CN110797532B - Composite positive electrode material of lithium-sulfur battery and preparation method thereof - Google Patents

Abstract

The invention belongs to the technical field of lithium-sulfur batteries, and specifically relates to a composite positive electrode material for lithium-sulfur batteries and a preparation method thereof. In the present invention, the polyaniline layer has a protective effect on the composite cathode material, has higher conductivity and a more complete structure, and is conducive to realizing charge and discharge under large current density; the PANI conductive layer does not participate in electrochemical reactions, improving its The conductive properties can well adapt to the charge and discharge process under high current density. At the same time, the PANI layer reduces the loss of active materials during the charge and discharge process and maintains structural integrity; the PANI layer makes silicon-containing compounds and VS ₄ better It can adapt to the volume change caused by Li ⁺ intercalation and deintercalation, and improve the charge and discharge cycle stability of the composite cathode material; the large specific surface area of the graphene material can give it a larger contact area with the sulfur element, which is beneficial to improving the electron transmission rate and reaction area, while containing silicon compounds and VS ₄ can further improve the specific capacity of the composite cathode material.

Description

A kind of composite cathode material for lithium-sulfur battery and preparation method thereof

Technical field

The invention belongs to the technical field of lithium-sulfur batteries, and specifically relates to a composite positive electrode material for lithium-sulfur batteries and a preparation method thereof.

Background technique

Lithium-sulfur batteries have the advantages of large specific energy, high operating voltage, long cycle life, low self-discharge rate, no memory effect and environmental friendliness, and are widely used in the field of portable electronic devices. The materials used as cathode materials are also constantly expanding. As an important component of lithium-sulfur batteries, they have always restricted the large-scale promotion and application of lithium-sulfur batteries. When lithium-sulfur batteries use sulfur as the cathode material, the ion conductivity and electronic conductivity of sulfur are very low, resulting in poor electrochemical performance and low utilization of sulfur in the electrode. During the charge and discharge process, a large amount of sulfur is generated. Lithium sulfide will irreversibly dissolve in the electrolyte, and the dispersed sulfur particles will agglomerate. In addition, the conductive structure of the electrode will change during the charge and discharge process. These factors cause the cycle charge and discharge performance of the battery to decrease and the specific capacity to decrease.

Contents of the invention

In view of this, the object of the present invention is to provide a composite cathode material for lithium-sulfur batteries and a preparation method thereof. The lithium-sulfur battery composite cathode material provided by the invention has good cycle charge and discharge performance and high specific capacity.

In order to achieve the purpose of the above invention, the present invention provides the following technical solutions:

The invention provides a composite cathode material for lithium-sulfur batteries, which includes a lithium iron phosphate layer, a graphene wrapping layer and a polyaniline layer laminated in sequence. The graphene wrapping layer includes a silicon-containing compound and VS ₄ , and the silicon-containing compound compounds and VS ₄ supported on graphene.

Preferably, the mass content of the graphene wrapping layer is 15% to 20%.

Preferably, the mass content of the polyaniline layer is 5-10%.

Preferably, the silicon-containing compound includes one or more of Li ₂ SiO ₃ , Li ₄ SiO ₄ and silicon dioxide.

Preferably, the loading amount of the silicon-containing compound and VS ₄ on the graphene is 30 to 40 wt%.

Preferably, the mass ratio of the silicon-containing compound and VS ₄ is 1:1-5.

The present invention also provides a method for preparing the composite cathode material for lithium-sulfur batteries described in the above technical solution, which includes the following steps:

Provide lithium iron phosphate cathode materials;

Graphene oxide, water, Na ₃ VO ₄ and thioacetamide are mixed and then subjected to a hydrothermal reaction to obtain a hydrothermal product;

The lithium iron phosphate cathode material, silicon-containing compound solution and hydrothermal product are mixed and calcined to obtain a graphene-wrapped cathode material;

The graphene-wrapped cathode material is soaked in aniline to perform a polymerization reaction to obtain the lithium-sulfur battery composite cathode material.

Preferably, the temperature of the hydrothermal reaction is 160-180°C, and the time of the hydrothermal reaction is 20-24 hours.

Preferably, the calcination temperature is 400-600°C and the calcination time is 1-2 hours.

Preferably, the aniline is polymerized in the presence of ammonium persulfate, and the molar ratio of the aniline to ammonium persulfate is 1:1.

The invention provides a composite cathode material for lithium-sulfur batteries, which includes a lithium iron phosphate layer, a graphene wrapping layer and a polyaniline layer laminated in sequence. The graphene wrapping layer includes a silicon-containing compound and VS ₄ , and the silicon-containing compound compounds and VS ₄ supported on graphene. In the present invention, the core-shell structural material with PANI (polyaniline) as the shell has the advantages of simple structure, low cost, stable cycle, and high cycle capacity. The polyaniline layer has a protective effect on the composite cathode material. Compared with direct exposure to electrolysis Composite materials in liquid environment, composite cathode materials coated with polyaniline layer have higher conductivity and more complete structure, which is conducive to charging and discharging under large current density, making the development and application of lithium-sulfur batteries have greater potential. Potential; the PANI conductive layer does not participate in electrochemical reactions, and coating the surface of silicon-containing compounds improves its conductive properties and can well adapt to the charge and discharge process under high current density. At the same time, the PANI layer reduces the charge and discharge process of silicon-containing compounds. The loss of active materials during the discharge process, such as the shedding and dissolution of active materials during charge and discharge, thereby maintaining structural integrity; the PANI layer enables silicon-containing compounds and VS ₄ to better adapt to the volume changes caused by Li ⁺ intercalation and deintercalation, and The loss of active material on the current collector is reduced, thereby significantly improving the charge and discharge cycle stability of the cathode material; silicon-containing compounds and VS ₄ are loaded on graphene, and the large specific surface area of the graphene material can make it more competitive with sulfur elements. The large contact area is conducive to increasing the electron transmission rate and reaction area, thereby improving the conductivity and cycle performance of the sulfur elemental cathode material. At the same time, silicon-containing compounds and VS ₄ can further increase the specific capacity of the composite cathode material. The data in the examples show that the lithium-sulfur battery composite cathode material provided by the present invention has a first discharge specific capacity of 1782-1947 mAh/g at a charge and discharge current density of 100 mA/g, and the discharge specific capacity remains at 90-94% after 50 cycles. .

Detailed ways

The invention provides a composite cathode material for lithium-sulfur batteries, which includes a lithium iron phosphate layer, a graphene wrapping layer and a polyaniline layer laminated in sequence. The graphene wrapping layer includes a silicon-containing compound and VS ₄ , and the silicon-containing compound compounds and VS ₄ supported on graphene.

In the present invention, the mass content of the graphene wrapping layer is preferably 15% to 20%.

In the present invention, the mass content of the polyaniline layer is preferably 5 to 10%.

In the present invention, the silicon-containing compound preferably includes one or more of Li ₂ SiO ₃ , Li ₄ SiO ₄ and silicon dioxide.

In the present invention, the loading amount of the silicon-containing compound and VS ₄ on graphene is preferably 30 to 40 wt%.

In the present invention, the mass ratio of the silicon-containing compound and VS ₄ is preferably 1:1 to 5.

The present invention also provides a method for preparing the composite cathode material for lithium-sulfur batteries described in the above technical solution, which includes the following steps:

Provide lithium iron phosphate cathode materials;

Graphene oxide, water, Na ₃ VO ₄ and thioacetamide are mixed and then subjected to a hydrothermal reaction to obtain a hydrothermal product;

The lithium iron phosphate cathode material, silicon-containing compound solution and hydrothermal product are mixed and calcined to obtain a graphene-wrapped cathode material;

The graphene-wrapped cathode material is soaked in aniline to perform a polymerization reaction to obtain the lithium-sulfur battery composite cathode material.

The invention provides lithium iron phosphate cathode materials. In the present invention, the lithium iron phosphate cathode material is preferably spherical. In the present invention, the lithium iron phosphate cathode material is preferably pretreated before use. The pretreatment preferably includes screening and calcination. The present invention has no special limitations on the specific methods of screening and calcination. In the present invention In the embodiment, it is preferably sieved to obtain material particles with a particle size of 1 to 10 μm, and then calcined at 400 to 600°C to remove impurities.

In the present invention, graphene oxide, water, Na ₃ VO ₄ and thioacetamide are mixed and then subjected to a hydrothermal reaction to obtain a hydrothermal product.

In the present invention, the temperature of the hydrothermal reaction is preferably 160-180°C, and the time of the hydrothermal reaction is preferably 20-24 hours.

In the present invention, the molar ratio of Na ₃ VO ₄ and thioacetamide is preferably 1:5 to 1:6.

In the present invention, the mass ratio of graphene oxide to Na ₃ VO ₄ is preferably 1:27 to 1:28, and more preferably 1:27.5 to 1:28.

After the hydrothermal reaction is completed, the present invention preferably uses deionized water and ethanol to alternately wash the obtained hydrothermal reaction product, and then vacuum-dries it to obtain the hydrothermal product. In the present invention, the number of alternating cleanings is independently preferably 4 to 6 times. In the present invention, the vacuum drying temperature is preferably 60 to 80°C, more preferably 65 to 85°C, and the vacuum drying time is preferably 10 to 12 hours.

After obtaining the lithium iron phosphate cathode material and the hydrothermal product, the present invention mixes the lithium iron phosphate cathode material, the silicon-containing compound solution and the hydrothermal product for calcination to obtain the graphene-wrapped cathode material.

In the present invention, the calcination temperature is preferably 400-600°C, the time is preferably 1-2 h, and the temperature rise rate to the calcination temperature is preferably 5-10°C/min. In the present invention, the calcination is preferably carried out in air. In the present invention, the silicon-containing compound solution preferably includes one or more of Li ₂ SiO ₃ aqueous solution, Li ₄ SiO ₄ aqueous solution and anhydrous ethanol solution of ethyl orthosilicate. In the present invention, the concentration of the silicon-containing compound solution is preferably 0.01 to 1 mmol/L.

In the present invention, the mixing is preferably ultrasonic dispersion for 0.5 to 2 hours.

After obtaining the graphene-wrapped cathode material, the present invention soaks the graphene-wrapped cathode material in aniline to perform a polymerization reaction to obtain the lithium-sulfur battery composite cathode material.

In the present invention, the aniline is preferably polymerized in the presence of ammonium persulfate, and the molar ratio of the aniline to ammonium persulfate is preferably 1:1.

In the present invention, the ammonium persulfate is preferably added in the form of an HCl solution of ammonium persulfate, and the HCl solution of ammonium persulfate is preferably added dropwise, and the dropwise addition is performed in an ice-water bath and under stirring conditions. In the present invention, the dropwise addition is preferably dropwise addition.

In the present invention, the polymerization reaction is preferably carried out in an ice-water bath, and the polymerization reaction time is preferably 8 to 10 hours.

After the polymerization reaction is completed, the present invention preferably uses deionized water and ethanol to alternately clean the polymerization reaction product, and then vacuum-dries the product to obtain the lithium-sulfur battery composite cathode material.

In order to further illustrate the present invention, the lithium-sulfur battery composite cathode material and its preparation method provided by the present invention are described in detail below with reference to the examples, but they should not be understood as limiting the protection scope of the present invention. Obviously, the described embodiments are only some of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

Example 1

(1) After the Fe ₃ (PO ₄ ) ₂ powder and Li ₂ CO ₃ are thoroughly mixed in a stoichiometric ratio, they are calcined at 400°C for 10 hours under a nitrogen atmosphere to obtain LiFePO ₄ .

(2) Weigh the graphene oxide powder into 150 mL of deionized water, disperse it ultrasonically until the solution turns golden yellow, then add Na ₃ VO ₄ (2.0125g) and thioacetamide with a metering ratio of 1:5, and stir for 1 hour. . Transfer the above solution to a hydrothermal reaction kettle and react at 160°C for 24 hours. At the end of the reaction, wash with deionized water and ethanol alternately 4 to 6 times, and vacuum dry at 60°C for 12 hours to obtain a hydrothermal product, in which the loading amount of VS ₄ on graphene is 32wt%;

According to the Li ₄ SiO ₄ content accounting for 8 wt% of the graphene mass, weigh the Li ₄ SiO ₄ aqueous solution to obtain a 0.01 mmol/L Li ₄ SiO ₄ aqueous solution. Ultrasonically disperse LiFePO ₄ , hydrothermal product and Li ₄ SiO ₄ aqueous solution for 1 hour to obtain a uniformly dispersed suspension. Move it to a magnetic heating stirrer. Continue magnetic stirring and heating at 30°C until the material is dry. Move the dry material into a corundum boat. In the muffle furnace, the temperature is programmed to 600°C and kept for 4 hours to obtain a layer of graphene-coated LiFePO ₄ . The mass of graphene accounts for 20% of the total mass of the composite cathode material;

(3) According to the mass content of the polyaniline layer accounting for 10wt% of the total mass of the composite cathode material, weigh the aniline, weigh the ammonium persulfate according to the molar ratio of aniline to ammonium persulfate of 1:1, and combine the ammonium persulfate and ammonium persulfate. Mix the HCl solution to obtain a HCl solution of ammonium persulfate. Add the HCl solution of ammonium persulfate dropwise to aniline in an ice-water bath and stirring conditions. Soak _{Li4SiO4 -} coated _LiFePO4 in aniline for polymerization reaction. In 8 hours, the composite cathode material for lithium-sulfur battery was obtained.

The electrochemical properties of the lithium-sulfur battery composite cathode material prepared in this example were tested. The results are as follows: the first discharge specific capacity was 1947mAh/g at a charge and discharge current density of 100mA/g, and the discharge specific capacity remained at 1947mAh/g after 50 cycles. 94%.

Comparative example 1

The same as Example 1, the only difference is that the lithium-sulfur battery composite cathode material does not contain a polyaniline layer.

The electrochemical properties of the lithium-sulfur battery composite cathode material prepared in this comparative example were tested. The results are as follows: the first discharge specific capacity was 1647mAh/g at a charge and discharge current density of 100mA/g, and the discharge specific capacity remained at 1647mAh/g after 50 cycles. 70%.

Example 2

(1) After the Fe ₃ (PO ₄ ) ₂ powder and Li ₂ CO ₃ are thoroughly mixed in a stoichiometric ratio, they are calcined at 400°C for 10 hours under a nitrogen atmosphere to obtain LiFePO ₄ .

(2) Weigh the graphene oxide powder into 150 mL of deionized water, disperse it ultrasonically until the solution turns golden yellow, then add Na ₃ VO ₄ (2.0125g) and thioacetamide with a metering ratio of 1:5, and stir for 1 hour. . Transfer the above solution to a hydrothermal reaction kettle and react at 160°C for 24 hours. At the end of the reaction, wash with deionized water and ethanol alternately 4 to 6 times, and vacuum dry at 60°C for 12 hours to obtain a hydrothermal product, in which the loading amount of VS ₄ on graphene is 15wt%;

According to the Li ₄ SiO ₄ content accounting for 15 wt% of the graphene mass, weigh the Li ₄ SiO ₄ aqueous solution to obtain a 0.01 mmol/L Li ₄ SiO ₄ aqueous solution. Ultrasonically disperse LiFePO ₄ , hydrothermal product and Li ₄ SiO ₄ aqueous solution for 1 hour to obtain a uniformly dispersed suspension. Move it to a magnetic heating stirrer. Continue magnetic stirring and heating at 30°C until the material is dry. Move the dry material into a corundum boat. In the muffle furnace, the temperature is programmed to 600°C and kept for 4 hours to obtain a layer of graphene-coated LiFePO ₄ . The mass of graphene accounts for 15% of the total mass of the composite cathode material;

(3) According to the mass content of the polyaniline layer accounting for 5wt% of the total mass of the composite cathode material, weigh the aniline, weigh the ammonium persulfate according to the molar ratio of aniline to ammonium persulfate of 1:1, and combine the ammonium persulfate and ammonium persulfate. The HCl solution is mixed to obtain an HCl solution of ammonium persulfate. The HCl solution of ammonium persulfate is added dropwise to aniline in an ice water bath and stirring conditions. The graphene-coated LiFePO ₄ is soaked in aniline for polymerization reaction for 8 hours. A composite cathode material for lithium-sulfur batteries was obtained.

The electrochemical properties of the lithium-sulfur battery composite cathode material prepared in this example were tested, and the results are as follows: the first discharge specific capacity was 1782 mAh/g at a charge and discharge current density of 100 mA/g, and the discharge specific capacity remained at 1782 mAh/g after 50 cycles. 90%.

Comparative example 2

Same as Example 2, the only difference is that VS ₄ is not added.

The electrochemical performance of the lithium-sulfur battery composite cathode material prepared in this comparative example was tested. The results are as follows: the first discharge specific capacity is 1467mAh/g at a charge and discharge current density of 100mA/g, and the discharge specific capacity remains at 1467mAh/g after 50 cycles. 78%.

Example 3

(1) After the Fe ₃ (PO ₄ ) 2 powder and Li ₂ CO ₃ are thoroughly mixed in a stoichiometric ratio, they are calcined at 400°C for 10 hours under a nitrogen atmosphere to obtain LiFePO ₄ .

(2) Weigh the graphene oxide powder into 150 mL of deionized water, disperse it ultrasonically until the solution turns golden yellow, then add Na ₃ VO ₄ (2.0125g) and thioacetamide with a metering ratio of 1:5, and stir for 1 hour. . Transfer the above solution to a hydrothermal reaction kettle and react at 160°C for 24 hours. After the reaction is completed, wash with deionized water and ethanol alternately 4 to 6 times, and vacuum dry at 60°C for 12 hours to obtain a hydrothermal product, in which the loading amount of VS ₄ on graphene is 30wt%;

According to the Li ₄ SiO ₄ content accounting for 6 wt% of the graphene mass, weigh the Li ₄ SiO ₄ aqueous solution to obtain a 0.01 mmol/L Li ₄ SiO ₄ aqueous solution. Ultrasonically disperse LiFePO ₄ , hydrothermal product and Li ₄ SiO ₄ aqueous solution for 1 hour to obtain a uniformly dispersed suspension. Move it to a magnetic heating stirrer. Continue magnetic stirring and heating at 30°C until the material is dry. Move the dry material into a corundum boat. In the muffle furnace, the temperature is programmed to 600°C and kept for 4 hours to obtain a layer of graphene-coated LiFePO ₄ . The mass of graphene accounts for 18% of the total mass of the composite cathode material;

(3) According to the mass content of the polyaniline layer accounting for 6wt% of the total mass of the composite cathode material, weigh the aniline, weigh the ammonium persulfate according to the molar ratio of aniline to ammonium persulfate of 1:1, and combine the ammonium persulfate and ammonium persulfate. The HCl solution is mixed to obtain an HCl solution of ammonium persulfate. The HCl solution of ammonium persulfate is added dropwise to aniline in an ice water bath and stirring conditions. The graphene-coated LiFePO ₄ is soaked in aniline for polymerization reaction for 8 hours. A composite cathode material for lithium-sulfur batteries was obtained.

The electrochemical properties of the lithium-sulfur battery composite cathode material prepared in this example were tested, and the results are as follows: the first discharge specific capacity was 1872 mAh/g at a charge and discharge current density of 100 mA/g, and the discharge specific capacity remained at 1872 mAh/g after 50 cycles. 92%.

Comparative example 3

Same as Example 3, except that no silicon-containing compound is added.

The electrochemical properties of the lithium-sulfur battery composite cathode material prepared in this comparative example were tested. The results are as follows: the first discharge specific capacity was 1501mAh/g at a charge and discharge current density of 100mA/g, and the discharge specific capacity remained at 1501mAh/g after 50 cycles. 76%.

The above are only preferred embodiments of the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the principles of the present invention. These improvements and modifications can also be made. should be regarded as the protection scope of the present invention.

Claims (5)

1. A composite cathode material for lithium-sulfur batteries, characterized in that it includes a lithium iron phosphate layer, a graphene wrapping layer and a polyaniline layer laminated in sequence, the graphene wrapping layer includes a silicon-containing compound and VS ₄ , the Silicon-containing compounds and VS ₄ are supported on graphene; The mass content of the graphene wrapping layer is 20%; The mass content of the polyaniline layer is 10%; The silicon-containing compound is Li ₄ SiO ₄ , the mass ratio of Li ₄ SiO ₄ and VS ₄ is 1:4, and the loading amount of Li ₄ SiO ₄ and VS ₄ on the graphene is 40wt%. 2. The preparation method of composite cathode material for lithium-sulfur batteries according to claim 1, characterized in that it includes the following steps: Provide lithium iron phosphate cathode materials; Graphene oxide, water, Na ₃ VO ₄ and thioacetamide are mixed and then subjected to a hydrothermal reaction to obtain a hydrothermal product; The lithium iron phosphate cathode material, silicon-containing compound solution and hydrothermal product are mixed and calcined to obtain a graphene-wrapped cathode material; The graphene-wrapped cathode material is soaked in aniline to perform a polymerization reaction to obtain the lithium-sulfur battery composite cathode material. 3. The preparation method according to claim 2, characterized in that the temperature of the hydrothermal reaction is 160-180°C, and the time of the hydrothermal reaction is 20-24 hours. 4. The preparation method according to claim 2, characterized in that the calcination temperature is 400-600°C and the calcination time is 1-2 hours. 5. The preparation method according to claim 2, characterized in that the aniline undergoes a polymerization reaction in the presence of ammonium persulfate, and the molar ratio of the aniline to the ammonium persulfate is 1:1.