CN115763731B

CN115763731B - Bi/C composite material with high tap density and preparation method and application thereof

Info

Publication number: CN115763731B
Application number: CN202211427088.6A
Authority: CN
Inventors: 吴昊; 张云; 武开鹏; 王倩; 邱小玲
Original assignee: Sichuan University
Current assignee: Sichuan University
Priority date: 2022-11-15
Filing date: 2022-11-15
Publication date: 2023-07-21
Anticipated expiration: 2042-11-15
Also published as: CN115763731A

Abstract

The invention discloses a Bi/C composite material with high tap density and a preparation method and application thereof, wherein the preparation method comprises the following steps: s1: adding bismuth nitrate or bismuth nitrate pentahydrate into a mixed solvent of ethylene glycol and isopropanol, and obtaining a mixed solution A after complete dissolution; s2: adding anhydrous glucose into the mixed solution A, and dissolving until the mixed solution A is clear and transparent to obtain a mixed solution B; s3: preserving the temperature of the mixed solution B for 6-24 hours at 150-200 ℃ to obtain a mixed solution C; s4: and cooling the mixed solution C, collecting the precipitate in the mixed solution C after cooling to room temperature, washing and drying the precipitate, and carbonizing under the protection of inert atmosphere to obtain the Bi/C composite material with high tap density. The preparation method disclosed by the invention is simple in process, suitable for large-scale production, and the product has high tap density, can realize high reversible capacity as a negative electrode of a sodium ion battery, and has excellent multiplying power performance and long-cycle stability.

Description

Bi/C composite material with high tap density and preparation method and application thereof

Technical Field

The invention relates to the technical field of ion batteries and electrode materials, in particular to a Bi/C composite material with high tap density, and a preparation method and application thereof.

Background

Lithium ion batteries have been widely used in the fields of electric vehicles, portable electronic products, and the like because of their excellent cycle stability and high energy density. However, due to lithium resources and cost factors, the ever-expanding energy storage field requirements cannot be met. Sodium ion batteries are considered as one of the powerful candidates for large-scale energy storage because of the advantages of abundant sodium reserves, low price and the like. Unlike lithium ion batteries, suitable anode materials have been one of the keys limiting the large-scale application of sodium ion batteries. The negative electrode material of the sodium ion battery is mainly made of hard carbon and alloy materials at present. However, there is a major disadvantage to each of these two materials: for hard carbon, the voltage platform is close to the deposition potential of metallic sodium, so that sodium metal dendrite is easy to generate under the condition of high-current charge, and serious safety problem is caused. For alloy materials, the solid-electrolyte interface film on the surface of the metal alloy is continuously crushed and generated in the charge and discharge process due to the severe volume deformation, so that the coulomb efficiency is low, and the cycle life of the full battery is greatly limited.

With the development and intensive research of sodium ion battery technology, bismuth-based materials are becoming one of ideal choices for negative electrode materials of sodium ion batteries. At present, the main modification strategies of bismuth-based materials comprise reducing the microscopic size of active materials, compounding with materials with high conductivity, morphology regulation, core-shell or multi-core-shell structure design and the like, and the main purposes are to promote effective transmission of electrons and ions by increasing the specific surface area and porosity of the materials, improving the conductivity and the like, so that excellent cycle stability is realized. However, these methods are often complicated, time-consuming and difficult to prepare in large quantities at the expense of the tap density of the material. Therefore, there is a need for a bismuth-based composite material having a high tap density while also ensuring excellent dynamic behavior and cycle stability, so as to fully exploit the advantages of Bi itself in terms of high volumetric energy density.

Disclosure of Invention

Aiming at the problems, the invention aims to provide a Bi/C composite material with high tap density, and a preparation method and application thereof.

The technical scheme of the invention is as follows:

in one aspect, a method for preparing a Bi/C composite material with high tap density is provided, comprising the steps of:

s1: adding bismuth nitrate or bismuth nitrate pentahydrate into a mixed solvent of ethylene glycol and isopropanol, and obtaining a mixed solution A after complete dissolution;

s2: adding anhydrous glucose into the mixed solution A, and dissolving until the mixed solution A is clear and transparent to obtain a mixed solution B;

s3: preserving the temperature of the mixed solution B for 6-24 hours at 150-200 ℃ to obtain a mixed solution C;

s4: and cooling the mixed solution C, collecting the precipitate in the mixed solution C after cooling to room temperature, washing and drying the precipitate, and carbonizing under the protection of inert atmosphere to obtain the Bi/C composite material with high tap density.

Preferably, in the step S1, the bismuth nitrate or bismuth nitrate pentahydrate is used in an amount of 4-16g/L.

Preferably, in step S1, the volume ratio of ethylene glycol to isopropanol in the mixed solvent is 1:11-2:1.

Preferably, in the step S2, the dosage of the anhydrous glucose is 7.2-36g/L.

Preferably, in the step S2, when the anhydrous glucose is dissolved, heating, stirring and dissolving are carried out in a constant-temperature water bath, wherein the temperature of the constant-temperature water bath is 60-85 ℃, the stirring speed is 200-500r/min, and the stirring time is 0.1-0.5h.

Preferably, in step S4, the precipitate is dried at a drying temperature of 50 to 90 ℃ for a drying time of 4 to 24 hours.

Preferably, in step S4, the inert atmosphere is argon or nitrogen.

Preferably, in step S4, carbonization is performed at 600-900 ℃ for 1-5 hours.

On the other hand, the Bi/C composite material with high tap density prepared by the preparation method of the Bi/C composite material with high tap density and the application of the Bi/C composite material with high tap density as a negative electrode material of a sodium ion battery are also provided.

The beneficial effects of the invention are as follows:

the preparation method provided by the invention has the advantages that the raw materials and the preparation process are simple, the Bi/C composite material can be prepared by carbon thermal reduction after a one-step solvothermal method, the repeatability is good, the industrialized production can be realized, and the popularization and the application are convenient. The obtained Bi/C composite material has the characteristics of low specific surface area and porosity, compact structure, high tap density and the like. And in the carbonization process, the composite sphere with Bi and C elements distributed in a concentration gradient manner is obtained through the Kendall effect, and the inner Bi nano particles are uniformly wrapped by the outer carbon layer, so that no obvious aggregation occurs. The negative electrode material of the sodium ion battery has special structure, good conductivity, excellent multiplying power performance, long-cycle stability and high volume/area specific capacity, and is a sodium ion battery negative electrode material with very high application potential.

Drawings

In order to more clearly illustrate the embodiments of the invention or the technical solutions of the prior art, the drawings which are used in the description of the embodiments or the prior art will be briefly described, it being obvious that the drawings in the description below are only some embodiments of the invention, and that other drawings can be obtained according to these drawings without inventive faculty for a person skilled in the art.

FIG. 1 is a scanning electron microscope image of the Bi-salt@CP precursor of example 1;

FIGS. 2a and 2b are, respectively, a scanning electron microscope image and a corresponding particle size distribution diagram of the CGD-Bi/C composite material prepared in example 1, FIG. 2C is a transmission electron microscope image of a single sphere of the sample, and FIG. 2d is a focused ion beam microscope image of the sample;

FIG. 3 is a schematic diagram showing the results of electrochemical performance test of the CGD-Bi/C composite material prepared in example 1; wherein FIG. 3a shows the charge-discharge curves of the first three turns, and FIG. 3b shows 5.0. 5.0A g ^-1 Long cycle performance at current density and corresponding volumetric energy density plot, FIG. 3c is 0.1-100A g ^-1 A ratio performance and corresponding volumetric energy density map over a range of current densities;

FIG. 4 is a schematic diagram showing the results of electrochemical performance tests of the CGD-Bi/C composite material prepared in example 1 at different surface loadings; wherein FIG. 4a is 8.52mg cm ^-2 The cycle performance and corresponding surface capacity at high surface loading are plotted, the inset of fig. 4a is a cross-sectional thickness scan image of the corresponding electrode, fig. 4b is a comparison plot of the rate performance of commercial hard carbon and CGD-Bi/C composites at different current densities, and the inset of fig. 4b is a cross-sectional thickness scan image of the two electrodes;

FIGS. 5a and 5b are scanning electron microscope images of the composite material prepared in example 2 at 5 μm and 1 μm dimensions, respectively; FIGS. 5c and 5d are scanning electron microscope images of the composite material prepared in example 3 at 5 μm and 1 μm dimensions, respectively; FIG. 5e shows the samples prepared in example 2 and example 3 at 5.0. 5.0A g ^-1 A schematic diagram of a long-cycle performance test result under current density;

FIGS. 6a and 6b are scanning electron microscope images of the composite material prepared in example 4 at 5 μm and 1 μm dimensions, respectively; FIGS. 6c and 6d are scanning electron microscope images of the composite material prepared in example 5 at 5 μm and 1 μm dimensions, respectively; FIG. 6e shows the samples prepared in example 4 and example 5 at 5.0. 5.0A g ^-1 A schematic diagram of a long-cycle performance test result under current density;

FIG. 7 is a scanning electron microscope image of the Bi-salt@CP precursor of example 6;

FIG. 8 is a scanning electron microscope image of the Bi-salt@CP precursor of example 7;

FIG. 9 is a scanning electron microscope image of the Bi-salt@CP precursor of comparative example 1;

FIG. 10 is a scanning electron microscope image of the Bi-salt@CP precursor of comparative example 2;

FIG. 11 is a schematic diagram showing the tap density test results of the Bi/C composite materials prepared in examples 6-7 and comparative examples 1-2.

Detailed Description

The invention will be further described with reference to the drawings and examples. It should be noted that, without conflict, the embodiments and technical features of the embodiments in the present application may be combined with each other. It is noted that all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs unless otherwise indicated. The use of the terms "comprising" or "includes" and the like in this disclosure is intended to cover a member or article listed after that term and equivalents thereof without precluding other members or articles.

In one aspect, the invention provides a method for preparing a Bi/C composite material with high tap density, comprising the following steps:

s1: bismuth nitrate or bismuth nitrate pentahydrate is added into a mixed solvent of ethylene glycol and isopropanol, and mixed solution A is obtained after complete dissolution.

In a specific embodiment, the bismuth nitrate or bismuth nitrate pentahydrate is used in an amount of 4-16g/L. Optionally, the bismuth nitrate or bismuth nitrate pentahydrate may be used in an amount of 5g/L, 8g/L, 10g/L, 12g/L, 15g/L, etc.

In a specific embodiment, the volume ratio of ethylene glycol to isopropanol in the mixed solvent is 1:11-2:1. Optionally, the volume ratio of the ethylene glycol to the isopropanol can be 1:5, 1:2, 1:1, etc.

In a specific embodiment, the bismuth nitrate or bismuth nitrate pentahydrate is completely dissolved into the mixed solvent by ultrasonic dispersion. The method of mixing and dissolving is known in the art, and other methods such as stirring may be applied to the present invention in addition to the ultrasonic dispersion.

S2: and adding anhydrous glucose into the mixed solution A, and dissolving until the mixed solution A is clear and transparent to obtain a mixed solution B.

In a specific embodiment, the anhydrous dextrose is used in an amount of 7.2 to 36g/L. When the anhydrous glucose is dissolved, heating, stirring and dissolving are carried out in a constant-temperature water bath, wherein the temperature of the constant-temperature water bath is 60-85 ℃, the stirring speed is 200-500r/min, and the stirring time is 0.1-0.5h. Optionally, the dosage of the anhydrous glucose can be 8g/Lg/L, 10g/L, 15g/L, 20g/L, 25g/L, 30g/L, 32g/L, etc., the temperature of the constant-temperature water bath can be 65 ℃, 70 ℃, 75 ℃, 80 ℃ and the like, the stirring rotating speed can be 250r/min, 300r/min, 350r/min, 400r/min, 450r/min and the like, and the stirring time can be 0.2h, 0.3h, 0.4h and the like.

S3: and (3) preserving the temperature of the mixed solution B for 6-24 hours at the temperature of 150-200 ℃ to obtain a mixed solution C.

In a specific embodiment, the temperature is raised from room temperature to 150-200deg.C at a heating rate of 1-10deg.C/min, optionally 2 deg.C/min, 3 deg.C/min, 4 deg.C/min, 5 deg.C/min, 6 deg.C/min, 7 deg.C/min, 8 deg.C/min, 9 deg.C/min. It should be noted that the present invention may be used in maintaining the temperature of mixed solution B at 150-200 deg.c.

In a specific embodiment, the incubation temperature may also be 160 ℃, 170 ℃, 180 ℃, 190 ℃, and the incubation time may also be 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, 20h, 21h, 22h, 23h.

In a specific embodiment, the mixed liquor C is cooled by adopting a natural cooling method. And when the precipitate is dried, the drying temperature is 50-90 ℃ and the drying time is 4-24h. The inert atmosphere is argon or nitrogen, and when carbonization is carried out, the carbonization temperature is 600-900 ℃ and the carbonization time is 1-5h. Optionally, when the temperature is raised to the carbonization temperature, the heating rate is 1-5 ℃/min, and 2 ℃/min, 3 ℃/min and 4 ℃/min can be selected. The carbonization temperature can be 650 ℃, 700 ℃, 750 ℃, 800 ℃ and 850 ℃ and the heat preservation time can be 2 hours, 3 hours and 4 hours.

The Bi/C composite material prepared by solvothermal reaction and carbothermic reduction is subjected to the Kendall effect in the carbonization process to obtain the composite sphere with Bi and C elements distributed in a concentration gradient manner, and the inner Bi nano particles are uniformly wrapped by the outer carbon layer without obvious aggregation. So the Bi/C composite material finally prepared is Bi/C composite balls with concentration gradient. The diameter of the composite sphere is about 500 nm-2 mu m, and the tap density can reach 3.5-6g cm ^-3 。

In the invention, the mixed solvent is selected from the mixed solvent of ethylene glycol and isopropanol, wherein the effect of the ethylene glycol is mainly to regulate and control sphericity, and the addition of the isopropanol can regulate the size and uniformity of spheres. The uniformity and growth rate of nucleation can be regulated and controlled by regulating the dosage of ethylene glycol and isopropanol, so that spherical particles with different particle sizes and regular degrees are obtained. In addition, the final prepared composite material can be greatly improved in tap density by adding isopropanol.

On the other hand, the invention also provides the Bi/C composite material with high tap density prepared by the preparation method of the Bi/C composite material with high tap density, and the application of the Bi/C composite material with high tap density as a negative electrode material of a sodium ion battery.

In a specific embodiment, the Bi/C composite material with high tap density is adopted as a negative electrode material of the sodium ion battery electrode, fully ground, uniformly mixed with acetylene black and sodium carboxymethylcellulose according to the mass ratio of 80:10:10, and vacuum-dried at 80 ℃ for 12 hours after film coating, so that the sodium ion battery electrode can be prepared.

In the above embodiment, the Bi/C composite material as the negative electrode material has an outer carbon layer capable of well accommodating the volume expansion of Bi particles, ensuring the stability of the electrode structure, and further improving the cycling stability of the electrode. In addition, the fine particles of the material can shorten the transmission paths of ions and electrons, so that the material has excellent rate capability.

Example 1

The Bi/C composite material with high tap density serving as the negative electrode material of the sodium ion battery is prepared by the following steps:

(1) 1.0mmol bismuth nitrate pentahydrate is added into a mixed solvent of 20mL Ethylene Glycol (EG) and 40mL isopropyl alcohol (IPA), and the mixture is dispersed by ultrasonic until the mixture is completely dissolved;

(2) Adding 0.432g of anhydrous glucose into the solution obtained in the step (1), placing the solution into a constant-temperature water bath kettle at 60 ℃, heating and stirring the solution at a stirring speed of 450r/min for 0.5h, and dissolving the solution into a clear and transparent solution;

(3) Placing the mixed solution prepared in the step (2) in a 100mL reaction kettle, heating to 150 ℃ from room temperature at a heating rate of 5 ℃/min, preserving heat for 16 hours, carrying out solvothermal reaction, naturally cooling to room temperature, collecting precipitate in the reaction kettle through suction filtration, washing with 3L of deionized water, and drying in a blast oven at 60 ℃ for 12 hours to finally obtain a beige powdery carbon-containing polysaccharide coated bismuth salt (Bi-salt@CP) precursor;

(4) Heating the precursor prepared in the step (3) to 800 ℃ at a speed of 5 ℃/min under the protection of argon atmosphere, preserving heat for 2 hours, and naturally cooling to room temperature after the heat preservation is finished, thereby finally preparing the Bi/C sodium ion battery anode material (CGD-Bi/C) with concentration gradient.

Example 2

Unlike example 1, bismuth nitrate pentahydrate in step (1) of this example was used in an amount of 1.5mmol, and the product obtained was designated CGD-Bi/C-2.

Example 3

Unlike example 1, bismuth nitrate pentahydrate in step (1) of this example was used in an amount of 2.0mmol, and the resultant product was designated CGD-Bi/C-3.

Example 4

Unlike example 1, the amount of anhydrous glucose in step (2) of this example was 0.864g, and the product obtained was designated CGD-Bi/C-4.

Example 5

Unlike example 1, the amount of anhydrous glucose in step (2) of this example was 1.296g, and the product obtained was designated CGD-Bi/C-5.

Example 6

Unlike example 1, the mixed solvent in step (1) of this example was obtained by mixing 5mL of ethylene glycol with 55mL of isopropyl alcohol.

Example 7

Unlike example 1, the mixed solvent in step (1) of this example was obtained by mixing 30mL of ethylene glycol with 30mL of isopropyl alcohol.

Comparative example 1

Unlike example 1, bismuth nitrate pentahydrate was added to 60mL of isopropanol solvent in step (1) of this comparative example, and no ethylene glycol was added.

Comparative example 2

Unlike example 1, bismuth nitrate pentahydrate was added to 60mL of ethylene glycol solvent in step (1) of this comparative example, and isopropyl alcohol was not added.

Test example 1

And carrying out electron microscope analysis on the Bi-salt@CP precursor prepared in the embodiment 1 and the carbonized CGD-Bi/C composite material, wherein the results are shown in fig. 1 and fig. 2 respectively. FIG. 1 is a scanning electron microscope image of a Bi-salt@CP precursor, according to which it can be seen that the particle size is large and uniformly distributed, and most of the particle size is in the range of 500-700 nm. FIG. 2a is a scanning electron microscope image of a CGD-Bi/C sample prepared in example 1, with spherical particles remaining intact after carbonization, with a corresponding average particle size of 688nm (FIG. 2 b); FIG. 2C is a transmission electron microscope image of a single sphere of a CGD-Bi/C sample, and it can be seen from FIG. 2C that the sphere is compact in structure and has almost no pores; from the elemental distribution of the focused ion beam microscope image of FIG. 2d, it can be seen that the CGD-Bi/C sample consists of three elements, C, bi, O, and that the C and Bi elements are distributed in a concentration gradient, which is caused by the Kendall effect during carbonization.

Test example 2

The CGD-Bi/C composite material prepared in example 1 was subjected to tap density measurement, resulting in 5.24g cm ^-3 . The cathode material of the sodium ion battery was used as a cathode material, and the electrochemical performance was tested, and the results are shown in fig. 3. Half cell with its working electrode assembled at 0.1A g ^-1 The initial coulombic efficiency was 70.6%, the charge-discharge curve in fig. 3a exhibited the principal characteristics of Bi, two pairs of alloyed and de-alloyed plateaus appeared in the range of 0.5-0.75V, and the charge-discharge curves at the 2 nd and 3 rd turns were substantially coincident, indicating that the electrode reaction was highly reversible. As shown in fig. 3b, at 5.0A g ^-1 The specific discharge capacity after 3500 long cycles is 325mAh g at the current density of (3) ^-1 The volume specific capacity is 1710mAh cm ^-3 The capacity retention rate is as high as 96%. As can be seen from FIG. 3C, the CGD-Bi/C composite material exhibits excellent rate performance at 0.2A g ^-1 Has 363mAh g at current density ^-1 Even at 20A g ^-1 And 50A g ^-1 333 and 291mAh g still remained at high current density ^-1 Is a stable discharge capacity. When the current density increases to 100A g ^-1 At the time of discharge, the specific discharge capacity was 213mAh g ^-1 The corresponding volume specific capacity is 1115mAh cm ^-3 This corresponds to about 5.4 seconds for one charge/discharge. This fully demonstrates that the material still has very stable cycle performance and excellent rate performance under the conditions of low porosity and high tap density.

Test example 3

The electrochemical performance of the samples prepared in example 1 was tested at different surface loads, and the results are shown in FIG. 4, when the surface load of the CGD-Bi/C composite material reached 8.52mg cm ^-2 The thickness of the electrode is only 32.9 mu m, and the compaction density of the corresponding pole piece is 2.59g cm ^-3 At 0.5mA cm ^-2 After 135 cycles of surface current, 836mAh cm still exists ^-3 High specific volumetric capacity of 2.75mAh cm ^-2 Is a surface capacity of the lens. In addition, the commercial hard carbon material (HC) was used as a reference at 7.06mg cm ^-2 The thickness of the pole piece is 111.4 mu m under the surface loading, and the CGD-Bi/C electrode material is 6.28mg cm ^-2 The electrode thickness was only 29.9 μm under the surface loading. The rate performance test of HC and CGD-Bi/C samples at different face currents showed that even at 5.0mA cm ^-2 At the surface current, the CGD-Bi/C composite material shows 341mAh g ^-1 High specific capacity of 724mAh cm ^-3 Volumetric specific capacity of 2.14mAh cm ^-2 Is a surface capacity of the lens.

Test example 4

Scanning electron microscopy tests on the sample prepared in example 2 (fig. 5a, 5 b) and the sample prepared in example 3 (fig. 5c, 5 d) show that the samples are spherical in shape. For both samples at 5.0A g ^-1 The cycle performance test was performed at current density, and the results are shown in fig. 5 e: after 3400 circles of circulation, the discharge specific capacities of the CGD-Bi/C-2 composite material and the CGD-Bi/C-3 composite material are 280 mAh g and 251mAh g respectively ^-1 。

Test example 5

Scanning electron microscopy tests on the samples prepared in example 4 (fig. 6a and 6 b) and the samples prepared in example 5 (fig. 6c and 6 d) show that the samples are spherical in shape. For both samples at 5.0A g ^-1 The cycle performance test was performed at current density, and the results are shown in fig. 6 e: after 3400 and 1700 cycles of the CGD-Bi/C-4 composite material and the CGD-Bi/C-5 composite material are circulated, the specific discharge capacities are 294 mAh g and 283mAh g respectively ^-1 。

Test example 6

The precursors prepared in examples 6 to 7 and comparative examples 1 to 2 were subjected to scanning electron microscopy, and the results are shown in FIGS. 7 to 10, respectively. The Bi/C composite materials prepared in examples 6 to 7 and comparative examples 1 to 2 were subjected to tap density tests, and the results are shown in fig. 11 and table 1:

table 1 tap density test results for some examples of the invention and comparative examples

Comparing the scanning electron microscope images of the embodiment of the invention with the comparative example, it can be found that when isopropanol is used as a solvent only, the precursor is formed by aggregation of irregular nano particles; when only ethylene glycol is used as a solvent, the precursor mainly consists of spheres with the particle size of about 900nm, and a plurality of nano-spheres are distributed around the precursor. In the invention, the function of the ethylene glycol is mainly to regulate and control the sphericity, and the addition of the isopropanol can regulate the size and uniformity of the sphere. The uniformity and growth rate of nucleation can be regulated and controlled by regulating the dosage of ethylene glycol and isopropanol, so that spherical particles with different particle sizes and regular degrees are obtained. In addition, when only ethylene glycol or isopropanol was used alone as the solvent, the tap densities of the prepared composite materials were 2.56 and 2.95g cm, respectively ^-3 The method comprises the steps of carrying out a first treatment on the surface of the When the invention uses two mixed solvents, the tap density of the material can be raised to 3.5-6.0g cm ^-3 Thereby increasing the volumetric energy density of the anode material.

In conclusion, the invention can not only improve the tap density of the Bi/C composite material, but also ensure the excellent dynamic behavior and the cycling stability of the Bi/C composite material, thereby fully playing the advantages of Bi per se in high volume energy density and solving the problem that the advantages of Bi-based anode material in volume energy density are difficult to play in the prior art. Compared with the prior art, the invention has obvious progress.

The present invention is not limited to the above-mentioned embodiments, but is intended to be limited to the following embodiments, and any modifications, equivalents and modifications can be made to the above-mentioned embodiments without departing from the scope of the invention.

Claims

1. The preparation method of the Bi/C composite material with high tap density is characterized by comprising the following steps of:

s1: adding bismuth nitrate or bismuth nitrate pentahydrate into a mixed solvent of ethylene glycol and isopropanol, and obtaining a mixed solution A after complete dissolution; in the mixed solvent, the volume ratio of the glycol to the isopropanol is 1:11-2:1;

2. The method for preparing a Bi/C composite material with high tap density according to claim 1, wherein the amount of bismuth nitrate or bismuth nitrate pentahydrate in step S1 is 4-16g/L.

3. The method for preparing a Bi/C composite material with high tap density according to claim 1, wherein the anhydrous glucose is used in an amount of 7.2 to 36g/L in step S2.

4. The method for preparing a Bi/C composite material with high tap density according to claim 1 or 3, wherein in step S2, when the anhydrous glucose is dissolved, the dissolution is performed by heating and stirring in a constant temperature water bath, the temperature of the constant temperature water bath is 60-85 ℃, the stirring speed is 200-500r/min, and the stirring time is 0.1-0.5h.

5. The method for preparing a Bi/C composite material having a high tap density according to claim 1, wherein the drying temperature is 50 to 90 ℃ and the drying time is 4 to 24 hours when the precipitate is dried in step S4.

6. The method for producing a Bi/C composite material having a high tap density according to claim 1, wherein in step S4, the inert atmosphere is argon or nitrogen.

7. The method for producing a Bi/C composite material having a high tap density according to claim 1 or 6, wherein in the step S4, carbonization is performed at 600 to 900 ℃ for 1 to 5 hours.

8. The high tap density Bi/C composite material prepared by the method for preparing a high tap density Bi/C composite material according to any one of claims 1 to 7.

9. The use of the Bi/C composite material of high tap density according to claim 8 as a negative electrode material for sodium ion batteries.