040] The present invention also provides a negative electrode active material prepared by the preparation method described above;

[0041] Preferably, it includes a hollow carbon shell, in which zinc nanoparticles are embedded in situ and have pores for zinc metal volatilization residues. The inner wall and cavity of the hollow carbon shell also contain nano-zinc particles.

[0042] Preferably, the thickness of the negative electrode active material is 80-120 nm, and the diameter is 280-520 nm;

[0043] Preferably, the zinc content in the negative electrode active material is 5-30 at%.

[0044] Preferably, the specific surface area of ​​the negative electrode active material is 600–800 m². 2 ·g -1 .

[0045] The present invention also provides a negative electrode material for a sodium metal battery, including the aforementioned negative electrode active material.

[0046] The negative electrode material of the metal battery of the present invention further includes a binder and a conductive agent.

[0047] Preferably, the content of the negative electrode active material in the negative electrode material is above 70 wt%, and more preferably 75 to 95 wt%.

[0048] The present invention also provides a sodium metal anode, comprising a current collector and an anode material loaded on its surface, characterized in that the anode material is the anode material described in the present invention.

[0049] The present invention also provides a sodium metal battery, comprising a positive electrode, a separator, and a negative electrode sequentially stacked, wherein the negative electrode is the negative electrode described in the present invention.

[0050] The sodium metal battery of the present invention, except for containing or using the negative electrode active material of the present invention, may have conventional components, contents, component structures and preparation methods.

[0051] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0052] This invention pre-forms ZnO particles, which then undergo a surface reaction with ligands in a required molar amount to form a Zn-based framework shell. This is followed by heat treatment at a specified temperature. Based on the combined control of the molar ratio in step 2 and the temperature in step 3, in-situ carbonization of the framework material within the shell and simultaneous embedding of zinc can be achieved, forming a carbon shell with embedded zinc particles. Furthermore, it simultaneously promotes the thermal reduction and ablation-controlled volatilization of zinc oxide in the core, creating a hollow structure and forming volatilization pores in the carbon shell, as well as controllable loading on the inner wall and cavity of the carbon shell. This synergistic effect reduces local current density, induces uniform sodium metal deposition, and avoids sodium dendrite growth. Moreover, it reduces side reactions between metallic sodium and the electrolyte, thereby improving the coulombic efficiency and cycle performance of sodium metal batteries. Detailed Implementation

[0053] The present invention will now be described in further detail with reference to embodiments, but these are not intended to limit the scope of protection of the invention. An example of the preparation of a typical negative electrode active material for a sodium metal battery according to the present invention includes, for example, the following steps:

[0054] Step 1: Dissolve zinc acetate dihydrate in Formula 1, heat and reflux in a flask, pyrolyze and then centrifuge to obtain spherical zinc oxide particles; the reaction temperature is above 150℃; the reaction time is, for example, 1 to 3 hours.

[0055] Step 2: Add the spherical zinc oxide obtained in Step 1 to an ethanol solution containing Formula 2, disperse ultrasonically, stir and heat to obtain a material with a zinc-based framework material (e.g., zif / 8)@zinc oxide coated on the surface. The molar ratio of zinc oxide to Formula 2 is 1:(15-25).

[0056] Step 3: Calcine the product obtained in Step 2 at 900-1250℃ to obtain the final product.

[0057] In this invention, the zinc oxide prepared by the preferred method in step 1 can be combined with subsequent processes to help synergistically improve the compatibility of the prepared material with sodium electrodes, thereby further improving its performance.

[0058] In this invention, pre-calcination treatment is performed before calcination in step 3, or gases including hydrogen and CO are introduced during the calcination stage, which can further synergistically improve the performance of sodium metal batteries.

[0059] Example 1

[0060] Step 1: Add 5.49 g of zinc acetate dihydrate to 250 mL of Formula 1, stir to dissolve, and heat to 160 °C in an atmospheric pressure reaction apparatus equipped with a condenser, maintaining this temperature for 2 hours (marked as t1). After cooling, wash three times with deionized water or ethanol and centrifuge; dry in a 60 °C oven to obtain spherical zinc oxide particles;

[0061] Step 2: Add 0.45g of spherical zinc oxide obtained in Step 1 to 37mL of ethanol solution, disperse by ultrasonication, add 9.05g of Formula 2 (the molar ratio of zinc oxide to Formula 2 is 1:20), stir to dissolve, and heat the suspension in an oil bath at 60℃ for 2h to obtain spherical zinc oxide with a zinc-based framework coated on the surface (also known as the precursor).

[0062] Step 3: Transfer the precursor obtained in Step 2 to an alumina crucible, place it in a tube furnace, purge with argon gas for 30 min, and purge the air from the tube; set the heating rate to 5℃·min. -1The target temperature was 1200℃ (marked as T1), and the calcination time was 2 hours; thus, a negative electrode active material was obtained. This material is a hollow carbon shell material formed by nuclear reduction ablation, with an in-situ carbonization of the zinc-based framework to form a zinc-embedded carbon shell. Furthermore, under the nuclear reduction ablation mechanism, volatilization pores and residual zinc particles on the inner wall and chamber are simultaneously formed on the shell surface. Characterization of the material's physicochemical parameters revealed that the prepared negative electrode active material has a diameter of 280–520 nm, uniform particle size, a pore size of 4–6 nm on the carbon shell surface, and a specific surface area of ​​660 m². 2 ·g -1 The zinc content is 7.1 at%.

[0063] Example 2

[0064] Compared with Example 1, the only difference is that the processing time t1 of step 1 is changed, as follows:

[0065] Group A: t1 is 1 hour;

[0066] Group B: t1 is 3 hours;

[0067] Other operations and parameters are the same as in Example 1;

[0068] The result is:

[0069] In Group A, the diameter of the negative electrode active material is 50–100 nm, the pore size of the mesopores on the carbon shell surface is 4–6 nm, and the specific surface area of ​​the hollow carbon spheres is 932 m². 2 ·g -1 .

[0070] In Group B, the diameter of the negative electrode active material is 280–520 nm, the pore size of the mesopores on the carbon shell surface is 4–6 nm, and the specific surface area of ​​the hollow carbon spheres is 645 m². 2 ·g -1 .

[0071] Sodium metal anode assembly was performed on the anode active materials obtained in Examples 1, 2A, and 2B: The anode active material, acetylene black, and polyvinylidene fluoride (PVDF) were uniformly mixed in a mass ratio of 80:10:10 and dispersed in a certain mass of N-methylpyrrolidone (NMP) to form a slurry. This slurry was then coated onto copper foil and vacuum dried at 80°C to obtain the sodium metal battery anode sheet. Battery assembly and testing were performed as follows: The anode sheet was stamped into an electrode sheet with a diameter of 10 mm, using metallic sodium as the counter electrode and 1 M NaPF6 / DGE as the electrolyte. A CR2032 coin cell was assembled in an argon-filled glove box. The battery was tested at 2 mA·cm⁻¹. -2 The current density, and the charge is 1 mA h·cm. -2 Under the specified conditions, charge-discharge cycle tests were conducted, and the test results are shown in Table 1 below:

[0072] Table 1

[0073]

[0074] The results show that controlling the solvothermal reaction time in step 1 to be above 1.5 h, especially 2 to 3 h, can yield a negative electrode active material with a structure and size suitable for sodium metal batteries. This material can exhibit better coulombic efficiency and cycle performance when used as a sodium metal negative electrode.

[0075] Example 3

[0076] Compared with Example 1, the only difference is that, while keeping the amount of Formula 2 constant, the amount of zinc oxide particles in step 2 is changed, thereby controlling the molar ratio of zinc oxide to Formula 2. The experimental groups are as follows:

[0077] Compared to Group A: the molar ratio of zinc oxide to Equation 2 is 1:5;

[0078] Compared to Group B: the molar ratio of zinc oxide to Equation 2 is 1:10;

[0079] Compared to Group C: the molar ratio of zinc oxide to Equation 2 is 1:30;

[0080] Group D: The molar ratio of zinc oxide to Equation 2 is 1:15;

[0081] Group E: The molar ratio of zinc oxide to Equation 2 is 1:25;

[0082] Compared to Group A, the materials that failed to form a complete hollow carbon shell structure.

[0083] Compared with the negative electrode active material obtained in group B, which is mainly composed of irregular carbon particles with a small number of hollow carbon spheres.

[0084] Compared to Group C: it mainly consists of hollow carbon spheres with a diameter of 280–520 nm, with a small amount of blocky solid carbon particles.

[0085] Groups D and E yielded material particles with a uniform particle diameter distribution of 280–520 nm, similar to the structure of Example 1. Performance tests were conducted using the method of Example 2, and the results are as follows:

[0086] Group D: The coulomb efficiency in the first lap was 87.3%, the coulomb efficiency in 100 laps was 99.1%, and the number of laps with a coulomb efficiency below 70% was 261.

[0087] Group E: The first lap coulomb efficiency was 88.1%, the 100th lap coulomb efficiency was 99.4%, and the number of laps with a coulomb efficiency below 70% was 272.

[0088] Based on Examples 1 and 3, it can be seen that controlling the appropriate ratio of zinc oxide to Formula 2 (e.g., zinc oxide to Formula 2 in a ratio of 1:15 to 25) helps to obtain materials with a structure suitable for sodium metal batteries.

[0089] Example 4

[0090] Compared with Example 1, the only difference is that the temperature of T1 in step 3 is changed, as follows:

[0091] Group A: T1 was 1000℃; other operations and parameters were the same as in Example 1. The zinc content of the negative electrode active material prepared in Group A was 28.6 at%.

[0092] Group B: T1 was 1100℃; other operations and parameters were the same as in Example 1. The zinc content of the negative electrode active material prepared in Group B was 10.6 at%.

[0093] Group C: T1 is 900℃;

[0094] Group D: Pre-treated at 500℃ for 1 hour, then sintered at 1200℃ for 1 hour;

[0095] Group E: Based on Group D, the atmosphere for both the pretreatment and calcination stages is a 5v% H2-Ar mixture.

[0096] Sodium metal anode assembly was performed on the negative electrode active materials (hollow carbon spheres) obtained in Examples 1 and 4: hollow carbon spheres, acetylene black, and polyvinylidene fluoride (PVDF) were uniformly mixed in a mass ratio of 80:10:10 and dispersed in a certain mass of N-methylpyrrolidone (NMP) to form a slurry. This slurry was then coated onto copper foil and vacuum dried at 80°C to obtain the sodium metal battery anode sheet. Battery assembly and testing were performed as follows: the anode sheet was stamped into an electrode sheet with a diameter of 10 mm, using metallic sodium as the counter electrode and 1 M NaPF6 / DGE as the electrolyte. A CR2032 coin cell was assembled in an argon-filled glove box. The battery was tested at 1 mA·cm⁻¹. -2 The current density, and the charge is 1 mA h·cm. -2 Sodium metal deposition overpotential was tested under the condition of 2 mA·cm. -2 The current density, and the charge is 1 mA h·cm. -2 Under the specified conditions, charge-discharge cycle tests were conducted, and the test results are shown in Table 2 below:

[0097] Table 2

[0098]

[0099]

[0100] Table 2 shows that controlling the processing temperature within the range of 1000–1250℃, especially 1100–1200℃, unexpectedly further facilitates the formation of active sites that induce sodium metal deposition and promotes the construction of sodium metal-compatible microstructures, resulting in superior performance. In particular, the preferred two-stage processing and the combined two-stage processing under a hydrogen-containing atmosphere can further synergistically improve the compatibility of the prepared material with sodium metal, contributing to further improvements in the performance of the prepared material in sodium metal batteries.

[0101] Example 5

[0102] Compared to Example 1, the only difference is that step 1 is carried out in a closed pressure vessel, and the initial volume of the raw material solution in the pressure vessel is 50%. Other operations and parameters are the same as in Example 1.

[0103] The performance of the prepared negative electrode active material was tested using the test method described in Example 2, and the results are as follows:

[0104] The coulomb efficiency for the first lap is 85.3%, the coulomb efficiency for 100 laps is 98.9%, and the number of laps with a coulomb efficiency below 70% is 260.

[0105] Example 6

[0106] Compared with Example 1, the only difference is that in step 1, a hydrothermal reaction is used to prepare the zinc oxide. The steps are as follows:

[0107] Take 50 mL of a 0.02 g / mL zinc acetate aqueous solution, then add 5–6 mL of saturated ammonia water dropwise and stir to mix. Perform hydrothermal treatment in a hydrothermal reactor at 210 °C for 30 h. After the reaction is complete, cool to room temperature, separate the solid and liquid phases, wash with water and ethanol, and vacuum dry to obtain ZnO. Subsequent steps are the same as in Example 1.

[0108] The performance of the prepared negative electrode active material was tested using the test method described in Example 2, and the results are as follows:

[0109] The coulomb efficiency for the first lap was 81.5%, the coulomb efficiency for 100 laps was 98.6%, and the number of laps with a coulomb efficiency below 70% was 239.

[0110] As can be seen from Examples 1, 5 and 6, the zinc oxide obtained by the method of the present invention, especially the atmospheric pressure preparation method, can achieve better synergy with subsequent processes, further improving the compatibility between the material and sodium metal, and further improving the performance of sodium metal batteries prepared by the material.

[0111] Comparative Example 1

[0112] Compared to Example 1, the only difference is that in step 3, T1 is controlled within the range required by the present invention, for example:

[0113] A: T1 is 800℃;

[0114] B: T1 is 1350℃;

[0115] The performance test was conducted using the method described in Example 4, and the results are as follows:

[0116] Group A: The Coulomb efficiency in the first lap was 83.2%, and the number of laps with a Coulomb efficiency below 70% was 221.

[0117] Group B: The Coulomb efficiency in the first lap was 81.6%, and the number of laps with a Coulomb efficiency below 70% was 202.

[0118] Comparative Example 2

[0119] Compared with Comparative Example 1A, the only difference is that after being treated at a temperature of 800°C for 2 hours, the material was cooled in the furnace, then immersed in a 1M hydrochloric acid solution at room temperature for 1 hour, then washed with water until the filtrate was neutral, and dried to obtain the material described above.

[0120] The performance was tested using the method in Example 4, and the results were as follows: the coulomb efficiency of the first cycle was 83.5%, and the number of cycles with a coulomb efficiency below 70% was 227.

Claims

1. A method for preparing a negative electrode active material for a sodium metal battery, characterized in that the steps include... include: Step (1): Preparation of ZnO particles; Step (2): ZnO particles and ligands with a molar ratio of 1:15~25 were subjected to surface reaction to in situ coat Zn-based framework materials on the surface of zinc oxide, resulting in a core-shell structured Zn framework@ZnO composite precursor. The ligands are compounds that can undergo coordination reactions with Zn; Step (3): The negative electrode active material was prepared by heat-treating the Zn framework@ZnO composite precursor at a temperature of 900~1250℃.

2. The preparation method according to claim 1, characterized in that, In step (1), the zinc source is reacted in a polyol to obtain the ZnO particles.

3. The preparation method according to claim 2, characterized in that, In step (1), the zinc source is at least one of zinc ion sulfate, nitrate, hydrochloride, and carboxylate; The polyol is a compound having the structure of Formula 1; Formula 1.

4. The preparation method according to claim 3, characterized in that, In step (1), the liquid-to-solid ratio of the zinc source and the polyol is 10~100 ml / g; The reaction process is either an atmospheric pressure reaction process or a solvothermal reaction process; The temperature during the reaction stage is above 140℃; The reaction time is more than 1 hour; The D50 particle size of ZnO particles is 250 ~ 550 nm.

5. The preparation method according to claim 4, characterized in that, In step (1), the temperature during the reaction stage is 150~190℃; The reaction time is 1 to 5 hours.

6. The preparation method according to claim 1, characterized in that, In step (2), the ligand is a compound having the structural formula 2; Equation 2.

7. The preparation method according to claim 6, characterized in that, In step (2), the surface reaction temperature is 40~80℃; The surface reaction time is 1 to 3 hours.

8. The preparation method according to claim 1, characterized in that, In step (3), the heat treatment atmosphere is a protective atmosphere; The protective atmosphere is at least one of nitrogen and an inert gas.

9. The preparation method according to claim 8, characterized in that, In step (3), an auxiliary gas is added to the atmosphere during the heat treatment stage. The auxiliary gas is at least one of hydrogen, CO, and CH4.

10. The preparation method according to claim 1, characterized in that, In step (3), the Zn framework@ZnO composite precursor is preheated at a temperature of 400~600℃ before subsequent heat treatment.

11. The preparation method according to claim 10, characterized in that, In step (3), the preheating time is 0.5~1.5h; The heat treatment stage lasts for 1 to 3 hours.

12. A negative electrode active material prepared by the preparation method according to any one of claims 1 to 11.

13. The negative electrode active material prepared by the preparation method according to claim 12, characterized in that, It includes a hollow carbon shell, in which zinc nanoparticles are embedded in situ and have pores for residual zinc metal volatilization. The inner wall and cavity of the hollow carbon shell also contain nano-zinc particles. The thickness of the negative electrode active material is 80~120 nm, and the diameter is 280~520 nm; In the aforementioned negative electrode active material, the zinc content is 5-30 at% The specific surface area of ​​the negative electrode active material is 600~800 m². 2 ·g -1 .

14. A negative electrode material for a sodium metal battery, characterized in that, Includes the negative electrode active material as described in claim 12 or 13.

15. The negative electrode material of the sodium metal battery as described in claim 14, characterized in that, The negative electrode material also includes a binder and a conductive agent.

16. The negative electrode material of the sodium metal battery as described in claim 15, characterized in that, In the aforementioned negative electrode material, the content of the negative electrode active material is above 70 wt%.

17. The negative electrode material of the sodium metal battery as described in claim 16, characterized in that, The negative electrode material contains 75-95 wt% of the negative electrode active material.

18. A sodium metal anode, comprising a current collector and an anode material loaded on its surface, characterized in that, The negative electrode material is the negative electrode material according to any one of claims 14 to 17.

19. A sodium metal battery, comprising a positive electrode, a separator, and a negative electrode sequentially laminated together, characterized in that, The negative electrode is as described in claim 18.

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