CN113422055B - Lithium-philic graphene quantum dot/lithium composite material and preparation method and application thereof - Google Patents

Abstract

The invention belongs to the technical field of lithium battery electrode materials, and particularly relates to a lithium-philic graphene quantum dot/lithium composite material and a preparation method and application thereof. The lithium-philic graphene quantum dot/lithium composite material provided by the invention has a main body of graphene quantum dots, wherein the graphene quantum dots are composed of a carbon core of a graphite phase and a polymer short-chain shell rich in elements such as oxygen, nitrogen and sulfur, and show strong lithium ion affinity. The coating is coated on the surface of metal lithium, so that the problem of lithium ion depletion on the surface of a lithium metal negative electrode under large current can be solved, the deposition behavior of the metal lithium is changed, and the deposition stripping of the metal lithium is stabilized. The lithium-philic graphene quantum dot/lithium cathode is applied to a lithium-air full battery, and the lithium-air battery with greatly improved rate capability and cycle life can be obtained. The preparation process is simple, can realize the lithium metal cathode with long cycle life under large current and large capacity, and has good application prospect in the field of quick charge of high-energy density batteries.

Description

Lithophilic graphene quantum dots/lithium composite material, preparation method and application thereof

technical field

The invention belongs to the technical field of lithium battery electrode materials, and particularly relates to a lithiophilic graphene quantum dot/lithium composite material and a preparation method and application thereof.

Background technique

With the development of society, human's demand for resources is becoming more and more urgent. After 30 years of development, the energy density of lithium-ion batteries is close to the theoretical value (350 Wh/kg), which is difficult to meet the needs of development. The lithium metal negative electrode has an ultra-high theoretical specific capacity and low electrode potential, which matches the positive electrode with high areal capacity (such as oxygen, sulfur, metal fluoride, etc.), and can break through the energy density limit of existing lithium-ion batteries. In order to achieve high energy output and fast charging of such high energy density batteries, extremely high battery densities, such as more than 30 mA/cm2 or 3C charging rate, are often required to achieve the goal of fully charging within 20 minutes. However, at high current density, lithium ions are rapidly depleted at the metal lithium anode interface, resulting in concentration polarization inside the huge battery, resulting in uneven distribution of lithium ion flow, concentrated deposition at defects, and the formation of dendrites. The formation of dendrites can lead to rapid deterioration of battery performance, and even cause internal short circuits in the battery, which in turn lead to safety accidents such as fire and explosion.

In order to solve the dendrite growth caused by the uneven distribution of lithium ions on the surface of the above-mentioned lithium metal anode, there have been some research reports. On the one hand, the electrode structure is designed to increase the specific surface area of the electrode, which can evenly disperse the lithium ion flow. On the other hand, optimizing the electrolyte solution, such as using a high-concentration electrolyte solution, can reduce the concentration polarization inside the battery and promote the uniform deposition of lithium. However, these methods mainly focus on the macroscopic distribution of lithium ion concentration in the whole electrolyte, and do not focus on the nanoscale distribution of lithium ion concentration at the interface between lithium metal anode and electrolyte. The non-uniform distribution and depletion of lithium ions mainly occur at the electrolyte-electrode interface, and the depletion of lithium ions at the interface is more serious under high current. Therefore, the current lithium metal anodes can still only be cycled at lower current densities, which is difficult to meet the needs of next-generation high-energy-density batteries.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a lithiophilic graphene quantum dot/lithium composite material that can effectively stabilize metal lithium deposition and stripping, and a preparation method and application thereof.

The lithiophilic graphene quantum dot/lithium composite material provided by the present invention, wherein the lithiophilic graphene quantum dot has a core ^- shell structure, the core part is a graphite crystal phase composed of sp hybridized carbon, and the shell part is rich in carbon. Polymer short chains of highly electronegative elements such as oxygen, nitrogen, and sulfur: The lithophilic graphene quantum dots can adsorb positively charged lithium ions, thereby relieving the depletion of lithium ion concentration on the surface of the lithium anode and promoting lithium metal Rapid and uniform deposition at high current, and the adsorption of lithium ions can still be maintained at high capacity, achieving stable deposition and stripping of lithium metal at high capacity; in addition, the ultra-thin coating of graphene quantum dots has a high modulus, It can effectively inhibit the growth of lithium dendrites and realize the dendrite-free deposition and stripping of lithium metal.

The preparation method of the lithiophilic graphene quantum dots/lithium composite material provided by the present invention adopts a hydrothermal method, uses citric acid and thiourea as raw materials, and undergoes a polycondensation reaction in the hydrothermal process to generate lithiophilic graphene quantum dots ; Then spin-coat the lithiophilic graphene quantum dot solution onto the lithium metal surface; the specific steps are as follows:

(1) Preparation of lithiophilic graphene quantum dots:

Dissolve 0.1-0.3 g of citric acid and 0.2-0.7 g of thiourea into 5-15 ml of deionized water to obtain a reaction solution; pour the reaction solution into a hydrothermal kettle, seal it and place it in an oven, and put it in an oven at 140- Under the condition of 180 degrees Celsius, the hydrothermal reaction is carried out for 4-6 hours; after the reaction, the reaction solution is lowered to room temperature, and the pH is adjusted to 5-6 with 0.5-1.0 mol/L sodium hydroxide solution, and then 3000-4000 Daltons are used. The molecular weight cut-off dialysis bag is dialyzed for 4-8 hours, and then the lithophilic graphene quantum dots are freeze-dried for 24-48 hours to obtain the target product lithiophilic graphene quantum dot material, which is stored in an anhydrous and oxygen-free environment;

(2) Preparation of lithiophilic graphene quantum dots/lithium composites:

The lithiophilic graphene quantum dot material was dissolved in a mixed solution of organic solvent dimethyl sulfoxide and acetonitrile, and the volume ratio of dimethyl sulfoxide and acetonitrile was 3:(4-6); The dot solution is spin-coated onto the surface of lithium metal, the speed is 1000-2000 r/s, the time is 60-90 seconds, and the spin-coating is repeated 1-5 times to obtain a lithiophilic graphene quantum dot/lithium composite material; The graphene quantum dot coating is ultra-thin, with a thickness of 20-100 nanometers.

The lithiophilic graphene quantum dot/lithium composite material prepared by the invention has excellent electrochemical performance and can be used as a negative electrode in a lithium ion battery.

The lithiophilic graphene quantum dot/lithium composite material prepared by the present invention is used as a negative electrode in a symmetrical battery, and the specific steps are as follows:

The battery electrolyte is composed of lithium bistrifluoromethanesulfonimide as electrolyte salt, 1,3-dioxolane and 1,2-ethylene glycol dimethyl ether as electrolyte solvent, and lithium nitrate as electrolyte additive. 2400 is used as the separator, and the lithiophilic graphene quantum dot/lithium composite material is used as the negative electrode to assemble a symmetrical battery. Among them, the volume ratio of 1,3-dioxolane and 1,2-ethylene glycol dimethyl ether is 1:1; the concentration of lithium bis-trifluoromethanesulfonimide is 0.5-1.5 mol/L, and the mass of lithium nitrate The concentration is 2-5%;

After standing for 6-12 hours, the galvanostatic charge-discharge cycle test was performed at a current density of 1-60 mA/cm2 and a surface capacity of 1-60 mAH/cm2. The lithiophilic graphene quantum dot/lithium composite negative electrode prepared by the invention exhibits stable deposition/stripping cycle performance under high current and high capacity, and no dendrite growth.

The lithiophilic graphene quantum dot/lithium composite material prepared by the present invention can be used as a negative electrode in a lithium-air battery, and the specific steps are as follows:

The lithium metal composite electrode modified with lithiophilic graphene quantum dots is used as the negative electrode, the oriented carbon nanotube film is used as the positive electrode, lithium trifluoromethanesulfonate is used as the electrolyte salt, tetraethylene glycol dimethyl ether is used as the electrolyte solvent, and lithium iodide is used as the electrolyte solvent. As an electrolyte additive, a Waterman glass fiber filter paper was used as the separator, and a Swagelok lithium-air full battery was assembled. Wherein, the concentration of lithium trifluoromethanesulfonate is 0.5-1.5 mol/liter.

The assembled battery was placed directly in the air for testing. The electrochemical test parameters were a current density of 1000 mA/g and a specific capacity of 500 mA/g. Due to the stabilizing effect of the ultrathin coating of lithiophilic graphene quantum dots on the deposition and stripping process of metallic lithium, the growth of lithium dendrites is suppressed, and the rate performance and cycle life of lithium-air batteries are greatly improved.

Description of drawings

Figure 1 is a schematic diagram of the surface deposition process of pure lithium negative electrode and lithiophilic graphene quantum dots/lithium composite material. Among them, a, is the deposition/stripping process of lithium on the pure lithium electrode; b, is the deposition/stripping process of lithium on the lithiophilic graphene quantum dots/lithium composite negative electrode.

Figure 2 shows the characterization of lithiophilic graphene quantum dots. Among them, a, is the transmission electron microscope image of the prepared graphene quantum dots; b, is the scanning electron microscope image and optical photo of the lithiophilic graphene quantum dots/lithium composite negative electrode; c, is the lithiophilic graphene quantum dots size distribution; d, is the infrared spectrum of graphene quantum dots.

Figure 3 is an electron micrograph of the lithiophilic graphene quantum dots/lithium composite anode after cycling. Among them, a, there is no dendrite growth on the surface of lithiophilic graphene quantum dots/lithium composite negative electrode; b, dendrite growth is serious on the surface of pure lithium electrode.

Figure 4 shows the cycling performance of lithiophilic graphene quantum dots/lithium composite electrodes. Among them, a, is the cycle performance of the lithiophilic graphene quantum dot/lithium composite electrode and pure lithium anode at a current density of 20 mA/cm2 and a capacity of 40 mAh/cm2; b, is the Cycling performance of lithiophilic graphene quantum dots/lithium composite electrodes and pure lithium anodes at a current density of 60 mA/cm2 and a capacity of 60 mAh/cm2.

Figure 5 is a lithium-air battery based on lithiophilic graphene quantum dots/lithium composite anode. Among them, a, Cycling performance of a lithium-air battery based on lithiophilic graphene quantum dots/lithium composite anode. b, Rate capability of Li-air battery based on lithiophilic graphene quantum dots/lithium composite anode.

Detailed ways

Example 1

(1) Preparation of lithiophilic graphene quantum dots

Preparation of lithophilic graphene quantum dots by hydrothermal method: Dissolve 0.21 g of citric acid and 0.46 g of thiourea in 10 ml of deionized water; then pour the reaction solution into a hydrothermal kettle, seal it and place it in a vacuum oven The reaction was carried out under the condition of 160 degrees Celsius for 4 hours; after the reaction, the reaction solution was lowered to room temperature, and the pH was adjusted to 6 with 1 mol/L sodium hydroxide solution; then dialysed 6 with a dialysis bag with a molecular weight cutoff of 3500 Daltons. hours; then lyophilized graphene quantum dots were freeze-dried for 24 hours and stored in an anhydrous and oxygen-free environment;

(2) Preparation of lithiophilic graphene quantum dots/lithium composite anode

The lithiophilic graphene quantum dots were dissolved in a mixture of dimethyl sulfoxide and acetonitrile (3:4 volume ratio) at a concentration of 0.5 mg/ml, and the lithiophilic graphene quantum dot solution was spin-coated to lithium metal Surface, the speed is 1000 rpm, the time is 60 seconds, and the spin coating is 3 times;

(3) Application of lithiophilic graphene quantum dots/lithium composite anode in symmetrical batteries

Using 1 mol/L lithium bis-trifluoromethanesulfonimide as the electrolyte salt, 1,3-dioxolane and 1,2-ethylene glycol dimethyl ether (1:1 volume ratio) as the electrolyte solvent, nitric acid Lithium was used as the electrolyte additive, and Celgard 2400 was used as the separator to assemble a symmetrical battery with lithiophilic graphene quantum dot-modified lithium metal negative electrode; Under the capacity of 60 mA/cm2, the charge-discharge cycle test was carried out; the results are shown in Figure 4, under the current density of 20 mA/cm2 and the capacity of 40 mA/cm2, the lithiophilic graphite The ene quantum dot/lithium composite electrode can be cycled stably for 1200 hours, while the pure lithium electrode shows a sharp increase in voltage after 28 hours of cycling; at a current density of 60 mA/cm2 and a capacity of 60 mAh/cm2, the lithiophilic The lithiophilic graphene quantum dot/lithium composite electrode can be cycled stably for 1000 hours, while the pure lithium negative electrode cannot work normally; the lithiophilic graphene quantum dot/lithium composite negative electrode prepared by the present invention shows stable performance under high current and high capacity. Deposition/stripping cycle performance, deposition/stripping behavior without dendrite growth.

Example 2

(1) Preparation of lithiophilic graphene quantum dots

Preparation of lithiophilic graphene quantum dots by hydrothermal method: Dissolve 0.1 g of citric acid and 0.2 g of thiourea in 5 ml of deionized water; then pour the reaction solution into a hydrothermal kettle, seal it and place it in a vacuum oven The reaction was carried out under the condition of 160 degrees Celsius for 4 hours; after the reaction, the reaction solution was lowered to room temperature, and the pH was adjusted to 6 with 1 mol/L sodium hydroxide solution; then dialysed 6 with a dialysis bag with a molecular weight cutoff of 3500 Daltons. hours; then lyophilized graphene quantum dots were freeze-dried for 24 hours and stored in an anhydrous and oxygen-free environment;

(2) Preparation of lithiophilic graphene quantum dots/lithium composite anode

The lithiophilic graphene quantum dots were dissolved in a mixture of dimethyl sulfoxide and acetonitrile (3:4 volume ratio) at a concentration of 1 mg/ml, and the lithiophilic graphene quantum dot solution was spin-coated to lithium metal Surface, the speed is 1000 rpm, the time is 60 seconds, and the spin coating is 3 times;

(3) Application of lithiophilic graphene quantum dots/lithium composite anode in lithium-air batteries

The lithium metal composite electrode modified with lithiophilic graphene quantum dots was used as the negative electrode, the oriented carbon nanotube film was used as the positive electrode, 1 mol/L lithium triflate was used as the electrolyte salt, and tetraethylene glycol dimethyl ether was used as the electrolyte. Solvent, 0.5 mmol/L lithium iodide as electrolyte additive, Waterman glass fiber filter paper for diaphragm, Swagelok lithium-air full battery was assembled, and tested directly in air; electrochemical test parameters are: The current density of 1000 mA/g, the specific capacity of 500 mA/g; the results are shown in Figure 5, the lithium-air battery modified with lithiophilic graphene quantum dots can be operated at a current of 1000 mA/g The density and the specific capacity of 500 mAh/g are stable for more than 400 cycles, an increase of 6 times; due to the stabilizing effect of the ultra-thin coating of lithiophilic graphene quantum dots on the deposition and stripping process of metal lithium, the growth of lithium dendrites is affected by Inhibition, the cycle life of Li-air batteries is greatly improved.

Claims (5)

1. A preparation method of a lithium-philic graphene quantum dot/lithium composite material is characterized in that citric acid and thiourea are used as raw materials, and a polycondensation reaction is carried out in a hydrothermal process to generate the lithium-philic graphene quantum dot; spin-coating the lithium-philic graphene quantum dot solution on the surface of lithium metal; the method comprises the following specific steps:

(1) preparing lithium-philic graphene quantum dots:

dissolving 0.1-0.3 g of citric acid and 0.2-0.7 g of thiourea in 5-15 ml of deionized water to obtain a reaction solution; pouring the reaction liquid into a hydrothermal kettle, sealing and then placing the hydrothermal kettle into an oven, and carrying out hydrothermal reaction for 4-6 hours at the temperature of 140-; after the reaction is finished, cooling the reaction solution to room temperature, adjusting the pH value to 5-6 by using 0.5-1.0 mol/L sodium hydroxide solution, dialyzing for 4-8 hours by using a 3000-plus-4000 Dalton dialysis bag with molecular weight cut-off, and freeze-drying the lithium-philic graphene quantum dots for 24-48 hours to obtain a target product lithium-philic graphene quantum dot material;

(2) preparing a lithium-philic graphene quantum dot/lithium composite material:

dissolving a lithium-philic graphene quantum dot material in a mixed solution of an organic solvent dimethyl sulfoxide and acetonitrile, wherein the volume ratio of the dimethyl sulfoxide to the acetonitrile is 3 (4-6); spin-coating the lithium-philic graphene quantum dot solution on the surface of the lithium metal at the rotation speed of 1000-2000 rpm for 60-90 seconds, and repeating the spin-coating for 1-5 times to obtain the lithium-philic graphene quantum dot/lithium composite material; wherein the thickness of the lithium-philic graphene quantum dot coating is 20-100 nanometers.

2. The lithium-philic graphene quantum dot/lithium composite material prepared by the preparation method of claim 1, wherein the graphene quantum dot has a core-shell structure, and the core part is sp²A graphite crystal phase consisting of hybridized carbon, and a shell part is a polymer short chain rich in oxygen, nitrogen and sulfur high electronegativity elements: the graphene quantum dot has high lithium ion affinity and can adsorb lithium ions, so that lithium on the surface of a lithium metal negative electrode is improvedThe problem of ion concentration depletion is solved, and the uniform and rapid deposition of the metal lithium is promoted; in addition, the lithium-philic graphene quantum dot ultrathin coating has high mechanical modulus and can effectively inhibit the growth of lithium dendrites.

3. The lithium-philic graphene quantum dot/lithium composite material as claimed in claim 2, which is used as a negative electrode material of a lithium ion battery.

4. The use according to claim 3, in a symmetrical battery, comprising the following steps:

the battery electrolyte is formed by taking lithium bistrifluoromethanesulfonimide as electrolyte salt, 1, 3-dioxolane and 1, 2-glycol dimethyl ether as electrolyte solvents and lithium nitrate as electrolyte additives; the method comprises the following steps of (1) assembling a symmetrical battery by taking Celgard 2400 as a diaphragm and taking a lithium-philic graphene quantum dot/lithium composite material as a negative electrode; wherein the volume ratio of the 1, 3-dioxolane to the 1, 2-glycol dimethyl ether is 1 (1-2); the concentration of the lithium bis (trifluoromethanesulfonyl) imide is 0.5-1.5 mol/L, and the mass concentration of the lithium nitrate is 2-5%.

5. The use according to claim 3, in a lithium-air battery, comprising the following steps:

assembling a diaphragm by using Woltmann glass fiber filter paper to obtain a Shiviaoke lithium-air full battery by using a lithium-philic graphene quantum dot/lithium composite material as a negative electrode, an oriented carbon nanotube film as a positive electrode, lithium trifluoromethanesulfonate as an electrolyte, tetraethylene glycol dimethyl ether as an electrolyte solvent and lithium iodide as an electrolyte additive; wherein the concentration of the lithium trifluoromethanesulfonate is 0.5-1.5 mol/L.

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