CN105406034A - Three-dimensional porous graphene-supported carbon-coated lithium sulfide cathode material as well as preparation method and application thereof - Google Patents

Abstract

The invention discloses a three-dimensional porous graphene-loaded carbon-coated lithium sulfide positive electrode material and its preparation method and application. The preparation method comprises: dispersing graphene oxide in water with magnetic stirring, then adding a reducing agent and stirring to dissolve to obtain brown solution; place the solution at 120°C-200°C for hydrothermal reaction for 4-12h to obtain a solution of columnar three-dimensional porous graphene; add lithium sulfate and carbon source to the solution of columnar three-dimensional porous graphene to form columnar three-dimensional porous graphene The soaking solution; the soaking solution is freeze-dried to obtain a precursor; the precursor is calcined at 800-1000° C. for 2-12 hours under a protective atmosphere to obtain a three-dimensional porous graphene-supported carbon-coated lithium sulfide material. The material prepared by the invention can be directly sliced to prepare the positive electrode of the battery, the steps of slurry preparation, coating and drying are omitted, the process is simpler, the method is suitable for large-scale production, and has excellent electrical properties.

Description

A three-dimensional porous graphene-supported carbon-coated lithium sulfide positive electrode material and its preparation method and application

technical field

The invention relates to the field of lithium-sulfur battery materials, in particular to a three-dimensional porous graphene-supported carbon-coated lithium sulfide positive electrode material and a preparation method and application thereof.

Background technique

Environmental issues and energy crisis are two major challenges facing human society today, and the development of clean and renewable new energy has become an urgent need of today's society. Lithium-ion secondary batteries that are currently widely used with embedded lithium-containing transition metal oxide-based (lithium manganese oxide, lithium cobalt oxide, ternary, lithium iron phosphate, layered lithium-rich lithium manganese oxide) materials as positive electrodes, due to their The limitation of theoretical capacity (the energy density of the current material system is difficult to break through the energy density bottleneck of 250Wh/kg), has been unable to meet the current requirements for higher energy density power supplies. Therefore, the research and development of new energy technologies for efficient energy conversion and storage has become a major demand for national energy development strategies.

Lithium-sulfur batteries have become a research hotspot for next-generation high-energy-density secondary batteries due to their high theoretical capacity. Compared with the traditional lithium-ion battery, the theoretical specific capacity of the lithium-sulfur battery reaches 1675mAh/g, and the specific energy reaches 2600wh/Kg. It also has the advantages of abundant storage, low price and environmental friendliness, which meets the requirements of electric vehicles for power batteries. One of the most promising cathode materials for lithium-ion batteries.

However, there are still some problems to be solved when sulfur is put into practical application as a positive electrode material. First, elemental sulfur is an insulator, and the conductivity of sulfur at room temperature is only 5*10 ^-30 S/cm, which leads to low utilization of active materials; Secondly, lithium-sulfur batteries have a shuttle effect during charging and discharging, and various intermediate products lithium polysulfide (Li ₂ S _n , 2<n≤8) produced during charging and discharging are easily soluble in the electrolyte and will pass through the separator. Migrating to the negative electrode, reacting with the lithium negative electrode leads to the consumption of active materials, and even forms insoluble Li ₂ S ₂ or Li ₂ S, which is deposited on the surface of the Li electrode, seriously affecting the performance of the electrode; in addition, when S ₈ is discharged to generate Li ₂ S , due to the difference in density between the two, it will be accompanied by a volume expansion of about 76%, which will easily destroy the structural stability of the porous conductive network and reduce the cycle stability of the electrode material; more importantly, the lithium-sulfur battery uses metal lithium as the negative electrode. During the charge and discharge process, metal lithium is prone to produce dendrites, which can pierce the separator and cause a short circuit to cause an explosion, which poses a safety problem.

Lithium sulfide can use non-lithium materials such as silicon (Si) and tin (Sn) as the negative electrode, which is safer; at the same time, when Li ₂ S is charged to generate S, the volume is reduced and the conductive network remains intact; and Li ₂ S is used as the positive electrode material , and its specific capacity is as high as 1166mAh/g, so it has attracted extensive research. However, Li ₂ S also faces some problems as a positive electrode material. Li ₂ S is also electrically insulating, and it also faces the dissolution of polysulfides and the shuttle effect during the battery cycle. At present, the most common research on the modification of Li ₂ S materials is to use ball milling method to disperse Li ₂ S in the conductive network to reduce the particle size of Li ₂ S, and at the same time, the conductive network is modified on the surface of Li ₂ S particles, thereby improving the electrochemical performance. performance. Although the above methods can improve the electrochemical performance of lithium-sulfur batteries to a certain extent, the electrochemical performance such as discharge specific capacity and cycle performance is still far from commercialization and needs to be improved. Moreover, lithium sulfide is easy to react with water, and the direct modification of lithium sulfide needs to be carried out under a protective atmosphere, which is cumbersome to operate.

Contents of the invention

The invention provides a three-dimensional porous graphene-supported carbon-coated lithium sulfide positive electrode material and its preparation method and application. The precursor of three-dimensional porous graphene-supported carbon-coated lithium sulfate is prepared by a wet chemical method, and then synthesized in situ at high temperature The three-dimensional porous graphene-loaded carbon-coated lithium sulfide cathode material is directly sliced as the cathode of the battery, eliminating the steps of slurry preparation, coating, and drying, and reducing the process flow.

A preparation method for in-situ synthesis of three-dimensional porous graphene-loaded carbon-coated lithium sulfide cathode material, comprising the following steps:

(1) Dispersing graphene oxide in water with magnetic stirring, and ultrasonically forming a uniform and stable solution to obtain a graphene oxide solution, then adding a reducing agent and stirring to dissolve to obtain a brown solution;

(2) Transfer the brown solution obtained in step (1) to an autoclave, seal the autoclave and place it at 120°C-200°C for hydrothermal reaction for 4-12h, then lower the temperature to obtain a columnar three-dimensional porous graphene solution ;

(3) adding lithium sulfate and carbon source to the solution of the columnar three-dimensional porous graphene obtained in step (2), soaking the columnar three-dimensional porous graphene after stirring and dissolving, forming the soaking solution of columnar three-dimensional porous graphene;

(4) freeze-drying the soaking solution of columnar three-dimensional porous graphene to obtain a precursor;

(5) Calcining the precursor obtained in step (4) at 800-1000° C. for 2-12 hours under a protective atmosphere to obtain a three-dimensional porous graphene-supported carbon-coated lithium sulfide material.

In step (1), as a preference, the graphene oxide concentration in the graphene oxide solution is 1-5 mg/mL, and the reducing agent is one or two or more of thiourea, citric acid, vitamins, etc. , the concentration of the reducing agent in the brown solution is 1-20 mg/mL, more preferably, 1-10 mg/mL.

In step (2), the temperature for forming columnar three-dimensional porous graphene by hydrothermal reaction is 120°C-200°C, and the hydrothermal time is 4-12h. Preferably, the hydrothermal reaction is carried out at 170°C-190°C for 8-12h. More preferably, the hydrothermal reaction is carried out at 180° C. for 10 h.

In step (3), preferably, the lithium sulfate is Li ₂ SO ₄ or Li ₂ SO ₄ ·H ₂ O, and the carbon source is glucose, sucrose, maltose, soluble starch, polyvinylpyrrolidone, etc. One or more than two kinds of carbon compounds, the concentration of lithium sulfate in the soaking solution of the columnar three-dimensional porous graphene is 50-200mg/mL (more preferably 50-100mg/mL), and the concentration of carbon source is 50-200mg/mL. 600mg/mL (further preferably 100-400mg/mL).

In step (4), preferably, the freezing method in the freeze-drying is liquid nitrogen freezing or refrigerator freezing, and the drying method in the freeze-drying is vacuum freeze-drying.

In step (5), under the protection of nitrogen or argon, the calcination temperature is 800-1000°C, and the calcination time is 2-12h. Preferably, it is calcined at 800-1000° C. for 2-6 hours under a protective atmosphere. More preferably, it is calcined at 900° C. for 2 hours under a protective atmosphere.

The three-dimensional porous graphene-supported carbon-coated lithium sulfide positive electrode material prepared by the preparation method, in the three-dimensional porous graphene carbon-coated lithium sulfide positive electrode material, the lithium sulfide content (mass percentage) is 30%-75%, Graphene and carbon content (mass percentage) are between 25% and 70%. The three-dimensional porous graphene-supported carbon-coated lithium sulfide cathode material can be directly cut into slices as the battery cathode and assembled into the battery.

The preparation of the positive electrode of the battery is to slice the three-dimensional porous graphene-loaded carbon-coated lithium sulfide positive electrode material. (LiTFSI) is the solute, the solvent is 1,3-dioxolane (DOL) and ethylene glycol dimethyl ether (DME) with a volume ratio of 1:1, and 1.0-5.0wt% LiNO ₃ is added, and the negative electrode is used Lithium, silicon, graphite, tin, etc. are the most important, and the battery assembly process is all completed in a glove box filled with argon and with a water and oxygen content of less than 0.1ppm.

After the assembled lithium-ion battery is placed for 24 hours, a constant current charge-discharge test is carried out. The first charge-discharge voltage is 1.5V-4V, and the subsequent charge-discharge voltage is 1.5V-3V. The positive electrode of the lithium-ion battery is cyclically measured in an environment of 25±2°C. Capacity, charge-discharge cycle performance and rate characteristics.

Compared with prior art, the present invention has following advantage:

The invention adopts graphene, lithium sulfate and carbon source as raw materials to generate three-dimensional porous graphene-loaded carbon-coated lithium sulfide positive electrode material at high temperature. Compared with the direct use of commercial lithium sulfide, the cost is low, and the material is directly sliced into a battery positive electrode. , the steps of slurry preparation, coating and drying are omitted, the preparation process is simple, and it is suitable for large-scale production. Three-dimensional porous graphene provides reaction sites for the reaction between lithium sulfate and carbon source. At the same time, three-dimensional porous graphene acts as a conductive network to improve the conductivity of the material. The in-situ generation of carbon-coated lithium sulfide structure can improve the conductivity of lithium sulfide electrodes. At the same time, it can reduce the dissolution of polysulfides during the electrochemical reaction, inhibit the shuttle effect, and maintain the stability of the coating structure, thereby improving the battery capacity and cycle life. The in-situ synthesis of the three-dimensional porous graphene-loaded carbon-coated lithium sulfide cathode material is suitable for high-energy-density energy storage devices.

Description of drawings

1 is a schematic diagram of the process of in-situ synthesis of three-dimensional porous graphene-loaded carbon-coated lithium sulfide cathode material in Example 1;

Fig. 2 is the XRD spectrum of the three-dimensional porous graphene-loaded carbon-coated lithium sulfide cathode material prepared in Example 1;

Fig. 3 is the SEM photograph of different magnifications of the in-situ synthesis of three-dimensional porous graphene-loaded carbon-coated lithium sulfide cathode material in Example 1, wherein (a) in Fig. 3 is the SEM morphology at low magnification, and (b) in Fig. 3 ) is the SEM morphology under high magnification;

Fig. 4 is the cyclic voltammetry curve of the battery prepared by adopting the three-dimensional porous graphene-loaded carbon-coated lithium sulfide cathode material of Example 1;

Figure 5 is the cycle performance of the battery prepared by using the three-dimensional porous graphene-supported carbon-coated lithium sulfide cathode material of Example 1 at 1C.

detailed description

The present invention will be described in detail below in conjunction with the embodiments and drawings, but the present invention is not limited thereto.

Example 1

(1) Disperse graphene oxide in 140g deionized water with magnetic stirring, and ultrasonically 1h to form a uniform and stable solution, the concentration of graphene oxide in the graphene oxide solution is 2mg/ml, then add reducing agent thiourea and stir to dissolve, Obtain a brown solution, the concentration of reducing agent thiourea in the brown solution is 1.5mg/mL;

(2) Transfer the brown solution obtained above to an autoclave, seal the autoclave and place it at 180° C. for 10 hours, then wait for the temperature of the autoclave to drop to room temperature 25° C. to obtain a columnar three-dimensional porous graphene solution ;

(3) add lithium sulfate monohydrate and glucose in the solution of the columnar three-dimensional porous graphene of gained, after stirring and dissolving, columnar three-dimensional porous graphene is soaked wherein, form the soaking liquid of columnar three-dimensional porous graphene, the columnar three-dimensional porous graphene The concentration of lithium sulfate monohydrate in the soaking solution is 70mg/mL, and the concentration of carbon source glucose is 280mg/mL;

(4) freezing the fully soaked columnar three-dimensional porous graphene with liquid nitrogen, and then vacuum freeze-drying to obtain a precursor;

(5) Calcining the precursor at 900°C for 2 hours under an argon protective atmosphere to obtain a three-dimensional porous graphene-supported carbon-coated lithium sulfide material;

(6) The three-dimensional porous graphene-loaded carbon-coated lithium sulfide material slice is used as the positive electrode of the battery, and the metal lithium sheet is used as the negative electrode of the battery to assemble a lithium-ion battery. What the separator uses is a polypropylene microporous membrane (Cellgard2300), and the electrolyte is With 1mol/L bis(trifluoromethane)sulfonamide lithium salt (LiTFSI) as the solute, 1,3-dioxolane (DOL) and ethylene glycol dimethyl ether (DME) at a volume ratio of 1:1 as the solvent , and adding 1.0wt% LiNO ₃ , the battery assembly process was all completed in a glove box filled with argon and with a water and oxygen content below 0.1ppm.

The process of in-situ synthesis of three-dimensional graphene-supported carbon-coated lithium sulfide cathode material and its battery is shown in Figure 1.

In-situ synthesis of three-dimensional graphene-supported carbon-coated lithium sulfide positive electrode material is tested by X-ray photoelectron spectroscopy (X-rayphotoelectronspectroscopy), as shown in Figure 2, for the preparation of in-situ synthesized three-dimensional porous graphene-loaded carbon-coated lithium sulfide for this example XRD patterns of cathode materials. According to Fig. 2, it can be known that the in situ synthesized in situ synthesized three-dimensional porous graphene-supported carbon-coated lithium sulfide material prepared in this example generates a standard phase of lithium sulfide (JCPDS Card No. 04-0664). The SEM pictures of the obtained final product are shown in Figures 3a and 3b. Figure 3a is the morphology of the product at a low magnification. It can be seen from the figure that the thickness of the three-dimensional porous graphene is increased compared with that of the simple three-dimensional porous graphene. , Figure 3b is the SEM morphology of the sample under high magnification. It can be seen that there are some particles loaded on the graphene, which are carbon-coated lithium sulfide, indicating that the thickening of graphene is caused by the carbon-coated lithium sulfide. According to the thermogravimetry, it can be calculated that the content of lithium sulfide is about 40%, and the content of graphene and carbon is 60%.

After the assembled lithium-ion battery is placed for 24 hours, a constant current charge-discharge test is carried out. The first charge-discharge voltage is 1.5V-4V, and the subsequent charge-discharge voltage is 1.5V-3V. The positive electrode of the lithium-ion battery is cyclically measured in an environment of 25±2°C. Capacity, charge-discharge cycle performance and rate characteristics.

After being assembled into a lithium-ion battery, various electrochemical performance tests were performed. Figure 4 is a cyclic voltammogram, the first charge, first charge to 4V, this is due to the need to overcome the potential barrier in the process of converting lithium sulfide into polysulfide, in the subsequent process, the charge and discharge voltage is 1.5V to 3V , is a typical lithium-sulfur battery charge-discharge curve. During charging, an oxidation peak appears near 2.4V, corresponding to the oxidation process of Li ₂ S ₂ or Li ₂ S to polysulfide Li ₂ S _n . During discharging, there are two peaks at 2.3V and 2.0V. two reduction peaks, corresponding to the conversion of sulfur to long-chain Li ₂ S _n (4≤n≤8) and the reduction from Li ₂ S _n (4≤n≤8) to short-chain Li ₂ S ₂ or Li ₂ S process. Figure 5 shows the cycle performance diagram of the battery prepared in this example at 1C. The first cycle is the activation process of charging to 4V. The first discharge capacity of the battery is 510mAh/g at a current density of 1C. After 100 After the second cycle, the discharge capacity was maintained at 414mAh/g, and the retention rate was 81%, which showed good cycle performance, and the Coulombic efficiency remained above 95%.

Example 2

(1) Disperse graphene oxide in 140g deionized water with magnetic stirring, and ultrasonically 1h to form a uniform and stable solution, the concentration of graphene oxide in the graphene oxide solution is 3mg/ml, then add reducing agent thiourea and stir to dissolve, Obtain a brown solution, and the concentration of reducing agent thiourea in the brown solution is 5 mg/mL;

(2) Transfer the brown solution obtained above to an autoclave, seal the autoclave and place it at 180° C. for 10 hours, then wait for the temperature of the autoclave to drop to room temperature 25° C. to obtain a columnar three-dimensional porous graphene solution ;

(3) Add lithium sulfate and polyvinylpyrrolidone with a number average molecular weight of 58,000 to the obtained columnar three-dimensional porous graphene and the solution, and soak the columnar three-dimensional porous graphene to obtain the soaking solution of columnar three-dimensional porous graphene, columnar three-dimensional porous graphene The concentration of lithium sulfate in the soaking solution of porous graphene is 50mg/mL, and the concentration of carbon source polyvinylpyrrolidone is 200mg/mL;

(4) freezing the soaking solution of columnar three-dimensional porous graphene with liquid nitrogen, and then vacuum freeze-drying to obtain a precursor;

(5) Calcining the precursor at 900°C for 2 hours under an argon protective atmosphere to obtain a three-dimensional porous graphene-supported carbon-coated lithium sulfide material;

(6) The three-dimensional graphene-loaded carbon-coated lithium sulfide positive electrode material slice is used as the positive electrode of the battery, and the metal lithium sheet is used as the negative electrode of the battery to assemble a lithium-ion battery. What the separator uses is a polypropylene microporous membrane (Cellgard2300), and the electrolyte is With 1mol/L bis(trifluoromethane)sulfonamide lithium salt (LiTFSI) as the solute, 1,3-dioxolane (DOL) and ethylene glycol dimethyl ether (DME) with a volume ratio of 1:1 are Solvent, and 1.0wt% LiNO ₃ was added, and the battery assembly process was all completed in a glove box filled with argon and the water and oxygen content was lower than 0.1ppm.

In-situ synthesis of three-dimensional graphene-supported carbon-coated lithium sulfide cathode material Through X-ray photoelectron spectroscopy (X-ray photoelectronspectroscopy) test, it can be confirmed that the in-situ synthesized three-dimensional graphene-supported carbon-coated lithium sulfide material generates lithium sulfide. According to the thermogravimetry, it can be calculated that the lithium sulfide content is about 35%, and the carbon content is 65%.

After the assembled lithium-ion battery is placed for 24 hours, a constant current charge-discharge test is carried out. The first charge-discharge voltage is 1.5V-4V, and the subsequent charge-discharge voltage is 1.5V-3V. The positive electrode of the lithium-ion battery is cyclically measured in an environment of 25±2°C. Capacity, charge-discharge cycle performance and rate characteristics. The initial discharge capacity of the battery is 680mAh/g when the current density is 1C, and the discharge capacity after 100 cycles is 450mAh/g, showing good performance.

Claims (10)

1. a preparation method for the coated lithium sulfide positive electrode of the three-dimensional porous graphene-supported carbon of fabricated in situ, is characterized in that, comprise the following steps:

(1) graphene oxide is dispersed in water magnetic agitation, and the solution of ultrasonic formation stable homogeneous, obtain graphene oxide solution, then add reducing agent and stirring and dissolving, obtain the solution of brown;

(2) solution of step (1) gained brown is transferred in autoclave, by airtight for autoclave be placed on 120 DEG C-200 DEG C at hydro-thermal reaction 4-12h, then lower the temperature, obtain the solution of columnar three-dimensional porous graphene;

(3) in the solution of the columnar three-dimensional porous graphene of step (2) gained, add lithium sulfate and carbon source, after stirring and dissolving, columnar three-dimensional porous graphene is soaked wherein, form the soak of columnar three-dimensional porous graphene;

(4) by the soak freeze drying of columnar three-dimensional porous graphene, presoma is obtained;

(5) step (4) gained presoma is calcined 2-12h at 800-1000 DEG C under protective atmosphere, obtain the coated lithium sulfide material of three-dimensional porous graphene-supported carbon.

2. preparation method according to claim 1, is characterized in that, in step (1), in described graphene oxide solution, graphene oxide concentration is 1-5mg/mL.

3. preparation method according to claim 1, is characterized in that, in step (1), described reducing agent is one or more in thiocarbamide, citric acid, vitamin.

4. preparation method according to claim 1, is characterized in that, in step (1), in the solution of described brown, the concentration of reducing agent is 1-20mg/mL.

5. preparation method according to claim 1, is characterized in that, in step (2), and hydro-thermal reaction 8-12h at 170 DEG C-190 DEG C.

6. preparation method according to claim 1, is characterized in that, in step (3), described carbon source is one or more in glucose, sucrose, maltose, soluble starch, polyvinylpyrrolidone.

7. preparation method according to claim 1, is characterized in that, in step (3), in the soak of described columnar three-dimensional porous graphene, the concentration of lithium sulfate is at 50-200mg/mL, and the concentration of carbon source is at 50-600mg/mL.

8. the coated lithium sulfide positive electrode of three-dimensional porous graphene-supported carbon prepared by preparation method according to claims 1 to 7.

9. the coated lithium sulfide positive electrode of three-dimensional porous graphene-supported carbon according to claim 8, it is characterized in that, in the coated lithium sulfide positive electrode of described three-dimensional porous graphene carbon, lithium sulfide mass percentage is at 30%-75%, and Graphene and carbon mass percentage are at 25%-70%.

10. the coated lithium sulfide positive electrode of three-dimensional porous graphene-supported carbon is according to claim 8 or claim 9 as the application in anode.