CN110407193B

CN110407193B - Negative electrode material, preparation method thereof and sodium ion battery containing negative electrode material

Info

Publication number: CN110407193B
Application number: CN201910766292.2A
Authority: CN
Inventors: 吴川; 王兆华; 吴锋; 白莹
Original assignee: Beijing Institute of Technology BIT
Current assignee: Beijing Institute of Technology BIT
Priority date: 2019-08-20
Filing date: 2019-08-20
Publication date: 2021-05-14
Anticipated expiration: 2039-08-20
Also published as: CN110407193A

Abstract

The invention discloses a negative electrode material, a preparation method thereof and a sodium ion battery containing the negative electrode material, wherein the preparation method comprises the following steps: (1) firstly, preparing inorganic salt water solution, then adding chitin or chitin derivative powder, slowly adding cosolvent, continuously stirring or ultrasonically treating to obtain uniform sol; (2) freezing the sol obtained in the step (1) and drying in vacuum to obtain a fluffy precursor; (3) and (3) performing high-temperature calcination on the precursor obtained in the step (2) in an inert atmosphere, and preparing the porous hard carbon material in situ by using the melting evaporation characteristic of inorganic salt in the carbonization process of the chitin or the chitin derivative. The negative electrode material has a cross-linked pore structure, a large specific surface area and rich nitrogen elements. The high electronic conductivity of the material is ensured by the rich nitrogen element, and meanwhile, the three-dimensional cross-linked network is beneficial to the rapid de-intercalation of sodium ions in the material, so that the high-capacity and high-rate charge and discharge of the battery are realized, and the cycle stability of the battery is improved.

Description

Negative electrode material, preparation method thereof and sodium ion battery containing negative electrode material

Technical Field

The invention relates to the field of battery materials, in particular to a negative electrode material, a preparation method of the negative electrode material and a sodium ion battery containing the negative electrode material.

Background

Traditional fossil energy such as coal, petroleum, natural gas and the like are gradually exhausted, the pollution to the environment is obvious, and the energy crisis and the environmental pollution problem become main problems facing human beings in the 21 st century. The vigorous development and efficient utilization of renewable clean energy sources such as wind energy, solar energy, tidal energy, nuclear energy, geothermal energy and the like are the most effective ways to solve the two problems. Secondary batteries serve as carriers for energy storage and conversion, bearing the hope of changing the energy structure and reducing environmental pollution.

The secondary battery can be used as energy storage and supply equipment to solve the problem well. Among them, the lithium ion battery has many advantages such as high energy density, long cycle life, little environmental damage, high average output voltage, large output power, small self-discharge, no memory effect, etc. However, the cost of the lithium ion battery is high due to the limitation of resources. Sodium is similar in chemical properties to lithium, and the sodium resource is nearly 400 times that of lithium, and the cost is only 1/80 times that of lithium. Meanwhile, sodium and lithium are positioned in the same main group, the potential of the sodium is-2.71V vs. SHE, the potential is very close to-3.04V of the lithium, and the chemical properties are similar, so that the possibility is provided for establishing a battery system.

In recent years, the positive electrode material has been studied more sufficiently in the electrode material of a sodium ion battery, and there have been various positive electrode materials having stable intercalation and deintercalation of sodium ions, such as a layered material, a polyanion compound, and a prussian blue analog. However, the negative electrode material is still subject to various restrictions in many respects. The current negative electrode materials mainly comprise carbon-based materials, metal oxides/sulfides, alloying materials, titanium-based materials and the like. Among them, the use of hard carbon materials as a sodium ion negative electrode material has a significant advantage in view of cost and cycle stability. Hard carbon materials are usually made from different carbon sources through high temperature carbonization, and thus the selection of the carbon source becomes an important component thereof. Chitin is one of the most abundant natural polymers in nature, and has the advantages of low price, easy acquisition, recoverability and the like. Therefore, the substance or the derivative thereof is selected as a carbon source, thereby expanding the advantage of low price of the sodium-ion battery.

Porous carbon materials are generally obtained by etching with a strong base in reaction with carbon at high temperature, such as the most commonly used pore-forming agent KOH. The method has wide application range, but has strong corrosivity, and a large amount of acid is needed in the method subsequently, so that the method causes great pollution to the environment. At present, a pore-forming material having environmental friendliness is also gradually attracting attention, for example, NaCl is used as a pore-forming agent because it is very soluble in water, but only the effect of physical occupation is utilized in the process, and NaCl prepared below the melting point has a dispersed pore structure and is all single macropore. In the subsequent process, a large amount of water is still needed for carrying out ultrasonic cleaning and other complex steps. Therefore, the method for preparing the cross-linked porous carbon negative electrode material by using the method is extremely simple, convenient and economical and has important significance.

Disclosure of Invention

The invention aims to overcome the defects of the existing pore-forming material and the preparation method thereof, and provides a cathode material which is very easy to prepare, a preparation method of the cathode material and a sodium-ion battery containing the cathode material. The negative electrode material has a three-dimensional cross-linked network structure, is large in specific surface area, is provided with the interconnected pores and contains abundant nitrogen elements, so that the rapid desorption of sodium ions in the material is facilitated, the high-capacity and high-rate charge and discharge of a battery are realized, and the cycling stability of the battery can be improved.

The inventor of the invention finds that the inorganic salt is dissolved in the deionized water to form a salt solution, then the chitin or chitin derivative powder is added into the salt solution, and the cosolvent is added after the mixture is uniformly stirred. Stirring was continued to give a translucent gel-like solution. And then transferring the solution to a low-temperature environment for freezing, thereby keeping the original shape of the material. And drying the water in the material on the premise of keeping the shape, and finally obtaining the fluffy block material. The material is calcined at high temperature under the protection of inert atmosphere. The three-dimensional porous network-shaped cathode material can be obtained without subsequent treatment. The material shows a blocky structure formed by a three-dimensional porous network by controlling the addition sequence of the solvent, the calcining temperature and the calcining time, and further controlling the melting and evaporation rate of the inorganic salt and other technologies. The special structure not only provides a quick channel for sodium ion transmission, but also can keep extremely stable performance in the circulating process of the whole block structure. And further, high-capacity and high-rate charge and discharge of the battery are realized, and the cycling stability of the battery is improved.

The invention provides a method for preparing a porous hard carbon negative electrode material in situ by melting and evaporating inorganic salt, wherein the method comprises the following steps:

(1) firstly, preparing inorganic salt water solution, then adding chitin or chitin derivative powder, slowly adding cosolvent, continuously stirring or ultrasonically treating to obtain uniform sol;

(2) freezing the sol obtained in the step (1) and drying in vacuum to obtain a fluffy precursor;

(3) and (3) performing high-temperature calcination on the precursor obtained in the step (2) in an inert atmosphere, and preparing the porous hard carbon material in situ by using the melting evaporation characteristic of inorganic salt in the carbonization process of the chitin or the chitin derivative.

In particular, the invention adopts chitin or chitin derivatives as carbon sources, is one of the most abundant natural polymers in nature, and has the advantages of low price, easy acquisition, recoverability and the like. The inventor also finds that the material has a natural rich nitrogen source, and the rich nitrogen element can ensure high electronic conductance of the material, thereby providing a foundation for high-capacity and high-rate charge and discharge of the battery.

Preferably, chitosan derivative chitosan is used as carbon source, the chitosan is obtained by removing acetyl from chitin through concentrated alkali treatment, and the chitin after removing acetyl becomes soluble chitosan_，And thus is well suited for uniform mixing with the inorganic salt solution of the present invention. More preferably, the viscosity of chitosan is selected to be at 100-200 mpa.s.

Preferably, in step (1), the inorganic salt is selected from NaCl, KCl, CaCl₂、MgCl₂The addition amount of the inorganic salt is 1:2-1:0.5 by mass according to the mass ratio of chitin or chitin derivatives. More preferably, the inorganic salt is NaCl.

Preferably, in step (1), the co-solvent is selected from H₂CO₃、CH₃COOH, HClO and cosolvent in 0.1-10 wt%. More preferably, the cosolvent is selected from CH₃COOH。

Preferably, in the step (2), the mixture is frozen by a refrigerator or quick frozen by liquid nitrogen, and the vacuum degree of vacuum drying is between 10 and 20Pa, and the drying time is between 60 and 72 hours.

Preferably, in the step (3), the inert atmosphere is nitrogen or argon or a mixed gas of the nitrogen and the argon, the calcination temperature is 1200-1400 ℃, and the calcination time is 2-4 h.

The invention provides a negative electrode material in a second aspect, wherein the negative electrode material is prepared by the method in the first aspect, the microstructure of the negative electrode material is a three-dimensional cross-linked network structure, the pore size range of the negative electrode material is wider and is represented as 1-200nm, and the overall structure size is 50-100 mu m.

The third aspect of the invention provides a sodium-ion battery, wherein the current collector of the sodium-ion battery is coated with the negative electrode material of the second aspect of the invention. The negative electrode material is mixed with conductive carbon black and a binder to form electrode slurry for coating, and finally, the electrode slurry, a diaphragm, electrolyte and a positive electrode are assembled into the sodium ion battery containing the conductive carbon black.

Compared with the prior art, the product and the method have at least the following advantages:

(1) the cathode material has a three-dimensional cross-linked network form, has a large specific surface, and the pores in the structure are mutually connected, so that the polarization phenomenon of an electrode in the electrochemical process is reduced, the contact between the electrode and electrolyte is increased, and the rapid diffusion of sodium ions in the electrode material is facilitated;

(2) the cathode material of the invention has the special performance of nano pores, and the embedding depth of sodium ions in the nano material is shallow, the diffusion path is short, thus being beneficial to the rapid de-embedding of the sodium ions in the material;

(3) the preparation method is extremely simple and low in cost, fully applies the characteristics of salt melting and evaporation in our life, and constructs the porous material in situ;

(4) the prepared negative electrode material can realize reversible charge and discharge, has good cycle performance, has advanced advantages in the current report of capacity, outstanding rate performance and stable large-current charge and discharge performance.

Additional features and advantages of the invention will be set forth in the detailed description which follows.

Drawings

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention and not to limit the invention. In the drawings:

fig. 1 is a scanning electron microscope image of the negative electrode material powder prepared in example 1.

Fig. 2 is a scanning electron microscope image of the negative electrode material powder prepared in example 2.

Fig. 3 is a scanning electron microscope image of the negative electrode material powder prepared in example 3.

Fig. 4 is a scanning electron microscope image of the negative electrode material powder prepared in example 4.

Fig. 5 is a scanning electron microscope image of the negative electrode material powder prepared in comparative example 1.

Fig. 6 is a scanning electron microscope image of the negative electrode material powder prepared in comparative example 2.

Fig. 7 is a scanning electron microscope image of the negative electrode material powder prepared in comparative example 3.

Fig. 8 is a scanning electron microscope image of the negative electrode material powder prepared in comparative example 4.

Fig. 9 is a scanning electron microscope image of the negative electrode material powder prepared in comparative example 5.

Fig. 10 is a scanning electron microscope image of the negative electrode material powder prepared in comparative example 6.

Fig. 11 is a scanning electron microscope image of the negative electrode material powder prepared in comparative example 7.

FIG. 12 shows the voltage at 0.01 to 3.0V and 1A g of the battery prepared in test example 3^-1Current density of (c) cycle performance plot of 300 cycles.

Detailed Description

The following describes in detail specific embodiments of the present invention. It should be understood that the detailed description and specific examples, while indicating the present invention, are given by way of illustration and explanation only, not limitation.

The endpoints of the ranges and any values disclosed herein are not limited to the precise range or value, and such ranges or values should be understood to encompass values close to those ranges or values. For ranges of values, between the endpoints of each of the ranges and the individual points, and between the individual points may be combined with each other to give one or more new ranges of values, and these ranges of values should be considered as specifically disclosed herein.

The method for preparing the cathode material in situ by melting and evaporating the inorganic salt comprises the following steps:

(1) dissolving sodium chloride in deionized water, adding chitosan into the solution, and dripping glacial acetic acid until the chitosan is completely dissolved to form transparent colloid.

In the present invention, the chitosan is selected to have a viscosity of 100-200 mpa.s.

In the invention, the mass concentration of the glacial acetic acid solvent in the system is 0.1-10%.

The concentration of the sodium chloride in the system is 1:2-1:0.5 by mass of chitosan.

The order of addition of the four substances has an important influence on the system.

Preferably, NaCl is firstly added into deionized water and stirred until the NaCl is completely dissolved, then chitosan powder is added, and after the solutions are uniformly mixed, glacial acetic acid solution is added dropwise and stirred continuously.

The stirring mode is strong magnetic stirring, and the stirring speed is 200-500 rpm.

Preferably, the stirring time is 12 to 24 hours.

(2) And (2) removing water from the transparent colloid obtained in the step (1) at high vacuum degree and ultralow temperature, and calcining the transparent colloid in a tubular furnace at 1200-1400 ℃ for 2-4 hours in an inert gas atmosphere.

In the invention, the temperature of the high vacuum ultralow temperature is-80 to-100 ℃, and the vacuum degree is maintained at

10-20 Pa. In the invention, the heating rate is 2-5 ℃/min.

(3) Grinding the carbonized material obtained in the step (2) into particles with the diameter of about 50 mu m, and using the particles as electrode materials.

The cathode material prepared by the method can be used for preparing electrode slurry, wherein the electrode slurry contains the cathode material. And coating the electrode slurry on a battery current collector to assemble a battery containing the negative electrode material.

In the present invention, the composition of the electrode paste may be determined in a manner conventional in the art, for example, the electrode paste may include: the negative electrode material, the conductive carbon black and the binder are provided. The electrode slurry can be obtained by mixing the above components.

In the present invention, the negative electrode material may be contained in an amount of 70 to 90 wt%, the conductive carbon black may be contained in an amount of 5 to 15 wt%, and the binder may be contained in an amount of 5 to 15 wt%, based on the weight of the electrode slurry; preferably, the negative electrode material may be contained in an amount of 75 to 85 wt%, the conductive carbon black may be contained in an amount of 7 to 13 wt%, and the binder may be contained in an amount of 7 to 13 wt%, based on the weight of the electrode paste.

In the present invention, both the conductive carbon black and the binder may be conductive carbon black and a binder conventionally used in the art for manufacturing a battery electrode paste. Wherein the binder may be, for example, 5 wt% polyvinylidene fluoride.

In the present invention, the separator of the sodium ion battery is not particularly limited, and may be a separator of a sodium ion battery that is conventional in the art, for example, a separator of Whatman GF/C.

In the present invention, the electrolyte may be an electrolyte conventional in the art, for example, NaPF₆A mixed solution of Ethylene Carbonate (EC) and diethyl carbonate (DEC), wherein EC: the volume ratio of DEC may be 1: 0.9-1.1, NaPF₆The concentration of (B) may be 0.8 to 1.1 mol/L.

The sodium ion battery provided by the invention has higher specific discharge capacity and capacity retention rate.

The present invention will be described in detail below by way of examples. In the following examples and comparative examples, scanning electron microscopy was performed using a scanning electron microscope of type QUANTA FEG250 (available from Kevlar (China) Co., Ltd.).

Examples 1 to 4 and comparative examples 1 to 7 are for explaining the anode material of the present invention.

Example 1

(1) According to the mass ratio of the materials, chitosan: sodium chloride ═ 1:1, weighing 2g of chitosan and 2g of sodium chloride respectively.

(2) 80mL of deionized water was added to the beaker, and weighed sodium chloride was added and stirred at room temperature until completely dissolved.

(3) Adding chitosan powder into sodium chloride solution, fully stirring, measuring 1mL of glacial acetic acid, dropwise adding into the solution, wherein the solution becomes very viscous, and enhancing magnetic stirring. Stirring was continued for 12 hours until the solution was translucent.

(4) Transferring the colloid obtained in the step (3) to a freezing environment for 12 hours, and then carrying out vacuum drying for 72 hours to obtain a precursor.

(5) The substance is placed in a porcelain boat, heated to 1200 ℃ under the argon atmosphere, and kept warm for 2 hours to obtain the carbonized material.

(6) And (5) fully grinding the carbonized material obtained in the step (5) in a mortar, wherein the mark is S1.

The powder of the negative electrode material obtained in example 1 was observed under a scanning electron microscope, and the scanning electron microscope (SEM image) is shown in fig. 1.

Example 2

(1) The procedure is as in example 1 except that the heat treatment time is 4 hours at 1200 c to give the corresponding negative powder material, designated S2.

Scanning electron microscopy analysis of example 2 is shown in fig. 2, wherein the material structure prepared under the conditions is similar to that of example 1, and has a porous network-like structure.

Example 3

(1) The procedure is as in example 1, except that the temperature of the heat treatment is 1400 ℃ and the duration is 2 hours. The corresponding negative powder material is obtained, labeled S3.

The negative electrode material powder obtained in example 3 was observed under a scanning electron microscope, and the scanning electron microscope image is shown in fig. 3.

Example 4

(1) The procedure is as in example 1, except that the temperature of the heat treatment is 1400 ℃ and the duration is 4 hours. The corresponding negative powder material is obtained, labeled S4.

The negative electrode material powder obtained in example 4 was observed under a scanning electron microscope, and the scanning electron microscope image is shown in fig. 4.

It can be observed from fig. 1-4 that under the conditions of the above examples, complete melt evaporation of sodium chloride occurs and the chitosan material is fully carbonized, creating a porous network structure that is rich and exhibits three-dimensional cross-linking.

Comparative example 1

(1) 2g of chitosan was weighed according to the mass of the substance.

(2) 80mL of deionized water was added to the beaker.

(3) Adding chitosan powder into deionized water solution, fully stirring, measuring 1mL of glacial acetic acid, dropwise adding into the solution, wherein the solution becomes very viscous in the process, and enhancing magnetic stirring. Stirring was continued for 12 hours until the solution was a clear gum.

(4) Transferring the solution obtained in the step (3) to a freezing environment for 12 hours, and then carrying out vacuum drying for 72 hours to obtain a precursor.

(5) The material was placed in a porcelain boat and heated to 800 ℃ under argon atmosphere and held for 2 hours to give a carbonized material, labeled D1.

The negative electrode material powder prepared in comparative example 1 was observed under a scanning electron microscope, and the Scanning Electron Microscope (SEM) image is shown in fig. 5. As can be seen from FIG. 5, the negative electrode material prepared in comparative example 1 has a solid micron sheet structure, the thickness of the sheet is 10-20 μm, and the surface is smooth and has no pore structure.

Comparative example 2

The procedure of comparative example 1 was followed, except that the heat treatment temperature was 1000 ℃ and the heat preservation was carried out for 2 hours, to obtain the corresponding negative electrode powder material, labeled D2.

Analysis of comparative examples 1 and 2 we predict that an increase in temperature will favor an increase in material order. The scanning electron micrograph of comparative example 2 is shown in FIG. 6.

Comparative example 3

The procedure of comparative example 1 was followed, except that the temperature of the heat treatment was 1200 ℃ and the heat was maintained for 2 hours. The corresponding negative powder material was obtained, labeled D3.

Comparative example 3 is in regular agreement with comparative example 1 and comparative example 2, the material interlamellar spacing gradually increasing with increasing temperature. The negative electrode material powder obtained in comparative example 3 was observed under a scanning electron microscope, and the scanning electron microscope image is shown in fig. 7.

Comparative example 4

The procedure of comparative example 1 was followed, except that the heat treatment temperature was 1400 ℃ and the temperature was maintained for 2 hours. The final negative electrode material powder was obtained and was designated as D4.

The interlayer spacing in the comparative D1/D2/D3 material gradually increased. The temperature therefore has a great influence on the degree of graphitization and also on the layer spacing.

The negative electrode material powder prepared in comparative example 4 was observed under a scanning electron microscope, and the scanning electron microscope image (SEM image) is shown in fig. 8.

As can be seen from fig. 5-8, the materials treated at the four temperatures have similar overall structures and all have thick sheet-like compositions. In the absence of sodium chloride, no pores appear on the surface and in the interior of the structure.

Comparative example 5

The procedure is as in example 1, except that the temperature of the heat treatment is 1400 ℃ and the duration is 0.5 hour. The corresponding negative powder material was obtained, labeled D5.

The negative electrode material powder obtained in comparative example 5 was observed under a scanning electron microscope, and the scanning electron microscope image is shown in fig. 9.

Comparative example 6

The procedure is as in example 1, except that the temperature of the heat treatment is 1400 ℃ and the duration is 1 hour. The corresponding negative powder material was obtained, labeled D6.

The negative electrode material powder obtained in comparative example 6 was observed under a scanning electron microscope, and the scanning electron microscope image is shown in fig. 10.

Comparative example 7

(1) According to the mass ratio of the materials, chitosan: potassium hydroxide 1:1, weighing 2g of chitosan and potassium hydroxide respectively.

(2) Adding 80mL of deionized water into a beaker, adding chitosan powder into the deionized water, fully stirring, measuring 1mL of glacial acetic acid, dropwise adding the glacial acetic acid into the solution, wherein the solution becomes very viscous in the process, and enhancing magnetic stirring. Stirring is continued for 12 hours until the colloid is transparent.

(3) Adding the weighed potassium hydroxide into the mixture obtained in the step (2), and uniformly stirring at room temperature.

(5) The substance is placed in a porcelain boat, heated to 1400 ℃ under the argon atmosphere, and kept warm for 2 hours to obtain the carbonized material.

(6) And (5) putting the carbonized material obtained in the step (5) into a beaker, adding hydrochloric acid and deionized water respectively, fully stirring and carrying out ultrasonic treatment, and obtaining the material by a high-speed centrifugation method. Oven drying is then carried out to obtain the desired material, labeled D7.

The negative electrode material powder prepared in comparative example 7 was observed under a scanning electron microscope, and the Scanning Electron Microscope (SEM) image is shown in fig. 11. As can be seen from fig. 11, the electrode material obtained by the conventional alkalization pore-forming method has a significantly different pore-forming mechanism from that of the molten salt in the present invention. The pores of the material prepared by the conventional method are shown as holes corroded by the chemical reaction, and the surface and the interior of the whole material are not obviously changed. However, when the pore is formed by using sodium chloride, the structure of the whole material is obviously and uniformly changed, and the internal structure of the material and the external part of the material are in a three-dimensional cross-linked network shape. Therefore, the negative electrode material obtained by molten salt treatment has a larger specific surface and better ion diffusion performance in terms of material treatment mode.

Test example 1

(1) Mixing the negative electrode material S1 obtained in example 1 with conductive carbon black (trade name SP, manufactured by TIMCAL, the same below) and polyvinylidene fluoride PVDF (5 wt% mixed solution, trade name US Suwei 1015, manufactured by gold Taoyi plastics raw material company, manufactured by Dongguan, the same below) according to the weight ratio of 8:1:1 to obtain electrode slurry;

(2) coating the electrode slurry obtained in the step (1) on copper foil to prepare a negative plate, taking a sodium plate as a negative electrode, taking Whatman GF/C (Whatman company of manufacturers) as a diaphragm, and using NaPF as electrolyte₆Mixed solution of + Ethylene Carbonate (EC) + diethyl carbonate (DEC) (wherein EC: DEC is 1:1 by volume, NaPF₆At a concentration of 1mol/L)And assembling the button cell into a CR2025 experimental button cell in a glove box filled with argon atmosphere.

Test examples 2 to 4

A battery was assembled in accordance with the method of test example 1, except that the negative electrode materials S2 to S4 obtained in examples 2 to 4 were used, respectively.

Comparative test examples 1 to 7

A battery was assembled in the same manner as in test example 1, except that the negative electrode materials D1 to D7 obtained in comparative examples 1 to 7 were used, respectively.

Test example

(1) After the sodium ion batteries obtained in test examples 1 to 4 and comparative test examples 1 to 7 were allowed to stand for 12 hours, the batteries were subjected to 50 cycles of charge and discharge on a LAND CT-2001A tester (blue electronics, Inc., Wuhan City): at a voltage of 0.01-3.0V and 20mA g^-1At the current density of (3), the battery is charged and discharged for 50 times; detecting first charge specific capacity (mAh g)^-1) And specific charging capacity (mAh g) after 50 cycles of charging and discharging^-1) The capacity retention ratio after 50 cycles of charge and discharge (charge specific capacity after 30 cycles of charge and discharge ÷ first charge specific capacity × 100%) was calculated, and the results are shown in table 1.

(2) Taking test example 3 as an example, the prepared sodium ion battery is subjected to the voltage of 0.01-3.0V and the voltage of 1A g^-1The cycle performance of 300 cycles at the current density of (A) is recorded in FIG. 12, and it can be seen from FIG. 12 that the voltage is 0.01-3.0V and 1A g^-1Under the current density of the battery, the charging and discharging specific capacity of the battery can still be kept in a very high and very stable state, and the capacity retention rate reaches 85 percent.

TABLE 1

As can be seen from table 1, the sodium ion battery obtained according to test example 3 of the present invention has a first charge specific capacity, a discharge specific capacity after 50 cycles, and a capacity retention rate significantly higher than those of the comparative examples.

It can be seen from comparative test example 1, test example 2, test example 3, and test example 4 that the materials treated under the optimum conditions all exhibited excellent electrochemical performance.

As can be seen from comparative test example 1, comparative test example 2, comparative test example 3 and comparative test example 4, the chitosan materials treated at different temperatures are very similar in appearance and show a solid sheet-like structure with a smooth surface on a microscopic scale. However, from the electrochemical data, the charging specific capacity gradually increases along with the increase of the temperature in the charging and discharging process. Therefore, the order degree of the material is continuously improved along with the temperature rise, and the defects in the material are gradually reduced. Therefore, 1400 ℃ is selected for modification treatment in subsequent treatment.

By comparing the test example 3, the comparative test example 5 and the comparative test example 6, it can be seen that the pores inside the material gradually decrease and the pore diameter becomes larger and irregular in the process of gradually decreasing the holding time from 2 hours to 1 hour. Therefore, the specific surface area of each of comparative examples 5 and 6 is small during the electrochemical reaction, and the penetration of the inner pore diameter is poor, thereby being disadvantageous to the diffusion of ions.

By comparing test example 3 with comparative test example 7, it can be seen that when different salts are selected for treatment, the materials exhibit a completely different pore pattern. The obtained product by simple sodium chloride treatment has uniform aperture, large specific surface area and three-dimensional connection mode of internal pores. Whereas the pores of the material obtained by the conventional potassium hydroxide treatment appear in the form of pores that leave traces after etching. A penetrating network structure is not formed around the holes, and the whole structure still presents a solid block structure, so that the specific surface of the structure is small, the infiltration of electrolyte is not facilitated, and the number of active sites is small. Compared with a simple sodium chloride treatment mode, the method has no strong corrosivity and is not beneficial to storing sodium in the material.

The preferred embodiments of the present invention have been described in detail, however, the present invention is not limited to the specific details of the above embodiments, and various simple modifications may be made to the technical solution of the present invention within the technical idea of the present invention, and these simple modifications are within the protective scope of the present invention. It should be noted that the various technical features described in the above embodiments can be combined in any suitable manner without contradiction, and the invention is not described in any way for the possible combinations in order to avoid unnecessary repetition. In addition, any combination of the various embodiments of the present invention is also possible, and the same should be considered as the disclosure of the present invention as long as it does not depart from the spirit of the present invention.

Claims

1. A method for preparing a porous hard carbon negative electrode material in situ by melting and evaporating inorganic salt is characterized by comprising the following steps:

(1) firstly, preparing NaCl aqueous solution, then adding chitin or chitin derivative powder, slowly adding cosolvent, continuously stirring or ultrasonically treating to obtain uniform sol; the adding amount of NaCl takes chitin or chitin derivatives as raw materials according to the mass ratio of 1:2-1: 0.5;

(2) freezing the sol obtained in the step (1) and drying in vacuum to obtain a fluffy precursor; freezing with refrigerator or quick freezing with liquid nitrogen, vacuum drying at vacuum degree of 10-20Pa for 60-72 hr;

(3) calcining the precursor obtained in the step (2) at high temperature in an inert atmosphere of nitrogen or argon or a mixed gas of the nitrogen and the argon at the temperature of 1200-1400 ℃ for 2-4h, and preparing the porous hard carbon material in situ by using the melting evaporation characteristic of inorganic salt in the carbonization process of the chitin or the chitin derivative;

(4) and (4) grinding the porous hard carbon material obtained in the step (3) into particles of 50-100 mu m, and using the particles as electrode materials.

2. The process of claim 1, wherein in step (1), the co-solvent is selected from H₂CO₃、CH₃COOH, HClO and cosolvent in 0.1-10 wt%.

3. A negative electrode material, characterized in that it is prepared by the method of any one of claims 1-2, and its micro-morphology is a three-dimensional cross-linked network structure.

4. A sodium ion battery, characterized in that the current collector of the sodium ion battery is coated with the negative electrode material according to claim 3.