Sodium-ion battery negative electrode material and preparation method and application thereof

Abstract

The invention discloses a sodium ion battery negative electrode material and a preparation method and application thereof, and belongs to the technical field of batteries, and the sodium ion battery negative electrode material comprises a porous material and a heterogeneous electron accepting material compounded with the porous material. The preparation method comprises the following steps: providing polyoxometallate; and mixing the polyoxometallate with a porous material source solution, adjusting the system to be alkaline, and stirring and reacting at room temperature to obtain the sodium-ion battery negative electrode material. According to the sodium-ion battery negative electrode material disclosed by the invention, the porous material and the heterogeneous electron accepting material are compounded, so that the separation of an ion diffusion path and an electron conduction path is realized, and the sodium-ion battery containing the negative electrode material has obviously improved battery capacity and long cycle stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries, and particularly relates to a negative electrode material for a sodium-ion battery, a preparation method thereof, and an application thereof. Background Art

[0002] With the continuous growth of global energy demand and the urgent need for environmentally friendly energy storage technologies, in the family of alkali metal ion batteries, compared with traditional lithium-ion batteries, potassium-ion batteries and sodium-ion batteries have become important research directions for the next-generation energy storage systems due to their advantages of rich resources and low cost. Alkali metal ion batteries achieve charge storage and release through the insertion and extraction of alkali metal ions between the positive and negative electrodes. The behavior process is mainly responsible for the negative electrode material, and the negative electrode material is also considered to be one of the main factors restricting the development of battery power density. Improving the process of metal ion insertion and extraction is the core of battery development.

[0003] According to the ion insertion / extraction theory, porous materials are widely used in the negative electrode because they can reduce the ion insertion / extraction energy barrier. The negative electrode of porous materials can greatly improve the capacity and rate performance of the battery passing through. Currently, most of the widely reported negative electrode materials are mainly transition metal compounds ( Figure 1 a in). However, the stability of the currently widely used porous material negative electrode battery is difficult to meet the industrial requirements. In the process of realizing the present invention, the inventors found that there are at least the following problems in the prior art: The early transition metals have strong reactivity, high coordination environment, and have more empty non-bonding / anti-bonding orbitals, providing sufficient orbital space for electron storage / release behavior. However, most strong metal-nonmetal interactions will generate strong crystal field effects, energy level splitting, and wide bandgap, restricting electron storage / release behavior. The late transition metals are rich in d electrons and have a low coordination number, which can provide action gaps for ion insertion / extraction. However, their rich d electrons will also occupy most of the non-bonding / bonding orbitals ( Figure 1 b- Figure 1 c in, where M n+ represents metal ions), and the charge and discharge electron behavior will cause a sharp increase in the non-bonding interaction in the negative electrode material and a decrease in stability. For porous materials, the electron storage / release behavior is the key factor affecting the material stability. Especially in the industrial environment of high current / high capacity, the negative electrode material itself needs to store a large amount of electrons / ions, and the more intense electron behavior impact brings more severe challenges to the stability of the negative electrode material. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a negative electrode material for a sodium-ion battery, a preparation method and an application thereof in view of the deficiencies of the above-mentioned prior art. By compounding a porous material with a hetero electron-accepting material, the separation of the ion diffusion path and the electron conduction path is realized, and the sodium-ion battery containing the negative electrode material has significantly improved battery capacity and long cycle stability.

[0005] The present invention has the following advantages compared with the prior art:

[0006] 1. In view of the problem of the coupled storage of electrons and ions in the negative electrode material, the present invention creatively proposes a novel negative electrode material for a sodium-ion battery based on the separation of the ion diffusion path and the electron transport path, and specifically solves the problem that the ion insertion / extraction and the charging / discharging behavior of electrons in the negative electrode material during the charge and discharge process of the traditional negative electrode material have an adverse impact on the structure and orbit of the negative electrode material. By compounding a porous material with a hetero electron-accepting material, the separation of the ion diffusion path and the electron conduction path is realized. The porous material does not store electrons, the negative electrode material has a high capacity, and the structure is stable in the long-cycle electrochemical test. The corresponding sodium-ion battery still maintains a high capacity after 2000 charge and discharge cycles.

[0007] 2. In the sodium-ion negative electrode material composed of a porous material and a hetero electron-accepting material, the porous material is preferably Cu2NCN, which can fully combine its open crystal structure and low packing density, as well as the σ-donation and π-electron precipitation of the [NCN] 2- ligand to achieve efficient diffusion of sodium ions and enhanced conductivity.

[0008] 3. In the sodium-ion negative electrode material composed of a porous material and a hetero electron-accepting material, a polyoxometalate is preferably used as the hetero electron-accepting material. Its anion center conjugated framework can accept electrons, and reversible electron transfer can be carried out at various potentials by adjusting the oxidation state. Its unique three-dimensional cluster structure can effectively avoid the storage of sodium ions.

[0009] 4. For the sodium-ion negative electrode material of the present invention, by compounding a porous material and a hetero electron-accepting material, the crystal structure of the porous material is stable during the charge and discharge cycle. The battery capacity retention rate is as high as 99.93% after 2000 charge and discharge cycles, and it has the characteristic of long cycle life.

[0010] The technical solution of the present invention will be further described in detail below with reference to the drawings and embodiments. Description of the Drawings

[0011] Figure 1 It is the battery cycle life and the schematic diagram of the charge and discharge function of the negative electrode with a transition metal compound as the negative electrode material in the background technology;

[0012] Figure 2 Schematic diagram of the charge and discharge principle of the anode material of the sodium-ion battery of the present invention;

[0013] Figure 3 Scanning electron microscope pictures of the anode material of the sodium-ion battery in Example 1 and Cu2NCN obtained in Step 2 of Comparative Example 1; among them Figure 3 a of Figure 3 b of Figure 3 c of Figure 3 d of

[0014] Figure 4 Element surface scanning imaging diagram of the anode material of the sodium-ion battery in Example 1;

[0015] Figure 5 XRD pattern, FTIR pattern, transmission electron microscope image before cycling, and transmission electron microscope image after cycling of the anode material of the sodium-ion battery in Example 1; among them Figure 5 a of Figure 5 b of Figure 5 c of Figure 5 d of

[0016] Figure 6 Schematic diagram of the long cycle test results of the sodium-ion battery assembled with the anode material of the sodium-ion battery in Example 1;

[0017] Figure 7 Schematic diagram of the electrochemical performance test results of the sodium-ion battery corresponding to the POM and Cu2NCN composite material of Comparative Example 1 at different rates;

[0018] Figure 8 Electrochemical CV curve diagram of the sodium battery assembled with POM of Comparative Example 2 as the anode material. Detailed implementation manners

[0019] Next, the technical solutions will be described clearly and completely in combination with the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0020] In the following description, the term "and / or" is used to describe the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B may represent: the case of A existing alone, the case of B existing alone, and the case of A and B existing simultaneously. Among them, A and B may be singular or plural.

[0021] In the following description, terms such as "comprising", "including", "having", and "containing" are all open-ended terms, meaning including but not limited to.

[0022] Those skilled in the art should understand that in the following description of the embodiments of the present application, the sequence numbers do not imply the order of execution. Some or all of the steps can be executed in parallel or successively. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation to the implementation process of the embodiments of the present application.

[0023] Those skilled in the art should understand that the numerical ranges in the embodiments of the present application should be understood as specifically disclosing each intermediate value between the upper and lower limits of the range. The intermediate values within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, are also included in the present application. The upper and lower limits of these smaller ranges can be independently included or excluded from the range.

[0024] Unless otherwise specified, the technical / scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which this application belongs. Although this application only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein can also be used in the implementation or testing of this application. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0025] The technical principle adopted by the present invention: The present invention creatively provides a sodium-ion battery anode material composed of two materials, where each material separately provides an ion transport path and an electron conduction path, such as Figure 2 shown, a sodium-ion battery anode material composed of a porous material and a hetero electron acceptor material. The hetero electron acceptor material hydrolyzes under alkaline conditions, inducing self-assembly with the porous material to form a regular nanoscale flower structure. During charge and discharge cycles, the porous material provides an ion transport path, and the hetero electron acceptor material undergoes reversible electron transfer.

[0026] On the one hand, a sodium-ion battery anode material is provided, including a porous material and a hetero electron acceptor material compounded with the porous material.

[0027] In some embodiments, the porous material is a transition metal compound crystalline material; and / or, the hetero electron acceptor material is a polyoxometalate; in some preferred embodiments, the transition metal in the transition metal compound crystalline material is Cu; in some specific embodiments, the porous material is Cu2NCN; in some preferred embodiments, the polyoxometalate topological skeleton is of the Keggin type, and the heteroatoms include P; in some specific embodiments, the polyoxometalate is [Ni4(H2O)2(PW9O 34 )2] 10- .

[0028] In the present invention, the preferred polyoxometalate is [Ni4(H2O)2(PW9O 34 )2] 10- , which contains an anion center composed of high-oxidation-state transition metals and has a highly conjugated framework structure formed by oxygen bridges, can absorb electrons to stabilize the charge distribution. At a certain potential, the oxidation state of the anion center changes, absorbing or releasing electrons to achieve reversible electron transfer.

[0029] In some embodiments, the morphology of the negative electrode material of the sodium-ion battery is nanoflowers, and the thickness of the petal layer of the nanoflowers is 1.5 - 2.0 nm.

[0030] On the one hand, a method for preparing the above-mentioned negative electrode material of the sodium-ion battery is provided, including:

[0031] Step 1: Provide a polyoxometalate;

[0032] Step 2: Mix the polyoxometalate with a porous material source solution, adjust the system to be alkaline, and stir at room temperature to react to obtain the negative electrode material of the sodium-ion battery.

[0033] In some specific embodiments, the preparation method of the negative electrode material of the sodium-ion battery specifically includes:

[0034] Step 1: Provide a polyoxometalate, including:

[0035] Step 101: Dissolve Na2WO4·2H2O, NaHPO4·7H2O, and Ni(NO3)2·6H2O in deionized water according to a mass ratio of 1:(0.02 - 0.2):(0.05 - 0.5), and adjust the pH to 6 - 8 with hydrochloric acid to obtain a light green suspension;

[0036] Step 102: Reflux the light green suspension; in some specific embodiments, the reflux temperature is 80 - 100 °C, and the time is 0.5 - 5 h;

[0037] Step 103: After the reflux ends, filter, and crystallize the filtrate to obtain a polyoxometalate;

[0038] Step 2. Mix the polyoxometalate with the porous material source solution, adjust the system to be alkaline, and stir and react at room temperature to obtain the anode material for sodium-ion batteries, including:

[0039] Step 201. At room temperature, dissolve CuCl2·2H2O and the polyoxometalate in deionized water according to a mass ratio of 1:(0.1 - 1). Subsequently, add KOH solution and aqueous cyanamide solution in sequence. After stirring, add hydrazine hydrate and continue stirring to allow the reaction; the mass of the deionized water is 100 - 1000 times the mass of the polyoxometalate; the volume in mL of the KOH solution is 5 - 50 times the mass in g of the polyoxometalate, and the concentration of the KOH solution is 1 - 5 M; the volume in mL of the aqueous cyanamide solution is 5 - 50 times the mass in g of the polyoxometalate, and the concentration of the aqueous cyanamide solution is 1 - 5 M; adding the KOH solution and the aqueous cyanamide solution in sequence and stirring for 3 minutes serves to fully react; the volume in mL of the hydrazine hydrate is 10 - 80 times the mass in g of the polyoxometalate; the time for continuing to stir and react is 0.5 - 3 h;

[0040] Step 202. Centrifuge, wash, centrifuge again, and freeze-dry the reaction system to obtain the Cu2NCN / POM composite material.

[0041] In the method for preparing the Cu2NCN / POM composite material of the present invention, it is preferably to stir and react CuCl2·2H2O and the polyoxometalate with a mass ratio of 1:(0.1 - 1) with an alkaline aqueous cyanamide solution in the presence of hydrazine hydrate for 0.5 - 3 h to form a nanoscale flower structure with a clear interface between Cu2NCN and POM and a stable Cu2NCN crystal phase.

[0042] On the other hand, a sodium-ion battery is provided, including the above-mentioned anode material for sodium-ion batteries.

[0043] Before the application of the present invention, a series of experiments were carried out. Now, a part of the test results are listed to further describe the invention in detail. The following is a detailed description in combination with examples.

[0044] Example 1

[0045] This example provides an anode material for sodium-ion batteries, including a porous material and a hetero electron-accepting material composite with the porous material, where the porous material is Cu2NCN and the hetero electron-accepting material is a polyoxometalate.

[0046] This example also provides a preparation method for the above-mentioned anode material for sodium-ion batteries, including:

[0047] Step 1. Provide the polyoxometalate Ni4(H2O)2(PW9O 34 )2] 10-, including:

[0048] Step 101: Dissolve 17.81 g of Na2WO4·2H2O, 1.61 g of NaHPO4·7H2O, and 3.48 g of Ni(NO3)2·6H2O in 50 mL of deionized water, and adjust the pH to 7.0 with hydrochloric acid to obtain a light green suspension;

[0049] Step 102: Reflux the light green suspension at 100 °C for 2 hours;

[0050] Step 103: After the reaction, filter to remove the precipitate, and add 20 g of sodium chloride to the filtrate to promote crystallization;

[0051] Step 104: Recrystallize the crystallization product in 50 mL of hot water, and purify to obtain a light green product, denoted as POM;

[0052] Step Two: Mix the polyoxometalate with the porous material source solution, and induce self-assembly under alkaline conditions to synthesize the Cu2NCN / POM composite material, including:

[0053] Step 201: At room temperature, dissolve 426.6 mg of CuCl2·2H2O and 100 mg of the POM in 45 mL of deionized water. Subsequently, add 2.5 mL of a 3.5 M KOH solution and 3 mL of a 2 M cyanamide aqueous solution in sequence. After stirring for 3 minutes, quickly add 5 mL of hydrazine hydrate, and continue stirring to allow the reaction to proceed for 2 hours;

[0054] Step 202: Centrifuge the reaction product, wash it with deionized water, centrifuge again, and freeze-dry to obtain the Cu2NCN / POM composite material.

[0055] Comparative Example 1

[0056] This comparative example examines the influence of the POM binding mode on the structure and performance of the anode material for sodium-ion batteries. The preparation method is as follows:

[0057] Step One: Provide the polyoxometalate Ni4(H2O)2(PW9O 34 )2] 10- , including:

[0058] Step 101: Dissolve 17.81 g of Na2WO4·2H2O, 1.61 g of NaHPO4·7H2O, and 3.48 g of Ni(NO3)2·6H2O in 50 mL of deionized water, and adjust the pH to 7.0 with hydrochloric acid to obtain a light green suspension;

[0059] Step 102: Reflux the light green suspension at 100 °C for 2 hours;

[0060] Step 103: After the reaction is completed, filter to remove the precipitate, and add 20 g of sodium chloride to the filtrate to promote crystallization;

[0061] Step 104: Recrystallize the crystallization product in 50 mL of hot water to obtain a light green product after purification, denoted as POM;

[0062] Step Two: Provide Cu2NCN, including:

[0063] Step 201: At room temperature, dissolve 426.6 mg of CuCl2·2H2O in 45 mL of deionized water, then sequentially add 2.5 mL of a KOH solution with a concentration of 3.5 M and 3 mL of a cyanamide aqueous solution with a concentration of 2 M. After stirring for 3 minutes, quickly add 5 mL of hydrazine hydrate and continue stirring to allow the reaction to proceed for 2 hours;

[0064] Step 202: Centrifuge the product after the reaction, wash it with deionized water, centrifuge again, and freeze-dry to obtain Cu2NCN;

[0065] Step Three: Mix the POM described in Step One and the Cu2NCN described in Step Two evenly according to a mass ratio of 1:1 to obtain a POM and Cu2NCN composite material.

[0066] Comparative Example 2

[0067] This comparative example provides a preparation method of polyoxometalate, including:

[0068] Step One: Dissolve 17.81 g of Na2WO4·2H2O, 1.61 g of NaHPO4·7H2O, and 3.48 g of Ni(NO3)2·6H2O in 50 mL of deionized water, and adjust the pH to 7.0 with hydrochloric acid to obtain a light green suspension;

[0069] Step Two: Reflux the light green suspension at 100 °C for 2 hours;

[0070] Step Three: After the reaction is completed, filter to remove the precipitate, and add 20 g of sodium chloride to the filtrate to promote crystallization;

[0071] Step Four: Recrystallize the crystallization product in 50 mL of hot water to obtain a light green product after purification, which is POM.

[0072] Performance Evaluation

[0073] 1. Structure and performance evaluation of the anode material of sodium-ion battery

[0074] Scanning electron microscope images of the anode material of sodium-ion battery in Example 1 and Cu2NCN obtained in Step Two of Comparative Example 1 are asFigure 3 As shown, it can be seen that the Cu2NCN obtained in the second step of Comparative Example 1 is dispersed in the form of nanosheets ( Figure 3 a), and at a small scale ( Figure 3 b), it can be observed that the shape of the nanosheets is irregularly arranged and the edges are not clear. The Cu2NCN / POM composite material of Example 1 has a nanoflower structure ( Figure 3 c), and at a small scale ( Figure 3 d), it can be observed that petal-shaped nanosheets aggregate to form a nanoflower structure. The thickness of the petal layer is about 1.7 nm, and the overall thickness is uniform. The elemental surface scanning imaging diagram of the negative electrode material of the sodium ion battery in Example 1 is as shown in Figure 4 . The Cu, N, and C elements come from Cu2NCN, and the P, W, and Ni elements come from POM, indicating that the Cu2NCN / POM composite material is successfully prepared in the present invention.

[0075] Figure 5 a in is the XRD diagram of the negative electrode material of the sodium ion battery in Example 1. The Cu2NCN spectrum is the spectrum of Cu2NCN prepared according to the second step of Comparative Example 1. It can be seen that, compared with the Cu2NCN standard card, obvious peaks attributed to Cu2NCN appear in both the Cu2NCN / POM of Example 1 and the Cu2NCN of Comparative Example 1, indicating the formation of the Cu2NCN structure and good crystallinity. Figure 5 b in is the FTIR diagram. In Cu2NCN and Cu2NCN / POM, the characteristic peaks at 2196 cm -1 , 2135 cm -1 , 1185 cm -1 and 1998 cm -1 are attributed to cuprous cyanamide. In Cu2NCN / POM, the characteristic peaks at 1034 cm -1 , 940 cm -1 and 887 cm -1 are attributed to POM. Combining with the high-resolution transmission electron microscope ( Figure 5 c) of the negative electrode material of the sodium ion battery in Example 1, it can be seen that there is a clear contact interface between Cu2NCN and POM in the Cu2NCN / POM (before cycling) of Example 1, and clear lattice fringes attributed to the (021) crystal plane appear in Cu2NCN. Combining with the XRD diffraction pattern ( Figure 5 a), it can be known that Cu2NCN in this negative electrode material has good crystallinity. Figure 5 d in is the high-resolution transmission electron microscope image of Cu2NCN / POM after 2000 cycles. Clear crystal stripe patterns can still be observed in Cu2NCN. It can be seen that after long cycling, Cu2NCN / POM still has a stable crystal phase and structure, indicating that the negative electrode material of the present invention has a stable structure during long charge-discharge cycles.

[0076] Using the anode material of the sodium-ion battery in Example 1 as the working electrode and a sodium foil as the counter electrode, the method for assembling a button-type sodium half-cell includes: taking the anode material of the sodium-ion battery in Example 1 as the active material, mixing it with Super P and sodium alginate according to a mass ratio of 8:1:1, placing the mixture in deionized water to prepare an anode slurry, coating the anode slurry on a copper foil, drying it in vacuum at 90 °C for 12 h, and punching it into circular discs with a diameter of 12 mm, where the loading amount of the active material is 1.2 mg cm -2 Transfer the circular discs to a glove box under an argon atmosphere. Using the sodium foil as the counter electrode, a glass fiber GF / D as the separator, and a 1.0 M sodium hexafluorophosphate solution in DEGDME as the electrolyte, perform charge-discharge cycling and galvanostatic intermittent titration technique (GITT) tests using a LAND-CT2001A test system. In the GITT test, apply a pulsed current of 100 mA g -1 for 10 minutes and relax for 40 minutes. Test the electrochemical performance using a CHI760E electrochemical workstation (Shanghai Chenhua). The results are as Figure 6 shown. It can be seen that during 2000 cycles, the specific capacity of the battery corresponding to Example 1 remains stable at ~300 mAh·g -1 . For the battery with the pure Cu2NCN in Step 2 of Comparative Example 1 as the anode material, it decreases from the initial ~180 mAh·g -1 to ~30 mAh·g -1 . It can be seen that the battery cycle stability of the anode material of the sodium-ion battery in Example 1 is higher. After 2000 cycles, the capacity retention rate of the battery corresponding to pure Cu2NCN is 17.10%, while the capacity retention rate of the battery corresponding to the Cu2NCN / POM composite material in Example 1 is as high as 99.93%, showing significantly improved long-cycle stability, indicating that the Cu2NCN / POM composite material prepared by the method of the present invention can significantly accelerate the charge-discharge kinetic reaction process and improve the specific capacity and cycle performance of the battery. Figure 7 It is a schematic diagram of the test results of the electrochemical performance of the battery corresponding to the POM and Cu2NCN composite material in Comparative Example 1 at different rates. It can be seen that after 80 cycles at a rate of 0.1C - 5C, the capacity retention of the battery corresponding to the POM and Cu2NCN composite material in Comparative Example 1 is about 85.3%. Its rate performance and reversibility are poor, indicating that it is difficult for the POM and Cu2NCN composite material prepared by physical mixing to support the stable cycling of the battery as the anode material.

[0077] Figure 8 It is the electrochemical CV curve of the battery assembled with POM in Comparative Example 2. It can be seen that POM has no capacity under the test conditions. Combining Figure 6 and Figure 8It can be seen that the long-cycle capacity retention rate of the battery corresponding to the negative electrode material obtained by the preparation method of the present invention exceeds 99.3%, which is significantly higher than that of POM or Cu2NCN, indicating that the composite material with a partitioned responsible ion transport path and electron conduction path obtained by the method of the present invention can significantly improve the electrochemical performance of the negative electrode material.

Claims

1. A negative electrode material for a sodium-ion battery, characterized in that It includes a porous material and a heterogeneous electron-accepting material composite with the porous material; the porous material is a transition metal compound crystalline material Cu2NCN, and the heterogeneous electron-accepting material is a polyoxometalate; the composite includes hydrolyzing the heterogeneous electron-accepting material under alkaline conditions to induce self-assembly with the porous material.

2. The negative electrode material of the sodium-ion battery according to claim 1, characterized in that The topological skeleton of the polyoxometalate is of the Keggin type.

3. The negative electrode material of the sodium ion battery according to claim 1, characterized in that, The morphology of the negative electrode material of the sodium-ion battery is nanoflowers, and the thickness of the petal layer of the nanoflowers is 1.5 - 2.0 nm.

4. A method for preparing the anode material of a sodium-ion battery as described in any one of claims 1 to 3, characterized in that, It includes: Providing a polyoxometalate; Mixing the polyoxometalate with a porous material source solution, adjusting the system to be alkaline, and stirring and reacting at room temperature to obtain a negative electrode material for a sodium-ion battery.

5. The method according to claim 4, wherein Providing a polyoxometalate specifically includes: Step 101: Dissolve Na2WO4·2H2O, NaHPO4·7H2O, and Ni(NO3)2·6H2O in deionized water, and adjust the pH to 6 - 8 with hydrochloric acid to obtain a light green suspension; Step 102: Reflux the light green suspension; Step 103: After the reflux ends, filter, and crystallize the filtrate to obtain a polyoxometalate.

6. The method according to claim 5, characterized in that, In step 101, the mass ratio of Na2WO4·2H2O, NaHPO4·7H2O, and Ni(NO3)2·6H2O is 1:(0.02 - 0.2):(0.05 - 0.5); and / or, in step 102, the reflux temperature is 80 - 100 °C, and the time is 0.5 - 5 h.

7. The method according to claim 4, wherein Mixing the polyoxometalate with a porous material source solution, adjusting the system to be alkaline, and stirring and reacting at room temperature to obtain a negative electrode material for a sodium-ion battery specifically includes: Step 201: At room temperature, dissolve CuCl2·2H2O and the polyoxometalate in deionized water, then successively add a KOH solution and an aqueous cyanamide solution, stir, and then add hydrazine hydrate, and continue stirring to make the reaction; Step 202: Centrifuge, wash the reaction system, centrifuge again, and freeze-dry to obtain a negative electrode material for a sodium-ion battery.

8. The method according to claim 7, wherein In step 201, the mass ratio of CuCl2·2H2O to the polyoxometalate is 1:(0.1 - 1); and / or, in step 201, the mass of the deionized water is 100 - 1000 times the mass of the polyoxometalate; and / or, in step 201, the volume of the KOH solution is 5 - 50 times the mass of the polyoxometalate, the volume unit of the KOH solution is mL, the mass unit of the polyoxometalate is g, and the concentration of the KOH solution is 1 - 5 M; and / or, in step 201, the volume of the aqueous cyanamide solution is 5 - 50 times the mass of the polyoxometalate, the volume unit of the aqueous cyanamide solution is mL, the mass unit of the polyoxometalate is g, and the concentration of the aqueous cyanamide solution is 1 - 5 M; and / or, in step 201, the volume of the hydrazine hydrate is 10 - 80 times the mass of the polyoxometalate, the volume unit of the hydrazine hydrate is mL, the mass unit of the polyoxometalate is g; and / or, in step 201, the time for continuing stirring and reacting is 0.5 - 3 h.

9. A sodium-ion battery, characterized in that, It includes the negative electrode material for a sodium-ion battery according to any one of claims 1 - 3.

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